Phosphate Complexation Model and Its Implications for Chemical Phosphorus Removal

Abstract

A phosphate complexation model is developed, in an attempt to understand the mechanistic basis of chemically mediated phosphate removal. The model presented here is based on geochemical reaction modeling techniques and uses known surface reactions possible on hydrous ferric oxide (HFO). The types of surface reactions and their reaction stoichiometry and binding energies (logK values) are taken from literature models of phosphate interactions with iron oxides. The most important modeling parameter is the proportionality of converting moles of precipitated HFO to reactive site density. For well-mixed systems and phosphate exposed to ferric chloride during HFO precipitation, there is a phosphate capacity of 1.18 phosphate ions per iron atom. In poorly mixed systems with phosphate exposed to iron after HFO formation, the capacity decreased to 25% of the well-mixed value. The same surface complexation model can describe multiple data sets, by varying only a single parameter proportional to the availability of reactive oxygen functional groups. This reflects the unavailability of reactive oxygen groups to bind phosphate. Electron microscope images and dye adsorption experiments demonstrate changes in reactive surface area with aging of HFO particles. Engineering implications of the model/mechanism are highlighted. Water Environ. Res., 80, 428 (2008).

Phosphorous removal in batch systems using ferric chloride in the presence of activated sludges

“…Other authors also demonstrated that equilibrium chemistry is not always fully applicable to the precipitation of phosphorus using ferric chloride. In most cases authors are forced to postulate new ferric phosphate species [24], adsorption [23] or complexation mechanisms [26] to better represent their results.…”

Molecular Variability in Wastewater Organic Matter and Implications for Phosphorus Removal Across a Range of Treatment Technologies

“…In orthophosphate removal, co-precipitates are formed between the soluble phosphorus and a metal salt, such as aluminum of iron salts (Smith et al 2008). Solids are removed in with the sludge and taken to landfill .…”

Comparison of Phosphorus Fractionation in Effluents from Different Wastewater Treatment Processes

“…calcium, magnesium, iron, or aluminum phosphates, or hydroxyphosphates). The direct precipitation of pure ferric phosphate does not appear to occur to any significant extent at pH values above 5 (Takacs et al, 2006;Smith et al, 2008;Szabo et al, 2008). This indicates that the components of pRP can be complex due to different operating conditions and other mechanisms, such as adsorption and co-precipitation of phosphate, can contribute more to the formation of pRP rather than precipitation of ferric phosphate.…”