Co-Pyrolysis of Sewage Sludge and Zeolitic Basalt: Physicochemical Characterization, Stability and Carbon Sequestration Potential

Mining and sewage treatment wastes have been accumulating at growing rates in urban areas. Recycling these wastes can be used to generate safe products for various agro-environmental uses, including the synthesis of fertilizers with the potential to sequester carbon (C) in the soil. Therefore, this study evaluated the physicochemical characteristics and C sequestration potential of biochar obtained by co-pyrolysis (500 °C) of sewage sludge (SS) individually or combined at a 1:1 (w:w) ratio with zeolitic basalt (ZB), referred to as SS + ZBBC. Subsequently, the raw materials and biochars were characterized by X-ray diffraction analysis, proximate analysis, elemental analysis, and FTIR spectroscopy, as well as pH and electrical conductivity (EC) determination. The results show that pyrolysis optimized material properties, especially SS biochar (SSB), which exhibited high stability with the highest fixed C content (13.6%) and thermostable fraction (TSF) of 43%. On the other hand, ZB had a higher pH and a lower EC than SS. Co-pyrolysis promoted complementary effects on the chemical and C stability properties of the SS + ZBBC combination. The combination raised the pH to a value close to neutrality (6.5), indicating potential corrective action for acidic soils. Furthermore, after co-pyrolysis, the TSF remained high (25.2%) and was classified as a high-longevity material (>1000 years), indicating high aromaticity and C condensation. Therefore, the co-pyrolysis of SS and ZB optimized the individual characteristics of the materials, thereby providing a promising and sustainable alternative for agro-environmental use that addresses the need to reduce C emissions and promote waste recycling.