Comparison of methylene blue adsorption on bentonite measured using the spot and colorimetric methods

The maximum amount of methylene blue (MB) adsorbed on montmorillonite in bentonite can be determined using a spot or colorimetric method to evaluate the quality of bentonite. The spot method differs from the colorimetric method in terms of the procedure used to add MB solution to a bentonite dispersion and the procedure to determine the maximum amount of MB. In the spot method, MB solution is added in a step-by-step process and the maximum amount is determined by observing a spot of MB and bentonite on filter paper. In the colorimetric method, MB is added only once and the maximum amount is determined using a spectrophotometer. This study revealed that the maximum amount of MB adsorbed onto bentonite measured using the spot method differs from that measured using the colorimetric method. The maximum amount measured using the colorimetric method can be estimated by multiplying the amount measured using the spot method by 0.937. To ascertain why the maximum amounts measured using the two methods differ, we examined bentonites using a newly proposed method (a step-by-step colorimetric method), in which MB is added in a step-by-step manner and the maximum amount is determined using a spectrophotometer. The maximum amount measured using the step-by-step colorimetric method showed that step-by-step addition of MB decreases the maximum amount. The decrease can be explained by the re-flocculation of montmorillonite particles in bentonite.