Removal and recovery of phosphorus from secondary effluent using layered double hydroxide-biochar composites

Removal of phosphorus (P) from wastewater and its recovery as a fertilizer are solutions to both P pollution control and resource recycling for agriculture. In this study, various layered double hydroxide biochar composites (LDH/BCs), namely, Zn-Al-LDH/BC, Mg-Al-LDH/BC, and Mg-Fe-LDH/BC, were synthesized to remove P from secondary effluents and then applied as fertilizers. Batch experiments showed that LDH/BCs could adsorb P in fast kinetics, with adsorption capacities ranging 35.19-55.76 mg P/g. A dynamic experiment was performed under different column heights and flow rates, and the results fitted well with Thomas model (R2 > 0.90). These LDH/BCs effectively removed P in the continuous mode, even when treating secondary effluents. Furthermore, when the used LDH/BCs applied as fertilizers, the adsorbed Mg-Al-LDH/BC and Mg-Fe-LDH/BC stimulated crop growth; however, Zn-Al-LDH/BC did not. These differences were attributed to not only the availability of P, but also the stimulation or inhibition of photosynthetic pigment synthesis in crops by adsorbents. Overall, we synthesized LDH/BCs, which effectively removed and recovered P from secondary effluents, and investigated the factors influencing the effects of LDH/BCs on crops. We suggest that both P availability and physiological influences of adsorbents on crops should be considered when using adsorbents as fertilizers.