Waste-to-resources: Exploratory surface modification of sludge-based activated carbon by nitric acid for heavy metal adsorption.

Sewage sludge has the potential to be utilised as a sorbent for dissolved pollutant removal. This study investigated metal removal efficiency of carbonised sewage sludges obtained via pyrolysis at 500 °C: biochar/carbonised sludge (CS), ZnCl2-activated sludge-based active carbon (SBAC), and three SBACs modified by nitric acid at different concentration and temperature (MSBACs). Batch adsorption kinetic tests indicated that the Pb2+ adsorption equilibrated in <5 min. Efficiencies of these sludge-based sorbents for Pb2+ adsorption followed the order MSBAC > SBAC > CS. Metal leachabilities were low or negligible for the SBAC and MSBACs. A comparison of 5-min adsorption of the best-performing MSBAC with different sorbents indicates that performance followed the trend MSBAC > grundite (illite) > zeolite (clinoptilolite) > commercial activated carbon (CAC) > kaolinite > perlite. Furthermore, MSBAC achieved 98.9%, 42.6%, and 34.6% removal of Cu, Zn, and Al, respectively, from spiked natural acid rock drainage in <5 min. The modified sludge-based activated carbon is an effective and sustainable sorbent for removing metals from aqueous environments.