Cd and pb Co-Pollution Increased Ecological Risk and Changed Rhizosphere Characteristics of Arabidopsis Thaliana During Phytoremediation

Previous studies have reported that co-contamination can result in more complex effects on the phytoremediation efficiency of plants relative to those of a single pollutant. However, the effect of co-contamination on plant rhizosphere characteristics has rarely been revealed. This study was carried out to assess the changes in soil pH, the content and fractionation of dissolved organic matter (DOM), and the metal solubility in the rhizosphere of Arabidopsis thaliana when treated with Cd and Pb simultaneously. The results showed that co-contamination increased the concentrations of DOM by 24.8% and 30.9% in the rhizosphere soil of A. thaliana relative to individual Cd or Pb pollution, respectively. At the end of the experiment, co-contamination significantly decreased the initial soil pH from 6.6 ± 0.3 to 5.5 ± 0.4, whereas a decrease was not observed under Pb pollution alone. Variations in soil pH and DOM can change the fractions of the two metals in the rhizosphere soil of A. thaliana. DOM in co-contaminated soil showed a higher Cd (1.05 mg L−1) and Pb (0.75 mg L−1) extraction ability relative to that in the Cd-polluted (0.89 mg Cd L−1 and 0.59 mg Pb L−1) or Pb-polluted (0.68 mg Cd L−1 and 0.63 mg Pb L−1) soils. The soluble Cd content in the co-contaminated (0.44 mg L−1) soil was significantly lower than that in the Cd-polluted (0.71 mg L−1) soil because A. thaliana is a Cd accumulator, whereas the soluble Pb content showed the opposite trend (47.0 mg L−1 vs. 37.4 mg L−1) because the species is a Pb excluder. Therefore, A. thaliana in co-contaminated soil would pose a leaching risk for the non-hyperaccumulated metals, thereby increasing the potential ecological risk during the phytoremediation process.