Saving the earth
One way of cleaning up contaminated land is to stabilise and solidify pollutants. Simon Tillotson reports on his company's solutionThe stabilisation and solidification of pollutants is now recognised as a viable and cost-effective way of achieving contaminated land clean-up.
Regulators accept that these approaches to treatment can reliably prevent contaminants from reaching receptors by preventing leaching, a key pathway in contaminated land risk assessment. Stabilisation involves the prevention of pollutant migration away from the contamination source; solidification improves the physical condition of the treated area.
Readily available materials, including lime and cement, have traditionally been used to achieve stabilisation and solidification. But effective use of these materials comes with significant drawbacks. There are difficulties with their use under certain soil conditions. Health and safety concerns can arise from the use of the materials themselves. And there can be issues with the long-term effectiveness of treatment, and the capacity of treated areas to recover ecologically.
Alternative solutions are, though, becoming available which avoid many of the drawbacks associated with conventional solutions. Virotec's ViroSoil Technology is one such innovative solution. This technology permanently fixes metals (including arsenic) to the surface of its proprietary ViroBind reagents. The immobilisation of metal contaminants reduces their leachability to below 0.1mg/kg, resulting in metal concentrations in leachates at low part per billion concentrations. ViroBind reagent, unlike lime, is itself highly insoluble. This ensures that the immobilised contaminants are not subsequently leached by rainwater, and will remain non-bioavailable.
ViroSoil Technology solutions also effectively neutralise acid contamination directly in the application zone and extensively below this. And they are also highly effective at treating acid sulphate soils without acceleration of sulphide oxidation or the formation of gypsum.
This innovative approach to stabilisation remediates more effectively than conventional approaches as it permanently binds metals and neutralises current and future acidity. But it also increases the potential for soils to remain on-site, particularly if the receptors at risk are groundwater or surface water. If off-site disposal is required, treatment with ViroBind reagents has the potential to reclassify the soil to a less costly waste type based on minimising the leachability of metals.
ViroSoil combines project know-how, plant and equipment, and a range of reagents to provide a solution to soil, solid waste and sediment contamination problems.
ViroBind reagents have unique physical and chemical attributes, which remove dissolved phase metals from leachates, fix metals in soils, neutralise acidity and control odour.
Our reagents are chemically benign and ecologically safe, and that they possess excellent sorption properties and a high acid neutralisation capacity.
The reagent is a fine-grained red powder and has a high surface-area-to-mass ratio. Its mineralogy gives it a high surface-charge-to-mass ratio, giving rise to the reagent's sorptive capacity of about 1,500meq/kg for metals.
Soils respond rapidly to treatment. Dissolved metals in leachate are significantly reduced within hours, and binding efficiency continues to improve by 40-50% over six months, even under acidic (pH 3) soil conditions.
Retained phosphates are semi-permanently bound, and are re-released slowly, remaining bioavailable. The presence of other nutrients in the media, and the moisture-retention attributes of the reagent, mean that ViroSoil Technology can also be used for soil improvement.
It is operationally straightforward to employ the technology. The most effective way to treat contaminated soils or sediments is by mixing them with ViroBind reagent in the form of a dry powder.
Virotec has developed more than 150 reagent formulations, each with unique physical characteristics to ensure that the optimal solution for each specific situation can be chosen. Soil leachate and groundwater can be decontaminated by passive flow-through treatment using pelletised reagents placed in packed treatment columns. The highly porous, open structure of the pelletised reagents facilitates its use in large-scale leachate and groundwater treatment for metals by constructing permeable reactive barriers.
Although conventional stabilisation media can be effective, ViroSoil Technology offers several advantages for the stabilisation of metals in contaminated soils and sediments, and in the decontamination of soil leachate and groundwater.
The treatment is more effective than, for example, lime treatment. This was demonstrated in long-term trials at Mount Carrington Mine, in Australia, where these two approaches were directly compared. Metal decontamination in spoil leachate was more extensive and more rapidly affected using ViroSoil Technology, which also outperformed liming in acid neutralisation. Furthermore, spoil treated by the leachable lime deteriorated after 18 months. In contrast, improvements to land and leachate quality after ViroSoil Technology treatment are maintained after five years without the need for reapplication of treatment media.
ViroSoil Technology solutions are not adversely affected by soil conditions in the way that conventional alternatives can be. Organic soil contaminants do not impair the ionic sorption mechanism to the ViroBind reagent surface, nor is the reagent inhibited or deactivated by aluminium or iron salts.
ViroBind reagent is effective without requiring soil hydration. And treatment with this does not generate heat and gases, even in acid environments, which can cause problems in some locations when using exothermic lime treatment. ViroSoil Technology can be adopted across a range of soil conditions without impacting adversely on its performance characteristics.
Conventional stabilisation media can be operationally difficult to use due to health and safety concerns. In contrast to more hazardous substances, such as hydrated lime, PFA or many thermoplastic media, ViroBind reagent is safe to use. Quantities required are typically modest and formulations are easy to handle.
ViroSoil Technology is effective for the treatment of acidic (sulphidic) soils, dredgings and sediments. The neutralisation capacity of ViroBind reagents (up to 14 moles/kg), its high iron sorption characteristics, and the absence of hydrogen carbonate minimises the catalytic oxidation of sulphides, while coating of sulphidic minerals with sorbed components excludes oxygen further reducing acid generation.
Toxicological studies have identified no adverse ecological impacts from the reagents used in ViroSoil Technology. There are many examples where the successful ecological recovery of both flora and fauna has been observed in the field after treatment of contaminated soils and sediments using the product.
Virotec's approach, using the highly insoluble ViroBind reagent, ensures long-term metal immobilisation and biological unavailability, and represents an improvement over conventional liming and other stabilisation treatments (such as pulverised fuel ash or thermoplastics) with regard to the capacity for ecological recovery of the treated zone.
Even as soil conditions vary, for example, with seasonal changes in moisture content and temperature, immobilised contaminants remain fixed to the ViroBind reagent.
The Gladstone Port Authority, in Queensland, Australia, used the technology to treat 30,000m3 sulphidic and acidic marine dredge spoil. Prior to treatment, when disturbed and exposed to oxygen, the spoil generated large amounts of acidic leachate. After treatment, the acid generation potential of the spoil was rapidly (within 24 hours) reduced to only 0.02mole/kg. Ensuing soil improvements, using benign ViroBind reagent, allowed the rapid re-establishment of vegetation from formerly very poor conditions.
Fauna can also benefit from improved soil or sediment conditions, not being adversely affected by the reagent. At prawn farms near Brisbane, Australia, poor yields of this sensitive species were the result of contamination of overlying waters from iron, aluminium and sulphuric acid contamination in sediments. Successful treatment of the metals, sulphates and neutralisation of the acidity using ViroSoil Technology, which also improved the growth of the natural foods of the prawns, dramatically improved the survival rates and improved farm yields.
Stabilisation is a cost-effective approach to land remediation, increasingly accepted by regulators and engineers as the best in-situ solution in many cases.
Conventional approaches, while sometimes effective, can pose problems under certain soil conditions, and with safety concerns for engineers and the long-term scope for ecological recovery of the treated area.
Simon Tillotson is general manager at Virotec Europe. Call 01865 304060
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