Functionalization of coffee waste biochars with Ca/Fe layered double hydroxides for enhanced removal and selectivity of phosphate ions: Mechanisms and reusability

초록

This study comprehensively estimated the feasibility of the Ca/Fe layered double hydroxides (LDH) functionalization to reinforce the adsorption performance and selectivity of coffee waste biochars (CWB) for phosphate (PO43) ions. Ca/Fe LDH-functionalized CWB (LDHCF@CWB) exhibited the higher adsorption capacity (Q(e)) of PO43- ions (34.5 mg/g) than CWB (2.4 mg/g), Mg/Fe LDH-functionalized CWB (6.5 mg/g), and Mg/Al LDH-functionalized CWB (19.1 mg/g) due to the high reactivity and low energy requirements of Ca contents for the interaction with PO43- ions. The adsorption behavior of PO43- ions by CWB and LDHCF@CWB was obeyed by the intra-particle diffusion (constant of intra-particle diffusion > constant of liquid film diffusion) and chemisorption. Furthermore, the endothermic and spontaneous reactions chiefly controlled the adsorptive removal of PO43- ions by CWB (triangle G degrees = -13.3 kJ/mol to -13.5 kJ/mol; triangle H degrees = 3.0 kJ/mol) and LDHCF@CWB (triangle G degrees = -21.6 kJ/mol to -23.5 kJ/mol; triangle H degrees = 28.9 kJ/mol). The decreasing ratio of the Q(e) values of PO43- ions using CWB under the co-presenting anions (decreasing ratio of Q(e) = 27.9-74.9%) was considerably reduced by changing the main adsorption mechanisms via the Ca/Fe LDH functionalization (decreasing ratio of Q(e) of LDHCF@CWB = 0.7-2.8%) from ligand exchange to anion exchange and electrostatic surface complexation. Since LDHCF@CWB maintained a high reuse efficiency (>= 88.4%) until four adsorption-desorption cycles, LDHCF@CWB might be a potential adsorbent for the elimination of PO43- ions in real water matrices.

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