Chemical and radiometric characterization of bentonite mineral used as raw material in different sectors in Turkey

Bentonite mineral is a soft, porous and easily shaped and absorbent mineral rich in aluminum, sodium and potassium. Bentonites are used as drilling mud, ore pelletizing, absorbent, adsorbent, bleaching agent, water impedance, coating and raw material or filling material in various industries (ceramic, cosmetics, paint, textile, casting, construction, etc.). The effective and efficient use of bentonites in these sectors depends on their chemical components. In this study, bentonite samples collected from twenty-one quarries located in different geographical regions of Turkey were analyzed to determine the natural radionuclide, major and minor element distributions of bentonite quarries. The radiological risks that may be caused by use of bentonites as building materials and to which workers may be exposed were evaluated by estimating the activity concentration index (I), indoor absorbed gamma dose rate (DRI) and the corresponding annual effective radiation dose (EI); outdoor absorbed gamma dose rate (DRO) and the corresponding annual effective radiation dose (EO) and the lifetime cancer risk (CR). The average activity concentrations of 226Ra, 232Th and 40K radionuclide measured in bentonite samples using gamma-ray spectrometric method were found as 50.3 ± 4.7 Bq/kg 76.2 ± 3.9 Bq/kg and 372.8 ± 18.6 Bq/kg, respectively. The average concentrations of SiO2, Al2O3, CaO, Fe2O3, MgO, Na2O, K2O, TiO2, SO3, P2O5 and MnO oxides in bentonite samples analyzed by energy dispersion X-ray fluorescence spectrometer were found as 54.12%, 15.24%, 4.65%, 4.36%, 3.25%, 2.78%, 1.40%, 0.58%, 0.35%, 0.20% and 0.10%, respectively. The highest concentrations of SiO2, Fe2O3, K2O and SO3, and TiO2 were analyzed in samples collected from bentonite quarries located in Ordu, Çorum, Trabzon, and Çankırı provinces, respectively; the bentonite quarries in Konya province have the highest concentrations of Al2O3, MgO, P2O5, and MnO, while the bentonite quarry in Trabzon province has the highest concentrations of Na2O and CaO. The average values of I, DRI, DRO, EI, EO and CR calculated for each bentonite sample were estimated as 0.7, 159.1 nGy/h, 84.8 nGy/h, 0.2 mSv/y, 0.8 mSv/y and 7.0 x 10-4, respectively. The radiological evaluation results reveal that the examined bentonites can be used safely as raw materials in building materials and that there is a low level of radiological risk for workers working in bentonite quarries.