Novel Iron-based Polyoxometalates For Catalytic Degradation Of Organic Contaminant In Heterogeneous Fenton-like Systems

Chlorophenols are a group of very significant organic synthesis intermediates, which have been widely employed in the production of herbicides, insecticides and wood preservatives. Chlorophenols in water are characterized as high toxicity, structural stability,non-biodegradability and suspected carcinogenicity, and thus they are seriously harmful to human health and environment. Therefore, the study on the techniques for efficient degradation of chlorophenols in wastewater is of great importance.In this work, two novel iron-based polyoxometalates, called FeIIIAsp Si W and Fc Si W,were successfully synthesized at ambient temperature in aqueous solution and used as the heterogeneous Fenton-like catalysts for organic contaminant removal. The former was synthesized from ferric chloride（Fe III）, aspartic acid（Asp） and silicotungstic acid（Si W）, and the later was produced from ferrocene（Fc） and silicotungstic acid（Si W）. The as-prepared two catalysts were characterized by FT-IR, TG, ICP-AES, UV-vis DRS, XRD and FE-SEM techniques.Most of heterogeneous Fenton-like catalysts reported previously show good performances only in acidic conditions（p H³）. As a consequence, acidification and neutralization are usually required before and after treatment, leading to an increased cost for wastewater treatment. Considering the practical feasibility, the catalytic performances of Fe IIIAsp Si W and Fc Si W catalysts were evaluated by the oxidation of 4-chlorophenol（4-CP）at the circumneutral p H（p H 6.5） of the reaction mixture, and no p H control was performed throughout the procedure. The results revealed that both the two catalysts showed high activity for the degradation of 4-CP at the circumneutral p H, and the reaction could significantly enhanced under irradiation. Under the optimum conditions, for Fe IIIAsp Si W catalyst, after 2 h of reaction, 88.6% TOC removal was achieved under irradiation, and 40.7%TOC removal was obtained in the dark. For Fc Si W catalyst, after 6 h of reaction, 89.5% TOC removal was achieved under irradiation, and in contrast, only 20.5% was obtained in the dark.The obtained data showed that the catalytic performances of as-prepared two catalysts at circumneutral p H have exceeded that of most of other heterogeneous Fenton-like iron-based catalysts reported previously, such as zero-valent iron and iron oxides. Although lowering p H value could further increase the reaction rate, the removal efficiency of 4-CP without p H adjustment was still considerable. In addition, the initial H₂O₂ concentration, catalyst dosage and temperature showed obvious effect on the degradation of 4-CP. Both the two catalystshave the considerable stability and reusability, and no obvious performance drop was observed when the recovered catalysts were reused for several times.Control experiments confirmed that hydroxyl radical（^·OH） was the dominant oxidative species generated in both the reaction systems and was responsible to the oxidation of 4-CP.The catalysis as-prepared iron-based polyoxometalates was believed to be related to the co-existence of iron and heteropoly anion in the catalysts. A plausible catalytic mechanism was proposed, which involved the synergetic effect of Fenton-like catalysis of iron and photocatalysis of the heteropoly anion.The catalytic performances of FeIIIAsp Si W and Fc Si W were also assessed by treating municipal wastewater. The results showed that both the catalysts showed obvious catalytic activity for TOC removal from the municipal wastewater in the presence of H₂O₂. Fixing municipal wastewater concentration, H₂O₂ concentration, temperature and p H value at 21.2mg L^-1（TOC value）, 20 mmol L^-1, 20 °C, 3.0, and FeIIIAsp Si W dose of 0.2 gL^-1, Fc Si W dose of 0.1 g L^-1, TOC removal efficiency after 8 h of reaction attained 19.3% and 17.0%,catalyzed by FeIIIAsp Si W and Fc Si W, respectively. Clearly, TOC removal efficiency is lower than that observed in the degradation of 4-CP, due to the complicated compositions in municipal wastewater. Possibly, Some inorganic quenchers for^·OH radical, such as Cl^-, PO₄³-and CO₃^2- usually found in municipal wastewater, may considerably depress the oxidation of organic contaminants. Much work should be done to improve the performance of the catalysts for removal of organic pollutants from wastewater with complicate components.