Perchlorate reduction by autotrophic bacteria attached to zerovalent iron in a flow-through reactor

Biological reduction of perchlorate by autotrophic microorganisms attached to zerovalent iron (ZVI) was studied in flow-through columns. The effects of pH, flow rate, and influent perchlorate and nitrate concentrations on perchlorate reduction were investigated. Excellent perchlorate removal performance (> or = 99%) was achieved at empty bed residence times (EBRTs) ranging from 0.3 to 63 h and an influent perchlorate concentration of 40-600 microg L(-1). At the longest liquid residence times, when the influent pH was above 7.5, a significant increase of the effluent pH was observed (pH > 10.0), which led to a decrease of perchlorate removal. Experiments at short residence times revealed that the ZVI column inoculated with local soil (Colton, CA) containing a mixed culture of denitrifiers exhibited much better performance than the columns inoculated with Dechloromonas sp. HZ for reduction of both perchlorate and nitrate. As the flow rate was varied between 2 and 50 mL min(-1), corresponding to empty bed contact times of 0.15-3.8 h, a maximum perchlorate elimination capacity of 3.0 +/- 0.7 g m(-3) h(-1) was obtained in a soil-inoculated column. At an EBRT of 0.3 h and an influent perchlorate concentration of 30 microg L(-1), breakthrough (> 6 ppb) of perchlorate in the effluent did not occur until the nitrate concentration in the influent was 1500 times (molar) greater than that of perchlorate. The mass of microorganisms attached on the solid ZVI/sand was found to be 3 orders of magnitude greater than that in the pore liquid, indicating that perchlorate was primarily reduced by bacteria attached to ZVI. Overall, the process appears to be a promising alternative for perchlorate remediation.     

Rate and extent of aqueous perchlorate removal by iron surfaces

The rate and extent of perchlorate reduction on several types of iron metal was studied in batch and column reactors. Mass balances performed on the batch experiments indicate that perchlorate is initially sorbed to the iron surface, followed by a reduction to chloride. Perchlorate removal was proportional to the iron dosage in the batch reactors, with up to 66% removal in 336 h in the highest dosage system (1.25 g mL(-1)). Surface-normalized reaction rates among three commercial sources of iron filings were similar for acid-washed samples. The most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial pH values. Surface mediation of the reaction is supported by the absence of reduction in batch experiments with soluble Fe2+ and also by the similarity in specific reaction rate constants (kSA) determined for three different iron types. Elevated soluble chloride concentrations significantly inhibited perchlorate reduction, and lower removal rates were observed for iron samples with higher amounts of background chloride contamination. Perchlorate reduction was not observed on electrolytic sources of iron or on a mixed-phase oxide (Fe3O4), suggesting that the reactive iron phase is neither pure zerovalent iron nor the mixed oxide alone. A mixed valence iron hydr(oxide) coating or a sorbed Fe2+ surface complex represent the most likely sites for the reaction. The observed reaction rates are too slow for immediate use in remediation system design, but the findings may provide a basis for future development of cost-effective abiotic perchlorate removal techniques.     

Efficient heterogeneous catalytic reduction of perchlorate in water

A new heterogeneous catalyst that promotes the reduction by hydrogen of perchlorate ion in water under mild conditions has been developed. The catalyst is prepared by adsorption of a rhenium(VII) precursor (either ammonium perrhenate or methylrhenium trioxide) onto carbon powder containing 5% palladium by weight. Under standard batch conditions of room temperature, 1 bar of hydrogen, and 200 ppm perchlorate (as HClO4), reduction proceeded to less than 1 ppm in as little as 5 h. Extended reaction times led to residual perchlorate at low parts per billion levels. Chloride was the only observed product, with good material balance. Catalytic materials ranging from 3% to 13% Re showed (pseudo) first-order rates linearly dependent on the Re content. Representative normalized rate constants for catalysts with 5-9% Re were in the range 0.1-0.3 L h(-1) (g of cat.)(-1). Inhibition by chloride was not significant, with little change in perchlorate reduction rate in the presence of excess chloride to 1000 ppm. However, optimal activity occurred in acidic solutions (pH ca. 3), and both the rate and extent of reaction decreased at higher values of pH. In its current form the catalyst might be best applied to destroy perchlorate in the acidic regeneration stream from selective ion exchange columns.     

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.     

Investigating the role of atomic hydrogen on chloroethene reactions with iron using tafel analysis and electrochemical impedance spectroscopy

Metallic iron filings are commonly employed as reducing agents in permeable barriers used for remediating groundwater contaminated by chlorinated solvents. Reactions of trichloroethylene (TCE) and tetrachloroethylene (PCE) with zerovalent iron were investigated to determine the role of atomic hydrogen in their reductive dechlorination. Experiments simultaneously measuring dechlorination and iron corrosion rates were performed to determine the fractions of the total current going toward dechlorination and hydrogen evolution. Corrosion rates were determined using Tafel analysis, and dechlorination rates were determined from rates of byproduct generation. Electrochemical impedance spectroscopy (EIS) was used to determine the number of reactions that controlled the observed rates of chlorocarbon disappearance, as well as the role of atomic hydrogen in TCE and PCE reduction. Comparison of iron corrosion rates with those for TCE reaction showed that TCE reduction occurred almost exclusively via atomic hydrogen at low pH values and via atomic hydrogen and direct electron transfer at neutral pH values. In contrast, reduction of PCE occurred primarily via direct electron transfer at both low and neutral pH values. At low pH values and micromolar concentrations, TCE reaction rates were faster than those for PCE due to more rapid reduction of TCE by atomic hydrogen. At neutral pH values and millimolar concentrations, PCE reaction rates were faster than those for TCE. This shift in relative reaction rates was attributed to a decreasing contribution of the atomic hydrogen reaction mechanism with increasing halocarbon concentrations and pH values. The EIS data showed that all the rate limitations for TCE and PCE dechlorination occurred during the transfer of the first two electrons. Results from this study show that differences in relative reaction rates of TCE and PCE with iron are dependent on the significance of the reduction pathway involving atomic hydrogen.     

毕赤酵母表面展示米黑根毛霉脂肪酶发酵条件

在50 L发酵罐水平对影响毕赤酵母表面展示米黑根毛霉脂肪酶(Rhizomucor miehei lipase,RML)发酵因素进行了研究。最适初始菌体密度(OD600)约为250,最佳诱导pH为6.0;降低诱导温度能有效提高单位甲醇转化率,诱导温度为20℃和25℃时的转化率分别是30℃的1.56倍和1.32倍。在以上最适条件下单位干菌体展示酶活及生产强度分别达275.0 U/g和462.5 U/(L.h),比优化前提高54.5%。研究发现,山梨醇和甲醇混合补料有效地提高了诱导前期(t=12h)的产酶效率,当山梨醇和甲醇混合比例为1∶4及1∶5是以甲醇为单一碳源时酶活力的1.7倍。     

Major anion effects on the kinetics and reactivity of granular iron in glass-encased magnet batch reactor experiments

The relative effects of sulfate (SO4(2-)), chloride (Cl-), nitrate (NO3-), and bicarbonate (HCO3-) (8 mM ionic strength solutions, adjusted to pH 10) on the reactivity of Master Builders iron was investigated using a low-abrasion batch reactor with a glass-encased magnet (GEM). Reactivity of the granular iron surface was assessed by measuring the reduction rate of 4-chloronitrobenzene (4ClNB) as a function of initial 4CINB concentration and anion type. Relative to a similarly prepared perchlorate (ClO4-) solution, in which perchlorate was assumed not to interact with the iron surface, nitrate and bicarbonate inhibited the reduction of the probe compound (4ClNB). Chloride and sulfate enhanced reactivity. Thus, the anions were ranked SO4(2-) > Cl- > or = ClO4- > NO3- > HCO3 (from most enhanced to most inhibited) in their influence on granular iron reactivity toward 4ClNB. Kinetic studies of 4CINB were conducted under conditions that caused the iron surface to saturate with the reacting compound (saturation kinetic studies). These experiments, conducted in the various anion solutions indicated above, showed that the gains in reactivity that occurred in the presence of Cl- and SO4(2-) were due to either increased surface reactivity or sorption capacity. The losses in reactivity that occurred in the presence of NO3- were due to decreases in one or both of these same two factors. However, reactivity declines in the presence of CO3(2-) appear to have been due, in large part, to a reduced affinity of 4ClNB for the iron surface.     

In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination

In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.     

Effects of soluble ferri-hydroxide complexes on microbial neutralization of acid mine drainage

Heterotrophic respiration of ferric iron by Acidiphilium cryptum was investigated in anoxic microcosms with initial media pH values from 1.5 to 3.5. No organic carbon consumption or iron reduction was observed with an initial pH of 1.5, indicating that A. cryptum may not be capable of iron respiration at this pH. Significant iron reduction was observed at pH 2.5 and 3.5, with different effects. When the initial pH was 3.5, pH increased to 4.7-5.5 over 60 days of incubation with simultaneous production of 0.4 g L(-1) Fe2+. However, at an initial pH of 2.5, no significant change in pH was observed during iron respiration, although the accumulation of soluble ferrous iron was significantly higher, averaging 1.1 g L(-1) Fe2+. The speciation of the ferric iron electron acceptor may explain these results. At pH values of 3.5 and higher, precipitated ferric hydroxide Fe- (OH)3 would have been the primary source of ferric iron, with reduction resulting in net production of OH- ions and the significant increases in media pH observed. However at pH 2.5, soluble complexes, FeOH2+ and Fe(OH)2+, may have been the more prevalent electron acceptors, and the alkalinity generated by reduction of complexed iron was low. The existence of charged ferri-hydroxide complexes at pH 2.5 was verified by voltammetry. Results suggest that initiation of bacterial iron reduction may result in neutralization of acid mine drainage. However, this effect is extremely sensitive to iron speciation within a relatively small and critical pH range.     

干酪乳杆菌LZ183E高密度培养条件优化

为提高干酪乳杆菌LZ183E在发酵培养液中的菌体密度,首先通过在不同温度下培养菌株,测定48 h内菌液的OD600 nm值并作出生长曲线,得到菌株最适培养温度为37 ℃、接种时间为16 h及收获时间30 h。随后通过单因素试验和正交试验优化,探究不同碳源、氮源、生长因子、初始pH值以及接种量对菌株LZ183E活菌数和OD600 nm值的影响。结果表明,菌株LZ183E的最佳培养条件为葡萄糖25 g/L、酵母膏20 g/L、南瓜汁24 g/L、初始pH 6.5以及接种量2%。此优化条件下,干酪乳杆菌LZ183E的活菌数对数值达到了9.20±0.04,满足高密度培养要求,并且比原始MRS培养基活菌数对数值（8.12±0.06）高出一个数量级,为干酪乳杆菌LZ183E的冻干保护提供了足够的活菌数。     

Processes at the sediment water interface after addition of organic matter and lime to an Acid Mine Pit Lake mesocosm

A strategy to neutralize acidic pit lakes was tested in a field mesocosm of 4500 m(3) volume in the Acidic Pit Mine Lake 111 in Germany. Carbokalk, a byproduct from sugar production, and wheat straw was applied near to the sediment surface to stimulate in lake microbial alkalinity generation by sulfate and iron reduction. The biogeochemical processes at the sediment-water interface were studied over 3 years by geochemical monitoring and an in situ microprofiler. Substrate addition generated a reactive zone at the sediment surface where sulfate and iron reduction proceeded. Gross sulfate reduction reached values up to 10 mmol m(-2) d(-1). The neutralization rates between 27 and 0 meq m(-2) d(-1) were considerably lower than in previous laboratory experiments. The precipitation of ferric iron minerals resulted in a growing acidic sediment layer on top of the neutral sediment. In this layer sulfate reduction was observed but iron sulfides could not precipitate. In the anoxic sediment H2S was oxidized by ferric iron minerals. H2S partly diffused to the water column where it was oxidized. As a result the net formation of iron sulfides decreased after 1 year although gross sulfate reduction rates continued to be high. The rate of iron reduction exceeded the sulfate reduction rate, which resulted in high fluxes of ferrous iron out of the sediment.     

重组环糊精葡萄糖基转移酶温控型工程菌的构建及其培养条件的优化

采用聚合酶链式反应（polymerase chain reaction,PCR）法从蜡状芽孢杆菌（Bacillus cereus）中扩增了β-环糊精葡萄糖基转移酶（β-cyclodextrin glucanotransferase,β-CGTase）基因,将该基因克隆到pBV220质粒中,转化E.coli DH5α,经氨苄青霉素抗性筛选和酶切验证后,得到能表达β-CGTase的重组大肠杆菌E.coliDH5α/pBVcgt。通过对工程菌产酶条件的优化,得到最佳产酶条件为：OD600 nm值达到1.0、初始培养温度30 ℃、发酵培养基初始pH 8.0、温度梯度诱导39 ℃培养0.5 h,40 ℃培养0.5 h,41 ℃培养1 h,42 ℃培养2 h。酶活力由优化前的445 U/mL提高到956 U/mL,酶活力提高了1.15 倍。温度梯度诱导比直接诱导酶活力提高20%。该酶的克隆表达以及发酵条件的研究表明,该酶能在大肠杆菌原核表达体系中高效表达。     

Kinetics of soluble chromium removal from contaminated water by zerovalent iron media: corrosion inhibition and passive oxide effects

Permeable reactive barriers containing zerovalent iron are being increasingly employed for in situ remediation of groundwater contaminated with redox active metals and chlorinated organic compounds. This research investigated the effect of chromate concentration on its removal from solution by zerovalent iron. Removal rates of aqueous Cr(VI) by iron wires were measured in batch experiments for initial chromium concentrations ranging from 100 to 10 000 microg/L. Chromate removal was also measured in columns packed with zerovalent iron filings over this same concentration range. Electrochemical measurements were made to determine the free corrosion potential and corrosion rate of the iron reactants. In both the batch and column reactors, absolute rates of chromium removal declined with increasing chromate concentration. Corrosion current measurements indicated that the rate of iron corrosion decreased with increasing Cr(VI) concentrations between 0 and 5000 microg/L. At a Cr(VI) concentration of 10 000 microg/L, Tafel polarization diagrams showed that chromium removal was affected by its diffusion rate through a passivating oxide film and by the ability of iron to release Fe2+ at anodic sites. In contrast, water reduction was not mass transfer limited, but chromium did decrease the exchange current for the hydrogen evolution reaction. Even at the most passivating concentration of 10 000 microng/L, effluent Cr(VI) concentrations in the column reactors reached a steady state, indicating that passivation had also reached a steady state. Although chromate contributes to iron surface passivation, the removal rates are still sufficiently fast for in situ iron barriers to be effective for Cr(VI) removal at most environmentally relevant concentrations.     

Quantification of the oxidizing capacity of nanoparticulate zero-valent iron

Addition of nanoparticulate zero-valent iron (nZVI) to oxygen-containing water results in oxidation of organic compounds. To assess the potential application of nZVI for oxidative transformation of organic contaminants, the conversion of benzoic acid (BA) to p-hydroxybenzoic acid (p-HBA) was used as a probe reaction. When nZVI was added to BA-containing water, an initial pulse of p-HBA was detected during the first 30 min, followed by the slow generation of additional p-HBA over periods of at least 24 h. The yield of p-HBA increased with increasing BA concentration, presumably due to the increasing 'ability of BA to compete with alternate oxidant sinks, such as ferrous iron. At pH 3, maximum yields of p-HBA during the initial phase of the reaction of up to 25% were observed. The initial rate of nZVI-mediated oxidation of BA exhibited a marked reduction at pH values above 3. Despite the decrease in oxidant production rate, p-HBA was observed during the initial reaction phase at pH values up to 8. Competition experiments with probe compounds expected to exhibit different affinities for the nZVI surface (phenol, aniline, o-hydroxybenzoic acid, and synthetic humic acids) indicated relative rates of reaction that were similar to those observed in competition experiments in which hydroxyl radicals were generated in solution. Examination of the oxidizing capacity of a range of Fe0 particles reveals a capacity in all cases to induce oxidative transformation of benzoic acid, but the high surface areas that can be achieved with nanosized particles renders such particles particularly effective oxidants.     

重组大肠杆菌P84A/MC1061诱导和表达卤醇脱卤酶的条件优化

考察重组工程菌E.coli P84A/MC1061的生长特性,并优化其在卤醇脱卤酶表达过程中的诱导条件和培养条件。在摇瓶发酵中,以阿拉伯糖诱导重组大肠杆菌P84A/MC1061表达卤醇脱卤酶。重点研究培养温度、装液量、摇床转速、培养基初始pH值、诱导时机、诱导剂添加量、诱导剂作用时间对卤醇脱卤酶的表达影响。在培养温度为30℃,装液量为25mL/250mL,摇床转速为200r/min,初始pH值为7.1时,当菌体量OD600值达到2时,按照1‰体积比添加L-阿拉伯糖,诱导培养22h后,发酵液OD值达到21.3,酶活力达到138555.3U/mL。研究发现,培养温度和初始pH值是基因工程菌E.coli P84A/MC1061诱导表达卤醇脱卤酶的显著影响因子,可为卤醇脱卤酶发酵生产的深入研究提供依据。     

Effect of pH on aqueous Se(IV) reduction by pyrite

Interaction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free (<1 ppm) conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reduction of aqueous Se(IV) by pyrite is dominated by the following reaction: FeS2+3.5HSeO3?+1.5H+=2SO4(2?)+Fe2++3.5Se(0)+2.5H2O. The released Fe(II) was partitioned between the bulk solution and pyrite surface at pH≈4.5 and 4.8, with the Fe2+ density at pyrite-solution interface about 4 orders of magnitude higher than that in the bulk solution, while iron oxyhydroxide precipitated at pH≈6.6, resulting in the decrease of dissolved iron. In the Se(IV) concentration range of the experiments, aqueous Se(IV) reduction rate follows the pseudofirst order which is in the form of ln mSe(IV)=?k′t+ln mSe(IV)0, where k′ is apparent rate constant combining the rate constant k and pyrite surface area to mass of solution ratio (A/M). And the aqueous Se(IV) reduction rate constant for a standard system (k) with 1 m2 pyrite surface area per 1 kg solution was obtained to be 1.65×10(?4) h(?1), 3.28×10(?4) h(?1), and 4.76×10(?4) h(?1) at pH around 4.5, 4.8, and 5.1, respectively. The positive correlation between reaction rate and pH disagrees with the theories that protons are consumed when HSeO3? is reduced to Se0, and negative charge density on pyrite surface increases as pH increases. Thus, a ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product.     

透析培养法稳定出芽短梗霉摇瓶发酵条件的研究

以出芽短梗霉色素低产株G58-2为生产菌株,研究了其透析培养下的菌体形态,及普鲁兰多糖摇瓶发酵的最佳条件。结果表明,通过透析培养制备的发酵接种液优于普通培养,在其OD600为0.442,接种龄36h,初始pH7.0,装液量50 mL/250 mL,摇床转速200 r/min条件下培养84 h出芽短梗霉G58-2不仅色素含量低,粗多糖产量可达到4.84 g/100 mL,并且实验稳定性有效提高。     

Effect of particle age (Fe0 content) and solution pH on NZVI reactivity: H2 evolution and TCE dechlorination

Subsurface injection of nanoscale zerovalent iron (NZVI) has been used for the in situ remediation of chlorinated solvent plumes and DNAPL source zones. Due to the cost of materials and placement,the efficacy of this approach depends on the NZVI reactivity and longevity, selectivity for the target contaminant relative to nonspecific corrosion to yield H2, and access to the Fe0 in the particles. Both the reaction pH and the age of the particles (i.e., Fe0 content) could affect NZVI reactivity and longevity. Here, the rates of H2 evolution and trichloroethene (TCE) reduction are measured over the lifetime of the particles and at solution pH ranging from 6.5 to 8.9. Crystalline reactive nanoscale iron particles (RNIP) with different initial Fe0 weight percent (48%, 36%, 34%, 27%, and 9.6%) but similar specific surface area were studied. At the equilibrium pH for a Fe(OH)2/H2O system (pH = 8.9), RNIP exhibited first-order decay for Fe0 corrosion (H2 evolution) with respect to Fe0 content with a Fe0 half-life time of 90-180 days. A stable surface area-normalized TCE reduction rate constant 1.0 x 10(-3)L x hr(-1) x m(-2) was observed after 20 days and remained constant for 160 days, while the Fe0 content of the particles decreased by half, suggesting that TCE reduction is zero-order with respect to the Fe0 content of the particle. Solution pH affected H2 evolution and TCE reduction to a different extent. Decreasing pH from 8.9 to 6.5 increased the H2 evolution rate constant 27 fold from 0.008 to 0.22 day(-1), but the TCE dechlorination rate constant only doubled. The dissimilarities between the reaction orders of H2 evolution and TCE dechlorination with respect to both Fe0 content and H+ concentration suggest that different rate controlling steps are involved for the reduction reactions.     

耐酸耐胆盐乳酸菌的鉴定及筛选

从自然发酵的酸奶中分离出2 株细菌,经16S rDNA分子鉴定为Lactobacillus plantarum SN1和Lactobacillusrhamnosus SN6,并对其生长曲线、产酸速率、耐酸耐胆盐能力进行了研究。L. plantarum SN1和L. rhamnosus SN6在2 h后进入对数期,16 h后达到稳定期,其OD600 nm值分别为8.47、7.43。L. plantarum SN1和L. rhamnosus SN6的产酸速率较快,pH值在8 h后就降到了4.2以下,48 h后降到3.3左右。L. plantarum SN1和L. rhamnosus SN6在pH 4的培养基培养16 h后,其相对OD600 nm值分别为49.29%、47.14%,具有较强的耐酸能力。L. plantarum SN1和L. rhamnosus SN6在0.3 g/L胆盐质量浓度下培养16 h后,相对OD600 nm值分别为57.7%、69.48%;在0.6 g/L胆盐质量浓度下的相对OD600 nm值分别为48.22%、29.56%,具有较强的耐胆盐能力。结果表明：L. plantarum SN1和L. rhamnosus SN6是生长性能好、产酸能力强、耐酸耐胆盐的益生菌株。     

Electrochemical study of arsenate and water reduction on iron media used for arsenic removal from potable water

Zerovalent iron filings have been proposed as a filter medium for removing As(III) and As(V) compounds from potable water. The removal mechanism involves complex formation of arsenite and arsenate with the iron surface and with iron oxides produced from iron corrosion. There is conflicting evidence in the literature on whether As(V) can be reduced to As(III) by iron filter media. This research uses electrochemical methods to investigate the redox reactions that occur on the surface of zerovalent iron in arsenic solutions. The effect of arsenic on the corrosion rate of zerovalent iron was investigated by analysis of Tafel diagrams for iron wire electrodes in anaerobic solutions with As(V) concentrations between 100 and 20,000 microg/L. As(V) reduction in the absence of surface oxides was investigated by analysis of chronoamperometry profiles for iron wire electrodes in solutions with As(V) concentrations ranging from 10000 to 106 microg/L. The effect of pH on As(V) reduction was investigated by analyses of chronopotentiometry profiles for iron wire electrodes at pH values of 2, 6.5, and 11. For freely corroding iron, the presence of As(III) and As(V) decreased the iron corrosion rate by a factor of 5 as compared to that in a 3 mM CaSO4 blank electrolyte solution. The decrease in corrosion rate was independent of the arsenic concentration and was due to the blocking of cathodic sites for water reduction by arsenic compounds chemisorbed to the iron surface. The chronoamperometry and chronopotentiometry experiments showed that elevated pH and increased As(III) to As(V) ratios near the iron surface decreased the thermodynamic favorability for As(V) reduction. Therefore, reduction of As(V) occurred only at potentials that were significantly below the apparent equilibrium potentials based on bulk solution pH values and As(III) to As(V) ratios. The potentials required to reduce more than 1% of the As(V) to As(III) were below those that are obtainable in freely corroding iron media. This indicates that there will be minimal or no reduction of As(V) in iron media filters under conditions relevant to potable water treatment.