Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration–Bipolar Membrane Electrodialysis-Based Closed-Loop Process

Abstract

Traditional technologies such as precipitation and coagulation have been adopted for fluoride-rich and silica-rich wastewater treatment, respectively, but waste solid generation and low wastewater processing efficiency are still the looming concern. Efficient resource recovery technologies for different wastewater treatments are scarce for environment and industry sustainability. Herein, a resource capture ultrafiltration–bipolar membrane electrodialysis (RCUF-BMED) system was designed into a closed-loop process for simultaneous capture and recovery of fluoride and silica as sodium silicofluoride (Na2SiF6) from mixed fluoride-rich and silica-rich wastewaters, as well as achieving zero liquid discharge. This RCUF-BMED system comprised two key parts: (1) capture of fluoride and silica from two wastewaters using acid, and recovery of the Na2SiF6 using base by UF and (2) UF permeate conversion for acid/base and freshwater generation by BMED. With the optimized RCUF-BMED system, fluoride and silica can be selectively captured from wastewater with removal efficiencies higher than 99%. The Na2SiF6 recovery was around 72% with a high purity of 99.1%. The aging and cyclic experiments demonstrated the high stability and recyclability of the RCUF-BMED system. This RCUF-BMED system has successfully achieved the conversion of toxic fluoride and silica into valuable Na2SiF6 from mixed wastewaters, which shows great application potential in the industry–resource–environment nexus.

Progress of Ultrafiltration-Based Technology in Ion Removal and Recovery: Enhanced Membranes and Integrated Processes

“…Membrane-based process is a promising separation technology with high efficiency, scalability, low operation cost, and low carbon footprint. − In recent years, the research and industrial application of membrane-based technology has experienced a rapid growth in ion removal. Among them, ultrafiltration (UF), , nanofiltration (NF), , and reverse osmosis (RO) , are widely utilized in pressure-driven membrane separation processes (Figure e). The RO membrane only allows water molecules to pass through membrane, resulting in a high rejection efficiency of above 99% for ions.…”