Adsorptive removal of cationic dye from aqueous solutions by ZnO/ZnMn2O4 nanocomposite

The ZnO/ZnMn₂O₄ nanocomposite (ZnMn) was used as adsorbent for the removal of cationic dye Basic Yellow 28 (BY28) from aqueous solutions. The adsorbent was characterized by X-ray diffraction, scanning electron microscope, TEM, Fourier transform infrared ray, BET, particle size distribution and zeta potential measurements. The adsorption parameters, such as temperature, pH and initial dye concentration, were studied. Kinetic adsorption data were analyzed using the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. The Langmuir and Freundlich isotherm models were applied to fit the equilibrium data. The maximum adsorption capacity of BY28 was 48.8 mg g^?1. Various thermodynamic parameters, such as ΔG°, ΔH° and ΔS°, were calculated.     

Removal of Boron from Aqueous Solutions by Adsorption Using Fly Ash,Zeolite, and Demineralized Lignite

In the present study for the purpose of removal of boron from water by adsorption using adsorbents like fly ash, natural zeolite, and demineralized lignite was investigated. Boron in water was removed with fly ash, zeolite, and demineralized lignite with different capacities. Ninety-four percent boron was removed using fly ash. Batch experiments were conducted to test the removal capacity, to obtain adsorption isotherms, thermodynamic and kinetic parameters. Boron removal by all adsorbents was affected by pH of solution; maximum adsorption was achieved at pH 10. Adsorption of boron on fly ash was investigated by the Langmuir, Freundlich, and the Dubinin-Radushkevich models. Standard entropy and enthalpy changes of adsorption of boron on fly ash were, ΔS ⁰  = ?0.69 kJ/mol K and ΔH ⁰  = ?215.34 kJ/mol, respectively. The negative value of ΔS ⁰ indicated decreased randomness at the solid/solution interface during the adsorption boron on the fly ash sample. Negative values of ΔH ⁰ showed the exothermic nature of the process. The negative values of ΔG ⁰ implied that the adsorption of boron on fly ash samples was spontaneous. Adsorption of boron on fly ash occurred with a pseudo-second order kinetic model, and intraparticle diffusion of boron species had also some effect in adsorption kinetics.     

Comparison of Basic Dye Crystal Violet Removal from Aqueous Solution by Low-Cost Biosorbents

Abstract

The removal of basic dye crystal violet by low-cost biosorbents was investigated in this study using a batch experimental system. The adsorption of crystal violet onto various adsorbents was solution pH-dependent and the maximum removal occurred at basic pH 10.0. The kinetic experimental data were analyzed using pseudo-first-order and pseudo-second-order equations to examine the adsorption mechanism and the intraparticle diffusion model to identify the potential rate controlling step. These results suggested that the adsorption of crystal violet onto various adsorbents was best represented by the pseudo-second-order equation. The suitability of the Langmuir and Freundich adsorption isotherms to the equilibrium data was also investigated at various temperatures for all four sorbents and the adsorption isotherms exhibited Freundlich behavior. The Freundlich constant K_f was 1.55 for alligator weed, 2.33 for Laminaria japonica, 9.59 for rice bran and 5.38 (mg/g)/(mg/L)^1/n for wheat bran, respectively at adsorbent concentration 5 g/L, pH 10.0 and 20°C. The thermodynamic parameters (ΔH, ΔG, and ΔS) were calculated and the results showed that the adsorption process for various adsorbents was spontaneous, endothermic, with an increased randomness, respectively. The particle size and the reaction temperature exhibited an insignificant impact on the adsorption equilibrium of crystal violet. The adsorbents investigated could serve as low-cost adsorbents for removing the crystal violet from aqueous solution.     

INHIBITING EFFECT OF CRYSTAL VIOLET DYE ON ALUMINUM CORROSION IN ACIDIC AND ALKALINE MEDIA

Crystal violet dye (CV) was studied as a corrosion inhibitor for aluminum in hydrochloric acid (1 mol dm^?3) and potassium hydroxide (0.5 mol dm^?3) solutions in the temperature range 30°–60°C using the gravimetric technique. The effect of iodide ions on the inhibiting efficacy of CV was also assessed. CV was found to inhibit Al corrosion in both aggressive media, exhibiting greater inhibition efficiency in 1 M HCl (83.6%) than in 0.5 M KOH (23.0%). Inhibition efficiency in 0.5 M KOH was, however, synergistically increased in the presence of iodide ions to attain values up to 85.3%. Inhibition efficiency in the acidic and alkaline solutions increased with CV concentration but decreased with rise in temperature, suggesting physical adsorption of CV on the Al surface. The calculated values of activation energy (E_a), free energy of adsorption (ΔG_ads), and heat of adsorption (Q_ads) confirm the physisorption mechanism. The inhibitor adsorption characteristics were approximated by the Freundlich adsorption isotherm.     

Evaluation of a functionalized copolymer as adsorbent on direct dyes removal process: Kinetics and equilibrium studies

Functionalized polymeric microbeads were investigated as adsorbent for the removal of three direct dyes from aqueous solutions. The effects of different experimental parameters, such as initial dye concentration, temperature, and solution pH on the adsorption process were investigated. The adsorption process can be conducted with very good result at normal working conditions: neutral pH and normal temperature. The maximum percentage removal obtained was 99.11% for the symmetrical disazo dye, 90.14% for asymmetrical disazo dye, and 98.53% for trisazo dye. The adsorption kinetics followed the pseudo‐second‐order equation for all three investigated dyes in all working conditions. The experimental data were fitted to Langmuir, Freundlich, Sips, and Redlich–Peterson isotherm models, and the best fit was obtained with Sips model. Thermodynamic parameters (ΔH°, ΔS°, and ΔG°) revealed that dye adsorption is an endothermic and spontaneous process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Equilibrium adsorption studies on removal of diclofenac sodium from aqueous solution using sawdust–polyaniline (SD–PAn) composites

In this work, equilibrium adsorption studies were carried out for the removal of antiinflamatory drug diclofenac sodium (DS) from aqueous solutions using sawdust–polyaniline(SD-PAn) composite as a potential sorbent at 20, 30, and 40°C. The composite sorbent was characterized by FTIR and SEM analysis. The particles exhibited porous and rough surface. The equilibrium uptake data was interpreted by the Langmuir, the Freundlic, the Temkin, and the Dubinin–Redushkevich (D -R) isotherm models. The maximum adsorption capacity was found to decrease with increase in temperature, thus indicating exothermic nature of the adsorption process. The thermodynamic parameters were evaluated using Langmuir, Flory–Huggins, Frumkin, and modified-Frumkin models. The negative values of ΔG_ads⁰ and ΔH⁰ indicated spontaneous and exothermic nature of the adsorption process, respectively. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125:1382–1390, 2012     

Synthesis of High Surface Area MgAl2O4 Nanopowder as Adsorbent for Leather Dye Removal

Abstract

MgAl₂O₄ nanopowder has been prepared by alkoxides hydrolysis with further calcination at temperature of 700°C. The adsorption of a leather dye, Direct Black 38, onto this material was investigated. The sample was characterized by X-ray-diffraction (XRD), N₂ adsorption–desorption isotherm and Fourier transform infrared spectroscopy. The results showed that sample present a pure phase, and the average nanocrystal size of 8 nm, the BET surface area is about 206.5 m² · g^?1 and total pore volume is about 1.44 cm³ · g^?1. Adsorption kinetics data were modeled by film and pore diffusion model. The experimental isotherm was described by the Langmuir model. MgAl₂O₄ nanopowder presented a great removal efficiency of leather dye by adsorption process, with a maximum adsorption capacity of 833 mg of dye per gram of adsorbent.     

Effect of Temperature on Kinetics and Adsorption Profile of Endothermic Chemisorption Process: –Tm(III)–PAN Loaded PUF System

Abstract

The sorption behavior of 3.18×10^?6 mol l^?1 solution of Tm(III) metal ions onto 7.25 mg l^?1 of 1‐(2‐pyridylazo)‐2‐naphthol (PAN) loaded polyurethane foam (PUF) has been investigated at different temperatures i.e. 303 K, 313 K, and 323 K. The maximum equilibration time of sorption was 30 minutes from pH 7.5 buffer solution at all temperatures. The various rate parameters of adsorption process have been investigated. The diffusional activation energy (ΔE_ads) and activation entropy (ΔS_ads) of the system were found to be 22.1±2.6 kJ mol^?1 and 52.7±6.2 J mol^?1 K^?1, respectively. The thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were calculated and interpreted. The positive value of ΔH and negative value of ΔG indicate that sorption is endothermic and spontaneous in nature, respectively. The adsorption isotherms such as Freundlich, Langmuir, and Dubinin–Radushkevich isotherm were tested experimentally at different temperatures. The changes in adsorption isotherm constants were discussed. The binding energy constant (b) of Langmuir isotherm increases with temperature. The differential heat of adsorption (ΔH_diff), entropy of adsorption (ΔS_diff) and adsorption free energy (ΔG_ads) at 313 K were determined and found to be 38±2 kJ mol^?1, 249±3 J mol^?1 K^?1 and –40.1±1.1 kJ mol^?1, respectively. The stability of sorbed complex and mechanism involved in adsorption process has been discussed using different thermodynamic parameters and sorption free energy.     

Response Surface Modeling and Optimization of Remazol Brilliant Blue Reactive Dye Removal Using Periwinkle Shell-Based Activated Carbon

This work investigates both batch and optimization studies of adsorption of Remazol Brilliant Blue Reactive (RBBR) dye onto activated carbon prepared from periwinkle shells (PSAC). The effects of three preparation variables: CO₂ activation temperature, CO₂ activation time, and KOH: char impregnation ratio (IR) were studied using Response Surface Modeling (RSM). Based on the central composite design (CCD), a quadratic model and two-factor interaction models (2FI) were developed to correlate the three preparation variables to the two responses: RBBR dye removal and PSAC yield. The optimum conditions for preparing PSAC for adsorption of RBBR dye were found as follows: CO₂ activation temperature of 811°C, CO₂ activation time of 1.7 h and IR of 2.95, which resulted in 82.76% of RBBR dye removal and 35.83% of PSAC yield. Experimental results obtained agreed satisfactorily well with the model predictions. The activated carbon prepared under optimum conditions was mesoporous with BET surface area of 1894 m²/g, total pore volume of 1.107 cm³/g and average pore diameter of 2.32 nm. The surface morphology and functional groups of PSAC were respectively determined from the scanning electron microscopy (SEM) and Fourier transform infrared analysis (FTIR).     

Removal of Rhodamine B Dye Using Activated Carbon Prepared from Palm Kernel Shell and Coated with Iron Oxide Nanoparticles

The efficacy of activated carbon prepared from Palm Kernel Shell (PKSAC) from agriculture biomass and coated with magnetic nanoparticle (Fe₃O₄) in the removal of Rhodamine B dye was investigated. Adsorption experiments were carried out at various initial pH, adsorbent dosage, initial dye concentration, particle size, and temperature. Kinetic analyses were conducted using pseudo first order, pseudo second order and intra particle diffusion models. However, the regression results showed that the adsorption kinetics was represented more accurately by the pseudo second order model. The pseudo second order kinetic constant obtained was 1.7 × 10^?4 min^?1 at 323 K when 200 mg L^?1 dye concentration was used. The equilibrium data were well described by both Langmuir and Freundlich isotherm models. The Langmuir adsorption capacity was 625 mgg^?1. The rate of adsorption improved with increasing temperature and the process was endothermic with ΔH value assessed at 80 kJmol^?1. Results obtained reveal that activated carbon prepared from Palm Kernel Shell coated with magnetic nanoparticle from agriculture biomass can be an attractive option for dye removal from industrial effluent.     

Coconut (Cocos nucifera) Shell Based Activated Carbon for the Removal of Malachite Green Dye from Aqueous Solutions

The adsorption of malachite green (MG) dye using coconut shell based activated carbon (CSAC) was investigated. Operational factors such as the effect of pH, initial dye concentration, adsorbent dosage, contact time, and solution temperature on the adsorption process were studied. Solution pH strongly affected the chemistry of both the dye molecule and CSAC in solution. Optimum dye removal was obtained at pH ≥ 8.0. Equilibrium was reached in 120 minutes contact time. The Langmuir, Freundlich, and Dubinin–Radushkevich (D-R) isotherm models were used to evaluate the adsorption data. The adsorption data fitted the Langmuir model most with maximum adsorption monolayer coverage of 214.63 mg/g. Pseudo-first-order, pseudo second-order, and intraparticle diffusion models were also used to fit the experimental data. Kinetic parameters, rate constants, equilibrium sorption capacities, and related correlation coefficients, for each model were calculated and discussed. Thermodynamic parameters such as ΔG⁰, ΔH⁰, and ΔS⁰ were evaluated and it was found that the sorption process was feasible, spontaneous, and exothermic in nature. The mean free energy obtained from D-R isotherm suggests that the adsorption process follows physiosorption mechanism. The results showed that coconut shells could be employed as a low-cost precursor in activated carbon preparation for the removal of MG dye from wastewaters.     

SYNERGISTIC INFLUENCE OF POLYVINYL ALCOHOL AND SURFACTANTS ON THE CORROSION INHIBITION OF MILD STEEL IN 0.1 M H2SO4

The corrosion inhibition behavior of mild steel in 0.1 M H₂SO₄ in the presence of polyvinyl alcohol (PVA) was investigated using weight loss and potentiodynamic polarization measurements in the temperature range of 30°–60°C. The inhibition efficiency (IE) increased with increasing PVA concentration, showing a maximum IE of 81.41% at 30°C at 100 mg/L and decreased with increasing temperature. The inhibiting action of PVA is synergistically enhanced on addition of very small amounts of surfactants sodium dodecyl sulfate (SDS) and cetyl pyridinium chloride (CPC). The adsorption of PVA alone or in combination with surfactants on the metal surface is found to obey the Langmuir adsorption isotherm from the fit of the experimental data of all concentrations and temperatures studied. The synergism parameter evaluated is found to be greater than unity, indicating that the enhanced IE of PVA caused by addition of surfactants is due only to synergism. The phenomenon of physical adsorption is proposed from the trend of the IE with temperature as well as the values of E_a, ΔH_ads, and ΔG_ads obtained. Thermodynamic parameters reveal that the adsorption process is spontaneous. The results obtained by weight loss measurements are consistent with the results of the potentiodynamic polarization measurements. The surface morphology of the corroded steel samples in the presence and absence of inhibitors was also evaluated using scanning electron microscopy (SEM).     

Applicability of Activated Carbon to Treatment of Waste Containing Iodine-Labeled Compounds

Abstract

A timber industry waste was transformed to activated carbon by a one-step chemical activation process using H₃PO₄ (H). The used activated carbon (SDH) was characterized by N₂ adsorption, FTIR, density, pH, point of zero charge pH_pzc, moisture and ash content. Methylene blue (MB) and the iodine number were calculated by adsorption from the solution. The applicability of the different activated carbon produced was carried out to treatment of aqueous waste contaminated with iodine-labeled prolactin (I-PRL) Treatment processes were performed under the varying conditions; contact time, temperature, carbon type, carbon dosage, and different particle size of the activated carbon (SDH). The results indicated that 5 hours are sufficient to reach a plateau, and the amount of I-PRL adsorbed on SDH activated carbons increase with the solution temperature with thermodynamic parameter of ΔG° = ?7.962 (kJ/mol), ΔH° = 28.869 (kJ/mol) and ΔS° = 109.94 (J/mol K). The optimum adsorption results were reached using carbon dose of 0.1 gm with particle size of <0.25 mm, and a batch factor (V/M) of 7.14 mlg^?1. First- and second-order equations, intraparticle diffusion equation, and the Elovich equation have been used to test experimental data. The experimental data was found to fit the second-order model and a chemisorptions mechanism. 0.7 M NaOH can be used for regeneration of spent SDH activated carbon with the efficiency of 99.6% and the regenerated carbon can be reused for five cycles effectively.     

Removal of Copper(II) and Zinc(II) from Aqueous Solutions Using a Lignocellulosic-Based Polymeric Adsorbent Containing Amidoxime Chelating Functional Groups

In this study, the adsorption of Cu(II) and Zn(II) ions from aqueous solutions onto amidoximated polymerized banana stem (APBS) has been investigated. Infrared spectroscopy was used to confirm graft copolymer formation and amidoxime functionalization. The different variables affecting the sorption capacity such as pH of the solution, adsorption time, initial metal ion concentration, and temperature have been investigated. The optimum pH for maximum adsorption was 10.5 (99.99%) for Zn²⁺ and 6.0 (99.0%) for Cu²⁺ at an initial concentration of 10 mg L^?1. Equilibrium was achieved approximately within 3 h. The experimental kinetic data were analyzed using pseudo-first-order and pseudo-second-order kinetic models and are well fitted with pseudo- second-order kinetics. The thermodynamic activation parameters such as ΔG^o, ΔH^o, and ΔS^o were determined to predict the nature of adsorption. The temperature dependence indicates an exothermic process. The experimental isotherm data were well fitted to the Langmuir model with maximum adsorption capacities of 42.32 and 85.89 mg g^?1 for Cu(II) and Zn(II), respectively, at 20°C. The adsorption efficiency was tested using industrial effluents. Repeated adsorption/regeneration cycles show the feasibility of the APBS for the removal of Cu(II) and Zn(II) ions from water and industrial effluents.     

Self‐assembly of alginate/polyethyleneimine multilayer onto magnetic microspheres as an effective adsorbent for removal of anionic dyes

A novel magnetic adsorbent alginate/polyethyleneimine (ALG/PEI)_n/MN was developed for removal of anionic dyes from aqueous solution in this study. (ALG/PEI)_n/MN was prepared by depositing ALG/PEI multi‐layers onto amine‐modified Fe₃O₄ microspheres through layer‐by‐layer method. The morphologies and structures of the adsorbent were characterized by scanning electron microscopy, X‐ray diffractometer, and Fourier transform infrared spectrometer, respectively, and its performance in adsorption of anionic dye (acid orange 10, AO10) under varied experimental conditions were also investigated. The results revealed that the uptake capacity of AO10 by (ALG/PEI)_n/MN increased with the number of coated (ALG/PEI) bilayer on the adsorbents, and the maximum adsorption capacity for AO10 by (ALG/PEI)₄MN was 246.3 mg g^?1 at 25 °C. The adsorption process was exothermic and well described by the pseudo‐second order kinetic model and the Langmuir isothermal model. Moreover, (ALG/PEI)₄/MN showed good reusability and excellent magnetic separability. All the results demonstrate that (ALG/PEI)₄/MN is a potential recyclable adsorbent for removal of anionic dyes from wastewater. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45876.     

SYNTHESIS OF ACTIVATED CARBON FROM JATROPHA SEED COAT AND APPLICATION TO ADSORPTION OF IODINE AND METHYLENE BLUE

This article provides evidence that jatropha seed coat residues can be used as a carbon source for preparing activated carbons that have good adsorption properties for iodine and methylene blue. Activated carbons were prepared using three different methods of activation, physical, chemical, and physico-chemical, for a range of activation temperatures (600°, 700°, 800°, and 900°C) and activation hold times (1, 2, and 3 h). The highest BET surface area (1479 m² g^?1) and the highest iodine adsorption (1511 mg g^?1) were obtained with physico-chemical activation at a temperature of 900°C and a hold time of 2 h. This activated carbon gave higher BET surface area and iodine adsorption than commercial activated carbon (1169.1 m² g^?1 and 1076 mg g^?1). The activated carbons prepared by physico-chemical activation at 900°C and 2 h were then tested for adsorption of methylene blue at a range of concentrations of methylene blue (100, 200, 300, 400, and 500 mg L^?1). It was found that a Langmuir isotherm gave a better fit (R ² = 0.999) to the observed adsorptions than a Freundlich isotherm (R ² = 0.884). For the adsorption kinetics, a pseudo-second-order model gave a better fit (R ² > 0.998, Δq _e  = 3.7%) than a pseudo-first-order model (R ² ≈ 0.95, Δq _e  = 85.6%). These results suggest that chemisorption is the rate-controlling step for the adsorption of methylene blue. The experimental results show that jatropha seed coat is a lignocellulosic waste precursor for preparation of activated carbon that is an alternative source for preparation of commercial-grade activated carbons.     

EFFECT OF TEMPERATURE ON THE ADSORPTION OF METHYLENE BLUE DYE ONTO SULFURIC ACID–TREATED ORANGE PEEL

The present study explains the preparation and application of sulfuric acid–treated orange peel (STOP) as a new low-cost adsorbent in the removal of methylene blue (MB) dye from its aqueous solution. The effects of temperature on the operating parameters such as solution pH, adsorbent dose, initial MB dye concentration, and contact time were investigated for the removal of MB dye using STOP. The maximum adsorption of MB dye onto STOP took place in the following experimental conditions: pH of 8.0, adsorbent dose of 0.4 g, contact time of 45 min, and temperature of 30°C. The adsorption equilibrium data were tested by applying both the Langmuir and Freundlich isotherm models. It is observed that the Freundlich isotherm model fitted better than the Langmuir isotherm model, indicating multilayer adsorption, at all studied temperatures. The adsorption kinetic results showed that the pseudo-second-order model was more suitable to explain the adsorption of MB dye onto STOP. The adsorption mechanism results showed that the adsorption process was controlled by both the internal and external diffusion of MB dye molecules. The values of free energy change (ΔG ^o) and enthalpy change (ΔH ^o) indicated the spontaneous, feasible, and exothermic nature of the adsorption process. The maximum monolayer adsorption capacity of STOP was also compared with other low-cost adsorbents, and it was found that STOP was a better adsorbent for MB dye removal.     

Biosorption of a Basic Dye from Aqueous Solutions by Euphorbia rigida

Abstract

In this study, the biosorption of Basic Blue 9 (BB9) dye from aqueous solutions onto a biomass of Euphorbia rigida was examined by means of the initial biosorbate concentration, biosorbent amount, particle size, and pH. Biosorption of BB9 onto E. rigida increases with both the initial biosorbate concentration and biosorbent amount, whereas decreases with the increasing particle size. The experimental data indicated that the biosorption isotherms are well‐described by the Langmuir equilibrium isotherm equation at 20, 30, and 40°C. Maximum biosorption capacity was 3.28×10^?4 mol g^?1 at 40°C. The biosorption kinetics of BB9 obeys the pseudo‐second‐order kinetic model. The thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated to estimate the nature of biosorption. These experimental results have indicated that E. rigida has the potential to act as a biosorbent for the removal of Basic Blue 9 from aqueous solutions.     

The Removal of Hg (II) Ions from Laboratory Wastewater onto Phosphorylated Haloxylon ammodendron: Kinetic and Equilibrium Studies

Haloxylon ammodendron (HA), a desert plant residue, has been utilized as adsorbent material for the removal of Hg (II) ions from laboratory wastewater after treatment with phosphoric acid to form Haloxylon ammodendron cellulose phosphate (HACP). Three levels of HACP having different phosphorous content were prepared. The HACP samples were characterized by estimating the phosphorous content as well as FT-IR spectra. Using the batch experimental systems, the removal of Hg (II) on the HACP particles was investigated. The data of the adsorption isotherm was tested by the Langmuir, Freundlich and Temkin models. The removal processes of Hg (II) onto HACP particles could be well described by pseudo-second order model. The adsorption rate of mercury was affected by the initial heavy metal concentration, initial pH, adsorbent dose and agitation time and temperature as well as extent of modification. The adsorption experiments indicated that the HACP particles have great potential for the removal of Hg (II) from laboratory wastewater. The maximum adsorption capacity (Q_max) of the HACP towards Hg (II) ions was found to be 384.6 and 416.7 and 476.2 mg/g at 30, 40 and 50°C, respectively. Similarly, the Freundlich constant, n values were found to be 6.6, 4.4 and 3.8 at 30, 40 and 50°C, respectively. The thermodynamics constants of the adsorption process: ΔH°, ΔS° and ΔG° were evaluated.     

Removal of Chromium(VI) and Chromium(III) from Aqueous Solution by Grainless Stalk of Corn

Abstract

Grainless stalk of corn (GLSC) was tested for removal of Cr(VI) and Cr(III) from aqueous solution at different pH, contact time, temperature, and chromium/adsorbent ratio. The results show that the optimum pH for removal of Cr(VI) is 0.84, while the optimum pH for removal of Cr(III) is 4.6. The adsorption processes of both Cr(VI) and Cr(III) onto GLSC were found to follow first-order kinetics. Values of k _ads of 0.037 and 0.018 min^?1 were obtained for Cr(VI) and Cr(III), respectively. The adsorption capacity of GLSC was calculated from the Langmuir isotherm as 7.1 mg g^?1 at pH 0.84 for Cr(VI), and as 7.3 mg g^?1 at pH 4.6 for Cr(III), at 20°C. At the optimum pH for Cr(VI) removal, Cr(VI) reduces to Cr(III). EPR spectroscopy shows the presence of Cr(V) + Cr(III)-bound-GLSC at short contact times and adsorbed Cr(III) as the final oxidation state of Cr(VI)-treated GLSC. The results indicate that, at pH ≈ 1, GLSC can completely remove Cr(VI) from aqueous solution through an adsorption-coupled reduction mechanism to yield adsorbed Cr(III) and the less toxic aqueous Cr(III), which can be further removed at pH 4.6.