Characterization of mixtures of Brazilian Ultisol with urban pruning waste biochar at two different proportions

This paper aims to assess the effect of incorporating biochar derived from urban pruning waste (abundantly generated, poorly managed, often disposed of in landfills) into a Brazilian Ultisol on its physical, chemical, and geotechnical attributes, for geoenvironmental applications.

Urban pruning waste-derived biochar produced through the pyrolysis at 500°C was added (at 2.5% and 5.0% w/w concentrations) to a composite sample of Ultisols, representative of Ribeira Valley (Brazil) and homogenized. The mixtures were compared regarding pH, electrical conductivity, cation exchange capacity, specific surface area, chemical composition, Atterberg limits, compaction parameters, permeability, and unconfined compressive strength.

The addition of biochar to soil increased electrical conductivity (from 24.8 µS cm⁻¹ to 95.0 µS cm⁻¹) and sum of exchangeable bases (from 5.8 mmol_c dm⁻³ to 35.0 mmol_c dm⁻³), which is a positive effect for agronomic applications, but it raises a concern regarding potential leaching when applied to soil remediation. The observed increase in pH (4.8 to 5.6) and unconfined compressive strength (208 to 240 kPa) is a positive effect for geoenvironmental applications. Compacted specimens of the soil-biochar mixtures exhibited higher permeability compared to those of unamended soil, which constitutes a potential concern for application in contaminated areas.

The biochar addition in the soil appears to be a promising alternative for the use of urban pruning waste, with potential to enhance soil properties for geoenvironmental applications. However, future research should focus on clarifying the leaching risks and long-term effects of biochar aging.