Decontamination of heavy metals (HMs), especially cadmium (Cd) which has high mobility in the soil, is very important due to the effects of HMs pollution on the soil, environment, and human. Numerous efforts have been made to develop technologies for the remediation of contaminated soils, including ex-situ washing with physical-chemical methods, and the in-situ immobilization of metal pollutants. These methods of clean up are generally very costly, and often harmful to properties of the soil (i.e., texture, organic matter, microorganisms). Recently, the phytoremediation of HMs from contaminated soils has attracted attention for its low cost of implementation and many environmental benefits. Several chelating agents, such as DTPA, EDTA, and NTA, have been studied for their ability to dissolve metals, leach heavy metals, and enhance the uptake of metals by plants. Although many researchers have reported that EDTA is excellent solubilizing agents for HMs from contaminated soils, it is quite persistent in the environment due to the low biodegradability. Hence recently the easily biodegradable chelating agent NTA has been proposed to enhance the uptake of HMs in phytoremediation as well as the leaching of HMs from the soil. Therefore, in the present study attempts are made to investigate the effect of applicability NTA in Cd leaching and the refining of Cd from contaminated-soil by maize.
In this research, the effect of NTA on Cd leaching and its absorption by maize in contaminated-soil in a greenhouse experiment were investigated. The experiment was a factorial experiment based on a completely randomized design. The treatments consisted of three levels of Cd contamination (0, 25 and 50 mg kg-1soil) and three levels of NTA (0, 15 and 30 mmol per pot) in loamy soil and in the cultured and non-cultured conditions under three irrigation conditions. The soil was contaminated with cadmium chloride (CdCl2.2.5H2O). Nitrogen, phosphorus, and potassium (in the form of urea, triple superphosphate and potassium phosphate, respectively) were added to the pots. NTA was added in three steps to the pots. The first step of adding NTA was beginning 4 weeks after cultivation, occurring approximately once in 14 days. Also, 7 days after adding NTA, the pots were irrigated with an amount corresponding to 20% more water than the moisture of soil saturation condition. The drainage water collected from each irrigation event was kept in a refrigerator at 5°C prior to Cd analysis. The plants were cut about 5 mm above the soil surface after 10 weeks of maize growth and were dried for analyzing Cd in the plant. Analysis of variance was used to study the effects of different treatments of Cd and NTA on Cd contents in drainage water, plant, and soil. Statistical analysis were performed using SPSS. Means of treatments were compared using Duncan’s Multiple Range Test (DMRT) and the graphs were plotted in Excel.
The contrasting impact between irrigation rounds and Cd treatments, as well as NTA treatments on Cdtotal leached was significant (P<0.05). The highest Cd leached was in 50 mgCd kg-1soil (Cd50) and 30 mmol NTA (NTA30) in the first irrigation round. In the next two rounds, the Cd leached from the soil was inconsiderable. Different levels of Cd and NTA showed a significant difference in Cd concentration in the first round of leaching. In non-cultivated pots, the amount of Cd leaching in Cd50NTA15 and Cd50NTA30 treatments increased by 8 and 15 times, respectively than that in Cd50NTA0 treatment. In the case of similar treatments in the presence of maize, the Cd leaching rate increased by 5.8 and 6 times, respectively, than that in (NTA0). Cd absorbed by maize in (Cd50, NTA30) was maximum and that measured 58% more than that in (Cd50, NTA0), while dry weight decreased significantly (30% in the shoot and 40% in the root). After the cultivation and leaching process, the maximum amount of DTPA-extractable Cd was observed in (Cd50, NTA0). While using (NTA15, NTA30) at the same level of Cd-contamination (Cd50), there was a significant decrease in DTPA-extractable Cd (due to the increase in Cd dissolved, Cd leached and Cd absorbed by plants). Due to pH between 2-3 and EC about 2.5-3.5 in NTA solutions, the application of NTA in soil decreased pH and increase EC in the soil. On the other hand, the decrease in pH of soil increased solubility of calcium carbonate equivalent (CCE), thereby reduced CCE in the soil. The results of this study showed that the soil pH was effective on HMs absorption by plants, therefore the availability of Cd after the use of NTA may be due to the decrease of alkalinity in the soil. The presence of organic-metal bonds in chelate-metal compounds causes metals to be less exposed to colloids, hydroxides, and oxides thus will prevent their stabilization in the soil. So it can be said that one of the effective methods for increasing the absorption of HMs from the soil by the plant is to reduce the pH of the soil. Some of the soil properties, such as pH and total heavy metal concentration, improves the efficiency of the chelator agent.
The results showed that an increase in the amount of Cd contamination and NTA applied increased Cd content in drainage water and Cd which was uptake by maize. Also, results showed well, the combined of maize planting and the use of NTA is successful in refining Cd from contaminated-soil. It seems that Adding NTA as a natural chelator in Iranian calcareous soils can increase the dissolution of Cd and extract it from the soil during a leaching period without contamination of the environment, as well as increase the efficiency of removing Cd by maize.
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