Evaluation of land farming and chemico-biological stabilization for treatment of heavily contaminated sediments in a tropical environment

Conventional and experimental methods were studied for the remediation of petroleum contaminated sediments from a dam previously used to collect acid run-off from a sulfur mine. The man-made lake had been neutralized, but bentonite rich sediments remained contaminated with very weathered hydrocarbons (sediments with ～50,000–60,000 mg/kg Total Petroleum Hydrocarbons were used in this study). Biostimulation, bioaugmentation (with native microorganisms) and chemico-biological stabilization, all resulted in similar reductions (14–16%) in the TPH concentration over a three month period. The land farming treatments resulted in variable reductions in toxicity, ranging from nil to complete, while the chemico-biological stabilization treatment, not only eliminated acute toxicity but also resulted in a slight stimulation (～103–109%) of the test organism in the bioassay (Microtox). All three treatments reduced polyaromatic hydrocarbons of probable carcinogenicity to below or nearly below the Mexican norms, reduced Toxic Characteristic Leaching Proceedure leachates to <1 mg/L, and left the material in a pH range of 7.0–7.8. The chemico-biological stabilization has the advantage of only requiring initial mixing of the chemical and organic reagents instead of daily aeration, thereby reducing operating costs. This method is also able to treat very difficult sites at low cost, relying on biological humification processes which are accelerated in a humid tropical and semitropical environment. The total unit cost of the chemico-biological stabilization treatment was estimated to be ～60% of that for land farming in the southern Gulf of Mexico region.