Phosphate removal by ZIF-8@MWCNT hybrids in presence of effluent organic matter: Adsorbent structure, wastewater quality, and DFT analysis

Abstract

Recently, zeolitic imidazolate framework-8 (ZIF8) and its derivatives have been applied in aqueous contaminant removal. Herein, three types of ZIF8@carbon nanotube (CNT) hybrids harvesting different pore structures and chemical bonding information are utilized for phosphate removal in the typical wastewater of activated sludge system (SW) and partial nitrification-denitrification treatment system (PND). Effluent organic matter (EfOM) is found to compete with phosphate for adsorption sites on adsorbents, resulting in reducing adsorptive capacities for phosphate, and the negative effect trend to become severer with increasing EfOM concentrations. Thus adverse impact are highly to be relieved by using ZIF8@CNT-2 (hybrids with CNT dosage of 120 mg) with novel structure design, the hybrid of which harvests the highest phosphate removal of 92.8–100%, the largest Partition coefficient (PC) of 9119.05 mg g−1 μM with initial concentration of 0.96 mg L−1, pH independence in the range from 4 to 10. Analyses of the XPS characterization and first-principles calculations demonstrate the dominant interactions of Zn-O-P and H-bond during phosphate adsorption process by ZIF8@CNT hybrids. Such interactions are suppressed in presence of EfOM by weakening the above-stated binding energy at different adsorption sites according to first-principles simulation, resulting in declined phosphate adsorption capacity. In this regard, the less sensitivity to co-existing EfOM of ZIF8@CNT-2 may be due to the increased P=O, Zn-O-P and P-OH and the strengthened tolerance of nanostructure. These results suggest the promising enhanced phosphate removal in presence of EfOM could be obtained by specifically designing adsorbent structure.