Influence of pH, electrical conductivity and ageing on the extractability of benzo[a]pyrene in two contrasting soils

Higher soil pH and electrical conductivity (EC) were suspected to result in higher extractability and bioavailability of benzo[a]pyrene (B[a]P) in soils. In this study, we investigated the influence of pH, EC and ageing on the extractability of B[a]P in two contracting soils (varied largely in soil texture, clay mineralogy and organic carbon content) over 4 months. Dilute sodium hydroxide (0.2 mol L^-1) and sodium chloride (0.1 mol L^-1) solutions were used to adjust soil pH and EC either separately or simultaneously. Extractability of B[a]P in these soils was monitored using a mild solvent extraction using butanol (BuOH, end-over-end shake over 24 h), and an exhaustive mix-solvent extraction using dichloromethane/acetone (DCM/Ace, v:v = 1:1) facilitated by sonication and a subsequent NaOH saponification method following the DCM/Ace extraction. Results showed that increased pH and/or EC significantly increased the B[a]P extractability in the sandy soil (GIA). Variance analysis of contribution of pH and/or EC modification and ageing time on changes in B[a]P extractability indicated that in GIA > 55% and over 25% of the changes in B[a]P extractability was attributed to increased pH&EC and pH only respectively. While ageing resulted in >85% of the change in B[a]P extractability in the clayey soil (BDA), following by increased pH&EC (contribution < 15%). Large amount of non-extractable residue (NER) were formed over the ageing period, up to 95% and 79% in GIA/BDA and its modified soils, respectively. Significant correlations were observed between B[a]P BuOH extractability and the exhaustive sequential extraction using DCM/Ace followed by NaOH saponification for all soils (p < 0.001). With slopes of the correlations close to 1, our results indicated that the simple mild solvent BuOH extraction was equivalent to the complex sequential DCM/Ace and NaOH saponification extraction in these soils.