Evaluation of quicklime mixing for the remediation of petroleum contaminated soils
Introduction
The recent implementation of the Landfill Directive (Council Directive 1999/31/EEC) in the UK has substantially increased the costs for disposal of contaminated soils. On-site remediation or pre-treatment prior to disposal may offer a more viable and economical alternative to direct disposal of contaminated soil.
Solidification/stabilization with addition of cement-based binders is an established treatment technology for soils contaminated with inorganic compounds [1] whereas a relatively limited experience exists for its applications to organic compounds, in particular petroleum hydrocarbons.
The mixing of contaminated soils with calcium oxide (quicklime) promotes a number of reactions in the soil/porewater including flocculation, ion exchange, lime carbonation, dissolution of clay minerals and pozzolanic reactions [2], [3], [4], [5], [6], [7], [8]. A cation linking mechanism, in which Ca ions link clay particles through their negative edges, or binding of the clay particles by the silicate hydrates gels produced by the pozzolanic reactions may result in formation of clay macroaggregates [9], [6], [10]. The macro-pores of these aggregates may encapsulate large volumes of porewater.
The hydration reaction of lime in the porewater is highly exothermic and generates significant amounts of heat energy, which results in evaporation of porewater and drying of soil [11].
As a result of the above reactions, lime mixing, brings about significant time-dependent changes in soil physico-chemical properties such as pH, cation exchange capacity [8], [12], [13] specific surface area [5] and Atterberg limits [9], [14], [15].
The effects of the above reactions may be useful to remediate soils contaminated with organic compounds such as petroleum hydrocarbons, whereby the more volatile compounds may be removed from the soils primarily by volatilization during the CaO exothermic reaction and the less mobile compounds may be immobilised by encapsulation in the clay/pozzolanic products aggregates. Furthermore, introduction of oxygen into the soil together with the heat generated during the quicklime mixing may lead to destruction of the hydrocarbon compounds [16]. Therefore, the mixing of soils contaminated with petroleum hydrocarbons with quicklime appears to be a promising remediation as well as pre-disposal treatment technique for soils contaminated with organic compounds.
A relatively limited number of studies exist in the literature, to the authors’ knowledge, about quicklime remediation of soils contaminated with organic compounds. A laboratory study on the treatment of PCB contaminated soils with calcium oxide has been carried out by USEPA [17]. Temperatures of 180–200 °C were measured in the soils after mixing them with quicklime. PCB removal was mostly attributed to dusting, vaporization and steam stripping and about 7% of removal attributed to chemical dechlorination and destruction. Marion and Payne [18] and Marion et al. [19] evaluated the mixing of quicklime for remediating soils contaminated with pesticides and petroleum hydrocarbons. The soils were mixed with quicklime in large on-site treatment units. The quicklime was pre-treated using natural fatty acid to make it hydrophobic and oleophilic. The authors observed a major decrease in the concentration of many hydrocarbons. They explained the removal of volatile BTEX constituents as due to volatilization and that of higher molecular weight constituents as the result of encapsulation in a Ca(OH)2 matrix. Dean [20] mixed quicklime to treat about 10,000 cubic yards of deep clayey soils. Trichloroethene and 1,1,1-trichloroethane soil concentrations were reduced to below the leachability soil cleanup target levels.
The aim of the present study was to assess the feasibility of quicklime mixing as a viable technique to remediate soils contaminated with low and high levels of petroleum hydrocarbon compounds and identify the different mechanisms responsible for reducing the concentration and the leachability of the compounds in soils. The study examined the effects of quicklime mixing on the concentrations of petroleum hydrocarbon compounds in two natural soils and a number of artificial sand/kaolinite mixtures. Several independent variables, such as clay content, mixing moisture content and quicklime content were considered in the study.
Section snippets
Materials
Natural samples of a clayey sandy silt of the London clay formation from Hampshire, UK and a clay of the Lower Coal Measures from Sheffield, UK and a number of artificial mixtures of sand and kaolinite were used in the experiments.
The silt and clay samples were taken from trial pits in disused petrol filling station sites. Due to the previous use of the sites, the soils were contaminated with petroleum hydrocarbon compounds. Composition, moisture content and Atterberg limits were determined on
Temperature measurements
Temperature measurements were carried out to evaluate the magnitude and rate of change of temperature upon quicklime mixing.
Plots of temperatures measured during the quicklime mixing of sand samples (S100)15-w5, (S100)30-w10, and (S100)45-w15 and clay samples SC5, SC10, and SC15 are presented in Fig. 1a and b. The temperature increased rapidly during the first few minutes of mixing, then reached a maximum value and eventually decreased gradually.
Temperatures were also measured in the
Discussion
Decreases in concentrations of petroleum hydrocarbon compounds were generally observed in soil samples contaminated with low (silt samples) and high levels (clay and sand/kaolinite samples) of petroleum hydrocarbon compounds treated with quicklime and in the leachates extracted from them. This observation can be explained as a result of removal of petroleum hydrocarbons from the soils upon quicklime mixing and/or a strong association between the petroleum hydrocarbon compounds and the solid
Conclusions
The study confirmed that quicklime mixing is a viable technique for remediation and pre-treatment prior to disposal of soils contaminated with either low or high levels of petroleum hydrocarbons.
Mixing soils with quicklime resulted in significant decreases in concentrations and leachability of petroleum hydrocarbon compounds in soils.
Large decreases in petroleum hydrocarbons concentrations occurred rapidly upon quicklime mixing. Smaller decreases may continue to take place progressively with
Acknowledgements
The authors whish to thank Dr. A.W.L. Dudeney, who supervised the work of Mr. N. Hadlow and Mr. R. Dudeney at Imperial College, ALcontrol Geochem (Chester, UK) for carrying out the chemical analyses and Total UK Ltd. the support provided.
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