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A Fast Ion Technique

A promising line of research in Denmark suggests heavy metal ion removal from soils could be drastically speeded up with a new processing technique, reports Paul Grad

Under laboratory conditions, heavy metals can be removed from contaminated soils in a matter of weeks, instead of the many months it can take using traditional methods.

The faster process is being developed by a team from the Technical University of Denmark, at Lyngby, led by associate professor Lisbeth M Ottosen, and at the Universidad Técnica Federico Santa Maria, Valparaiso, Chile.

It has already shown surprisingly good results.

Depending on the type of treated soil, the team achieved removal percentages for lead of between 85% and 92% in two to three weeks.

Removal of up to 96% of copper was achieved in about 70 days.

Chrome has been harder to remove and the team achieved only 18% removal after about 20 days.

The method being used is a novel combination of the traditional electrokinetic soil remediation method with conventional electrodialysis.

Most remediation work previously, including much of the earlier research by the team, has been carried out electrokinetically, insitu with the electrodes placed in electrode wells directly into the contaminated soil.

In dialysis, however, soil is treated in suspension and ion exchange membranes separate the soil suspension from the processing solutions at the electrodes.

In dialysis, membranes ensure that the main direction for electromigration within the soil is out of the soil.

To investigate a combination method the team used cylindrical cells made of polymethyl methacrylate, usually called Plexiglass.

There is a central compartment and two electrode compartments.

EDR_Cell

An anion exchange membrane separates one electrode compartment from the central compartment, and a cation exchange membrane separates the central compartment from the other electrode compartment.

Platinum coated electrodes came from Sweden’s Permascand.

During the process the soil is kept suspended by constant stirring with a plastic flap attached to a glass stick and connected to an overhead stirrer motor.

In the traditional electrokinetic method, the fraction of heavy metals present in ionic form is removed from contaminated soils by electromigration, where the ions are made to flow in the pores of a moist porous material by an applied electric field.

The process causes the ions to concentrate around the electrode of opposite polarity.

At the electrodes, the current carried by ions is transferred to current carried by electrons in the metallic electrodes.

Oxidation processes occur at the anode and reduction processes occur at the cathode. In some cases electro-osmosis occurs - that is, the transport of water in the porous medium under the action of the applied electric field.

Electrolysis of water is often a major process at both anode and cathode.

Thus there is a process of acidification at the anode whereas the material becomes alkaline at the cathode.

Next to electrolytic transport, several other processes occur within the porous material during the application of the electric field: pH changes, ion exchange, redox changes, adsorption/desorption, precipitation/dissolution, mineral degradation, and structural changes.

There can also be complexation, whereby ions, particularly calcium and magnesium, form chemical groupings.

All these processes can limit the electrokinetic extraction of contaminants.

the water content in the medium can vary significantly, even over short distances.

On the other hand, those processes can, if properly used, be turned to good effect to optimise the extraction of contaminants.

In most cases an acid is used at the cathode to hinder alkalinisation of the soil because that causes the heavy metals to precipitate and become immobile for electromigration.

On the other hand, the acidic front developing from the anode is often used for desorption and mobilisation of the metals.

One drawback of electrokinetic processes is that only free ions in the pore water are directly mobile for electromigration.

Release of the bound contaminants is necessary for successful removal.

This can mean strong acidification or use of complexing agents is often necessary to mobilise the heavy metals for electromigration.

Even then, because the ions are not in a free solution, but a porous material, they cannot move by the shortest route to the opposite pole.

Instead, they must find their way around the particles or air-filled voids blocking their direct path.

The electromigration rate of ions in porous media depends on the pore volume, geometry and water content.

In addtion the water content in the medium can vary significantly, even over short distances.

Thus the electric field can be expected to vary as well, because the water content affects the resistivity of the material.

The electric field distribution in those porous materials with varying water content has a significant bearing on the overall success of the electrokinetic contaminant removal.

The applied electric field causes matrix changes which can be physicochemical and hydrological.

Some solid phases are weathered and new ones can be formed.

rensningafjord_1_

OLYMPUS DIGITAL CAMERA

For soils, it is difficult to mobilise the heavy metals by acidification without changing the matrix.

This suggests using complexing agents which attack the soil less.

For example, in a copper-polluted calcareous soil, the addition of ammonia leads to the formation of Cu(NH3)2+ complexes and this copper can electromigrate even in an alkaline soil.

The main reason for adding ammonia is to prevent dissolution of carbonates which would be highly acid demanding and increased electric current would be wasted in removing Ca2+ instead of Cu2+.

The pH level at which desorption starts depends strongly on the type of soil treated.

Usually, in the team’s experience with various types of soil, as the pH decreased, desorption started with zinc, followed by copper and then lead.

The relationship between pH and the start of desorption was most significant for lead.

In one soil type, less than 10% of the lead was desorbed at pH of 2.5, and in another soil type, 60% of lead was desorbed.

The team verified that copper, lead and zinc started to desorb at a higher pH from calcareous soils than from soils with low carbonate content.

This is due to co-precipitation of heavy metals in the carbonates.

Thus, the crucial factor in desorption of heavy metals is the medium’s pH.

The team verified that copper, lead and zinc started to desorb at a higher pH from calcareous soils than from soils with low carbonate content.

Soil pH is the single parameter which most influences the remediation result.

In stationary setups, the acidification of the soil occurs gradually from the anode toward the cathode end. In the stirred setup, all of the soil volume is acidified at the same time.

An advantage of the electrodialytic remediation is that it can be combined with soil washing.

In this case the clean coarser fraction and the highly polluted fine fraction are separated during the soil washing and the electrodialytic treatment is used only for remediation of the fine fraction.

Another advantage is fast distribution of enhancement solution.

Increased understanding of electrokinetics in homogeneous matrices at pore level could help optimise the remediation process.

The experimental work is continuing on the process and has not yet been developed commercially, but that is the intent of the university teams.

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