Investigating the activation of hydrochar from sewage sludge for the removal of terbuthylazine from aqueous solutions

Hydrothermal carbonization offers the opportunity to reduce the issues related to the management of sewage sludge, enhancing dewaterability and avoiding thermal treatment prior landfill disposal or incineration, as water acts as a subcritical medium which catalyzes the process. The hydrochar obtained can find sustainable application as fuel, sorbent and soil amendment. Here, the effects of activation temperature (550–750 °C), time (1–4 h) and impregnation ratio potassium hydroxide: hydrochar (1–3), as well as their interactions, were investigated by Response Surface Methodology. The carbonaceous sorbent was then applied for the removal of terbuthylazine from aqueous solutions. Solid yields up to 32.31% were obtained, with corresponding ash content ranging from 13.05 to 35.06%. The combined effect of time and temperature significantly affects the ash content. An increase of impregnation ratio or temperature leads to low yields but high terbuthylazine adsorption, while the effect of the activation time on the pollutant removal is negligible. Up to 63.87% of the herbicide was adsorbed by the activated material, at terbuthylazine concentration of 2.5 mg L⁻¹. The developed sorbent well fits the circular economy approach, offering a promising potential for sewage sludge management, as well as for water remediation.