Electron Transfer Budgets and Kinetics of Abiotic Oxidation and Incorporation of Aqueous Sulfide by Dissolved Organic Matter

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227–0.325 h–1. The main transformation products were elemental S (S0) and thiosulfate (S2O32–), yielding electron accepting capacities of 2.82–1.75 μmol e– (mg C)−1. Native iron contents in the HA could account for only 6–9% of this electron transfer. About 22–37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.

天然溶解性有机质（natural dissolved organic matter, DOM）与硫化物的反应活性，在电子转移、产物生成及反应动力学方面尚未得到充分研究。为此，本研究在缺氧条件、pH=6的环境下，探究了硫化物分别与还原态及非还原态西格玛奥德里奇（Sigma-Aldrich）腐殖酸（humic acid, HA）反应时的非生物转化过程。硫化物与非还原态HA的条件速率常数为0.227~0.325 h⁻¹。主要转化产物为单质硫（elemental S, S⁰）与硫代硫酸盐（thiosulfate, S₂O₃²⁻），其电子接受容量达2.82~1.75 μmol e⁻·(mg C)⁻¹。腐殖酸中天然铁的含量仅能解释该电子转移过程的6%~9%。约22%~37%的硫与HA结合生成有机硫（organic S, Sorg）。在还原态HA参与的反应中仅生成Sorg，未检测到任何无机转化产物。X射线吸收近边结构光谱（X-ray absorption near edge structure spectroscopy, XANES）分析结果证实，Sorg主要为零价硫形态，包括巯基、有机二硫化物、多硫化物及杂环类化合物。综上，本研究结果表明，HA可在缺氧环境中通过非生物途径氧化硫化物，其反应速率可与分子氧或铁氧化物介导的硫化物氧化速率相媲美。