Column study of the effects of operational and water quality factors on bromate reduction by zerovalent iron

Zerovalent iron (ZVI) is not expensive and can be obtained by re-using scrap iron filings. It has been widely used in treatment water contaminated with organic and inorganic compounds. Through oxidation, ZVI donates electron to reduce the compounds. Although it has been demonstrated that bromate can be reduced by ZVI in batch study, the capacity of ZVI packed columns for bromate removal under different water quality and water treatment parameters is less clear.In this study, a series of column experiments were conducted to investigate the effects of flow rates; pH; sand admixture; and concentrations of dissolved oxygen, bromate, nitrate, sulfate, phosphate and bicarbonate on removal capacity of ZVI for bromate in synthetic water. A long term column experiment was also conducted in the synthetic water. In addition, the effect of particles on the bromate removal capacity was also examined in real raw water.Results of this study showed that the bromate concentration profile did not stabilize, but migrated into the columns with time. Migration rates (cm/PV) and hence removal capacities (mg BrO3-/g Fe) can be obtained from the migration of the concentration profiles. The removal capacity was observed to decrease with increases in flow rate, which is probably a consequence of the decrease in hydraulic residence time. Increasing initial pH from 6 to 8 had no significant effect on the removal capacity, in contrast to a pH adjustment of the solution to pH 4, which noticeably increased the removal capacity. The higher removal capacity was accompanied by a considerably higher soluble iron concentration at early time of the column run, which is presumably attributable to the dissolution of the hydrous ferrous oxide at such low pH. In the case of 20 wt-% iron was replaced by sand, the removal capacity increased, but it inevitably decreased when the weight percentage was doubled. Dissolved oxygen played a minor role in decreasing the removal capacity, depassivating iron near the proximal end only. The removal capacity remained nearly unchanged at different inflowing bromate concentrations reflecting a fixed ratio between bromate and iron. The removal capacities were also found to decrease with increases in nitrate concentration. It was found that more than 30% of nitrate was removed with 90% or higher high nitrogen recoveries. This shows that nitrate competes with bromate for electrons. The removal capacity increased in the presence of 100 mg/L sulfate concentration but decreased when the concentration further increased, depending on the dominance of its promotion and inhibition roles. Comparing phosphate to sulfate, the inhibiting effect of phosphate on the bromate removal capacity was larger. The removal capacity increased with increasing bicarbonate concentration, which can probably be accounted for by the formation of aqueous complexes. In long term column test, no bromate breakthrough was observed throughout the column run. Last, raw water pretreated with sand filtration resulted in the slower pressure buildup at the entrance of column although the breakthrough time of the filter and unfiltered raw water were similar.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005