ENGI 9621 Soil Remediation Engineering

Spring 2012Faculty of Engineering & Applied Science

Lecture 7: Soil Flushing

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Also called as “soil washing” if the contaminants are treated on-situ

An innovative treatment technology that floods soils with a solution to move the contaminants out

A developing technology that has had limited useAccomplished by passing the flushing solution through

in-place soils using an injection or infiltration processExtraction fluids must be recovered from the underlying

aquifer and, when possible, they are recycled

7.1 Introduction(1) Definition of soil flushing

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The flushing solution typically one of two types of fluids: 1) water only; or 2) water plus additives such as acids (low pH), bases (high pH) or surfactants (like detergents)

If injecting a solvent mixture into either vadose zone, saturated zone, or both to extract organic contaminants cosolvent flushing

The cosolvent mixture normally injected upgradientof the contaminated area, and the solvent with dissolved contaminants is extracted downgradient and treated above ground

(2) Cosolvent flushing

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(3) Operation processesDrilling of injection /extraction wells into the contaminated

site number, location, and depth of the wells depend on geological factors and engineering considerations

Transportation or built up the site equipments (such as a wastewater treatment system)

Pumping the flushing solution into the injection wells the solution passes through the soil, picking up contaminants along its way as it moves toward the extraction wells the extraction wells collect the elutriate (the flushing solution mixed with the contaminants)

The elutriate is pumped out of the ground then treated by a wastewater treatment system to remove the contaminants

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Source: EPA, 1996Typical in-situ soil flushing in vadose zone

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Recovered flushing fluids and groundwater with the desorbed contaminants need treatment to meet appropriate standards before reuse in the flushing process or discharge

Separation of surfactants from recovered flushing fluid for reuse a major factor in the cost of soil flushingTreatment of the recovered fluids results in process sludges

and residual solids, such as spent carbon and spent ion exchangeresin must be appropriately treated before disposal

Air emissions of volatile contaminants from recovered flushing fluids should be collected and treated to meet applicable regulatory standards

(4) Recovered fluid treatment

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The target contaminant group for soil flushing inorganics including radioactive contaminants

It can be used to treat VOCs, SVOCs, fuels, and pesticides, but it may be less cost-effective than alternative

The addition of environmentally compatible surfactants may be used to increase the effective solubility of some organic compounds

The technology offers the potential for recovery of metals and can mobilize a wide range of organic and inorganic contaminants from coarse-grained soils

7.2 Applicability

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Contaminants Considered for Treatment by In Situ Soil Flushing

Source: EPA, 19968

Since in situ soil flushing is tailored to treat specific Contaminants it is not highly effective with soils contaminated with a mixture of hazardous substances, for example, metals and oils It would be difficult to prepare a flushing solution that would effectively remove several different types of contaminants at the same time

7.3 Limitations

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Low permeability or heterogeneous soils difficult to treat

Surfactants can adhere to soil and reduce effective soil porosity

Reactions of flushing fluids with soil can reduce contaminant mobility

Permits are required for wastewater and air treatment systems

Aboveground separation and treatment costs for recovered fluids can drive the economics of the process

More limitations…

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Approximate costs: $50 to $200 per tonKey cost drivers (1) Soil Permeability soils with lower permeability are

more recalcitrant to soil flushing thus remediation time can be significantly increased which increases costs

(2) Depth to Groundwater soils with a deeper water table causing a higher cost to complete

7.4 Economic consideration

Scenarios A B C DSite sizes Small LargeSite conditions Easy Difficult Easy DifficultCost per cubic yard $32 $49 $18 $27

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Spring 2012 Faculty of Engineering & Applied Science

Lecture 8: Soil Fracturing

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ENGI 9621 Soil Remediation Engineering

Fracturing creating fractures in dense soils and making existing fractures larger to enhance the mass transfer of contaminants

The fractures increase the effective permeability and change paths of fluid flow, thus making in situ remediation more effective and economical

Fracturing also reduces the number of extraction wells required, trimming labor and material costs

Two types of fracturing Pneumatic fracturing + Hydraulic fracturing

8.1 Introduction(1) Fracturing

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injects highly pressurized air or other gas into consolidated, contaminated sediments to extend existing fractures and to create a secondary network of fissures and channels

accelerates the removal of contaminants by soil vaporextraction, bioventing, and enhanced in situ biodegradation

(2) Pneumatic fracturing

(3) Hydraulic fracturing involves injecting a fluid, usually water, at modest rates

and high pressures into the soil matrix to be fractureda slurry mixture of sand and biodegradable gel is then

pumped at high pressure to create a distinct fracture as the gel degrades, it leaves a highly permeable sand-lined fracture with the sand acting as a propping agent preventing the fracture from collapsing 14

Source: Sharma and Reddy, 2004

Two types of soil fracturing

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Fracturing is most appropriately applied to soils where the natural permeability is insufficient to allow adequate movement of fluids to achieve the remediation objectives in the desired time frame.

8.2 Applicability

• silty clay/clayey silt• sandy silt/silty sand• clayey sand• sandstone• siltstone• limestone• shale

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Fracturing techniques are equally applicable to both vadose zone (unsaturated) soils and saturated zone soils to improve the flow of air and water, respectively fracture formation in the range of from 20 to 35 ft or more is possible for near-surface soils

Fracturing, by itself, is not a remediation technique has to be combined with other technologies to facilitate the reduction of contaminant mass and concentration e.g.

• in situ biodegradation (by enhancing the delivery of oxygen and nutrients into inaccessible locations)• in situ air sparging (by creating fractured pathways to collect the injected air laden with contaminants)

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The selection between hydraulic (water-based) and pneumatic (air-based) fracturing are based on the following considerations:

8.3 Description of the process

• soil structure and stress fields• the need to deliver solid compounds into the fractures• target depth• desired areal influence• contractor availability• acceptability of fluid injection by regulatory agencies

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Hydraulic Soil Fracturing Pneumatic Soil FracturingEffective in soils and rock Primarily effective in rock

Long term permeability enhancementShort term permeability enhancement in unconsolidated sediments

Specialized equipment and fluid chemistry expertise required

Less equipment and expertise required

Low leak-off prevents spreading of subsurface contaminants

Injected air can potentially spread soil vapour phase contaminants

Fracture clogging by fines is minimized because frac sand is designed to act as a geotechnical filter while maintaining enhanced permeability

Fractures are unsupported; migration of fines quickly clogs fractures

Greater range of adaptability with remediation technologies (e.g. SVE, Bioremediation)

Not readily adaptable to many remediation technologies

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injecting a fluid into a borehole at a constant rate until the pressure exceeds a critical value and a fracture is nucleated

The most widely used fracture fluid for environmentalapplication the continuous mix grade of guar gum

The injection pressure required to create hydraulic fractures is remarkably modest (less than 100 psi)

(1) Hydraulic fracturing

Injection pressure as a function of time during hydraulic fracturing

Source: Suthersan, 1997 20

Method for creating hydraulic fractures in soilSource: Suthersan, 1997

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Hydraulic FracturesSource: Slack , 1998 22

advancing a borehole to the desired depth of exploration and withdrawing the auger

positioning the injector at the desired fracture elevationsealing off a discrete 1 or 2 ft interval by inflating the flexible

packers on the injector with nitrogen gasapplying pressurized air for approximately 30 srepositioning the injector to the next elevation and repeating

the procedure a typical fracture cycle approximately 15 mina production rate with one rig 15 to 20 fractures per day

(2) Pneumatic fracturing

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Schematic of pneumatic fracturing processInjection rates of up to 1000 scfm sufficient to create

satisfactory fracture networks in low permeability formations

Source: Suthersan, 1997

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8.4 LimitationsThe technology should not be used in areas of high

seismic activityFractures will close in non-clayey soils Investigation of possible underground utilities, structures,

or trapped free product is requiredThe potential exists to open new pathways for the

unwanted spread of contaminants (e.g., dense nonaqueousphase liquids)

Pneumatic fracturing $8 to $12 per tonHydaulic fracturing 160 to $180 per ton for remediation in a

1-year treatment and $100 to $120 per ton in a 3-year remediation

8.5 Cost

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Spring 2012 Faculty of Engineering & Applied Science

Lecture 9: Phytoremediation

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ENGI 9621 Soil Remediation Engineering

Use of plants remediate contaminated soil or groundwater

Most of the activity in phytoremediation takes place in the rhizosphere – in other words, the root zone

Can be used for the remediation of inorganic contaminants as well as organic contaminants

Most suited for sites with moderately hydrophobic contaminants e.g.benzene, toluene, ethylbenzene, xylenes, chlorinated solvents, PAHs, excess nutrients such as nitrate, ammonium, and phosphate, and heavy metals

9.1 Introduction

(1) Definition of Phytoremediation

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low capital costs the operational cost of phytoremediation is substantially less and involves mainly fertilization and watering for maintaining plant growth

aesthetic benefits minimization of leaching of contaminants and soil

stabilization

(2) Advantages

(3) Limitationscontaminants present below rooting depth will not be extractedplant may not be able to grow in the soil at every contaminated

site due to toxicityremediation process can take years for contaminant

concentrations to reach regulatory levels requires a long-term commitment to maintain the system

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Plants remove organic contaminants utilizing two major mechanisms (1) direct uptake of contaminants and subsequent accumulation of nonphytotoxic metabolites into the plant tissue + (2) release of exudates and enzymes that stimulate microbial activity and the resulting enhancement of microbial transformations in the rhizosphere (the root zone)

9.2 Phytoremediation mechanisms of organic contaminants

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Not all organic compounds are equally accessible to plant roots in the soil environment

The inherent ability of the roots to take up organic compounds can be described by the hydrophobicity (or lipophilicity) of the target compounds

Hydrophobicity = log KOW (KOW octanol–water partitioning coefficient)

The higher a compound’s log KOW the greater the root uptake

If compounds are quite water soluble (log KOW <0.5) they are not sufficiently sorbed to the roots or actively transported through plant membranes

(1) Direct uptake (Phytotransformation) -- Prerequisite

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Wood is composed of thousands of hollow tubes, like the bed of a hollow fiber chromatography column, with transpirational water serving as the moving phase

Lignification Once an organic chemical is taken up, a plant can store (sequestration) the chemical in new plant structures

Metabolism Detoxificate a parent chemical to nonphytotoxic metabolites, including lignin, that are stored in plant cells different plants exhibit different metabolic capacities

Mineralization Mineralize the chemical to carbon dioxide, water, and chlorides

(1) Direct uptake (Phytotransformation) -- Mechanism

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(2) Degradation in rhizosphere (Rhizosphere bioremediation)Roots of plants exude a wide spectrum of compounds

including sugars, amino acids, carbohydrates, and essential vitamins may act as growth and energy-yielding substrates for the microbial consortia in the root zone

Exudates may also include compounds such as acetates, esters, benzene derivatives, and enzymes

In situ microbial populations in rhizosphere enhanced degradation by provision of appropriate beneficial primary substrates for cometabolic transformations of the target contaminants

Typical microbial population in rhizosphere 5 ×106

bacteria, 9×105 actinomycetes, and 2×103 fungi per gram of air-dried soil

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Oxygen, CO2 , water, and contaminate cycling through a tree

Source: Suthersan, 1997 33

Most heavy metals have multiple chemical and physical forms in soil all forms are not equally hazardous, nor are all forms equally amenable to uptake by plants

Phytoremediation of heavy metal contaminated soils can be divided into phytostabilization, phytoextraction, phytosorption and phytofiltrationapproaches

9.3 Phytoremediation mechanisms of heavy metals

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It involves the reduction in the mobility of heavy metals by minimizing soil erodibility, decreasing the potential for wind-blown dust, and reduction in contaminant solubility by the addition of soil amendments

Eroded material is often transported over long distances extending the effects of contamination and increasing

the risk to the environmentPlanting of vegetation at contaminated sites

significantly reduce the erodibility of the soils both by water and wind effectively hold the soil and provide a stable cover against erosion

(1) Phytostabilization

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The use of unusual plants that have the ability to accumulate very high (2 to 5%) concentrations of metals from contaminated soils in their biomass metals are translocated to the shoot and tissue via the roots

Hyperaccumulator plants they exhibit the ability to tolerate high concentrations of toxic metals in above-ground plant tissues

After harvesting a biomass processing step or disposal method that meets regulatory requirements should be implemented

(2) Phytoextraction

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Source: Suthersan, 1997

Phytoextraction of heavy metals37

Hybrid poplar tree for phytoextractionSource: Chappell, 199738

Aquatic plants and algae are known to accumulate metals and other toxic elements from solution Plant roots acting to sorb, concentrate, or precipitate metals

e.g. one blue-green filamentous algae of the genus Phormidium and one aquatic rooted plant, water milfoil (Myriophyllum spicatum) exhibited high specific adsorption for Cd, Zn, Ph, Ni, and Cu

(3) Phytosorption and phytofiltration

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More information: http://www.abydoz.com/tech.html

9.4 Filed application: a case studyAbydoz technology Abydoz systems uses plants capable of purifying a wide variety of domestic, municipal and industrial wastewater's. The treatment area is a stable, engineered ecosystem and is based on complex inter relationships between plants, soils and microorganisms.

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Spring 2012 Faculty of Engineering & Applied Science

Lecture 10: Stabilization and Solidification

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ENGI 9621 Soil Remediation Engineering

Also called as immobilization, fixation, or encapsulationS/S uses additives or processes to chemically bind and

immobilize contaminants or to microencapsulate the contaminants in a matrix that physically prevents mobility

Stabilization refers to a chemical processes that actually converts the contaminants into a less soluble, mobile, or toxic form

Solidification refers to a physical process where a semisolid material or sludge is treated to render it more solid

10.1 Introduction(1) Stabilization and solidification (S/S)

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S/S neither removes the contaminant from soils (such as soil flushing) nor degrades the contaminants, (such as bioremediation) it eliminates or impedes the mobility of contaminants

S/S can be implemented under ex-situ or in-situ conditions

In situ S/S involves the injecting and/or mixing of stabilizing agents into subsurface soils to immobilize the contaminant, to prevent them from leaching into groundwater

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Application of reagents to the S/S treatment siteSource: Jones, 2009 44

S/S applicable to soils contaminated with metals, radionuclides, and other inorganics as well as non-volatile and semi-volatile organic compounds

S/S not appropriate to treat soils contaminated solely with volatile or organic compounds they may be volatized and released during mixing and curing operations

S/S applicable to all types of soils (clay, silts, or sands)

10.2 Applicability

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Low cost due to the use of widely available and relatively inexpensive addictive and reagents

Applicable to a wide variety of contaminants, including organic compounds and heavy metals

Applicable to deferent types of soils

Uses readily available equipments and is simple

High throughput rates compared to other technologies

(1) Advantages

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Contaminants are not destroyed or removedThe volume of treated soil may be increased significantly

with the addition of reagents

Emissions of VOCs and particulates may occur during mixing procedures requiring extensive emission controls

Delivery of reagents to the subsurface and achieving uniform mixing for in-situ treatment may be difficult

In-situ solidification may hinder future site use

Long-term efficiency of the process may be uncertain

(2) Disadvantages

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10.3.1 S/S processes depend on the type of stabilization reagents used can be grouped into

10.3 Description of the process

(1) Cement-based S/S (inorganic) contaminated soils are mixed with Portland cement

water is added to the mixture if soil water content is lowthe high pH value of the cement hydroxides of the metals

are formed are much less soluble than other ionic metal species

small amounts of fly ash, sodium silicate and bentonites are added to the cement to enhance processing

it is applicable for metals, PCBs, oils, and other organic compounds

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physical entrapment of the contaminate in the pozzolanmatrix

commonly used pozzolans fly ash, pumice, lime kiln dusts and blast furnace slag

pozzolanic reactions are generally slower than cement reactions

it is applicable for metals, waste acids, and creosote

(2) Pozzolanic S/S (inorganic) uses siliceous and aluminosilicate materials + lime or cement + water

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(3) Thermoplastic S/S (organic) a microencapsulationprocess soil contaminants do not react chemically with the encapsulating materials

a thermoplastic material such as asphalt or polyethylene is used to bind the contaminants into as S/S mass

it is applicable for metals, organics and radionuclides

(4) Organic polymerization S/S (organic) relies on polymer formation to immobilize the soil contaminantsurea-formaldehyde the most commonly usedit is applicable for special wastes such as radionuclides

also applicable for metals and organic contaminants

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10.3.2 Detailed S/S processes

The stabilizing reagents fed into the auger and then into the contaminated soil through a hollow stem

Inside the caisson the auger mixes the reagents with the soil by a lifting and turning action a large diameter (≥ 6 ft) “plug” of the contaminated soil is mixed in place

After thorough mixing the auger is removed and the setting slurry is left in place

The auger is advanced to overlap the last plug slightly the process is repeated until the contaminated area is covered

Physical and chemical testing is required for the characterization of soils prior to and after treatment

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Source: Suthersan, 1997 In-situ S/S process

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10.4 Modified or complementary technologiesIn situ S/S named as in-situ immobilization or in-situ

fixationVitrification heat metls and converts contaminated

soils into glass or other crystalline productsS/S enhanced through combined with vapor extraction,

hot air injection, or hydrogen peroxide injection to remediate organic compounds effectively

$ 40 to $ 60 per cubic yard for shallow application$150 to $250 per cubic yard for deeper application$100 per ton for ex-situ S/S treatment

10.5 Cost

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