Chemical And Electrochemical Removal Of Nitrate And Disinfection Byproducts By Al-Fe Composites Materials

In recent years, zero-valent metals such as aluminum, iron,magnesium, zinc, etc. have been used in pollutants removal because of their rapid reductivity. Zero-valent iron has been used in permeated reactive barrier for groundwater remediation. However, zero-valent iron in the process was corrosed and caused the dissolution of the metal iron and increase of pH values. Because ferrous oxides or hydroxides deposition on the iron surface limitted its application, researchers found out a effective nitrate reduction in acidic conditions. The deposition of a second metal such as Ni, Pd, Cu onto the iron surface was found to be efficient in improvement for the resistance of iron. A1 is the highest ratio of metals in the earth crust. In previous study, bimetallic Fe@Al was successfully used in pollutants removal. However, uneven distribution of the surface metal and uncoherence of the second metal usually leads to unstable removal of pollutants. In this study, we synthesized a seris of Al-Fe alloys composites for nitrate and disinfection byproducts removal by melting certain weight ratio of metallic aluminum and iron in electric furnace.The contents and main results of this study was as the following goes:1. A primary experiment showed that dionized water (D.I. water)can activate the alloys materials and thus we compared several chemicals such as KBH4, HC1, D.I. water in pretreatment of Al-Fe alloys.By comparing the reactivity for nitrate removal, integrating with the SEM-EDS and the XRD analysis, we declared the relation between the alloys composition and the reactivity for nitrate removal. Furthermore,factors such as pretreatment temperatures and longevity were also investigated. Water-bathed at 45 ? for 12 h was the optimal condition for activation of the material. The D.I. water pretreatment was better than HC1 and KBH4. Adjusting the water-bath temperature to higher or lower than 45 ? can effect the reductivity of the alloys. Nevertheless,decrease or increase the water-bath time from 12 h was also disadvantageous for nitrate removal.2. Aluminum-iron alloys were compared with zero-valent iron and aluminum for the reduction of aqueous nitrate in column studies. The Al-Fe alloy materials were efficient to reduce nitrate in water in an entire pH range of 2-12. At near neutral pH, nitrate reduction proceeded in a pseudo-first order. The observed reaction rate constant (Kobs) of the Al-Fe10 alloy was 3 times higher than that of Fe and the Kobs of Al-Fe20 doubled that of Al-Fe10. The final product of the reduction by Fe was ammonia whereas 17.6%, 23.9% and 40.3% of nitrate were reduced to nitrogen gas by Al-Fe10, Al-Fe20 and Al-Fe58, respectively. The enhanced selectivity of nitrogen gas product was likely due to the presence of intermetallic Al-Fe phases. It was also found that the nitrogen gas selectivity increased with increasing the pH of the solution and controlling the intermediate product (nitrite) of the reduction and the reduction kinetics was important to adjust the selectivity of the reduction.3. Aluminum-iron alloys can remove DBPs in very short time (<3 min) and maintained effective even in 30-day use. EDS and XRD analysis showed that Al and Fe were integrated in Al-Fe alloys and intermetallic compounds such as Al13Fe4, FeAl2 and Fe3Al were formed in the melting process. The electric potential of Al-Fe alloys was much lower than Al and Fe, which is mainly attributed to the galvanic couple between Al-rich phase and Fe-rich phase in Al-Fe alloys. Fe-rich phase catalyzed the hydrogenation of DBPs and depassivated the Al-rich phase in Al-Fe alloys. Al-rich phase provided electrons to Fe-rich phase (Fe and Al13Fe4) and maintained low open circuit potential of less than-0.90V. Al-Fe20 containing abundant A113Fe4 phase can maintain highly active in removing DBPs and the density was convenient for backwash.Furthermore, Al-Fe alloys have no adverse effects on drinking water quality. Al-Fe20 is a favorable alternate for degradation of DBPs because its comparable density and backwash performance to quartz sand. In the last part of this article, affecting factors such as initial CCl4 concentration, reaction temperature, filter heights and additional organic ions were investigated for the removal of CCl4 by Al-Fe20 and can provide instructions for practical operation.4. The removal efficiency of different Fe-Al electrodes, applied voltages, electrolytes and pollutant concentration were also studied in this thesis. Al-Fe 10 electrode had better degradation effect than Al-Fe20 electrode. Among the voltages chosen in the experiment, maximum nitrate removal rate is achieved when the voltage is -1.2V, and maximum carbon tetrachloride removal rate is achieved when the voltage is -1.5V. Among the electrolytes chosen in this experiment,maximum nitrate and carbon tetrachloride removal rate is achieved in 10 mmo1·L-1 Na2SO4 electrolyte, followed by 10 mmol·L-1 KCl electrolyte,and 5.77 mmol·L-1 Na2SO4 electrolyte is at the last. When nitrate and carbon tetrachloride are reduced together, the removal of carbon tetrachloride is not affected by nitrate, while the removal of nitrate is not affected by 200 ?g·L-1 carbon tetrachloride, but prohibited by the 600?g·L-1 and 1200 ?g·L-1 carbon tetrachloride. Its prohibition is independent of concentration of carbon tetrachloride.