# Temperature effects on vanadium speciation and adsorption to biochar alone and biochar–metal oxide nanoparticle composites

## TL;DR

This study shows that mixing biochar with aluminum nanoparticles is effective at trapping vanadium, a toxic metal, and works well in both cold and warm temperatures.

## Contribution

The study introduces a novel biochar–aluminum oxide composite that effectively immobilizes vanadium under varying temperature conditions.

## Key findings

- BCAl composites showed the highest vanadium adsorption capacity at both cold and warm temperatures.
- Vanadium was found to form inner-sphere complexes with aluminum on BCAl surfaces.
- Temperature influenced vanadium speciation, with orthovanadate being more prevalent at warmer temperatures.

## Abstract

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 (r
2 ∼0.99) and Langmuir (r
2 = 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.

Vanadium is a toxic metal that can pollute soil and water, especially near factories and mines. It is important to find safe and effective ways to stop it from spreading. One option is biochar, a black, charcoal‐like material made from plants, which can trap harmful metals. Another option is using tiny metal particles, called nanoparticles, but using too many can be expensive and risky for the environment. This study tested a new idea: mixing small amounts of metal nanoparticles with biochar to remove vanadium more safely and effectively. The researchers tested this mix at cold and warm temperatures, like those found in northern climates. They found that the mix with aluminum particles (called BCAl) was the most effective. It held onto vanadium better than the others. Using special imaging tools, the researchers saw that vanadium was sticking to the aluminum areas on the biochar. The study also showed that temperature affects how vanadium moves and reacts in cold and warm temperatures.