Fe and Cu in humic acid extracts modify bacterial inactivation pathways during solar disinfection and photo-Fenton processes in water
Graphical abstract
Introduction
Solar disinfection of drinking water (SODIS) is a well-established process for water sterilization in developing countries [1]. Despite the relative success of this process, its limitations, namely its requirement for low turbidity, low treatment volumes (up to 2 L), temperature dependence, and the risk of microbial regrowth during storage after treatment [2], have prompted an array of works on its enhancement. One of the most promising improvements is the photo-Fenton process. This method not only builds upon the base process (the use of solar light) [3], but also takes advantage the ubiquitous presence of iron and the photo-generation of hydrogen peroxide to produce the highly oxidative hydroxyl radical [4], which can readily and effectively inactivate the majority of microorganisms. The addition of Fe and H2O2 enhances the overall efficiency of the system, but the elucidation of the exact inactivation pathways is far from a simple process; when near-neutral (natural) water is involved, the presence of inorganic substances, such as carbonate and sulfate, and organic substances, such as natural organic matter (NOM) [5] induce further pathways, for which information is scarce.
Humic substances (HS) comprise the majority of dissolved organic matter (DOM) in aquatic systems. HS may also be considered the main precursor of excited triplet states (exogenous photosensitizers), which are able to degrade organic compounds and inactivate microorganisms such as bacteria and viruses [[6], [7], [8], [9], [10], [11]]. Humic acids absorb light in the UVA and visible spectrum to form excited states that can attack biomolecules or react with oxygen to generate reactive oxygen species (ROS). More specifically, under UV–vis irradiation and in the presence of O2, the substances can produce ROS such as singlet oxygen (1O2) and superoxides (HO2/O2−) (Eqs. (1)–(4)) [[12], [13], [14]].
The literature describes the different roles of organic matter in photosensitization events, which depending on the type of functional groups present in the HS. For instance, ketonic [15,16] and quinonic groups [17,18] have been shown to be the most photo-chemically active. Various methods, including spectroscopic and fluorescence methods, have been used to characterize the organic matter and predict or correlate its photo-activity with specific characteristics [[19], [20], [21], [22]]. Additionally, the quantity of HAs present also plays an important role in the net outcome; in addition to its action as a light screen [23,24], Porras et al. investigated the photo-transformation of drugs and pesticides using different humic substances in different quantities, and showed a correlation between the structural characteristics of the humic substances and their photo-transformation activity. An antagonistic effect was noted at high HA concentrations, indicative of light attenuation or self-inhibition [25,26].
The organic substances present in natural water can also affect solar-based treatment processes, including SODIS [27,28], photo-catalysis [29,30], and photo-Fenton [31,32] processes. For instance, Giannakis et al. described both the beneficial action of HAs in maintaining a homogeneous photo-Fenton reaction [33] and the implications when a heterogeneous mode was employed for the inactivation of viruses [34]. The organic matter could enhance or inhibit photo-assisted disinfection processes, regardless of the target microorganism [3]. In all the aforementioned studies, pure NOM isolates or model NOM compounds were used. Their ability to complex iron is the source of the enhanced activity (Eq. (5)) [35,36], but the literature lacks studies of naturally metal- and mineral-bearing humic substances and their effects on solar-related disinfection processes, in which they could initiate Fenton (Fe-based) and Fenton-like (Cu-based reactions) as follows (Eqs. (6)–(19)):
Fenton
Fenton-like
The aim of this research was to study the role of HA, as a model of DOM, on bacterial disinfection in several solar-based processes. The presence of metals (i.e., Fe and Cu) and their photo-chemical implications were investigated. After characterization of the HA samples, their effects on SODIS (hv), hv/H2O2, and hv/H2O2/Fe (photo-Fenton) processes were considered. Special attention has been given to the use of solar light alone, as it is the process on which the other two are based. The complex photo-Fenton process has been broken down into its constituent steps, and the use of HAs that inherently contain metals have been assessed. A detailed disinfection and spectroscopic study to investigate the variables involved, namely the quantity (initial concentration) and composition of the HS, was attempted using three HAs (HA1, HA5, and HA16) containing different amounts of mineral matter (1, 5, and 16%, respectively). The results indicate the pitfalls and opportunities for enhancement of the photo-chemical disinfection of water when HAs are naturally present or intentionally added.
Section snippets
Chemicals and reagents
All chemicals were of reagent grade. Hydrogen peroxide 30% and iron (II) sulfate heptahydrate (FeSO4·7H2O) were provided by Merck. Titanium (IV) oxysulfate (TiOSO4) for the spectrophotometric determination of H2O2 was provided by Fluka. All solutions were prepared with Milli-Q water (18.2 MΩ cm) immediately prior to irradiation. Reagent grade NaOH, HCl, and HF were provided by Sigma-Aldrich.
Preparation of humic acid (HA) fractions
The terrestrial humic acid sample referred to as HAMC (humic acid mineral content) was obtained from an
Characterization of the humic acid extracts (HAMC)
In this study, three humic acid samples with different mineral contents were evaluated. The samples HA1, HA5, and HA16 contained humic acids extracted from a carbonaceous material, and had mineral matter contents of 1, 5, and 16%, respectively. Fig. 1 shows the SEM results for HA16 and HA5 and demonstrates the surface topography of the samples. The differences between the two samples and their metal contents are clearly indicated by the bright spots in Fig. 1; it is evident that the HA16 sample
Conclusions
In this work, the use of naturally extracted humic acids to enhance solar-based disinfection methods was investigated. HAs containing metals (Fe, Cu) were evaluated in pure SODIS, with added H2O2, and with the simultaneous addition of Fe and H2O2 to induce the photo-Fenton process. The main differences in these processes lie in the mechanisms by which the process is enhanced, which in turn depend on whether H2O2 and/or Fe are added. The SODIS enhancement is related to the organic matter
Acknowledgements
The authors gratefully acknowledge the Universidad de Antioquia (UdeA) for funding the research. Jazmin Porras would like to acknowledge the Colombian Administrative Department of Science, Technology and Innovation (COLCIENCIAS) for her Ph.D. Stefanos Giannakis and Cesar Pulgarin would like to acknowledge the European project WATERSPOUTT H2020-Water-5c-2015 (GA 688928) for the financial support.
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