SOIL TREATMENT
in-situ chemical oxidation of contaminated soils
(using hydrogen peroxide)
Introduction
Soils contaminated with hydrocarbons (petroleum residues, solvents, pesticides, wood preservatives, etc.) present one of the more difficult challenges for remediation specialists. This is particularly true when the contamination has spread deep below the surface, making excavation costly or impractical. The technologies currently in use all have serious shortcomings.
Technology Option / Major ShortcomingsSoil washing / Produces an aqueous effluent needing treatment/disposal
Not effective for DNAPL뭩
Vapor extraction / Requires treatment of the extracted vapors
Not effective for DNAPL뭩
Biotreatment / Lengthy process (years)
Difficult to monitor and control
Inconsistent results associated with toxicity, biodegradability, solubility, and nutrient availability
Landfilling / Decreasing availability of suitable disposal sites
Increasing transportation and disposal costs
Incineration / Lengthy and difficult permitting process
High capital costs
The use of H2O2 to chemically oxidize the contaminants into mineralized products (CO2 , salts, and readily biodegradable organic fragments) offers a number of advantages over these current treatment methods.
Benefits of Soil Decontamination by Chemical Oxidation:
· The reagent used (H2O2) is inexpensive, readily available, and has as its reaction products water and oxygen;
· The chemistry of the process (Fenton뭩 Reagent) has been known for over 50 years and its applicability to a wide variety of contaminants is well documented;
· The process is easily applied and controlled;
· The treatment occurs rapidly, lasting from a few hours to a few weeks;
· The process produces no by-product wastestreams, and can be tuned for the degree of contaminant removal desired. This allows it to be used synergistically with other technologies such as biotreatment or soil washing.
Background/ Principles
The basis of the process involves Fenton뭩 Reagent, whereby the transition metal catalyst is either provided by iron oxides within soil or added separately as a solubilized iron salt. The oxidation occurs both at the soil interface (in the case of DNAPL뭩) and within the interstitial groundwater (in the case of desorbed organics). The process is related to UV photolysis methods used in groundwater remediation in that both processes are considered Advanced Oxidation Technologies (AOTs). By definition, AOT뭩 generate highly reactive, short-lived hydroxyl radicals (.OH) which degrade even the most recalcitrant organics.
The application of chemical oxidation technologies for degrading toxic, inhibitory or biorefractory organics has also been well studied of late. These properties are highly correlated to aromatic ring structures and non-polarity (low water solubility). Chemical digestion of these substances by AOT뭩 results in ring cleavage and insertion of oxygen into the digested fragments. This reduces their toxicity and increases their water solubility, thereby facilitating their degradation in the natural environment.
Discussion
The following is an overview of the variables which affect the technology뭩 application.
1. Effect of H2O2 : contaminant dose ratio
This parameter is closely related to the efficiency of treatment since H2O2 is generally the primary cost component in the application. And because of the complexity of soil matrices, the process is inherently less efficient than that in aqueous reactions. This results in effective weight ratios of 5-50:1 for soil treatments versus 1-5:1 for aqueous treatments. Typical of most chemical reactions, efficiency increases with higher contaminant concentrations and decreases as target treatment levels become more stringent.
2. Effect of H2O2 concentration
Unlike aqueous applications of Fenton뭩 Reagent where the strength of the H2O2 feed solution has no effect, in soil applications the amount of water present in the reaction mix (termed liquid:soil ratio) is critical. This aspect was characterized for the treatment of PAH-contaminated soils by Richard Watts at Washington State University. Two systems were studied: 1) a low (water) volume/high (solids) concentration, which used 25-50% H2O2; and 2) a high volume/low concentration, which used 0.5 - 2% H2O2. It was observed that the lower water volume (higher H2O2 concentration) provided faster reaction times (hours versus days/weeks) but less efficient H2O2 use. A second important finding was that the lower water volume (higher H2O2 concentration) provided significantly greater removal of DNAPL-type contaminants. The following general guidelines were produced for applying the process in an ex-situ mode.
High volume/low concentration systemInitial TPH Conc. (mg/Kg) / H2O2 Conc. / H2O2 Volume (gallons / ton of soil)
300 - 800 / 0.5% / 800
800 -1300 / 1% / 2000
1300 -1800 / 1% / 2400
1800 -2300 / 1% / 3200
Low volume/high concentration system
Contaminant / Initial TPH Conc. (mg/Kg) / H2O2 Conc. / H2O2 Volume (gallons / ton of soil)
Gasoline/diesel / 200 - 1000 / 25% / 48
Gasoline/diesel / 1000 - 2000 / 35% / 48
Motor/Hydraulic oils / 200 - 1000 / 50% / 96
Motor/Hydraulic oils / 1000 - 2000 / 50% / 192
3. Effect of soil type
The type of soil affects the process in many ways, the most obvious being the availability of iron. Various studies have shown that crystalline soils (especially those containing goethite and hematite iron oxides) effectively catalyze the reaction. Silica sand was even used in one study. Amorphous soils (e.g., clays) are less effective. If supplemental iron is needed, application rates of 0.5 - 2 gms-Fe2+ per Kg-soil are indicated.
A second effect of soil type relates to its buffering capacity (carbonate alkalinity). This has two effects: 1) Fenton reactions perform best at pH 2-4 and not at all at > pH 7-8 (except if certain chelated iron complexes are used); and 2) carbonate ions are strong free radical scavengers and effectively retard Fenton reactions. Thus, highly alkaline soils (e.g., those containing limestone) may require mineral acid to be added to the zone of contamination in order to bring the pH into the optimal range. Such pH adjustment is not needed for low alkalinity soils since the desired pH is quickly obtained by the organic acid fragments once the reaction gets underway.
A third effect of soil type relates to its organic carbon (humus) content. Since the hydroxyl radical is relatively non-selective in its reactions, we should expect competition between the natural background organics and the hydrocarbon contaminants. As the ratio of background-to-contaminate organics increases, so will the required H2O2 : contaminate dose ratio. However, Watts, et.al. observed no significant effect with contaminant levels of 500 - 2000 mg/Kg against background organic carbon contents of 0.1 - 1.3%.
4. Effect of reaction time
Theoretically, applications of Fenton Reagent follow a first-order rate equation (i.e., contaminant removal is a linear relationship to H2O2 dose). However, there are a couple of factors which cause results to deviate from the model. The first is the concentration of iron catalyst. In most cases, supplemental iron addition to the soil will provide savings in both H2O2 efficiency and project length. The second factor relates to the polarity of the organic contaminants. There is evidence that reaction times may depend on the desorption of adsorbed organics from the soil particles into the aqueous phase. This is consistent with the known properties of Fenton뭩 Reagent, and explains why, all things being equal, DNAPL-type organics react more slowly than light hydrocarbons (weeks versus hours). However, as discussed above, this can be compensated for by lowering the liquid:soil ratio (increasing the feed concentration of H2O2). Incidentally, this work suggested that other (non-Fenton-type, heterogenous) reaction mechanisms may be partially responsible for the oxidation of such non-polar contaminants.
In summary, the following factors may be used to reduce the reaction times: higher H2O2 : contaminant dose ratios, higher iron catalyst doses, and lower liquid:soil ratios (higher H2O2 feed concentrations).
5. Effect of contaminant type.
Soil decontamination with H2O2 has been demonstrated on the following types of organics:
Contaminants Treated
· Chlorinated solvents (TCE, PCE)
· Munitions (TNT, RDX)
· Pesticides (chlorophenoxy, atrazine, pendimethalin)
· Petroleum residues (BTEX, PAH, TPH, MTBE, diesel fuel)
· Wood preservatives (PCP, creosote)
· Misc. (PCB뭩, phenolics)
6. Effect of other parameters
Although much work remains to identify and characterize these other variables, we can gain some insight from aqueous reactions of Fenton뭩 Reagent. For example, the optimal pH is 2-4, and the optimal temperature is 10-20 deg-C. We also know that in highly contaminated systems (where the potential for unproductive side-reactions are great), H2O2 efficiency is improved by performing the oxidation either in a stepwise fashion or (optimally) in a slow, continuous mode ?as opposed to adding the H2O2 all at once.
Practical considerations
Although soil decontamination with H2O2 is relatively easy to employ, for the following reasons the process is rather complex to evaluate:
· The major cost component (H2O2 requirement) is not a linear function of contaminate levels. As discussed above, there are a number of parameters to consider -- soil type, liquid:soil ratios, contaminant type, and reaction times. Profiling these variables requires carefully designed column studies (using multivariable, multilevel factorial design experiments).
· The costs incurred as a result of project length, H2O2 use, and special site constraints will inevitably present trade-offs. These must be carefully evaluated to develop the process parameters which provide the most benefit at the least cost.
· Given the diminishing returns in treating to ppb levels of residual contaminant, there may be synergy in combining this technology with other technologies more suitable to removing trace levels of organics. Examples of complementary technologies include biotreatment, soil washing, and UV photolysis (of the groundwater). Choosing an appropriate combination and determining the optimal transition point requires careful study.
· Site constraints, especially geohydraulic considerations, will impact how the technology is applied. In-situ applications (treating the soil in place using sprayers, sprinklers or nozzle injectors) have the advantages of less exposure of workers, less impact of site constraints, and typically less cost. Ex-situ applications (excavating the soil and treating it batchwise in a surface mixer, leach pile, etc.) have the advantages of greater process control and reduced risk of leaching contaminants or degradation products into the groundwater. NB. To reduce these risks, known subsurface process delivery and control methods have been adapted for this technology. These include: vertical/horizontal sparging, recirculation wells, permeable treatment walls, hydraulic fracturing, enhanced soil mixing, and injection/permeation dispersal. Choosing and applying the best method(s) requires the services of a geohydrologist specializing in the field.
Another practical consideration relates to the safety of applying concentrated H2O2 solutions to soils. The reaction can be vigorous, with the rapid evolution of oxygen, steam and carbon dioxide. Allowances should be made to ensure adequate venting of these gases. If possible, the H2O2 should be added stepwise or in a slow, controlled continuous fashion.
Conclusions
Adaptation of Fenton뭩 Reagent from treating aqueous wastes to treating contaminated soils has been successfully demonstrated for number hydrocarbons, and the primary variables affecting its performance have been identified and characterized. The technology has been shown to remove organic contaminants adsorbed onto soils and degrade them to CO2 or more polar compounds which are amenable to subsequent treatments. Compared to currently used technologies, the process offers advantages related to simplicity, cost, and range of effectiveness.
Literature References
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Gates, D.D. et.al., "Laboratory Evaluation of the In-Situ Chemical Oxidation of Volatile and Semi-Volatile Organic Compounds Using Hydrogen Peroxide and Potassium Permanganate," Oak Ridge National Laboratory (1994).
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