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

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 (R2 > 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 CO2 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.