Use of Ferrate and Ferrites for Water Disinfection - ACS Symposium

Chapter 6

Use of Ferrate and Ferrites for Water Disinfection Downloaded by UNIV OF GEORGIA on December 27, 2016 | http://pubs.acs.org Publication Date (Web): December 19, 2016 | doi: 10.1021/bk-2016-1238.ch006

Irwing M. Ramírez-Sánchez1 and Erick R. Bandala2,* 1Department

of Civil and Environmental Engineering, Universidad de las Americas, Puebla. Sta. Catarina Martir, Cholula, Puebla, Mexico 72810 2Division of Hydrologic Sciences, Desert Research Institute (DRI), 755 E. Flamingo Road, Las Vegas, Nevada 89119-7363, United States *E-mail: [email protected]

Water treatment using iron-based materials is an emerging, highly attractive research field that has gained increasing interest over the few last years, particularly for inactivating pathogenic microorganisms in water. It is also a field that relatively few studies have been conducted. This chapter provides a review of the applications of iron-based materials, ferrate and ferrites in particular, for water disinfection. The reviewed material includes some information highlighting the main proposed mechanisms that occur during the inactivation processes at the lab or pilot scale. This chapter use of discusses using iron-based materials to inactivate bacteria, viruses and other pathogenic microorganisms and reviews the main perspectives on developing these technologies.

Introduction Over the last few years, iron-based nanoparticles have gained increased interest for environmental remediation applications (1). In particular, their use in water treatment has been identified as a very attractive alternative for removing biotic and abiotic contaminants (2). Among these, ferrates and ferrites (MFe2O4) are considered an important class of iron-based materials that have unique physical-chemical properties with significant potential for water treatment (3).

© 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by UNIV OF GEORGIA on December 27, 2016 | http://pubs.acs.org Publication Date (Web): December 19, 2016 | doi: 10.1021/bk-2016-1238.ch006

Ferrate has extraordinary redox potential and its use has been determined to increase under acidic conditions. It has been reported to be an excellent oxidizing agent that is capable of carrying out coagulation functions by forming ferric hydroxide in the reaction mixture (4). Ferrite also generates an excellent redox pair and has a conducting solid structure that can disperse electrons. It can be doped with different transition metals to allow a refinement of its catalytic/redox activity and magnetic properties (5, 6). Different transition metals (e.g., Cu2+, Co2+, Mn2+, Ni2+, Fe2+) have been tested for the synthesis of ferrite-based nanoparticles in the search for the best properties to be used for water treatment (3, 7, 8). The aims of this chapter is to overview the use of ferrate- and ferrite-like compounds as disinfectants and mechanisms involved in their actions. The perspectives and main challenges to practically apply ferrate and ferrite are also presented.

Use of Ferrate for Water Disinfection Ferrate has been extensively studied in water disinfection and has been proven to be an excellent disinfectant (8). Besides using it to inactivate bacteria (Escherichia coli, Salmonella, Staphylococcus aureus, Bacillus sp., Pseudomonas sp., Enterococcus faecalis) in water (9–11), ferrate has also been used for interesting applications such as inactivating fish parasites (Ichthyophthirius multifiliis) (12) and removing harmful algae (e.g., Microcystis aeruginosa) (11) and viruses (13). Previous reports show that the disinfection capacity of ferrate increases significantly at pH values lower than 8.0 due to protonated ferrate species (Figure 1), which means that it can inactivate most of the pathogens mentioned above using concentrations as low as 1 mg/L (14, 15). For water disinfection assessments, ferrate was shown to be significantly affected by temperature. Hu et al. (13) found that the ferrate inactivation rate constants for coliphage MS2 increased by up to fourfold by increasing the temperature from 5 to 30˚C (Figure 2). The effect of temperature on the inactivation process of MS2 was found to fit with the Arrhenius equation, and a dependence resulting from varying oxidation mechanisms and/or initial attack sites on the MS2 phage protein or genome components has been suggested. However, the effect of temperature is complicated because other studies have reported marginal or no effect of temperature for the inactivation of MS2 and Bacillus subtilis spores using ferrate (16). Water pH was also found to highly affect ferrate-based disinfection processes. Higher inactivation rate constants were identified for the disinfection process of MS2 at a lower pH and the same was found to be true for E. coli inactivation (17). It has been suggested that the effect of pH on the disinfection process may be related to the dependence of the reactivity of Fe(VI) on acid-base speciation. In this pH interval (e.g., between 5 and 8), ferrate is characterized by two conjugate pairs (Eqs. 1 and 2):

146 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Previous reports suggest that protonated ferrate species (Eq. 1) are stronger oxidants than nonprotonated species (18–20) because protonation reduces the electron-donating capacity of the coordinated oxygen ligands promoting metal center to act with a higher oxidative potential (21). Using ferrate as a preoxidant for natural organic matter (NOM), studies have been reported to decrease the formation of THMs during water disinfection when it is followed by chlorination (22, 23). However, it has also been reported that low doses of ferrate (1 mg/L as Fe) may generate chloral hydrate and halo ketones, whereas higher doses (20 mg/L Fe) greatly reduce the formation of such by-products (24).

Figure 1. Dependence of different ferrate-related species on solution pH. (Reproduced with permission from reference (26). Copyright 2006 Elsevier).



Figure 2. Effect of (A) temperature and (B) pH on the kinetics of MS2 inactivation using ferrate. (Reproduced with permission from reference (19). Copyright 2008 American Chemical Society).



Large scale ferrate disinfection has been reported more recently in tertiary treated wastewater to simultaneously remove total organic carbon (TOC) and disinfect water (25). A treatment was suggested to promote coagulation in addition to the oxidation reaction because of its reduction potential, as shown in reaction 3 and 4 (25).

Figure 3A shows the results obtained for TOC removal using Fe(VI) compared with using hypochlorite as a conventional oxidizer. As shown, TOC removal was directly dependent on the oxidant concentration and a higher ferrate concentration resulted in higher TOC removal, probably because of the simultaneous occurrence of ferrate-produced coagulation and an oxidative/reductive process (25, 26). The higher TOC removal was achieved using 14 mg/L of ferrate (e.g., 48% TOC removal) after one hour of reaction, whereas less than half of the TOC was removed using the same reaction conditions for chlorine. The most interesting result was that ferrate was also able to simultaneously inactivate microorganisms despite reacting with organic matter in the wastewater. Figure 3B shows the inactivation of total and fecal coliforms using ferrate (circles) and hypochlorite (triangles). Figure 3B shows that ferrate was able to inactivate up to 4-log units using disinfectant dose (C×t) values under 20 (mg/L).min, whereas C×t values higher than 120 (mg/L).min of hypochlorite where needed to achieve the same inactivation values.

Use of Ferrites for Water Disinfection Using magnetic photocatalysts in water disinfection has become increasingly attractive because of the simple recovery and reuse of photoactive materials (27, 28). In some cases, ferrites have been used as magnetic photocatalysts, which demonstrates their capability to inactivate pathogens and effectively adsorb inorganic pollutants in water (29, 30). Different possible mechanisms have been suggested for the antibacterial action of ferrite-type nanoparticles. It has been proposed that nanoparticles adhere to the bacteria’s cell wall and penetrate it or cause degradation and lysis of the cytoplasm, which leads to cell death (31, 32). Other authors (33) have tested the antibacterial activity of nickel ferrite/poly acrylonitrile maleic anhydride (PAMA) against gram-positive and gram-negative bacteria using a disk diffusion assay. They found excellent antibacterial activity and the possibility of removing the material from the water solution after the disinfection process by applying an external magnetic field (34). Recently, the efficiency of CoFe2O4/SiO2/Ag to inactivate bacteria using S. aureus, Bacillus subtilis, E. coli and Pseudomonas aeruginosa has been tested (35). The results suggested a synergistic effect of the synthesized composite in combination with streptomycin and showed excellent antibacterial activity. 149 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 3. Simultaneous removal of TOC (A) and total and fecal coliforms (B) in wastewater using ferrate (circles) and hypochlorite (triangles). (Reproduced with permission from reference (25). Copyright 2009 IWA Publishing).



Figure 4 shows the proposed mechanism occurring on the ferrite’s surface associated with bacteria inactivation, which has been suggested to be similar to the mechanism previously proposed for other semiconductors. Once the charge carriers are produced, oxidation of the cell membrane is proposed to occur following reactive oxygen species (ROS) oxidation. As reported previously for nitrogen doped TiO2 (36), photoproduced ROS (e.g., hydroxyl radical, superoxide, peroxides, singlet oxygen) attack the cell wall far beyond triggered self-defense mechanisms and repair machineries, which increase cell wall permeability and lead to cell inactivation and ultimately complete cell decomposition if the required time is provided, as shown in Figure 5. Other authors have proposed direct oxidation as an alternative inactivation mechanism (35). Although it is unlikely, the latter should be considered a possible research avenue when little is known about the actual mechanism. The antibacterial effect of metal oxide has been proposed to be related with surface charge interactions, dissolved ions, and particle size (37). However, more research is needed to provide accurate information on this reaction mechanism.

Figure 4. Proposed mechanism for ferrite-based materials in disinfection processes.



Figure 5. Different stages of cell damage in E. coli K-12 after (A) 0 h, (B) 6 h, (C) 12 h, (D) 30 h of treatment using ferrite-related photocatalysts. (Reproduced with permission from reference (34). Copyright 2013 American Chemical Society).

A few studies on the use of ferrite for photocatalytic water disinfection are reported. The bacteria most often tested for using ferrites to disinfect water is Escherichia coli which is often used for photocatalytic studies. This bacteria has been tested, alone or with other bacterial strains, for its inactivation using several types of ferrite-based materials. E. coli and Staphylococcus aureus have been successfully inactivated using ferrite under LED lamps, but the presence of certain ions affect the inactivation process positively or negatively depending on their nature. It has also been suggested that the ferrite material used was capable of working at a high performance under different conditions and after up to five consecutive cycles of use (27). Cobalt-ferrite nanoparticles (1 g/L) were also tested for their inactivation of the same strains (e.g., E. coli and S. aureus). It was found that S. aureus was slightly more resistant than E. coli. Other ferrite-like compounds that included different increasing concentrations of manganese were also tested, which shows that including manganese decreased the inactivation rate. Zinc and copper ferrite nanoparticles have been tested for E. coli inactivation and also showed positive results for water disinfection (31).



Figure 5 shows the effect of the interaction of the cell wall with the ROS. It is worth noting that the amount of time required to observe damage in the cell wall is significantly higher than the time usually required for inactivation processes. After 30 hours of treatment, little of the cell remains (Figure 5D), but it is very likely that the cell was inactivated after a few minutes of treatment. Considering the well-known efficiency of TiO2 disinfection, composite nanoparticles consisting of a shell of TiO2 and a magnetic core of ferrite have been developed. Despite the interesting characteristics of ferrite, relatively few studies have been reported that use it for photocatalytic water disinfection (Table 1). The effectiveness of TiO2-NiFe2O4 composite nanoparticles for E. coli inactivation using only UV radiation has been demonstrated (38). Another enhanced TiO2-doped shell has also been developed: W4+-doped TiO2 on NiFe2O4 hounding particles were tested and shown to be more efficient than undoped TiO2 composite nanoparticles for decreasing the concentration of E. coli in water (39). It is suggested that the greater bacterial inactivation is a consequence of W4+ doping of TiO2, which reduces the band gap and the probability of electron-hole recombination processes. The reduction on the band gap by TiO2 metal doping coated NiFe2O4 has been suggested to increase E. coli inactivation effectiveness in the following order: W4+>Nd3+>Zn2+>undoped TiO2 (40). It was also found that nanoparticles containing 75% of the total weight of nickel-ferrite core have significantly improved effectiveness compared with 100% pure TiO2. Because photocatalytic applications using solar radiation are a promising method for water disinfection, visible-light photocatalysis has been increasingly studied. The ZnFe2O4-SnO2 composite nanoparticles were demonstrated to be magnetically separable and had visible-light (using a tungsten halogen lamp and UV filter) photocatalytic-antibacterial activity (41). Under sunlight irradiation, graphene-ZnFe2O4-polyaniline exhibited antibacterial activity for S. Aureus, E. Coli, and Candida albicans (42). Both SrTi and SrTi1-xFexO3-δ showed effective antibacterial action under dark and visible-light conditions (37). Recently, magnesium ferrite nanoparticles (MFNs) were used as a magnetic core to support SiO2-Ag4SiW12O40-Ag (43). These MFNs-SiO2-Ag4SiW12O40-Ag were used as a photocatalyst under visible light and showed a reduction in E. coli that was five orders of magnitude more than Degussa P25 nanoparticles under visible-light photocatalyst activation.

Conclusions Iron-based materials, ferrates and ferrites in particular, were explored for their use in water disinfection. Although only a few reports were found, both ferrates and ferrites showed significant applications for inactivating common water-related bacteria, as well as viruses and other pathogens.


Table 1. Photocatalytic experimental condition for antibacterial ferrite composite evaluation.a

154


Nanoparticle composite

a

Model microorganism

Photoreactor

Radiation source

Optical filter

Catalyst load

Reference

TiO2-NiFe2O4

E. coli

4 mL quartz cubic cell

UV spectrophotometer at 270 nm

ND

1 mg/mL

(38)

(W4+, Nd3+, Zn2+) doped TiO2-NiFe2O4

E. coli

4 mL quartz cubic cell

UV spectrophotometer at 270 nm

ND

1 mg/mL

(40)

ZnFe2O4-SnO2

E. coli

Double walled photoreactor

150 W tungsten halogen lamp

ND

0.02 g/L

(41)

Graphene- ZnFe2O4polyaniline

E. coli, Candida albicans, Staphylococcus aureus

50x10 mm Petri dish

Sunlight

ND

ND

(42)

MFNs- SiO2Ag4SiW12O40-Ag

E. coli

50x10 mm Petri dish

300 W xenon lamp, PLS-SXE300

Glass filter < 400 nm

0.05 mg/mL

(43)

SrTi1-xFexO3- δ

E. coil

20 mL stirred suspension

40 W fluorescent lamp

Glass filter, GG435

1 g/L

(37)

Note: ND = no data.

Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Ferrates and ferrites were also shown to be highly cost-effective for water disinfection because both can be used more than once in the treatment procedure and have been reported to have relatively low preparation costs. However, more research is needed for the development of new materials and processes that may improve the results reported so far. Only few upscaling processes were reported for any of the cases, which shows the need for further technological approaches that can provide data beyond the lab scale and allow estimates of the suitability of using iron-based materials for pilot- or full-scale applications in water disinfection processes.


Challenges and Perspectives Despite the significant efficiency of ferrate and ferrites at disinfecting water, few reports have been published information regarding their application or assessed further kinetic, upscaling, or mechanistic parameters. Further research is required to systematically determine several unknown characteristics and implications for their use in producing pathogen-free drinking water. For example, little is known about the effects of the different metals included in the chemical structure of ferrites (MFe2O4) or the components of the water used in ferrate disinfection processes. Using a variety of metallic ions in ferrite formulations might lead to unknown characteristics or help control the size and morphology of the resulting particles, which is considered a highly important parameter for enhancing their photocatalytic activity. Although it has been suggested that ferrate may reduce TOC and microorganism load at the same time, there are no reports on the effects that other water components—such as ions (e.g., chloride, carbonate, phosphate) or the presence of nitrogen- or phosphorus-related compounds (e.g., nitrate and nitrite)—might have on its efficiency in disinfection processes. Using the iron-based metals studied here to inactivate highly resistant microorganisms (e.g., helminthes eggs and bacterial or fungal spores) is another field in which no reports have been published. Using highly resistant microorganisms in a conservative model for water disinfection is very important to ensure the production of safe drinking water even in areas where pathogens such as giardia cysts or Cryptosporidium parvum oocysts may present a threat to consumers if the proper measurements for secure drinking water are not taken. Additionally, their use for inactivating other nonpathogenic but very important microorganisms in water (e.g., harmful algal blooms, phytopathogens, aquatic invasive species) may be a research field that is worth exploring. From an application point of view, the capability of ferrites to enhance the reaction rate by radiation absorption make these materials a very interesting alternative in the photocatalytic processes field. Additionally, the paramagnetic properties of some of these materials provides a considerable advantage because no further posttreatment of the effluent is required, except for using a magnetic field to remove the photocatalyst. The capability of ferrates to work as a coagulant after the oxidation process provides a very interesting posttreatment possibility. For both ferrates and ferrites, their high efficiency and use as low toxic elements 155 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

for water treatment suggests that these technologies are potentially feasible for use as pre- or posttreatment in conventional wastewater facilities.

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Use of Ferrate and Ferrites for Water Disinfection - ACS Symposium

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