Abstract
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.
Highlights
As a consequence of ongoing rapid economic development and expanding urbanization, increasingly large amounts of waste are being discharged into the soil
We used quasi-first-order and quasi-second-order kinetic models to simulate the activation process of heavy metals by chelating agents [40,41], the results showed that pseudo-second-order kinetic model fits quite well to the activation of Pb, Cd, and Tl for the six selected chelating agents with the goodness of fit more than 0.9878, thereby reflecting the chemical activation characteristics of the chelating agents [42]
Under the present experimental conditions, the maximum equilibrium activation effect (MEAE) of Tl is below 15%, which is mainly related to the existential speciation of Tl, the stability constant of complexation reaction and the activation of non-target element Fe in red soil
Summary
As a consequence of ongoing rapid economic development and expanding urbanization, increasingly large amounts of waste are being discharged into the soil. Among these wastes, heavy-metal pollutants, which are well known as high bioaccumulation, recalcitrant, and toxic, are serious [1,2,3]. Heavy metals can enter into the human body via the food chain, posing serious threats to the environment and human health. It is essentially important to remove and control the heavy metals from contaminated soil for its remediation, which has become an urgent problem to be solved around the world [4,5]. Public Health 2020, 17, 497; doi:10.3390/ijerph17020497 www.mdpi.com/journal/ijerph
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