Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland.
Environ Sci Technol. 2012 Aug 21;46(16):8808-16. doi: 10.1021/es301520s. Epub 2012 Aug 10.
Naturally occurring variations of redox conditions are considered to affect the interactions between trace metals and humic substances in a 2-fold manner. First, additional proton binding sites of humic substances formed under reducing conditions may also act as binding sites for trace metals. Second, reduced humic substances may transfer electrons to redox-active trace metals. In this study, we investigated the influence of electrochemical reduction of a purified soil humic acid on the binding of two chalcophile metal cations of environmental concern, Cd(2+) and Ag(+), with metal titrations conducted under monitored redox conditions. The binding of cadmium to reduced humic acid was slightly enhanced compared to humic acid reoxidized by O(2) and quantitatively in excellent agreement with the increase in binding sites formed upon reduction. Competitive experiments with calcium indicated that sulfur-containing sites played a minor role in cadmium binding, although sulfur K-edge XANES revealed that 36% of humic sulfur was in a reduced oxidation state. In all experiments with silver, the formation of Ag(0) was detected with transmission electron microscopy. Free Ag(+) activities under reducing conditions were controlled by Ag(0) formation rather than by binding to humic acid.
自然发生的氧化还原条件变化被认为以两种方式影响痕量金属和腐殖物质之间的相互作用。首先，在还原条件下形成的腐殖物质的额外质子结合位点也可能作为痕量金属的结合位点。其次，还原的腐殖物质可能将电子转移到氧化还原活性的痕量金属上。在这项研究中，我们研究了电化学还原纯化的土壤腐殖酸对两种环境关注的亲硫金属阳离子 Cd(2+)和 Ag(+)的结合的影响，在监测的氧化还原条件下进行金属滴定。与氧化还原由 O(2)再氧化的腐殖酸相比，还原腐殖酸对镉的结合略有增强，并且与还原形成的结合位点增加完全一致。与钙的竞争实验表明，含硫位点在镉结合中作用较小，尽管腐殖质硫 K 边 XANES 表明 36%的腐殖质硫处于还原氧化态。在所有与银的实验中，通过透射电子显微镜检测到 Ag(0)的形成。在还原条件下，Ag(+)的自由活性受 Ag(0)形成的控制，而不是与腐殖酸的结合。
