Determination of soluble aluminium concentration in alkaline humic water using atomic absorption spectrophotometry

Introduction

The AAS with a nitrous - acetylene flame

High level of soluble aluminium in water bodies has detrimental effects to the environment. Therefore the soluble aluminium concentration in wastewater discharged by plants in the aluminium industry must be monitored and, subsequently, controlled within the permitted range.

The standard method to determine trace concentration of soluble aluminium begins with filtering followed by acidifying then analysing with the Atomic Absorption Spectrophotometer (AAS). When applied to brown drainwater flowing through a historical red mud pond of an aluminium smelter, acidification gives rise to the formation of a brown particulate matter.

The project addresses the following questions:

• What is the particulate matter?
• How does the particulate matter affect the performance of the AAS?
• How to determine precisely the soluble aluminium concentration in the presence of the particulate matter?

Methods

113 drainwater samples were studied with the AAS, Scanning Electron Microscopy (SEM), proton Nuclear Magnetic Resonance (1H - NMR) spectroscopy and Infrared (IR) spectroscopy techniques.

Results and Discussion

Genesis of the particulate matter

The plant bark covering the historical red mud pond and the temperate climate in the region enable the formation of humic substances under a series of complex activities of soil micro-organisms. Humic acid is a class of humic substances that is soluble in alkaline condition and precipitates at acidic pH.

Water in contact with red mud becomes highly alkaline, therefore extracts a large amount of humic acid from the bark layer. When the water is analysed by acidification, humic acid precipitates out of the solution in the form of a brown particulate matter.

Elemental analysis

The SEM spectrum reveals a significant amount of aluminium bound to the particulate humic acid.

Spectroscopic studies

1H - NMR and IR spectra of the particulate matter are compared with those of humic acid in the literature. The presence of typical peaks of the humic acid in the spectra confirms that the particulate matter is humic acid.

Limitation of the current method

The particulate nature of humic acid is prone to induce inaccuracy in the AAS readings as a consequence of the non-uniform distribution of the particulate humic acid during the analysis.

Development of a new method

Experiments with non-humic waters show that soluble aluminium concentration can be determined accurately in basicified solutions. Hence basicification is an alternative to the conventional acidification method.

When applied to humic water, basicification keeps the humic acid in the soluble form, hence maintaining the homogeneity of the samples during analysis.

Conclusion

The following conclusions were drawn from this study:

• The particulate matter was identified as humic acid.
• The analytical results obtained from the acidified solutions of alkaline humic water were adversely affected by the tendency of humic acid to settle.
• Basicification was found to be able to give more accurate results than acidification when quantifying soluble aluminium concentration in the presence of humic acid.

Acknowledgement

The work was supported by Comalco Aluminium (Bell Bay) Limited and the School of Chemical Engineering, University of Adelaide. The authors would like to thank the Department of Chemistry and Adelaide Microscopy at the University of Adelaide for their help with the spectroscopic and microscopic studies, respectively.

Publication

Nguyen, L., Lewis, D., Jolly, M., Robinson, J., Determination of Soluble Aluminium Concentration in Alkaline Humic Water Using Atomic Absorption Spectrophotometry, Water Research, submitted 2 September 2003.