Mechanism of hexavalent chromium removal by dead fungal biomass of Aspergillus niger

When synthetic wastewater containing Cr(VI) was placed in contact with the dead fungal biomass of Aspergillus niger, the Cr(VI) was completely removed from aqueous solution, whereas Cr(III), which was not initially present, appeared in aqueous solution. Desorption and X-ray photoelectron spectroscopy (XPS) studies showed that most of the Cr bound on the biomass was in trivalent form. These results indicated that the main mechanism of Cr(VI) removal was a redox reaction between Cr(VI) and the dead fungal biomass, which is quite different from previously reported mechanisms. The influences of contact time, pH, Cr(VI) concentration, biomass concentration and temperature on Cr(VI) removal were also evaluated. The Cr(VI) removal rate increased with a decrease in pH and with increases in Cr(VI) concentration, biomass concentration and temperature. Although removal kinetics was dependent on the experimental conditions, Cr(VI) was completely removed in the aqueous solution. In conclusion, a new mechanism of Cr(VI) removal by the dead fungal biomass has been proposed. From a practical viewpoint, this abundant and inexpensive dead fungal biomass has potential application in the conversion of toxic Cr(VI) into less toxic or nontoxic Cr(III).     

Purification of chromium(VI) finishing wastewaters using calcined and uncalcined Mg-Al-CO3-hydrotalcite

The applicability of calcined and uncalcined hydrotalcite for the purification of industrial effluents has been studied using chromium finishing wastewaters. Using a batch method, the influence of the initial concentration of chromium (10-450mg/l), hydrotalcite (HT) dose (0.5-5g/l) and time (0.5-72h) has been evaluated. The process could be described by the Langmuir model and gave a maximum removal of chromium of 16.3mg Cr(VI)/g on uncalcined HT and 128mg Cr(VI)/g on calcined hydrotalcite (C-HT). Removal using the calcined product provided an effective system to treat chromium finishing wastewaters with the most stringent discharge limit for such industrial streams being achieved with between two and four consecutive removal cycles on C-HT at a dose of 2g/l.     

Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide

A series of sols consisting of aluminum magnesium mixed hydroxide (AMH) nanoparticles with various Mg/Al molar ratios were prepared by coprecipitation. The use of AMH as adsorbent to remove Cr(VI) from aqueous solution was investigated. Adsorption experiments were carried out as a function of the Mg/Al molar ratio, pH, contact time, concentration of Cr(VI) and temperature. It was found that AMH with Mg/Al molar ratio 3 has the largest adsorption efficiency due to the smallest average particle diameter and the highest zeta potential; AMH was particularly effective for the Cr(VI) removal in a pH range from acid to slightly alkaline, even though the most effective pH range was between 2.5 and 5.0. The adsorption of Cr(VI) on AMH reached equilibrium within 150 min. The saturated adsorption capacities of AMH for Cr(VI) were 105.3-112.0 mg/g at 20-40 °C. The interaction between the surface sites of AMH and the Cr(VI) ions may be a combination of both anion exchange and surface complexation. The pseudo-second-order model best described the adsorption kinetics of Cr(VI) onto AMH. The results showed that AMH can be used as a new adsorbent for Cr(VI) removal which has higher adsorption capacity and faster adsorption rate at pH values close to that at which pollutants are usually found in the environment.     

Remediation of hexavalent chromium by consortia of indigenous bacteria from tannery waste-contaminated biotopes in Fez,Morocco

Bioremediation is a promising, safe and economical technology widely used to clean up both soils and wastewaters containing Cr(VI). Most work uses pure microbial cultures. Little research has been done with mixed microbial cultures. In this study, consortia of indigenous bacteria isolated from a polluted site have been used to study the effect of different conditions such as Cr(VI) concentration, metal ions, electron donors and pH on Cr(VI) reduction by the consortia. We also evaluate the microbial ability to detoxify repeatedly and continuously in a non-modified medium. Results show that consortia of indigenous bacteria are resistant to greater than 200 mg/L Cr(VI). However, mixed cultures exhibited decreasing diversity with increasing levels of chromate. The consortia show high Cr(VI) removal capacity under various conditions and exhibit an ability for continuous reduction of Cr(VI) up to three consecutive inputs. The consortia may be appropriate for environmental applications for Cr(VI) remediation.     

Removal of chromium from aqueous solutions and plating bath rinse by an electrochemical method

In this work, a bipolar packed bed electrolytic cell having steel Raschig rings behaving as electrodes has been used to remove chromium in the form of Cr(VI) from aqueous solutions and a sample of plating bath rinse.

Fe²⁺ ions generated at the anode side of the electrodes have reacted with OH^‐ ions generated at the cathode side forming Fe(OH)₂ in this electrolytic cell.Cr(VI) have coprecipitated as Cr(OH)₃ with Fe(OH)₃ forming after the redox reaction between Fe(OH)₂ and Cr(VI). Meanwhile other impurities have also removed besides Cr(VI) using this process. Removal rate of 100% for Cr(VI) has been achieved in the experiments done with different initial Cr(VI) concentration, duration of electrolysis and applied potential. Fully removal of Cr(VI) has also been achieved from the sample of plating bath rinse.     

Analysis for chromium traces in the aquatic ecosystem II. A study of Cr(III) and Cr(VI) in the susquehanna river basin of New York and Pennsylvania

A new procedure for the differential analysis of ppb concentrations of Cr(III) and Cr(VI) in natural waters has been developed. In this method, three-liter samples are filtered, acidified to pH 6.0, and divided into three parts. One liter is passed through an anion-exchange resin bed (AG-1X4, 100–200 mesh, Cl^? form), one liter is passed through a cation-exchange resin bed (50 WX4, 100–200 mesh, Na⁺ form), and one liter remains untreated. 10 ml of 1.0 M HNO₃ are then added to each of the three aliquots and they are individually reduced in volume by evaporation to 10 ml. Analysis by A.A. employing the method of standard addition follows. By difference, cationic, anionic, and non-ionic Cr concentrations in the sample become available. Cr(III) concentration is probably closely related to (cationic + non-ionic), and Cr(VI) concentration in the original sample corresponds to the anionic portion. Precision is ±20 % or better for total, cationic, and anionic Cr at the 1 ppb level. Precision of the non-ionic analyses is closely linked to these other precisions as the concentration of non-ionic Cr species in the original sample is obtained by the mass balance requirement set up by the other three analyses.Application of this technique to the analysis of natural water samples revealed the predominance of trivalent Cr in uncontaminated waters of the Upper Susquehanna River Basin (1–2 ppb). River water samples taken downstream of Binghamton, N.Y. area sewage outfalls revealed an increase in the mean Cr(VI) content of the river from ～0.5 ppb to ～1.6 ppb. The persistence of the Cr(VI) at least as far as 60 km downstream of the outfall area leads to the conclusion that at these levels, little dissolved Cr(VI) is removed from the river as it flows away from a point of contamination. Concentrations of Cr in samples of river sediments also revealed Cr contamination below the outfall area, and Cr contamination extended at least as far as 60 km downstream of the outfall area.     

Batch Cr(VI) removal by polyacrylamide-grafted sawdust: Kinetics and thermodynamics

Batch sorption studies have been carried out to determine the effect of adsorbent dose, initial sorbate concentration and pH on the adsorption of Cr(VI) on polymer-grafted sawdust. The process was found to be pH, temperature and concentration dependent. An empirical relationship has been obtained to predict the percentage Cr(VI) removal at any time for known values of sorbent and initial sorbate concentration under observed test conditions. The effect of diverse ions has been studied and it is found that there is very little effect on the sorption of Cr(VI). The process was found to be exothermic with a maximum adsorption of 91.0% at 30°C for an initial concentration of 100 mg l⁻¹ at pH 3. The process follows first-order kinetics and the data fits the Freundlich adsorption isotherm. Thermodynamic parameters were also evaluated. Desorption studies confirmed that adsorbent can be effectively regenerated using 0.2 M NaOH and 0.5 M NaCl and can then be reused.     

南水北调中线工程京石段干渠底泥吸附铬研究

近些年水质重金属污染事件频发,南水北调中线工程京石段干渠沿程交叉建筑物较多,对突发意外事故可能产生的水质重金属铬迁移转化规律进行了实验室模拟研究.结果表明,在模拟水流振荡过程中,Cr（Ⅲ）、Cr （VI）之间难以相互转化.酸性条件下底泥对Cr（Ⅲ）的吸附去除效果较好,在pH=5时,底泥对Cr（Ⅲ）的吸附效率最高,为97％;且吸附速度很快,10 min吸附率可达97％左右.Cr（Ⅲ）浓度为1～20 mg/L时,底泥对Cr（Ⅲ）的吸附去除率随铬浓度升高而略有下降,但均在94％以上,底泥对Cr（Ⅲ）的最大吸附量为0.952 mg/g.底泥吸附去除Cr （VI）的效率较低,不同pH、振荡时间条件下,底泥对Cr（VI）的最大去除率为12.50％.不同有机物浓度、吸附时间条件下,底泥对Cr（VI）的最大去除率为20.96％.     

Biosorption of chromium(VI) from aqueous solutions by green algae Spirogyra species.

Biosorption of heavy metals is an effective technology for the treatment of industrial wastewaters. Results are presented showing the sorption of Cr(VI) from solutions by biomass of filamentous algae Spirogyra species. Batch experiments were conducted to determine the adsorption properties of the biomass and it was observed that the adsorption capacity of the biomass strongly depends on equilibrium pH. Equilibrium isotherms were also obtained and maximum removal of Cr(VI) was around 14.7 x 10(3) mg metal, kg of dry weight biomass at a pH of 2.0 in 120 min with 5 mg/l of initial concentration. The results indicated that the biomass of Spirogyra species is suitable for the development of efficient biosorbent for the removal and recovery of Cr(VI) from wastewater.     

Polyacrylamide derivatives as reagents for purification of waters of U(VI) and Cr(VI)

The paper has investigated the impact of water-soluble macromolecular polyelectrolytes on the ultra- and nanofiltration process of purification of waters polluted by U(VI) and Cr(VI). Derivatives of polyacrylamide—cationic and anionic Praestols were used as such compounds. The relationship between the degree of extraction of U(VI) and Cr(VI) and the pH and concentration of Praestols was found. It has been shown that when using the latter the maximum retention coefficient of U(VI) both by ultra- and nanofiltration membranes constituted 0.999; the maximum retention coefficient of Cr(VI) by the ultrafiltration membrane—0.65     

Removal of co-present chromate and arsenate by zero-valent iron in groundwater with humic acid and bicarbonate

The interactions of co-present Cr(VI) and As(V), and the influences of humic acid and bicarbonate in the process of Cr(VI) and As(V) removal by Fe⁰ were investigated in a batch setting using simulated groundwater with 5 mM NaCl, 1 mM Na₂SO₄, and 0.8 mM CaCl₂ as background electrolytes at an initial pH value of 7. Cr(VI) and As(V) were observed to be subject to different impacts induced by co-existing As(V) or Cr(VI), humic acid and bicarbonate, originating from their distinct removal mechanisms by Fe⁰. Cr(VI) removal is a reduction-dominated process, whereas As(V) removal principally involves adsorption onto iron corrosion products. Experimental results showed that Cr(VI) removal was not affected by the presence of As(V) and humic acid. However, As(V) removal appeared to be inhibited by co-present Cr(VI). When the Cr(VI) concentration was 2, 5, and 10 mg/L, in the absence of humic acid and bicarbonate, As(V) removal rate constants were decreased by 27.9%, 49.0%, and 61.2%, respectively, which probably resulted from competition between Cr(VI) and As(V) for adsorption sites of the iron corrosion products. Furthermore, the presence of humic acid significantly varied As(V) removal kinetics by delaying the formation and aggregation of iron hydroxides due to the formation of soluble Fe-humate complexes and stably dispersed fine iron hydroxides colloids. In the presence of bicarbonate, both Cr(VI) and As(V) removal was increased and the inhibitory effect of Cr(VI) on As(V) removal was suppressed, resulting from the buffering effects and the promoted iron corrosion induced by bicarbonate, and the formation of CaCO₃ in solution, which enhanced As(V) adsorption.     

Chromium removal from ion-exchange waste brines with calcium polysulfide

Chromium removal from ion-exchange (IX) brines presents a serious challenge to the water industry. Although chromium removal with calcium polysulfide (CaS₅) from drinking waters has been investigated somewhat, its removal from ion-exchange brines has not been evaluated to date. In this study, a Central Composite Design as well as experimental coagulation tests were performed to investigate the influence of pH, CaS₅/Cr(VI) molar ratio, alkalinity, and ionic strength in the removal of chromium from IX brines. The optimal pH range for the process was found to be pH 8-10.3 and brine alkalinity did not affect coagulation. The efficiency of chromium removal improved only slightly when the ionic strength increased from 0.1 M to 1.5 M; no significant difference was observed for an ionic strength change from 1.5 to 2.1 M. For chromium (VI) concentrations typically found in ion-exchange brines, a CaS₅/Cr(VI) molar ratio varying from 0.6 to 1.4 was needed to obtain a final chromium concentration <5 mg/L. Maximum efficiency for total chromium removal was obtained when oxidation reduction potentials were between −0.1 and 0 (V). Solids concentrations (0.2-1.5 g/L) were found to increase proportionally with CaS₅ dosage. The results of this research are directly applicable to the treatment of residual waste brines containing chromium.     

A study of Cr(VI) and NH4+adsorption using greensand (glauconite) as a low‐cost adsorbent from aqueous solutions

This paper investigates the removal of Cr(VI) and NH⁴⁺ from aqueous solutions using greensand (glauconite). The effects of adsorbent dosage, contact time, initial Cr(VI) and NH⁴⁺ concentrations, and pH were investigated in batch experiments. The results show that these parameters influenced Cr(VI) and NH⁴⁺ removal using glauconite considerably. Pseudo‐first‐order, pseudo‐second‐order and intraparticle diffusion kinetic models were applied to explain the kinetic data, and the pseudo‐second‐order model achieved good agreement. The equilibrium isotherm data are coordinated with the Freundlich, Langmuir models, Temkin and Dubinin–Radushkevich; the Freundlich model proved most best suitable. The removal efficiency of Cr(VI) and NH₄⁺ were 12.21 mg/g (54% at pH 3) and 19.24 mg/g (77.08% at pH 7). All in all, the results showed that the adsorption process on glauconite could be used as an effective method for removing Cr(VI) and NH⁴⁺ from aqueous solutions.     

Comparisons of low-cost adsorbents for treating wastewaters laden with heavy metals

In this article, the removal performance and cost-effectiveness of various low-cost adsorbents derived from agricultural waste, industrial by-product or natural material are evaluated and compared to those of activated carbon for the removal of heavy metals (Cd(II), Cr(III), Cr(VI), Cu(II), Ni(II) and Zn(II)) from metals-contaminated wastewater. To highlight their technical applicability, selected information on pH, dose required, initial metal concentration, adsorption capacity and the price of the adsorbents is presented. It is evident from the survey of 102 published studies (1984-2005) that low cost adsorbents derived from agricultural waste have demonstrated outstanding capabilities for the removal of heavy metal (Cr(VI): 170 mg/g of hazelnut shell activated carbon, Ni(II): 158 mg/g of orange peel, Cu(II): 154.9 mg/g of soybean hull treated with NaOH and citric acid, Cd(II): 52.08 mg/g of jackfruit), compared to activated carbon (Cd(II): 146 mg/g, Cr(VI): 145 mg/g, Cr(III): 30 mg/g, Zn(II): 20 mg/g). Therefore, low-cost adsorbents can be viable alternatives to activated carbon for the treatment of metals-contaminated wastewater. It is important to note that the adsorption capacities presented in this paper vary, depending on the characteristics of the individual adsorbent, the extent of surface modification and the initial concentration of the adsorbate. In general, technical applicability and cost-effectiveness are the key factors that play major roles in the selection of the most suitable adsorbent to treat inorganic effluent.     

Removal of chromium (VI) from wastewater using bentonite-supported nanoscale zero-valent iron

Bentonite-supported nanoscale zero-valent iron (B-nZVI) was synthesized using liquid-phase reduction. The orthogonal method was used to evaluate the factors impacting Cr(VI) removal and this showed that the initial concentration of Cr(VI), pH, temperature, and B-nZVI loading were all importance factors. Characterization with scanning electron microscopy (SEM) validated the hypothesis that the presence of bentonite led to a decrease in aggregation of iron nanoparticles and a corresponding increase in the specific surface area (SSA) of the iron particles. B-nZVI with a 50% bentonite mass fraction had a SSA of 39.94 m²/g, while the SSA of nZVI and bentonite was 54.04 and 6.03 m²/g, respectively. X-ray diffraction (XRD) confirmed the existence of Fe⁰ before the reaction and the presence of Fe(II), Fe(III) and Cr(III) after the reaction. Batch experiments revealed that the removal of Cr (VI) using B-nZVI was consistent with pseudo first-order reaction kinetics. Finally, B-nZVI was used to remediate electroplating wastewater with removal efficiencies for Cr, Pb and Cu > 90%. Reuse of B-nZVI after washing with ethylenediaminetetraacetic acid (EDTA) solution was possible but the capacity of B-nZVI for Cr(VI) removal decreased by approximately 70%.     

Hardness and carbonate effects on the reactivity of zero-valent iron for Cr(VI) removal

Zero-valent iron (Fe0) was used to remove hexavalent chromium, Cr(VI), in groundwater via a coupled reduction-oxidation reaction. Nine columns were set up under various groundwater geochemistry to investigate the effects of hardness and carbonate on Cr(VI) removal. The Cr(VI) removal capacity of Fe0 was found to be about 4 mgCr/g Fe0 in the control column (i.e., column 1). A slight decrease in the Cr(VI) removal capacity was found in the presence of calcium hardness. However, there was a 17% drop in the Cr(VI) removal capacity when magnesium hardness was present at low to moderately hard level. Results also revealed that carbonate changed the morphology of the Fe0 by formation of pale green precipitates on the iron filings. Furthermore, there was a 33% decrease in the Cr(VI) removal capacity of Fe0 when both carbonate and hardness ions were present. In general, the presence of hardness ions and carbonate in groundwater have great impact on the Fe0 by formation of passivated precipitates, such as CaCO3, on the Fe0 surface resulting in a diminished lifespan of the Fe0 by blocking electron transfer.     

Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles

Hexavalent chromium existing in the effluent is a major concern for the metal-processing plant. In this study, a new method combining nanoparticle adsorption and magnetic separation was developed for the removal and recovery of Cr(VI) from wastewater. The nanoscale maghemite was synthesized, characterized, and evaluated as adsorbents of Cr(VI). Various factors influencing the adsorption of Cr(VI), e.g., pH, temperature, initial concentration, and coexisting common ions were studied. Adsorption reached equilibrium within 15 min and was independent of initial Cr concentration. The maximum adsorption occurred at pH 2.5. The adsorption data were analyzed and fitted well by Freundlich isotherm. Cr(VI) adsorption capacity of maghemite nanoparticles was compared favorably with other adsorbents like activated carbon and clay. Competition from common coexisting ions such as Na+, Ca2+, Mg2+, Cu2+, Ni2+, NO3-, and Cl- was ignorable, which illustrated the selective adsorption of Cr(VI) from wastewater. Regeneration studies verified that the maghemite nanoparticles, which underwent six successive adsorption-desorption processes, still retained the original metal removal capacity. In addition, the adsorption mechanisms were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopic techniques.     

Studies on Mg/Fe hydrotalcite‐like – compound (HTlc): removal of Chromium (VI) from aqueous solution

The affinity of Mg/Fe hydrotalcite‐like‐compound (HTlc) for the removal of Cr (VI) from aqueous medium was studied as a function of pH, contact time, temperature, HTlc dose and Cr (VI) concentration. The fraction of Cr (VI) removal decreases with increase in pH of 3 to 10. The reaction kinetic study was undertaken by considering adsorption of Cr (VI) on the outer surface as well as diffusion within the pores of the adsorbent. The adsorption follows first order kinetics. The adsorption data fit well with the Langmuir isotherm model in the temperature range 30–50°C and the thermodynamic parameters viz. ΔG°, ΔH° and ΔS° were calculated to predict the nature of adsorption. The positive value of ΔH° indicates that the adsorption process is endothermic in nature.     

Enhanced chitosan beads-supported Fe-nanoparticles for removal of heavy metals from electroplating wastewater in permeable reactive barriers

The removal of heavy metals from electroplating wastewater is a matter of paramount importance due to their high toxicity causing major environmental pollution problems. Nanoscale zero-valent iron (NZVI) became more effective to remove heavy metals from electroplating wastewater when enhanced chitosan (CS) beads were introduced as a support material in permeable reactive barriers (PRBs). The removal rate of Cr (VI) decreased with an increase of pH and initial Cr (VI) concentration. However, the removal rates of Cu (II), Cd (II) and Pb (II) increased with an increase of pH while decreased with an increase of their initial concentrations. The initial concentrations of heavy metals showed an effect on their removal sequence. Scanning electron microscope images showed that CS-NZVI beads enhanced by ethylene glycol diglycidyl ether (EGDE) had a loose and porous surface with a nucleus-shell structure. The pore size of the nucleus ranged from 19.2 to 138.6 μm with an average aperture size of around 58.6 μm. The shell showed a tube structure and electroplating wastewaters may reach NZVI through these tubes. X-ray photoelectron spectroscope (XPS) demonstrated that the reduction of Cr (VI) to Cr (III) was complete in less than 2 h. Cu (II) and Pb (II) were removed via predominant reduction and auxiliary adsorption. However, main adsorption and auxiliary reduction worked for the removal of Cd (II). The removal rate of total Cr, Cu (II), Cd (II) and Pb (II) from actual electroplating wastewater was 89.4%, 98.9%, 94.9% and 99.4%, respectively. The findings revealed that EGDE-CS-NZVI-beads PRBs had the capacity to remediate actual electroplating wastewater and may become an effective and promising technology for in situ remediation of heavy metals.     

改性膨润土吸附处理含Cr（Ⅵ）废水的研究

用阳离子表面活性剂十六烷基三甲基溴化铵（CTMAB）对钙基膨润土进行活化改性,并用制备的改性膨润土对含Cr（VI）模拟废水进行吸附实验,研究了改性膨润土去除模拟水样中重金属Cr（VI）的适宜条件。结果表明,用质量分数为5%的CTMAB溶液改性后的膨润土去除Cr（VI）效果较好,当改性膨润土用量为10g/L、搅拌时间30min、pH值为3～5时、有机膨润土对含Cr（VI）废水的去除率超过85%。