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Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor
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JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(12) 2403–2409
www.jesc.ac.cn
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor Jinjing Tang1,2 , Jinsong Guo1, ∗, Fang Fang1 , Youpeng Chen1 , Lijing Lei1 , Lin Yang1 1. Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment (Ministry of Education), Chongqing University, Chongqing 400045, China. E-mail: [email protected] 2. School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Received 09 January 2013; revised 24 April 2013; accepted 02 May 2013
Abstract Excess nitrogenous compounds are detrimental to natural water systems and to human health. To completely realize autohydrogenotrophic nitrogen removal, a novel 3-dimensional biofilm-electrode reactor was designed. Titanium was electroplated with ruthenium and used as the anode. Activated carbon fiber felt was used as the cathode. The reactor was separated into two chambers by a permeable membrane. The cathode chamber was filled with granular graphite and glass beads. The cathode and cathode chamber were inhabited with domesticated biofilm. In the absence of organic substances, a nitrogen removal efficiency of up to 91% was achieved at DO levels of 3.42 ± 0.37 mg/L when the applied current density was only 0.02 mA/cm2 . The oxidation of ammonium in biofilmelectrode reactors was also investigated. It was found that ammonium could be oxidized not only on the anode but also on particle electrodes in the cathode chamber of the biofilm-electrode reactor. Oxidation rates of ammonium and nitrogen removal efficiency were found to be affected by the electric current loading on the biofilm-electrode reactor. The kinetic model of ammonium at different electric currents was analyzed by a first-order reaction kinetics equation. The regression analysis implied that when the current density was less than 0.02 mA/cm2 , ammonium removal was positively correlated to the current density. However, when the current density was more than 0.02 mA/cm2 , the electric current became a limiting factor for the oxidation rate of ammonium and nitrogen removal efficiency. Key words: autotrophic; biofilm-electrode; nitrogen removal; ammonium; oxidation behavior DOI: 10.1016/S1001-0742(12)60300-3
Introduction When ammonium is inappropriately discharged into natural waters, the resulting eutrophication of lakes and rivers causes excessive growth of algae and decreases the quality of drinking water resources (Zanetti et al., 2012; Anup et al., 2011). Diverse abiotic techniques and biological processes have been exploited to reduce the ammonium content in natural water systems and wastewater (Tee et al., 2012; Rafael et al., 2012). In 1980, it was reported that ammonium could be effectively removed by electrochemical process (Monica et al., 1980). Since then, many kinds of electrodes, such as Pt electrodes and Pt-Me (Me = Ni, Ir, Ru, Cu) binary alloys (Vooys et al., 2001), metal oxide electrodes (RuO2 or IrO2 ) (Kim et al., 2006; Kapałka et al., 2011), nickel electrodes (Ni/Ni(OH)2 ) (Kapałka et al., 2010a), and a boron-doped diamond electrode (Kapałka et al., 2010b) have been used to electro-oxidize ammonium. In terms of mechanism, the majority of research on ammonium oxidation has involved * Corresponding author. E-mail: [email protected]
platinum anodes (Bunce and Bejan, 2011). On Pt-based electrodes, adsorbed ammonium is dehydrogenated and N2 as a final product is formed by the following Reactions (1)–(4) (Gerischer and Mauerer, 1970; Black & Veatch Corporation, 2010; Bunce and Bejan, 2011). NH3,ads −→ NH2,ads + H+ + e− +
−
(1)
NH2,ads −→ NHads + H + e NH x,ads + NHy,ads −→ N2 + (x + y) H+ + (x + y) e−
(2) (3)
NHads −→ Nads + H+ + e−
(4)
Besides N2 , oxygenated nitrogen species (such as NO and N2 O) may also be formed when the electrode surface becomes oxidized (Vooys et al., 2001). The course of oxidation is now well understood on Pt-based anodes. However, the removal mechanism of ammonium in the electrochemical process is poorly understood in terms of the oxidation route and reaction kinetics (Li and Liu, 2009) and the oxidation process of ammonium in the cathode chamber of electrochemical cells has not been discussed.
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1 Materials and methods 1.1 Three-dimensional biofilm-electrode reactor set-up A 3-dimensional BER was separated into two chambers (an anode chamber and a cathode chamber) by a cellulose acetate membrane (0.02 µm average aperture; Hangzhou Water Treatment Technology Development Center, China). Titanium electroplated with ruthenium (10 cm × 10 cm
+
DC Power
-
Effluent Effluent Anode Biofilm
Influent
Fig. 1
Membrane
In electrochemical processes for the elimination of pollutants, no bacteria are required, and only electrical energy is consumed. In fact, a certain amount of Cl− is required to form an oxidizing agent (HOCl) through electrochemical reaction in this technique. That is to say, the wastewater itself contains a high concentration of Cl− , ranging from 2000 to 18,500 mg/L (Vlyssides and Israilides, 1997; Chiang et al., 1995), or an extra amount of 4800–6600 mg/L (Vanlangendonck et al., 2005; Vlyssides et al., 2002) is added to the wastewater. Furthermore, ammonium concentrations have been relatively high, ranging from 150 (Vlyssides et al., 2002) to 3000 mg/L (Chiang et al., 1995). In this study, a biofilm-electrode reactor (BER) was designed to remove low concentrations of ammonium (less than 50 mg/L) and the oxidation behavior of ammonium in the cathode chamber was researched. In the course of nitrogen removal, Cl− was not required. In the BER, denitrifying microorganisms were immobilized on a novel electrode system and hydrogen that is required by the microorganisms was simultaneously produced from the electrolysis of H2 O on the surface of the electrodes. Autotrophic denitrification is stimulated by the electrochemical process (Ghafaria et al., 2010). Hydrogen exploitation in BERs is more efficient than in hydrogensparged bioreactors for hydrogenotrophic denitrification (Mousavi et al., 2012). Sakakibara and Kuroda (1993) found that nitrate could be selectively reduced to N2 using a cathode with immobilized denitrifying microorganisms. Since then, BERs have come into notice (Felekea et al., 1998; Prosnansky et al., 2002; Park et al., 2006; Zhao et al., 2011). However, attention has primarily focused on using the cathode chamber of a BER to reduce NO3 − to N2 (denitrification) (Zhao et al., 2012). A few reports (Kuroda et al., 1996; Virdis et al., 2011) found that oxidation and reduction could occur simultaneously in a BER where nitrifying and denitrifying microorganisms were immobilized on the electrodes. However, there have not been any comprehensive reports regarding the oxidation of ammonium in BERs. In the present article, completely autohydrogenotrophic nitrogen removal in the cathode chamber of a 3dimensional BER was successfully realized. In addition to the reported reduction processes, oxidation was also observed in the cathode chamber of the BER. The oxidation site as well as overall oxidation behavior was comprehensively investigated.
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Cathode Biofilm Fillings
Influent Schematic of 3-dimensional biofilm-electrode reactor (BER).
× 10 cm) was used as the anode material while activated carbon fiber felt (10 cm × 10 cm × 10 cm) was used as the cathode material. The cathode chamber with an empty volume of 1000 cm3 was filled with granular graphite (2– 3 mm diameter, 2–7 mm column length) and glass beads (2–3 mm diameter). The remaining volume was approximately 600 cm3 for the cathode chamber. The schematic of the BER is illustrated in Fig. 1 and was a suitable site for nitrifying bacteria and denitrifying bacteria. The start-up process (Guo et al., 2012) of the BER was performed in two stages; biofilm cultivation and biofilm domestication. The cathode and cathode chamber were inhabited with domesticated microbes from a wastewater treatment plant. In the biofilm cultivation period, no electric current was loaded on the BER. After the biofilm on the electrodes and packing particles had stabilized, the domestication period began. In the domestication period, the electric current (I = 0.5, 1, 1.5, 2 mA) was gradually increased, with sources of organic carbon (C/N = 3, 1, 0.5, 0) decreased by degrees. In the process of start-up, the conversion rate of ammonium and the removal rate of total nitrogen (TN) in the BER were gradually improved and stabilized. 1.2 Microorganisms and synthetic water Nitrifying bacteria and denitrifying bacteria that formed a biofilm on the surface of the cathode and filled matrix were collected from the Jiguanshi Wastewater Treatment Plant (Chongqing, China). The biofilm grown on the cathode and filled matrix was acclimated to an electric current of 2 mA over a period of 60 days. The synthetic wastewater consisted of a concentrated stock solution of NH4 HCO3 , Na2 CO3 , phosphate buffer and trace element solution dissolved in tap water. 1.3 Experimental design 1.3.1 Oxidation behavior of ammonium in cathode chamber To clarify that ammonium could be oxidized in the cathode chamber of the BER, a separate compartmentalized biofilm-electrode reactor (CBER) was designed to exclude the influence of oxidation in the anode chamber. The design of the CBER is similar to the BER. The only difference is that the cellulose acetate membrane
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor
separating the anode and cathode chambers in the BER, which is permeable, was substituted by an airtight clapboard. The anode chamber and cathode chamber were connected only by a salt-bridge acting as the electron channel. Diffusion caused by the concentration differential and electro-migration resulting from the electric field were excluded. The operating conditions of the CBER was the same as the BER, where I = 2 mA, C/N = 0, HRT (hydraulic detention time) = 24 hr. When the influent only contained NH4 + but no NO2 − and NO3 − , effluent from the cathode chamber of the CBER was characterized. If NO2 − or NO3 − was detected in the effluent, this indicated that oxidation occurred in the cathode chamber. 1.3.2 Oxidation site of ammonium in the cathode chamber If oxidation occurs in the cathode chamber of the BER, the oxidation site can be at the cathode or on the packing particles. To determine if ammonium was oxidized on the cathode, the electrode potential was measured. SCE (potential at 25°C: +0.2415 V vs. standard hydrogen electrode) was used as reference electrodes and placed in both chambers. SCE and the working electrodes were connected to a data acquisition unit (Agilent 34970A), which recorded the voltage of the BER (measured as the voltage difference between the working electrodes) and electrode potentials (measured as the voltage difference between the reference electrode and working electrode of each chamber). To determine if ammonium was oxidized on the packing particles, the following experimental design was used. The packing particles in the cathode chamber of the CBER were removed. This formed a conventional 2-dimensional BER. Conditions were unchanged. If oxidation of ammonium occurred on the packing particles, nitrite and nitrate should not be detected in the effluent. To find the exact oxidation site, the 2 dimesional BER and CBER were operated under the same conditions for 15 days. 1.3.3 Oxidation behavior of ammonium in the cathode chamber of the BER at different current densities Under the same operational conditions, the BER was operated with different current densities (IE). The IE was varied from 0 to 0.04 mA/cm 2 . The reactor was operated continuously for one week whenever the loading IE was changed. The nitrogenous concentration in the cathode chamber was measured. The BER was operated steadily over 24 hr at different IE (0.01, 0.02, 0.03, 0.04 mA/cm2 ). Nitrogenous compounds in the effluent were tested at 2 hr intervals. 1.4 Analytical methods Samples obtained from each chamber were immediately filtered through a 0.22 µm sterile filter. Ammonium (NH4 + -N), nitrite (NO2 − -N) and nitrate (NO3 − -N) were determined by ultraviolet spectrophotometer (Purkinje
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General, T6, Beijing, China) and UV-Vis spectrophotometer (Purkinje General, TU1901, Beijing, China). The pH of the influent and effluent was determined using a pH meter (YSI, PH100). Dissolved oxygen (DO) was measured using a DO meter (HACH, LDOTMHQ10) with a DO probe placed in the cathode chamber. The CODCr was also measured using standard methodology.
2 Results and discussion 2.1 Completely autohydrogenotrophic nitrogen removal in cathode chamber of BER After the BER was acclimated to C/N = 0, excess ammonium over 91% was removed steadily by complete autohydrogenotrophic nitrogen removal in the cathode chamber. Virdis et al. (2011) removed ammonium by SND at a DO level of 5.73 ± 0.03 mg/L in the cathode chamber. The current system was operated successfully at a DO level of 3.42 ± 0.37 mg/L. The denitrification profiles are shown in Fig. 2. In the anode and cathode chambers, ammonium was reduced from 45.63 mg/L to 15.63 mg/L and 4.03 mg/L respectively. Nitrate was accumulated in both the anode and cathode chambers. A small quantity of nitrite was accumulated in the cathode chamber. However, little nitrite accumulated in the anode chamber. Ammonium was removed not only in the anode chamber but also in the cathode chamber of the BER. It has been postulated that ammonium is oxidized only in the anode chamber. Diffusion caused by concentration differential, and electro-migration resulting from electric field and nitrification are considered while illustrating the nitrogen evolution associated with ammonium, nitrate and nitrite (Fig. 3). In the anode chamber, the concentration of ammonium will decrease with oxidation of ammonium and electro-migration of positively charged ammonium to the cathode chamber. The diffusion of ammonium from the cathode chamber to the anode chamber is caused by concentration differential, which is the only factor that will increase the concentration of ammonium in the anode 50
Concentration (mg/L)
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45
Ammonium
40
Nitrite
35
Nitrate
30 25 20 15 10 5 0
Influent Anode chamber Cathode chamber Fig. 2 Nitrogenous concentration in the anode chamber and cathode chamber of a 3-dimensional BER.
Journal of Environmental Sciences 2013, 25(12) 2403–2409 / Jinjing Tang et al.
Electromigrate Diffuse
-
NO2 NO3-
Diffuse Electromigrate
N2
Denitrification
NH4+
NO NO
2 3
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Cathode
Concentration (mg/L)
Membrane
Nitrification
Biofilm
Anode
Biofilm
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45
Ammonium
40
Nitrite
35
Nitrate
30 25 20 15 10 5 0
Influent Anode chamber Cathode chamber Fig. 4 Nitrogenous concentrations in the anode chamber and the cathode chamber of a completely separated biofilm-electrode reactor.
Fig. 3 Illustration of the nitrogen evolution in the BER assuming oxidation only occurs in the anode chamber.
chamber. The final results of diffusion caused by concentration differential is that ammonium in the anode chamber was less than or equivalent to that in the cathode chamber. Based on deduction, it can be surmised that ammonium in the anode chamber will be less than or equivalent to that in the cathode chamber if ammonium is oxidized only in the anode chamber. However, in our BER, it was found that ammonium in the anode chamber was more than that in the cathode chamber. This study shows that ammonium may be oxidized in cathode chamber. 2.2 Oxidation behavior of ammonium in the cathode chamber of the BER To confirm that ammonium can be oxidized in the cathode chamber, an airtight clapboard replaced the separating membrane in the original BER. Excluding electrons, there was not any other substance exchange correlated to nitrogen between the two chambers. As nitrite or nitrate was detected in the cathode chamber, this shows that oxidation of ammonium in the cathode chamber occurred. Under the same conditions, the denitrification profile of the CBER is depicted in Fig. 4. After the two chambers were completely separated, nitrite and nitrate were measured in the cathode chamber while no noticeable amounts of nitrite and nitrate were detected in the influent. The results also reveal that ammonium was oxidized in the cathode chamber. Compared with the BER, the amount of ammonium in the anode chamber of the CBER increased, but the results in the cathode chamber followed an opposing trend. Because of the clapboard, ammonium with positive charge could not migrate from the anode chamber to the cathode chamber. This resulted in a higher concentration of ammonium in the anode chamber.
The electric potential of the anode and the cathode were found to be 1.22 V and –0.79 V respectively. Possible stoichiometric redox equations related to nitrogen are as follows: Anode: HNO2 + H2 O − 2e = NO−3 + H+ Cathode: NO−3 + H2 O + 2e = NO−2 + 2OH−
ϕΘ = 0.94 V
(5)
ϕΘ = 0.01 V (6)
On the anode, nitrite tended to be oxidized to nitrate, while on the cathode, nitrate was more likely to be reduced to nitrite. It has been shown that nitrite can be oxidized on the anode and that nitrate can be reduced on the cathode, while ammonium can be oxidized in the anode chamber as well as the cathode chamber of the BER. 2.3 Oxidation site of ammonium in the cathode chamber of the BER It has been shown that ammonium can be oxidized in the cathode chamber of the BER. The oxidation site is still unknown, and could be the surface of the cathode or the packing particles. To investigate whether the packing particles in the cathode chamber were the oxidation site, a 2-dimensional BER (with no packing particles in the cathode chamber) was studied. The denitrification profiles for the 2-dimensional BER are shown in Fig. 5. After the packing particles in the cathode chamber were removed, ammonium in the anode chamber remained the same while ammonium in the cathode chamber increased rapidly. In the cathode chamber of the 2-dimensional (BER), no noticeable amount of nitrite and nitrate was found to be accumulated in the cathode chamber. This indicated that ammonium was not oxidized in the cathode chamber when there were no packing particles. It was further shown that the oxidation site of ammonium is on the packing particles in the cathode chamber, not the surface of the cathode.
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor
Ammonium: Nitrite:
Anode chamber
Cathode chamber
Anode chamber
Cathode chamber
10
40
8
35 30
6
25 20
4
15 2
10 5
50 Ammonium concentratio (mg/L)
a
45
Influent
Cathode chamber
Nitrate:
Nitrite (nitrate) concentration (mg/L)
Ammonium concentratio (mg/L)
50
Anode chamber
45
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10
b
8
40 35
6
30 25 20
4
15 2
10 5
Nitrite (nitrate) concentration (mg/L)
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0 0 0 0 5 10 15 10 15 Time (day) Time (day) Fig. 5 Nitrogenous concentration in the anode chamber and cathode chamber of a 2-dimensional BER (a) and a 3-dimensional BER (b). 0 0
5
model was fitted by the following mathematical equation to data after 22 hr:
The nitrogen transformation profiles shown in Fig. 6 were observed for the cathode chamber of the BER at different electric currents. When IE < 0.02 mA/cm2 , the ammonium concentration in the effluent decreased as IE loading on the BER increased. However, when IE > 0.02 mA/cm2 , ammonium concentration changed in the opposite direction. The nitrite concentration in the cathode chamber of the BER was almost constant at different electric currents. The nitrate concentration in the effluent, which is equivalent to the concentration in the influent, did not vary with IE. The profile of TN was similar to that of ammonium. It can be noted that ammonium in the effluent was affected by the IE. It was found by periodic testing that the ammonium concentration sharply declined in the initial 2 hr, while it decreased at a relatively slow rate over 22 hr. A kinetic
ln (C2 /Ct ) = kt
Ammonium Nitrite Nitrate
Influent Influent Influent
Effluent Effluent Effluent
TN
Influent
Effluent
(7)
where, C2 (mg/L) is ammonium concentration in the effluent as the experiment proceeded to 2 hr; Ct (mg/L) is ammonium concentration in the effluent at 2 hr intervals after 2 hr of experiment run; k is reaction rate constant and t (hr) is reaction time. Regression analysis profiles are shown in Fig. 7. The reaction rate constant (k) did not increase with increased IE. When IE < 0.02 mA/cm2 , k is positively correlated to IE. However, k is limited by IE when IE > 0.02 mA/cm2 . H2 from electrolysis is the limiting factor for ammonium removal at low IE. Therefore, H2 from electrolysis increases and denitrification occurs at a higher rate when IE is increased (Flora et al., 1993). When IE > 0.02 mA/cm2 , H2 from electrolysis is no longer the limiting factor for ammonium removal. Superfluous hydrogen from 1.8 1.6
IE = 0.01 mA/cm2, k = 0.014, R2 = 0.985 IE = 0.02 mA/cm2, k = 0.065, R2 = 0.980
40
1.4
IE = 0.03 mA/cm2, k = 0.055, R2 = 0.980 IE = 0.04 mA/cm2, k = 0.032, R2 = 0.982
35
1.2 ln(C2/Ct)
Concentration (mg/L)
2.4 Oxidation behavior of ammonium in cathode chamber of BER at different electric currents
30 25
1.0 0.8
20 0.6
15 10
0.4
5
0.2
0 0.01
0.02
0.03 2
0.04
IE (mA/cm ) Fig. 6 Nitrogenous concentration in the effluent of the BER at different currents.
0.0
2
6
10
14 18 22 Time (hr) Fig. 7 Regression analysis of ammonium in the effluent of the BER at different currents.
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high IE electrolysis causes the hydrogen inhibition effect (Flora et al., 1993). Furthermore, high IE will deactivate microorganisms (Parvanova-Mancheva et al., 2009; Gong et al., 2008). The nitrogen transformation profiles of the BER at different electric current show that IE loading on the reactor affected the amount of electron donors and the activation of microorganisms. When IE < 0.02 mA/cm2 , the reaction rate of ammonium and nitrogen removal efficiency could be enhanced with increased IE. However, when IE > 0.02 mA/cm2 , this had the opposite effect.
3 Conclusions With low current density (0.02 mA/cm2 ), a new type of 3-dimensional BER was successfully developed to realize complete autohydrogenotrophic nitrogen removal. Ammonium could be oxidized not only on the anode, but also on the packing particles filling the cathode chamber of the BER. In the BER, the electric field can improve nitrogen removal by providing electron donors and activating the microorganisms cultivated on the electrode. Within a certain IE range, the ammonium removal had a linear relation with IE, but this was not consistent throughout the IE range applied. Acknowledgments This work was supported by the Water Special Project (No. 2009ZX07104 002) and the Fundamental Research Funds for the Central Universities of China (No. CQDXWL2012-040).
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JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(12) 2403–2409
www.jesc.ac.cn
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor Jinjing Tang1,2 , Jinsong Guo1, ∗, Fang Fang1 , Youpeng Chen1 , Lijing Lei1 , Lin Yang1 1. Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment (Ministry of Education), Chongqing University, Chongqing 400045, China. E-mail: [email protected] 2. School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Received 09 January 2013; revised 24 April 2013; accepted 02 May 2013
Abstract Excess nitrogenous compounds are detrimental to natural water systems and to human health. To completely realize autohydrogenotrophic nitrogen removal, a novel 3-dimensional biofilm-electrode reactor was designed. Titanium was electroplated with ruthenium and used as the anode. Activated carbon fiber felt was used as the cathode. The reactor was separated into two chambers by a permeable membrane. The cathode chamber was filled with granular graphite and glass beads. The cathode and cathode chamber were inhabited with domesticated biofilm. In the absence of organic substances, a nitrogen removal efficiency of up to 91% was achieved at DO levels of 3.42 ± 0.37 mg/L when the applied current density was only 0.02 mA/cm2 . The oxidation of ammonium in biofilmelectrode reactors was also investigated. It was found that ammonium could be oxidized not only on the anode but also on particle electrodes in the cathode chamber of the biofilm-electrode reactor. Oxidation rates of ammonium and nitrogen removal efficiency were found to be affected by the electric current loading on the biofilm-electrode reactor. The kinetic model of ammonium at different electric currents was analyzed by a first-order reaction kinetics equation. The regression analysis implied that when the current density was less than 0.02 mA/cm2 , ammonium removal was positively correlated to the current density. However, when the current density was more than 0.02 mA/cm2 , the electric current became a limiting factor for the oxidation rate of ammonium and nitrogen removal efficiency. Key words: autotrophic; biofilm-electrode; nitrogen removal; ammonium; oxidation behavior DOI: 10.1016/S1001-0742(12)60300-3
Introduction When ammonium is inappropriately discharged into natural waters, the resulting eutrophication of lakes and rivers causes excessive growth of algae and decreases the quality of drinking water resources (Zanetti et al., 2012; Anup et al., 2011). Diverse abiotic techniques and biological processes have been exploited to reduce the ammonium content in natural water systems and wastewater (Tee et al., 2012; Rafael et al., 2012). In 1980, it was reported that ammonium could be effectively removed by electrochemical process (Monica et al., 1980). Since then, many kinds of electrodes, such as Pt electrodes and Pt-Me (Me = Ni, Ir, Ru, Cu) binary alloys (Vooys et al., 2001), metal oxide electrodes (RuO2 or IrO2 ) (Kim et al., 2006; Kapałka et al., 2011), nickel electrodes (Ni/Ni(OH)2 ) (Kapałka et al., 2010a), and a boron-doped diamond electrode (Kapałka et al., 2010b) have been used to electro-oxidize ammonium. In terms of mechanism, the majority of research on ammonium oxidation has involved * Corresponding author. E-mail: [email protected]
platinum anodes (Bunce and Bejan, 2011). On Pt-based electrodes, adsorbed ammonium is dehydrogenated and N2 as a final product is formed by the following Reactions (1)–(4) (Gerischer and Mauerer, 1970; Black & Veatch Corporation, 2010; Bunce and Bejan, 2011). NH3,ads −→ NH2,ads + H+ + e− +
−
(1)
NH2,ads −→ NHads + H + e NH x,ads + NHy,ads −→ N2 + (x + y) H+ + (x + y) e−
(2) (3)
NHads −→ Nads + H+ + e−
(4)
Besides N2 , oxygenated nitrogen species (such as NO and N2 O) may also be formed when the electrode surface becomes oxidized (Vooys et al., 2001). The course of oxidation is now well understood on Pt-based anodes. However, the removal mechanism of ammonium in the electrochemical process is poorly understood in terms of the oxidation route and reaction kinetics (Li and Liu, 2009) and the oxidation process of ammonium in the cathode chamber of electrochemical cells has not been discussed.
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1 Materials and methods 1.1 Three-dimensional biofilm-electrode reactor set-up A 3-dimensional BER was separated into two chambers (an anode chamber and a cathode chamber) by a cellulose acetate membrane (0.02 µm average aperture; Hangzhou Water Treatment Technology Development Center, China). Titanium electroplated with ruthenium (10 cm × 10 cm
+
DC Power
-
Effluent Effluent Anode Biofilm
Influent
Fig. 1
Membrane
In electrochemical processes for the elimination of pollutants, no bacteria are required, and only electrical energy is consumed. In fact, a certain amount of Cl− is required to form an oxidizing agent (HOCl) through electrochemical reaction in this technique. That is to say, the wastewater itself contains a high concentration of Cl− , ranging from 2000 to 18,500 mg/L (Vlyssides and Israilides, 1997; Chiang et al., 1995), or an extra amount of 4800–6600 mg/L (Vanlangendonck et al., 2005; Vlyssides et al., 2002) is added to the wastewater. Furthermore, ammonium concentrations have been relatively high, ranging from 150 (Vlyssides et al., 2002) to 3000 mg/L (Chiang et al., 1995). In this study, a biofilm-electrode reactor (BER) was designed to remove low concentrations of ammonium (less than 50 mg/L) and the oxidation behavior of ammonium in the cathode chamber was researched. In the course of nitrogen removal, Cl− was not required. In the BER, denitrifying microorganisms were immobilized on a novel electrode system and hydrogen that is required by the microorganisms was simultaneously produced from the electrolysis of H2 O on the surface of the electrodes. Autotrophic denitrification is stimulated by the electrochemical process (Ghafaria et al., 2010). Hydrogen exploitation in BERs is more efficient than in hydrogensparged bioreactors for hydrogenotrophic denitrification (Mousavi et al., 2012). Sakakibara and Kuroda (1993) found that nitrate could be selectively reduced to N2 using a cathode with immobilized denitrifying microorganisms. Since then, BERs have come into notice (Felekea et al., 1998; Prosnansky et al., 2002; Park et al., 2006; Zhao et al., 2011). However, attention has primarily focused on using the cathode chamber of a BER to reduce NO3 − to N2 (denitrification) (Zhao et al., 2012). A few reports (Kuroda et al., 1996; Virdis et al., 2011) found that oxidation and reduction could occur simultaneously in a BER where nitrifying and denitrifying microorganisms were immobilized on the electrodes. However, there have not been any comprehensive reports regarding the oxidation of ammonium in BERs. In the present article, completely autohydrogenotrophic nitrogen removal in the cathode chamber of a 3dimensional BER was successfully realized. In addition to the reported reduction processes, oxidation was also observed in the cathode chamber of the BER. The oxidation site as well as overall oxidation behavior was comprehensively investigated.
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Cathode Biofilm Fillings
Influent Schematic of 3-dimensional biofilm-electrode reactor (BER).
× 10 cm) was used as the anode material while activated carbon fiber felt (10 cm × 10 cm × 10 cm) was used as the cathode material. The cathode chamber with an empty volume of 1000 cm3 was filled with granular graphite (2– 3 mm diameter, 2–7 mm column length) and glass beads (2–3 mm diameter). The remaining volume was approximately 600 cm3 for the cathode chamber. The schematic of the BER is illustrated in Fig. 1 and was a suitable site for nitrifying bacteria and denitrifying bacteria. The start-up process (Guo et al., 2012) of the BER was performed in two stages; biofilm cultivation and biofilm domestication. The cathode and cathode chamber were inhabited with domesticated microbes from a wastewater treatment plant. In the biofilm cultivation period, no electric current was loaded on the BER. After the biofilm on the electrodes and packing particles had stabilized, the domestication period began. In the domestication period, the electric current (I = 0.5, 1, 1.5, 2 mA) was gradually increased, with sources of organic carbon (C/N = 3, 1, 0.5, 0) decreased by degrees. In the process of start-up, the conversion rate of ammonium and the removal rate of total nitrogen (TN) in the BER were gradually improved and stabilized. 1.2 Microorganisms and synthetic water Nitrifying bacteria and denitrifying bacteria that formed a biofilm on the surface of the cathode and filled matrix were collected from the Jiguanshi Wastewater Treatment Plant (Chongqing, China). The biofilm grown on the cathode and filled matrix was acclimated to an electric current of 2 mA over a period of 60 days. The synthetic wastewater consisted of a concentrated stock solution of NH4 HCO3 , Na2 CO3 , phosphate buffer and trace element solution dissolved in tap water. 1.3 Experimental design 1.3.1 Oxidation behavior of ammonium in cathode chamber To clarify that ammonium could be oxidized in the cathode chamber of the BER, a separate compartmentalized biofilm-electrode reactor (CBER) was designed to exclude the influence of oxidation in the anode chamber. The design of the CBER is similar to the BER. The only difference is that the cellulose acetate membrane
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor
separating the anode and cathode chambers in the BER, which is permeable, was substituted by an airtight clapboard. The anode chamber and cathode chamber were connected only by a salt-bridge acting as the electron channel. Diffusion caused by the concentration differential and electro-migration resulting from the electric field were excluded. The operating conditions of the CBER was the same as the BER, where I = 2 mA, C/N = 0, HRT (hydraulic detention time) = 24 hr. When the influent only contained NH4 + but no NO2 − and NO3 − , effluent from the cathode chamber of the CBER was characterized. If NO2 − or NO3 − was detected in the effluent, this indicated that oxidation occurred in the cathode chamber. 1.3.2 Oxidation site of ammonium in the cathode chamber If oxidation occurs in the cathode chamber of the BER, the oxidation site can be at the cathode or on the packing particles. To determine if ammonium was oxidized on the cathode, the electrode potential was measured. SCE (potential at 25°C: +0.2415 V vs. standard hydrogen electrode) was used as reference electrodes and placed in both chambers. SCE and the working electrodes were connected to a data acquisition unit (Agilent 34970A), which recorded the voltage of the BER (measured as the voltage difference between the working electrodes) and electrode potentials (measured as the voltage difference between the reference electrode and working electrode of each chamber). To determine if ammonium was oxidized on the packing particles, the following experimental design was used. The packing particles in the cathode chamber of the CBER were removed. This formed a conventional 2-dimensional BER. Conditions were unchanged. If oxidation of ammonium occurred on the packing particles, nitrite and nitrate should not be detected in the effluent. To find the exact oxidation site, the 2 dimesional BER and CBER were operated under the same conditions for 15 days. 1.3.3 Oxidation behavior of ammonium in the cathode chamber of the BER at different current densities Under the same operational conditions, the BER was operated with different current densities (IE). The IE was varied from 0 to 0.04 mA/cm 2 . The reactor was operated continuously for one week whenever the loading IE was changed. The nitrogenous concentration in the cathode chamber was measured. The BER was operated steadily over 24 hr at different IE (0.01, 0.02, 0.03, 0.04 mA/cm2 ). Nitrogenous compounds in the effluent were tested at 2 hr intervals. 1.4 Analytical methods Samples obtained from each chamber were immediately filtered through a 0.22 µm sterile filter. Ammonium (NH4 + -N), nitrite (NO2 − -N) and nitrate (NO3 − -N) were determined by ultraviolet spectrophotometer (Purkinje
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General, T6, Beijing, China) and UV-Vis spectrophotometer (Purkinje General, TU1901, Beijing, China). The pH of the influent and effluent was determined using a pH meter (YSI, PH100). Dissolved oxygen (DO) was measured using a DO meter (HACH, LDOTMHQ10) with a DO probe placed in the cathode chamber. The CODCr was also measured using standard methodology.
2 Results and discussion 2.1 Completely autohydrogenotrophic nitrogen removal in cathode chamber of BER After the BER was acclimated to C/N = 0, excess ammonium over 91% was removed steadily by complete autohydrogenotrophic nitrogen removal in the cathode chamber. Virdis et al. (2011) removed ammonium by SND at a DO level of 5.73 ± 0.03 mg/L in the cathode chamber. The current system was operated successfully at a DO level of 3.42 ± 0.37 mg/L. The denitrification profiles are shown in Fig. 2. In the anode and cathode chambers, ammonium was reduced from 45.63 mg/L to 15.63 mg/L and 4.03 mg/L respectively. Nitrate was accumulated in both the anode and cathode chambers. A small quantity of nitrite was accumulated in the cathode chamber. However, little nitrite accumulated in the anode chamber. Ammonium was removed not only in the anode chamber but also in the cathode chamber of the BER. It has been postulated that ammonium is oxidized only in the anode chamber. Diffusion caused by concentration differential, and electro-migration resulting from electric field and nitrification are considered while illustrating the nitrogen evolution associated with ammonium, nitrate and nitrite (Fig. 3). In the anode chamber, the concentration of ammonium will decrease with oxidation of ammonium and electro-migration of positively charged ammonium to the cathode chamber. The diffusion of ammonium from the cathode chamber to the anode chamber is caused by concentration differential, which is the only factor that will increase the concentration of ammonium in the anode 50
Concentration (mg/L)
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45
Ammonium
40
Nitrite
35
Nitrate
30 25 20 15 10 5 0
Influent Anode chamber Cathode chamber Fig. 2 Nitrogenous concentration in the anode chamber and cathode chamber of a 3-dimensional BER.
Journal of Environmental Sciences 2013, 25(12) 2403–2409 / Jinjing Tang et al.
Electromigrate Diffuse
-
NO2 NO3-
Diffuse Electromigrate
N2
Denitrification
NH4+
NO NO
2 3
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Cathode
Concentration (mg/L)
Membrane
Nitrification
Biofilm
Anode
Biofilm
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45
Ammonium
40
Nitrite
35
Nitrate
30 25 20 15 10 5 0
Influent Anode chamber Cathode chamber Fig. 4 Nitrogenous concentrations in the anode chamber and the cathode chamber of a completely separated biofilm-electrode reactor.
Fig. 3 Illustration of the nitrogen evolution in the BER assuming oxidation only occurs in the anode chamber.
chamber. The final results of diffusion caused by concentration differential is that ammonium in the anode chamber was less than or equivalent to that in the cathode chamber. Based on deduction, it can be surmised that ammonium in the anode chamber will be less than or equivalent to that in the cathode chamber if ammonium is oxidized only in the anode chamber. However, in our BER, it was found that ammonium in the anode chamber was more than that in the cathode chamber. This study shows that ammonium may be oxidized in cathode chamber. 2.2 Oxidation behavior of ammonium in the cathode chamber of the BER To confirm that ammonium can be oxidized in the cathode chamber, an airtight clapboard replaced the separating membrane in the original BER. Excluding electrons, there was not any other substance exchange correlated to nitrogen between the two chambers. As nitrite or nitrate was detected in the cathode chamber, this shows that oxidation of ammonium in the cathode chamber occurred. Under the same conditions, the denitrification profile of the CBER is depicted in Fig. 4. After the two chambers were completely separated, nitrite and nitrate were measured in the cathode chamber while no noticeable amounts of nitrite and nitrate were detected in the influent. The results also reveal that ammonium was oxidized in the cathode chamber. Compared with the BER, the amount of ammonium in the anode chamber of the CBER increased, but the results in the cathode chamber followed an opposing trend. Because of the clapboard, ammonium with positive charge could not migrate from the anode chamber to the cathode chamber. This resulted in a higher concentration of ammonium in the anode chamber.
The electric potential of the anode and the cathode were found to be 1.22 V and –0.79 V respectively. Possible stoichiometric redox equations related to nitrogen are as follows: Anode: HNO2 + H2 O − 2e = NO−3 + H+ Cathode: NO−3 + H2 O + 2e = NO−2 + 2OH−
ϕΘ = 0.94 V
(5)
ϕΘ = 0.01 V (6)
On the anode, nitrite tended to be oxidized to nitrate, while on the cathode, nitrate was more likely to be reduced to nitrite. It has been shown that nitrite can be oxidized on the anode and that nitrate can be reduced on the cathode, while ammonium can be oxidized in the anode chamber as well as the cathode chamber of the BER. 2.3 Oxidation site of ammonium in the cathode chamber of the BER It has been shown that ammonium can be oxidized in the cathode chamber of the BER. The oxidation site is still unknown, and could be the surface of the cathode or the packing particles. To investigate whether the packing particles in the cathode chamber were the oxidation site, a 2-dimensional BER (with no packing particles in the cathode chamber) was studied. The denitrification profiles for the 2-dimensional BER are shown in Fig. 5. After the packing particles in the cathode chamber were removed, ammonium in the anode chamber remained the same while ammonium in the cathode chamber increased rapidly. In the cathode chamber of the 2-dimensional (BER), no noticeable amount of nitrite and nitrate was found to be accumulated in the cathode chamber. This indicated that ammonium was not oxidized in the cathode chamber when there were no packing particles. It was further shown that the oxidation site of ammonium is on the packing particles in the cathode chamber, not the surface of the cathode.
Oxidation behavior of ammonium in a 3-dimensional biofilm-electrode reactor
Ammonium: Nitrite:
Anode chamber
Cathode chamber
Anode chamber
Cathode chamber
10
40
8
35 30
6
25 20
4
15 2
10 5
50 Ammonium concentratio (mg/L)
a
45
Influent
Cathode chamber
Nitrate:
Nitrite (nitrate) concentration (mg/L)
Ammonium concentratio (mg/L)
50
Anode chamber
45
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10
b
8
40 35
6
30 25 20
4
15 2
10 5
Nitrite (nitrate) concentration (mg/L)
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0 0 0 0 5 10 15 10 15 Time (day) Time (day) Fig. 5 Nitrogenous concentration in the anode chamber and cathode chamber of a 2-dimensional BER (a) and a 3-dimensional BER (b). 0 0
5
model was fitted by the following mathematical equation to data after 22 hr:
The nitrogen transformation profiles shown in Fig. 6 were observed for the cathode chamber of the BER at different electric currents. When IE < 0.02 mA/cm2 , the ammonium concentration in the effluent decreased as IE loading on the BER increased. However, when IE > 0.02 mA/cm2 , ammonium concentration changed in the opposite direction. The nitrite concentration in the cathode chamber of the BER was almost constant at different electric currents. The nitrate concentration in the effluent, which is equivalent to the concentration in the influent, did not vary with IE. The profile of TN was similar to that of ammonium. It can be noted that ammonium in the effluent was affected by the IE. It was found by periodic testing that the ammonium concentration sharply declined in the initial 2 hr, while it decreased at a relatively slow rate over 22 hr. A kinetic
ln (C2 /Ct ) = kt
Ammonium Nitrite Nitrate
Influent Influent Influent
Effluent Effluent Effluent
TN
Influent
Effluent
(7)
where, C2 (mg/L) is ammonium concentration in the effluent as the experiment proceeded to 2 hr; Ct (mg/L) is ammonium concentration in the effluent at 2 hr intervals after 2 hr of experiment run; k is reaction rate constant and t (hr) is reaction time. Regression analysis profiles are shown in Fig. 7. The reaction rate constant (k) did not increase with increased IE. When IE < 0.02 mA/cm2 , k is positively correlated to IE. However, k is limited by IE when IE > 0.02 mA/cm2 . H2 from electrolysis is the limiting factor for ammonium removal at low IE. Therefore, H2 from electrolysis increases and denitrification occurs at a higher rate when IE is increased (Flora et al., 1993). When IE > 0.02 mA/cm2 , H2 from electrolysis is no longer the limiting factor for ammonium removal. Superfluous hydrogen from 1.8 1.6
IE = 0.01 mA/cm2, k = 0.014, R2 = 0.985 IE = 0.02 mA/cm2, k = 0.065, R2 = 0.980
40
1.4
IE = 0.03 mA/cm2, k = 0.055, R2 = 0.980 IE = 0.04 mA/cm2, k = 0.032, R2 = 0.982
35
1.2 ln(C2/Ct)
Concentration (mg/L)
2.4 Oxidation behavior of ammonium in cathode chamber of BER at different electric currents
30 25
1.0 0.8
20 0.6
15 10
0.4
5
0.2
0 0.01
0.02
0.03 2
0.04
IE (mA/cm ) Fig. 6 Nitrogenous concentration in the effluent of the BER at different currents.
0.0
2
6
10
14 18 22 Time (hr) Fig. 7 Regression analysis of ammonium in the effluent of the BER at different currents.
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high IE electrolysis causes the hydrogen inhibition effect (Flora et al., 1993). Furthermore, high IE will deactivate microorganisms (Parvanova-Mancheva et al., 2009; Gong et al., 2008). The nitrogen transformation profiles of the BER at different electric current show that IE loading on the reactor affected the amount of electron donors and the activation of microorganisms. When IE < 0.02 mA/cm2 , the reaction rate of ammonium and nitrogen removal efficiency could be enhanced with increased IE. However, when IE > 0.02 mA/cm2 , this had the opposite effect.
3 Conclusions With low current density (0.02 mA/cm2 ), a new type of 3-dimensional BER was successfully developed to realize complete autohydrogenotrophic nitrogen removal. Ammonium could be oxidized not only on the anode, but also on the packing particles filling the cathode chamber of the BER. In the BER, the electric field can improve nitrogen removal by providing electron donors and activating the microorganisms cultivated on the electrode. Within a certain IE range, the ammonium removal had a linear relation with IE, but this was not consistent throughout the IE range applied. Acknowledgments This work was supported by the Water Special Project (No. 2009ZX07104 002) and the Fundamental Research Funds for the Central Universities of China (No. CQDXWL2012-040).
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