Ecofriendly Application of Calabrese Broccoli Stalk Waste as a Biosorbent for the Removal of Pb(II) Ions from Aqueous Media

Abstract

A new biosorbent obtained from Calabrese broccoli stalks has been prepared, characterised and used as an effective, low-cost and ecofriendly biomass to remove Pb(II) from aqueous solutions, without any complicated pretreatment. Structural and morphological characterisation were performed by TGA/DGT, FTIR and SEM/EDX; the main components are hemicellulose, starches, pectin, cellulose, lignin and phytochemicals, with important electron donor elements (such as S from glucosinolates of broccoli) involved in Pb(II) sorption. The biosorbent showed values of 0.52 and 0.65 g mL−1 for bulk and apparent densities, 20.6% porosity, a specific surface area of 15.3 m2 g−1, pHpzc 6.25, iodine capacity of 619 mg g−1 and a cation exchange capacity of 30.7 cmol kg−1. Very good sorption (88.3 ± 0.8%) occurred at pH 4.8 with a biomass dose of 10 g L−1 after 8 h. The Freundlich and Dubinin–Radushkevich isotherms and the pseudo-second-order kinetic models explained with good fits the favourable Pb(II) sorption on the heterogeneous surface of broccoli biomass. The maximum adsorption capacity was 586.7 mg g−1. The thermodynamic parameters evaluated showed the endothermic and spontaneous nature of the Pb(II) biosorption. The chemical mechanisms mainly involved complexation, ligand exchange and cation–π interaction, with possible precipitation.

Full text

Citation: Granado-Castro, M.D.;
Galindo-Riaño, M.D.; Gestoso-Rojas,
J.; Sánchez-Ponce, L.;
Casanueva-Marenco, M.J.;
Díaz-de-Alba, M. Ecofriendly
Application of Calabrese Broccoli Stalk
Waste as a Biosorbent for the Removal
of Pb(II) Ions from Aqueous Media.
Agronomy 2024,14, 554. https://
doi.org/10.3390/agronomy14030554
Academic Editors: Zhen Li, Da Tian
and Haoming Chen
Received: 17 February 2024
Revised: 5 March 2024
Accepted: 6 March 2024
Published: 8 March 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
agronomy
Article
Ecofriendly Application of Calabrese Broccoli Stalk Waste as a
Biosorbent for the Removal of Pb(II) Ions from Aqueous Media
María Dolores Granado-Castro , María Dolores Galindo-Riaño , Jesús Gestoso-Rojas, Lorena Sánchez-Ponce,
María JoséCasanueva-Marenco * and Margarita Díaz-de-Alba
Department of Analytical Chemistry, Institute of Biomolecules (INBIO), Faculty of Sciences, International
Campus of Excellence of the Sea (CEI-MAR), University of Cadiz, Campus Rio San Pedro, 11510 Puerto Real,
Cadiz, Spain; dolores.granado@uca.es (M.D.G.-C.); dolores.galindo@uca.es (M.D.G.-R.);
jesus.gestoso@gmail.com (J.G.-R.); lorena.sanchezponce@alum.uca.es (L.S.-P.); margarita.diaz@uca.es (M.D.-d.-A.)
*Correspondence: mariajose.casanueva@uca.es; Tel.: +34-956-016-358
Abstract: A new biosorbent obtained from Calabrese broccoli stalks has been prepared, characterised
and used as an effective, low-cost and ecofriendly biomass to remove Pb(II) from aqueous solu-
tions, without any complicated pretreatment. Structural and morphological characterisation were
performed by TGA/DGT, FTIR and SEM/EDX; the main components are hemicellulose, starches,
pectin, cellulose, lignin and phytochemicals, with important electron donor elements (such as S from
glucosinolates of broccoli) involved in Pb(II) sorption. The biosorbent showed values of 0.52 and
0.65 g mL
−1
for bulk and apparent densities, 20.6% porosity, a specific surface area of
15.3 m2g−1
,
pH
pzc
6.25, iodine capacity of 619 mg g
−1
and a cation exchange capacity of 30.7 cmol kg
−1
. Very
good sorption (88.3
±
0.8%) occurred at pH 4.8 with a biomass dose of 10 g L
−1
after 8 h. The
Freundlich and Dubinin–Radushkevich isotherms and the pseudo-second-order kinetic models ex-
plained with good fits the favourable Pb(II) sorption on the heterogeneous surface of broccoli biomass.
The maximum adsorption capacity was 586.7 mg g
−1
. The thermodynamic parameters evaluated
showed the endothermic and spontaneous nature of the Pb(II) biosorption. The chemical mechanisms
mainly involved complexation, ligand exchange and cation–
π
interaction, with possible precipitation.
Keywords: lead; broccoli stalk; reuse of agricultural waste; biosorption; green; sustainability; sorption
mechanisms; aqueous polluted samples
1. Introduction
The handling, storage and/or disposal of the large amount of food and agro-industrial
waste generated at different stages of the production process is a well-known global
problem. In most cases, they are neither processed nor disposed of properly, a situation
that contributes to the increase in environmental pollution. Paradoxically, these wastes
have an intrinsic potential to be exploited in different processes, including the manufacture
of new products (such as in animal feed production), the reuse as feedstock for biofuels
(bioethanol, biodiesel, biogas, energy biomass) and, alternatively, their use as sorbents in
bioremediation for the removal of contaminants (for example, in the removal of heavy
metals, dyes or hydrocarbons) in order to recover the state of the environment that has
been damaged [
1
–
7
]. The production of fertilizers or micronutrient dietary supplements
for animals or plants is also an attractive recent use of biomass, which is used when the
enrichment of biomass is carried out in nutritionally significant elements [8].
The presence of heavy metals in aqueous media such as aquatic environments, wastew-
ater or industrial effluents is an important ecological problem due to their toxic nature and
their ability to accumulate in the food chain. These elements affect the water quality and
tend to accumulate in living organisms, resulting in various disorders and diseases in the
ecosystems. Among them, lead is a prominent pollutant with drastic health consequences.
It is a cumulative toxicant that affects multiple body systems, principally harmful to young
Agronomy 2024,14, 554. https://doi.org/10.3390/agronomy14030554 https://www.mdpi.com/journal/agronomy

Agronomy 2024,14, 554 2 of 22
children, and can cause anaemia, kidney malfunction, brain tissue damage, and even death
in cases of severe poisoning [
9
]. Lead pollution in water not only occurs naturally but can
also be due to anthropogenic activities, such as battery manufacturing, metal plating and
finishing, glass manufacturing, anti-knocking synthesis, ceramic industries and others [
10
].
According to the WHO guidelines for drinking-water quality, the maximum tolerable
limit is 0.005 mg L
−1
(24 nmol L
−1
) for total lead amounts; moreover, the Environmental
Protection Agency (EPA) allows the permissible limit of 0.05 mg L
−1
(0.24
µ
mol L
−1
) for
lead in wastewater. However, the usual content of lead in industrial wastewater ranges
from 200 to 500 mg L−1, exceeding water quality standards [11].
The use of ecofriendly biomaterials for sorption processes applied in the cleaning of
waters contaminated by heavy metals, such as lead, has gained huge attention [
12
]. The
ideal sorbent for wide application needs to be inexpensive, available in large quantities,
non-toxic, with little or no processing, and with known kinetic parameters and good
sorption characteristics [
13
]. In recent years, these studies have been focused on the use
of waste materials, especially those obtained from food and agricultural waste as citrus
peel, olive stone, groundnut shell, potato or cucumber peels, barley straw, cranberry kernel
shell, tea waste, potato, strawberry or canola stems, among others [
2
,
7
,
14
–
20
]. This type of
sorbents offers great potential for innovation in metal removal from aqueous media at a
very low cost [21].
Agro-materials are usually composed of lignin and cellulose as main constituents
and may also include other polar functional groups, such as alcohols, aldehydes, ketones,
carboxylic, phenolic and ether groups, with an average composition of 40–50% cellulose,
20–30% hemicellulose, 20–25% lignin and 1–5% ash [
22
–
24
]. The removal of heavy metals
in aqueous media will depend on the sorption capacity of these functional groups towards
metal ions, among other different factors.
Broccoli (Brassica oleracea var. italica) is a valuable plant from the Brassicaceae (or
Cruciferae) family (genus Brassica); the floret is the main edible part of this vegetable.
The broccoli plant has several components, standing out the glucosinolates (mainly gluco-
raphanin and sulforaphane) [
25
], like other cruciferous vegetables; these sulphur-containing
compounds give broccoli its characteristic odour and taste. The chemical composition of
broccoli stalk also includes soluble and insoluble fibre (pectins, hemicellulose, cellulose,
sugars, starch, lignin, etc.), minerals (K, Ca, Mg, P, Fe, etc.), vitamins (A, C (ascorbic acid), K
and some B vitamins like B9 (folate)) and other phytochemicals (organic acids, organophos-
phorus and phenolic compounds such as flavonoids, phytates, etc.) and aminoacids
(particularly, phenylalanine, tryptophan and cysteine) [
26
–
30
]. It has acquired a consider-
able relevance in the last few years as a health-promoting food, rich in antioxidants and
anti-inflammatories. However, broccoli marketable florets (flower heads) represent only
a minor part of the total above-ground plant biomass (<25% of the total yield), which
generates a huge amount of agricultural waste [
31
,
32
]. So far, the use of broccoli waste is
restricted to flour and fibre, standard extraction or characterisation of glucosinolates, and
dairy cattle feed production [
33
–
36
]. The waste produced from broccoli by horticulture
represents around 60–75% of the broccoli production [
37
,
38
]. Considering the worldwide
production in 2017 (25,984,758 tonnes, reports combined with cauliflower) and if assuming
that 60% of production is wasted, more than 15 million tonnes of waste were generated
in 2017 [
28
]. Research studies that used broccoli stalk waste as sorbents of heavy metals
are scarce in the literature. So far, there is only one study that focuses in the application of
broccoli stalk waste (as raw material and as a precursor of carbon-based sorbents) in the
removal of Cd(II), Ni(II), Cu(II) and Zn(II) [39].
Therefore, this study aims to add value to broccoli stalk waste by using it as a biosor-
bent for the removal of toxic metals, such as Pb(II) ions, from aqueous solutions. The
application of this biomass in sorption processes can be an interesting and ecofriendly reme-
diation strategy to clean polluted environments, to be applied to treat industrial wastewater
or effluents, or for any other separation purposes.

Agronomy 2024,14, 554 3 of 22
2. Experimental
2.1. Chemical Reagents and Equipment
Different chemical reagents of analytical grade were used according to the methodolo-
gies to be applied or the parameters to be analysed: (a) iodine adsorption capacity: iodine
(0.01 mol L
−1
I
2
solution, Panreac, Barcelona, Spain), sodium thiosulfate pentahydrate
(Na
2
S
2
O
3·
5H
2
O, Panreac, Barcelona, Spain), starch (1% (C
6
H
10
O
5
)
n
for HPLC, Panreac,
Barcelona, Spain), potassium iodide (KI, Merck, Darmstadt, Germany) and potassium
dichromate (K
2
Cr
2
O
7
, Merck, Darmstadt, Germany); (b) acid and basic surface groups,
the pH value at the point of zero charge (pH
pzc
) and the cation exchange capacity of the
biomass: sodium hydroxide (NaOH), potassium hydrogen phthalate (KC
8
H
5
O
4
), sodium
carbonate (Na
2
CO
3
), sodium hydrogen carbonate (NaHCO
3
), hydrochloric acid (HCl, 37%),
barium acetate (Ba(CH
3
COO)
2
), silver nitrate (AgNO
3
) and sodium chloride (NaCl) (Pan-
reac, Barcelona, Spain); (c) extraction of natural fats and oils: n-hexane (hexane mixture of
isomers RS for HPLC Isocratic Grade, Carbo-Erba, Sabadell, Spain); (d) sorption experi-
ments: solutions of Pb(NO
3
)
2
(100%, Panreac, Barcelona, Spain) as Pb(II) precursor and
acetate buffer (prepared conventionally using acetic acid (CH
3
COOH, 96%, Merck, Darm-
stadt, Germany) and sodium hydroxide (NaOH, Panreac, Barcelona, Spain)); (e) standard
solutions of the calibration curves for metal analysis: dilutions of an ICP standard solution
of 1000 mg L−1Pb in 2–3% HNO3(Certipur, Merck, Darmstadt, Germany).
Pure (type II, <1
µ
S cm
−1
) or ultrapure water (type I, 18.2 M
Ω
cm) was used as
required for aqueous solutions and obtained by an Autwomatic system coupled with an
Ultramatic Plus system (Wasserlab, Barbatáin, Spain).
A wide variety of instrumental equipment was used: (a) basic equipment: a J.P. Selecta
oven (Selecta, Barcelona, Spain), a Crison Basic 20+ pH-meter with a 50–10 T combined
glass-Ag/AgCl wire (Crison Instruments, Barcelona, Spain), an IKA HS 501 D open air
laboratory shaker platform (Labortechnik, Wasserburg am Bodensee, Germany) and a D95
Dinko vacuum pump (Dinko Instruments, Barcelona, Spain); (b) advanced instrumentation:
a TGA7 Thermogravimetric Analyzer (PerkinElmer, Waltham, MA, USA), a Shimadzu
IRAffinity-1S Fourier transform infrared spectrophotometer (Shimadzu Corporation, Kyoto,
Japan) with the PIKE MIRacle™ ATR sampling accessory (PIKE Technologies, Fitchburg,
WI, USA), the SEM-FEI Nova NanoSEM 450 with the secondary electrons detector (TLD-SE)
(Nova, Fort Worth, TX, USA) and the EDAX detector 100 mm
2
surface (AMETEK
®
, Newark,
DE, USA), the Microtrac NANOTRAC Wave DLS (dynamic light scattering) analyser, and
an iCE 3000 series Atomic Absorption Spectrometer (Thermo Scientific, Waltham, MA,
USA).
The data modelling for isotherm, kinetic and thermodynamic studies was processed
on Excel 2016 program (Microsoft, Redmond, DC, USA).
2.2. Preparation of Broccoli Stalks as a Natural Sorbent
Calabrese broccoli was purchased at a local market. The broccoli stalk and big stems
present in the vegetable were separated, washed several times with pure water and chopped
into small chunks. After that, they were oven-dried at 45
◦
C until constant weight, crushed
and ground in a mortar. Biomass particles of 125 < x < 249
µ
m and x < 125
µ
m sizes
were obtained by using 60-mesh and 120-mesh nylon sieves (CISA, Barcelona, Spain),
respectively. A portion of the biomass was defatted applying the Soxhlet method. For that,
6 g of dry broccoli biomass were extracted by heating to reflux with 130 mL of n-hexane for
8 h. The defatted and non-defatted biomass obtained were stored dry until further studies.
2.3. Methodologies for Biomass Characterisation
The following methodologies and techniques were applied for the characterisation
of the biomass using the 125 < x < 249
µ
m particle size fraction. Thermogravimetric and
derivative thermogravimetric analyses (TGA and DTG) were performed under nitrogen
atmosphere at 10
◦
C min
−1
until 850
◦
C. The FTIR spectra in attenuated total reflection
mode (ATR) were obtained with a 4 cm
−1
resolution in the region from 4000 to 650 cm
−1
. A

Agronomy 2024,14, 554 4 of 22
scanning electron microscope (SEM) (operated at an accelerating voltage of 5 kV) provided
images of the biomass surface and a coupled energy-dispersive X-ray spectrometer (EDXS)
allowed the microanalysis of the raw and Pb-loaded biomass after the sorption process.
The micrographs were taken with the sample previously deposited onto a carbon grid and
coated by sputtering with a 15 nm gold layer.
The procedures for the determinations of the iodine adsorption capacity [
40
], cation
exchange capacity (CEC) [
41
], functional groups present in the biomass [
42
,
43
], biomass
pH and conductivity [
44
], point of zero charge (pH
pzc
) [
45
], bulk density (BD) [
40
] and
apparent density (AD) [
46
] are described in Table S1 of the Supplementary Material. The
percentage of porosity was calculated according to the following equation [47]:
porosity (%)=1−BD
AD ×100 (1)
The specific surface area (SSA) was evaluated by dynamic light scattering measure-
ments. All the determinations were performed in duplicate.
2.4. Metal Removal Experiments
The capacity of the biomass for Pb(II) removal was determined by batch experiments.
The broccoli biomass was added to 50 mL of an aqueous solution containing different
1
metal concentrations according to each study. The pH of the solution was adjusted to
the selected value with 0.1 mol L
−1
acetate buffer for each experiment. Experiments
were performed in duplicate using 100 mL polypropylene containers with a screw cap
and the suspensions were shaken on an orbital shaker at 200 rpm under a controlled
temperature. The suspensions were filtered using a vacuum pump, a glass filter holder and
4.7 cm (
ϕ
) glass microfiber filters (Whatman
®
, Maidstone, UK). After the experiments, the
concentrations of Pb(II) in the filtered suspensions were analysed by atomic absorption
spectroscopy.
The percentage of Pb(II) removal by the biomass was determined as follows:
%Pb removal =C0−Ct
C0
×100 (2)
where C
0
and C
t
were the initial concentration and t-time concentration of Pb(II) ions
(mg L−1) in the aqueous solution, respectively.
The sorption capacity (q
t
, mg g
−1
), defined as the amount of Pb(II) ions (mg) sorbed
per unit weight of sorbent (g), was calculated from the following equation:
qt=(C0−Ct)×V
m(3)
where mwas the mass of the biosorbent (g) and Vwas the volume of the solution (L).
Finally, the equilibrium sorption capacity (q
e
, mg g
−1
) was obtained according to the
following equation:
qe=(C0−Ce)×V
m(4)
where Ceis the concentration of Pb(II) (mg L−1) in the aqueous solution at equilibrium.
2.5. Sorption Isotherms
The sorption isotherms were performed by adding 0.5 g of biomass (particle size of
0.125 mm < x < 0.249 mm) to 50 mL of aqueous metal solution at pH 4.8 and 23
◦
C for 8 h
(equilibrium time) following the procedure described in Section 2.4. The initial concentra-
tions of Pb(II) used were 0.25; 0.5; 1 and 1.5 mmol L
−1
. The fit of the experimental data with
the linear forms of the Langmuir, Freundlich, Dubinin–Radushkevich (D-R) and Temkin
isotherm models was studied (equations in Supplementary Material Table S2) [48–51].

Agronomy 2024,14, 554 5 of 22
2.6. Sorption Kinetics
Pseudo-first-order (or Lagergen kinetic model), pseudo-second-order, Elovich, Ritchie’s
second-order, first-order reversible and intraparticle diffusion models were applied to study
the dynamic sorption process by broccoli biomass. The kinetic equations, their linear forms and
the axes of the plots are summarised in Table S3 of the
Supplementary Material [49,52–56]
.
The experiments were carried out for different time periods (n= 10) under the same
conditions as the sorption isotherms study using 0.25, 0.5, 1 and 1.5 mmol L
−1
Pb(II)
solutions.
2.7. Thermodynamic Studies
The thermodynamic parameters of the sorption process were studied through batch
experiments carried out at different temperatures (18, 23 and 30
◦
C) under the same
conditions as the study of sorption isotherms using 1 mmol L
−1
Pb(II) solutions for 8 h.
The equations for the changes of standard Gibbs free energy, enthalpy and entropy, and
also the activation energy and the sticking probability of biomass [57], are shown in Table
S4 of the Supplementary Material.
3. Results and Discussion
3.1. Previous Studies to Select the Biomass Preparation
The aim of this study was to propose an ecofriendly use of broccoli stalks waste as a
biosorbent for Pb(II) ions in aqueous solution. One aspect to take into account was to eval-
uate the suitability of a short preparation. To achieve this, the drying of biomass (particle
size: x < 125
µ
m) was studied using two different treatments: (1) at low temperature, oven-
dried at 45
◦
C until constant weight and (2) applying a freeze-drying process. The batch
Pb(II) sorption experiments, as described in 2.4, were carried out during 24 h and a ratio
biomass/aqueous solution of 0.5 g/50 mL (10 g L
−1
) was applied. The results obtained were
comparable, yielding removal rates of 83.2
±
0.8% and 84.3
±
1.3%, respectively. A compar-
ison was drawn between the removal efficiencies of particles sizes of
125 < x < 249 µm
and
x < 125
µ
m; these were oven-dried in both cases. The percentage of Pb(II) sorption exhib-
ited negligible variations with particle size. However, a slightly higher removal efficiency
of
88.5 ±0.5%
was achieved with the larger particle size (
0.125 mm < x < 0.249 mm
). On
the other hand, it was demonstrated that the defatting process did not contribute to the
enhancement of metal sorption. As a result, the amount of Pb(II) sorbed by the broccoli
biomass after fat and oil extraction was found to be 88.0
±
0.8%. Therefore, further ex-
periments were carried out employing the oven-dried broccoli biomass at 45
◦
C using a
particle size of 125 < x < 249
µ
m. In this way, the pretreatment was very efficient, marked
by a minimal consumption of resources, with only a few steps, being a simple process.
3.2. Sorbent Characterisation
The results for the characterisation of the sorbent are shown in Figures 1–4and Table 1.
Figure 1shows the TGA/DTG profiles obtained for broccoli biomass where different
processes can be distinguished, contributing to a total weight loss of 89.9%: (a) the first loss
of 11% due to moisture removal, meaning the presence of significant hydrophilic functional
groups until 148
◦
C [
58
]; (b) the loss of 47.9% up to 400
◦
C and 16.4% up to 500
◦
C due to
the removal of volatile materials (principally hemicellulose and cellulose, respectively); and
(c) the pyrolysis of the more stable lignins in the range of 500–600
◦
C with a loss of 8.0%;
this high thermal stability of lignin may be due to the formation of pseudolignin [
59
]. Also,
the pyrolysis of char produced from the thermal degradation of pectins and hemicelluloses
may be involved in this loss of weight [
60
]. The pyrolysis analysis was over at 850
◦
C with
a low weight loss of 6.7%. The ash weight at 850 ◦C was 9.7% of the initial biomass.

Agronomy 2024,14, 554 6 of 22
Agronomy 2024, 14, x FOR PEER REVIEW 7 of 23
could be attributed to the aromatic C=C bonds, such as those in glucosinolates, and
showed a small shift after lead sorption to a lower wavenumber [25]. The peak at 2335
cm−1 was attributed to N–H stretching vibrations or to C=O stretching vibrations where
oxygen or nitrogen could be involved in Pb(II) sorption, increasing the intensities of the
IR spectrum after metal exposure [67].
Figure 1. Thermogravimetric analysis of broccoli stalk biomass.
Figure 3 shows SEM micrographs of broccoli stalk biomass before and after Pb(II)
ions sorption and Figure 4 includes the EDX analysis. The SEM analysis revealed the
presence of particles with irregular sizes and non-rounded shapes, featuring fibrous
structures with internal voids. After the metal sorption by the biomass, a slightly
degraded and fluffy surface was observed. This degradation can be attributed to the
erosion caused by solution during agitation, likely associated with the hydrolysis of
natural fibers, the exchange of Na ions from the buffer solution, and the interactions
with the Pb(II) ions. The weight percentages of the chemical composition of the raw
biomass were as follows: 45.06% of C, 40.84% of O, 6.06% of K, 3.46% of N, 2.18% of Cl,
1.13% of Na, 0.38% of S, 0.33% of Ca, 0.25% of P, 0.2% of Mg and 0.11% of Cu (Figure 4a),
according to the nature of broccoli stalks described in the introduction (e.g., sulfur-,
nitrogen- and phosphorus-containing organic compounds (glucosinolates, proteins,
phytic acid), minerals (Ca, K, Mg, P), etc.) [68]. In the same sense, the O/C ratio of 0.91
was closed to the unit, showing good hydrophilic properties of the biomass due to
carboxylic, alcohols, ethers and aldehydes groups with a high degree of chemical
functionality. This value can be provided from broccoli stalk components such as
cellulose, hemicellulose, starch and pectin with an O/C ratio of 0.83, sugars (fructose,
sucrose, mannose…) with O/C ratio of 1, or different carboxylic acids (lactic, gallic,
malic, citric, succinic…) with an O/C of ≈1. The lower content of lignin in broccoli
biomass is consistent with the lower O/C ratio of 0.35 described in the literature for this
organic polymer [69,70]. These results suggest the adequate use of raw broccoli stalk
biomass to produce chemical sorbents for ions in aqueous solutions rather than to
produce bioenergy. The N/S ratio of 9.1 obtained was similar to that found in crops of
Brassica species and associated with the glucosinolates concentration when fertilization
treatments were used [71]. The comparison of the elemental analysis before and after the
sorption of Pb(II) ions showed the presence of the loaded metal in the biomass (see
spectra in Figure 4) and the effect of the presence of Na(I) ions from the acetate buffer.
-10
-7,5
-5
-2,5
00
25
50
75
100
0 200 400 600 800
DTG: Weight (%/min)
TGA: Weight (%)
Temperature (
o
C)
TGA
DTG
−
10
−
7.5
−
5
−
2.5
0
Figure 1. Thermogravimetric analysis of broccoli stalk biomass.
Table 1. Characterisation of broccoli stalk biomass.
Parameter Average Value SD
Iodine adsorption capacity (mg I2g−1biomass) 619.7 33.3
Cation exchange capacity, CEC
(meq H+exchange 100 g−1biomass = cmol kg−1
biomass)
30.7 0.01
Functional groups (mmol g−1)
Basic 1.085 0.021
Acid 1.257 0.012
Carboxylic group 1.216 0.012
Lactonic group 0.006 0.013
Phenolic group 0.035 0.013
pH of biomass 6.88 0.02
pHpzc 6.25 0.02
Conductivity (µS cm−1)1566 22
Bulk density, BD (g mL−1)0.52 0.01
Apparent density, AD (g mL−1)0.65 0.03
Porosity (%) 20.6 3.2
Specific surface area, SSA (m2g−1)15.3 2.7
Several characteristics of FTIR spectra were observed related to the main compounds
of broccoli stalk: hemicellulose, starches, pectin, cellulose, lignin and some phytochem-
icals. The absorption intensities generally increased for Pb-loaded biomass (decreasing
transmittance) (Figure 2). Aldehyde and ketone C–O stretching, alkyl C–H stretching, and
alcohol O–H stretching from carbohydrates were associated with peaks at 1022 cm
−1
and
2910 cm
−1
, and with a broad band around 3320 cm
−1
, respectively [
25
]. The alteration of
this broad band suggests that Pb(II) ions compete with the hydrogen bonding between OH
groups associated with cellulose and proteins of the biomass [
61
]. This behaviour has been
considered as a ligand exchange process and it occurs when hydroxyl groups exist on the
adsorbent as covalent bonds and not ions [
62
]. The peak around 1080 cm
−1
, corresponding
to the symmetrical stretching vibrations of the C–O–C ether bond and related to the glyco-
sidic linkage in polysaccharides, was affected by Pb(II) sorption, as well as the small peak at
900 cm
−1
. Similarly, the peak around 1730 cm
−1
was attributed to C=O stretching present
in some carbohydrates, such as acetyl groups in hemicellulose, in flavonoids or phenolic
acids. The peaks between 2130 and 2058 cm
−1
were correlated with asymmetric stretching

Agronomy 2024,14, 554 7 of 22
of N=C=S of glucosinolates [
63
,
64
]. The C=S stretching vibration was also identified by the
peak at 1022 cm
−1
that shifted after Pb(II) sorption [
65
]. The peaks between 1200 cm
−1
and
1400 cm
−1
related to the symmetric and asymmetric stretch of S=O were also found to be
characteristic of this type of glucosides and the interaction of this group with the metal was
shown with higher absorption intensities [
25
]; also, they could be related to the P=O stretch
of organophosphorus compounds such as phytates [
66
]. FTIR analysis also showed the
absorption bands of C–H bending vibrations at 1352 cm
−1
of alkyl, aliphatic and aromatic
groups from cellulose, hemicellulose and lignin [
29
]. In addition, the peak at 1600 cm
−1
could be attributed to the aromatic C=C bonds, such as those in glucosinolates, and showed
a small shift after lead sorption to a lower wavenumber [
25
]. The peak at
2335 cm−1
was
attributed to N–H stretching vibrations or to C=O stretching vibrations where oxygen or
nitrogen could be involved in Pb(II) sorption, increasing the intensities of the IR spectrum
after metal exposure [67].
Agronomy 2024, 14, x FOR PEER REVIEW 8 of 23
K(I) ions were exchanged by Na(I) ions, increasing the Na(I) weight % from 1.13 to 5.83
and decreasing the K(I) weight % from 6.06 to 1.33. A slight exchange of Mg(II) and
Ca(II) ions by Pb(II) could also take place, resulting in the decrease in the weight % from
0.2 to 0.01 and from 0.33 to 0.25, respectively.
Figure 2. FTIR spectra of broccoli stalk biomass before and after Pb(II) sorption.
The iodine capacity (I, mg g−1) can provide significant information about the surface
of powder materials. It is defined as the mass of iodine in mg that is consumed by grams
of a chemical substance and is often used to determine the amount of unsaturation in the
form of double bonds; consequently, the higher the iodine capacity, the higher the
number of C=C bonds present. In the present work, the iodine capacity of the biomass
was found to be 619.7 ± 33.3 mg g−1 (Table 1). This result is higher than that of other
biosorbents used for Pb(II) removal, such as coconut-shell-activated carbon (334.2 mg
g−1) [72], pine cone powder (23.7 mg g−1) [40] or rice husk (68 mg g−1) [73], showing the
great potential of the Calabrese broccoli stalk biomass as a sorbent.
An intermediate CEC value of 30.7 ± 0.01 cmol kg−1 was found for broccoli biomass,
being suitable for the sorption of divalent ions by exchange. This value is comparable to
the CEC range of 11.7–34.8 cmol kg−1 described for agricultural biochar [74]. Higher
values can be found for other biomass such as green compost (67.8 cmol kg−1) and peat
(84.6 cmol kg−1) [75] with significant exchange properties for Pb(II). Lower values can
also be found for biochar from mushroom compost (5.76 cmol kg−1) [76] or for woody
biomass (1.3–3.0 cmol kg−1) with lower mineral contents [74], indicating the minor role of
ion exchange during the metal adsorption.
92
96
100
65013201990266033304000
Transmittance % (a.u.)
Wavenumber (cm
-1
)
Biomass
Biomass + Pb
Wave number (cm−1)
Figure 2. FTIR spectra of broccoli stalk biomass before and after Pb(II) sorption.
Figure 3shows SEM micrographs of broccoli stalk biomass before and after Pb(II) ions
sorption and Figure 4includes the EDX analysis. The SEM analysis revealed the presence
of particles with irregular sizes and non-rounded shapes, featuring fibrous structures with
internal voids. After the metal sorption by the biomass, a slightly degraded and fluffy
surface was observed. This degradation can be attributed to the erosion caused by solution
during agitation, likely associated with the hydrolysis of natural fibers, the exchange of
Na ions from the buffer solution, and the interactions with the Pb(II) ions. The weight
percentages of the chemical composition of the raw biomass were as follows: 45.06% of C,
40.84% of O, 6.06% of K, 3.46% of N, 2.18% of Cl, 1.13% of Na, 0.38% of S, 0.33% of Ca, 0.25%
of P, 0.2% of Mg and 0.11% of Cu (Figure 4a), according to the nature of broccoli stalks
described in the introduction (e.g., sulfur-, nitrogen- and phosphorus-containing organic
compounds (glucosinolates, proteins, phytic acid), minerals (Ca, K, Mg, P), etc.) [
68
]. In
the same sense, the O/C ratio of 0.91 was closed to the unit, showing good hydrophilic
properties of the biomass due to carboxylic, alcohols, ethers and aldehydes groups with
a high degree of chemical functionality. This value can be provided from broccoli stalk
components such as cellulose, hemicellulose, starch and pectin with an O/C ratio of 0.83,
sugars (fructose, sucrose, mannose
. . .
) with O/C ratio of 1, or different carboxylic acids
(lactic, gallic, malic, citric, succinic
. . .
) with an O/C of
≈
1. The lower content of lignin in
broccoli biomass is consistent with the lower O/C ratio of 0.35 described in the literature
for this organic polymer [
69
,
70
]. These results suggest the adequate use of raw broccoli

Agronomy 2024,14, 554 8 of 22
stalk biomass to produce chemical sorbents for ions in aqueous solutions rather than to
produce bioenergy. The N/S ratio of 9.1 obtained was similar to that found in crops of
Brassica species and associated with the glucosinolates concentration when fertilization
treatments were used [
71
]. The comparison of the elemental analysis before and after
the sorption of Pb(II) ions showed the presence of the loaded metal in the biomass (see
spectra in Figure 4) and the effect of the presence of Na(I) ions from the acetate buffer. K(I)
ions were exchanged by Na(I) ions, increasing the Na(I) weight % from 1.13 to 5.83 and
decreasing the K(I) weight % from 6.06 to 1.33. A slight exchange of Mg(II) and Ca(II) ions
by Pb(II) could also take place, resulting in the decrease in the weight % from 0.2 to 0.01
and from 0.33 to 0.25, respectively.
Agronomy 2024, 14, x FOR PEER REVIEW 9 of 23
Figure 3. SEM micrographs of the surface morphology of broccoli stalk biomass before (a) and
after (b) Pb(II) sorption.
Regarding the functional groups on the biomass surface, the results showed that the
acid groups (ag: 1.257 ± 0.012 mmol g
−1
) were slightly higher than the basic ones (bg:
1.085 ± 0.021 mmol g
−1
) (Table 1). Carboxylic groups (1.216 ± 0.012 mmol g
−1
) were
predominant compared to phenolic (0.035 ± 0.013 mmol g
−1
) and lactonic groups (0.006 ±
0.013 mmol g
−1
). Other biomasses recently used for Pb removal showed higher values for
acid groups such as blue-green algae (ag: 0.52/bg: 0.02 mmol g
−1
) [77], corncob-activated
carbon (ag: 1.22/bg: 0.57 mmol g
−1
) [78], olive stone (ag: 0.96/bg: 0.41 mmol g
−1
), pine nut
shell biochar (ag: 0.206–0.266/bg: 0.020–0.029 mmol g
−1
) [79] or nanche stone (ag:
0.1037/bg: 0.046 mmol g
−1
) [80]. The carboxylic groups are Pearson hard basic sites on the
biomass surface, while Pb(II) is a borderline softer acid ion [81]. The biomass–metal ionic
electrostatic interaction with these groups might not be the main one, and the Pb(II) ions
could also show covalent complexation, cation–π interactions [74,82,83] or other
biosorption mechanisms. The large configuration of electron cloud ([Xe]6s
2
4f
14
5d
10
) of
Pb(II), with a more diffuse electron distribution, increases the probability and the
intensity of interactions between the ions and the sorbent, facilitating the sorption
processes and explaining this type of interactions [83]. In this sense, it has been reported
that Pb(II) was readily bound by cation–π interactions, attributing the adsorption to
electron donor ligands of aromatic functional groups such as the content found within
the broccoli biomass [84,85].
Figure 3. SEM micrographs of the surface morphology of broccoli stalk biomass before (a) and after
(b) Pb(II) sorption.
The iodine capacity (I, mg g
−1
) can provide significant information about the surface
of powder materials. It is defined as the mass of iodine in mg that is consumed by grams
of a chemical substance and is often used to determine the amount of unsaturation in the
form of double bonds; consequently, the higher the iodine capacity, the higher the number
of C=C bonds present. In the present work, the iodine capacity of the biomass was found
to be 619.7
±
33.3 mg g
−1
(Table 1). This result is higher than that of other biosorbents used
for Pb(II) removal, such as coconut-shell-activated carbon (334.2 mg g
−1
) [
72
], pine cone
powder (23.7 mg g
−1
) [
40
] or rice husk (68 mg g
−1
) [
73
], showing the great potential of the
Calabrese broccoli stalk biomass as a sorbent.

Agronomy 2024,14, 554 9 of 22
Agronomy 2024, 14, x FOR PEER REVIEW 10 of 23
Figure 4. EDX spectra of broccoli stalk biomass before (a) and after (b) Pb(II) sorption.
In relation to the functional groups, the pH of the biomass was found to be 6.88 ±
0.02, the pH
pzc
value was 6.25 ± 0.02 (data in Table S5 of Supplementary Material), and
the conductivity of the biomass was 1566 ± 22 µS cm
−1
(Table 1). These pH values have a
great importance in metal sorption because they determine the ionization of the
chemically active sites in the sorbent and the charge of the sorbent surface during
sorption processes, affecting the sorption mechanism. Its quantification is of great
importance when the main process is physisorption and provides information on the
affinity of the biomass for cationic or anionic species depending on the pH of the liquid
phase [86]. Thus, at solution pH higher than pH
pzc
= 6.25, the biomass surface will be
negatively charged, facilitating the interaction with positive species, while at pH lower
than this value, the behaviour of the solid surface will be the opposite. Lower or similar
values can be found in the literature for other biomasses used for Pb biosorption: blue-
green algae (pH
pzc
1.3) [77], corncob-activated carbon (pH
pzc
3.8) [78], nanche stone
(pH
pzc
6) [80], mixture of coffee grounds and orange barks residues (pH
pzc
5.2) [87], olive
Figure 4. EDX spectra of broccoli stalk biomass before (a) and after (b) Pb(II) sorption.
An intermediate CEC value of 30.7
±
0.01 cmol kg
−1
was found for broccoli biomass,
being suitable for the sorption of divalent ions by exchange. This value is comparable
to the CEC range of 11.7–34.8 cmol kg
−1
described for agricultural biochar [
74
]. Higher
values can be found for other biomass such as green compost (67.8 cmol kg
−1
) and peat
(
84.6 cmol kg−1) [75]
with significant exchange properties for Pb(II). Lower values can also
be found for biochar from mushroom compost (5.76 cmol kg
−1
) [
76
] or for woody biomass
(1.3–3.0 cmol kg
−1
) with lower mineral contents [
74
], indicating the minor role of ion
exchange during the metal adsorption.
Regarding the functional groups on the biomass surface, the results showed that
the acid groups (ag: 1.257
±
0.012 mmol g
−1
) were slightly higher than the basic ones
(bg: 1.085 ±0.021 mmol g−1) (Table 1)
. Carboxylic groups (1.216
±
0.012 mmol g
−1
)

Agronomy 2024,14, 554 10 of 22
were predominant compared to phenolic (0.035
±
0.013 mmol g
−1
) and lactonic groups
(
0.006 ±0.013 mmol g−1)
. Other biomasses recently used for Pb removal showed higher
values for acid groups such as blue-green algae (ag: 0.52/bg: 0.02 mmol g
−1
) [
77
], corncob-
activated carbon (ag: 1.22/bg: 0.57 mmol g
−1
) [
78
], olive stone (
ag: 0.96/bg: 0.41 mmol g−1)
,
pine nut shell biochar (ag: 0.206–0.266/bg: 0.020–0.029 mmol g
−1
) [
79
] or nanche stone
(ag: 0.1037/bg: 0.046 mmol g
−1
) [
80
]. The carboxylic groups are Pearson hard basic sites
on the biomass surface, while Pb(II) is a borderline softer acid ion [
81
]. The biomass–
metal ionic electrostatic interaction with these groups might not be the main one, and the
Pb(II) ions could also show covalent complexation, cation–
π
interactions [
74
,
82
,
83
] or other
biosorption mechanisms. The large configuration of electron cloud ([Xe]6s
2
4f
14
5d
10
) of
Pb(II), with a more diffuse electron distribution, increases the probability and the intensity
of interactions between the ions and the sorbent, facilitating the sorption processes and
explaining this type of interactions [
83
]. In this sense, it has been reported that Pb(II) was
readily bound by cation–
π
interactions, attributing the adsorption to electron donor ligands
of aromatic functional groups such as the content found within the broccoli biomass [
84
,
85
].
In relation to the functional groups, the pH of the biomass was found to be
6.88 ±0.02
,
the pH
pzc
value was 6.25
±
0.02 (data in Table S5 of Supplementary Material), and the
conductivity of the biomass was 1566
±
22
µ
S cm
−1
(Table 1). These pH values have a
great importance in metal sorption because they determine the ionization of the chemically
active sites in the sorbent and the charge of the sorbent surface during sorption processes,
affecting the sorption mechanism. Its quantification is of great importance when the main
process is physisorption and provides information on the affinity of the biomass for cationic
or anionic species depending on the pH of the liquid phase [
86
]. Thus, at solution pH
higher than pH
pzc
= 6.25, the biomass surface will be negatively charged, facilitating the
interaction with positive species, while at pH lower than this value, the behaviour of the
solid surface will be the opposite. Lower or similar values can be found in the literature
for other biomasses used for Pb biosorption: blue-green algae (pH
pzc
1.3) [
77
], corncob-
activated carbon (pH
pzc
3.8) [
78
], nanche stone (pH
pzc
6) [
80
], mixture of coffee grounds
and orange barks residues (pH
pzc
5.2) [
87
], olive stone (pH
pzc
6.61) [
88
] or untreated and
alkaline-treated apricot shell (pHpzc 4.9 and 5.7, respectively) [89].
Other physico-chemical parameters such as bulk density (BD) and apparent density
(AD) were measured, with values of 0.52
±
0.01 g mL
−1
and 0.65
±
0.03 g mL
−1
, re-
spectively (Table 1), with a porosity of 20.6
±
3.2% and a specific surface area (SSA) of
15.3 ±2.7 m2g−1
. These values were comparable with those of other biomasses found in
the literature: mixture of coffee grounds and orange barks residues (
BD = 0.43 g mL−1) [87]
,
untreated and alkaline-treated apricot shell (BD = 1.15 g mL
−1
and 1.10 g mL
−1
;
AD = 1.50 g mL−1
and 1.58 g mL
−1
; SSA = 15.4 m
2
g
−1
and 20.0 m
2
g
−1
, respectively) [
89
],
green compost (BD = 0.38 g mL
−1
) and peat (BD = 0.25 g mL
−1
) [
75
] or olive leave biomass
(BD = 0.25 g mL
−1
) [
90
]. As defined by American Water Work association, bulk density
of biomass higher than 0.25 g mL
−1
is adequate for metal removal [
91
]; thus, the BD for
broccoli was adequate for this purpose. Additionally, the comparison of the SSA of some
biochars derived from agricultural biomass such as: corncob (SSA = 53.71 m
2
g
−1
), rice
husk (SSA = 51.39 m
2
g
−1
) or wheat straw (130.14 m
2
g
−1
) [
92
,
93
], showed higher values
for the surface area, although these biochars require greater pretreatments. The physical
adsorption process is mainly affected by porosity and specific surface area which provide
more adsorption sites. Therefore, it seems unlikely that the mechanism by which broccoli
removes Pb(II) is mainly based on physical adsorption if it has a lower number of voids [
62
].
3.3. Metal Removal Experiments
3.3.1. Effect of Ratio Mass of Biosorbent/Volume of Pb(II) Solution
In order to reduce the amount of biomass used in the sorption experiments, studies on
the ratio of mass of biosorbent/volume of aqueous metal solution were carried out in the
range of 5–10 g L
−1
. The yield obtained with the lowest ratio was 67.9
±
0.8%, while the
ratio of 10 g L
−1
yielded 88.5
±
0.5% after 24 h of exposure (Figure S1 of Supplementary

Agronomy 2024,14, 554 11 of 22
Material). Therefore, a ratio of 10 g L
−1
was used for further experiments, i.e., 0.5 g of
biomass were used in 50 mL of aqueous solution loaded with Pb(II).
3.3.2. Effect of pH
The effect of the solution pH on the effectiveness of Pb(II) ions removal from aqueous
solutions using broccoli biomass as sorbent was studied at pH values of 3, 4.8 and 7. Higher
pH values were not studied due to the rapid precipitation of the lead-loaded solution.
Lower pH values could lead to the breaking of some biomass molecules. The results for
24 h were 85.8
±
2.6, 88.5
±
0.5, and 40.8
±
1.2, respectively (Figure S1 of Supplementary
Material). The pH dependence observed suggested that the removal of Pb(II) ions was
favoured by a slightly acid pH value of 4.8.
The influence of pH on the sorption of metal ions is complex and depends mainly on
the acidic properties of the cation and the acid-base character of the biomass surface. Con-
cerning the cation, the pH speciation of Pb(II) indicates Pb(OH)
2
as the dominant species at
pH > 5 and Pb(OH)
+
at pH < 5. Thus, precipitation processes occur at pH values higher
than 5, and the insoluble Pb(OH)
2
particles may be deposited from the bulk solution; these
particles may also block the biomass pores and be
absorbed [65,75]
. The values obtained in
the broccoli experiments were in agreement with the Pb(II) speciation, suggesting that it is
more appropriate not to perform the experiments at cation precipitation values.
Related to the broccoli biomass, the value of pH
pzc
determines when the net charge is
0. Below this value, it is positive, and the H
+
ions competes for the active sites with Pb(II)
ions, thereby protonating the oxygen-containing organic groups (carboxylate, phenolic
hydroxyl groups, glucosinolates, phytates, etc.) leading to repulsion which increases as
the pH decreases [
94
]. The pH
pzc
value of 6.25 for broccoli biomass was higher than the
pH of 4.8, at which the net charge of the biomass was positive. Nevertheless, the sorption
was satisfactory. This phenomenon has been previously described in the literature and can
be explained by considering other sorption mechanisms. The adsorption mechanisms of
Pb(II) include: ion exchange (with negative surface of biomass), ligand exchange (with
group providing hydrogen bonding such –OH group (phenolic hydroxyl)), precipitation
(depending on the metal speciation), complexation (with electron donor elements in the
complexing groups), cation–
π
interactions (with electron donor functional groups with
delocalized
π
-electrons), physical sorption on surface sites (contact adsorption) and intra-
particle diffusion [
62
,
85
,
93
]. Therefore, based on the pH of the aqueous solution, both ion
exchange and physical electrostatic adsorption were mechanisms that were less favoured
using broccoli biomass at pH 4.8, as already mentioned in Section 3.2. Microprecipitation
on the biomass surface could also occur given the proximity to the pH value of 5.
3.3.3. Effect of Sorption Time and Initial Concentration of Pb(II) on the Removal
The removal of Pb(II) at different concentrations of 0.25, 0.5, 1, and 1.5 mmol L
−1
was studied over time. The experiments were conducted under the conditions mentioned
in Section 2.4, at pH 4.8 and 23
◦
C, using 0.5 g of biomass (particle sizes: 0.125 mm
< x < 0.249 mm) in 50 mL of aqueous solution. The percentage of Pb(II) removal was
calculated with Equation (2) and the experimental data obtained are shown in Figure 5. It
can be observed that Pb(II) removal increased rapidly with time during the initial intervals,
followed by a slower increase until equilibrium was reached. This effect decreased at higher
concentrations, requiring more time to reach equilibrium. The removal efficiency does
not seem to increase with the increasing concentration gradient from the solution to the
sorbent, as occurs mainly in physical processes, which rely more on diffusion. Equilibrium
was reached after 6–8 h for all initial concentrations, making it unnecessary to extend the
removal times. The percentage of removal at equilibrium was similar in the range of 0.25–1
mmol L
−1
Pb(II), with an average of 88.1
±
1% after 6 h and 88.3
±
0.8% after 8 h, with
lower yields for 1.5 mmol L
−1
Pb(II) of 83.2
±
0.1% after 6 h and 83.4
±
0.3% after 8 h.
When the Pb(II) concentration was too high, this efficiency was slightly lower, probably
due to the saturation of the binding sites on the biomass. The precision of the method was

Agronomy 2024,14, 554 12 of 22
evaluated using a concentration of 1 mmol L
−1
of Pb(II) by six replicate measurements at a
confidence level of 95%, and was found to be 0.23%.
Agronomy 2024, 14, x FOR PEER REVIEW 12 of 23
Nevertheless, the sorption was satisfactory. This phenomenon has been previously
described in the literature and can be explained by considering other sorption
mechanisms. The adsorption mechanisms of Pb(II) include: ion exchange (with negative
surface of biomass), ligand exchange (with group providing hydrogen bonding such –
OH group (phenolic hydroxyl)), precipitation (depending on the metal speciation),
complexation (with electron donor elements in the complexing groups), cation–π
interactions (with electron donor functional groups with delocalized π-electrons),
physical sorption on surface sites (contact adsorption) and intraparticle diffusion
[62,85,93]. Therefore, based on the pH of the aqueous solution, both ion exchange and
physical electrostatic adsorption were mechanisms that were less favoured using
broccoli biomass at pH 4.8, as already mentioned in Section 3.2. Microprecipitation on
the biomass surface could also occur given the proximity to the pH value of 5.
3.3.3. Effect of Sorption Time and Initial Concentration of Pb(II) on the Removal
The removal of Pb(II) at different concentrations of 0.25, 0.5, 1, and 1.5 mmol L
−1
was studied over time. The experiments were conducted under the conditions
mentioned in Section 2.4, at pH 4.8 and 23 °C, using 0.5 g of biomass (particle sizes: 0.125
mm < x < 0.249 mm) in 50 mL of aqueous solution. The percentage of Pb(II) removal was
calculated with Equation (2) and the experimental data obtained are shown in Figure 5.
It can be observed that Pb(II) removal increased rapidly with time during the initial
intervals, followed by a slower increase until equilibrium was reached. This effect
decreased at higher concentrations, requiring more time to reach equilibrium. The
removal efficiency does not seem to increase with the increasing concentration gradient
from the solution to the sorbent, as occurs mainly in physical processes, which rely more
on diffusion. Equilibrium was reached after 6–8 h for all initial concentrations, making it
unnecessary to extend the removal times. The percentage of removal at equilibrium was
similar in the range of 0.25–1 mmol L
−1
Pb(II), with an average of 88.1 ± 1% after 6 h and
88.3 ± 0.8% after 8 h, with lower yields for 1.5 mmol L
−1
Pb(II) of 83.2 ± 0.1% after 6 h and
83.4 ± 0.3% after 8 h. When the Pb(II) concentration was too high, this efficiency was
slightly lower, probably due to the saturation of the binding sites on the biomass. The
precision of the method was evaluated using a concentration of 1 mmol L
−1
of Pb(II) by
six replicate measurements at a confidence level of 95%, and was found to be 0.23%.
Figure 5. Removal of Pb(II) at different initial concentrations over the time (as percentage).
Figure 5. Removal of Pb(II) at different initial concentrations over the time (as percentage).
3.4. Mechanisms of the Pb(II) Sorption
3.4.1. Sorption Isotherm
Adsorption isotherm models are used as a tool to elucidate the interactions between
sorbents and target substances. An isotherm describes the relationship between the amount
of substance sorbed per unit mass of sorbent and the remaining concentration in the
aqueous phase at equilibrium, under controlled temperature conditions. Experimental data
on this relationship and their fitting to different predictive models can be used to define the
nature of the interactions in the sorption process. The mechanisms of adsorption, sorption
favourability, and sorbate–sorbent affinity can be explained by the isotherm models [
95
].
Four models were applied by using the equations described in the Supplementary Material
(Table S2). The constants and determination coefficients of the models, calculated from the
linear plot, are shown in Table 2at the equilibrium time of 8 h.
The Langmuir isotherm is widely used and applies to the monolayer sorption pro-
cess on a surface with a finite number of identical sites and equivalent sorption energies.
Once the sites are occupied by the sorbate, no further sorption can take place. Addition-
ally, it assumes that there are no interactions between the adsorbed species [
51
,
96
]. K
L
is the Langmuir isotherm constant and q
max
is the maximum adsorption capacity. The
model predicts the suitability of the sorption process as a function of the values of the
dimensionless constant separation factor R
L
, which depends on K
L
(Table S2). The sorp-
tion is favoured within the range of 0 (more favourable) to 1 (less favourable), while
sorption processes with out-of-range values are not suitable. In this work, the fit of the
Langmuir isotherm provided the lowest R
2
value (R
2
= 0.873) among the four models
applied for the Pb(II) removal experiments. The values of R
L
varied significantly de-
pending on the initial concentration of Pb(II): 0.62 (for C
0
= 0.25 mmol L
−1
); 0.39 (for
C0= 0.5 mmol L−1); 0.33 (for C0= 1 mmol L−1); and 0.19 (for C0= 1.5 mmol L−1), indicat-
ing that the Pb(II) sorption with broccoli biomass was suitable at all concentrations and more
favourable at higher concentrations. The calculated q
max
was significant, with a value of
64.9 mg g
−1
compared to other similar Langmuir values for stalk or stem biomass with Pb
(II) (corn stalks (biochar): 40.98 mg g
−1
[
97
]; grape stalks: 37.7 mg g
−1
[
98
]; rooibos shoot:
19.8 mg g−1[99]; and tobacco stems: 5.54 mg g−1[100]).

Agronomy 2024,14, 554 13 of 22
Table 2. Results of the fits of parameters and determination coefficients for the different isotherm models.
Isotherm Models Isotherm Constants and Determination Coefficients a
Parameter Values at 23 ◦C
Langmuir
KL(L mg−1//L mmol−1)0.013//2.78
qmax (mg g−1//mmol g−1)64.9//0.31
RL0.19–0.62
R20.873
MSE 0.023
Freundlich
KF(mg g−1//mmol g−1)1.11//0.005
1/n0.82
R20.969
MSE 0.005
Dubinin–Radushkevich
B(mol2J−2) 7.28 ×10−9
qmax (mg g−1//mmol g−1)587//2.83
E(kJ mol−1)8.29
R20.981
MSE 0.018
Temkin
B9.93
KT(L mg−1//L mmol−1)0.24//50.4
bT(J mol−1)248
R20.980
MSE 2.69
aThe model equations and parameters are defined in Table S2.
The Freundlich isotherm model assumes multilayer adsorption on the heterogeneous
sorbent surface characterised by non-uniform sorption energies. The Freundlich isotherm
constant K
F
is an indicator of adsorption capacity related to the surface heterogeneity
and activity. Additionally, the parameter 1/nindicates the strength of adsorption: the
smaller the 1/nvalue, the greater the heterogeneity. If the value of 1/nis less than 1,
it indicates normal adsorption. Values of nin the range of 1 < n< 10 indicate good
adsorption [
21
]. From the plot of experimental data using the Freundlich model, the value of
K
F
was 1.11 mg g
−1
and 1/nwas 0.823 with a good coefficient of determination
(R2= 0.969)
(Table S2), indicating a favourable Pb(II) sorption on the heterogeneous surface of the
broccoli biomass. These results were similar to those determined for some stalk or stem
biomass mentioned above, such as rooibos shoot: K
F
= 1.47 mg g
−1
and 1/
n= 0.34 [99]
,
and tobacco stems:
KF= 0.95 mg g−1
and 1/n= 0.723 [
100
]. The values for corn stalks
(biochar):
KF= 16.44 mg g−1
and 1/n= 7.25 [
97
] did not result in favourable sorption, and
the Freundlich model was not studied for grape stalks [98].
The Langmuir and Freundlich models can be used to determine whether the adsorp-
tion process is monolayer or multilayer, but they do not really describe the nature of the
sorption. The Dubinin–Radushkevich model (D-R) can be used for this purpose as it allows
the determination of the type of adsorption by calculating the average free energy of ad-
sorption (E) per mole of sorbate. Energy values in the range of 8–16 kJ mol
−1
are indicative
of chemical sorption, whereas values below 8 kJ mol
−1
suggest physical sorption [
95
]. The
study of the experimental data with broccoli biomass using the D-R model yielded a very
good fit (R
2
= 0.981) (Table S2), similar to that obtained with the Freundlich model. The
value of q
max
was significantly high (q
max
= 586.7 mg g
−1
) and the nature of the sorption
was defined as chemisorption based on the calculated Evalue (E= 8.29 kJ mol
−1
). No data
were found for other stalk or stem biomasses mentioned above using the D-R model.
By ignoring extremely low and high concentration values, the Temkin isotherm con-
siders the fact that the heat of sorption for all molecules on the sorbent surface decreases
linearly with increasing coverage due to interactions between the sorbent and the sorbate.
It also suggests that sorption is defined by a uniform distribution of binding energies up
to a maximum binding energy [
51
,
95
,
101
]. K
T
is the equilibrium binding constant, corre-
sponding to the maximum binding energy, and it indicates the strength of the interaction.

Agronomy 2024,14, 554 14 of 22
b
T
is the Temkin isotherm constant and is related to the heat of sorption. Although the
Temkin equation is more suitable for predicting gas phase equilibria [
102
], a determina-
tion coefficient was obtained with the experimental data of Pb(II) sorption using broccoli
biomass (R
2
= 0.980) (Table S2). This value was as good as that obtained for the Freundlich
and D-R models. However, the value of the MSE (mean square error) was significantly
higher than the values for other isotherms (MSE = 2.69 compared with MSE values of 0.005
and 0.018 for the Freundlich and D-R fits, respectively). This discrepancy can be attributed
to the complexity of the liquid–solid interphase when different mechanisms coexist, which
is not assumed by the Temkin model. For this reason, the Freundlich and D-R models
provide better explanations for Pb(II) sorption with broccoli biomass.
It can be concluded that the Freundlich and D-R models can better explain the
favourable Pb(II) sorption on the heterogeneous surface of the broccoli biomass, with
different possible interactions and binding energy, preferably by chemical mechanisms and
with a very high q
max
of 586.7 mg g
−1
. Based on the percentage of Pb(II) removal (Figure 3),
the adsorption process did not encounter any resistance with a substantial adsorption
force, demonstrating rapid kinetics at beginning of the process and reaching a high average
removal of 88.3
±
0.8% after 8 h for the Pb(II) concentrations range of
0.25–1 mmol L−1.
Increasing the number of adsorbed subsequent layers resulted in only a slightly decrease in
the adsorption force, particularly notable for the highest Pb(II) concentrations of
1.5 mg L−1
,
with an efficiency of 83.4 ±0.3% after 8 h.
3.4.2. Sorption Kinetics
Kinetic models allow for the determination of the sorbate removal rate from solution
by the biosorbent, as well as the elucidation of the mechanisms of the process. The
kinetic behaviour of Pb(II) removal on broccoli biomass was investigated by six common
models. The experimental data obtained for several concentrations of Pb(II) (0.25, 0.5, 1
and
1.5 mmol L−1
) were fitted to each model’s linear equation form. The parameters of
these models were then calculated from the slopes and intercepts of the plots, as described
in Table S3.
As can be seen in Table 3, the pseudo-second-order model proved to be the most
effective in describing the sorption of Pb(II), withvery good R
2
values for all concentrations of
Pb(II). In addition, the fitted equilibrium adsorption capacities (q
e
(
0.25 mg g−1) = 3.95 mg g−1
;
(q
e
(0.5 mg g
−1
) = 10.4 mg g
−1
; (q
e
(1 mg g
−1
) = 13.5 mg g
−1
; (q
e
(1.5 mg g
−1
) = 27.0 mg g
−1
)
were very close to the experimental data (q
e
exp (0.25 mg g
−1
) = 3.95 mg g
−1
; (q
e
exp (0.5 mg
g
−1
) = 10.3 mg g
−1
; (q
e
exp (1 mg g
−1
) = 13.4 mg g
−1
; (q
e
exp (1.5 mg g
−1
) = 27.9 mg g
−1
).
The suitability of the pseudo-second-order kinetic was further demonstrated by the plot of
t/q
t
versus t, showing a linear relationship (Figure 6). The equilibrium sorption capacity
(q
e
) of the broccoli biomass increased when increasing the initial Pb(II) concentration from
0.25 to 1.5 mmol L
−1
as the slope (1/q
e
) decreased with the concentration (Figure 6). This
increase indicated favourable interactions between the Pb(II) ions and the sites on the
biomass. These results suggest that the adsorption processes of Pb
2+
onto broccoli biomass
may involve chemical interactions with electron sharing or exchange [
103
]. Previous studies
(Table 1) have indicated that the CEC of broccoli biomass was intermediate, implying that
ion exchange plays a less significant role. Therefore, interactions such as ligand exchange
(with –OH group of the polysaccharide matrix), complexation with key donor elements
(e.g., O, S, N, P) found in main compounds of broccoli stalks (e.g., cellulose, hemicellulose,
glucosinolates, phytates, amino acids), and cation–
π
interactions (with aromatic functional
groups) appear to be the main mechanisms controlling the Pb(II) sorption [
104
,
105
]. In the
literature, the frequency of the different binding groups on biomass surface involved in the
complexation of metal ions has been reported: carboxylates: 40.8%, aromatic ring: 15.5%,
hydroxyl: 15.5%, amine: 12.7%, phosphate: 4.2%, carbonyl: 4.2%, thiol: 2.8%, amide: 2.8%,
and sulfonate 1.4%. Pb ranks among the top heavy metals of concern for water pollution
when complexed by ligands with S and –COOH groups, such as some of the broccoli
biomass, due to the Pb(II) configuration and Pearson properties [106].

Agronomy 2024,14, 554 15 of 22
Table 3. Results of the fits of the kinetic models, parameters and determination coefficients for
different Pb(II) concentrations.
Kinetic Models Kinetic Parameters and Determination Coefficients b
Parameter a0.25 mmol L−10.5 mmol L−11 mmol L−11.5 mmol L−1
Pseudo-first-order
k1(min−1) 3.78 ×10−47.73 ×10−30.01 0.03
qe(mg g−1//mmol g−1) 0.02//1.14 ×10−4
0.30//1.43
×
10
−3
1.11//5.35
×
10
−312.2//0.06
qeexp (mg g−1//mmol g−1)3.95//0.02 10.3//0.05 13.4//0.07 27.9//0.14
R20.008 0.989 0.942 0.968
MSE 0.05 1.43 ×10−30.03 0.05
Pseudo-second-order
k2(g mg−1min−1//mmol−1
min−1)0.85//1.76 ×1020.09 0.04//7.96 4.22 ×10−3//0.87
qe(mg g−1//mmol g−1)3.95//0.02 10.4//0.05 13.5//0.07 27.0//0.13
qeexp (mg g−1//mmol g−1)3.95//0.02 10.3//0.05 13.4//0.07 27.9//0.14
h(mg g−1min−1//mmol g−1
min−1)13.2//0.06 10.1//0.05 6.95//0.03 3.08//0.02
R20.999 0.999 0.999 0.999
MSE 0.03 1.86 ×10−32.11 ×10−30.03
Elovich
a(mg g−1min−1)
non fitted
2.86 ×1048 1.26 ×1012 0.57
b(g mg−1)11.6 2.57 0.32
R20.985 0.876 0.768
MSE 2.10 ×10−40.05 5.55
Ritchie’s second
order
k2_R (min−1)1.00 0.88 1.19 3.55
R20.056 0.965 0.755 0.751
MSE 23.3 ×1031.85 ×1021.05 ×1045.66 ×104
First order reversible
k1(min−1)0.02 0.03 0.02 0.03
k−1(min−1) 3.34 ×10−33.20 ×10−32.73 ×10−36.71 ×10−3
R2R2< 0 R2< 0 R2< 0 0.947
MSE 13.3 6.83 3.78 0.15
Intraparticle
diffusion
Kd(mg g−1min−1/2)9.0 ×10−50.02 0.07 0.50
C(mg g−1)3.94 10.0 12.1 17.7
R20.0007 0.9417 0.712 0.582
MSE 6.74 ×10−48.19 ×10−40.11 10.0
a
The model equations and parameters are defined in Table S3.
b
R
2
< 0: the model is not able to adequately
explain the adsorption behaviour observed in the experimental data.
Agronomy 2024, 14, x FOR PEER REVIEW 16 of 23
R
2
0.999 0.999 0.999 0.999
MSE 0.03 1.86 × 10
−3
2.11 × 10
−3
0.03
Elovich
a (mg g
−1
min
−1
)
non fitted
2.86 × 10
48
1.26 × 10
12
0.57
b (g mg
−1
) 11.6 2.57 0.32
R
2
0.985 0.876 0.768
MSE 2.10 × 10
−4
0.05 5.55
Ritchie’s
second order
k
2_R
(min
−1
) 1.00 0.88 1.19 3.55
R
2
0.056 0.965 0.755 0.751
MSE 23.3 × 10
3
1.85 × 10
2
1.05 × 10
4
5.66 × 10
4
First order
reversible
k
1
(min
−1
) 0.02 0.03 0.02 0.03
k
−1
(min
−1
) 3.34 × 10
−3
3.20 × 10
−3
2.73 × 10
−3
6.71 × 10
−3
R
2
R
2
< 0 R
2
< 0 R
2
< 0 0.947
MSE 13.3 6.83 3.78 0.15
Intraparticle
diffusion
K
d
(mg g
−1
min
−1/2
) 9.0 × 10
−5
0.02 0.07 0.50
C (mg g
−1
) 3.94 10.0 12.1 17.7
R
2
0.0007 0.9417 0.712 0.582
MSE 6.74 × 10
−4
8.19 × 10
−4
0.11 10.0
a
The model equations and parameters are defined in Table S3.
b
R
2
< 0: the model is not able to
adequately explain the adsorption behaviour observed in the experimental data.
Figure 6. Pseudo-second-order kinetics for Pb(I) removal onto broccoli biomass.
Other models provided good fits but not for all Pb(II) concentrations, which could
be related to other concurrent sorption mechanisms that varied in effectiveness
depending on the concentration. In this sense, the pseudo-first-order model exhibited
good R
2
values except for the lowest Pb(II) concentration of 0.25 mmol L
−1
. In addition,
the predicted values of q
e
differed significantly from the experimental values. This could
also indicate that physical sorption was less efficient than chemical interactions.
3.4.3. Sorption Thermodynamic Studies
Figure 6. Pseudo-second-order kinetics for Pb(I) removal onto broccoli biomass.

Agronomy 2024,14, 554 16 of 22
Other models provided good fits but not for all Pb(II) concentrations, which could be
related to other concurrent sorption mechanisms that varied in effectiveness depending
on the concentration. In this sense, the pseudo-first-order model exhibited good R
2
values
except for the lowest Pb(II) concentration of 0.25 mmol L
−1
. In addition, the predicted
values of q
e
differed significantly from the experimental values. This could also indicate
that physical sorption was less efficient than chemical interactions.
3.4.3. Sorption Thermodynamic Studies
Thermodynamic studies are used to investigate the effect of temperature on the sorp-
tion capacity and to understand whether the sorption is spontaneous or not. The standard
Gibbs free energy change (
∆
G
◦
), the standard enthalpy change (
∆
H
◦
), the standard entropy
change (
∆
S
◦
), the activation energy (E
a
) and the sticking probability (S*) are thermody-
namic parameters that can be obtained by the equations detailed in
Table S4 [57,107]
. The
values of enthalpy and entropy variation were calculated using the graphical representation
of ln K
c
vs. 1/T, being (
−∆
H
◦
/R) the slope of the curve and (
∆
S
◦
/R) the ordinate at the
origin. From the modified Arrhenius and its subsequent linearization, the parameters E
a
and S* could be determined. Thus, the representation of ln (1
−θ
) vs. 1/Tallowed the
calculation of Ea/R as the slope of the curve and ln S* as the ordinate at the origin.
Biosorption experiments were carried out as described in Section 2.7 to evaluate the
thermodynamic properties, and the results are presented in Table 4. The values of enthalpy
(
∆
H
◦
) and entropy (
∆
S
◦
) were determined to be 8.98 kJ mol
−1
and 45.20 J mol
−1
K
−1
,
respectively. The positive value of
∆
H
◦
was consistent with the endothermic nature of the
sorption process and slightly exceeded the indicative energy value for physical sorption
(<8 kJ mol
−1
). The positive value of
∆
S
◦
suggested an increase in randomness at the solid–
liquid interface, while a low value of this parameter indicated that not remarkable change
in entropy occurred. The values of the standard Gibbs energy change were calculated and
found to be
−
4.17,
−
4.40 and
−
4.72 kJ mol
−1
at 18
◦
C, 23
◦
C and 30
◦
C, respectively. The
negative values of
∆
G
◦
increased with increasing temperature, indicating the spontaneity
of the sorption process and that the process was favoured by the increase in temperature.
Table 4. Thermodynamic parameters
a
for the Pb(II) sorption onto broccoli Calabresse biosorbent
(50 mL of 1 mmol L−1Pb(II) solution at pH 4.8; biosorbent mass = 0.5 g).
Temperature (◦C/K) ∆G◦(kJ mol−1)∆H◦(kJ mol−1)∆S◦(J mol−1K−1)S* Ea(kJ mol−1)
18/291 −4.17
8.98 45.20 0.006 7.66
23/296 −4.40
30/303 −4.72
aThe model equations and parameters are defined in Table S4.
The value of the activation energy (E
a
) was 7.66 kJ mol
−1
, which was in agreement with
the positive value of
∆
H
◦
for the endothermic Pb(II) sorption. Understanding activation
energy provides insight into the nature of sorption. Physisorption entails weak forces, with
activation energy values below 4.2 kJ mol
−1
. Conversely, chemisorption involves stronger
and specific forces compared to physical adsorption, with activation energy exceeding this
value [
108
]. The activation energy (E
a
) value determined in this study indicates that the
sorption of Pb onto the broccoli biosorbent is predominantly a chemisorption process. The
sticking probability (S*) was 0.006, a very low value. The interpretation of this parameter
can be described as follows: (a) S* > 1: sorbate unsticking to sorbent—no sorption; (b)
S* = 1:
linear sticking relationship between sorbate and sorbent—possible mixture of physical and
chemical sorption; (c) S* = 0: indefinite sticking of sorbate to sorbent—predominance of
chemical sorption; (d) 0 < S* < 1: favourable sticking of sorbate to sorbent—predominance
of physical sorption [
109
]. The obtained value of S* suggests that the sorption of Pb(II) was
governed by a mechanism on the borderline between chemisorption and physisorption.
This value, being very close to zero, highlights the significance of chemical sorption, and
also corroborates the findings from the kinetic study.

Agronomy 2024,14, 554 17 of 22
4. Conclusions
A low-cost and ecofriendly biosorbent obtained from Calabrese broccoli stalks showed
significant potential for the removal of Pb(II) from aqueous solutions, exhibiting efficient
adsorption performance without requiring complicated pretreatment processes. The elec-
tronic configuration of Pb(II) ions, along with the metal speciation at a pH of 4.8 in aqueous
solution, and the nature of broccoli biomass, synergistically supported the sorption process.
The Freundlich and Dubinin–Radushkevich models explained with good fits the favourable
Pb(II) sorption on the heterogeneous surface of the broccoli biomass, with different possible
interactions and binding energies, preferably by chemical mechanisms. The pseudo-second-
order kinetic model was the best fit, further confirming the chemical nature of the sorption
for the Pb(II) removal using broccoli biomass. These chemical mechanisms mainly involved
complexation, ligand exchange, and cation–
π
interaction. Ion exchange and physical sorp-
tion occurred to a lesser extent, as did physical sorption. The pH of the solution could
also induce microprecipitation on the biomass surface. The thermodynamic parameters
evaluated showed the endothermic and spontaneous nature of the Pb(II) biosorption, and
the sticking probability confirmed the borderline mechanism between chemisorption and
physisorption. Hence, this novel biomass has proven to be a promising sorbent for the
removal of Pb(II) ions from aqueous solutions.
Supplementary Materials: The following supporting information can be downloaded at https:
//www.mdpi.com/article/10.3390/agronomy14030554/s1: Table S1: Methodologies applied for
biomass characterisation; Table S2: Sorption isotherm models applied in this study; Table S3: Kinetic
models applied in this study; Table S4: Equations for thermodynamics and surface parameters;
Table S5: Experimental data obtained in the study of pH
pzc
; Figure S1: Effect of the mass of biosor-
bent/volume ratio and pH of Pb(II) solution on the removal percentage of Pb(II).
Author Contributions: Conceptualization, M.D.G.-R.; Methodology, M.D.G.-R., M.D.-d.-A., M.J.C.-M.
and M.D.G.-C.; Software, M.D.G.-R.; Validation, M.D.G.-R., M.D.-d.-A., M.J.C.-M. and M.D.G.-C.;
Formal Analysis, M.D.G.-R., M.D.-d.-A., M.J.C.-M. and M.D.G.-C.; Investigation, J.G.-R. and L.S.-P.;
Resources, M.D.G.-R.; Data Curation, M.D.G.-R.; Writing—Original Draft Preparation, M.D.G.-C.
and M.D.-d.-A.; Writing—Review and Editing, M.D.G.-R., M.D.-d.-A., M.J.C.-M. and M.D.G.-C.;
Visualization, M.D.-d.-A. and M.J.C.-M.; Supervision, M.D.G.-R., M.D.-d.-A. and M.D.G.-C.; Project
Administration, M.D.G.-R.; Funding Acquisition, M.D.G.-R. All authors have read and agreed to the
published version of the manuscript.
Funding: This work was supported by the supports of the Programme of “Fomento e Impulso de
la Investigación y de la Transferencia” from the University of Cadiz (Spain) (Project PR2022-017),
the Programme of “Plan Propio de estímulo y apoyo a la Investigación y Transferencia” for the use
of equipment from the Central Research Service for Science and Technology from the University of
Cadiz (Spain) (Ref: SC2022-002) and the Programme of “Plan Propio of the Institute of Biomolecules
(INBIO)” from the University of Cadiz (Spain) (INBIO 2022 and 2023).
Data Availability Statement: Data are contained within the article or the Supplementary Material.
Acknowledgments: The authors would like to thank the Central Research Service for Science and
Technology from the University of Cadiz (Spain) for the use of their equipment, the Research Group
“FQM 249-Instrumentación y Ciencias Ambientales” from University of Cádiz for their assistance
and supply of the Microtrac NANOTRAC Wave DLS analyser and A. Gil Montero for FTIR analysis.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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