Pyrolysis temperature governs heavy metal immobilization by sewage sludge biochar in tropical multi-contaminated Vertisol: a multivariate assessment

Abstract

This study evaluated the effectiveness of sewage sludge biochar (SSB) produced at 350, 450, and 600 °C for immobilizing potentially toxic elements (PTEs) in a tropical multi-contaminated Vertisol. Chromium, copper, cobalt, nickel, cadmium, lead, and zinc were assessed through sequential extraction, ecological risk indices, and principal component analysis (PCA). Pyrolysis temperature markedly altered biochar properties and, consequently, remediation performance. Among the treatments, SSB450 provided the most balanced and consistent response. It reduced the potential ecological risk index (RI) from 139.06 in the untreated soil to 65.23, representing an approximately 53.1% decrease, and produced the lowest RAC, Cf, PLI, and mCd values among the amended treatments. Increasing pyrolysis temperature promoted the redistribution of metals from mobile fractions (F1 + F2) to more stable fractions (F3 + F4). For example, the mobile fraction of Pb declined from 69.5% in the control soil to 35.2% after application of SSB450 and SSB600, while the mobile fraction of Cd decreased from 52.1% to 28.4% with SSB450. PCA confirmed clear multivariate separation among treatments and identified fixed carbon and organic carbon as the biochar attributes most strongly associated with lower contamination risk, with PC1 loadings of -0.916 and − 0.922, respectively. In contrast, SSB350 showed comparatively limited Cd stabilization, indicating that low-temperature biochar may be less suitable for uniform remediation of multi-contaminated Vertisols. Overall, the results demonstrate that careful calibration of pyrolysis temperature is essential to optimize the performance of sewage sludge biochar for heavy metal immobilization in tropical soils.

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