Adsorption of tungstate (WO4) on birnessite, ferrihydrite, gibbsite, goethite and montmorillonite as affected by pH and competitive phosphate (PO4) and molybdate (MoO4) oxyanions

The knowledge about adsorption characteristics of tungstate (WO4) on soil mineral components is critical to elucidate WO4 mobility in the soil and aqueous systems. The objectives of this study were to characterize WO4 adsorption on selected soil clay minerals (gibbsite, ferrihydrite, goethite, birnessite and montmorillonite), and to investigate the competition in adsorption of WO4 with PO4 and MoO4 on these clay minerals as affected by pH. Based on an adsorption isotherm study, adsorption affinity for WO4 follows the order of gibbsite = Fe (oxyhydr) oxides > birnessite > montmorillonite in acid to alkaline pH ranges. The adsorption maximum of WO4 was greater in the acid pH for all minerals with an exception of ferrihydrite whose adsorption characteristics were independent on solution pH. We found that WO4 has higher affinity for clay minerals than PO4 under acid pH, and than MoO4 under alkaline pH. This is evidenced by the result that, by adding WO4 and oxyanions with the same molar ratio, more WO4 than PO4 and MoO4 was usually adsorbed on soil minerals in acid and alkaline pH, respectively. In a case of gibbsite, for example, the adsorbed WO4/adsorbed PO4 molar ratio was 1.10 at pH 4, and 0.49 at pH 8, whereas the adsorbed WO4/adsorbed MoO4 molar ratio was 1.16 at pH 4, and 8.19 at pH 8. Unlike other clay minerals, birnessite and montmorillonite showed a unique adsorption behavior that the presence of PO4 and MoO4 in solution enhanced the WO4 adsorption along with the increase of pH. The information reported herein provides for better understanding of WO4 adsorption on major soil clay minerals in relation to the solution pH and competitive oxyanions.