Removal of chromium(III) from aqueous solutions using Lewatit S 100: the effect of pH, time, metal concentration and temperature

The removal of the Cr(III) ion from aqueous solutions with the Lewatit S 100 ion-exchange resin is described; and the performance of this resin was compared with Chelex-100 resin. The effect of adsorbent dose, initial metal concentration, contact time, pH and temperature on the removal of Cr(III) was investigated. Lewatit S 100 shows a remarkable increase in sorption capacity for Cr(III). The Batch ion-exchange process was relatively fast; and it reached equilibrium after about 150 min of contact. The ion-exchange process, which is pH dependent show maximum removal of Cr(III) in the pH range 2.8-4.0 for an initial Cr(III) concentration of 1.0 x 10(-3)M. The equilibrium constants were 36.67 at pH value 3.5 for Lewatit S 100 and 6.64 at pH value 4.5 for Chelex-100 resin. Both of the resins had high-bonding constants. The equilibrium related to their ion-exchange capacity and the amount of the ion exchange was obtained by using the plots of the Langmuir adsorption isotherm. It was observed that the maximum ion-exchange capacity of 0.39 mmol of Cr(III)/g for Lewatit S 100 and 0.29 mmol of Cr(III)/g for Chelex-100 was achieved at optimum pH values of 3.5 and 4.5, respectively. The thermodynamic equilibrium constant and the Gibbs free energy flow were calculated for each system. The ion exchange of Cr(III) on these cation-exchange resins followed first-order reversible kinetics. The intra-particle diffusion of Cr(III) on ion-exchange resin represented the rate-limiting step. The rise in temperature caused a slight increase in the value of the equilibrium constant (K(c)) for the sorption of Cr(III) ion for both resins.     

Sorption of uranium from underground leaching solutions with highly basic anion exchangers

Sorption of U on strongly basic anion exchangers AMP, AM-p, Lewatit K6267, Lewatit MonoPlus MP600XL, Purolite A500u, and Purolite A600u was studied at various temperatures in batch and dynamic experiments. The dynamic exchange capacities, (g U) dm^?3, of the anion exchangers to a breakthrough of 0.5 (mg U) dm^?3 at 6–8°C were determined: AMP (9.56) > Purolite A600u (9.26) > Lewatit K6267 (9.13) > Purolite A500u, Lewatit MonoPlus MP600XL (8.87) > AM-p (5.88). Also, the total dynamic exchange capacities, (g U) dm^?3, of the anion exchangers at 6–8°C were determined: AMP (37.13) > Purolite A500u (31.79) > Lewatit MonoPlus MP600XL (28.88) > Lewatit K6267 (27.74) > AM-p (27.58) > Purolite A600u (25.24). The concentration equilibrium distribution coefficients (K _d) of U on these resins were determined in batch experiments. The highest value of K _d = 2869 at 6°C and initial uranium concentration in solution of 65 mg dm^?3 was attained with AMP of commercial grain-size distribution. At 24°C and the same uranium concentration, the highest value of K _d = 5195 was obtained with separated 0.8–1.0-mm fraction of AMP. The kinetic experiments were performed, and the internal diffusion step was found to be the limiting step of the U sorption with all the resins under consideration. The internal diffusion coefficients in ion exchanger grains were calculated to compare the resins with each other.     

Kinetics of removal of chromium from water and electronic process wastewater by ion exchange resins: 1200H, 1500H and IRN97H

The removal of chromium from aqueous solution by an ion exchange resin is described. Ion exchange resins 1200H, 1500H and IRN97H show a remarkable increase in sorption capacity for chromium, compared to other adsorbents. The adsorption process, which is pH dependent show maximum removal of chromium in the pH range 2-6 for an initial chromium concentration of 10mg/l. The metal ion adsorption obeyed linear, Langmuir and Freundlich isotherms. The adsorption of chromium on these cation exchange resins follows first-order reversible kinetics and pseudo-first-order kinetics. The intraparticle diffusion of chromium on ion exchange resins represents the rate-limiting step. The uptake of chromium by the ion exchange resins was reversible and thus have good potential for the removal/recovery of chromium from aqueous solutions. We conclude that such ion exchange resins can be used for the efficient removal of chromium from water and wastewater.     

Evaluation of iron-based hybrid materials for heavy metal ions removal

The article presents in detail the results of the sorption of heavy metal ions such as Cu(II), Zn(II), Cd(II), and Zn(II) in the presence of ethylenediamine-N,N′-disuccinic acid as well as chromium(VI) on Purolite Arsen X^np and Lewatit FO36. Factors affecting the sorption equilibrium (sorbent dose, contact time, temperature, pH, and the presence of interfering ions) were studied. To compare the surface morphologies of the studied ion exchangers, scans using atomic force microscope were also recorded, and attenuated total reflectance infrared spectroscopy was applied to establish the adsorption mechanism. The main parameters affecting sorption are initial concentration of the solution, pH, and phase contact time. Temperature has only a slight influence. The kinetic data were fitted using the pseudo-first-order and pseudo-second-order models. In the case of Cr(VI) adsorption, the equilibriums on Purolite Arsen X^np and Lewatit FO36 were established within 60 min, in the case of Cu(II) and Zn(II) within 30–40 min and for Cd(II) and Pb(II) even less than 30 min. Moreover, it was found that the effectiveness of adsorption in the case of Cr(VI), Cu(II), Zn(II), Cd(II), and Pb(II) on Purolite ArsenX^np was higher than that on Lewatit FO36.     

A comparative study of two chelating ion-exchange resins for the removal of chromium(III) from aqueous solution

Macroporous resins containing iminodiacetic acid (IDA) groups (Lewatit TP 207 and Chelex-100) were investigated as a function of concentration, temperature and pH for their sorption properties towards chromium(III). The chromium(III) ions sorbed onto the resin and in the equilibrium concentration were determined by inductively coupled plasma spectrophotometer. The maximum sorption for chromium ions was observed at pH 4.5. Solution pH had a strong effect on the equilibrium constant of Cr(III). The equilibrium constants were 320 and 7 at pH value 4.5 for Lewatit TP 207 and Chelex-100 resin, respectively. The Langmuir isotherm was used to describe observed sorption phenomena. Both the sorbents had high bonding constants with Lewatit TP 207 showing stronger binding. The equilibrium related to adsorption capacity and energy of adsorption was obtained by using plots of Langmuir adsorption isotherm. It was observed that the maximum adsorption capacity of 0.288 mmol of Cr(III)/g for Chelex-100 and 0.341 mmol of Cr(III)/g for Lewatit TP 207 was achieved at pH of 4.5. The rise in temperature caused a slight increase in the value of the equilibrium constant (K_c) for the sorption of chromium(III) ion.     

Removal of Cr(VI) from aqueous solutions using modified red pine sawdust

The adsorption of Cr(VI) from aqueous solutions on sawdust (SD), base extracted sawdust (BESD) and tartaric acid modified sawdust (TASD) of Turkish red pine tree (Pinus nigra), a timber industry waste, was studied at varying Cr(VI) concentrations, adsorbent dose, modifier concentration and pH. Batch adsorption studies have been carried out. Sawdust was collected from waste timber industry and modified with various amount of tartaric acid (TA) (0.1-1.5M). The batch sorption kinetics has been tested and the applicability of the Langmuir and Freundlich adsorption isotherms for the present system has been tested at 25+/-2 degrees C. Under observed test conditions, the equilibrium adsorption data fits the linear Freundlich isotherms. An initial pH of 3.0 was most favorable for Cr(VI) removal by all adsorbents. Maximum Cr(VI) was sequestered from the solution within 120 min after the beginning for every experiment. The experimental result inferred that chelation and ion exchange is one of the major adsorption mechanisms for binding metal ions to the SD. Percentage removal of Cr(VI) was maximum at the initial pH of 3.0 (87.7, 70.6 and 55.2% by TASD, BESD, and SD, respectively). Adsorption capacities range from 8.3 to 22.6 mg/g for SD samples.     

Deep oxidation of pollutants using gold deposited on a high surface area cobalt oxide prepared by a nanocasting route

A new acrylic anion exchanger with both tertiary and quaternary ammonium as well as ketone groups in the structural unit has been prepared by the nucleophilic substitution reaction of aminolyzed vinylacetate:acrylonitrile:divinylbenzene copolymer of porosity structure in the swelling state with 2-chloroacetone as a halogenated compound. The new compound exhibits better qualities of strong base exchange capacity than the weak base anion exchangers. The obtained acrylic anion exchanger was used to remove Cr(VI) from the aqueous solution. Batch adsorption studies have been carried out to determine the effect of contact time, concentration of hexavalent chromium in the solution and pH on the sorption capacity. The kinetic parameters were determined on the basis of the static results. The thermodynamic parameters of Cr(VI) sorption process on the anion exchanger were calculated based on the Langmuir and Freundlich isotherms. Sorption was studied in the pH range of 1.5-7 and it was found that it depends on the solution acidity. At the pH values of 3.5 and 7 the anion exchanger exhibited large values of chromium sorption capacity. The speciation of chromium was investigated in the studied pH range by the Diffuse Reflectance Spectroscopy (DRS) method. Reduction of chromium(VI) to chromium(III) under acidic conditions was observed. The performed acrylic strong base anion exchanger is superior compared to the conventional one based on the styrene:divinylbenzene matrix due to its ability for reposition of the long spacer arm for providing exchange sites, hydrophilic character of matrix, and possible hydrogen bonds provided by carbonyl functional groups.     

Biosorption of chromium(VI) ion from aqueous solutions using walnut, hazelnut and almond shell

The potential to remove Cr(VI) ion from aqueous solutions through biosorption using, the shells of Walnut (WNS) (Juglans regia), Hazelnut (HNS) (Corylus avellana) and Almond (AS) (Prunus dulcis) was investigated in batch experiments. The equilibrium adsorption level was determined to be a function of the solution contact time and concentration. Kinetic experiments revealed that the dilute chromium solutions reached equilibrium within 100 min. The biosorptive capacity of the shells was dependent on the pH of the chromium solution, with pH 3.5 being optimal. Adsorption of Cr(VI) ion uptake is in all cases pH-dependent showing a maximum at equilibrium pH values between 2.0 and 3.5, depending on the biomaterial, that correspond to equilibrium pH values of 3.5 for (WNS), 3.5 for (HNS) and 3.2 for (AS). The adsorption data fit well with the Langmuir isotherm model. The sorption process conformed to the Langmuir isotherm with maximum Cr(VI) ion sorption capacities of 8.01, 8.28, and 3.40 mg/g for WNS, HNS and AS, respectively. Percentage removal by WNS, HNS and AS was 85.32, 88.46 and 55.00%, respectively at a concentration of 0.5 mM. HNS presented the highest adsorption capacities for the Cr(VI) ion.     

Characteristics and mechanisms of hexavalent chromium removal by biochar from sugar beet tailing

Removal of Cr(VI) from aqueous solutions using biochar from sugar beet tailing (SBT) was investigated as a function of pH, contact time, and biochar mass via batch experiments. The surface characteristics of SBT biochar before and after Cr(VI) sorption was investigated with scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. Desorption and X-ray photoelectron spectroscopy studies showed that most of the Cr bound to SBT biochar was Cr(III). These results indicated that the electrostatic attraction of Cr(VI) to positively charged biochar surface, reduction of Cr(VI) to Cr(III) ion, and complexation between Cr(III) ion and SBT's function groups were probably responsible for Cr(VI) removal by SBT biochar. An initial solution with a pH of 2.0 was most favorable for Cr(VI) removal. The sorption process can be described by the pseudo-second order equation and Langmuir isotherm. The maximum sorption capacity for Cr(VI) was 123 mg/g under an acidic medium, which was comparable to other low-cost sorbents.     

Synthesis of some ferromagnetic composite resins and their metal removal characteristics in aqueous solutions

In this study, a procedure for synthesis of new organic-inorganic magnetic composite resins was established. The procedure was based upon immobilization of magnetite (Mag) as a ferromagnetic material within the polymer poly(acrylic acid acrylonitrile) P(AA-AN) and the ion exchange resin (Amberlite IR120). The produced magnetic resins, IR120-PAN-Mag (R1) and P(AA-AN)-Mag (R2) were assessed as sorbents for Cr(VI). Various factors influencing the sorption of Cr(VI), e.g., pH, equilibrium time, initial concentration and temperature were studied. The sorption process was very fast initially and maximum sorption was achieved within 3 h and pH 5.1. The kinetic of the system has been evaluated with pseudo first order model, second order model, Elovich model, intra-particle diffusion model and liquid film diffusion model. Chromium interaction with composite particles followed second-order kinetics with a correlation coefficient extremely high and closer to unity and rate constant (k_s) has the values 1.68 × 10⁻⁴ and 1.9 × 10⁻⁴ g (mg⁻¹ min⁻¹) for R1 and R2, respectively. The values of equilibrium sorption capacity (q_e) are consistent with the modeled data and attain the range 893–951 mg g⁻¹. Kinetically, both pore diffusion and film diffusion are participating in ruling the diffusion of Cr(VI) ions. The sorption data gave good fits with Temkin and Flory–Huggins isotherm models. The isotherm parameters related to the heat of sorption are in the range 8–16 kJ mol⁻¹ which is the range of bonding energy for ion exchange interactions and so suggest an ion exchange mechanism for removal of Cr(VI) by the composite sorbents. The adsorption process was exothermic with ΔH in the range of −73 to −97 kJ mol⁻¹. The negative values of Gibbs free energy confirm the feasibility and the spontaneous nature of Cr(VI) removal with these novel composites.     

Ion-exchange of Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ ions from aqueous solution by Lewatit CNP 80

Removal of trace amounts of heavy metals can be achieved by means of selective ion-exchange processes. The newly developed resins offered a high resin capacity and faster sorption kinetics for the metal ions such as Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ions. In the present study, the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ions from aqueous solutions was investigated. Experimental investigations were undertaken using the ion-exchange resin Lewatit CNP 80 (weakly acidic) and were compared with Lewatit TP 207 (weakly acidic and chelating). The optimum pH range for the ion-exchange of the above mentioned metal ions on Lewatit CNP 80 and Lewatit TP 207 were 7.0-9.0 and 4.5-5.5, respectively. The influence of pH, contact time, metal concentration and amount of ion-exchanger on the removal process was investigated. For investigations of the exchange equilibrium, different amounts of resin were contacted with a fixed volume of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ion containing solution. The obtained sorption affinity sequence in the presented work was Ni(2+)>Cu(2+)>Cd(2+)>Zn(2+)>Pb(2+). The metal ion concentrations were measured by AAS methods. The distribution coefficient values for metal ions of 10(-3)M initial concentration at 0.1mol/L ionic strength show that the Lewatit CNP 80 was more selective for Ni(2+), Cu(2+) than it was for Cd(2+), Zn(2+) and Pb(2+). Langmuir isotherm was applicable to the ion-exchange process and its contents were calculated. The uptake of metal ions by the ion-exchange resins was reversible and thus has good potential for the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) from aqueous solutions. The amount of sorbed metal ion per gram dry were calculated as 4.1, 4.6, 4.7, 4.8, and 4.7mequiv./g dry resin for Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+), respectively. Selectivity increased in the series: Cd(2+)>Pb(2+)>Cu(2+)>Ni(2+)>Zn(2+). The results obtained showed that Lewatit CNP 80 weakly acidic resin had shown better performance than Lewatit TP 207 resin for the removal of metals. The change of the ionic strength of the solution exerts a slight influence on the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+). The presence of low ionic strength or low concentration of NaNO(3) does not have a significant effect on the ion-exchange of these metals by the resins. We conclude that Lewatit CNP 80 can be used for the efficient removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) from aqueous solutions.     

Untreated coffee husks as biosorbents for the removal of heavy metals from aqueous solutions

The objective of this work was to propose an alternative use for coffee husks (CH), a coffee processing residue, as untreated sorbents for the removal of heavy metal ions from aqueous solutions. Biosorption studies were conducted in a batch system as a function of contact time, initial metal ion concentration, biosorbent concentration and pH of the solution. A contact time of 72 h assured attainment of equilibrium for Cu(II), Cd(II) and Zn(II). The sorption efficiency after equilibrium was higher for Cu(II) (89-98% adsorption), followed by Cd(II) (65-85%) and Zn(II) (48-79%). Even though equilibrium was not attained in the case of Cr(VI) ions, sorption efficiency ranged from 79 to 86%. Sorption performance improved as metal ions concentrations were lowered. The experimental sorption equilibrium data were fitted by both Langmuir and Freundlich sorption models, with Langmuir providing the best fit (R2>0.95). The biosorption kinetics was determined by fitting first and second-order kinetic models to the experimental data, being better described by the pseudo-second-order model (R2>0.99). The amount of metal ions sorbed increased with the biosorbent concentration in the case of Cu(II) and Cr(VI) and did not present significant variations for the other metal ions. The effect of the initial pH in the biosorption efficiency was verified in the pH range of 4-7, and the results showed that the highest adsorption capacity occurred at distinct pH values for each metal ion. A comparison of the maximum sorption capacity of several untreated biomaterial-based residues showed that coffee husks are suitable candidates for use as biosorbents in the removal of heavy metals from aqueous solutions.     

The influence of ferric iron in calcined nano-Mg/Al hydrotalcite on adsorption of Cr (VI) from aqueous solution

The influence of ferric iron in calcined nano-Mg/Al hydrotalcite on removal of Cr (VI) from aqueous solution was studied from aspects of structure characteristics, adsorption properties and mechanism discussions. The calcined hydrotalcites (CH-Mg/Al and CH-Mg/Al/Fe) were obtained by thermal decomposition of their corresponding precursors and characterized by XRD, TEM, pH(PZC) and FTIR. The adsorption properties were studied as a function of pH, initial Cr (VI) concentration and contact time. The results showed that the nature of adsorption is endothermic and spontaneous for both CH-Mg/Al and CH-Mg/Al/Fe, but the thermodynamic parameter value changes revealed the addition of Fe(3+) is disadvantage to adsorption process and the theoretical saturated adsorption capacity decreased by approximately 10.2mg/g at tested temperatures. The removal mechanism involved not only intercalation but adsorption on external surface of the layers and interlayer anion exchange for both CH-Mg/Al and CH-Mg/Al/Fe. Furthermore, the results also indicated that intercalation accounts for a large proportion during removal process whatever for CH-Mg/Al, or for CH-Mg/Al/Fe. Additionally, the replacement of Al(3+) by Fe(3+) in CH-Mg/Al led to the interlayer anion exchange more difficult. On the basis of the results, it is concluded that the existence of ferric iron in calcined Mg/Al hydrotalcite is unfavorable to removal of Cr (VI) from aqueous solution.     

Removal of chromium from water and wastewater by ion exchange resins

Removal of chromium from water and wastewater is obligatory in order to avoid water pollution. Batch shaking adsorption experiments were carried out to evaluate the performance of IRN77 and SKN1 cation exchange resins in the removal of chromium from aqueous solutions. The percentage removal of chromium was examined by varying experimental conditions viz., dosage of adsorbent, pH of the solution and contact time. It was found that more than 95% removal was achieved under optimal conditions. The adsorption capacity (k) for chromium calculated from the Freundlich adsorption isotherm was found to be 35.38 and 46.34 mg/g for IRN77 and SKN1 resins, respectively. The adsorption of chromium on these cation exchange resins follows the first-order reversible kinetics. The ion exchange resins investigated in this study showed reversible uptake of chromium and, thus, have good application potential for the removal/recovery of chromium from aqueous solutions.     

Iminodiacetic acid functionalized cation exchange resin for adsorptive removal of Cr(VI), Cd(II), Ni(II) and Pb(II) from their aqueous solutions

Iminodiacetic acid functionality has been introduced on styrene-divinyl benzene co-polymeric beads and characterized by FT-IR in order to develop weak acid based cation exchange resin. This resin was evaluated for the removal of different heavy metal ions namely Cd(II), Cr(VI), Ni(II) and Pb(II) from their aqueous solutions. The results showed greater affinity of resin towards Cr(VI) for which 99.7% removal achieved in optimal conditions following the order Ni(II)>Pb(II)>Cd(II) with 65%, 59% and 28% removal. Experiments were also directed towards kinetic studies of adsorption and found to follow first order reversible kinetic model with the overall rate constants 0.3250, 0.2393, 0.4290 and 0.2968 for Cr(VI), Ni(II), Pb(II) and Cd(II) removal respectively. Detailed studies of Cr(VI) removal has been carried out to see the effect of pH, resin dose and metal ion concentration on adsorption and concluded that complexation enhanced the chromium removal efficacy of resin drastically, which is strongly pH dependent. The findings were also supported by the comparison of FT-IR spectra of neat resin with the chromium-adsorbed resin.     

Chromium (VI) adsorption on boehmite

Boehmite was synthesized and characterized in order to study the adsorption behavior and the removal of Cr(VI) ions from aqueous solutions as a function of contact time, initial pH solution, amount of adsorbent and initial metal ion concentration, using batch technique. Adsorption data of Cr(VI) on the boehmite were analyzed according to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) adsorption models. Thermodynamic parameters for the adsorption system were determinated at 293, 303, 313 and 323K temperatures. The kinetic values and thermodynamic parameters from the adsorption process show that the Cr(VI) ions adsorption on boehmite is an endothermic and spontaneous process. These results show that the boehmite could be considered as a potential adsorbent for chromium ions in aqueous solutions.     

Removal of chromium and toxic ions present in mine drainage by Ectodermis of Opuntia

This work presents conditions for hexavalent and trivalent chromium removal from aqueous solutions using natural, protonated and thermally treated Ectodermis of Opuntia. A removal of 77% of Cr(VI) and 99% of Cr(III) can be achieved. The sorbent material is characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, infrared spectroscopy, thermogravimetric analysis, before and after the contact with the chromium containing aqueous media. The results obtained from the characterization techniques indicate that the metal ion remains on the surface of the sorbent material. The percentage removal is found to depend on the initial chromium concentration and pH. The Cr(VI) and Cr(III) uptake process is maximum at pH 4, using 0.1g of sorbent per liter of aqueous solution. The natural Ectodermis of Opuntia showed a chromium adsorption capacity that was adequately described by the Langmuir adsorption isotherm. Finally, an actual mine drainage sample that contained Cd, Cr, Cu, Fe Zn, Ni and Pb was tested under optimal conditions for chromium removal and Ectodermis of Opuntia was found to be a suitable sorbent material. The use of this waste material for the treatment of metal-containing aqueous solutions as well as mine drainage is effective and economical.     

Removal of hexavalent chromium from aqueous solutions by D301, D314 and D354 anion-exchange resins

Removal of hexavalent chromium from electroplating industry wastewater is obligatory in order to avoid pollution. Batch shaking experiments were carried out to evaluate the adsorption capacity of resins (D301, D314 and D354) in the removal of chromium from aqueous solutions. Varying experimental conditions were studied, including Cr(6+) concentrations, resin amounts, initial pH, contact time and temperatures. The ion-exchange process, which is pH-dependent, indicated the maximum removal of Cr(6+) in the pH range of 1-5 for an initial concentration 100 ppm of Cr(6+). It was found that more than 99.4% of the removal was achieved under optimal conditions. High adsorption rates of chromium for the three resins were observed at the onset, and then plateau values were gradually reached within 30 min. The experimental results obtained at various concentrations (27+/-1 degrees C) showed that the adsorption pattern on the resins have followed Langmuir isotherms and the calculated maximum sorption capacities of D301, D314 and D354 were 152.52, 120.48 and 156.25mg/g, respectively. The thermodynamic parameters (free energy change DeltaG, enthalpy change DeltaS and entropy change DeltaH) for the sorption have been evaluated. It was also found that the adsorption of chromium on these anion-exchange resins follows first-order reversible kinetics.     

Removal of chromium from aqueous solution by using oxidized multiwalled carbon nanotubes

The batch removal of hexavalent chromium (Cr(VI)) from aqueous solution by using oxidized multiwalled carbon nanotubes (MWCNTs) was studied under ambient conditions. The effect of pH, initial concentration of Cr(VI), MWCNT content, contact time and ionic strength on the removal of Cr(VI) was also investigated. The removal was favored at low pH with maximum removal at pH <2. The adsorption kinetics was modeled by first-order reversible kinetics, pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion models, respectively. The rate constants for all these kinetic models were calculated, and the results indicate that pseudo-second-order kinetics model was well suitable to model the kinetic adsorption of Cr(VI). The removal of chromium mainly depends on the occurrence of redox reaction of adsorbed Cr(VI) on the surface of oxidized MWCNTs to the formation of Cr(III), and subsequent the sorption of Cr(III) on MWCNTs appears as the leading mechanism for chromium uptake to MWCNTs. The presence of Cr(III) and Cr(VI) on oxidized MWCNTs was confirmed by the X-ray photoelectron spectroscopic analysis. The application of Langmuir and Freundlich isotherms are applied to fit the adsorption data of Cr(VI). Equilibrium data were well described by the typical Langmuir adsorption isotherm. Overall, the study demonstrated that MWCNTs can effectively remove Cr(VI) from aqueous solution under a wide range of experimental conditions, without significant Cr(III) release.     

Characterization of Cr(VI) removal from aqueous solutions by a surplus agricultural waste--rice straw

The removal of Cr(VI) from aqueous solution by rice straw, a surplus agricultural byproduct was investigated. The optimal pH was 2.0 and Cr(VI) removal rate increased with decreased Cr(VI) concentration and with increased temperature. Decrease in straw particle size led to an increase in Cr(VI) removal. Equilibrium was achieved in about 48 h under standard conditions, and Cr(III), which appeared in the solution and remained stable thereafter, indicating that both reduction and adsorption played a part in the Cr(VI) removal. The increase of the solution pH suggested that protons were needed for the Cr(VI) removal. A relatively high level of NO(3)(-) notably restrained the reduction of Cr(VI) to Cr(III), while high level of SO(4)(2-) supported it. The promotion of the tartaric acid modified rice straw (TARS) and the slight inhibition of the esterified rice straw (ERS) on Cr(VI) removal indicated that carboxyl groups present on the biomass played an important role in chromium remediation even though were not fully responsible for it. Isotherm tests showed that equilibrium sorption data were better represented by Langmuir model and the sorption capacity of rice straw was found to be 3.15 mg/g.