The role of iron in hexavalent chromium reduction by municipal landfill leachate

The function of iron (ferric (Fe(III)) and ferrous (Fe(II))) in the hexavalent chromium (Cr(VI)) reduction mechanism by bacteria in municipal landfill leachate (MLL) was assessed. Evidence of an "electron shuttle" mechanism was observed, whereby the Cr(VI) was reduced to trivalent chromium (Cr(III)) by Fe(II) with the resulting Fe(III) bacterially re-reduced to Fe(II). Typically, investigations on this electron shuttle mechanism have been performed in an artificial medium. As MLL comprises an elaborate mixture of bacteria, humic materials and organic and inorganic species, additional complexities were evident within the cycle in this study. Bioavailability of the Fe(III) for bacterial reduction, availability of bacterially produced Fe(II) for chemical Cr(VI) reduction and hydrolysis of Fe(II) and Fe(III) become prevalent during each phase of the shuttle cycle when MLL is present. Each of these factors contributes to the overall rate of bacterial Cr(VI) reduction in this media. This work highlights the need to consider local environmental conditions when assessing the bacterial reduction of Cr(VI).     

Effects of tri-valent (Cr(III)) and hexa-valent (Cr(VI)) chromium on the growth of activated sludge

The effects of Cr(VI) and Cr(III) species on the activated sludge growth rate have been assessed for a batch growth system, for a range of chromium concentration between 0 and 320 mg l(-1). Cr(VI) was found to stimulate microbial growth for concentrations up to about 25 mg l(-1), exhibiting maximum growth stimulation at 10 mg l(-1), whilst the lethal dose was found to be between 80 and 160 mg l(-1). On the other hand, Cr(III) was also found to stimulate microbial growth for concentrations up to about 15 mg l(-1), (with a maximum stimulation concentration at 10 mg l(-1)), whilst the lethal dose was found to lie between 160 and 320 mg l(-1). The results indicate that Cr(VI) is more toxic to biomass at relatively high concentrations (higher than 70 mg l(-1)) whilst it has a more pronounced growth stimulation effect at relatively smaller concentrations (less than 25 mg l(-1)), compared with Cr(III).     

Chromate reduction by Arthrobacter CR47 in biofilm packed bed reactors

Bacterial strain Cr47 was isolated from a landfarming process soil sample. It was identified, by 16s rDNA sequencing, as Arthrobacter sp. The time course of the Cr(VI) reduction was monitored in batch operated packed bed biofilm reactors (12 mL void volume) and in recirculating packed bed biofilm reactors (100 mL void volume) inoculated with bacterial strain Cr47. The reduction was evaluated with, 30 mg L(-1) Cr(VI) laboratory solutions prepared with K2Cr2O7 and enriched with glucose-minimal medium, and with 30 mg L(-1) Cr(VI) industrial model solutions prepared with chrome plating waste waters enriched with sucrose-minimal medium. Under batch mode the reduction reaction by the biofilm seemed to fit well an exponential-decay model with a first order kinetic parameter of 0.071 mg(L h)(-1) Cr(VI). In the recirculating reactor, monitored after 4 weeks from inoculation and fed with laboratory solutions the removal rate was 0.79 mg(L h)(-1). In the reactor fed with the industrial model solutions the maximum Cr(VI) removal rate attained was 0.49 mg(L h)(-1). Artrobacter sp. packed bed biofilm reactors achieved Cr(VI) reduction rates comparable to other aerobic and anaerobic fixed film bioreactors previously reported.     

A kinetic study of biological Cr(VI) reduction in trickling filters with different filter media types

Two pilot-scale trickling filters were used in order to estimate Cr(VI) reduction through biological mechanisms in biofilm reactors operated in SBR mode with recirculation using different filter media types, i.e. plastic media and calcitic gravel. The feed concentrations of Cr(VI) examined were about 5, 10, 20, 30, 50 and 100mg/l, while the concentration of the organic carbon was constant at 400mg/l, in order to avoid carbon limitations in the bulk liquid. Maximum reduction rates of 4.8 and 4.7gCr(VI)/d were observed for feed Cr(VI) concentration of about 5mgCr(VI)/l, for the filters with the plastic support material and the gravel media, respectively. The reduction rates were significantly affected by the feed Cr(VI) concentration in both bioreactors. A dual-enzyme kinetic model was used in order to describe Cr(VI) reduction by aerobically grown mixed cultures. Model predictions were found to correspond very closely to experimental quantitative observations of Cr(VI) reduction at both pilot-scale trickling filters used.     

Reduction of hexavalent chromium by Sphaerotilus natans a filamentous micro-organism present in activated sludges

Wastewaters produced by various industries may contain undesirable amounts of hexavalent chromium (Cr(VI)), as chromate and dichromate, a hazardous metal affecting flora and animals of aquatic ecosystems as well as human health. One removal strategy comprises the microbial reduction of Cr(VI) to Cr(III), a less soluble chemical species that is less toxic than Cr(VI). In this work, the ability to reduce Cr(VI) of Sphaerotilus natans, a filamentous bacterium usually found in activated sludge systems, was evaluated. In aerobic conditions, S. natans was able to efficiently reduce Cr(VI) to Cr(III) from dichromate solutions ranging between 4.5 and 80 mg Cr(VI)l(-1) in the presence of a carbonaceous source. A simultaneous evaluation of the microbial respiratory activity inhibition was also carried out to analyze the toxic effect of Cr(VI). Cr(VI) reduction by S. natans was mathematically modeled; chromium(VI) reduction rate depended on both Cr(VI) concentration and active biomass concentration. Although it is known that S. natans removes heavy metal cations such as Cr(III) by biosorption, the ability of this micro-organism to reduce Cr(VI), which behaves as an oxyanion in aqueous solutions, is a novel finding. The distinctive capacity to reduce Cr(VI) to Cr(III) than remain soluble or precipitated becomes S. natans a potential micro-organism to decontaminate wastewaters.     

Degradation of pyridine by one Rhodococcus strain in the presence of chromium (VI) or phenol

A Rhodococcus strain, Chr-9, which has the ability to degrade pyridine and phenol and reduce chromium (VI) (Cr (VI)) was isolated. The strain could grow with pyridine as the sole carbon and nitrogen source, and its pyridine-degradation capability was enhanced by 100 mg l(-1) phenol; however, the degradation of pyridine was inhibited when the phenol concentration was greater than 400 mg l(-1). The hydroxylation of pyridine suggested that the stimulation and inhibition of phenol to the pyridine degradation may be attributed to competition of phenol and pyridine for the hydroxylase gene. Strain Chr-9 was also able to reduce Cr (VI) when glucose and LB was used as the carbon source; however, the Cr (VI) reduction did not occur when pyridine was the sole carbon and energy source. In addition, strain Chr-9 could reduce Cr (VI) and simultaneously degrade pyridine in the presence of glucose. To the best of our knowledge, strain Chr-9 is the first Rhodococcus strain reported to degrade pyridine in the presence of Cr (VI), and the first strain with the pyridine degradation being stimulated by low concentrations of phenol.     

Statistical design of experiments as a tool for optimizing the batch conditions to Cr(VI) biosorption on Araucaria angustifolia wastes

In order to reduce the total number of experiments for achieving the best conditions for Cr(VI) uptake using Araucaria angustifolia (named pinh?o) wastes as a biosorbent, three statistical design of experiments were carried out. A full 2(4) factorial design with two blocks and two central points (20 experiments) was experimented (pH, initial metallic ion concentration-C(o), biosorbent concentration-X and time of contact-t), showing that all the factors were significant; besides, several interactions among the factors were also significant. These results led to the performance of a Box-Behnken surface analysis design with three factors (X, C(o) and t) and three central points and just one block (15 experiments). The performance of these two statistical designs of experiments led to the best conditions for Cr(VI) biosorption on the pinh?o wastes using a batch system, where: pH 2.0; C(o) = 1200 mg l(-1) Cr(VI); X = 1.5 g l(-1) of biosorbent; t = 8 h. The maximum Cr(VI) uptake in these conditions was 125 mg g(-1). After evaluating the best Cr(VI) biosorption conditions on pinh?o wastes, a new Box-Behnken surface analysis design was employed in order to verify the effects of three concomitant ions (Cl(-), NO(3)(-) and PO(4)(3-)) on the biosorption of Cr(VI) as a dichromate on the biosorbent (15 experiments). These results showed that the tested anions did not show any significant effect on the Cr(VI) uptake by pinh?o wastes. In order to evaluate the pinh?o wastes as a biosorbent in dynamic system, a glass column was fulfilled with pinh?o wastes (4.00 g) as biosorbent, and it was fed with 25.0 mg l(-1) Cr(VI) at pH 2.0 and 2.5 ml min(-1). The breakpoint was attained when concentrations of effluent of the column attained the value of 0.05 mg l(-1) Cr(VI) using 5550 ml of the metallic ion solution. In these conditions, the biosorbent was able to remove completely Cr(VI) from aqueous solution with a ratio of Cr(VI) effluent volume/biosorbent volume of 252.3.     

Hexavalent chromium reduction by Acinetobacter haemolyticus isolated from heavy-metal contaminated wastewater

Possible application of a locally isolated environmental isolate, Acinetobacter haemolyticus to remediate Cr(VI) contamination in water system was demonstrated. Cr(VI) reduction by A. haemolyticus seems to favour the lower concentrations (10-30 mg/L). However, incomplete Cr(VI) reduction occurred at 70-100 mg/L Cr(VI). Initial specific reduction rate increased with Cr(VI) concentrations. Cr(VI) reduction was not affected by 1 or 10 mM sodium azide (metabolic inhibitor), 10 mM of PO(4)3-, SO4(2-), SO(3)2-, NO3- or 30 mg/L of Pb(II), Zn(II), Cd(II) ions. However, heat treatment caused significant dropped in Cr(VI) reduction to less than 20% only. A. haemolyticus cells loses its shape and size after exposure to 10 and 50 mg Cr(VI)/L as revealed from TEM examination. The presence of electron-dense particles in the cytoplasmic region of the bacteria suggested deposition of chromium in the cells.     

Isolation and characterization of a Cr(VI)-reduction Ochrobactrum sp. strain CSCr-3 from chromium landfill

A strain CSCr-3 with high Cr(VI)-reducing ability under alkaline conditions was isolated from a chromium landfill and identified as Ochrobactrum sp. on the basis of 16S rRNA gene sequence analysis. The cells were rod shaped, Gram-negative and motile. The physiological characteristics and Cr(VI)-reduction of the strain were also studied. The results showed that the Ochrobactrum sp. strain CSCr-3 was tolerant to very high concentration of Cr(VI) (800 mg/L) and capable of reducing different forms of Cr(VI) (chromate and dichromate), under a wide range of temperatures (25-40 degrees C) and pH (7-11) with optimum at 35 degrees C and initial pH 10. Higher rates of Cr(VI)-reduction were observed with higher initial cell and Cr(VI) concentrations. Strain CSCr-3 could reduce Cr(VI) very efficiently over a wide range of Cr(VI) concentrations (100-800 mg/L). The addition of glucose caused a dramatic increase in Cr(VI)-reduction by Ochrobactrum sp. CSCr-3, while the presence of sulfate or nitrate had no influence. The presence of other metals, such as Cu, Co, Mn, etc., significantly stimulated Cr(VI)-reduction ability by the strain CSCr-3. The results obtained in this study have significance for the bioremediation of chromate pollution.     

Hexavalent chromium removal by ferrochromium slag

The aim of this study is to demonstrate the potential of Cr(VI) generation during jigging operation applied for ferrochrome recovery from slag. The Cr(VI) concentrations of water contacted with ferrochromium slag (W/FS=10) in a closed cycle after 50 batches were found as 0.61 mg/l. Also, reduction of Cr(VI) to Cr(III) and a subsequent precipitation of Cr(III) by using ferrochrome slag (FS) in a model solution has been aimed. The effects of amount of acid, contact time, FS dosage, initial Cr(VI) concentration and temperature on the Cr(VI) reduction have been studied through the batch runs. The amount of acid has been found to be the most effective parameter affecting the Cr(VI) reduction. A 10 g/l FS dosage and 3.5 ml/l H2SO4 (5M) are sufficient to reduce all Cr(VI) in the model solution containing 10mg/l Cr(VI) and for contact time of 60 min at 25 degrees C. In reduced solutions, precipitation of metal ions has been studied by using extra FS. A 60 g/l dosage of fresh FS can precipitate all Cr(III) and Fe ions in the reduced solution. Thus, it has been demonstrated that the treatment of jigging water stream to be generated in a ferrochrome plant containing Cr(VI) can be accomplished by using ferrochromium slag and sulphuric acid. Also, it has been determined that solid residues of the process are environmentally stable by applying TCLP test.     

Stabilization of chromium ore processing residue (COPR) with nanoscale iron particles

Laboratory batch experiments were conducted on heavily contaminated groundwater and chromium ore processing residue (COPR) samples to determine the rate and extent of hexavalent chromium [Cr(VI)] reduction and immobilization by nanoscale iron particles. Laboratory synthesized nanoscale iron particles (<100 nm, specific surface area 35 m(2)/g) were used for this work. Groundwater ([Cr(VI)]=42.83 +/- 0.52 mg/L, pH 11.0+/-0.5) and COPR samples ([Cr(VI)] = 3280 +/- 90 mg/kg) were collected from an industrial site in New Jersey. Cr(VI) in the water and COPR samples was quickly reduced and precipitated out of the aqueous solution. The surface area normalized reaction rate constant of Cr(VI) reduction by nanoscale iron particles was 0.157 +/- 0.018 mg m(-2) min(-1), about 25 times greater than that by iron powders (100 mesh). One gram of nanoparticles can reduce 84.4-109.3mg Cr(VI) in the groundwater and 69.3-72.7 mg Cr(VI) in the COPR. This reduction capacity is 50-70 times greater than that of iron powders under the same experimental conditions.     

Reduction of hexavalent chromium by Pannonibacter phragmitetus LSSE-09 stimulated with external electron donors under alkaline conditions

A novel Cr (VI) resistant bacterial strain LSSE-09, identified as Pannonibacter phragmitetus, was isolated from industrial sludge. It has strong aerobic and anaerobic Cr (VI)-reduction potential under alkaline conditions. At 37 °C and pH 9.0, growing cells of strain LSSE-09 could completely reduce 100 and 1000 mg L(-1) Cr (VI)-Cr (III) within 9 and 24h, respectively under aerobic condition. Resting cells showed higher anaerobic reduction potential with the rate of 1.46 mg g(-1)((dry weight))min(-1), comparing with their aerobic reduction rate, 0.21 mg g(-1)min(-1). External electron donors, such as lactate, acetate, formate, pyruvate, citrate and glucose could highly increase the reduction rate, especially for aerobic reduction. The presence of 3000 mg L(-1) acetate enhanced anaerobic and aerobic Cr (VI)-reduction rates up to 9.47 mg g(-1)min(-1) and 4.42 mg g(-1)min(-1), respectively, which were 5 and 20 times faster than those without it. Strain LSSE-09 retained high activities over six batch cycles and NO(3)(-) and SO(4)(2-) had slightly negative effects on Cr (VI)-reduction rates. The results suggest that strain LSSE-09 has potential application for Cr (VI) detoxification in alkaline wastewater.     

Cr(VI) reduction in aqueous solutions by siderite

Hexavalent chromium is a common and toxic pollutant in soils and wastewaters. Reduction of the mobile Cr(VI) to less mobile and less toxic Cr(III) is a solution for decontamination of industrial effluents. In this study, the reduction of hexavalent chromium in aqueous solutions by siderite was investigated. The influences of amount of acid, contact time, siderite dosage, initial Cr(VI) concentration, temperature and particle size of siderite have been tested in batch runs. The process was found to be acid, temperature and concentration dependent. The amount of acid is the most effective parameter affecting the Cr(VI) reduction since carbonaceous gangue minerals consume acid by side reactions. The highest Cr(VI) reduction efficiency (100%) occurred in the 50 mg/l Cr(VI) solution containing two times acid with respect to stoichiometric amount of Cr(VI) and at the conditions of siderite dosage 20 g/l, contact time 120 min and temperature 25 degrees C. Reduction efficiency increased with increase in temperature and decrease in particle size. The reduction capacity of siderite was found to be 17 mg-Cr(VI)/g.     

A mechanism study of light-induced Cr(VI) reduction in an acidic solution

The mechanisms of photo-catalytic reduction of Cr(VI) were investigated in acidic solutions with and without Fe(III). In a system without Fe(III), no Cr(VI) reduction was observed in dark conditions; conversely, under light conditions, the reduction reaction rate increased to 0.011 and 0.020microM min(-1) at pH 2 and pH 1, respectively, indicating the occurrence of Cr(VI) photo-reduction. The Cr(VI) photo-reduction reaction was induced by the photolysis of water molecules, leading to O(2) production. Upon the addition of Fe(III), the photo-reduction rate of Cr(VI) was significantly enhanced due to the formation of Fe(II), which is the photolytic product of FeCl(2)(+) and the electron donor for Cr(VI) reduction. However, with the same concentration of FeCl complexes, a strong inhibition of Cr(VI) reduction at pH 2 was observed, compared with pH 1. A possible explanation is that FeOH(2+) becomes predominant with increasing pH and that its photolytic product, the OH free radical, is an oxidant for Fe(II) and Cr(III) and can compromise Cr(VI) reduction. The kinetic result of each photo-reduction reaction pathway shows zero-order kinetics, suggesting that the photolysis reaction of H(2)O or FeCl(2+) is the rate-determining step in each pathway. The results also show the potential of developing a homogeneous photo-catalytic method to treat Cr(VI)-containing water.     

Adsorption of Cr(VI) from synthetic solutions and electroplating wastewaters on amorphous aluminium oxide

The adsorption behaviour of amorphous aluminium oxide was studied with respect to Cr(VI) in order to consider its application to purify electroplating wastewaters. A batch method was employed using Cr(VI) concentrations ranged from 10 to 200mg/l. The Langmuir model was found to describe the adsorption process well, offering a maximum adsorption capacity of 78.1mg/g. The effect of ionic strength (0-0.1M KNO(3)), pH (3-9) and competitive solutes (molar ratio [Cr(VI)]/[SO(4)(2-)]=1 and 100) on the retention process was evaluated. Cr(VI) adsorption on amorphous aluminium oxide appeared to be dependent on ionic strength with a more pronounced effect in acid conditions. Conversely, adsorption was not affected by pH in acid medium, but decreased when pH sifted to alkaline values. The presence of SO(4)(2-) greatly reduced Cr(VI) removal across the entire pH range when both solutes were present in similar concentrations. Amorphous aluminium oxide also showed a high adsorption capacity when used in the purification of Cr(VI) electroplating wastewaters. The adsorbent doses required to attain more than 90% of Cr(VI) removal varied between 1 and 5 g/l depending on Cr(VI) concentration in wastewaters.     

Column study of chromium(VI) adsorption from electroplating industry by coconut coir pith

The removal of Cr(VI) from electroplating wastewater by coir pith was investigated in a fixed-bed column. The experiments were conducted to study the effect of important parameters such as bed depth (40-60cm) and flow rate (10-30ml min(-1)). At 0.05 C(t)/C(0), the breakthrough volume increased as flow rate decreased or a bed depth increased due to an increase in empty bed contact time (EBCT). The bed depth service time model (BDST) fit well with the experimental data in the initial region of the breakthrough curve, while the simulation of the whole curve using non-linear regression analysis was effective using the Thomas model. The adsorption capacity estimated from the BDST model was reduced with increasing flow rate, which was 16.40mg cm(-3) or 137.91mg Cr(VI)g(-1) coir pith for the flow rates of 10ml min(-1) and 14.05mg cm(-3) or 118.20mg Cr(VI)g(-1) coir pith for the flow rates of 30ml min(-1). At the highest bed depth (60cm) and the lowest flow rate (10mlmin(-1)), the maximum adsorption reached 201.47mg Cr(VI)g(-1) adsorbent according to the Thomas model. The column was regenerated by eluting chromium using 2M HNO(3) after adsorption studies. The desorption of Cr(III) in each of three cycles was about 67-70%. The desorption of Cr(III) in each cycle did not reach 100% due to the fact that Cr(V) was present through the reduction of Cr(VI), and was still in coir pith, possibly bound to glucose in the cellulose part of coir pith. Therefore, the Cr(V) complex cannot be desorbed in solution. The evidence of Cr(V) signal was observed in coir pith, alpha-cellulose and holocellulose extracted from coir pith using electron spin resonance (ESR).     

Biosorption of Cr(VI) by immobilized biomass of two indigenous strains of cyanobacteria isolated from metal contaminated soil

Biosorption of Cr(VI) using native strains of cyanobacteria from metal contaminated soil in the premises of textile mill has been reported in this paper. Biosorption was studied as a function of pH (1-5), contact time (5-180 min) and initial chromium ion concentration (5-20mg/l) to find out the maximum biosorption capacity of alginate immobilized Nostoc calcicola HH-12 and Chroococcus sp. HH-11. The optimum conditions for Cr(VI) biosorption are almost same for the two strains (pH 3-4, contact time 30 min and initial chromium concentration of 20mg/l) however, the biomass of Chroococcus sp. HH-11 was found to be more suitable for the development of an efficient biosorbent for the removal of Cr(VI) from wastewater, as it showed higher values of q(m) and K(f), the Langmuir and Freundlich isotherm parameters. Both the isotherm models were suitable for describing the biosorption of Cr(VI) by the cyanobacterial biosorbents.     

Implication of chromium speciation on disposal of discarded CCA-treated wood

The wood preservative chromated copper arsenate (CCA) contains hexavalent chromium [Cr(VI)] and the conversion of Cr(VI) to trivalent chromium [Cr(III)] drives fixation of the treatment chemicals to the wood fibers. Since the toxicity of Cr depends on its valence state, an assessment of the Cr species occurring in CCA-treated wood, as well as leachates and ashes from CCA-treated wood, is helpful when assessing implications for disposal. In this study, both new and weathered wood samples of CCA-treated wood and their ashes were evaluated for total Cr and Cr(VI) within the solid matrices and within leachates. Results show that for both new and weathered CCA-treated wood, Cr(VI) occurred in the range of 0.7-4% of the total Cr. Greater Cr leaching occurred at the pH extremes, with Cr(VI) only measured under alkaline pH values (pH > 9.0). Total chromium concentrations from synthetic precipitation leaching procedure (SPLP) leachates from CCA-treated wood were consistently less than 3mg/L with Cr(VI) below detection limits. The results suggest that leaching of Cr(VI) from discarded CCA-treated wood should not be a concern in most landfill environments. One exception would be disposal in landfills with alkaline leachate; Cr(VI) was observed to leach from CCA-treated wood in the presence of alkaline leachate from crushed concrete. When CCA-treated wood is combusted, chromium becomes concentrated in the ash. Cr(VI) in ash from the combustion of CCA-treated wood was found between 4 and 7% of the total chromium. In ash from the combustion of wood recovered from construction and demolition (C&D) debris (which contained some CCA-treated wood), Cr(VI) accounted for as much as 43% of the total Cr. Nearly, all of the Cr in SPLP leachates produced from the ash was in the Cr(VI) form. The degree of Cr(VI) leaching from the ash was highly dependent upon the alkalinity of the ash, with higher ash leachate pH resulting in greater concentrations of Cr(VI).     

Hexavalent chromium reduction with scrap iron in continuous-flow system Part 1: effect of feed solution pH

The reduction of hexavalent chromium by scrap iron was investigated in continuous system, using long-term column experiments, for aqueous Cr(VI) solutions having low buffering capacities, over the pH range of 2.00-7.30. The results showed that the initial pH of Cr(VI) solution significantly affects the reduction capacity of scrap iron. The highest reduction capacity was determined to be 19.2 mg Cr(VI)/g scrap iron, at pH 2.50, and decreased with increasing the initial pH of Cr(VI) solution. A considerable decrease in scrap iron reduction capacity (25%) was also observed at pH 2.00, as compared to pH 2.50, due to the increased contribution of H(+) ions to the corrosion of scrap iron, which leads to a rapid decrease in time of the scrap iron volume. Over the pH range of 2.50-7.30, hexavalent chromium concentration increases slowly in time after its breakthrough in column effluent, until a steady-state concentration was observed; similarly, over the same pH range, the amount of solubilized Cr(III) in treated column effluent decreases in time, until a steady-state concentration was observed. The steady-state concentration in column effluent decreased for Cr(VI) and increased for Cr(III) with decreasing the initial pH of Cr(VI) solution. No steady-state Cr(VI) or Cr(III) concentrations in column effluent were observed at pH 2.00. Over the entire studied pH range, the amount of Fe(total) in treated solution increases as the initial pH of column influent is decreased; the results show also a continuously decrease in time of Fe(total) concentration, for a constant initial pH, due to a decrease in time of iron corrosion rate. Cr(III) concentration in column effluent also continuously decreased in time, for a constant initial pH, over the pH range of 2.50-7.30. This represents an advantage, because the amount of precipitant agent used to remove Fe(total) and Cr(III) from the column effluent will also decrease in time. The optimum pH for Cr(VI) reduction with scrap iron in continuous-flow system was established at the value of 2.50.     

Hexavalent chromium removal from wastewater using aniline formaldehyde condensate coated silica gel

A resinous polymer, aniline formaldehyde condensate (AFC) coated on silica gel was used as an adsorbent in batch system for removal of hexavalent chromium from aqueous solution by considering the effects of various parameters like reaction pH, dose of AFC coated silica gel, initial Cr(VI) concentration and aniline to formaldehyde ratio in AFC synthesis. The optimum pH for total chromium [Cr(VI) and Cr(III)] adsorption was observed as 3. Total chromium adsorption was second order and equilibrium was achieved within 90-120 min. Aniline to formaldehyde ratio of 1.6:1 during AFC synthesis was ideal for chromium removal. Total chromium adsorption followed Freundlich's isotherm with adsorption capacity of 65 mg/g at initial Cr(VI) 200mg/L. Total chromium removal was explained as combinations of electrostatic attraction of acid chromate ion by protonated AFC, reduction of Cr(VI) to Cr(III) and bond formation of Cr(III) with nitrogen atom of AFC. Almost 40-84% of adsorbed chromium was recovered during desorption by NaOH, EDTA and mineral acids. AFC coated silica gel can be effectively used for treatment of chromium containing wastewaters as an alternative.