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Cation exchange capacity (CEC) is known as the main geo-environmental characteristic of the soil in which in uncontaminated soils it is a direct function of soil swelling, cohesion, and specific surface area. According to the results of the several researches the current method for determination cation exchange capacity and exchangeable cations in carbonated soils has faced to several problems due to the solubility of calcium carbonate. Carbonated soils can be found in many parts of Iran, therefore, in this type of soil, it is essential to study the impact of carbonate calcium concentration of soil upon the measures quantity 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 sample of bentonite with the initial weight of 0.3, 0.5, and 0.7 grams were mixed with 30 ml of 0.1 M BaCl2 for two hours period. Then, samples was centrifuged at 2500 rpm for a 5 minutes period. The achieved electrolytes was 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 cation exchange capacity 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 is decrease 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 for the current barium chloride method.

Keywords: Cation Exchange Capacity, Bentonite, Carbonate Calcium, Barium Chloride, Calcium Ion

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