Lodo de foto-Fenton como recurso catalítico sustentável : aplicações em processos eletroquímicos visando o tratamento de lixiviado e a formação de produtos de alto valor agregado

Abstract

Leachate is a highly complex aqueous matrix resulting from the combination of solid waste degradation and the water cycle in a landfill and its treatment is considered a challenge for researchers, engineers and managers seeking to improve conventional processes and optimize new treatment technologies. Among the technologies for its treatment, the solar photo-Fenton process stands out for its efficiency in reducing recalcitrant organic load and color. However, a significant drawback is the generation of sludge due to the precipitation of ferric ions. Given this, the present research aimed to evaluate the potential of this sludge to be transformed into an attractive resource for use as a raw material in new treatment processes. To achieve this, the ProKnow-C (Knowledge Development Process – Constructivist) method was initially used to develop a bibliometric and systemic study on the alternatives adopted in the literature for the reuse of ferric sludge. The main finding of this study highlights the transformation of ferric sludge into a catalyst for use in the heterogeneous Fenton reaction, which accounted for about 73% of the articles in the final bibliographic portfolio. Subsequently, the catalytic performance of ferric sludge dried at 100°C and after thermal treatment at 600 and 750°C was evaluated in the heterogeneous electro-Fenton process under different pH conditions and current densities. The results showed that treating leachate using the heterogeneous electro-Fenton process with sludge dried at 100°C and under pH 3 conditions achieved better performance compared to treatment without pH adjustment. Specifically, under the evaluated current densities (30, 60, and 90 mA/cm²), the COD removal efficiency was 47%, 48%, and 50% (pH 3) - 15%, 25%, and 36% (no pH adjustment), and the TOC removal efficiency was 36%, 36%, and 42% (pH 3) - 16%, 20%, and 30% (no pH adjustment). It was observed that under acidic conditions, current density did not significantly influence the degradation and mineralization of compounds after 240 minutes of operation. However, high-value-added products (carboxylic acids) were formed, with the possibility of selective electroconversion into acetic and formic acids. Regarding decolorization, successful results were observed under all evaluated conditions, especially for treatments applying a current density of 90 mA/cm², achieving a minimum decolorization of 80% and a maximum of 94%. Additionally, ferric sludge was evaluated as a catalyst in electrochemical oxidation with simultaneous production of green hydrogen, achieving 61% COD removal, 44% TOC removal, 94% decolorization, and an average green hydrogen production rate of 14.64 mL/min. These results are promising compared to the same treatment process without the use of sludge. The proposed treatment offers environmental benefits and represents an innovative, efficient, and sustainable pathway for circular economy and achieving SDGs 6 and 12, as a treatment residue was reintegrated into a new hazardous effluent treatment process, paving the way for further scientific investigations.