Nano-heterostructures for tetracycline degradation in wastewater: Interfacial charge transfer, reactive species, transformation pathways and environmental safety

The relationship between tetracycline (TC) contamination, pharmaceutical application, wastewater discharge, ecological exposure and the spread of antimicrobial resistance is complex. Nanomaterial-based treatment is of interest because adsorption, light harvesting, charge separation, redox cycling and catalyst recovery can potentially be integrated within a single material system. The performance of conventional treatment methods for TC at low concentrations and in the presence of dissolved minerals requires careful evaluation. However, details about mineralization, the toxicity of byproducts, metal leaching, energy needs and ongoing operation are not reported as clearly in literature. In this review, the metal and metal-oxide nanoparticles, carbonaceous materials, two-dimensional materials and MXenes, metal-organic frameworks, semiconductor photocatalysts, nano-Fenton catalysts, persulfate/peroxymonosulfate activators and multifunctional hybrids for TC removal and degradation are discussed critically. Special emphasis is placed on heterointerfaces because they can influence band bending, internal electric fields, carrier lifetimes, adsorption microenvironments and oxidant accessibility. These interfacial effects can promote the formation of hydroxyl radicals, superoxide, sulfate radicals, singlet oxygen and reactive holes. Mechanistic assignments, based only on band-edge diagrams, are distinguished from perceived benefits of type-II, Z-scheme, S-scheme, Schottky, carbon-mediated and redox-coupled architectures. It is emphasized in this review that the reported performance values of different works from literature are not directly comparable without considering the concentration, catalyst dose, irradiation, oxidant input, reaction time and water matrix. Moreover, the paper addresses the issue of ambiguity between adsorption and degradation, parent disappearance and mineralization, transformation pathways, residual toxicity, stability of the catalyst, recovery and regeneration of the catalyst, actual wastewater, immobilized systems, membranes and flow-through reactors. The conclusion is that interface engineering can overcome the bottlenecks identified in specific situations. However, practical progress will depend on (i) prioritizing mass-normalized mineralization over maximum batch removal, (ii) quantifying and controlling the release of bound or particulate material, and (iii) improving catalyst recovery and maintaining structural integrity.