Sustainable adsorbents from sewage sludge: Efficient removal of cytostatic compounds in single and complex aqueous matrices

The present study investigates several sludge-based activated carbons (SACs) synthesised by physical activation using urban WWTP sludge as a precursor for their use as adsorbents of cytostatic drugs. The synthesis was initiated in all cases with a pyrolysis step, N 2 flow, at 750ºC, and two other subsequent steps were accomplished, a thermal step at 850ºC with CO 2 and an acid wash with HCl. The SACs were characterised by N 2 physisorption, elemental analysis, FTIR, XRF, isoelectric point and SEM. The activated carbons were used to remove three cytostatic drugs: methotrexate (MTX), cyclophosphamide (CYC) and 5-fluorouracil (5FU) in single, multicomponent and real matrix tests. The results underlined that the use of HCl for rinsing proved to be a crucial step in the synthesis, and its placement before CO 2 activation gave the best properties of the material (SAC3, S BET = 443 m 2 ·g -1 ), showing the best adsorption results in single tests (35.7, 23.3 and 210.9 mg·g -1 for 5FU, CYC and MTX, respectively). pH studies showed that the adsorption of all three compounds onto both carbons was disadvantaged at basic pH. In this work, the materials were additionally tested in multicomponent and hospital wastewater adsorption experiments, maintaining a high adsorption capacity. Following the circular economy principles, the results of this research highlighted the possibility of valorising sewage sludge wastes as adsorbents for the removal of cytostatic drugs, emerging contaminants listed as hazardous pollutants in the 2014/955/EU Decision of the Waste List (2008/98/EC Directive of the European Parliament). • SACs effectively removed cytostatic drugs, finding remarkable adsorption capacities for SAC3 (25-200 mg·g -1 ). • Kinetic data best fitted to the pseudo-second-order model, and a mechanism combined of film and intra-particle diffusion was found. • Equilibrium data fitted best to the Freundlich model, suggesting a heterogeneous surface with multilayer adsorption. • Adsorption was found to be disfavoured at alkaline pH; thus, electrostatic forces played a major role in the process. • The adsorption capacity was maintained at 22.82 mg·g -1 for MTX with SAC3 with the hospital sewage water matrix.