Unveiling the adsorption mechanisms and key influencing factors of cyclic acetals on powdered activated carbon.

Abstract

Cyclic acetals (CAs), such as 2-ethyl-5,5-dimethyl-1,3-dioxane (2-EDD) and 2-ethyl-4-methyl-1,3-dioxolane (2-EMD), are emerging odourants in drinking water, raising significant concerns due to their extremely low odour thresholds, high stability, and potential health risks. This study investigated 2-EDD and 2-EMD adsorption performance on six powdered activated carbons (PACs). The adsorption isotherms fitted well with Freundlich (R²= 0.907∼0.996) and Temkin models (R²= 0.874-0.997). The adsorption efficiency of 2-EDD (the Freundlich constant K_F = 0.0847-0.802) was higher than 2-EMD (K_F = 0.0435-0.239), because of its greater molecular mass and higher hydrophobicity. All PACs reached equilibrium in about 120 minutes, and the adsorption kinetics fitted better with the pseudo-second-order model (R²= 0.920∼0.997), indicating that chemical adsorption significantly contributed to CAs' adsorption. The adsorption rates for 2-EDD (k₂ = 0.123-1.235) were lower compared to 2-EMD (k₂ = 0.245-4.770). Results from correlation analysis revealed that average pore size, pore volume, and mesoporous fraction were the key PAC properties in controlling CAs' adsorption. Diffusion-chemisorption model, Weber and Morris intraparticle diffusion kinetic model, and Boyd kinetic model were employed to elucidate the adsorption mechanism. The results indicated that the two CAs were interacted mainly through chemical adsorption, with film diffusion serving as the step controlling the rate. PACs exhibited effective performance under neutral to slightly alkaline conditions, as well as in source water and tap water. Meanwhile, 20 mg·L^-1 PAC could reduce CAs' concentration from 40 ng·L^-1 to 5 ng·L^-1. This study provides a benchmark for selecting effective carbon to address odour issues caused by CAs.