Adsorption of Cr(VI) using activated neem leaves: kinetic studies

In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The dimensionless equilibrium parameter, R _L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R _L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg) (min).     

Synthesis of Calcium Silicate Hydrate from Coal Gangue for Cr(VI) and Cu(II) Removal from Aqueous Solution

Both the accumulation of coal gangue and potentially toxic elements in aqueous solution have caused biological damage to the surrounding ecosystem of the Huainan coal mining field. In this study, coal gangue was used to synthesize calcium silicate hydrate (C-S-H) to remove Cr(VI) and Cu(II)from aqueous solutions and aqueous solution. The optimum parameters for C-S-H synthesis were 700 °C for 1 h and a Ca/Si molar ratio of 1.0. Quantitative sorption analysis was done at variable temperature, C-S-H dosages, solution pH, initial concentrations of metals, and reaction time. The solution pH was precisely controlled by a pH meter. The adsorption temperature was controlled by a thermostatic gas bath oscillator. The error of solution temperature was controlled at ± 0.3, compared with the adsorption temperature. For Cr(VI) and Cu(II), the optimum initial concentration, temperature, and reaction time were 200 mg/L, 40 °C and 90 min, pH 2 and 0.1 g C-S-H for Cr(VI), pH 6 and 0.07 g C-S-H for Cu(II), respectively. The maximum adsorption capacities of Cr(VI) and Cu(II) were 68.03 and 70.42 mg·g⁻¹, respectively. Furthermore, the concentrations of Cu(II) and Cr(VI) in aqueous solution could meet the surface water quality standards in China. The adsorption mechanism of Cu(II) and Cr(VI) onto C-S-H were reduction, electrostatic interaction, chelation interaction, and surface complexation. It was found that C-S-H is an environmentally friendly adsorbent for effective removal of metals from aqueous solution through different mechanisms.     

Adsorption of Cr(VI) using cellulose microsphere-based adsorbent prepared by radiation-induced grafting

Cellulose microsphere (CMS) adsorbent was prepared by radiation-induced grafting of dimethylaminoethyl methacrylate (DMAEMA) onto CMS followed by a protonation process. The FTIR spectra analysis proved that PDMAEMA was grafted successfully onto CMS. The adsorption of Cr(VI) onto the resulting adsorbent was very fast, the equilibrium adsorption could be achieved within 15 min. The adsorption capacity strongly depended on the pH of the solution, which was attributed to the change of both the existed forms of Cr(VI) and the tertiary-ammonium group of PDMAEMA grafted CMS with the pH. A maximum Cr(VI) uptake (ca. 78 mg g^?1) was obtained as the pH was in the range of 3.0–6.0. However, even in strong acid media (pH 1.3), the adsorbents still showed a Cr(VI) uptake of 30 mg g^?1. The adsorption behavior of the resultant absorbent could be described with the Langmuir mode. This adsorbent has potential application for removing heavy metal ion pollutants (e.g. Cr(VI)) from wastewater.     

Kinetics and equilibrium studies of adsorption of chromium(VI) ion from industrial wastewater using Chrysophyllum albidum (Sapotaceae) seed shells

A new biosorbent has been prepared by coating Chrysophyllum albidum (Sapotaceae) seed shells with chitosan and/or oxidizing agents such as sulfuric acid. This study investigated the technical feasibility of activated and modified activated C. albidum seed shells carbons for the adsorption of chromium(VI) from aqueous solution. The sorption process with respect to its equilibria and kinetics as well as the effects of pH, contact time, adsorbent mass, adsorbate concentration and particle size on adsorption was also studied. The most effective pH range was found to be between 4.5 and 5 for the sorption of the metal ion. The pseudo-first-order rate equation by Lagergren and pseudo-second-order rate equation were tested on the kinetic data, the adsorption process followed pseudo-second-order rate kinetics, also, isotherm data was analyzed for possible agreement with the Langmuir and Freundlich adsorption isotherms, the Freundlich and Langmuir models for dynamics of metal ion uptake proposed in this work fitted the experimental data reasonably well. However, equilibrium sorption data were better represented by Langmuir model than Freundlich. The adsorption capacity calculated from Langmuir isotherm was 84.31, 76.23 and 59.63 mg Cr(VI)/g at initial pH of 3.0 at 30 °C for the particle size of 1.00–1.25 mm with the use of 12.5, 16.5 and 2.1 g/L of CACASC, CCASC and ACASC adsorbent mass, respectively. This readily available adsorbent is efficient in the uptake of Cr(VI) ion in aqueous solution, thus, it could be an excellent alternative for the removal of heavy metals and organic matter from water and wastewater.     

Highly efficient adsorption of Cr(VI) from aqueous solution by Fe3+ impregnated biochar

Biochar (BC) has been widely used as a low-cost adsorbent for the removal of contaminants from water and soil. However, the adsorption ability of BC towards heavy metal oxyanions (e.g., Cr(VI)) is relatively low due to the negatively charged surface of BC. In this study, pristine BC was impregnated with Fe³⁺ to improve its Cr(VI) adsorption capability. Fe³⁺-impregnated BC (Fe³⁺-BC) was successfully synthesized by a simple impregnation method and used for the removal of Cr(VI) from aqueous solution. Various factors affecting the adsorption, such as impregnation ratio, pH, adsorbent dosage, contact time, temperature, and the presence of humic acid, were investigated in detail. Results showed that Fe³⁺-BC had strong adsorption ability to Cr(VI) with a maximum adsorption capacity of 197.80 mg/g, which were not only significantly higher than that of the pristine BC, but also were superior to many previously reported adsorbents. It was favorable for Cr(VI) adsorption under the condition of acidic and high temperature. The adsorption data obeyed Sips and Langmuir isotherms and the kinetic data were well described by the pseudo-first-order kinetic model. The results herein revealed that the Fe³⁺-impregnated BC had a good potential as a highly efficient material for adsorption of Cr(VI) from aqueous solution.     

Adsorption of metal anions of vanadium(V) and chromium(VI) on Zr(IV)-impregnated collagen fiber

The metal anions of vanadium (V) and chromium (VI) in aqueous solution can be effectively adsorbed by Zr(IV)-impregnated collagen fiber (ZrICF). The maximum adsorption capacity of V(V) takes place within the pH range of 5.0 to 8.0, while that of Cr(VI) is within the pH range of 6.0 to 9.0. When the initial concentration of metal ions was 2.00 mmol L⁻¹ and the temperature was 303 K, the adsorption capacity of V(V) on Zr-ICF was 1.92 mmol g⁻¹ at pH 5.0, and the adsorption capacity of Cr(VI) was 0.53 mmol g⁻¹ at pH 7.0. As temperature increased, the adsorption capacity of V(V) increased, while that of Cr(VI) was almost unchanged. The adsorption isotherms of the anionic species of V(V) and Cr(VI) can be fit by the Langmuir equation. The adsorption rate of V(V) follows the pseudo-first-order rate model, while the adsorption rate of Cr(VI) follows the pseudo-second-order rate model. Furthermore, ZrICF shows high adsorption selectivity to V(V) in the mixture solution of V(V) and Cr(VI). Practical applications of ZrICF could be expected in consideration of its performance in adsorption of V(V) and Cr(VI).     

Adsorption of Cr (VI) by treated weed <Emphasis Type="Italic">Salvinia cucullata</Emphasis>: kinetics and mechanism

In the present study a new low cost, easily available and environmentally friendly adsorbent was used for removal of Cr (VI). The Cr (VI) removal efficiency of the adsorbent was studied as a function of contact time, pH, adsorbent dose, adsorbate concentration, temperature and stirring speed. Different adsorption model equations for kinetics, isotherm and rate mechanism of the process were used to find the best model, which fit well to the experimental data. A full factorial design of n^k type was used to find a mathematical relation between the percentage of adsorption and variables affecting the adsorption process such as time, pH, adsorbate concentration and temperature. Using the Students ‘t’ test, the significance of each term of the derived equation was tested. The insignificant terms were removed from the derived equation. The adequacy of the equation after removing the insignificant terms was tested using the Fisher adequacy test. From the factorial design analysis it is found that pH has the most pronounced effect followed by time, temperature and the adsorbate concentration. A column study was performed using the optimum operating conditions.     

Adsorption and thermodynamic behavior of uranium(VI) on <Emphasis Type="Italic">Ulva sp</Emphasis>.-Na bentonite composite adsorbent

Summary The algae-clay composite adsorbent was tested for its ability to recover U(VI) from diluted aqueous solutions. Macro marine algae (Ulva sp.) and clay (Na bentonite) were used to prepare composite adsorbent. The ability of the composite adsorbent to adsorp uranium(VI) from aqueous solution has been studied at different optimized conditions of pH, concentration of U(VI), temperature, contact time. Parameters of desorption were also investigated to recover the adsorbed uranium. The adsorption patterns of uranium on the composite adsorbent followed the Freundlich and Dubinin-Radushkevich isotherms. The thermodynamic parameters such as the enthalpy ΔH, entropy ΔS and Gibbs free energy ΔG were calculated from the slope and intercept of lnK_d vs. 1/T plots. The results suggested that the Ulva sp.-Na bentonite composite adsorbent is suitable as sorbent material for recovery and biosorption/adsorption of uranium ions from aqueous solutions.     

Thermodynamic analysis of sorption isotherms of chromium(VI) anionic species on reed biomass

A new sorbent material for removing Cr(VI) anionic species from aqueous solutions has been investigated. Adsorption equilibrium and thermodynamics of Cr(VI) anionic species onto reed biomass were studied at different initial concentrations, sorbent concentrations, pH levels, temperatures, and ionic strength. Equilibrium isotherm was analyzed by Langmuir model. The experimental sorption data fit the model very well. The maximum sorption capacity of Cr(VI) onto reed biomass was found to be 33 mg · g^?1. It was noted that the Cr(VI) adsorption by reed biomass decreased with increase in pH. An increase in temperature resulted in a higher Cr(VI) loading per unit weight of the adsorbent. Removal of Cr(VI) by reed biomass seems to be mainly by chemisorption. The change in entropy (ΔS^°) and heat of adsorption (ΔH^°) for Cr(VI) adsorption on reed biomass were estimated as 2205 kJ · kg^?1 · K^?1 and 822 kJ · kg^?1, respectively. The values of isosteric heat of adsorption varied with the surface loading of Cr(VI).     

Optimization of Parameters for Cr(VI) Adsorption on Used Black Tea Leaves

Dynamic characteristics of Cr(VI) sorption on used black tea leaves (UBTLs) as a low-cost adsorbent are studied. Batch experiments were conducted to evaluate the effects of Cr(VI) concentration, solution pH and temperature on the removal process. Both of adsorption and reduction, involved in the process, are affected by the processing parameters. The adsorption kinetics is described successfully using pseudo-second order rate equation and the rate constant decreases with increasing the initial concentration of Cr(VI) up to 150 mg/L (for 0.1 g/L UBTLs) then becomes slow. Experimental and calculated kinetic data for equilibrium are well expressed by Langmuir isotherm. The solution pH has a profound effect on the adsorption rate. The rate constant increases linearly with an increase in temperature, and the low value of activation energy of adsorption, 16.3 kJ/mol, indicates that Cr(VI) is easily adsorbed on UBTLs. The maximum Cr(VI) adsorptive conditions, with a minimum reduction, were achieved from the dynamics of operational parameters: the initial Cr(VI) concentration < 150 mg/L (for 0.1 g/L UBTLs); the initial solution pH = 1.54–2.00 and the processing temperature < 50 ^∘C, for the possibility of its practical application.     

Removal of Cr(VI) from Aqueous Solution Using Activated Cow Dung Carbon

Removal of hexavalent chromium from aqueous medium by using activated cow dung carbon was studied. Cow dung was carbonized and activated by treating with concentrated H(2)SO(4) followed by heating for 24 h at 120 degrees C. The extent of adsorption was studied as a function of pH, contact time, amount of adsorbent, concentration of adsorbate, and temperature. At lower pH (<3.5), the prepared sorbent was capable of removing approximately 90% Cr(VI) at 5 ppm concentration from aqueous synthetic solution. The dynamics of migration of the sorbate ions from the bulk onto the sorbent surface was studied and the results obtained under various experimental conditions were found to follow standard adsorption isotherms. The reaction kinetics was found to be of first order. Copyright 2000 Academic Press.     

The kinetic and thermodynamic study of the removal of Cr(VI) ion from aqueous solution by human hair waste

The removal of Cr(VI) ions from aqueous solution by human hair waste is investigated by using UV–Vis spectrophotometer technique. The morphological analysis of the human hair was also investigated by the scanning electron microscopy, Fourier transforms infrared spectroscopy and X-ray photoelectron spectroscopy. The influence of various physicochemical effective parameters such as pH, ionic strength, adsorbent amount, contact time, initial concentration of metal ion on removal of Cr(VI) ions by human hair process was also studied. The optimum conditions for this adsorption process were obtained at pH = 2 and contact time of 150 min while the highest Cr(VI) uptake is recorded for 0.5 g of the adsorbent per 100 ml of solution. Three isotherms models including Langmuir, Freundlich and Temkin were applied to describe the equilibrium data. It was found that the experimental data were well described by Freundlich isothermal model. The maximum adsorption capacity was found to be 11.64 mg g^?1.The thermodynamic study data showed that the adsorption process of Cr(VI) on human hair is an endothermic, spontaneous and physisorption reaction. The kinetics of the adsorption process was studied using three kinetics models including Lagergren-first-order, pseudo-second-order and Elovich model. The obtained data are indicated that the adsorption processes of Cr(VI) over human hair could be described by the pseudo-second-order kinetic model.     

Removal of Cr (VI) from aqueous solutions by the nonliving biomass of Alligator weed: kinetics and equilibrium

The removal of Cr (VI) from aqueous solutions using Alligator weed, a freshwater macrophyte, was investigated in batch studies. Various factors including solution pH, Cr (VI) concentrations, agitation time, and temperature were taken into account and promising results obtained. An initial solution pH of 1.0 was most favorable for Cr (VI) removal. The kinetic data were analyzed using several models, including the pseudo-second-order equation, external diffusion model, and intraparticle diffusion model. The comparison gave insight about the mechanism of adsorption and potential rate controlling step. The results suggested that the Cr (VI) adsorption at all temperatures was best represented by the pseudo-second-order equation. The external film diffusion played an important role in the adsorption mechanism. The Freundlich, Langmuir and Langmuir-Freundlich isotherms for the present system were analyzed. The best interpretation for the equilibrium data at different temperatures was given by the Langmuir-Freundlich isotherm. The Alligator weed could serve as low-cost adsorbent to remove Cr (VI) from aqueous solutions.     

Removal, preconcentration and determination of Mo(VI) from water and wastewater samples using maghemite nanoparticles

Removal and recovery of Mo(VI) from aqueous solutions were investigated using maghemite (γ-Fe₂O₃) nanoparticles. Combination of nanoparticle adsorption and magnetic separation was used to the removal and recovery of Mo(VI) from water and wastewater solutions. The nanoscale maghemite with mean diameter of 50 nm was synthesized by reduction coprecipitation method followed by aeration oxidation. Various factors influencing the adsorption of Mo(VI), e.g. pH, temperature, initial concentration, and coexisting common ions were studied. Adsorption reached equilibrium within <10 min and was independent of initial concentration of Mo(VI). Studies were performed at different pH values to find out the pH at which maximum adsorption occurred. The maximum adsorption occurred at pHs between 4.0 and 6.0. The Langmuir adsorption capacity (q_max) was found to be 33.4 mg Mo(VI)/g of the adsorbent. The results showed that nanoparticle (γ-Fe₂O₃) is suitable for the removal of Mo(VI), as molybdate, from water and wastewater samples. The adsorbed Mo(VI) was then desorbed and determined spectrophotometrically using bromopyrogallol red as a complexation reagent. This allows the determination of Mo(VI) in the range 1.0–86.0 ng mL⁻¹.     

Separation and spectrophotometric determination of very low levels of Cr(VI) in water samples by novel pyridine-functionalized mesoporous silica

A modified SBA-15 mesoporous silica was developed, as an adsorbent, for the removal of Cr(VI) ions from natural-water samples. The effects of experimental parameters, including pH of solution, sample and eluent flow rate, the eluent composition, the eluent volume, and the effect of coexisting ions on the separation and determination of Cr(VI), were investigated. It was shown that Cr(VI) was selectively adsorbed from aqueous solution at pH 3, but Cr(III) could be adsorbed from solution at alkaline pH range. The retained Cr(VI) was eluted with 0.5?mol?L^?1 KCl solution in 0.1?mol?L^?1 Na₂CO₃ subsequently. Under the optimum conditions, the modified mesoporous silica (py-SBA-15) with a high pore diameter exhibited an adsorption capacity of 136?mg?g^?1 and a lower limit of detection than 2.3?µg?L^?1 by using diphenylcarbazide as a chromophorous reagent for the determination of Cr(VI) ions. A preconcentration factor as high as 200 was calculated for Cr(VI). The loaded py-SBA-15 can be reactivated with recovery of more than 98.5% over at least eight cycles. The relative standard deviation (RSD) for Cr(VI) ion recovery was less than 1.8%. Validation of the outlined method was performed by analysing a certified reference material (BCR 544). The proposed method was applied to determine Cr(VI) value in natural and waste water samples successfully.     

Adsorption of chromium from aqueous solution using chitosan beads

A basic investigation on the removal of Cr(III) and Cr(VI) ions from aqueous solution by chitosan beads was conducted in a batch adsorption system. The chitosan beads were prepared by casting an acidic chitosan solution into an alkaline solution. The influence of different experimental parameters; pH, agitation period and different concentration of Cr(III) and Cr(VI) ions was evaluated. A pH 5.0 was found to be an optimum pH for Cr(III) adsorption, and meanwhile pH 3.0 was the optimum pH for the adsorption of Cr(VI) onto chitosan beads. The Langmuir and Freundlich adsorption isotherm models were applied to describe the isotherms and isotherm constants for the adsorption of Cr(III) and Cr(VI) onto chitosan beads. Results indicated that Cr(III) and Cr(VI) uptake could be described by the Langmuir adsorption model. The maximum adsorption capacities of Cr(III) and Cr(VI) ions onto chitosan beads were 30.03 and 76.92 mg g⁻¹, respectively. Results showed that chitosan beads are favourable adsorbents. The Cr(III) and Cr(VI) ions can be removed from the chitosan beads by treatment with an aqueous EDTA solution.     

On the use of a natural peat for the removal of Cr(VI) from aqueous solutions

A natural peat has been used as an adsorbent for the removal of hexavalent chromium from aqueous solution. The peat was firstly characterized in terms of particle size and chemical composition (ash content, pH of the point of zero charge, FT-IR and thermal analysis). Next, the kinetic and equilibrium aspects of the adsorption of Cr(VI) by this adsorbent were studied. The kinetic data were satisfactorily fitted to a kinetic law of partial order in C equal to one. The specific adsorption rates are around 10(-4)s(-1), increasing as temperature does. A noticeable influence of diffusion on the global adsorption process has been demonstrated. Finally, the equilibrium isotherms were satisfactorily fitted to a previously proposed model. The adsorption capacity of Cr(VI) was similar to some other previously reported and the affinity of Cr(VI) towards the active sites of the adsorbent increases as temperature rises.     

Removal of chromium(VI) from wastewater using Sorel's cement

Summary Synthetic Sorel's cement [3Mg(OH)₂ ^. MgCl₂ ^. 8H₂O], is used as a new adsorbent material for removal of chromium(VI) ion from wastewater effluents. Parameters including contact time, adsorbent dosage and pH are examined and optimized. The equilibrium data are fitted very well to the Langmuir and Freundlich isotherms rather than linear. The adsorption isotherm indicates that the monolayer coverage is 21.4 mg Cr(VI) ion per g of Sorel's cement. The adsorbent is considered as a better replacement technology for removal of Cr(VI) ion from aqueous solutions due to its low cost, good efficiency, fast kinetics, and simple preparation. It offers remarkable efficiency for Cr(VI) removal from wastewater compared with many other natural and synthetic adsorbents.     

Stability of fluoroquinolone antibiotics in river water samples and in octadecyl silica solid-phase extraction cartridges

Airborne particulate matter has been sampled at a location close to a metallurgical plant in North-Rhine-Westphalia, Germany, and first results on the chromium(VI) content in the collected dust are presented. A special procedure using a scrubber as sampling device was used to preserve Cr(VI) during the sampling procedure. The scrubber solution which consisted of 0.1 mol L^–1 TRIS-buffer solution was adjusted to a slightly alkaline pH of 8.6 to reduce the oxidation potential of Cr(VI) and to avoid possible oxidation of Cr(III) to Cr(VI), for example by oxygen (or ozone at ambient concentrations). After sampling Cr(VI) was pre-concentrated on an anion-exchange material and eluted with aqueous 0.6 mol L^–1 sodium perchlorate solution. After elution, a species-selective complex of Cr(VI) with diphenylcarbazide (DPC) was prepared; this was extracted into n-hexanol and quantified by UV–visible spectrophotometry. A detection limit of 0.9 ng m^–3 for Cr(VI) in ambient aerosols can be achieved with this method.     

Removal of trace Cr(VI) from water using chitosan-iron nanowires in porous anodic alumina

Chitosan-iron nanowires in porous anodic alumina (PAA) have been successfully prepared under ambient conditions as an adsorbent. The adsorbent was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and N₂-BET surface area. The results showed that PAA can disperse and protect Fe⁰ nanorods from oxidation. The adsorption characteristics of trace Cr(VI) onto adsorbent have been examined at different initial Cr(VI) concentrations with pH 5. Batch adsorption studies show that the removal percentage of adsorbent for the removal of trace Cr(VI) is strongly dependent on the initial Cr(VI) concentrations. Langmuir and Freundlich isotherm models were used to analyze the experiment data. The adsorption of trace Cr(VI) by adsorbent is well modeled by the Langmuir isotherm and the maximum adsorption capacity of Cr(VI) is calculated as 123.95 mg/g which is very closed to the experiment results. Intraparticle diffusion study shows that the intraparticle diffusion of adsorbent is not the sole rate-controlling step. The negative value of Gibbs free energy change, ΔG^o, indicated that the process of Cr(VI) onto adsorbent was spontaneous. This work has demonstrated that chitosan-iron nanowires in porous anodic alumina as an adsorbent has promising potential for heavy metal removal at trace level.