Organic amendment optimization for treatment of hydrocarbon contaminated soil using the chemicalbiological stabilization process

Abstract

Sugar cane cachasse was tested as an organic soil amendment at 0, 2, 4 and 9% (dry weight), for the remediation of hydrocarbon contaminated soil (with an average initial concentration of 14,356 mg/Kg), which had been pre-treated by the incorporation of 4% (dry weight) calcium hydroxide according to the chemical-biological stabilization treatment method. Remediation efficiency was measured in terms of overall hydrocarbon reduction, hydrocarbon stabilization, soil leachates, microbial activity, acute toxicity and biomass production in a tropical forage grass (Brachiaria humidicola). Compared to the control, the over all half life for hydrocarbon degradation was optimal with 2 - 4% cachasse, reducing the half life from 301 days to about 195 days. The treatment with 9% cachasse presented reduced respiration rates, probably due to fermentation conditions, and a longer half life. Hydrocarbon availability (versus stabilization), and thus potential toxicity and leachability, was lowest in the treatments with 4 and 9% cachasse. In these treatments, there were no methanol extractable hydrocarbons after 19 months, although the TPH concentration was 1,000 - 1,500 mg/kg. In less than four months, toxicity, as  determined by the Microtox method, was reduced to regional background levels (Effective Concentration 50 > 100,000 mg/L), and soil leachates (TCLP) were reduced to < 1 mg/L in all treatments. Grass biomass production was related to the amendment concentration, being two to three times greater in the treatment with 9% cachasse during the major part of the treatment. According to these results, a 4% application rate is recommended to optimize the microbial response, with an additional 4% added after one year to further stimulate pasture growth.

Key words: Soil remediation, petroleum, biodegradation, toxicity, biomass production, pasture.