Soil decontamination
December 1st 2006

In situ soil decontamination is a difficult to achieve without any adverse effect on safety and ecological suitability. Simon Tillotson discusses a new approach

Stabilisation and solidification techniques are an effective approach to the treatment of soils contaminated with metals.

Conventional approaches use cement, lime, pulverised fuel ash (PFA), ground granulated blastfurnace slag (GGBS) or thermoplastics, which, although they can be effective, all have significant draw-backs in terms of their safety and ecological suitability. New solutions are now available which remove many of the problems associated with conventional approaches.

Virotec's ViroSoil Technology is one such solution. It incorporates a range of toxicologically safe reagent formulations (ViroBind reagents) suited to the permanent fixing of metals (including arsenic) effectively immobilising any metals present to the reagent surface. In this manner metals are permanently immobilised reducing leachability of metals to <0.1mg/kg and concentrations in soil leachates to low part per billion concentrations.ViroBind reagent also neutralises acidity in an extensive area within and below the application zone.The reagent is itself highly insoluble; removed contaminants cannot subsequently be leached and are not bioavailable.

A new approach – ViroSoil Technology combines plant and equipment, project know-how, and a suite of reagents to provide a total solution to soil contamination problems.ViroBind reagent is chemically benign and ecologically safe. Appearing as a fine red powder, the reagent has a high surface areato- mass ratio to provide its excellent sorption characteristics. Physical and chemical reactions immobilise pollutants to the reagent surface, removing them from the contaminated soil by out-competing soil sorption sites and strongly retaining the contaminants.

Soils treated with ViroBind reagent significantly reduce metal concentrations in leachate within a time-scale of days; metals are nonleachable even under acidic (ca.pH 3) conditions.ViroBind reagent has high acid neutralisation properties (neutralisation capacity of up to 14 moles/kg), and also good sulphate sorption characteristics.

How is it different? – ViroSoil Technology solutions offer many advantages over the conventional alternatives.These include: Outperforming conventional liming both in how rapidly leachate contamination is reduced, and in the extent and longevity of acid neutralisation. Superior improvement in soil pH after ViroSoil Technology solutions remain evident after five years, where neutralisation in the lime treated soils had deteriorated again after 18 months.

ViroBind reagent is safe to use, in contrast to health and safety concerns when using hydrated lime, cement, PFA, GGBS or thermoplasticbased treatments.Application of the ViroBind reagent is operationally simple,being most effective when it is intimately mixed with the material to be treated as a dry powder (Figure 1). Spray irrigation with reagent slurries has also proved successful.

Treated soils will be classified as inert or non-hazardous waste in the event that its disposal is necessary.

Alternative formulations of Virotec technology, each with unique physical characteristics to suit specific environmental situations, can be applied.

Soil conditions do not impair treatment using ViroSoil Technology solutions. Inorganic ViroBind reagent does not alter soil oxygenation (as PFA can), and its efficacy is not impaired by organic contaminants,or by aluminium and iron salts in the soil.No hydration is necessary (a serious issue for quicklime treatment in certain soils), and treatment with ViroBind reagent does not generate the heat and gases that are problematic when using liming agents in acid soils.

Toxicological studies and field experience have shown that ViroSoil Technology reagents have no adverse ecological impacts. Soils and sediments treated using ViroSoil Technology are rapidly re-colonised by flora (Figure 2), and growing waters overlying treated sediments are ecologically improved, as shown at a shellfish farm (Brisbane,Australia), resulting in ten-fold increased yields of highly sensitive prawns.The longterm capability of treated soils to recover ecologically is of particular concern for some conventional treatment approaches (notably PFA, GGBS and thermoplastics).

With proven success in contaminated soil and leachate/groundwater treatment, ViroSoil Technology offers an alternative total solution approach to in situ land decontamination that is efficient, cost effective, safe and convenient to use.