Capacity of Cr(VI) reduction in an aqueous solution using different sources of zerovalent irons

Zerovalent iron (ZVI) has drawn intense interest as an effective and inexpensive tool to enhance degradation of various environmental contaminants. Reduction of Cr(VI) to Cr(III) by ZVI merits environmental concern as a hazardous species is transformed into a non-hazardous one. Objectives of this research were to assess kinetics and capacity of Cr(VI) reduction by different sources of ZVIs, of which chemical parameters can base in situ application of ZVI to treat Cr(VI) contaminated water. Reduction kinetics were first-order and rapid showing that 50% of the initial Cr(VI) was reduced within 7.0 to 347 min depending on Cr(VI) concentration, temperature and ZVI source. The reduction rates were increased with decreasing the initial Cr(VI) concentrations and increasing the reaction temperatures. The J ZVI (Shinyo Pure Chemical Co., Japan) was more effective in Cr(VI) reduction than PU (Peerless Metal Powders, USA). The maximum reduction capacities of J and PU ZVIs at 25°C were 0.045 and 0.042 mmol g⁻¹ Fe⁰, respectively. A relatively higher value of the net reaction energy (E _a ) indicated that Cr(VI) reduction by ZVI was temperature dependent and controlled by surface properties of ZVI. Chemical parameters involved in the Cr(VI) reduction by ZVI such as temperature quotient, kinetic rates, and stoichiometry indicated that the ZVI might be effective for in situ treatment of the Cr(VI) containing wastewater.     

Removal of Cr(VI) from Model Solutions by a Combined Electrocoagulation Sorption Process

The elimination of hexavalent chromium from synthetic effluents in a batch stirred electrocoagulation cell with aluminum electrodes coupled with a sorption process using red onion skin adsorbent is addressed. The effect of process variables such as medium pH and adsorbent concentration was investigated in an attempt to optimize the removal capacity. Adsorption and electrocoagulation were first considered separately, with maximum Cr(VI) removal yields of 47 % and 78 %, respectively, at pH 2. When combining the two processes, a removal of 97 % of the total Cr(VI) in a pH range of 3–6 was achieved. The almost total elimination observed for the considered pollutant demonstrated the feasibility and the synergistic effect of the coupled process.     

光催化法处理含Cr(VI)废水的研究

采用P 25 TiO2作为光催化剂,研究了废水的pH值、Cr(VI)的初始浓度、气氛及有机物等因素对含铬废水中Cr(VI)去解率的影响。结果表明,在pH值为3.0时,光催化反应速率最大;反应气氛对该体系中Cr(VI)的光催化还原无明显影响;苯酚、葡萄糖等有机物的存在能有效地促进Cr(VI)的光催化还原,当加入与Cr(VI)等物质的量的苯酚或葡萄糖时,150 mL反应液[Cr(VI)浓度为0.96 mmol/L],0.15 g光催化剂,经12 W紫外灯照射反应120 m in,Cr(VI)完全被去除,相对于在反应体系中不加有机物时,Cr(VI)光催化还原效率提高了近100%;Cr(VI)的光催化还原符合L-H动力学规律。     

Adsorption of Cr (VI) Oxyanions onto Modified Wood Pulp

A new adsorbent was prepared from wood pulp (WP) after reaction with epichlorohydrin and dimethylamine in the presence of pyridine and N,N-dimethylformamide (DMF). The adsorption of Cr (VI) from aqueous solutions by the so-prepared wood pulp adsorbent (WP-A) was investigated. Various factors affecting adsorption, such as pH, adsorbent concentration (1–5 g/L), agitation time (5–60 min), and Cr (VI) concentration (50–700), were taken into consideration. The adsorption of Cr (VI) onto (WP-A) was found to be pH-dependent and maximum adsorption was obtained at pH 3. The adsorption data obeyed Langmuir and Freundlich adsorption isotherms. The Langmuir adsorption capacity (Q_max) was found to be 588.24 mg/g. Freundlich constants, K_F and n, were found to be 55.03 and 2.835, respectively.     

Enhanced adsorptive removal of Cr(VI) in aqueous solution by polyethyleneimine modified palygorskite

Polyethyleneimine (PEI) modified palygorskite (Pal) was used for the adsorption of Cr(VI) in aqueous solution. The absorbent was characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Characterized results confirmed that the Pal has been successfully modified by PEI. The modification of PEI increased the Cr(VI) adsorption performance of the Pal by the adsorption combined reduction mechanism, and amino groups of the adsorbent play the main role in the enhanced Cr(VI) adsorption. The maximum adsorption capacity was 51.10 mg·g⁻¹ at pH 4.0 and 25 °C. The adsorption kinetics of Cr(VI) on the adsorbent conforms to the Langmuir isotherm model. The maximum adsorption occurs at pH 3, and then the adsorption capacity of PEI-Pal was decreased with the increase of pH values. The adsorption kinetics of Cr(VI) on PEI-Pal was modeled with pseudo-second-order model. The addition of Cl⁻, SO₄²⁻ and PO₄³⁻ reduced the Cr(VI) adsorption by competition with Cr(VI) for the active sites of PEI-Pal. The Cr(VI) saturated PEI-Pal can be regenerated in alkaline solution, and the adsorption capacity can still be maintained at 30.44 mg·g⁻¹ after 4 cycles. The results demonstrate that PEI-Pal can be used as a potential adsorbent of Cr(VI) in aqueous solutions.     

Adsorption of Cr(VI) using thermally activated weed Salvinia cucullata

Cr(VI) removal studies were carried out by using activated carbon obtained from waste weed, Salvinia cucullata. Effects of various parameters, such as pH, contact time, temperature, adsorbate concentration, adsorbent dose and particle size of the adsorbent on percentage of adsorption were studied. The adsorption studies were carried out at an agitation speed of 600 rpm to minimize the film diffusion. The adsorption kinetics followed dual rate; it was fast during a first stage and then it was reduced. The equilibrium was achieved in 12 h. The kinetics increased with decrease in pH. Adsorbate and adsorbent concentration also influenced the kinetics. The adsorption process was endothermic in nature. The reaction kinetics followed pseudo-second-order kinetic equation. Empirical rate equation developed, which explained the effect of various adsorption parameters, was studied. Theoretical numbers of stages were calculated based on the results. Intra-particle diffusion was found to be the rate-controlling step. Optimization studies were also carried out to establish the upper and lower breakthrough points.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Removal of Cr(VI) from aqueous solution by surfactant-modified kaolinite

Removal of Cr(VI) from aqueous solution by hexadecyltrimethylammonium chloride (HDTMA) modified kaolinite (HMK) was investigated, where the maximum adsorptive capacity reached 27.8 mg/g Cr(VI) using HMK compared with only 0.7 mg/g using unmodified natural kaolinite (NK). The adsorption of Cr(VI) on HMK can be well described by the Langmuir isotherm, and the kinetic adsorption of Cr(VI) on both HMK and NK fitted a pseudo-second-order model. FTIR analysis showed that surface modified HDTMA was responsible for the high adsorptive capacity of Cr(VI). HMK was used to remove Cr(VI) from an electroplating wastewater.     

Removal behavior of Cu(II) during Cr(VI) reduction by cast iron powder in absence and presence of ultrasound

ABSTRACT

Cu(II) is an important and typical heavy metal ion in the wastewater containing Cr(VI), and its removal during Cr(VI) reduction by zero valent iron (ZVI) may make it separately be recovered as a kind of copper resource. In this study, the removal behavior of Cu(II) during Cr(VI) reduction by cast iron powder in absence and presence of ultrasound was investigated by atomic absorption spectrometry (AAS), X-ray powder diffractometer (XRD), scanning electron microscope-energy dispersion spectrum (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The AAS tests indicated that the ultrasound could not only obviously enhance the removal of Cu(II) but also improve the reduction rate of Cr(VI). The XRD and SEM-EDS analyses showed that Cu(II) in the solution was reduced to metallic copper and then was deposited at the surface of ZVI. The ultrasound could remove the Fe-Cr oxides and hydroxides at the ZVI surfaces, resulting in the active surfaces of iron increased. The XPS analyses demonstrated that the surface of metallic copper would be transformed into the film of copper oxide (CuO) in the ultrasound system. The obtained metallic copper and copper oxide could be recovered alone by traditional method of the acid pickling.     

A Study in the Adsorption Behaviors of Cr (VI) on Crosslinked Cationic Starches

The adsorption behaviors between Cr (VI) and crosslinked cationic starch with quaternary ammonium group were investigated in various conditions. The adsorption processes are found to be dependent on the initial pH of the solution, the dose of the absorbents, initial concentration of Cr (VI), and adsorption temperature. The adsorption reaches equilibrium in a short time, and follows the Langmuir isotherm. The adsorption capacity increases with the increasing cationic groups. The adsorption thermodynamics study shows that adsorption processes are exothermic, and lower temperature is favorable to the adsorption of Cr (VI).     

Well-designed ZnIn2S4/TiO2 Z-type heterojunction for efficient photocatalytic reduction of Cr(VI)

ZnIn₂S₄/TiO₂ photocatalyst was obtained by a facile hydrothermal method. Various techniques were used to characterize the ZnIn₂S₄/TiO₂, crystal structure and optical properties of ZnIn₂S₄/TiO₂. Cr (Ⅵ) as highly-toxic pollutant was used as the target reduction product to evaluate the catalytic performance of ZnIn₂S₄/TiO₂ under visible light irradiation. According to the experiment results, the reduction rate of Cr(VI) in the presence of ZnIn₂S₄/TiO₂ reaches 99% within 60 min, which is much better than ZnIn₂S₄ and TiO₂, respectively. At the same time, ZnIn₂S₄/TiO₂ also performs good stability for reduction rate hardly changes after 5 recycling experiments.     

Removal of Cr(VI) from aqueous solution using magnetite/non-oxidative graphene composites: Synergetic effect of Cr(VI) on dyes removal

Non-oxidative graphene (nOG) synthesized from natural graphite powder was modified with magnetite (Fe₃O₄) for removal of Cr(VI) and dyes in aqueous solution. The adsorption behavior of Cr(VI) on Fe₃O₄/nOG (M-nOG) was systematically investigated, and the simultaneous adsorption of Cr(VI) and dyes such as methylene blue (MB) and rhodamine B (RhB) was evaluated. Adsorption kinetic and isotherm of Cr(VI) were fitted well with pseudo-second-order model and Sips model, respectively. For the binary system, Cr(VI) removal was not affected with increasing the dye concentration, whereas the adsorption capacity of both MB and RhB was enhanced with increasing the concentrations of Cr(VI).     

Studies of adsorption behavior of crosslinked chitosan for Cr(VI), Se(VI)

In this work, it was found that crosslinked chitosan (CCTS) had strong adsorption ability for some anions under certain conditions. Cr(VI) and Se(VI) existed in anion forms in aqueous solution, and their adsorption rates by CCTS were 97% for Cr(VI) at pH 3.0 and 95% for Se(VI) at pH 4.0. In addition, the adsorption balance time and isotherm of CCTS for Cr(VI) and Se (VI) were discussed and adsorption mechanism was explained. This research will be useful for designing CCTS‐based adsorption for metallic toxin removal and preconcentrating Cr(VI) and Se(VI) in their trace analysis. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3216–3219, 2000     

A new bioinorganic process for the remediation of Cr(VI)

Palladised biomass of Desulfovibrio desulfuricans ATCC 29577 (bio‐Pd(0)) effected reduction of Cr(VI) to Cr(III) under conditions where biomass alone or chemically‐prepared Pd(0) were ineffective. Reduction of 500 µmol dm^?3 Cr(VI) by 0.4 mg cm^?3 bio‐Pd(0) (Pd : biomass ratio of 1:1) was achieved from 1 mol dm^?3 formate/acetate buffer at pH 1–7 at room temperature; the optimum pH was 3.0. The ratio of mass of Pd : dry mass of biomass, and the need for finely ground bio‐Pd(0) were important parameters for optimal Cr(VI) reduction, with a ratio of 1:1 giving 100% reduction of 500 µmol dm^?3 Cr(VI) within 6 h at room temperature, decreasing to 30 min following heat treatment of the Pd(0)‐loaded biomass. The reduced Cr was recovered quantitatively as soluble Cr(III) at pH 3.0 with no poisoning of the bioinorganic catalyst with respect to continued reduction of Cr(VI). © 2002 Society of Chemical Industry     

Studies on the removal of Cr(VI) from waste-water by feldspar

The efficacy of feldspar in the removal of Cr(VI) from representative waste-water from a plating industry has been investigated in a completely mixed batch reactor at different concentrations, rate of agitation and particle size. The data obey the Langmuir isotherm for the present system and the process of uptake follows first-order kinetics. The maximum removal (91%) was observed at 40°C and pH 2.5 with initial concentration of 19.23 μmol dm^?3 Cr(VI) and 40 g dm^?3 feldspar. The process involves both film and pore diffusion to different extents. Column studies have also been carried out using a certain concentration of waste-water. More than 92% recovery has been achieved and the column can be used for 10 cycles before regeneration. The present technique seems to be quite attractive.     

Nanorod-pillared mesoporous rGO/ZnO/Au hybrids for photocatalytic Cr (VI) reduction: Enhanced Cr(VI) adsorption and solar energy harvest

Herein, we fabricated mesoporous ternary hybrids composed of Au-functionalized ZnO nanorods grown on rGO nanosheets. The ZnO nanorods distributed on the rGO nanosheets can act as a pillaring layer to avoid the agglomeration of rGO, leading to the formation of abundant mesopores within the hybrids. The high-surface-area, mesoporous hybrids can offer sufficient active sites and transport channels for efficient adsorption and diffusion of Cr(VI), which was favorable for its photocatalytic reduction due to the adsorption enrichment effects. Moreover, the plasmonic-induced visible-light absorption of Au nanoparticles and efficient charge separation by rGO synergistically contributed to the significantly improved solar-light-driven photocatalytic Cr(VI) reduction properties of the ternary hybrids.     

Cr(VI)-乙酸复合体系中硫酸盐掺杂的含钛高炉渣光催化还原Cr(VI)

采用煅烧的硫酸盐掺杂的含钛高炉渣作光催化剂,研究了Cr(VI)-乙酸(AA)复合体系中Cr(VI)的光催化还原效率,考察了初始pH值、Cr(VI)初始浓度、AA/Cr(VI)体积比、协同效率因子、光催化剂使用寿命等因素的影响. 结果表明,增大AA/Cr(VI)比到0.2%,Cr(VI)的还原效率先增大到27.55%随后逐渐降低. 酸性条件下,Cr(VI)单一体系和Cr(VI)-AA复合体系中Cr(VI)的还原率和吸附率都明显提高;相同反应时间下(110 min),初始pH 1.5时,2种体系中Cr(VI)的还原效率分别为76.32%(单一体系)和100%(复合体系). 复合体系中协同效率因子始终大于0. 循环使用5次后催化剂对Cr(VI)的光催化还原率为92.2%. Cr(VI)在Cr(VI)-AA体系中的光催化还原遵循Langmuir-Hinshelwood动力学规律.     

Inhibited Cr(VI) formation in Cr2O3-containing refractory castable using reactive MgO as hydraulic binder

Reactive MgO was used in the first time as alternative hydraulic binder of calcium aluminate cement (CAC) to prepare Cr₂O₃-bearing refractory castable. The formation of Cr(VI), and physical and mechanical properties of MgO-bonded refractory castables after heat-treating were investigated. Microstructural characterization and phase composition analyses on the heat-treated MgO-bonded refractory castable matrices resulted in a comprehensive understanding of the mechanism for the inhibition of Cr(VI), and of the strength development during firing. The results indicate that compared with CAC, Cr(VI) levels were 6.7–28.1 times lower using reactive MgO after firing at 700–1300 °C. The in situ Mg(Cr,Al)₂O₄ spinel formed from the preferential interactions among MgO and Cr₂O₃ and Al₂O₃ would be the main reason leading to the inhibited Cr(VI) formation and strength development during firing.     

Removal of Cr(VI) From Aqueous Solution by Turkish Vermiculite: Equilibrium,Thermodynamic and Kinetic Studies

Abstract

The adsorption of Cr(VI) from aqueous solution by Turkish vermiculite were investigated in terms of equilibrium, kinetics, and thermodynamics. Experimental parameters affecting the removal process such as pH of solution, adsorbent dosage, contact time, and temperature were studied. Equilibrium adsorption data were evaluated by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. Langmuir model fitted the equilibrium data better than the Freundlich model. The monolayer adsorption capacity of Turkish vermiculite for Cr(VI) was found to be 87.7 mg/g at pH 1.5, 10 g/L adsorbent dosage and 20°C. The mean free energy of adsorption (5.9 kJ/mol) obtained from the D–R isotherm indicated that the type of sorption was essentially physical. The calculated thermodynamic parameters (ΔG ^o , ΔH ^o and ΔS ^o ) showed that the removal of Cr(VI) ions from aqueous solution by the vermiculite was feasible, spontaneous and exothermic at 20–50°C. Equilibrium data were also tested using the adsorption kinetic models and the results showed that the adsorption processes of Cr(VI) onto Turkish vermiculite followed well pseudo-second order kinetics.