TiO2 Nanoparticles in Soil: Adsorption, Transformation, and Environmental Risks

Titanium-containing nanoparticles have emerged as materials of significant technological importance due to their multifunctional properties and excellent performance. With their expanding applications, the amount of TiO2 nanoparticles (TNPs) being released into the soil environment has increased significantly. This review addresses the gap in current research, which has predominantly focused on the environmental behavior of TNPs in aquatic systems while lacking systematic integration of the synergetic mechanism of adsorption–transformation–ecological effects in soil systems and its guiding value for practical applications. It deeply reveals the interaction mechanisms between TNPs and environmental pollutants. TNPs exhibit outstanding adsorption performance towards environmental pollutants such as heavy metals and organic compounds. Specifically, the maximum adsorption capacities of titanate nanowhiskers for the heavy metal ions Cu(II), Pb(II), and Cr(III) are 143.9 mg·g−1, 384.6 mg·g−1, and 190.8 mg·g−1, respectively. Additionally, 1-hydroxydinaphthoic acid surface-modified nano-TiO2 exhibits an adsorption rate of up to 98.6% for p-nitrophenol, with an enrichment factor of 50-fold. The transformation process of TNPs after pollutant adsorption profoundly affects their environmental fate, among which pH is a critical controlling factor: when the environmental pH is close to the point of zero charge (pHpzc = 5.88), TNPs exhibit significant aggregation behavior and macroscopic sedimentation. Meanwhile, factors such as soil solution chemistry, dissolved organic matter, and microbial activities collectively regulate the aggregation, aging, and chemical/biological transformation of TNPs. In the soil ecosystem, TNPs can exert both beneficial and detrimental impacts on various soil organisms, including bacteria, plants, nematodes, and earthworms. The beneficial effects include alleviating heavy metal stress, serving as a nano-fertilizer to supply titanium elements, and acting as a nano-pesticide to enhance plants’ antiviral capabilities. However, excessively high concentrations of TiO2 can stimulate plants, induce oxidative stress damage, and impair plant growth. This review also highlights promising research directions for future studies, including the development of safer-by-design TNPs, strategic surface modifications to enhance functionality and reduce risks, and a deeper understanding of TNP–soil microbiome interactions. These avenues are crucial for guiding the sustainable application of TNPs in soil environments.