Sustainable phosphate removal using Mg/Ca-biochar hybrids: Current trends and future outlooks

The Ca/Mg-biochar hybrids have outstanding phosphate adsorption abilities over most modified adsorbents when applied in stoichiometric amounts; recent MgO-functionalized lignin-based bio-charcoal reported the highest phosphate uptake (906.8 mg/g). Apart from large phosphate uptake, Ca/Mg-rich biochars offer a variety of advantages like non-renewable phosphate recovery, soil amendment, resource utilization, waste management and conversion, etc. However, complex preparation methods, expensive metallic reagents, and/or the need for energy-intensive equipment limit the economic interest for these composites. Therefore, in this review, the latest Mg/Ca-biochar synthesis methods are evaluated, emphasizing the value of utilizing green/ waste materials and reagent-free fabrication methods leading to sustainable development. Among the loaded elements, Ca and Mg are employed to provide a prospect of high phosphate uptake as well as increased phosphate recovery. The spent Ca/Mg-biochar hybrids with large phosphate loadings can be implemented as safe slow-release phosphate fertilizers or can be recycled using practical methods; a recent study found that basification followed by tap water addition recovered over 82% phosphate, after reusing the absorbent for 4 cycles, without affecting its stability. At the end of the review, the real-time applications, and limitations of these biochars, important knowledge gaps in the literature, and future directions are summarized, aiming to promote the sustainable utilization of Ca/Mg-modified biochar in various environmental applications.