Application of a novel zirconium coagulant in the coagulation-ultrafiltration process: Fluoride removal and membrane fouling alleviation

Groundwater fluoride pollution is a worldwide problem that threatens human health and drinking water. Economical and efficient coagulation is the most widely used defluorination technology. The fluoride removal efficiency depends on the complexation and adsorption capacities of the hydrolyzed metal salt coagulant, so the development of new metal coagulants is the key to coagulation defluorination technology. In this study, novel polymeric zirconium salt coagulants (ZXCs) were prepared by the sol-gel method. Their capabilities in coagulation and coagulation-ultrafiltration processes were evaluated and compared with those of the traditional polyaluminum chloride (PAC) and polyferric sulfate (PFS) coagulants. ZXC exhibited more effective fluoride removal than PAC and PFS, and the fluoride removal rates of ZXC, PAC and PFS were 81.0 %, 24.7 % and 5.4 % with the optimized dosages of 0.30 mM and pHs of 5.0, 6.0 and 5.0, respectively. With an initial fluoride concentration of 4.0 mg/L, 0.30 and 1.5 mM doses of ZXC reduced the fluoride concentrations to 0.76 and 0.13 mg/L. The structures and properties of ZXC and the flocs were characterized with Fourier Transform Infrared spectroscopy (FT-IR), X-ray Diffraction (XRD), X-ray photoelectron Spectroscopy (XPS), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area determinations. The series characterization and hydrolysis precipitation area map showed that adsorption onto hydroxyl groups and ion exchange with surface hydroxyl groups were the main pathways for fluoride removal by ZXC. In the coagulation-ultrafiltration defluorination process, the highly polymerized ZXC was fully hydrolyzed under acidic conditions to form larger and looser flocs with almost no dissolved residue, which substantially reduced membrane fouling. This work provides new insights into the development of materials and technologies for fluoride removal from drinking water.