Ca2+ complexation of dissolved organic matter in arid inland lakes is significantly affected by drastic seasonal change of salinity

CaCO3 precipitation is one of the most common and important geochemical processes in the arid inland waters and it can be significantly affected by interaction of DOM with Ca2+. Effects of the drastic seasonal change of water salinity on interaction of DOM with Ca2+ in the arid inland waters were completely unknown. In the present study, complexation of DOM with Ca2+ in the freshwater (0.5‰ salinity) and hypersaline water (70‰ salinity) were comparatively examined by excitation-emission matrix (EEM) fluorescence quenching titration and isothermal titration calorimetry (ITC). The complexation of DOM with Ca2+ was significantly influenced by the drastic change of salinity. The ITC complexation is exothermic at 0.5‰ salinity but turns to an endothermic process at 70‰ salinity. More energy is needed for the complex reaction between DOM and Ca2+ under the hypersaline condition than in the fresh water. Fluorescence quenching titration indicates that DOM has stronger binding ability toward Ca2+ in the freshwater than in the saline water, and more fractions of DOM in the freshwater are accessible to Ca2+ than in the saline water. Ca2+ complexation in the DOM is dominated by the tryptophan-like components at both salinities and the complexation of Ca2+ with fulvic acid-like components is ignorable. The findings is important for understanding the aquatic geochemical processes in some lakes that seriously affected by irrigation water use in the arid zone.