| With the widespread application of ozonation-biological activated carbon, ozonation of bromide- bearing raw water produces the by-product, bromate, which is classified as a potential carcinogen (Group 2B) by the IARC (International Agency for the Research on Cancer).And the the drinking water quality standard (GB 5749-2006) in China set the maximum contaminant level (MCL) of bromate to 10μg/L. So it is urgent to develop effective technology in bromate removal. In this study, based on the comparation of different removal technologies of bromate, activated carbon (AC) has been served as a supporting material for nZVI (nZVI/GAC) and applied in bromate removal in the presence of oxygen.Firstly, ethonal and PEG-6000 were selected for the modification of nZVI/GAC. Different modified nZVI/GACs were synthesised, and batch experiments were conducted to compare the effect on bromate removal. The results indicated that ethonal and PEG-6000 could both improve the efficiency of bromate removal in the presence of oxygen. And when the concentration of ethonal and the dosage of PEG-6000 was 25% and 0.25 g, respectively, the effect on bromate removal was the best. The effect of ethonal and PEG-6000 together would not as good as independent one. On the basis of comparison of different modified nZVI/GAC, ethonal concentration of 25% was selected. And nearly 90% of bromate was removed under a DO value of 6.5±0.5 mg/L with an appropriate nZVI/GAC dosage of 40 mg/L.On the basis of comparison of different modified nZVI/GAC, a series of influencing factors, such as the dosage of nZVI/GAC, mass ratio of nZVI, initial bromate concentration, pH, reaction temperature, humic acid (HA) and trichloroacetic acid (TCAA) on bromate removal at a DO value of 6.5±0.5 mg/L were discussed. The results showed that the removal efficiency would increase with the increase of dosage and temperature. Bromate removal rate decreased at first with an increase of initial bromate concentration. pH had a dual effect, acidic condition could enhance bromate removal, and alkaline condition inhibited the reaction due to the formation of iron-hydroxide precipitation on the surface of nZVI. The presence of HA decreased the bromate removal efficiency. The result could be explained that a number of available sites of nZVI were occupied by HA and complexation happened between HA andthe Fe2+ produced. The presence of TCAA could inhibit the bromate removal due to the competition between TCAA and bromate, and bromate could be removed faster than TCCA. Comparing with nZVI, nZVI/GAC performs well through a broad pH range for bromate removal, while the removal of bromate by nZVI was more pH dependent. And, nZVI/GAC showed an anti-fouling effect against HA and TCAA compared with nZVI.Moreover, in the column test, nZVI/GAC performed excellent bromate removal capacity as filler. The column was operated with the EBCT of 7.5 min, bed height of 15 cm and initial bromate concentration of 100μg/L. At the breakthrougu point, the bed volume passed was 1157.In addition, the analysis of mechanism revealed that nZVI/GAC showed excellent bromate removal efficiency, which is likely due to its synergistic effects of adsorption by GAC and reduction by nZVI. And bromate removal by nZVI/GAC could be concluded in three steps:firstly, bromate was adsorbed on GAC; secondly, reduced by nZVI; finally, the products desorped from the surface of nZVI. The most important step was the reduction on the surface of nZVI. The concentration of Fe3+ in the solution for bromate removal by nZVI/GAC was not more than 0.1 mg/L and remained relatively stable during the whole reaction period. Therefore, there was no risk of secondary pollution of Fe3+ by using nZVI/GAC for bromate reduction. Moreover, the whole removal process fits well with the pseudo-second-order reaction kinetics model.Therefore, nZVI/GAC could be an effective material for bromate removal in the presence of oxygen. |
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