Compost extract enhances desorption of α-naphthol and naphthalene from pristine and contaminated soils

Enhancing desorption of hydrophobic organic compounds (HOCs) may be necessary to achieve bioremediation of soils. We measured desorption of added α-naphthol and naphthalene in compost extract (200 mg C in 10 mmol l−1 CaCl2) and in 10 mmol l−1 CaCl2 alone from (i) soils varying in age, diagenesis, and quantity and quality of organic matter, and (ii) wettable and non-wettable soil samples from a 20-y-old oil spill. Compost extract increased desorption of α-naphthol and naphthalene from all soils compared to desorption in CaCl2 alone, despite the wide range of values among the soils for linear partition coefficients (Kd varied 40-fold for α-naphthol and 50-fold for naphthalene) and partition coefficients normalized for organic C content (Koc varied 3-fold for α-naphthol and 2.5-fold for naphthalene). The proportional enhancement of desorption tended to be higher for α-naphthol than for naphthalene. No consistent relationship, however, was detected between the character of the soil organic matter, as measured by CPMAS-13C NMR, and the magnitude of enhanced desorption. Furthermore, enhanced desorption could not be attributed to surfactant-like activity of the compost extract as measured by a Du Noüy tensiometer. Sorption of the compost extract on soil, however, was substantial (foc = 0.047, Kd = 0.42) and may have contributed to enhanced desorption of HOCs. Our results reinforce previous reports that dissolved organic C can enhance desorption of HOCs from soils. Our study identified sorption of compost extract C and displacement of sorbed HOCs as a possible mechanism of desorption enhancement meriting further investigation. Furthermore, the enhanced desorption of HOCs in the presence of co-sorbed organic contaminants needs to be addressed.