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 Engineer

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

The Effects of Chloride Ions on the Hydrogen Gas Generation and TCE Degradation in Aqueous Solutions Containing Zero Valent

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

CONTENTS

Abstract i

CONTENTS ii

LIST OF FIGURES iv

LIST OF TABLE

1 INTRODUCTION 1

2 LITERATURE REVIEW 5

2.1 Dechlorination reaction by Zero valent iron (ZVI) 5

2.1.1 Mechanism 5

2.1.2 Kinetic 9

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

3 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

4 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

5 CONCLUSIONS 42

REFERENCES 44

Figure 3 Concept of Fe(0)-supported denitrification (Till et al., 1998) 16

Figure 4 Experiments for the investigation of hydrogen gas generation 20 Figure 5 Experimental procedure for determining effect of pH on TCE degradation

by ZVI in the presence of chloride ions 22

Figure 6 Experimental procedure for determining effect of pH on TCE degradation

by ZVI in the presence of chloride ions 23

Figure 7 Hydrogen gas generation from iron corrosion 25 Figure 8 Hydrogen gas generation from iron corrosion in the presence of TCE 27 Figure 9 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

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

LIST OF TABLES

Table 1 Properties of iron samples 18 Table 2 The surface area normalized reaction rate constant at different chloride

concentrations 40

1 INTRODUCTION

The use of halogenated compounds in industry has led the groundwater to be polluted These compounds such as Trichloroethylene (TCE), Perchloroethylene (PCE) are generally toxic to human health and can cause some serious diseases (ATSDR, 2006) The study on degradation of halogenated organic compounds by zero valent metals has been carried out extensively recently Among zero valent metals, iron is the most popular metal used for the dechlorination because of its availability, low cost, and low toxicity to the environment Zero valent iron (ZVI) reactive barriers have been shown to be a cost-effective alternative to conventional pump-and-treat technologies for in situ remediation of groundwater contaminated with chlorinated hydrocarbons

Dechlorination of TCE by ZVI is a reductive reaction in which TCE receives electrons released from the iron surface to produce compounds with lesser chlorine

such as ethane, ethylene, cis and trans Dichloroethylene (DCE), etc (Arnold and

Robert, 2000)

The degradation of TCE by ZVI is a surface based reaction with three major steps: (a) adsorption of TCE to the iron surface, (b) reaction at the surface and (c) desorption of the products The rate of the degradation depends mainly on the active surface area of iron, solution pH, the mixing rate, temperature, the competitions of other electron acceptors (water, reaction products, etc), the presence of corrosion promoters or inhibitors, etc

pH is the most important factor that affects the degradation of TCE The study of

Chen et al (2001) showed that the increase of pH resulted in lower surface

normalized reaction rate constant of TCE degradation and iron corrosion In addition, pH also involves the precipitation of iron hydroxides on the iron surface due to the reduction of water Long term observation showed that the higher pH, the

smaller active areas on the iron surface are available for releasing and transferring electrons to TCE The degradation process will decrease, consequently (Orth and Gillham, 1996)

Chloride salts are well-known as corrosion promoters Their main effect on iron corrosion is the production of pits containing acidic microenvironments on the iron surface, which can stimulate metal dissolution However, the acceleration in iron corrosion by chloride ions occurs just in a certain range of concentrations Chloride ions decreased the corrosion of iron in the study carried out by Reardon (1995), which showed that the corrosion rate of iron appeared lower for the saline groundwater compared to de-ionized water They also decreased with the

increasing NaCl concentration According to Podobaev (2005) and Aleksanyan et

al (2007), chloride ions can promote or inhibit iron dissolution depending on their

concentrations and the pH value of solution

Previous studies reported that the degradation of halogenated compounds can be improved in the presence of chloride The complete degradation of TNT by ZVI in the presence of 5mM NaCl was in 90 minutes When ZVI was used alone, only

40 % of TNT was degraded after 120 minutes of reaction (Kim et al., 2007) On the other hand, Johnson et al (1998) investigated the effects of chloride ions on the

degradation of CCl4 without pH control In this study, CCl4 removal rate was maximum with the concentration of NaCl about 0.06 mM and became lower when the NaCl conentration was increased

Hydrogen gas is a product of iron corrosion in anaerobic aqeous solution Water receives electrons from iron and relases H2, together with OH- Hydrogen gas evolution is important in developing permeable reactive barrier (PRB) remediation technology If the rate of hydrogen gas production exceeds the rate of its removal in the dissolved state, a two phase flow could develop The consequence could be the

restriction or short-circuiting of the ground water through the PRB On the positive side, the produced hydrogen gas is a potential fuel source for various microorganisms, whose biochemical activity can potentially carry out other organic degradation reactions at contaminated sites (Reardon, 2005)

De-nitrification in groundwater by autotrophic bacteria using hydrogen gas from anaerobic corrosion of iron as electron donors was carried out by some researchers

(Till et al., 1998; Biswas and Bose, 2005) The results demonstrated that 0.5g of

steel wood and hydrogenotrophic denitrifying microorganisms were sufficient to reduce nitrate concentrations from 40mg/L (as NO3

N) to less than 2mg/L (as NO3

N) with a retention time of 13 days (Biswas and Bose, 2005)

TCE and water are competitors for receiving electrons from iron As a result, the hydrogen gas generation rate in the solution containing TCE may be lower than that

in the case of iron corrosion only However, it is possible to combine dechlorination and denitrification processes in one treatment process using ZVI For the occurrence of biotic denitrification using hydrogen gas as an electron donor, hydrogen gas should be supplied adequately to the microorganisms Thus, the rate

of hydrogen gas generation is a significant parameter in this reaction

Researches showed that elevated concentration of chloride ions (Cl-) in the surface

and ground water were common in the United States and other countries (Panno et al., 2006) Approximately 14% of 244 Texas counties had median chloride

concentration exceeding 250 mg/L Several counties had very high median of chloride levels, above 500 mg/L Factors contributing to high chloride levels can include mineral constituents of the aquifers, seepage of saline water from nearby formations, coastal salt water intrusion, irrigation return flow, and oil/gas

TCE degradation by zero valent iron is important in designing the remediation facility for groundwater treatment at the site containing high chloride level However, this topic has not received sufficient coverage in the literature to date

Based on the aforementioned facts, it is important to evaluate the effects of chloride ions on TCE degradation by ZVI as well as the generation of hydrogen gas from that reaction These issues are the main objectives for this research The following experiments were performed to assist the study:

 Estimating the generation rate of hydrogen gas from

+ Iron corrosion reaction,

+ TCE degradation reaction by ZVI,

+ TCE degradation reaction by ZVI in the presence of chloride ions

 Investigating the effects of chloride ions on TCE degradation by ZVI

+ The effect of pH on TCE degradation in the presence of chloride ions + The effect of chloride concentrations on TCE degradation

The study is based on the hypothesis that chloride ions can enhance the degradation

of TCE by ZVI due to the increase in iron corrosion

2 LITERATURE REVIEW

This chapter gives brief definitions, mechanisms and kinetics of dechlorination reaction In addition, the effects of chloride ions on the corrosion of iron and chlorinated compounds are introduced as well This chapter also describes the generation of hydrogen gas from dechlorination reaction and the possibility for the combination of TCE degradation and denitrification in one process using zero valent iron

2.1 Dechlorination reaction by Zero valent iron (ZVI)

The study about dechlorination reaction of TCE by zero valent iron was performed extensively during last decade It is confirmed that the use of zero valent is effective in in-situ remediation of groundwater contaminated with chlorinated hydrocarbons, nitrate or chromate

2.1.1 Mechanism

Reduction of halogenated organic compounds by ZVI is an an-biotic, heterogeneous, and electrochemical reaction in which iron is oxidized and halogenated compounds are reduced

The corrosion of iron in the aqueous phase releases electrons to the solution, which

is shown in the following half reactions (1 and 2)

(1)

(2)

In the absence of oxygen, chlorinated aliphatic hydrocarbons and water are in competition to receive the electrons released from the iron surface

(3) TCE + (xH+) + ye- → product + (y-x)Cl- E0=0.37-0.63 V

(4)

From the value of E0 in above equations, it can be concluded that dechlorination reactions are thermodynamically favorable The produced hydroxide ions (OH-) from the reaction (3) result in increasing pH in weakly buffered system The precipitates iron hydroxide can be on the metal surface and prevent the dissolution

of iron (Matheson and Tratnyek, 1994)

Recent studies showed that -reductive elimination (in which two halide ions are released), hydrogenolysis (replacement of a halogen by a hydrogen atom) and hydrogenation (the addition of hydrogen to unsaturated compounds) are the main mechanisms for dechlorination reaction (Matheson and Tratnyek, 1994; Arnold and Roberts, 2000) as shown in the flowing reactions, respectively

RHCl = RHCl + 2e- + H+  RH2=RHCl + Cl-

(5) RHCl = RHCl + 2e-  + 2Cl-

(6)

Arnold and Roberts (2000) indicated that reductive -elimination accounts for a 97% TCE degradation A 100% degradation of highly reactive chloro- and dichloro- acetylene mediates produced from reductive elimination of TCE occurs via hydrogenolysis

Observed products of the dechlorination reactions are ethylene, acetylene, cis-, trans- DCE Trace amounts of longer chain hydrocarbons (C3, C5, C6) were also detected (Arnold and Roberts, 2000)

Cl H

Cl Cl

Cl H

H Cl

Cl H

Cl H

H H

Cl Cl

H H

Cl H

H H

H H

H

H H

H H

C

TCE

ethane

Figure 1 Reaction pathways for TCE reduction by Fe0 proposed by Arnold and Roberts (2000)

(c)

H2 + RX → RH + H+ + X

-Figure 2 Proposed hydrogenolysis of TCE in anoxic Fe0-H2O system (Matheson and Tratnyek, 1994) (a) Direct electron transfer from iron metal at the iron surface (b)

Reduction of Fe2+, which results from iron corrosion (c) Catalyzed hydrogenolysis by the

H2 from anaerobic corrosion

2.1.2 Kinetic

The dechlorination reaction by Fe0 is a surface based reaction in which close contact of reactive media and contaminants is required for the electron transfer to occur Therefore, the rate of dechlorination is mainly depended upon the rate of mass transfer of contaminant between the bulk solution and the Fe0 surface via stagnant fluid layer surrounding Fe0 and the rate of electron transfer from Fe0 to the contaminants (Lai and Lo, 2007)

The reaction kinetics was studied extensively Although there are many degradation pathways and various processes occurring at the iron surface, such as adsorption of the TCE, chemical reaction at the surface, desorption of the products, most of the studies showed that the reaction was generally a second order reaction with respect

to the concentration of substrate (TCE) and concentration of specific environmental

reactant (iron) as described by the equation below (Tratnyek et al., 1997)

In this equation, kSA is the specific rate constant (L hr-1 m-2), a is the concentration

of iron surface area (m2 L-1 of solution) and P is the concentration of TCE By using surface area normalized rate constants, it is possible to make generalizations about degradation rates by iron metals that apply over a wide range of laboratory

conditions (Tratnyek et al., 1997)

When the amount of iron in the solution is abundant, we can assume that a does not decrease during the reaction In this case, the reaction can be called a pseudo-first order reaction with respect to TCE concentration

(8) With kobs = kSA a, which is called a pseudo-first order rate constant

The kinetic model represented by equations 7 and 8 is formulated to reflect only a single pathway of contaminant transformations However, it now appears that the degradation of hydrocarbons by iron metal can be due to a variety of degradation pathways The various possibilities can be accommodated by expanding kSA into the sum of rate constants for each individual degradation pathway Thus, we can write,

kSA = ket + kre + kother,

Where ket represents hydrogenolysis, kre represents reductive elimination and kother

is included to accommodate other possibilities whose importance remains to be

demonstrated (Tratnyek et al., 1997)

2.2 Effects of chloride ions on dechlorination reaction

2.2.1 Effect of chloride ions on iron corrosion

Corrosion is an electrochemical process involving the transfer of electrons between

a metal surface and an electrolyte solution It results from the reaction of metals with oxygen, water and other substances in an aqueous environmental or humid air This process creates an electro-chemical cell in which metal serves as the anode (electron donor) and oxygen as the cathode (electron acceptor) with the solution of ions serving as a salt bridge Dissolution of the metal occurs at anodic sites, releasing ions which may then react to form insoluble corrosion products (rust)

The thin film of hydroxides that develops on ZVI evolves into a complex layer structure as a result of oxidation, re-crystallization, and precipitation

Chlorides penetrate pores or defects in the oxide film more readily than other ions and can disperse the oxidation film, increasing permeability The halide ion also adsorbs on the film in competition with dissolve oxygen or OH- This process favors hydration of the metal ions and increases the ease with which the ions enter solution, in contrast to adsorbed oxygen, which decrease the rate of metal dissolution

From an electrochemical perspective, the absorbed chloride increases the exchange current (decreases overvoltage) for anodic dissolution from that of surfaces covered with oxygen Thus the iron is not readily passive in solution containing appreciate

Cl- and the metal continues to dissolve at high rate

The corrosion rate increases with the salt concentration, reaching a maximum at approximately 3% NaCl (average salt content of sea water), then decreases, falling below that of distilled water when the saturation is reached (26 % NaCl) The decrease at very high salt concentration occurs because the solubility of oxygen in water decreases with increasing salt concentration

Breakdown of passivity with chloride occurs locally, with sites determined by variations in the oxide film structure and thickness Pitting corrosion is a form of localized corrosion that does not spread laterally across an exposed surface but penetrates into the metal

Pit initiation occurs when an electrochemical or chemical reaction exposes a local site on a metal surface to aggressive halide ions Precipitates of metal ions usually cover the pit area, causing a buildup of protons (and thus positive charge) with the pit The electrical charge imbalanced between the high positive charge within the pit and the negative charge outside causes negative halide ions to diffuse into the pit

to maintain charge neutrality The halide ions migrate to anodic sites and metal ions hydrolyze, producing HCl which inhibits the formation of a passive oxide/hydroxide film Instead of reacting only with hydroxyl ions, the iron reacts with halide ions Iron is very soluble in the chloride form Anions that form solute complexes tend to extend the zone of corrosion while those that form insoluble compounds tend to extend the zone of passivity The HCl reacts with the metal to produce ferrous chloride and hydrogen gas, or ferric chloride and water Hydrolysis

of the metal chloride forms Fe(OH)2 (which is not passive) and H+ causing pH to fall about 3 and facilitating further anodic dissolution The gradual increase in metal ion concentration stimulates further migration of chloride and the process becomes autocatalytic While anodes and cathodes must be relatively near each other in distilled water, they can be much further apart in salt solution because of greater conductivity Passivity is decreased because the oxide will tend to form away from the metal surface

Generally, chloride ions can increase or decrease the corrosion rate of iron denpeding on their concentrations, and the solution pH According to Podobaev

(2005) and Aleksanyan et al (2007), chloride ions can promote or inhibit iron

dissolution depending on their concentrations and the pH value of solution

In a study about the measurement of hydrogen gas generation in anaerobic corrosion, Reardon (1995) found that corrosion rates decrease with increasing NaCl concentration from 0.02 to 3.0 M

2.2.2 Effect of chloride ions on degradation of halogenated compounds

Previous studies presented that the degradation of halogenated compounds can be improved in the presence of chloride ions

Hernandez et al (2004) reported that addition of chloride accelerated TNT

degradation during treatment of contaminated water with ZVI

The presence of 1% w/v chloride salts remarkably increased the degradation rate of

high explosives in aqueous solution such as TNT, RDX and HMX (Kim et al.,

2007) The complete degradation of TNT in the company of 5mM NaCl was in 90 min and only 40 % of TNT was degraded after 120min with zero valent alone Furthermore, the degradation rate was faster with 50 mM of NaCl The study also indicated that presence of Cl- maintained a highly reactive iron surface and sustained a high rate of destruction for a much longer period than ZVI alone

On the other hand, NaCl was seemed to decrease the corrosion rate of iron in the study carried out by Reardon (1995) which showed that the corrosion rate of iron appeared lower for the saline groundwater compared to de-ionized water And the

corrosion rates decreased with the increasing NaCl concentration Johnson et al

(1998) investigated the effects of chloride ions on the degradation of CCl4 without

pH control In this study, CCl4 removal rate was maximum with the concentration

of NaCl about 0.06 mM and became lower when the NaCl conentration was increased However, the pH measurement was not performed

2.3 Hydrogen gas generation from dechlorination reactions

2.3.1 Hydrogen gas generation from iron corrosion and dechlorination reactions

Even in the absence of oxygen, iron still corrodes by the oxidative action of water itself as shown in half reaction 1 and 3

Fe(s) + 2H2O(l) → Fe2+ + 2OH- + H2(g)

(9)

In this reaction, iron is a reductant (electron donor) and water is an oxidant (electron acceptor) And one mol of hydrogen gas generated for every mole of iron corroded As a result, hydrogen gas generation is directly related to iron corrosion Hydrogen gas evolution is considerable important in developing PRB remediation technology If the rate of hydrogen gas production exceeds the rate of its removal in the dissolved state, a two phase flow could develop The consequence could be the restriction or short-circuiting of the ground water through the PRB On the plus side, the produced hydrogen gas is a potential fuel source for various microorganisms, whose biochemical activity can potentially carry out other organic degradation reactions at contaminated sites Bacterial utilization of hydrogen, however, can lead

to bio-film formation in the pore network This can lead to decrease in porosity and permeability within the PRB (Reardon, 2005)

There are two kinds of approach when consider hydrogen gas generation by ZVI They are from electro-chemical standing point and environmental standing point

 Electro-chemical standing point: estimates the hydrogen gas evolution by measuring some electro-chemical parameters such as Tafel analysis, electrochemical impedance spectroscopy (Wang and Farrell, 2003)

 Environmental standing point: due to the complex understanding and measurement equipment of electrochemical, the hydrogen gas generation in the reactor can be measured by the change of pressure inside the reactor due to hydrogen gas built up

Two of these methods both have advantages and disadvantages It’s will be better when combine two methods together Due to the lack of equipment as well as deeply understanding about other fields, environmental standing point is preferred However, the researchers from electro-chemical standing point are good references for discussing and explaining the results from the studies

2.3.2 The possibility of the combination between TCE dechlorination and biotic denitrification by ZVI

Reduction of nitrate by ZVI is a highly exergonic reaction and has long been known

to occur Previous studies demonstrated that nitrate can be completely reduced by metallic iron under anoxic and aerobic conditions, with the major product being ammonia

4Fe(0) + NO3- + 7H2O → 4Fe2+ + NH4 + 10OH- (10)

However, ammonia has an adverse aesthetic impact on drinking water and may interfere with subsequent disinfection process

De-nitrification in groundwater by autotrophic bacteria using hydrogen gas from anaerobic corrosion of iron as electron donors was carried out by some researchers

(Till et al., 1998; Biswas and Bose, 2005) The results demonstrated that 0.5g of

steel wood and hydrogenotrophic denitrifying microorganisms were sufficient to

reduce nitrate concentrations from 40mg/L (as NO3-N) to less than 2mg/L (as NO3

-N) with a retention time of 13 days (Biswas and Bose, 2005)

Figure 3 Concept of Fe(0)-supported denitrification (Till et al., 1998)

From the above information, it is possible to combine dechlorination and denitrification in one process using ZVI ZVI will undergo anaerobic corrosion to supply electron for dechlorination reaction as well as produce hydrogen gas by reaction with water for denitrification process The important thing for hydrogen gas generation as well as TCE dechlorination to be happened is the presence of electron released from the iron surface

After electrons are released to the bulk solution, they will combine with TCE for dechlorination or water molecules for hydrogen gas generation And the proportion

of electron going to TCE or H2O molecules is not clear Furthermore, according to Matheson and Tratnyek (1994), the formation of bubbles of hydrogen gas on the ZVI surface may affect the TCE degradation First, it could hinder the transport and diffusion of TCE from the solution to ZVI surface and vice versa for the TCE degradation products → decrease the degradation rate of TCE Secondly, hydrogen gas could serve as an electron donor for TCE reduction → resulting in higher TCE degradation rate

For the occurrence of biotic denitrification using hydrogen gas as an electron donor, hydrogen gas should be supply adequately to the microorganisms Thus, the rate of hydrogen gas generation is also a significant parameter in this reaction

3 MATERIALS AND METHODS

The chapter gives description about the material (iron) used in the experiment Name of the experiments, their meanings and procedure are also presented in this chapter

3.1 Iron Powders

The irons used in this study were purchase from Samchun Chemical (Seoul, Korea) The BET surface analysis of the iron samples were performed by an ASAP 2010 (Micromeritics Inc USA)

Table 1 Properties of iron samples

Iron sample Particle size (mesh) Specific surface are

(m2/g)

3.2 Experimental apparatus

3.2.1 Estimation of hydrogen gas generation

The hydrogen gas generation rates in anaerobic condition were investigated in the experiment It includes three smaller parts

 Hydrogen gas generation from iron corrosion: this experiment was done to evaluate the hydrogen gas evolution rate in anaerobic aqueous solution at different pH This data is useful in designing permeable reactive barrier for groundwater treatment