Enhanced selectivity and recovery of phosphate and nitrate ions onto coffee ground waste biochars via co-precipitation of Mg/Al layered double hydroxides: A potential slow-release fertilizer.

In this study, the feasibility of Mg/Al layered double hydroxides (LDH) functionalized coffee ground waste biochars (LDH MgAl @CWGB) as a potential adsorbent to selectively recover phosphate (PO 4 3−) and nitrate (NO 3 −) ions in aqueous phases and their consecutive uses as a slow-release fertilizer for stimulating the plant growth were identified. The higher adsorption capacity of PO 4 3− and NO 3 − ions by LDH MgAl @CWGB (PO 4 3− = 6.98 mgP/g, NO 3 − = 2.82 mgN/g) compared with pristine coffee ground waste biochars (CWGB; PO 4 3− = 0.19 mgP/g, NO 3 − = 0.32 mgN/g) was mainly due to the incorporation of Mg/Al mixed oxides and Cl contents. Chemisorption and intra-particle mainly controlled the adsorptive recovery of PO 4 3− and NO 3 − ions by CWGB and LDH MgAl @CWGB in aqueous phases and their adsorption toward CWGB and LDH MgAl @CWGB proceeded endothermically and spontaneously. The changes in the major adsorption mechanisms of PO 4 3− and NO 3 − ions from ligand exchange (CWGB) to electrostatic surface complexation and anion-exchange (LDH MgAl @CWGB) supported the conclusion that the alternation of the surface features through Mg/Al LDH functionalization might improve selectivity and adsorption capacity of PO 4 3− and NO 3 − ions onto CWGB under the co-existence of Cl−, SO 4 2−, and HCO 3 − ions. Since PO 4 3-- and NO 3 −-loaded LDH MgAl @CWGB exhibited much higher seed germination, root and shoot growth rates of garden cress seeds (Lepidium sativum L) than other liquid and solid matrices, including 5 mgP/L PO 4 3− and 5 mgN/L NO 3 −, 10 mgP/L PO 4 3− and 10 mgN/L NO 3 −, and LDH MgAl @CWGB, it can be postulated that PO 4 3-- and NO 3 −-loaded LDH MgAl @CWGB could be practically applicable to the agricultural field as a slow-release fertilizer to facilitate the seed germination, root and shoot growth of the plants. [Display omitted] • LDH MgAl @CWGB more efficiently captured PO 4 3− and NO 3 − ions than CWGB. • LE played a critical role in adsorption of PO 4 3− and NO 3 − ions onto CWGB. • Mg/Al LDH functionalization increased selectivity of CWGB for PO 4 3− and NO 3 − ions. • ESC and AE mainly governed adsorption of PO 4 3− and NO 3 − ions toward LDH MgAl @CWGB. • PO 4 3-- and NO 3 −-loaded LDH MgAl @CWGB promoted seed germination and plant growth. [ABSTRACT FROM AUTHOR]