Catechol‑iron-clay surface complexation enriches radical formation and efficiency in heterogeneous Fenton reactions

Heterogeneous Fenton reactions are complex and to date, the role of surface confinement (of the pollutant and oxidant) on the reaction rates, the parameters that control the type and concentration of radical species produced, and the properties that impact the extent of degradation are still not fully understood. Here, the degradation behavior of several phenolic and carboxylic compounds was monitored in the presence of H₂O₂ and an iron-oxide-coated clay catalyst. The role of compound structure on the extent of surface complexation, iron-oxide reduction, H₂O₂ activation, and phenol degradation was examined. The two compounds carrying a catechol group, catechin (CC) and 3,4-dihydroxybenzaldehyde (DHB), had the highest adsorption affinity towards the catalyst (K_L = 34 and 29 L∙g⁻¹, respectively), induced iron-reduction on the surface, and were rapidly degraded within 5 min. In contrast, the compounds with single phenol or carboxylic groups exhibited low surface complexation affinity, did not reduce the surface-bound iron(III), and needed 24 h to completely degrade. Radical probe experiments showed the formation of hydroxyl, superoxide, and singlet oxygen reactive species for CC and DHB. In contrast, mainly hydroxyl radicals were produced in the presence of the latter compounds. Furthermore, when the catechol-carrying compounds were added alongside the less reactive compounds, they helped induced rapid degradation, reducing the less reactive compounds' half-lifetime by an order of magnitude. These results suggest that wastewaters that are rich in catechol groups may be highly suited for this type of heterogeneous Fenton remediation process. In addition, the basic insight gained regarding surface complexation with the iron(III)-oxide-coated clay and the structure-activity relationship controlling radical formation can aid in the design of improved, efficient heterogeneous Fenton systems for recalcitrant pollutants in soils and water.