Utilization of waste product (tamarind seeds) for the removal of Cr(VI) from aqueous solutions: Equilibrium, kinetics, and regeneration studies

In the present study, an adsorbent was prepared from tamarind seeds and used after activation for the removal of Cr(VI) from aqueous solutions. The tamarind seeds were activated by treating them with concentrated sulfuric acid (98% w/w) at a temperature of 150 °C. The adsorption of Cr(VI) was found to be maximum at low values of initial pH in the range of 1–3. The adsorption process of Cr(VI) was tested with Langmuir, Freundlich, Redlich–Peterson, Koble–Corrigan, Tempkin, Dubinin–Radushkevich and Generalized isotherm models. Application of the Langmuir isotherm to the system yielded a maximum adsorption capacity of 29.7 mg/g at an equilibrium pH value ranging from 1.12 to 1.46. The adsorption process followed second-order kinetics and the corresponding rate constants obtained were 2.605 × 10⁻³, 0.818 × 10⁻³, 0.557 × 10⁻³ and 0.811 × 10⁻³ g/mg min⁻¹ for 50, 200, 300 and 400 mg/L of initial Cr(VI) concentration, respectively. The regenerated activated tamarind seeds showed more than 95% Cr(VI) removal of that obtained using the fresh activated tamarind seeds. A feasible solution is proposed for the disposal of the contaminants (acid and base solutions) containing high concentrations of Cr(VI) obtained during the regeneration (desorption) process.     

The treatment of textile wastewater including chromium(VI) and reactive dye by sulfate-reducing bacterial enrichment

Sulfate-reducing bacteria (SRB) that could grow on modified Postgate C medium (PC) containing chromium(VI) were isolated from industrial wastewaters and their chromium(VI) reduction capacities were investigated as a function of changes in the initial pH values, chromium, sulfate, NaCl and reactive dye concentrations. The optimum pH value at 50 mg l(-1) initial chromium(VI) concentration was determined to be 8. Chromium(VI) reduction by SRB was investigated at 22.7-98.4 mg l(-1) initial chromium(VI) concentrations. At the end of the experiments, the mixed cultures of SRB were found to reduce within 2-6 days more than 99% of the initial chromium(VI) levels, which ranged from 22.7 to 74.9 mg l(-1). The effects of the initial 0-9.0 g l(-1) concentrations of disodium sulfate and 0-6% (w/v) concentrations of NaCI to chromium reduction showed that the lowest concentrations of sulfate and NaCI were the best for chromium reduction in the PC medium including 50 mg l(-1) chromium(VI). Chromium(VI) reduction in 50 mg l(-1) and 25-100 mg l(-1) Remazol Blue, Reactive Black B or Reactive Red RB containing media were also investigated. In the experiments, 25-30% of the initial dye concentrations and 95% of the chromium(VI) was removed from the medium at the end of 72-h and 24-h incubation periods, respectively.     

Structure and thermal stability of toxic chromium(VI) species doped onto TiO2 powders through heat treatment

Chromium(VI)-containing sorbents in the form of sludge or solid residue from treatment processes are often landfilled or used as fill materials, therefore the long-term stability of metal binding is important. The reduction of Cr(VI)–Cr(III) through heat treatment may be a useful detoxification method. After heating at 500, 900, 1000, and 1100 °C for 4 h, the transformation of chemical states of chromium on 105 °C-dried, 7.9% Cr(VI)-doped TiO₂ powders was studied on the basis of surface area measurements, scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) spectra. It was shown that Cr(VI) was reduced to Cr(III) in the Cr(VI)-doped samples after heating within 500–900 °C. The present results also suggested that the chromium octahedral was bridged to the titanium tetrahedral and was incorporated in TiO₂ minerals formed after 1000 °C treatment.     

含Cr（Ⅵ）废水生物处理技术及其影响因素

本文综述了微生物还原处理含价铬的废水的研究进展。讨论了影响微生物还原Cr(Ⅵ）因素包括生物体密度、初始Cr（Ⅵ）的浓度、碳源、pH、温度、溶解氧、氧化还原电位、含氧阴离子和金属离子。微生物还原Cr（Ⅵ）技术作为一种富有创新的研究应用于Cr（Ⅵ）污染的环境恢复。     

Removal of Pb(II) ions from aqueous solution using activated tea waste: Adsorption on a fixed-bed column

An inexpensive and effective adsorbent was developed from waste tea leaves for the dynamic uptake of Pb(II). Characterization of the adsorbents showed a clear change between physico-chemical properties of activated tea waste and simply tea waste. The purpose of this work was to evaluate the potential of activated tea waste in continuous flow removal of Pb(II) ions from synthetic aqueous effluents. The performance of the system was evaluated to assess the effect of various process variables, viz., of bed height, hydraulic loading rate and initial feed concentration on breakthrough time and adsorption capacity. The shape of the breakthrough curves was determined for the adsorption of Pb(II) by varying different operating parameters like hydraulic loading rate (2.3–9.17 m³/h m²), bed height (0.3–0.5 m) and feed concentration (2–10 mg/l). An attempt has also been made to model the data generated from column studies using the empirical relationship based on the Bohart–Adams model. There was an acceptable degree of agreement between the data for breakthrough time calculated from the Bohart–Adams model and the present experimental study with average absolute deviation of less than 5.0%. The activated tea waste in this study showed very good promise as compared with the other adsorbents available in the literature. The adsorbent could be suitable for repeated use (for more than four cycles) without noticeable loss of capacity.     

Removal of Se(IV) from aqueous solution using sulphuric acid-treated peanut shell

A carbonaceous sorbent was prepared from peanut shell via sulphuric acid treatment. Se(IV) removal from aqueous solution on the sorbent was studied varying time, pH, Se(IV) concentration, temperature and sorbent status (wet and dry). Se(IV) removal was faster using the wet sorbent than the dry sorbent following a pseudo-first-order model. Se(IV) removal increases at low pH values, and decreases as pH increases until pH 7. Sorption was found to fit the Langmuir equation and sorption capacity for the wet sorbent was higher than that for the dry one. Both sorbents showed an increased selenium sorption by rising the temperature. Redox processes between Se(IV) and the carbon sorbent are involved. Analysis by scanning electron microscope and X-ray powder diffraction for the sorbent after the reaction with acidified Se(IV) confirmed the availability of elemental selenium as particles on the sorbent surface as a result of Se(IV) reduction. Physicochemical tests showed an increase in sorbent acidity, cation exchange capacity (CEC) and surface functionality after the reaction with acidified Se(IV), indicating the oxidation processes occurring on the sorbent surface. Due to its reduction properties, the sorbent seems efficient for Se(IV) removal from aqueous solution.     

Removal of copper (II) from aqueous solution by adsorption onto low-cost adsorbents

The use of low-cost adsorbents was investigated as a replacement for current costly methods of removing metals from aqueous solution. Removal of copper (II) from aqueous solution by different adsorbents such as shells of lentil (LS), wheat (WS), and rice (RS) was investigated. The equilibrium adsorption level was determined as a function of the solution pH, temperature, contact time, initial adsorbate concentration and adsorbent doses. Adsorption isotherms of Cu (II) on adsorbents were determined and correlated with common isotherm equations such as Langmuir and Freundlich models. The maximum adsorption capacities for Cu (II) on LS, WS and RS adsorbents at 293, 313 and 333 K temperature were found to be 8.977, 9.510, and 9.588; 7.391, 16.077, and 17.422; 1.854, 2.314, and 2.954 mg g(-1), respectively. The thermodynamic parameters such as free energy (delta G0), enthalpy (delta H0) and entropy changes (delta S0) for the adsorption of Cu (II) were computed to predict the nature of adsorption process. The kinetics and the factors controlling the adsorption process were also studied. Locally available adsorbents were found to be low-cost and promising for the removal of Cu (II) from aqueous solution.     

活性污泥吸附重金属Cr6+的研究

以活性污泥为材料，采用不同时间、温度、pH进行吸附重金属Cr^6 的研究。实验结果表明，当吸附时间为15min，吸附温度为28℃，吸附pH=7时具有较好吸附效果。在此条件下，当重金属Cr^6 浓度为50mg/L，其吸附率可达97．2％。     

Removal of Pb(II) from wastewater using wheat bran

The adsorption of Pb(II) ions from aqueous solutions on wheat bran (WB) has been investigated as a function of initial concentration, adsorbent dose, adsorbent particle size, agitation speed, temperature, contact time and pH of solution. The equilibrium process was described well by the Langmuir isotherm model with maximum sorption capacities of 69.0, 80.7 and 87.0 mgg(-1) of Pb(II) on wheat bran at 20, 40 and 60 degrees C, respectively. Thermodynamic parameters, i.e. DeltaG(0), DeltaH(0) and DeltaS(0) have also been calculated for the system and the sorption process was found to be endothermic. Good correlation coefficients were obtained for the pseudo second-order kinetic model. The metal ion could be stripped by addition of 0.5M HCl, making the adsorbent regeneration and its reutilization possible.     

Adsorptive removal of Cd(II) and Pb(II) ions from aqueous solutions by using Turkish illitic clay

The ability of Turkish illitic clay (TIC) in removal of Cd(II) and Pb(II) ions from aqueous solutions has been examined in a batch adsorption process with respect to several experimental conditions including initial solution pH, contact time, initial metal ions concentration, temperature, ionic strength, and TIC concentration, etc. The characterization of TIC was performed by using FTIR, XRD and XRF techniques. The maximum uptake of Cd(II) (11.25 mg g⁻¹) and Pb(II) (238.98 mg g⁻¹) was observed when used 1.0 g L⁻¹ of TIC suspension, 50 mg L⁻¹ of initial Cd(II) and 250 mg L⁻¹ of initial Pb(II) concentration at initial pH 4.0 and contact time of 240 min at room temperature. The experimental data were analyzed by the Langmuir, Freundlich, Temkin and Dubinin Radushkevich (D-R) isotherm models. The monolayer adsorption capacity of TIC was found to be 13.09 mg g⁻¹ and 53.76 mg g⁻¹ for Cd(II) and Pb(II) ions, respectively. The kinetics of the adsorption was tested using pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion models. The results showed that the adsorption of Cd(II) and Pb(II) ions onto TIC proceeds according to the pseudo-second-order model. Thermodynamic parameters including the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) changes indicated that the present adsorption process was feasible, spontaneous and endothermic in the temperature range of 5–40 °C.     

Removal and recycling of copper from aqueous solutions using treated Indian barks

Removal of copper from aqueous solutions containing 100–1000 ppm, using different Indian bark species, was performed on laboratory scale. The percentage removal of metal ions depends on the solution pH, bark species and time. The efficiency of copper removal by the used raw barks increases with a rise of solution pH and reaches a maximum of about 65–78% around pH 4–5. However, the decontaminated aqueous solutions were colored due to the dissolution of soluble organic compounds contained in the raw bark. This increases the biological and chemical oxygen demand (BOD and COD) of the solutions as well as the total organic carbon content (TOC). For this reason, raw bark should be treated either by chemical or biological means. Such treatment will allow the extraction of the soluble organic compounds and increase the chelating capacity and efficiency of the treated bark. Depending on the pH value, the chelating efficiency of treated barks is about 1.2–2.2 times that of the raw ones. Moreover, the retention capacity of the Indian treated bark varies from about 42–51 mg/g of dry bark. It is equal to or higher than that of common European species. About 1.8 mols of H₃O⁺ are released, by the treated barks, for every mol of chelated copper ions. Moreover, scanning electron microscopy (SEM) observations show uniform distribution of metal ions throughout the copper saturated bark. Infra red (IR) spectra suggest that the copper ions are chelated to hydroxyl and/or carboxyl functional groups of organic compounds contained in the treated bark. It seems that the interaction of the copper ions with the bark follows a cation exchange mechanism. This hypothesis is supported by elution experiments that allow recovery of about 99% of the contained copper. The retention capacity of the treated bark is almost constant after five cycles of chelation–elution, suggesting that the ‘life time cycle' is sufficiently long for continuous industrial application. The spent copper loaded barks can either be incinerated or pyrolysed. It generates solids containing either ≈80% of CuO or ≈14% of Cu°, respectively. Such materials can be used either in the secondary or primary copper production, thus offering a friendly environmental solution of effluents' treatment. The suggested process can be used as an alternative to the classical technologies for effluent decontamination. It is also efficient for polishing effluents treated by other methods.     

Biosorption of Cu(II), Zn(II), Ni(II) and Pb(II) ions by cross-linked metal-imprinted chitosans with epichlorohydrin

Cross-linked metal-imprinted chitosan microparticles were prepared from chitosan, using four metals (Cu(II), Zn(II), Ni(II), and Pb(II)) as templates, and epichlorohydrin as the cross-linker. The microparticles were characterized by Fourier transform infrared spectroscopy, solid state (13)C nuclear magnetic resonance spectroscopy, and energy-dispersive X-ray spectroscopy. They were used for comparative biosorption of Cu(II), Zn(II), Ni(II) and Pb(II) ions in an aqueous solution. The results showed that the sorption capacities of Cu(II), Zn(II), Ni(II), and Pb(II) on the templated microparticles increased from 25 to 74%, 13 to 46%, 41 to 57%, and 12 to 43%, respectively, as compared to the microparticles without metal ion templates. The dynamic study showed that the sorption process followed the second-order kinetic equation. Three sorption models, Langmuir, Freundlich, and Dubinin-Radushkevich, were applied to the equilibrium isotherm data. The result showed that the Langmuir isotherm equation best fitted for monolayer sorption processes. Furthermore, the microparticles can be regenerated and reused for the metal removal.     

Adsorption of basic dye from aqueous solution onto fly ash

The fly ash treated by H₂SO₄ was used as a low-cost adsorbent for the removal of a typical dye, methylene blue, from aqueous solution. An increase in the specific surface area and dye-adsorption capacity was observed after the acid treatment. The adsorption isotherm and kinetics of the treated fly ash were studied. The experimental results were fitted using Langmuir and Freundlich isotherms. It shows that the Freundlich isotherm is better in describing the adsorption process. Two kinetic models, pseudo-first order and pseudo-second order, were employed to analyze the kinetic data. It was found that the pseudo-second-order model is the better choice to describe the adsorption behavior. The thermodynamic study reveals that the enthalpy (ΔH⁰) value is positive (5.63 kJ/mol), suggesting an endothermic nature of the adsorption.     

活性污泥吸附重金属Cr6+的研究

以活性污泥为材料,采用不同时间、温度、pH进行吸附重金属Cr^6 的研究。实验结果表明,当吸附时间为15min,吸附温度为28℃,吸附pH=7时具有较好吸附效果。在此条件下,当重金属Cr^6 浓度为50mg/L,其吸附率可达97．2％。     

Removal of uranium(VI) from contaminated sediments by surfactants

Uranium(VI) sorption onto a soil collected at the Melton Branch Watershed (Oak Ridge National Laboratory, TN) is strongly influenced by the pH of the soil solution and, to a lesser extent, by the presence of calcium, suggesting specific chemical interactions between U(VI) and the soil matrix. Batch experiments designed to evaluate factors controlling desorption indicate that two anionic surfactants, AOK and T77, at concentrations ranging from 60 to 200 mM, are most suitable for U(VI) removal from acidic soils such as the Oak Ridge sediment. These surfactants are very efficient solubilizing agents at low uranium concentrations: ca. 100% U(VI) removal for [U(VI)]o,sorbed = 10(-6) mol kg-1. At greater uranium concentrations (e.g., [U(VI)]o,sorbed = ca. 10(-5) mol kg-1), the desorption efficiency of the surfactant solutions increases with an increase in surfactant concentration and reaches a plateau of 75 to 80% of the U(VI) initially sorbed. The most probable mechanisms responsible for U(VI) desorption include cation exchange in the electric double layer surrounding the micelles and, to a lesser extent, dissolution of the soil matrix. Limitations associated with the surfactant treatment include loss of surfactants onto the soil (sorption) and greater affinity between U(VI) and the soil matrix at large soil to liquid ratios. Parallel experiments with H2SO4 and carbonate-bicarbonate (CB) solutions indicate that these more conventional methods suffer from strong matrix dissolution with the acid and reduced desorption efficiency with CB due to the buffering capacity of the acidic soil.     

Application of cupuassu shell as biosorbent for the removal of textile dyes from aqueous solution

The cupuassu shell (Theobroma grandiflorum) which is a food residue was used in its natural form as biosorbent for the removal of C.I. Reactive Red 194 and C.I. Direct Blue 53 dyes from aqueous solutions. This biosorbent was characterized by infrared spectroscopy, scanning electron microscopy, and nitrogen adsorption/desorption curves. The effects of pH, biosorbent dosage and shaking time on biosorption capacities were studied. In acidic pH region (pH 2.0) the biosorption of the dyes were favorable. The contact time required to obtain the equilibrium was 8 and 18 h at 298 K, for Reactive Red 194 and Direct Blue 53, respectively. The Avrami fractionary-order kinetic model provided the best fit to experimental data compared with pseudo-first-order, pseudo-second-order and chemisorption kinetic adsorption models. The equilibrium data were fitted to Langmuir, Freundlich, Sips and Radke-Prausnitz isotherm models. For both dyes the equilibrium data were best fitted to the Sips isotherm model.     

Removal of phenol from aqueous solution by adsorption onto OTMAC-modified attapulgite

The potential of octodecyl trimethyl ammonium chloride (OTMAC)-modified attapulgite (AT) for phenol adsorption from aqueous solutions was studied. The comparison of natural AT and modified AT showed that it is possible to utilize the sonication-modified OTMAC-AT in the treatment of phenol-contaminated wastewaters. Batch sorption studies were carried out to evaluate the effect of contact time, shaking frequency, temperature and the amount of AT. The results showed that in a lab-scale reactor, at room temperature, with an amount of the modified AT added (2.5 g), and a shaking frequency of 140 rev/min, the adsorption rate of phenol could be 60.4% for a duration of 60 min. The sorption kinetics were described by a pseudo-second-order model, and the values of k and q(e) were 1.367 mg/ig min and 0.7901 ig/mg, respectively. The analysis of equilibrium data showed that the Freundlich isotherms were found to be applicable for the adsorption equilibrium data. K and 1/n were estimated to be 14.53 and 0.8438, respectively.     

Kinetic and equilibrium characterization of uranium(VI) adsorption onto carboxylate-functionalized poly(hydroxyethylmethacrylate)-grafted lignocellulosics

This study investigated the feasibility of using a new adsorbent prepared from coconut coir pith, CP (a coir industry-based lignocellulosic residue), for the removal of uranium [U(VI)] from aqueous solutions. The adsorbent (PGCP-COOH) having a carboxylate functional group at the chain end was synthesized by grafting poly(hydroxyethylmethacrylate) onto CP using potassium peroxydisulphate-sodium thiosulphite as a redox initiator and in the presence of N,N'-methylenebisacrylamide as a crosslinking agent. IR spectroscopy results confirm the graft copolymer formation and carboxylate functionalization. XRD studies confirm the decrease of crystallinity in PGCP-COOH compared to CP, and it favors the protrusion of the functional group into the aqueous medium. The thermal stability of the samples was studied using thermogravimetry (TG). Surface charge density of the samples as a function of pH was determined using potentiometric titration. The ability of PGCP-COOH to remove U(VI) from aqueous solutions was assessed using a batch adsorption technique. The maximum adsorption capacity was observed at the pH range 4.0-6.0. Maximum removal of 99.2% was observed for an initial concentration of 25mg/L at pH 6.0 and an adsorbent dose of 2g/L. Equilibrium was achieved in approximately 3h. The experimental kinetic data were analyzed using a first-order kinetic model. The temperature dependence indicates an endothermic process. U(VI) adsorption was found to decrease with an increase in ionic strength due to the formation of outer-sphere surface complexes on PGCP-COOH. Equilibrium data were best modeled by the Langmuir isotherm. The thermodynamic parameters such as DeltaG(0), DeltaH(0) and DeltaS(0) were derived to predict the nature of adsorption. Adsorption experiments were also conducted using a commercial cation exchanger, Ceralite IRC-50, with carboxylate functionality for comparison. Utility of the adsorbent was tested by removing U(VI) from simulated nuclear industry wastewater. Adsorbed U(VI) ions were desorbed effectively (about 96.2+/-3.3%) by 0.1M HCl. The adsorbent was suitable for repeated use (more than four cycles) without any noticeable loss of capacity.     

Removal of basic dye (Astrazon Blue FGRL) using macroalga Caulerpa lentillifera

The macroalga Caulerpa lentillifera was found to have adsorption capacity for a basic dye, Astrazon Blue FGRL. For the whole range of concentrations employed in this work (20-1280 mgl(-1)), the adsorption reached equilibrium within the first hour. The kinetic data corresponded well with the pseudo second-order kinetic model where the rate constants decreased as initial dye concentrations increased. At low dye concentrations (20-80 mgl(-1)), an increase in the adsorbent dosage resulted in a higher removal percentage of the dye, but a lower amount of dye adsorbed per unit mass (q). The adsorption isotherm followed both the Langmuir and Freundlich models within the temperature range employed in this work (18-70 degrees C). The highest maximum adsorption capacity (q(m)) was obtained at 50 degrees C. The enthalpy of adsorption was estimated at 14.87 kJmol(-1) suggesting a chemical adsorption mechanism.     

Removal of Cr(VI) from Cr-contaminated groundwater through electrochemical addition of Fe(II)

The conventional chemical reduction-precipitation technique in the removal of Cr(VI) from contaminated groundwater involves a two-step process whereby Cr(VI) is first reduced to Cr(III) at an acidic pH by a reducing agent and in a subsequent step, Cr(III) is precipitated as insoluble hydroxide at an alkaline pH. In a variation of this method, Fe(II) is added electrochemically to the Cr(VI) containing water. From a pure iron electrode, Fe(2+) ions are released into the solution and bring forth the reduction of Cr(VI). At the cathode, H(2)O is reduced whereby the OH(-) ions entering the solution keep the pH of the solution in the alkaline range. This latter fact greatly facilitates simultaneous reduction of Cr(VI) to Cr(III) and co-precipitation of hydroxides of trivalent Cr and Fe. On the basis of a set of experimental data, it is shown that this process is both thermodynamically and kinetically efficient, meaning, with the electrochemical method, rapid and nearly complete removal of Cr(VI) from a groundwater source with both high and low levels of Cr-contamination can be achieved. These factors make the electrochemical process superior to the conventional chemical process in remediation of Cr-contaminated groundwater.