Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA.
Environ Sci Pollut Res Int. 2010 Aug;17(7):1362-70. doi: 10.1007/s11356-010-0322-2. Epub 2010 Mar 31.
Dissolved humic acids abiotically reduced inorganic arsenic to varying degrees, depending on solution pH, ionic strength, and type of humate used. The functionalities of dissolved organic matter responsible for these redox reactions remained in question, but quinoid moieties undoubtedly played an important role. It is not fully understood whether the quinoids are solely responsible for arsenate reduction, and what the kinetics of the relevant processes are.
Electron spin resonance (ESR) spectroscopy was used to monitor the radical content of the humates, both as bulk material and as size fractions. Information on the redox status of the humates was obtained from fluorescence excitation-emission matrices and correlated with the observed spin count. Size data were obtained from fractionation and UV-Vis spectrometry. Arsenic speciation was carried out by ion chromatography.
ESR spectroscopy showed a free radical content of 3.4 x 1,017-20 x 1,017 spins/g for bulk and fractionated aqueous humic acids. The number of electrons corresponding to these counts could not account for the entire charge transferred to arsenate during abiotic reduction. The rate constants of the reactions were found to be independent of the humic concentration. Leonardite humic acid separated on a XAD-8 resin yielded fractions that on the short time frame (0-5 h) had rate constants of 0.035 h(-1) for the hydrophobic fraction compared to 0.0052 h(-1) for the hydrophilic fraction. The rate constants for the hydrophobic and hydrophilic fractions over the longer time frame (100-200 h) were similar-7.3 x 10(-4) and 7.2 x 10(-4) h(-1), respectively. Fluorescence excitation-emission matrices of humates involved in arsenate reduction exhibited shifts typical of quinoid components undergoing redox transformations. These gradual shifts took place during the first 24 h of reduction process, after which the spectra no longer changed. The reduction of arsenate, however, continued after this period, indicating that species other than quinoids were involved. This was consistent with the fact that the rate constants for the later processes were smaller.
The existence of different redox pools within the humate was confirmed, with the quinoid-centered redox entities showing the fastest kinetics. The results pertained to all size and polarity fractions.
根据溶液 pH 值、离子强度和所用腐殖酸的类型，溶解的腐殖酸将无机砷不同程度地还原。负责这些氧化还原反应的溶解有机物的功能仍存在疑问，但醌型部分无疑起着重要作用。目前尚不完全清楚醌类物质是否仅负责砷酸盐的还原，以及相关过程的动力学如何。
电子顺磁共振（ESR）光谱法用于监测腐殖酸的自由基含量，包括作为块状材料和作为大小分数。从荧光激发-发射矩阵获得有关腐殖酸氧化还原状态的信息，并与观察到的自旋数相关联。大小数据是通过分级和紫外-可见光谱法获得的。采用离子色谱法进行砷形态分析。
ESR 光谱显示，水相腐殖酸的自由基含量为 3.4 x 1,017-20 x 1,017 自旋/g。这些计数对应的电子数不能说明在非生物还原过程中转移到砷酸盐的全部电荷。发现反应的速率常数与腐殖酸的浓度无关。用 XAD-8 树脂分离的莱奥尼蒂腐殖酸得到的分数在短时间（0-5 h）内，疏水性部分的速率常数为 0.035 h(-1)，而亲水性部分的速率常数为 0.0052 h(-1)。较长时间（100-200 h）内疏水性和亲水性部分的速率常数相似，分别为 7.3 x 10(-4) 和 7.2 x 10(-4) h(-1)。参与砷酸盐还原的腐殖酸的荧光激发-发射矩阵显示出醌型成分经历氧化还原转化的典型位移。这些逐渐的位移发生在还原过程的前 24 小时内，此后光谱不再变化。然而，在此期间砷酸盐的还原仍在继续，表明除了醌类物质之外，还有其他物质参与其中。这与后期过程的速率常数较小的事实一致。
证实了腐殖酸中存在不同的氧化还原库，以醌型为中心的氧化还原实体具有最快的动力学。结果适用于所有大小和极性分数。
