Effects of acidification on the optical properties of dissolved organic matter from high and low arsenic groundwater and surface water

Abstract The optical properties of bulk dissolved organic matter (DOM) at ambient pH and upon acidification (pH ~2) by hydrochloric acid (HCl) or nitric acid (HNO 3 ) were examined in groundwater and surface water samples from the Bengal Basin. Samples of shallow high arsenic (As) and deep low As groundwaters and surface waters from the same geographic area were collected and preserved with HCl and HNO 3 . The optical properties of groundwater samples responded to acidification differently than those of the surface water samples. The intensity of humic-like and protein-like fluorescence decreased by 47% and 80%, respectively, upon acidification with HCl in groundwater but remained unchanged in surface water samples. Similarly, the humification index (HIX) decreased only in surface waters (from 6.6 to 3.7) and remain unchanged in groundwaters upon HCl acidification. The absorbance at 254 nm (Abs 254 ) was not affected by HCl acidification; however, HNO 3 acidification increased Abs 254 in groundwater (by 9-fold) as well as in surface water samples (by 3-fold), possibly due to inherent absorbance of HNO 3 at 254 nm. Humic- and protein-like fluorescence intensities decreased by HNO 3 acidification by 49% and 78% respectively, which may be attributed to aggregation losses and changes in the protonation states of amines, hydroxyls and carboxylic functional groups. Parallel factor (PARAFAC) analysis revealed a unique component that resulted from the acidification of samples with HNO 3 . The other fluorescence indices such as fluorescence index (FI) and freshness index (β:α) remained unchanged upon either type of acidification of surface water as well as groundwater samples. These results reflect the effects of pH perturbation in groundwater environments where DOC concentrations may be lower as compared to surface water environments and provide insights into the structural, molecular and reactive properties of DOM in these environments.