Wang Qihuang, Wang Jiajia, Wang Xingxing, Kumar Naresh, Pan Zezhen, Peiffer Stefan, Wang Zimeng
Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China.
Soil Chemistry and Chemical Soil Quality, Wageningen University and Research, Wageningen 6708, The Netherlands.
Environ Sci Technol. 2023 Mar 14;57(10):4342-4353. doi: 10.1021/acs.est.2c06921. Epub 2023 Mar 2.
The association of poorly crystalline iron (hydr)oxides with organic matter (OM), such as extracellular polymeric substances (EPS), exerts a profound effect on Fe and C cycles in soils and sediments, and their behaviors under sulfate-reducing conditions involve complicated mineralogical transformations. However, how different loadings and types of EPS and water chemistry conditions affect the sulfidation still lacks quantitative and systematic investigation. We here synthesized a set of ferrihydrite-organic matter (Fh-OM) coprecipitates with various model compounds for plant and microbial exopolysaccharides (polygalacturonic acids, alginic acid, and xanthan gum) and bacteriogenic EPS (extracted from ). Combining wet chemical analysis, X-ray diffraction, and X-ray absorption spectroscopic techniques, we systematically studied the impacts of C and S loadings by tracing the temporal evolution of Fe mineralogy and speciation in aqueous and solid phases. Our results showed that the effect of added OM on sulfidation of Fh-OM coprecipitates is interrelated with the amount of loaded sulfide. Under low sulfide loadings (S(-II)/Fe < 0.5), transformation to goethite and lepidocrocite was the main pathway of ferrihydrite sulfidation, which occurs more strongly at pH 6 compared to that at pH 7.5, and it was promoted and inhibited at low and high C/Fe ratios, respectively. While under high sulfide loadings (S(-II)/Fe > 0.5), the formation of secondary Fe-S minerals such as mackinawite and pyrite dominated ferrihydrite sulfidation, and it was inhibited with increasing C/Fe ratios. Furthermore, all three synthetic EPS proxies unanimously inhibited mineral transformation, while the microbiogenic EPS has a more potent inhibitory effect than synthetic EPS proxies compared at equivalent C/Fe loadings. Collectively, our results suggest that the quantity and chemical characteristics of the associated OM have a strong and nonlinear influence on the extent and pathways of mineralogical transformations of Fh-OM sulfidation.
结晶性差的铁（氢）氧化物与有机物质（OM）（如胞外聚合物（EPS））的结合，对土壤和沉积物中的铁和碳循环产生深远影响，其在硫酸盐还原条件下的行为涉及复杂的矿物学转变。然而，不同负荷和类型的EPS以及水化学条件如何影响硫化作用仍缺乏定量和系统的研究。我们在此合成了一组铁水合物 - 有机物质（Fh - OM）共沉淀物，其中包含用于植物和微生物胞外多糖（聚半乳糖醛酸、海藻酸和黄原胶）以及细菌源EPS（从 中提取）的各种模型化合物。结合湿化学分析、X射线衍射和X射线吸收光谱技术，我们通过追踪水相和固相中铁矿物学和形态的时间演变，系统地研究了碳和硫负荷的影响。我们的结果表明，添加的OM对Fh - OM共沉淀物硫化作用的影响与加载的硫化物量相关。在低硫化物负荷（S(-II)/Fe < 0.5）下，向针铁矿和纤铁矿的转变是铁水合物硫化的主要途径，与pH 7.5相比，在pH 6时这种转变更强烈，并且在低和高C/Fe比时分别受到促进和抑制。而在高硫化物负荷（S(-II)/Fe > 0.5）下，次生Fe - S矿物（如马基诺矿和黄铁矿）的形成主导了铁水合物的硫化作用，并且随着C/Fe比的增加而受到抑制。此外，所有三种合成EPS替代物均一致抑制矿物转化，而在等效C/Fe负荷下比较时，微生物源EPS比合成EPS替代物具有更强的抑制作用。总体而言，我们的结果表明，相关OM的数量和化学特性对Fh - OM硫化作用的矿物学转变程度和途径具有强烈的非线性影响。
