Performance and mechanism of ‘coagulation co-adsorption’ on simultaneous reduction of turbidity and low-fluoride pollution by a polymer metal-based defluorination and turbidity-removal agent (PMDTA)

Abstract

ABSTRACT A polymer metal-based defluorination and turbidity-removal agent (PMDTA) was prepared using a poly-Si–Fe coagulant (PSF) and aluminum. Simultaneous reduction of turbidity and fluoride by PMDTA was studied using the ‘coagulation co-adsorption (CcA)’ method for the simulated and real low-fluoride waters, respectively, compared with that by PSF. The surface and bond structures, charged properties, and size of PMDTA and its flocs were explored, and the mechanism of removing turbidity and fluoride was analyzed. The results showed PMDTA decreased fluoride to <1 mg/L (PSF could not), but both decreased turbidity to <0.5 NTU. PMDTA adapted to a wider initial turbidity, humid acid (HA), NaCl, and temperature. PMDTA had similar bond structures to PSF, more complex surface structures than PSF, and higher surface space and adsorption sites than PSF. Filtration had almost no impact on fluoride removal, but gave positive impact on turbidity removal. The removal mechanisms of fluoride and turbidity were different: the former was mainly removed through adsorption on PMDTA-flocs (controlled by their forming conditions) having efficient adsorption properties, and the latter was mainly cleared through a synergistic effect of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory (classic coagulation mechanism). The efficient fluoride removal by PMDTA can fully utilize the existing facilities in water plants.