Advanced Oxidation Technologies Based On Activated Persulfate Using Iron-contained Compounds For Organic Pollutants Remediation

Persulfate is a wildey used oxidant for in situ chemical oxidation in the remediation of organic polluted soil and groundwater. Persulfate (S2O82-), a strong oxidant (E0=2.01V), is an attractive candidate and its activation results in generation of strong oxidizing sulfate radical anion (SO4·-, E0=2.6V). Because of its high solubility, better stability than hydrogen peroxide, relative long life time of SO4·-radicals (>4s) and SO4·-radicals may yield a greater mineralization than·OH radicals, persulfate has been used as a source potential for remediation of contaminated water and soil successfully. The key point of using persulfate for advanced oxidation technology is to activate persulfate to generate persulfate radical, and the activation of persulfate may be achieved with physical and chemical modes. The physical activation ways include heating, light, gamma and microwave irradiation et al.(S2O82-→2SO4·-). The chemical activation of persulfate is generally carried out by using transition metal ions as activators, such as Fe2+, Ag+, Cu2+, Mn2+,Ce2+and Co2+. However, disadvatanges of high energy consuming when using thermal to activate persulfate and it should be operated at pH<3.0to prevent the precipitation of Fe(Ⅱ) and Fe(Ⅲ) when using Fe2+as activator hinder its widely use. Moreover, excess Fe2+themselves will react with SO4·-radicals, resulting in poor utilization of persulfate. Therefore, the goals of the present work are to develop the activators or methods to improve the activating ability and the oxidant ultilization efficiency of persulfate. After that, highly efficient oxidation systems were constructed for remediation of organic contaminated water and soil. Related activating mechanism, organic contaminants degradation process and the efficiency of utilization of persulfate were also investigated. The major contents are described as follows:(1) A method of thermal activating persulfate and then used for sulfamonomethoxine (SMM) degradation was developed. It can be seen from the results that thermal can activate persulfate effectively, which can induce fast SMM degradation. Increasing temperature and persulfate concentration can enhance degradation rate constant. Under the conditions of pH6.8,70℃and2.4mmol L-1 persulfate, complete removal of SMM (c0=0.06mmol L-1) was observed within30min. Major radical types in solution was detected using radical caption experiment and found that when the initial solution pH was greater than8.0, the dominant free radicals are SO4·-and·OH, when the initial solution pH was not higher than pH7.0, the major free radicals were found to be SO4·-. Based on the LC-MS analysis of the degradation intermediates, a full mechanism was proposed for the SMM degradation.(2) An oxidative method was investigated for SMM degradation in heterogeneous activating system of nano-Fe3O4and persulfate to avoid high energy-consumption when using thermal activte persulfate and to extend its operation pH values instead of using Fe2+activting persulfate. Nano-Fe3O4was synthesized through a modified reverse co-precipitation method and then characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). In persulfate/Fe3O4system, reactive free radicals were generated through nano-Fe3O4mediated activation of persulfate, leading to immediate degradation of SMM in relative wide pH values, and side reaction between surplus Fe2+and SO4·-was decreased in this system. In selected system of S2O82-:Fe3O4MNPs:SMM was20:40:1, complete removal of SMM (c0=0.06mmol L-1) was observed within15min. Activating mechanism of persulfate was investigated based on the XRD and atomic absorption spectrum (AAS) analysis results. That is, persulfate was decomposed into SO4·-through one electron transfer process by the effect of Fe2+in inverse spinel structured nano-Fe3O4. After activation reaction, Fe3O4was oxidized to be γ-Fe2O3.(3) To improve the Fe2+content of the activator and particle specific surface area, highly-dispersed colloidal activator of ferrous hydroxide colloid was synthesized using ammonia water as precipitant. The ferrous hydroxide colloid was characterized using TEM, zeta potential measuring instrument and laser particle size analyzer and then used as an activator of persulfate for organic contaminates degradation. It was confirmed that reactive radicals were generated on the surface of ferrous hydroxide colloids, which mediated the activation of persulfate, leading to immediate degradation of RhB within1min. Ferrous hydroxide colloids prepared using NH3·H2O with [NH3]/[Fe2+] molar ratio2.0was found to be the most efficient. In selected system of1.2mmol L-1ferrous hydroxide colloids and1.2mmol L-1S2O82-at pH6.8, almost complete removal of SMM (c0=0.06mmol L-1) with the apparent rate constant for SMM degradation was evaluated to be2.21min-1, being5.3folds of that with Fe2+ion (0.42min-1) as the homogeneous activator at pH3.0, and was much superior to nano-Fe3O4(1.09min-1), which were tested as good activator of persulfate under similar reaction conditions. The highly efficient activation effect of the colloids is attributed to the intrinsic nature of colloid particles with large specific surface area, which simultaneously increase the chemical attacking of persulfate and the adsorption of organic pollutants on the colloidal surface. Such an efficient system can be successfully applied for RhB and4-NP degradation.(4) The possibility of soil washing enhanced by surfactants and followed by advanced oxidation process based on persulfate was developed to treat nitrobenzene (NB) polluted kaolin. Aqueous solutions containing sodium dodecylbenzen sulfonate (SDBS) or/and tween80were used to extract NB from the soil samples, and the NB desorption efficiency can reach76.8%, which was superior to Tween80(29.3%) and SDBS/Tween80mixed surfactant with surfactant concentrations24mmol L-1and solution to soil20:1. The washing effluent containing NB and SDBS was successively collected and treated using persulfate-activating method. The data obtained here strongly suggested the generated SO4·-radicals preferably remove electrons with aromatic compound of NB over long alkyl chain compound of SDBS through hydrogen abstraction reactions due to the lone pair electron of SO4·-. A high concentration of SDBS remaining in the effluent after NB was complete degraded, which can be reused for antother new soil washing process.