Examination of Cd Accumulation Within Sunflowers Enhanced by Low Molecular Weight Organic Acids in Alkaline Soil Utilizing an Improved Freundlich Model

This research aims to improve the efficiency of phytoremediation of Cd in soil using sunflowers and optimizing the model of Cd uptake by plants. Five low molecular weight organic acids (LMWOAs), oxalic acid (OA), acetic acid (AA), tartaric acid (TA), malic acid (MA), and citric acid (CA) at 2, 4, and 6 mmol/kg, were applied to sunflowers at 20, 30, and 40 days after seed emergence. The pH, electric conductivity (EC), total nitrogen (TN), total phosphorus (TP), catalase activity, amylase activity, invertase activity, and Cd content in rhizosphere soil and Cd in plant tissues were analyzed. Plant biomass was greater at the application rates of 2 and 4 mmol/kg. For each acid treatment, the Cd content in roots and leaves increased, while that of the stem and bud decreased. The total Cd accumulation increased due to the increase in plant biomass, especially root tissues; LMWOAs applied at 20 or 30 days after seedling emergence achieved higher Cd accumulations by expanding the Cd absorption capacity of rhizosphere soil. Application time had a more significant influence on plant biomass, Cd content, and accumulation in plant tissues than the variation of LMWOAs. Soil invertase activity and pH are the main factors causing the change in Cd form. An improved Freundlich model, using soil EC and pH, had higher accuracy than a model using pH alone. LMWOAs can increase Cd accumulation in plants, and a lower application rate (2 mmol/kg) and earlier application date (20 days after seedling emergence) are beneficial for improving the phytoremediation efficiency of sunflower. The increase of root biomass expanded the contact area between plants and soil, which was one of the reasons for the increase of Cd accumulation. EC could be used as a supplementary index to improve the Freundlich model when using pH.