Effect of pore size distribution on iron oxide coated granular activated carbons for phosphate adsorption – Importance of mesopores

Adsorption is often suggested for to reach very low phosphate levels in municipal wastewater effluent and even to recover phosphate. Adsorbent performance is usually associated with surface area but the exact role of the pore size distribution (PSD) is unclear. Here, we show the effect of the PSD on phosphate adsorption. Granular activated carbons (GACs) with varying PSDs were treated with potassium permanganate followed by reaction with ferric chloride to form iron oxide coated GACs (Fe-GACs). Energy dispersive X-ray and kinetics experiments confirmed that manganese anchored on the GAC is important for subsequent iron attachment. Mössbauer spectroscopy showed presence of ferrihydrite in Fe-GAC. Transmission electron microscopy showed that the iron oxide particles are not present in the micropores of the GACs. Phosphate adsorption isotherms were performed with the Fe-GACs and adsorption at lower phosphate concentrations correlated with the porous area of >3 nm of the adsorbents, a high fraction of which is contributed by mesopores. These results show that high surface areas of GACs resulting from micropores do not contribute to adsorption at low phosphate concentrations. This can be explained by the micropores being difficult to coat with iron oxide nanoparticles, but in addition the diffusion of phosphate into these pores could also be hindered. It is therefore recommended to use backbones having high mesoporous areas. This information is useful for developing adsorbents particularly for applications treating low phosphate concentrations, for e.g. in municipal wastewater effluent polishing.