Investigation of the effect of phosphate on iron(ii) sorption to iron oxides

Iron is ubiquitous in the environment mostly as stable iron hydr(oxides) such as hematite (α-Fe2O3) and goethite (α-FeOOH). The Fe(II)-Fe(III) redox couple plays a vital role in nutrient cycling, bacteria respiration, and contaminant removal. This redox couple, however, can be affected by external influences such as anion adsorption of sulfate, oxalate, and phosphate which can influence various goethite properties including the point of zero charge. This study attempts to determine the effect of phosphate sorption to a goethite surface and its subsequent influence on Fe(II) sorption. The objectives, specifically, were to quantify phosphate sorption on a goethite surface using colorimetric methods and to use Mössbauer spectrometry to determine if electron transfer occurred after a layer of phosphate was adsorbed to the goethite surface. The hypothesis of this study is as follows: an adsorbed layer of phosphate on a goethite surface will inhibit the electron transfer between the Fe(II) and Fe(III) phases at the surface.

The results of the study showed that phosphate follows typical anion sorption as seen in previous works, where more phosphate sorbed at lower pH values. In addition, with increasing aqueous phosphate concentrations there is increasing phosphate adsorption to the goethite surface. However, phosphate sorption was not significantly affected by reaction time after 20 hours or by changes in Fe(II) concentrations. Fe(II) sorption pH edges showed characteristic cation adsorption, where more Fe(II) sorbed at higher pH values. Fe(II) sorption was not affected by the presence or absence of phosphate, but was affected by an increase in the aqueous Fe(II) concentration. With increased Fe(II) there was a pH edge shift to a higher pH, which is consistent with Ca2+ sorption results on goethite. An Fe(II) isotherm was also conducted and showed that as Fe(II) concentration increased so did Fe(II) sorption, however the isotherm appeared to be approaching a plateau where the goethite surface sites would be saturated, below this limit the surface sites where not saturated.

Mössbauer analysis was conducted on a sample by Drew Latta, spectra showed that electron transfer was still occurring despite the adsorbed phosphate layer, disproving our initial hypothesis. It is possible that a higher concentration of phosphate could inhibit electron transfer, but at 500 μM PO43- and 100 μM Fe(II), electron transfer between the adsorbed Fe(II) and bulk phase Fe(III) still occurred.