Change in phytoextraction of Cd by rapeseed (Brassica napus L.) with application rate of organic acids and the impact of Cd migration from bulk soil to the rhizosphere

Adding exogenous low-molecular weight organic acids is an effective technique to improve phytoremediation of Cd-contaminated soil and has been well documented, but how acid application rate affects remediation efficiency and its underlying limiting factors remains elusive. We investigated this using pot experiments with rapeseed (Brassica napus L.) as the model plant. Plastic pots packed with a sandy loam contaminated by Cd at 4.838 mg/kg were amended with acetic acid, oxalic acid, citric acid, malic acid and tartaric acid, respectively, at an application rate gradient varying from 0.0 to 12.0 mmol/kg. Plants in each pot were harvested after growing for five months, and we then measured the exchangeable, carbonate, Fe–Mn oxide, organic and residual Cd in the rhizosphere, as well as Cd in both roots and shoots. The results showed that all organic acids improved plant uptake of Cd and, compared with the control without acid addition, they could improve Cd uptake by more than 100%. The enhanced Cd extraction was due to the increase in exchangeable Cd in the rhizosphere. Plant Cd was weakly correlated to the amount of Cd lost from a unit volume of the rhizosphere due to root extraction (R2 = 0.06), but a good negative correlation was found between them after normalizing the lost Cd by root biomass (R2 = 0.36). Mass balance analysis revealed that the average Cd content in soil (rhizosphere and bulk soils combined) was much higher than the Cd content in the rhizosphere, and the improved Cd mobility after acid addition was thus due to the increased chelation. As diffusion of ligands in water is one order in magnitude smaller than diffusion of Cd ions, our results suggested that Cd migration from the bulk soil into the rhizosphere was a major factor limiting Cd phytoextraction by rapeseed after adding the exogenous organic acids.