Use of Methylene Blue and Crystal Violet for Determination of Exchangeable Cations in Montmorillonite

Abstract

Ahstract--A procedure for the determination of cation exchange capacity (CEC) and the amounts of exchangeable cations adsorbed to montmorillonite is proposed. The method consists of a single incubation of the clay in a suspension containing a low concentration of an organic dye of large binding affinity, followed by analysis of the displaced inorganic cations by inductively-coupled plasma emission spectrometry (ICPES). The CEC is obtained by taking the largest sum of displaced exchangeable cations. Montmorillonite suspensions were incubated with methylene blue (MB) or crystal violet (CV) at dye concentrations below 4 mM, for one, three or fourteen days. For total dye concentrations up to the CEC, all the dye was adsorbed and equivalent amounts of exchangeable cations were released. Both dyes could adsorb to the clay in excess of the CEC.After one day of incubation in the presence of dye concentrations of about 50% in excess of the CEC, the total amounts of cations released were reduced to below the CEC. This reduction was interpreted as due to massive aggregation of the clay particles induced by the dye. With CV the total amounts of cations released after three or fourteen days of incubation increased and became equal to the CEC.The same CEC was found for Na-, Ca-and SWyl crude-montmorillonite, by employing either of the dyes.

Exchange Reactions in the Ca-Mg-Na-Montmorillonite System

“…The concentrations of the soluble cations were determined in supernatant A, while the amount of cations adsorbed were determined in supernatant B. Elemental concentrations were determined using a Spectra inductively coupled plasma atomic emission spectrometer (ICPAES). Measurements of standards of inorganic cations dissolved in MB solutions of different concentrations showed that the presence of the dye did not interfere with the analysis of the inorganic ions (Rytwo et al 1991).…”

“…The precipitate was washed three times, by the centrifugation method with distilled water and freezedried in a Christ Alpha 4-1 lyophilizer. The cation exchange capacity of the Na saturated montmorillonite was 0.81 mmole g l (Rytwo et al 1991). With the SSA specified above, it yields an area per charged site of 1.55 nmL Clay suspensions were prepared by dispersing 5 g of Na-montmorillonite in 200 mL of distilled water.…”

“…Experimentally, we have determined the amounts of adsorbed cations in clay treated with various proportions of the ions. A solution with low concentration of an organic cation of large binding affinity, was used to displace the adsorbed ions in a single desorption step (Rytwo et al 1991). This procedure increased significantly the precision of the ion exchange measurements.…”

Sorption of Selected Cationic and Neutral Organic Molecules on Palygorskite and Sepiolite

“…The Kag may be determined from the adsorption spectrum of pure dye in solution. The adsorption of dye dimers or higher order aggregates may be ignored (Rytwo et al 1991). Dye aggregation is significant when the dye addition exceeds the CEC of the clay, because below the CEC, almost all the dye is adsorbed.…”

“…The higher affinity of this compound to form charged complexes, PXi~, on negative sorption sites may be a factor explaining the higher sorption of this cation, but the main reason seems to be the tendency of MB to form aggregates in the solution, thereby reducing the monomer concentration (Cenes and Shoonheydt 1988; Rytwo et al 1995). Rytwo et al (1991) also reported that CV shows a better diffusion into the clay aggregates that leads to sorption of this cation, even after flocculation of clay particles that may occur due to sorption of cations. Inclusion of Equation [9], which accounts for aggregation of MB in the solution, in the iterative procedure, successfully described the lower sorption of MB for both clay minerals.…”