Variations in the redox state of As and Fe measured by X-ray absorption spectroscopy in aquifers of Bangladesh and their effect on As adsorption
Variations in the redox state of As and Fe measured by X-ray absorption spectroscopy in aquifers of Bangladesh and their effect on As adsorption
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DOI：
10.1016/j.apgeochem.2009.09.026
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发表时间：
2010-01-01
影响因子：
3.4
通讯作者：
Mitamura, Muneki
中科院分区：
文献类型：
--
作者：
Itai, Takaaki;Takahashi, Yoshio;Mitamura, Muneki
地下水富集天然存在的砷是一个非常严重的问题。在孟加拉国，成千上万的人饮用这种地下水。家庭井水沃茨中砷的浓度变化很大，即使在一个小村庄。这种差异给缓解战略的设计带来了困难。据推测，在每个井中的As浓度是由沉积物和地下水之间的吸附-解吸平衡控制。为了验证这一假设，本研究主要关注两个因素：（i）As和Fe在固相中的形态;（ii）As（III）和As（V）在沉积物上的吸附特性。通过这种方法，它是可能的了解在地下水中的水相砷的变化。沉积物岩心样品收集自孟加拉国中东部Sonargaon的富砷含水层。X射线吸收近边结构（XANES）分析表明，沉积物中As和Fe的氧化态在距地表5 m以下有一个明显的氧化还原边界，而溶解态As的浓度峰值出现在15 m以下。As（V）的表观分配系数（Kd = C-固/C-固溶）在各深度均大于As（III）。考虑As的氧化态，用磷提取的As量乘以K-d（-1）得到的地下水As模拟浓度与地下水As的深度剖面一致。这表明，溶解态砷的浓度是由沉积物和地下水之间的吸附-解吸平衡控制。Kd的变化主要是由Fe羟基氧化物的浓度控制，而表面积也是重要的As（III）。氧化还原边界的深度和溶解As的峰值之间的差异归因于P-提取As的丰度的差异，而不是Kd的变化。本文提出的吸附平衡模型可用于评价全新世地下水中As含量的大幅度变化。(C)2009爱思唯尔有限公司保留所有权利。
Groundwater enrichment with naturally occurring As is a very serious problem. In Bangladesh, many thousands of people are drinking such groundwater. Household well waters generally show a large variation of concentration of As even within a small village. This variation creates difficulty in the design of mitigation strategies. It was hypothesized that the concentration of As in each well is controlled by an adsorption-desorption equilibrium between sediment and groundwater. To verify the hypothesis, two factors are focused upon in this study: (i) speciation of As and Fe in the solid phase, and (ii) the adsorption properties of As(III) and As(V) to sediment. By this approach, it is possible to understand the variation of aqueous As in groundwater. Sediment core samples were collected from an As-enriched aquifer at Sonargaon, central eastern Bangladesh. The oxidation states of As and Fe in the sediments determined by Xray absorption near edge structure (XANES) showed a distinct redox boundary below 5 m from the ground surface, whereas the peak of the concentration of dissolved As is observed below 15 m. The apparent distribution coefficient (K-d = C-solid/C-solution) of As(V) is always larger than that of As(III) at all the depths. Simulated concentrations of As in the groundwater obtained by multiplying the amount of P-extracted As and K-d(-1) with consideration of the oxidation state of As is consistent with the depth profile of As in the groundwater. This suggests that the concentration of dissolved As is controlled by an adsorption-desorption equilibrium between sediment and groundwater. Variation in Kd is primarily controlled by the concentration of Fe oxyhydroxides, whereas surface area is also important for As(III). The discrepancy between the depth of the redox boundary and the peak of dissolved As is attributed to the difference in the abundance of P-extracted As rather than to a variation of Kd. The adsorption equilibrium model proposed in this study can be applied to the evaluation of the large variation in aqueous As concentration in groundwater from Holocene aquifers. (C) 2009 Elsevier Ltd. All rights reserved.