Impact of co-contamination by PAHs and heavy metals on micro-ecosystem in bioretention systems with soil, sand, and water treatment residuals

Rainfall runoff contained toxic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Bioretention systems can effectively remove PAHs and heavy metals from runoff. When PAHs and heavy metals accumulated in the media of bioretention systems, co-contamination occurred. To reveal the response processes of co-contamination in the media micro-ecosystem, bioretention systems were constructed and simulated rainfall tests were carried out to reveal the media micro-ecosystem evolution in the bioretention system under the co-contamination of PAHs and heavy metals. The simulated rainfall tests showed that the bioretention systems with different media had good purification effects with load reduction rates >90% (PAHs) and >40% (heavy metals). The microbial α-diversities in different media decreased by 6.67%–32.27%, and 9.21%–18.69% after the tests. The most dominant species of microorganisms in the media were Proteobacteria (all relative abundances >62%). As a typical PAHs-heavy metal degrading bacteria, the relative abundances of Pseudomonas increased significantly after the tests (both >9%). The co-contamination of PAHs and heavy metals inhibited all enzyme activities. The long-term operation of the bioretention system gradually increased the level of co-contamination in the media, and the stability of the media micro-ecosystem gradually deteriorated. The results of this paper can provide a theoretical basis and technical support for maintaining the operational efficiency of the bioretention system with modified-media and avoiding the risk of potential contamination.