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Removal of Humic Substances from Drinking Water Using GAC and Iron-Coated Adsorbents: Consideration of Two Kinetic Models and the Influence of Mixing
Abstract
This paper investigated the removal of humic substances (HS) from drinking water supplies by adsorption onto granular activated carbon (GAC), iron-coated alumina (AAFS) and ferric oxihydroxide (β-FeOOH). The determination of the rate limiting step of the adsorption process was tested using the pseudo-first and second order kinetic models. Two mixing speeds were considered namely; 200 and 300 rpm. Disintegration (attrition) of the adsorbents occurred above 300 rpm while 200 rpm represented the minimum mixing speed required to keep the solution in suspension. During the first few minutes of adsorption at 300 rpm, strong instantaneous adsorption was observed. Among the three adsorbents, AAFS showed the least affinity for the HS with no instantaneous adsorption at all at 200 rpm. Furthermore, the pseudo-first order model failed to represent the whole range of data while the second order model was the best fit for all the experimental range considered in this study.
... It can also have a detrimental effect on fertility of soil, causes water acidity, influence pollutants' transport and mitigation and reduces the efficiency of chemical disinfection. Therefore, it is important to characterize the HS present in water to ensure suitable drinking water treatment and improvement of water quality [12][13][14][15]. HS may comprise of various components with different molecular weights. ...
... The heterogeneity of HS causes them to have broader distribution of adsorption affinities. It was demonstrated that HS with higher molecular weights and hydrophobic nature possess greater potential for adsorption on mineral adsorbents [12,16]. On the other hand, enhanced iron oxide mineral-containing carboxylic functions groups were used [17] to adsorb fulvic acid components, pattern of fractionation which is in consistence with surface complexation processes. ...
... All experiments were run in duplicate and were found to be reproducible. The results of the kinetic studies [12], indicated that equilibrium is reached in 4 days when the adsorbents are mixed with the HS solution. Therefore, results presented in this paper correspond to equilibrium analyses performed, using a centrifuge at 200 rpm, after 4 days contact between the adsorbate (HS) and the abovementioned adsorbents. ...
The adsorption of humic substances on three different adsorbents was investigated and adsorption isotherms were applied in this research. The three adsorbents studied include granular activated carbon (GAC), ferric oxihydroxide in its beta form (β-FeOOH) and iron-coated activated alumina (AAFS). Physical and chemical characteristics of the adsorbents were also fully investigated. Calcium was added to the HS solution in order to represent water with a hardness equivalent to water hardness typically found in London (UK). The examination of the GAC indicated a large microporous area with lower surface area associated with meso- and macropores. The AAFS and β-FeOOH did not present any microporous area. The overall surface area was high for GAC (980 m2 g−1) but lower for AAFS (286 m2 g−1) and β-FeOOH (360 m2 g−1). The Freundlich isotherm model was fitted to all adsorbent–adsorbate systems. It was shown that GAC offered a large adsorption capacity for removal of low molecular weight humics F1 (MW 0–5 kDa) but not for substances with molecular weight larger than 10 kDa (F3). The β-FeOOH adsorption capacity was only 0.43 mg g−1, compared with 9.11 and 2.55 mg g−1 on GAC and AAFS, respectively. On the contrary, F1 is not well adsorbed and only F2 (5–10 kDa) can be efficiently removed by AAFS and β-FeOOH. It was strongly suggested that precipitation/condensation occurred on the adsorbent surface.
... Humic substances are heterogeneous mixtures of polydispersed materials formed by biochemical reactions through both decay and transformation of plant and microbial remains [1,2]. Humic substances usually enter the surface waters through rainwater runoff from the surrounding land [2,3] and are also found in the groundwater, aquatic environment, and sewage plant effluents [4]. Humic acid (HA) is a principal component of humic substances, and the presence of HA in drinking water has been reported to produce serious problems, such as undesired color and taste, membrane fouling, biofilm in pipe line, and reduction of the lifetime of granular activated carbon [3]. ...
... Humic substances usually enter the surface waters through rainwater runoff from the surrounding land [2,3] and are also found in the groundwater, aquatic environment, and sewage plant effluents [4]. Humic acid (HA) is a principal component of humic substances, and the presence of HA in drinking water has been reported to produce serious problems, such as undesired color and taste, membrane fouling, biofilm in pipe line, and reduction of the lifetime of granular activated carbon [3]. Moreover, HA can react with chlorine during disinfection processes and produce carcinogenic by-products like trihalomethanes [3]. ...
... Humic acid (HA) is a principal component of humic substances, and the presence of HA in drinking water has been reported to produce serious problems, such as undesired color and taste, membrane fouling, biofilm in pipe line, and reduction of the lifetime of granular activated carbon [3]. Moreover, HA can react with chlorine during disinfection processes and produce carcinogenic by-products like trihalomethanes [3]. ...
The feasibility of applying nanoscale TiO2 with coconut shell powder (TCNSP) composite to remove HA in aqueous solution was evaluated, and the optimization of the photocatalytic systems using newly-developed TCNSP composite was carried out. Newly-developed TCNSP composite has high specific surface area (i.e., 454 m2/g) and great porosity (i.e., 66.9%) with pore size of less than 5μm. High removal efficiencies (≥ 95%) of HA were observed due to the significant synergistic effects by coupling adsorption and photocatalytic reaction of TCNSP composite. As the initial concentration of HA increased, the degradation rate (Kapp) decreased due to HA sorption saturation to the surface of TCNSP composite and the photon interception by HA molecules in aqueous solution. Since the increased loading amount of TCNSP composite enhanced the number of active sites, Kapp values increased until the optimum loading amount of TCNSP composite. As pH values increased, HA removal efficiency decreased due to increasing electrostatic repulsion between HA and TCNSP composite. Based on the response surface methodology, higher HA removal efficiencies were obtained with acidic condition, longer reaction time and appropriated loading amount of TCNSP. Further pilot-scale study is in progress using TCNSP composite combined with UVC to remove HA from large amounts of surface water (i.e., 200 m3/d).
... For a system in which new solid phases of various size and density are present, used mixing designs and mixing intensities should be one of the major sources of reported discrepancies. In fact mixing may hold original and new formed particles in suspension [24,25], thereby disturbing the natural layered disposition of Fe 0 and Fe-oxides. ...
... In laboratory batch experiments mixing operations are mainly used for two purposes [24,25]: [20,21]. Moreover, vigorous mixing can also induce two negative effects: (i) breaking the material grains subject to friability (attrition) [25], and (ii) eliminating the diffusion inhibition of surface processes [26]. ...
... In laboratory batch experiments mixing operations are mainly used for two purposes [24,25]: [20,21]. Moreover, vigorous mixing can also induce two negative effects: (i) breaking the material grains subject to friability (attrition) [25], and (ii) eliminating the diffusion inhibition of surface processes [26]. Tomashov and Vershinina [26] have shown that a sufficiently vigorous stirring and continuous renewal of the Fe 0 surface (scouring) eliminates the inhibition of the electrode-process step associated with adsorption or the formation of surface layers. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... Consequently, they can coagulate and be filtered through biofilters or membranes. However, they cover such a large molecular mass range (from one to several thousands daltons) that a satisfactory removal mechanism has been difficult to determine, prompting an investigation into this particular problem [1]. ...
... Equilibrium data from batch experiments provided valuable information regarding the adsorption of humic substances (HS) on granular activated carbon (GAC), iron coated activated alumina, AAFS, and ferric oxihydroxide in its beta form, akaganeite -FeOOH [1]. However, water treatment works and industries usually operate on a continuous flow basis and the batch tanks are replaced by columns, filled with the adsorbent. ...
... Solutions F1 (HS < 5 kDa), F12 (HS < 10 kDa) and F123 (HS < 50 kDa) were produced, stored in a dark fridge at 4 • C and were used within 3-4 weeks. Further details on the experiments can be found in [1]. The solution was diluted with deionised water to reach a dissolved organic content (DOC) of approximately 10 mg L −1 . ...
The efficiency of iron coated activated alumina, ferric oxihydroxide and granular activated carbon for the removal of humic substance (HS) from water and wastewater was demonstrated in batch studies. This paper investigated the use of these successful adsorption–adsorbate systems in a continuous flow mode in an effort to obtain the required design and operational data; essential for successful application in water treatment works. The adsorbents were used individually. The influence of the various molecular mass fractions on the adsorption ability of the various systems was considered. Fractionation of humic substances was carried out and low, medium and high molecular mass fractions were produced. Two different column diameters (2.5 and 1 cm) were used in the experimental runs. The smaller diameter column was used for experiments dealing with the lower molecular weigh humics as these are very difficult to produce in large enough quantities to carry out continuous adsorption runs in the traditional 2.5 cm diameter laboratory adsorption columns. The dissolved organic content (DOC), ultraviolet (UV) absorbance and specific ultraviolet absorbance (SUVA) values were measure and analysed under different operating conditions. Furthermore, the experimental results were modelled using the Thomas model and the empty bed contact time model (EBCT). The Mass Transfer Zone was also analysed for all cases. The results indicated strongly the influence of molecular mass in the adsorption of humic substances. In addition, two distinctive features appear from these results; GAC is able to remove high MM although lacking of mesoporosity (attributed to precipitation and alteration of HS conformation) and β-FeOOH does not show such a high adsorption capacity as previously predicted, attributed to the non-equilibrium state and to the lack of surface under the media compaction.
... The existence of carboxylic and phenolic groups results in predominantly carrying negative charges in aqueous solutions [1]. Soluble humic substances such as humic acid come from the leaching of soils, sediments, aquatic animal and vegetal life, as well as from the effluents of sewage treatment works [6,7]. ...
... Several factors affecting the adsorption process, such as pH (3)(4)(5)(6)(7)(8)(9), initial concentration (20-80 mg/l), and carbon type (AC-0 and AC-Fe) were studied. In most of experiments, three samples were prepared and tested and the average values are reported. ...
Humic acid come to water sources from the leaching of soils, sediments, aquatic animal and vegetal life and effluents. The high humic acid concentration in drinking water has the potential for some serious diseases, because it produces disinfection by-products such as trihalomethanes. So removal of humic from water supply is very important.The tests of humic acid adsorption onto iron-coated activated carbon (AC-Fe) samples were conducted in batch mode. The humic acid content values were measure and analysed under different operating conditions. The effect of various factors, e.g., pH, initial concentration, and carbon type on the adsorption capacity, was quantitatively determined.From the batch experiment results, the humic acid could be removed by the iron-coated activated carbon. These studies show that humic acid adsorption on AC-0 (activated carbon without carbon as blank) and AC-Fe was 40.56 mg/g and 60.72 mg/g respectively with 30 mg/l initial concentration, pH 5, and 28 oC temperatures. The maximum adsorption conducted at AC-Fe was 80.13 mg/g with 80 mg/l initial concentration. The efficiency of iron-coated activated carbon for the removal of Humic acid adsorption will decrease with increasing pH. Iron-coated activated carbon is effective adsorbents for removal of humic acid from water.
... Humic substances are ubiquitous throughout aquatic and terrestrial ecosystems and are formed by biochemical and chemical reactions during the decay and transformation of plant and microbial remains (Stevenson 1994;Andre and Khraisheh 2009). Generally, humic acid (HA) is considered to be the major fraction and extractable component of humic substances, and consist of heterogeneous mixtures of macromolecules with a wide range of molecular weights and different functional groups (Stevenson 1994). ...
Humic acids (HA) are known as the precursors of carcinogenic compounds formed by the disinfection of drinking water. While conventional treatments were found to be inefficient HA removal processes in drinking water, advanced oxidation processes have been proven to have a significant effect in the treatment of HA. The degradation of HA was investigated using nano-sized zinc oxide (ZnO)/laponite composite (NZLC). The reactions occurred in a UVC reactor by considering following variables: pH, initial HA concentration, catalyst loading, addition of hydrogen peroxide (H2O2), and catalyst reuse. Water samples containing HA were analysed by ultraviolet/visible spectrophotometer and high-performance size-exclusion chromatography. Initial HA concentrations were tested by the Langmuir–Hinshelwood model with k and K
ads values, determined to be 0.126 mg/L.min and 0.0257 L/mg, respectively. The change in pH affected the HA degradation efficiency by the photocatalytic activity where it was higher under acidic conditions rather than alkaline ones. Optimal catalyst loading was proved to be a constrained factor in influencing the photocatalytic efficiency: the increase of catalyst concentration enhanced the HA decomposition efficiency up to an optimum value of 20 g/L, where there was no further degradation with excess loading. The addition of H2O2 was investigated through homogenous and heterogeneous photocatalysis, and, heterogeneous photocatalysis showed higher removal efficiency due to the combined effect of both catalysts and H2O2. Finally, NZLC was effective for reuse and exhibited an excellent stability after six times of usage.
... For a system in which new solid phases of various size and density are present, used mixing designs and mixing intensities should be one of the major sources of reported discrepancies. In fact mixing may hold original and new-formed particles in suspension [24] [25], thereby disturbing the natural layered disposition of Fe 0 and Fe-oxides. The two objectives of this work were: (i) to characterise the effect of shaking intensity on the process of methylene blue (MB) discoloration from the aqueous solution by metallic iron (Fe 0 ), and (ii) to identify the critical shaking intensity above which the process of MB discoloration by Fe 0 is significantly disturbed to be representative for natural situations. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).
Attrition resistance in an adsorbent is one measure of its mechanical strength. It quantifies its ability to resist the impact of frictional forces generated in a milieu where hydrodynamic agitation exists. However, attrition resistance has been only sporadically examined in adsorbents used to remove aqueous adsorbates. Since attrition is relevant in aqueous adsorption process design, this review discusses the quantification of attrition resistance, variability in loss due to attrition across adsorbents, and the implications of attrition on adsorption systems. Finally, some key research opportunities that could be explored for a better understanding of attrition in real-scale water purification are presented. It is inferred that substantial research and development still needs to be accomplished to better understand the attrition resistance-adsorbent behavior within real-scale aqueous adsorption environments. The results can be harnessed to design and produce more robust, efficient and cost-effective adsorbents.
The feasibility of several technologies (pH adjustment-PAC enhanced coagulation, KMnO4 pre-oxidation/coagulation, coagulation/flocculation, activated carbon absorption/coagulation, Fenton oxidation) on the treatment of ground water with high content humic acid was investigated in this study. The effect of different processes on organic composition was characterized by three-dimensional excitation emission matrix (3-DEEM) spectra and high performance size-exclusion chromatography analysis. The results indicated that dissolved organic matters (DOMs) were mainly composed by two humic acids whose molecular weights (MWs) were 1 600 and 3 500, respectively. Yellow color in water was caused by humic-like organics. PAC enhanced coagulation, KMnO4 pre-oxidation/coagulation had rather limited effect on DOC removal. However, organic matters could be effectively removed by using pH adjustment-PAC enhanced coagulation, Fenton oxidation and activated carbon absorption. DOC concentration decreased to 5.99 mg/L when the dosage of PAC and pH were 300 mg/L and 6.5, respectively. The content of DOC reduced to 8 mg/L by adding micro-size activated carbon at the dosage of 0.6 g/L, and then the highly dispersive powdered activated carbon could be rapidly removed through PAC coagulation followed by settling process. Again, effluent DOC concentration was 5.6 mg/L when the dosage of hydrogen peroxide, molar ratio of ferrous and hydrogen peroxide, pH were 0.5%(v/v), 0.05, 3, respectively. Some by-products with low MWs were observed after Fenton oxidation treatment.
Application of kinetic models to represent the removal of humic substances from water and wastewater was investigated. An adsorbate-adsorbent system was represented by humic substances and three adsorbents, namely granular activated carbon (GAC), iron-coated activated alumina (AAFS), and ferric oxihydroxide in its beta form. The Elovich, internal diffusion, and external diffusion models were considered. The first model represents fairly well the experimental data for GAC and AAFS. The intraparticle diffusion model applies well for the second stage (stage 2) of adsorption only. Reliable diffusion coefficients were estimated for GAC and beta ferric oxihydroxide. A different phenomenon occurred with AAFS, as adsorption seemed to follow a second stage instead of stage 3, which was attributed to the flaking of the coating, thus generating more bare surface. As a result, the dominant mechanism on AAFS varies, starting with the adsorption being limited by internal diffusion, then diffusion accompanied by a chemical reaction and later by external diffusion. The external diffusion model applies well in different stages of adsorption, most likely because of attrition generating new grains and hence exposing a new external surface. Therefore, it is not applicable as such over the whole range of data, which is to be expected, because the mixing is not likely to limit external adsorption. The calculated diffusion coefficient is between 8.34 and 9 x 10(-11) m(2)s(-1), representing molecular weights up to 50 kDa.
The removal of natural organic matter (NOM) by four different granular sorbent media, activated carbon, an anion exchange resin, activated alumina and granular ferric hydroxide, respectively, was investigated, isotherm data, kinetic parameters and column breakthrough curves were determined. Adsorption analysis was applied to describe sorption equilibria, and the film-homogeneous surface diffusion model was used to predict NOM uptake in fixed-bed columns. The results show that NOM adsorption by activated carbon and activated alumina could be modelled quite well by the approach used. NOM uptake by the anion exchange resin could be predicted with respect to capacity while two kinetic parameters were obviously not sufficient to describe the rate of uptake correctly. NOM adsorption by granular ferric hydroxide proved to be a very slow process that could not be predicted using parameters derived from batch data.
A pseudo‐second order and a pseudo‐first order mechanism for the sorption of Omega Chrome Red ME as well as o‐cresol and p‐nitrophenol onto fly ash have been compared. Intraparticle diffusion processes and chemical sorption processes for the sorption also have been studied. The batch data for the sorption of Omega Chrome Red ME, o‐cresol and p‐nitrophenol onto fly ash have been analysed to predict the rate constant of sorption based on the assumption of a pseudo‐second order mechanism. The equilibrium capacity and initial sorption rate have also been determined to study the effect of initial solute concentration and temperature on the sorption process. An activation energy of sorption has also been evaluated using the pseudo‐second order rate constants.
Most models that have been developed to describe multicomponent adsorption are based on the assumption that different absorbates interact with regard to equilibrium only. This approach is examined at low adsorbate concentrations. Multicomponent data obtained by the minicolumn method prove that the external mass transfer of one substance is not affected by the simultaneous diffusion of other adsorbates. Bisolute adsorption rate experiments conducted in a batch system show two different impacts of multicomponent adsorption on the intraparticle mass transport. The surface diffusion of the more weakly adsorbed substance can be accelerated when both species are adsorbed simultaneously. On the other hand, surface diffusion of both adsorbates is hindered when displacement processes take place. The experimental results are also used to investigate procedures for lumping individual substances into pseudocomponents with regard to kinetic parameters.
Adsorption kinetics of a variety of single substances on activated carbon is investigated at low liquid-phase concentrations. Experimental data obtained by the minicolumn method show the influence of dissociation and molecular weight of the adsorbates on their external mass transfer behavior. Modeling of minicolumn breakthrough curves proves that internal mass transport resistances have to be taken into account at low concentrations as well as high. A differential fixed-bed reactor technique was used to determine intraparticle mass transport parameters. For strongly adsorbable substances, diffusion in the adsorbed phase proves to be prevailing, whereas for weakly adsorbable or high molecular weight adsorbates, pore diffusion also contributes to the internal mass transport. However, the surface diffusivities depend on the carbon loading in an adsorbate-specific manner.
A solution to the homogeneous surface diffusion model has been developed and incorporated into a batch adsorption model based on external boundary layer mass transport and homogeneous diffusion. The model has been extensively tested using three experimental adsorption systems, namely, phenol on carbon, basic yellow dye on carbon and basic blue dye on silica. The effect of initial solute concentration and adsorbent mass has been studied in 23 batch experiments, which have been modelled using the collocation solution method to solve the homogeneous surface diffusion equation. The theoretical concentration decay curves show a high degree of correlation with experimental data.
Adsorption kinetics of a fulvic acid (FA) were investigated in detail. Based on experimental equilibrum data, the mixture was subdivided into two pseudocomponents with different adsorbabilities and one nonadsorbable fraction. Thus the impact of the initial concentration on the overall isotherm could be described well. However, equilibrium data of the FA after preadsorption were poorly predicted. Minicolumn experiments allowed for the determination of external mass transfer coefficients of the adsorbable pseudocomponents. The results also yielded an estimation of the effective diffusivity and the average molecular size of the FA in the bulk liquid phase. Intraparticle diffusivities were evaluated from batch kinetic experiments. The overall concentration change with time was well described by the film-homogeneous diffusion model, while the behavior of the pseudocomponents could be described more closely by the filmheterogeneous diffusion model. The applicability of the models to predict a minicolumn breakthrough curve is discussed.
The adsorption isotherms and rates of two dyes and humic acid from aqueous solutions onto chitosan‐encapsulated activated carbon (CEAC) beads were measured at 30 °C. Such beads were prepared by mixing different weight percents of cuttlefish‐based chitosan (100%, 80%, 67%, and 55%) and rice‐based activated carbons. It was shown that the isotherms of dyes and humic acid were well fitted by the Freundlich equation. The adsorption capacity and rate could be enhanced when activated carbon was encapsulated with chitosan. Four simplified kinetic models including the pseudo‐first‐order equation, pseudo‐second‐order equation, intraparticle diffusion model, and the Elovich equation were tested to follow the adsorption processes. The adsorption of dyes was best described by the Elovich equation, but that of humic acid was best described by the intraparticle diffusion model. The kinetic parameters of each best‐fit model were calculated and are discussed in this paper.
© 2002 Society of Chemical Industry
The removal of cadmium ions from water by sorption onto bone char has been studied. Bone char, traditionally used for colour removal in sugar refining, is produced by the carbonisation of animal bone. Batch experiments were carried out to study the effects of initial cadmium ion concentration and bone char mass on the sorption rate. The experimental contact data were analysed using the Elovich model, previously used to describe the kinetics of gas adsorption on solids, and the Ritchie model, a modified second-order kinetics equation. Initial analysis of the data, using the Ritchie model, was poor and therefore a modification was incorporated to make the Ritchie model predictions correlate the experimental data more accurately. Both the Elovich and modified Ritchie equations accurately predict the sorption capacity of cadmium on bone char, however, the sorption kinetics, derived from the differential forms of the two equations, were correlated better using the Elovich equation.© 2000 Society of Chemical Industry
The adsorption of cadmium ions onto bone char has been studied using a batch adsorber. The experimental data was analyzed using four sorption kinetic models--the pseudo-first order, the Ritchie second order, the modified second order and the Elovich equations--to determine the best-fit equation for the sorption of metal ions onto bone char. The best-fit equation was identified using the sum of the errors squared (SSE). Finally, equilibrium studies were used to evaluate the sorption capacity of bone char for cadmium ions and experimental results showed this to be 0.57 mmol g-1 at an equilibrium solution concentration of 3.0 mmol dm-3. Since the sorption capacity is relatively high, bone char can be considered as a suitable sorbent for the adsorption of cadmium in wastewater treatment systems.
The sorption of three metal ions, namely, copper, nickel and lead onto sphagnum peat moss has been studied using an agitated batch sorber system. The equilibrium isotherms were determined and kinetic runs were performed over a range of concentrations for each metal ion. A film-pore diffusion mass transfer model has been developed based on a single effective diffusion coefficient for each system. Error analysis of the experimental and theoretical data indicated relatively large errors at low initial metal ion concentrations. Therefore the model was modified to introduce a surface coverage concentration dependent effective diffusivity to account for a contribution from surface diffusion.
Biosorption capacities and rates of different kinds of dried yeasts (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces marxianus, Candida sp., C. tropicalis, C. lipolytica, C. utilis, C. quilliermendii and C. membranaefaciens) for Remazol Blue reactive dye from aqueous solutions were compared under laboratory conditions as a function of initial pH and initial dye concentration. Optimum initial biosorption pH was determined as 2 for all the yeasts. All the yeast species showed comparable and very high dye sorption at 100 mg/l initial dye concentration. The equilibrium sorption capacity of the biomass increased with increasing initial dye concentration up to 400 mg/l for Candida sp. C. lipolytica and C. tropicalis; up to 300 mg/l for C. quilliermendii and C. utilis and up to 200 mg/l for S. cerevisiae, S. pombe, K. marxianus and C. membranaefaciens while the adsorption yield of dye showed the opposite trend for all the yeasts. Among the nine yeast species, C. lipolytica exhibited the highest dye uptake capacity (Q(0) = 250 mg/g). Both the Freundlich and Langmuir adsorption models were found suitable for describing the biosorption of the dye by all the Candida yeasts (except C. membranaefaciens). The results indicated that the dye uptake process followed the pseudo-second-order kinetics for each dye-yeast system.
Chitosan, a naturally abundant biopolymer, has widely been studied for metal adsorption from various aqueous solutions, but the extension of chitosan as an adsorbent to remove humic substances from water has seldom been explored. In this study, chitosan was coated on the surface of polyethyleneterephthalate (PET) granules through a dip and phase inversion process and was examined for humic acid removal in a series of batch adsorption experiments. Scanning electron microscopic (SEM) images showed that the PET granules were uniformly covered with a layer of chitosan and the chitosan layer possessed numerous open pores on the surface. Zeta potential study indicated that the chitosan-coated granules had positive zeta potentials at pH < 6.6 and negative zeta potentials at pH > 6.6. Adsorption of humic acid onto the chitosan-coated granules was found to be strongly pH-dependent. Significant amounts of humic acid were adsorbed under acidic and neutral pH conditions, but the adsorption capacity was reduced remarkably with increasing solution pH values. The adsorption isothermal data under various initial humic acid concentrations (at the same solution pH value) can be adequately modeled by the Langmuir and Freundlich models. X-ray photoelectron spectroscopy (XPS) revealed that the amino groups of the chitosan layer were protonated due to humic acid adsorption, suggesting the formation of organic complex between the protonated amino groups and humic acid. Kinetic study indicated that the adsorption process was transport-limited at low solution pH values, but became both transport- and attachment-limited at high solution pH values.
Porous iron oxides are being evaluated and selected for arsenic removal in potable water systems. Granular ferric hydroxide, a typical porous iron adsorbent, is commercially available and frequently considered in evaluation of arsenic removal methods. GFH is a highly porous (micropore volume ∼0.0394±0.0056 cm3 g−1, mesopore volume ∼0.0995±0.0096 cm3 g−1) adsorbent with a BET surface area of 235±8 m2 g−1. The purpose of this paper is to quantify arsenate adsorption kinetics on GFH and to determine if intraparticle diffusion is a rate-limiting step for arsenic removal in packed-bed treatment systems. Data from bottle-point isotherm and differential column batch reactor (DCBR) experiments were used to estimate Freundlich isotherm parameters (K and ) as well as kinetic parameters describing mass transfer resistances due to film diffusion (kf) and intraparticle surface diffusion (Ds). The pseudo-equilibrium (18 days of contact time) arsenate adsorption density at pH 7 was 8 μg As/mg dry GFH at a liquid phase arsenate concentration of 10 μg As/L. The homogeneous surface diffusion model (HSDM) was used to describe the DCBR data. A non-linear relationship (DS=3.0−9×Rp1.4) was observed between Ds and GFH particle radius (RP) with Ds values ranging from 2.98×10−12 cm2 s−1 for the smallest GFH mesh size (100×140) to 64×10−11 cm2 s−1 for the largest GFH mesh size (10×30). The rate-limiting process of intraparticle surface diffusion for arsenate adsorption by porous iron oxides appears analogous to organic compound adsorption by activated carbon despite differences in adsorption mechanisms (inner-sphere complexes for As versus hydrophobic interactions for organic contaminants). The findings are discussed in the context of intraparticle surface diffusion affecting packed-bed treatment system design and application of rapid small-scale column tests (RSSCTs) to simulate the performance of pilot- or full-scale systems at the bench-scale.
The adsorption by a coal-based mesoporous activated carbon of humic acids (HAs) isolated from two Polish lignites was studied. For comparison, a commercial Aldrich humic acid was also included into this study. The differences in chemical structure and functional groups of HAs were determined by elemental analysis and infrared spectroscopy DRIFT. Two activated carbons used differed in terms of mesopore volume, mesopore size distribution, and chemical properties of the surface. The kinetics of adsorption of HAs have been discussed using three kinetic models, i.e., the first-order Lagergren model, the pseudo-second-order model, and the intraparticle diffusion model. It was found that the adsorption of HAs from alkaline solution on mesoporous activated carbon proceeds according to the pseudo-second-order model. The correlation coefficients were close to 1. The intraparticle diffusion of HA molecules within the carbon particle was identified to be the rate-limiting step. Comparing the two activated carbons, the carbon with a higher volume of pores with widths of 10-50 nm showed a greater removal efficiency of HA. An increase in the Freundlich adsorption capacity with decreasing carbon content of HA was observed. Among the HAs studied, S-HA shows characteristics indicating the highest contribution of small-size fraction. The S-HA was removed by both activated carbons to the highest extent. The effect of pH solution on the adsorption of HA was examined over the range pH 5.4-12.2. It was found that the extent of adsorption decreased with decreasing pH of the solution.
The effect of solution temperature and the determination of the thermodynamic parameters of adsorption of methylene blue (MB), Cibacron Reactive black C-NN (RB) and Cibacron Reactive golden yellow MI-2RN (RY) onto manganese-oxides-modified diatomite (MOMD), such as activation energy, E, enthalpy of activation, DeltaH*, entropy of activation, DeltaS*, and free energy of activation, DeltaG*, on the adsorption rates is important in understanding the adsorption mechanism. The rate and the transport/kinetic processes of dye adsorption onto the adsorbents were described by applying various kinetic adsorption models. This would lead to a better understanding of the mechanisms controlling the adsorption rate. The pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behaviour of RB onto MOMD, suggesting that the adsorption mechanism might be a chemisorption process. The activation energies, E, for RB, RY and MB were -6.74, 56.65 and 99.80 kJ/mol, respectively. The negative value of the activation energy suggested that the rise in the solution temperature did not favour RB adsorption onto MOMD. Moreover, the activation energy of the diffusion process, E', for RB, RY and MB increased as MB > RY > RB. It means that the RB molecules are much faster moving and a lower energy is needed to diffuse into MOMD than RY and MB molecules. E', the activation energy for adsorption into pores, of RY is higher than E, indicating that the rate-limiting step of RY adsorption onto MOMD might be diffusion controlled, while the activation energy of the diffusion process, E', of MB is slightly lower than E, suggesting that the rate-limiting step is a combination of chemical and diffusion adsorption.