Polymer hydrogels with enhanced stability and heterogeneous Fenton activity in organic pollutant removal

Polymer hydrogel-based materials have been shown to act as novel Fenton catalysts for water treatment, but the rational design of hydrogel-based catalysts with good stability has been a great challenge. To increase the stability and activity of polymer-based Fenton catalysts, uniform urchin-like α-Fe₂O₃ was grown in situ in a PVA carrier matrix here. PVA molecules promoted the growth of urchin-like α-Fe₂O₃, and then the PVA hydrogel acted as a barrier and carrier to reduce agglomeration. Through coordination by hydroxyl groups, PVA had good combination with Fe ions and α-Fe₂O₃. The formation of Fe-O-C bonds between iron oxides and polymers was reported for the first time, enhancing the material stability during catalysis. Under higher PVA concentrations, the resulting composite hydrogel could generate more ˙OH due to the increase in the number of active sites because of the hairy urchin-like structure. In tetracycline degradation through a heterogeneous Fenton reaction, the resulting material had good catalytic activity from pH 2 to pH 10 with low iron leaching, good reusability and remained at a level of nearly 90% after five consecutive cycles. Density functional theory calculations were used to further prove the mechanism of structural change of the iron oxides. The HOMO and LUMO energies of the iron oxides changed from 5.428 and 4.899 eV to 5.926 and 5.310 eV, indicating that the presence of PVA could influence the charge of the iron atom. The results provide new insights into the preparation of polymer hydrogel-based heterogeneous Fenton catalysts with enhanced stability for water treatment.