Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich , 8092 Zurich, Switzerland.
Section for Environmental Chemistry and Physics, Department of Plant and Environmental Sciences, University of Copenhagen , DK-1871 Frederiksberg C, Denmark.
Environ Sci Technol. 2016 Apr 5;50(7):3607-16. doi: 10.1021/acs.est.5b05791. Epub 2016 Mar 11.
Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low-flow stream conditions, flocs may settle, become buried, and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study, we reacted freshwater flocs (704-1280 mg As/kg) from a minerotrophic peatland (Gola di Lago, Switzerland) with sulfide (5.2 mM, S(-II)spike/Fe = 0.75-1.62 mol/mol) at neutral pH and studied the speciation changes of Fe, S, and As at 25 ± 1 °C over 1 week through a combination of synchrotron X-ray techniques and wet-chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52-81% of Fesolid at day 7) as well as solid-phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite, although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide-reacted flocs contained primarily arsenate (47-72%) which preferentially adsorbed to Fe(III)-(oxyhydr)oxides, despite abundant mackinawite precipitation. At higher S(-II)spike/Fe molar ratios (≥1.0), the formation of an orpiment-like phase accounted for up to 35% of solid-phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III)-(oxyhydr)oxides, mobilization of As was recorded in all samples (Asaq = 0.45-7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of Asaq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate-reducing conditions and emphasize the pivotal role Fe-rich organic freshwater flocs play in trace metal(loid) cycling in S-rich wetlands characterized by oscillating redox conditions.
富铁有机絮体经常在湿地的地表水层中被观察到，并且对痕量金属（类）具有高亲和力。在低流量溪流条件下，絮体可能会沉降、埋藏，并最终处于还原条件下，有利于痕量金属（类）的释放。在这项研究中，我们用硫化物（5.2 mM，S(-II) 脉冲/Fe = 0.75-1.62 mol/mol）与来自矿营养泥炭地（瑞士拉哥湖）的淡水絮体（704-1280 mg As/kg）反应，在中性 pH 下，在 25 ± 1°C 下通过同步加速器 X 射线技术和湿化学分析相结合的方法研究了 Fe、S 和 As 的形态变化在一周内。絮体的水铁矿和纳米晶纤铁矿的硫化作用导致了磁黄铁矿的快速形成（第 7 天时 Fesolid 的 52-81%）以及固相间的 S(0)和多硫化物。尽管不能排除从水铁矿新形成纤铁矿，但水铁矿优先于纤铁矿被还原。尽管有大量磁黄铁矿沉淀，但硫化物反应后的絮体主要含有砷酸盐（47-72%），其优先吸附到 Fe(III)-(氧氢)氧化物上。在较高的 S(-II) 脉冲/Fe 摩尔比（≥1.0）下，高达 35%的固相 As 形成了雄黄样相。尽管有 Fe 和 As 硫化物沉淀和残留的 Fe(III)-(氧氢)氧化物存在，但在所有样品中都记录到了 As 的迁移（7 天时 Asaq = 0.45-7.0 μM）。水相 As 形态分析记录了硫代砷酸盐的形成，其占 Asaq 的高达 33%。我们的研究结果表明，来自拉哥湖泥炭地的淡水絮体在硫酸盐还原条件下可能成为 As 的来源，并强调了富含铁的有机淡水絮体在富含硫的湿地中痕量金属（类）循环中所起的关键作用，这些湿地的氧化还原条件波动。
