地下水系统中砷活化的钼同位素地球化学指示

砷在天然环境中的迁移富集与氧化还原状态密切相关.盆地环境地下水中砷的活化迁移机制主要为沉积物中铁/锰氢氧化物由氧化还原条件变化导致发生还原性溶解进而释放吸附在其表面的砷.钼及钼同位素为氧化还原环境的重要指示参数, 且铁/锰氢氧化物对钼同位素分馏有着重要的控制作用.将地下水的钼同位素应用于砷的活化迁移规律研究.大同盆地地下水中钼同位素比值(δ98Mo)范围为-0.12‰~+2.17‰, 相比于淡水中钼同位素组成偏重.桑干河河水的δ98Mo为+0.72‰, 与文献报道的河水平均钼同位素比值+0.7‰相当.大同盆地地下水中δ98Mo与硫化物之间存在正相关关系, 表明Mo-Fe-S复合物可能形成于特定条件下, 并优先利用水溶液中轻的钼使地下水中δ98Mo比值升高.砷浓度与钼浓度之间的微弱负相关以及砷浓度与钼同位素之间的正相关说明, Mo-Fe-S的形成过程可能与同环境中As-Fe-S的复合物的形成存在竞争关系, 进而使得地下水中砷富集.地下水中相对偏高的δ98Mo可能来源于铁的氢氧化物对溶液中轻的钼的吸附速率高于先前吸附在铁的氢氧化物的钼的释放, 且铁的氢氧化物对水溶液中钼的再吸附这一循环过程会导致地下水中钼浓度降低及钼同位素比值的升高.钼同位素指示的循环性的铁的氢氧化物的还原溶解及再氧化过程对砷的富集也有重要影响. 

Molybdenum Isotope Geochemistry of Arsenic Mobilization in Groundwater System

Arsenic (AS) mobilization is closely linked to redox state in nature.In basin environment, the primary mechanism governing arsenic mobility is the reductive dissolution of Fe/Mn-(hydr)oxides which results in the subsequent As released into groundwater. Molybdenum (Mo) and Mo isotope can be informative of the redox conditions. Moreover, Mo isotope fractionation is mainly controlled by the adsorption and desorption onto/from Fe/Mn-(hydr)oxides. This study applies Mo isotope ratio(δ98Mo) of dissolved Mo in groundwater to arsenic mobilization in groundwater system for the very first time. The Mo isotope ratios (δ98Mo) in groundwater in Datong basin range from -0.12‰ to 2.17‰, which are relatively heavier than those reported in fresh waters. δ98Mo of Sanggan River shows a value of 0.72‰, comparable to the average δ98Mo of riverine Mo isotopic composition of 0.7‰.δ98Mo ratios of groundwater in Datong basin are positively correlated to dissolved sulfide, indicating that the formation of Mo-Fe-S complex preferentially co-precipitated the light Mo in groundwater resulting in the gradually increased δ98Mo values under certain condition. The formation of Mo-Fe-S complex might be competitive to the similar formation of As-Fe-S complex, as is further confirmed by the weak correlation between As and Mo concentrations and the positive relationship between As and δ98Mo ratios. This process leads to an elevation of As content in groundwater.The relatively heavier δ98Mo ratio of groundwater might be a consequence of the faster rate of adsorption of light Mo from groundwater than the rate of desorption of Mo from Fe-(hydr)oxides and the re-adsorption of dissolved Mo in groundwater.The progressive processes decrease Mo content and elevate δ98Mo ratio in groundwater, which is consistent with the observation in groundwater in Datong basin.The indicative δ98Mo ratio of groundwater indicates that the reductive dissolution of Fe-(hydr)oxides also has important influence on arsenic mobilization in groundwater.