Anaerobic Corrosion Of Iron Biology Essay

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Anaerobic corrosion of iron generates H2, hydroxide ions and ferrous iron. The H2 generated serves as an electron donor in many important natural processes such as sulfate reduction and methanogenesis. Hydrogen is an excellent energy source for many anaerobes, including methanogens, homoacetogens, sulfate reducers, and dehalorespirers (Wu et al, 1998). Dehalorespiration is an environmentally significant process which utilizes molecular H2 as the primary electron donor. Under low energy electron accepting conditions, minimal energy may be available for microbes thus favoring a reductive attack where the hydrogen serves as an excellent electron donor for highly electrophilic pollutants like PCBs (Rhee et al, 1999). Production of cathodic hydrogen from the anaerobic corrosion of iron is of great advantage to halorespiring bacteria, because H2 is one of the most favorable electron donors for dehalorespiration (Loffler et al., 1999). Elemental iron in the subsurface sediments rapidly depletes dissolved oxygen and generates anaerobic corrosion products such as ferrous and ferric species and hydrogen (Braun et al, 1997).

Figure 6.1.1 Pathway for reductive dehalogenation in Fe0-H2O systems through the generation of H2 (Matheson and Tratnyek, 1994).

Low quantities of iron have been proven to be effective achieving biodegradation of PCBs in sediments containing syntrophic communities of various microbial consortia. Sokol et al., (1994) showed H2 to be the primary electron donor for the dechlorination of 2,3,4-trichlorobiphenyl in Hudson river sediment microcosms. Therefore for the design of bioremediation systems using iron as an indirect source of electrons, it is important to study the parameters affecting the corrosion of iron in natural waters which contain various anionic species and the prevalence of different pH conditions. Corrosion of iron in anaerobic systems has been shown to be both abiotic and biotic. In the absence of oxygen, iron corrodes according to the reaction

Fe0(s) + 2H2O(l) Fe2+ + 2OH- + H2(g)

Iron oxidizes to ferrous ion and produces H2 with a redox potential of 0.44 V making the reaction thermodynamically feasible (Rysavy et al, 2005). The generation of Fe2+ and OH- is significant because these species participate in some very important cycles in nature. The generation of OH- ­­ results in an increase in the pH of the system which in turn leads to the formation of CO32- ions from HCO3-. The CO32- precipitates as FeCO3 and carbonates of other cationic species such as Ca2+ and Na+. This may result in a decrease in the alkalinity of the system. Apart from the formation of FeCO3, Fe (OH)2 may also precipitate. The aqueous matrix under which anaerobic corrosion occurs also affects the rate of corrosion of elemental iron. Whitman et al., (1924) and Reardon, (1995) showed that pH does not directly increase iron corrosion rates unless it is below pH 4.0. On of the major factors affecting the corrosion of iron is the fouling of the surface by the precipitation of salts in natural systems. This reduces the surface area for the cathodic-depolarization reaction to occur thereby inhibiting corrosion. Wieckowski et al., (1983) showed that the anaerobic corrosion of iron is accelerated by the presence of HCO3- and H2CO3 as the oxidants. This is consistent with the observations made by Reardon, (1995) who reported that the rate of corrosion of granular iron showed the following ion effect under strictly anaerobic conditions: HCO3- > SO42- > Cl-. In natural systems the formation of FeCO3 and Fe(OH)2 is thermodynamically more favorable and this results in the passivation of the surface of iron particles. Conversely, at pH below 6.0, precipitation of Fe2+ ions does not occur, as a result of which the iron particles continue to corrode. Since the formation of FeCO­3 coats the surface of iron and inhibits corrosion, it is an important factor in determining the effectiveness of iron for bioremediation. Davenport et al., (2000), showed that in simple Fe0-H2O systems thin films of oxyhydroxides coat the bare surface of iron which gradually develops into complex, crystalline and multilayered structures. Agrawal et al., (2002) reported a decreasing rate of dechlorination of 1,1,1-Trichloroethane and the surface coverage of FeCO3 on the surface or iron. Biotic corrosion of elemental iron in anaerobic systems has been shown to be accelerated by sulfate-reducing bacteria. Sulfate reducing bacteria have been thoroughly studied for their role in the corrosion of iron by either cathodic depolarization or by the formation of sulfides.

Figure 6.1.2 Mechanism of iron corrosion by sulfate-reducing bacteria in anaerobic systems (Potekhina et al., 1999).

Tiller and Booth, (1962) studied the effect of FeS on the corrosion of iron in a sulfate free environment in the presence of Desulfovibrio desulfuricans. The authors reported the rate of iron corrosion to be proportional to the amount of FeS added and hypothesized the formation of an Fe0-FeS electrochemical cell. Conversely, McNeil and Little, (1990) attributed the corrosion of iron to the utilization of bacterial hydrogenases. They proposed that the effectiveness of the electrochemical couple is dependent on the rate of consumption of molecular hydrogen formed at the surface of iron. Under strictly anaerobic conditions, bacterial hydrogenases are primarily responsible for the corrosion of iron by the consumption of hydrogen, but in the presence of other substrates, complex parameters govern the corrosion of elemental iron (Videla, 1996). Boopathy and Daniels, (1991) showed that methanogens are capable of accelerating cathodic depolarization-mediated oxidation of elemental iron under anaerobic conditions. Daniels et al., (1987) showed that high dosages of iron in an anaerobic system inhibit further cathodic hydrogen production and therefore inhibit methanogenesis. Dissimilatory Fe (III) reduction is known to compete successfully with methanogenesis and sulfate-reduction for hydrogen (Lovley and Phillipis, 1988). Fe (III) reducing bacteria like Geobacteraceae and Shewanella reduce Fe (III) to Fe (II) ions thereby removing corrosion deposits on the surface of iron particles. Therefore, apart from natural abiotic processes, biotic processes also play a significant role in accelerating or inhibiting corrosion of elemental iron under anaerobic conditions.

6.1.1 Research Objectives

An indirect mechanism to enhance microbially mediated reductive dechlorination of PCBs is the addition of elemental iron. The corrosion of iron in water under anerobic conditions generates H2 which depends on the properties of the medium in which corrosion occurs. Different sediment sites have different physical and chemical properties owing to the geographical location and other topographical and geological features. The research objectives thus focus on studying the effect of natural water samples from Lake Hartwell, New Bedford Harbor and Roxana Marsh on the anaerobic corrosion of iron and evaluate the effectiveness of iron recharge in sustaining hydrogen generation via continuous corrosion of iron. Iron acts as an indirect electron donor for the enhancement of PCB dechlorination. Apart from the generation of H2, the byproducts of corrosion play a significant role in other natural processes in a mixed microbial consortium. The concentration of H2 and the rate of cathodic hydrogen production in the microcosms are vital for the enrichment of a dechlorinating population and the subsequent dechlorination of PCBs. This chapter discusses the corrosion of iron in natural site waters and an evaluation of the factors that dictate the corrosion of iron in anaerobic systems over a period of 12 months

6.2 MATERIALS AND METHODS

The samples used in this study were natural water samples from the water column overlying the sediment at Lake Hartwell, New Bedford Harbor and Roxana Marsh. The water samples were homogenized prior to experimental setup. The chemical properties of the water samples before setup are given in Table 6.2.1. Fisher iron powder (99% purity and 325 mesh) was used in this study.

Table 6.2.1 Properties of water samples from different sediment sites.

Sample Site

pH

Total Dissolved Solids (TDS)

(mg/l)

Conductivity (μS)

Alkalinity

(Mg/l CaCO3)

Chloride (mg/l)

Sulfate(mg/l)

Lake Hartwell

6.85

146.80

302.56

91.26

3.44

1.79

New Bedford Harbor

7.17

NA

58300

97.00

21259

2238

Roxana Marsh

7.43

154

333

104.33

15.05

21.89

6.2.1 Iron corrosion experiments

The protocol consists of two experiments.

(a) Iron

The anaerobic corrosion of iron generates H2 and raises the pH of the system. The rate of corrosion is affected by both biotic and abiotic processes in natural systems. The natural water samples are mixed with a given quantity of iron. One level of iron, 0.3g:100ml of water sample (corresponding to 0.1g/g wet weight of sediment or 0.03g/g dry weight of sediment used to study PCB degradation) was added to the system and the effect of chemical parameters characteristic of each water sample on the anaerobic corrosion of iron was studied.

(b) Periodic amendment of iron (iron recharge)

Observations by Reardon, (1995) and Srirangam, 2007 made on the anaerobic corrosion of iron show a progressive decrease in the generation of H2 by the corrosion of iron in natural samples after 6 months. Therefore periodic amendments of fixed amounts iron to the microcosm will aid in hydrogen generation by continuous corrosion of iron. This will prolong the growth phase of the microorganisms by providing a continuous supply of hydrogen as the electron donor. One level of iron, 0.3g:100ml of water sample (corresponding to 0.1g/g wet weight of sediment or 0.03g/g dry weight of sediment used to study PCB degradation) were added to two systems, with amendments at every 3 month and 6 month period respectively. The interaction between periodic addition of iron and the chemical properties of the water samples was studied.

6.2.2 Experimental Procedure

The iron corrosion experiments were carried out in serum bottles (microcosms). Volume of natural water corresponding to the sediment water ratio (1:4) used in the PCB degradation study was used for corrosion study as well. A known weight of iron powder was added to the water samples and the remaining head space was filled with nitrogen gas to make sure that conditions are oxygen free. The intermediate loading of iron 0.03g/g dry weight of sediment (based on the PCB biodegradation study) was used for the water samples as well. Experiments were carried out in triplicates and for a span of twelve months. The serum bottles were crimped with Teflon faced butyl rubber septa inside aluminum seals and was allowed to tumble for the corresponding time period. Backup for the various experiments were prepared to account for any damage during handling.

In order to achieve representative conditions, steps were taken to ensure strict anaerobic conditions. Natural water obtained from the sediment site was purged for 15 hours with nitrogen before the day of experimental setup. After adding the iron powder to the bottle, the corresponding site water was kept ready. The bottles were taken near a nitrogen gas cylinder connected to a thin needle for outlet. As soon as the natural water was added, the bottles were immediately purged with nitrogen gas for about 10 seconds to drive out the oxygen that is present inside the bottles and were immediately sealed with air tight Teflon faced Butyl rubber septa. After this procedure was completed for all serum bottles, they bottles were transferred inside the controlled atmosphere chamber for purging.

6.2.3 Controlled Atmosphere Chamber

The controlled atmosphere chamber was used to ensure strict anaerobic conditions for each microcosm loaded with the site water. In order to drive out the oxygen that would have crept inside the serum bottles, a sequence of purging and vacuuming was done. The samples were segregated according to the three experiments designed (Fe, Fe recharge). The cycle was started by purging the chamber with the headspace gas (N2) after which the Teflon faced butyl rubber septa were removed. At least ten cycles of alternate vacuuming and purging of anaerobic gas mixture (10% H2, 5% CO2, 85% N2) was performed to enhance the reaction and to consume the residual oxygen present inside the system. During the cycle, the system temperature was maintained at 370C by a thermostat placed inside. During the course of the reaction, fogging of the chamber was a direct indication of oxygen still present in the system, so the cycles are further extended until no fogging was observed. After ensuring that the anaerobic reaction had taken place, the thermostat was switched off and five cycles of corresponding head space gas were vacuumed and purged. By following this protocol, the headspace of the microcosms was filled up with anaerobic gases thus simulating a real time system where anaerobic pockets like these are created. The bottles were then crimped with Teflon faced butyl rubber septa and sealed with aluminum caps, after which they were taken out from the chamber. The sealed bottles were loaded inside a shaker for several months and were drawn out at regular intervals of time for analysis.

For the microcosms to be periodically amended with iron, the required dosage of iron for each sample was weighed in a small HDPE weigh dish and transferred to the controlled atmosphere chamber along with the sealed samples from the original loading. The cycle was started by purging the chamber with the headspace gas (N2) after which the crimped seals were removed. At least ten cycles of alternate vacuuming and purging of anaerobic mixture (10% H2, 5% CO2, and 85% N2) was performed at a constant temperature of 370C to enhance the reaction and to consume the residual oxygen present inside the system. Following the fogging-defogging cycles, the iron was transferred into the serum bottles and the headspace purged with 5 cycles of N2 gas. The bottles were then crimped with Teflon faced butyl rubber septa and sealed with aluminum caps. The sealed bottles were loaded inside a shaker for several months and were drawn out at regular intervals of time for analysis.

6.2.4 Routine Analysis

The samples that were taken out for analysis every month were opened and transferred to

centrifuge tubes. Since our protocol consists of several experiments and all samples were prepared in triplicates, several bottles were taken out per site every month. After centrifugation, the natural water was decanted and stored for measuring the pH, total dissolved solids and the oxidation-reduction potential of the system. To avoid cross contamination, water was stored separately in 20ml vials for auxiliary measurements.

6.2.5 Hydrogen Analysis

Prior to opening the microcosms for water sample withdrawal, volumes (500 µL) for headspace H2 analysis were removed using a gas-tight locking syringe and analyzed using a GC equipped with a thermal conductivity detector (TCD). A molecular sieve column was used for hydrogen analysis. The carrier gas used was N2. External calibration standards for H2 were prepared using standard hydrogen gas concentrations.

6.2.6 Methane Analysis

Prior to opening the microcosms for water sample withdrawal, volumes (500 µL) for headspace methane analysis were removed using a gas-tight locking syringe and analyzed using a GC equipped with a thermal conductivity detector (TCD). A silica gel column was used for hydrogen analysis. The carrier gas used was Helium. Single point calibration standard for methane was prepared using standard methane gas concentrations.

6.2.7 Soluble Iron Analysis

Soluble iron in the corrosion experiments were measured using an atomic absorption spectrometer. A five point external calibration in the concentration range of standards for iron was prepared from a 1000 mg/L standard stock solution. The calibration standards were freshly prepared in a mixture of DI water 2% HNO3 solution.

6.2.8 Sulfate Levels

The concentration of sulfate in the biodegradation experiments was determined using an Ion Chromatograph (IC25, Dionex).

6.2.9 Quality Control

The following precautions were taken to ensure the accuracy and precision of the test results.

All chemical analyses was carried out in triplicates.

All apparatus was washed with soap and rinsed with 95% ethanol and water and then oven dried at 1600C to avoid cross contamination.

The TCD/GC was calibrated frequently using 3-point calibration method with standards covering a wide range of concentration to ensure the quality of the data. The linearity in the calibration graph was routinely checked by injecting check standards.

6.3 RESULTS AND DISCUSSION

6.3.1 Lake Hartwell

Figure 6.3.1 and 6.3.2 show the variation in the pH and hydrogen production with respect to time for the 0.3g iron no recharge, 3 month and 6 month recharge systems in Lake Hartwell site water. A gradual increase in the pH is observed in the system accompanied by a decrease in the rate of increase in pH. The recharge systems show a corresponding increase in the pH based on the amount of iron added to the system.

Figure 6.3.1 Variation of pH in Lake Hartwell water as function of time and iron recharge.

As shown in Figure 6.3.2, the no recharge system exhibits hydrogen production till the 6 month period after which hydrogen production is negligible. Conversely, the recharge systems exhibit hydrogen generation proportional to the amount of iron added to the system. At every 3 month and 6 month period in the recharge systems increase in hydrogen generation is observed corresponding to the amendment of iron. The increase in pH and the continuous generation of hydrogen in the recharge systems show that the periodic amendment of iron helps in sustaining the generation of hydrogen vital for dehalorespiring microbial populations.

Figure 6.3.2 Variation of hydrogen production in Lake Hartwell water as function of time and iron recharge.

Figure 6.3.3 and 6.3.4 show the ORP profile and soluble iron present in the system. The addition of iron to the system reduces the ORP by consuming any residual oxygen in the system with the simultaneous generation of H2. Highly reducing conditions conducive for reductive dechlorination of PCBs and other halogenated organics are achieved within a span of 5 months. The recharge systems exhibit marginally lower ORP than the no recharge system. The similarity in ORP values in all the systems could be attributed to the accelerated rates of corrosion on the addition of more iron and a subsequent precipitation of salts coating the surface of iron. As shown in Figure 6.3.4, the concentration of Fe2+ initially increases in the system till the 2 month period which corresponds to a pH of 7.5. Increase in the pH of the system (>7.5), results in the precipitation of iron as Fe3+ with a sharp decrease in soluble iron concentration which is clearly evident from Figure 6.3.4.

Figure 6.3.3 Variation of ORP in Lake Hartwell water as function of time and iron recharge.

Figure 6.3.4 Variation of soluble iron in Lake Hartwell water as function of time and iron recharge.

The recharge systems exhibit slightly higher soluble iron concentration at each amendment period following which a sharp decrease in soluble iron concentration is observed with a corresponding increase in the pH of the system. This shows that the corrosion of iron is sustained in the iron amendment systems.

Figure 6.3.5 Variation of sulfate concentration in Lake Hartwell water as function of time and iron recharge.

Very low concentrations of sulfate were detected in Lake Hartwell site water. Figure 6.3.5 shows a gradual decrease in the sulfate concentrations in the aqueous phase on the addition of iron with greater sulfate reduction in the recharge systems. The extent to sulfate reduction is proportional to the amount of iron added to the system. The addition of iron increases the pH and reduces the ORP of the system, thereby creating conditions ideal for the abiotic reduction of sulfate to sulfides in the presence of hydrogen. As opposed to the site water samples, no sulfate was detected in the sediment slurry microcosm studies with Lake Hartwell sediment. This can be attributed to the involvement of sulfate reducing bacteria in the sediment samples in addition to the kinetically unfavorable abiotic sulfate reduction process alone in site water samples

Figure 6.3.6 Variation of Total Dissolved Solids in Lake Hartwell water as function of time and iron recharge.

Figure 6.3.7 Variation of alkalinity in Lake Hartwell water as function of time and iron recharge.

Figure 6.3.6 and 6.3.7 show the variation in TDS and alkalinity of the site water as function of time and iron recharge. A gradual decline in the TDS of the system is observed with the recharge systems showing an increased rate of TDS removal. This could be attributed to the precipitation of dissolved anionic and cationic species with an increase in the pH and precipitation of hydroxides and carbonates of iron. The periodic addition of iron results in the generation of more Fe2+ which in turns results in a greater reduction in TDS. The precipitation of carbonates and bicarbonates in the site water is evident from the decrease in alkalinity of the system observed with the addition of iron (Figure 6.3.7).

6.3.2 New Bedford Harbor

Figure 6.3.8 Variation of pH in New Bedford Harbor water as function of time and iron recharge.

Figure 6.3.9 Variation of hydrogen production in New Bedford Harbor water as function of time and iron recharge.

Figures 6.3.8 and 6.3.9 show the variation in the pH and hydrogen production with respect to time for the 0.3g iron no recharge, 3 month and 6 month recharge systems in New Bedford Harbor site water. A steady increase in pH of the system is observed. With the recharge systems show a corresponding increase in the pH based on the amount of iron added to the system. As shown in Figure 6.3.9, the no recharge system exhibits hydrogen production only till the 3 month period after which hydrogen production is negligible. Conversely, the recharge systems exhibit hydrogen generation proportional to the amount of iron added to the system. At every 3 month and 6 month period in the recharge systems increase in hydrogen generation is observed corresponding to the amendment of iron. It is interesting to note that in both the recharge systems in New Bedford Harbor site water hydrogen generation following the addition of iron is the highest for a period of 1 month after which the profile remains flat. This can be attributed to the passivation of the surface of the iron particles therefore gradually inhibiting iron corrosion. Therefore the periodic amendment of iron sustains hydrogen production in the system compared to the passivation and cessation of hydrogen generation in the no recharge system.

Figure 6.3.10 Variation of ORP in New Bedford Harbor water as function of time and iron recharge.

Figures 6.3.10 and 6.3.11 show the ORP profile and soluble iron present in the New Bedford Harbor system. Highly reducing conditions conducive for reductive dechlorination of PCBs and other halogenated organics are achieved within a span of 2 months. No difference was observed in the ORP of the no recharge and recharge systems possibly due to the prevalence of highly reducing conditions even in the control set. Initially, the concentration of Fe2+ increases in the system till the 1 month period (Figure 6.3.11) corresponding to a pH of 7.5. Increase in the pH of the system (>7.5), results in the precipitation of iron as Fe3+ with a sharp decrease in soluble iron concentration which is clearly evident from Figure 6.3.11. The recharge sets exhibit a temporary rise in Fe2+ concentration with each recharge which subsequently results in a decline in Fe2+ again with an accompanying increase in pH.

Figure 6.3.11 Variation of soluble iron in New Bedford Harbor water as function of time and iron recharge.

Figure 6.3.12 Variation of sulfate concentration in New Bedford Harbor water as function of time and iron recharge.

High sulfate concentrations were detected in New Bedford Harbor site water. Significant amount of sulfate reduction was observed as shown in Figure 6.3.12. The no recharge set exhibited gradual sulfate reduction till the 4 month period followed by cessation in sulfate reduction. In the recharge systems sulfate reduction was sustained with highest sulfate reduction in the 3 month recharge set which also contains the highest amount of iron. The cessation in sulfate reduction in the no recharge set could be attributed to the passivation of iron particles by precipitation of iron salts. The rate and extent of sulfate reduction observed in the New Bedford Harbor sediment microcosm study was much greater than that observed in the site water samples. This can again be explained by significant involvement of sulfate reducing bacteria in the reduction of sulfate compared to abiotic reduction of sulfate under highly reducing conditions.

Figure 6.3.13 Variation of alkalinity in New Bedford Harbor water as function of time and iron recharge.

Figure 6.3.13 shows the variation in alkalinity in New Bedford Harbor site water as function of time and iron recharge. A lag is observed in the alkalinity profile preceding a drastic drop in alkalinity at the 4 month period. The drop in alkalinity could be associated with the precipitation of carbonates, bicarbonates and hydroxides of iron and other cations like calcium and magnesium due to the high pH and low ORP conditions prevalent in the system. This could have resulted in the passivation of iron particles in the no recharge system thus resulting in a decline in rate of hydrogen production and cessation of sulfate reduction observed earlier in the New Bedford Harbor site water.

6.3.3 Roxana Marsh

Figure 6.3.14 Variation of pH in Roxana Marsh water as function of time and iron recharge.

Figure 6.3.15 Variation of hydrogen production in Roxana Marsh water as function of time and iron recharge.

Figures 6.3.14 and 6.3.15 show the variation in the pH and hydrogen production with respect to time for the 0.3g iron no recharge, 3 month and 6 month recharge systems in Roxana Marsh site water. A drastic increase in pH of the system is observed between months 1 and 8. The recharge systems show a corresponding increase in the pH based on the amount of iron added to the system. As shown in Figure 6.3.15, the no recharge system exhibits hydrogen production only till the 2 month period after which hydrogen production is negligible. Conversely, the recharge systems continue to exhibit hydrogen generation proportional to the amount of iron added to the system. At every 3 month and 6 month period in the recharge systems a surge in hydrogen generation is observed corresponding to the amendment of iron. There is a ten-fold increase in H2 generation in the 3 month recharge system when compared to the no recharge set. It is interesting to note that both the recharge systems in Roxana Marsh site water exhibit, hydrogen generation for only a 1 month period following the periodic addition of iron after which the concentration profile remains flat. This can be attributed to rapid passivation of the surface of the iron particles therefore gradually inhibiting iron corrosion. Therefore the periodic amendment of iron sustains hydrogen production in the system compared to the passivation and cessation of hydrogen generation in the no recharge system.

Figure 6.3.16 Variation of ORP in Roxana Marsh water as function of time and iron recharge.

Figures 6.3.16 and 6.3.17 show the ORP profile and soluble iron present in the Roxana Marsh system. Highly reducing conditions conducive for reductive dechlorination of PCBs and other halogenated organics are achieved within a span of 4 months. No significant difference was observed in the ORP of the no recharge and recharge systems possibly due to the prevalence of highly reducing conditions even in the control set (-40mV). Initially, the concentration of Fe2+ increases in the system till the 2 month period (Figure 6.3.17) corresponding to a pH of 7.7. Increase in the pH of the system (>7.5), results in the precipitation of iron as Fe3+ with a sharp decrease in soluble iron concentration which is clearly evident from Figure 6.3.17. The recharge sets exhibit a temporary rise in Fe2+ concentration with each recharge which subsequently results in a decline in Fe2+ with to an accompanying increase in pH. Increase in soluble iron concentration is not observed beyond 4 months since the pH of the system is already greater than 8.5 at the 4 month period.

Figure 6.3.17 Variation of soluble iron in Roxana Marsh water as function of time and iron recharge.

Uncharacteristic of most freshwater systems high sulfate concentrations (22mg/l) were detected in Roxana Marsh site water. Slow and gradual sulfate reduction was observed in the no recharge systems throughout the period of study (Figure 6.3.18). No significant change in the rate of sulfate reduction was observed in the recharge systems when compared to the no recharge set. This shows that apart from the amount of iron added to the system and the creation of reducing conditions other factors may also play a role in sulfate reduction. The rate and extent of sulfate reduction observed in the Roxana Marsh sediment microcosm study was much greater than that observed in the site water samples. This can be explained by significant involvement of sulfate reducing bacteria in the reduction of sulfate compared to abiotic reduction of sulfate under highly reducing conditions prevalent in the microcosms.

Figure 6.3.18 Variation of sulfate concentration in Roxana Marsh water as function of time and iron recharge.

Figures 6.3.19 and 6.3.20 show the variation in TDS and alkalinity in Roxana Marsh site water as function of time and iron recharge. A lag is observed in the TDS profile preceding a drastic drop in TDS at the 4 month period followed by a gradual drop in TDS. The alkalinity profile on the other hand exhibits a gradual decline in alkalinity over time with no significant difference between the no recharge and recharge sets. The gradual decline in TDS and alkalinity could be associated with the precipitation of carbonates, bicarbonates and hydroxides of iron and other cations like calcium and magnesium due to the high pH and reducing conditions prevalent in the system. This could have resulted in slow but gradual passivation of iron particles in Roxana Marsh site water since only a gradual increase in pH, hydrogen generation and sulfate reduction is observed for the system being studied.

Figure 6.3.19 Variation of Total Dissolved Solids in Roxana Marsh water as function of time and iron recharge.

Figure 6.3.20 Variation of alkalinity in Roxana Marsh water as function of time and iron recharge.

6.4 CONCLUSIONS

The effect of water parameters on the corrosion of iron and iron recharge under anaerobic conditions was studied for all three sediment site waters. It was determined that a combination of factors like composition of water and pH significantly affects the corrosion of iron in water. In the case of Lake Hartwell site water, a gradual increase in pH and a cessation in H2 generation is observed after 4 months whereas the recharge systems continue generating H2 with each amendment. In the case of New Bedford Harbor site water, a similar trend is observed with the recharge systems sustaining H2 production and a cessation in H2 generation in the no recharge system. The Roxana Marsh site water exhibits higher pH values and greater H2 generation in the recharge sets when compared to Lake Hartwell and New Bedford Harbor. This suggests that iron corrosion occurred at a faster rate in the Roxana Marsh site water than the other two sites. In the case of Lake Hartwell, corrosion of iron is initially enhanced by the presence of HCO3- ions which gradually transform into CO32- ions at higher pH therefore passivating the surface of iron by precipitating as FeCO3. Research has shown the presence of sulfides to be associated with an increase in the corrosion rate of iron. Since relatively higher rates of sulfate reduction was observed in New Bedford and Roxana Marsh site water, the formation of FeS would have enhanced iron corrosion. At the same time, the presence of Cl- has been shown to be inhibitory to iron corrosion. Therefore, Roxana Marsh site water exhibits higher H2 generation via corrosion of iron when compared to New Bedford Harbor site water due to the presence of high concentrations of inhibitory Cl- ions in New Bedford Harbor. The effect of sulfides in enhancing corrosion is also evident in the lag times observed preceding a decrease in the alkalinity in the New Bedford Harbor and Roxana Marsh site waters.

In summary, the periodic amendment of iron to the site waters from three sediments sustained the corrosion of iron in water and resulted in continuous generation of H2, which is the primary electron donor for dehalorespiration. The periodic amendment of iron also resulted in a partial decrease in sulfate concentrations and soluble iron by the formation of insoluble sulfides, which aids in removing toxic HS- from the system.

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