The role of oxalic acid in nutrient solubilization, potentially toxic elements from sewage sludge biochar, and remineralizers for sustainable local fertilizers

Abstract

Global population growth presents serious challenges to food security and sustainable agricultural production. Most countries rely heavily on imports of conventional fertilizers to sustain food production. This reliance emphasizes the importance of finding local and environmentally safe options. In this context, sewage sludge biochar (SSB) and remineralizers (REMs) emerge as promising nutrient sources. However, these materials have low natural solubility, which requires alternative methods to improve nutrient release and make them suitable for producing sustainable fertilizers. Thus, this study evaluated the effects of oxalic acid (OA) on the solubilization of nutrients (P, K, Ca, Mg, and S) and potentially toxic elements (Al, Cd, and Cr) from SSB pyrolyzed at 300 °C (SSB300) and 500 °C (SSB500) and from two potassium rocks, mica schist and phonolite. After chemical and mineralogical characterization, the materials were incubated in different concentrations of OA (0, 0.33, 0.67, and 1 mol L−1) for 10 days. The results demonstrated that the solubilization of the elements was dependent on the OA concentration, the aromaticity of the biochars, and the mineral structure of the REMs. Principal component analysis (PCA) showed that mixtures of SSB300 with mica schist or phonolite, at intermediate OA concentrations (0.33 and 0.67 mol L−1), released more essential elements for plant growth, such as P, K, and Ca, without significantly increasing potentially toxic elements, making them promising options for developing sustainable multinutrient fertilizers. Future research on how these materials perform in the soil-plant system should be conducted.