Adsorption of tungstate (WO 4 ) on birnessite, ferrihydrite, gibbsite, goethite and montmorillonite as affected by pH and competitive phosphate (PO 4 ) and molybdate (MoO 4 ) oxyanions

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