Uptake, desorption, and hysteresis of heavy metals and PAHs with environmental concerns onto quick clays: effects of salinity and temperature.

Abstract

This study investigated the sorption, desorption, and trapping of 9,10 dimethylated anthracene (DMA), copper (Cu), and cadmium (Cd) onto quick clay (QC), focusing on the effects of temperature, salinity, and their environmental relevance. Sorption isotherms were generated at different temperatures (4, 10, and 20 °C) and salinities (1 and 25 g·L^-1). Thermodynamic parameters were calculated to elucidate the underlying mechanisms. Isosteric heat of adsorption (ΔH_X) was determined to assess the heterogeneity of adsorption sites. Isotherms results were processed using the Freundlich model to assess sorption and hysteresis parameters of QC. Kinetic studies revealed a rapid initial uptake of DMA followed by a slower logarithmic phase, reaching equilibrium within 1440 min. The presence of the methyl group in DMA compared to non-methylated PAHs from other studies likely influences its adsorption rate. Temperature and salinity significantly impacted both the adsorption and desorption processes. Notably, Cd adsorption was nearly made ineffective with increasing salinity. Interestingly, Cu hysteresis index (HI) decreased from 1.57 to - 0.08 with increasing salinity, suggesting a shift from inner-sphere complexation at low salinity to outer-sphere complexation at high salinity. Conversely, DMA adsorption increased by 1.83-fold with increasing salinity, likely due to the salting-out effect. Thermodynamic analysis indicated a spontaneous and endothermic adsorption process driven by a positive entropy change (ΔS⁰). The ΔH_X values supported physical adsorption as the dominant mechanism. The observed homogeneity in ΔH_X values for DMA and Cd suggests consistent interaction with the clay surface, while the heterogeneity observed for Cu signifies a variation in adsorption site energies.