CHAPTER 8 Remediation and Pollution Mitigation 8.1 INTRODUCTION Implementation of effective techniques and procedures for treatment of contami- nated sites to remove or minimize the concentration of pollutants constitutes the fundamental aim of remediation and pollution mitigation programs. The previous chapter has addressed the need for development of effective and compatible techniques for site decontamination (pollutant removal) based upon a proper understanding of the nature of the problem, and the processes involved in pollutant fate determination. In this chapter, some of the generic procedures for pollutant removal will be exam- ined insofar as they relate to the pollutant-removal and pollution-mitigation issues. In addition to the present standard procedures available for treatment of contaminated sites, innovative procedures and technologies are continuously being developed. It is recognized that it is not always necessary to completely remove all pollutants from a contaminated site. It is not unusual to find that complete pollutant removal could be prohibitively expensive, and may not be necessary since residual pollutant concentra- tions (i.e., pollutants remaining after clean-up) would be considerably below regulatory limits and limits defined by health-protection standards. Reduction of pollutant concen- tration below critical limits (i.e., pollution mitigation) is therefore a serious alternative. There are many ways of approaching site remediation implementation. It is useful to follow a protocol of procedures that would eliminate inefficient procedures, as shown, for example by the requirements and procedures developed in Figures 7.1 and 7.18. These are captured in Figure 8.1. The sets of general information and protocols needed for assessment of site contamination, and treatment required to provide for effective clean-up are shown in the diagram. 8.2 POLLUTANTS AND SITE CONTAMINATION Experience shows that very few contaminated sites are contaminated by one species (type) of pollutants. Generally, one finds various kinds of organic or inorganic contaminants (pollutants) or mixtures of these in contaminated soil, thus making it © 2001 by CRC Press LLC difficult to structure a one-step remedial treatment technique that can effectively remove the spectrum of pollutants in the contaminated soil. For remedial treatments to be effective, it is essential to match the treatment technique with the nature of the pollutants in the site and their bonding with the soil fractions. The use of treatment procedures as “black-box procedures” is not prudent since it is likely that this: • Would limit improvement of decontamination capability; • Would limit introduction of innovative techniques; • Could lead to application of inappropriate and incompatible technology; and • Could develop unexpected and perhaps adverse reactions or treatment products. 8.2.1 Pollution Mitigation, Elimination, and Management The first and foremost requirement in remedial treatment of a contaminated site is to eliminate the health and environmental threats posed by the presence of pol- lutants in the contaminated site. This requires management of the pollutants in the contaminated site, and can be achieved by: • Total removal of all the pollutants — This meets the requirement of a pristine site. Both aggressive remedial treatment and the traditional “dig and dump” (to be replaced by clean fill) are likely candidate procedures. Removal of all sorbed pollutants and also all pollutants transferred to (and originally in) porewater will Figure 8.1 Requirements and procedures in assessment of remediation-treatment of a con- taminated site. © 2001 by CRC Press LLC be required. Measurements of likelihood of presence of residual pollutants is required (Figure 8.2); • Reduction of concentration of the pollutants to levels below critical (allowable) levels — This requires remedial treatments and measurements of “residual” con- centrations of pollutants and assurance that they would not become environmentally mobile; • Immobilizing the pollutants to ensure no movement of the pollutants from their fixed (immobilized) positions — Solidification and stabilization procedures are the most likely candidate procedures. Monitoring is a key requirement; and • Containment of the pollutants in situ — By constructing impermeable cells or barriers to contain the pollutants. Management of pollutant transport through the cell walls or barriers is a prime requirement, together with monitoring. It is clear that “return to pristine conditions” is an objective that will never be easily met. This is due to either one or both of the following: (a) technical require- ments and available technology, and (b) economics of required treatment. The basic elements shown in Figure 8.2 demonstrate that in the initial stages, detached pollut- ants (from soil solids) will be transferred to the porewater. Removal of all pollutants from the porewater will be required as an integral element of the total remedial treatment process. It should be fairly clear that the remedial treatment process will Figure 8.2 Principal elements in consideration of in situ and ex situ remedial treatment. © 2001 by CRC Press LLC not be a one-step process. “Return to pristine conditions” and even the “pollutant concentration reduction” objective are treatment objectives that require integration of multi-step processes. For these reasons, and for reasons associated with require- ments for long-term performance predictions, risk assessment and risk management are necessary tools in pollution management. 8.2.2 In situ and Ex situ Remedial Treatment The choice of in situ and/or ex situ remedial treatment options is most often dictated by such considerations as: (a) requirements and objectives set forth by land use policies; (b) regulatory requirements; (c) site specificities; (d) land capability; (e) ownership objectives, requirements and expectations; (f) timing; and (g) eco- nomics — as illustrated in Figure 8.3. As will be seen, there are basically three options: (a) total remediation in situ; (b) removal of the contaminated soil substrate material for treatment elsewhere (off-site); and (c) removal of the contaminated soil material for treatment above ground but remaining on-site. There are other ramifi- cations to the basic three options. These will be evident when the generic techniques are addressed. The basic factors considered in determining whether on-site ex situ, off-site ex situ, or in situ remediation technology and procedures for remedial treatment of contaminated sites should be used include: Figure 8.3 Principal elements in in situ and ex situ remedial treatment of a contaminated site. © 2001 by CRC Press LLC • Contaminants/Pollutants — Type, concentration, and distribution in the ground; • Site — Site specificities, i.e., location, site constraints, substrate soil material, lithography, stratigraphy, geology, hydrogeology, fluid transmission properties, etc.; • Rehabilitation — Intended land use, land suitability/capability, local zoning reg- ulations, and requirements for clean-up remediation; • Economics and Timing — Economics and compatible technology, efficiency, time and penalties; • Regulatory Requirements — Regulations, constraints, etc.; and • Risks — Risk management. The first three factors are required in the evaluation of the technical feasibility for site decontamination, and determination of the best available technology for site decontamination and rehabilitation. The final choice is generally made in accord with other governing considerations, e.g., risk, treatment effectiveness, benefits, and permanency of treatment. Regulations and requirements become very important considerations. In summary, we note that the choice of remediation/decontamination technique requires one not only to consider the many scientific and technological aspects of the problem, but also hazard identification, toxicity and exposure, and risk characterization or evaluation. 8.3 BASIC SOIL DECONTAMINATION CONSIDERATIONS The simplest basic requirement in in situ clean-up of contaminated sites pays attention to remedial treatment procedures that will: (a) remove the offending con- taminants (pollutants) in the substrate, and/or (b) immobilise the pollutants in the substrate — to prevent them from moving in the substrate. In the first case, removal of the pollutants can be achieved either by treatment processes which will remove (detach) them from the soil solids and subsequently from the porewater, or by physically removing the substrate material. At the very least, ex situ treatment requirements pay attention to the first case (removal of pollutants). Immobilization of contaminants is generally achieved by processes that fix the pollutants in the substrate (i.e., stabilization and solidification), or by virtual thermal destruction. If the end-point objectives specified in regulatory requirements for remediation and rehabilitation of the contaminated sites are known, the required treatment technology can be developed in conjunction with geotechnical engineering input to produce the desired sets of actions. The general techniques that support the end-point objectives can be broadly grouped as follows: • (Group 1) Physico-Chemical — e.g., techniques relying on physical and/or chem- ical procedures for removal of the pollutants, such as precipitation, desorption, soil washing ion exchange, flotation, air stripping, vapour/vacuum extraction, demulsi- fication, solidification stabilization, electrochemical oxidation, reverse osmosis, etc.; • (Group 2) Biological — i.e., generally bacterial degradation of organic chemical compounds, biological detoxification; bioventing, aeration, fermentation, in situ biorestoration; © 2001 by CRC Press LLC • (Group 3) Thermal — e.g., vitrification, closed-loop detoxification, thermal fixa- tion, pyrolysis, super critical water oxidation, circulating fluidized-bed combustion; • (Group 4) Electrical-Acoustic-Magnetic — e.g., techniques involving electrical, acoustic, and/or magnetic, procedures for decontamination such as electrokinetics, electrocoagulation, ultrasonic, electroacoustics, etc; and • (Group 5) Combination of any or all of the preceding four groups, e.g., laser- induced photochemical, photolytic/biological, multi-treatment processes, treatment trains, reactive walls, etc. 8.4 PHYSICO-CHEMICAL TECHNIQUES 8.4.1 Contaminated Soil Removal and Treatment The simplest physical procedure for decontamination of a contaminated site is an ex situ procedure which involves removal of the contaminated soil in the affected region, and replacement with clean soil — i.e., the “dig, dump, and replace” proce- dure. For contaminated sites that are limited in spatial size and depth, this procedure is very popular because of the obvious simplicity in site rehabilitation. The removed contaminated soil is relocated in a prepared waste containment (landfill) site, or is treated by any of the means covered under Groups 1 through 4 listed in the preceding section. The simplest general treatment procedure for dislocated contaminated soil is a soil washing procedure as shown in Figure 8.4. This is best suited for contam- inated soils that do not have significant clay contents. Granular soils with little clay contents, which are contaminated with inorganic pollutants, will present the best candidates for washing procedures. Using heavy metal (HM) pollutants as an example, we note that HM sorption mechanisms associated with the reactive surfaces of clay fractions, such as those listed in Table 5.1, render the washing-extraction procedure more difficult — in the sense that chemical treatments will need to be introduced to detach the sorbed pollutants from the surfaces of the clay soil solids. The retention mechanisms listed in Table 5.1 make it very difficult to remove the HM pollutants without resorting to aggressive chemical treatments in the wash process. In addition to the preceding set of problems, dispersants will need to be introduced in the grinding and wet slurry preparation stage of the process shown in Figure 8.4 to disperse the soil solids for chemical washing to achieve effective HM pollutant removal. For soils contaminated with organics, incineration of the soil is most often recommended — for destruction of the contaminants. However, if removal of the organic chemicals is warranted, as, for example, in instances where the organic chemical contents are high, extraction of the chemicals using the process shown in Figure 8.5 may be necessary. For soils containing soil fractions with little reactive surfaces, the product leaving the extractor should contain little extractant residue. For soils where the reactive surfaces of the soil solids are a significant factor, the choice of extractant(s) used becomes very critical. Two particular actions can be considered: (a) use of solvents, surfactants, biosurfactants, etc. as extractants, and (b) use of a secondary washing process that would remove the residual extractants. © 2001 by CRC Press LLC Option (a) is the more useful course of action. The merits of choosing an effective biosurfactant have been shown in Chapter 7. 8.4.2 Vacuum Extraction — Water and Vapour Vacuum extraction, which is commonly used to obtain contaminated groundwa- ter for cleaning, is generally classed as a physical technique, in the same manner of reasoning as physical removal of contaminated soil. For obvious reasons, application of this extraction technique is limited in respect to subsurface depth. The treatment of the extracted groundwater, which is required before discharge, can be achieved by several means, not the least of which are the standard wastewater chemical and biological treatment techniques and air stripping. Standard wastewater treatment will not be discussed herein. Application of the vacuum technique for soil vapour extraction is sometimes identified as air sparging when it includes extraction of volatilized groundwater pollutants, i.e., volatilized VOCs in the groundwater. This technique is best suited for treatment of soils contaminated by volatile and semi-volatile organic compounds. Biosparging, which is sometimes included with air sparging, relies on enhanced biodegradation as a contribution to the total vapour product being removed. The biodegradation of the less volatile and higher molecular weight of the VOCs and Figure 8.4 Basic elements in ex situ soil washing treatment of granular soils contaminated by inorganic pollutants. © 2001 by CRC Press LLC the removal of the vapour phase allows for a degree of remedial treatment of the VOC-contaminated soil. Soil venting and bioventing are considered to be essentially similar to air sparging and biosparging in respect to the removal or mass transfer of the volatile compounds from the VOCs. The basic elements for soil water and vapour extraction of VOCs (volatile organic compounds) is shown in Figure 8.6. The extrac- tion probe is located in the vadose zone. The tendency of the VOCs to volatilize from water into air is an important factor in the structuring of the remediation technique. If oxygen is used in place of nitrogen as the injecting medium, it not only promotes volatilization, but also contributes to the aerobic biodegradation processes. The first part of the technique is considered to be a physical technique (i.e., soil water and soil vapour extraction), and the second part of the technique where cleaning of the soil water and soil vapour occurs is not necessarily “physical” since one generally uses water treatment procedures (for water) and a packed tower containing activated carbon or synthetic resins to facilitate interphase mass transfer. Soil-structural features that impede flow of fluid and vapour can be significant. Not only must the delivery of the injected nitrogen or oxygen be effective, but the exiting conditions for the products must also be minimally impeded. Once again, granular soils permit better transmissivity, and soils with high clay and SOM content will present difficulties in transmission of both fluid and vapour. High density soils and high water contents in the unsaturated zone do not provide for good transmission properties. In particular, soils with SOM will show good VOC retention capability. Figure 8.5 Multi-step process for removal of a soil heavily contaminated with organic chemicals. © 2001 by CRC Press LLC In other words, complexes formed between the organic chemicals and soil fractions (particularly SOM) will inhibit volatilization. Properties of the VOCs are also important considerations. Solubility, sorption and partitioning coefficients, vapour pressure and Henry’s law constant, and con- centration of the VOCs are important factors which will affect withdrawal of the vapours. Preconditioning of the contaminated soil to obtain better transmission of water and vapour, and also to obtain release of the VOC will provide for a better treatment process. 8.4.3 Electrokinetic Application The use of electrokinetics for containment or treatment of sites with inorganic contaminants has attracted considerable attention, partly because of previous expe- riences with electro-osmotic procedures in soil dewatering, and partly because of the relatively “simplicity” of the field application method. This is generally consid- ered a physico-chemical technique because of the field application methods, i.e., the use of electrodes and current energy. For the more granular types of soils (silts), the procedure can be effective. However, in the case of clay soils, diffuse double-layer mechanisms developed in the soils can pose several problems, not the least of which are the energy requirements needed to maintain ionic movement. Figure 8.6 Elements of vacuum extraction of water and vapour in a VOC contaminated site. Treatments of contaminated water and air are not shown in the diagram. © 2001 by CRC Press LLC The basic principles involved in the use of electrokinetics in pollutant-removal processes have been discussed in Section 7.4 and will not be repeated here. In application of electrokinetic technology, one introduces similar procedures used in electro-osmotic dewatering, i.e., anodes and cathodes are inserted into the soil to produce movement of cations and anions to their respective receiving electrodes. In soils that have significant surface activity, i.e., where interpenetration of diffuse double layers are prominent, one needs to move the pollutants from the region dominated by diffuse double layers. The amount of energy required will need to be greater than the interaction energies established between the contaminant ions and the soil particles. Development of dissociation reactions (see Section 7.4) can seri- ously impair the useful life of the electrodes. Capitalizing on the electro-osmosis and ion migration effects when the direct current is established between electrode pairs, and benefitting from pre-conditioning of the soil to permit easier release of pollutants, in-field electrokinetics can be successfully applied. However, treatability studies are necessary for determination of the necessary pre-treatment procedures and the reagents to be used at the elec- trodes to facilitate removal of the pollutants. These can take the form of conditioning fluids that will improve the electrochemistry (of interactions) at the electrodes, as discussed in Section 7.4. “Fouling” of the electrodes is a serious consideration. 8.4.4 Solidification and Stabilization Techniques for “fixing” pollutants in their sorbed environment, i.e., pollutants sorbed to the soil solids and pollutants in the porewater, require an end product that ensures the pollutants are totally immobilized. Present application of stabilization- solidification (SS) techniques are either single-step or two-step processes. In the two-step process, the first step is the stabilization process where the polluted soil is rendered insoluble. This is followed by the second procedure which is a solidification process — to render the insoluble soil-pollutant mass solid. The single-step process uses a “binder-fix” that is designed to produce the same effect as the two-step process. The economics of the remedial treatment is best justified for toxic pollutants. In situ SS process application is limited by the permeability of the soil substrate being treated. Since application of the binder mixture is generally made with the aid of injectors which work similarly to a hollow-stem auger, penetration (propaga- tion) of the binder mixture into the surrounding soil will be controlled by the transmission characteristics of the penetrated soil mass, the viscosity of the binder, and the “set” time of the binder. High densities, clay soils, presence of soil organic matter and amorphous oxides all render application of in situ SS application highly problematic. Ex situ application of SS processes are more effective if the contaminated soil is in a dispersed state. As in soil washing processes, the excavated material is broken up by grinders, pulverizers, etc. prior to application of the binder mixture. The greater the cohesive nature of the soil, the greater will be the effort needed to grind the material to the kinds of sizes needed for best application of the binder mixture. Disposal of the resultant SS material will still be needed. Since the solidified or © 2001 by CRC Press LLC [...]... 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Removal of all sorbed pollutants and also all pollutants transferred to (and originally in) porewater will Figure 8. 1 Requirements and procedures. clean-up of contaminated sites pays attention to remedial treatment procedures that will: (a) remove the offending con- taminants (pollutants) in the substrate, and/ or (b) immobilise the pollutants