Studies on arsenic transforming groundwater bacteria and their role in arsenic release from subsurface sediment

Ten different Gram-negative arsenic (As)-resistant and As-transforming bacteria isolated from As-rich groundwater of West Bengal were characterized to assess their role in As mobilization. 16S rRNA gene analysis confirmed the affiliation of these bacteria to genera Achromobacter, Brevundimonas, Rhizobium, Ochrobactrum, and Pseudoxanthomonas. Along with superior As-resistance and As-transformation abilities, the isolates showed broad metabolic capacity in terms of utilizing a variety of electron donors and acceptors (including As) under aerobic and anaerobic conditions, respectively. Arsenic transformation studies performed under various conditions indicated highly efficient As³⁺ oxidation or As⁵⁺ reduction kinetics. Genes encoding As³⁺ oxidase (aioA), cytosolic As⁵⁺ reductase (arsC), and As³⁺ efflux pump (arsB and acr3) were detected within the test isolates. Sequence analyses suggested that As homeostasis genes (particularly arsC, arsB, and acr3) were acquired by most of the bacteria through horizontal gene transfer. A strong correlation between As resistance phenotype and the presence of As³⁺ transporter genes was observed. Microcosm study showed that bacterial strain having cytosolic As⁵⁺ reductase property could play important role in mobilizing As (as As³⁺) from subsurface sediment.     

Removal of arsenic from groundwater by granular titanium dioxide adsorbent

A novel granular titanium dioxide (TiO2) was evaluated for the removal of arsenic from groundwater. Laboratory experiments were carried out to investigate the adsorption capacity of the adsorbent and the effect of anions on arsenic removal. Batch experimental results showed that more arsenate [As(V)] was adsorbed on TiO2 than arsenite [As(III)] in US groundwater at pH 7.0. The adsorption capacities for As(V) and As(III) were 41.4 and 32.4 mgg(-1) TiO2, respectively. However, the adsorbent had a similar adsorption capacity for As(V) and As(III) (approximately 40 mgg(-1)) when simulated Bangladesh groundwater was used. Silica (20 mgl(-1)) and phosphate (5.8 mgl(-1)) had no obvious effect on the removal of As(V) and As(III) by TiO2 at neutral pH. Point-of-entry (POE) filters containing 3 l of the granular adsorbent were tested for the removal of arsenic from groundwater in central New Jersey, USA. Groundwater was continuously passed through the filters at an empty bed contact time (EBCT) of 3 min. Approximately 45,000 bed volumes of groundwater containing an average of 39 microgl(-1) of As(V) was treated by the POE filter before the effluent arsenic concentration increased to 10 microgl(-1). The total treated water volumes per weight of adsorbent were about 60,000 l per 1 kg of adsorbent. The field filtration results demonstrated that the granular TiO2 adsorbent was very effective for the removal of arsenic in groundwater.     

Removal of arsenic from contaminated groundwater by solar-driven membrane distillation

Experimental investigations were carried out on removal of arsenic from contaminated groundwater by employing a new flat-sheet cross flow membrane module fitted with a hydrophobic polyvinylidenefluoride (PVDF) microfiltration membrane. The new design of the solar-driven membrane module in direct contact membrane distillation (DCMD) configuration successfully produced almost 100 per cent arsenic-free water from contaminated groundwater in a largely fouling-free operation while permitting high fluxes under reduced temperature polarization. For a feed flow rate of 0.120 m³/h, the 0.13 μm PVDF membrane yielded a high flux of 74 kg/(m² h) at a feed water temperature of 40 °C and, 95 kg/m² h at a feed water temperature of 60 °C. The encouraging results show that the design could be effectively exploited in the vast arsenic-affected rural areas of South-East Asian countries blessed with abundant sunlight particularly during the critical dry season.     

Removal of arsenic from groundwater by micellar-enhanced ultrafiltration (MEUF)

The removal characteristics of arsenate using micellar-enhanced ultrafiltration (MEUF) were investigated. Among four different cationic surfactants used, hexadecylpyridinium chloride (CPC) showed the highest removal efficiency of arsenic (96%), and the removal efficiency with hexadecyltrimethylammonium bromide (CTAB) was 94%. But the removal efficiency with benzalkonium chloride (BC) was the lowest (57%) due to higher critical micelle concentration (CMC) of BC than those of other surfactants. Over 80% of arsenic was removed with octadecylamine acetate (ODA). On the effect of solution pH on the arsenic removal, since the valance of arsenate decreases from trivalent to monovalent as pH decreases, the removal was reduced at lower pH. The presence of 0.45mM of nitrate and 0.01mM of phosphate reduced the removal efficiency by 5-8%. This decrease was because of the competition between the arsenate, nitrate and phosphate for the binding sites of the surfactant micelle. Similar decrease in the removal of arsenate was observed with CPC, CTAB and ODA in the presence of these anions. In cross-flow filtration, the removal efficiency of arsenic was similar to that in the dead-end system. However, the decline in flux was less than that in dead-end filtration. In order to lower the concentration of the surfactant in the effluent, the effluent was treated with powdered activated carbon (PAC) before discharging to the environment. Over 98% surfactant was removed with 1gl(-1) of PAC. In conclusions, the MEUF is considered as a feasible process using CPC or CTAB to remove the arsenate from groundwater compared with the other solid based adsorbent processes.     

纳滤膜技术在地下水除砷应用中的研究进展

砷污染突发事件的频发严重威胁着地下水饮用水水源的水质安全,加之饮用水控制标准的提高,就对饮用水除砷技术提出了更高的要求,而纳滤(NF)膜分离技术为饮用水除砷提供了新的思路。首先概述了地下水中砷的存在形态、化学性质以及我国高砷地下水地区分布,然后分析了NF膜特点、除砷机理与性能,系统地阐述了各种因素包括膜操作因素（操作压力、膜回收率、膜排布方式等）和原水水质因素（pH、水温、共存离子、共存有机物及砷浓度与砷价态等）,对NF膜除砷性能的影响。此外,对NF除砷的关键问题,如原水预处理、膜浓水处理、膜污染及其清洗等,也作了探讨。最后,总结了目前NF除砷应用中所面临的问题,探索性提出了NF膜技术在除砷应用中的研究方向。     

PRB技术对地下水中重金属离子的处理研究

实验模拟地下环境,以受重金属离子Pb(Ⅱ)、As(Ⅲ)、Cd(Ⅱ)、Cr(Ⅵ)、Fe(Ⅱ)和总Mn污染的地下水为研究对象,利用还原铁粉、铸铁粉、铸铁粉与颗粒活性炭的混合物为可渗透反应墙(PRB)的主要介质,石英砂为辅助介质,设计了3种反应器.在有效孔隙率为60%～65%、水力停留时间为12.0～14.4 h的条件下,考察其对污染物的去除效果.结果表明:3种反应器对Pb(Ⅱ)、As(Ⅲ)、Cd(Ⅱ)、Cr(Ⅵ)均有较高的去除效果,去除率达98%以上;总Mn的去除率分别达98%、89%和66%;Fe(Ⅱ)的去除率分别达83%、56%和49%.考察了3种反应器内pH、Eh、DO的关系及对重金属离子去除效果的影响,分析了污染物的去除机理.综合考虑处理效果与成本,笔者认为以铸铁粉与石英砂的混合物为PRB的反应介质,应用PRB技术原位处理受上述重金属离子污染的地下水是可行的.     

Concentrations and speciation of arsenic in groundwater polluted by warfare agents

Groundwater polluted with phenylarsenicals from former warfare agent deposits and their metabolites was investigated with respect to the behavior of relevant arsenic species. Depth profiles at the estimated source and at about 1 km downgradient from the source zone were sampled. The source zone is characterized by high total arsenic concentrations up to 16 mg L⁻¹ and is dominated by organic arsenic compounds. The concentrations in the downgradient region are much lower (up to 400 μg L⁻¹) and show a high proportion of inorganic arsenic species. Iron precipitation seems to be an effective mechanism to prevent dispersion of inorganic arsenic as well as phenylarsonic acid. Reductive conditions were observed in the deeper zone with predominant occurrence of trivalent arsenic species. The inorganic species are in redox equilibrium, whereas the phenylarsenic compounds have variable proportions. Methylphenylarsinic acid was identified in groundwater in traces which indicates microbial degradation activity.     

Microbial physiology-based model of ethanol metabolism in subsurface sediments

Dense non-aqueous phase liquids (DNAPLs) present in the subsurface may contain surface active compounds that impact DNAPL migration and distribution. While a number of studies have revealed the role surface active compounds play in altering the wettability of quartz sand, few have considered the implications for other minerals common to contaminated sites. This study extends understanding of DNAPL/surfactant wettability to iron oxide surfaces. Specifically, quartz and iron oxide-coated sands in a tetrachloroethene (PCE)/water system containing the organic base (an organic molecule that acts as a base) dodecylamine (DDA) were compared at a variety of scales. Wettability of the minerals' surfaces, and the impact of wettability on capillary resistance to DNAPL entry, were assessed as a function of pH through: (i) advancing and receding contact angles, (ii) primary drainage capillary pressure-saturation experiments, and (iii) small, two-dimensional, flow cell experiments. The work revealed that, at neutral pH and under identical boundary capillary pressures, DNAPL invaded quartz sand but not iron oxide-coated sand; however, at low pH, DNAPL invaded both sands equally. These differences were demonstrated to be due to wettability alterations associated with the strength of attractive forces between DDA and the mineral surface, dictated by the isolectric point of the minerals and system pH. Observed differences in DNAPL invasion behavior were consistent with measured intrinsic contact angles and P(c)-S relationships, the latter requiring scaling by the operative contact angle inside the porous medium for a meaningful comparison. This study suggests that the distribution of minerals (and, more specifically, their isoelectric points), as well as the aqueous phase pH at a given site, may have a significant impact on the DNAPL source zone architecture.     

Bacterial release of arsenic ions and organoarsenic compounds from soil contaminated by chemical warfare agents

The objective of this paper was to investigate possible participation of microorganisms in the release of soluble arsenical compounds from organoarsenic warfare agents in contaminated soil. A number of bacterial strains were isolated with high resistance against As3+ and As5+ ions which are able to degrade the water insoluble compounds triphenylarsine (TP) and triphenylarsineoxide (TPO). These strains belong to different genera of bacteria. Release of arsenic ions and soluble organoarsenic compounds from soil by the activity of autochthonic soil bacteria and a mixture of the isolated pure cultures was demonstrated by percolation experiments with undisturbed soil samples (core drills) from the contaminated site. This release increased after addition of nutrients (mineral nitrogen and phosphorus, sodium acetate and ethanol) and is nearly independent of the percolation temperature (5 degrees C and 22 degrees C). These results show that bacteria play an important role in the release of arsenical compounds from organoarsenic warfare agent contaminated soil. This release is limited by shortage of water and, above all, of nutrients for the microorganisms in the sandy forest soil. These results are important both for the management and security and possibly for bioremediation of military waste sites containing similar contaminations. Furthermore, this is the first report on bacterial degradation of organoarsenic warfare compounds.     

Mobilization of arsenic in aquifers from the Datong Basin,China: Evidence from geochemical and iron isotopic data

Iron isotope compositions of various Fe pools in aquifer sediments were measured at a known As-contaminated site in the Datong Basin, China. The δ⁵⁶Fe values of HCl-extracted poor-crystalline Fe(III) range widely from ?0.41‰ to 0.36‰. We interpret the low Fe(II)/Fe_Extractable ratios (<50%) and the negative correlation between Fe(II)/Fe_Extractable and δ⁵⁶Fe values in HCl-extracted poor-crystalline Fe to be best explained by redox cycling of Fe induced by microbial Fe(III) reduction. However, the high Fe(II)/Fe_Extractable ratios (?70%) and positive correlation between Fe(II)/Fe_Extractable and δ⁵⁶Fe values for HCl-extracted poor-crystalline Fe indicates production of sulfides (FeSs). The δ⁵⁶Fe values of crystalline Fe(III) extracted by reductant appears to be comparatively small varying from ?0.01‰ to 0.24‰, which is consistent with the δ⁵⁶Fe values for ferric oxides/hydroxides having undergone microbial Fe(III) reduction. The Fe isotope composition of various Fe pools shows the transformation between crystalline Fe(III) and poor-crystalline crystalline Fe(III) and the secondary Fe(II) phases has already occurred or is occurring in aquifer sediments. More importantly, there is a significant difference in the As concentrations in crystalline Fe(III) oxides/hydroxides and HCl-extracted Fe phases. The concentrations of As range from 1.6 to 29.9 mg kg^?1 and from 0.6 to 3.0 mg kg^?1, for crystalline Fe(III) and HCl-extracted Fe phases respectively. Accordingly, the transformation of Fe minerals induced by microbial Fe(III) reduction can contribute to the mobilization of As. This study is the first to examine the Fe isotope compositions in high As aquifer sediments; the results show that the Fe isotope would be an important tool in demonstrating the enrichment of As in groundwater.     

A novel low-cost detection method for screening of arsenic in groundwater

In the present work, a novel and simple detection system for As inorganic species contained in groundwater is presented. To reach the required detection limit, the proposed methodology is based on two steps: first is the transport and preconcentration of the inorganic arsenic species using a polymer inclusion membrane (PIM) system and second is the formation of a coloured complex, the  absorbance of which is measured. Different parameters related to the membrane composition and the transport kinetics have been studied, and it was found that membranes made of polyvinyl chloride (PVC) as a polymer, and Aliquat 336 as a carrier, ensured efficient arsenic transport when the carrier content was at least 31 % (w/w). The implementation of the designed PIM in a special device that contained only 5 mL of the stripping solution (0.1 M NaCl) allowed As preconcentration from a 100-mL water sample, thus facilitating its detection with the colorimetric method. The new method developed here was validated, and its analytical figures of merit were determined, i.e. limit of detection of 4.5 μg L^?1 at 820 nm and a relative standard deviation within the range 8–10 %. Finally, the method was successfully applied to the analysis of different water samples from Catalonia region with naturally occurring As.     

Removal of arsenic from water by electrocoagulation

In the present study electrocoagulation (EC) has been evaluated as a treatment technology for arsenite [As(III)] and arsenate [As(V)] removal from water. Laboratory scale experiments were conducted with three electrode materials namely, iron, aluminum and titanium to assess their efficiency. Arsenic removal obtained was highest with iron electrodes. EC was able to bring down aqueous phase arsenic concentration to less than 10 microgl(-1) with iron electrodes. Current density was varied from 0.65 to 1.53 mAcm(-2) and it was observed that higher current density achieved rapid arsenic removal. Experimental results at different current densities indicated that arsenic removal was normalized with respect to total charge passed and therefore charge density has been used to compare the results. Effect of pH on arsenic removal was not significant in the pH range 6-8. Comparative evaluation of As(III) and As(V) removal by chemical coagulation (with ferric chloride) and electrocoagulation has been done. The comparison revealed that EC has better removal efficiency for As(III), whereas As(V) removal by both processes was nearly same. The removal mechanism of As(III) by EC seems to be oxidation of As(III) to As(V) and subsequent removal by adsorption/complexation with metal hydroxides generated in the process.     

Geochemistry of high arsenic groundwater in Chia-Nan plain, Southwestern Taiwan: possible sources and reactive transport of arsenic

Major ion, trace element, and stable isotope analyses were performed on groundwater samples collected during November 2005 and 2006 in Chia-Nan plain of southwestern Taiwan to examine As mobilization in aquifers. The high concentrations of As, Fe and Mn in the groundwater is consistent with low Eh values (under moderately reduced state). Moreover, the observed Na/Cl and SO(4)/Cl molar ratios in groundwater demonstrate the influence of seawater intrusion. Seawater intrusion could provide required electron acceptors (i.e., SO(4)) for bacterial sulfate reduction and promote reducing conditions that are favorable for As mobilization. The concurrent increases in the concentrations of Fe and Mn from 2005 to 2006 may be caused by bacterial Fe(III) and Mn(IV) reduction. Geochemical modeling demonstrate that As(III) is the dominant As species and the presence of Fe-bearing carbonates, sulfides, and oxide phases may locally act as potential sinks for As. Mud volcano fluids were also collected and analyzed to assess the possible source of As in the Chia-Nan plain groundwater. The oxygen and hydrogen isotopic signatures indicate that the As-rich mud volcano fluids may have been modified by chemical exchange with (18)O-rich crustal rocks and possibly originated from mixing of deep brines with circulating meteoric water. Thus As in the Chia-Nan plain groundwater may have been evolved from deep crustal fluids or rock sources. The hydrogeochemistry and widespread As enrichment in groundwater of Chia-Nan plain result from multiple processes, e.g., de-watering of deep crustal fluids, seawater intrusion, and biogeochemical cycling of Fe, As, and S in alluvial sediments.     

Speciation of arsenic and chromium metal ions by reversed phase high performance liquid chromatography

The speciation of arsenic [As(III) and As(V)] and chromium [Cr(III) and Cr(VI)] was carried out by high performance liquid chromatography. The column used was Econosil C18 (250 x 4.6 mm i.d., particle size 10 microm). The mobile phases consisted of water-acetonitrile (80:20, v/v) for arsenic and 10 mM ammonium acetate buffer (6.0 pH)-acetonitrile (10:90, v/v) for chromium speciation separately and respectively. The detection was carried out by UV-Vis at 410 nm and atomic absorption spectrometer (AAS) respectively and separately. The values of alpha and Rs of As(III) and As(V) species were 1.4 and 1.5 respectively while the values of alpha and Rs for Cr(III) and Cr(VI) were 1.35 and 0.2 respectively. The effect of the acetonitrile percentages was also carried out on the speciation of arsenic only. The relative standard deviation and limit of detection were in the range of 0.01-0.02 and 0.4-1.0 microg/ml respectively.     

Sorption and oxic degradation of the explosive CL-20 during transport in subsurface sediments

The abiotic sorption and oxic degradation processes that control the fate of the explosive CL-20, Hexanitrohexaazaisowurtzitane, in the subsurface environment were investigated to determine the potential for vadose and groundwater contamination. Sorption of aqueous CL-20 is relatively small (K(d) = 0.02-3.83 cm3 g(-1) for 7 sediments and 12 minerals), which results in only slight retardation relative to water movement. Thus, CL-20 could move quickly through unsaturated and saturated sediments of comparable composition to groundwater, similar to the subsurface behavior of RDX. CL-20 sorption was mainly to mineral surfaces of the sediments, and the resulting isotherm was nonlinear. CL-20 abiotically degrades in oxic environments at slow rates (i.e., 10s to 100s of hours) with a wide variety of minerals, but at fast rates (i.e., minutes) in the presence of 2:1 phyllosilicate clays (hectorite, montmorillonite, nontronite), micas (biotite, illite), and specific oxides (MnO2 and the ferrous-ferric iron oxide magnetite). High concentrations of surface ferrous iron in a dithionite reduced sediment degraded CL-20 the fastest (half-life < 0.05 h), but 2:1 clays containing no structural or adsorbed ferrous iron (hectorite) could also quickly degrade CL-20 (half-life < 0.2 h). CL-20 degradation rates were slower in natural sediments (half-life 3-800 h) compared to minerals. Sediments with slow degradation rates and small sorption would exhibit the highest potential for deep subsurface migration. Products of CL-20 oxic degradation included three high molecular weight compounds and anions (nitrite and formate). The 2-3.5 moles of nitrite produced suggest CL-20 nitro-groups are degraded, and the amount of formate produced (0.2-1.2 moles) suggests the CL-20 cage structure is broken in some sediments. Identification of further degradation products and CL-20 mineralization rates is needed to fully assess the impact of these CL-20 transformation rates on the risk of CL-20 (and degradation product) subsurface movement.     

Degradation of the herbicide dichlobenil and its metabolite BAM in soils and subsurface sediments

The worldwide used herbicide dichlobenil (2,6-dichlorobenzonitrile) has resulted in widespread presence of its metabolite 2,6-dichlorobenzamide (BAM) in surface water and groundwater. To evaluate the potential for natural attenuation of this BAM pollution in groundwater, we studied the degradation of BAM and dichlobenil in 16 samples of clayey till, unconsolidated sand and limestone, including sediments from both oxidized and reduced conditions. The degradation of dichlobenil occurred primarily in the upper few meters below surface, although dichlobenil was strongly sorbed to these sediments. However, the degradation of dichlobenil to BAM could not be correlated to either sorption, water chemistry, composition of soils or sediments. Degradation of dichlobenil to BAM was limited (<2% degraded) in the deeper unsaturated zones, and no degradation was observed in aquifer sediments. This illustrates, that dichlobenil transported to aquifers does not contribute to the BAM-contamination in aquifers. A small, but significant degradation of BAM was observed in the upper part of the unsaturated zones in sandy sediments, but no degradation was observed in the clayey till sediment or in the deeper unsaturated zones. The insignificant degradation of BAM in aquifer systems shows that BAM pollution detected in aquifers will appear for a long time; and consequently the potential for natural attenuation of BAM in aquifer systems is limited.     

Mobilization of colloids in groundwater due to infiltration of water at a coal ash disposal site

We investigated groundwaters in the vicinity of a coal ash site near an electric generating station in the western U.S.A. The purpose of the study was to ascertain why fine particles or colloids appear in some subsurface water samples there. If such fine particles are merely introduced during bailing or pumping operations which suspend otherwise immobile soil colloids, we should exclude these particulate materials from the water samples before analysis intended to quantify what is moving through the aquifer. However, if the colloids were truly suspended and moving with the groundwater flow in situ, then we should includes their contribution to our assessment of the mobile loads.Application of very careful sampling techniques (slow pumping rates, no atmospheric exposure) did not cause the large quantities of colloids observed previously to disappear from well water in which they occured. Additionally, the same sampling procedures did not cause similar abundances of colloids to appear in waters collected from neighboring wells installed and developed in the same manner and in the same geologic strata. Thus we believe sampling artifacts do not explain the colloids' presence in the groundwater samples.On the other hand, the groundwater chemistry and the nature of the suspended colloids (size, composition) strongly suggest these fine particles were suspended and therefore moving with the groundwater flow. At wells exhibiting large amounts of suspended colloids (≈10–100 mg L⁻¹), the water was enriched in CO₂ and depleted in O₂ relative to nearby locations. The colloids were typically between 0.1 and 2 μm in size and were primarily silicates. These results suggest to us that, where infiltrating water is percolating through a site that has been mixed with coal ash, the secondary carbonate mineral in the soils are being dissolved; removal of this cementing carbonate phase may consequently release soil silicate colloids to be carried in the flowing water.Such processes may enhance contaminant transport in groundwater by augmenting the pollutant load moving in the groundwater, and increasing the permeability of the porous medium to pollutant infiltration with water water and/or rainwater.     

二段铁盐沉淀深度脱除高浓度含砷废水中的砷

探索高浓度含砷废水中砷的深度脱除方法及脱除机理。采用预氧化-二段铁盐沉淀的方法脱除炼铅烟尘浸出液中的砷,研究影响砷脱除效率的控制条件和因素,砷脱除机理。结果表明,用双氧水氧化原料液中的低价铁和砷,加FeCl3调节铁砷比=1,在pH=4~5、T=60~70℃反应60 min,砷的脱除率达到99.95%,XRD分析结果表明,沉淀物为非晶态砷酸铁形态;在二段除砷中,控制铁砷比=5、pH=7~8,除砷后液中的砷含量小于0.014 mg/L,远低于允许的排放限值。采用二段铁盐脱砷工艺可以将高砷溶液中的砷深度脱除。     

Conceptual and numerical model of uranium(VI) reductive immobilization in fractured subsurface sediments

A conceptual model and numerical simulations of bacterial U(VI) reduction in fractured subsurface sediments were developed to assess the potential feasibility of biomineralization at the fracture/matrix interface as a mechanism for immobilization of uranium in structured subsurface media. The model envisions flow of anaerobic groundwater, with or without acetate as an electron donor for stimulation of U(VI) reduction by dissimilatory metal-reducing bacteria (DMRB), within mobile macropores along a one-dimensional flow path. As the groundwater moves along the flow path, U(VI) trapped in the immobile mesopore and micropore domains (the sediment matrix) becomes desorbed and transferred to the mobile macropores (fractures) via a first-order exchange mechanism. By allowing bacterial U(VI) reduction to occur in the mesopore domain (assumed to account for 12% of total sediment pore volume) according to experimentally-determined kinetic parameters and an assumed DMRB abundance of 10(7) cells per cm3 bulk sediment (equivalent to 4 mg of cells per dm3 bulk sediment), the concentration of U(VI) in the macropore domain was reduced ca. 10-fold compared to that predicted in the absence of mesopore DMRB activity after a 6-month simulation period. The results suggest that input of soluble electron donors over a period of years could lead to a major redistribution of uranium in fractured subsurface sediments, converting potentially mobile sorbed U(VI) to an insoluble reduced phase (i.e. uraninite) in the mesopore domain that is expected to be permanently immobile under sustained anaerobic conditions.     

山阴地区浅层沉积物中砷的分布特征及吸附行为

本文通过室内实验的方法,对山阴高砷地下水地区不同浅层沉积物中砷的分布特征及吸附行为进行了研究。结果表明：研究区内饮用水浓度高,病情重。浅层承压含水层中的砷含量平均值为290 mg/kg,在17.4～22.7 m段的平均值达643 mg/kg,22.7～25.8 m段的平均值为115 mg/kg,25.8 m以下的平均值为212 mg/kg。As（Ⅲ）、As（Ⅴ）在4种沉积物表面上的动力学吸附过程均符合Lagergren模型,相关系数达0.9以上。沉积物对As（Ⅴ）的吸附速率较As（Ⅲ）快,但As（Ⅲ）在沉积物中的吸附比As（Ⅴ）稳定。4种含水层介质对As（Ⅲ）、As（Ⅴ）的吸附速率依次为粘土〉亚粘土〉粉砂〉中砂。吸附动力学吸附过程符合二级速率方程。Freundlich方程和Langmuir方程对As（Ⅲ）、As（Ⅴ）的吸附过程拟合性最佳,相关系数均在0.95以上。沉积物对As（Ⅲ）吸附的最佳pH在7～8范围内,As（Ⅴ）吸附的最佳pH在5～6左右。此外,沉积物对砷的吸附性能主要受沉积物颗粒大小、矿物成分等因素综合作用的影响,而非各组分吸附砷的简单加和。