Characterisation of dissolved organic matter in water treatment using ultra-high resolution techniques

Abstract

Dissolved Organic Matter (DOM) is an extremely heterogeneous complex mixture, consisting of thousands of chemical species. The composition of DOM greatly influences the potable water production process. Current methods employed by industry for the characterisation of DOM are relatively low resolution; namely Total Organic Carbon (TOC) analysis or molecular weight analysis via Liquid Chromatography – Organic Carbon Detection (LC-OCD). In this thesis, high-resolution spectrometric and spectroscopic techniques were used to investigate the complexity of DOM and to characterise changes that occurred throughout various treatment processes.

A pilot plant evaluating the use of a Suspended Ion Exchange (SIX) system, followed by coagulation and ceramic membrane filtration, for the removal of DOM was sampled and investigated. Nuclear Magnetic Resonance (NMR) and negative mode electrospray – Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI (-) FT-ICR-MS) were used to characterise the organic species present within the inlet water, post ion-exchange and the final outlet water. The samples from this pilot plant were also compared to those of the existing, full-scale water treatment works present on the same site. This study found via mass spectrometry that the SIX treatment was non-selective in the removal of compounds, with the composition of samples being highly similar to those of the raw water. The coagulation and ceramic membrane filtration treatment, however, was selectively removing aromatic and phenolic species. These findings were also corroborated with the use of NMR and Fourier Transform Infrared Spectroscopy (FT-IR). Compounds with these characteristics have been shown to result in the production of by-products in later treatment stages, so their removal is desirable.

A second pilot plant was investigated to evaluate the use of granular activated carbon (GAC) and ion-exchange (IEX) filtration as post coagulation treatments for the removal of DOM over the course of 6 months. Throughout the 6-month period, the IEX consistently outperformed the GAC treatment in terms of number of species removed, reduction of aromatic compounds and performance stability over time. The GAC treatment however, also resulted in the removal of aromatic compounds and is a less expensive system to implement and maintain. DOM has been shown to negatively impact many of the processes used to create potable water, increasing coagulation loads, reducing the lifetime of any filtration systems, and creating by-products in the disinfection stage. This makes DOM removal essential for improving the quality of the produced drinking water and the efficiency of a treatment works.

Photocatalysis is a highly promising method for the oxidation of DOM and its mineralisation. TiO2 is a well-known photocatalyst capable of degrading DOM, when activated by UV light. The composition of Suwannee River Fulvic Acid (SRFA), a common international DOM standard, throughout photocatalytic degradation under various wavelengths was characterised across a period of three hours and three irradiation wavelengths (370 nm, 410 nm and white LED). The performance of pristine TiO2 was compared to that of a TiO2-based catalyst that had been doped with bismuth. We show that the mechanism of degradation is highly likely to be the same for both catalysts and that the performance of the doped catalyst is superior to that of pristine TiO2; doping of TiO2 has enabled more efficient utilisation of softer irradiation. Based on a detailed analysis of ESI (-) FT-ICR-MS data, the effects of photocatalysis were monitored in terms of molecular weight distribution, double-bond equivalent, aromaticity, oxygen numbers and a susceptibility of compound classes. Chemometric analysis of 1H NMR data highlighted the existence of long chain fatty acids as products of the photolytic degradation. This work represents the most detailed molecular level analysis of photocatalytic degradation of DOM to date.

Disinfection of potable water is essential to providing the public with a safe drinking source. DOM has the potential to react with chemical disinfectants such as chlorine, the most widely used primary disinfectant. This results in the production of a variety of disinfection by-products (DBPs), some of which are regulated. To reduce the production of DBPs, water suppliers are exploring alternative disinfection processes. One process gaining traction is chloramination. In this thesis, we explore chloramination via high-resolution NMR, employing 19F as an NMR probe into the complexity of DBPs. To effectively achieve this, existing 19F NMR experiments were optimised, and new pulse sequences developed. These include methods for establishing the 1H - 19F and 19F - 13C correlation utilising far-reaching proton-fluorine and carbon-fluorine scalar coupling constants. The obtained coupling constant and correlated 1H, 13C and 19F chemical shifts were used to propose structures of a dozen chloramination DBPs produced from 3-Fluoro-4-hydroxybenzoic acid, a compound with functional groups known to be prevalent in DOM. Decarboxylation, dehydroxylation, chlorination and an addition of nitrogen were observed. The developed methodology will assist in developing chloramination as an industrial process.