Coprecipitation facilitates the incorporation of Zn into the lattice of Fe (hydr)oxide, which is a crucial mechanism causing low Zn bioavailability in widespread Zn-deficient soils. Zn isotopes are a potential indicator of the coprecipitation of Zn and Fe (hydr)oxides in soils. However, the Zn isotope fractionation caused by coprecipitation with amorphous Fe (hydr)oxides (e.g., ferrihydrite) and the impact of environmentally relevant conditions remain unknown. Here, the molecular mechanism and impact factor of Zn isotope fractionation during Zn-Fe coprecipitation under natural soil conditions (including different pH values and initial Zn/Fe ratios) were investigated by combining isotope ratio measurements, extended X-ray absorption fine structure (EXAFS) analyses, and density functional theory (DFT) geometry optimization. The results show that aqueous Zn (Zn) can be complexed on the ferrihydrite surface (Zn) with positive isotope fractionation (ΔZn = 0.42 ± 0.09 ‰) or directly incorporated into the ferrihydrite lattice with slightly negative isotope fractionation (ΔZn = −0.08 ± 0.03 ‰). In addition, over time, the surface-complexed Zn can further transform to a layered structure on the surface but exhibits limited Zn isotope fractionation. According to Zn K-edge EXAFS, the Zn isotope fractionation of surface complexation and incorporation of Zn are related to the decrease in the Zn–O bond strength in the order surface complexed Zn (R = 2.00 Å) > aqueous Zn (R = 2.08 Å) > directly incorporated Zn (R = 2.06–2.07 Å with slight distortion). The limited Zn isotope fractionation associated with the transformation of Zn is related to the reconfiguration of Zn during the nucleation of octahedral Zn layer structures. Furthermore, the primary process controlling Zn isotope fractionation is different under varying experimental conditions, leading to distinct Zn isotope fractionation between the bulk solid and solution during Zn–Fe coprecipitation. Low pH values (5.0–6.0) and initial Zn/Fe ratios (0.01–0.02) favor the direct precipitation of Zn, resulting in slightly negative Zn isotope fractionation between the bulk solid and solution. In contrast, high pH values (7.0–7.5) and initial Zn/Fe ratios (0.1–0.2) favor surface complexation and the transformation of Zn, leading to heavy Zn isotope enrichment in the bulk solid. These findings offer novel insights into the primary mechanism through which Fe (hydr)oxides regulate Zn bioavailability in Zn-deficient soils and provide experimental evidence supporting the potential application of Zn isotopes as an indicator for comprehending the fate of Zn controlled by Fe (hydr)oxides in soils.

中文翻译：

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与无定形铁（氢）氧化物共沉淀过程中的锌同位素分馏


共沉淀促进锌结合到铁（氢）氧化物晶格中，这是导致广泛缺锌土壤中锌生物利用度低的关键机制。锌同位素是土壤中锌和铁（氢）氧化物共沉淀的潜在指标。然而，由与无定形铁（氢）氧化物（例如水铁矿）共沉淀引起的锌同位素分馏以及环境相关条件的影响仍然未知。本文通过结合同位素比测量、扩展X射线吸收精细结构（EXAFS）研究了自然土壤条件（包括不同pH值和初始Zn/Fe比）下Zn-Fe共沉淀过程中Zn同位素分馏的分子机制和影响因素。 ）分析和密度泛函理论（DFT）几何优化。结果表明，水相锌（Zn）可以络合在具有正同位素分馏（ΔZn = 0.42 ± 0.09 ‰）的水铁矿表面（Zn）上，或者直接掺入到具有轻微负同位素分馏（ΔZn = -0.08 ± 0.03）的水铁矿晶格中。 ‰）。此外，随着时间的推移，表面络合的锌可以进一步转变为表面的层状结构，但表现出有限的锌同位素分馏。根据 Zn K-edge EXAFS，表面络合的 Zn 同位素分馏和 Zn 的掺入与 Zn-O 键强度的降低有关，顺序为表面络合 Zn (R = 2.00 Å) > 水相 Zn (R = 2.08 Å) > 直接掺入 Zn（R = 2.06–2.07 Å，略有变形）。与 Zn 转变相关的有限 Zn 同位素分馏与八面体 Zn 层结构成核过程中 Zn 的重构有关。 此外，在不同的实验条件下，控制锌同位素分馏的主要过程是不同的，导致在 Zn-Fe 共沉淀过程中，块体固体和溶液之间存在不同的锌同位素分馏。低 pH 值 (5.0–6.0) 和初始 Zn/Fe 比 (0.01–0.02) 有利于 Zn 的直接沉淀，导致本体固体和溶液之间的 Zn 同位素分馏略为负值。相反，高pH值（7.0-7.5）和初始Zn/Fe比率（0.1-0.2）有利于表面络合和Zn的转化，导致固体中重Zn同位素富集。这些发现为铁（氢）氧化物调节缺锌土壤中锌生物利用度的主要机制提供了新的见解，并提供了实验证据，支持锌同位素作为理解铁（氢）控制的锌命运的潜在应用的实验证据土壤中的氧化物。