Adsorptive Remediation of Atrazine-Contaminated Soil Using Rice Husk Biochar

Abstract

Atrazine is a widely used chloro-s-triazine herbicide whose persistence and mobility in agricultural soils raise concerns about groundwater contamination, ecological toxicity, and human exposure. This study evaluated rice husk biochar (RHB) produced by slow pyrolysis at 500, 600, and 700 °C for the remediation of atrazine-contaminated soil under laboratory-scale batch conditions. Soil from an uncontaminated secondary forest in Awka, Anambra State, Nigeria, was amended with 2% (w/w) RHB and contacted with 825 mg L⁻¹ atrazine solution. Atrazine remaining in the solution phase was quantified by UV–Vis spectrophotometry at 255 nm after calibration, and adsorption kinetics were interpreted using the pseudo-second-order (PSO) model. Increasing pyrolysis temperature decreased biochar yield from 43% at 500 °C to 29% at 700 °C, but increased BET specific surface area from 168.202 to 330.186 m² g⁻¹ and total pore volume from 0.056 to 0.095 cm³ g⁻¹. FTIR interpretation indicated progressive dehydration, loss of aliphatic groups, persistence of aromatic C=C structures, and strong Si–O–Si/Si–O bands typical of rice-husk-derived chars. Apparent atrazine removal after 180 min was 69.1%, 69.7%, and 73.3% for RHB500, RHB600, and RHB700, respectively, corresponding to qₜ values of 285.0, 287.5, and 302.3 mg g⁻¹. PSO fits were strong for RHB500 and RHB700 (R² = 0.997 and 0.999), while the RHB600 regression produced a physically inadmissible negative rate constant, suggesting rapid equilibration, slight desorption, or mixed transport-controlled behaviour. Overall, RHB700 was the most effective amendment, attributable to enhanced surface area, microporosity, and aromaticity. The findings support valorisation of rice husk into biochar as a low-cost, circular-economy material for pesticide-contaminated tropical soils, while highlighting the need for field validation, lower environmentally realistic atrazine concentrations, and desorption/leaching tests before full-scale deployment.