كليدواژه :
ظرفيت تبادل كاتيوني , بنتونيت , كربنات كلسيم , باريم كلريد , يون كلسيم
چكيده فارسي :
ظرفيت تبادل كاتيوني به عنوان مهمترين سرشتي زيست محيطي خاك ها است. براساس تحقيقات ارائه شده توسط محققين مختلف، روش هاي رايج تعيين كاتيون هاي تبادلي و ظرفيت تبادل كاتيوني (CEC) خاك هاي رسي كربنات دار به دليل انحلال كربنات كلسيم با مشكلات زيادي همراه است. با توجه به گستردگي خاك هاي حاوي كربنات كلسيم در ايران، مطالعه تاثير كربنات بر مقادير اندازه گيري شده CEC در اين خاك ها ضروري است. همچنين انتخاب وزن بهينه و مناسب خاك براي تعيين CEC مشكل رايج در تحقيقات است. هدف اين مقاله توسعه و اعتبارسنجي روش اصلاح شده باريم كلريد براي تعيين دقيق ظرفيت تبادل كاتيوني خاك هاي حاوي كربنات كلسيم و يا حاوي مقادير قابل توجه كاتيون تبادلي Ca2+ است. به اين منظور روش باريم كلريد، با اشباع نمودن محلول تبادلي با كربنات كلسيم اصلاح شده است. براي اعتبارسنجي روش اصلاح شده پيشنهادي، بنتونيت مورد مطالعه با استفاده از اسيد، كربنات زدائي شده و مقدار ظرفيت تبادل كاتيوني اين نمونه، قبل و بعد از كربنات زدائي و نيز براي نمونه هاي داراي درصد مختلف كربنات طبيعي و آزمايشگاهي مورد اندازه گيري قرار گرفت. نتايج بدست آمده بيانگر كاهش 60% در مقدار اندازه گيري شده ظرفيت تبادل كاتيوني نمونه بنتونيت در روش اصلاح شده در مقايسه با مقادير بدست آمده از روش متداول بوده است. علت اين موضوع به عدم رسوب باريم در روش اصلاح شده نسبت داده شده است. نتايج نشان مي دهد كه در روش اصلاح شده، مقدار CEC اندازه گيري شده و مجموع كاتيون-هاي تبادلي، به درصد كربنات در هر وزن خاك وابسته نبوده و همچنين مستقل از نسبت محلول- جامد مي باشد.
چكيده لاتين :
Cation exchange capacity (CEC) is known as the main geo-environmental characteristic of soil. According to the results of several researches, the current method for determination of CEC and exchangeable cations in carbonated soils has faced several problems due to the solubility of calcium carbonate. In this type of soil, it is essential to study the impact of calcium carbonate concentration of soil through the quantity measurements of soil CEC. In addition, the selection of optimum soil weight for CEC experiment is a common problem in previous researches. The objective of this research is to develop and to validate the modified barium chloride method for CEC measurement and determination of accurate exchangeable calcium cation concentration in carbonated clayey soils. To achieve this objective, the current method of barium chloride is modified by saturation of exchangeable electrolyte with calcium carbonate. In this step, the dried samples of bentonite with the initial weights of 0.3, 0.5, and 0.7 grams were mixed with 30 ml of 0.1 M BaCl2 for two hours. Then, samples were centrifuged at 2500 rpm for a 5 minutes period. The achieved electrolytes were used for analysis of exchangeable cations and the measurement of barium concentration. Generally, by the use of the current barium chloride method for CEC measurement of carbonated soils, one observes an increase in the measured quantity of CEC of a single soil sample as its carbonate concentration increases. This can be attributed to the solubility of calcium ions and precipitation of barium ions which cause a wrong measured quantity for CEC. Furthermore, for validation of the proposed modified method, a bentonite sample was de-carbonated with acid. The CEC of this sample was measured before and after de-carbonation. Moreover, the CEC was measured for laboratory added calcium carbonate to the de-carbonated bentonite. The achieved results show around 60% reduction in the measured quantity of CEC of bentonite in the proposed modified method in comparison to the common barium chloride method. This is attributed to the prevention of the precipitation of barium in the modified method. In addition, according to the achieved results in the validation section of this research, the reduction of the deviation of CEC and summation of exchangeable cations, in some cases was around 80%. For instance, for a sample with initial weight of 0.3 grams, the measured quantity of calcium ion decreases from 66.45 to 8.09 cmol/kg-soil. The achieved results show that in the modified method the measured quantity of CEC and the summation of exchangeable cations was independent to the weight of carbonate in the soil sample and soil: electrolyte ratio. It should be emphasized that according to the results of this paper, the standard deviation and coefficient of variation of the proposed modified method show 5 to 12 times reduction in comparison to that of the current barium chloride method.
