| Arsenic(As) is a toxic metalloid element ubiquitous in water, soil and air, and is considered as one of the primary pollutants. World Health Organization(WHO), U.S. Environmental Protection Agency(EPA) and other organizations report As as a severe carcinogenic, teratogenic and mutagenic substance. Since As pollution occurred frequently in recent years, the prevention, treatment and control of As has aroused high attention and become a research hotspot worldwide. The toxicity of As is largely dependent on its forms. Its toxicity and migration in the nature can be efficiently reduced through adsorption, oxidation, reduction and methylation of As. Iron oxide and iron hydroxide minerals, with very high specific surface area and strong oxidizability, can achieve these purposes through adsorption, oxidization and coprecipitation of As in media. These minerals are ideal materials for treatment of As-bearing soil and wastewater. In the nature, the migration and conversion of As are affected by the oxidation-reduction of Fe. The Fe form transformation by microorganisms also affects the form changes and environmental behaviors of As. So far, relevant research is focused on the separation of microbial strains from As-polluted environment, and the adsorption, oxidation and coprecipitation of As by iron oxides. However, there is few research about the correlations between the microbial-mediated Fe oxidation- reduction and As oxidation-reduction in the environment.Here we studied the adsorption dynamics and adsorption quantities of two As forms [As(Ⅲ), As(V)] by three Fe minerals(lepidocrocite, ferrihydrite and magnetite) under varying pH, as well as the pH changes during the adsorption, coprecipitation and reaction of Fe and As. Scanning electron microscopy(SEM) and X-ray diffractometry(XRD) were used to characterize the three iron minerals before and after reactions. The adsorption and coprecipitation of As(Ⅲ), As(V) by the three iron minerals were comprehensively investigated. Under laboratory conditions, we simulated(1) how natural As-bearing iron minerals(arsenopyrite and scorodite) would affect the growth of typical iron-reducing bacteria Shewanella oneidensis MR-1 and Shewanella sp. strain MR-4;(2) the As- and Fe-release dynamics of arsenopyrite and scorodite under microbial activities;(3) the dissolution-adsorption-coprecipitation of Fe and As released during reactions and the underlying mechanisms. Liquid Chromatography- Atomic Fluorescence Spectrometry(LC-AFS), SEM and XRD were used to systematically measure and characterize the aqueous concentrations of Fe, As(Ⅲ) and As(V), and the form changes of microorganisms and minerals before and after reactions. The findings will provide direct evidences underlying how iron-reducing bacteria are involved in the oxidation-reduction of As in natural water and soil ecosystems, and offer a new method for treatment of As pollution. The main contents and results are listed below.1. The dynamics and influence factors on the adsorption of As(Ⅲ) and As(V) by the three iron minerals were studied. The three iron minerals all could well absorb As(Ⅲ) and As(V) from solutions, but the absorption abilities differ among three iron minerals. The kinetic data were well fitted by Lagergren quasi-second-order kinetic equations. The aqueous pH, HPO42- concentration and HA concentration significantly affected each iron mineral in absorption of As(Ⅲ) and As(V).The adsorption of As(Ⅲ) or As(V) by iron minerals was balanced after 24 h of reaction at initial pH 7.0, dosage of 5.0 g/L, and initial concentrations of As(Ⅲ) or As(V) of 200 mg/L. The absorption ability ranks by mineral type as follows: ferrihydrite-As(Ⅲ) > magnetite-As(Ⅲ)> ferrihydrite-As(V) >lepidocrocite-As(V) > lepidocrocite-As(Ⅲ) > magnetite-As(V). The equilibrium adsorption quantities(mg/g) by ferrihydrite, magnetite and lepidocrocite are 39.422, 29.230, 14.488 [As(Ⅲ)], respectively, and 15.764, 9.216, 13.32 [As(V)], respectively. At initial pH = 3.0, 5.0, 7.0, 9.0 and 11.0, the equilibrium adsorption quantities(mg/g) of As(Ⅲ) are 35.47, 39.49, 39.48, 39.27, 30.50, respectively(ferrihydrite); 17.79, 29.57, 29.36, 29.16 and 22.04, respectively(magnetite); 12.14, 17.56, 15.17, 13.23 and 10.33, respectively(lepidocrocite). The equilibrium adsorption quantities(mg/g) of As(V) are 39.86, 26.66, 18.77, 12.21, 9.77, respectively(ferrihydrite); 31.09, 22.66, 12.96, 8.22 and 6.17, respectively(magnetite); 13.38, 21.17, 16.83, 10.17 and 7.94, respectively(lepidocrocite). Because of similar adsorption sites, HPO42- will complete with As O2- and As O43-, so the equilibrium adsorption quantities of As(Ⅲ) and As(V) by any iron mineral dropped with the gradual increase of phosphate concentration. With the increase of humic acid(HA) concentration, the equilibrium adsorption quantities of As(Ⅲ) and As(V) both declined gradually. When HA concentration was 1 mg/L, iron minerals could not largely adsorb As(Ⅲ) or As(V).2. The leaching kinetics and influence factors of As(Ⅲ) and As(V) in iron mineral-loaded quartz sand columns were studied. During the leaching process, the quartz sand columns reserved As ions to different degrees depending on the type of iron mineral. The reservation abilities of quartz sand columns were significantly affected by the pH, HPO42- concentration, and HA concentration in the leaching stock solutions.At pH =7.0, initially 10 mg/L As(Ⅲ), Na Cl(ionic strength 0.1 mol/L) as the supporting electrolyte, and dosage of iron mineral = 1 g/100 g quartz sand, the As reserving ability of packed columns ranked by the type of iron mineral as follows: ferrihydrite > magnetite > lepidocrocite, which is consistent with the preliminary adsorption experiments. When pH was changed, the As reservation ability by ferrihydrite-loaded quartz sand column ranked as: pH 5.0 > 7.0 > 9.0 > 11.0 > 3.0 [As(Ⅲ)]; pH 3.0 > 7.0 > 5.0 > 9.0 > 11.0 [As(V)]. As the phosphate concentration in the leaching solutions gradually increased, the reservation abilities of ferrihydrite-loaded quartz sand column over As(Ⅲ) and As(V) were both obviously reduced. The reservation abilities of ferrihydrite-loaded quartz sand column over As(Ⅲ) and As(V) were efficiently enhanced by adding a small amount of HA, but were hindered by addition of excessive HA.3. The oxidation of As(Ⅲ) by three iron minerals separately and the reduction of As(V) by pyrite or siderite were studied. Results show the three iron minerals all could oxidize a part of high-toxicity As(Ⅲ) to low-toxicity As(V). Pyrite and siderite could both reduce a part of As(V) into As(Ⅲ).At the dosage of each iron mineral = 5 g/L, initial As(Ⅲ) concentration = 200 mg/L, and initial pH = 7.0, the As(Ⅲ) concentration gradually dropped with time, but the As(V) concentration increased, reaching equilibrium at d 21. The equilibrium concentrations(mg/L) of As(Ⅲ) and As(V) were 1.33 vs. 1.41(ferrihydrite); 33.47 vs. 14.65(magnetite); 74.42 vs. 18.43(lepidocrocite).At the dosage of pyrite or siderite = 5 g/L, initial As(V) concentration = 10 mg/L, and initial pH = 7.0, the of As(Ⅲ) concentration first increased and then dropped with time, but the As(V) first declined and then increased.4. The reductive dissolution of natural As-bearing iron minerals(arsenopyrite and scorodite) by iron-reducing bacteria(Shewanella oneidensis MR-1 and Shewanella sp. strain MR-4), as well as the As re-release mechanisms were studied. Results show the metal release abilities over natural As-bearing iron minerals are different between bacteria. MR-1 is more effective at releasing As and Fe from scorodite versus arsenopyrite; the Fe concentration in the scorodite-treated solutions increased with the rise of As concentration. MR-4 shows opposite results to MR-1.At the dosage of arsenopyrite or scorodite = 2.5 g/L, inoculation dosage of iron-reducing bacterium = 5.0E+8 cfu/m L, and oscillation incubator temperature at 30 oC, the As and Fe concentrations under action of either MR-1 or MR-4 both gradually increased and became balanced at d 14. The balanced total As and total Fe concentrations(mg/L) in the MR-1-treated solutions are 1.19 vs. 1.07(arsenopyrite), and 3.73 vs. 19.51(scorodite). The balanced total As and total Fe concentrations(mg/L) in the MR-4-treated solutions are 25.88 vs. 9.13(arsenopyrite), and 0.58 vs. 6.33(scorodite). When environmental temperature, pH in liquid medium, exotic Fe(Ⅲ) concentration, or dosage of methyl donor was changed, the reductive dissolution abilities were different between MR-1 and MR-4 and between arsenopyrite and scorodite. After 14-d culture, the concentrations(mg/L) of As(Ⅲ), As(V), monomethylated arsenic(MMA) and dimethylated arsenic(DMA) in MR-1 inoculated solutions are 0.189, 1.332, 0.321, 0.0835(20 oC); 0.265, 3.670, 0.402, 0.175(30 oC); 0.2323, 2.234, 0.303, 0.1402(40 oC). The concentrations(mg/L) of As(Ⅲ), As(V), MMA and DMA in MR-4 inoculated solutions are 5.392, 10.049, 1.988, 0.988(20 oC), 8.480, 22.383, 4.710, 1.610(30 oC); 7.988, 18.880, 3.440, 1.550(40 oC). Generally the reductive dissolution and methylation efficiencies of iron-reducing bacteria over natural As-bearing iron minerals change with pH as follows: pH 7.0 > 8.0 > 6.0. In particular, the reductive dissolution ability of MR-4 over arsenopyrite is significantly higher than that of MR-1 over scorodite. The addition of external Fe(Ⅲ) significantly enhanced the reductive dissolution and methylation abilities of bacteria. The concentrations of MMA and DMA are different depending on the type of methyl donor. Generally the productions of methylated As change with methyl donor as follows: B12> glutathione(GSH) > S-adenosylmethione(SAM). XRD shows several new peaks at some As positions under the action of MR-1 on scorodite or the action of MR-4 on arsenopyrite, and the intensities and areas of these peaks are both enhanced with time. |
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