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JSCSEN 7 ( ) – (20 )9 1129 11310 217
ISSN 0352–5139(Print)
UDC 54:66
BELGRADE 2012
No 9
VOLUME 77 Available on line at
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J. Serb. Chem. Soc. 77 (9) 1287–1300 (2012) UDC 66.061+669.24/.26:631.422+541.45–36
JSCS–4352 Original scientific paper
1287
Ultrasound-assisted extraction of matrix elements and
heavy metal fractions associated with Fe, Al and Mn
oxyhydroxides from soil
SVETLANA M. STANIŠIĆ1, LJUBIŠA M. IGNJATOVIĆ1*#, IVAN ANĐELKOVIĆ2,
MILICA C. STEVIĆ1#, ALEKSANDRA M. TASIĆ1 and MARJETKA SAVIĆ BISERČIĆ1
1Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12–16,
Belgrade, Serbia and 2Faculty of Chemistry, University of Belgrade,
Studentski Trg 12–16, Belgrade, Serbia
(Received 29 September, revised 17 November 2011)
Abstract: Single agent extractions of major and trace metals from soil samples
were conducted by means of a rotary mixer and an ultrasonic bath with soni-
cation times of 10, 20, 30, 40 and 50 min. The sequential extraction was under-
taken according to the European Community Bureau of Reference. The ob-
tained soil extracts were analyzed by inductively coupled plasma-optical emis-
sion spectrometry and according to the results, the rotary mixer-assisted ex-
traction was more efficient in the case of alkaline earth elements. However, by
use of ultrasound, several times higher amounts of matrix elements (Fe, Al and
Mn) and heavy metals predominantly associated with Fe, Al and Mn oxyhyd-
roxides were extracted. The increase of the sonication time failed to improve
the extraction yields. The changes in the conductivity, pH, redox potential, par-
ticle size diameter and zeta potential of colloid particles with increasing soni-
cation time were measured. The extraction mechanism and expressed selecti-
vity of ultrasound is discussed and an explanation is suggested.
Keywords: soil analysis; extraction mechanism; sequential extraction; soil phases.
INTRODUCTION
The measurements of major elements in soil samples are required to expand
knowledge of the elemental composition of soil, while the assessment of the soil
trace metal content is of major importance nowadays, due to their toxic effects
and bio-accumulative nature. An excessive presence of metals in soils and sedi-
ments of industrial regions, particularly, and their potential leakage into surface
and groundwaters could pose environmental problems. Thus, the determination
of the metal amounts that are bound within soil solid phases and knowledge the
* Corresponding author. E-mail: ljignjatovic@ffh.bg.ac.rs
# Serbian Chemical Society member.
doi: 10.2298/JSC110929209S
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1288 STANIŠIĆ et al.
chemical mechanism of metal binding are important for predicting possible metal
transfer to the aquatic systems.
The total metal content in soil is partitioned between the solid phases, i.e.,
phyllosilicate minerals, carbonates, sulfides, Fe, Al and Mn oxyhydroxides and
organic matter.1 In addition, the different mechanisms of binding of metal ions to
the different phases (ion exchange, outer- and inner-sphere surface complexation
(adsorption), precipitation or co-precipitation) influence their mobility and bio-
availability to a great extent.2 In order to determine the manner in which the total
metal content is subdivided between the soil phases, fractionation of the metal
content, either by ion exchange processes or by dissolution of selected soil phase,
is required. For this purpose, a sequential extraction (SE) procedure was intro-
duced and widely accepted, and subsequently, numerous extraction schemes, i.e.,
according to Tessier, The European Community Bureau of Reference (BCR),
Gibbs, Ure, Campanella, etc.,3 in which different extraction agents and condi-
tions were suggested, were adopted. Many studies have been conducted in an at-
tempt to define the optimal extraction conditions and to harmonize operational
extraction procedure, since the results obtained by application of different extrac-
tion schemes are often non-comparable. In addition, SEs often give unreliable re-
sults because of the non-selectivity of the extraction agents simultaneously for
the selected phase and ions, precipitation of new mineral phases or redistribution
of ions between already existing soil phases during extraction.4 However, a ma-
jor disadvantage of the SEs is related to the fact that they are time and labor con-
suming, and according to the Tessier or BCR scheme require an overall operation
time of about 18 and 51 h, respectively. Thus, there exists considerable interest in
the development of ultrasound-assisted extractions (UAE) or microwave-assisted
extractions, which provide the same information as conventional SEs, but are
faster to realize.
Initially, ultrasonic energy was used for the dispersion of soil aggregates5 or
the disintegration of sewage sludge,6 since the ultrasonic cavitation phenomenon
together with the turbulent flow of aqueous suspension of soil and acoustic
streaming result in friction, stress and dispersion of soil aggregates. In addition,
ultrasonic energy has been widely used for accelerating the extraction of metal,7
aliphatic and polycyclic aromatic hydrocarbons8 and organochlorine pesticides9
from soil or other solid samples. Thereby, the ultrasonic energy was applied by
the use of an ultrasonic probe,10 an ultrasonic bath11 or cup-horn sonoreactors12
with the conclusion that the probe provides shorter extraction time (up to 100
times), while the ultrasonic bath enables simultaneous replicate extractions. Pe-
rez-Cid et al.13 described an ultrasound-assisted BCR SE of Cu, Cr, Ni, Pb and
Zn ions from sewage sludge, whereby the duration was only 22 min, while reco-
veries were similar to those obtained by the conventional BCR extraction. A si-
milar study was conducted by Davidson and Delevoye14 and as they reported, the
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1289
recoveries were similar to those of conventional shaking for all metals (Cu, Mn
and Zn) except from the important matrix element Fe. Väisänen and Kiljunen15
performed a five-step ultrasound assisted SE of As, Cd, Cu, Pb and Zn ions from
soil sample, according to the Tessier scheme. With the exception of the As con-
centrations, which were too high, the results of the UAE procedure were highly
comparable with the results obtained by the conventional procedure. Arain et
al.16 showed that by the use of optimized sonication conditions, a three-step BCR
UAE could be successfully completed in 15–30 min, thus providing considerable
time saving, with a high treatment rate and low sample and reagent usage. The
acceleration of the first step of the BCR extraction for trace and matrix elements
was investigated by Rusnák et al.,17 whereby the experiment included soil,
sediment and gravitation dust samples. The results showed that the effect of
ultrasound was different for all the studied sample types and each element. Ac-
cording to Filgueiras et al.,18 the ultrasound-assisted versions of the BCR extrac-
tion scheme showed a better performance than the Tessier ones in terms of ob-
taining good agreement with the conventional SE, with the best results being
found for metal partitioning in sewage sludge. To summarize, according to some
studies, UAEs have proved to be successful for achieving quantitative recoveries
from various environmental matrices, such as soil, sewage sludge, marine and
lake sediments and reference materials.19 However, according to others, ultra-
sonic energy did not affect all the types of solid samples in precisely the same
way as conventional shaking. When it comes to soil sample, the difficulty of de-
veloping a rapid version of SEs is related to the different fractionation patterns
obtained by UAE in comparison to conventional ones, mainly for the matrix ele-
ments, such as Fe.4
The aim of this study was to investigate the influence of ultrasonic energy on
aqueous suspensions of soil, through changes in different physico-chemical para-
meters, in order to determine whether ultrasound could be used for accelerating
the extraction of major and trace elemental from soil samples.
A sample of serpentinite soil type Ranker was used as the substrate in this
research. Rotary mixer-assisted extraction (RAE) and UAE were performed as
single extractions using deionized water as the only extracting agent. A SE was
performed in order to assess the amounts of heavy metals associated with diffe-
rent soil phases. For the determination of the cation concentrations in the soil ex-
tracts obtained by RAE, UAE and SE, inductively coupled plasma-optical emis-
sion spectrometry (ICP-OES) measurements were performed. For all the soil sus-
pension during the UAEs, conductivity (
κ
) measurements were continuously per-
formed. Additionally, the UAEs were repeated in order to measure the suspension
parameters, i.e., the oxido-reduction potential (ORP), the particle size diameter
(PSD), the zeta potential of the colloidal particles (ZP) and the pH, during the
ultrasonic treatment.
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EXPERIMENTAL
The soil sample was taken from a site covered with natural vegetation, at the location
Bubanj Potok, near Belgrade, Serbia, which had been exposed to minimal influences of an-
thropogenic pollution. The geographical coordinates of the location are 44° 44’ 4” North, 20°
32’ 36” East and the height is 157 m above sea level. The soil sample, weighing 1 kg in total,
was obtained by combining samples taken from the surface horizon, rich in humus, from 30
different sites, at a depth of 20 cm. The depth of the total soil profile at this location is 50 cm.
The sample was air dried for 72 h. Subsequently, the large fractions were removed, crushed in
a mortar and sieved through a 1 mm pore diameter sieve. The basic pedological analysis in-
cluded: the potentiometric determination of the pH in H2O and 1.0 mol L-1 KCl, the humus
content after the Turin method, the adsorptive complex of the soil (H, T, S) after Kappen,
determination of the soil texture by the pipette method, determination of the hygroscopic
moisture by drying at 105 °C and determination of the mass loss during heating at 700 °C for
30 min.
The extraction suspensions were prepared in 50 mL volumetric flasks by mixing the soil
sample with deionized water in a ratio 1:10, i.e., 2 g:20 mL. Two series of three extractions
each were performed using an Overhead Mixer Reax 20/8 (Carl Roth, Germany) rotary mixer
in which the suspension was processed for 22 h mixing at 10 rpm at room temperature (20 °C).
The second technique involved the use of an ultrasonic bath with the suspension positioned at
the same place in the bath and at the same initial water temperature, 17 °C. Two extractions
were made for each of the following extraction times: 10, 20, 30, 40 and 50 min. A Transsonic
T 760 DH (Elma, Germany) ultrasonic bath operated at an ultrasonic frequency of 40 kHz and
effective ultrasound power of 170 W was employed for these experiments. Deionized water
(18.2 MΩ cm) produced by a Milli-Q Reagent Grade system (Phenomenex, USA) was used
for the extractions and preparation of all suspensions. The substances used for the analyses
were of high analytical purity. After the extraction processes were completed, each of the
extraction suspensions was first centrifuged, then filtered through medium pore size filter
paper and finally through 0.2 µm pore size syringe membrane filter (Phenomenex, USA). The
thus obtained soil extract was acidified by addition of 1 µl of 70 % concentrated perchloric
acid (Merck, Germany) per 1 mL of extract and preserved at 4 °C in a laboratory refrigerator
for further analysis.
Furthermore, the soil sample was subjected to SE according the scheme suggested by the
Standards, Measurements and Testing Program of the European Commission (BCR, for-
merly).3 The last step of the extraction procedure using aqua regia, according to EPA 3050B
digest extract method was added in order to determine the total metal contents. Additionally,
in order to distinguish between the easily reducible fraction bound to Mn oxyhydroxides and
the moderately and poorly reducible one bound to amorphous and crystalline Fe and Al
oxyhydroxides, the procedure was modified by the inclusion of a third extraction step.20 Dry
soil samples (1 g) were weighed into 50 mL polystyrene flasks and after addition of the ex-
traction agents, the flasks were shaken on a rotary mixer at 15 rpm. The residue was washed
with 25 mL of deionized water, and centrifuged at 3000 rpm, prior to the next extraction step.
The extraction was performed in triplicate, according to the procedure summary (Table I).
A Thermo Scientific iCAP-6500 DUO ICP (Thermo Fisher Scientific, UK) spectrometer,
with continuous wavelength coverage ranging from 166 to 847 nm, equipped with RACID86
charge injector device (CID) detector, pneumatic cross-flow type nebulizer and quartz torch,
was used for the ICP-OES measurements. The instrumental conditions were set at: input
power, 1150 W; auxiliary gas flow, 0.5 L min-1; coolant gas flow, 12 L min-1 and nebulizer
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1291
flow, 0.7 L min-1. For ICP-OES calibration, multi-element plasma standard solution 4, Spec-
pure (Alfa Aesar, Germany, Cat. No. 42885) was used and two series of standard solutions
were prepared: for alkali and alkaline earth metal elements of concentration 0.01, 0.05, 1, 10
and 50 mg L-1 and for the transition elements of concentration 0.1, 1, 10, 100 and 500 µg L-1.
The relative standard deviation was calculated automatically, RSD = (SD/µ)×100, where SD is
standard deviation and µ represents the mean value of three measurements. After accuracy
evaluation, the relative standard deviations of the ICP-OES measurements were determined to
be: Al, 1.06 %; Ca, 0.21 %, Cd, 8.69 %; Co, 6.91 %; Cr, 7.17 %; Cu, 6.21 %; Fe, 0.63 %; K,
2.68 %; Mg, 0.79 %; Mn, 4.86 %; Na, 0.82 %; Ni, 8.42 %; Pb, 7.62 % and Zn, 8.67 %.
TABLE I. A summary of the SE operating conditions
Elements fraction Extraction
time, h
Agitation
method
Extractant
amount, mL Extraction reagent
Water-exchangeable,
weakly adsorbed
16 Shaking,
room temp.
40 0.11 mol L-1 acetic acid (HOAc)
Easily reducible (Mn
oxyhydroxide phase
bound)
16 Shaking,
room temp.
40 0.1 mol L-1 NH2OH·HCl/HNO3
pH 2
Moderately reducible
(Fe, Al oxyhydroxide
p
hase bound)
10 Shaking,
room temp.
40 0.2 mol L-1 ammonium oxalate/
0.2 mol L-1 oxalic acid
Oxidizable
(organically bound)
3 Occasional
agitation,
85 °C
2x10 30 % (8.8 mol L-1) H2O2/HNO3
pH 2
16 Shaking,
room temp.
40 1 mol L-1 NH4OAc, pH 5
Residual 0.5 Water bath,
95 °C
10 Aqua regia, HNO3/HCl (1:3)
With the exception of
κ
, for the determination of each suspension parameter, additional
UAEs were performed in five replicates with an aqueous soil suspension. In order to adjust
ionic strength for the
κ
measurement, the extraction solutions were prepared with an ionic
strength buffer (1 mol L-1 KCl) instead of deionized water. The conductivity, ORP and pH
measurements were performed using a 3540 Conductivity/pH meter (Jenway, UK). The PSD
and ZP measurements were performed by dynamic light scattering spectroscopy using a Zeta-
sizer Nano Red ZS, with a 633 nm He–Ne laser (Malvern, UK). The instrumental conditions
were set at: run duration, 10 s; temperature, 25 °C; refractive index of material, 1.600 and ab-
sorption index, 0.01. The measurements were conducted during the extraction process every
10 min for the colloid PSD and ZP, every 5 min for ORP and pH, while the value of
κ
was
measured continuously. For the duration of the pH and ORP measurements, the ultrasonic bath
was turned off.
RESULTS AND DISCUSSION
The measured pH value in 1 mol L–1 KCl was 6.0 and in deionized water,
the pH value was (5 g soil:10 mL water) 6.9. The results of the other pedological
analyses are given in Table II.
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TABLE II. The results of the basic pedological analysis
Soil parameter No. 1 No. 2 No. 3 Mean value
Humus, % 5.15 5.23 5.12 5.16
Total C, % 2.99 3.03 2.97 2.99
The sum of base cations, mEq 100 g-1 34.6 33.5 39.6 35.9
Large sand particles, % 4.87 6.22 6.83 5.97
Small sand particles, % 30.10 30.8 31.58 30.85
Colloid clay, % 45.75 43.61 40.99 43.45
Silt, % 19.28 19.28 20.60 19.72
Hygroscopic moisture, % 3.1 3.1 3.1 3.1
Heating loss, % 14.2 14.6 14.4 14.4
According to the results, the investigated soil sample contained low levels of
some elements; thus, the extracted amounts of Li were in range from 0.003 to
0.024 mg 100 g–1 soil, the extracted amounts of Cd were in the range of 0.386 to
0.963 µg 100 g–1 soil and for Co the values ranged from 0.101 to 0.969 µg 100
g–1 soil. Since the amounts of these elements extracted by both extraction tech-
niques were too small for conclusions to be drawn, these elements were not taken
into further consideration.
A comparison of the amounts of the elements extracted by UAE and RAE is
shown in Fig. 1, from which it can be seen that significantly higher amounts of
the matrix elements (Fe, Al and Mn) were extracted in the UAE than in the RAE.
Compared to the amounts extracted using the rotary mixer, the average amounts
of Fe and Al ions extracted during the UAE were 7.5 and 8.2 times higher, res-
pectively. However, in the case of the alkaline earth elements (Ca and Mg), higher
amounts were extracted in the RAE than in the UAE. Considering the concentra-
tions of Mg ion, the high amounts extracted using both techniques could be ex-
plained by the properties of the soil itself, since Ranker over serpentinite type of
soil has a ratio Ca:Mg<1, unlike other soil types.21 The lower amounts of the al-
kaline earth elements were obtained using UAE compared to those extracted by
RAE could be explained either by the significantly longer agitation time of the
RAE (22 h) and subsequent dissolution, or by the re-adsorption of extracted cat-
ions onto the sorption sites newly exposed by the influence of ultrasound.
The amounts of all elements extracted in the UAE as a function of sonication
time are shown in Fig. 2, from which it can be seen that the extracted amounts of
all elements changed, which was most noticeable in the case of the matrix ele-
ments (Fe, Al and Mn) and some trace elements (Cr, Cu and Zn).
During the UAE, the pH also changed in the range of 5.60 to 8.12 (Fig. 3).
As it can be seen in Fig. 3, the largest increase in pH to 8.12 was registered after
20 min of sonication, at the same time a decrease of the acid (Al and Fe) and the
increase of the base cation (Mg and Ca) concentrations were observed in the ex-
tract. At an extraction time of 40 min, the extracted amounts of both base and
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1293
acid cations were the largest observed, but the pH fell simultaneously to 7.58, in-
dicating that the total content of acid cations (Al, Fe and Mn) exceeded the total
content of base cations (Ca and Mg). With further increase in the sonication time,
the extracted amounts of Fe, Al and Mn decreased and a subsequent increase in
the pH of the soil suspension was observed. It is assumed that the changes in the
pH value were caused by adsorption and desorption processes, but were not the
cause of these processes. Large amounts of various cations were released into the
solution, influencing the pH change. This pH change can influence the adsorption
and desorption processes to some extent, but these processes are mainly related
to competitive cation interactions for the sorbing phase and the exchange of sor-
bed cations with those from solution is influenced by ultrasound.
Fig. 1. Comparison of the average amounts extracted using a rotary mixer and ultrasound.
The results of the SE procedure, obtained for Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe,
Al and Mn, are shown in Table III. Considering that deionized water was used as
the extraction agent in the UAE and RAE processes, the extracted amounts are
significantly lower compared to the amounts extracted by means of the chemical
agents proposed in the SE procedure. Thus, the results present the percentage of
the total content of elements contained in the water-exchangeable, easily and
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1294 STANIŠIĆ et al.
moderately reducible, oxidizable and residual fraction. Although not the most
abundant in the soil, the oxyhydroxides of Fe, Al and Mn have a large surface
area, which makes them important reactive phases with respect to metal sorption.
Concerning the other fractions, the largest amounts of Cr and Cu ions were con-
tained within the moderately reducible fraction and hence associated with the
amorphous and crystalline Fe, Al oxyhydroxides.
Fig. 2. The effectiveness of the UAE for the extraction of Na, K, Ca, Mg, Pb, Mn,
Cr, Zn, Cu and Ni ions as a function of sonication time.
Fig. 3. The effectiveness of the UAE for the extraction of Fe and Al ions (left)
and changes in pH value (right) as a function of sonication time.
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1295
Furthermore, the largest amounts of Cd, Co, Ni, Pb and Zn ions were con-
tained within the easily reducible fraction of elements, which are associated with
Mn oxyhydroxides. The manner in which metals are retained in the soil refers to
the type of surface sorption complex, which is influenced by surface loading, as
well as by pH, ionic strength and type of sorbing phase. Hence, outer-sphere
complexation is a rapid and reversible process that involves electrostatic interac-
tions and occurs on surfaces of opposite charge. On the contrary, inner-sphere
complexation can increase or reduce the surface sorptive charge regardless of the
original charge.22 Since sorption includes adsorption, precipitation and polymeri-
zation, the sorption mechanisms can be determined only through molecular in-
vestigations by use of spectroscopic techniques.
TABLE III. The results obtained by the SE procedure. Participations of different metal frac-
tions in total metal content, %. Bold values present the largest non-residual fraction for each
element
Fraction Cd Co Cr Cu Ni Pb Zn Fe Al Mn
Water-exchangeable – 0.05 – – 1.4 0.01 – – – 1.4
Easily reducible (Mn oxyhydroxides) 90.3 49.9 0.9 1.2 17.1 18.9 15.4 1.2 1.6 73.5
Moderately reducible (Fe and Al
oxyhydroxides)
9.7 14.7 17.8 22.7 7.1 1.3 6.5 13.2 6.2 10.3
Oxidizable – 3.9 16.3 0.05 15.8 5.9 5.1 0.1 0.3 1.6
Residual – 31.5 65.0 76.0 58.6 73.9 72.9 85.5 92.0 13.2
The changes in extracted amounts of some trace elements (Cr, Cu and Zn) as
a function of sonication time are dependant on the cation sorption mechanism, as
well as on the type of mineral or organic matter with which they are associated.
According to Charlet and Manceau,23 Cr (III) can be found on goethite (iron oxy-
hydroxide) surfaces, either adsorbed as an inner-sphere complex or in the form of
Cr hydroxide surface precipitates. The results of the SE showed that 17.8 % of
the total Cr content (50.8 % of the non-residual content) was associated with Fe
and Al oxyhydroxides. In keeping with this, as shown in Figs. 2 and 3, the in-
creases in the extracted amounts of Fe and Cr with the extraction time follow the
same change pattern. The same pattern of change was observed in the case of Mn
and Zn. Scheinost et al.24 demonstrated that Zn(II) can form both inner-sphere
surface complexes and a Zn hydrotalcite-hydroxide phase upon sorption to Al-
bearing minerals, inner-sphere surface complexes on goethite and both inner-
sphere and multinuclear hydroxo-complexes on manganite surfaces. In the stu-
died soil sample, Zn had predominantly formed complexes on manganite sur-
faces since, according to the SE results, 70.3 % of the non-residual content of Zn
was associated with Mn oxyhydroxides within the easily reducible fraction. Ac-
cording to Weesner and Bleam,25 Cu(II) can form outer- and inner-sphere surface
complexes upon sorption to boehmite (Al oxyhydroxide). This observation was
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1296 STANIŠIĆ et al.
confirmed by the results of the SE, which showed that 94.8 % of non-residual
content of Cu was found within the moderately reducible fraction. However, the
variations in the changes of pattern of the extracted amounts of Cu and Al were
not the same, which implies that certain portions of the metal released by ultra-
sonic treatment were re-adsorbed onto the other remaining soil phases.
An explanation could be suggested for the high amounts of extracted matrix
elements (Fe, Al and Mn), as well as to the variations of the amounts of extracted
cations with increasing sonication time. First, the influence of the ultrasound on
the aqueous soil solution could be attributed to the cavitation effect. The cavi-
tation is caused by interaction of the ultrasound with soil solution, by implosion
of cavitation bubbles, subsequent increases in the local pressure and the for-
mation of elastic shock waves. This leads to the production of localized high tem-
peratures in the solution and thus creates extreme conditions for chemical reac-
tions to occur. At the beginning of the ultrasonic extraction, the temperature of
the water was 17 °C and the increase in temperature was 11 degrees, which makes
28 °C at the end of the extraction process. This rise in the temperature influences
on the amounts extracted, however it is hard to measure to what extent. The ma-
ximum temperature Tmax and maximum pressure Pmax in the cavitation bubble
before collapse is defined as:
T
max = Tin [Pa (γ – 1)/Pin] (1)
P
max = Pin [Pa (γ – 1)/Pin]γ/(γ – 1) (2)
where Tin and Pin are the initial temperature and pressure in the bubbles, res-
pectively, Pa is the acoustic pressure at the beginning of collapse and γ is the ave-
rage specific heat ratio at constant volume of the gas in the bubble.26 Further-
more, the temperature and pressure rise inside cavitation bubbles leads to the
formation of free radicals. Related to this, the application of ultrasound for de-
composition of different organic contaminants in water has been widely re-
searched. In these reactions, the production of highly reactive oxygen species by
sonolysis of water molecules, i.e., hydroxyl-, peroxy- and superoxide- radicals, is
considered to play an important role. Their generation is supposed to occur on the
inside/interface of the adiabatic cavitation bubbles that are formed in aqueous
suspensions during sonication.27 In the present case, the effectiveness of the
sonochemical decomposition of water was also highly dependent on the presence
of soil particles, which could facilitate the cavity formation process and thereby
intensify the generation of radical species. According to Tuziuti et al.,28 the pre-
sence of soil particles can also influence the scattering of the ultrasound waves
followed by subsequent attenuation of the cavitation effect and decrease in for-
mation of reactive species. Once generated, highly reactive species influence
changes in the redox status in the soil suspension. The changes in the redox status
have a large influence on the mobility of certain metal species, such as Fe, Mn
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1297
and Cr. Under reductive conditions, oxyhydroxides of Fe and Mn are subjected to
reductive dissolution and hence metal species, associated with these phases can
have changes in solubility. Related to this, the conducted measurements showed
that the ORP of the extraction suspension increased within 5 min after stopping
sonication from 30 to 130 mV vs. Ag/AgCl, 3 mol L–1 KCl reference electrode,
probably due to the generation of multiple redox couples.
In addition to this, ultrasound has an influence on desorption processes and
the ability of ultrasonic energy to intensify desorption processes and to release
metals from soil, activated carbon or other materials with high sorption capacity
is well documented. Hwang et al.29 showed that the efficiency of leaching solu-
tions (both citrate and EDTA) for heavy metal removal from soil was increased
with sonication compared to those of soil washing. Hamdaoui et al.30 explored
the effects of ultrasound on the desorption of metal ions from activated carbon,
and the results of the conducted study indicated that the desorption rates of
Cu(II), Mn(II), Hg(II) and Cr(VI) were significantly improved by ultrasonic irra-
diation.
Beside desorption, re-adsorption processes are also intensified under the in-
fluence of ultrasound. The re-adsorption process and subsequent decrease of the
extracted amount after 40 min of sonication occurred in the cases of all other
mentioned elements, also of Fe, Al, Mn, Cr and Zn. The rate of adsorption and
re-adsorption in soils is dependent on the type and quantity of inorganic and
organic components and the charge and radius of the ion being considered.31 Sur-
face reactive sites of the soil phases comprise permanent and variable charge
sites, depending on their origin. For example, various functional groups, such as
phenolic, carboxyl and alcoholic, are found on organic molecules. The major in-
organic surface functional groups are the siloxane, associated with the silica tet-
rahedral layer of phyllosilicates and hydroxyl groups, such as silanol and alu-
minol, originating from broken mineral lattice of clay minerals or associated with
the edges of metal oxyhydroxides. However, besides the mentioned reactive
groups, ultrasonic energy can cause fragmentation of the soil particle and disper-
sion of soil aggregate, thus increasing the surface area available for reaction with
the extraction agent. Sonication produces a significant increase of the specific
surface area due to particle size reduction; hence, the number of sites available
for adsorption is increased. Factors affecting soil aggregate dispersion during ul-
trasound application are the soil–water ratio, the total applied energy and the
power output per volume of the extracting suspension. This was explained by en-
hanced surface diffusivity, which is related to the phenomena induced by acoustic
cavitation, such as acoustic vortex micro-streaming, high-speed microjets, high-
pressure shock waves and intense localized heating. In the aqueous soil suspen-
sion, ultrasonic treatment modifies the particle size, morphology and structural
order of clay minerals. The elongated crystals are broken up into smaller units
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1298 STANIŠIĆ et al.
that retain the typical lamellar morphology of the starting crystals.32 The results
of PSD analysis of the sonicated suspensions are given in Table IV, from which it
could be seen that the mean particle size decreased with increasing sonication time.
Thereby, the ZP of colloid particles ranged from –49.54 to –11.08 mV with
the largest change being observed between 40 and 50 min of sonication. ZP va-
lues more negative than –30 mV are considered to represent sufficient mutual
repulsion to ensure the stability of a suspension.33 It could be assumed that due to
Fe, Al and Mn sorption processes in the last 10 min of sonication, the surface
charge of colloidal particles decreased, and simultaneously the PSD showed a
slight increase. As the results showed (Table IV), the suspension
κ
increased
during sonication, partly due to the heating of the suspension and partly due to an
increase in the number of total charged particles in the suspension.
TABLE IV. Changes in the colloid PSD, ZP and conductivity of extraction suspension as a
function of sonication time
Extraction
time,min
Detected particle
size fraction, nm
Fraction par-
ticipation, %
Average par-
ticle size, nm
Zeta potential
mV
Conductivity
µS cm-1
10 4843 14.2 1258 –25.42 18
1127 39.8
265 46.0
20 4853 12.7 1090 –35.82 20
860 47.8
158 39.5
30 4886 10.2 1074 –28.04 23
690 81.4
163 8.4
40 4294 9.3 966 –49.54 28
653 85.7
141 5.0
50 4731 7.7 970 –11.08 38
691 86.3
156 6.0
CONCLUSIONS
As indicated above, the influence of ultrasonic energy on the soil sample
preparation was found to be significant. However, not only due to a possible de-
crease in the extraction time, but mainly due to the expressed selectivity for the
matrix elements (Fe, Al and Mn) and heavy metal portions associated with Fe, Al
and Mn oxyhydroxides. As shown, the interaction of ultrasonic energy with the
aqueous soil suspensions, alternately, influenced the processes of cation adsorp-
tion and desorption, thereby leading to a change in the extracted amounts of cat-
ions as function of sonication time. In addition to the aforementioned, the inter-
action of ultrasonic energy with the soil suspension created extreme conditions
for chemical reactions to occur, some of them resulting in the generation of
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EXTRACTION OF METAL OXYHYDROXIDES ASSOCIATED FRACTION 1299
highly reactive species. Therefore, it could be concluded that ultrasound cannot
simply replace conventional treatments, such as conductive heating, in each ex-
traction step of the sequential procedure. The introduction of ultrasound requires
further investigation, first, to determine whether it has the same effect on diffe-
rent soil samples. Further investigations are required in order to explore whether
ultrasound can be used to assess the heavy metal fraction associated with the
easily reducible Mn oxyhydroxides and moderately reducible Fe and Al oxyhyd-
roxides. For this purpose, ultrasound could be combined with deionized water
only in particular SE steps for the efficient enhancement of the SEs, since com-
pared to salt and acid solutions, deionized water is a preferable extractant, lead-
ing to avoidance of sample contamination.
Acknowledgments. The authors acknowledge the financial support of the Ministry of
Education, Science and Technological Development of the Republic of Serbia (Grant 172030/
/2011) for funding this research.
ИЗВОД
ЕКСТРАКЦИЈА ГЛАВНИХ ЕЛЕМЕНАТА И ФРАКЦИЈЕ ТЕШКИХ МЕТАЛА
ВЕЗАНИХ У СКЛОПУ Fe, Al И Mn ОКСИДНИХ ФАЗА ЗЕМЉИШТА
ПОТПОМОГНУТА УЛТРАЗВУКОМ
СВЕТЛАНА М. СТАНИШИЋ1, ЉУБИША М. ИГЊАТОВИЋ1, ИВАН АНЂЕЛКОВИЋ2,
МИЛИЦА Ц. СТЕВИЋ1 и АЛЕКСАНДРА М. ТАСИЋ1
1Факултет за физичку хемију, Универзитет у Београду, Студентски Трг 12–16, Београд и
2Хемијски факултет, Универзитет у Београду, Студентски Трг 12–16, Београд
Вршена је екстракција главних катјона и катјона елемената у траговима из узорка
земљишта помоћу дејонизоване воде употребом ротационе мућкалице и ултразвучне
каде са екстракционим временом од 10, 20, 30, 40 и 50 min. Узорак земљишта је под-
вргнут секвенционалној екстракцији према BCR процедури. Садржај добијених екстрак-
та земљишта је одређен оптичке емисионе спектрометрије са индуктивно спрегнутом
плазмом, и према резултатима екстракција изведена помоћу ротационе мућкалице се
показала ефикасном у случају земноалкалних елемената. Употребом ултразвука екстра-
ховане су неколико пута веће количине матрикс елемената (Fe, Al и Mn) и тешких ме-
тала који се у земљишту претежно налазе у склопу Fe, Al и Mn оксидне фазе. Продужење
екстракционог времена није резултирало повећањем екстраховане количине. Током ул-
тразвучне екстракције вршена су мерења проводљивости, pH, оксидоредукционог потен-
цијала суспензије земљишта, величине и зета потенцијала колоидних честица.
Предложено је објашњење механизма екстракције и утицаја ултразвука на екстракцију
главних елемената и тешких метала из оксидних фаза земљишта.
(Примљено 29. септембра, ревидирано 17. новембра 2011)
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