New technique for isolation of very dangerous chemical waste

Citation

(2003), "New technique for isolation of very dangerous chemical waste", Pigment & Resin Technology, Vol. 32 No. 2. https://doi.org/10.1108/prt.2003.12932bab.009

New technique for isolation of very dangerous chemical waste

New technique for isolation of very dangerous chemical waste

Keywords: Hazardous materials, Chemicals, Waste

A meeting was recently held in Southampton of the partners in the project LowRiskDT who are working to find a safe method of disposing of Toxic Waste. The project, partly funded by the EU Framework V programme particularly focussed on the concept of disposing of toxic waste in abandoned mines.

Very recently, the Swedish government decided that all mercury-containing waste in Sweden that cannot be reprocessed shall be deposited deep down in the rock. This is not a matter of concealing things but a way to make use of the very slow groundwater flow at depth and to create very long transport paths for mercury that can be released from i.a. batteries to levels where drinking water can be extracted.

The decision of demanding “geological” disposal of mercury waste is logical since the Swedish Environmental Protection Agency has carried out several studies and has shown that this option offers very good opportunities to isolate this and other types of toxic waste very effectively. This option is also favoured by the European Commission, which finances a comprehensive project for investigating how abandoned mines can be used for final disposal of chemical waste. This project is conducted by five partners, i.e. the Swedish company Geodevelopment AB (also co-ordinating the project) and the German enterprise DURTEC GmbH, which are both engaged in advanced research and development in several geoscientific fields, the British company Computational Mechanics, which is famous for its expertise in numerical calculation, and the technical universities Wessex Technical Institute in Southampton (WIT), UK, and the Mining department of the National Technical University of Athens (NTUA), Greece. WIT has a high reputation in developing numerical codes and techniques for calculating all sorts of flow, stress and strain, and NTUA has unique competence and experience in mining techniques and waste handling.

Parallel to the mine disposal project, complementary studies are being performed to develop techniques for preparing waste packages of highly compacted clay powder in which the waste objects are embedded. The Research Council of the Swedish company Ångpanneföreningen sponsors this study which is made by the research staff of Geodevelopment AB that has extensive experience from designing engineered barriers in repositories for highly radioactive waste in co-operation with the Swedish companies ÅF-Energikonsult AB and Tekedo AB. It is in fact a good example of how efforts in one technological field can provide solutions to other areas and how this can lead to application of very advanced industrial techniques, like compaction of powder under very high pressures. This technique was worked out by the Swedish company ASEA (now ABB) several decades ago for manufacturing artificial diamonds.

The compacted clay packages form a very effective engineered barrier. This is because of the extremely low hydraulic conductivity and low content of movable porewater which mean that the average percolation rate down in the backfilled mine is only about 0.01 mm per year and that other migration processes like diffusion are extremely slow as well. The dense clay, which will have smectite minerals as major constituent, retards the transport of hazardous elements also by adsorbing them through ion exchange mechanisms and provides pH conditions that strongly reduce dissolution and release of such elements from the embedded waste.

The clay material in the confined compacted blocks has a density after saturation and expansion of about 2,000 kg/m³. This yields a hydraulic conductivity of E-14 to E-12 m/s depending on the porewater chemistry and it is less than 1/1,000 of that of the confining rock. Under the hydraulic gradient situation that prevails after restoring the groundwater pressure conditions the amount of water that is transported through a waste block with a volume of 1 m³ is less than 0.1 milliliter per year. Not until after about 10,000 years the block has been permeated by 1l of water.

Diffusion of dissolved matter in ionic form is therefore the most important transport mechanism but also this sort of migration is very slow. Thus, an ionic concentration of 10 per cent of the maximum concentration in the waste elements will not develop at the boundaries of the waste packages until after 1,000 years and further transportation into and through the surrounding rock, which will take place by flow in fractures, will also be very slow because the porosity of the rock mass is usually lower than 1-2 per cent. Therefore, the concentration of toxic elements like Hg in groundwater used for drinking purposes will be acceptably low even after tens of thousands of years. In fact, chemical complexation will preserve the condition of insignificant contamination of such water for any period of time provided that comprehensive rock disturbance by seismic events will not take place. The selection of suitable disposal sites should therefore be made with respect to tectonics.

The longevity of clay materials expressed in per cent of degraded clay minerals is more than 90 in the pH and temperature ranges in ordinary rock down to several hundred meters. According to recent geochemical modelling, the dominant part of the clay mineral content of suitable smectitic clays will be intact for at least one million years.

For further information, visit the Web site at: www.beasy.com