Dulnee Siriwan, Scheinost Andreas C
Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.
Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany; The Rossendorf Beamline at ESRF, F-38043 Grenoble, France.
J Contam Hydrol. 2015 Jun-Jul;177-178:183-93. doi: 10.1016/j.jconhyd.2015.03.012. Epub 2015 Apr 6.
The interaction of Sn(II) with metastable, highly reactive mackinawite is a complex process due to transient changes of the mackinawite surface in the sorption process. In this work, we show that tin redox state and local structure as investigated by Sn-K X-ray absorption spectroscopy (XAS) change with pH. We observe at pH<7 that divalent Sn forms two short (2.38 Å) Sn-S bonds to the S-terminated surface of mackinawite, and two longer (2.59 Å) Sn-S bonds pointing most likely towards the solution phase, in line with a SnS4 innersphere sorption complex. Precipitation of SnS or formation of a solid solution with mackinawite could be excluded. At pH>9, Sn(II) is completely oxidized to Sn(IV) by an Fe(II)/Fe(III) (hydr)oxide, most likely green rust, forming on the surface of mackinawite. Six O atoms at 2.04 Å and 6 Fe atoms at 3.29 Å indicate a structural incorporation by green rust, with Sn(IV) substituting for Fe in the crystal structure. The transition between Sn(II) and Sn(IV) and between sulfur and oxygen coordination takes place at a pH of 7 to 8 and an Eh of -250 mV, close to the thermodynamically predicted transitions from mackinawite to Fe (hydr)oxide and from sulfide to sulfate. The uptake processes of Sn(II) by mackinawite are largely in line with the uptake processes of divalent cations with soft Lewis-acid character like Cd, Hg and Pb, and lead to a strong retention of Sn with logRd values from 5 to 7 across the investigated pH range of 5 to 11.
由于在吸附过程中马基诺矿表面的瞬态变化，Sn(II)与亚稳态、高反应性的马基诺矿之间的相互作用是一个复杂的过程。在这项工作中，我们表明，通过Sn-K X射线吸收光谱（XAS）研究的锡氧化态和局部结构随pH值变化。我们观察到，在pH<7时，二价Sn与马基诺矿的S端表面形成两个短（2.38 Å）的Sn-S键，以及两个较长（2.59 Å）的Sn-S键，最有可能指向溶液相，这与SnS4内球吸附络合物一致。可以排除SnS的沉淀或与马基诺矿形成固溶体的情况。在pH>9时，Sn(II)被马基诺矿表面形成的Fe(II)/Fe(III)（氢）氧化物（很可能是绿锈）完全氧化为Sn(IV)。2.04 Å处的六个O原子和3.29 Å处的六个Fe原子表明绿锈在结构上发生了掺入，Sn(IV)在晶体结构中取代了Fe。Sn(II)和Sn(IV)之间以及硫和氧配位之间的转变发生在pH为7至8和Eh为-250 mV时，接近从马基诺矿到Fe（氢）氧化物以及从硫化物到硫酸盐的热力学预测转变。马基诺矿对Sn(II)的吸收过程在很大程度上与具有软路易斯酸性质的二价阳离子（如Cd、Hg和Pb）的吸收过程一致，并导致在5至11的研究pH范围内对Sn有很强的保留，logRd值为5至7。
