Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Development of instrumental procedures for the analysis of toxic forms of arsenic in environmental waters and sediments

Danny McDonald

Supervisor: Dr Terry Cardwell

Arsenic is a naturally occurring element in waterways. Levels may be increased by anthropogenic activities that enhance the rate of leaching from rocks and sediments, smelting ores, and the use of pesticides and dyes. It usually occurs in the 3+ or 5+ oxidation state, but can also occur uncharged. Arsenic also occurs in the organic form as monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and Arsenobetaine. It is necessary to be able to measure arsenic speciation as the toxicity of different arsenic species is not the same. Inorganic arsenic is both more toxic and more prevalent in the environment than organic arsenic. As such, this thesis focuses on the analysis of inorganic arsenic.

Currently, HPLC-ICP-MS is the best method for analysing arsenic speciation. Unfortunately, this method is not ideal as it has very high setup and running costs. Many literature articles talk about using chromatography (HPLC or IC) followed by HG-AAS for arsenic speciation. This technique is appreciably cheaper, but is still a fairly complex technique.

This thesis then investigates the suitability of three different techniques to measure arsenic speciation at the levels required for natural waters (detection limit of 0.01 mg/l or lower). These techniques are IC with conductivity detection, HPLC with uv/vis detection at 200 nm and HG-AAS. Conditions were found for IC where arsenate could be separated from common anions (fluoride, chloride, nitrite, nitrate, phosphate and sulphate). The detection limit of 0.1 mg/l was not suitable for the analysis of environmental samples, and arsenite was unable to be detected. HPLC was also unsuitable with a detection limit of 0.5 mg/l for both arsenite and arsenate. The separation achieved in 12 minutes is so good that conditions may be changed to provide a shorter analysis time. HG-AAS is able to correctly analyse arsenite and total arsenic down to 0.1 mg/l however the procedure is fiddly, time consuming and suffers from instrumental drift.

This project was carried out in collaboration with the Victorian EPA.