ENGI 9621 Soil Remediation Engineering Lecture 7: Soil Flushing Spring 2012 Faculty of Engineering & Applied Science 1
7.1 Introduction (1) Definition of soil flushing 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 use Accomplished by passing the flushing solution through in-place soils using an injection or infiltration process Extraction fluids must be recovered from the underlying aquifer and, when possible, they are recycled 2
(2) Cosolvent flushing 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 upgradient of the contaminated area, and the solvent with dissolved contaminants is extracted downgradient and treated above ground 3
(3) Operation processes Drilling 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 4
Source: EPA, 1996 Typical in-situ soil flushing in vadose zone 5
(4) Recovered fluid treatment 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 flushing Treatment of the recovered fluids results in process sludges and residual solids, such as spent carbon and spent ion exchange resin 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 6
7.2 Applicability 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
Contaminants Considered for Treatment by In Situ Soil Flushing Source: EPA, 1996 8
7.3 Limitations 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 9
More limitations 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 10
7.4 Economic consideration Approximate costs: $50 to $200 per ton Key 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 Scenarios Site sizes A B C D Small Large Site conditions Easy Difficult Easy Difficult Cost per cubic yard $32 $49 $18 $27 11
ENGI 9621 Soil Remediation Engineering Lecture 8: Soil Fracturing Spring 2012 Faculty of Engineering & Applied Science 12
8.1 Introduction (1) Fracturing 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 13
(2) Pneumatic fracturing 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 vapor extraction, bioventing, and enhanced in situ biodegradation (3) Hydraulic fracturing involves injecting a fluid, usually water, at modest rates and high pressures into the soil matrix to be fractured a 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
Two types of soil fracturing Source: Sharma and Reddy, 2004 15
8.2 Applicability 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. silty clay/clayey silt sandy silt/silty sand clayey sand sandstone siltstone limestone shale 16
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) 17
8.3 Description of the process The selection between hydraulic (water-based) and pneumatic (air-based) fracturing are based on the following considerations: 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 18
Hydraulic Soil Fracturing Effective in soils and rock Long term permeability enhancement Specialized equipment and fluid chemistry expertise required Low leak-off prevents spreading of subsurface contaminants Pneumatic Soil Fracturing Primarily effective in rock Short term permeability enhancement in unconsolidated sediments Less equipment and expertise required Injected air can potentially spread soil vapour phase contaminants Fracture clogging by fines is minimized Fractures are unsupported; because frac sand is designed to act as a migration of fines quickly clogs geotechnical filter while maintaining enhanced fractures permeability Greater range of adaptability with remediation technologies (e.g. SVE, Bioremediation) Not readily adaptable to many remediation technologies 19
(1) Hydraulic fracturing 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 environmental application the continuous mix grade of guar gum The injection pressure required to create hydraulic fractures is remarkably modest (less than 100 psi) Injection pressure as a function of time during hydraulic fracturing Source: Suthersan, 1997 20
Method for creating hydraulic fractures in soil Source: Suthersan, 1997 21
Hydraulic Fractures Source: Slack, 1998 22
(2) Pneumatic fracturing advancing a borehole to the desired depth of exploration and withdrawing the auger positioning the injector at the desired fracture elevation sealing off a discrete 1 or 2 ft interval by inflating the flexible packers on the injector with nitrogen gas applying pressurized air for approximately 30 s repositioning the injector to the next elevation and repeating the procedure a typical fracture cycle approximately 15 min a production rate with one rig 15 to 20 fractures per day 23
Schematic of pneumatic fracturing process Source: Suthersan, 1997 Injection rates of up to 1000 scfm sufficient to create satisfactory fracture networks in low permeability formations 24
8.4 Limitations The technology should not be used in areas of high seismic activity Fractures will close in non-clayey soils Investigation of possible underground utilities, structures, or trapped free product is required The potential exists to open new pathways for the unwanted spread of contaminants (e.g., dense nonaqueous phase liquids) 8.5 Cost Pneumatic fracturing $8 to $12 per ton Hydaulic fracturing 160 to $180 per ton for remediation in a 1-year treatment and $100 to $120 per ton in a 3-year remediation 25
ENGI 9621 Soil Remediation Engineering Lecture 9: Phytoremediation Spring 2012 Faculty of Engineering & Applied Science 26
9.1 Introduction (1) Definition of Phytoremediation 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 27
(2) Advantages 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 (3) Limitations contaminants present below rooting depth will not be extracted plant may not be able to grow in the soil at every contaminated site due to toxicity remediation process can take years for contaminant concentrations to reach regulatory levels requires a long-term commitment to maintain the system 28
9.2 Phytoremediation mechanisms of organic contaminants 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) 29
(1) Direct uptake (Phytotransformation) -- Prerequisite 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 K OW (K OW octanol water partitioning coefficient) The higher a compound s log K OW the greater the root uptake If compounds are quite water soluble (log K OW <0.5) they are not sufficiently sorbed to the roots or actively transported through plant membranes 30
(1) Direct uptake (Phytotransformation) -- Mechanism 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 31
(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 10 6 bacteria, 9 10 5 actinomycetes, and 2 10 3 fungi per gram of air-dried soil 32
Oxygen, CO 2, water, and contaminate cycling through a tree Source: Suthersan, 1997 33
9.3 Phytoremediation mechanisms of heavy metals 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 phytofiltration approaches 34
(1) Phytostabilization 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 environment Planting 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 35
(2) Phytoextraction 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 aboveground plant tissues After harvesting a biomass processing step or disposal method that meets regulatory requirements should be implemented 36
Source: Suthersan, 1997 Phytoextraction of heavy metals 37
Source: Chappell, 1997 Hybrid poplar tree for phytoextraction 38
(3) Phytosorption and phytofiltration 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 39
9.4 Filed application: a case study Abydoz 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. More information: http://www.abydoz.com/tech.html 40
ENGI 9621 Soil Remediation Engineering Lecture 10: Stabilization and Solidification Spring 2012 Faculty of Engineering & Applied Science 41
10.1 Introduction (1) Stabilization and solidification (S/S) Also called as immobilization, fixation, or encapsulation S/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 42
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 43
Application of reagents to the S/S treatment site Source: Jones, 2009 44
10.2 Applicability S/S applicable to soils contaminated with metals, radionuclides, and other inorganics as well as nonvolatile 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) 45
(1) Advantages 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 46
(2) Disadvantages Contaminants are not destroyed or removed The 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 47
10.3 Description of the process 10.3.1 S/S processes depend on the type of stabilization reagents used can be grouped into (1) Cement-based S/S (inorganic) contaminated soils are mixed with Portland cement water is added to the mixture if soil water content is low the 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 48
(2) Pozzolanic S/S (inorganic) uses siliceous and aluminosilicate materials + lime or cement + water physical entrapment of the contaminate in the pozzolan matrix 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 49
(3) Thermoplastic S/S (organic) a microencapsulation process 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 contaminants urea-formaldehyde the most commonly used it is applicable for special wastes such as radionuclides also applicable for metals and organic contaminants 50
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 51
In-situ S/S process Source: Suthersan, 1997 52
10.4 Modified or complementary technologies In situ S/S named as in-situ immobilization or in-situ fixation Vitrification heat metls and converts contaminated soils into glass or other crystalline products S/S enhanced through combined with vapor extraction, hot air injection, or hydrogen peroxide injection to remediate organic compounds effectively 10.5 Cost $ 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 53