Characterization of the sewage sludge derived biochar and evaluation of its effect on growth of Indian mustard [Brassica juncea (L.) Czern. & Coss.]

Abstract

Sewage sludge management remains a pressing environmental concern due to heavy metals and pathogens that limit its direct agricultural application. This study investigates the production of biochar from sewage sludge through pyrolysis under varying thermal conditions (300°C, 500°C, and 700°C for 1–3 h) and evaluates its effectiveness as a soil amendment. Biochars were characterized for physicochemical properties, functional groups, morphology, crystallinity, and BCR heavy metal fractionation. Elevated pyrolysis temperature and extended residence time increased pH (6.2–9.8), water holding capacity (WHC: 26.5–55.6 %), and total ash content, while reducing volatile matter (VM: 30.1–6.0 %) and fixed carbon (FC: 18.1–3.7 %), indicating greater thermal stability. Further facilitated the formation of C–H and CC bonds, along with deformation of C–O (carboxyl, ester) functional groups. Among nine treatment regimes, the 500°C treatment for 2 h (5T2H) was identified as optimal, with a yield of 65.4 %, pH 8.2, WHC 54.5 %, FC 14 %, and low VM (15.9 %). Ecological risk assessment showed a substantial global ecological risk (GER) reduction from 411 (high risk, raw sludge) to 245.9 (moderate risk, 5T2H). The 5T2H biochar was applied to soil at 5–50 % (w/w) and its effect on the growth and developemnt of Brassica juncea was evaluated. Significant improvements were observed in soil pH (7.1→7.7), WHC (22.9→38.4 %), and plant metrics, including germination (97 %), shoot length (6.4 cm), root length (6.6 cm), dry biomass (8.3 g), and chlorophyll content (8.5 mg/g FW). This study integrates pyrolysis optimization, metal speciation, and plant performance assessment, offering a comprehensive approach for transforming sewage sludge into an effective and low-risk soil amendment.