Effects of pH and salinity on adsorption of hypersaline photosynthetic microbial mat exopolymers to goethite: A study using a quartz crystal microbalance and fluorescence spectroscopy

Salinity and pH are two water chemical factors that vary drastically—seasonally or daily—in a variety of aquatic environments. Drastic change of salinity and pH may have a significant impact on adsorption of microbial extracellular polymers (EPS) to sediment minerals and thus influence many other important elemental geochemical processes. However, how salinity and pH changes affect EPS adsorption to minerals is poorly known. In the present study, adsorption of EPS from a hypersaline microbial mat to goethite at different pH values (4, 7 and 10) and salinity (0.15‰, 35‰ and 70‰) was monitored online using a quartz crystal microbalance (QCM) with EEM fluorescence spectroscopy as a complementary method. The adsorption kinetics were well described by an exponential function. The adsorption capacity of EPS to goethite significantly increased as the pH and salinity increased. The pH and salinity dependence of EPS adsorption to goethite is of great importance for understanding the environmental geochemistry of heavy metals in aquatic environments. In inland lakes and estuary zones, the seasonal or daily pulse of hydrological events will drastically alter water salinity and pH and consequently exert significant influences on the mobility, chemical species and ecotoxicity of heavy metals through the EPS pathway. It is necessary to systematically investigate the geochemical behavior of EPS in such dynamic aquatic environments and their potential effects on other geochemical processes.