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

• Published in: The Science of the total environment Vol. 653; pp. 1326 - 1332
• Main Authors: Kulkarni, Harshad, Mladenov, Natalie, Datta, Saugata
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 25.02.2019
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.11.040

The optical properties of bulk dissolved organic matter (DOM) at ambient pH and upon acidification (pH ~2) by hydrochloric acid (HCl) or nitric acid (HNO3) 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 HNO3. 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 (Abs254) was not affected by HCl acidification; however, HNO3 acidification increased Abs254 in groundwater (by 9-fold) as well as in surface water samples (by 3-fold), possibly due to inherent absorbance of HNO3 at 254 nm. Humic- and protein-like fluorescence intensities decreased by HNO3 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 HNO3. 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. [Display omitted] •Response of DOM optical properties to acidification varies between surface and groundwater•Acidification quenches humic-like as well as protein-like fluorescence intensities in groundwater but not in surface water•Fluorescence index and freshness index remain unchanged upon acidification in all samples•Acidification by HNO3 results in the production of a new PARAFAC component