Bacteria and fungi inactivation using Fe3+/sunlight, H2O2/sunlight and near neutral photo-Fenton: A comparative study
Graphical abstract
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Highlights
► The inactivation of a bacterium and a fungus was evaluated under sunlight with H2O2, Fe3+ and H2O2/Fe3+. ► The inactivation pattern and rate is observed to be highly dependent on the type of microorganism. ► The order of disinfecting efficacy was: photo-Fenton > H2O2/sunlight > Fe3+/sunlight.
Introduction
According to United Nations, the world human population is expected to reach over 10 billion in the next few decades. One of the most serious global problems will be water scarcity and lack of access to safe water. Although, access to clean, safe water for human consumption was declared a human right by the United Nations in July 2010, the perspective to 2025 is that 1.8 billion people will be living in countries or regions with absolute water scarcity, and two thirds of the world's population could be living under water stressed-conditions [1]. The most important issue is the disinfection of drinking water. According to the WHO and UNICEF, polluted drinking water and lack of sanitation is responsible for the death of approximately 4500–5000 children every day, and 884 million people still lack access to improved sources of drinking water [2].
The second most critical issue is the disinfection of water for agriculture. According to the Food and Agriculture Organization of the United Nations (FAO), agriculture is the largest global consumer of water. The 80% of land cultivated is today still exclusively rain fed, and supplies over 60% of the world's food. However, this activity could triple or quadruple in the coming decades to provide food for the growing human population [3].
The effective and sustainable treatment of polluted water is one of the most attractive strategies to combat water quality problems. Wastewater must be treated before discharge because it contains domestic, industrial and agriculture chemical pollutants and also is loaded with a wide range of pathogenic microorganisms. These pathogens may belong to completely different kingdoms like the human bacterial pathogen Escherichia coli, which is an indicator of fecal contamination and fungal phytopathogens like Fusarium solani, which causes a significant crop loss. The guidelines of safety standards for wastewater reuse are different depending on the intended final use for the treated water. Standards for water reuse for agriculture irrigation are more tolerant than for drinking water. Taking into account that agriculture is the largest global consumer of fresh water, wastewater reuse for agriculture can reduce the pressure exerted by human activities on existing fresh water resources and augment water supply in water-scarce and semi-arid zones.
Irrigation water and wastewater effluents accumulate pathogens like bacteria and fungi almost everywhere. The water is a vehicle for these pathogens generating plant and human diseases. Agriculture is probably the most affected field by fungal pathogens like Fusarium spp. [4], which is especially harmful in intensive greenhouse agriculture due to the optimal conditions of both humidity and temperature found [5]. Fusarium spp. is a ubiquitous soil borne filamentous fungi which can be transmitted via water, and is known to be plant, animal, and human pathogen [6], [7]. Fusarium genus produces three forms of spore: macroconidia, microconidia and chlamydospores. This genus has been reported to be highly resistant to chemical and photocatalytic treatments due to the formation of a resistant spore [8]. Furthermore, Fusarium spp. also produces human diseases e.g., skin diseases, especially in immunodeficient, or eye infections due to fungal contamination of contact lenses [9]. Some Fusarium species can increase their virulence producing fumonisins and trichothecenes toxins in water. These micotoxins are associated with a variety of respiratory neurological and other systemic symptoms, causing several human fungal infections [10].
E. coli is a faecal indicator organism and its presence in water indicates possible contamination with other enteric pathogens like Salmonella spp., Yersinia spp., Shigella sp., etc. which are found in the gastrointestinal tract of infected mammals. These pathogens can produce diseases when contaminate fruit or other agricultural products are consumed. Diarrhea is the main symptom of enteric bacteria, however depending on the strain virulence or the person immunologic state, severe illness or death can occur. Therefore it is important to develop efficient methods to inactivate Fusarium spores and enteric bacteria prior to water reuse.
Traditional methods for water disinfection like boiling, filtering and chlorine tablets have been shown to be inefficient against some resistant pathogens and cannot guarantee complete disinfection. Some disinfection treatments can produce disinfection by-products (chlorination [11] and ozonation [12]), which are phytotoxic to plants and hazardous for human. Solar disinfection has been used in developing countries to disinfect drinking water and is effective against low resistantance microorganisms like bacteria. This method, so called SODIS (solar disinfection), consists of exposing the water to solar radiation in transparent containers (1–2 L) and the combination of UVA and heat leads to inactivation of pathogenic microorganisms in the water [16]. However, resistant microorganisms like sporulated fungi and bacteria, or protozoa are more resistant to solar disinfection [13], [14], [15]. The biocidal effect of sunlight is due to optical and thermal processes, and a strong synergistic effect occurs for water temperatures exceeding 45 °C and UV radiation. The ROS generated in water by UV light can cause oxidative damage in proteins, lipids and nucleic acids [17].
The inactivation of microorganisms using UV light can be enhanced using Advanced Oxidation Processes (AOPs). These processes involve the generation of hydroxyl radicals (OH). AOPs have been reported as promising techniques to remove hazardous organic compounds and microorganisms from contaminated water. Solar driven AOPs should be lower cost and may be applied for the sustainable treatment of drinking water and irrigation water [18]. With solar driven AOPs, the inactivation of microorganisms by UVA light is accelerated by the formation of reactive oxygen species, such as OH radicals. Photo-induced AOPs can be divided into heterogeneous and homogeneous processes. Titanium dioxide photocatalysis is an example of a heterogeneous process and has been the most studied for the inactivation of microorganisms [18]. However, photo-Fenton has attracted great interest due to its high efficiency for OH generation. The Fenton process is described by the following equations [19]:Fe2+ + H2O2 → Fe3+ + OH− + OH (k = 63 L mol−1 s−1)Fe3+ + H2O2 → Fe2+ + HO2 + H+ (k = 3.1 × 10−3 L mol−1 s−1)OH + H2O2 → HO2 + H2O (k = 3.3 × 107 L mol−1 s−1)OH + Fe2+ → Fe3+ + OH− (k = 3.0 × 108 L mol−1 s−1)Fe3+ + HO2 → Fe2+ + O2 + H+ (k = 2.0 × 103 L mol−1 s−1)Fe2+ + HO2 + H+ → Fe3+ + H2O2 (k = 1.2 × 106 L mol−1 s−1)HO2 + HO2 → H2O2 + O2 (k = 8.3 × 105 L mol−1 s−1)where k is the second order rate constant.
The production of OH is greatly increased by UV–vis radiation up to a wavelength of 600 nm. This reaction (Eq. (8)) closes the catalytic cycle and is called photo-Fenton [20]:
Under these conditions, the photolysis of Fe3+ complexes promotes Fe2+ regeneration and iron may be considered a true catalyst [19].
Another photo-induced process which has more recently generated interest for the inactivation of pathogens in water is the synergistic effect of H2O2 and solar radiation. It is well known that the photolysis of H2O2 occurs when it is irradiated by photons of wavelengths lower than 300 nm yielding OH as shown in Eq. (9) [21], [22]:
However, solar radiation at Earth's surface does not contain photons with wavelengths below 280 nm, so solar energy is inefficient for OH generation by this pathway. The combined effect of H2O2 with solar radiation was reported for first time for phage T7 inactivation in 1977 [23]. Up to now, very few contributions have reported the damaging effects of H2O2/solar light on microorganisms in water. There is experimental evidence of disinfection capacity of near UV or visible light and hydrogen peroxide using different targets such as E. coli and Streptococcus mutans [24], [25]. Nevertheless, recent studies have shown very good disinfection efficiencies in resistant fungal spores (Fusarium equiseti and F. solani) using low concentrations of hydrogen peroxide (≤10 mg/L) in the presence of natural solar radiation in solar reactors with compound parabolic collectors (CPCs) [4], [26].
The synergistic effect between hydrogen peroxide and solar photons is attributed to the generation of OH through Fenton reactions inside microbial cells due to the natural iron content and diffusion of H2O2 across the cell membranes [26]. Increased ferrous ion concentrations may occur in cells irradiated with near UV photons due to increased membrane permeability to Fe2+ [27]. The critical factor seems to be the availability of the cellular labile iron pool (LIP), which also may be favored by cells under UV light irradiation [28]. The trace concentrations of “free” iron catalyse the production of hydroxyl radicals via Fenton/Habber–Weiss reaction cycle.
The purpose of the current study was to evaluate and compare the effectiveness of three solar treatments; Fe3+/sunlight, H2O2/sunlight and solar photo-Fenton at near-neutral pH for the inactivation of F. solani and E. coli in distilled water. For this, different concentrations of Fe3+ (0–50 mg/L), H2O2 (0–10 mg/L), and Fe3+/H2O2 (1/2.5, 5/10, 10/10, 50/10 mg/L) were evaluated in bottle reactors (200 mL) for 5 h under natural solar light in the Southeast of Spain. In the literature, it has been shown that the use of photo-Fenton system is efficient for inactivation of bacteria like E. coli [27], and the denaturation of prion protein [29] using UV-lamps. To our knowledge the solar photo-Fenton system has not been applied to fungi spores inactivation. Thus, the main novelty of this study is to demonstrate the disinfection capacity of solar photo-Fenton at near neutral pH treatment and to compare with H2O2/sunlight for the inactivation of E. coli and F. solani spores in water.
Section snippets
Fungal strain enumeration and quantification
A wild strain of F. solani belonging to fungal library of the University of Almería, isolated from the Andarax River in Almeria, Spain, was used like fungal spore model to conduct experiments. This fungus was chosen because it is a common phytopathogen in soil and the water distribution system affecting crops and it has been demonstrated to be highly resistant to photocatalytic treatment. The same strain and enumeration–quantification methods have been described elsewhere [4], [5], [8]. Fungal
E. coli
Fig. 1 shows inactivation of E. coli in water with different added concentrations of Fe3+ (0, 1, 5, 10, 50 mg/L) during 5 h of exposure to natural solar irradiation. Viable bacteria cells in distilled water with 50 mg/L of Fe3+ remained constant in the dark for 5 h (data not shown). Consequently, the mere presence of iron did not affect E. coli cells cultivability under the experimental conditions of this work. Therefore, the bacterial inactivation observed in Fig. 1 was due to the joint effect of
Discussion
Exposure to oxidative stress can induce a wide series of responses in microorganisms ranging from increase mitosis to apoptosis, and finally, necrosis [31]. This stress is induced by reagent oxidative species (ROS) such as O2, H2O2 or derived oxygen species generated during photolysis [32]. Different inactivation mechanisms are involved in the cell death during photolysis [27]. Furthermore, when an advanced oxidation treatment like solar photo-Fenton is used for inactivation of microorganisms
Conclusions
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Solar/Fe3+, solar/H2O2, and solar photo-Fenton process have been demonstrated to have detrimental effects over the vegetative cells of E. coli and F. solani microconidia in distilled water.
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E. coli inactivation with solar light was enhanced by adding Fe3+, H2O2, and photo-Fenton process. The best bacterial disinfection results were obtained with photo-Fenton for 5 mg/L of Fe3+–10 mg/L of H2O2, for which complete inactivation was attained with 0.96 kJ/L.
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F. solani spores were also inactivated with
Acknowledgments
The authors wish to thank the Spanish Ministry of Science and Innovation for financial support under the AQUASUN project (reference: CTM2011-29143-C03-03). Irene García-Fernández would like to thank the University of Almeria and CIEMAT-PSA for her Ph.D. research grant.
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