The effects of chloride ions on the hydrogen gas generation and TCE degradation in aqueous solutions containing zero valent iron

THE EFFECTS OF CHLORIDE IONS ON THE HYDROGEN GAS GENERATION AND TCE DEGRADATION IN AQUEOUS SOLUTIONS CONTAINING ZERO VALENT IRON Thesis for the Degree of Master of Science in Engineering Khanh An Huynh 2007 Department of Civil and Environmental Engineering Korea University Thesis for the Degree of Master of Science in Engineering The Effects of Chloride Ions on the Hydrogen Gas Generation and TCE Degradation in Aqueous Solutions Containing Zero Valent Iron By Huynh, Khanh An Department of Civil and Environmental Engineering The Graduate School of Korea University December 2007 金 之 瀅 敎授指導 博士學位論文 The Effects of Chloride Ions on the Hydrogen Gas Generation and TCE Degradation in Aqueous Solutions Containing Zero Valent Iron 이 論文을 工學 博士學位 論文으로 提出함 2007 年 12 月 高麗大學校 大學院 社會環境시스템工學科 Huynh, Khanh An The Effects of Chloride Ions on the Hydrogen Gas Generation and TCE Degradation in Aqueous Solutions Containing Zero Valent Iron Huynh, Khanh An Dept of Civil and Environmental Engineering, Graduate school of Korea University Abstract The effects of chloride ions on the hydrogen gas generation and trichloroethylene (TCE) degradation in aqueous solutions containing zero valent iron were investigated in this study TCE degradation rate followed pseudo-first order kinetic with respect to TCE concentration Chloride ions had strong effects on TCE degradation rates at low pH They were 32.93%, 19.51%, 7.54% higher at pH of 4.84, 6.41 and 7.16, respectively, in the presence of 20mM NaCl The degradation of TCE depended on the chloride concentrations The increase of chloride concentrations increased TCE degradation rates until the concentration of 40 mM NaCl was reached Degradation rate constant increased to 7.80E-05 h-1m-2L which was 40% higher than using iron only Further increase in chloride concentrations resulted in the decrease of degradation rate constants The hydrogen gas generation of iron in the anaerobic aqueous solutions was also determined It depended on solution pH, the presence of electron receiving competitor such as TCE, the amount of iron corrosion promoter such as NaCl In the solutions containing chloride ions, TCE, the generation rate of hydrogen gas could be characterized as Riron corrosion > Riron+TCE+Cl > Riron+TCE i CONTENTS Abstract i CONTENTS ii LIST OF FIGURES iv LIST OF TABLE INTRODUCTION LITERATURE REVIEW 2.1 Dechlorination reaction by Zero valent iron (ZVI) 2.1.1 Mechanism 2.1.2 Kinetic 2.2 Effects of chloride ions on dechlorination reaction 10 2.2.1 Effect of chloride ions on iron corrosion 10 2.2.2 Effect of chloride ions on degradation of halogenated compounds 13 2.3 Hydrogen gas generation from dechlorination reactions 14 2.3.1 Hydrogen gas generation from iron corrosion and dechlorination reactions 14 2.3.2 The possibility of the combination between TCE dechlorination and biotic denitrification by ZVI 15 MATERIALS AND METHODS .18 3.1 Iron Powders 18 3.2 Experimental apparatus 18 3.2.1 Estimation of hydrogen gas generation 18 3.2.2 The effect of chloride ions on TCE degradation by ZVI 21 ii RESULTS AND DISCUSSIONS .25 4.1 Estimation of hydrogen gas generation 25 4.1.1 Hydrogen gas generation from iron corrosion 25 4.1.2 Hydrogen gas generation in the presence of TCE 26 4.1.3 Hydrogen gas generation in the presence of TCE and chloride ions .28 4.1.4 Conclusion about the hydrogen gas generation 29 4.2 Effects of chloride ions on TCE degradation by ZVI .31 4.2.1 The effect of pH on TCE degradation 31 4.2.2 The effects of chloride concentrations on TCE degradation 38 4.2.3 Conclusion about the effect of chloride on TCE degradation .40 CONCLUSIONS .42 REFERENCES 44 iii LIST OF FIGURES Figure Reaction pathways for TCE reduction by Fe0 proposed by Arnold and Roberts (2000) Figure Proposed hydrogenolysis of TCE in anoxic Fe0-H2O system (Matheson and Tratnyek, 1994) Figure Concept of Fe(0)-supported denitrification (Till et al., 1998) 16 Figure Experiments for the investigation of hydrogen gas generation .20 Figure Experimental procedure for determining effect of pH on TCE degradation by ZVI in the presence of chloride ions 22 Figure Experimental procedure for determining effect of pH on TCE degradation by ZVI in the presence of chloride ions 23 Figure Hydrogen gas generation from iron corrosion 25 Figure Hydrogen gas generation from iron corrosion in the presence of TCE 27 Figure Hydrogen gas generation in the presence of TCE and chloride ions 28 Figure 10 Hydrogen gas generation at pH 4.8 .30 Figure 11 Hydrogen gas generation at pH 6.4 .31 Figure 12 TCE degradation by ZVI at pH 4.84 in the presence of chloride ions 32 Figure 13 TCE degradation by ZVI at pH 6.41 in the presence of chloride ions 32 Figure 14 TCE degradation by ZVI at pH 7.14 in the presence of chloride ions 33 Figure 15 (a) Surface area normalized degradation rate constant of TCE at different pH; (b) The increase of TCE degradation rate in the solution containing Fe and 20 mM NaCl 34 iv Figure 16 Hydrogen gas generation from iron corrosion at pH 4.8 .36 Figure 17 Hydrogen gas generation from iron corrosion at pH 6.4 .37 Figure 18 Observed rate constant of degradation reactions at different NaCl concentrations 39 v LIST OF TABLES Table Properties of iron samples 18 Table The surface area normalized reaction rate constant at different chloride concentrations 40 Relative concentration of TCE (C/C0) 1.2 1.0 0.8 0.6 0.4 Control samples Without NaCl NaCl 20mM 0.2 0.0 20 40 60 80 100 120 140 160 180 Time (h) Figure 14 TCE degradation by ZVI at pH 7.14 in the presence of chloride ions (Solid lines and dash lines are the regression curves of the samples, vertical bars indicate standard deviation) 33 5e-5 Iron only Iron and NaCl kSA (h-1m-2L) 4e-5 3e-5 2e-5 1e-5 6.4 4.8 7.1 pH (a) % increase in degradation rate 40 30 20 10 4.8 6.4 7.1 pH (b) Figure 15 (a) Surface area normalized degradation rate constant of TCE at different pH; (b) The increase of TCE degradation rate in the solution containing Fe and 20 mM NaCl 34 Due to the large amount of iron powder (10 g), the pH buffer capacity was not enough to maintain initial pH values during the experiment However, no significant difference was observed in final pH of the samples containing iron together with NaCl and iron only HEPES and phosphate buffer were used to control pH of the solutions in previous studies The use of HEPES requires adding acid (H2SO4, HCl) or base (NaOH) to obtain desired pH, which results in the presence of unwanted ions in the solutions Controlling pH by phosphate buffer needs the addition of EDTA to prevent iron precipitation also changes the chemical properties of the solution Although acetate has inhibitor effects on iron corrosion, small capacity acetate buffer solutions seemed to be the most appropriate way to control pH in some preliminary experiments The rise of pH due to small buffer capacity resulted in creating iron hydroxide on the iron surface and thus decreased the degradation rate Interestingly, the drop in reaction rate was not shown in this experiment Regression analysis indicated that the increase in pH did not have significant impact on the dechlorination reaction in this study although final pH was about 2-3 unit higher than initial value Final pH was far lower than the pH for ferrous precipitation may be the possibly reason (Farrel et al 2000) The column experiment studying about the long-term performance of zero-valent iron for reductive dechlorination of TCE showed that there was no difference in reaction rate constant along the length of the column despite the changes of pH Degradation reaction of TCE followed pseudo first order kinetic with respect to TCE concentration for the samples containing iron with chloride ions and iron only,which were in consistent with many previous studies The surface area normalized reaction rate constants at different pH obtaining from regression analysis are shown in figure 15b 35 Chloride ions were supposed to increase the iron corrosion by de-passivation the oxide films on the iron surface and therefore make the corrosion happen continuously (Kim et al., 2007) From figure 12, 13, 14 and 15a, the addition of chloride ions resulted in the increase of TCE degradation rate This can be explained by the fact that more electrons were generated in the solutions containing iron and chloride due to enhancement of iron corrosion rate than those in the solutions containing iron only Do prove this assumption, another experiment about the corrosion of iron in the solution containing 20 mM NaCl was also conducted The results of this experiment are presented in figure 16 and 17 The presence of 20 mM NaCl improved the hydrogen gas generation rate, which is similar to iron corrosion at both pH 4.8 and 6.4 Because the solution contained only iron and NaCl, it could be conclude that the increase of hydrogen gas generation rate was due to the increase of active area for the release of electrons from the iron surface The more active surface, the more electron can be generated and the more H2 can be generated, also H2 generation rate (mmol.kg-1d-1) 100 80 60 40 20 NaCl = mM NaCl = 20 mM 0 20 40 60 80 100 Time (h) Figure 16 Hydrogen gas generation from iron corrosion at pH 4.8 36 H2 generation rate (mmol.kg-1d-1) 40 30 20 10 NaCl = mM NaCl = 20 mM 0 20 40 60 80 100 Time (h) Figure 17 Hydrogen gas generation from iron corrosion at pH 6.4 In the previous part (4.1.3 and figure 9), the effect of TCE on the generation of hydrogen gas in the solutions containing 20 mM NaCl was investigated The result showed that the hydrogen gas generation was higher in the solution contained chloride ions From the figure 15b, 16 and 17, the presence of chloride in the solution gave more hydrogen gas amount and made more TCE degraded which meant that more electrons were generated From that, it can be concluded that chloride ions (at concentration of 20 mM) can make more active area on the iron surface In addition, the results data also showed that degradation rate of TCE by zero valent iron depended strongly on the solution pH The lower pH, the higher degradation rate could be obtained with the same amount of chloride ions added Many previous researches showed that there is a value of chloride concentration that can make the iron corrosion happen at the maximal rate (Cmax) Aleksanyan et al (2007) showed that Cmax increased with an increase in pH value, which means that the iron corrosion rate (or TCE degradation rate) at high pH in the experiments (pH 6.41 37 and 7.16) would be higher if more amount of NaCl was present The difference in amount of active area on the iron surface layer at different pH may be the reason In low pH solution, there is more active area available on the iron surface layer than that in solution having high pH value due to the precipitation of corrosion products on the iron surface (iron hydroxide/ iron oxide) In addition, their stability increases with the increase of solution pH And thus it may be difficult to breakdown the passive layer at high pH With the same amount of chloride ions in the solutions, it can be assumed that the areas which can be re-active (or de-passive) by chloride ions at higher pH are smaller than those at lower pH due to the stability of the passive layer Consequently, the active area on the iron surface will be higher in solution having lower pH It can also due to formation rate of passive layer on the iron surface is higher in higher solution pH 4.2.2 The effects of chloride concentrations on TCE degradation The amount of chloride ions in the solution has great influence on TCE degradation From the range of to 40 mM NaCl, the more amount of chloride in the solution, the more increase in TCE degradation rate was obtained The degradation rate decreased when the concentration of chloride was higher than 40 mM The observed and surface area normalized degradation rate constants are shown on figure 18 and table The results are in agreement with similar previous studies Kim et al (2007) illustrated that degradation of high explosives such as TNT, RDX and HMX was faster when the concentration of chloride was increase to 50 mM Timothy (1998) investigated the effects of chloride ions on the degradation of CCl4 without pH control In this study, CCl4 removal rate was the highest with the concentration of NaCl about 0.06 mM and became lower when the NaCl concentration was increased However, the pH measurement was not performed 38 As mentioned on the previous part, the presence of chloride ions can increase the corrosion of iron Thus, more active area will be available for releasing electrons from the iron surface And more amount of TCE can receive electrons for reductive dechlorination reaction Consequently, more TCE will be degraded in the in solution containing chloride ions The great improvement of chloride ions on the TCE degradation was observed when it reached the concentration of 40 mM 0.025 kobs (1/h) 0.020 0.015 0.010 0.005 0.000 20 40 60 80 100 120 NaCl concentration (mM) Figure 18 Observed rate constant of degradation reactions at different NaCl concentrations (Vertical bars indicate error values from regression analysis) However, the degradation rate of TCE decreased when the concentration of chloride exceeded 40 mM Podobaev (2005) and Aleksanyan et al (2007) proposed that the enter chloride ion into the metal-water adsorption complex (Fe-H2OadsClads) improved iron dissolution and the adsorption of chloride ion on the metal surface forming Fe-Cl-ads complex, otherwise, would decrease the iron dissolution rate The increase of chloride ions in the solution could result in raising the 39 probability of intermediate interaction of chloride ions with the metal, thus decrease the iron corrosion rate and TCE dechlorination rate consequently Furthermore, the dissolution of organic compounds into aqueous solution depends strongly on the salt concentration The presence of predominant inorganic ionic species generally decreases the aqueous solubility of non-polar or weak polar organic compound The solubility of phenantherene in seawater at 250C and 0.44M salt decreased 30% as compared to pure water (Schwarzenbach et al., 2003) In the experiment, at small NaCl concentration, its effect on the dissolution of TCE was negligible However, when the concentration of chloride ions were more than 40 mM, the effect on TCE dissolution became significant and affect the degradation of TCE Table The surface area normalized reaction rate constant at different chloride concentrations Chloride concentration (mM) Normalized reaction rate constant kSA (h-1m-2L) 4.61E-05 4.66E-05 10 5.37E-05 20 5.37E-05 30 5.45E-05 40 7.80E-05 50 4.78E-05 100 4.91E-05 4.2.3 Conclusion about the effect of chloride on TCE degradation Higher increase in TCE degradation rate by chloride ions were observed at lower pH The degradation rate was faster with higher concentrations of chloride ions at 40 in the range of to 40 mM chloride However, the degradation rate of TCE decreased when the chloride concentration exceeded 40 mM 41 CONCLUSIONS This paper presented the studies about the hydrogen gas generation of iron in aqueous solutions containing TCE and chloride ions The investigation about the effect of chloride ions on the degradation of TCE by zero valent iron was also performed From the results, the following conclusions can be drawn:  pH is the most important parameters affects the degradation rate of TCE by ZVI in solutions containing other chemicals It also plays an important role in the generation of hydrogen gas due to the corrosion of iron in anaerobic aqueous solutions  Chloride ion has significant effects on the corrosion of iron as well as the degradation of TCE by ZVI For details, the improvement of TCE degradation strongly depends on its concentration The optimal chloride ions for TCE degradation is 40 mM which equivalents to 2340 mg/L At this concentration, the TCE degradation rate was the highest (kSA = 7.80E-05 h-1m-2L)  The hydrogen gas generation data from this experiment is an important source in designing groundwater treatment facilities employ denitrification and dechlorination process using ZVI for the removal of TCE and nitrate in contaminated groundwater  In practical application, the adding of chloride ions into the groundwater for the enhancement of TCE degradation by ZVI at the optimal concentration (40 mM or 2340 mg/L) is not a wise decision The standard of chloride ions level for drinking water is 250 mg/L according to US EPA (1996) The adding of chloride ions can enhance the degradation of TCE However, the treated groundwater will have to undergo desalination process prior used It can increase the total cost for using groundwater Furthermore, introduce high 42 concentration of chloride in the groundwater can make significant changes for the environmental ecology related directly to that aquifer As a result the adding of chloride ions into the aquifer or into the 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NaCl was investigated The result showed that the hydrogen gas generation was higher in the solution contained