Faculty of Material Sciences and Chemistry, China University of Geosciences, Wuhan 430074, China.
Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Institute of Eco-environmental and Soil Sciences, Guangdong Academy of Sciences, Guangzhou 510650, China; National-Regional Joint Engineering Research Center for Soil Pollution Control and Remediation in South China, Guangzhou 510650, China.
J Environ Sci (China). 2022 Mar;113:260-268. doi: 10.1016/j.jes.2021.06.011. Epub 2021 Jul 2.
Electron shuttles such cysteine play an important role in Fe cycle and its availability in soils, while the roles of pH and organic ligands in this process are poorly understood. Herein, the reductive dissolution process of goethite by cysteine were explored in the presence of organic ligands. Our results showed that cysteine exhibited a strong reactivity towards goethite - a typical iron minerals in paddy soils with a rate constant ranging from 0.01 to 0.1 hr. However, a large portion of Fe(II) appeared to be "structural species" retained on the surface. The decline of pH was favorable to generate more Fe(II) ions and enhancing tendency of Fe(II) release to solution. The decline of generation of Fe(II) by increasing pH was likely to be caused by a lower redox potential and the nature of cysteine pH-dependent adsorption towards goethite. Interestingly, the co-existence of oxalate and citrate ligands also enhanced the rate constant of Fe(II) release from 0.09 to 0.15 hr; nevertheless, they negligibly affected the overall generation of Fe(II) in opposition to the pH effect. Further spectroscopic evidence demonstrated that two molecules of cysteine could form disulfide bonds (S-S) to generate cystine through oxidative dehydration, and subsequently, inducing electron transfer from cysteine to the structural Fe(III) on goethite; meanwhile, those organic ligands act as Fe(II) "strippers". The findings of this work provide new insights into the understanding of the different roles of pH and organic ligands on the generation and release of Fe induced by electron shuttles in soils.
半胱氨酸等电子穿梭体在铁循环及其在土壤中的有效性中发挥着重要作用，而 pH 值和有机配体在这一过程中的作用却知之甚少。在此，研究了在有机配体存在的情况下半胱氨酸对针铁矿的还原溶解过程。结果表明，半胱氨酸对半胱氨酸 - 典型的水稻土铁矿物具有很强的反应活性，其速率常数范围为 0.01 至 0.1 小时。然而，大部分 Fe(II)似乎是“结构物种”保留在表面上。pH 值的下降有利于生成更多的 Fe(II)离子，并增强 Fe(II)向溶液中释放的趋势。pH 值增加时生成的 Fe(II)减少可能是由于还原电位较低以及半胱氨酸对针铁矿的 pH 依赖性吸附的性质所致。有趣的是，草酸盐和柠檬酸盐配体的共存也将 Fe(II)释放的速率常数从 0.09 提高到 0.15 小时；然而，与 pH 值的影响相反，它们对总体 Fe(II)的生成几乎没有影响。进一步的光谱证据表明，两个半胱氨酸分子可以形成二硫键 (S-S) 通过氧化脱水生成胱氨酸，随后诱导电子从半胱氨酸转移到针铁矿上的结构 Fe(III)；同时，这些有机配体作为 Fe(II)“剥离剂”。这项工作的发现为理解 pH 值和有机配体在土壤中电子穿梭诱导的 Fe 生成和释放中的不同作用提供了新的见解。
