Year

2002

Degree Name

Doctor of Philosophy

Department

Department of Environmental Science

Abstract

This study was undertaken in response to the potential release of a MoO42- -PO43-containing waste effluent into aquatic ecosystems. The environmental behaviour, associated fate and subsequently, identification of a suitable treatment technique for removal of molybdenum from waste process solutions was investigated. A detailed literature review and experimental studies were completed.

This study investigated the adsorption of MoO42- on a number of naturally occurring particulates, amorphous ferric oxyhydroxide [Fe2O3 · H2O(amorphous)], goethite, hematite, hydrous manganese dioxide and an aluminosilicate, kaolin. MoO42- adsorption on these particulates was examined as a function of pH, particle concentration and initial anion concentration. MoO42- adsorption increased with decreasing pH (with a maximum at ~4.5) and increasing particle concentration via the formation of an inner-sphere (surface co-ordination) complex. This was determined using a combination of a series of macroscopic (electrolyte variations) techniques and spectroscopic data.

The effects of competitive anion adsorption on the adsorption behaviour of MoO42- on Fe2O3 · H2O(amorphous) as a function of anion and particle concentration was also investigated for a range of anions. These experiments showed that VO43- and PO4 3-both illustrated a greater affinity for Fe2O3 · H2O(amorphous) than MoO42- when present in equimolar amounts. Additionally, it was showed that MoO42- displayed a greater affinity than CrO42-, SeO42- or SO42- (in that order).

A literature review concluded that due to its environmental behaviour, a suitable treatment technique for the removal of molybdenum from discharge waters was required. The review indicated that traditional wastewater treatment techniques for heavy metals, which primarily include the dosing of wastewaters with CaO or NaOH to precipitate insoluble phases, are ineffective for MoO42- . Therefore, a number of alternative treatment regimes were tested for the removal of MoO42- from the waste stream.

The most appropriate treatment identified was adsorption/co-precipitation, which was then studied experimentally to determine its effectiveness in MoO42- and PO4 3-containing systems with freshly precipitated Fe2O3 • H2O(amorphous). However, the inhibition of MoO42- removal from solution in the presence of high initial concentrations of PO43-, necessitated the introduction of a preliminary treatment step.

PO43- was selectively precipitated as a calcium phosphate phase by the addition of either CaCl2 or Ca(OH)2. Additionally, a selective precipitation technique utilising MgCl2 was demonstrated. Following the use of an initial selective pretreatment step, more than 99% of the MoO42- was removed by adsorption/co-precipitation on Fe2O3 · H2O(amorphous). This was incorporated within a Fe2O3 · H2O(amorphous) recycling process. The adsorption/co-precipitation process with Fe2O3 • H2O(amorphous), when used with the preliminary PO43-removal step is feasible for the efficient removal of MoO42- from an industrial effluent. The system developed also provides for recycling/reuse options for both molybdenum and phosphate to be included.

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