WO2000062949A1

WO2000062949A1 - Treatment of polluted or contaminated substrates

Info

Publication number: WO2000062949A1
Application number: PCT/GB2000/001480
Authority: WO
Inventors: James Donald Hillcoat; Clive John Hillcoat; Mark James Hillcoat
Original assignee: UPPERFORCE Ltd
Current assignee: UPPERFORCE Ltd
Priority date: 1999-04-15
Filing date: 2000-04-17
Publication date: 2000-10-26
Anticipated expiration: 2001-10-15
Also published as: GB9908632D0

Abstract

A method of treating a polluted or contaminated substrate comprising creating a plurality of treatment columns in the substrate, each treatment column having the form of a permeable matrix bearing a silicic acid composition and a solution of an alkaline metal salt, the size, location and spacing of the columns being selected such that leachates in the substrate contact a matrix in their natural migration through the substrate.

Description

TREATMENT OF POLLUTED OR CONTAMINATED SUBSTRATES

This invention relates to the treatment of substrates contaminated or polluted by leachates derived from industrial wastes or residues. Such substrates are commonly found in "brown field" sites, particularly those where industry has previously been active.

Pollution or contamination is commonly caused by spillage of raw materials or bi-products into the ground or lagoons. Typical residues are the metal salts of arsenic, cyanide, chromium (in various forms) , potassium, lead, cadmium and zinc, and many sulphates and chlorides which can be particularly damaging. These metal salts together with rainfall and carbonic acids occurring naturally in soil produce solutions which become toxic leachates. Such leachates percolate through the ground and may enter water courses and can contaminate land over much greater areas than those of original sites where the contamination originated.

The manner in which alkaline salts and silicates can stabilize toxic metal salts is well documented in the literature. Reference is directed in this respect to O92/21409 and 093/16797; EP-A-088, 587; US-A-3 , 837 , 872 ; US-A-4, 952,242; US-A-5 , 037, 479 ; Soviet Patent Publication No. 1,583,405; JP 53 086364, JP 56 018682; JP 01 132318; DE-3,517,645 and GB-2, 277, 515.

Various soil stabilising techniques are known and reference is directed in this respect to WO97/20012 and US-A-5, 122, 012. It is also known that silica compounds can be used to stabilise metal sulphates in soil, and reference is directed in this respect to O93/11203. EP-A-0902077 discloses a method is described of stabilising soluble sulphates in substratum materials using a modified silicate solution. We have found that sites can be effectively treated by creating in the substrate a plurality of treatment columns which contact leachates in their natural migration through the substrate. According to one aspect of the present invention there is provided a method of treating a polluted or contaminated substrate comprising creating a plurality of treatment columns in the substrate, each treatment column having the form of a permeable matrix bearing a silicic acid composition and a solution of an alkaline metal salt: the size, location and spacing of the columns being selected such that leachates in the substrate contact a matrix in their natural migration through the substrate. The invention effectively provides reservoirs of silicic acid positioned in the path of leachates in their natural migration through the substrate. Thus, as the leachates move around and within the site, they make contact with an react with the treatment materials in the columns to form inert or non-reactive products, eventually rendering the site non-hazardous.

It will be appreciated the size, location and spacing of the treatment columns will be selected for each particular site. Typically, a treatment column will have a diameter in the range of 200 to 500mm with a preferred minimum diameter being around 300mm. While the columns can be of considerable depth, this will not normally exceed 16m. They can be set at a wide variety of spacings, typically from as close as 0.5m to 5m. The columns would normally be created in a vertical orientation, but they could be inclined. This enables them to extend beneath existing building structures without or with minimal interference in their foundations.

The respective treatment columns can be created in a number of ways. One preferred technique uses the material of the substrate itself, fractured to form the permeable matrix. In one such technique, a pilot hole is first drilled for each column, and compressed air then delivered to the pilot hole to fracture the substrate therearound. Alternatively a hydro fracture treatment may be employed using water under pressure. The treatment materials can then be delivered to the pilot hole, whence they dissipate into the permeable matrix comprising the fractured substrate to form the treatment column.

In an alternative technique, a hole for the entire treatment column is excavated and filled with a neutral material to form the matrix. The treatment materials are either mixed with the matrix material before filling or applied thereafter and remain captivated in the matrix. Typical neutral materials are sand, mica, starch, cellulose or waste paper bi-products, polystyrene grains or granules, or mixtures thereof.

In either technique it is normally desirable to flush the hole with limewater, preferably under pressure. The reaction of silicic acid with the contaminant metals requires a pH of >6.5 and most brown field sites tend to be more acidic. Pumping limewater or other alkaline metal salt solution under pressure into the drill hole will flood an area beyond the drill altering the pH of the substrate. The introduction of limewater or other alkaline metal salt solution in this manner can form part of a deliberate hydrofracture technique to form a permeable matrix in the substrate itself.

Thereafter silicic acid solution may be pumped into the drill hole/excavation hole to react with the limewater and contaminant metal salts to convert the latter to stable insoluble silicate form. Thus within the vicinity of the bore hole/excavation hole are immediately stabilised. The silicic acid solution may be in the form of a modified silicic acid solution as disclosed in EP-A-0902077. Such a modified alkali silicic acid generally has a Me₂ Oi : Si 0₂ ratio of 1 : 2 or 1 : 3. In the general formula Me₂0 for the oxide part of the solution Me is a metal selected from lithium, sodium and potassium. The latter is preferred in the mixture with sodium.

A typical composition as disclosed in EP-A-09020777 would contain a 20kg of solution:

Silicic acid (calculated as Si 0₂) 30 to 32 moles Alkali metal ions (calculated as Na⁺) 23 to 24.3 moles C-_ - C₃ alcohol (calculated as ethanol) 2 to 2.1 moles Carbonate (calculated as C0₃ ^2") 1.6 to 2.1 moles Water calculated as at least 400 moles The viscosity of the solution is generally 10 to 12 in Pa. Sec. Ethanol is the preferred alcohol.

An example of such a composition is commercially available under the trade designation ISO 110K.

Although the above described modified silicic acid solution provides beneficial results surprisingly, better results were achieved with lower concentrations of the silicic acid solution. Specifically, while satisfactory results were achieved with concentrations of the above defined modified silicic acid in the solution at around

20%, at least as good results were achieved when the concentration was reduced to 10% or lower, and indeed some improvement was found, with equally good results being achieved with concentrations of 5%, in some tests concentration levels of the order of 1% have been equally effective and in other tests the efficacity against many metal contaminants is retained at concentrations of as low as 0.10% While we cannot sure of the reason for this, it is believed that the more dilute solutions permeate through the substrate more effectively than the higher concentrations . The present invention applies the technique more generally using the material of the substrate itself as the permeable matrix. Particularly with the low concentrations of a silicic acid in the solution, it appears that the dilute solution itself migrates within the substrate to contact and react with the leachates to form stable compounds from active contaminants.

After introduction of the silicic acid solution finely composed amorphous silicic acid, preferably with at least 90% by weight Si0₂ or finely composed precipitate active silicates of magnesium calcium or aluminium may be introduced into the column to provide a reservoir of active material which will be contacted by leachates doing their natural migration through the substrate. Such silicic acids have a BET surface of 50 to 200 m²/g and d_50% value in accordance with a grain analysis of Cilas of below 20μm. The finely composed amorphous silicic acid is generally introduced into the drill hole/excavation hole and compacted under pressure. It may be achieved with neutral matrix materials such as sand, mica, starch, cellulose or waste paper bi-products, polystyrene grains or granules, permeable concrete mix etc. Alternatively, the silicic acid may be preformed into a shaped article for introduction into the hole e.g. in a similar manner to plasterboard .

In sites of extreme contamination, one or more of the treatment columns may need replacement or replenishment with treatment material. In such sites, treatment columns formed in excavated holes are preferred as they are more readily removable. In some circumstances, a treatment column can be effectively enclosed within a permeable sleeve to facilitate its removal and replacement. It will be appreciated that the treatment of the invention may need to be conducted over prolonged periods of time e.g. several years. It is desirable to have monitoring stations at various points on a site to sample leachate and test for contaminants and determine whether any of the treatment columns need replenishment. Replenishment of treatment columns may involve removal and replacement if they are in areas which are readily accessible. Alternatively, when the original columns are installed ducting or pipework may be laid which allows additional treatment material to be pumped under pressure into the columns .

The present invention has been described with reference particularly to industrial pollutants or contaminants in "brown field" sites. It will be appreciated, though, that the invention is equally applicable to the treatment of sites polluted or contaminated in other ways, by using different treatment materials in the columns. Particularly, it can be effective in cleansing or purification systems for the treatment of organic pollutants.

We have found the modified silicic acid solutions described above can be useful not only in stabilising soluble sulphates in substratum materials beneath concrete, but also to stabilise other metal salt contaminants in open soil .

According to a second aspect of the present invention, a modified silicic acid solution is introduced into an open substrate to stabilise metal salt into contaminants therein. The modified silicic acid solution is preferably used at a concentration of less than 5%, preferably less than 1% as described above. In one embodiment the open substrate may comprise contaminated soil which has been excavated and stored. Dilute modified silicic acid may be used to treat the contaminated soil to stabilise the contaminants e.g. by a continuous irrigation system which applies the solution to the soil allowing it to percolate through the soil and recirculating the solution with the addition of fresh solution as necessasry.

The invention will be described with reference to the following Example. Example

In the following Example, a control solution comprising a number of single element ICP spectroscopic standards of known concentrations was prepared. The analysis of the control solution is as follows:

Two treatment solutions were prepared, each containing a proportion of control water, a volume of 1 : 3 ISO 110, and a solution containing alkaline metal ions. The solutions have the following composition: Solution 1

Control water 1 litre

Saturated Calcium Oxide Solution 950ml 1 : 3 ISO 100 solution 50ml

Solution 2

Control water 500ml

25g/l Calcium Chloride Solution 474ml 1 : 3 ISO 110 solution 25ml

Each of the above solutions was added to a prepared solution of control water, and after a period of approximately four hours, precipitates were filtered off and the filtrate analysed for the component metal ions using direct ICP-OES or hydride generation. The results of the analysis are set out in the following Table:

Contaminant Concentration (Solution 1) (Solution 2) as μ/Litre

Arsenic 110 35 22

Cadmium 12 less than 5 less than 5

Chromium 50 less than 10 less than 10

Copper 160 less than 10 less than 10

Lead 150 10 less than 10

Nickel 160 less than 10 less than 10

Zinc 1700 less than 10 less than 10

A number of ISO 110K solutions were prepared from concentrations, ranging from 1% to 20% The same tests were than repeated, each solution being added to a solution of control water and calcium ions as before. In each test to one litre of control water was added 950ml of saturated calcium oxide solution and 50ml of ISO 110K solution. In seven separate experiments, seven different concentrations of ISO 110K solution were used 0.10%, 0.15%, 0.5%, 1%, 5%, 10% and 20% Each sample of treated control water was then analysed after standing for four hours. Results are reported on the following Table.

The results clearly demonstrate the effectiveness of the ISO 110K solution even at concentrations below 1%

In addition to analysing the filtrate in the above experiments, the precipitate was also analysed to determine its pH value, which was found to be 11.4 in each case. The precipitate seemed relatively stable, but tests were made to determine the extent of its stability by lowering its pH value. Stability was maintained until the pH value dropped below 7.

Consideration was also given to the effect of the treatment described above on a control solution in which water is contaminated with 100 mg/1 of mercury ions. The treatment resulted in the reduction of active mercury from a level of 100,000 μg/1 to 21,000.

Claims

CLAIMS :

1. A method of treating a polluted or contaminated substrate comprising creating a plurality of treatment columns in the substrate, each treatment column having the form of a permeable matrix bearing a silicic acid composition and a solution of an alkaline metal salt: the size, location and spacing of the columns being selected such that leachates in the substrate contact a matrix in their natural migration through the substrate.

2. A method according to Claim 1 wherein each treatment column has a diameter in the range 20 to 500 mm, a depth of no more than 16m and the treatment columns are set at a spacing in the range 0.5 to 5.0 metres.

3. A method according to Claim 1 or Claim 2 wherein a treatment column is formed by drilling under pressure a pilot hole and compressed air or aqueous liquid is delivered to the pilot hole to fracture the substrate around the pilot hole, the fractured substrate forming the permeable matrix.

4. A method as claimed in Claim 1 or Claim 2 wherein a hole is excavated for each treatment column, and filled with neutral material to form the matrix.

5. A method as claimed in Claim 4 wherein the neutral material is selected from sand, mica, starch, cellulose or waste paper bi-products, polystyrene grains or granules and porous concrete .

6. A method as claimed in any one of Claims 3 to 5 in which an aqueous solution of an alkaline metal salt is introduced under pressure into the pilot hole or excavation prior to introduction of silicic acid composition.

7. A method as claimed in Claim 6 in which a solution of silicic acid is introduced under pressure into the pilot hole or excavation after said alkaline metal salt solution.

8. A method as claimed in Claim 7 in which said solution of silicic acid is obtained by providing a composition comprising: silicic acid (calculated as Si 0₂) 30 to 32 moles alkali metal ions (calculated as Na⁺) 23 to 24.3 moles C - C₃ alcohol (calculated as ethanol) 2 to 2.1 moles carbonate (calculated as C0₃ ^2~) 1.6 to 2.1 moles water calculated as at least 400 moles in 10kg of solution which solution is used directly or diluted.

9. A method as claimed in any preceding Claim in which finely divided amorphous silicic acid is introduced into the column and compacted under pressure.

10. A method as claimed in any preceding Claim wherein the treatment columns are created with their axis inclined to the vertical .

11. A method as claimed in Claim 10 wherein the treatment columns extend obliquely to the vertical, and under an existing building.

12. A method as claimed in any preceding Claim wherein each treatment column is surrounded by a permeable sleeve.

13. A method as claimed in any preceding Claim comprising the additional step of sampling and analysing the leachate from the site over a period of time and replenishing the silicic acid composition in one or more of the columns when the toxic levels of the leachate rise.

14. A method of treating substrates containing metal salt contaminants including salts other than sulphates in which method modified silicic acid solution is introduced into the substrate to stabilise said metal salt contaminants.

15. A method as claimed in Claim 14 in which said modified silicic acid is obtained by providing a composition comprising: silicic acid (calculated as Si 0₂) 30 to 32 moles alkali metal ions (calculated as Na⁺) 23 to 24.3 moles C-_L - C₃ alcohol (calculated as ethanol) 2 to 2.1 moles carbonate (calculated as C0₃ ^2~) 1.6 to 2.1 moles water calculated as at least 400 moles in 10kg of solution which solution is used directly or diluted.

16. A method as claimed in Claim 15 in which said solution is diluted to a concentration of less than 5%

17. A method as claimed in Claim 15 in which said solution is diluted to a concentration of less than 1%.

18. A method as claimed in any one of Claims 14 to 18 including the step of adding alkaline metal ions to the substrate prior to introducing said modified silicic acid solution.

19. A method as claimed in Claims 14 to 19 in which the substrate is contaminated soil which has been excavated and stored.

20. A method as claimed in Claim 19 in which the modified silicic acid solution is applied by a continuous irrigation system which recirculates the modified silicic acid solution.