Combining fluidized activated carbon with weak alternatingelectric fields for disinfection Justina Racyte , Jalal-Al-Din Sharabati , Astrid H. Paulitsch-Fuchs ,Doekle R. Yntema Mateo J.J. Mayer , Harry Bruning Huub H.M. Rijnaarts a Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlandsb Sub-Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlandsc Faculty of Chemistry, University Duisburg-Essen, Universita¨tsstraße 2, 45141 Essen, Germanyd EasyMeasure B.V., Breestraat 22, 3811 BJ Amersfoort, The Netherlands This study presents fluidized bed electrodes as a new device for disinfection. In the fluidized Received 25 May 2011 bed electrodes system, granular activated carbon particles were suspended, and an alternat- Accepted 31 July 2011 ing radio frequency electric field was applied over the suspended bed. Proof-of-principle Available online 5 August 2011 studies with the luminescent non-pathogenic bacterium Escherichia coli YMc10 demon-strated that disinfection with fluidized bed electrodes requires both the presence of granularactivated carbon particles and the application of radio frequency electric field. Disinfectionwas investigated at various frequencies in range from 80 to 200 kHz at electric field strengthof 6 ± 0.5 V/cm during 6 h. The largest decrease of E. coli viable cell concentration in theliquid (from 108 to 106 CFU/mL) was obtained at an optimum frequency of 140 kHz. Possiblemechanisms of this electromediated disinfection are discussed in the manuscript. Theresults are promising for development of a new disinfection process with fluidized bedelectrodes.
Ó 2011 Elsevier Ltd. All rights reserved.
pathogens; the major issue with these methods is the ratherhigh capital and maintenance costs . Electrical and electro- The quality of drinking water and treated domestic and indus- chemical methods for disinfection are widely investigated trial wastewater is worldwide an issue of concern Increased because they produce effluents less harmful for biological requirements of water reuse ask for effective disinfection consumers than chemical treatment methods, and they are methods to assure public health, health of cattle and plants cheaper than UV treatment . Electrochemical disinfection in agricultural production systems and to prevent damage methods treat water streams by electrically produced active to natural ecosystems . The oldest and most reliable species such as radicals . The use of solely electric fields chemical water treatment methods such as chlorination and for disinfection needs high electric field densities up to ozonation are being widely applied for water disinfection Although effective, these methods require post treat- Microorganisms have a dielectric nature meaning that ment, mainly due to formation of by-products occurring in they can be polarized in an electric field Electromagnetic the treated water stream Photolytic methods employing fields are reported to affect several microbial life phenomena UV radiation are proven to be very effective for killing such as: microbial growth cell fusion (PEF) * Corresponding author at: Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands. Fax: +31 582 843001.
E-mail address: (J. Racyte).
0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
voltage gated channels ; and can even cause cell death GAC physical characteristics All mentioned effects employ direct current (DC) ora low frequency electromagnetic field (LF-EMF). The main Activated carbon was analyzed before (GACr) and after the problem of these DC or LF-EMF methods is the high power pretreatment (GACpr). Point of zero charge (pHpzc), surface consumption and degradation of electrode materials. There- area (BET) and macropore content were determined.
fore, the treated liquid has to be without ions, particles orgas bubbles to avoid electrical short-circuiting or fouling pH point of zero charge determination between electrodes spaced on a short distance .
The pHpzc was determined by an immersion technique using Optionally, alternating current (AC) treatment can be used NaCl as the electrolyte The 0.05 M NaCl solution was To reach high disinfection efficiency with AC, either degassed by stirring in N2 environment for 48 h. Two grams of strong electric fields ranging from 3 to 100 kV/cm, or high pretreated and untreated GAC were mixed with 100 mL of frequencies (60–1000 kHz) are required . For wastewater decarbonized 0.05 M NaCl and stirred mechanically in N2 envi- that generally has a low electrical resistance, the application ronment at constant room temperature for 24 h (both GACs in of high voltages results in high currents that contribute to triplicate), so that the carbon charges reach a balance (zero high power consumption. The large power required has charge). After 24 h, the GAC was filtered from the liquid with restrained the large scale application of AC disinfection .
a 0.45 lm hydrophobic syringe filter and pHpzc was determined To reduce electric field strength, carbon particles can be with a pH electrode (Liquisys M CPM 253, Endress + Hauser, added in-between two electrodes of an electrical cell, which The Netherlands). The used GAC is brittle, therefore the pHpzc results in a so called three-dimensional electrode .
of powder activated carbon NORIT RX 3 EXTRA (PAC) was The carbon morphology allows bacteria to adhere , determined as well. PAC for pHpzc measurement was made by which may play a role in the inactivation of bacteria.
grinding the GAC in a ball mill grinder (PM100, Retsch, Preliminary experiments indicated that a low amplitude radio Germany) and the above described procedure was applied for frequency electric field (RF-AC) combined with granular acti- determination of pHpzc for PAC.
vated carbon (GAC) can result in disinfection. In the presentstudy a ‘‘fluidized bed electrode'' (FBE) was constructed that BET determination consisted of a RF-AC applied to a stirred GAC particle suspen- The GAC pore structure properties were determined using sion. The FBE was operated in batch mode. The proof-of- nitrogen adsorption on GAC surfaces. GAC was dried and de- principle for disinfection by using the FBE system is presented gassed in a N2 environment for 24 h at 350 °C (VacPrep 061 LB, and key experimental factors that determine the disinfection Micrometrics, Germany). Nitrogen adsorption isotherms were performance were identified. For disinfection experiments measured (Tristar 3000, Micromeritics, USA) to obtain total non-pathogenic and bioluminescent Escherichia coli YMc10 BET surface area (m2/g). The macropore area (Amacropore) was were used as test microorganisms, which are suitable as a measured using mercury intrusion porosimetry (Autopore II representative for enteric pathogens (E. coli O157:H7) which 9220 porosimeter, Micromeritics, USA).
can cause waterborne diseases The use of non-pathogenic E. coli YMc10 made experiments possible under Bacteria – Escherichia coli YMc10 normal laboratory conditions. In this paper mechanisms thatmay play a role in FBE disinfection process are distinguished and discussed.
bacteria E. coli YMc10 (Belgian Coordinated Collections ofMicroorganisms, Belgium) served as target microorganismfor FBE disinfection experiments . This non-pathogenic strain carries a plasmid (pJE202) that contains Vibrio fischerigenes on a vector: luxR, luxI, luxC, luxD, luxA, luxB, luxE, luxG Granular activated carbon specifying the luminescence enzymes and encoding regula-tory functions for bioluminescence It possesses an Commercially available GAC NORIT RX 3 EXTRA (Norit BV, The ampicillin resistance on the same vector to prevent the Netherlands) was used. This GAC is extruded, steam activated multiplication in an environment without ampicillin and and acid washed. Prior to the experiments 140 g of GAC was cross-transfer of the vector to the other microorganisms.
fluidized in a beaker with 1 L Milli-Q water (18.2 MXÆcm Lysogeny broth (LB) medium was prepared in Milli-Q to cul- 25 °C, 0.22 lm, Millipore Biocel SAS 67120, France) and wet- tivate E. coli YMc10 (NaCl 10 g/L; BactoTM Tryptone 10 g/L; ted (4 h stirring). After this the GAC particles were washed BBLTM Yeast extract 5 g/L; Ampicillin 0.1 g/L; pH 7). The pre- with Milli-Q water and autoclaved at 0.1 MPa, 121 °C for pared medium was then autoclaved for 25 min at 0.1 MPa, 30 min to avoid contamination of the microbial culture by 121 °C to sterilize the media. A part of the autoclaved medium other bacteria. Subsequently, GAC particles were washed with (100 mL) was inoculated with 1 mL of E. coli YMc10 stock solu- Milli-Q water again and left standing for 24 h to remove tion and incubated for 18 h at 25 °C. After incubation, 90 mL of remaining air from GAC pores Before the experiment the bacterial suspension were centrifuged at 3273 g for 15 min.
the particles were submerged for 24 h in 1/4 LB medium The supernatant was discarded and the pellet was resus- (section 2.3) to saturate the GAC with electrolyte. For the pended in 400 mL diluted (1:4) LB medium (1/4 LB medium) FBE disinfection experiment 400 mL of the prepared medium which was used in the FBE disinfection experiments. The LB (1/4 LB media containing ampicillin) and 140 g of pretreated dilution ratio was determined experimentally (data not GAC (GACpr) were used.
shown), as the minimum amount of substrate concentration
to maintain a stable culture and achieve active luminescence with a spectrophotometer measuring the luminescence but inhibit exponential growth of the bacteria. The 1/4 LB (1420 Multilabel Counter Victor3; Perkin Elmer, USA). There- medium concentration is comparable to a lightly polluted fore 200 lL of each sample was pipetted into a well of a 96 well wastewater concentration black optical bottom plate (MicroWell, Nunc, Denmark). Theintensity of luminescence as photon emissions from a sample Experimental set-up in counts per second (CPS) was measured by a extra highscale luminescence detection method. However, lumines- The fluidized bed electrode (FBE) set-up (consisted of a cence intensity is not only dependent on the amount of bac- 1 L beaker glass, 2 stainless steel plate electrodes (35.6 cm2) and teria, but also on factors such as bacteria quorum, a magnetic stirrer (Heidolph instruments D91126, Germany). A autoinducers and Lux gene regulators . Bacterial concen- trations corresponding to a luminescence level below 10 CPS (fmax = 600 kHz, Imax = 5 A, Vmaxpp = 30 V) connected to a func- cannot be detected. Therefore parallel to luminescence mea- tion generator (TG 2000 DDS Thurlby-Thandar instruments, surements, samples were plated on LB agar to confirm that UK) provided an alternating voltage, that was connected to the change in CPS value corresponded to the viable and culti- the two stainless steel electrodes. The 400 mL of liquid medium vable E.coli concentration – CFU value. The relation between to be disinfected was placed in the beaker into which the elec- CPS values and CFU values was experimentally determined trodes were immersed and the GAC particles were suspended.
to be DCFC ¼ 1 ðDlogðCPSÞÞ with r2 ranging from 0.68 to 0.92.
The system was stirred at 650 rpm, which resulted in a fluid- The measured CPS values do not always correspond with ized bed with turbulent character. Power was turned on at the relation; therefore consequently samples were plated to t = 0 min and 4 mL samples were taken every 30 min for lumi- determine the number of viable and cultivable E. coli in CFU/ nescence analysis and every 60 min for viable cell colony form- mL. The drop technique was used to determine the ing unit (CFU) analysis. The duration of an experiment was CFU/mL. Instead of plating different dilutions, three drops 360 min. Both temperature and pH were measured with a pH of 10 lL of the same dilution were plated (by allowing the drops to run down the agar surface) on the same petri dish Endress + Hauser, Naarden, The Netherlands). Stirring led to as triplicate. The plates were incubated for 18 h at 37 °C.
a minor pH increase, likely due to release of remaining oxygencomplexes from the GAC pores. During operation, pH was Testing the influence of experimental factors on maintained at values between 7 and 7.5, by feeding 1 M HCl bacterial survival and inactivation in FBE system solution. Temperature was controlled at a value of 23 ± 2 °C.
A set of blanks was designed ) to test the relevance of Microbiological analysis each experimental factor involved in FBE disinfection process.
During each experiment different combinations of experi- To determine the concentration of E. coli YMc10 in the samples mental factors were investigated and the disinfection was from the FBE experiments a fast screening was performed measured by the changes in luminescence intensity level.
Experimental factors are the parts of the FBE reactor system:Electrodes, RF-AC electric field (100 kHz; 6 ± 0.5 V/cm) andGAC; and the liquid composition: LB medium and bacteria(E.coli YMc10). Samples were compared using a hypothesis t-test (compared means with a = 0.05). Before the hypothesistests, samples were verified to be normally distributed.
The effect of RF-AC on FBE disinfection The relation between AC frequency and FBE disinfection effi-ciency was investigated. Duplicate experiments were per-formed at a various frequencies in a radio frequency rangefrom 80 to 200 kHz, with a step size of 20 kHz. The voltage ap-plied was 6 ± 0.5 V/cm. The reaction media containing 1/4 LBmedia had a conductivity of 5.2 ± 0.05 mS/cm and the average Fig. 1 – The FBE disinfection research set-up. (1) Alternating current measured was 163 ± 5 mA/cm2 electrode. For each power source (2) Stainless steel electrode plates. (3) experiment conditions such as temperature, pH, GAC Granulated activated carbon (GAC). (4) 1 L beaker glass. (5) amount, stirring intensity by magnetic stirrer, RF-AC ampli- Magnetic stirring bar. (6) Magnetic stirrer. Beaker A: blank 7 tude and media composition were kept constant. A control ) (includes 1/4 LB media and bacteria, and excludes without AC electric field blank 6; was run- electrodes, GAC, applied RF-AC electric field); Beaker B: ning parallel to the disinfection sample disinfection blank 6 ) (includes 1/4 LB media, bacteria, electrodes, GAC, and excludes RF-AC electric field); Beaker C: Samples were taken every hour and plated on agar for CFU disinfection sample 0 ((includes all experimental counts. The obtained data was statistically analyzed (t-test) to factors: 1/4 LB media, electrodes, GAC, RF-AC electric field, compare the two independent runs from the same frequency (duplicates). Statistically equal samples were averaged. The Table 1 – Experimental scheme of different experimental factors (LB medium, Electrode plates, GAC, RF-AC, E.coli YMc10)investigated for their influence on the performance of FBE disinfection. ‘‘+'' experimental factor is present during experiment,‘‘'' experimental factor is absent during experiment.
Granulated activated carbon (GAC) Radio frequency alternating el.
field (RF-AC 100 kHz; 6 ± 0.5 V/cm)E.coli YMc10 (bacteria) a Stainless steel electrodes are present but no RF-AC is applied (B).
b Stainless steel electrodes are present and RF-AC is applied normal distribution of the samples was verified using STAT- experiments, but apparently it also changes GAC properties DISK 10.4.0 .
so that E. coli YMc10 adhere in lesser extent on GAC.
Results and discussion Luminescence measured in a set of blanks In the luminescence intensity change in counts persecond (CPS) of the luminescent E. coli YMc10 was used to The properties of pretreated (autoclaved) carbon (GAC quantify the disinfection during 360 min: the disinfection carbon before pretreatment (GAC sample and different blanks were compared; r) are compared in .
Analyzed carbons have moderately high surface areas (BET) disinfection sample is presented in all the (Figs. 2–4) to show The pretreatment and milling does not influence the the relation between sample and the blanks. In case an change in BET area ().
electric field was applied, the field was 100 kHz; 6 ± 0.5 V/cm.
The macropore area represents a small percentage of total For blanks 2 and 3 no nutrients were added, and an surface area (This small macropore area is not in fa- exponential decrease of the CPS value was observed with no vor for bacterial adhesion. The size of macropores is defined difference between RF-AC electric field applied (blank 2) or as larger than 50 nm. The E. coli size is in the range of 1 lm, in absence of an RF-AC electric field (blank 3). The decrease therefore they can only adhere at the external surface and of the CPS value in these blanks is much faster (from 106 in the larger macropores, and not in meso or micro pores .
CPS to 101 CPS in 30 min) than for the disinfection sample (from 106 CPS to 101 CPS in 180 min). The fast decrease of pzc values () indicate that GACr is basic, but basicity was diminished after pretreatment (GAC the CPS value is caused by physiological stress due to nutrient the pretreatment steps consisted of autoclaving at a pressure limitation and unfavorable osmotic conditions This indi- of 0.1 MPa and a temperature of 121 °C. These autoclaving cates the necessity to add salt to maintain the isotonic pres- conditions may oxidize the GAC surface slightly Through sure necessary for an intact cell membrane and nutrients to oxidation of the GAC, the increase of the amount of surface keep bacteria luminescence active.
oxygen complexes influences the amphiphilic character of For the blanks 4, 5 and 7 that did not contain GAC A), the solid surface and the pH no decay in the CPS value could be observed in comparison to pzc . The pHpzc together with amphiphilic properties of bacteria and solid surfaces are the disinfection sample (0) parameters that determine the extent of bacterial adhesion For the solution with GAC a decrease in E. coli YMc10 lumi- on the surfaces E. coli pH nescence value (CPS) was observed in both cases (with pzc is in range 2.1 and 4.3 Therefore E. coli adhere better on hydrophobic than the applied RF-AC electric field and without the RF-AC electric on hydrophilic surfaces The increased hydrophilic prop- field blank 6 and blank 1).
erties of GAC after pretreatment comparing to GAC In the presence of an RF-AC electric field and after 150 min pected to decrease the bacterial adhesion. The pretreatment the CPS values decreased from 106 to 101, whereas in the was applied for the sterilization of the materials prior to the absence of an RF-AC electric field CPS values decreased from Table 2 – GAC physical characteristics (Carbon type, surface area (BET), macropore area (Amacropore) percentage in total BETarea, pH point of zero charge (pHpzc) of GAC and PAC before pretreatment (GACr; PACr); and GAC and PAC pretreated (GACpr;PACpr)). Values are given with ± standard deviation.
Activated carbon RX 3 EXTRA type Amacropore, in% of BET Granulated activated carbon before pretreatment (GACr) Granulated activated carbon pretreated (GACpr) Powder activated carbon before pretreatment (PACr) Powder activated carbon pretreated (PACpr)
Fig. 2 – The effect of nutrients absence on luminescence Fig. 4 – The effect of RF-AC on luminescence intensity.
intensity. Comparison of disinfection sample 0 m (1/4 LB Comparison of disinfection sample 0 m (1/4 LB media, media, electrodes, GAC, 100 kHz; 6 ± 0.5 V/cm RF-AC electric electrodes, GAC, 100 kHz; 6 ± 0.5 V/cm RF-AC electric field, field, bacteria), Blank 2 d (no LB media), Blank 3 s (no LB bacteria), Blank 1 s (no electrodes, no RF-AC electric field), media, no electrodes, no RF-AC electric field).
Blank 6 d (no RF-AC electric field).
handy method for FBE parameter screening, but viable cellconcentration determination by plating (CFU) is needed toaccurately quantify disinfection as was done in the experi-ments discussed below. Thus, by using bioluminescentE. coli YMc10, we were able to show that disinfection withthe FBE is dependent on the combination of RF-AC and GACparticles.
The effect of RF-AC on disinfection The previous experiments were performed at a constant100 kHz RF-AC. In this experiment, the effect of various fre-quencies (in range 80–200 kHz) RF-AC on the FBE disinfectionwas investigated. The effect of frequency was compared to acontrol (blank 6) without applied electric field. Preli-minary experiments (data not shown) showed that lumines- Fig. 3 – The effect of GAC on luminescence intensity.
cence intensity decrease of E. coli YMc10 is frequency Comparison of disinfection sample 0 m (1/4 LB media, dependent. The decrease in E. coli YMc10 viability was subse- electrodes, GAC, 100 kHz; 6 ± 0.5 V/cm RF-AC electric field, quently confirmed with the CFU method as disinfection in bacteria), Blank 4 j (no GAC, no RF-AC electric field), Blank 5 time (top to bottom in The largest decrease of E. coli D (no GAC), Blank 7 s (no electrodes, no GAC, no RF-AC YMc10 concentration was detected at a frequency of electric field).
140 kHz. A similar effect of electric field frequency on redblood cell fusion has been reported by Chang at 0.5–5 kV/cm in the range of 80 kHz to 1000 kHz, however without 106 to 104 (). The difference in decline of CPS intensity be- the use of carbon particles. The time required for E. coli YMc10 tween with and without applied RF-AC electric field demon- concentration (CFU/mL) to decrease is 1 to 5 h in the FBE. This strates that the RF-AC electric field combined with GAC is longer than disinfection with DC in range 3–110 kV disinfects the fluid. However, the presence of GAC alone also or low frequency electric field at 16–60 Hz with electric cur- leads to the decrease of the CPS values over time. Moreover, rent 125–370 mA/cm2 electrode . Nevertheless the FBE CPS measurement was found to be influenced by GAC powder system operates at a low electric field strength which enables by light scattering Powder of activated carbon (PAC) is the use of a conductive solution that contains particles or produced whilst fluidizing the reactor, as was confirmed by other impurities next to bacteria. This is not the case for DC microscopy studies of fluid samples (data not shown). The GAC powder and the bacteria are likely to form GAC-bacteria The disinfection efficiency depends on the applied fre- aggregates. These aggregates and the presence of GAC powder quency The highest efficiency within 5 h time was interfered with the luminescence measurement (CPS), which reached at 140 kHz within the frequency range tested at an leads to the observed decrease in the CPS intensity in the ab- average power consumption of 99 ± 1 W per 1 L of disinfected sence of RF-AC fields. The CPS monitoring is a valuable and solution. To confirm if the disinfection sample is significantly
without an applied RF-AC electric field, 0.5 log CFU/mL) of expected adhesion was observed. The low adhesionlevel can be explained with the relatively short experimentaltime (360 min) and the turbulent character of the fluidizedbed. The weak magnetic field from the magnetic stirrer couldhave a synergistic effect with the alternating current andcause the decrease of E. coli YMc10 concentration in the con-trol and in the sample. Nevertheless in this study the mag-netic field is not considered as experimental factor as all theblanks and samples were stirred with the magnetic stirrer.
Therefore the effect of magnetic field is not investigated inthis study. The measured decrease of viable E. coli YMc10concentration by CFU counts and photometric measurement(in CPS) shows the actual disinfection rate due to the appliedconditions.
Fig. 5 – Disinfection with FBE system at 6 ± 0.5 V/cm and163 ± 5 mA/cm2 electrode at different frequencies. E. coli General discussion on possible electrical YMc10 viable cell concentration (log(CFU0/CFUt)) within disinfection mechanisms 360 min (series). *Dashed lines connect the same timepoints from different frequencies.
Electric fields can lead to induced currents in microorgan-isms. Different frequencies can lead to phenomena such as:surface polarization ; increased cell membrane perme-ability and change in shape . At the resonance frequencyfor each microorganism the polarization effect will be maxi-mal. Due to the polarization effect on the outer cell mem-brane, a potential difference can build up between theinside of the cell membrane and the outer wall similar to adiode. By this effect cells can be damaged through the block-age of the voltage gated channels The effect of the res-onance frequency on cell wall permeability has beenestablished in cell fusion by induced increase in membranepotential An increased membrane potential could alsocause membrane disruption followed by leakage of cellularcontent Cells are found to rotate in alternating electrical fields possibly causing mechanical cell fatigue Cell rotationcharacteristics are dependent on a lot of factors such as: cell Fig. 6 – Comparison of the effect of GAC (Control 6 d) and the aging, viability etc. For the pathogenic E. coli K12 rota- GAC combined with RF-AC (disinfection sample (0) m) at tion torque arises at the frequency between 100 and 500 kHz 140 kHz. Error bars indicate standard deviations.
which is consistent with the frequency range applied inour study.
In RF-AC electric fields a time dependent effect of cavita- different from a control (without applied RF-AC), the standard tion can cause changes in cell shape Moreover it in- deviations of control and the disinfection sample were calcu- duces physical disintegration of microbial cells and e.g. E. coli lated (Bacterial concentration (CFU/mL) in the control cells are highly susceptible to it .
remains constant during 360 min. In the disinfection sample Furthermore, the magnetic field from the magnetic stirrer the bacterial concentration (CFU/mL) exponentially decrease may play a role in FBE disinfection. Studies have shown that after 120 min of FBE disinfection. The FBE disinfection perfor- weak magnetic fields in combination with electric fields could mance at 140 kHz differs significantly (p = 0.05) between dis- disturb the biochemical equilibrium in microbial cells e.g. by infection sample and control after 120 min of treatment.
the formation of radical species The weak magnetic The variation of E. coli YMc10 concentration in the control fields, in combination with alternating electric fields influ- (can be explained by the following two phenomena: ence the dissociation probability of Ca, Mg, Zn ion-proteins adhesion of E. coli YMc10 on GAC particles and/or the effect from their carrier DNA strands in E.coli cells of the weak magnetic field from the stirrer on the E.coli .
The cause of microorganism death in most above men- When bacteria adhere on GAC they might loose the viability tioned cases is affected by pH, temperature, radical formation by membrane disruption Bacteria have cell structures and conductivity of the disinfected medium Granu- that make adherence on rough surfaces possible . To lated activated carbon seems to serve as fluidized conductive completely adhere E. coli on GAC particles at least 3 days of component with a high surface area that enhances these cell- shaking with GAC at 100 rpm is necessary . In our case electric field interactions.
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