pH值对雨水中溶解有机物荧光光谱特征的影响

利用三维荧光光谱研究了pH值改变对雨水中溶解有机物(DOM)荧光组分及荧光指数的影响。2009年夏季在厦门湾采集了一个具有正常pH值的雨水样品,利用平行因子分析(PARAFAC)识别出雨水DOM具有6个荧光组分,包括4个类腐殖质(C1,C3,C4和C5)、1个类蛋白质(C2)以及1个未知组分(C6)。当pH值从初始的5.7增加到10.7,C1,C3和C5的荧光强度均呈增加趋势,而C4及C6的荧光强度则不断减小,导致类腐殖质组分所占比例不断增大;而当pH值减小、酸性增强时则呈相反的变化趋势,且未知组分C6所占比例占优势。类蛋白质组分C2的荧光强度及所占比例随pH变化基本保持不变。荧光指数(FI)、腐殖化指数(HIX)和自生源指标(BIX)也都受到pH值变化的影响。本研究表明,在雨水DOM光谱研究中,应同时测定、报道样品的pH值以便评估雨水荧光特性的"pH效应"。

Effect of pH perturbation on rainwater dissolved organic matter fluorescence characteristics

The influence of pH variation on the fluorescence characteristics of rainwater dissolved organic matter (DOM) was studied via excitation emission-matrix spectroscopy. A rainwater sample (pH 5.7) collected in Xiamen (China) in the summer of 2009 was adjusted to different pH values ranging from 3.2 to 10.7. Parallel factor analysis (PARAFAC) decomposed the fluorescence matrices into four humic-like components (C1, C3, C4, and C5), one protein-like component (C2) and an unknown component (C6). As pH values of the sample increased from the initial value of 5.7 to 10.7, the fluorescence intensities of C1, C3, and C5 also increased, while those of C4 and C6 decreased gradually, resulting in an increasing contribution of humic-like components to total fluorescence. The opposite trend was observed when pH decreased to more acidic conditions and the unknown component (< 250, 330/386 nm) became dominant. The relative contribution of the protein-like component to the total fluorescence remained more or less constant as pH varied. In addition, the fluorescence index (FI), humification index (HIX) and the index of recent autochthonous contribution (BIX) were also affected by pH perturbation. It is therefore recommended that the pH of rainwater samples should be measured and reported in future studies to effectively evaluate such "pH effects" on rainwater DOM fluorescence characteristics.