PHOSPHATE SORPTION BY CALCIUM-BENTONITE AS DESCRIBED BY COMMONLY USED ISOTHERMS

The one and two Langmuir, the Freundlich, and the Temkin isotherms were fitted to phsophorus (P) sorption data for P sorption by calcium (Ca)-bentonite at different initial concentration and pH values of 3.8, 4.8, 6.0, 7.0, 8.0, and 9.0. Each was found to describe P sorption by Ca-bentonite with comparable success. The effect of pH on P adsorption by Ca-bentonite was studied and Langmuir, Temkin, and Freundlich isotherms were converted to the forms: Langmuir: (Co-X)X = 1/(0.0275-0.0025pH(12.323-1.061pH) + (Co-X)(12.323-1.016pH) Temkin: X = (2.45-0.21lpH)In(AC) Freundlich: X = (1.324-0.146pH)C(0.172+0.044pH) where: X = (mmol P/kg) the amount of P sorbed per unit weight of soil, C (mumol P/L) the P concentration in the equilibrium solution, and Co = (mumol P/L) initial P concentration. It is noteworthy, that the maximum amount (Xm) of P that can be sorbed in a monolayer decreases by increasing of pH. Finally, the B constant of Temkin isotherms is indepented from pH changes.