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Distinct roles of pH and organic ligands in the dissolution of goethite by cysteine.

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Title: Distinct roles of pH and organic ligands in the dissolution of goethite by cysteine.
Authors: Li, Ji1 (AUTHOR), Shi, Chenlu1,2,3 (AUTHOR), Zeng, Wenbin1 (AUTHOR), Wang, Yaru1 (AUTHOR), Hong, Zebin2,3 (AUTHOR), Ma, Yibing4 (AUTHOR), Fang, Liping1,2,3 (AUTHOR) jegerfang@gmail.com
Source: Journal of Environmental Sciences (Elsevier). Mar2022, Vol. 113, p260-268. 9p.
Subject Terms: *pH effect, *Soil mineralogy, Goethite, Cysteine, Ligands (Chemistry), Charge exchange, Reduction potential
Abstract: • The rate constant of Fe(II) release and generation correlated with cysteine. • A low pH thermodynamically enhanced electron transfer from cysteine to goethite. • Organic ligands only kinetically promoted release of the less available Fe(II). • Electron transfer from cysteine to goethite was via S-S complexation on goethite. 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−1. 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−1; 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. [Display omitted] [ABSTRACT FROM AUTHOR]
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Abstract:• The rate constant of Fe(II) release and generation correlated with cysteine. • A low pH thermodynamically enhanced electron transfer from cysteine to goethite. • Organic ligands only kinetically promoted release of the less available Fe(II). • Electron transfer from cysteine to goethite was via S-S complexation on goethite. 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−1. 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−1; 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. [Display omitted] [ABSTRACT FROM AUTHOR]
ISSN:10010742
DOI:10.1016/j.jes.2021.06.011