Efficiacy of activated PMS/PDS by different methods in inactivating E.coli and P.aeroginosa

doi:10.21203/rs.3.rs-3003793/v1

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Efficiacy of activated PMS/PDS by different methods in inactivating E.coli and P.aeroginosa

https://doi.org/10.21203/rs.3.rs-3003793/v1

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The aim of the present study was to evaluate to efficacy of peroxydisulfate(PDS)/peroxymonosulfate(PMS) activated by different methods in inactivating E.coli and P.aeruginosa. The UV-A, UV-A/PDS or PMS and NaOH/PDS processes were used to inactivate E.coli and P.aeruginosa. Three different sulfate salts (K₂S₂O₈, Na₂S₂O₈ and Oxone) were used to activate with UV-A radiation and the inactivation effects were determined. During 30 minutes of UV A radiation, the addition of 3 mM K₂S₂O₈, Na₂S₂O₈, and Oxone increased the removal efficiency of E. coli by 1.76, 1.51, and 2.47 log, respectively. For P. aeruginosa, there was an increase of 1.44, 1.62, and 2.55 logs, respectively. The highest removal efficiency was obtained with oxone+UV-A on E.coli and P.aeroginosa. In order to examine the inactivation effect of alkaline (NaOH) activated peroxydisulfate on microorganisms, studies were carried out using two different doses of K₂S₂O₈ (2 and 3 mmol/L). With the increase of PDS concentration, the removal efficiency for both bacteria increased. Higher removal efficiency was obtained at high base/PDS ratios. When the PDS concentration is increased from 2mM to 3mM at the same NaOH/PDS (0.25) ratio, the time required for 4 log bacteria removal is reduced by 33% for E.coli, while there is a decrease of more than 44.2% for P.aeroginosa.

Alkaline activation

Inactivation effect

PDS

PMS

UV-A

Increasing water use due to the increasing world population and the effects of climate change, the amount and quality of water resources are significantly affected. Water crises, which may be one of the biggest global risks of the future, will bring along difficulties in accessing quality drinking water. Problems in hygienic water supply will gain more importance in the coming years. Level of accessibility to safe drinking water was around the 21.3% in developing countries, while this rate remained at only 52.4% in developed countries. These data clearly show that access to safe drinking water is a problem not only for developing countries, but also for developed countries, and that water crises may occur in the world in the future (Fogden and Wood 2009). This problem will cause significant water-borne diseases (80% of all diseases) to occur even in developing countries (Blanco et al. 2009).

Microorganism disinfection is one of the most important points in supplying drinkable quality water. Disinfection is a critical step designed to control microbial pathogens and thus prevent waterborne diseases (Crittenden et al. 2012; Moreno-Andrés et al. 2019b). By-products of disinfection such as trihalomethane and haloacetic acid which are stated to be harmful to human health occur by traditional methods used for disinfection. Recently, many alternative methods have been developed recently for the purification and disinfection of drinking water due to these disinfection by-products, which are considered mutagenic and/or carcinogenic (Mohd Zainudin et al. 2018; Zhang et al. 2013) for the aquatic ecosystem and humans. A promising alternative to traditional disinfection methods is the generation of highly reactive radicals via advanced oxidation processes (AOPs) (Anipsitakis et al. 2008; Bianco et al. 2017; Gosselin et al. 2013; Guerra-Rodríguez et al. 2018; Ruales-Lonfat et al. 2016; Verma et al. 2016). The potential use of AOPs for the treatment of water and wastewater has been attractive to scientists and engineers, as the radical species formed are effective and mimic naturally occurring oxidation processes at accelerated rates (Bolton and Cotton 2008).

In recent years, studies with the sulfate radical-based AOP (SR-AOP) process have shown a significant increase. It has advantages such as strong oxidation ability of SR-AOP (2.5 to 3.1 V) (Ghanbari and Moradi 2017), high performance at a wide range of pH (2 to 8) (Ghanbari and Moradi 2017; Hammouda et al. 2018; Lutze et al. 2015; Rodríguez-Chueca et al. 2018), more stable, more selective (Moreno-Andrés et al. 2019b; Wacławek et al. 2017), longer half-life (30–40 µs) (Hu and Long 2016; Matta et al. 2011), relatively low cost and environmentally friendly (Ren et al. 2015). Sulfate radicals-based inactivation of microorganisms has been recognized as an effective alternative to traditional disinfection processes (Xiao et al. 2019) due to these powerful properties.

Sulfate radicals (SO₄^●−) are generally produced via the activation of peroxymonosulfate (PMS) and persulfate (PS) parent peroxides (Han et al. 2015; Shi et al. 2012). Studies have been carried out with chemical oxidants used as persulfate salts such as Na₂S₂O₈, K₂S₂O₈ and KHSO₅ (Oxone) in advanced oxidation methods based on sulfate radicals (Rodríguez-Chueca et al. 2017b; Wacławek et al. 2017; Wei et al. 2015). SO₄^●− can be generated by heat (thermal), UV radiation (photolysis), ionizing radiation (radiological), or chemical electron transfer activation (catalysis; base and metal activation) (Antoniou et al. 2010; Hori et al. 2005; Johnson et al. 2008; Lau et al. 2007; Liang et al. 2004; Liang and Guo 2010; Wang et al. 2018; Wei et al. 2015; Wordofa et al. 2017; Yuan et al. 2014). The sulfate radical anion is applied to inactivate microorganisms by causing irreversible damage to cell structures and/or functions, as with the hydroxyl radical (OH^●) (Mamane et al. 2007; Sun et al. 2016; Wordofa et al. 2017; Zhang et al. 2014). Compared to OH^●, SO₄^●− has a stronger selective oxidation ability with electron-rich moieties on macromolecules/biomolecules ( Buxton et al. 1999; Neta et al. 1988).

Previous studies have demonstrated that persulfates activated by different methods have been effective in disinfecting water contaminated with various microorganisms, including Pseudomonas putida, Listeria monocytogenes, bacteriophage MS215, cyanobacteria Microcystis aeruginosa, Escherichia coli and Enterococcus faecalis (Anipsitakis et al. 2008; Bianco et al. 2017; Marjanovic et al. 2018; Qi et al. 2018; Tsitonaki et al. 2008; Sun et al. 2016; Wang et al. 2016; Xia et al. 2017).

In this study, the effects of sodium persulfate, potassium persulfate and oxon activated by UV-A (λ = 368 nm) on the inactivation of E. coli and P. aeroginosa were evaluated. Additionaly, the effect of base/PDS ratio on the inactivation of E. coli and P.aeroginosa by alkali-activated persulfate was also assessed.

Microbial strains and preparation of microorganisms cultures

E. coli (ATCC 25922) and P. aeruginosa (ATCC 15442) cultures were purchased from the American Type Culture Collection (ATCC). E. coli and P. aeruginosa bacteria to be inactivated were used from lyophilized strains by reconstitution as specified by ATCC. Bacterial cultures were incubated in tryptic soy broth (Oxoid) for 21 hours at 37 ± 0.5 ^oC in an orbital incubator. A portion of the 21-hour culture was transferred to sterile tryptic soy broth (Oxoid) to determine the growth curve of each bacterium. Bacterial growth was monitored by measuring optical densities (OD₅₉₅) during the incubation period. Bacterial cells collected in stationary phase were collected by centrifugation at 8000 rpm at 26ºC for 10 minutes and the bacterial pellet was washed twice with sterile phosphate buffer. Finally, the precipitate formed at the bottom of the tube with the washing solution was dissolved by shaking and transferred to the stock bottle. Experiments were conducted with bacteria in the stationary phase (Rincon et al, 2004), which are expected to be more resistant to irradiation. The time to reach the stationary phase in the growth curves were determined 180 min and 840 min for E. coli and P. aeruginosa, respectively. The resulting stock cell suspensions had a bacterial concentration of approximately 10⁹ CFU/ml and were stored at 4ºC.

Enumeration of microorganisms

E.coli (Çalışkan Eleren et al. 2014; Rincón and Pulgarin 2004) and P.aeroginosa (Maes et al. 2019) were enumerated by the pour-plate method using the PCA as non-selective medium (Plate Count Agar; Merck), since the experiments were performed with bacterial suspensions prepared from lyophilized strains (Barbeau et al. 2005; Çalışkan Eleren et al. 2014; Maes et al. 2019; Rincón and Pulgarin 2004). The plates were incubated at 37 ^oC for 24 h, after the colonies were counted. Then, typical colonies were subjected to confirmatory tests. Typical E. coli and P. aeruginosa colonies were confirmed with EC broth (Oxoid, Hampshire, UK) (APHA/AWWA/WEF 1998) and milk agar medium (Teksoy et al. 2011), respectively.

Experimental set-up

UV-A-assisted experiments were carried out in batch mode in a lab-scale reactor. The UV-lamp is mounted on the upper part in a closed dark area with dimensions of 54 cm x 44 cm x 33 cm (WxDxH). UV-A irradiation was performed by using a lamp (Sylvania Blacklight), producing predominantly 368 nm wavelength UV radiation. UV-A radiation experiments were carried out in 150 ml a glass beaker that had a diameter of 6.5 cm and a height of 4.5 cm. All experiments were performed at room temperature (23 ± 2 ^oC) and under constant stirring. The light intensity of lamp was 466 µW.cm^− 2. A schematic diagram of the UV reactor radiation experiment set-up is shown in Fig. 1. Before each exposure, the UV lamp was turned on for at least 30 min to ensure a uniform lamp output and to sterilize the closed experiment area. One hundred ml of water was placed into a sterile container. Bacteria were inoculated in required numbers (i.e. 5*10⁸ CFU/100 ml), mixed thoroughly with a magnetic bar. Following this, the required concentrations (2 and 3 mM) of different sulfate salts (K₂S₂O₈, Na₂S₂O₈ or Oxone) were immediately added. The UV reactor was mixed with a magnetic bar and was irradiated for various times in order to achieve required UV doses.

Samples were collected at 0, 1, 3, 6, 10, and 30 minutes for the analysis of E. coli and P. aeruginosa. Disinfection experiments were carried out separately for E.coli and P. aeruginosa in the presence of different sulfate salts (K₂S₂O₈, Na₂S₂O₈ or Oxone). Each set of experiments was repeated twice and each dilution was plated in triplicate. Blank experiments without UV-A were carried out in order to determine the effect of each of three sulfate salts alone (3 mM) on inactivation of bacteria in dark conditions. The results of these experiments showed that no significant bacterial inactivation occurred.

Alkaline activation experiments were performed in 2 l a glass cylinder reactor that had a diameter of 9 cm and a hight of 41 cm (Fig. 2). Pre-calculated amount bacteria suspension (initial bacterial count of approximately 5*10⁸ CFU/ml) was added into sterile water containing potassium persulfate (2 or 3 mM K₂S₂O₈). Then, the inactivation process was started by adding the calculated amounts of activator (0.25, 0.5, 0.75 and 1.5 mM sodium hydroxide). The effect of different activator amounts on inactivation was determined. The samples were taken at 0, 10, 20, 30, 60 and 90 seconds and was added PBS to neutralize the residual persulfate and the pH. Then, E.coli and P. aeruginosa analysis was performed All experiments were performed at room temperature (23 ± 2 ^oC) and under constant stirring. Each set of experiments was repeated twice and each dilution was plated in triplicate.

Kinetic modelling

Kinetic models of experimental values were made with the Microsoft® Excel add-in tool GInaFiT (Geeraerd and Van Impe Inactivation Fitting Tool) (Geeraerd et al. 2005; Rodríguez-Chueca et al. 2017b).

In this study, the inactivation effect of PDS and PMS activated by UV-A radiation and PDS activated by alkaline environment on E.coli and P. aeruginosa bacteria was investigated.

Bacterial inactivation with non-activated PMS and PDS has been described in the literature with different results. While a group of researchers stated that non-activated sulfate salts have some disinfection capacity (Moreno-Andrés et al. 2019a; Rodríguez-Chueca et al. 2017a), some researchers have stated that this capacity is either absent (Guerra-Rodríguez et al. 2022; Sánchez-Montes et al. 2020) or at a negligible level (Zhang et al. 2022). For this reason, the corresponding blank experiments were carried out with the maximum concentration of each reagent to determine whether PMS and PDS alone had an inactivation effect in the dark without the activation process. For the doses evaluated, bacterial inactivations were negligible during the inactivation experiment (30 min) (results not provided).

Disinfection efficiency of activated peroxdisulfate/peroxmonosulphate by UV-A on E.coli

Three different sulfate salts (K₂S₂O₈, Na₂S₂O₈ and Oxone) were used to activate with UV-A radiation and the inactivation effects of different salts were demonstrated. Log reduction of E. coli was determined according to different concentrations of PDS/PMS during UV-A radiation. Figure 3 shows that inactivations of E. coli in varying experimental conditions (K₂S₂O₈/UV-A, Na₂S₂O₈/UV-A and Oxone/UV-A) at different PDS or PMS concentrations.

Figure 3a illustrates the impact of UV-A and UV-A-activated K₂S₂O₈ on E. coli inactivation. It is clearly seen from the figure that a reduction of 2.89 logs in E. coli was achieved within 30 minutes only through UV-A radiation without the addition of K₂S₂O₈. The reduction of E. coli after 30 min exposure was approximately 3.65 log with 2 mM K₂S₂O₈ activated by UV-A whereas 4.65 log was achieved with 3 mM K₂S₂O₈ activated by UV-A. When 3 mM K₂S₂O₈ was used in combination with UV-A, there was a 60.9% increase in E. coli log reduction.

Figure 3b shows the effect of increasing Na₂S₂O₈ concentration used as PDS source under UV-A radiation. E.coli removal was achieved 3.50 log for 2 mM and 4.40 log for 3 mM with activated Na₂S₂O₈ by UV-A, adding Na₂S₂O₈ increased log reduction by 21.11% and 52.25%, respectively.

Figure 3c indicates that inactivation of E. coli increased with increasing PMS concentration in UV-A/PMS experiments. When the oxone concentration was increased from 2 mM to 3 mM, the logarithmic removal increased from 4.03 to 5.36.

The inactivation effect of different sulfate salts on P. aeroginosa is given in Fig. 4. Figure 4a shows the effect of increasing K₂S₂O₈ concentration in PDS + UV-A processes using K₂S₂O₈ as PDS. P. aeroginosa reduction was approximately 1.8 log after 30 min of UV-A exposure without K₂S₂O₈, whereas 3.24 log reduction was achieved with 3 mM K₂S₂O₈. When 3 mM K₂S₂O₈ was used in combination with UV-A, there was a 80% increase in P. aeroginosa log reduction.

Figure 4b and c show the inactivation of P. aeroginosa by the action of sulfate radicals produced by UV-A radiation from different sulfate sources. After 30 minutes of exposure to 2 mM Na₂S₂O₈ (Fig. 4b) and oxone (Fig. 4c) which activated by UV-A, a reduction of 2.18 log and 3.18 log was observed, respectively. The corresponding reductions increased to 3.42 log and 4.35 log when using 3mM Na₂S₂O₈ and oxone with UV-A radiation.

Although peroxydisulfate (PDS) and peroxymonosulfate (PMS), which are activated to generate sulfate radicals, are both characterized by the presence of O-O bonds (Xia et al. 2020), they have different structures (Guerra-Rodríguez et al. 2018; Xia et al. 2020) and activation mechanisms are also different (Tang et al. 2020). Generally, both PDS and PMS are widely applied due to their excellent water solubility, low cost, ease of storage, and low environmental impact. Na₂S₂O₈ and K₂S₂O₈ are used as PDS (Wordofa et al. 2017); oxone as PMS. Although there are many studies on bacteria removal by PDS and PMS activation (Guerra-Rodríguez et al. 2021; Moreno-Andrés et al. 2019a; Rodríguez-Chueca et al. 2017a, 2019b;), research on the comparison of different sulfate salts in bacteria removal is rare (Moreno-Andrés et al. 2019a).

Increasing the concentrations of sulfate salts enhances the efficiency of bacteria removal. It is clear from Fig. 5 that the highest removal efficiency was obtained with oxone, when comparing the inactivation effect of different sulfate salts activated by UV-A (different disinfection processes) on E.coli and P. aeroginosa. PMS is more easily activated than PDS due to its asymmetric structure (Guan et al. 2013; Luo et al. 2021). Oxone decomposes in water to a strong oxidant monopersulphate, which can be converted to the SO₄^●− (Wacławek et al. 2015). SO₄^●− shows a higher oxidation-reduction potential at neutral pH and is more selective for electron transfer compared to OH^●. This may result in a higher selectivity towards electron-rich moieties (Neta et al. 1977). SO₄^●− can selectively oxidize biomolecules and macromolecules with electron-rich groups, making SO₄^●− more suitable for inactivating microorganisms (Neta et al. 1988; Rodríguez-Chueca et al. 2019c; Zhou et al. 2023) determined that PMS activation under UV radiation had better inactivation effect on bacteria than PDS system. The different behaviors of both oxidants are clearly seen, when the photolytic activations formed by UV energy input are examined. Photolysis of the PMS molecule produces a sulfate radical as well as a hydroxyl radical, while PDS two sulfate radicals are produced (Rodríguez-Chueca et al. 2019c; Wang and Wang 2018). Although SO₄^●− has more selective oxidizing properties, an electrostatic repulsion occurs between negative charge of sulfate radicals and the surface of negatively charged bacteria (Rodríguez-Chueca et al. 2019c; Sun et al. 2016). The bacterial surface is approached more easily with the HO^● radicals produced by PMS and this repulsion force disadvantage is overcome. Therefore, additional production of hydroxyl radicals from PMS increases inactivation efficiency (Rodríguez-Chueca et al. 2019c). It is also necessary to mention the sulfur pentoxide radical (SO₅^•−) produced during the activation of PMS with UV radiation (Moreno-Andrés et al. 2019a). It has a low oxidation potential (1.1V), but it has been reported that it can cause significant damage to bacteria (Rodríguez-Chueca et al. 2019b). It can be said that oxone is the most effective sulfate salt when the results are examined, and this is supported by the literature mentioned above. Also, he higher efficiency of PMS could be attributed to its higher oxidant potential (Eº=2.51 V) in comparison with PS (Eº=2.01 V)(Rodríguez-Chueca et al. 2019a).

Kinetic modeling of disinfection processes was performed with the Microsoft® Excel add-in tool GInaFiT (Geeraerd and Van Impe Inactivation Fitting Tool) (Geeraerd et al. 2005; Rodríguez-Chueca et al. 2017b). By evaluating the inactivation curves corresponding to the experimental values, the mathematical kinetic models that can be applied according to the different disinfection methods used in the GInaFiT content were examined and the evaluation of their compliance with the inactivation results was controlled using two parameters (R², RMSD). Inactivation results for both bacteria were found to be compatible with the Biphasic Model (Cerf 1977).

After the application of PMS and PDS which activated by UV-A, there was no significant difference between sulfate salts (especially between K₂S₂O₈ and Na₂S₂O₈), especially in the first 10 minutes of disinfection. This finding is consistent with literature (Rodríguez-Chueca et al. 2017a), in which they examined the effect of UV-A Led + PMS on E.coli. In our experimental studies, it has been observed that inactivation occurs rapidly at first minutes and then slows down. Biphasic model fitted the inactivation results (k₁) for PMS (3 mM oxone)/UV-A inactivation of E.coli and P.aeroginosa were obtained 0.97 min^− 1 for E.coli and 1.50 min^− 1 for P.aeroginosa. This constant for PDS/UV-A inactivation of E.coli and P.aeroginosa were obtained 0.92 min^− 1, 0.86 min^− 1 for 3 mM K₂S₂O₈/UV-A and 1.19 min^− 1, 1.86 min^− 1 for 3 mM Na₂S₂O₈/UV-A, respectively.

In addition, the required times for 4 logs of E.coli and P.aeroginosa inactivation were calculated with GInafiT software. While the required time for a 4-log reduction in E.coli removal with UV-A alone was grater than 30 minutes, the addition of 3 mM K₂S₂O₈, 3 mM Na₂S₂O₈, and 3 mM Oxone was reduced this time to 12.6, 26.4, and 11.4 minutes, respectively. For P.aeruginosa removal, using only 3 mM Oxone with UV-A, the required time could be reduced to 28.2 minutes.

Disinfection efficiency of alkaline activated peroxdisulfate

Few studies have focused on the efficacy of alkali-activated persulfate on microbial inactivation. Most of these studies have been conducted on foodborne pathogen inactivation on high alkali concentrations (Qi et al. 2018, 2019; Qi and Hung 2019). Studies in the field of environmental engineering are those that focus on chemical degradation. These studies showed that the degradation rate of diesel in diesel-contaminated soils (Lominchar et al. 2018) and the degradation kinetics of decabromodiphenyl ether (Peng et al. 2017) increase with increasing persulfate concentration. In this study, the effects of peroxydisulfate activated with low concentration of NaOH on bacteria that can be found in drinking water on E.coli, and P.aeruginosa were investigated. In order to examine the inactivation effect of alkaline (NaOH) activated peroxydisulfate on microorganisms, studies were carried out using two different doses of K₂S₂O₈ (2 and 3 mmol/L) alone. Alkali/persulfate-peroxymonosulfate doses (Table 1) were determined by considering the ratios in studies on foodborne pathogens.

Table 1

Bacterial removal efficiencies with variation of NaOH and PDS concentrations

	2 mM K₂S₂O₈				3 mM K₂S₂O₈
NaOH concentration (mM)	0.25	0.50	0.75	1.5	0.25	0.50	0.75	1.5
NaOH/ PDS	0.125	0.25	0.375	0.75	0.083	0.166	0.25	0.5
E.coli	3.92	5.33	5.49	6.03	4.5	5.88	6.03	6.25
P.aeroginosa	1.99	3.18	3.43	4.1	2.95	3.56	4.01	4.43

K₂S₂O₈ (2 and 3 mM) were activated with four different doses of NaOH (0.25, 0.5, 0.75 and 1.5 mM) to evaluate the effect of different PDS/base concentrations on bacteria removal. Blank experiments were conducted at the highest concentration of NaOH (1.5 mM) to assess whether NaOH alone has a removal effect on E.coli and P. aeruginosa and no inactivation was observed. Qi et al. (2018) stated that in the experiments performed for E.coli O157:H7, no significant bacterial decrease was observed until NaOH increased to 10 mmol/L. The bacterial removal efficiencies of E.coli (Fig. 6a), and P. aeruginosa (Fig. 6b) increased with increasing sodium hydroxide concentration in this study. Qi et al. (2018) reported that base-activated persulfate systems become increasingly reactive with higher base/persulfate molar ratios. Furman et al. (2011) also stated that the alkali activation reaction of persulfate will increase with the increase in basicity. This explains the increase in log removal as the PDS/NaOH ratios decrease. At the same time, they stated that alkali activation may be advantageous over iron activation due to the long activation period.

The highest removal efficiencies, the highest peroxydisulfate concentration (3mM) and the longest time (90 seconds) were obtained for E.coli and P. aeruginosa bacteria. At the same base/PDS ratio (0.25), increasing peroxydisulphate concentration resulted in higher removal efficiencies for both bacteria (Fig. 7). Qi et al. (2019) stated in their study that the bacteria removal efficiency increased with the increase of PS concentration, and the reason for this was the increase in sulfate radicals produced with increasing persulfate concentration. The removal efficiency of both bacteria increased when the NaOH concentration was kept constant at certain values (0.25, 0.5, 0.75 and 1.5 mM) and the PDS concentration was increased from 2 mM to 3 mM. When the NaOH concentration was kept constant and the PDS concentration was increased, the removal efficiencies for E.coli increased at rates ranging from 0.22 log to 0.58 log. This value occurred as an increase in removal rates of 0.33 log to 0.96 log for P. aeruginosa. Qi et al. (2019) obtained similar rates of increase (approximately 1 log increase) by increasing the initial PS concentration from 50 mM to 80 mM. Table 1 demonstrates that the removal efficiency increases as the NaOH levels increase.

pH is important to maintain maximum inactivation with alkaline activation. Furman et al. (2011) stated that persulfate-induced oxidation occurs more rapidly around pH 12 than at lower pH regimes. The neutralization effect can cause significant pH change in treatment with low NaOH levels. This may reduce the activation power on persulfate. As a result, fewer free radicals may be produced and lower bacterial reduction may occur (Qi et al. 2019). pH at low NaOH values resulted in values below 12 (11.25 ± 0.44), when the pH change was monitored in our study. At high concentration, the pH remained around 12 (11.95 ± 0.38). Furman et al. (2011) reported that an additional benefit of high base/persulfate ratios is that it provides a lower pH drop potential as persulfate decomposes into sulfuric acid. Therefore, it can be said that high base/persulfate molar ratios become more reactive in base-activated persulfate systems. In summary, the inactivation efficiency by alkaline activation can be increased with higher basicity (higher NaOH/PDS).

Kinetic rate constants of E.coli and P.aeroginosa at NaOH/PS = 0.25, were as follows: k₁ and k₂ constants, 0.40, 0.02 for E.coli and 0.14, 0.02 for P.aeroginosa at using 2 mM K₂S₂O₈ with NaOH, respectively. k₁ and k₂ constants at using 3 mM K₂S₂O₈ with NaOH: 0.59, 0.04 and 0.30, 0.01 for E.coli and P.aeroginosa, respectively.

Similar to kinetic coefficients at the same NaOH/PS values (0.25), when the K₂S₂O₈ concentration was increased from 2 mM to 3 mM, the time required for 4 log bacteria removal showed a decrease from 24.3 seconds to 16.2 seconds for E. coli. Required times for 4 log P.aeroginosa removal decreased from > 100 seconds to 55.8 seconds (Fig. 6).

This study presents the effects of different sulfate sources used to generate sulfate radicals on the removal efficiencies of two bacteria. In addition to the use of different sulfate sources to generate sulfate radicals, different activation methods have also been tried. An overall evaluation of the present study showed that;

PMS showed higher efficiency than PDS in removal of both bacteria. Increasing concentrations of PMS and PDS activated by UV-A increased the removal efficiency of E.coli and P.aeruginosa. The addition of 3 mM K₂S₂O₈, Na₂S2O₈, and Oxone during a 30-minute UV-A radiation period increased the E.coli removal efficiency by 1.76, 1.51, and 2.47 logs, respectively. Similarly, for P.aeruginosa, these values increased by 1.44, 1.62, and 2.55 logs, respectively.
While the time required for 4 log bacteria removal was > 30 minutes for both bacteria, these times decreased to 11.4 minutes for E.coli and 28.2 minutes for P.aeroginosa with the addition of 3mM oxone to UV-A radiation.
K₂S₂O₈ was activated through alkaline activation, and disinfection was conducted. As the PDS concentration increased, the removal efficiency for both bacteria also increased. The highest removal efficiency for both bacteria was achieved with 3 mM K₂S₂O₈.
High base/PDS ratios provided higher reactivity. Therefore, higher removal efficiency was obtained at higher base/PDS ratios. When the PDS concentration was increased from 2mM to 3mM at the same NaOH/PDS (0.25) ratio, the time required for 4 log bacteria removal was reduced by 33% for E.coli. On the other hand, there was a decrease of more than 44.2% for P. aeruginosa.
An overall evaluation of the present study showed that E.coli is more sensitive than P.aeroginosa to radicals obtained by both UV-A activation and alkaline activation.
The biphasic model saticfactorily described the inactivation curves of E.coli and P.aeruginosa.

Author Contributions

Sevil Çalışkan Eleren contributed to the formulation of overarching research goals and aims and designed the research All authors contributed to the study conception and design. Data collection and analysis were performed by Ebru Yavaş. The first draft of the manuscript was written by Sevil Çalışkan Eleren and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

The authors declare that no funds, grants, or other supports were received during the preparation of this manuscript.

Data Availability

All data generated or analyzed during this study are included in this published article.

Competing Interests

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.

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Efficiacy of activated PMS/PDS by different methods in inactivating E.coli and P.aeroginosa

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Microbial strains and preparation of microorganisms cultures

Enumeration of microorganisms

Experimental set-up

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Results and discussion

Disinfection efficiency of activated peroxdisulfate/peroxmonosulphate by UV-A on E.coli

Disinfection efficiency of alkaline activated peroxdisulfate

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