Evaluation of dissipation mechanisms by Lolium perenne L, and Raphanus sativus for pentachlorophenol (PCP) in copper co-contaminated soil

Though phytoremediation is widely studied in remediation of metal contaminated soils or organic contaminated soils, little information is available regarding the effectiveness and processes of phytoremediation of sites co-contaminated with organic and metal pollutants. Sites co-contaminated with organic and metal pollutants are common and considered to be a more complex problem as the two components often cause a synergistic effect on cytotoxicity as measured both by growth inhibition and colony-forming ability. In this paper, the dissipation mechanisms for pentachlorophenol (PCP) in copper co-contaminated soil by Lolium perenne L, and Raphanus sativus was investigated in a greenhouse experiment by monitoring the growth response of plants, evaluating the removal efficiency of extractable PCP, differentiating PCP residuals in strongly and loosely adhering rhizosphere soils, and analyzing the microbial activity in the rhizosphere. In copper co-contaminated soil with the initial PCP concentration of 50 mg/kg, plants grew better with the increment of soil Cu level (0, 150, 300 mg/kg), which implied that combinations of inorganic and organic pollutants sometimes exerted antagonistic effects on plant cytotoxicity. The observed higher PCP dissipation in soil spiked with 50 mg/kg PCP in the presence of Cu and the less difference of PCP residual between strongly and loosely adhering soils further suggests the occurrence of Cu–PCP interaction and the enhanced degradation and mass flow are two possible explanations. In copper co-contaminated soil with the initial PCP concentration of 100 mg/kg, however, both plant growth and microbial activity were inhibited with the increment of soil Cu level. The lowered degrading activity of microorganisms and the reduced mass flow were probably responsible for the significantly lower levels of PCP dissipation in copper co-contaminated soil. These results showed that remediation of sites co-contaminated with organic and metal pollutants is a complex problem and a more thorough understanding of the extent and mechanisms by which metals inhibit organic degradation is needed to develop phytoremediation of co-contaminated sites.