Degradation of antibiotic sulfamethoxazole by biochar-activated persulfate: Factors affecting the activation and degradation processes

Spent malt rootlets were valorized to synthesize biochars for water treatment. In particular, the ability of biochar (BC) to activate sodium persulfate (SPS) for the oxidation of sulfamethoxazole (SMX), a representative antibiotic belonging to the family of emerging micro-contaminants, was tested. The biochar was fully characterized by nitrogen adsorption, FTIR spectroscopy, SEM, XRD, TGA and potentiometric mass titration (PMT); the biochar has a specific surface area of 100 m2/g, a point of zero charge value of 8.4 and its surface contains, amongst others, a considerable amount of hydroxyl groups probably in phenolic structure. The effect of various parameters such as SPS concentration (100–600 mg/L), SMX concentration (125–500 μg/L) and the water matrix (pure water, bottled water, treated domestic wastewater, alcohols as radical scavengers and humic acid) on degradation was investigated. The actual matrix effect on degradation was minor and so was the effect of radical scavengers. Based on the PMT curves, persulfate activation seems to occur on the biochar surface through interactions with the surface functional groups, generating radicals that are not released in the solution; this mechanism is promoted by the presence of humic acid. The biochar-induced SPS activation was also coupled with either 20 kHz ultrasound irradiation or simulated solar light. In either case, the combined activation was more efficient than the individual ones, leading to process synergy; the degree of synergy, based on pseudo-first order rate constants, was 23–35%.