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

• Digestate hydrochar was activated using KOH (DHK), H 3 PO 4 (DHH) and CO 2 (DHC). • ∼95 % of Diuron removal was achieved, maximum adsorption capacity of 20.2 mgg -1 . • Adsorption occurred via chemisorption. • Adsorption of Diuron by DHC and DHH best fitted by Freundlich and DHK by Langmuir model. • 80 % diuron removal after 5 cycles by H 3 PO 4 activated hydrochar. This study reports the impacts of H 3 PO 4 , KOH and CO 2 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 m 2 g -1 ) and Raman Id/Ig ratio (6.36) compared to CO 2 and H 3 PO 4 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 CO 2 and H 3 PO 4 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 CO 2 activated hydrochar, an increase in temperature decreased the removal efficiency and this is attributed to being an exothermic adsorption. Using the H 3 PO 4 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 H 3 PO 4 activated material having up to 80 % removal of diuron during the 5th cycle.