Leachability of 17-year old stabilised/solidified contaminated site soils

摘要

Increased acceptance of in-situ stabilisation/solidification (S/S) depends on its long-term effectiveness and field performance at contaminated sites. Field trials of deep soil mixing application of S/S were performed at a contaminated site in West Drayton, near Heathrow Airport, in the UK back in 1996. Core samples were taken from this site at 2 months and 4.5 years and tested. This paper addresses the leaching properties of the contaminated site soils at 17 years in comparison to the properties at 0.24, 2.4 and 5 years. Seven different cement-based binders were used, including cement, PFA, lime and bentonite to the soils which were contaminated with a cocktail of heavy metals and organics. Two leaching methods: Toxicity Characteristic Leaching Procedure (TCLP) and the BS EN 12457 were applied to study the leaching behaviour of the heavy metals in stablised/solidified soils. The leaching test results showed that leachate pH decreased with time in the first 5 years and then leveled off at 17 years. The BS EN 12457 leachate concentrations were lower than drinking water standards, suggesting that the performance of the S/S at 17 years' service time is satisfactory. The leaching behaviour of the heavy metals in these S/S soils was mainly controlled by the alkalinity of the binders, with some of these heavy metals being absorbed into calcium silicate hydrates (C-S-H) gel phases. The binding mechanisms within the applied cementitious binders may be dominated by their leachate pH which correlated with their corresponding metal hydroxide solubility.