Institute of Geosciences, Burgweg 11, Friedrich-Schiller University, D-07749 Jena, Germany.
Environ Sci Technol. 2011 Jun 1;45(11):4726-32. doi: 10.1021/es1040249. Epub 2011 May 10.
Hydrous ferric oxide (HFO) is an X-ray amorphous compound with a high affinity for anions under strongly or mildly acidic conditions. Because of the usually small particle size of HFO, the adsorption capacity is high and adsorption may significantly impact the thermodynamic properties of such materials. Here we show that adsorption of phosphate and arsenate stabilizes HFO by experimental determination of enthalpies of formation (by acid-solution calorimetry) and estimates of standard entropies for six phosphate- or arsenate-enriched HFO samples. At pH values lower than ∼5, the phosphate-doped HFO is not only less soluble than ferrihydrite (anion-free HFO) but also crystalline FeOOH polymorphs feroxyhyte and lepidocrocite. The arsenate-doped HFO is also stabilized with respect to the ferrihydrite. Phosphate availability in soils can be controlled by the phosphate-enriched HFO which is many orders of magnitude less soluble than apatite or crystalline Fe(III) phosphates, for example strengite (FePO(4)·2H(2)O). Thermodynamic dissolution models for scorodite (FeAsO(4)·2H(2)O) and As-enriched HFO show that under mildly acidic or circumneutral conditions, scorodite dissolves, As-HFO precipitates, and a substantial amount of As(V) is released into the aqueous solution (at pH 7, log m(As) ∼ -2.5). The data presented in this paper can be used to model the equilibrium concentration of Fe(III), P(V), or As(V) in soil solutions or in natural or anthropogenic sediments polluted by arsenic.
水合氧化铁（HFO）是一种 X 射线非晶态化合物，在强酸或弱酸条件下对阴离子具有高亲和力。由于 HFO 的通常粒径较小，因此吸附容量较高，吸附可能会显著影响这些材料的热力学性质。在这里，我们通过形成焓的实验测定（通过酸溶液量热法）和对六种富含磷酸盐或砷酸盐的 HFO 样品的标准熵估计，表明磷酸盐和砷酸盐的吸附稳定了 HFO。在 pH 值低于约 5 的情况下，与无阴离子的 HFO（即水铁矿）相比，磷酸掺杂的 HFO不仅溶解度更低，而且还结晶成 FeOOH 多晶型体纤铁矿和水羟铁矿。砷酸盐掺杂的 HFO 相对于水铁矿也更稳定。土壤中磷酸盐的可用性可以通过富磷酸盐的 HFO 来控制，其溶解度比磷灰石或结晶态 Fe（III）磷酸盐，例如羟磷灰石（FePO4·2H2O）低几个数量级。辉锑矿（FeAsO4·2H2O）和 As 富集 HFO 的热力学溶解模型表明，在弱酸或近中性条件下，辉锑矿溶解，As-HFO 沉淀，大量 As（V）释放到水溶液中（在 pH 值为 7 时，log m（As）∼-2.5）。本文提供的数据可用于模拟土壤溶液中或受砷污染的天然或人为沉积物中 Fe（III）、P（V）或 As（V）的平衡浓度。
