PROCESS TO REMEDIATE CONTAMINATED SOILS USING CARBON
DIOXIDE-ASSISTED HYPOCHLORITE OXIDATION
The present disclosure relates to the remediation of contaminated soils that require treatment.
BACKGROUND OF THE DISCLOSURE
Contaminated soils are a concern in urban areas and various processes exist to remediate these soils. In urban areas, it is generally necessary to excavate the contaminated soils. Therefore, a regional soil treatment centre performs the ex-situ remediation treatment. The most common soil contaminants are heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) and hydrocarbon compounds (C10-C50, HAP). These contaminants are either alone or together in a same soil.
This section discusses the background art of commercial treatment of hydrocarbon-contaminated soils.
Commercial-scale treatments can use chemical oxidation in soil piles for the remediation of contaminated soils. The treatment uses strong oxidants, such as hydrogen peroxide, potassium permanganate, ozone or persulfate salts. In this process, the main problems are the high oxidant consumption, the low substrate selectivity and the strong oxidant ineffectiveness in oxidizing recalcitrant organic contaminants, namely C25-050 and heavy polycyclic aromatic hydrocarbons (HAP). In most cases, these treatments are too lengthy for an efficient ex-situ treatment.
One observation regarding existing commercial oxidation treatments is that the stronger the oxidant is, the less cost-efficient the oxidation reaction. Finally, the more severe the decontamination needs to be, the higher the oxidant consumption per contaminant mass will be.
Another observation is that the oxidative treatment could necessitate the dosing of a surfactant.
The surfactant eases the release of the hydrocarbon contaminants and facilitate the remediation process. The surfactant is loss in the process.
SUMMARY OF THE DISCLOSURE
In one aspect, there is provided a soil decontamination process comprising:
providing a solution comprising an hypochlorite salt;
providing a slurry comprising said solution and contaminated soils comprising hydrocarbon-based compounds, wherein said slurry is maintained at a pH from 6 to 10 by contacting carbon dioxide with said slurry; and recovering soils, wherein said recovered soils are comprising an amount of hydrocarbon-based compounds lower than that of said contaminated soils.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 represents the residual C10-050 concentration in a contaminated soil following NaCIO, 1-1202 or KMn04 oxidation at 25 C in a single or double batch operation;
Fig. 2 schematically illustrates an embodiment of how this disclosure may be incorporated in a soil decontamination process;
Figs. 3A, 3B and 3C illustrate side and front sectional views as well as a top view of an embodiment for a treatment centre that may incorporate a soil decontamination process as defined herein; and Fig. 4 illustrates the influence of the NaCIO initial concentration with a CO2 overhead on the decontamination of a soil comprising CIO-050 contaminants.
DETAILED DESCRIPTION OF THE DISCLOSURE
In accordance with the present description there is now provided a hypochlorite (CIO-) oxidation process assisted with carbon dioxide to remediate contaminated soils.
In one embodiment, the soil is contaminated with hydrocarbon aliphatic contaminants ranging from C10 to C50 alkanes. In this embodiment, the present disclosure addresses soils with an initial contamination level that is no more than about 15 000 mg/kg. In this embodiment, the decontamination target is a residual Cl 0-050 content ranging from 300 to 3 500 mg/kg. In one embodiment, the target C10-050 content is lower than 3 500 mg/kg, lower than about or lower than about 700 mg/kg. It is also a target to provide C25-050 amount in similar ranges in the decontaminated soils. The legislation determines which decontamination level to reach.
In a further embodiment, the soil is contaminated with polycyclic aromatic hydrocarbons (MI 1). The legislation lists about two dozen carcinogenic PAH including, for example, anthracene and phenanthrene. In this embodiment, the initial contamination content is up to 5 000 mg/kg with a typical initial contamination content up to 500 mg/kg. In this embodiment, the decontamination target is a residual PAI-I content up to 100 mg/kg. The legislation determines which decontamination level to reach for each PAH. PALL may be present in contaminated soils in addition to C10-050 or independently present on their own. A list of PAH
may be comprising the following: acenaphtene, acenaphtylene, anthracene, benzo (a) anthracene, benzo (a) pyrene, benzo (b+j+k) fluoranthene, benzo (c) phenanthrene, benzo (g,h,i) perylene, chrysene, dibenzo (a,h) anthracene, dibenzo (a,i) pyrene, dibenzo (a,h) pyrane, dibenzo (a,l) pyrene, dimethy1-7,12 benzo (a) anthracene, fluoranthene, fluorene, indeno (1,2,3-ed) pyrene, methyl-3 cholanthrene, naphtalene, methyl-1 naphtalene, methyl-2 naphtalene, dimethyl-1,3 naphtalene, trimethy1-2,3,5 naphtalene, phenanthrene, and pyrene.
In both these embodiments, an accredited protocol measures the extent of the contaminant oxidation. This protocol includes a solvent extraction on a soil sample and a chromatographic scan (GC-FID or GC-MS) on the extracted solvent phase. The solvent can be, for example, hexane or dichloromethane. The solvent extracts part of the natural soil organic matter even though it is no contamination. Thus, the solvent extraction reduces the soil sample mass. This mass reduction is not proportional and sometimes does not even correlate to the decontamination yield. For this reason, it is necessary to compare a chromatographic scan prior and after treatment to measure the oxidation yield.
In one embodiment, the soil is sifted to remove the largest oversized material which is inconvenient for the reaction. A trommel removes these oversized materials (branches, bricks, etc.) prior to the oxidation treatment. The residual oversized materials which diameter is greater than 2.5 mm are not an inconvenience for the reaction.
The reaction suits all soil types (clay, sand, silt, peat, etc.). The easiness to remediate a contaminated soil depends on the contamination age and less on the soil type.
In one embodiment, the present disclosure provides a hypochlorite aqueous solution, referred to as a solution, comprising 0.1 wt% to 15 wt%, or 0.1 wt% to 3 wt%, or 1 wt% to 7 wt%, or 1 wt% to 3 wt% of one or a mixture of the following components: sodium hypochlorite. lithium hypochlorite or calcium hypochlorite. Sodium hypochlorite and/or lithium hypochlorite are preferred and sodium hypochlorite is most preferred. Calcium hypochlorite is the least preferred because it is a partially soluble powder difficult to handle.
In one embodiment, the process is comprising adding a second, or further, amount of the hypochlorite salt solution before said step of recovering the soils.
Preferentially, an electrolytic hypochlorite generator provides on-line the addition of hypochlorite.
The contaminated soil mixes with a solution in a soil-to-solution mass ratio ranging from 1:1 to 1:10, preferably 1:5 or more preferably 1:3 to optimize the mixing and to minimize the required quantity of solution. The soil and solution mix is referred to as a slurry.
In another embodiment, carbon dioxide contacts with a slurry as a gas overhead in a closed slurry reactor or as a bubbling gas in an open slurry reactor. Flue gas from a direct combustion burner or a carbon dioxide cylinder supply the carbon dioxide to the reaction.
Carbon dioxide increases the hypochlorite reaction rate by maintaining the slurry at the required pH. A
sufficient amount of carbon dioxide contacts the slurry to provide the required pH. The amount of carbon dioxide can be determined by the skilled person as it will vary based on the remediation conditions, contamination and soils. The pH of the slurry will guide the amount of carbon dioxide to be used.
In one embodiment, the pH of the slurry (i.e. soil and solution mix) is maintained from 6 to 10.
Preferably the pH is from 7 to 10 or more preferably about 8.
In one embodiment, the oxidation reaction is conducted on the soil at a temperature of 5 to 30 C, preferentially 15 to 25 C. The process can use a heating device to raise the slurry temperature to the desired value if necessary.
In another embodiment, the reactor comprises a partially fluidized soil column, a solution recirculation loop, a flue gas sparger and a scrubber to neutralize the hypochlorite smell.
In another embodiment, the recovered solution is either regenerated into the original hypochlorite solution or sent to a water treatment unit. The solution regeneration takes place in an electrolytic cell and can be performed while the oxidation proceeds. The water treatment unit consists in a reverse osmosis membrane to concentrate the salts. In the treatment unit, water is recycled back to the process and hypochlorite and chloride salts are either discarded or regenerated into a hypochlorite solution.
One advantage of the present disclosure is that it is possible to keep low and fairly constant the specific oxidant consumption. In this regard, NaCIO compares favorably to the common oxidants used in soil oxidation processes (Fig. 1). The specific oxidant consumption is the mass of oxidant required to oxidize a given mass of contaminant. (e.g. kg oxidant /
kg oxidized contaminant). Typically, the specific NaCIO consumption stays below 40 kg NaCIO / kg oxidized CI 0-050 to decontaminate up to the decontamination target. According to the prior art, the specific strong oxidant consumption (e.g. 11202) can increase many folds as the oxidation reaction proceeds. This increases almost exponentially the oxidation cost.
Therefore, the advantage of the present disclosure is to keep the oxidation cost proportional to the mass of contaminant to oxidize.
A further advantage of the present process is that organochlorides are not likely produced due to the neutral-to-basic reaction p1-I. An alkaline reagent, preferentially sodium hydroxide, may be added to the slurry if initial acidic conditions prevail prior to the oxidation treatment. The alkaline reagent may also be added to the slurry at the end of the treatment to ensure a neutral to basic soil p1-I.
A further advantage of the present process may also be to free the heavy metal contamination from the natural soil organic matter. As a result, this facilitates the subsequent heavy metal decontamination of the resulting soil.
The process of the present disclosure is effective in the ex-situ remediation of contaminated soils.
Fig. 2 schematically illustrates how this disclosure may be incorporated in a soil decontamination process: soil excavation (1), particle size segregation (2), preparation of the oxidative solution (3), oxidation reaction (4), hypochlorous acid recovery (5), soil dewatering (6) and regeneration of the solution (7). The oxidant regeneration is preferentially performed on the residual salt (sodium chloride) recovered in the spent solution or performed on a fresh sodium chloride supply.
Soil excavation (section 1) First, standard analysis (Centre d'expertise et d'analyse environnementale du Quebec (CEAEQ), methods MA.200-Met1.2 (heavy metals), MA.400-HAP1.1 (HAP), MA.400-1-IYD.1.1 (C10-050)) determine the contamination type and level in a contaminated land. Then, the soil on this land is excavated and loaded on trucks to a treatment centre.
The treatment center can be a permanent centre or a mobile centre. The soil is unloaded from the trucks on a dumping pad awaiting for treatment.
Particle size segre2ation (section 2) The excavated contaminated soil on the dumping pad contains large uncontaminated debris and rocks. A trommel with 1-to-5 inch openings separates these debris and rocks from the soil because they are uncontaminated material and inconvenient to carry in the reaction process.
These debris and rocks are discarded.
At the exit of the trommcl, the soil is once again stacked awaiting for treatment. At this point, the soil contains maximum 1 to 5-inch soil particles. However, contamination is distributed in the fine soil particles. The fine particles are the soil particles with an average diameter less than
2.5 mm, according to the standard analysis protocols previously listed.
Optionally, the soil could be segregated once again in a different trommel to remove the coarser soil fraction. The decision would depend on the soil particle distribution (e.g. a high gravel proportion). This coarser fraction is not inconvenient for the reaction.
Preparation of the oxidative solution (section 3) The oxidative solution comprises 0.1 wt% to 15 wt% of one or a mixture of the following components: sodium hypochlorite, lithium hypochlorite or calcium hypochlorite and is prepared prior to the soil oxidation. The balance is tap water. A covered reservoir can be used to stock this solution. Regeneration of the solution could use the same reservoir.
Oxidation reaction (section 4) A loader carries the soil from the dumping pad to the soil oxidation set-up (Fig. 3). The design of this set-up rests on the principle that partial soil fluidization causes the soil and solution to form a slurry in which the reaction proceeds. The set-up comprises a reaction section (8), a solution recirculation section (9) and a solution distribution section (10) which are all three located below the ground level. The paragraphs below describes the three sections of the set-up.
The reaction section comprises a 15 slope (11) and a soil pile (12). The slope allows the loader to stack the soil in a soil pile. The loader stacks about 200 tons of soil in a rectangular pile whose height is maximum 2-meter. The depth of the reaction section is maximum 6 meters. A
removable wall (13) isolates the soil pile from the slope.
The solution recirculation section (9) shares the same liquid level with the reaction section (8).
This allows the fluidized soil particles to slow down. Therefore, the circulating solution carries only the -finest soil particles (clay and silt) and the heavier soil particles (fine sand) loop in the reaction section. The section is designed to minimize the required solution-to-soil ratio while ensuring an efficient soil mixing.
-)0 The solution distribution section (10) is beneath the reaction section. The section comprises perforated pipe (14) buried in 0-3/4-inch gravel. The number of perforations shown on Fig. 3 is for illustration purposes only. The section distributes the solution flow rate as uniformly as possible at the entrance of the reaction section. The uniform solution flux dictates the achievable level of mixing in the reaction section.
The oxidation reaction takes place when the solution contacts the soil.
To increase the oxidation reaction rate, carbon dioxide is injected as flue gases or as a pure gas in the solution recirculation section (9). This carbon dioxide reacts with the hypochlorite salts to lower the p1-1 from 10 to 12 down to p1-1 6 to 10. Consequently, the hypochlorite oxidation potential is increased due to the formation of hypochlorous acid.
The oxidation reaction time can last from 30 min up to several hours but preferentially less than 2 hours. The oxidation reaction operates batchwise, e.g. one pile of soil at a time. At the end of the reaction, sodium hydroxide raises the slurry pH to p1-1 10 to 12 to neutralize all remaining hypochlorite odor.
Hypochlorous acid recovery (section 5) Hypochlorous acid is a vapor and can be entrained out of the slurry by bubbling carbon dioxide or flue gases. This active oxidation component may be recovered. This recovery can be performed, for example, in a scrubber in which the gases exiting the soil column contact with a pH 6 to 12 caustic solution. This neutralization reaction recovers hypochlorous acid as a sodium hypochlorite that can recycle back to the soil oxidation set-up.
Soil dewatering (section 6) Once the oxidation is complete, the solution circulation stops. The slurry decants in the reaction section (8). Two phases form, the aqueous form on top comprising most of the spent solution and a soil phase at the bottom. The soil phase has to be dewatered.
The spent solution is pumped to the covered reservoir. The spent solution in the aqueous phase on top is pumped from the top. The soil phase which contains part of the spent solution is drained from the perforated pipes in the solution distribution section. To do this, the pump is either inverted or, preferentially, the spent solution in the reaction section is syphoned through the perforated pipes. A sump pit collects the drained spent solution (15). The soil phase is dewatered sufficiently to be shoveled out of the reaction section with a loader.
Regeneration of the solution (section 7) As described in the previous steps, the spent solution is collected in the supernatant and syphoned through the perforated pipes. The spent solution is pumped to the covered reservoir.
Two options are possible to regenerate the oxidative solution.
The first option is to use a membrane to recycle the water and discard a concentrated salt solution. The recycled water is fed with fresh hypochlorite and the solution is regenerated. The fresh hypochlorite is either bought or generated in-situ using an electrochemical cell and a fresh chloride salt supply. This is the less preferred option.
The second option is to regenerate the entire spent solution in an electrochemical cell.
Ilypochlorite. preferentially sodium hypochlorite, can be regenerated from its corresponding chloride salt. In the case of sodium hypochlorite, the corresponding salt is sodium chloride.
Sodium chloride is the oxidation by-product of sodium hypochlorite. In this option, the regeneration can also be performed in the oxidation set-up while the oxidation proceeds. This is the most preferred option.
The examples illustrate the typical performance of the disclosure and provide a better understanding of the disclosure.
Example 1 The procedure was initiated with filling a bucket with about 15 kg of a real Cl 0-050 contaminated calcareous soil. The soil came from the dumping pad in a soil treatment center.
The soil was identified Soil B and was highly contaminated (Fig. 1). Soil B
contained a low carbonate content. Sun exposition dried the soil for over a week to facilitate the dry sifting of the soil. A 2.5-mm sieve sifted the soil to recover the fines. The fines were mostly sand with a low organic content (3 wt% hexane extractible content). Analysis (ICP-MS, hexane extraction, GC-MS) quantified the contamination and soil elemental composition on a homogenized fines lot. These analysis determined the initial contamination level. The reaction in the example used only these fines. Each reaction used 25 g of the fines from the homogenized lot.
For each reaction, 25 g of soil fines were put in a glass autoclave. A 20-psi CO2 overhead prevailed in the autoclave for the CO2-assisted experiments. Each reaction used 50 ml of oxidative solution with a 3-wt% oxidant content. The experiments compared the oxidants NaC10. KMn04 and I-1707. Each oxidant was reagent grade and was bought from Sigma-Aldrich. The reactions lasted from 2 h up to 10 h.
At the end of the reaction, filtration with a Whatman #1 paper then recovered the solids from the spent solution. The filtration cake was washed and the spent solution was discarded. The cake was air-dried at ambient temperature and its hydrocarbon content analysed according to procedure MA.400-HYD.1.1.
The performance parameters are the oxidation yield (Y) and the specific oxidant dosage expressed as g dosed oxidant : g oxidized C 1 0-050 (D). For soil B, NaCIO
provided the lowest specific oxidant consumption D compared to KMn04 and H202 (Fig. 1). NaCIO kept its better oxidation yield with varying reaction time or double reaction treatment. H202 was particularly I 0 inefficient even though the soil did not contain a high carbonate content. It is well known in the art that carbonates consume uselessly the peroxide free radicals.
Example 2 15 The procedure was initiated with filling a bucket with about 15 kg of a real C 10-050 contaminated calcareous soil. The soil came from the dumping pad in a soil treatment center.
The soil was identified Soil A. The soil preparation prior to the oxidation reaction was conducted as described in example 1. The fines were mostly fine sand and clay with a high organic content (5 wt% hexane extractible content). The reaction in the example used only these 20 fines. Each reaction used 25 g of the fines from the homogenized lot.
For each reaction, 25 g of soil fines were put in a glass autoclave. A 20-psi CO2 overhead prevailed in the autoclave for the CO,)-assisted experiments. Each reaction used 50 ml of oxidative solution but the NaCIO content varied from 2.5 wt% to 7.0 wt%. The NaCIO supply 25 was reagent-grade I0-15wt% NaCIO solution bought from Sigma-Aldrich.
Each reaction lasted 2 h.
At the end of the reaction, filtration with a Whatman #1 paper then recovered the solids from the spent solution. The filtration cake was not washed and the spent solution was discarded. The 30 cake was air-dried at ambient temperature and its hydrocarbon content analysed according to procedure MA.400-HYD.1.1.
For this soil, the oxidation yield correlated linearly with the oxidant content in the solution for the CO2-assisted reactions (Fig. 4). Consequently, the specific oxidant dosage (D) was constant.
Example 3 The procedure initiates with a real HAP and heavy-metal contaminated calcareous soil. The soil was identified Soil C. The soil preparation prior to the oxidation reaction was conducted as described in example 1. The fines were mostly clay with a high organic content (5 wt% hexane extractible content). In this example, each reaction used 50 g of the fines from the homogenized lot.
For each reaction, 50 g of soil fines were put in a PVC bench-scale fluidized bed with a similar hydrodynamic design than the oxidation set-up shown on Fig. 3. In this design, 150 ml of solution ensured a homogeneous mixing in the reaction zone.
The oxidation reaction used 1-wt% or 3-wt% NaCIO solutions. The NaCIO supply was a commercial maintenance product (brand: La Parisienne). CO2 bubbled at 10-20 ml/min in the slurry to lower the reaction pH to 8 before the reaction begins. The p11 lowered to a value from 7 to 8 over the reaction time. The recirculation flow rate was set to maintain a uniform mixing in the slurry. Each reaction lasted 2 h.
At the end of each reaction, filtration with a Whatman N paper recovered the solids from the spent solution. The filtration cake was washed and the spent solution was discarded. At the very end of the experiment, a Naafi solution raised the slurry pH to 11 to neutralize any remaining hypochlorous acid. The cake was air-dried at ambient temperature and its hydrocarbon content analysed according to procedure MA.400-HAP 1. 1.
Part of the dried cake (25 g) underwent heavy metal extraction. This was to evaluate the contribution of the oxidation treatment on the heavy metal extraction yield.
The extraction treatment consisted in 3 successive washes with 50 ml of a 2-wt%
nitrilotriacetic acid solution (NTA) at 25 C. Each chelation reaction lasted 2 h. At the end of each reaction, filtration with a Whatman #1 paper recovered the solids from the leachate. The filtration cake was washed and the leachate and wash water were analyzed for heavy metal content by ICP-MS
For soil C. the oxidation yield correlated with the oxidant concentration in the oxidation process with NaCIO (Table I). The PAH oxidation yield is the overall oxidation yield of all listed PAH
and is indicative of the yield for each PAH. NaCIO can form sodium complexes with the soil organic matter. These complexes adsorb on the soil surface and may limit the oxidation extent.
Washing away these complexes is a technique that may be used in such case. The oxidation protocol also favors the heavy metal extraction by chelation in a subsequent soil treatment.
Table 1. Enhanced remediation of soil C heavy metal and PAH content by NaCIO-oxidation.
Soil remediation treatmentNTA recovery oxidation Cu Pb Zn PAH
SOM oxidation Heavy metal cake wash Wash 1 Wash 2 Wash 3 extraction oxidation extraction
3 x 1 g NTA /
70% 65% 65% 40% 0%
cake 2 x 1.3 g (3 wt%) 3 x 1 g NTA /
NaCIO-CO, / 25 g no 25% 0%
cake soil C
2 x 1.3 g (3 wt%) 3 x 1 g NTA!
NaCIO-CO, /25 g yes 87% 84% 84% 40% 43%
cake soil C
3 x 0.8 (1 wt%) NaCIO-CO, /25 g no 0% 0%
3 x 0.5 g (1 wt%) NaCIO-CO, /25 g yes 0% 0% 0% 0% 22%