Groundwater Remediation: Risk Management and Contaminant Removal with GRAFTA™

The challenge of groundwater remediation

Groundwater remediation remains one of the most challenging undertakings, particularly where the contaminant plume is found to have migrated extensively in both horizontal and vertical dimensions. In such cases, conventional in-situ remediation techniques may not be effective and can be of significant cost.

Given the advancement of technological capabilities in recent years, risk management and passive remediation approaches are preferred over conventional active “clean-up” of contaminated groundwater.

The concept and use of Permeable Reactive Barriers (PRBs) has gained considerable recognition as an effective and relatively low-cost method to ensure reduced risk of contaminant migration and exposure control.

PRBs can be installed in different forms and configurations including in-situ soil mixing, excavated trenches and injection of the reactive materials into deeper sections of aquifers.

A wide variety of materials are used in PRBs to tackle a broad spectrum of contaminants including heavy metals, organic compounds and inorganics. The most common materials used in PRBs are either carbon-based (i.e. different forms of activated carbon) which may be augmented with reducing agents such as Zero Valent Iron (ZVI) to target organic compounds such as chlorinated solvents or bacterial culture to promote bacterial degradation of organic compounds.

PRB Treatment Processes

The reactive material typically removes or manages contaminants through:

Destructive Processes that enhance chemical transformation or biological degradation of the contaminant, or
Non-Destructive Processes that retard contaminant migration by sorption or immobilisation.

Destructive processes transform the contaminants permanently to less harmful substances (in most cases) by the manipulation of pH, reduction-oxidation (redox) potential and biodegradation.

Non-destructive processes immobilize the contaminant to the reactive material(s), and include sorption, precipitation, volatilisation, and changing the contaminant's chemical state (for example, valence).

The main characteristics of PRB reactive materials include:

Contaminant compatibility and reactivity of the material;
Stability and durability of the reactive material;
Availability and cost;
Hydraulic characteristics of the material;
Flexibility of field application in the form of trench, soil mixing or injection;
Environmental compatibility.

Very importantly, the remedial material should not create byproducts or cause undesirable chemical reactions when interacting with the contaminant constituents. This could lead to loss of reactivity or impede the permeability of the reactive material, thus impacting PRB performance.

Application of GRAFTA™ in PRBs

Key attributes of GRAFTA™ that are critical to PRB performance:

Contaminant compatibility and reactivity of the material
Stability and durability of the reactive material
Hydraulic characteristics of the material
Flexibility of field application in form of trench, soil mixing and injection
Environmental compatibility
Availability and cost

Contaminant compatibility and reactivity of the material

GRAFTA™ is currently supplied in two forms:

GRAFTA™1.0 (G1) ; and

GRAFTA™2.0 (G2).

G1 is a micro-scale inert particle housing hundreds of thousands of graphene and graphene oxide layers. The particle distribution ranges from about 150 µm to over 450 µm. G1 has shown outstanding performance in adsorptive removal of a wide range of contaminants from water including but not limited to a variety of organic compounds, heavy metals and inorganics such as ammonia. Graphene layers in G1 adsorb organic compounds whereas metals are adsorbed on graphene oxide layers.

G2, having the same structure of G1, is amended with a reducing agent that chemically reduces the hard to adsorb contaminants (such as selenium in the form of selenate) to ensure a high rate of adsorption by the graphene and graphene oxide layers of GRAFTA™.

GRAFTA™ products are well suited to adsorb contaminants in PRB applications passively having shown considerable adsorption capacity for a variety of contaminants, with a strong bond.

Minimal leaching of contaminants is a key attribute that makes GRAFTA™ an adsorbent of choice to ensure Risk Management goals of a given impacted site are persistently met over the design life of the PRB. The longer adsorption cycles and increased length of time to refresh the GRAFTA materials should reduce the operating costs of the PRB over the lifetime of the project.

The following is a list of contaminants in various groups that have proven to be effectively adsorbed/removed from groundwater within a PRB treatment zone (performance rating; Moderate: <60%, Good: 60% to 80% and Outstanding: over 95%):

Hydrocarbons

Heavy Metals

Inorganics

Stability and durability of the reactive material

GRAFTA™ has been demonstrated to be physically stable under typical conditions encountered in common groundwater remediation scenarios.

Chemical composition of water, including pH and ionic strength, does not affect the physical integrity and stability of GRAFTA™ products.

Once in place, GRAFTA™ can provide a considerable long-term performance as per the design lifetime of the PRB, which is estimated based on modeling the performance of GRAFTA™ using the parameters obtained in bench-scale tests (i.e. adsorption or removal capacity) for any given contaminate water. Typical design lifetimes of PRBs can range from a few years to decades. In cases where reinjection or replacement of reactive material is not practically possible, materials with higher longevity, like GRAFTA™ will be preferred in an optimally sized PRB.

GRAFTA™ has shown significant longevity in adsorption (G1) and chemical reduction-adsorption (G2). This attribute combined with the physical stability of GRAFTA™ minimizes the rate of material washout, promoting longer-term performance compared to highly reactive (e.g. nano-scale ZVI), low density carbon based slurries and other materials. The relatively higher density of GRAFTA™ compared to other carbon based materials, promotes the physical durability of the material within a PRB treatment zone.

Hydraulic characteristics of the material

The initial and long-term permeability of the reactive material is a key factor to be considered during the design process to facilitate groundwater flow through the PRB under typically low natural gradients. In general, the permeability of the media should be higher than the native formation to reduce the risk of bypass around the PRB. The particle size and reactivity of the material and site geochemistry factor into the long-term hydraulic performance of the system.

The following factors should be considered when selecting the PRB material:

• Particle size ― GRAFTA™ particle size ranges from 150 µm to about 450 µm. Each GRAFTA™ particle houses hundreds of thousands of nano-scale single or multiple layers of graphene and graphene oxide that provide a substantial surface area promoting reactivity (i.e. adsorption) while maintaining hydraulic performance minimizing susceptibility to long-term clogging. Relatively smaller grain size media (e.g. nano-scale ZVI, which has a nominal diameter that is less than 10 microns) typically have higher specific surface areas; as a result, they are more reactive than larger particles but may be more susceptible to long-term clogging issues or may become inert sooner. An optimum range of particle size providing the desired reactivity should be considered.
• Permeability ― GRAFTA™ range of particle size and the large surface area of the graphemic structure, makes it an optimal choice in terms of hydraulic performance and reactivity. Relatively larger grain size media (for example, granular iron, which has a nominal diameter that is more than 300 microns) typically have higher permeabilities and may be less susceptible to long-term clogging, but may be less reactive than their smaller counterparts.

Flexibility of field applications in form of trench, soil mixing and injection

There are a number of construction methods ranging from standard excavation with a wheeled or track-mounted excavator to pneumatic fracturing and injection. The construction methods have depth, and media limitations that must be considered during the PRB design.

While excavated trench methods are limited to relatively shallow depths, fracturing (not favored for a variety of reasons), direct push injection and injection wells can provide a treatment zone of over 40m deep which in many cases is critical to achieve (e.g. plumes with initial DNAPL impact).

GRAFTA™, having a wide range of particle sizes, can be flexibly used in various methods corresponding to the site geology and groundwater conditions. Injection trials of GRAFTA™ have shown that it can be injected in most cases with a more viscous carrier (e.g. guar gum) and that injections can be achieved at sub-fracturing pressures.

Environmental compatibility

The use of selected PRB material should not generate harmful byproducts or result in chemical or biological changes in the subsurface environment. GRAFTA™ provides an adsorptive platform with possible chemical reduction where necessary. Adsorption, as the main function of GRAFTA™ is a non-destructive reaction with no byproducts and no alteration of natural subsurface environment.

Availability and cost

GRAFTA™ is manufactured in Canada, distributed, and sold both domestically and internationally. Typically, the product is purchased on a per tonne basis and deployed by the end-user in its operations.

The cost of GRAFTA™ when deployed in a passive system is competitive to other materials and systems and potentially lower in cost over the life of PRB due to the higher longevity of the product and the multiple contaminants adsorbed.