Comparative Activation Process of Pb, Cd and Tl Using Chelating Agents from Contaminated Red Soils.

Adding chelating agents is a critical technique of heavy metal activation for enhancing phytoextraction through the formation of soluble metal complexes which will be more readily available for extraction. The preliminary, dynamic, equilibrium activation experiments and speciation analysis of Pb, Cd and Tl in contaminated red soils were used to select six chelates with relatively good activation performance from nine chelates, and the effects of dosage and pH on the heavy metals activation were studied systematically. Results showed that the activation of Pb, Cd and Tl by chelates reached equilibrium within 2 h, and the activation process showed three stages. Under neutral conditions, chelates had better activation performance on Pb- and Cd-contaminated soils. Except for S,S-ethylenediamine disuccinic acid (S,S-EDDS) and citric acid (CA), the maximum equilibrium activation effect (MEAE) of ethylenediaminetetraacetic acid (EDTA), N,N-bis (carboxymethyl) glutamic acid (GLDA), diethylenetriaminepentaacetic acid (DTPA) and aminotriacetic acid (NTA) was over 81%. The MEAE of Tl-contaminated soil was less than 15%. The decreasing order of the dosage of chelating agents corresponding to MEAE for three types of contaminated soils was Pb-, Cd- and Tl-contaminated soil, relating to the forms of heavy metals, the stability constants of metal-chelates and the activation of non-target elements Fe in red soil. Under acidic conditions, the activation efficiencies of chelates decreased to differing degrees in Pb- and Cd-contaminated soils, whereas the activation efficiencies of chelating agents in Tl-contaminated soils were slightly enhanced.

Evaluation method for the measuring comprehensive suitability of chelating agents: a study of the temporal dynamics of heavy metal activation.

The effects of chelating agents on heavy metal activation in Cd- and Pb-contaminated soils were studied through a dynamic activation experiment. An evaluation method for the measuring comprehensive suitability of chelating agent was established by calculating indexes for the degree of activation effect suitability and activated heavy metals' half-life suitability. The following results were obtained: in Cd- and Pb-contaminated soils, heavy metal activation effects reached or approached maximum activating effects within 1 d and subsequently showed different levels of decline in all chelating agent treatment conditions. Declines in activation effects similarly subjected to the law of exponents over time and to the goodness of fit in DTPA, NTA, and GLDA ranged from 0.80 to 0.98. For Cd- and Pb-contaminated soils, chelating agents' levels of comprehensive suitability (H) were recorded as follows: NTA(1.40) > GLDA(1.31) > DTPA(1.14) > EDTA(1.00) > EDDS(0.14) > CA(0.06) and GLDA(1.56) > DTPA(1.48) > EDTA(1.00) > NTA(0.78) > EDDS(0.26) > CA (0.02). GLDA and DTPA are both suitable for Cd and Pb phytoextraction. Moreover, NTA and GLDA are optimal chelating agents for Cd and Pb phytoextraction, respectively.

Insights into Heavy Metals Leakage in Chelator-Induced Phytoextraction of Pb- and Tl-Contaminated Soil.

Chelators including DTPA (diethylene triamine pentaacetic acid) and oxalic acid were selected for inducing phytoextraction of heavy metals (HMs) from Pb-, Tl-, and Pb-Tl- contaminated soil, in which heavy metals leakage was highly remarkable. Results showed that compared with the control group without chelating agent under planting conditions, the extraction efficiency (i.e., uptake coefficient) of Pb, Tl increased by 86%, 43% from Pb-Tl- contaminated soil in the presence of oxalic acid, and there was no significant change in heavy metal leakage under rainfall conditions. It was the best phytoremediation scheme in this work. Under rainfall conditions, the HMs concentration in the leachate showed a linear decreasing trend. Acid rain promoted the leakage of heavy metals, and the average leached amount of Tl increased by 1.47 times under acid rain conditions. However, for Pb, DTPA was the main influencing factor, followed by acid rain.