Impact of a phosphate compound on plant metal uptake when low molecular weight organic acids are present in artificially contaminated soils

Abstract

A phosphate compound can stabilize cadmium (Cd), lead (Pb), and zinc (Zn)-contaminated soils, although its effectiveness in reducing metal availability and plant uptake when low molecular weight organic acids (LMWOAs) are present is uncertain but nonetheless worth evaluating. Here, 3 % NaH2PO4 (a P compound), tartaric acid (TA), oxalic acid (OA), and mixtures of P and TA/OA are used to evaluate Indian spinach (Basella Alba L.) for soil Cd, Pb, and Zn availability and absorption. Additionally, soil metal immobilization options were examined. Of these treatments, P plus low TA (P-TA2), followed by P, decreased shoot Cd, Pb, and Zn the most, with values of 1.03, 2.52, and 104.50 mg kg−1, respectively, somewhat greater than threshold values of WHO, particularly for Pb and Zn. After harvest, P-TA2 contained >2.44, >321.70, and >290.22 mg kg−1 of total oxidizable and residual Cd, Pb, and Zn, showing that 48 %, 80 %, and 58 % of soil metals were immobilized and not taken up by the plant. P-TA2′s zeta potential, followed by P, became steadily more negative as pH increased, confirming that electrostatic adsorption was the main metal immobilization process, notably in P-TA2. However, in XRD studies, P alone generated Cd phosphate, pyromorphite, and hopeite, showing that sorption and precipitation were the predominant metal immobilization mechanisms. Finally, P-TA2 is the best in immobilizing and inhibiting metals in contaminated soils for plant absorption. Since rhizotropic LMWOAs may impact Cd, Pb, and Zn mineral stability, they should be addressed when treating Cd, Pb, and Zn-contaminated soils with P for plant growth studies.