Time dependency and irreversibility of water desorption by drinking-water treatment residuals: Implications for sorption mechanisms

Abstract

Drinking-water treatment residuals (WTRs) are being evaluated as cost-effective sorption media for use in environmental remediation. Data from previous work have suggested that intraparticle phosphorus (P) diffusion into micropores is the rate-limiting mechanism of P sorption by WTRs. We used isothermal thermogravimetric analysis (TG) to study water desorption/resorption dynamics as they relate to steric diffusion rate limitations for prospective sorbates. Results showed that air-dried WTR particles contain significant amounts of water. Only about 40% of water desorbed isothermally (70 °C) for 10 h was readsorbed when particles were reexposed to ambient temperature and moisture conditions. This hysteresis related closely with time dependency of water loss, suggesting steric diffusional hindrance of water re-adsorption by meso- and micropores. The irreversibly desorbed water may be the component requiring increased kinetic energy to overcome diffusional resistance. Another possible factor in irreversibility could be pore shrinkage. Samples incubated for 12 months at 70 °C prior to TG analysis showed no hysteresis at 70 °C. Isothermal water losses with time fit well (r2=0.95) the diffusion model of Kabai. These results are consistent with an aqueous pore network that would account for high phosphorus sorption capacity and hysteresis that has been recently documented for WTRs.