Adsorption Efficiency of Sustainable Activated Carbons: A Malathion and Malaoxon Study

Viscose fibers were converted into activated carbon materials by physical activation with water vapor or CO2, yielding VFH2O and VFCO2 with similar textural properties but markedly different surface chemistries. Their adsorption performance toward malathion (MLT) and its more toxic oxidation product, malaoxon (MOX), was systematically evaluated. Adsorption equilibrium was reached within approximately 60 min, and the pseudo-second-order model provided the most consistent description of the kinetic data. Equilibrium adsorption was analyzed using the Freundlich, Langmuir, Temkin, and Dubinin–Radushkevich models. For MLT, both Freundlich and Langmuir models showed excellent agreement with the experimental data, whereas MOX model fits were more system-dependent, particularly on VFCO2. Temperature effects were also system-dependent: for MLT, Langmuir qmax decreased with temperature on VFH2O but increased on VFCO2, despite a simultaneous decrease in KL on the latter. Thermodynamic analysis over 25–40 °C yielded negative ΔG° values for all systems and negative apparent ΔH° values, indicating thermodynamically favorable adsorption with an exothermic contribution over the investigated range. Under flow-through conditions, useful removal was observed only for MLT. The results highlight the importance of adsorbent surface chemistry, pesticide structure, and operating conditions in organophosphate adsorption.