Ca2+ and SO42− interactions with dissolved organic matter: Implications of groundwater quality for CKDu incidence in Sri Lanka

Abstract

It has recently been proposed that recalcitrant dissolved organic carbon (DOC) in groundwater plays a potent etiological role in the peculiar distribution of chronic kidney disease of unknown etiology (CKDu). This study aims to elucidate the interactions of Ca2+ and SO42− with a model organic fraction of humic acid (SHA) to determine the possible relationship of CKDu incidence with the DOC in drinking water. XPS and FT-IR methods respectively determined the surface functional groups and chemical composition of protonated dissolved organic carbon (HDOC) in a CKDu high-risk zone (HR) of Sri Lanka and in SHA. Higher surface C composition (87.9%) and lower O (11.4%) were observed for HDOC from the HR region than for SHA (C: 73.8%, O: 24.7%). Aromatic C with less O-containing functional groups was observed in HDOC. The IR band at 1170 cm−1 confirms the formation of organic sulfonate (C–SO3-) on SHA. A band at 1213 cm−1 due to organic sulfonate in HDOC from the CKDu HR region was also identified. The IR band at 866 cm−1 evidenced the formation of CaCO3 on SHA above pH 7.4. XPS data confirmed the presence of sulfur oxidation states corresponding to SO32− and SO42− at 168.9 eV and 170.1 eV binding energies, respectively. The binding energies at 347 eV and 351 eV for Ca 2p3/2 and Ca 2p1/2 eV, respectively, confirmed the bidentate complexation of Ca2+ with COO− and sulfonate groups on SHA. The organic sulfonate formed is postulated as a uremic toxicant.