Equilibrium data and its analysis with the Freundlich model in the adsorption of arsenic(V) on granular ferric hydroxide

An accurate understanding of equilibrium conditions is an essential starting point for dimensioning and modelling efforts in adsorption systems. However, key assumptions and methodologies in the generation and analysis of adsorption data sets might be contributing to the error in the description of the equilibrium. This study investigates the potential impacts of the equilibration time, test adsorbent particle size selection, and the regression technique applied, on the fitted coefficients of the Freundlich equilibrium model in the adsorption of arsenate on Granular Ferric Hydroxide. The choice of regression algorithm was found to impact primarily the exponent describing the energetic heterogeneity of the adsorption surface and resulting non-linearity of the isotherm, n_f. Non-linear regression with a hybrid error function on the observed solute concentration rather than loading was found to be a suitable approach. Insufficient equilibration time was found to impact predominantly the strength of adsorption, given by K_f, when applying a small adsorbent size fraction, and both coefficients when applying a coarser technical size fraction. Application of the small size fractions in lieu of the larger in accelerated trials appeared to over-estimate both coefficients, but is a reasonable alternative to long contact times.