Phytoremediation of soil co-contaminated with heavy metals (HMs) and tetracyclines: effect of the co-contamination and HM bioavailability analysis

Purpose The co-contamination of soil with heavy metals (HMs) and tetracyclines (TCs) is an environmental issue of great concern globally. The interactive effects of HMs (Cd and Cu) and TCs (tetracycline, oxytetracycline, and chlortetracycline, labeled with TC, OTC, and CTC, respectively) on contaminant removal from soil planted with Mirabilis jalapa L. were evaluated. Materials and methods Pot experiment was carried out in co-contaminated soils with HMs at four rates (Cd/Cu: 0/0, 2.5/200, 5.0/400, and 10/600 mg kg(-1)) and TCs at four rates (0, 0.5, 2.5, and 5.0 mg kg(-1)). The bioavailability of HM was evaluated via subsequent planting with Zea mays L., ex situ extraction by diethylenetriaminepentaacetic acid (DTPA) and in situ extraction (diffusive gradients in thin films, DGT). Results and discussion The results showed that a low level of contaminants increased the growth of M. jalapa. The biomass of M. jalapa increased by 30.31% and 38.70% in treatments with TC concentrations of 0.5 mg kg(-1) and Cd/Cu concentrations of 2.5/200 mg kg(-1), respectively. The concentration and accumulation of HMs in the plant tissue were significantly higher (p < 0.05) in the co-contamination treatments, especially in the low HM treatments. The mean concentrations of the residual TC, OTC, and CTC in the presence of M. jalapa decreased by 28.18-36.86%, 20.93-42.27%, and 16.36-28.94%, respectively. However, phytoremediation increased the bioavailability of Cd and Cu. In treatments with addition of HMs, the Cd concentrations in subsequent Z. mays increased from 14.25 to 18.04 mg kg(-1) in roots and 2.62-3.31 mg kg(-1) in shoots in treatments without phytoremediation to 14.67-22.44 mg kg(-1) in roots and 2.77-4.18 mg kg(-1) in shoots in phytoremediation treatments; Cu concentrations increased from 288.77 to 372.61 mg kg(-1) in roots and 27.76 to 32.49 mg kg(-1) in shoots in treatments without phytoremediation to 291.47-384.33 mg kg(-1) in roots and 32.61-49.69 mg kg(-1) in shoots in phytoremediation treatments. In addition, the HM concentrations in soils of phytoremediation treatments extracted by DTPA and measured by DGT increased by 2.86-3.76% and 11.35-15.24% for Cd, respectively, and 1.22-4.56% and 3.13-10.95% for Cu, respectively, compared with non-phytoremediation treatments with same concentrations of pollutants. Conclusions Our results indicated that co-contamination with TCs increased the adsorption of HMs and that phytoremediation improved the degradation of TCs. However, one season of phytoremediation also increased the bioavailability of HMs. Thus, phytoremediation should be applied continuously.