Application of molybdenum adsorption phenomena in natural and waste water treatment systems

Author

David Phillip Blackmore, University of Wollongong

Year

2002

Degree Name

Doctor of Philosophy

Department

Department of Environmental Science

Recommended Citation

Blackmore, David Phillip, Application of molybdenum adsorption phenomena in natural and waste water treatment systems, Doctor of Philosophy thesis, Department of Environmental Science, University of Wollongong, 2002. https://ro.uow.edu.au/theses/1380

Abstract

This study was undertaken in response to the potential release of a MoO₄^2- -PO₄^3-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 MoO₄^2- on a number of naturally occurring particulates, amorphous ferric oxyhydroxide [Fe₂O₃ · H₂O(amorphous)], goethite, hematite, hydrous manganese dioxide and an aluminosilicate, kaolin. MoO₄^2- adsorption on these particulates was examined as a function of pH, particle concentration and initial anion concentration. MoO₄^2- 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 MoO₄^2- on Fe₂O₃ · H₂O(amorphous) as a function of anion and particle concentration was also investigated for a range of anions. These experiments showed that VO₄^3- and PO₄ ^3-both illustrated a greater affinity for Fe₂O₃ · H₂O(amorphous) than MoO₄^2- when present in equimolar amounts. Additionally, it was showed that MoO₄^2- displayed a greater affinity than CrO₄^2-, SeO₄^2- or SO₄^2- (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 MoO₄^2- . Therefore, a number of alternative treatment regimes were tested for the removal of MoO₄^2- from the waste stream.

The most appropriate treatment identified was adsorption/co-precipitation, which was then studied experimentally to determine its effectiveness in MoO₄^2- and PO₄ ^3-containing systems with freshly precipitated Fe₂O₃ • H₂O(amorphous). However, the inhibition of MoO₄^2- removal from solution in the presence of high initial concentrations of PO₄^3-, necessitated the introduction of a preliminary treatment step.

PO₄^3- 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 MoO₄^2- 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 PO₄^3-removal step is feasible for the efficient removal of MoO₄^2- from an industrial effluent. The system developed also provides for recycling/reuse options for both molybdenum and phosphate to be included.