Stabilisation and Solidification of Contaminated Soil and Waste Part 3:


Treatment of Inorganic Contaminants By Dr Collin Hills, Director, Centre for Contaminated Land Remediation, University of Greenwich


Scope


This is the third part of an occasional series of articles on stabilisation/solidification (s/s) technology. Previously, s/s has been introduced as a risk management strategy and the interaction of cement-based binders and soil has been discussed. Dr Colin Hills Director of the Centre for Contaminated Land Remediation at the University of Greenwich is joined in this article by Edward Bates, recently retired from the US Environmental Protection Agency, who was deeply involved in the application and regulation of s/s in that country for 30 years, and Dr Peter Gunning of Carbonate Systems Ltd. In this article, stabilisation/solidification (s/s) is presented in respect to the fixation of inorganic compounds in s/s waste forms.


Introduction The establishment of a stable chemical environment during s/s is achieved through the appropriate selection of binding agents, uniquely formulated for each application. Contaminants are immobilised by chemical processes including adsorption, hydrolysis and precipitation and are physically trapped by (the waste forms) low permeability, thereby isolating them from leaching solutions, such as groundwater or rainfall.


The effective use of s/s has been demonstrated with a range of inorganic contaminant groups, as shown in Table 1, produced by the Interstate Technology and Regulatory Council (ITRC, 2011). The s/s of organics is discussed further in the next article in this series. A thorough review of the literature on the use of s/s to immobilise contaminants is available (Environment Agency, 2004)


The majority of sites remediated by s/s under the Superfund program have been contaminated with metals or mixtures of metals and inorganic compounds, as shown in Figure 1. A useful update to source treatment technologies (including s/s) to 2008, for both in-situ and ex-situ remedial operations is given by the US EPA (2010). A recent examination of aged waste forms has demonstrated the efficacy of treatment for both organic and inorganic contaminants over time (PASSiFy, 2010).


The physical and chemical processes by which anionic and cationic contaminants interact with cement-based binders assists in the choice of treatment of waste and soil, although consideration of the contaminated matrix is needed if remedial targets are to be met and maintained.


The interaction of contaminants with soil and/or binder involves a number of ‘fixation’ mechanisms, which include:


• adsorption to binder-soil matrices; • pH-dependent precipitation; • redox-controlled precipitation of insoluble compounds;


• absorption/encapsulation into and onto nano-porous C-S-H gel; and


• incorporation into crystalline components of the cement matrix.


The interaction of metal contaminants and clayey soils involves cation exchange (see article 2) in accordance with the lyotropic series. This relationship is significantly influenced by a soils organic content, mineral composition, and the prevailing pH and Eh conditions. However, soil clay fraction/binder-relationship provides the long-term stabilising process primarily centred upon adsorption and metal precipitation.


The alkaline pore fluid of cement-based systems promotes the precipitation of insoluble compounds such as metal hydroxides, but the longer-term maintenance of pH is dependent on the impact of environmental loads experienced in service. As such, the long-term release of contaminants from treated soil or waste will eventually occur but should not present a significant risk.


Adsorption in binder-soil matrices Adsorption in binder-soil matrices is achieved at high pH when metals have a strong affinity to the surface of clays. However, as natural groundwater tends to be acidic (rather than alkaline), the long-term neutralising capacity of a binder system is important. For example, the work of Astrup et al. (2001, 2003) concluded that a particular s/s system was capable of performing (i.e. retaining a pH >9) for ca. 100,000 years, illustrating the resilience of s/s waste forms to withstand environmental loading.


pH-dependent precipitation Precipitation of salts from solution is pH dependent (see article 2). As well as facilitating the surface adsorption of metal ions, high concentration of OH- ions in solution can promote the formation of metal complexes and the precipitation of metal salts. Factors such as pH, Eh ion exchange capacity, and complexation/chelation with organic matter may directly affect metal solubility in cement-bound system.


|106| ENVIRONMENT INDUSTRY MAGAZINE


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