Reverse osmosis reject water management by immobilization into alkali-activated materials

Water-intensive industries face challenges due to water scarcity and pollution. In the management of these challenges, membrane processes play an important role. However, they produce significant amounts of reject waters, in which the separated salts and pollutants are concentrated. This study aims to develop a novel management concept for reject waters using alkali activation to immobilize salts in a solid phase using metakaolin, blast furnace slag (BFS), or their mixture as precursors and to create alkali-activated materials with sufficient properties to be potentially used in construction applications. Seven different waters were used to prepare the NaOH-based alkali activator solution: deionized water, three simulated seawaters with increasing salinity, and three reverse osmosis (RO) reject waters from mining or pulp and paper industries. Overall, BFS-based samples had the highest immobilization efficiency, likely due to the formation of layered double hydroxide phases (hydrotalcite, with anion exchange capacity) and hydrocalumite (chloride-containing mineral). Moreover, high-salinity water enhanced the dissolution of precursors, prolonged the setting time, and increased the compressive strength compared with nonsaline water. Thus, the obtained materials could be used in construction applications, such as backfilling material at mines where RO concentrates are commonly produced.