Tailoring the porosity of chemically activated carbons derived from the HTC treatment of sewage sludge for the removal of pollutants from gaseous and aqueous phases

Abstract

The management of sewage sludge is currently an open issue due to the large volume of waste to be treated and the necessity to avoid incineration or landfill disposal. Hydrothermal carbonization (HTC) has been recognized as a promising thermochemical technique to convert sewage sludge into value-added products. The hydrochar (HC) obtained can be suitable for environmental application as fuel, fertilizer, and sorbent. In this study, activated hydrochars (AHs) were prepared from sewage sludge through HTC followed by chemical activation with potassium hydroxide (KOH) and tested for the removal of pollutants in gaseous and aqueous environments, investigating carbon dioxide (CO₂) and ciprofloxacin (CIP) adsorption capacity. The effects of activation temperature (550-750 °C) and KOH/HC impregnation ratio (1-3) on the produced AHs morphology and adsorption capacity were studied by Response Surface Methodology (RSM). The results of RSM analysis evidenced a maximum CO₂ uptake of 71.47 mg/g for mild activation conditions (600-650 °C and KOH/HC = 1 ÷ 2), whereas the best CIP uptake of 628.61 mg/g was reached for the most severe conditions (750 °C, KOH/HC = 3). The prepared AHs were also applied for the removal of methylene blue (MB) from aqueous solutions, and the MB uptake results were used for estimating the specific surface area of AHs. High surface areas up to 1902.49 m²/g were obtained for the highest activation temperature and impregnation ratio investigated. Predictive models of CO₂ and CIP uptake were developed by RSM analysis, and the optimum activation conditions for maximizing the adsorption performance together with high AH yield were identified: 586 °C and KOH/HC ratio = 1.34 for maximum yield (26.33 %) and CO₂ uptake (67.31 mg/g); 715 °C and KOH/HC ratio = 1.78 for maximum yield (18.75 %) and CIP uptake (370.77 mg/g). The obtained results evidenced that chemical activation of previously HTC-treated sewage sludge is a promising way to convert waste into valuable low-cost adsorbents.

Keywords: Adsorption; Chemical activation; Design of Experiments; Hydrothermal carbonization; Sewage sludge; Sludge-based activated carbons.