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

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 Ocontaining functional groups was observed in HDOC.The IR band at 1170 cm-1confirms the formation of organic sulfonate(C–SO3~-) on SHA.A band at 1213 cm-1due to organic sulfonate in HDOC from the CKDu HR region was also identified.The IR band at 866 cm-1evidenced the formation of CaCO3on 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 2p3/2and Ca 2p1/2eV,respectively, confirmed the bidentate complexation of Ca2+with COO-and sulfonate groups on SHA.The organic sulfonate formed is postulated as a uremic toxicant.