Retention of inorganic arsenic by coryneform mutant strains

The natural resistance mechanisms of corynebacteria to respond to the environments containing high levels of arsenic were successfully adopted to develop inexpensive and selective extractants for submicrogram amounts of arsenic. Kinetic and equilibrium characteristics were evaluated, and a preliminary exploration of the capability of these strains to be used for arsenic speciation was also made in this work. Three kinetics models were used to fit the experimental data. It was found that the pseudo-first-order kinetics model was not quite adequate to describe the retention process, while the intraparticle diffusion and the pseudo-second-order kinetics models provide the best fits. The equilibrium isotherm showed that the retention of arsenic was consistent with the Langmuir equation and that the Freundlich and Dubinin-Radushkevich models provided poorer fits to the experimental data. The maximum effective retention capacity for arsenic was about 15.4 ng As/mg biomass. The amount of arsenic retained was directly measured in the biomass by forward planning a slurry electrothermal atomic absorption spectrometric procedure.     

纤维素吸附剂的制备及其含砷水吸附性能

基于生物吸附法快速、经济、环境友好等特性以及纤维素量大易得的特点,同时为了顺应我国《生活饮用水卫生标准》(GB5749-2006)中砷含量0.01 mg/L的要求,选取麦糟作为吸附剂原料,采用了NaOH预处理-环氧氯丙烷交联-三甲胺季胺化的改性方法制备三价砷(As(III))阴离子吸附剂,并确定了最佳参数。利用扫描电镜、表面酸碱特性等表征了麦糟和阴离子吸附剂的结构特征和物理化学性质。静态吸附条件下考察了阴离子吸附剂吸附As(III)的影响因素和行为机理。与麦糟相比,阴离子吸附剂对As(III)的去除率大大提高。     

Urinary arsenic methylation capability and carotid atherosclerosis risk in subjects living in arsenicosis-hyperendemic areas in southwestern Taiwan

Long-term exposure to inorganic arsenic from artesian drinking well water is associated with carotid atherosclerosis in the Blackfoot Disease (BFD)-hyperendemic area in Taiwan. The current study examined the arsenic methylation capacity and its risk on carotid atherosclerosis. A total of 304 adults (158 men and 146 women) residing in the BFD-hyperendemic area were included. The extent of carotid atherosclerosis was assessed by duplex ultrasonography. Chronic arsenic exposure was estimated by an index of cumulative arsenic exposure (CAE) and the duration of artesian well water consumption. Urinary levels of inorganic arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] were determined by high performance liquid chromatography linked on-line to a hydride generator and atomic absorption spectrometry (HPLC-HG-AAS). The percentage of arsenic species, primary methylation index [PMI = MMA(V) / (As(III) + As(V)] and secondary methylation index [SMI = DMA(V) / MMA(V)] were calculated and employed as indicators of arsenic methylation capacity. Results showed that women and younger subjects had a more efficient arsenic methylation capacity than did men and the elderly. Carotid atherosclerosis cases had a significantly greater percentage of MMA(V) [%MMA(V)] and a lower percentage of DMA [%DMA (V)] compared to controls. Subjects in the highest two tertiles of PMI with a median of CAE > 0 mg/L-year had an odds ratio (OR) and a 95% confidence interval (CI) of carotid atherosclerosis of 2.61 and 0.98-6.90 compared to those in the highest two tertiles of PMI with a CAE = 0 mg/L-year. We conclude that individuals with greater exposure to arsenic and lower capacity to methylate inorganic arsenic may be at a higher risk to carotid atherosclerosis.     

As(III) removal from drinking water using manganese oxide-coated-alumina: Performance evaluation and mechanistic details of surface binding

This paper describes the arsenite [As(III)] removal performance of manganese oxide-coated-alumina (MOCA) and its interaction with As(III) in drinking water. MOCA was characterized by XRD, SEM, EDAX, gas adsorption porosimetry, and point of zero charge (pH_pzc) measurements. Raman spectroscopy coupled with sorption experiments were carried out to understand the As(III) interaction with MOCA. As(III) sorption onto MOCA was pH dependent and the optimum removal was observed between a pH of 4 and 7.5. The Sips isotherm model described the experimental equilibrium data well and the predicted maximum As(III) sorption capacity was 42.48 mg g⁻¹, which is considerably higher than that of activated alumina (20.78 mg g⁻¹). The sorption kinetics followed a pseudo-second-order equation. Based on sorption and spectroscopic measurements, the mechanism of As(III) removal by MOCA was found to be a two-step process, i.e. oxidation of As(III) to arsenate (As(V)) and retention of As(V) on MOCA surface, with As(V) forming an inner surface complex with MOCA. The results of this study indicated that MOCA is a promising alternative sorbent for As(III) removal from drinking water.     

Feasibility analysis of As(III) removal in a continuous flow fixed bed system by modified calcined bauxite (MCB)

This study examine the feasibility of As(III) removal from aqueous environment by an adsorbent, modified calcined bauxite (MCB) in a continuous flow fixed bed system. MCB exhibited excellent adsorption capacity of 520.2 mg/L (0.39 mg/g) with an adsorption rate constant 0.7658 L/mgh for an influent As(III) concentration of 1mg/L. In a 2 cm diameter continuous flow fixed MCB bed, a depth of only 1.765 cm was found necessary to produce effluent As(III) concentration of 0.01 mg/L, from an influent of 1 mg/L at a flow rate of 8 mL/min. Also, bed heights of 10, 20, and 30 cm could treat 427.85, 473.88 and 489.17 bed volumes of water, respectively, to breakthrough. A reduction in adsorption capacity of MCB was observed with increase in flow rates. The theoretical service times evaluated from bed depth service time (BDST) approach for different flow rates and influent As(III) concentrations had shown good correlation with the corresponding experimental values. The theoretical breakthrough curve developed from constantly mixed batch reactor (CMBR) isotherm data also correlated well with experimental breakthrough curve.     

Indirect photocatalytic reduction of arsenate to arsenite in aqueous solution with TiO2 in the presence of hole scavengers

The indirect photocatalytic reduction of arsenate to arsenite in aqueous solution with titanium dioxide(TiO_2)was investigated with various hole scavengers such as methanol, ethanol, 2-propanol, formaldehyde, acetone,formic acid and acetic acid. Although the direct photocatalytic reduction of arsenate to arsenite with TiO_2 was impossible, an indirect reduction of As(V) was possible in the presence of sacrificial electron donors to form strongly reductive radicals. The addition of ethanol was very effective for indirect photocatalytic reduction of As(V) in aqueous solution with TiO_2 photocatalyst. The indirect photocatalytic reduction rate of As(V) may be related with both the reaction rate constants of reaction of hydroxyl radicals with hole scavenger and the reactivities for the radicals M· which are produced by the reaction of ·OH with hole scavenger.     

荧光法同时测定环境水中的As(Ⅲ)和As(V)

研究了一种快速、灵敏的同时测定水中的As(Ⅲ)和As(V)的荧光新方法。在pH=6.5~7.5的缓冲介质中,利用2’,7’-二氯荧光素(DCF)作为荧光试剂,激发波长λex=510nm,发射波长λem=528nm下,As(Ⅲ)和DCF竞争碘,引起荧光强度的增强,从而测定痕量As(Ⅲ)。同时利用L一半胱氨酸还原剂将水中的As(V)还原成As(Ⅲ),从而测定As(Ⅲ)和As(v)的总量,差减间接测定As(V)。As(Ⅲ)浓度在4~180ng/mL范围内,相对荧光强度差值与As(Ⅲ)浓度呈线性关系,线性方程△F=7.82C+0.76,相关系数为o.9991,本法快速、简便、灵敏度高,已用于检测自来水和池塘水中痕量的As(Ⅲ)和As(V),回收率在96%~105%,结果令人满意。     

Biotransformation of arsenic species by activated sludge and removal of bio-oxidised arsenate from wastewater by coagulation with ferric chloride

The potential of activated sludge to catalyse bio-oxidation of arsenite [As(III)] to arsenate [As(V)] and bio-reduction of As(V) to As(III) was investigated. In batch experiments (pH 7, 25 degrees C) using activated sludge taken from a treatment plant receiving municipal wastewater non-contaminated with As, As(III) and As(V) were rapidly biotransformed to As(V) under aerobic condition and As(III) under anaerobic one without acclimatisation, respectively. Sub-culture of the activated sludge using a minimal liquid medium containing 100mg As(III)/L and no organic carbon source showed that aerobic arsenic-resistant bacteria were present in the activated sludge and one of the isolated bacteria was able to chemoautotrophically oxidise As(III) to As(V). Analysis of arsenic species in a full-scale oxidation ditch plant receiving As-contaminated wastewater revealed that both As(III) and As(V) were present in the influent, As(III) was almost completely oxidised to As(V) after supply of oxygen by the aerator in the oxidation ditch, As(V) oxidised was reduced to As(III) in the anaerobic zone in the ditch and in the return sludge pipe, and As(V) was the dominant species in the effluent. Furthermore, co-precipitation of As(V) bio-oxidised by activated sludge in the plant with ferric hydroxide was assessed by jar tests. It was shown that the addition of ferric chloride to mixed liquor as well as effluent achieved high removal efficiencies (>95%) of As and could decrease the residual total As concentrations in the supernatant from about 200 microg/L to less than 5 microg/L. It was concluded that a treatment process combining bio-oxidation with activated sludge and coagulation with ferric chloride could be applied as an alternative technology to treat As-contaminated wastewater.     

A comparative study of As(III) and As(V) in aqueous solutions and adsorbed on iron oxy-hydroxides by Raman spectroscopy

The sorption of the arsenite (AsO₃³⁻) and the arsenate (AsO₄³⁻) ions and their conjugate acids onto iron oxides is one of main processes controlling the distribution of arsenic in the environment. The present work intends to provide a large vibrational spectroscopic database for comparison of As(III) and As(V) speciation in aqueous solutions and at the iron oxide - solution interface. With this purpose, ferrihydrite, feroxyhyte, goethite and hematite were firstly synthesized, characterized in detail and used for adsorption experiments. Raman spectra were recorded from As(III) and As(V) aqueous solutions at various pH conditions selected in order to highlight arsenic speciation. Raman Scattering and Diffuse Reflectance Infrared Fourier Transform (DRIFT) studies were carried out to examine the respective As-bonding mechanisms. The collected data were curve-fitted and discussed according to molecular symmetry concepts. X-ray Absorption Near Edge Spectroscopy (XANES) was applied to confirm the oxidation state of the sorbed species. The comprehensive spectroscopic investigation contributes to a better understanding of arsenic complexation by iron oxides.