Sources and toxicity of hexavalent chromium

Nanoencapsulation of hexavalent chromium with nanoscale zero-valent iron: High resolution chemical mapping of the passivation layer

Solid phase reactions of Cr(VI) with Fe(0) were investigated with spherical-aberration-corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy-dispersive spectroscopy (XEDS). Near-atomic resolution elemental mappings of Cr(VI)-Fe(0) reactions were acquired. Experimental results show that rate and extent of Cr(VI) encapsulation are strongly dependent on the initial concentration of Cr(VI) in solution. Low Cr loading in nZVI (<1.0wt%) promotes the electrochemical oxidation and continuous corrosion of nZVI while high Cr loading (>1.0wt%) can quickly shut down the Cr uptake. With the progress of iron oxidation and dissolution, elements of Cr and O counter-diffuse into the nanoparticles and accumulate in the core region at low levels of Cr(VI) (e.g., <10mg/L). Whereas the reacted nZVI is quickly coated with a newly-formed layer of 2-4nm in the presence of concentrated Cr(VI) (e.g., >100mg/L). The passivation structure is stable over a wide range of pH unless pH is low enough to dissolve the passivation layer. X-ray photoelectron spectroscopy (XPS) depth profiling reconfirms that the composition of the newly-formed surface layer consists of Fe(III)-Cr(III) (oxy)hydroxides with Cr(VI) adsorbed on the outside surface. The insoluble and insulating Fe(III)-Cr(III) (oxy)hydroxide layer can completely cover the nZVI surface above the critical Cr loading and shield the electron transfer. Thus, the fast passivation of nZVI in high Cr(VI) solution is detrimental to the performance of nZVI for Cr(VI) treatment and remediation.

Trace Metals in the Urine and Hair of a Population in an Endemic Arsenism Area

There have been few investigations of trace elements in the urine and hair of populations exposed to high levels of arsenic (As) in drinking water. Therefore, concentrations of selected metals in urine and hair samples from a population in a study area where arsenism was endemic and a control area were determined. It was found that the median concentrations of barium (Ba), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), zinc (Zn), and As in the urine samples from the population in the study area were 3.87, 0.47, 0.50, 61.84, 26.82, 1.33, 128.45, 7.05, 1.10, 233.75, and 339.63 μg/L, respectively. The corresponding values in the urine samples from a population in the control area were 29.08, 0.19, 0.21, 27.77, 10.32, 4.61, 14.01, 2.19, 3.90, 113.92, and 20.28 μg/L, respectively. In the study area, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, and Zn excreted in the urine were likely to be mainly derived from drinking water with high levels of arsenic. The median concentrations of Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Zn, and As in the hair samples from the study area were 4.16, 0.03, 0.09, 1.09, 6.54, 1.97, 0.06, 0.53, 1.64, 144.28, and 1.67 mg/kg, respectively. The corresponding values from the control area were 4.76, 0.03, 0.02, 1.41, 8.31, 1.34, 0.07, 0.39, 0.86, 154.58, and 0.29 mg/kg, respectively. Significant positive correlations were observed between As and Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, and Zn in the urine in the study area. However, As was not positively associated with these metals in the hair samples. Exposure to high levels of As in drinking water increased the accumulation of Ba and Mn in the hair and the excretion of Cd, Cu, and Mo in the urine in the study area. The population in the study area might experience Cu and Mo deficiencies for an increasing excretion of Cu and Mo.

Callitriche cophocarpa (water starwort) proteome under chromate stress: evidence for induction of a quinone reductase

Chromate-induced physiological stress in a water-submerged macrophyte Callitriche cophocarpa Sendtn. (water starwort) was tested at the proteomic level. The oxidative stress status of the plant treated with 1 mM Cr(VI) for 3 days revealed stimulation of peroxidases whereas catalase and superoxide dismutase activities were similar to the control levels. Employing two-dimensional electrophoresis, comparative proteomics enabled to detect five differentiating proteins subjected to identification with mass spectrometry followed by an NCBI database search. Cr(VI) incubation led to induction of light harvesting chlorophyll a/b binding protein with a concomitant decrease of accumulation of ribulose bisphosphate carboxylase (RuBisCO). The main finding was, however, the identification of an NAD(P)H-dependent dehydrogenase FQR1, detectable only in Cr(VI)-treated plants. The FQR1 flavoenzyme is known to be responsive to oxidative stress and to act as a detoxification protein by protecting the cells against oxidative damage. It exhibits the in vitro quinone reductase activity and is capable of catalyzing two-electron transfer from NAD(P)H to several substrates, presumably including Cr(VI). The enhanced accumulation of FQR1 was chromate-specific since other stressful conditions, such as salt, temperature, and oxidative stresses, all failed to induce the protein. Zymographic analysis of chromate-treated Callitriche shoots showed a novel enzymatic protein band whose activity was attributed to the newly identified enzyme. We suggest that Cr(VI) phytoremediation with C. cophocarpa can be promoted by chromate reductase activity produced by the induced quinone oxidoreductase which might take part in Cr(VI) → Cr(III) bioreduction process and thus enable the plant to cope with the chromate-generated oxidative stress.

Quinic acid and hypervalent chromium: a spectroscopic and kinetic study

The redox reaction between an excess of quinic acid (QA) and Cr^VI involves the formation of intermediates, namely, Cr^IV and Cr^V species, which in turn react with the organic substrates. As observed with other substrates that have already been studied, Cr^IV does not accumulate during this reaction because of the rate of the reaction. Its rate of disappearance is several times higher than that of the reaction of Cr^VI or Cr^V with QA. Kinetic studies indicate that the redox reaction proceeds via a combined mechanism that involves the pathways Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III, which is supported by the observation of superoxo-Cr^III (CrO₂²⁺) ions, free radicals, and oxo-Cr^V species as intermediates and the detection of Cr^VI ester species. The present study reports the complete rate laws for the QA/chromium redox reaction.

The redox reaction between an excess of quinic acid (QA) and Cr^VI involves the formation of intermediates, namely, Cr^IV and Cr^V species, which in turn react with the organic substrates.

Improving the capability of UiO-66 for Cr(vi) adsorption from aqueous solutions by introducing isonicotinate N-oxide as the functional group

In this study, we showed that a conceptual design can be helpful in synthesizing environmentally safe and stable adsorbents along with suitable adsorption parameters.

Water quality parameters, biomarkers and metal bioaccumulation in native fish captured in the Ilha River, southern Brazil

The Ilha River is one of the main tributaries of the Sinos River, southern Brazil, and it is located in an area characterized by low population density and presence of agricultural activities. Thus, this study aimed to assess the water quality of two sites of the Ilha River (source and mouth, S1 and S2 respectively) in five sampling periods using water physicochemical and microbiological analyses, biomarkers, such as condition factor, micronucleus test, gill histopathological analysis, and metal bioaccumulation in the native fish Bryconamericus iheringii. Mean values of BOD₅, thermotolerant coliforms, aluminum, iron and lead exceeded the limits established by the Brazilian legislation for surface waters at both sampling sites. Significant higher micronucleus, nuclear abnormalities and mucous cells frequencies were found at S2 in, at least, one sampling period, whereas fish from S1 presented significant lower condition factor, higher frequencies of lamellar alterations and higher concentrations of chromium and nickel in muscle. Additionally, concentrations of cadmium, chromium and lead in fish muscle exceeded the limits considered safe for human consumption at both sites in at least one sampling period. Data from our study evidenced the mouth of the Ilha River suffers from point genotoxic effects, whereas the source is also contaminated by metals, despite being located in an area under minor anthropic activities.

Extraction of Natural Surfactant Saponin from Soapnut (Sapindus mukorossi) and its Utilization in the Remediation of Hexavalent Chromium from Contaminated Water

In this study optimization of extraction conditions for saponin from soapnut (Sapindus mukorossi) has been investigated. This investigation showed that a better extraction of saponin can be achieved with increasing the dielectric constant of solvent employed. The best yield was attained in a 50% v/v aqueous ethanol medium. Another objective of the present study is to assess the thermodynamics of the uptake-reduction of hexavalent chromium in contaminated water samples using saponin. Pyrene has been employed to determine the critical micellar concentration (CMC) (in the UV-vis method of determination) of saponin as a spectroscopic probe. Thus, the effectiveness of the bio-surfactant as a self-motivated adsorbent for hexavalent chromium is investigated. Several physico-chemical parameters like contact time, sorbate concentration, pH and temperature have been determined. The findings of the investigation invoked a very efficient uptake of 213.48 mg g−1 of hexavalent chromium from the contaminated water sample at a lower value of pH 2 and temperature 35°C. It is observed that the method followed a pseudo-first order kinetics, where the evaluated ΔG0 has supported the sorption as a feasible and spontaneous process. The HR-MS, FTIR, steady state Fluorescence spectroscopy, HR-TEM and UV-Vis spectroscopy have been employed for the current scientific studies.

Evaluation of activated carbon fiber supported nanoscale zero-valent iron for chromium (VI) removal from groundwater in a permeable reactive column

An activated carbon fiber supported nanoscale zero-valent iron (ACF-nZVI) composite for Cr(VI) removal from groundwater was synthesized according to the liquid phase reduction method. The techniques of N₂ adsorption/desorption, FESEM, EDX, XRD and XPS were used to characterize the ACF-nZVI composite and the interaction between the ACF-nZVI composite and Cr(VI) ions. Batch experiments were conducted to evaluate the effects of several factors, including the amount of nZVI on activated carbon fiber (ACF), pH value, initial Cr(VI) concentration, and co-existing ions on Cr(VI) removal. The results indicate that presence of ACF can inhibit the aggregation of nanoscale zero-valent iron (nZVI) particles and increase its reactivity, and the Cr(VI) removal efficiency increases with increasing amounts of nZVI on ACF and a decrease in the initial Cr(VI) concentration. In acidic conditions, almost 100% of Cr(VI) in solution can be removed after 60 min of reaction, and the removal efficiency decreases with increasing initial pH values. The Cr(VI) removal is also dependent on the co-existing ions. Reusability experiments on ACF-nZVI demonstrate that the ACF-nZVI composite can keep a high reactivity after five successive reduction cycles. The removal mechanisms are proposed as a two-step interaction including the physical adsorption of Cr(VI) on the surface or inner layers of the ACF-nZVI composite and the subsequent reduction of Cr(VI) to Cr(III) by nZVI.

Stepwise Deprotonation of Magnetite-Supported Gallic Acid Modulates Oxidation State and Adsorption-Assisted Translocation of Hexavalent Chromium

Recently, a synergistic strategy involving reduction of carcinogenic Cr(VI) into less toxic Cr(III) followed by Cr(III) adsorption and subsequent separation by surface-engineered magnetite nanoparticles has emerged as a promising alternative to address the environmental hazards associated with Cr(VI)-contaminated water. Despite several previous attempts exploiting this synergy, modulating the oxidation state and translocation of Cr(VI) with high spatiotemporal precision remains a major challenge. Here, we report how Cr(VI) responds accordingly in a well-defined manner to deprotonation of gallic acid covalently immobilized on magnetite nanoparticles, which proceeds through a fixed spatial sequence of distinct stages. To the best of our knowledge, this proof-of-principle study, for the first time, demonstrates that accurate spatiotemporal control over the cascading reduction-adsorption process of Cr(VI) by magnetic adsorbents is feasible, which provides guidance for rational design of more exquisite, magnetite-supported surfaces, where a predictable, and hence controllable, synergy can manifest for Cr(VI) detoxification.

Selective removal of toxic Cr(VI) from aqueous solution by adsorption combined with reduction at a magnetic nanocomposite surface

The adsorption of toxic hexavalent chromium (Cr(VI)) and its reduction to trivalent chromium (Cr(III)) are important processes for the treatment of industrial wastewater. Conducting polymers can adsorb and reduce Cr(VI) to less toxic Cr(III) but have low adsorption capacities due to agglomeration of particles and are difficult to separate from treated water. In this study, magnetic polypyrrole (PPy)-polyaniline (PANI)/iron oxide (Fe₃O₄) nanocomposite was synthesized for the selective removal of Cr(VI) in aqueous solution. PPy-PANI/Fe₃O₄ nanocomposite was characterized using various techniques including ATR-FTIR, FE-SEM, HR-TEM, EDX, TGA, XRD, VSM and XPS analyses. PPy-PANI/Fe₃O₄ nanocomposite (0.05g) removed 99% of Cr(VI) from aqueous solution (100mg/L, pH 2). Speciation studies confirmed Cr(VI) adsorption and reduction to Cr(III) by the PPy-PANI/Fe₃O₄ nanocomposite in solutions with initial pH of 2 and 3 and that no Cr(VI) reduction occurred at pH values of 4 and above. The Langmuir maximum adsorption capacity for Cr(VI) removal by PPy-PANI/Fe₃O₄ nanocomposite at pH 2 was 303mg/g at 25°C. PPy-PANI/Fe₃O₄ nanocomposite was highly selective for Cr(VI) removal and could be used for three consecutive treatment cycles without loss of adsorption capacity. Moreover, the magnetic nanocomposite could be separated from the reaction fluid using an external magnet. PPy-PANI/Fe₃O₄ nanocomposite is therefore a promising magnetic adsorbent for the treatment of industrial wastewater.

Characterization of Product and Potential Mechanism of Cr(VI) Reduction by Anaerobic Activated Sludge in a Sequencing Batch Reactor

Bioremediation of Cr(VI) and nitrate is considered as a promising and cost-effective alternative to chemical and physical methods. However, organo-Cr(III) complexes in effluent generally causes environmental concerns due to second-pollution. Here, Cr(VI) reduction and immobilization efficiencies of anaerobic activated sludge were investigated. Anaerobic activated sludge showed strong reduction ability of Cr(VI) and possessed a great potential of Cr(III) immobilization. Almost 100.0 mg l⁻¹ Cr(VI) could be completely reduced and immobilized by anaerobic activated sludge in a sequencing batch reactor in 24 h. And most generated Cr(III) was accumulated outside of sludge cells. Extracellular polymeric substances (EPS) could bind to Cr(VI) and form EPS-Cr(VI) interaction to reduce the toxic effect of Cr(VI) and promote the Cr(VI) reduction. Protein-like and humic-like substances were responsible for binding with Cr(VI), meanwhile the process was a thermodynamically favorable binding reaction. Then Cr(VI) was reduced to Cr(III) by membrane-associated chromate reductase of sludge. Eventually, the generated Cr(III) might exist as poly-nuclear Cr(III) complexes adhered to sludge surfaces.

Influences of chromium and cadmium on the development of black soldier fly larvae

The black soldier fly Hermetia illucens is a good candidate for waste management. The harvested insects are rich in protein and have the potential to be used in animal feed. However, people are wary about heavy metals in waste. Therefore, it is necessary to understand how the uptake of heavy metals could affect H. illucens and where and to what extent metals are accumulated by the black soldier fly. Based on these considerations, developmental parameters were investigated in the different life stages of H. illucens fed an increasing concentration gradient of cadmium (Cd) and chromium (Cr); additionally, Cd and Cr distribution in the body parts of H. illucens at the different life stages was monitored. We found that Cd and Cr have no effects on larvae survival and eclosion rate, but they do have effects on larvae duration and pupation rate. Both Cd and Cr were transferred into larvae, prepupae, and pupae. While the concentrations of Cd in larvae and prepupae were much higher than that in their diets, the opposite case was observed with Cr. The concentrations of Cd and Cr in H. illucens decreased in later development stages. In individual larva and prepupa, Cd and Cr were mainly included in the body and not in the integument. In the pupa, the puparium contained higher Cd and Cr concentrations than the pupa body. The distribution of Cd and Cr in the different life stages and body parts may present a potential strategy for how H. illucens tolerate and remove heavy metal stress.

The Origin of Hexavalent Chromium as a Critical Parameter for Remediation of Contaminated Aquifers

Two different cases of Cr(VI) contaminated ophiolitic aquifers are presented herein. The first is located at Vergina (Northern Greece), where the maximum Cr(VI) concentration measured was 64 μg/L, being one of the highest geogenic concentrations recorded globally in areas with similar geological background. The second case is located at Inofyta (Central Greece), a makeshift industrial area, where the maximum detected Cr(VI) concentration exceeds 10,000 μg/L, indicating clearly anthropogenic activities as the main source. Although for the Vergina aquifer, area-wide institutional measures and in some cases pump and treat systems might be sufficient to ensure a safe water supply for domestic and agricultural use, this is not the case for the contaminated aquifer of Inofyta. There a comprehensive remediation scheme should be properly implemented adopting, however, realistic remediation targets, that should also take into account the geogenic contribution to the high Cr(VI) groundwater concentrations measured.

A simple model to predict chromate partitioning in selected soils from China

Due to its mobility and toxicity, chromate [Cr(VI)] partitioning in soils, especially in the vadose zone, is an important environmental concern. The aim of this study was to develop a mechanism-based multi-surface complexation model using published parameters to predict the soil/water partitioning of Cr(VI) in 12 soils (previously depleted of organic matter) from China. The retention of Cr(VI) in soils was attributed to two reactive oxide surfaces: goethite and hydrous ferric oxide (HFO); however, modeling results showed that the best prediction was obtained with goethite alone, whereas the addition of HFO resulted in an overestimation of adsorption in some soils. Cr(VI) adsorption onto goethite could be described using our previously proposed CD-MUSIC model. In the absence of a specific value for the soil-reactive surface area of goethite, a general value of 45.8m²/g was used. The available phosphate in soils was identified as a strong competitor for Cr(VI) adsorption; thus, for soils with a low Fe/P ratio (<1) the effect of phosphate on Cr(VI) retention should not be neglected. The simple method presented herein can be applied to soils with a wide range of properties, pH values, and Cr(VI) loading concentrations.

In vitro cytotoxicity and genotoxicity of composite mixtures of natural rubber and leather residues used for textile applications

A novel composite material has been developed from natural rubber and leather waste, and a corresponding patent has been filed. This new material may be incorporated into textile and footwear products. However, as leather waste contains chromium, the biocompatibility of this new material and its safety for use in humans must be investigated. The aim of the present study was to investigate the presence of chromium in this new material, determine the amount of each form of chromium present (trivalent or hexavalent), and evaluate the potential cytotoxic and genotoxic effects of the novel composite in two cell lines. The cellular viability was quantified using the MTT3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction method and neutral red uptake assay, and genotoxic damage was analyzed using the comet assay. Our findings indicated that the extracts obtained from the composite were severely cytotoxic to both cell lines tested, and additionally highly genotoxic to MRC-5 cells. These biological responses do not appear to be attributable to the presence of chromium, as the trivalent form was predominantly found to be present in the extracts, indicating that hexavalent chromium is not formed during the production of the novel composite. The incorporation of this new material in applications that do not involve direct contact with the human skin is thus indicated, and it is suggested that the chain of production of this material be studied in order to improve its biocompatibility so that it may safely be used in the textile and footwear industries.

Influence of organic acids on kinetic release of chromium in soil contaminated with leather factory waste in the presence of some adsorbents

In this study, batch experiments were conducted to investigate the effects of nanoparticles (NPs) (MgO, ZnO, TiO2) and clay minerals (bentonite, zeolite) on the release of chromium (Cr) from leather factory waste (LFW) and LFW treated soil using organic acids. Chromium release from all treatments was studied in the presence of citric acid, oxalic acid and CaCl2 solutions. The results showed that, in all treatments, organic acids released more Cr than inorganic salt (CaCl2). The release of Cr by citric acid was higher than that by oxalic acid. In LFW treated soil and LFW, the release of Cr from the all treatments with NPs was less than that from the clay mineral treatments. On the other hand, in the presence of organic acids, Cr release by NPs and clay minerals decreased. Two kinetic models including pseudo-first- and pseudo-second-order model were tested to describe the time dependent Cr release data. Among the kinetic models used, the pseudo-second-order model generally gave the best fits to experimental data. Before and after release experiments, Cr in LFW, treated LFW, control soil and LFW treated soils were fractionated. In all treatments, the greatest amounts of Cr were found in the residual fraction (RES). The organic acids were effective in reducing the exchangeable (EXC), bound to organic matter (OM) and bound to carbonate (CAR) fractions of Cr in all treatments, whereas, after release of Cr from treated soils, Cr remained mainly in the RES fraction. The application of NPs and clay minerals in soil led to a significant transformation of Cr from mobile fractions to the RES fraction. Therefore, organic ligands played a dominant role in mobility and bioavailability of Cr and the removal of Cr by adsorbents.

Chromium Distribution and Spatial Variations in the Finer Sediment Grain Size Fraction and Unfractioned Surficial Sediments on Nyanza Gulf, of Lake Victoria (East Africa)

Surficial sediments collected from the Nyanza Gulf of Lake Victoria (East Africa) were used to determine spatial concentrations of Cr and determine differences in contents of the unfractioned (whole sediment) and the finer grain size sediments, establishing any changes in Cr enrichment and potential ecological risks using sediment quality guidelines. A single pollution index was also used to evaluate level of Cr contamination. The spatial mean Cr contents in the <63 µm (silt-clay) fraction were found to be significantly lower than those in the unfractioned sediments, but with a strong linear positive correlation. The study results show decreasing spatial amounts of Cr in surficial sediments of the Nyanza Gulf, when compared to a study done 20 years earlier. However, the 95% confidence limits of the overall mean Cr in unfractioned sediments exceed the threshold effect concentration (TEC), indicating the potential for Cr remobilization from sediments. In general the sediment enrichment is evidence of possible dominance of lithogenous sources of Cr in the surface lake sediments, with potential anthropogenic sources from the drainage system and nearshore urban areas. The sediments are unpolluted with respect to geoaccumulation index, and sediment enrichment factors suggest a minor to moderate enrichment of Cr in surficial sediments of three sites around the Nyanza Gulf zones and around the river mouth in the main lake.

Immobilization in cement mortar of chromium removed from water using titania nanoparticles

Because of the high toxicity of chromium, particularly as Cr (VI), it is removed from industrial effluents before their discharge into water bodies by a variety of techniques, including adsorption. Ultimate disposal of the sludge or the adsorbate, however, is a serious problem. While titania, in nanoparticle form, serves as a very good adsorbent for chromium, as an additive, it also helps to increase the compressive strength of mortar and concrete. Combining these two properties of the material, titania nanoparticles were used to adsorb chromium and then added to mortar up to a concentration of 20% by weight. The compressive strength of the resulting mortar specimens that replaced 15% of cement with chromium laden titania showed an improved strength than that without titania, thus confirming that this material had positive effect on the mortar strength. Leachate tests using the Toxicity Characteristics Leaching Procedure (TCLP) confirmed that the mortar sample chromium leachate was well within the permissible limits. The proposed technique thus offers a safe and viable method for the ultimate disposal of toxic metal wastes, in general, and those laden waste chromium, in particular.

On the removal of hexavalent chromium from a Class F fly ash

Coarse and fine samples of a Class F fly ash obtained from commercial combustion of Illinois bituminous coal have been exposed to two long-term leaching tests designed to simulate conditions in waste impoundments. ICP-AES analysis indicated that the coarse and fine fly ash samples contained 135 and 171mg/kg Cr, respectively. Measurements by XAFS spectroscopy showed that the ash samples originally contained 5 and 8% of the chromium, respectively, in the hexavalent oxidation state, Cr(VI). After exposure to water for more than four months, the percentage of chromium as Cr(VI) in the fly-ash decreased significantly for the coarse and fine fly-ash in both tests. Combining the XAFS data with ICP-AES data on the concentration of chromium in the leachates indicated that, after the nineteen-week-long, more aggressive, kinetic test on the coarse fly ash, approximately 60% of the Cr(VI) had been leached, 20% had been reduced to Cr(III) and retained in the ash, and 20% remained as Cr(VI) in the ash. In contrast, during the six-month-long baseline test, very little Cr was actually leached from either the coarse or the fine fly-ash (<0.1mg/kg); rather, about 66% and 20%, respectively, of the original Cr(VI) in the coarse and fine fly-ash was retained in the ash in that form, while the remainder, 34% and 80%, respectively, was reduced and retained in the ash as Cr(III). The results are interpreted as indicating that Cr(VI) present in Class F fly-ash can be reduced to Cr(III) when in contact with water and that such chemical reduction can compete with physical removal of Cr(VI) from the ash by aqueous leaching.

Dual enhancement-inhibition roles of polycarboxylates in Cr(VI) reduction and organic pollutant oxidation in electrical plasma system

In this study, the roles of polycarboxylates in synergistic Cr(VI) reduction and organic pollutant oxidation are investigated in glow discharge electrolysis (GDE). H2O2 generated in GDE plays a primary role for Cr(VI) reduction, and the presence of polycarboxylates can significantly enhance the reduction of Cr(VI) to Cr(III) with less value of [H2O2](consumption)/[Cr(VI)](reduction). Simultaneously, polycarboxylates inhibit the production of ·OH via chromium-based Fenton-like reaction, leading to the retarded oxidation of other pollutant oxidation, i.e., RhB. The formation of peroxochromate(V) is a requisite both for Cr(VI) reduction to Cr(III) and ·OH formation via Fenton-like reaction. Polycarboxylates can form complexes with peroxochromate(V), which can transform to Cr(III) spontaneously, thereby interrupting the pathway for additional ·OH production. These influences induced by polycarboxylate were found closely relative to the number and position of -OH group in polycarboxylates. Besides, 162.7 mg L(-1) Cr(VI) in actual electroplating effluent can be rapidly and almost completely reduced in GDE with introducing polycarboxylate containing nickel electroplating effluent. Generally, the present study provides a versatile strategy for Cr(VI) reduction, exhibiting a bright application future for real wastewater treatment.

Accumulation patterns of Cr in Callitriche organs--qualitative and quantitative analysis

The aims of this study were both the qualitative and quantitative analysis of chromium accumulation in the shoots of Callitriche cophocarpa. This globally distributed, submersed macrophyte exhibits outstanding Cr phytoremediation capacity in an aquatic environment. Cr was applied separately for 7 days at two stable forms as Cr(VI) and Cr(III), known from their diverse physicochemical properties and toxicities. The maps of Cr depositions in young leaves, mature leaves, and stems were obtained by micro X-ray fluorescence spectroscopy (μXRF). The detailed analysis of XRF maps was done based on Image-Pro PLUS (Media Cybernetics) software. Cr was accumulated either in trichomes or vascular bundles in respect to the element speciation and the plant organ. The concentration of Cr significantly increased in the following order: Cr(VI) mature leaves < Cr(VI) young leaves = Cr(VI) stems < Cr(III) young leaves ≤ Cr(III) mature leaves ≤ Cr(III) stems. The observed differences in distribution and accumulation of Cr were correlated with the different reduction potential of Cr(VI) by particular plant organs. The reduction of Cr(VI) is considered the main detoxification mechanism of the highly toxic Cr(VI) form. The unique L-band electron resonance spectrometer (L-band EPR) was applied to follow the reduction of Cr(VI) to Cr(III) in the studied material.

Mobility of as, Cu, Cr, and Zn from tailings covered with sealing materials using alkaline industrial residues: a comparison between two leaching methods

Different alkaline residue materials (fly ash, green liquor dregs, and lime mud) generated from the pulp and paper industry as sealing materials were evaluated to cover aged mine waste tailings (<1% sulfur content, primarily pyrite). The mobility of four selected trace elements (Cr, Cu, Zn, and As) was compared based on batch and column leaching studies to assess the effectiveness of these alkaline materials as sealing agents. Based on the leaching results, Cr, Cu, and Zn were immobilized by the alkaline amendments. In the amended tailings in the batch system only As dramatically exceeded the limit values at L/S 10 L/kg. The leaching results showed similar patterns to the batch results, though leached Cr, Cu, and Zn showed higher levels in the column tests than in the batch tests. However, when the columns were compared with the batches, the trend for Cu was opposite for the unamended tailings. By contrast, both batch and column results showed that the amendment caused mobilization of As compared with the unamended tailings in the ash-amended tailings. The amount of As released was greatest in the ash column and decreased from the dregs to the lime columns. The leaching of As at high levels can be a potential problem whenever alkaline materials (especially for fly ash) are used as sealing materials over tailings. The column test was considered by the authors to be a more informative method in remediation of the aged tailings with low sulfur content, since it mimics better actual situation in a field.

Combined abiotic and biotic in-situ reduction of hexavalent chromium in groundwater using nZVI and whey: A remedial pilot test

The paper describes a pilot remediation test combining two Cr(VI) geofixation methods - chemical reduction by nanoscale zero-valent iron (nZVI) and subsequent biotic reduction supported by whey. Combination of the methods exploited the advantages of both - a rapid decrease in Cr(VI) concentrations by nZVI, which prevented further spreading of the contamination and facilitated subsequent use of the cheaper biological method. Successive application of whey as an organic substrate to promote biotic reduction of Cr(VI) after application of nZVI resulted in a further and long-term decrease in the Cr(VI) contents in the groundwater. The effect of biotic reduction was observed even in a monitoring well located at a distance of 22 m from the substrate injection wells after 10 months. The results indicated a reciprocal effect of both the phases - nZVI oxidized to Fe(III) during the abiotic phase was microbially reduced back to Fe(II) and acted as a reducing agent for Cr(VI) even when the microbial density was already low due to the consumed substrate. Community analysis with pyrosequencing of the 16S rRNA genes further confirmed partial recycling of nZVI in the form of Fe(II), where the results showed that the Cr(VI) reducing process was mediated mainly by iron-reducing and sulfate-reducing bacteria.

Effect of clay minerals and nanoparticles on chromium fractionation in soil contaminated with leather factory waste

This study was conducted to investigate the effect of time, clay minerals and nanoparticles (NPs) on chromium (Cr) fractionation in a soil contaminated with leather factory waste (LFW). Soil was mixed with LFW, then, the contaminated soils were treated with clay minerals (bentonite and zeolite) and nanoparticles (MgO, TiO2 and ZnO) at 5% and 1%, respectively. The samples were incubated for 15-180 days at 25 °C and constant moisture. After incubation, Cr in control and treated soils was fractionated by the sequential extraction procedure. The distribution of various Cr fractions in control soil indicated that the greatest amounts of Cr were found in the residual fraction (RES) followed by the carbonate (CAR), organic matter (OM) and exchangeable (EXC) fractions. The addition of LFW in soils increased Cr concentration in all fractions. The higher proportion of EXC fraction in the soil treated with LFW indicates its higher potential of leaching and runoff transport. In all treated soils, the RES fraction was increased, while EXC and OM fractions were decreased during incubation. The results indicated that NPs are effective adsorbent for the removal of Cr ions from LFW treated soil, and they could be useful in reducing their environment risk.

Synthesis and Spectrophotometric Study of Toxic Metals Extraction by Novel Thio-Based Non-Ionic Surfactant

A new thio-based non-ionic surfactant 1-(3-chlorophenyl)-3-te-tradecanoylthiourea has been synthesized from potassium thiocyanate, tetradecanoyl chloride and 3-chloroanaline. The purity of the compound was characterized by techniques like 1H NMR, 13C NMR, and FT-IR. The compound was used to detect the toxic metals like copper, mercury and manganese by using UV-Visible spectrophotometric technique. The compound is able to act as a ligand as well to form micelles. Due to this the compound extracts these toxic metals in form of solubilization and complexation. The extracted metals settle down to the bottom in a water tank. The visible change in color with time proves the interaction of the compound with the metals. The compound having carbonyl as well as sulfur groups have soft corner to metal for complexation. This will be used to remove toxic metals from polluted soil and soft drinking surface and underground water. The surfactant is easily synthesized, very economical and environmentally acceptable. The CMC of the surfactant is also determined.

Adsorption behavior comparison of trivalent and hexavalent chromium on biochar derived from municipal sludge

In this work, static equilibrium experiments were conducted to distinguish the adsorption performance between the two valence states of chromium on biochar derived from municipal sludge. The removal capacity of Cr(VI) is lower than 7mg/g at the initial chromium concentration range of 50-200mg/L, whereas that of Cr(III) higher than 20mg/g. It indicates that Cr(III) is much easier to be stabilized than Cr(VI). No significant changes in the biochar surface functional groups are observed before and after the adsorption equilibrium, demonstrating the poor contribution of organic matter in chromium adsorption. The main mechanism of heavy metal adsorption by biochar involves (1) surface precipitation through pH increase caused by biochar buffer ability, and (2) exchange between cations in solution (Cd(2+)) and in biochar matrix (e.g. Ca(2+) and Mg(2+)). The reduction of Cr(VI) to Cr(III) is necessary to improve removal efficiency of chromium.

Best Combination of Promoter and Micellar Catalyst for Room Temperature Rapid Conversion of D-Lyxose to D-Lyxonic Acid in Aqueous Medium

The kinetic study of catalytic oxidation of D-lyxose by hexavalent chromium has been investigated spectrophotometrically under pseudo first order condition at temperature 313 K. The rate of oxidation of D-lyxose is very slow. Picolinic acid (PA), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) are used as promoter to accelerate the rate of the reaction. Phen acts as the most effective promoter in aqueous medium. The rate of the reaction is also increased in presence of nonionic surfactant Triton-X-100 (TX-100) and anionic surfactant sodium dodecyl sulphate (SDS). They are used as catalyst in this reaction. Thus the observed micellar effects have been explained by considering the hydrophobic and electrostatic interactions between the reactants and surfactants in terms of the proposed mechanism. However, the combination of promoter and surfactants produces a better result. The maximum rate enhancement is obtained in presence of the combination of phen and TX-100.

Potential for chromium (VI) bioremediation by the aquatic carnivorous plant Utricularia gibba L. (Lentibulariaceae)

The aquatic carnivorous plant Utricularia gibba has one of the smallest known genomes among flowering plants, and therefore, it is an excellent model organism for physiological and developmental studies. The main aim of our work was to check whether the ubiquitous U. gibba might be useful for the phytoremediation of the highly toxic and mobile hexavalent chromium in waters. Plants were incubated for 1 week in a 50 μM (2.6 mg dm(-3)) Cr(VI) solution in laboratory conditions. Our results revealed that the plant exhibits a very high accumulation capacity for Cr. The accumulation level was higher than 780 mg kg(-1) and a bioconcentration factor >300. On the other hand, the plants showed a low tolerance to the elevated Cr concentration, which was expressed in a significant decrease of the photosystem II activity. However, the most pronounced negative influence of chromate was found on the morphology and activity of the traps. Due to its high accumulation capacity, we suggest that U. gibba may be efficient in the removal of chromate over a short time scale. It can also provide a new molecular resource for studying the mechanisms of Cr(VI) detoxification.

Long-term Coexposure to Hexavalent Chromium and B[a]P Causes Tissue-Specific Differential Biological Effects in Liver and Gastrointestinal Tract of Mice

Complex mixtures of environmental agents often cause mixture-specific health effects that cannot be accounted for by a single mechanism. To study the biological effects of exposure to a mixture of chromium-VI and benzo[a]pyrene (B[a]P), often found together in the environment, we exposed mice for 60 days to 0, 55, 550, or 5500 ppb Cr(VI) in drinking water followed by 90 days of coexposure to B[a]P at 0, 1.25, 12.5, or 125 mg/kg/day and examined liver and gastrointestinal (GI) tract for exposure effects. In the liver, the mixture caused more significant histopathology than expected from the sum of effects of the individual components, while in the GI tract, Cr(VI) alone caused significant enterocyte hypertrophy and increases in cell proliferation and DNA damage that were also observed in mice coexposed to B[a]P. Expression of genes involved in drug metabolism, tumor suppression, oxidative stress, and inflammation was altered in mixed exposures relative to control and to singly exposed mice. Drug metabolism and oxidative stress genes were upregulated and tumor suppressor and inflammation genes downregulated in the proximal GI tract, whereas most markers were upregulated in the distal GI tract and downregulated in the liver. Oral exposure to Cr(VI) and B[a]P mixtures appears to have tissue-specific differential consequences in liver and GI tract that cannot be predicted from the effects of each individual toxicant. Tissue specificity may be particularly critical in cases of extended exposure to mixtures of these agents, as may happen in the occupational setting or in areas where drinking water contains elevated levels of Cr(VI).

Room Temperature Micellar Catalysis on Permanganate Oxidation of Butanol to Butanal in Aqueous Medium at Atmospheric Pressure

Oxidation kinetics of 1-butanol by permanganate have been carried out in the presence of surfactant in aqueous acidic medium to observe the micellar effect on rate. The oxidation kinetics were studied by UV–VIS spectrophotometry. The presence of cationic N-cetylpyridinium chloride (CPC) micelles was found to accelerate the reaction rate and this effect has been explained by the partitioning of the reactants in micelle and also by considering the hydrophobic and electrostatic interactions between the surfactant and reactants. The alcohol was quantitatively oxidized to the carbonyl compound, which was identified by the preparation of 2,4-DNP derivatives. The probable mechanism is proposed and pseudo-first-order rate constants are calculated. The cationic surfactant (CPC) accelerates the reaction. The critical micelle concentration (CMC) value of the CPC micelle has been determined from the plot of rate constant values at different CPC concentration.

A magnetically-separable Fe3O4nanoparticle surface grafted with polyacrylic acid for chromium(iii) removal from tannery effluents

A magnetically-separable nanoadsorbent was synthesized, and evaluated as an alternative for chromium(iii) removal from tannery effluent.

Surfactant Assisted Enhancement of Bioremediation Rate for Hexavalent Chromium by Water Extract of Siris (Albizia lebbeck) Sawdust

Cr(VI) is introduced into environment as a byproduct of industries. It is highly toxic. Biosorption of hexavalent chromium by various types of sawdust appears as a very cost-effective alternative for decontamination of Cr(VI) bearing effluents. In this work water extract of siris sawdust is used for the bioremediation of hexavalent chromium. Cr(VI) ions were reduced to Cr(III) ions as a result of oxidation of organic components present in the water extract of siris sawdust. Formation of Cr(III) is proved by UV-VIS spectroscopy. Functional groups involved in the reduction of Cr(VI) are characterized by FTIR spectroscopy. Bioremediation rate is increased by the use of anionic surfactant sodium dodecylsulphate (SDS) and neutral surfactant Triton-X-100 (TX-100). Here they act as micellar catalyst. Formation of micelles which is responsible for the catalysis of the process is proved by SEM and optical images of the solution. In absence of surfactants 39 % of the total chromium(VI) is reduced within 531 h whereas removal percentage increases upto 54 % in presence of TX-100. Again in presence of SDS the reduction process is almost 99 % complete within 531 h.