Citric acid-assisted MnOx/biochar for periodate activation and diclofenac degradation: A non-radical pathway driven by Mn(III) stabilization.

The performance and mechanism of a citric acid (CA)-enhanced MnOx/biochar (MnOx/BC) system for activating periodate (PI) to degrade diclofenac (DCF) have been investigated in the present study. This study has involved (i) the synthesis of a MnOx/BC composite, (ii) evaluation of its catalytic efficiency under various conditions and with real-water matrices, and (iii) a comprehensive investigation of the underlying reaction pathways by using spectroscopic, electrochemical, and probe-based techniques. Furthermore, four machine learning (ML) models have been developed for the prediction of the degradation performance, using shapely additive explanations (SHAP) and a bidirectional partial dependence analysis (PDP) for quantification of the feature importance and optimization of the operational parameters. Under the optimized conditions (MnOx/BC = 0.20 g L-1, PI = 0.20 g L-1, CA = 0.20 g L-1, initial pH = 9.8), the MnOx-CA/PI system achieved >99 % DCF removal within 10 min with a pseudo-first-order rate constant (Kobs) of 0.2287 min-1. These results indicated that the MnOx-CA/PI system achieved a rapid DCF removal with high stability. Also, the introduction of CA triggered a fundamental mechanistic shift from •OH-dominated radical pathways to a non-radical regime. The X-ray photoelectron spectroscopy (XPS), UV-Vis, and methyl phenyl sulfoxide (PMSO) probe analyses indicated that CA sequestered and stabilized in situ-generated Mn(III) intermediates, thereby facilitating the formation of high-valent Mn(V)=O and 1O2 species through a two-electron transfer process. The ML analysis identified the ligand type as the most influential determinant of the catalytic efficacy (24.6 %), followed by the catalyst and oxidant dosages. Based on the optimal experimental conditions obtained from the ML model and PDP analyses, the theoretically optimized MnOx-CA/PI system achieved the highest Kobs for DCF degradation and consistently exhibited higher removal rates across various initial concentrations. These findings have provided a systematic framework for an understanding of the ligand-enhanced PI activation, and have indicated its potential for efficient, matrix-tolerant water remediation.