Application of Fe(III)-EDDS complex in advanced oxidation processes : 4-ter-butylphenol degradation

Advanced Oxidation Processes (AOPs) have been proved to be successfully applied in the treatment of sewage. It can decolorize the wastewater, reduce the toxicity of pollutants, convert the pollutants to be a biodegradable by-product and achieve the completed mineralization of the organic pollutants. The Fenton technologies which are performed by iron-activated hydrogen peroxide (H2O2) to produce hydroxyl radical (HO•) has been widely investigated in the past few decades. Recently, Sulfate radical (SO4•-) which was produced by the activation of persulfate (S2O82-) is applied to the degradation of organic pollutants in water and soil. It is a new technology recently developed. It is also believed to be one of the most promising advanced oxidation technologies.In this study, a new iron complex is introduced to the traditional Fenton reaction. The ferric iron (Fe(III)) and Ethylene diamine-N,N′-disuccinic acid (EDDS) formed the complex named Fe(III)-EDDS. It can overcome the main disadvantage of traditional Fenton technology, which is the fact that traditional Fenton technology can only perform high efficiency in acidic condition. Simultaneously, EDDS is biodegradable and it is one of the best environment-friendly complexing agents. On the other hand, the transition metal is able to activate S2O82- to generate SO4•-. Therefore, Fe(III)-EDDS will also be applied to activate S2O82- in the present study. 4-tert-Butylphenol (4-t-BP) has been chosen as a target pollutant in this study. It is widely used as a chemical raw material and is classified as endocrine disrupting chemicals due to the estrogenic effects. The 4-t-BP degradation rate (R4-t-BP) is used to indicate the efficiency of the advanced oxidation processes which are based on Fe(III)-EDDS utilization. The main contents and conclusions of this research are shown as follows:In the first part, the chemical structure and properties of Fe(III)-EDDS and the 4-t-BP degradation efficiency in UV/Fe(III)-EDDS system were studied. The results showed that Fe(III)-EDDS was a stable complex which was formed by the Fe(III) and EDDS with the molar ratio 1:1. From the photoredox process of Fe(III)-EDDS, the formation of hydroxyl radical was confirmed including that HO• is the main species responsible for the degradation of 4-t-BP in aqueous solution. Ferrous ion (Fe(II)) was also formed during the reaction. With the increasing Fe(III)-EDDS concentration, 4-t-BP degradation rate increased but is inhibited when the Fe(III)-EDDS concentration was too high. Indeed, Fe(III)-EDDS is the scavenger of HO•. pH value had a significant effect on the degradation efficiency of 4-t-BP that was enhanced under neutral or alkaline conditions. On the one hand, Fe(III)-EDDS presented in the FeL-, Fe(OH)L2-, Fe(OH)2L3-, Fe(OH)4- four different forms under different pH conditions and they had different sensitivity to the UV light. On the other hand, pH value affected the cycle between Fe(III) and Fe(II ). The formation of hydroperoxy radicals (HO2•) and superoxide radical anions (O2•-) (pka = 4.88) as a function of pH was also one of the reasons. It was observed that O2 was an important parameter affecting the efficiency of this process. This effect of O2 is mainly due to its important role during the oxidation of the first radical formed on the pollutant. (...)