The application of biochars for the remediation of water and soils contaminated with potentially toxic elements (PTEs) has seen a recent growing interest. The mechanisms of chemical immobilization of PTEs with biochars in aqueous media have been well defined. However, immobilization mechanisms by which biochars interact with PTEs in soil matrix are more complex. The biological immobilization mechanisms and their interactions with PTEs in biochar-amended soils are not as well defined. This review presents an overview of factors governing interactions of biochars with PTEs as well as the synergistic effect of biochars and microorganisms in biological immobilization processes. The effectiveness of biochars in improvement of microbial immobilization of PTEs mainly depends on biochar properties, application rates, and soil environments such as organic matter content, clay type and content, pH and redox potential. Although some modified biochars appear to be better than the pristine biochars for immobilization of PTEs, their potential adverse impacts on soil microbial activity should be considered. This review highlights the most common analytical methods to discover molecular interaction mechanisms between biochar and PTEs and future research areas required for the understanding of biochar-PTE interactions in polluted soil systems. Graphical Abstract

Background Phosphorus (P), iron (Fe) and zinc (Zn) are essential elements for plant growth and development, but their availability in soil is often limited. Intercropping contributes to increased P, Fe and Zn uptake and thereby increases yield and improves grain nutritional quality and ultimately human health. A better understanding of how intercropping leads to increased plant P, Fe and Zn availability will help to improve P-fertilizer-use efficiency and agronomic Fe and Zn biofortification.