Deep-eutectic-solvent synthesis of ferrocene-derived Fe-functionalized activated carbon for adsorption- assisted heterogeneous fenton oxidation in complex dye matrices.

This study presents an organic-solvent-free, deep-eutectic-solvent (DES)-assisted route to prepare a ferrocene-loaded powdered activated carbon (GFePAC) that couples adsorption with heterogeneous Fenton catalysis for methylene blue (MB) and synthetic textile wastewater, including Congo red (CR). In a choline chloride/glycerol DES, ferrocene-derived iron was uniformly immobilized on PAC, increasing the bulk Fe content from 0.0270 ± 0.0004 to 0.1992 ± 0.0017 wt% (7.4-fold). Multimodal characterization (ICP-OES, SEM-EDS, BET, FTIR, XRD, and XPS) confirmed preservation of the carbon framework and the formation of mixed-valent Fe(II/III) sites. In batch degradation tests, GFePAC + H2O2 consistently outperformed homogeneous FeCl2 + H2O2, showing higher apparent pseudo-first-order (Kapp) rate constants at 10 mg L-1 MB (k = 0.3731, 0.2651, 0.2925 min-1 at pH 4/7/9) compared with FeCl2 (k = 0.1803, 0.1265, 0.1111 min-1). Even at 50 mg L-1, GFePAC maintained clear kinetic advantages under acidic conditions favorable for Fenton oxidation (e.g., pH 4: 0.2317 vs. 0.0586 min-1), which was attributed to adsorption-assisted enrichment near redox-active Fe centers. Radical scavenger tests confirmed •OH as the dominant oxidant, whereas five-replicate ICP-OES analyses showed negligible Fe leaching, demonstrating catalyst stability. As stand-alone adsorbents, PAC showed higher MB uptake than GFePAC due to partial pore occupation by iron; however, GFePAC preserved substantial adsorption capacity while delivering superior catalytic performance. In a synthetic textile wastewater matrix containing MB and CR, both adsorption and Fenton degradation were attenuated by competitive dye uptake and ionic-strength effects; nevertheless, GFePAC still outperformed FeCl2 with 20 min degradation efficiencies of 96.8%, 88.0%, and 88.1% at pH 4, 7, and 9, respectively, compared with 78.9%, 64.3%, and 56.2% for FeCl2. Overall, the DES-enabled GFePAC system provides reproducible, mechanism-supported enhancement of dye degradation through synergistic adsorption-Fenton action, highlighting its potential as a sustainable catalytic platform for treating dye-laden industrial wastewaters.