Mechanistic investigation of NiAl-layered double hydroxide activated peroxymonosulfate for tetracycline degradation: Feasibility of an integrated ultrafiltration system

Abstract

Transition metal catalysts have shown great potential as peroxymonosulfate (PMS) activators in sulfate radical-based advanced oxidation processes (SR-AOPs). However, their practical application in water remediation is often limited by resource scarcity and the toxicity of certain transition metals. In this study, we synthesized a cost-effective and highly efficient NiAl-layered double hydroxide (NiAl-LDH) catalyst for PMS activation and systematically investigated its mechanism in tetracycline (TC) degradation. The synthesized NiAl-LDH exhibited a well-crystalline, plate-like CO₃^2--type structure with an average particle size of ∼100 nm. Despite having a smaller surface area than pure Ni(OH)₂, the NiAl-LDH/PMS system demonstrated superior TC degradation efficiency. The initial pH significantly influenced both TC degradation performance and LDH structural stability. While CO₃²⁻ and PO₄³⁻ slightly inhibited TC degradation, Cl⁻, NO₃⁻, and SO₄²⁻ had negligible effects. Ultrafiltration with the NiAl-LDH/PMS system enabled the removal of 500 mg/L TC while maintaining a high permeate flux of 2680 LMH. In real wastewater matrices, TC removal efficiency slightly decreased to 85.1 %, with a reduction in flux (1990 LMH) due to organic matters and concentrated anions. The DFT calculations proved that enhanced electron conductivity, strong PMS adsorption, and reduced endothermic energy of PMS activation resulted in improved catalytic activity of NiAl-LDH compared to pure Ni(OH)₂. Mechanistic investigations revealed that hydroxyl radicals (·OH) were generated via the redox cycling of Ni²⁺/Ni³⁺ on NiAl-LDH with PMS and rapidly transformed into singlet oxygen (¹O₂) through superoxide (O₂⁻·) intermediates. Furthermore, TC degradation pathways were elucidated, providing deeper insights into its catalytic performance for water treatment applications.