Remediation of soils, sediments and sludges by extraction with organic solvents

Remediation of contaminated soils, sediments and sludges by extraction with organic solvents is still in the initial stages of development. So far hardly any scientific research has been carried out into this approach. Therefore, the main objective of the present investigation was to study the effect of several process parameters on the efficiency of the extraction step of a solvent extraction process for soils, sediments and sludges (generally indicated as solids) contaminated with polycyclic aromatic hydrocarbons (PAHs). The process parameters of interest were: type of solvent, extraction time, extraction procedure, water content of the extraction solvent, type of solids, size of the solid particles, and water content of the solids.

In the development of a remediation method, a critical factor is the analysis of the PAH concentration in the solids. Therefore, several extraction methods were investigated to remove PAHs from an aged sandy soil, a harbour sediment and an extraction sludge. Extraction with N-methyl-2-pyrrolidinone in a microwave oven at 130°C for one hour was the most efficient method for the extraction of PAHs. The second best method was extraction with a mixture of 80 vol.% acetone and 20 vol.% water in a microwave oven at 100°C for one hour.

Acetone is suitable for use in a solvent extraction process, because it is less toxic than most other organic solvents, easily biodegradable, and socially acceptable in the Netherlands. In addition, it showed high removal efficiencies in the extraction of spiked pyrene and benzo[a]pyrene from sandy, silty and clayey soils, even when water was present in the soil or extraction agent. In the extraction of air-dried soil with mixtures of acetone and water, maximum efficiencies were reached with 10 to 20 vol.% water in the mixture.

To determine the extraction efficiency of a mixture of 80 vol.% acetone and 20 vol.% water, a procedure comprising seven extraction steps was carried out at 20°C using a rotary tumbler for mixing. In this way, an aged extraction sludge and an aged harbour sediment were remediated. After seven extractions, the Dutch target level established for clean soil was reached in the sludge and almost reached in the sediment. The PAH concentrations decreased from about 1,000 mg kg ^-1in the sludge and about 650 mg kg ^-1in the sediment to about 2 mg kg ^-1in both the sludge and the sediment. Of the PAHs studied, those of low molecular weight (phenanthrene, anthracene and fluoranthene) were found to be most difficult to remove from the solids.

Another important aspect of the extraction process is the rate at which PAHs are removed. Experiments revealed that about 90% of the PAHs concerned were desorbed from the aged sludge within 10 minutes of extraction with a mixture of 80 vol.% acetone and 20 vol.% water. Within 40 minutes, at least 95% of the PAHs concerned were removed from the sludge and the extraction was then assumed to be complete. The experimental desorption curves were fitted by means of a radial diffusion model and a first-order reaction model. The diffusion model fitted the curves best for a situation in which it is assumed that the PAH contamination is accumulated in the core of the sludge particles.

Because of the high extraction efficiencies and high desorption rates attained, realisation of the solvent extraction process with acetone is probably worthwhile. This process is especially suitable for the remediation of sediments and sludges containing high amounts of water and clay and contaminated with barely (bio)available or biodegradable organic contaminants.