High-Efficiency Reclaiming Phosphate from an Aqueous Solution by Bentonite Modified Biochars: A Slow Release Fertilizer with a Precise Rate Regulation

Biochar can be used to reclaim nutrients from wastewater, and the nutrient loaded biochar can be regarded as a slow release fertilizer; however, the prerequisite of this concept is that the used biochar should have high adsorption capacities for nutrients. In addition, it remains challenging to tune the release rate of nutrients to coordinate with the uptake rate of plants. Herein, we report bentonite modified biochars derived from the copyrolysis of biomass and bentonite, which exhibit much higher phosphate adsorption capacities and superior P-slow release kinetics compared to that of the biochar without the modification of bentonite. The mechanistic study reveals that the improved adsorption and release performance of the as-prepared biochars originates from the presence of Ca and Mg in bentonite which leads to (1) the formation of desirable porous structure, (2) the reduced negative charge on the surface of the derived biochars, and (3) the formation of Ca and Mg related precipitations. Moreover, we demonstrate that the P-release kinetics of the as-prepared biochars can be precisely tuned by controlling the amount of bentonite, and a modified Fick model is developed to establish a quantitative relationship between biochars with different formulations and their P-release kinetics.