Schwertmannite is a metastable sulfate-rich ferric iron Fe(III) (oxyhydr)oxide and a common mineral in acid mine drainage sites and acid sulfate soils. Schwertmannite is also used as a sorbent in various industrial applications, including phosphate removal in water treatment and environmental remediation. Phosphate sorption to schwertmannite, however, is complex and likely involves ligand exchange for inner- and outer-spherically coordinated sulfate groups, both on the surface and in the tunnel structure of the mineral. Here, we investigated phosphate sorption, concomitant sulfate release and their impact on the structure of schwertmannite as a function of pH and phosphate concentration. Kinetic and equilibrium batch experiments with synthetic schwertmannite were carried out at pH 3, 6, and 8, and the solid-phase was analyzed using a combination of microscopic, spectroscopic, and X-ray diffraction techniques. We found a strong correlation between phosphate sorption and sulfate release, with both following a two-step sorption model. The kinetics of phosphate sorption and sulfate release were faster at higher pH levels. Maximum phosphate sorption was found at pH 6 (1.7 mmol PO g), which decreased to 1.5 and 1.2 mmol PO g at pH 3 and 8, respectively. Fourier transform infrared spectroscopy revealed a shift from inner- to outer-spherical coordination of sulfate with increasing pH. Fe-Mössbauer analyses of schwertmannite demonstrated an initial transformation of schwertmannite at neutral to alkaline pH, characterized by a rise in a partially ordered sextet area. This change was interpreted as an increase in crystallinity resulting from the transition from Fe-SO to FeO domains. The emerging phase differed from the original schwertmannite, but did not represent a complete change to a new crystalline phase, thus indicating a proto-transformation of schwertmannite. This pH-induced proto-transformation was inhibited in the presence of phosphate. We concluded that the phosphate sorption rate and maximum as well as the proto-transformation of schwertmannite were strongly affected by the mineral's affinity for sulfate. Sorption to schwertmannite should primarily be regarded as a competitive exchange reaction between the sorbing oxyanion and the bound sulfate. As a result, the highest phosphate sorption occurred at circumneutral pH, in stark contrast to non-sulfate-containing Fe(III) (oxyhydr)oxides, where phosphate sorption is highest at acidic pH. Our findings are important for a fundamental understanding of the sorption properties of schwertmannite in phosphate-rich environments as they point towards the central role of sulfate coordination for phosphate immobilization.

中文翻译：

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硫酸盐亲和力控制磷酸盐吸附和施威特曼石的原始转化

施威特曼石是一种亚稳态富含硫酸盐的三价铁 Fe(III)（羟基）氧化物，是酸性矿山排水场所和酸性硫酸盐土壤中的常见矿物。施威特曼石还用作各种工业应用中的吸附剂，包括水处理和环境修复中的磷酸盐去除。然而，磷酸盐对施威特曼石的吸附是复杂的，并且可能涉及矿物表面和隧道结构中的内球和外球配位硫酸盐基团的配体交换。在这里，我们研究了磷酸盐吸附、伴随的硫酸盐释放及其对施威特曼石结构的影响（作为 pH 值和磷酸盐浓度的函数）。在 pH 3、6 和 8 下进行了合成施威特曼石的动力学和平衡批量实验，并结合显微镜、光谱和 X 射线衍射技术对固相进行了分析。我们发现磷酸盐吸附和硫酸盐释放之间存在很强的相关性，两者都遵循两步吸附模型。 pH 值越高，磷酸盐吸附和硫酸盐释放的动力学越快。在 pH 6 时发现最大磷酸盐吸附（1.7 mmol PO g），在 pH 3 和 8 时分别降至 1.5 和 1.2 mmol PO g。傅里叶变换红外光谱显示，随着 pH 值的增加，硫酸盐的配位从内球到外球的转变。施威特曼石的 Fe-穆斯堡尔分析表明施威特曼石在中性至碱性 pH 值下发生初始转变，其特征是部分有序六重峰区域的上升。这种变化被解释为由于从 Fe-SO 域转变为 FeO 域而导致结晶度增加。出现的相与原始施威特曼石不同，但并不代表完全转变为新的结晶相，因此表明施威特曼石的原始转变。这种 pH 诱导的原转化在磷酸盐存在下受到抑制。我们得出的结论是，磷酸盐吸附率和最大值以及施威特曼石的原始转化受到矿物对硫酸盐的亲和力的强烈影响。对施威特曼石的吸附主要应被视为吸附氧阴离子和结合的硫酸盐之间的竞争性交换反应。因此，最高的磷酸盐吸附发生在中性 pH 值附近，与不含硫酸盐的 Fe(III)（羟基）氧化物形成鲜明对比，其中磷酸盐吸附在酸性 pH 条件下最高。我们的研究结果对于从根本上理解施威特曼石在富含磷酸盐的环境中的吸附特性非常重要，因为它们指出了硫酸盐配位对于磷酸盐固定的核心作用。