Degradation of sulfamonomethoxine with Fe3O4 magnetic nanoparticles as heterogeneous activator of persulfate

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Abstract

Iron oxide magnetic nanoparticles (Fe3O4 MNPs) can effectively activate persulfate anions (S2O82−) to produce sulfate free radicals (SO4radical dot), which are a powerful oxidant with promising applications to degrade organic contaminants. The kinetics of sulfamonomethoxine (SMM) degradation was studied in the system of Fe3O4 MNPs and S2O82−. A complete removal of the added SMM (0.06 mmol L−1) was achieved within 15 min with the addition of 1.20 mmol L−1 S2O82− and 2.40 mmol L−1 Fe3O4 MNPs. There is an optimum concentration of Fe3O4 MNPs because Fe3O4 MNPs may also act as a SO4radical dot scavenger at higher concentrations. It was further observed that the addition of Fe3O4 MNPs in several batches for a given total amount of the activator is favorable to enhancing the degradation of SMM. A degradation mechanism was proposed on the basis of identification of the degradation intermediates of SMM with liquid chromatography combined with mass spectroscopy.

Introduction

The pollution of PPCPs (Pharmaceuticals and Personal Care Products) in surface and ground water has been an environmental concern in recent years [1], [2], [3], [4], [5], [6]. PPCPs are generally resistant to biodegradation. Although their levels in waters may be low, their continuous discharge and low doze exposure in the environment will cause terrible effects in terrestrial and aquatic organisms in long term. Sulfamonomethoxine (SMM, N1-(6-methoxyl-4-pyrimidinyl) sulfanilamide) is one of the most popular PPCPs normally administered via food, and is widely used for therapeutic or prophylactic proposes for food-producing animal diseases due to its wide spectrum of antibacterial activity and economical advantage [7]. However, its residues generated by unmetabolized excretion or active metabolites being discharged from municipal wastewater treatment plants and agricultural runoff can lead to antibiotic resistant genes, which may be built up and widely transferred among microorganisms. It proves that SMM has a potential impact on environment such as the effects on fertility and thyroid hormone homeostasis in organisms [8]. Therefore, it is important to develop new efficient ways of treating SMM-containing wastewaters.

Advanced oxidation processes (AOPs) have received much attention in recent years due to their potential effectiveness in the degradation and mineralization of organic pollutants [9]. Fenton and Fenton-like processes are one group of the AOPs [10], [11]. The Fenton reagent (H2O2/Fe2+) has to be used at low pH values (pH < ∼3.0) to avoid hydrolysis and precipitation of Fe3+, which is one of the demerits of the Fenton process for the treatment of wastewater. Persulfate (S2O82−) is an alternative oxidant, because its activation results in generation of strongly oxidizing sulfate free radicals (SO4radical dot, E0 = 2.6 V). With the advantages of high solubility, longer residence time in subsurface than peroxide and wide operative pH range [12], persulfate has been successfully used for environmental applications of remediation of trichloroethylene, benzene, toluene, ethylbenzene, xylene, and polychlorinated biphenyls in aqueous and sediment systems [13], [14], [15], [16].

S2O82− has to be effectively activated to generate SO4radical dot when it is activated by ultraviolet light, heat or transition metal (such as Fe2+ and Co2+) [13], [14]. Zero-valent iron, supported cobalt catalysts and iron–cobalt mixed oxide were introduced for heterogeneous activation [17], [18], [19]. Some of reactions during the activation of S2O82− may be expressed as follows:S2O82− + 2e  2SO42− E0 = 2.01 VS2O82heatorUV2SO4S2O82− + Men+  Me(n+1)+ + SO4radical dot + SO42−SO4radical dot + e  SO42− E0 = 2.6 VHere, Me represents Fe2+ or Co2+. It should be noted that the initiation by UV irradiation is unfavorable to the treatment of UV-absorbing contaminants due to the light filtering effect [20], [21]. The activation way of heating consumes a great amount of heat energy. The use of Fe2+ for activating persulfate is limited in a narrow pH range. Excess Fe2+ ions can also react with SO4radical dot, independent of the presence of organic substrates, which results in a low efficiency for the utilization of persulfate. The above problems may be partly resolved by using chelating agent such as citric acid and EDTA because the use of chelating agent controls the formation rate of SO4radical dot [14].

Fe3O4 magnetic nanoparticles (Fe3O4 MNPs) have been used as peroxidase mimetic instead of Fe2+ to activate H2O2 for the removal of organic pollutants [22], [23], [24]. Because H2O2 and S2O82− are similar in structure of having O–O bond, we anticipate that Fe3O4 MNPs are a good candidate for activation of S2O82−. The present work aimed at providing insight into the activating ability of Fe3O4 MNPs to enhance the decomposition of S2O82− and then degrade organic pollutants: not only an efficient oxidation process was developed to degrade SMM, but also the activation mechanism of persulfate and the degradation mechanism of SMM were clarified.

Section snippets

Materials

SMM with a purity of higher than 99% was purchased from Acros (Geel, Belgium). Ferrous sulfate (FeSO4·7H2O), ferric chloride (FeCl3·6H2O), persulfates and ammonia water were obtained from Sinopharm Chemical Reagents (Shanghai, China). All the chemical reagents were of analytical grade and used as received. All solutions were prepared with deionized water. The pH of solution was adjusted with 0.1 mol L−1 H2SO4 or NaOH.

Preparation of Fe3O4 MNPs

Fe3O4 MNPs were synthesized by a modified reverse co-precipitation process

Characterizations of Fe3O4 MNPs

XRD patterns of as-prepared Fe3O4 MNPs are shown in Fig. 1a. The peaks at 2θ values of 30.1, 35.4, 43.1, 53.6, 57.1 and 62.7° can be indexed as the diffractions of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0), respectively, which are almost the same as the previously reported data for Fe3O4 nanoparticles (JCPDS 79-0419) [27]. On the basis of XRD patterns, the average size of the particles can be evaluated with the Debye–Sherrer formula D = /(β cos θ), where K is the Sherrer constant (0.89), λ is

Conclusions

An oxidative method was investigated for SMM degradation in heterogeneous activation system of Fe3O4 MNPs and persulfate. Reactive free radicals generated through Fe3O4 MNPs mediated activation of persulfate, leading to immediate degradation of SMM. In aqueous system, the effects of oxidant and Fe3O4 MNPs concentrations were studied for the Fe3O4/S2O82− system and their molar ratio of Fe3O4 MNPs:S2O82− was optimized at 2:1. The degradation removal of SMM was slightly dependent on solution pH

Acknowledgments

Financial supports from the National Science Foundation of China (Grants Nos. 20877031 and 21077037) and the National Science Foundation of Hubei Province (Grants No. 2009CDB078) are gratefully acknowledged. The Analytical and Testing Center of Huazhong University of Science and Technology is also thanked for its help in the characterization of the activator.

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