Retention of molybdenum(IV), vanadium(V) and tungsten(VI) by kaolin and three Finnish mineral soils

Abstract

Adsorption of aqueous species of Mo(VI) and V(V) from 10ˉ⁴ M and 10ˉ⁵ M and W(VI) from 10ˉ⁴ M sodium salt solutions by kaolin and three Finnish mineral soils was measured at room temperature using a solid:solution ratio 1:100 (w:v), pH range 2.3-7.5 (pH 2.8-7.5 for W) and an equilibration time of 72 h. Potassium chloride (0.02 M) was used as ionic strength adjustor. In desorption experiments, 0.80 g of soil or kaolin was extracted with 50 ml of 0.02 M KCl. A shaking time of 17 h was used. Kaolin H₂Al₂Si₂O₈•H₂O (Kaolin.pond Ph.Eur.III 180 μm) was provided by the Helsinki University Pharmacy. Cation exchange capacity and pH of kaolin were determined, as well as the powder X-ray diffraction pattern recorded. The test soils, a clay and two finesand samples with organic matter content less than 5 %, were provided by the Agricultural Research Centre, Jokioinen. The characteristics of soils necessary for explaining adsorption properties are given, and the possibility of the reduction of V(V) in contact with these soils is discussed. Results are given in the form of adsorption and desorption envelopes (adsorption or desorption vs. pH). For molybdate, protonation constants determined from a 10ˉ⁴ M Na₂MoO₄ solution in 0.02 M KCl were used in explaining adsorption and desorption of different ionic species initially present in solution. Retention of V(V) was interpreted using protonation constants taken from the literature. The aqueous chemistry of W(VI) proved to be so inadequately known that it was not possible to explain retention of W(VI) by identifying different ionic species initially present in solution. Time-dependence of the adsorption of V(V), Mo(VI) and W(VI) at the natural pH of each soil, as well that of Mo(VI) from solutions with initial pH values 2.0 and 3.0 was followed for 70 h. Amounts of phosphorus released at the natural pH of each soil, in addition to those of Ca, Mg, Mn, P, Al, and Fe released in acidic conditions were measured. All elemental analyses were performed by plasma emission spectrometry. The possibility of utilizing ¹⁷O and ⁵¹V NMR spectrometry in identifying mono- and polyionic species in aqueous solutions is discussed.