The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite-water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7-9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In-situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8 - 9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0 mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27 Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)-Fe bond distances of ~2.92-2.94 and 3.41-3.44 Å, respectively. The As-Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As-Fe bonding mechanisms.

中文翻译：

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硅酸对 6-L 水铁矿的砷酸盐和亚砷酸盐保留机制的影响：光谱和批量吸附方法

使用分批吸附实验、衰减全反射傅里叶变换红外 (ATR-FTIR) 光谱、扩展 X 射线吸收精细结构 (EXAFS) 在宽 pH 范围内研究了砷酸盐和亚砷酸盐与硅酸在水铁矿-水界面上的竞争吸附) 光谱学和密度泛函理论 (DFT) 建模。批量吸附结果表明砷酸盐和亚砷酸盐在 6-L 水铁矿表面的吸附表现出强烈的 pH 依赖性，并且 pH 对砷吸附的影响在砷酸盐和亚砷酸盐之间存在差异。砷酸盐吸附随着 pH 值的增加而持续降低；而亚砷酸盐的吸附最初随着 pH 值的增加而增加，在 pH 值 7-9 时达到最大吸附，之后吸附随着 pH 值的进一步增加而减少。结果表明，在实验条件下，硅酸和砷酸盐之间的竞争吸附可以忽略不计；而在硅酸和亚砷酸盐之间观察到强烈的竞争吸附，特别是在低和高 pH 值下。原位、流通式 ATR-FTIR 数据显示，在没有硅酸的情况下，砷酸盐在整个 pH 范围内在水铁矿表面形成球内双核双齿复合物。在本研究探测的整个 pH 范围内（pH 2.8 - 9.0），硅酸还在水铁矿表面形成内球复合物。ATR-FTIR 数据还表明，在所研究的环境相关浓度（例如，1.0 mM）下，硅酸在水铁矿表面发生聚合。根据 ATR-FTIR 数据，砷酸盐络合模式不受硅酸存在的影响。EXAFS 分析和 DFT 模型证实，砷酸四面体通过双核双齿络合与 Fe 金属中心键合，平均 As(V)-Fe 键距为 3.27 Å。EXAFS 数据表明亚砷酸盐与 6-L 水铁矿形成单核双齿和双核双齿复合物，如两个 As(III)-Fe 键距分别为 ~2.92-2.94 和 3.41-3.44 Å 所示。在 Si 存在下，砷酸盐和亚砷酸盐 EXAFS 光谱中的 As-Fe 键距保持不变，这表明虽然 Si 优先减少亚砷酸盐吸附，但它对 As-Fe 键合机制的影响可以忽略不计。EXAFS 分析和 DFT 模型证实，砷酸四面体通过双核双齿络合与 Fe 金属中心键合，平均 As(V)-Fe 键距为 3.27 Å。EXAFS 数据表明亚砷酸盐与 6-L 水铁矿形成单核双齿和双核双齿复合物，如两个 As(III)-Fe 键距分别为 ~2.92-2.94 和 3.41-3.44 Å 所示。在 Si 存在下，砷酸盐和亚砷酸盐 EXAFS 光谱中的 As-Fe 键距保持不变，这表明虽然 Si 优先减少亚砷酸盐吸附，但它对 As-Fe 键合机制的影响可以忽略不计。EXAFS 分析和 DFT 模型证实，砷酸四面体通过双核双齿络合与 Fe 金属中心键合，平均 As(V)-Fe 键距为 3.27 Å。EXAFS 数据表明亚砷酸盐与 6-L 水铁矿形成单核双齿和双核双齿复合物，如两个 As(III)-Fe 键距分别为 ~2.92-2.94 和 3.41-3.44 Å 所示。在 Si 存在下，砷酸盐和亚砷酸盐 EXAFS 光谱中的 As-Fe 键距保持不变，这表明虽然 Si 优先减少亚砷酸盐吸附，但它对 As-Fe 键合机制的影响可以忽略不计。EXAFS 数据表明亚砷酸盐与 6-L 水铁矿形成单核双齿和双核双齿复合物，如两个 As(III)-Fe 键距分别为 ~2.92-2.94 和 3.41-3.44 Å 所示。在 Si 存在下，砷酸盐和亚砷酸盐 EXAFS 光谱中的 As-Fe 键距保持不变，这表明虽然 Si 优先减少亚砷酸盐吸附，但它对 As-Fe 键合机制的影响可以忽略不计。EXAFS 数据表明亚砷酸盐与 6-L 水铁矿形成单核双齿和双核双齿复合物，如两个 As(III)-Fe 键距分别为 ~2.92-2.94 和 3.41-3.44 Å 所示。在 Si 存在下，砷酸盐和亚砷酸盐 EXAFS 光谱中的 As-Fe 键距保持不变，这表明虽然 Si 优先减少亚砷酸盐吸附，但它对 As-Fe 键合机制的影响可以忽略不计。