- Email: [email protected]
A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes
Accepted Manuscript Title: A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes Authors: Ali Reza Rahmani, Mohammad Reza Samarghandi, Davood Nematollahi, Fahime Zamani PII: DOI: Reference:
S2213-3437(18)30710-3 https://doi.org/10.1016/j.jece.2018.11.030 JECE 2785
To appear in: Received date: Revised date: Accepted date:
31 August 2018 11 November 2018 13 November 2018
Please cite this article as: Rahmani AR, Samarghandi MR, Nematollahi D, Zamani F, A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.11.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes
Ali Reza Rahmania, Mohammad Reza Samarghandia, Davood Nematollahib, Fahime
a
SC RI PT
Zamania*
Department of Environmental Health Engineering, Faculty of Health and Research Center for Health Sciences,
Hamadan University of Medical Sciences, Hamadan, Iran b
Faculty of Chemistry, Bu-Ali-Sina University, Hamadan, Iran
N
U
* Corresponding Author: E-mail address: [email protected], Tel: + 989351712739
A
ABSTRACT
M
The electrochemical disinfection (ECD) has achieved much attention due to its environmental compatibility, simplicity and production of oxidizing species. In the present study, the ECD of total coliform (TC) and fecal coliform (FC) was investigated with a focus on direct and indirect
D
oxidation using a series of anodes (stainless steel (SS)/lead (Pb) O2 (SS/PbO2), stainless steel,
TE
titanium (Ti), platinum (Pt), graphite (GP) and Pb/PbO2). The influence of electrode material, current density (CD), charge passed, initial pH values, different concentrations of NaCl, TDS,
EP
electrical conductivity (EC) and energy consumption on process performance was examined. The amount of in situ generated active chlorine during the process under the optimal conditions for
CC
each anode was found to be effective in removal of bacterial. The disinfection system was very efficient in bacterial elimination in presence of 0.01 M NaCl. An increase in CD from 0.16 to 0.5 mA/cm2 resulted in a decrease more than 60% in log 10 bacterial load for all electrodes except Pt.
A
Among anodes investigated, SS/PbO2 demonstrated the highest efficiency (complete inactivation) in 5 min by applying 0.01 M NaCl in current density of 0.5 mA/cm2 when charge passed was > 15 C. The efficiency of the anodes in inactivation of TC and FC was in the order of SS/PbO2 > SS > Ti> Pt > GP > Pb/PbO2. In spite of the high efficiency of the electrochemical method in removing TC and FC, indirect oxidation has a higher efficiency due to the production of strong oxidants such as radical hydroxyl and active chlorine.
Keywords: Electrochemical disinfection, Constant current electrolysis, A set of anodes, Total and fecal coliform, Surface water 1. Introduction One of the concerns in developing countries which have been highly regarded is the diseases and
SC RI PT
deaths due to pathogenic agents in water. According to reports by the World Health Organization (WHO), the cause of two million deaths in 2014 were the lack of access to healthy drinking water in rural areas [1]. Therefore, access to a water treatment process based on low energy consumption to provide healthy drinking water seems necessary [2]. The first step in water disinfection is the removal of pathogenic microorganisms from various water sources [3]. In spite of numerous disinfection methods, each of them has limitations. The use of advanced oxidation processes
U
(AOPs) despite the production of radical hydroxyl, as a strong oxidizing agent for many resistant
N
compounds, has limitations including: complexity, high treatment expense and chemicals utilization [4]. Filtration is one of the common physical processes which its usage has been limited
A
due to membrane fouling and high power consumption. Ultraviolet radiation (UV), although does
M
not generate disinfection by-products, its effectiveness depends on the depth of light penetration in the water affected by the turbidity. Ozone (O3) is capable of deactivating pathogenic
D
microorganisms but, toxic intermediates are produced during disinfection [4]. However, the electrochemical disinfection (ECD) has solved these limitations. The ECD which is also called
TE
green technology by disinfectant production on site is compatible with the environment and human [5]. The principal reactant that takes part in the process is electron, which is considered as a
EP
harmless and clean component [6]. Other observed advantages of this method are as follows: portable equipment, less requirement land [7], easy operation, simplicity, cost-effectiveness,
CC
generation of oxidizing agents and no harmful chemicals [5, 8]. The mechanism of the ECD reaction is presented in two stages: (1) direct oxidation that occurs on the surface of the electrode and is specified by rapid killing of microorganisms (2) indirect oxidation of water caused by
A
oxidizing species generation [3] like hydroxyl radical (●OH), atomic oxygen (●O), ozone (O3) and hydrogen peroxide (H2O2) via intermediate reactions (Eqs. 1-5) [3, 5]. M + H2O → M (HO•) + H+ + e-
(1)
HO• → O• + H+ + e-
(2)
2O• → O2
(3)
2HO• →H2O2
(4)
O• + O2 → O3
(5)
These powerful oxidizers are also called reactive oxygen species (ROS). Physisorbed hydroxyl
SC RI PT
radical M (HO•), with high redox potential (Eº = 2.80 V), is the most important ROS [6]. A broad range of pathogenic microorganisms could be inactivated by the ECD through ROS generated according to (Eqs. 1-5) and also in the presence of chloride which maybe exist naturally in water or be added to water, reactive chorine species (RCS) such as radical chlorine (•Cl, .Cl2-) and active chlorine species (Cl2, HOCl, ClO-) are produced and are known as main disinfectants [5]. In other words, if chloride ions are used in the process, disinfection is done more quickly due to rapid attack
U
to microorganisms by active chlorine species [6]. As indicated in (Eqs. 6-8) hypochlorous acid /
N
hypochlorite ion and acid hydrochloric are produced by dissolved chlorine hydrolysis. After
A
chlorination, total chlorine concentration is considered as active chlorine and the average pH of
M
solution determines the contribution of each active chlorine species [3]. 2Cl- → Cl2 +2e-
TE
HClO → ClO- + H+
D
Cl2 + H2O →Cl- + HClO + H+
(6) (7) (8)
It should be noted, in ECD, the applied potentials between electrodes also causes to bacterial cell
EP
wall destruction [9]. The variables commonly used to evaluate an electrochemical disinfection system include voltage, current intensity, pH and reaction time [5]. The operating conditions and
CC
anode material determine the type of produced oxidants [10]. Thus, anode selection is one of the most important factors affecting the electrochemical processes [3, 6, 11]. The difference among
A
electrode materials is pertinent to the substance and the amount of oxidants generated on the surface of anode [8]. Therefore, the electrode material has a significant effect on microbial load reduction. Hence, the comparison study on performance of a series of electrodes as anodes in the inactivation of microorganisms seems to be an interesting subject. In previous studies, a limited number of researches have tried to study the performance of one or more anodes in the inactivation of microorganisms. In present study, we conducted a comparative study on the efficiency of a set
of anodes in the removal of indicator microorganisms in water. Stainless steel (SS), titanium (Ti), platinum (Pt) and graphite (GP) with purpose of direct oxidation were applied for indirect and direct anodic oxidation by using lead (Pb)/PbO2 and SS/ PbO2 electrodes. The dimensionally stable anodes (DSAs) are desirable for lots of electro-organic processes and are categorized to active and inactive in terms of their chemical nature. Active electrodes are not very beneficial in
SC RI PT
mineralization of organic pollutants because of a lot of M (HO•) change to chemisorbed “superoxide” MO which has weak oxidation power, whereas, in inactive anodes, this conversion decreases and they therefore exhibit high ability to mineralize organic pollutants [6]. In inactive anodes, there is no oxidation location available but adsorbed hydroxyl radical with high redox potential (Eº = 2.80 V) oxidizes organic materials (such as SnO2 and PbO2) [12]. It has been proved that hydroxyl radicals react with a large number of compounds [13]. The usage of the EO process
U
has been studied in inactivation of pathogenic microorganisms available in water. Inactivation of
N
pathogenic microorganisms occurs through the destruction of their cell wall [6]. Escherichia coli and fecal coliform are commonly considered as the organisms listed in the water quality standards
A
to determine the effectiveness of electrochemical disinfection technology [5]. The aim of this study
M
was to research the ECD of microbial indicators (TC and FC) by using direct and indirect anodic oxidation. Also, the effect of active chlorine presence on the process was tested by applying a
D
series of anodes under optimal conditions. The main variables of this study included current
TE
density (CD), charge passed, different pH values, electrode material, various concentrations of NaCl, TDS, electrical conductivity (EC) and energy consumption.
EP
2. Experimental 2.1. Chemicals
CC
Lactose Broth, Brilliant Green Bile Broth (BGB), EC Broth, Sulphuric acid (H2SO4) and sodium hydroxide (NaOH) were used in the current study. All chemical materials used in this study (with
A
purity of 99.5%) were purchased from Merck Company, Germany. Sodium chloride (NaCl) was utilized in electrolytic tests. 2.2. Microbiological cultures TC were first cultured in Lactose Broth and then cultured at confirmatory phase in Brilliant Green Bile Broth. Incubator was used to provide temperature of 37 °C for 48 hours within probable and
confirmatory phase. The identification of FC in EC Broth culture and in water bath at 44.5 °C for 48 hours was accomplished. 2.3. Experimental unit The components of the applied electrochemical pilot included electrodes, magnetic stirrer, glass
SC RI PT
reactor and DC power source. An 800 ml glass reactor was filled with 600 mL of the sample for all electrochemical trials. During the experiments, the temperature of the samples was controlled using a thermometer to maintain the temperature of the samples close to the ambient temperature. The pH adjustment was performed using sulfuric acid and sodium hydroxide .Six types of electrodes: SS, Ti, Pt, GP, Pb/PbO2 and SS/PbO2 as anodes with dimensions of 4×15×0.1 cm were applied. In the case of GP and Pb/PbO2, the dimensions were 4×15×1 cm. SS was selected as
U
cathode for all trials. The electrodes were fixed parallelly in 2-cm gap from each other. Before
N
each experiment, all the electrodes with the aim of cleaning were immersed in the HCl solution (15% Wt.) and then washed with distilled water. To supply direct current, the electrodes were to
a
DC
power
source
A
connected
(Aram
Tronik
CO.
M
Iran). A magnetic stirrer was used to mix the contents of the reactor. Immediately after each sampling in order to prevent every contamination, the microorganisms were measured in a sterile
𝑁𝑖−𝑁𝑢 𝑁𝑖
× 100
TE
(RTC %) or (RFC %):
D
room. The removal efficiency of TC and FC were calculated using the following equation: (1)
Where, RTC% or RFC% present the removal percentage of TC or FC, Ni and Nu are the initial and
EP
ultimate amount of microorganisms. In this study, the variables pH (6-8), current density (0.16 to 0.5 mA/cm2), charge passed (10-150 C), electrode type were optimized by the one factor at a time.
CC
First, constant current electrolysis was used. After optimization the values of the mentioned variables, the NaCl concentration (0.01-0.12 M) was optimized.
A
2.4. Instrumentation Different values of pH in range 6, 7 and 8 were measured using pH meter (HACH CO, USA). To determinate TDS and EC, a conductivity meter was applied (Hach conductivity meter, model 44600). Microbiological tests were carried out according to the standard methods. Active chlorine
was quantified based on the colorimetric method by N, N-diethyl-p-phenylenediamine (DPD) (DR 5000 UV–Vis spectrophotometer at = 515 nm) [3]. 2.5. Electrode preparation and characterization To build the Pb/PbO2 electrode, firstly, the Pb rods were placed in sulfuric acid solution10% then
SC RI PT
the current density of 10 mA/cm2 was applied for 90 min, while the temperature was maintained at 25 °C. (Eqs. 9 and 10) Pb + SO42- → PbSO4 + 2e-
(9)
PbSO4 + 2H2O → PbO2 + SO42- + 4H+ + 2e-
(10)
The preparation steps of the SS/PbO2 electrode are as follows: the surface of SS electrode was
U
cleaned in 10% sulfuric acid solution and then in acetone solution was sonicated for 10 min. After
N
initial preparation, it was placed in Pb (NO3)2 solution 0.5 N at current density of 1 mA/cm2 for 5 hours. The structure and morphology of the composite material was specified via analysis
A
technique X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray
M
diffraction (XRD) of SS/PbO2 and Pb/PbO2 was restarted using scanning electron microscopy (SEM, Philips XL-30) and X-ray diffraction (XRD, Thermo ARL SCINTAG X'TRA, 45 kV). As
D
can be seen in Fig. 1, the regular cube-shaped crystals formed on the surface of the SS electrode
TE
are mainly of –β -PbO2 with the fine size of 10 μm. 3. Results and Discussion
EP
3.1. The effect of pH
The initial pH in the range of 6 to 8 was investigated as one of the most important factors in
CC
electrochemical process with current density of 0.1 A/cm2 and using a pair of SS electrodes as anode and cathode (Fig. 2). As can be seen in Fig. 2, at pH 6, the TC and FC removal efficiency
A
were 87% and 84%, respectively and by raising the pH to 7, the rate of removal reached to 28% and 34%, respectively. Hence, by enhancing the pH from weak acidic to neutral conditions, the inactivation rate decreased so that the highest bacterial inactivation efficiency was obtained in weak acidic pH and with trend to alkaline pH, microbial inactivation decreased because at pH 8 the TC and FC were removed 14% and 30%, respectively. This can be deduced that, by enhancing acidity, radical scavengers have less scavenging effect [3]. Thus, it is expected that the system
exhibits a higher disinfection power in weak acidic pHs close to neutral conditions. Since the production of H• increases in acidic conditions, ROS such as •OH and H2O2 are generated because of the conversion of H• previous (Eq. 11 - 16) [3, 14]. k = 2.3× 1010 M-1 S-1
(11)
H• + H2O2 → H2O + • OH
k = 5× 107 M-1 S-1
(12)
H• + H2O → H2 + • OH
k = 1× 1010 M-1 S-1
H• + O2 → HO2•
k = 1.2× 1010 M-1 S-1
H• + HO2• → H2O2
k = 1× 1010 M-1 S-1
H2O2 + • OH → HO2• + H2O
k = 2.7× 107 M-1 S-1
SC RI PT
H+ + e-→ H• + H2O
(13)
(14)
(15)
U
(16)
N
It should be mentioned that another role of H2O2 is radical hydroxyl scavenger. But the constant rate of it is less compared to the other reactions that are favorable for ROS production, as
OH + • OH → H2O2
(17)
k = 2× 106 M-1 S-1
(18)
D
HO2• + HO2• → H2O2 + O2;
k = 4× 1010 M-1 S-1
M
•
A
mentioned in Eq. 17 and 18 [3, 14].
TE
H2O2 is formed by intermediate compounds: HO2• and •OH. In addition, alkaline pH can greatly contribute to self-destruction of H2O2 by Eq. 19. The results of this study are in accordance with
EP
the study by Singh and Philip [14]. 2H2O2 → H2O + O2
(19)
CC
3.2. The effect of current density Current density, which can control the reaction, is one of the most significant parameters in EO
A
process. Moreover, the efficiency of the electrodes is affected by this factor. Fig. 3a and b indicate the effect of applying various current densities in reduction of TC and FC. The findings of the experiments confirmed the decrement of microbial rate by enhancing the current density; this is in accordance whit past studies [3, 15]. An enhancement in current density from 0.16 to 0.5 mA/cm2 led to a decrease in log 10 bacterial load (more than 60%) for all electrodes except for Pt. Referring to Fig. 3 the optimized current density in constant charge passed of 72 C, for SS, Ti, Pt, GP,
Pb/PbO2 and SS/PbO2 0.83 mA/cm2, 0.83 mA/cm2 , 0.83 mA/cm2 , 0.66 mA/cm2 , 0.5 mA/cm2 and 0.5 mA/cm2 were obtained, respectively. In these situation the most removal rate for TC and FC were (99.99%, 99.99%), (99.98%, 99.94%), (99.92%, 99%), (97.71%, 97.71%), (98.66%, 99.99%) and (99.96%, 99.93%), respectively. The close correlation between the used current density in the electrochemical process and the synergistic reactions of hydroxyl radicals has been
SC RI PT
proven [16]. Enhancing the current density up to the optimal value causes to electrical production of oxidizing species such as radical hydroxyl. In addition, if the current density value exceeds than the optimum value owing to enhancement of H2O hydrolysis, the production of oxygen occurs instead of radical hydroxyl. Thus, there is no improvement in high current density so that at too high current densities there is the possibility of destruction of both hydroxyl radical and ozone and subsequently the performance of the disinfection process is diminished [15]. In the case of SS, Ti
U
and Pt, by raising the current density up to 0.83 mA/cm2 resulted in a decrease in bacterial
N
concentration whereas, in the case of GP, Pb/PbO2 and SS/PbO2, the performance did not change with increasing current density after the optimum value. Before reaching the optimum value of
A
current density, the energy consumed is spent to produce oxidizing species, but then it is spent to
M
generate oxygen. At a lower level of the optimum value of current density, a lower amount of oxidative compounds is generated which is consistent with electrochemical theory [3].
D
3.3. The effect of electrode materials
TE
In Fig. 4, the impact of using different types of anode on the removal of TC and FC has been demonstrated. The performance of the electrodes in reduction of TC and FC were in the order of
EP
SS/PbO2 > SS > Ti> Pt > GP > Pb/PbO2. The efficiency of SS/PbO2 compatible with indirect oxidation was greater than that of Pb/PbO2. It seems that Pb/PbO2 as an inactive electrode has a
CC
high efficiency in oxidation of organic compounds because this electrode due to generation of oxygen evolution intermediates especially •OH with non-selective oxidizing function oxidizes
A
organic compounds [17], but it is less effective in reduction of bacterial concentration, which confirms the results of previous studies [18]. SS/PbO2 had the highest efficiency among the other electrodes; SS/PbO2, as inactive electrode, releases •OH which is stronger than MO that is released during oxidation reaction by active electrodes (like SS, Ti, Pt, GP) (reaction 1 and 20-21). This illustrated that hydroxyl radicals have the principal responsibility for inactivating microorganisms; this is consistent with previous results [19].
M (HO•) + R → M + products
(20)
M (HO•) → MO + H+ + e-
(21)
In the case of active electrodes, more oxide MO (Eq. 21) is produced by interaction between adsorbed hydroxyl radical and anode surface and if higher oxidation sites exist, depending on the
SC RI PT
type of electrode that has a high thermodynamic potential in order to oxygen evolution, MO/M plays the role of mediator in the oxidation of compounds (Eq. 22)[20]. MO + R → M + RO
(22)
SS, consistent with direct oxidation, demonstrated better performance in removal of TC and FC in comparison with other active electrodes. The microbial reduction rate of Ti was lower compared
U
to SS but this difference was low [19]. Among the low-performance anodes, the power oxidation of Pt was better than that of GP which agrees with the results of previous studies [8]. The most
N
probable reason for the inactivation of TC and FC by Pt is direct electron transmission reaction of
A
microorganisms adsorbed on the surface of Pt [19]. In high current density and application of GP
M
the turbidity of the sample increased, which is because of separation from the anode surface following the electrode corrosion phenomenon [21]. Therefore, the applied electrodes depending
D
on their type showed different efficiencies in reduction of microbial concentration.
TE
3.4. Effect of Nacl concentration and assessment of EC and TDS variations Fig.5 represents the influence of various Nacl concentrations on the inactivation of
EP
microorganisms through ECD under the optimal conditions for each electrode. The results indicated that, by elevating chloride concentration, the removal efficiency improved. For the trials
CC
without the use of Nacl, the inactivation rate of microorganisms was very low compared with when Nacl was employed. Addition of 0.01M and 0.04 M indicated the same effects. Thus, by reduction of operation time compared to when Nacl was not used, the removal efficiency of microorganisms
A
achieved 99.99%. These findings demonstrate that Nacl, even at very low concentrations, could raise the disinfection power. Increasing the Nacl concentration enhances the amount of ROS production so it accelerates the disinfection. Among the investigated electrodes, SS/PbO2 anode (the most efficient anode) had the highest performance in the shortest reaction time (5 min). That is, active chlorine concentration in this situation was sufficient to destroy the cell wall of microorganisms in short period of time. In other words, the disinfection power of electro-generated
active chlorine is much more than that of ROS. Referring to latest studies, the inactivation tendency of microorganisms is highly influenced by their adsorption on the anode surface since M (OH) just acts close to the anode while active chlorine mostly acts in whole mass. It should be noted that, except active chlorine as principal oxidant, other species such as Cl2 , ClOH and Cl are entered in the process which are also effective in disinfection [6]. In accordance with the reactions
SC RI PT
previously mentioned (Eqs. 1-8) H2O2, OH, HOCl, OCl- and O3 are able to remove microorganisms. Because of the intricacy of the involved factors, it is hard to recognize the most remarkable disinfectant and its action mechanism. Conforming to the literature, electro generated chlorine and hydroxyl radicals act original mechanism [11]. Another efficient parameters on ECD is the conductivity of solution for this purpose; Nacl is often used to obtain the desirable amount of EC [22]. Changes in the EC and TDS values were measured before and after usage of Nacl. As
U
shown in Fig. 6, by increasing Nacl concentration, the value of EC and TDS went up. The amounts
N
of EC and TDS in the sample before oxidation were 800 μS/cm and 391mg/l and with the available chloride concentration of 0.01M (as the optimum amount) in electrolyte solution, these values for
A
SS/PbO2, SS, Ti, Pt, GP and Pb/PbO2 reached to 1994 μS/cm and 997mg/l, 1904 μS/cm and
M
950mg/l, 1930 μS/cm and 964mg/l,1857μS/cm and 926mg/l, 2 mS/cm and 1023mg/l, 1578 μS/cm
D
and 784mg/l, respectively.
3.5. Effect of electrolysis time on the chloride conversion rate and formation of active
TE
chlorine
Increasing the operation time improved the amount of free and total chlorine production. The total
EP
of reaction time was 50 min, and the sampling was done at intervals of 10 min. The experiments were carried out in optimum conditions for each electrode. The amount of generated active
CC
chlorine was different for each electrode. As can be seen in Fig.7, the concentration of active chlorine increased up to 40 min and, in the remaining 10 min of reaction time, the amount of active
A
chlorine production was almost constant and no significant increase was observed. In the presence of 35 mg/l chloride concentration in electrolyte solution, the concentration of active chlorine in 40 min for SS, Ti, GP, SS/PbO2, Pt and Pb/PbO2 obtained 8.5, 3, 0.58, 0.45, 0.3 and 0.22 mg/l. Therefore, it could be inferred in initial chloride concentration of 35 mg/l in 40 min of reaction time, the optimized concentration of active chlorine attained. In a research conducted by Mezule et al., it was reported that the highest active chlorine concentration with 10 mg/l of available
chloride concentration was 1.65 mg/l in 30 min. In the study by Kerwick et al., in 60 min the most concentration of active chlorine 1.2 mg/l was gained [23]. Therefore, our findings are consistent with prior studies. 3.6. Effect of charge passed
SC RI PT
The optimum values of current density obtained from the former steps were used to determine the optimal amount of time. As shown in Fig. 8a and b, when charge passed went up, the rate of bacterial concentration reduced. In the other words, an enhancement in reaction time increased the efficiency of microbial inactivation. SS/PbO2 with an increase in charge passed up to 15 C, 2-log reduced TC and FC concentration. A further increase in charge passed resulted in 100% removal (3.3 log reduction) of TC and FC concentration. Pb/PbO2 with increasing charge passed up to 15
U
C showed similar performance of SS/PbO2 but 0.95% removal efficiency was achieved by rising
N
to 150 C. In the case of Pt in charge passed of 150 C, 1.8 log reduction of TC was obtained and FC concentration decreased to 1.8 log in charge passed of 120 C, whereas the performances of GP
A
on removal of TC and FC in charge passed of 60 C were attained 97% and 99.99%, respectively.
M
SS with a further increase from 60 C, illustrated 3.3 log reduction in bacterial concentration. When Ti anode was applied in charge passed of 60 C, the rate of TC and FC removal were attained 1.7
D
and 1.85 log, respectively. As can be seen from Fig. 9, three pathways of oxidation are involved in the removal of microorganisms. The first way represents the direct oxidation of microorganisms
TE
that occurs on the surface of the anode (pathway I). Indirect electro oxidation using active chlorine species (Cl2, HOCl, ClO-) occurs through pathway II (Eqs. 6-8). During pathway III,
EP
microorganisms are eliminated based on indirect oxidation through physisorbed hydroxyl radical M (HO•) according to Eq. 1 and 20. Pathway I indicates the oxidation of water molecules resulting
CC
in radical hydroxyl production, and occurs in both active and inactive electrodes; the difference is because of the fact that the interaction between radical hydroxyl and the anode surface in direct
A
oxidation is stronger than indirect oxidation. The three pathways mentioned above are progressed by increasing the current density and reaction time; this has been confirmed in previous literatures [15].
3.7. Electrochemical disinfection energy consumption The following equation was used to calculate energy consumption during the ECD process.
𝑘𝑊ℎ
E ( 𝑚3 ) =
IVt
(2)
V
Where I is average current applied (A), V is the voltage of the cell (V), t is the time of reaction (h), V is volume of tested water (m3). As noted earlier, although in high current densities, the rate of microbial inactivation increased, but it is resulted in more energy consumption [3]. Since
SC RI PT
energy consumption is influenced by current density, reaction time and cell voltage, so as the amount of energy consumption also increases with increasing these parameters. As shown in Table1, complete inactivation of microorganisms (100%) was performed using the SS/PbO2 electrode for 5 min and consumed 16.6 Wh/m3 energy and within 2.5 min showed 1.2 log
bactericidal reduction with 7.5 Wh/m3 energy consumption. For each m3 of water sample used to remove TC and FC, the energy consumed for SS/ PbO2 was less than those for other electrodes, while, for Ti, the amount of energy consumed was higher.
N
U
4. Conclusions
A comparative study was conducted on the performance of a series of electrode (SS, Ti, Pt, GP,
A
Pb/PbO2 and SS/PbO2) through investigation the electrochemical parameters in the inactivation of
M
TC and FC. This study illustrates the significant effect of increasing current density and the use of NaCl in increasing of microbial inactivation. The progressive role of ROS (like ●OH, O3, H2O2)
D
and RCS (Cl2, HOCl, ClO¯) in microbial inactivation can explain it well. The order of the electrode's efficiency in the removal of microorganisms is as follows: SS/PbO2 > SS> Ti> Pt > GP
TE
> Pb/PbO2. Among these anodes, SS/PbO2, due to release of hydroxyl radicals, was known as the most efficient electrode because ●OH is more powerful than MO released by active electrodes
EP
(like; SS, Ti, Pt, GP). The highest bacterial inactivation efficiency was obtained in weak acidic pH. Complete inactivation (3.3 log reduction) of TC and FC was demonstrated by using SS/PbO2
CC
within 5 min in current density of 0.5 mA/cm2 and the energy consumption of 16.66 Wh/m3. Increasing current density up to the optimal value caused to electro generation of radical hydroxyls
A
and, in turn, the rate of microbial inactivation enhanced. This article concludes that all of the selected anodes could be effective in removal of TC and FC but by using SS/PbO2 the electrolyse time and energy consumption reduced. Acknowledgement This study is part of thesis No. 960129693 at Hamadan University of Medical Sciences.
Disclosure statement
A
CC
EP
TE
D
M
A
N
U
SC RI PT
No potential conflict of interest was reported by the authors.
References [1] Wen J, Tan X, Hu Y, Guo Q, Hong X. Filtration and Electrochemical Disinfection Performance of PAN/PANI/AgNWs-CC Composite Nanofiber Membrane. Environmental science & technology. 2017;51(11):6395-403.
SC RI PT
[2] Clayton GE, Thorn RM, Reynolds DM. Development of a novel off-grid drinking water production system integrating electrochemically activated solutions and ultrafiltration membranes. Journal of Water Process Engineering. 2017.
[3] Ghasemian S, Asadishad B, Omanovic S, Tufenkji N. Electrochemical disinfection of bacterialaden water using antimony-doped tin-tungsten-oxide electrodes. Water research. 2017;126:299307.
U
[4] Drogui P, Blais J-F, Mercier G. Review of electrochemical technologies for environmental applications. Recent patents on engineering. 2007;1(3):257-72.
N
[5] Huang X, Qu Y, Cid CA, Finke C, Hoffmann MR, Lim K, et al. Electrochemical disinfection
A
of toilet wastewater using wastewater electrolysis cell. Water research. 2016;92:164-72.
M
[6] Bruguera-Casamada C, Sirés I, Brillas E, Araujo RM. Effect of electrogenerated hydroxyl radicals, active chlorine and organic matter on the electrochemical inactivation of Pseudomonas aeruginosa using BDD and dimensionally stable anodes. Separation and Purification Technology.
D
2017;178:224-31.
TE
[7] Long Y, Ni J, Wang Z. Subcellular mechanism of Escherichia coli inactivation during electrochemical disinfection with boron-doped diamond anode: a comparative study of three
EP
electrolytes. Water research. 2015;84:198-206. [8] Sopaj F, Rodrigo MA, Oturan N, Podvorica FI, Pinson J, Oturan MA. Influence of the anode
CC
materials on the electrochemical oxidation efficiency. Application to oxidative degradation of the pharmaceutical amoxicillin. Chemical Engineering Journal. 2015;262:286-94. [9] Jeong J, Kim JY, Cho M, Choi W, Yoon J. Inactivation of Escherichia coli in the
A
electrochemical disinfection process using a Pt anode. Chemosphere. 2007;67(4):652-9. [10] Coria G, Sirés I, Brillas E, Nava JL. Influence of the anode material on the degradation of naproxen by Fenton-based electrochemical processes. Chemical Engineering Journal. 2016;304:817-25. [11] Chen S, Hu W, Hong J, Sandoe S. Electrochemical disinfection of simulated ballast water on PbO2/graphite felt electrode. Marine pollution bulletin. 2016;105(1):319-23.
[12] Basha CA, Chithra E, Sripriyalakshmi N. Electro-degradation and biological oxidation of non-biodegradable organic contaminants. Chemical engineering journal. 2009;149(1-3):25-34. [13] Liu S, Wang Y, Zhou X, Han W, Li J, Sun X, et al. Improved degradation of the aqueous flutriafol using a nanostructure macroporous PbO2 as reactive electrochemical membrane. Electrochimica Acta. 2017;253:357-67.
SC RI PT
[14] Singh RK, Babu V, Philip L, Ramanujam S. Disinfection of water using pulsed power technique: Effect of system parameters and kinetic study. Engineering: Springer; 2017. p. 307-36.
Sustainability Issues in Civil
[15] Rahmani AR, Nematollahi D, Samarghandi MR, Samadi MT, Azarian G. A combined advanced oxidation process: Electrooxidation-ozonation for antibiotic ciprofloxacin removal from aqueous solution. Journal of Electroanalytical Chemistry. 2018;808:82-9.
U
[16] Li H, Long Y, Zhu X, Tian Y, Ye J. Influencing factors and chlorinated byproducts in
N
electrochemical oxidation of bisphenol A with boron-doped diamond anodes. Electrochimica Acta. 2017;246:1121-30.
A
[17] Ghernaout D, Naceur MW, Aouabed A. On the dependence of chlorine by-products generated
M
species formation of the electrode material and applied charge during electrochemical water treatment. Desalination. 2011;270(1-3):9-22.
D
[18] Curteanu S, Godini K, Piuleac CG, Azarian G, Rahmani AR, Butnariu C. Electro-oxidation
TE
method applied for activated sludge treatment: experiment and simulation based on supervised machine learning methods. Industrial & Engineering Chemistry Research. 2014;53(12):4902-12. [19] Jeong J, Kim C, Yoon J. The effect of electrode material on the generation of oxidants and
EP
microbial inactivation in the electrochemical disinfection processes. Water Research. 2009;43(4):895-901.
CC
[20] Martinez-Huitle CA, Ferro S. Electrochemical oxidation of organic pollutants for the wastewater
treatment:
direct
and
indirect
processes.
Chemical
Society
Reviews.
A
2006;35(12):1324-40. [21] Rahmani AR, Nematollahi D, Godini K, Azarian G. Continuous thickening of activated sludge by electro-flotation. Separation and Purification Technology. 2013;107:166-71. [22] Hiwarkar AD, Singh S, Srivastava VC, Mall ID. Mineralization of pyrrole, a recalcitrant heterocyclic compound, by electrochemical method: Multi-response optimization and degradation mechanism. Journal of environmental management. 2017;198:144-52.
[23] Saha J, Gupta SK. A novel electro-chlorinator using low cost graphite electrode for drinking
A
CC
EP
TE
D
M
A
N
U
SC RI PT
water disinfection. Ionics. 2017;23(7):1903-13.
SC RI PT U N A M D TE EP CC A
Fig. 1. (a) SEM image, (b) XRD pattern of SS/ PbO2
SC RI PT U N A
M
Fig. 2. Effect of pH on logarithmic reduction of TC and FC during ECD of 600 mL sample using
A
CC
EP
TE
D
a pair of stainless steel electrodes as anode and cathode at current density of 0.1 A/cm2
SC RI PT U N A M D TE EP CC A
Fig. 3. Effect of (a) current density on logarithmic reduction of TC and (b) FC at pH 6 in constant charge passed of 72 C
SC RI PT U
N
Fig. 4. Influence of electrode materials on removal efficiency of TC and FC in optimum current
A
density of each electrode, stainless steel-0.83 mA/cm2, titanium-0.83 mA/cm2, platinum-0.83
A
CC
EP
TE
D
M
mA/cm2, graphite-0.66 mA/cm2, Pb/ PbO2- 0.5 mA/cm2, SS/PbO2-0.5 mA/cm2
SC RI PT U N A M D TE EP CC A
Fig.5.Influence of various Nacl concentrations on the inactivation of (a) TC and (b) FC through ECD, stainless steel-0.83 mA/cm2, titanium-0.83 mA/cm2, platinum-0.83 mA/cm2, graphite-0.66 mA/cm2, Pb/ PbO2- 0.5 mA/cm2 , SS/PbO2-0.5 mA/cm2
SC RI PT U N A M D TE EP CC A
Fig.6. Effect of TDS and EC variation by using 0.01M Nacl at pH 6
SC RI PT U N
A
Fig.7. Effect of electrolysis time on the chloride conversion rate and formation of active chlorine
A
CC
EP
TE
D
M
with available chloride concentration of 35 mg/l in electrolyte solution
SC RI PT U N A M D TE EP CC
A
Fig. 8.Effect of charge passed on inactivation of (a) TC and (b) FC in operation time of (2.5 – 25 min) at pH 6
SC RI PT U N A M D TE A
CC
EP
Fig.9.The pathway of TC and FC inactivation by ECD
Table 1. Energy consumption of electrochemical disinfection (ECD) for 1.2 log reduction and 3.3 log reduction (complete inactivation) of TC and FC 1.2 log reduction Energy consumption (Wh/m3)
Electrodes
7.5
SC RI PT
SS/PbO2 Pb/ PbO2
47.5
Pt
83.33
GP
97.22
SS
157.77
277.77
U
Ti
3.3 log reduction (complete inactivation)
N
Electrodes
A
SS/ PbO2
M
Pb/ PbO2 Pt
D
GP
TE
SS
A
CC
EP
Ti
Energy consumption (Wh/m3) 16.66 95.83 112.5 141.66 297.77 1
S2213-3437(18)30710-3 https://doi.org/10.1016/j.jece.2018.11.030 JECE 2785
To appear in: Received date: Revised date: Accepted date:
31 August 2018 11 November 2018 13 November 2018
Please cite this article as: Rahmani AR, Samarghandi MR, Nematollahi D, Zamani F, A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.11.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A comprehensive study of electrochemical disinfection of water using direct and indirect oxidation processes
Ali Reza Rahmania, Mohammad Reza Samarghandia, Davood Nematollahib, Fahime
a
SC RI PT
Zamania*
Department of Environmental Health Engineering, Faculty of Health and Research Center for Health Sciences,
Hamadan University of Medical Sciences, Hamadan, Iran b
Faculty of Chemistry, Bu-Ali-Sina University, Hamadan, Iran
N
U
* Corresponding Author: E-mail address: [email protected], Tel: + 989351712739
A
ABSTRACT
M
The electrochemical disinfection (ECD) has achieved much attention due to its environmental compatibility, simplicity and production of oxidizing species. In the present study, the ECD of total coliform (TC) and fecal coliform (FC) was investigated with a focus on direct and indirect
D
oxidation using a series of anodes (stainless steel (SS)/lead (Pb) O2 (SS/PbO2), stainless steel,
TE
titanium (Ti), platinum (Pt), graphite (GP) and Pb/PbO2). The influence of electrode material, current density (CD), charge passed, initial pH values, different concentrations of NaCl, TDS,
EP
electrical conductivity (EC) and energy consumption on process performance was examined. The amount of in situ generated active chlorine during the process under the optimal conditions for
CC
each anode was found to be effective in removal of bacterial. The disinfection system was very efficient in bacterial elimination in presence of 0.01 M NaCl. An increase in CD from 0.16 to 0.5 mA/cm2 resulted in a decrease more than 60% in log 10 bacterial load for all electrodes except Pt.
A
Among anodes investigated, SS/PbO2 demonstrated the highest efficiency (complete inactivation) in 5 min by applying 0.01 M NaCl in current density of 0.5 mA/cm2 when charge passed was > 15 C. The efficiency of the anodes in inactivation of TC and FC was in the order of SS/PbO2 > SS > Ti> Pt > GP > Pb/PbO2. In spite of the high efficiency of the electrochemical method in removing TC and FC, indirect oxidation has a higher efficiency due to the production of strong oxidants such as radical hydroxyl and active chlorine.
Keywords: Electrochemical disinfection, Constant current electrolysis, A set of anodes, Total and fecal coliform, Surface water 1. Introduction One of the concerns in developing countries which have been highly regarded is the diseases and
SC RI PT
deaths due to pathogenic agents in water. According to reports by the World Health Organization (WHO), the cause of two million deaths in 2014 were the lack of access to healthy drinking water in rural areas [1]. Therefore, access to a water treatment process based on low energy consumption to provide healthy drinking water seems necessary [2]. The first step in water disinfection is the removal of pathogenic microorganisms from various water sources [3]. In spite of numerous disinfection methods, each of them has limitations. The use of advanced oxidation processes
U
(AOPs) despite the production of radical hydroxyl, as a strong oxidizing agent for many resistant
N
compounds, has limitations including: complexity, high treatment expense and chemicals utilization [4]. Filtration is one of the common physical processes which its usage has been limited
A
due to membrane fouling and high power consumption. Ultraviolet radiation (UV), although does
M
not generate disinfection by-products, its effectiveness depends on the depth of light penetration in the water affected by the turbidity. Ozone (O3) is capable of deactivating pathogenic
D
microorganisms but, toxic intermediates are produced during disinfection [4]. However, the electrochemical disinfection (ECD) has solved these limitations. The ECD which is also called
TE
green technology by disinfectant production on site is compatible with the environment and human [5]. The principal reactant that takes part in the process is electron, which is considered as a
EP
harmless and clean component [6]. Other observed advantages of this method are as follows: portable equipment, less requirement land [7], easy operation, simplicity, cost-effectiveness,
CC
generation of oxidizing agents and no harmful chemicals [5, 8]. The mechanism of the ECD reaction is presented in two stages: (1) direct oxidation that occurs on the surface of the electrode and is specified by rapid killing of microorganisms (2) indirect oxidation of water caused by
A
oxidizing species generation [3] like hydroxyl radical (●OH), atomic oxygen (●O), ozone (O3) and hydrogen peroxide (H2O2) via intermediate reactions (Eqs. 1-5) [3, 5]. M + H2O → M (HO•) + H+ + e-
(1)
HO• → O• + H+ + e-
(2)
2O• → O2
(3)
2HO• →H2O2
(4)
O• + O2 → O3
(5)
These powerful oxidizers are also called reactive oxygen species (ROS). Physisorbed hydroxyl
SC RI PT
radical M (HO•), with high redox potential (Eº = 2.80 V), is the most important ROS [6]. A broad range of pathogenic microorganisms could be inactivated by the ECD through ROS generated according to (Eqs. 1-5) and also in the presence of chloride which maybe exist naturally in water or be added to water, reactive chorine species (RCS) such as radical chlorine (•Cl, .Cl2-) and active chlorine species (Cl2, HOCl, ClO-) are produced and are known as main disinfectants [5]. In other words, if chloride ions are used in the process, disinfection is done more quickly due to rapid attack
U
to microorganisms by active chlorine species [6]. As indicated in (Eqs. 6-8) hypochlorous acid /
N
hypochlorite ion and acid hydrochloric are produced by dissolved chlorine hydrolysis. After
A
chlorination, total chlorine concentration is considered as active chlorine and the average pH of
M
solution determines the contribution of each active chlorine species [3]. 2Cl- → Cl2 +2e-
TE
HClO → ClO- + H+
D
Cl2 + H2O →Cl- + HClO + H+
(6) (7) (8)
It should be noted, in ECD, the applied potentials between electrodes also causes to bacterial cell
EP
wall destruction [9]. The variables commonly used to evaluate an electrochemical disinfection system include voltage, current intensity, pH and reaction time [5]. The operating conditions and
CC
anode material determine the type of produced oxidants [10]. Thus, anode selection is one of the most important factors affecting the electrochemical processes [3, 6, 11]. The difference among
A
electrode materials is pertinent to the substance and the amount of oxidants generated on the surface of anode [8]. Therefore, the electrode material has a significant effect on microbial load reduction. Hence, the comparison study on performance of a series of electrodes as anodes in the inactivation of microorganisms seems to be an interesting subject. In previous studies, a limited number of researches have tried to study the performance of one or more anodes in the inactivation of microorganisms. In present study, we conducted a comparative study on the efficiency of a set
of anodes in the removal of indicator microorganisms in water. Stainless steel (SS), titanium (Ti), platinum (Pt) and graphite (GP) with purpose of direct oxidation were applied for indirect and direct anodic oxidation by using lead (Pb)/PbO2 and SS/ PbO2 electrodes. The dimensionally stable anodes (DSAs) are desirable for lots of electro-organic processes and are categorized to active and inactive in terms of their chemical nature. Active electrodes are not very beneficial in
SC RI PT
mineralization of organic pollutants because of a lot of M (HO•) change to chemisorbed “superoxide” MO which has weak oxidation power, whereas, in inactive anodes, this conversion decreases and they therefore exhibit high ability to mineralize organic pollutants [6]. In inactive anodes, there is no oxidation location available but adsorbed hydroxyl radical with high redox potential (Eº = 2.80 V) oxidizes organic materials (such as SnO2 and PbO2) [12]. It has been proved that hydroxyl radicals react with a large number of compounds [13]. The usage of the EO process
U
has been studied in inactivation of pathogenic microorganisms available in water. Inactivation of
N
pathogenic microorganisms occurs through the destruction of their cell wall [6]. Escherichia coli and fecal coliform are commonly considered as the organisms listed in the water quality standards
A
to determine the effectiveness of electrochemical disinfection technology [5]. The aim of this study
M
was to research the ECD of microbial indicators (TC and FC) by using direct and indirect anodic oxidation. Also, the effect of active chlorine presence on the process was tested by applying a
D
series of anodes under optimal conditions. The main variables of this study included current
TE
density (CD), charge passed, different pH values, electrode material, various concentrations of NaCl, TDS, electrical conductivity (EC) and energy consumption.
EP
2. Experimental 2.1. Chemicals
CC
Lactose Broth, Brilliant Green Bile Broth (BGB), EC Broth, Sulphuric acid (H2SO4) and sodium hydroxide (NaOH) were used in the current study. All chemical materials used in this study (with
A
purity of 99.5%) were purchased from Merck Company, Germany. Sodium chloride (NaCl) was utilized in electrolytic tests. 2.2. Microbiological cultures TC were first cultured in Lactose Broth and then cultured at confirmatory phase in Brilliant Green Bile Broth. Incubator was used to provide temperature of 37 °C for 48 hours within probable and
confirmatory phase. The identification of FC in EC Broth culture and in water bath at 44.5 °C for 48 hours was accomplished. 2.3. Experimental unit The components of the applied electrochemical pilot included electrodes, magnetic stirrer, glass
SC RI PT
reactor and DC power source. An 800 ml glass reactor was filled with 600 mL of the sample for all electrochemical trials. During the experiments, the temperature of the samples was controlled using a thermometer to maintain the temperature of the samples close to the ambient temperature. The pH adjustment was performed using sulfuric acid and sodium hydroxide .Six types of electrodes: SS, Ti, Pt, GP, Pb/PbO2 and SS/PbO2 as anodes with dimensions of 4×15×0.1 cm were applied. In the case of GP and Pb/PbO2, the dimensions were 4×15×1 cm. SS was selected as
U
cathode for all trials. The electrodes were fixed parallelly in 2-cm gap from each other. Before
N
each experiment, all the electrodes with the aim of cleaning were immersed in the HCl solution (15% Wt.) and then washed with distilled water. To supply direct current, the electrodes were to
a
DC
power
source
A
connected
(Aram
Tronik
CO.
M
Iran). A magnetic stirrer was used to mix the contents of the reactor. Immediately after each sampling in order to prevent every contamination, the microorganisms were measured in a sterile
𝑁𝑖−𝑁𝑢 𝑁𝑖
× 100
TE
(RTC %) or (RFC %):
D
room. The removal efficiency of TC and FC were calculated using the following equation: (1)
Where, RTC% or RFC% present the removal percentage of TC or FC, Ni and Nu are the initial and
EP
ultimate amount of microorganisms. In this study, the variables pH (6-8), current density (0.16 to 0.5 mA/cm2), charge passed (10-150 C), electrode type were optimized by the one factor at a time.
CC
First, constant current electrolysis was used. After optimization the values of the mentioned variables, the NaCl concentration (0.01-0.12 M) was optimized.
A
2.4. Instrumentation Different values of pH in range 6, 7 and 8 were measured using pH meter (HACH CO, USA). To determinate TDS and EC, a conductivity meter was applied (Hach conductivity meter, model 44600). Microbiological tests were carried out according to the standard methods. Active chlorine
was quantified based on the colorimetric method by N, N-diethyl-p-phenylenediamine (DPD) (DR 5000 UV–Vis spectrophotometer at = 515 nm) [3]. 2.5. Electrode preparation and characterization To build the Pb/PbO2 electrode, firstly, the Pb rods were placed in sulfuric acid solution10% then
SC RI PT
the current density of 10 mA/cm2 was applied for 90 min, while the temperature was maintained at 25 °C. (Eqs. 9 and 10) Pb + SO42- → PbSO4 + 2e-
(9)
PbSO4 + 2H2O → PbO2 + SO42- + 4H+ + 2e-
(10)
The preparation steps of the SS/PbO2 electrode are as follows: the surface of SS electrode was
U
cleaned in 10% sulfuric acid solution and then in acetone solution was sonicated for 10 min. After
N
initial preparation, it was placed in Pb (NO3)2 solution 0.5 N at current density of 1 mA/cm2 for 5 hours. The structure and morphology of the composite material was specified via analysis
A
technique X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray
M
diffraction (XRD) of SS/PbO2 and Pb/PbO2 was restarted using scanning electron microscopy (SEM, Philips XL-30) and X-ray diffraction (XRD, Thermo ARL SCINTAG X'TRA, 45 kV). As
D
can be seen in Fig. 1, the regular cube-shaped crystals formed on the surface of the SS electrode
TE
are mainly of –β -PbO2 with the fine size of 10 μm. 3. Results and Discussion
EP
3.1. The effect of pH
The initial pH in the range of 6 to 8 was investigated as one of the most important factors in
CC
electrochemical process with current density of 0.1 A/cm2 and using a pair of SS electrodes as anode and cathode (Fig. 2). As can be seen in Fig. 2, at pH 6, the TC and FC removal efficiency
A
were 87% and 84%, respectively and by raising the pH to 7, the rate of removal reached to 28% and 34%, respectively. Hence, by enhancing the pH from weak acidic to neutral conditions, the inactivation rate decreased so that the highest bacterial inactivation efficiency was obtained in weak acidic pH and with trend to alkaline pH, microbial inactivation decreased because at pH 8 the TC and FC were removed 14% and 30%, respectively. This can be deduced that, by enhancing acidity, radical scavengers have less scavenging effect [3]. Thus, it is expected that the system
exhibits a higher disinfection power in weak acidic pHs close to neutral conditions. Since the production of H• increases in acidic conditions, ROS such as •OH and H2O2 are generated because of the conversion of H• previous (Eq. 11 - 16) [3, 14]. k = 2.3× 1010 M-1 S-1
(11)
H• + H2O2 → H2O + • OH
k = 5× 107 M-1 S-1
(12)
H• + H2O → H2 + • OH
k = 1× 1010 M-1 S-1
H• + O2 → HO2•
k = 1.2× 1010 M-1 S-1
H• + HO2• → H2O2
k = 1× 1010 M-1 S-1
H2O2 + • OH → HO2• + H2O
k = 2.7× 107 M-1 S-1
SC RI PT
H+ + e-→ H• + H2O
(13)
(14)
(15)
U
(16)
N
It should be mentioned that another role of H2O2 is radical hydroxyl scavenger. But the constant rate of it is less compared to the other reactions that are favorable for ROS production, as
OH + • OH → H2O2
(17)
k = 2× 106 M-1 S-1
(18)
D
HO2• + HO2• → H2O2 + O2;
k = 4× 1010 M-1 S-1
M
•
A
mentioned in Eq. 17 and 18 [3, 14].
TE
H2O2 is formed by intermediate compounds: HO2• and •OH. In addition, alkaline pH can greatly contribute to self-destruction of H2O2 by Eq. 19. The results of this study are in accordance with
EP
the study by Singh and Philip [14]. 2H2O2 → H2O + O2
(19)
CC
3.2. The effect of current density Current density, which can control the reaction, is one of the most significant parameters in EO
A
process. Moreover, the efficiency of the electrodes is affected by this factor. Fig. 3a and b indicate the effect of applying various current densities in reduction of TC and FC. The findings of the experiments confirmed the decrement of microbial rate by enhancing the current density; this is in accordance whit past studies [3, 15]. An enhancement in current density from 0.16 to 0.5 mA/cm2 led to a decrease in log 10 bacterial load (more than 60%) for all electrodes except for Pt. Referring to Fig. 3 the optimized current density in constant charge passed of 72 C, for SS, Ti, Pt, GP,
Pb/PbO2 and SS/PbO2 0.83 mA/cm2, 0.83 mA/cm2 , 0.83 mA/cm2 , 0.66 mA/cm2 , 0.5 mA/cm2 and 0.5 mA/cm2 were obtained, respectively. In these situation the most removal rate for TC and FC were (99.99%, 99.99%), (99.98%, 99.94%), (99.92%, 99%), (97.71%, 97.71%), (98.66%, 99.99%) and (99.96%, 99.93%), respectively. The close correlation between the used current density in the electrochemical process and the synergistic reactions of hydroxyl radicals has been
SC RI PT
proven [16]. Enhancing the current density up to the optimal value causes to electrical production of oxidizing species such as radical hydroxyl. In addition, if the current density value exceeds than the optimum value owing to enhancement of H2O hydrolysis, the production of oxygen occurs instead of radical hydroxyl. Thus, there is no improvement in high current density so that at too high current densities there is the possibility of destruction of both hydroxyl radical and ozone and subsequently the performance of the disinfection process is diminished [15]. In the case of SS, Ti
U
and Pt, by raising the current density up to 0.83 mA/cm2 resulted in a decrease in bacterial
N
concentration whereas, in the case of GP, Pb/PbO2 and SS/PbO2, the performance did not change with increasing current density after the optimum value. Before reaching the optimum value of
A
current density, the energy consumed is spent to produce oxidizing species, but then it is spent to
M
generate oxygen. At a lower level of the optimum value of current density, a lower amount of oxidative compounds is generated which is consistent with electrochemical theory [3].
D
3.3. The effect of electrode materials
TE
In Fig. 4, the impact of using different types of anode on the removal of TC and FC has been demonstrated. The performance of the electrodes in reduction of TC and FC were in the order of
EP
SS/PbO2 > SS > Ti> Pt > GP > Pb/PbO2. The efficiency of SS/PbO2 compatible with indirect oxidation was greater than that of Pb/PbO2. It seems that Pb/PbO2 as an inactive electrode has a
CC
high efficiency in oxidation of organic compounds because this electrode due to generation of oxygen evolution intermediates especially •OH with non-selective oxidizing function oxidizes
A
organic compounds [17], but it is less effective in reduction of bacterial concentration, which confirms the results of previous studies [18]. SS/PbO2 had the highest efficiency among the other electrodes; SS/PbO2, as inactive electrode, releases •OH which is stronger than MO that is released during oxidation reaction by active electrodes (like SS, Ti, Pt, GP) (reaction 1 and 20-21). This illustrated that hydroxyl radicals have the principal responsibility for inactivating microorganisms; this is consistent with previous results [19].
M (HO•) + R → M + products
(20)
M (HO•) → MO + H+ + e-
(21)
In the case of active electrodes, more oxide MO (Eq. 21) is produced by interaction between adsorbed hydroxyl radical and anode surface and if higher oxidation sites exist, depending on the
SC RI PT
type of electrode that has a high thermodynamic potential in order to oxygen evolution, MO/M plays the role of mediator in the oxidation of compounds (Eq. 22)[20]. MO + R → M + RO
(22)
SS, consistent with direct oxidation, demonstrated better performance in removal of TC and FC in comparison with other active electrodes. The microbial reduction rate of Ti was lower compared
U
to SS but this difference was low [19]. Among the low-performance anodes, the power oxidation of Pt was better than that of GP which agrees with the results of previous studies [8]. The most
N
probable reason for the inactivation of TC and FC by Pt is direct electron transmission reaction of
A
microorganisms adsorbed on the surface of Pt [19]. In high current density and application of GP
M
the turbidity of the sample increased, which is because of separation from the anode surface following the electrode corrosion phenomenon [21]. Therefore, the applied electrodes depending
D
on their type showed different efficiencies in reduction of microbial concentration.
TE
3.4. Effect of Nacl concentration and assessment of EC and TDS variations Fig.5 represents the influence of various Nacl concentrations on the inactivation of
EP
microorganisms through ECD under the optimal conditions for each electrode. The results indicated that, by elevating chloride concentration, the removal efficiency improved. For the trials
CC
without the use of Nacl, the inactivation rate of microorganisms was very low compared with when Nacl was employed. Addition of 0.01M and 0.04 M indicated the same effects. Thus, by reduction of operation time compared to when Nacl was not used, the removal efficiency of microorganisms
A
achieved 99.99%. These findings demonstrate that Nacl, even at very low concentrations, could raise the disinfection power. Increasing the Nacl concentration enhances the amount of ROS production so it accelerates the disinfection. Among the investigated electrodes, SS/PbO2 anode (the most efficient anode) had the highest performance in the shortest reaction time (5 min). That is, active chlorine concentration in this situation was sufficient to destroy the cell wall of microorganisms in short period of time. In other words, the disinfection power of electro-generated
active chlorine is much more than that of ROS. Referring to latest studies, the inactivation tendency of microorganisms is highly influenced by their adsorption on the anode surface since M (OH) just acts close to the anode while active chlorine mostly acts in whole mass. It should be noted that, except active chlorine as principal oxidant, other species such as Cl2 , ClOH and Cl are entered in the process which are also effective in disinfection [6]. In accordance with the reactions
SC RI PT
previously mentioned (Eqs. 1-8) H2O2, OH, HOCl, OCl- and O3 are able to remove microorganisms. Because of the intricacy of the involved factors, it is hard to recognize the most remarkable disinfectant and its action mechanism. Conforming to the literature, electro generated chlorine and hydroxyl radicals act original mechanism [11]. Another efficient parameters on ECD is the conductivity of solution for this purpose; Nacl is often used to obtain the desirable amount of EC [22]. Changes in the EC and TDS values were measured before and after usage of Nacl. As
U
shown in Fig. 6, by increasing Nacl concentration, the value of EC and TDS went up. The amounts
N
of EC and TDS in the sample before oxidation were 800 μS/cm and 391mg/l and with the available chloride concentration of 0.01M (as the optimum amount) in electrolyte solution, these values for
A
SS/PbO2, SS, Ti, Pt, GP and Pb/PbO2 reached to 1994 μS/cm and 997mg/l, 1904 μS/cm and
M
950mg/l, 1930 μS/cm and 964mg/l,1857μS/cm and 926mg/l, 2 mS/cm and 1023mg/l, 1578 μS/cm
D
and 784mg/l, respectively.
3.5. Effect of electrolysis time on the chloride conversion rate and formation of active
TE
chlorine
Increasing the operation time improved the amount of free and total chlorine production. The total
EP
of reaction time was 50 min, and the sampling was done at intervals of 10 min. The experiments were carried out in optimum conditions for each electrode. The amount of generated active
CC
chlorine was different for each electrode. As can be seen in Fig.7, the concentration of active chlorine increased up to 40 min and, in the remaining 10 min of reaction time, the amount of active
A
chlorine production was almost constant and no significant increase was observed. In the presence of 35 mg/l chloride concentration in electrolyte solution, the concentration of active chlorine in 40 min for SS, Ti, GP, SS/PbO2, Pt and Pb/PbO2 obtained 8.5, 3, 0.58, 0.45, 0.3 and 0.22 mg/l. Therefore, it could be inferred in initial chloride concentration of 35 mg/l in 40 min of reaction time, the optimized concentration of active chlorine attained. In a research conducted by Mezule et al., it was reported that the highest active chlorine concentration with 10 mg/l of available
chloride concentration was 1.65 mg/l in 30 min. In the study by Kerwick et al., in 60 min the most concentration of active chlorine 1.2 mg/l was gained [23]. Therefore, our findings are consistent with prior studies. 3.6. Effect of charge passed
SC RI PT
The optimum values of current density obtained from the former steps were used to determine the optimal amount of time. As shown in Fig. 8a and b, when charge passed went up, the rate of bacterial concentration reduced. In the other words, an enhancement in reaction time increased the efficiency of microbial inactivation. SS/PbO2 with an increase in charge passed up to 15 C, 2-log reduced TC and FC concentration. A further increase in charge passed resulted in 100% removal (3.3 log reduction) of TC and FC concentration. Pb/PbO2 with increasing charge passed up to 15
U
C showed similar performance of SS/PbO2 but 0.95% removal efficiency was achieved by rising
N
to 150 C. In the case of Pt in charge passed of 150 C, 1.8 log reduction of TC was obtained and FC concentration decreased to 1.8 log in charge passed of 120 C, whereas the performances of GP
A
on removal of TC and FC in charge passed of 60 C were attained 97% and 99.99%, respectively.
M
SS with a further increase from 60 C, illustrated 3.3 log reduction in bacterial concentration. When Ti anode was applied in charge passed of 60 C, the rate of TC and FC removal were attained 1.7
D
and 1.85 log, respectively. As can be seen from Fig. 9, three pathways of oxidation are involved in the removal of microorganisms. The first way represents the direct oxidation of microorganisms
TE
that occurs on the surface of the anode (pathway I). Indirect electro oxidation using active chlorine species (Cl2, HOCl, ClO-) occurs through pathway II (Eqs. 6-8). During pathway III,
EP
microorganisms are eliminated based on indirect oxidation through physisorbed hydroxyl radical M (HO•) according to Eq. 1 and 20. Pathway I indicates the oxidation of water molecules resulting
CC
in radical hydroxyl production, and occurs in both active and inactive electrodes; the difference is because of the fact that the interaction between radical hydroxyl and the anode surface in direct
A
oxidation is stronger than indirect oxidation. The three pathways mentioned above are progressed by increasing the current density and reaction time; this has been confirmed in previous literatures [15].
3.7. Electrochemical disinfection energy consumption The following equation was used to calculate energy consumption during the ECD process.
𝑘𝑊ℎ
E ( 𝑚3 ) =
IVt
(2)
V
Where I is average current applied (A), V is the voltage of the cell (V), t is the time of reaction (h), V is volume of tested water (m3). As noted earlier, although in high current densities, the rate of microbial inactivation increased, but it is resulted in more energy consumption [3]. Since
SC RI PT
energy consumption is influenced by current density, reaction time and cell voltage, so as the amount of energy consumption also increases with increasing these parameters. As shown in Table1, complete inactivation of microorganisms (100%) was performed using the SS/PbO2 electrode for 5 min and consumed 16.6 Wh/m3 energy and within 2.5 min showed 1.2 log
bactericidal reduction with 7.5 Wh/m3 energy consumption. For each m3 of water sample used to remove TC and FC, the energy consumed for SS/ PbO2 was less than those for other electrodes, while, for Ti, the amount of energy consumed was higher.
N
U
4. Conclusions
A comparative study was conducted on the performance of a series of electrode (SS, Ti, Pt, GP,
A
Pb/PbO2 and SS/PbO2) through investigation the electrochemical parameters in the inactivation of
M
TC and FC. This study illustrates the significant effect of increasing current density and the use of NaCl in increasing of microbial inactivation. The progressive role of ROS (like ●OH, O3, H2O2)
D
and RCS (Cl2, HOCl, ClO¯) in microbial inactivation can explain it well. The order of the electrode's efficiency in the removal of microorganisms is as follows: SS/PbO2 > SS> Ti> Pt > GP
TE
> Pb/PbO2. Among these anodes, SS/PbO2, due to release of hydroxyl radicals, was known as the most efficient electrode because ●OH is more powerful than MO released by active electrodes
EP
(like; SS, Ti, Pt, GP). The highest bacterial inactivation efficiency was obtained in weak acidic pH. Complete inactivation (3.3 log reduction) of TC and FC was demonstrated by using SS/PbO2
CC
within 5 min in current density of 0.5 mA/cm2 and the energy consumption of 16.66 Wh/m3. Increasing current density up to the optimal value caused to electro generation of radical hydroxyls
A
and, in turn, the rate of microbial inactivation enhanced. This article concludes that all of the selected anodes could be effective in removal of TC and FC but by using SS/PbO2 the electrolyse time and energy consumption reduced. Acknowledgement This study is part of thesis No. 960129693 at Hamadan University of Medical Sciences.
Disclosure statement
A
CC
EP
TE
D
M
A
N
U
SC RI PT
No potential conflict of interest was reported by the authors.
References [1] Wen J, Tan X, Hu Y, Guo Q, Hong X. Filtration and Electrochemical Disinfection Performance of PAN/PANI/AgNWs-CC Composite Nanofiber Membrane. Environmental science & technology. 2017;51(11):6395-403.
SC RI PT
[2] Clayton GE, Thorn RM, Reynolds DM. Development of a novel off-grid drinking water production system integrating electrochemically activated solutions and ultrafiltration membranes. Journal of Water Process Engineering. 2017.
[3] Ghasemian S, Asadishad B, Omanovic S, Tufenkji N. Electrochemical disinfection of bacterialaden water using antimony-doped tin-tungsten-oxide electrodes. Water research. 2017;126:299307.
U
[4] Drogui P, Blais J-F, Mercier G. Review of electrochemical technologies for environmental applications. Recent patents on engineering. 2007;1(3):257-72.
N
[5] Huang X, Qu Y, Cid CA, Finke C, Hoffmann MR, Lim K, et al. Electrochemical disinfection
A
of toilet wastewater using wastewater electrolysis cell. Water research. 2016;92:164-72.
M
[6] Bruguera-Casamada C, Sirés I, Brillas E, Araujo RM. Effect of electrogenerated hydroxyl radicals, active chlorine and organic matter on the electrochemical inactivation of Pseudomonas aeruginosa using BDD and dimensionally stable anodes. Separation and Purification Technology.
D
2017;178:224-31.
TE
[7] Long Y, Ni J, Wang Z. Subcellular mechanism of Escherichia coli inactivation during electrochemical disinfection with boron-doped diamond anode: a comparative study of three
EP
electrolytes. Water research. 2015;84:198-206. [8] Sopaj F, Rodrigo MA, Oturan N, Podvorica FI, Pinson J, Oturan MA. Influence of the anode
CC
materials on the electrochemical oxidation efficiency. Application to oxidative degradation of the pharmaceutical amoxicillin. Chemical Engineering Journal. 2015;262:286-94. [9] Jeong J, Kim JY, Cho M, Choi W, Yoon J. Inactivation of Escherichia coli in the
A
electrochemical disinfection process using a Pt anode. Chemosphere. 2007;67(4):652-9. [10] Coria G, Sirés I, Brillas E, Nava JL. Influence of the anode material on the degradation of naproxen by Fenton-based electrochemical processes. Chemical Engineering Journal. 2016;304:817-25. [11] Chen S, Hu W, Hong J, Sandoe S. Electrochemical disinfection of simulated ballast water on PbO2/graphite felt electrode. Marine pollution bulletin. 2016;105(1):319-23.
[12] Basha CA, Chithra E, Sripriyalakshmi N. Electro-degradation and biological oxidation of non-biodegradable organic contaminants. Chemical engineering journal. 2009;149(1-3):25-34. [13] Liu S, Wang Y, Zhou X, Han W, Li J, Sun X, et al. Improved degradation of the aqueous flutriafol using a nanostructure macroporous PbO2 as reactive electrochemical membrane. Electrochimica Acta. 2017;253:357-67.
SC RI PT
[14] Singh RK, Babu V, Philip L, Ramanujam S. Disinfection of water using pulsed power technique: Effect of system parameters and kinetic study. Engineering: Springer; 2017. p. 307-36.
Sustainability Issues in Civil
[15] Rahmani AR, Nematollahi D, Samarghandi MR, Samadi MT, Azarian G. A combined advanced oxidation process: Electrooxidation-ozonation for antibiotic ciprofloxacin removal from aqueous solution. Journal of Electroanalytical Chemistry. 2018;808:82-9.
U
[16] Li H, Long Y, Zhu X, Tian Y, Ye J. Influencing factors and chlorinated byproducts in
N
electrochemical oxidation of bisphenol A with boron-doped diamond anodes. Electrochimica Acta. 2017;246:1121-30.
A
[17] Ghernaout D, Naceur MW, Aouabed A. On the dependence of chlorine by-products generated
M
species formation of the electrode material and applied charge during electrochemical water treatment. Desalination. 2011;270(1-3):9-22.
D
[18] Curteanu S, Godini K, Piuleac CG, Azarian G, Rahmani AR, Butnariu C. Electro-oxidation
TE
method applied for activated sludge treatment: experiment and simulation based on supervised machine learning methods. Industrial & Engineering Chemistry Research. 2014;53(12):4902-12. [19] Jeong J, Kim C, Yoon J. The effect of electrode material on the generation of oxidants and
EP
microbial inactivation in the electrochemical disinfection processes. Water Research. 2009;43(4):895-901.
CC
[20] Martinez-Huitle CA, Ferro S. Electrochemical oxidation of organic pollutants for the wastewater
treatment:
direct
and
indirect
processes.
Chemical
Society
Reviews.
A
2006;35(12):1324-40. [21] Rahmani AR, Nematollahi D, Godini K, Azarian G. Continuous thickening of activated sludge by electro-flotation. Separation and Purification Technology. 2013;107:166-71. [22] Hiwarkar AD, Singh S, Srivastava VC, Mall ID. Mineralization of pyrrole, a recalcitrant heterocyclic compound, by electrochemical method: Multi-response optimization and degradation mechanism. Journal of environmental management. 2017;198:144-52.
[23] Saha J, Gupta SK. A novel electro-chlorinator using low cost graphite electrode for drinking
A
CC
EP
TE
D
M
A
N
U
SC RI PT
water disinfection. Ionics. 2017;23(7):1903-13.
SC RI PT U N A M D TE EP CC A
Fig. 1. (a) SEM image, (b) XRD pattern of SS/ PbO2
SC RI PT U N A
M
Fig. 2. Effect of pH on logarithmic reduction of TC and FC during ECD of 600 mL sample using
A
CC
EP
TE
D
a pair of stainless steel electrodes as anode and cathode at current density of 0.1 A/cm2
SC RI PT U N A M D TE EP CC A
Fig. 3. Effect of (a) current density on logarithmic reduction of TC and (b) FC at pH 6 in constant charge passed of 72 C
SC RI PT U
N
Fig. 4. Influence of electrode materials on removal efficiency of TC and FC in optimum current
A
density of each electrode, stainless steel-0.83 mA/cm2, titanium-0.83 mA/cm2, platinum-0.83
A
CC
EP
TE
D
M
mA/cm2, graphite-0.66 mA/cm2, Pb/ PbO2- 0.5 mA/cm2, SS/PbO2-0.5 mA/cm2
SC RI PT U N A M D TE EP CC A
Fig.5.Influence of various Nacl concentrations on the inactivation of (a) TC and (b) FC through ECD, stainless steel-0.83 mA/cm2, titanium-0.83 mA/cm2, platinum-0.83 mA/cm2, graphite-0.66 mA/cm2, Pb/ PbO2- 0.5 mA/cm2 , SS/PbO2-0.5 mA/cm2
SC RI PT U N A M D TE EP CC A
Fig.6. Effect of TDS and EC variation by using 0.01M Nacl at pH 6
SC RI PT U N
A
Fig.7. Effect of electrolysis time on the chloride conversion rate and formation of active chlorine
A
CC
EP
TE
D
M
with available chloride concentration of 35 mg/l in electrolyte solution
SC RI PT U N A M D TE EP CC
A
Fig. 8.Effect of charge passed on inactivation of (a) TC and (b) FC in operation time of (2.5 – 25 min) at pH 6
SC RI PT U N A M D TE A
CC
EP
Fig.9.The pathway of TC and FC inactivation by ECD
Table 1. Energy consumption of electrochemical disinfection (ECD) for 1.2 log reduction and 3.3 log reduction (complete inactivation) of TC and FC 1.2 log reduction Energy consumption (Wh/m3)
Electrodes
7.5
SC RI PT
SS/PbO2 Pb/ PbO2
47.5
Pt
83.33
GP
97.22
SS
157.77
277.77
U
Ti
3.3 log reduction (complete inactivation)
N
Electrodes
A
SS/ PbO2
M
Pb/ PbO2 Pt
D
GP
TE
SS
A
CC
EP
Ti
Energy consumption (Wh/m3) 16.66 95.83 112.5 141.66 297.77 1