Sewage sludge-derived hydrochar for the removal of color cocktails and pharmaceutical compounds from complex water matrices

In this study, hydrochar was synthesized from municipal sewage sludge collected from two sewage treatment plants (68 MLD SBR and 18 MLD ASR) using hydrothermal carbonization (HTC) at 180°C, 210°C, and 240°C. Structural characterization revealed that high-temperature hydrochars exhibited higher surface areas (~5.68 m²/g) and sharper micropore distributions, whereas low-temperature products (e.g., 18-180HC) retained abundant oxygenated functional groups. These hydrochars were evaluated for the removal of a contaminant cocktail comprising anionic dye (Congo Red), cationic dyes (Crystal Violet, Malachite Green, Methylene Blue), and the pharmaceutical ciprofloxacin. The results showed a maximum adsorption capacity of 102.25 mg/g for total dyes and 17.76 mg/g for ciprofloxacin, with low-temperature hydrochar (18-180HC) performing best. >90% removal for dyes and >75% for ciprofloxacin was observed across the studied pH range. Environmental factor studies revealed strong mechanistic contrasts: dye removal remained consistently high (>85%) across ionic strengths, natural organic matter (NOM), and water matrices, confirming robust inner-sphere complexation. In contrast, ciprofloxacin adsorption declined sharply with increasing ionic strength and NOM concentrations, and was strongly matrix-dependent, indicating weaker outer-sphere interactions. XPS analysis confirmed these pathways, showing pronounced surface restructuring and new nitrogen/oxygen environments after dye adsorption, while ciprofloxacin induced only minor spectral shifts. Reusability tests demonstrated >90% dye and >75% ciprofloxacin removal across multiple cycles. Overall, findings suggest that sewage sludge-derived hydrochar, particularly produced at lower HTC temperatures can be used as an effective, reusable, and sustainable adsorbent for the treatment of both anionic and cationic dyes and pharmaceutical contaminants in water treatment units. • Sewage sludge (18 MLD-ASR, 68-MLD, SBR) hydrochar exhibits multi-contaminant removal • Optimal hydrochar (18-180°C) achieves 102.25 mg/g dye and 17.76 mg/g ciprofloxacin uptake • Adsorption via π–π stacking, H-bonding, pore filling, and inner-sphere complexation • Dye removal remains >85% under pH, ionic strength, NOM, and matrix effects • Hydrochar enables sustainable reuse with high efficiency over multiple cycles