تغييرات ظرفيت نگهداري آلاينده در خاك‌هاي كربناتي و استفاده از EDTA در رفع آلودگي سرب از كايولينيت كربناتي آلوده

Buffering Capacity Variations in Calcareous Soils and the use of EDTA for Remediation of Pb-Contaminated Calcareous Kaolinite

در اين نوشتار، تاثير كلسيت در كايولينيت بر فرآيند نگهداري آلاينده در خاك‌ و رفع آلودگي از خاك توسط EDTA مطالعه شده است. EDTA در نسبت‌هاي EDTA/Pb برابر 2/0 تا 1 به كايولينيت داراي درصدهاي كربنات و غلظت‌هاي مختلف آلاينده افزوده و سپس مقدار Pb و Ca آزادشده توسط EDTA اندازه‌گيري شد. در اين فرآيند رفع آلودگي به‌واسطه‌ي كاهش pH خاك، بخشي از كربنات حل مي‌شود و در نتيجه، ظرفيت بافرينگ خاك كاهش مي يابد. اگر خاك مورد رفع آلودگي، مجدداً در معرض آلودگي قرار گيرد؛ اين موضوع اهميت ويژه ‌يي خواهد داشت. با انحلال كلسيت و آزادشدن يون كلسيم، رقابت بين كاتيون هاي سرب و كلسيم براي تشكيل كمپلكس با EDTA رخ مي‌دهد، كه باعث كاهش بازده‌ي رفع آلودگي مي‌شود. استفاده از محلول‌ EDTA با غلظت كم و انجام فرآيند شستشو در چند مرحله باعث كاهش انحلال كلسيم مي‌شود و افزايش بازده‌ي رفع آلودگي توسط EDTA را سبب خواهد شد.

In this paper, the influence of calcium carbonate (calcite) in kaolinite on the buffering capacity of clayey soils and soil contaminant remediation with EDTA is investigated. For this purpose, several mixtures of kaolinite and different percentages of carbonate were prepared and were laboratory contaminated with different concentrations of Pb(NO3)2. The kaolinite sample initially had 4% natural carbonate. To investigate the impact of carbonate on the soil interaction process with EDTA, 6, 11, 16, 21 and 26 percentages of calcite was added to soil samples. Therefore, different calcareous soil samples had 10, 15, 20, 25, and 30 percentage of calcite, respectively. Then, different concentrations of EDTA were applied to these samples in the EDTA/Pb ratios of 0.2 to 1. This range of EDTA is proposed by prior research. After the mixing period, the samples were centrifuged. Then, the concentration of Pb and Ca was measured in supernatant. The results show that in this method of soil remediation, due to the solubility of a part of carbonate in low pH, the buffering capacity of soil decreases. With the solubility of calcium carbonate and the release of calcium ions, both calcium and Pb ions will tend to interact with EDTA and form a Ca-EDTA or Pb-EDTA complex. This, in turn, will reduce the efficiency of soil remediation with EDTA. The results of this paper show that the use of EDTA, at low concentrations, with the application of several soil washing will decrease the solubility of carbonate and consequently increase the efficiency of contaminant removal by EDTA from calcareous soils. In addition, according to the results of this paper, several soil washing with low concentrations of EDTA will not significantly decrease the soil pH. Therefore, the soil buffering capacity will remain constant in this method of soil remediation. In fact, in such a case, the remediated soil will keep its retention capability after the remediation process.