Nature of ammonium ion adsorption by sepiolite: analysis of equilibrium data with several isotherms

Adsorption isotherms for an ammonium ion-sepiolite system have been established for initial ammonium ion solution concentrations in the range of 8.32-388.06 mmol NH(4)(+)/L (150-7000 ppm). The change of the "g sepiolite/mL solution" ratios from 1/10 to 1/70 caused an increase in the adsorption from 1.82 to 3.70 mmol NH(4)(+)/g for the average particle size of 2.8mm. The equilibrium data were fitted to "general-purpose isotherms" and "first-order adsorption model", and the adjustable parameters of the isotherms were estimated by nonlinear least-squares analysis. Langmuir and Freundlich models were found insufficient to explain the adsorption equilibrium, while Langmuir-Freundlich and Tooth isotherms explained the data well. Goodness of fit increased with the increased sepiolite quantity in the treated suspension.