From waste to resources: Integrating advanced oxidation and hydrochar production for potassium recovery and carbon retention

• Agro-industrial vinasse was transformed into stable, nutrient-rich hydrochar. • Hydrochar achieved K + retention up to 14.5 mg g − 1 following pseudo-second-order chemisorption. • Carbon retention indices confirm long-term sequestration and fertilizer applicability. The escalating demand for fertilizers and the scarcity of natural potassium (K) reserves highlight the need for sustainable recovery strategies. Vinasse, a by-product of sugarcane ethanol, contains exceptionally high organic loads (>100 g L −1 COD) and is discharged from the process at elevated temperature, conditions that enable efficient persulfate thermal activation (TAP) and justify its treatment with a combined TAP–Fenton process. Using substoichiometric doses of persulfate and hydrogen peroxide, the combined treatment promoted partial oxidation and coagulation, generating an iron-rich sludge (50 % Carbon) suitable for valorization. This sludge was transformed into hydrochar via hydrothermal carbonization (HTC) and subsequently activated with NaOH (HTC-A), which increased surface reactivity via deprotonated oxygenated groups. The activated hydrochar displayed strong K + retention (up to 14.5 mg g −1 ) with pseudo-second-order kinetics (R 2 > 0.99), indicating chemisorption by oxygenated functional groups. The spent hydrochar (HTC-S) also co-adsorbed phosphorus, supporting its use as a slow release, multinutrient soil amendment. Carbon retention analysis indicated a net sequestration potential of 1.25–1.45 t CO 2 eq per ton of hydrochar. This combined functionality, namely nutrient recovery and carbon retention, supports the potential of the TAP–Fenton/HTC route as a circular strategy for the treatment and valorization of high-load agro-industrial effluents.