Functionalization of coffee waste biochars with Ca/Fe layered double hydroxides for enhanced removal and selectivity of phosphate ions: Mechanisms and reusability
Jaegwan Shin, Hojung Rho, Y. J. Cho, Changgil Son, Jinwoo Kwak, Sang-Won Kim, Seohyun Ki, Hee-Joong Kim, Yong-Gu Lee, Yongeun Park, Sang-Ho Lee, Kangmin Chon
Abstract
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 (PO 4 3 ) ions. Ca/Fe LDH-functionalized CWB (LDH CF @CWB) exhibited the higher adsorption capacity (Q e ) of PO 4 3− 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 PO 4 3− ions. The adsorption behavior of PO 4 3− ions by CWB and LDH CF @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 PO 4 3− ions by CWB (∆G° = −13.3 kJ/mol to −13.5 kJ/mol; ∆H° = 3.0 kJ/mol) and LDH CF @CWB (∆G° = −21.6 kJ/mol to −23.5 kJ/mol; ∆H° = 28.9 kJ/mol). The decreasing ratio of the Q e values of PO 4 3− 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 LDH CF @CWB = 0.7–2.8%) from ligand exchange to anion exchange and electrostatic surface complexation. Since LDH CF @CWB maintained a high reuse efficiency (≥ 88.4%) until four adsorption-desorption cycles, LDH CF @CWB might be a potential adsorbent for the elimination of PO 4 3− ions in real water matrices.