Ratasuk Nopawan, Nanny Mark A
Faculty of Science, Silpakorn University, Nakorn Pathom, Thailand 73000.
Environ Sci Technol. 2007 Nov 15;41(22):7844-50. doi: 10.1021/es071389u.
Cyclic oxidation and reduction reactions using oxygen and palladium with H2, respectively, of dissolved humic and fulvic acids (HA and FA) and model quinone compounds were used to structurally characterize and quantify the electron-carrying capacity (ECC) of reversible redox sites present in humic substances. This technique was used to examine 8 quinone compounds and 14 HA and FA samples and identified 3 redox sites as a function of their stability against the Pd-catalyzed hydrogenolysis process. Six highly aliphatic HA and FA isolated from landfill leachate did not contain redox sites under any conditions; however, the other HA and FA demonstrated reversible redox properties characterized by a combination of three redox sites. On the basis of the model compound results, it is proposed that one site consists of a non-quinone structure (NQ) and the other two sites have quinone structures. The two quinone sites differ in that one group (Q1) has electron-withdrawing groups adjacent to the quinone functional group while the second group (Q2) contains either no substituents near the quinone or has nearby electron-donating groups with additional substitutents hindering hydrogenolysis through steric interactions. The reversible ECC of NQ sites ranged from 25 to 265 microequiv e- transferred/g HA or FA, representing 21-56% of the total ECC of the HA and FA when measured with the mildest reducing method (pH 8.0, pure Pd). Q1 redox sites resistant to hydrogenolysis at pH 8.0 using Pd/Al2O3 accounted for 13-58% of the total ECC and ranged from 40 to 120 microequiv e-/ g HA or FA. The most sensitive O2 reversible redox sites accounted for 8-50% of the total ECC (20-220 microequiv e-/ g HA or FA). These results directly demonstrate that HA and FA are capable of acting as reversible electron-transfer agents using different functional groups, some of which may not be quinones.
分别使用氧气和钯与氢气对溶解的腐殖酸和富里酸（HA和FA）以及模型醌化合物进行循环氧化和还原反应，以对腐殖物质中存在的可逆氧化还原位点的结构进行表征并量化其电子携带能力（ECC）。该技术用于检测8种醌化合物以及14个HA和FA样品，并根据它们对钯催化氢解过程的稳定性确定了3个氧化还原位点。从垃圾渗滤液中分离出的6种高度脂肪族HA和FA在任何条件下均不含有氧化还原位点；然而，其他HA和FA表现出可逆的氧化还原特性，其特征是由三个氧化还原位点组合而成。根据模型化合物的结果，提出一个位点由非醌结构（NQ）组成，另外两个位点具有醌结构。这两个醌位点的不同之处在于，一组（Q1）在醌官能团附近具有吸电子基团，而第二组（Q2）在醌附近要么没有取代基，要么具有附近的供电子基团以及通过空间相互作用阻碍氢解的额外取代基。NQ位点的可逆ECC范围为25至265微当量电子转移/克HA或FA，在用最温和的还原方法（pH 8.0，纯钯）测量时，占HA和FA总ECC的21 - 56%。使用Pd/Al2O3在pH 8.0下对氢解具有抗性的Q1氧化还原位点占总ECC的13 - 58%，范围为40至120微当量电子/克HA或FA。最敏感的O2可逆氧化还原位点占总ECC的8 - 50%（20至220微当量电子/克HA或FA）。这些结果直接表明，HA和FA能够利用不同的官能团作为可逆的电子转移剂，其中一些官能团可能不是醌。
