Enhancement And Mechanism Of Phytoremediation For Cd And PAHs Co-contaminated Soils

Soil contamination by heavy metals and persistent organic pollutants has been accelerated in China during the past decade because of rapid urbanization and industrialization. The output of heavy metals and organic pollutants into environment will stay in a comparatively high level in a long period. Soil contamination in China tends to be severe and complicated. Cadmium (Cd) and polycyclic aromatic hydrocarbons (PAHs) are of particular concern due to their persistence; potentially carcinogenic, mutagenic, and teratogenic properties; and their ubiquitous occurrence in the environment. Sites co-contaminated by heavy metals and PAHs, in many regions, e.g. Shen-Fu Irrigation Area located between Shenyang and Fushun City in Liaoning Province of China, have been documented in recent. Therefore, the need to develop environmental friendly, low-cost, and in situ strategies directed at co-contaminated sites is of great importance. This study investigated the interaction between Cd and PAHs during the process of soil co-contamination and their effects on phytoremediation of Cd-PAHs co-contaminated soil by a Cd hyperaccumulator Sedum alfredii native to China. Agronomic, chemical, and microbial strategies were also investigated to improve phytoremediation of Cd-PAHs co-contaminated soils. The main results are summarized in the following text:(1) A microcosmos incubation experiment was conducted to investigate the interaction between Cd and PAHs during the process of soil co-contamination. The dissipation of benzo[a]pyrene (B[a]P) in soil was mainly due to microbial degradation. High Cd concentration (25mg kg-1) significantly inhibited the degradation of B[a]P in the soil, and the addition of pyrene (PYR) with an initial concentration of250mg kg-1remarkably promoted B[a]P degradation. Both desorbing and non-desorbing fractions of B[a]P contributed to B[a]P degradation in soil; however, desorbing fraction contributed more as compared to non-desorbing fraction. The contribution of residual fraction to B[a]P degradation could be negligible. Desorbing fraction of B[a]P could transform into non-desorbing fraction and vice versa. The loss of B[a]P promoted by Addition of PYR was mainly attributed to desorbing fraction. During250days’ incubation, the addition of PYR did not affect the concentrations of water-extractable and available Cd (N H4OAc-extractable) in soils, whereas aging effects decreased the concentrations of both Cd fractions in high Cd treatments in the later incubation period.(2) A Pot experiment was conducted to investigate the growth of S. alfredii and removal of contaminants from Cd-PAHs co-contaminated soil. Soil slightly contaminated by Cd (0.92mg kg-1) was collected from a vegetable field in Hangzhou and was spiked with two levels (0and6mg kg-1) of Cd and three levels (0,25and150mg kg-1) of phenanthrene (PHE) or PYR. Elevated Cd level (6.38mg kg-1) increased S. alfredii growth. The presence of PAHs decreased the stimulatory effects of Cd on plant biomass and Cd concentrations in shoots in Cd spiked soil, thus decreasing Cd phytoextraction efficiency. Cadmium removal by S. alfredii after60d of growth varied from5.8to6.7%and from5.7to9.6%, in Cd unspiked and spiked soils, respectively. Removal rate of PAHs in the soil was similar with or without the plants. Pyrene removal rate in the soil with elevated Cd. This appears to be due to decrease in soil microbial activity, which is confirmed by the decrease in DHA, which is a good indicator of soil microbial activity. The results demonstrate that S. alfredii could effectively extract Cd from Cd-contaminated soils in the presence of PHE or PYR; however both PAHs exhibited negative effects on phytoextraction of Cd from Cd spiked soil (6.38mg kg-1). Sedum alfredii is not suitable for remediation of PAHs. Additional strategies are needed to accomplish simultaneous removal of Cd and PAHs from co-contaminated soils by S. alfredii. The effects of Cd and PAHs concentrations on the removal rate of PAHs appear to be attributed to the changes in microbial activities in the soil.(3) A pot experiment was conducted to investigate the potential for enhanced phytoextraction of Cd by S. alfredii and dissipation of PAHs in co-contaminated soil by application of pig manure vermicompost (PMVC). Soil contaminated by Cd (5.53mg kg-1) was spiked with PHE, anthracene (ANT) and PYR together (250mg kg-1for each PAH). The pot experiment was conducted in a greenhouse with four treatments:1) soil without plants and PMVC (Control),2) soil planted with S. alfredii (Plant),3) soil amended with PMVC at5%(w/w)(PMVC), and4) treatment2+3(Plant+PMVC). Application of PMVC to co-contaminated soil increased the shoot and root dry biomass of S. alfredii by2.27-and3.93-fold, respectively, and simultaneously increased Cd phytoextraction efficiency by1.97-fold without inhibiting soil microbial population and enzyme activities. The highest dissipation rate of PAHs was observed in Plant+PMVC treatment. The dissipation rates of PHE, PYR and ANT were significantly higher by0.26,3.21and4.00%, respectively, as compared to the control. However, neither S. alfredii nor PMVC enhanced PAHs dissipation when applied separately. Abundance of PAH-degraders in soil was not significantly related to PAHs dissipation rate. Plant+PMVC treatment significantly influenced the bacterial community structure. Enhanced PAHs dissipation in the Plant+PMVC treatment could be due to the improvement of plant root growth, which may result in increased root exudates, and subsequently change bacterial community structure to be favorable for PAHs dissipation. This study demonstrated that remediation of Cd-PAHs co-contaminated soil by S. alfredii can be enhanced by simultaneous application of PMVC.(4) A pot experiment was conducted to investigate the potential for phytoextraction of heavy metals and rhizoremediaiton of PAHs in co-contaminated soil by co-planting S. alfredii with ryegrass(Lolium perenne) or castor (Ricinus communis). Co-planting with castor decreased the shoot biomass of S. alfredii as compared to that in monoculture. Cadmium concentration in the S. alfredii shoot decreased by64.3and47.4%, respectively, in S-R and S-C treatments, as compared to that in S. alfredii monoculture. In contrast, Zn concentration in the S. alfredii shoot increased by19.8and25.6%, respectively, in S-R and S-C treatments, as compared to that in S. alfredii monoculture. Lead concentration in S. alfredii shoot in S-R treatment was significantly greater as compared to that in S. alfredii monoculture. Total removal of either Cd, Zn or Pb by plants was similar across S. alfredii monoculture or co-planting with ryegrass or castor, except for the enhanced Pb removal in S. alfredii and ryegrass co-planting treatment. Co-planting of S. alfredii with ryegrass or castor significantly enhanced the PYR and ANT dissipation as compared to that in the control soil or S. alfredii monoculture. The dissipation rate of PYR was greater by2.3and3.5%, respectively, in S-R and S-C treatments as compared to that in the control soil. The dissipation rate of ANT was greater by8.5and9.1%, respectively, in S-R and S-C treatments as compared to that in the control soil. This appears to be due to the increased soil microbial population and enzyme activities in both co-planting treatments. Co-planting of S. alfredii with ryegrass or castor provides a promising strategy to mitigate both metal and PAH contaminants from co-contaminated soils.(5) A pot experiment was conducted to investigate the single and combined effects of non-ionic surfactant (Tween80) and B[a]P-degrading bacteriums on phytoremediation of soils co-contaminated with Cd and high-molecular-weight PAH (B[a]P) by S. alfredii. Neither separate nor combined application of Tween80and B[a]P-degrading bacteriums significantly affected plant growth and Cd uptake and accumulation by plant. However, S. alfredii can effectively extract Cd from co-contaminated soil. Cadmium phytoextraction rate after120d of growth varied from16.8to20.8%and from6.4to7.6%, respectively, in Cd unspiked and spiked soils. In genaral, the application of B[a]P-degrading bacteriums can enhance B[a]P dissipation in soil. However, the effects of Tween80on B[a]P dissipation were influenced by the interaction between soil Cd concentration and plant. In the soil with high Cd concentration (4.71mg kg-1), the combined application of of Tween80and B[a]P-degrading bacteriums could remove B[a]P most effectively in both planted and unplanted treatments. These results demonstrated that decontaminaton of Cd and B[a]P can be achieved by S. alfredii associated with B[a]P-degrading bacteriums; however, the combined application of Tween80and B[a]P-degrading bacteriums can remove B[a]P more efficiently from soil with relatively higher Cd concetration (4.71mg kg-1). The complicated interactions among Tween80, B[a]P-degrading bacteriums and soil Cd concentration on the competition between B[a]P-degrading bacteriums and indigenous microbial community need future investigations.