Process envelopes for and biodegradation within stabilised/solidified contaminated soils

Reginald Kogbara, Cambridge University
Geotechnical Engineering Group


Stabilisation/solidification (S/S), which usually employs the addition of cementitious binders to contaminated soils in order to immobilise the contaminants present, is an obvious solution for dealing with soil contamination. As there are a large number of variables governing the performance of S/S treated contaminated soils, the development of process envelopes, which define the limits of operating variables that result in acceptable performance, is ideal to facilitate an understanding of correlations between composition and leaching and mechanical properties. Furthermore, it is desirable to combine biodegradation, which generally entails the transformation and degradation of organic compounds through metabolic processes of microorganisms, with S/S to bring about some form of organic contaminant attenuation over time, and hence address uncertainties regarding the long-term effectiveness of the S/S treatment since conventional S/S treatment does not remove the contamination.

In the light of the above, this research investigated two aspects of S/S technology, namely, development of operating envelopes for the S/S process and facilitation of biodegradation within S/S treated soils, aimed at improving the robustness and sustainability of the technology. Extensive data was generated in a series of laboratory tests in order to develop process envelopes for S/S treatment of contaminated soils using a real site soil contaminated with a cocktail of heavy metals and petroleum hydrocarbons, treated by blended binder systems incorporating Portland cement (CEMI), pulverised fuel ash (PFA), hydrated lime (hlime) and ground granulated blastfurnace slag (GGBS). The specific binder formulations were: CEMI, CEMI:PFA = 1:4, CEMI:GGBS = 1:9 and hlime:GGBS = 1:4. The second aspect on S/S integrated with biodegradation investigated the applicability of two relatively low-pH magnesium phosphate cement (MPC) mixes to facilitate the biodegradation of an organic contaminant, namely 2-chlorobenzoic acid (2CBA), within a model silty sand contaminated with lead (Pb) and zinc (Zn). One of the mixes was at the low pH end, ~6.5, and the other at the high pH end, ~10, based on different combinations of the cement constituents. The performance of the S/S treatments was assessed using compaction, bulk density, unconfined compressive strength (UCS), permeability, porosity, leachate pH, acid neutralisation capacity (ANC) and pH-dependent leachability of contaminants, monolithic leaching, contaminant extraction and dehydrogenase activity tests.

The results of the process envelopes studies demonstrated that compaction around the optimum water content can be used to achieve the best possible balance between acceptable mechanical and leaching properties. The 28-day UCS of the S/S treated contaminated soils ranged from ~ 50 2,500 kPa and was in the order, CEMI > CEMI-GGBS > hlime-GGBS > CEMI-PFA. The permeability of the mixes was generally between 10-8 and 10-9 m/s in the order, CEMI < CEMI-GGBS CEMI-PFA < hlime-GGBS. Generally, CEMI and hlime-GGBS showed better immobilisation capacities for the more soluble metals (Cd, Ni and Zn), while CEMI-PFA and CEMI-GGBS was better for the more amphoteric metals (Cu and Pb). Speciation of metal contaminants was the same in all binder systems. The pH-dependent leachability of the metals decreased over time and CEMI-GGBS mixes showed a unique trend in this regard. There was no significant effect of water content on leachability. Generally, = 20% dosage of the binders is required to fully satisfy acceptable mechanical and leaching criteria. Overall, the process envelopes for different performance criteria depend on the end-use of the treated material.

Contaminant recovery analysis in the combined S/S and biodegradation studies indicated a similar reduction in 2CBA concentration to ~56% in the different grout mixes. The cement constituents exhibited stimulatory and inhibitory effects on soil dehydrogenase activity. Heavy metal leachability as well as the mechanical behaviour of the treated soils conformed to acceptable standards. Hence, the results show considerable promise for the application of MPC in contaminated land remediation.