Adsorption of Diuron from aqueous solution by physically (CO2) and chemically (KOH and H3PO4) activated digestate hydrochar

Abstract

Abstract

This study reports the impacts of H3PO4, KOH and CO2 activation on hydrochar derived from digestate sludge. The impact is evaluated using BET, XRD, Raman, XRF, CHNSO, FESEM and FTIR-ATR analyses and Diuron adsorption experiments. Adsorption isotherms and kinetics are presented. KOH activation produced an adsorbent with two times higher specific surface area (134 m2g-1) and Raman Id/Ig ratio (6.36) compared to CO2 and H3PO4 activation. The pseudo second order kinetic model fitted the data best for all the data, and this was verified by the residual plots. The adsorption of the herbicides in the case of the CO2 and H3PO4 activated digestate was best described by the Freundlich adsorption isotherm indicating that adsorption was onto heterogenous surfaces and by the Langmuir isotherm in the case of the KOH activated digestate hydrochar. The thermodynamic studies showed that in the case of the KOH activated hydrochar temperature had a minimal effect on its efficiency. In the case of CO2 activated hydrochar, an increase in temperature decreased the removal efficiency and this is attributed to being an exothermic adsorption. Using the H3PO4 activated material, the adsorption was found to be slightly endothermic, as an increase in temperature led to a slight increase in the removal. About 95 % of diuron removal was achieved and maximum adsorption capacity of 20.2 mgg-1. Regeneration of the materials with ethanol worked best with the H3PO4 activated material having up to 80 % removal of diuron during the 5th cycle.