Mineralization of humic acids (HAs) by a solar photo-Fenton reaction mediated by ferrioxalate complexes: commercial HAs vs extracted from leachates

The mineralization of bio-recalcitrant humic acids (HAs) by a solar photo-Fenton (SPF) process was investigated in aqueous system, in order to understand its abatement in real high-HA content matrices, such as sanitary landfill leachates. SPF reactions were performed in tubular photoreactors with CPCs at lab-scale (simulated solar light) and pilot-scale (natural sunlight). Considering the experimental conditions selected for this work, the formation of insoluble HA-Fe³⁺ complexes was observed. Thus, to avoid HA precipitation, oxalic acid (Ox) was added, since Fe³⁺-Ox complexes present a higher stability constant. The effect of different process variables on the performance of SPF reaction mediated by ferrioxalate complexes (SPFF) was assessed with excess of H₂O₂ (50–250 mg L⁻¹), at lab-scale: (i) pH (2.8–4.0); (ii) initial iron concentration (20–60 mg Fe³⁺ L⁻¹); (iii) iron-oxalate molar ratio (Fe³⁺-Ox of 1:3 and 1:6); (iv) temperature (20–40 °C); (v) UV irradiance (21–58 WUV m⁻²); and (vi) commercial-HA concentration (50–200 mg C L⁻¹). At the best lab conditions (40 mg Fe³⁺ L⁻¹, pH 2.8, 30 °C, 1.6 Fe³⁺-Ox molar ratio, 41 WUV m⁻²), commercial HAs’ mineralization profile was also compared with HAs extracted from a sanitary landfill leachate, achieving 88 and 91% of dissolved organic carbon removal, respectively, after 3-h irradiation (8.7 kJUV L⁻¹). Both reactions followed the same trend, although a 2.1-fold increase in the reaction rate was observed for the leachate-HA experiment, due to its lower humification degree. At pilot-scale, under natural sunlight, 95% HA mineralization was obtained, consuming 42 mM of H₂O₂ and 5.9 kJUV L⁻¹ of accumulated UV energy. However, a pre-oxidation during 2.8 kJUV L⁻¹ (12 mM H₂O₂) was enough to obtain a biodegradability index of 89%, showing the strong feasibility to couple the SPFF process to a downstream biological oxidation, with low chemicals and energetic demands. Graphical abstract ᅟ