CA2176864C - Membrane-assisted leaching process and apparatus for the removal of metals from soil - Google Patents

Abstract

The present invention relates to a process and apparatus to effectively and efficiently remove metals from contaminated soil wherein a reactor and membrane extraction unit are utilized. The use of the subject system provides improved metal removal from soil over a batch process.

Description

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Membrane-Assisted Leaching Process and Apparatus for the Removal of Metals from Soil FIELD OF THE INVENTION
The present invention relates to a process and apparatus to effectively and efficiently remove metals from contaminated soil wherein a reactor and membrane extraction unit are utilized. The use of the subject system provides improved metal removal from soil over a batch process.
BACKGROUND OF THE INVENTION
For many years now, soils near the earth's surface have been used as a dumping ground for society's residual chemicals with little concern of the consequences. Although these activities have been greatly reduced through government regulation, the damage that has already been done cannot be ignored. In 1991, in the U. S. alone, the National Priority List contained 498 sites eligible for remediation funding. Of these, more than 60 percent identified heavy metals as the principal contaminant of concern (Yarlagadda, P.S., et al. 1995. "Characteristics of Heavy Metals in Contaminated Soils."
Journal of Environmental Engineering. 121(4): 276-285.).
Heavy metals can be described as those that are used or discharged by industrial enterprises or used by humans in various ways (Schalscha, E.B.
1989. "Heavy Metal Movement in Irrigated Soil. " Encyclopedia of Environmental Control Technology. Volume 3 - Wastewater Treatment Technology. Gulf Publishing Company. Houston, Texas: 543-555) including Cd, Cr, Co, Fe, Hg, Mn, Mo, Pb and Zn. All are toxic at high concentrations and some, including lead, are toxic at very low concentrations.
Heavy metals can reach toxic levels in soils due to natural processes and more importantly from human activities. These sources can be categorized as:
1. irrigation with non-treated industrial wastewater;

2. disposal of water-treatment plant sludges on land; and, 3 . dumping of wastes and refuse (Schalscha, 1989) .
The interactions between soil and heavy metals, as well as the Z176~~~

movement of heavy metals through soil, has been the focus of many studies over the past two decades. Nevertheless, these processes are still not well understood. Many different types of soil/heavy metal interactions have been studied. Difficulties arise in understanding these processes due to the simultaneous existence of more than one type of interaction.
The principal cause of heavy metal movement in soils is advection of ground water. Molecular diffusion is significant only if ground water movement is extremely slow. For soils of medium and high permeabilities such as silts, sands and gravels, the effect of molecular diffusion is very small.
For very low permeability clays such as clay liners used in modern landfills, molecular diffusion can become the dominant transport mechanism.
An inevitable consequence of advection in soil is a decrease in downstream contaminant concentrations and a subsequent increase in the volume of the contaminated area. This phenomenon will be referred to as dissipation.
Mechanisms responsible for dissipation are molecular diffusion and dispersion.
Molecular diffusion contributes to dissipation independently of advection, but is only significant when advection is small.
The tortuous paths taken by ground water through the soil medium causes turbulence and mixing. This is referred to as dispersion and is usually a significant contributor to dissipation.
Retardation and attenuation are two similar processes that affect the fate of contaminants in the subsurface.
Retardation slows the transport of contaminants as a result of a reversible interaction between the soil and the contaminant. This interaction removes portions of the contaminant from the aqueous solution and can take several forms. Precipitation and adsorption are~two such forms.
Precipitation is a well understood chemical process and can be modelled using solubility product expressions. Heavy metals will precipitate with hydrous oxides, carbonates, sulfides, phosphates and silicates that may be present in the soil or ground water (Schalscha, 1989). It is important to note that the solubility of heavy metals is strongly related to pH. Precipitates can be immobilized due to the filtration ability of soil and this causes retardation (LaGrega, M.D., Buckingham, P.L., and Evans, J.C. 1994. Hazardous Waste Management, McGraw-Hill, Inc.).
Processes that cause heavy metals to sorb onto the surface of soil particles include ion exchange, van der Waals and other electrostatic forces. Ion exchange of metallic ions with soil is a partially reversible process, in that saturated ion exchange sites may release cations in response to either a decrease in the concentration of cations or a change in pH. Thus, ion exchange is considered a retardation mechanism.
Attenuation reduces the concentration of aqueous phase contaminants in the plume by an irreversible biological or chemical reaction. Important attenuation mechanisms are chemical and biological oxidation-reduction reactions (LaGrega et al. , 1994) .
Studies have shown that contaminants sometimes bind preferentially to the finer fraction of soil (Griffiths, R.A. 1995. "Soil Washing Technology and Practice." Journal ofHazardous Materials. 40(2): 175-189).
Existing methods for heavy metal removal from soil include those that can be carried out in-situ and those that require the contaminated soil to be excavated and then treated. One conventional method of remediation is to excavate and landfill the contaminated soil. This method does not actually treat, but simply relocates the contaminated soil. In-situ methods might be a cost effective alternative, which reduce land disturbance. Other excavation methods require the soil to be treated, and the treated soil can then be replaced.
Some of the potential in-situ methods include those that immobilize the metals by precipitation and those that solubilize and remove metals from the system. The methods that solubilize the contaminants do not require long term monitoring because the metals are removed in the process. One method, which uses solubilization is soil flushing. The methods that bind the metals to the soil will require long term monitoring because the metals may be remobilised by environmental conditions and/or chemicals. One principle of immobilization of the metals is stabilization.
This method is similar to a process called "solution mining" used in the mining industry (Sabatini, D.A., and Knox, R.C. 1992. Transport and Remediation of Subsicrface Contaminants. ACS Symposium Series American Chemical Society). An extraction fluid is applied to an undisturbed soil and is 2i 76864 allowed to pass through the soil layer. The metals are solubilized from the soil matrix into the liquid phase of the extracting fluid. The extracting fluid carries the metals downward in the soil to a drainage or collection system. The advantage of this method is that large areas of land can be treated while being left relatively undisturbed. The disadvantages are: a) in that soil is anisotropic and non homogeneous, it results in incomplete exposure of soil to the extracting fluid, and therefore not all of the soil is treated, b) the potential for ground water contamination exists if not all extracting fluid is collected, c) the difficulty in having acceptable quality control quality assurance, and d) the extracting fluid may cause soils to swell or plug the aquifer (Sabatini, et al., 1992) .
Stabilization uses reagents to minimize the rate of migration into the environment and to reduce the toxicity of the contaminant (Lagrega, et al. , 1994). Fixation refers to the use of additives to improve handling and physical characteristics of waste, to decrease surface area over which mass transfer can occur, to limit solubility of metals and to reduce toxicity of the contaminant.
Solidification uses a solidifying material to increase the strength and decrease the compressibility and permeability of the contaminated soil (Lagrega, et al., 1994). The specific reagent chosen for a site depends on its ability to precipitate the contaminant. Some examples of stabilization methods include vitrification, and the use of cement and other chemicals.
Vitrification is another in-situ method used to stop the heavy metals from migrating further in the soil phase. Vitrification reduces the soil volume by 20-40% of its original size and produces an inert soil structure (O'Brien &
Gere Engineers, Inc. 1995. Innovative Engineering Technologies for Hazardous Waste Remediation. Van Nostrand Reinhold.). The process simultaneously reduces the volume, mobility and toxicity of the waste.
The use of cement is well suited for inorganic wastes such as heavy metals (Lagrega, et al . , 1994) . The most common cement employed is "Portland"
cement which is a mixture of calcium, silicate, aluminum and iron oxides.
Extract and treat techniques include extracting the contaminated soil and then treating it by different processes. After treatment the cleaned soil can be replaced and the contaminated portion can be land filled. In these processes the bulk of the heavy metals are completely removed from the system and therefore no long-term monitoring is required. One example of this method is _ 5 soil washing.
Soil washing is divided into two classes. One of these classes is referred to as the fines separation technique (Sabatini, et al., 1992). This process is based on the principle that the heavy metals are associated with the fine particles of the soil. In this process the clay and silt particles, the fines, are scrubbed from the soil, using a water based solution (Griffiths, 1995). The advantages of this process are that the volume of contaminated soil to be treated is greatly reduced and the quality control/quality assurance is excellent (Sabatini, et al. , 1992). The disadvantages are that the soil is not completely remediated, the mixing requires energy, and the equipment requires personnel with a large level of expertise (Sabatini, D.A., and Knox, R.C. 1992. Transport and Remediation of Subsurface Contaminants. ACS Symposium Series American Chemical Society).
The second class is a newer, related technology of the first class. This process is similar to vat leaching used in the mining industry (Sabatini, et al., 1992). The soil is extracted from the contaminated site and placed in an agitation vessel with an extracting solution. Extracting solutions can be acids, bases, chelating agents, alcohols or others. The principle for this process is that the metals will transfer from the soil to the extracting agent, as long as a concentration gradient exists. The contaminant must then be removed from the liquid phase. This process has good quality control/quality assurance but is also energy intensive and requires high level of operating skills (Sabatini, et al., 1992). This process is well suited for sandy soils while other types of soil may present chemical and physical problems.
As discussed above, there are many types of interactions that can bind metals to soil particles and it is likely that several types of soil metal interactions exist in the contaminated soil used in this experiment. Only some interactions are reversible by adjusting the pH of the extracting agent. For example, saturated ion exchange sites may release cations in response to a drop in pH. Also, precipitated metals may dissolve when the pH lowered.
Although the metal binding and release mechanisms that occur in a soil-acid slurry are likely extremely complex, they are assumed to behave as an equilibrium-partitioning process.
When a finite volume of acid is mixed with a batch of soil, an equilibrium state will be reached before all of the teachable lead is removed 2~~~86 from the solid phase, further limiting the removal of lead. However, the concentration gradient can be restored by removing metal from the slurry.
Thus, in consideration of the above methods of cleaning soils, there has been a need for a system which maintains a high concentration gradient between the contaminant and leaching agent to improve the system efficiency.
Membranes are thin films through which certain substances can pass and are vital components of filtration systems. Today, membranes are made from a wide variety of materials including polymers, ceramics, metals and even linings of animal and vegetable bodies. A useful application of filtering systems is to separate solids from liquids. With this application, membranes can be energy efficient since no phase change is required (Raycheba, J.M.T.
1990. Membranes Technology Reference Guide. Ontario Hydro). In the past, however, membranes may be easily or readily fouled when used with very fine particles to the extent that a complete loss in permeability results.
Accordingly, in the past, the use of membranes with fine particles has been rejected.
Filtration systems are designed as either dead-end or cross-flow. In dead-end filtration, the feed flow direction is perpendicular to the membrane surface and there is only one output stream; the permeate. In cross-flow filtration, the feed flows parallel to the membrane surface and there are two output streams;
the permeate and the concentrate. Cross-flow filtration reduces membrane fouling (plugging of the membrane pores) (Raycheba, 1990). In cross-flow Filtration, the linear speed is the average fluid velocity parallel to the membrane surface.
Tubular modules are extremely simple membrane configurations that employ cross-flow filtration. These are relatively easily cleaned when fouling occurs. Fouling is a problem because it leads to increased maintenance and operating costs. Fouling can be reduced by increasing linear speed but this increases operating costs.
Accordingly, in view of the above, there has been a need for a simple and effective process and apparatus to effect metal removal from soils.
Specifically, there has been a need for a process and apparatus utilizing a membrane unit in which acceptable permeation rates are obtained without fouling the membrane.

A review of the prior art reveals that, in the past, membranes have not been used to effect liquid/solid separation in the treatment of contaminated soils.
SUNINIARY OF THE INVENTION
In accordance with the invention, an apparatus for removing metals from soil is provided, the apparatus comprising:
a reactor for containing a slurry of leaching agent and soil;
a membrane separation unit in fluid communication with the reactor, the membrane unit having a semi-permeable membrane permitting the permeation of leachate therethrough;
means for circulating the slurry between the extractor and membrane separation unit.
In alternate embodiments of the invention, the membrane separation unit is preferably a polymeric or a ceramic cross flow filtration unit and the means for circulating includes a peristaltic pump and means for regulating pressure within the membrane separation unit. In a preferred form, the apparatus includes means for adding leaching agent to the reactor.
In another aspect of the invention, a process for removing metals from soil is provided comprising the steps of:
a. forming a soil/leaching agent slurry in a reactor;
b. circulating the soil/leaching agent slurry between the reactor and a membrane separation unit to effect separation of leachate from the soil;
wherein a constant volume of soil/leaching agent is maintained in the reactor through the addition of fresh leaching agent.
The invention contemplates the treatment of soils contaminated with a variety of metals wherein the metals may include any one of or a combination of cadmium, chromium, cobalt, iron, mercury, manganese, molydinum, lead or zinc. Still further, the leaching agent may be selected from any one, but is not limited to, acid, base, chelating agent or other suitable solvent or solvent s systems.
In the specific case of treating lead contaminated soil, the preferred leaching agent is hydrochloric acid and the pH in the reactor is 1.
Still further, it is preferred that the soil/leaching agent ratio (weight/volume) is approximately 1:10 and that the pressure within the membrane separation unit is between 2 and 20 psi.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be more apparent from the following description in which reference is made to the appended drawings wherein:
Figure 1 is a schematic view of the apparatus in accordance with the invention;
Figure 2 is a plot of the lead concentration in the leachate as a function of time comparing the batch and diafiltration processes;
Figure 3 is a plot of the percent lead removal as a function of time comparing the batch and diafiltration processes;
Figure 4 is a plot of the percent chromium removal as a function of time comparing the batch and diafiltration processes.

2~~6864 _ 9 DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, an apparatus and process is herein described for the removal of metals from soils. As described above, the leaching process transfers metals from the solid to liquid phase of a slurry, thereby removing the metals from the soil, this transfer being driven by the concentration gradient of the metal between the solid and liquid phases. A
comparison of the effectiveness of a membrane separation process (diafiltration) with a batch extraction process was investigated.
In the batch experiment, the soil and acid will come to equilibrium at which point no further mass transfer will occur. The diafiltration process overcomes this limitation by maintaining a high concentration gradient to achieve a cleaner soil by continuously adding fresh acid to the slurry, removing leachate and adding fresh leaching agent to maintain a constant volume. The addition of leaching agent reduces the concentration of metal in the liquid phase, therefore keeping the concentration gradient high. Various metals suitable for leaching include but are not limited to cadmium, chromium, cobalt, iron, mercury, manganese, molydinum, lead or zinc. The apparatus 100 of the diafiltration process is shown in Figure 1.
With reference to Figure 1, the apparatus 100 includes a reactor 1 in fluid communication with a membrane unit 3. Pump 11 is provided to maintain fluid flow from the reactor 1 to the membrane unit 3 and from the membrane unit 3 back to the reactor 1. Pressure valve 6 is used to control the system pressure and is monitored by pressure gauge 5. The reactor 1 is provided with stirring device 2 to maintain a soil/liquid slurry. Fresh leaching agent 9 is pumped to the reactor 1 by pump 4 to maintain a constant reactor volume to compensate for leachate removal 10 from the membrane unit 3.
The membrane unit 3 is preferably a tubular membrane unit such as an ENKA''M (Enka, Dusseldorf, Germany) 0.2 ~,m unit (filtration surface area-0.036 m2). This membrane unit has a braided polypropylene support with a polypropylene microfiltration membrane. A ceramic MEMBR.ALOX'~ (Alcoa Separation Technology Inc.) is also suitable. This membrane unit has an average pore diameter of 0.05 ,um and filtration surface area of 0.0044 m2.
The MEMBRALOX ultrafilter elements have an asymmetric ceramic structure composed of zirconium oxide. .
At the start of an experiment, a batch of clean leaching agent and contaminated soil are added to the reactor 1. Constant stirring is required to to maintain a suspended slurry. The peristaltic pump 11 draws slurry la out of the reactor 1 and passes it through the tubular membrane unit 3. The pressure on the membrane determines the quantity of leaching agent removed as leachate 10 wherein the higher the pressure, the higher the rate of permeation of leachate.
A soil sample, contaminated with lead, was obtained from a location outside Montreal, Quebec. A sieve analysis was conducted to determine the particle size distribution of the sample. The results of the sieve analysis are presented in Table 1. As stated previously, the lead tends to bind to the fines of the soil. Accordingly, an initial digestion was conducted on the different particle sizes to determine their respective lead concentrations. The results of the metal concentration as shown in Table 2 show that the concentration of lead increased as the particle size decreased.
For experimental purposes, a soil sample with an adequate lead concentration was required so that the removal of lead could be readily monitored. From the results of the digestion of the different particle sizes, fines passing an 80 mesh sieve were selected as containing a satisfactory amount of lead for testing. The soil was prepared by removing as much rock as possible as well as any other debris. The remaining soil was passed through a rock crusher to reduce an adequate amount of soil to the proper size.
Batch Experiments Batch experiments were conducted as a basis of comparison for the diafiltration process. These tests were done by placing 100 g of soil with 900 ml solution concentrated of hydrochloric acid (--- lOM) at a pH of 1 (~0.1) in a beaker. The pH of the slurry was maintained manually by adding strong hydrochloric acid at each sampling time. The mixture was continuously stirred and duplicate samples were taken using a syringe at approximate time intervals of 5 min, 15 min, 30 min, 1 hr, 2 hrs, 5 hrs, 8 hrs and 24 hrs. The samples were centrifuged to separate the acid and soil. The acid was decanted from the soil mass to a vial. As a small amount of acid still remained with the soil, leaching was thought to be still occurring. Accordingly, distilled water was added to this mixture and stirred to stop or at least reduce further leaching.
This new mixture was centrifuged to separate the soil and water. The water was decanted and the soil was removed and dried in an oven at 105°C
overnight. The soil was then digested and the liquid sample was analysed using flame atomic absorption spectroscopy to determine respective concentrations of the contaminant.

,;:
f 2 ~ 76864 Diafiltration Experiments The diafiltration experiments were conducted by placing 100 g of contaminated soil with 900m1 of hydrochloric acid at pH 1 into the reactor.
The mixture was continuously stirred to maintain a suspended slurry. As with the batch experiments, the pH rose slightly between the sampling times. The pH was monitored and maintained at 1 by adding a solution of hydrochloric acid at each sampling time using pump 4. Samples were taken at the same times and in the same fashion as the batch experiments. A number of runs were conducted at a linear speed of 1.7 - 3.7 m/s and at a pressure between 2 and 8 psi.. The change in pressure altered the permeate flow and therefore the hydraulic retention time (HRT). Hydraulic retention time is average fluid residence time in the system and is calculated on the basis of the system volume and permeate flow rate.
Test results reveal that in case of batch leaching, the concentration of leached metals grows steadily until it reaches an equilibrium between metal concentrations in the liquid and solid phases (Figure 2). In same experiments, a decrease in the metal concentration in the liquid phase was observed after several hours of leaching. This phenomenon should be attributed to the resorption of metal ions onto soil particles. In diafiltration tests, metal concentration first increased then decreased due to the removal of metals with the permeate. This greatly reduced chances for resorption.
In light of the above, the incorporation of a membrane into the leaching process is advantageous. When slurry is pumped through the module, heavy metal ions are continuously removed from the system; therefore, their accumulation is largely eliminated. Since the metal concentration in the aqueous phase remains at a relatively low level, the driving force of the membrane-assisted leaching (MASL) is higher than one of the batch leaching.
Figures 3 and 4 illustrate the concentration of metals remaining in the soil, as a function of time, for the batch and diafiltration modes. For both the lead and chromium, the initial rate of metal removal was substantially higher in case of diafiltration. After six hours of extraction, soil treated with MASL
had only 29 % of initial chromium and 30 % of initial lead, compared to 39 and 48 % achieved in the batch process.
As indicated previously, the concentration gradient is the driving force for the transfer of lead from soil to acid. The larger the concentration __ 21 T 686 gradient, the faster the rate of mass transfer. A large gradient can be maintained by keeping the concentration of lead in the liquid phase low. This was accomplished by lower HRTs and it was therefore hypothesized that those diafiltration experiments run at lower HRTs will have better removal of lead.
A table of the percent of lead removed versus time for a batch experiment and three diafiltration experiments run at different HRTs is presented in Table 3.
As predicted, the shortest HRT of 28.51 hours had the best removal of lead of approximately 80 % . The next shortest HRT had a removal of almost 70 while the longest HRT and batch had the lowest removals of about 40 % after 24 hours.
The terms and expressions which have been employed in this specification are used as terms of description and not of limitations, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claims.

~.1'~~8~4 Table 1- Sieve Analysis of Contaminated Soil Sieve Size Mass Retained (g) % Pass 4 70.3 ~ 95.56382912 16 602 57.57556635 40 566.4 21.83378557 80 203.8 8.973307251 120 45.4 ~ 6.108411687 200 33.7 3.981826213 P~ 63.1 S~ = 1 S 84.7 Table 2 - Metal Concentration of Different Particle Sizes Sieve Size Concentration (mg/g) 200-pan 33.3 200-pan 29.9 200 to 80 2g,g 200 to 80 19 80 to 120 18.5 80 to 120 14 40 to 80 40 to 80 g 16 to 40 16 to 40 14 ,S
Table 3 Remaining Lead Time (hours) Batch processDiafiltration ' HRT = 28.5 HRT = 40.9 HRT = 50'.5 0,25 ?2 64 55 70 0.5 70 52 53 65 1.0 61 55 SI 64~

2,0 40 60 47 63 4:0 55 41 45 57 6.0 56 SO 40 42 Note: Initial average lead concentration = 28 mg/g, pH =1, Pressure a 8 psi.

Claims (18)

1. An apparatus for removing metals from soil comprising:
a reactor for containing a slurry of leaching agent and soil;
a membrane separation unit in fluid communication with the reactor, the membrane unit having a semi-permeable membrane permitting the permeation of leachate therethrough;
means for circulating the slurry between the reactor and the membrane separation unit

2. The apparatus as in claim 1 wherein the membrane separation unit is a ceramic cross flow filtration unit.

3. The apparatus as in claim 1 wherein the means for circulating includes a peristaltic pump and means for regulating pressure within rise membrane separation unit.

4. The apparatus as in claim 1 further comprising means for adding the leaching agent to the reactor.

5. The apparatus as in claim 2 wherein the means for circulating includes a peristaltic pump and means for regulating pressure within the membrane separation unit.

6. An apparatus as in claim 5 further comprising means for adding leaching agent to the reactor.

7. A process for removing metals from soil comprising the steps of:
a. forming a slurry of soil and a leaching agent is a reactor;
b. circulating the slurry between the reactor and a membrane separation unit to effect separation of leachate from the soil;
wherein a constant volume of the slurry is maintained in the reactor through the addition of fresh leaching agent.

8. The process as in Claim 7 wherein the metals may include any one of or a combination of cadmium, chromium, cobalt, iron, mercury, manganese, molybdenum, lead or zinc.

9. The process as in claim 7 wherein the leaching agent is selected from any one of a strong acid, strong base, chelating agent or alcohol.

10. The process as in claim 7 wherein the leaching agent is hydrochloric acid and the pH in the reactor is 1.

11. The process as in claim 7 wherein the slurry formed within the reactor in step (a) has a soil:leaching agent ratio (weight:volume) of approximately 1:10.

12. The process as in claim 7 wherein the pressure within the membrane separation unit is between 2 and 20 psi.

13. The process as in claim 7 wherein the soil particle size passes an 80 mesh sleve.

14. The process as in claim 8 wherein the leaching agent is selected from any one of a strong acid, strong base, chelating agent or alcohol.

15. The process as in claim 14 wherein the leaching agent is hydrochloric acid and the pH in the reactor is 1.

16. The process as in claim 15 wherein the slurry formed within the reactor in stop (a) has a soil;leaching agent ratio (weight:volume) of approximately 1:10.

17. The process as in claim 16 wherein the pressure within the membrane separation unit is between 2 and 20 psi.

18. The process as in claim 7 wherein the soil particle size passes an 80 mesh sieve.