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.