Optimization electrophotocatalytic removal of Streptococcus faecalis from water by Taguchi model.
MATERIALS AND METHODS
The ZnO nanoparticles with special area 50 [m.sup.2] [g.sup.-1] and particle size 20 nm were supplied from Amohr Co. (Germany). Azide dextrose broth, PSE agar, brain heart infusion (BHI), nutrient agar, sodium chloride, sodium hydroxide, and nitric acid were purchased from Merck Co. (Germany). Nitric acid and sodium hydroxide (1 N) were applied for pH adjustment.
Preparation of ZnO nanoparticles
5 grams of ZnO nanoparticles were placed into 100 ml of distilled water. The suspension was mixed with a magnetic stirrer for 30 min and then sonicated in an ultrasonic bath (MATR. N.B., Italy) at a frequency 50 kHz for 22 min improved the dispersion of ZnO in distilled water. The weight of zinc electrode was measured after hydroxylation, and washing with distilled water.
Preparation of electrodes
The Zn electrode was used as the substrate for the immobilization of ZnO nanoparticles. The Zinc electrode was pre-treated by detergent and sodium hydroxide solution at 0.01 N to increase the number of OH groups.
Immobilization of ZnO nanoparticles
To prepare the ZnO films, dry methods were used (Malato et al, 2013: Zuolian et al, 2010). In this study a Zn plate was used for immobilization. After the pre-treatment, the Zn electrode was weighted, immersed in the colloidal solution, and dried in an oven at 35[degrees]C for 30 min. The coated particles were then calcined in a muffle furnace at 105 and 320[degrees]C for 60 min. The thermal treatment of immobilized ZnO films led to developing good mechanical stability of the films. For 2- and 3-layer coatings, the process were repeated twice and three times. They were washed with distilled water to remove any free ZnO nanoparticles.
Batch EPC reactor
The experimental setup was shown in Fig. 1. The batch reactor was a 360-ml glass vessel (10x6x6 cm). The characteristics of electrodes were as follows: two electrodes of thin layer ZnO nanoparticles immobilized on Zn (anode), and copper electrode (cathode). The area of each electrode was 36 [cm.sup.2] (9x4x0.1 cm). The distance between the bottom of the reactor and the electrodes was 1 cm, and the distance between the LED UV-A lamp and the Zn/ZnO electrode was adjusted 1.5 cm. The alternative current (AC) electrical source had an electrical energy production equal to 1-5 A, and a maximum electrical power of 60 W. The LED UV-A lamp had an electrical power of 1 W, radiation intensity of 120 mW [cm.sup.-2], a wavelength of 395 nm, and a voltage of 3.4 V. To evaluate the effect of the current densities, catalyst, and UV light on the disinfection process, samples underwent LED UV-A lamp treatments (at 360, 480, and 600 mW [cm.sup.-2]), with an electrode of thin layer ZnO nanoparticles immobilized on Zn (at 5%, 10%, and 15%), different current densities (at 3, 6, and 9 mW [cm.sup.-2]), different pHs (at 6, 7, and 8), and different radiation times (at 7.5, 15, and 30 min). A magnetic stirrer was used for homogeneous mixing of the contaminated water samples. The Log bacterial reduction was calculated using the equation below and was converted to percentage cell killed (K. Mwabi et al, 2012):
Log reduction = ([Log.sub.10] [bacterial count.sub.before treatment] - [Log.sub.10] [bacterial count.sub.after treatment] ... (1)
The kill % = 100 - Survivor count x Initial count x 100 ... (2)
The percentage cell reduction was calculated according to the following equation:
R (%) = (1 - [B.sub.t]/[B.sub.t0]) x 100 ... (3)
Where R was the percentage of cell kill, [B.sub.t0] and [B.sub.t] were the average of initial and survival count of live cells per milliliter.
The operational cost required to FS removal was calculated to the following Eqn (4):
Operational cost = [C.sub.energyt] + [C.sub.electrod] + [C.sub.UVlamp] ... (4)
Where the operational cost ([C.sub.operational], Rial kWh per kg of FS removed), the consumed electrical energy cost ([C.sub.energy], kWh [kg.sup.-1]), consumed electrode cost ([C.sub.electrode], Rial per kg of FS removed), and the consumed LED UV-A lamp cost ([C.sub.UV-A], kWh [kg.sup.-1]) expressed.
The electrical energy required to FS removal was calculated to the following Eqn (5):
EE([kWh/kg]FS removed %) = ([VIt x [1000.sub.t]]/[60 x ([C.sub.t0] - [C.sub.t])]) ... (5)
Where the consumed electrical energy cost (EE, kWh per kg FS removed), the electrical voltage (V, volt), the electrical current (I, A), and the electrochemical time (t, min) expressed.
Kinetics reaction models were calculated according to the following Equations (6) and (7):
In Bt = ln Bt0 - K1t ... (6)
1/Bt = K2t + 1/Bt0 ... (7)
Where [K.sub.1] and [K.sub.2] were the first, and second order reaction constants, respectively. Values of [K.sub.1] and [K.sub.2] could be calculated from the slope of the plots ln [B.sub.t] versus t, and 1/[B.sub.t] versus t, respectively (Ifelebuegu et al., 2013).
Preparation of FS
Suspension of FS (ATCC 29212) in water was obtained following the technique proposed by other researchers (Gholami et al, 2014). FS was reactivated from frozen stock (15% glycerinated azide dextrose broth) in a 100-ml Erlenmeyer flask containing 50 ml of azide dextrose broth (Merck). The sample was incubated at 37[degrees]C for 18-24 h. The bacterial cell was isolated at 5000 rpm for 15 min after inoculating strain in azide dextrose broth at 37[degrees]C. Strain was stored on trypticase soy agar at 4[degrees]C. The strain was grown on BHI agar at 35[degrees]C for 24-48 hrs. The optical density (OD) of the cell suspension was measured with a spectrophotometer at 650 nm. The described procedure resulted in suspensions with a cell concentration of 5 x [10.sup.1] and 5 x [10.sup.2] CFU/ml. The FS was measured by standard method 9230 B (APHA-AWWA-WEF, 2012). In this method, the bacteria could be confirmed by production of brownish-black colonies with brown halos. This method reported the number of microorganisms as most probable number (MPN). After each round of the study, reactor water was picked and cultured on zide dextrose broth tubes, and PSE agar to evaluate the efficiency of the removal process. After incubation at 35[degrees]C for 48h, the number of cells was counted, and the results were expressed as MPN. EPC reactor without microbe, and electrophoto was used as the test control. EPC experiments were at least duplicated and all samples are analyzed in triplicate.
Effect of initial number of FS
The effect of the initial number of FS on the removal efficiency of the EPC process was shown in figure 2. The removal efficiency was decreased by an increase in the cell number from 5 x [10.sup.1] to 5 x [10.sup.2] CFU/ml. The EPC reactor showed the removal percentage for FS cells (5 x [10.sup.1] in ml) increased from 90% to 100% as the pH increased from 6 to 8, with 7.5 min irradiation. The EPC reactor showed the removal percentage for FS cells (5 x [10.sup.2] in ml) increased from 84% to 99% as the pH increased from 6 to 8, with 7.5 min irradiation. This effect was attributed to increasing the number of FS cells accordingly fixed the number of photocatalytic sites and UV-A light. This phenomenon was the same as E. coli bacterium. They were investigated the effect of photocatalytic disinfection on E. coli. These experiments were performed an initial cell concentration in the range of [10.sup.2] to [10.sup.3] cells in ml at pH 7, radiation time 5 minute, distance between the UV-A lamp and Zn/ ZnO electrode 2 cm, voltage 10 V, ZnO nanoparticles 5% wt, and LED UV-A lamp power 240 mw [cm.sup.-2]. At higher concentration, the efficiency started to lessen (Rezaee et al, 2011). The EPC reactor reached the highest efficiency (100%) at pH 8, radiation time 7.5 minute, and a cell concentration of 5 x [10.sup.1] and 5 x [10.sup.2] CFU/ml. Photocatalytic exposure time required for complete cell inactivation (5 x [10.sup.1] and 5 x [10.sup.2] in ml) were 7.5 min. At lower concentration, photocatalytic exposure time required for complete cell inactivation started to lessen. The influence of ultrasound (US) dose on E. coli inactivation was studied at two different sonolysis time (15 and 30 minutes). The results showed a synergistic effect of US on E. coli reduction at 15 minute (Naddeo V et al, 2009). Dehgani et al. found that the fun-gi population decreased with increasing sonication time (Dehghani et al, 2007).
Effect of pH
The pH was a significant operating variable affecting the performance of the EPC process. The bactericidal effect of this method was strongly dependent on pH, and was enhanced by an increase in pH. In the EPC process, different concentrations of O[H.sup.-] radical from water were formed depending on the pH. These products played an important role in the removal of FS cells in the EPC process. This effect was attributed to an increase in the O[H.sup.-] concentration at a higher pH. This observation was consistent with other previously studies (Mendez-Arriaga et al, 2008). Endocrine disrupting chemicals (EDC) bisphenol-A however get increasingly very significant at alkaline pH. The initial and final pH values were measured in this study in order to investigate the effect of pH more effectively. The initial pH enhanced during EPC studies. The effect of the pH on the removal efficiency of the EPC process was shown in figure 2, and figure 3. The EPC reactor reaches the highed efficiency (100%) at pH 8, radiation time 15 minute, ZnO nanoparticles 10% wt, distance between the LED UV-A lamp and Zn/ ZnO electrodes 1.5 cm, LED UV-A lamp intensity 480 mw [cm.sup.-2], current density 3 mA [cm.sup.-2], and a cell concentration of 5 x [10.sup.1] and 5 x [10.sup.2] CFU/ml. The pH 8 needed lower current density, compared with the two other current densities. It was concluded that optimum pH for reaching to microbial standard (MPN 0) was pH 8. It was expected that positive surface charge of FS logarithmic growth phase could affected the solution pH during photocatalytic oxidation. This observation was consistent with the significant of the pH effect for certain organics destruction in a basic pH as reported by Torres et al. (Torres et al, 2008).
Effect of lamp intensity
The effect of the LED UV-A lamp intensity on the removal efficiency of the EPC process was shown in figure 4. The removal percentage for FS cells (5 x [10.sup.2] in ml) increased from 97 % to 100% as the LED UV-A lamp intensity increased from 360 to 480 mw [cm.sup.-2], with 7.5 min of radiation, and pH 8. The removal efficiency of FS was proportional to the LED UV-A lamp intensity and enhanced by an increase in the LED UV-A lamp intensity. This observation was consistent with previously published data. When primary wastewater samples was exposed to UV irradiation, the number of P. aeruginosa (ATCC 15442) cells decreased progressively from [10.sup.7] cells in ml to [10.sup.4] as the UV-C lamp dose increased from 0 to 500 mW x s [cm.sup.-2] (Mounaouer, Abdennaceur, 2012). It was also reported that the concentration of 2,4-dichlorophenoxyacetic acid (2,4-D) decreased from 45 to 20 mg [L.sup.-1] as the UV-C lamp dose increased from 150 to 400 W (Kundua et al, 2007). At higher lamp intensity, the exposure time, and current density started to lessen. Optimum UV-A lamp intensity for reaching to microbial standard (MPN 0) was 480 mw [cm.sup.-2]. The above increased optical activity was explained by higher formation of reactive oxygen species (ROS), such as electron donor OH- radical from hydroxide anion of water, and superoxide radical anion ([O.sub.2.sup.*-]). The linear increase trend of the degradation rate for FS at UV-A lamp intensity was explained by producing more electron/hole pairs due to being available more photons for excitation at the Zn/ZnO surface. This finding was consistent with photocatalytic experiments were performed using a ZnO nanoparticle (Kundua et al, 2007).
Effect of ZnO and LED UV-A lamp
The effect of the ZnO, and LED UV-A lamp intensity on the removal efficiency of the EPC process was shown in figure 5. The removal percentage for FS cells (5x [10.sup.1], and 5 x [10.sup.2] in ml) dramatically increased in presence of ZnO photocatalyst nanoparticles and the LED UV-A lamp. At higher lamp intensity along with higher amount of ZnO catalyst up to solution 10% wt, the exposure time, and current density started to lessen. At fixed lamp intensity, it was that an optimum catalyst amount would present where the photocatalyst would form a maximum concentration of ROS which could take part in reaction at the outer film surface. The optimum amount of ZnO catalyst solution, and optimum intensity of the LED UV-A lamp for reaching to microbial standard (MPN 0) were 10% wt., and 480 mw [cm.sup.-2], respectively. While the removal efficiency decreased at the 1- and 3-layer Zno nanoparticle films, it reached the highest value (100%) at 2-layer Zno nanoparticle film. This finding was attributed to an increase the surface area for inactivation of FS cells (5 x [10.sup.1], and 5 x [10.sup.2] in ml). This finding was consistent with photocatalytic experiments were performed using ZnO. They concluded that the decay rate constant of Congo red (CR) was proportional to the ZnO concentration. The decay rate enhanced from 68.73 to 90.02% as ZnO concentration was increased from 0.25 to 0.5 g [L.sup.-1]. However, increase in the ZnO concentration more than 0.5 g [L.sup.-1] led to decreasing the decay rate of CR (Elaziouti et al, 2011). However, a limiting value was observed at thick films due to increase in opacity and light scattering leading to a decrease in the passage of irradiation through the film. At higher catalyst loadings (i.e. more than two layer), the removal efficiency of FS started to lessen. This phenomenon was attributed to a decrease in UV penetration to the outer layers of the film, and a decrease in protection effect of clusters blocking UV from reach catalyst surface. The presence of ZnO photocatalyst nanoparticles and UV-A was led to increasing the removal efficiency of FS due to the generation of O[H.sup.-] radicals. This finding was consistent with photocatalytic experiments were carried out using Ti[O.sub.2] (Daghrir et al, 2012). O[H.sup.-] radicals led to fat peroxidation of cellular membrane and degradation of the different compounds of the cell. [O.sub.2.sup.*-], hydro peroxyl radical and hydrogen peroxide, generated by the reduction of dissolved oxygen in anode, could also feed into the photocatalytic disinfection mechanism. These species were responsible for decaying the FS. The photoelectrocatalytic application by Ti[O.sub.2] thin film photoanodes for disinfection process in water had been reported (Selcuk, 2010). UV dosage required for 99.9% destruction of FS was 8 mJ x [cm.sup.-2].
Effect of current density
A key variable parameter affecting the oxidation ability of EPC process was the applied current density since it regulated the amounts of generated OH- radicals acting as oxidizing agents. The effect of the current density on the removal efficiency of the EPC process was shown in figure 6. At lower current density, and lower radiation time, the removal efficiency of FS started to lessen. On the other hand, at higher current density, the radiation time started to lessen. The optimum current density for reaching to microbial standard (MPN 0) was 3 mA [cm.sup.-2]. At lower initial cell loadings, the photocatalytic treatment time required for complete cell inactivation started to lessen. The experimental results showed that the current density electrode enhanced the resulting gradient separated electron-hole, thereby diminishing its recombination rate, enhancing the photocurrent rate, and at length expediting the cell inactivation as shown in figure 3. Under higher applied current densities, the external electric field improved the direct and indirect electro-oxidation reactions at anode. The biocidal efficiency was proportional to the specific surface area of photocatalysts and the quantum yield of photocatalytic system because the number of O[H.sup.-] was proportional to the specific surface area and inversely proportional to the electron-hole recombination rate. The photoelectrocatalytic accelerated the mass transfer by electro-migration of negatively charged bacterium cells towards the electrode. The selection of current densities was depended in the removal efficiency of bacterial, and the consumed electrical energy cost. This finding was the same as photocatalytic experiments were carried out using N-doped Ti/Ti[O.sub.2] photoanode (Daghrir et al, 2012). The experimental results showed that the more intensity the radiation penetrating the photocatalytic electrode was, the faster the cell inactivation progresses. As expected, for the current density and exposure time was enhanced, accordingly the removal efficiency of FS was enhanced as shown in Figure 3. This finding was the same as photocatalytic experiments were carried out using Ti[O.sub.2] reactor (Mounaouer, Abdennaceur, 2012).
EPC decay pattern for a Gram-positive bacterium FS was distinguished by complex structure of cell wall, peptidoglycan layers, and teichoic acids made of alcohol and phosphate groups. From the viewpoint of the cell wall structure, gram-positive streptococci's cell walls were notably thicker (200 [Angstrom]) than gram-negative enterobacteria's. The negative charge of teichoic acids of the Gram-positive bacterium FS led to its absorption by the Zn/ZnO electrode and could be mineralized and disrupted by strong oxidants such as positive hole, and OH- radical or reduced by electron in the conduction band. The increase in current density, and exposure time led to faster generation of electrolysis products such as O[H.sup.-] and [Cl.sup.-] anions in cathode and anode electrodes, respectively. These products were responsible for FS inactivation. Increased current density led to an increased drift force on electrode surface, which was the main factor in electrochemical processes. Therefore, it was obvious that the generation adequate quantity of reactive oxygen species for FS oxidation needed optimum radiation time (7.5 min). This finding was the same as experiments were performed using N-doped Ti[O.sub.2] photoanode (Daghrir et al, 2014). Clearly, the band gap of the ZnO semiconductor (Eg = 3.2 eV) was meagerly equal to that of UV-A radiation LED lamp ([E.sub.UV-A] =3.4 eV). The photogenerated electron ([e.sup.-])-hole ([h.sup.+]) pairs could be facilely isolated and transferred to the semiconductor/adsorbate interface efficiently, therefore enhancing the photocatalytic activity. This finding was the same as photocatalytic experiments were carried out using UV light (Tomasevic et al, 2009). The oxygen produced in anode electrode led to higher bactericidal effect against FS, because oxygen molecule played an important role in photocatalysis stage, and transformed to [O.sub.2.sup.*-] radical in capacity bond of ZnO photocatalyst nanoparticles. This finding was the same as photocatalytic experiments were performed using Ti[O.sub.2] (Pelaez et al, 2012). The efficiency of FS absorption by Zn electrode layered by ZnO nanoparticles as positive pole (anode) was directly related to an increase in current density, and exposure time. This electrophotocatalytic mechanism was illustrated in the following equations:
ZnO + h( = 390 nm) [right arrow] ZnO ([e.sup.-.sub.(CB)] + [h.sup.+.sub.(VB)]) ... (4)
[O.sub.2] + [H.sub.2]O [right arrow] [O.sub.3] + 2[H.sup.+] + 2[e.sup.-] ... (5)
[e.sup.-.sub.(CB)](ZnO) + [O.sub.2ads] [right arrow] [O.sub.2ads.sup.*-] + ZnO ... (6)
[O.sub.2ads.sup.*-] + [H.sup.+] [right arrow] H[O.sub.2ads.sup.*-] ... (7)
H[O.sub.2ads.sup.*-] [right arrow] [O.sub.2] + [H.sub.2][O.sub.2] ... (8)
[H.sub.2][O.sub.2] + h [right arrow] 2 O[H.sup.*] ... (9)
[h.sup.+.sub.(VB)] + O[H.sup.-] [right arrow] O[H.sup.*] ... (10)
* OH + FS [right arrow] degradation of the FS ... (11)
The results of FS removal efficiency by Taguchi model showed that concentration was the most important variable. This finding was not consistent with experiments were performed using iron electrodes (Chandra Srivastaval et al, 2011). Figure 8 showed the plots of the kinetics first, and second order reaction models fitted with the FS removal experimental data in batch EPC reactor. The experimental data fitted better to the first order reaction. The regression coefficient for the fitted line was calculated to be [R.sup.2] = 0.9882 for FS. The apparent rate constant, [K.sub.1] and the half-life time, [t.sub.1/2] were calculated to be 0.028 [min.sup.-1] and 0.7 min. This finding was consistent with electrocoagulation experiments were performed using Iron-Steel electrodes. They concluded that the degradation of the CR followed first-order kinetics (Mohammadlou et al, 2014). Thereby, EPC reactor technology could be the basis of a point off use treatment system of water able to enhance water quality by producing high disinfection. According to optimum conditions (electrical current 0.03 A, electrical potential 30 V, reaction time 7.5 min, Zinc oxide concentration 10% wt, UV-A lamp intensity 480 mw [cm.sup.-2], and water need 40 L/day), it was calculated that the minimum operational cost of the EPC was initial bacterial number 5 x [10.sup.1] in mL with removal efficiency 100% (3860 = 370 (consumed electrode cost) + 2175 (consumed electrical energy cost) + 1315 (consumed LED UV-A lamp cost)) and the maximum operational cost of the EPC was initial bacterial number 5 x [10.sup.2] in mL with removal efficiency 99,5% (4538 = 1030 (consumed electrode cost) + 2186 (consumed electrical energy cost) + 1322 (consumed LED UV-A lamp cost)). Therefore, at higher efficiency, the operational cost started to lessen.
The experimental results suggested that ZnO thin layer nanoparticles immobilized on Zn photoanode in a batch EPC reactor was a promising method for the FS inactivation. The EPC was affected by pH, the number of bacteria, the lamp intensity, radiation time, the number of layers ZnO nanoparticles catalyst, and current density. The following conclusions were obtained from the experiments:
1. High removal efficiency of FS was obtained by the EPC reactor (97%) compared to the photoelrctrochemical (PEC) reactor (79%).
2. The EPC treatments were capable of FS removal at the pH value (8) investigated, with a radiation time less than 7.5 min.
3. Enhanced FS removal was obtained with an increase in the pH, the lamp intensity, radiation time, and current density.
The author thanked the Department Environmental Health of Islamic Azad University, Tehran Medical Sciences Branch for financial and instrumental supports.
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G. Kashi * and Sh. Aliannejad
Department of Environmental Health, Islamic Azad University, Tehran Medical Sciences Branch, Khaghani St. Shariati Ave, Tehran State, Postal code: 19395/1495, Iran.
(Received: 21 April 2015; accepted: 30 June 2015)
* To whom all correspondence should be addressed. Tel.: +9821 22006667; Fax: 226007141; E-mail: firstname.lastname@example.org
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|Author:||Kashi, G.; Aliannejad, Sh.|
|Publication:||Journal of Pure and Applied Microbiology|
|Date:||Sep 1, 2015|
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