Predicting anion breakthrough in granular ferric hydroxide (GFH) adsorption filters.

Adsorption of arsenate, phosphate, salicylic acid, and groundwater DOC onto granular ferric hydroxide (GFH) was studied in batch and column experiments. Breakthrough curves were experimentally determined and modelled using the homogeneous surface diffusion model (HSDM) and two of its derivatives, the constant pattern homogeneous surface diffusion model (CPHSDM) and the linear driving force model (LDF). Input parameters, the Freundlich isotherm constants, and mass transfer coefficients for liquid- and solid-phase diffusion were determined and analysed for their influence on the shape of the breakthrough curve. HSDM simulation results predict the breakthrough of all investigated substances satisfactorily, but LDF and CPHSDM could not describe arsenate breakthrough correctly. This is due to a very slow intraparticle diffusion and hence higher Biot numbers. Based on this observation, limits of applicability were defined for LDF and CPHSDM. When designing fixed-bed adsorbers, model selection based on known or estimated Biot and Stanton numbers is possible.