Prolonged exposure to particulate Cr(VI) is cytotoxic and genotoxic to fin whale cells

BackgroundHexavalent chromium [Cr(VI)] is a human lung carcinogen and global marine pollutant. High Cr concentrations, resembling the ones observed in occupationally exposed workers, have been observed in fin whales (Balaenoptera physalus) in the Gulf of Maine. This outcome suggests Cr might be disrupting the health of fin whale populations. Indeed, Cr in acute (24 h) exposure does cause toxicity in fin whale cells. However, human cell culture data indicate prolonged exposures (120 h) induce a higher amount of toxicity compared to 24 h exposure due to an inhibition of homologous recombination repair. However, whether prolonged exposure causes similar outcomes in fin whale cells is unknown.ObjectiveDue to the importance of assessing prolonged exposure toxicity, this study focuses on characterizing acute and prolonged exposure of Cr(VI) in male and female fin whale cells.MethodsCytotoxicity was measured by the clonogenic assay, also known as colony forming assay, which measures the ability of cells to proliferate and form colonies after the treatment. DNA double strand breaks were analyzed by neutral comet assay. Clastogenicity was measured using the chromosome aberration assay. Intracellular Cr levels were measured with Graphite Furnace Atomic Absorption Spectrometry (GFAAS) with Syngistix Software.ResultsIn this study, we demonstrate that particulate Cr(VI) induces cytotoxicity and genotoxicity in a treatment-dependent manner after 24 h and 120 h exposures. Cytotoxicity levels were generally low with relative survival above 64 %. DNA double strand break data and chromosome aberration data were elevated after a 24 h exposure, but decreased after a 120 h exposure. While cytotoxicity was similar after 24 h and 120 h exposures, less DNA double strand breaks and chromosomal instability occurred with prolonged exposure.ConclusionParticulate Cr(VI) is cytotoxic and genotoxic to fin whale cells after acute and prolonged exposures. The reduction of genotoxicity we have observed after 120 h exposure may be partly explained by lower intracellular Cr levels after 120 h. However, the decrease in intracellular levels is not reflected by a similar decrease in chromosome aberrations suggesting other mechanisms may be at play. Male fin whale cells appear to be more susceptible to the genotoxic effects of particulate Cr(VI) while female cells are less susceptible possibly due to increased cell death of damaged cells, but more work is needed to clarify if this outcome reflects a sex difference or interindividual variability. Overall, the study shows particulate Cr(VI) does induce toxicity at both acute and prolonged exposures in fin whales cells indicating Cr(VI) exposure is a health risk for this species.     

Influence of Cr(III) and Cr(VI) on the interaction between sparfloxacin and calf thymus DNA

Cr(III) and Cr(VI) have different binding capacity with sparfloxacin, and have different combination modes with calf thymus DNA. Selecting these two different metal ions, the influence of them on the binding constants between sparfloxacin (SPFX) and calf thymus DNA, as well as the related mechanism has been studied by using absorption and fluorescence spectroscopy. The result shows that Cr(III) has weaker binding capacity to SPFX in the SPFX-Cr(III) binary system, but influences the binding between SPFX and DNA obviously in SPFX-DNA-Cr(III) ternary system. However, although Cr(VI) has a stronger binding capacity to SPFX, it has no effect on the binding between SPFX and DNA. Referring to the different modes of Cr(III) and Cr(VI) binding to DNA, the mechanism of the influence of metal ions on the binding between SPFX and DNA has been proposed. SPFX can directly bind to DNA by chelating DNA base sites. If a metal ion at certain concentration binds mainly to DNA bases, it can decrease the binding constants between SPFX and DNA through competing with SPFX. While if a metal ion at certain concentration mainly binds to phosphate groups of DNA, it can increase the binding constants by building a bridge between SPFX and DNA. If a metal ion at certain concentrations binds neither to bases nor phosphate groups in DNA, it will have no effect on the binding constant between SPFX and DNA. Our result supports Palumbo's conclusion that the binding between SPFX and the phosphata groups is the precondition for the combination between SPFX and DNA, which is stabilized through stacking interactions between the condensed rings of SPFX and DNA bases.     

The clastogenic effects of chronic exposure to particulate and soluble Cr(VI) in human lung cells

Hexavalent chromium (Cr(VI)) is a well-designated human lung carcinogen, with solubility playing an important role in its carcinogenic potential. Although it is known that particulate or water-insoluble Cr(VI) compounds are more potent than the soluble species of this metal, the mechanisms of action are not fully elucidated. In this study, we investigated the hypothesis that the difference in potency between particulate and soluble Cr(VI) is due to more chronic exposures with particulate chromate because it can deposit and persist in the lungs while soluble chromate is rapidly cleared. Chronic exposure to both insoluble lead chromate and soluble sodium chromate induced a concentration and time-dependent increase in intracellular Cr ion concentrations in cultured human lung fibroblasts. Intracellular Pb levels after chronic exposure to lead chromate increased in a concentration-dependent manner but did not increase with longer exposure times up to 72 h. We also investigated the effects of chronic exposure to Cr(VI) on clastogenicity and found that chronic exposure to lead chromate induces persistent or increasing chromosome damage. Specifically, exposure to 0.5 microg/cm(2) lead chromate for 24, 48 and 72 h induced 23, 23 and 27% damaged metaphases, respectively. Contrary to lead chromate, the amount of chromosome damage after chronic exposure to sodium chromate decreased with time. For example, cells exposed to 1 microM sodium chromate for 24, 48 and 72 h induced 23, 13 and 17% damaged metaphases, respectively. Our data suggest a possible mechanism for the observed potency difference between soluble and insoluble Cr(VI) compounds is that chronic exposure to particulate Cr(VI) induces persistent chromosome damage and chromosome instability while chromosome damage is repaired with chronic exposure to soluble Cr(VI).     

The clastogenic effects of chronic exposure to particulate and soluble Cr(VI) in human lung cells

Hexavalent chromium (Cr(VI)) is a well-designated human lung carcinogen, with solubility playing an important role in its carcinogenic potential. Although it is known that particulate or water-insoluble Cr(VI) compounds are more potent than the soluble species of this metal, the mechanisms of action are not fully elucidated. In this study, we investigated the hypothesis that the difference in potency between particulate and soluble Cr(VI) is due to more chronic exposures with particulate chromate because it can deposit and persist in the lungs while soluble chromate is rapidly cleared. Chronic exposure to both insoluble lead chromate and soluble sodium chromate induced a concentration and time-dependent increase in intracellular Cr ion concentrations in cultured human lung fibroblasts. Intracellular Pb levels after chronic exposure to lead chromate increased in a concentration-dependent manner but did not increase with longer exposure times up to 72 h. We also investigated the effects of chronic exposure to Cr(VI) on clastogenicity and found that chronic exposure to lead chromate induces persistent or increasing chromosome damage. Specifically, exposure to 0.5 μg/cm² lead chromate for 24, 48 and 72 h induced 23, 23 and 27% damaged metaphases, respectively. Contrary to lead chromate, the amount of chromosome damage after chronic exposure to sodium chromate decreased with time. For example, cells exposed to 1 μM sodium chromate for 24, 48 and 72 h induced 23, 13 and 17% damaged metaphases, respectively. Our data suggest a possible mechanism for the observed potency difference between soluble and insoluble Cr(VI) compounds is that chronic exposure to particulate Cr(VI) induces persistent chromosome damage and chromosome instability while chromosome damage is repaired with chronic exposure to soluble Cr(VI).     

Environmental and Kinetic Parameters for Cr(VI) Bioreduction by a Bacterial Monoculture Purified from Cr(VI)-Resistant Consortium

Hexavalent chromium, Cr(VI), is toxic to living systems. Widespread contamination of water and soil by Cr(VI) present a serious public health problem. Chromium-resistant bacteria can reduce and detoxify Cr(VI). Twelve bacteria resistant to high concentrations of Cr(VI) were isolated from soil enrichment cultures. Environmental parameters and kinetic parameters of Cr(VI) bioreduction by one monoculture isolate, identified by 16S rRNA gene sequence as Bacillus sp. PB2, were studied. The optimal temperature for growth and Cr(VI) reduction was 35 degrees C. The isolate grew luxuriantly and substantially reduced Cr(VI) at initial pH 7.5 to 9. Maximal Cr(VI) bioreduction occurred at initial pH 8.0. Substantial Cr(VI) bioreduction was observed in salt media, but removal efficiency was inversely related to salt concentration (1-9%). Michaelis-Menten hyperbolic equation and the Lineweaver-Burk double reciprocal plot were comparatively employed to determine the k (m) and V (max) of Cr(VI) bioreduction. A k (m) of 82.5 microg mL(-1) and V (max) of 7.78 microg mL(-1) h(-1) were calculated by nonlinear regression analysis of the hyperbola curve. Linear regression analysis of the double reciprocal plot revealed k (m) and V (max) of 80.9 microg mL(-1) and 10.6 microg mL(-1) h(-1), respectively. Time course studies displayed about 90% reduction of Cr(VI) at an initial concentration of 8,000 microg L(-1) in 8 h, with an estimated t (1/2) of 4 h. Data from time course analysis of the rate of Cr(VI) bioreduction fitted zero-order model, and the kinetic constant k was calculated to be 840 microg L(-1) h(-1). The monoculture isolate, Bacillus sp. PB2, strongly reduces Cr(VI) and could be used for bioremediation of Cr(VI)-contaminated aquatic and terrestrial environments.     

Kinetics of Cr(III) and Cr(VI) removal from water by two floating macrophytes

The aim of this work was to compare Cr(III) and Cr(VI) removal kinetics from water by Pistia stratiotes and Salvinia herzogii. The accumulation in plant tissues and the effects of both Cr forms on plant growth were also evaluated. Plants were exposed to 2 and 6 mg L^?1 of Cr(III) or Cr(VI) during 30 days. At the end of the experiment, Cr(VI) removal percentages were significantly lower than those obtained for Cr(III) for both macrophytes. Cr(III) removal kinetics involved a fast and a slow component. The fast component was primarily responsible for Cr(III) removal while Cr(VI) removal kinetics involved only a slow process. Cr accumulated principally in the roots. In the Cr(VI) treatments a higher translocation from roots to aerial parts than in Cr(III) treatments was observed. Both macrophytes demonstrated a high ability to remove Cr(III) but not Cr(VI). Cr(III) inhibited the growth at the highest studied concentration of both macrophytes while Cr(VI) caused senescence. These results have important implications in the use of constructed wetlands for secondary industrial wastewater treatment. Common primary treatments of effluents containing Cr(VI) consists in its reduction to Cr(III). Cr(III) concentrations in these effluents are normally below the highest studied concentrations in this work.     

Non-homologous DNA end joining in normal and cancer cells and its dependence on break structures

DNA double-strand breaks (DSBs) are a serious threat to the cell, for if not or miss-repaired, they can lead to chromosomal aberration, mutation and cancer. DSBs in human cells are repaired via non-homologous DNA end joining (NHEJ) and homologous recombination repair pathways. In the former process, the structure of DNA termini plays an important role, as does the genetic constitution of the cells, through being different in normal and pathological cells. In order to investigate the dependence of NHEJ on DSB structure in normal and cancer cells, we used linearized plasmids with various, complementary or non-complementary, single-stranded or blunt DNA termini, as well as whole-cell extract isolated from normal human lymphocytes, chronic myeloid leukemia K562 cells and lung cancer A549 cells. We observed a pronounced variability in the efficacy of NHEJ reaction depending on the type of ends. Plasmids with complementary and blunt termini were more efficiently repaired than the substrate with 3' protruding single-strand ends. The hierarchy of the effectiveness of NHEJ was on average, from the most effective to the least, A549/ normal lymphocytes/ K562. Our results suggest that the genetic constitution of the cells together with the substrate terminal structure may contribute to the efficacy of the NHEJ reaction. This should be taken into account on considering its applicability in cancer chemo- or radiotherapy by pharmacologically modulating NHEJ cellular responses.     

DNA damage checkpoints in stem cells, ageing and cancer

DNA damage induces cell-intrinsic checkpoints, including p53 and retinoblastoma (RB), as well as upstream regulators (exonuclease 1 (EXO1), ataxia telangiectasia mutated (ATM), ATR, p16(INK4a) and p19(ARF)) and downstream targets (p21, PUMA (p53 upregulated modulator of apoptosis) and sestrins). Clearance of damaged cells by cell-intrinsic checkpoints suppresses carcinogenesis but as a downside may impair stem cell and tissue maintenance during ageing. Modulating the activity of DNA damage checkpoints can either accelerate or decelerate tissue ageing and age-related carcinogenesis. The outcome depends on cell-intrinsic and cell-extrinsic mechanisms that regulate the clearance of damaged cells and on the molecular context in ageing tissues, including the level of DNA damage accumulation itself.     

Addition of DNA to Cr(VI) and cytochrome b5 containing proteoliposomes leads to generation of DNA strand breaks and Cr(III) complexes

Chromium (Cr) is a cytotoxic metal that can be associated with a variety of types of DNA damage, including Cr-DNA adducts and strand breaks. Prior studies with purified human cytochrome b(5) and NADPH:P450 reductase in reconstituted proteoliposomes (PLs) demonstrated rapid reduction of Cr(VI) (hexavalent chromium, as CrO(4)(2-), and the generation of Cr(V), superoxide (O(2)(*-)), and hydroxyl radical (HO(*)). Studies reported here examined the potential for the species produced by this system to interact with DNA. Strand breaks of purified plasmid DNA increased over time aerobically, but were not observed in the absence of O(2). Cr(V) is formed under both conditions, so the breaks are not mediated directly by Cr(V). The aerobic strand breaks were significantly prevented by catalase and EtOH, but not by the metal chelator diethylenetriaminepentaacetic acid (DTPA), suggesting that they are largely due to HO(*) from Cr-mediated redox cycling. EPR was used to assess the formation of Cr-DNA complexes. Following a 10-min incubation of PLs, CrO(4)(2-), and plasmid DNA, intense EPR signals at g=5.7 and g=5.0 were observed. These signals are attributed to specific Cr(III) complexes with large zero field splitting (ZFS). Without DNA, the signals in the g=5 region were weak. The large ZFS signals were not seen, when Cr(III)Cl(3) was incubated with DNA, suggesting that the Cr(III)-DNA interactions are different when generated by the PLs. After 24 h, a broad signal at g=2 is attributed to Cr(III) complexes with a small ZFS. This g=2 signal was observed without DNA, but it was different from that seen with plasmid. It is concluded that EPR can detect specific Cr(III) complexes that depend on the presence of plasmid DNA and the manner in which the Cr(III) is formed.     

Enhanced clonogenic survival induced by protein tyrosine phosphatase (PTP) inhibition after Cr(VI) exposure is mediated by c-Raf and Ras activity

Our recent studies showed that maintenance of protein tyrosine phosphorylation by PTP inhibition enhanced cell growth, clonogenic survival, and mutagenesis after a single low-level Cr(VI) exposure, thereby suggesting that tyrosine phosphorylation-dependent signaling may govern inappropriate survival in human lung fibroblasts (HLFs). Our goal is to identify specific phospho-tyrosine regulator(s)/ downstream effectors involved in enhanced survival after Cr(VI) exposure and PTP inhibition. Phosphotyrosine profiling array showed that PTP inhibition following Cr(VI) exposure increased tyrosine phosphorylation of specific proteins, such as FGR and ABL, which are upstream regulators of both Erk and Akt pathways. To explore the roles of these pathways in the PTP-induced increase in clonogenic survival after Cr(VI) exposure, we examined the effect of combined Akt1 and Erk1/2 knockdown via siRNA technology. Akt1 and/or Erk1/2 silencing had no effect on the PTP inhibitor-induced increase in survival following Cr(VI) exposure, suggesting the presence of non-Akt/non-Erk-mediated survival signaling. Interestingly, geldanamycin, an HSP90 inhibitor and non-specific Raf inhibitor, abrogated the PTP inhibitor-mediated increase in survival following Cr(VI) exposure and abolished the expression/activity of c-Raf and activity of Mek. These findings prompted us to explore upstream regulators of Erk, i.e., Ras, c-Raf and Mek for their potential roles in clonogenic survival. GW5074, a specific c-Raf kinase inhibitor did not alter the effect of the PTP inhibitor but decreased Cr(VI)-mediated clonogenic lethality, potentially though Mek hyperactivation. A genetic approach with a c/a Mek1 mutant also showed that Mek activity was not directly associated with the PTP inhibitor effect. Finally, a genetic approach with d/n or c/a Ras and c-Raf mutants, showed that Ras and c-Raf activities play a substantive role in enhancing clonogenic survival by PTP inhibition following Cr(VI) insult. In conclusion, these studies highlight a novel pro-survival mechanism for clonogenic survival in the face of genotoxic stress in the presence of PTP inhibition via an Erk/Mek-independent and Ras/c-Raf-dependent regulation in normal human lung fibroblasts.     

FANCD2 monoubiquitination and activation by hexavalent chromium [Cr(VI)] exposure: Activation is not required for repair of Cr(VI)-induced DSBs

Fanconi anemia (FA) is a rare autosomal recessive disorder characterized by congenital abnormalities, progressive bone marrow failure, and cancer susceptibility. FA cells are hypersensitive to DNA crosslinking agents. FA is a genetically heterogeneous disease with at least 11 complementation groups. The eight cloned FA proteins interact in a common pathway with established DNA-damage-response proteins, including BRCA1 and ATM. Six FA proteins (A, C, E, F, G, and L) regulate the monoubiquitination of FANCD2 after DNA damage by crosslinking agents, which targets FANCD2 to BRCA1 nuclear foci containing BRCA2 (FANCD1) and RAD51. Some forms of hexavalent chromium [Cr(VI)] are implicated as respiratory carcinogens and induce several types of DNA lesions, including DNA interstrand crosslinks. We have shown that FA-A fibroblasts are hypersensitive to both Cr(VI)-induced apoptosis and clonogenic lethality. Here we show that Cr(VI) treatment induced monoubiquitination of FANCD2 in normal human fibroblasts, providing the first molecular evidence of Cr(VI)-induced activation of the FA pathway. FA-A fibroblasts demonstrated no FANCD2 monoubiquitination, in keeping with the requirement of FA-A for this modification. We also found that Cr(VI) treatment induced significantly more S-phase-dependent DNA double strand breaks (DSBs), as measured by γ-H2AX expression, in FA-A fibroblasts compared to normal cells. However, and notably, DSBs were repaired equally in both normal and FA-A fibroblasts during recovery from Cr(VI) treatment. While previous research on FA has defined the genetic causes of this disease, it is critical in terms of individual risk assessment to address how cells from FA patients respond to genotoxic insult.     

FANCD2 monoubiquitination and activation by hexavalent chromium [Cr(VI)] exposure: activation is not required for repair of Cr(VI)-induced DSBs

Fanconi anemia (FA) is a rare autosomal recessive disorder characterized by congenital abnormalities, progressive bone marrow failure, and cancer susceptibility. FA cells are hypersensitive to DNA crosslinking agents. FA is a genetically heterogeneous disease with at least 11 complementation groups. The eight cloned FA proteins interact in a common pathway with established DNA-damage-response proteins, including BRCA1 and ATM. Six FA proteins (A, C, E, F, G, and L) regulate the monoubiquitination of FANCD2 after DNA damage by crosslinking agents, which targets FANCD2 to BRCA1 nuclear foci containing BRCA2 (FANCD1) and RAD51. Some forms of hexavalent chromium [Cr(VI)] are implicated as respiratory carcinogens and induce several types of DNA lesions, including DNA interstrand crosslinks. We have shown that FA-A fibroblasts are hypersensitive to both Cr(VI)-induced apoptosis and clonogenic lethality. Here we show that Cr(VI) treatment induced monoubiquitination of FANCD2 in normal human fibroblasts, providing the first molecular evidence of Cr(VI)-induced activation of the FA pathway. FA-A fibroblasts demonstrated no FANCD2 monoubiquitination, in keeping with the requirement of FA-A for this modification. We also found that Cr(VI) treatment induced significantly more S-phase-dependent DNA double strand breaks (DSBs), as measured by gamma-H2AX expression, in FA-A fibroblasts compared to normal cells. However, and notably, DSBs were repaired equally in both normal and FA-A fibroblasts during recovery from Cr(VI) treatment. While previous research on FA has defined the genetic causes of this disease, it is critical in terms of individual risk assessment to address how cells from FA patients respond to genotoxic insult.     

Effects of Contaminant Concentration,Aging, and Soil Properties on the Bioaccessibility of Cr(III) and Cr(VI) in Soil

Contaminated soils at numerous U.S. Department of Defense, Department of Energy, and other industrial facilities often contain huge inventories of toxic metals such as chromium. Ingestion of soil by children is often the primary risk factor that drives the need for remediation. Site assessments are typically based solely on total soil-metal concentrations and do not consider the potential for decreased bioaccessibility due to metal sequestration by soil. The objectives of this research are to investigate the effect of soil properties on the bioaccessibility of Cr(III) and Cr(VI) as a function of contaminant concentration and aging. The A and upper B horizons of two well-characterized soils, representative of Cr-contaminated soils in the southeastern United States, were treated with varying concentration of Cr(III) and Cr(VI) and allowed to age. The bioaccessibility of the contaminated soils was measured over a 200-d time period using a physiologically based extraction test (PBET) that was designed to simulate the digestive process of the stomach. The sorption of Cr(III) and Cr(VI) varied significantly as a function of soil type and horizon, and the oxidation state of the contaminant. Solid phase concentrations with Cr(III) were significantly greater than Cr(VI) for any given initial Cr concentration. This is consistent with the mechanisms of Cr(III) vs. Cr(VI) sequestration by the soils, where the formation of Cr(III)-hydroxides can result in the accumulation of large mass fractions of contaminant on mineral surfaces. Overall, Cr bioaccessibility decreased with duration of exposure for all soils and at all solid phase concentrations, with aging effects being more pronounced for Cr(III). The decrease in Cr bioaccessibility was rapid for the first 50 d and then slowed dramatically between 50 and 200 d. In general, the effects of Cr solid phase concentration on bioaccessibility was small, with Cr(III) showing the most pronounced effect; higher solid phase concentrations resulted in a decrease in bioaccessibility. Chemical extraction methods and X-ray Adsorption Spectroscopy analyses suggested that the bioaccessibility of Cr(VI) was significantly influenced by reduction processes catalyzed by soil organic carbon. Soils with sufficient organic carbon had lower Cr bioaccessibility values (～10 to 20%) due to an enhanced reduction of Cr(VI) to Cr(III). In soils where organic carbon was limited and reduction processes were minimal, the bioaccessibility of Cr(VI) dramatically increased (～60 to 70%).     

Isolation of a novel chromium(III) binding protein from bovine liver tissue after chromium(VI) exposure

In the ongoing investigation into the biological importance and toxicity issues surrounding the bioinorganic chemistry of chromium, the accepted literature procedure for the isolation of the biological form of chromium, low molecular weight chromium binding protein (LMWCr) or chromodulin, was investigated for its specificity. When chromium(VI) is added to bovine liver homogenate, results presented here indicate at least four chromium(III) binding peptides and proteins are produced and that the process is non-specific for the isolation of LMWCr. A novel trivalent chromium containing protein (1) has been isolated to purity and initial characterization is reported here. Chromium(III) identification was determined by optical spectroscopy and diphenylcarbazide testing. This chromium binding protein has a molecular weight of 15.6kDa, which was determined from both gel-electrophoresis and mass spectrometry. The protein is comprised primarily of Asx, Glx, His, Gly/Thr, Ala, and Lys in a 1.00:2.51:0.37:2.09:0.39:1.17 ratio and is anionic at pH 7.4. In addition, the protein binds approximately 2.5 chromium(III) ions per molecule.     

The release of bystander factor(s) from tissue explant cultures of rainbow trout (Onchorhynchus mykiss) after exposure to gamma radiation

The bystander response has been documented in cell lines and cell cultures derived from aquatic species over the past several years. However, little work has been undertaken to identify a similar bystander response in tissue explant cultures from fish. In this study, indirect effects of ionizing gamma radiation on tissue explant cultures of fish were investigated. Tissue explants in culture were exposed to 0.5 Gy and 5 Gy gamma radiation from a 60Co teletherapy unit. A bystander response in Epithelioma papulosum cyprini (EPC) cells exposed to gamma-irradiated tissue conditioned medium from rainbow trout explants was investigated, and the effects on cell survival were quantified by the clonogenic survival assay. Dichlorofluorescein and rhodamine 123 fluorescent dyes were used to identify alterations in reactive oxygen species (ROS) and mitochondrial membrane potential (MMP), respectively. Results indicate a different response for the three tissue types investigated. Clonogenic assay results vary from a decrease in cell survival (gill) to no effect (skin) to a stimulatory effect (spleen). Results from fluorescence assays of ROS and MMP show similarities to clonogenic assay results. This study identifies a useful model for further studies relating to the bystander effect in aquatic organisms in vivo and ex vivo.     

Immobilized chitosan as biosorbent for the removal of Cd(II), Cr(III) and Cr(VI) from aqueous solutions

The generation of layer-by-layer silicate-chitosan composite biosorbent was studied. The films were evaluated on its stability regarding the polymer leakage and its capability in the removal of Cd(II), Cr(III) and Cr(VI) from an aqueous solution. SEM, EDAX and ATR-IR techniques were applied for material characterization. Silicate-chitosan films with a final layer of silicate demonstrated chitosan retention and had better sorption capacities than those without it. For metal species, such as Cd(II) and Cr(III), the greatest adsorption was obtained when the pH of the solution was 7. When Cr(VI) was evaluated, pH 4 was the optimal for its adsorption. Langmuir and Freundlich isotherms were modeled for the equilibrium data. An 80% of the adsorbed metal was recovered by HNO(3) incubation. This non-covalent immobilization method allowed chitosan surface retention and did not affect its adsorption properties. The use of a coated surface would facilitate sorbent removal from medium after adsorption.