Progress in Nuclear Magnetic Resonance Spectroscopy
NMR spectroscopy of wastewater: A review, case study, and future potential
Graphical abstract
Introduction
Water is a precious and limited resource, needed for everything from industry and agriculture to sustaining life as a whole. However, with over 97% of the Earth’s water being saltwater, and two thirds of freshwater being frozen in the ice caps and glaciers [1], there is a significant need to maintain and reuse this precious resource. Hence, modern society applies a number of techniques, collectively known as wastewater treatment, to prevent waterway pollution and even to recover water that has been previously utilized. This idea is not new, and, in fact, mankind has been employing wastewater treatment techniques since as early as 2500 BCE [2].Water treatment has (quite obviously) improved since then, and as our technology has improved, questions such as “how effective is this treatment technique?”, “what gets left behind?” and “what treatment technique would best fit wastewater from a specific source?” have begun to arise. Scientific understanding of wastewater treatment processes and their effects is necessary for the development of effective water treatment policies to help maintain our precious water supply.
Wastewater can span storm water runoff, as well as water comprised of industrial, domestic or commercial sewage [3]. This is quite a broad definition and exemplifies the considerable heterogeneity of wastewater. Depending on the source, wastewater varies in composition, which in turn determines its potential for pollution and subsequent environmental and human health risks [4]. As an example, the tannery industry, which is often considered a major source of pollution in general, employs a number of chromium salts to create high quality leather [5], [6]. Around 90% of the water used in the tanning process is released as wastewater [7], a total of around 300 million tons per year [8]. Prior to treatment, the wastewater may have concentrations of chromium hundreds to thousands of times greater than the recommended drinking water standards set by the United States Environmental Protection Agency [5], [6], [7], [9], [10]. On the other hand, storm water runoff, which picks up pollutants from buildings, roads and other sources as it flows into storm drains, is much more dilute, and commonly less biologically harmful than examples such as tannery effluent or raw sewage [4]. However, even storm water runoff may be harmful, and has been shown to have higher biological toxicity than properly treated wastewater [4]. In some instances, storm water can overwhelm treatment works, carrying raw sewage into watercourses or into the ocean. Clearly, if left untreated, all forms of wastewater can have a potentially detrimental impact on the environment.
Understanding the chemistry of wastewater is especially important if the wastewater is to be reused, for example in agricultural irrigation [11], [12], [13]. Inevitably, as environmental regulations shift, so too do the structures of chemical compounds being manufactured and used, many of which can end up in wastewater. Emerging contaminants are constantly being discovered [14], and, importantly, it is unclear if current wastewater treatments are effective in dealing with new compounds. As an example, there has been an increase in the concentrations of various antibiotics in waterways into which wastewater effluent flows, as many wastewater treatment plants are ill-equipped to handle pharmaceuticals [15], [16], [17], [18]. It is crucial to understand how these new (and old) contaminants are affected by wastewater treatment, as well as how they interact with the environment as a whole, in order to make effective decisions to protect our environment - an area in which NMR is an invaluable tool.
There have been a number of reviews published on the techniques used in wastewater treatment in general [19], [20], [21], [22], [23], [24], [25], [26], as well as reviews of treatment practices in specific industries [13], [27], [28], [29], [30], [31], [32]. Here, we aim to briefly mention the three basic phases of water treatment and their properties, in order to help demonstrate the applications of NMR in understanding these processes.
Modern wastewater treatment is typically divided into three phases: primary, secondary and tertiary treatments [33], [34]. A general schematic of wastewater treatment can be found in Fig. 1. Conventional primary treatment is made up of physical methods, involving screens and sedimentation chambers (often called clarifiers) that rely on gravity to settle large components [34]. This is used to remove large components such as sand, twigs, stones and some primary biosolids. However, removal of suspended particles smaller than 50 μm is typically inefficient, and chemical flocculants are often added to help settle smaller particles [35]. This is known as chemically enhanced primary treatment (CEPT), and typically involves the addition of metal salts such as FeCl3, Al(SO4)3 and FeSO4 and/or organic polyelectrolytes to aid in flocculation [35], [36]. Though the efficiencies of various flocculants vary with factors such as pH, CEPT has been shown to be effective in removing up to 90% of total suspended solids, and significant proportions of total phosphorus [34], [35], [37].
Following primary treatment, secondary treatment, which involves biological processes, is applied. In this case, the wastewater is added to one or more bioreactors, which have a controlled population of bacteria in the form of activated sludge [34]. This can be done under aerobic, anaerobic or hybrid conditions [38], [39], with the main goal of degrading organic matter. Following this, an additional sedimentation step is employed to remove any biosolids formed, often with the aid of flocculants [33]. At this point, the water is sufficiently treated to be released into the environment, and is commonly used in crop irrigation, especially in arid regions [11], [12].
For household and industrial use, however, additional treatment is required. This is known as tertiary treatment and is meant to reduce or remove components that were not effectively removed in the first two phases. These include excessive phosphorus and nitrogen (which can lead to eutrophication) [40], leftover heavy metals [41], and pathogenic viruses or bacteria [42]. Tertiary treatment employs a separation technique such as sand beds [43], active charcoal [22], ultrafiltration [44] or reverse osmosis [45] to help remove heavy metals and any additional flocculated components. Further, the water is disinfected to allow safe consumption. This is commonly done through chlorination [34], [46], [47], though due to the formation of potentially hazardous chlorinated by-products [48], [49], [50], [51], there has been a shift towards other disinfection techniques such as UV irradiation [42], ozonation [52] and oxidation processes based on radical reactions [53]. A general schematic of wastewater treatment can be found in Fig. 1.
Like many environmental samples, wastewater can contain various phases (solids, liquids, gels) and chemical categories including carbohydrates, lipids, proteins and xenobiotics [33], [34], [40]. Heterogeneous complex sample analysis is an area in which NMR excels in comparison to other commonly used techniques such as mass spectrometry [54]. NMR has the potential for analysis of solids, solutions and gels, even using a single NMR probe [54], [55], [56], non-destructively and in their native, unconcentrated state [54], [57], preventing any of the biases or losses associated with sample preparation or cleanup. Further, NMR is a versatile technique that is able to analyze any analyte with a spin active nucleus such as 1H or 13C, along with many heteronuclei (e.g. 19F, 15N, 31P) [54] – a constraint that is met by virtually all environmentally relevant compounds. Using multidimensional NMR techniques, NMR has been reported to have considerable resolving power [58], making it applicable to even the most complex matrices and samples. Not only is NMR useful for structure determination and compound identification, it can also be used to examine non-covalent interactions [59], [60], [61], [62] between the components of wastewater, such as pollutant-protein or pollutant-humic substance binding - which is very difficult to do using other techniques.
In this review, we aim to examine the use of and potential applications of NMR as a tool for understanding wastewater and the processes and effects of wastewater treatment. While there have been many studies involving the use of NMR for the study of wastewater, to the authors’ knowledge there has only been one published review [33] directly linking NMR with wastewater. A deeper exploration of NMR techniques that have been and can be applied to wastewater is therefore warranted. Though this review will focus primarily on wastewater and wastewater treatment, where key NMR studies from related fields (e.g. natural waters, soils, sediments, agriculture) afford important examples, they will be included to demonstrate the breadth of NMR and its application to environmental research. In addition, we aim not only to provide a review of the literature, but also a short case study showing that when various NMR characterization approaches are combined with NMR based toxicity evaluation, the result is a comprehensive understanding of both wastewater and its impacts, that cannot be matched by any other modern analytical approach.
Section snippets
Sample preparation
Though one of the advantages of NMR is the possibility of little/no sample preparation [55], whether or not sample preparation is necessary is dependent on the goal of the research, the available NMR hardware, the experiments employed, and the concentrations of the analytes of interest. Indeed, the Achilles’ heel of NMR – sensitivity – may make sample preparation preferable, or even unavoidable, for the analysis of trace components. It is important to note that the following sections focus
Water suppression
In the case that unaltered samples are directly analyzed by NMR, water, which is the largest constituent of wastewater samples, contributes a very large and broad signal to the 1H NMR spectrum. Not only can this overlap with and mask signals of interest, it can also prevent optimization of receiver settings, leading to a lower signal-to-noise ratio, along with baseline roll and other distortions [132], [133]. Consequently, water suppression techniques are commonly employed in such studies to
Applications of wastewater NMR organized by nucleus of study
1D analysis, specifically 1H NMR, is by far the most commonly used NMR technique. However, the field of NMR is vast and there are many different nuclei and techniques that can be used in the study of wastewater and its treatment. The use of these techniques, especially in combination, has excellent potential for structure elucidation and mixture deconvolution, as well as the study of non-covalent interactions. In this section we will discuss a number of NMR techniques, and follow up with
Non-targeted analysis
One of the major benefits of 1H NMR is that it is a completely non-selective detector. While techniques like MS need to be optimized for individual compounds, and other techniques such as fluorescence detect only fluorescent compounds, 1H NMR can, to an equal degree, analyze any compound that contains protons. Non-targeted analysis refers to the study of a mixture without the goal of studying a certain compound or group of compounds [268]. Commonly 1H NMR is applied along with statistical
Integrating NMR approaches into a cohesive framework
Thus far, the use of NMR in the context of wastewater has been introduced and applications to date have been reviewed. However, what makes NMR stand out amongst all other analytical approaches for environmental analysis is its great versatility [54].
For example, NMR can provide targeted and non-targeted analysis, including overall compositional profiling (fingerprinting) and individual chemical identification/quantification along with physico-chemical information (such as dynamics, molecular
Conclusions and future work
As demonstrated in this review and case study, NMR has considerable potential to provide an exceptional understanding of the structures, interactions, processes, and toxicity of compounds in wastewater. In this review, applications were organized based on the nucleus of study (1H, 13C, etc.) and at the end of each section, future directions were thoroughly discussed, therefore there is no need to revisit them here. As a field in general, NMR applications to wastewater will likely diverge along
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) (Strategic (STPGP 494273-16), Discovery (RGPIN-2019-04165) and Alliance (ALLRP 549399 and ALLRP 555452) programs), the Canada Foundation for Innovation (CFI), the Ontario Ministry of Research and Innovation (MRI), and the Krembil Foundation for providing funding. A. J. S. would like to thank the Government of Ontario for an Early Researcher Award. Flavio Kock would like to thank the Brazilian
Glossary
- ADP
- Adenosine diphosphate
- AMP
- Adenosine monophosphate
- ATP
- Adenosine triphosphate
- CDFA
- Chlorodifluoroacetic acid
- CEPT
- Chemically Enhanced Primary Treatment
- CFC
- Chlorofluorocarbon
- CMP
- Comprehensive Multiphase
- COD
- Chemical Oxygen Demand
- COSY
- COrrelated SpectroscopY
- CPC
- Cetylpyridinium chloride
- CP-MAS
- Cross-Polarization Magic Angle Spinning
- CRAM
- Carboxylic Rich Aliphatic Material
- DEAE
- (Diethylaminoethyl)
- DEPT
- Distortionless Enhancement by Polarization Transfer
- DFA
- Difluoroacetic acid
- DMSO‑d6
- Deuterated Dimethylsulfoxide
- DOC
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