Natural and Enhanced Remediation Systems - Chapter 4 pot

Suthersan, Suthan S. “Chapter 4: In Situ Reactive Zones” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 4 In Situ Reactive Zones CONTENTS 4.1 Introduction 4.2 Engineered Anaerobic Systems 4.2.1 Enhanced Reductive Dechlorination (ERD) Systems 4.2.1.1 Early Evidence 4.2.1.1.1 Biostimulation vs. Bioaugmentation 4.2.1.2 Mechanisms of Reductive Dechlorination 4.2.1.3 Microbiology of Reductive Dechlorination 4.2.1.3.1 Cometabolic Dechlorination 4.2.1.3.2 Dechlorination by Halorespiring Microorganisms 4.2.1.4 Electron Donors 4.2.1.4.1 Production of H 2 by Fermentation 4.2.1.4.2 Competition for H 2 4.2.1.5 Mixture of Compounds on Kinetics 4.2.1.6 Temperature Effects 4.2.1.7 Anaerobic Oxidation 4.2.1.8 Electron Acceptors and Nutrients 4.2.1.9 Field Implementation of IRZ for Enhanced Reductive Dechlorination 4.2.1.10 Lessons Learned 4.2.1.11 Derivation of a Completely Mixed System for Groundwater Solute Transport of Chlorinated Ethenes 4.2.1.12 IRZ Performance Data 4.2.2 In Situ Metals Precipitation 4.2.2.1 Principles of Heavy Metals Precipitation 4.2.2.2 Aquifer Parameters and Transport Mechanisms 4.2.2.3 Contaminant Removal Mechanisms ©2001 CRC Press LLC 4.2.3 In Situ Denitrification 4.2.4 Perchlorate Reduction 4.3 Engineered Aerobic Systems 4.3.1 Direct Aerobic Oxidation 4.3.1.1 Aerobic Cometabolic Oxidation 4.3.1.2 MTBE Degradation 4.4 In Situ Chemical Oxidation Systems 4.4.1 Advantages 4.4.2 Concerns 4.4.3 Oxidation Chemistry 4.4.3.1 Hydrogen Peroxide 4.4.3.2 Potassium Permanganate 4.4.3.3 Ozone 4.4.4 Application 4.4.4.1 Oxidation of 1,4-Dioxane by Ozone 4.4.4.2 Biodegradation Enhanced by Chemical Oxidation Pretreatment 4.5 Nano-Scale Fe (0) Colloid Injection within an IRZ 4.5.1 Production of Nano-Scale Iron Particles 4.5.2 Injection of Nano-Scale Particles in Permeable Sediments 4.5.3 Organic Contaminants Treatable by Fe (0) References Oxidation-reduction process plays a major role in the mobility, transport, and fate of inorganic and organic contaminants in natural waters. Hence, the manip- ulation of REDOX conditions to create an in situ reactive zone (IRZ) to meet the cleanup objectives was a predictable evolution … . 4.1 INTRODUCTION The concept of in situ reactive zones is based on the creation of a subsurface zone, where migrating contaminants are intercepted and permanently immobilized or degraded into harmless end products. Figures 4.1a and b pictorially describe the concept of in situ reactive zones (IRZ). The successful design of these reactive zones requires the ability to engineer two sets of reactions: 1) between the injected reagents and the migrating contaminants; and 2) between the injected reagents and the subsurface environment to manipulate the bio-geo-chemistry to optimize the required reactions, in order to effect remediation. These interactions will be different at each contaminated site and, in fact, may vary within a given site. Thus, the major challenge is to design an engineered system for the systematic control of these reactions under the naturally variable or heterogeneous conditions found in the field. The effectiveness of the reactive zone is determined largely by the relationship between the kinetics of the target reactions and the rate at which the mass flux of contaminants passes through it with the moving groundwater. Creation of a spatially ©2001 CRC Press LLC fixed reactive zone in an aquifer requires not only the proper selection of the reagents, but also the proper mixing of the injected reagents uniformly within the reactive zone. Furthermore, such reagents must cause few side reactions and be relatively nontoxic in both its original and treated forms. When dealing with dissolved inorganic contaminants such as heavy metals, the process sequence in a pump and treat system required to remove the dissolved heavy metals present in the groundwater becomes very complex, operation- and mainte- nance-intensive, and costly. In addition, the disposal of the metallic sludge, in most cases as a hazardous waste, is also very cost prohibitive. Therefore, in situ treatment methods capable of achieving the same mass removal reactions for dissolved con- taminants in an in situ environment are evolving and gradually gaining prominence in the remediation industry. The advantages of an in situ reactive zone to address the remediation of ground- water contamination are as follows: • An in situ technology enables implementation of most ground treatment processes and eliminates the expensive infrastructure required for a pump and treat system with no disposal of water or wastes Figure 4.1a Pictorial depiction of an in situ reactive zone (IRZ) formation. Figure 4.1b Cross sectional view of the creation of an IRZ around an individual injection well at a selected location. Plan View Source Area IRZ Grid Contaminant Plume Individual Reactive Zones Created by Individual Injection Points Providing a Collective In Situ Reactive Zone (IRZ) Curtain Cross Sectional View Contaminant Zone Reagent ©2001 CRC Press LLC • Inexpensive installation because primary capital expenditure for this technology is the installation of injection wells at appropriate locations • Inexpensive operation that allows inexpensive reagents to be injected at fairly low concentrations and, hence, should result in insignificant cost; only sampling required is for groundwater quality monitoring and performance monitoring parameters are usually done in the field; remediation of large volumes of contam- inated water without any pumping or disposal needs • Can be used to remediate deep sites because cluster injection wells or in-well mixing systems can be installed to address deeper sites • Unobtrusive because once the system is installed, site development and operations can continue with minimal obstructions • In situ degradation of contaminants because organic contaminants and a few inorganics such as NH 4 + , NO 3 – , and CIO 4 – can be degraded by implementing the appropriate reactions • Immobilization of contaminants because once the dissolved heavy metals are precipitated out, the capacity of the soils and sediments is utilized to adsorb, filter out, and retain inorganic contaminants Manipulation of the reduction-oxidation (REDOX) potential of an aquifer is a viable approach for in situ remediation of REDOX-sensitive groundwater contami- nants. In addition, various microbially induced or chemically induced reactions also can be achieved in an in situ environment. As noted earlier, creation of spatially fixed reactive zones to achieve these reactions is very cost effective in comparison to treating the entire plume as a reaction zone. Since the first IRZ for the precipitation and remediation of hexavalent chromium (Cr 6+ ), was installed in 1993, this technology has advanced by leaps and bounds. 1 Currently the application of this technology can be classified into three categories based on the creation of specific bio-geo-chemical and REDOX environments: 1) engineered anaerobic systems, 2) engineered aerobic systems, and 3) in situ chemical oxidation. The engineered anaerobic systems can be further divided into enhanced reductive dechlorination (ERD) systems, in situ denitrification, in situ perchlorate transforma- tion, and in situ heavy metals precipitation. The ERD application has been expanded to many contaminants since the first trichloroethene (TCE) application site. The IRZ technology has been successfully applied to remediate the following chlorinated compounds: • Chlorinated ethenes: tetrochloroethane (PCE), trichloroethane (TCE), dichloroet- hene (Cis 1,2 DCE, and 1,1 DCE), vinylchloride • Chlorinated ethanes: 1,1,2,2 tetrachloroethane (1,1,2,2 PCA), 1,1,1 trichloroet- hane, (1,1,1 TCA), 1,1,2 trichloroethane (1,1,2 TCA), 1,1 and 1,2 dichloroethane (DCA), chloroethane (CA) • Chlorinated phenols: pentachlorophenol (PCP), and tetrachlorophenol • Chlorinated pesticides • Perchlorate In addition, the IRZ technology has been successfully applied to precipitate the following dissolved metals at contaminated sites: Cr 6+ , Pb 2+ , Cd 2+ , Ni 2+ , Zn 2+ , Hg 2+ . ©2001 CRC Press LLC 4.2 ENGINEERED ANAEROBIC SYSTEMS 4.2.1Enhanced Reductive Dechlorination (ERD) Systems 4.2.1.1 Early Evidence The first microbially mediated reductive dechlorination of PCE and TCE was observed in the early 1980s, and this study 2,3 reported the degradation of PCE to nonchlorinated end products in an acetate-fed, continuous-flow methanogenic glass bead column. It appeared that the first step in the degradation pathway was dechlori- nation to TCE. Further anaerobic oxidation of TCE to carbon dioxide and hydrochloric acid was suggested. In 1984 4 , further evidence of dechlorination of PCE beyond TCE came in an experiment where sediments from an aquifer recharge basin were incubated with PCE and methanol as the electron donor. Significant concentrations of TCE, cis - 1,2 DCE and VC were observed after three weeks, whereas in sterile controls no dechlorination had occurred. Another study in the 1980s demonstrated that dechlori- nation of PCE to VC in a methanogenic column was achievable. 5 Similar studies using 13 C-TCE, showed that TCE was dechlorinated exclusively to cis -DCE in soil. 6 In 1989, the first evidence of complete dechlorination of PCE to ethene under methanogenic conditions with methanol as electron donor was demonstrated. 7 Another study found PCE reduction via ethene to ethane with lactate as electron donor in a flow-through column filled with a mixture of polluted sediment and anaerobic granular sludge. 8 Meanwhile, numerous publications showed that micro- organisms capable of reductively dechlorinating chlorinated ethenes are abundant in polluted anaerobic environments. (An overview of the biological reductive dechlo- rination pathway of chlorinated solvents is shown in Figure 4.2.) PCE and TCE are dechlorinated mainly to cis -DCE, although sometimes trans -DCE and 1,1-DCE have also been found as products. 9,10 However, the formation of the 1,1-DCE is believed to be a result of abiotic dechlorination in the presence of sulfide. 10 Evidence from the earlier studies indicated that the dechlorination of PCE to cis -DCE was found to be a relatively fast process, whereas, subsequent rates of dechlorination of cis -DCE to VC and ethene were significantly slower or even absent. 7,11 Dechlorination of 1,1-DCE and trans -DCE was less studied. In some of the earlier reports and studies the dechlorination of chlorinated ethenes was often found to be incomplete, both in the laboratory and in field experiments, resulting in an accumulation of cis -DCE and VC. It was not fully understood at that time why dechlorination beyond these compounds was problem- atic, other than raising valid questions regarding the required microbial consortia for complete dechlorination. During that time (late 1980s and early 1990s) micro- organisms capable of dechlorinating DCE and VC had not been isolated yet, although several enrichment cultures existed. Little was known about the substrate require- ments of these bacteria. Later studies reported that PCE dechlorination in a contam- inated soil down to ethene was only achieved by adding a complex mixture of organic electron donors. Significant research was focused, during the early to mid 1990s, on the microbial ecology that could perform complete dechlorination of PCE to ©2001 CRC Press LLC ethene and the biogeochemical conditions under which this biotransformation could be achieved. The choice of a suitable electron donor for the stimulation of in situ dechlori- nation is still a matter of discussion and may be dependent on local conditions; this will be discussed in detail in a later section. When hydrogen is assumed to be the major electron donor for dechlorination, its amendment can only be achieved by using substrates yielding hydrogen after anaerobic degradation. 12 Often, short-chain organic acids are produced as intermediate products, which may lead to acidification of the groundwater and soil. Additionally, electron donors that support dechlorination are generally readily degraded by nondechlorinating microorganisms, leading to competition for the substrate and excessive bacterial growth in soil pores near the injection well. As a result, significantly more electron donor mass will be needed than theoretically necessary to reduce all chlorinated ethenes present to ethene. 4.2.1.1.1 Biostimulation vs. Bioaugmentation The first level of the treatment hierarchy for chlorinated ethenes is intrinsic bioremediation, or natural biodegradation, whereby indigenous microflora destroy the contaminant(s) of concern without any stimulation or enhancements. The second choice in this hierarchy, biostimulation or enhanced biodegradation, involves stim- ulating the indigenous microbial populations and thus enhancing microbial activity Figure 4.2 Biological and abiotic degradation pathways of the common chlorinated com- pounds encountered at contaminated sites (adapted from McCarty and Semprini, 1994; after Vogel et al., 1987, and Wiedermeier et al., 1999). PCE CT TCE CF cis-1,2 DCE * DCM VC CM Ethene Ethane 1,1- DCE Acetic Acid Primary ReactionBiotic Reactions Abiotic Reactions CO 2 , H 2 O, CI - 1,1,1-TCE 1,1- DCA CA * ©2001 CRC Press LLC so that they destroy the target compounds at a rate that meets the cleanup objectives at the site. At almost every contaminated site a natural population of degradative microorganisms exists within the contaminated zone; however, specific nutrients, growth substrates inducers, electron donors, and electron acceptors may be required to create optimal microbial activity. 12 Thus, through the introduction of required additional reagents, the native degradative microbial population can be stimulated to grow, multiply, and destroy the target contaminants. Most environments contain microorganisms able to grow on and destroy a variety of chlorinated compounds; at some sites, the persistence of these compounds, is not a consequence of the absence of organisms but rather of the absence of the full set of conditions necessary for the indigenous species to function rapidly. 12 In the past there was a significant debate among remediation experts whether the microorganisms responsible for cometabolic degradation and dehalorespiration are ubiquitous. Current belief is that these organisms are nearly ubiquitous. When intrinsic bioremediation or biostimulation is not feasible at a given site due to the absence of an appropriate microbial population, bioaugmentation may be utilized. Bioaugmentation involves injection of selected exogenous microorganisms with the desired metabolic capabilities directly into the contaminated zones along with any required nutrients to effect the rapid biodegradation of target compounds. Two distinct bioaugmentation approaches have been developed for remediating chlorinated ethenes. In the first approach, degradative organisms are added to com- plement or replace the native microbial population. The added microorganisms can be selected for their ability to survive for extended periods or to occupy a specific niche within the contaminated environment. If needed, stimulants or selective cosub- strates can be added to improve survival or enhance the activity of the added organism. Thus, the goal of this approach is to achieve prolonged survival and growth of the added organisms and degradation of the target contaminants. In the second bioaugmentation approach, large numbers of degradative bacteria are added to a contaminated environment as biocatalysts which will degrade a significant amount of the target contaminant before becoming inactive or perishing. 12 Additional microbes can be added as needed to complete the remediation process. Attempts can be made to increase the production of the degradative enzymes or to maximize catalytic efficiency or stability, but long-term survival, growth, and estab- lishment of an active microbial population are not the primary goals of this treatment approach. In the past, bioaugmentation has been implemented frequently and successfully only in bioreactors. The conditions in these bioreactors are controlled and quite different from those in nature, and prior to start-up, no microorganisms are present anyway. Hence, the addition of enriched cultures is essential. Furthermore, bioreac- tors are engineered and controlled systems where conditions can be readily altered or optimized for a particular process and can be designed to promote the multipli- cation and activity of the inoculated species — in contrast to contaminated field sites. The record of success of in situ bioaugmentation systems for chlorinated com- pounds has been rather spotty. On the one hand, the initiation or enhancement of degradation has been reported (far more commonly in samples of the contaminated environments in simulated laboratory experiments) following the addition of ©2001 CRC Press LLC enriched bacterial cultures that can metabolize and grow on chlorinated ethenes. On the other hand, a number of failures in the field have been reported. Such reports of failure of bioaugmentation came as no surprise to microbial ecologists. Without question, a species with a substrate uniquely available to it has a distinct advantage, yet that advantage may not be sufficient to compensate for many other traits also necessary for survival, no less multiplication, in a natural ecosystem. Possessing the requisite enzymes to metabolize a novel compound is a necessary attribute for the organism, but it is not sufficient for the organism to succeed. Populations of introduced microorganisms are subject to a variety of abiotic and biotic stresses, and these must be overcome for these organisms to be able to express beneficial traits. The reasons for the frequent failures of bioaugmentation are many: 12 limiting nutrients and growth factors in the uncontrolled natural environment, suppression by predators and parasites, inability of the introduced bacteria to penetrate significant space, metabolism of other nontarget organic compounds present, concentration of the target chlorinated compound too low to support multiplication, and other inhib- itory biogeochemical conditions such as pH, temperature, salinity, and toxins. In summary, the problems usually encountered in scaling up the bioaugmentation successes achieved in laboratory experiments can be summarized as follows: 12 • Contaminant rates established in controlled laboratory studies may differ substan- tially from those in pilot-scale, full-scale, or even other laboratory studies. • Positive biotransformation results from small systems often are not reproduced in different systems. • Instantaneous biotransformation rates vary widely and in an apparently stochastic manner, even in well-operated, steady-state systems. 4.2.1.2 Mechanisms of Reductive Dechlorination Naturally occurring biological processes can degrade organic contaminants in situ or during transport in the subsurface under aerobic and/or anaerobic conditions. Microorganisms catalyze degradation reactions to obtain energy for growth, repro- duction, and cell maintenance. Useable energy is recovered through a series of REDOX reactions where the microorganisms act as “electron transport mediators” (Figure 4.3). Biologically mediated electron transfer couples the oxidation of an electron donor (organic compound) with the reduction of an electron acceptor (inor- ganic or organic) and results in the production of useable energy for microbial consortia. 12,13,14 The bulk electron donor acts as a fuel source for the reactions and the reactions proceed as long as there is a source of bioavailable electrons. Fuel sources can be the target chlorinated compounds, native organic carbon, co-contam- inants such as fuel hydrocarbons, or organic compounds such as carbohydrates. In aerobic environments, the chlorinated compounds act as electron donors and under anaerobic conditions they act as electron acceptors. There are two primary mechanisms involved in the biodegradation of chlorinated organic contaminants (Table 4.1). First, biodegradation may be growth-linked and provide carbon and energy to support growth when the compound is used as primary ©2001 CRC Press LLC substrate and directly utilized by the mediating organisms via the processes included in Category 1. Some chlorinated solvents are used as electron donors and some are used as electron acceptors when serving as primary growth substrates. When used as an electron donor (under aerobic and anaerobic conditions) the contaminant is oxidized. Conversely, when used as an electron acceptor, the contaminant is reduced via the reductive dechlorination process called halorespiration. 17 In addition to their use as a primary growth substrate, chlorinated solvents can also be degraded via cometabolic pathways. During cometabolism, microorganisms gain carbon and energy for growth from metabolism of a primary substrate, and chlorinated solvents are degraded fortuitously by enzymes present in the metabolic Figure 4.3 Description of microorganisms acting as electron transport mediators (after Schwarzenbach et al., 1993). Table 4.1Summary of the Categories of Degradation Pathways for Chlorinated Organic Compounds (Adapted from Wiedemeier et al., 1999) 14 Category 1 Category 2 (used as primary substrate) (used as cometabolic substrate) Chlorinated Compound Halo- respiration Direct Aerobic Oxidation Direct Anaerobic Oxidation Aerobic Cometabolism (co-oxidation) Anaerobic Cometabolism (reductive dechlorination) Tetrachloroethylene (PCE) XX Tr ichloroethylene (TCE) XXX Dichloroethene (DCE) XXX X X Vinyl Chloride (VC) X X X X X Tr ichchloroethane (1,1,1 TCA) XXX Dichloroethane (1,2 DCA) XX X X Carbontetrachloride (CT) XX Methylenechloride (MC) XX X (bulk) ox (bulk) red (mediator) ox (mediator) red + ne - + ne - (Contaminant) ox (Contaminant) red + ne - - ne - [...]... studied most extensively in Dehalobacter restrictus42 ,49 and Dehalospirillum multivorans.26 ,41 ,45 In general, a PCE respiration chain should contain an electron-donating enzyme, electron carriers, and a reductive dehalogenase as terminal reductase Studies with D multivorans and Desulfitobacterium strain PCE-S indicate that a proton gradient or a membrane potential may also be essential for chloroethene... requires careful engineering and a knowledge of the geologic parameters affecting groundwater flow and transport Different configurations, in plan view and cross sections, used for IRZ system designs are shown in Figures 4. 13a and b, and 4. 14a and b Creative engineering considerations have to be taken into account to accommodate the requirements of a smaller plume vs a larger plume and a shallower plume vs... convert H2 and CO2 into CH4 and H2O The process by which hydrogen is produced by one strain of bacteria and utilized by another is called interspecies hydrogen transfer It should be noted that the terminal products of anaerobic decomposition, CH4, and CO2, respectively, are the most reduced and the most oxidized carbon compounds There are a number of compounds besides the ones listed in Table 4. 4 that... only to cis-1,2-dichloroethene (cisDCE) .46 ,47 Dehalospirillum multivorans also dechlorinates PCE to cisDCE using H2, but has a much more widely varied biochemical repertoire: it is additionally able to use various organic substrates such as pyruvate, lactate, ethanol, formate, and glycerol as electron donors.7,37 ,44 Other PCE-dechlorinating organisms have been isolated that do not use H2. 34, 61 It was... reported.63 In this case, reduction of the 3-chlorobenzoate did not proceed until the 3,5-dichlorobenzoate was completely transformed This could not be explained by competing reaction rates, but was successfully described ©2001 CRC Press LLC with a competitive inhibition model Similar behavior for the reduction of 4- amino3,5-dichlorobenzoate to 4- amino-3-chlorobenzoate also was observed It is important... Electron Flow Perchloroethene e- CI CI C H2 C CI CI Trichloroethene CI CI C CI C CI CI CI C C H + H ion H (Predominant Biological Reaction) cis-1,2,-Dichloroethene H - C CI (Limited Biological Reaction) 1,1-Dichloroethene +CI (Limited Biological Reaction) trans-1,2,-Dichloroethene H C C H H CI CI C H Vinyl Chloride CI H C C H H Ethene H h C C H H Ethane H C H H Figure 4. 4 H H C H Hydrogenolysis reactions... Fe (III) - EDTA.72,73 Studies have focused on the possibility of oxidation of VC and DCE when they are used as a primary growth substrate under anaerobic environments.72,73, 74 These results show VC and DCE mineralization under methanogenic and iron reducing conditions in anaerobic streambed sediments without the accumulation of ethene or ethane and buildup of carbon dioxide. 14 Decreases of VC and DCE... electron donor and each electron acceptor present on site (Figure 4. 12) Higher levels of electron acceptor increase the oxidative poise and thus require more electron donor, therefore raising treatment costs A series of generic reactions is given in Table 4. 6 to illustrate some of the possible reactants and products Table 4. 6 Possible Reactants and Products of SpeciÞc Terminal Electron-Accepting Processes... estimated so its cost can be calculated Afterwards, a site-specific, cost-benefit analysis must be undertaken to determine if the site is a good candidate for enhanced reductive dechlorination (ERD) application 4. 2.1.9 Field Implementation of IRZ for Enhanced Reductive Dechlorination The author’s success and significant experience in creating an IRZ for enhanced reductive dechlorination is based purely on... molasses and cheese whey is based purely on economics as illustrated in Figure 4. 12 and Table 4. 4.1 The geologic and hydrogeologic setting in which an IRZ system is installed governs its successful application IRZ systems rely on the delivery of dissolved reagents, such as dilute molasses, throughout a contaminant plume; administering delivery of these amendments through both the vertical and horizontal . Degradation 4. 4 In Situ Chemical Oxidation Systems 4. 4.1 Advantages 4. 4.2 Concerns 4. 4.3 Oxidation Chemistry 4. 4.3.1 Hydrogen Peroxide 4. 4.3.2 Potassium Permanganate 4. 4.3.3 Ozone 4. 4 .4 Application 4. 4 .4. 1. Application 4. 4 .4. 1 Oxidation of 1 , 4- Dioxane by Ozone 4. 4 .4. 2 Biodegradation Enhanced by Chemical Oxidation Pretreatment 4. 5 Nano-Scale Fe (0) Colloid Injection within an IRZ 4. 5.1 Production of Nano-Scale. 4. 2.3 In Situ Denitrification 4. 2 .4 Perchlorate Reduction 4. 3 Engineered Aerobic Systems 4. 3.1 Direct Aerobic Oxidation 4. 3.1.1 Aerobic Cometabolic Oxidation 4. 3.1.2 MTBE Degradation 4. 4