Sheindorf-Rebhun-Sheintuch Research Articles

Competitive Adsorption of Drugs from a Multi-Component Mixture on Sugarcane Bagasse

This work examines the adsorption in single- and multi-solute adsorption experiments of antibiotic and non-steroidal anti-inflammatory drugs, i.e., ciprofloxacin (CPX), sulfamethoxazole (SMX), ibuprofen (IBU), and diclofenac (DCF), onto sugarcane bagasse. The maximum experimental adsorption capacities of single components CPX, DCF, IBU, and SMX, were 0.98, 0.77, 0.61, and 0.51 mg/g, respectively, with decreases between 5 and 28% in multi-solute mixtures, assuming competitive adsorption. The experimental data of a single drug fitted a pseudo-second-order model, while the experimental isotherms fit the Freundlich model. The presence of CPX did not interfere with the adsorption of other solutes. The adsorption of SMX was lower in the presence of adsorption competitors than SMX single solution. The adsorption of binary systems adequately fitted the Sheindorf–Rebhun–Sheintuch model. The results showed that the competition process depends on each adsorbate and that sugarcane bagasse can adsorb drugs in multi-component systems.

Sustainable approach of batch and continuous biosorptive systems for praseodymium and thulium ions removal in mono and binary aqueous solutions

With advancement of technologies in most of the sectors, the market requirement for the rare earth elements (REEs) increasing at a rapid rate. Hence, it is vital to recover REEs from wastewater. Thus, in the present study, biosorptive uptake of praseodymium (Pr(III)) and thulium (Tm(III)) ions from mono and binary solutions using polysulfone-immobilized Turbinaria conoides (PITC) were examined for the first time The sorption of Pr(III) and Tm(III) ions were performed in batch and column biosorption systems. The biosorptional uptakes of PITC for Pr(III) and Tm(III) in mono solution were found as 1.14 and 1.23 mmol/g, respectively. The equilibrium biosorption data of the mono system were examined through the Langmuir and Freundlich isotherms, whereas data of binary biosorption experiments were checked with Sheindorf–Rebhun–Sheintuch and extended Langmuir isotherm model. The desorption efficiency of PITC in batch system showed recovery of Pr(III) and Tm(III) more than 95.5% up to 5 cycles. Continuous biosorption models like the Thomas, Yoon–Nelson and modified dose–response were employed to understand continuous biosorption behavior, and model constants were determined. The column desorption efficiency were observed as 98.9% (Pm(III)) and 99.5% (Tm(III)). Good biosorption capacity, desorption and reuse potential of PITC illustrated the environmental friendliness and scalable features.

A low‐cost treatment system for underground water using Moringa oleifera seeds and Musa cavendish peels for remote communities

AbstractBACKGROUNDNemours plant‐based biomass exhibit higher removal capacities in comparison to chemical treatment methods. The present study aimed to investigate the potential of Moringa oleifera (MO) seeds and Musa cavendish (MC) peel biomass pre‐treated by HNO3 (1 N) and NaOH (0.1 M) for removing nickel (Ni), lead (Pb) and cadmium (Cd) ions from synthetic groundwater. It further aimed to establish the design and test the efficiency of an individual domestic groundwater treatment system with low‐cost using local materials. The batch tests with biomass (MC, MO and MC + MO) were conducted at pH 7. Adsorption was investigated using Freundlich and Langmuir models for single elements, and the Sheindorf–Rebhun–Sheintuch (SRS) model for multicontaminant elements. By contrast, the simulation of actual groundwater quality was studied using a Monte Carlo simulation model.RESULTSThe adsorption results fitted with the Freundlich model revealed that ≈91%, 98% and 84% of groundwater samples met WHO standards for Pb2+, Ni2+ and Cd2+, respectively. The FTIR analysis of MC, MO and MC + MO revealed that CC, CO and NH were the main functional groups playing roles in the removal of heavy metals from the water samples. The mechanism of adsorption took place by chemisorption and physisorption. The total cost of the water treatment system with a capacity of 50 L was US$17, and considered suitable for remote, lower income communities.CONCLUSIONThe MC + MO biomass was efficient in adsorbing heavy metal ions from water samples owing to the presence of abundant functional groups.

The modified Song isotherm model: application to multisolute sorption of phenols in organoclays using the ideal adsorbed solution theory

ABSTRACT The three-parameter (K, b, and n) Song isotherm model was slightly modified to make it possible to obtain analytical integration of the spreading pressure integral. The modified Song model (MSM) allows more efficient and accurate calculation of the ideal adsorbed solution theory (IAST). The MSM also satisfies the Henry’s law and the Freundlich model at low and high concentrations, respectively, and reverts to the Langmuir and the linear models when n equals zero and one, respectively. Approximate values of each parameter could be estimated from a plot of log (q/c) versus log c; the partition coefficient in the Henry’s law region (K) and the Freundlich index (n) can be estimated from the ordinate value of the low-concentration asymptote and the slope of the high-concentration asymptote, respectively, and the parameter (b) can be estimated from the solution-phase concentration of the intersection point of the two asymptotes. The MSM was fitted to the single-solute sorption of 2-chloro-, 3-cyano-, and 4-nitrophenol onto montmorillonites modified with either HDTMA cation or TMA/HDTMA dual cations. The ideal adsorbed solution theory (IAST) combined with either dual-mode model, Khan model or MSM as a single-solute isotherm model was used to predict three bisolute and one trisolute sorption to organoclays. The Sheindorf-Rebhun-Sheintuch (SRS) and Murali-Aylmore (M-A) were also used to predict bisolute sorption to organoclays. The IAST predictions were generally in good agreement with the multisolute sorption data. The advantages of MSM over other three-parameter models were fully discussed.

Sorption of antibiotics onto montmorillonite and kaolinite: competition modelling

ABSTRACTAntibiotic contaminants, which are generally present in bi-solute systems, can be competitively adsorbed onto clays. Single- and bi-solute sorptions of sulfadiazine (SDZ) and ciprofloxacin (CIP) onto montmorillonite and kaolinite were investigated at pH values of 5 and 8. Freundlich and Langmuir models were used and fit the experimental data well for single-solute sorption. The sorption isotherms were nonlinear (NF = 0.265–0.730), and the maximum sorption capacities (qmL) of the SDZ and CIP onto montmorillonite were higher than those onto kaolinite. The octanol–water distribution ratio (Dow), cation exchange capacity (CEC), Brunauer–Emmett–Teller (BET) surface area (ABET), pore size, point of zero charge (pHPZC), and basal spacing predominantly affected the Freundlich constant (KF) and qmL of SDZ0 and CIP+ at pH 5 more than SDZ− and CIP± at pH 8. For bi-solute sorption, the presence of CIP inhibited the SDZ sorption onto montmorillonite and kaolinite. Competitive sorption models such as Sheindorf–Rebhun–Sheintuch (SRS), Murali–Aylmore (M–A) and the modified extended Langmuir model (MELM) were used; of these, the MELM provided the best prediction with SDZ sorption onto montmorillonite at pH 8 and CIP onto kaolinite at pH 5 and 8 in SDZ/CIP system occurring synergistically, whereas others occurred antagonistically. The distribution coefficient (Kd) of the bi-solute sorption decreased with increasing pH in the order cationic > neutral > anionic for SDZ and cationic > zwitterionic > anionic for CIP, which resembled the Kd of single-solute sorption. Fourier transform infrared spectroscopy (FT-IR) spectra indicated that amine in SDZ and keto oxygen in CIP were responsible for the interactions with the montmorillonite and kaolinite.

Molybdenum-phosphate retention and transport in soils

Transport and retention experiments were carried out to quantify the mobility and sorption of molybdenum (Mo) and phosphate (P) in two different soils. Batch experiment was used to determine sorption isotherms over a wide range of concentrations for Mo and P. For both soils, sorption isotherms were nonlinear with higher affinity of P than Mo. Sorption of Mo was significantly reduced as the amount added P in solution increased. This was observed for both soils and is indicative of competitive sorption for available site. Miscible-displacement experiments were carried out using soil column where a pulse of Mo solution was introduced in each soil. Breakthrough curves (BTCs) indicated extensive sorption of Mo where as much as 50% of that applied was retained by a predominant kaolinatic soil. Subsequently, a pulse of mixed solution (of Mo and P) was introduced in each column. Results of BTCs for Mo indicated enhanced mobility of Mo in the presence of P for both soils. A five step sequential extraction procedure provided evidence that majority of applied Mo was strongly and/or irreversibly retained. A competitive transport model (CMRTM) based on the Sheindorf-Rebhun-Sheintuch (SRS) equation was capable of describing BTCs of Mo and to a lesser extent for P. Future research should focus on improvement of models that accounts for chemical mechanisms of competitive sorption.

Simultaneous sorption of 4-nitrophenol and 2-nitrophenol on a hybrid geocomposite based on surfactant-modified pillared-clay and activated carbon

New generations of hydrophobic composite geomaterials intended to sorb certain hazardous wastes are assembled by mixing surfactant-modified aluminum-pillared montmorillonite (Al-SMPM) with activated carbon (AC). The sorption of 4-nitrophenol (4NP) and 2-nitrophenol (2NP) on these sorbents was studied in both single and binary component systems from aqueous solutions using batch tests. Results showed the efficiency of these composite sorbents relative to the two phenolic compounds, with a dominant contribution from Al-SMPM. In single-component systems, sorption isotherms of 4NP and 2NP as a function of pH were analyzed with the Freundlich equation, whose statistical interpretation was also developed. Sorption capacities sharply decreased when the solution pH value was raised from 3 to 9. In binary-component systems, the fit between measured and predicted simultaneous sorption capacities of both 4NP and 2NP indicated that the Sheindorf–Rebhun–Sheintuch model, an extended Freundlich model, is fully applicable. The 2NP favorably influences the sorption of 4NP. The synergistic effect between these two nitrophenol compounds has been confirmed by the higher competition coefficients, which increase with increasing pH. A number of hypotheses, based on the Abraham’s solvation parameters model, have been advanced to discuss this mechanism.

Experimental characterisation and evaluation of perlite as a sorbent for heavy metal ions in single and quaternary solutions

Due to its unique properties such as low bulk density, high water retention capacity and high air-filled porosity, perlite has been recently employed as an important substrate additive to support vegetation in green roofs, vertical gardens and biofilters. Considering that the runoff quality of the above systems often comes under debate, it is important to explore the sorption capacity of substrate additives. Thus, the present study explores the potential of perlite to sorb important metal ions such as Cd2+, Cu2+, Ni2+ and Pb2+ in single and multi-component solutions. Characterisation of perlite indicated that the material possesses low bulk density (148kg/m3), high water holding capacity (27.5%) and high air-filled porosity (31.1%). Experimental results according to the Langmuir model revealed that perlite sorbed 0.130, 0.089, 0.057 and 0.025mmol/g of Pb2+, Cu2+, Ni2+ and Cd2+ respectively, at an optimum pH of 5. The kinetics data were well described by pseudo-first order model, followed by the pseudo-second order and Elovich models. Quaternary sorption experiments showed that the presence of multiple ions had an adverse effect on the sorption capacity of perlite towards individual ions. Application of Sheindorf–Rebhun–Sheintuch equation to experimental quaternary isotherm data confirmed that sorption of Cd2+ ions was severely affected whereas Pb2+ ions were favoured.

Competitive adsorption studies of caffeine and diclofenac aqueous solutions by activated carbon

Equilibrium and dynamic studies on the adsorption removal of caffeine and diclofenac onto granular activated carbon (GAC) were developed. In the competitive adsorption, lower adsorption capacities for caffeine and diclofenac, 190.9 and 233.9mgg−1, respectively, were reported compared with the single adsorption system. Sheindorf–Rebhun–Sheintuch and the extended Freundlich multi-component isotherm models were also used to describe the simultaneous adsorption of caffeine and diclofenac from binary system. The caffeine and diclofenac equilibrium adsorption data were best adjusted to the extended Freundlich model, providing lower percentage error values (ε) of 0.29 and 0.28 for caffeine and diclofenac, respectively.Breakthrough curves of caffeine and diclofenac aqueous solutions were obtained at different operational conditions (initial concentration, volumetric flow rate and column length). Therefore, adsorption parameters, such as adsorption capacities at breakthrough and saturation points, length of mass transfer zone, fractional bed utilization and micropollutant removal percentage were obtained. In general, the caffeine removal percentages at breakthrough time are higher than the diclofenac recovery. Pore and Surface Diffusion (PSDM) model has been used to predict the fixed-bed breakthrough curves for caffeine–diclofenac aqueous solutions. Bi number values, ranged from 1 to 100, were obtained, indicating that the surface diffusion inside the micropores represents the rate-controlling step in the process for the experimental conditions used in the study.

Mobility and Interaction of Heavy Metals in a Natural Soil

We study the mobility and interaction under competing conditions observed for copper (\(\text{ Cu}^{2+}\)) and zinc (\(\text{ Zn}^{2+}\)) ions in the context of laboratory-scale experiments performed in natural soil columns. The experiments focus on the analysis of solute breakthrough curves (BTCs) obtained after injection of an aqueous solution containing similar concentrations of the two metal ions into a soil column fully saturated with double deionized water. Transport of the competing ions is tested for the same soil under aerobic and anaerobic conditions. Measurements show that the species with lower affinity for the soil, \(\text{ Zn}^{2+}\), migrates occupying all available adsorption sites, and is then progressively replaced by the ion with higher affinity, \(\text{ Cu}^{2+}\). The two ions are displaced in the system with different effective retardation. The slowest species replaces the sorbed ions, resulting in observed \(\text{ Zn}^{2+}\) concentrations that display a non-monotonic behavior in time and which, for a certain period, are larger than the concentration supplied continuously at the inlet. In the absence of a complete geochemical characterization of the system, we show that the measured concentrations of both metals can be interpreted through simple models based on a set of coupled partial differential and algebraic equations, involving a small subset of aqueous and adsorbed species that are present in the system. Depending on the model considered, the relationship between aqueous and adsorbed ion concentrations is described at equilibrium by a Gaines–Thomas (GT) formulation, a competitive Sheindorf–Rebhun–Sheintuch (SRS) isotherm, or an Extended Langmuir (EL) isotherm, respectively. The GT formulation provides the best interpretation of the observed behavior among the models tested. We find that employing these simple models, which account only for the main governing reactive processes, allows reasonable estimation of the observed BTCs in experiments where only partial geochemical datasets are available.

Biosorption Behavior of Basic Red 46 and Violet 3 by Dead Pleurotus mutilus from Single‐ and Multicomponent Systems

The performance of nonviable P. mutilus for removal of Crystal Violet (CV) and Basic Red 46 (BR46) was investigated in single and binary systems. Batch kinetic studies were carried out as a function of pH, temperature, biomass amount, and dye concentration to determine the decolorization efficiency of biosorbent. In single system, the biosorption capacities of P. M. reached 166 and 76.92 mg/g for CV and BR46, respectively. A comparison of kinetic models applied to the adsorption of basic dyes onto P. Mutilus was evaluated for the pseudo‐second‐order and intraparticle diffusion kinetics models. The experimental data fitted very well the pseudo‐second‐order kinetic model, whereas diffusion is not only the rate‐controlling step. The thermodynamic study indicates that the adsorption of dyes is spontaneous and endothermic process. In binary system, the biosorption capacities of P. Mutilus for both dyes decreased significantly compared to that in single system. Competitive coefficients calculated on a concentration basis using Sheindorf‐Rebhun‐Sheintuch (SRS) equation were useful for describing the degree of competitive interaction in P. M.

Removal of cobalt and strontium from groundwater by sorption onto fishbone

Fishbone as a main backfill material of permeable reactive barrier to remediate groundwater contaminated with Co and Sr was investigated through single- and bi-solute competitive sorptions. The single-solute sorption data were fitted by Freundlich, Langmuir and Dubinin-Radushkevich models. The coefficients of determination (R2 > 0.91) indicated that all models fitted well. Maximum sorption capacities (qmL) of Co and Sr predicted by the Langmuir model were 0.55 mmol/g and 0.50 mmol/g, respectively. The bi-solute competitive sorption of the metals was analyzed by the Langmuir, competitive Langmuir, Sheindorf-Rebhun-Sheintuch (SRS) and P-factor models. The sorbed amount of one solute in bi-solute system decreased due to competition with the other solute. Langmuir model parameters for single- (qmL and bL) and bi-solute (qmL* and bL*) competitive sorptions were compared to analyze the effect of competition between the metals. The competitive Langmuir, SRS and P-factor models predicted the bi-solute competitive sorption data well (R2 > 0.93).

Estimation of Single-Metal and Competitive Sorption Isotherms through Maximum Likelihood and Model Quality Criteria

Metal sorption of single and binary (competitive) systems for several soils is analyzed to assess the ability of alternative isotherm models to interpret experimental observations. The analysis is performed within a Maximum Likelihood framework and on the basis of model identification (sometimes termed “quality” or “information”) criteria. These methodologies allow the assessment of the measurement error variance in the parameter estimation process and the uncertainty arising from the use of alternative (conceptual-mathematical) models. We first analyze Cu and Zn sorption in two Israeli soils, Bet Dagan and Yatir, which are slightly alkaline but with substantially different sorption capacities and perform an extensive set of batch experiments in single and binary systems. We then analyze the data set published by Liao and Selim (2009) where Ni and Cd sorption was studied in three different (one neutral and two acidic) soils. Single component data from both sets of experiments are interpreted on the basis of the Langmuir, Freundlich, and Redlich–Peterson (RP) models. The family of binary systems results is analyzed in light of the Sheindorf–Rebhun–Sheintuch (SRS) model, the modified RP model, and the modified and extended Langmuir models. All of the considered models are expressed in terms of initial and equilibrium concentrations, two variables that are measured independently. Maximum Likelihood and model identification criteria (such as Bayesian criteria BIC and KIC, and information theoretic criteria AIC, AICc, and HIC) are employed to (a) estimate model parameters, (b) rank alternative models, and (c) estimate the relative degree of likelihood of each model by means of a weight, or posterior probability. We show that modeling observation error variance either as a constant or as a function of concentration does not significantly affect parameter estimates for a given model. These different representations of measurement error variance impact the ranking of alternative models based on posterior probability weights. The weights associated with different models can be very similar when a uniform measurement error variance is considered, so that it is difficult to clearly identify a single best model.

Removal of Co2+, Sr2+ and Cs+ from aqueous solution by phosphate-modified montmorillonite (PMM)

Sorptive removal of Co2+, Sr2+ and Cs+ from aqueous solution by phosphate-modified montmorillonite (PMM) was studied considering the influencing factors (initial metal ion concentrations, initial solution pH and temperature). The Freundlich, Langmuir and Dubinin–Radushkevich (DR) models were used to fit single-solute sorption data. The Freundlich model was the best, indicating heterogeneous surface property of PMM. The maximum sorption capacity (qmL) of Langmuir model was in the order of Cs+>Co2+>Sr2+. The mean sorption energy (E) values of DR model (<8kJ/mol) at pH 5 for all metals indicated physical sorption. However, Cs+ sorption mainly occurred by chemical sorption. In bi-solute competitive sorption, the sorption of one metal ion was suppressed by the presence of competing metal ion. The Sheindorf–Rebhun–Sheintuch (SRS) model, the extended Freundlich model (EFM), the modified extended Langmuir model (MELM) and the IAST-Freundlich model predicted the competitive sorption adequately. Sorptions of Co2+ and Sr2+ were strongly dependent on the initial solution pH but that of Cs+ was not. The calculated thermodynamic parameters such as ΔH, ΔS, and ΔG showed that sorptions of Co2+ and Sr2+ onto PMM were endothermic, whereas that of Cs+ was exothermic and that all sorption occurred spontaneously.

Single and binary biosorption of cerium and europium onto crab shell particles

The ability of crab shell to biosorb two rare earth elements (REE), namely cerium(III) and europium(III) from single and binary systems has been studied. Crab shell majorly comprised of calcium carbonate, protein and chitin. Experiments to identify the role of these constituents in REE biosorption revealed that calcium carbonate was mainly responsible for REE removal. Microprecipitation of REE by carbonate ions and subsequent settling on the surface of crab shell was identified as major mechanism responsible for crab shell biosorption performance. At optimum pH of 6, in single component system, crab shell exhibited maximum Ce(III) and Eu(III) uptakes of 144.9 and 49.5 mg/g, respectively, according to the Langmuir model. In binary systems, both Ce(III) and Eu(III) compete with each other resulted in decreased uptake. In an attempt to model the binary biosorption data, Sheindorf–Rebhun–Sheintuch equation provided more accurate prediction of isotherm data compared to extended Langmuir model with constant interaction factor. Kinetic experiments revealed that equilibrium was attained in 60 min, followed by complete saturation in 2 h, for both Ce(III) and Eu(III). Pseudo-first order model better predicted the kinetic data with high correlation coefficients and low % error values than pseudo-second order model.

Cooperative coadsorption of 4-nitrophenol and basic yellow 28 dye onto an iron organo–inorgano pillared montmorillonite clay

Sorption properties of an iron surfactant-modified pillared montmorillonite (Fe-SMPM) toward two organic pollutants, basic yellow 28 dye (BY28) and 4-nitrophenol (4-NP), were studied at different pH values in both single component and binary pollutant systems. The pseudo-first-order model fits well with the kinetic data obtained in single component studies and sorption capacities of both BY28 and 4-NP increased with the pH value. A sorption synergetic mechanism was observed in binary systems; 4-nitrophenol adsorption was enhanced by the presence of BY28 in the mixture and increased with dye concentrations. Isotherms were described using the Freundlich model in single component systems and the Sheindorf–Rebhun–Sheintuch (SRS) model, an extended Freundlich model, in binary mixtures systems. Hydrophobic interactions between the surfactant-modified pillared clay and the pollutants were suggested to explain the sorption mechanisms.

Competitive Sorption of Nickel and Cadmium in Different Soils

Competing ions strongly affect heavy-metal retention and release in soils. In this study, we evaluated the sorption of Ni and Cd in single and binary Ni-Cd systems for one neutral and 2 acidic soils. Ni and Cd sorption isotherms were obtained using batch techniques over a wide range of concentrations. Both Ni and Cd adsorption isotherms exhibited strong nonlinear behavior with similar overall patterns for all soils. Sorption of Ni and Cd correlated well the cation exchange capacity for all soils. Moreover, Cd sorption by the two acidic soils was greater than Ni, whereas for the neutral soil, Ni sorption was greater than Cd. The Freundlich model was utilized to describe the family of Ni and Cd isotherms in the presence of competing ions. For both Ni and Cd isotherms, the Freundlich distribution coefficient K decreased with increasing concentration of the competing ion. The dimensionless parameter n was within a narrow range of 0.50 to 0.64 for both Ni and Cd and was not affected by the competing ion. Competitive sorption was described using the Sheindorf-Rebhun-Sheintuch (SRS) equation, which is based on multicomponent Freundlich-type models. Estimates of SRS competitive sorption coefficients indicated that for the neutral soil, Ni sorption was least affected by Cd. The SRS equation was also capable of describing the overall competitive Ni and Cd sorption patterns for all soils. However, SRS model modifications are needed to improve sorption prediction for the highest competing concentrations.

Modeling Competitive Arsenate‐Phosphate Retention and Transport in Soils: A Multi‐Component Multi‐Reaction Approach

This study was conducted to investigate the kinetics of arsenate [As(V)]‐phosphate [P] competitive retention during transport in soils. Time‐dependent batch experiments were performed to describe competitive As(V)‐P sorption kinetics in Olivier (fine‐silty, mixed, active, thermic Aquic Fraglossudalfs) and Windsor (mixed, mesic Typic Udipsamments) soils. Miscible‐displacement experiments were also performed to quantify As(V)‐P competition when anion pulses were introduced simultaneously or consecutively into water‐saturated soil columns. The results demonstrated that the rates and amounts of As(V) sorption are significantly reduced by increasing addition of P. Due to competitive sorption, the presence of P resulted in increased mobility of As(V) in the soil columns. Flow interruptions indicated the dominance of time‐dependent sorption during As(V) and P transport in soils. We extended the equilibrium‐kinetic multireaction model (MRM) to simulate competitive retention kinetics of multiple chemical species in soils. Competitive coefficients from Sheindorf–Rebhun–Sheintuch (SRS) equation were adopted to describe the extent of As(V)‐P competition. A multi‐component multireaction model (MCMRM) was coupled with the advection‐dispersion equation (ADE) to describe breakthrough curves (BTCs) of As(V) and P simultaneously. Model predictions of measured BTCs for competitive As(V)‐P transport were achieved using rate coefficients based on inverse modeling.

Quasi‐thermodynamic basis of competitive‐adsorption coefficients for anionic mixtures in soils

SUMMARYEquilibrium competitive‐adsorption coefficients derived from the application of the multi‐component Freundlich‐type Sheindorf‐Rebhun‐Sheintuch (SRS) equation appeared to have a soil‐specific, quasi‐thermodynamic basis. Using three soils, single‐solute adsorption isotherms at 288, 298, and 308 K were constructed with arsenate, phosphate, and molybdate. A Clausius‐Clapeyron‐type equation was used to estimate apparent isosteric heats of adsorption values (qiso). The adsorption of the three anions was characterized by different qiso values, and was generally in the order of H2PO4‐&gt;H2AsO4‐&gt;MoO42‐. The magnitudes of the SRS competitive coefficients derived from binary‐solute mixtures were in agreement with the distributions of the heats of adsorption of the individual anion‐soil systems. Higher competitive coefficients, indicating the preferential adsorption of solute i in a binary‐solute mixture containing solutes i and j, tended to associate with higher qiso values for solute i relative to solute j. However, the qiso values for the adsorption of molybdate by one soil underestimated the extent of competitive interactions between arsenate and molybdate.

Competitive Coefficients for the Adsorption of Arsenate, Molybdate, and Phosphate Mixtures by Soils

AbstractThe adsorption behavior of anionic mixtures (binary‐solute combinations of arsenate, molybdate, and phosphate) by three soils (Cecil clay [Typic Hapludults], EPA‐14 [Ultisol], and Catlin silt loam [Typic Argiudolls]) was investigated using the Sheindorf‐Rebhun‐Sheintuch (SRS) equation. The SRS equation is a multicomponent Freundlich‐type adsorption equation. Most of the isotherm data were found to conform to linear SRS expressions, making it possible to derive competitive coefficients on a concentration basis for each binary‐solute system. The adsorption of arsenate and molybdate by all three soils were significantly reduced by the presence of phosphate, which was attributed to competitive interactions. In each solute‐soil system, the adsorption of phosphate was not greatly suppressed by either arsenate or molybdate. The adsorption of arsenate by Cecil clay and EPA‐14 was reduced in the presence of molybdate, whereas arsenate did not compete strongly with molybdate adsorption. In contrast, the adsorption of arsenate by Catlin appeared to be independent of molybdate, while the presence of arsenate lowered molybdate adsorption. In general, the associated competitive coefficient for each solute‐soil system appeared to be consistent with qualitative observations, and was viewed as a single‐valued term indicative of the relative affinity of each soil for a given anion with respect to a competing anion from a binary‐solute mixture.