Temperature effects on vanadium speciation and adsorption to biochar alone and biochar-metal oxide nanoparticle...

Vanadium (V) is a potentially toxic metal widely distributed in the environment. This study investigates temperature effects on V adsorption and speciation in biochar (BC) and BC-metal oxide composites under conditions relevant to contaminated soils in temperate climates. While BC and metal oxide nanoparticles can individually immobilize V, limited information exists on temperature effects. This study investigates V adsorption and surface characteristics of BC alone and BC combined with iron (Fe), aluminum (Al), and titanium (Ti) oxide nanoparticles (BC: oxides at 5:1 ratio) at warm (22°C) and cold (4°C) temperatures. V adsorption was conducted at pH 7.5 using concentrations from 0 to 40 mg L^-1. Visual MINTEQ modeling software was used to predict dissolved V species at experimental conditions. Surface characteristics were examined using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy. Adsorption data were fitted to the Freundlich (r² ∼0.99) and Langmuir (r² = 0.66-0.96) isotherms. Maximum adsorption capacity followed the order: BCAl-cold = BCAl-warm > BCTi-cold > BCTi-warm = BC-cold = BC-warm > BCFe-warm = BCFe-cold. Predicted orthovanadate (%) was higher at warm temperatures. H₂VO₄ ^- was the dominant species at pH 7.5 under both temperatures. Microaggregates were observed in BCAl and BCTi, indicating greater surface area than BC or BCFe. SEM-EDS showed V and Al enrichment on BCAl surfaces suggesting the inner-sphere complexes between Al-oxygen (O) and H₂VO₄ ^-. These results offer mechanistic insight into V adsorption on BC-nano-oxide composites under varying climatic conditions and support their potential use in remediating V-contaminated soils.