Perez Jeffrey Paulo H, Chan A Li Han, Mosselmans J Frederick W, Benning Liane G
GFZ Helmholtz Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany.
Department of Microbiology, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States.
ACS Earth Space Chem. 2025 Jun 10;9(6):1642-1653. doi: 10.1021/acsearthspacechem.5c00061. eCollection 2025 Jun 19.
Arsenic immobilization in soils and sediments is primarily controlled by its sorption onto or incorporation into reactive soil minerals, such as iron (oxyhydr)-oxides. However, coexisting ions (e.g., dissolved bicarbonate, phosphate, silica, and organic matter) can negatively impact the interaction of the toxic arsenate species with iron (oxy)-hydroxides. Of special note is inorganic phosphate, which is a strong competitor for sorption sites due to its analogous chemical and structural nature to inorganic arsenate. Much of our understanding of this competing nature between phosphate and arsenate focuses on the impact on mineral sorption capacities and kinetics. However, we know very little about how coexisting phosphate will alter the stability and transformation pathways of arsenate-bearing Fe (oxyhydr)-oxides. In particular, the long-term fate and behavior regarding arsenate immobilization are unknown under anoxic conditions. Here, we document, through mineral transformation reactions, the immobilization of both phosphate (P) and arsenate [As-(V)] in secondary mineral products and characterize their changing compositions during the transformations. We did this while controlling the initial P/As-(V) ratios. Our results document that, in the absence or at low P/As-(V) ratios, the initial ferrihydrite rapidly transforms to green rust sulfate (GR ), which further transforms into magnetite after 180 days. Meanwhile, high P/As-(V) ratios resulted in a mixture of GR and vivianite, with magnetite as a minor fraction. Invariably, the speciation and partitioning of As-(V) were also affected by the P/As-(V) ratio. A higher P/As-(V) ratio also led to a faster partial reduction of mineral-bound As-(V) to As-(III). The most important finding is that the initial ferrihydrite-bound As-(V) became structurally incorporated into magnetite [low P/As-(V) ratio] or vivianite [high P/As-(V) ratio] and was thus immobilized and not labile. Overall, our results highlight the influence of coexisting phosphate in controlling the toxicity and mobility in anoxic, Fe-rich subsurface settings, such as contaminated aquifers.
土壤和沉积物中砷的固定主要受其在活性土壤矿物（如铁（氢）氧化物）上的吸附或掺入控制。然而，共存离子（如溶解的碳酸氢根、磷酸盐、二氧化硅和有机物）会对有毒砷酸盐物种与铁（氢）氧化物的相互作用产生负面影响。特别值得注意的是无机磷酸盐，由于其与无机砷酸盐类似的化学和结构性质，它是吸附位点的强竞争者。我们对磷酸盐和砷酸盐之间这种竞争性质的大部分理解集中在对矿物吸附容量和动力学的影响上。然而，我们对共存磷酸盐如何改变含砷铁（氢）氧化物的稳定性和转化途径知之甚少。特别是，在缺氧条件下，砷酸盐固定的长期归宿和行为尚不清楚。在这里，我们通过矿物转化反应记录了磷酸盐（P）和砷酸盐[As-(V)]在次生矿物产物中的固定，并表征了它们在转化过程中不断变化的组成。我们在控制初始P/As-(V) 比率的同时进行了这项工作。我们的结果表明，在不存在或低P/As-(V) 比率的情况下，初始水铁矿迅速转化为绿锈硫酸盐（GR），180天后进一步转化为磁铁矿。同时，高P/As-(V) 比率导致GR和蓝铁矿的混合物，磁铁矿占比很小。不可避免地，As-(V) 的形态和分配也受到P/As-(V) 比率的影响。较高的P/As-(V) 比率还导致矿物结合的As-(V) 更快地部分还原为As-(III)。最重要的发现是，初始水铁矿结合的As-(V) 在结构上被纳入磁铁矿[低P/As-(V) 比率] 或蓝铁矿[高P/As-(V) 比率] 中，因此被固定且不易分解。总体而言，我们的结果突出了共存磷酸盐在控制缺氧、富铁地下环境（如受污染的含水层）中的毒性和迁移性方面的影响。
