User:Debra Tabron/sandbox

Emplacement of activated carbon (AC) into the subsurface is an emerging technology for in situ remediation. The technology has two components: activated carbon, which has high adsorption capacity for contaminants, and secondary chemical or biological amendments to stimulate in situ contaminant transformation. AC-based technology has been primarily applied at petroleum and chlorinated solvent contaminated sites where complex hydrogeology results in persistent plumes.

Related Article(s):

CONTRIBUTOR(S): Dr. Dimin Fan

Key Resource(s):

• Remedial Technology Fact Sheet - Activated Carbon-Based Technology for In Situ Remediation.
• CLU-IN Technology Focus Area: Activated Carbon-Based Technology for In Situ Remediation

Introduction

AC-based technology involves in situ emplacements of AC-based amendments in the subsurface to form an adsorptive/reactive zone for contaminant remediation. As contaminated groundwater flows through this zone, the contaminants are retarded due to adsorption on to activated carbon. Sorption increases the residence time of contaminants within the reactive zone and therefore also increases contact with the reactive amendments, which has the potential to promote contaminant degradation.

Fundamental Processes

Adsorption

Common organic contaminants are sorbed by AC mainly through van der Waals forces. Adsorption is reversible under typical subsurface conditions, so contaminants may be released if water chemistry changes or competing solutes are present. The large adsorption capacity of AC is due to its highly porous internal structure and is influenced by many factors, including solution chemistry and presence of co-contaminants or dissolved organic matter (DOM) that compete for sorption sites. Chemical and biological processes can also alter the AC, potentially reducing adsorption capacity. Regeneration of AC with persulfate can increase the polar functional groups that contain oxygen on the surface, reducing adsorption capacity. Organic compounds released by microorganisms can compete for sorption sites, similar to DOM. These factors are more likely to impact in situ treatment performance than that of above-ground engineered reactors due to the lack of process controls. For example, pre-treatment to remove metals and periodic backwash are both commonly used in ex situ treatment to improve the performance of activated carbon reactors, but are not practical for in situ applications.

Effects of Activated Carbon on Degradation

A variety of commercially available AC-based technologies have been developed in which AC is combined with zero valent iron, chemical oxidants, or organic amendments to stimulate contaminant degradation.

AC has also been shown to increase the longevity of abiotic dechlorination mediated by ZVI when ZVI is impregnated within the AC pore matrix. This has been attributed to AC protecting ZVI from other side reactions that can consume Fe(0), such as reduction of water. The enhanced longevity of abiotic dechlorination has also been observed in one pilot test in which Carbo-Iron® ® was injected into a contaminated aquifer.

AC addition has the potential to both enhance and inhibit biotransformation reactions. In ex situ water treatment applications, the large surface area of AC can promote microbial attachment and biofilm formation, enhancing some biodegradation processes). Recently, AC-supported biofilms were shown to enhance the extent of dechlorination of polychlorinated biphenyls (PCBs) in sediment, and of RDX in granular activated carbon (Bio-GAC) reactors. Adsorption to AC also has the potential to decrease contaminant availability for degradation because (i) micropores, the major sorption sites, are located deep in the AC pore matrix, which are physically separated from reactive amendments, and (ii) most degradation reactions require contaminants in the dissolved phase. If degradation processes cause aqueous phase concentration to decline, contaminants can desorb from the AC allowing further degradation.

AC addition could potentially enhance some biotransformation processes by facilitating direct interspecies electron transfer (DIET). AC addition to anaerobic digesters has been shown to increase methane production. Electrons produced during fermentation processes or hydrogen oxidation can be transferred through the AC to methanogens co-located on the activated carbon). However, the impact of AC addition on DIET in the subsurface has not been studied.

AC can be produced from a variety of different materials with different activation methods, pretreatments, and impregnations. These differences result in differing physical and chemical properties which can impact degradation processes. In recent laboratory studies, complete biological reductive dechlorination was inhibited in the presence of two different kinds of AC. These results are in contrast with the laboratory and field data reported for a commercial AC product, which have shown positive effects of AC on biological reductive dechlorination. The mechanisms that cause such a contrast are currently unknown, highlighting the need for further research.

State of Practice

The first AC-based product for in situ groundwater remediation was developed in 2004. Since then, five more AC-based products have been developed (Table 1). Interests from practitioners and regulators have grown with increasing availability of commercial products. Together, these products have been applied at sites across North America and Europe. The largest proportion of applications have been at small retail gas station sites, focusing on treatment of petroleum constituents, primarily benzene, toluene, ethylbenzene and xylenes (BTEX). However, the technology has been increasingly applied at large federal sites, such as Superfund sites, where chlorinated solvents are the major contaminants of concern.

Table 1. Properties of seven AC-based products that have been used for in situ applications

Product	Property	Target Contaminants	Degradation Pathway
BOS-100®	Granular AC impregnated with ZVI	Chlorinated solvents	Abiotic reductive dechlorination
BOS-200®	Powder AC mixed with nutrients, electron acceptors, and facultative bacteria	Petroleum hydrocarbons	Aerobic and anaerobic biodegradation
CAT-100®	BOS-100® plus reductive dechlorination bacterial strains	Chlorinated solvents	Abiotic and biotic reductive dechlorination
COGAC®	Powder AC mixed with calcium peroxide and sodium persulfate	Chlorinated solvents or petroleum hydrocarbons	Chemical oxidation, aerobic and anaerobic biodegradation
PlumeStop®	Colloidal AC suspension with a proprietary organic stabilizer, co-applied with hydrogen or oxygen release compounds, and/or corresponding bacterial strains	Chlorinated solvents or petroleum hydrocarbons	Biotic reductive dechlorination of chlorinated solvents or aerobic biodegradation of petroleum hydrocarbons
Carbo-Iron®	Colloidal AC impregnated with ZVI	Chlorinated solvents	Abiotic reductive dechlorination
EHC-Plus®	35% (wt) microscale ZVI, 50% controlled-release organic carbon, 15% powder AC	Chlorinated solvents	Abiotic and biotic reductive dechlorination

Treatment Scenario

AC-based technology has been primarily applied for management of persistent plumes resulting from the slow release of contaminants from low permeability zones (see Dispersion and Diffusion). The combination of in situ adsorption and degradation is thought to be more cost-effective than pump & treat (P&T) and sustain longer treatment effectiveness than degradation alone. Several field applications have demonstrated long-term effectiveness of AC-based technology. However, questions remain about whether adsorption or degradation is the primary process responsible for contaminant removal.

This technology has also been used to limit contaminant migration from groundwater to surface water, where high groundwater velocity limited the effectiveness of soluble amendments.

Implementation

Implementation of in situ AC-based technologies follows similar design principles and engineering approaches as any other injection-based remediation technology. Subsurface hydrogeology and contaminant distribution, both vertically and horizontally, must be well understood and delineated. Colloidal AC products can be injected under low pressure and transported through high permeability zones to reduce mass flux. GAC and PAC-based products must be injected under pressure and are not expected to transport substantial distances. The design loading rate of GAC and PAC-based products is typically determined by total mass of a contaminant in both high and low permeability zones. The design loading rate of colloidal AC-based products is based on the dissolved contaminant mass flux.

Performance monitoring for in situ AC-based remedial technology requires multiple lines of evidence to confirm that contaminants are removed not only by adsorption but also by degradation. Reduction in concentration of parent compounds alone cannot differentiate degradation from adsorption. If reductive dechlorination is thought to be an important degradation process, treatment performance should be evaluated by monitoring for complete dechlorination products (e.g., ethene/ethane) and molecular indicators (e.g., Dehalococcoides population or presence of vinyl chloride reductase (vcrA)). For petroleum hydrocarbons, consumption of electron acceptors (e.g., nitrate or sulfate) or production of volatile fatty acids (VFAs) may serve as general indicators for biological degradation. Molecular diagnostic tools may provide more definitive evidence by targeting specific functional genes necessary for biodegradation.