Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia.
Environ Sci Technol. 2010 Mar 15;44(6):1968-73. doi: 10.1021/es903114z.
Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and S0(4)-reducing conditions in previously oxic-acidic sediments. This creates potential for mobilization of As during the redox transition. We explore the consequences for As by investigating the hydrology, porewater geochemistry, solid-phase speciation, and mineralogical partitioning of As across two tidal fringe toposequences. Seawater inundation induced a tidally controlled redox gradient Maximum porewater As (~400 μg/L) occurred in the shallow (<1 m), intertidal, redox transition zone between Fe-oxidizing and S0(4)-reducing conditions. Primary mechanisms of As mobilization include the reduction of solid-phase As(V) to As(lll), reductive dissolution of As(V)-bearing secondary Fe(lll) minerals and competitive anion desorption. Porewater As concentrations decreased in the zone of contemporary pyrite reformation. Oscillating hydraulic gradients caused by tidal pumping promote upward advection of As and Fe(2+)-enriched porewater in the intertidal zone, leading to accumulation of As(V)-enriched Fe(lll) (hydr)oxides at the oxic sediment-water interface. While this provides a natural reactive-Fe barrier, it does not completely retard the flux of porewater As to overtopping surface waters. Furthermore, the accumulated Fe minerals may be prone to future reductive dissolution. A conceptual model describing As hydro-geochemical coupling across an intertidal fringe is presented.
潮水淹没沿海酸性硫酸盐土壤会在先前的氧化酸性沉积物中产生铁和硫代硫酸盐还原条件。这在氧化还原转化过程中会产生砷的迁移潜力。我们通过调查两个潮间带地形序列中的水文、孔隙水地球化学、固相反位、砷的矿物学分配，来探索砷的后果。海水淹没会引起潮汐控制的氧化还原梯度，最大的孔隙水砷（~400μg/L）出现在浅层（<1m）、潮间带、铁氧化和硫代硫酸盐还原条件之间的氧化还原过渡带。砷迁移的主要机制包括固相砷（V）还原为砷（III）、含砷（V）的次生铁（III）矿物的还原溶解和竞争阴离子解吸。在当代黄铁矿再形成区，孔隙水砷浓度降低。潮汐抽吸引起的波动水力梯度促进了砷和富铁（II）孔隙水在潮间带的向上对流，导致在氧化沉积物-水界面处积累了富含砷（V）的铁（III）（氢）氧化物。虽然这提供了一个自然的反应性铁障碍，但它并不能完全阻止孔隙水砷向越顶地表水的通量。此外，积累的铁矿物可能容易发生未来的还原溶解。提出了一个描述潮间带边缘砷水文地球化学耦合的概念模型。
