Sustainable Rabbit-Manure-Based QL-RMB Nanocomposite for Mn(VII) Removal from Wastewater and Catalytic Reuse

Abstract

A sustainable strategy was developed to valorize rabbit manure waste by synthesizing a porous quaternary Ni-Co-Zn-Fe layered double hydroxide/biochar nanocomposite (QL-RMB) for the efficient removal of Mn(VII) in the form of permanganate (MnO4−) from aqueous solutions. The QL-RMB adsorbent exhibited a well-developed mesoporous structure with uniformly dispersed nanoparticles, achieving 73% MnO4− removal within 60 min under optimized conditions (pH 3.0; dosage 0.5 g L−1). Adsorption followed pseudo-second-order kinetics and was best described by the Freundlich isotherm model (R2 > 0.98), yielding a maximum Langmuir adsorption capacity (qmax) of 45.13 mg g−1. Statistical physics modeling confirmed a multi-ionic, vertically oriented adsorption configuration, while thermodynamic analysis demonstrated that the process was spontaneous and exothermic, governed by electrostatic attraction, anion exchange, and surface complexation. The QL-RMB composite exhibited excellent MnO4− selectivity in the presence of competing ions (selectivity coefficients: 24.96 for Fe3+, 31.59 for Ni2+, 23.56 for Zn2+) and retained significant removal efficiency (73.96%) after five regeneration cycles. In a circular economy approach, the Mn (VII)-spent adsorbent (QL-RMB/Mn) was valorized as an electrocatalyst for urea electro-oxidation, achieving a current density of ~127.19 mA cm−2 for pristine QL-RMB, which increased to ~217.07 mA cm−2 after Mn(VII) adsorption (QL-RMB/Mn) in 1 M KOH/1 M urea. Batch scale-up studies revealed an efficiency of 42.55 g or 95% MnO4− removal from 50 L water, with a low estimated production cost of 0.0602 USD g−1. Environmental sustainability was confirmed by the National Environmental Methods Index (NEMI), modified Green Analytical Procedure Index (Mo-GAPI), Eco-scale (score: 77), and Analytical GREEness (AGREE) assessment frameworks.