Selective Removal of Sulfachloropyridazine by Natural Manganese Sand via Mn–N Coordinative Adsorption Coupled with Mn(III)/Mn(IV) Surface Oxidation

Sulfachloropyridazine (SCP) is a sulfonamide antibiotic widely detected in aquatic environments. Its terminal pyridazine ring contains vicinal pyridinic nitrogen atoms (=N–N=) with strong metal-complexing ability, which may cause distinct interactions with metal-oxide filter media. Here, we systematically investigated the removal of sulfonamide antibiotics by natural manganese sand (NMS), a representative filtration medium in water treatment. NMS exhibited pronounced selectivity toward SCP, while showing negligible removal of sulfadiazine (SD) and sulfamethoxazole (SMX). Under optimal conditions (NMS = 5 g·L−1, pH 3), 99.38% of SCP (5 mg·L−1) was removed within 6 h; the TOC reduction only reached 42.65%, suggesting the partial transformation of SCP. Mechanistic evidence suggests that the vicinal pyridinic N–N motif of SCP provides dual electron-donating sites, enabling inner-sphere Mn–N complexation on NMS. This coordination-driven adsorption is strongly pH-dependent and is inhibited under neutral to alkaline conditions (pH > 5) due to electrostatic repulsion. After selective binding, surface Mn(III)/Mn(IV) species can act as electron acceptors, driving the surface oxidation of SCP. Although NMS induced mild oxidative transformation of SCP, some degradation products still exhibited potential ecotoxicity and therefore require further attention in practical water treatment applications. These findings link terminal functional-group structure to selective abiotic removal on Mn-oxide media and inform targeted control of sulfonamide micropollutants in filtration-based water treatment.