| Summary: | 碩士 === 國立宜蘭大學 === 環境工程學系碩士班 === 105 === Plow pan commonly appears under the surface layer in paddy soil in Taiwan. It is able to keep the soil surface with moisture for a longer time due to its firm and dens structure. Heavy metals found in the contaminated paddy are almost restricted within its surface layer, and therefore the circulation-enhanced electrokinetics technique is considered a suitable method for the soil remediation. The aim of this study was to conduct a series of laboratory-scale experiments for exploration of the dynamic changes of the heavy metal in soil profile during the treatment. The nitrogen, phosphorus, and potassium contents were also examined after remediation.
Soil sample was obtained from a fallow paddy where was contaminated with high level of copper, zinc, nickel, and chromium (with the following original concentrations: 777, 1162, 465, and 475 mg kg-1, respectively) in Changhua City. Citric acid, malic acid, and ascorbic acid, were employed for preparation of a 0.1 M electrolyte to compare their removals of heavy metals. Then, only citric acid was picked up for mixing with sodium sulfite and sodium dithionite, respectively, to prepare two mixture electrolyte solutions, 0.1 M citric acid/0.2 M sodium sulfite and 0.1 M citric acid/0.2 M dithionite, in order to get a higher removal efficiency. The circulation-enhanced electrokinetics experiments were accomplished for 5-, 14-, and 21-day treatments. After remediation the soil in reaction tank was divided into ten same sized blocks for sampling and analysis; whole soil block was previously divided into upper and lower layer equally and each of them was then horizontally subdivided into five same sized blocks.
After the 5-day citric acid treatment, remarkable accumulations of Cu, Zn, and Ni in the area close to the cathode trough were observed and their concentrations even became much higher than the initial levels, while their concentrations in the areas near the anode trough were shown significant declines. Similar accumulation of Cr was less significant and only concentrated in the middle section of the soil. The citric acid treatment showed the highest removal in comparison with malic acid and ascorbic acid. Average concentrations of Cu, Zn, Ni, and Cr in soil were still higher than the levels of control standard just through 5 days of remediation, but they could be declined below the levels in most areas when remediation was conducted 14 days. However, only 5% of heavy metals could further be eliminated away the soil through the 21-day treatment in comparison with the 14-day treatment. The removals of Cu, Zn, Ni, and Cr at the 21-day treatment were found to be 89, 91, 89, and 6%, respectively. The removals of Cu, Zn, Ni, and Cr in whole soil were to be 57%, 95%, 86%, and 11% by the 21-day remediation using citric acid/sodium sulfite electrolyte, respectively. The removals of Cu, Zn, Ni, and Cr were to be 78%, 96%, 93%, and 40% through the 21-day remediation using citric acid/sodium dithionite electrolyte, respectively. The utilization of chemical reductants actually raised the removals of Cr, Zn, and Ni, exclusive of Cu. On the other hand, more heavy metals would be accumulated in lower layer than in upper layer for all treatments during entire process, this revealing that gravity might affect the electromigration.
After the 21-day treatment, available N-NH3 and N-NO3 contents in soil increased 6.4 times and 3.0 times their initial levels using citric acid electrolyte, respectively; available N-NH3 and N-NO3 contents increased 3.0 times and 1.6 times using citric acid/sodium dithionite mixture electrolyte, respectively; available N-NH3 and N-NO3 contents decreased 20% and 40%, respectively, when using citric acid/sodium sulfite mixture electrolyte for treatment. In summary, the effective phosphorus and the exchangeable potassium contents in soil eventually decreased about 0.06–36 % and 28–74 %, respectively.
Keywords: Chemical reductant, Electrokinetic remediation, Heavy metals mobilization, Nitrogen, Phosphorus, Potassium
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