Rhizosphere effects of Loliumperenne L. and Beta vulgaris var. cicla L. on the immobilization of Cd by modified nanoscale black carbon in contaminated soil

The choice and stability of the immobilization agents in in situ remediation of heavy metal-polluted soil is particularly important. This study aimed at investigating the rhizosphere effects of different plants on the immobilization stability of Cd by modified nanoscale carbon black (MBC) in contaminated soil. Pot experiments using Cd-tolerant plant ryegrass (Loliumperenne L.) and Cd hyperaccumulator leaf red beet (Beta vulgaris var. cicla L.) were conducted. A rhizobag was applied to investigate the rhizosphere effect on the remediation of Cd by the MBC. After 60 days of cultivation, the pH, DTPA-extractable Cd concentrations, and the fractionation of Cd of rhizosphere and bulk soil were determined. The dry biomass and the concentrations of Cd in the shoots and roots of two plants were also measured. The results indicated that the biomass of ryegrass significantly increased by 63 and 65% in roots and shoots, respectively, with the addition of MBC. Significant change of the biomass of leaf red beet after adding MBC was not seen. Addition of MBC led to the decreases of soil pH, DTPA-extracted Cd in both rhizosphere and bulk soils, and declined bioavailability of Cd cultivated with either ryegrass or leaf red beet. Compared to the bulk soil, the concentration of DTPA-extracted Cd in rhizosphere reduced by 8.91 and 0.66%, for ryegrass and leaf red beet, respectively, and when MBC was added, decreased by 6.92 and 6.20%, respectively. The rhizosphere process of ryegrass had a negative effect on the immobilization of MBC on Cd. However, there was a promoting function of the immobilization by MBC for Cd in the rhizosphere of leaf red beet. Thus, the Cd-tolerant plant ryegrass should be avoided in the practical application of MBC.