腐殖酸三维荧光光谱特性研究

Characterization of humic acid by three dimensional excitation emission matrix fluorescence spectroscopy

腐殖质的荧光特性被广泛用来解析其在各种天然环境中的来源及分布。由于荧光光谱分析具有灵敏度高,选择性好,且不破坏样品结构的优点,非常适合用来研究腐殖质的结构和官能团等特征。利用三维荧光激发-发射光谱研究了Fluka腐殖酸的荧光光谱特性,结果显示,离子强度(0～0.05mol/LKClO4)对Fluka腐殖酸的三维荧光光谱特性影响非常小,而腐殖酸的浓度(5～100mg/L)和溶液pH(2～12)对其三维荧光光谱特性影响显著。当腐殖酸浓度增大时,荧光峰出现明显红移现象。荧光强度一般随着pH的升高而增大,当pH大于10后呈下降趋势,我们从垃圾渗滤液溶解有机质的实验结果中也得到相同结论;在浓度为50mg/L和100mg/L的Fluka腐殖酸中,荧光峰B(fulvic-like)的荧光强度却在pH=5.0左右时达到最大值,与前人报道的土壤富里酸的行为一致,质子化常数lgK'HL分别为3.57和3.13,与二羧基化合物接近,说明荧光峰B可能与Fluka腐殖酸结构中的羧基有关;荧光峰A与荧光峰B的荧光强度比值r(A/B)在0.61～2.59之间,并且在pH=2～11范围内,r(A/B)与pH具有较好的线性相关关系,表明荧光峰A和荧光峰B随着pH值的改变有着相同的变化趋势。

The fluorescence spectroscopy is widely used to elucidate the origin and structure of humic substances (HS) in natural environments. Due to its high sensitivity, good selectivity and nondestructive nature, fluorescence technique is suitable to the study of HS. In this study, three dimensional excitation emission matrix fluorescence spectroscopy (3DEEM) was applied to characterize a commercial Fluka humic acid. The results show that 3DEEMs of HA have several Ex/Em peaks. Ionic strength in the range of 0～ 0.05 mol/L KClO4 had little effect on the fluorescence spectra, while the concentrations (5～ 100 mg/L) of HA and pH (2～ 12) had significant effect. A red shift in the longer wavelength peak region was observed when concentration or pH value increased. The fluorescence intensity increased with increasing pH, but slightly decreased when pH≥ 10. Similar changing trend was also observed for the sample of landfill leachate dissolved organic matter. For the fulvic like peak B (HA: 50 mg/L or 100 mg/L), the maximum fluorescence occurred at pH 5.0. The protonation constants (lg K′ HL ) were calculated to be 3.57 and 3.13, indicating that peak B was associated with carboxyl groups. The r(A/B) values ranged from 0.61 to 2.59. A strong linear relationship between r(A/B) and pH was also observed. This indicates that the fluorescent molecules of peak A and B seem to be the same sensitive to pH variations.