Biochar derived from urban waste for carbon sequestration in degraded soils: potential and risks associated with plastic contamination

• Grant number: 25/22115-1
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: December 01, 2025
• End date: November 30, 2026
• Field of knowledge: Physical Sciences and Mathematics - Geosciences
• Principal Investigator: Matheus Bortolanza Soares
• Grantee: Joyce Oliveira da Silva Diatchuk
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil

Abstract

Urban solid waste (USW) mismanagement and degraded tropical soils create an urban-rural nexus of cumulative impacts: cities accumulate waste with low recycling rates, while degraded pastures and Oxisols poor in organic matter lose water, nutrients, and carbon. USW-derived biochar, produced by pyrolysis under limited oxygen, offers a pathway to stabilize carbon and improve soil structure and functionality. Such synergy fits into the logic of the circular economy by converting urban residual streams into a stable agronomic input, reducing disposal volume and adding value. However, the challenge when the feedstock is USW lies in the presence of plastics, which may generate microfragments after pyrolysis. In this study, we propose to produce USW biochar at 600°C and purify it through two complementary routes (dry sieving and ultrasound-assisted wet processing) to assess its effect on soil carbon dynamics. The biochar will be characterized by spectroscopy, microscopy, and spectrometry, covering functional groups, morphology, and elemental composition, in addition to quantifying plastic residues removed during the purification steps. Subsequently, the biochar will be applied (5% w/w) to a degraded pasture Oxisol and incubated for 90 days, during which we will evaluate soil carbon dynamics. Our hypothesis is that the wet/ultrasound route will remove more plastics than dry sieving without compromising the aromatic structure, reopening micropores, increasing surface area, and exposing oxygenated groups, conditions that favor microbial colonization and sorption. It is expected that the study will demonstrate the technical and environmental feasibility of purified USW biochar for recarbonizing degraded tropical soils, aligning waste reduction, soil quality, and climate mitigation. (AU)