Influence of pyrolysis temperature on chemical and physical properties of biochar from sewage sludge

The use of sewage sludge biochar (SSB) for agro-environmental purposes has been increasing. However, due to the strong influence of pyrolysis temperatures on its production, there is great variation in its final properties. In this regard, efforts to generate relationships among many correlated SSB properties may help to understand this influence. This study sought to evaluate the effect of pyrolysis temperature on agro-environmental physicochemical properties of SSB. Biochars from sewage sludge (SS) were produced at 300, 400 and 500°C and their physicochemical properties were evaluated in comparison to SS samples. The increase in pyrolysis temperature decreased C, N, and H contents and the H/C atomic ratio, while increasing the C/N ratio. The pyrolysis process increased pH values, the surface area and pore volume and enriched the SSB with macro and micronutrients. Considering all variables together, the biochar produced at 300°C was that which showed the greatest nutrients availability, such as N total, S, NO₃^?, NH₄⁺, Ca²⁺ and Mg²⁺. Conversely, SSB produced at 500°C showed higher recalcitrant organic matter and alkalinity, important properties for C sequestration and the correction of acidic soils. The combined application of SSB produced at lower and higher pyrolysis temperatures should be furthered studied.