Development of an automated trace analyser and a novel passive sampling device for the monitoring of ammonia in marine environments

Abstract

Ammonia is commonly used as an indicator of water quality, to assess the impacts of anthropogenic activity on ecosystem function and health. Water quality assessment often relies on the use of expensive equipment requiring a high degree of operator skill, or periodic, discrete, manual sampling and laboratory-based analysis, which is laborious, costly and without the guarantee that episodic pollution events will be detected. Low-cost, portable and/or field-deployable analytical tools are required to overcome this challenge. Hence, research conducted in the context of this thesis involves the development of novel analytical tools for the monitoring of ammonia in marine waters, covering both active and passive sampling.

A flow-based analytical method was designed and developed for the determination of total ammonia over a wide concentration range in marine waters using the gas-diffusion spectrophotometric method. Limits of detection similar to that of highly sensitive fluorometric methods was achieved. A novel flow approach was adopted whereby a continuous stream of sample was merged with the sodium hydroxide reagent stream and delivered to a gas-diffusion ammonia separation unit, allowing large sample volumes to be used, rather than being limited by the use of discrete samples. The working range and sensitivity of the method could be tailored by simple modification of the sample volumes used, and by minor adjustments to the program used to control the instrument, without the need to make changes to the manifold. Three working ranges were obtained, and the analytical figures of merit are described. This project was an enabling step in the development of an ammonia gas-diffusion passive sampling device, as it allowed the measurement of low concentrations often found in field samples, as well as high concentrations accumulated in the passive sampler’s receiving solution, using the same instrumentation and reagents.

A passive sampling device based on gas-diffusion across a hydrophobic membrane was developed and successfully applied for the determination of the time-weighted average concentration (Ctwa) of ammonia in marine waters for a period of 3 to 7 days. Molecular ammonia (NH3) present in the sampled source solution (SS) diffuses through a hydrophobic membrane into an acidic receiving solution where it is ionised and accumulated as NH4+ which is directly proportional to the NH3 concentration in the SS. Biofouling limited the application of the first gas-diffusion-based passive sampler (GD-PS) prototype to 3 days, and a number of antifouling strategies were therefore assessed, with a copper mesh enabling the sampling period to be extended to 7 days. The effects of environmental variables (temperature, pH and salinity) on NH3 accumulation were also investigated, and the Group Method of Data Handling (GMDH) Algorithm was used to develop a single calibration model for a range of environmental conditions (10 to 30 degrees C, pH 7.8 to 8.2, salinity 20 to 35). PSDs were deployed at four estuarine and marine sites in Nerm (Port Phillip Bay), south eastern Australia, achieving good agreement between passive and automated discrete sampling methods (maximum relative error between -12 % to -19 %). The GD-PS covers the revised water quality trigger value (160 ug L-1 NH3-N) and allows for episodic pollution events to be successfully detected, highlighting this as an exciting new tool for water quality assessment.