Sorption and desorption of chromium in bentonite and kaolinite mixtures

Chromium sorption in soil occurs under different conditions based on the oxidative state of the Cr ion. Variety of factors is influencing the sorption of Cationic Cr III in sub surface environments. In this study, sorption of Cr III has been studied in mixtures of different percentages of Benotonite and Kaolinite clay such as 0:100, 25:75, 50:50, 25:75 and 100:0. Experiments were conducted under various soil solution ratios 1:100, 1:90, 1:80 and 1:70 and pH values 4, 6, 8 and 10 for both sorption and desorption stages. Cr concentrations were applied as 10,20,30,40 and 50 ppm. Isotherms were fitted with Langmuir and Freundlich models and the processes are characterized by distribution coefficients for sorption and desorption stages. Mathematical Modeling was carried out through regression analysis for the distribution coefficients during sorption and desorption with various parameters pH, Soil solution ratio, ratio of surface area to volume of the mixtures of Bentonite and Kaolinite using SPSS -17.0. Principal component analysis was also carried out to determine correlations among the parameters using XLSTAT 2009.

Key words: Kaolinite, Bentonite, Sorption, desorption, Langmuir, Freundlich, Distribution coefficient, regression analysis

[...] Flame Atomic Adsorption Spectrophotometer (Chemito) was used to obtain the concentrations of the chromium in the supernatants under the operating parameters wave length 357.9 nm for concentrations 2-8 ppm and 425.4 nm for concentrations 10-20 ppm. The dissolved chromium (trivalent chromium) is converted to hexavalent state by oxidation with potassium permanganate. The time required to reach equilibrium concentration was observed as 24 hours. The mass of the sorbed contaminant per unit mass of soil is then obtained through the difference between initial concentration and equilibrium concentration following CS = (Co Ce) V / M Property Standard K B Table-1 Geotechnical properties of K&B (K-Kaolinite and B-Bentonite) Mixture LiquidL Plastic Exchangeable cations CEC proportion imit Limit (cmol/kg) cmol/kg % % B Table-4 Model summary for distribution coefficient for sorption Model Sum of Squares df Mean Square F 1 b. [...]

[...] Five mixture proportions Bentonite and Kaolinite in percentages 0:100, 25:75, 50:50, 75:25 & 100:0 pH ranges as & 10 Soil solution ratio 1:100,1:90,1:80, 1:70 Chromium concentrations 10,20,30,40 & 50mg/l 2.1 Sorption stage This test consists of suspending and stirring clay and contaminant solutions. Clay contaminant solutions were placed in 1000cc glass jar with synthetic rubber plugs and motor with shaft and blades provided for continuously stirring the solutions at 250 rpm. pH of the clay contaminant solution was maintained to the desired level with the addition of buffer solution and measured frequently during experiments. [...]