Chloride Removal from Stormwater and Lakes Impacted by Road Salt Application: Experimental Assessment of Friedel’s Salt Reactions

Road salt (e.g., NaCl) application in cold-temperature regions has increased Cl– concentrations in freshwater. Removing Cl– from water is challenging because it is highly soluble. This paper is the first to demonstrate and characterize the removal of Cl– from stormwater via Friedel’s salt (Ca4Al2(OH)12Cl2(H2O)4) precipitation (FSP), achieved by adding calcium oxide (CaO) and sodium aluminate (NaAlO2). Laboratory tests were done on Cl– removal with FSP using a range of Ca/Al/Cl ratios in deionized (DI) water and stormwater samples impacted by road salt. Even the lowest doses resulted in significant removal of Cl– with maximum removal rates for comparable Ca/Al/Cl ratios of 72% and 84% in stormwater and DI water, respectively. Temporal experiments in stormwater and DI water indicate that most of the Cl– removal occurred within the first 10 min of reaction time. Potential applications of FSP were demonstrated for two hypothetical scenarios based on a well-characterized, salt-impacted lake. The first scenario indicates that a 30% reduction in stormwater Cl– mass is sufficient to maintain concentrations below 150 mg/L. A second scenario with an initial 300 mg/L lake water concentration shows the potential for implementing FSP in low-flow pump and treat systems for the rehabilitation of road-salt-impaired lakes.