Preparation of adsorbent for phosphate recovery from aqueous solutions based on condensed tannin gel

We have synthesized an iron-loaded tannin gel as an adsorbent for phosphate recovery in aqueous solutions. The use of the tannin gel prepared from condensed tannin, which is a ubiquitous and inexpensive natural polymer, is not only cost effective and environment-friendly, but interesting because the phosphate-adsorbed gel can be expected to use directly as a fertilizer. The amount of iron loaded into the tannin gel oxidized by nitric acid was much larger than that into the non-oxidized tannin gel. This increase in the amount of the loaded iron resulted in the significant increase in the adsorption amount of phosphate onto the gel. Mössbauer spectroscopy indicated that the morphology of iron in the gel is a mono-type complex, which is formed as a result of the reaction between Fe(III) and the oxidized tannin gel with carbonyl groups. The iron-loaded tannin gel showed the adsorption selectivity for phosphate over other anions and the pH independence of phosphate adsorption in the wide range of initial pH 3-12. The phosphate adsorption isotherm for the iron-loaded tannin gel followed the Freundlich equation with constants of K(F)=2.66 and 1/n=0.31, rather than the Langmuir equation. The adsorption amount of phosphate on iron weight basis for the iron-loaded tannin gel is 31.3mg-P/g-Fe, which indicates that iron in the gel was efficiently used for the phosphate adsorption compared with other phosphate adsorbents, such as iron hydroxides.