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Flocculation monitoring of wastewater by using impedance spectroscopy
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
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Flocculation monitoring of wastewater by using impedance spectroscopy A. Mortadi a, *, A. Elmelouky a, M. Chahbi a, N. EL Ghyati a, S. Zaim a, O. Cherkaoui b, R. El Moznine a a b
Laboratory Physics of Condensed Matter (LPMC), University Chouaib Doukkali, El-Jadida, Morocco Higher School of Textile and Clothing Industries, Laboratory REMTEX, Casablanca, Morocco
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2019 Received in revised form 26 July 2019 Accepted 28 July 2019 Available online 29 July 2019
The aim of the present work is to monitor the flocculation process using the analysis of the electric and dielectric properties. Therefore the dielectric and electrical characteristics of wastewater with different cationic polymer concentrations were investigated via the impedance spectroscopy (IS) method. Impedance measurements were carried at different concentration of cationic polymer in the frequency range from 0.1 Hz to 100 kHz. The analysis of complex permittivity spectra was described by the superposition of a power law at a low frequency related to the diffusion process and Cole-Cole relaxation behavior at high frequency. Moreover, an equivalent circuit model was developed in order to analyze the experimental data and to further investigate both processes. The variation of the parameters extracted from the equivalent circuit with the increase of cationic polymer concentrations has shown a net transition at 10 mg/l. This behavior could reflect the flocculation of dispersed particles at 10 mg/l. The findings in this work could draw new attention toward the monitoring of the coagulation-flocculation process using impedance spectroscopy and could be extended to other kinds. © 2019 Elsevier B.V. All rights reserved.
Keywords: Wastewater treatment Cationic polymer Dielectric properties Hopping conductivity Flocculation process
1. Introduction Contamination of water with pollutants of various origins has become a global problem [1]. Wastewater is one of the most important sources of pollution of surface waters and groundwater, especially for agricultural soil. The textile industry is one of the industries the most water-consuming and generates a waste consisting of organic and inorganic molecules, generally presenting color problems, high concentrations of BOD5, COD, suspended solids, high toxicity and high conductivity [2]. The waste of the textile industry represents a great hazard for human health and the environment [2,3].In fact, the various dyes used in the different steps of clothing manufacturing cause problems in the quality of wastewater because of their stability and low biodegradability [4e6]. Thus, it is necessary to treat this waste before it gets discharged into the remediation system. These effluents to be treated contain non-biodegradable substances, inhibitory or toxic for most living microorganisms [7]. Moreover, the heterogeneity of their composition makes it difficult or almost impossible to obtain pollution thresholds lower than or equal to those imposed by environmental standards, after treatment with traditional techniques [8]. * Corresponding author. E-mail address: [email protected] (A. Mortadi). https://doi.org/10.1016/j.saa.2019.117437 1386-1425/© 2019 Elsevier B.V. All rights reserved.
Indeed, in order to improve the liquid-solid phase separation, flocculating agents are commonly used in various studies [8e10]. These flocculating agents react with wastewater components to induce flocculation which expedites the liquid-solid phase separation. The success of wastewater treatment using coagulationflocculation process depends mainly on this last step. Thus, in order to understand the mechanism of this process and to improve liquid-solid separation, many researchers have tried to monitor and explain this process following the addition of the flocculent agent, which is usually a high molecular weight cationic polymer [11,12]. Because coagulation-flocculation processes are usually based on electrostatic interaction and because of the strength of the aggregations formed during this process is believed to play a significant role in liquid-solid separation, therefore impedance spectroscopy has been assumed to be the appropriate method to monitor and to investigate the electrical and dielectric properties during treatment wastewater [13,14]. In our previous work [7], the coagulation process with the addition of the calcium oxide (CaO) was investigated using impedance spectroscopy. It was found that the dielectric properties are a good tool to control and to monitor the sludge coagulation process. This technique is also useful in several areas such as, on the corrosion inhibitors [15,16] solar cells [17] and to monitor the adsorption in the anionic clay [18].
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The main objective of the current work is to investigate the effect and the impact of the cationic polymer on the flocculation process. Impedance measurements were investigated based on the analysis of the complex permittivity and the electrical complex modulus M*(u) to further analyze the nature and the origin of relaxation observed in the evolution of the imaginary part of complex permittivity. 2. Material and methods 2.1. Samples The sample of wastewater was collected from a heterogeneous common effluent treatment plant located in Berrechid, Morocco. This wastewater contains effluents from textile industries. A cationic polymer was used as a flocculent. Seven samples at different concentrations (0, 2.5, 5,10,15, 20 and 25 mg/l) of cationic polymer were prepared and analyzed using impedance measurement. For each concentration, the samples were stirred at 20 C during 3 min with a suitable speed. The mixture was then stirred gently during 20 min in order to avoid contact between particles before the experiment. 2.2. Electrical measurement The electrical measurement was carried out, using (Voltalab 10, PGZ 301, Radiometer Analytical Instruments). The impedance data were measured using three electrodes, a saturated calomel electrode Ag/AgCl/KCl saturated, used as a reference electrode against a stainless steel electrode and a working electrode in glacier atoms. The measurements were carried out at room temperature in the frequency range from 10 mHz to 100 kHz and using AC potential amplitude of 10 mV. The experimental data were analyzed and investigated using the Z.View 2.2 software. 3. Results and discussion 3.1. Analysis of the complex permittivity The frequency dependence of dielectric measurement at different concentration of cationic polymer is shown in Fig. 1a, b. Fig. 1a and b depicts two representations of dielectric measurement using Bode and Nyquist diagrams respectively [19,20]. All compositions exhibited similar behavior. As can be seen from Bode representation (Fig. 1a), the imaginary part showed high values at low frequencies shown a linear decrease when the frequency increases. This behavior could reflect the
diffusion process due to the free charge carriers. In addition, Fig. 1b showed also high values at low frequencies. This behavior could be due to the accumulation of the charge carriers at the sample/ electrode interface forming the charged double layer or/and move from one site to another site causing an increase in the conduction behavior, what is usually called the low frequencies dispersion (LFD). On the other hand, when the frequency increases the imaginary part showed a maximum. It is well known that wastewater can be considered as heterogeneous materials. The complexation between the particles of wastewater (negative charges) and the cationic polymer (positive charges) lead to the formation of double-layer at the interface. This fact suggests that the induced dipole due to the accumulation of negative and positive sites can generate an interfacial polarization. The local and fast movements of the induced dipole lead to a dynamic behavior at high frequency (H.F.) resulting in the relaxation process. This kind of relaxation was revealed by Hiroyuki Sugimoto et al. [20] in their study on dielectric relaxation due to interfacial polarization for heat-treated wood Carbon. This similar behavior was also revealed by Roldan-Cruza et al. [19] to investigate the complication process between the gum Arabic as an anionic polymer and chitosan as cationic polymer. The variation of ε00 (u) at higher frequencies showed a relaxation peak as it is observed in both Bode and Nyquist diagrams. It is important to note that, in the Bode representation (Fig. 1a), the frequency corresponding to the relaxation peak in the imaginary part shifted to the low frequency side when the concentration increased from 0 to 10 mg/l. In contrast, this relaxation peak shifted to the high frequency side when the concentration increased from 10 to 25 mg/l. Therefore, the evolution of dielectric properties shown in Fig. 1 was reported in two graphs in order to better illustrate the effect and the impact of cationic polymer concentration on the dielectric properties as it is shown in Figs. 2aeb and 3aeb. Figs. 2ae3a represent the evolution of dielectric properties corresponding to the concentration range from 0 to 10 mg/l. In contrast, Figs. 2be3b represent the evolution of dielectric properties corresponding to the concentration range from 10 to 25 mg/l. consequently; the transition at 10 mg/l could be seen very clearly indicating the existence of the two regions. The first corresponds to the concentration below 10 mg/l. the second region corresponds to the concentration above 10 mg/l. In addition, this separated in two graphs could provide a significant change in the dielectric strength and relaxation time which has occurred with the increase of the cationic polymer concentration. The analysis of the dielectric permittivity data at different concentrations revealed the existence the relaxation process that
Fig. 1. (a) Bode plot and (b) ColeeCole plot at different cationic polymer concentration.
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
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Fig. 2. (aeb): Imaginary part vs. frequency at different polymer concentration. (a): Concentration below 10 mg/L. (b): Concentration beyond 10 mg/l.
manifests itself at high frequency (H.F.), wheel the diffusion process that manifests itself at low frequency (L.F.). However, it is wellknown that ‘electrode polarization’ effect, mainly due to the spatial charge that accumulates near the electrodes, represents a serious obstacle in the dielectric and electrical measurement of conductive solutions [21]. Therefore, more attention has been made in the analysis of dielectric properties to further investigate the origin of the relaxation peak observed in the imaginary part of the permittivity. Consequently, an alternative approach was used to overcome this situation which consists of the analysis of the electrical complex modulus M*(u). In our previous work [7], this approach was employed to further analysis and examine of the evolution of both complex modulus M*(u) and permittivity ε*(u) to further analysis the ‘electrode polarization’ effect. 3.2. Analysis complex modulus M * (u) The imaginary part of the complex permittivity as a function of frequency showed a very high value at low frequencies due to the low frequencies dispersion. This suggests that any relaxation process could be masked or hidden in the evolution of the imaginary part of the complex permittivity at low frequency. In this case; the relaxation process could be seen very clearly in the evolution of the imaginary part of the complex modulus M*(u) since this later M*(u) is defined as the inverse of the complex permittivity ε*(u) [8].
M*ðuÞ ¼
00 1 1 ¼ ¼ M 0 ðuÞ þ jM ðuÞ 00 ε*ðuÞ ε0 ðuÞ jε ðuÞ
(1)
versus frequency and the permittivity versus frequency is shown in Fig. 4a, b, c for three typical concentrations 0, 10 and 25 mg/l. The first concentration corresponds to the raw sample 0 mg/l taken from the range 0 to 10 mg/l in the first region. The second corresponds to the critical concentration (10 mg/l) where the transition has occurred. The third concentration was selected from the range of concentration (10-25 mg/l) in the second region. From Fig. 4, it is important to note that the plots of M00 (u) vs. log f do not have any significant relaxation peak over the investigated frequency (102e105 Hz) range. However, the spectra show a narrow peak which is located between 101 and 10 Hz. Since this later narrow peak appeared in the very low-frequency value, it appears difficult to ascribe this peak to bulk motions. Therefore, the relaxation peaks revealed by the M(u) indicated the occurrence of electrode polarization (EP) processes. The electrode polarization (EP) process was shown to be a participating process below 1 Hz due to the accumulated charges between the electrodes/sample [21]. The law magnitude of this peak could indicate that aggregation during the flocculation process. It is therefore suggested that the relaxation peak could reflect the phenomenon occurs inside the aggregation during the coagulation process. This relaxation behavior could be attributed to the localized relaxation process due to the short-range hopping motion of ions and water proton trapped inside the aggregations. From electrical/dielectric measurements and considering different parameters (permittivity and electrical modules) and different representations, we have been able to highlight the existence of two phenomena: relaxation phenomena and low frequency dispersion phenomena (H.F.).
The evolution of the imaginary parts of the electric module
Fig. 3. (aeb): Imaginary part vs. real part at different concentration. (a): Concentration below 10 mg/l. (b): Concentration beyond 10 mg/l.
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Fig. 4. (a, b and c): Bode diagram of M00 and ε00 versus frequency for the selected concentrations 0, 10 and 25 mg/l.
There is a small amount of the free ions which had moved up to the electrode. In contact; the relaxation peaks observed in the evolution of the imaginary part of the complex permittivity was attributed to the phenomenon occurs inside the cluster. 3.3. Origin of low frequency dispersion (LFD) According to Jonscher, an important feature of this lowfrequency dispersion situation is that of dipole systems that will show peaks in the imaginary part of the complex permittivity, while bearer-dominated systems will exhibit a low-frequency dispersion (LFD) response [14,19]. It is well known that the process of coagulation-flocculation by the addition of cationic polymer promotes aggregations of charged particles. These aggregates can be considered as clusters of different sizes under an applied electrical field, the ions in the clusters can move inside the clusters (intra-cluster movement) or jump outside the clusters (inter-cluster movement). Indeed, if the distance traveled by the ions is less than the size of the cluster, these ions remain in the cluster and leading to an intra-cluster movement. If the distance traveled by the ions is greater than the size of the cluster, the displacement of the loads takes place between the clusters and therefore leading to an inter-cluster movement. In this case, there are potentially mobile charges that contribute to the conduction process and also the low-frequency dispersion (LFD) [20].
relaxation process that manifests itself at high frequency (H.F.): This process can be attributed to the relaxation of the condensed or adsorbed ions on the polymer chains forming clusters. Therefore, if the distance traveled by the charges, is less than the size of the cluster; these charges remain in the cluster and we have intracluster movement. In this case, the clusters can be considered as ‘linked dipoles’. The condensation of the ions generates induced dipole moments which are translated by local and fast movements and consequently their dynamic behavior manifested at high frequency (H.F.). This kind of relaxation is known as MaxwelleWagnerseSillars (MWS) relaxation [22]. A schematic is shown in Fig. 5 to illustrate the flocculation process during wastewater treatment showing the aggregates resulting from the condensed of counter-ions and free ions [23,24].
3.4. Origin of the relaxation process As shown before, the detailed analysis of the dielectric permittivity data at different concentrations revealed the existence of a
Fig. 5. Illustration of suspensions in wastewater and resulting aggregates [18].
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
It is therefore suggested that the relaxation peak could reflect the phenomenon that has occurred inside the aggregation during the flocculation process. This relaxation behavior could be attributed to the localized relaxation process due to the short-range hopping motion of ions and water proton trapped inside the aggregations.
3.5. Modelisation of the dielectric responses using equivalent circuit Based on the above analysis of the dielectric responses of the wastewater samples at different concentrations of the cationic polymer showed: - The presence of a peak between 300 Hz and 105 Hz in the spectrum of the complex permittivity ε*(u) indicates the appearance of the process of high frequency relaxation (H.F.). - Low frequency dispersion process (LFD), this process has appeared in spectra of complex permittivity with the simultaneous increase of the real and imaginary part. - The presence of a peak between 0.1 Hz to 200 Hz with low amplitude in the evolution of the imaginary part of complex modulus M*(u) indicating the onset of the electrode polarization process (E.P.). Since the magnitude of this later relaxation related to the electrode polarization (E.P.) process was very small as it is shown in Fig. 4aec; the contribution of the relaxation corresponding to the EP was neglected. The use of the Cole-Cole relaxation function is a useful model to describe MaxwelleWagnerseSillars (MWS) relaxation process in the complex permittivity ε*(u) [25].
ε* ðuÞ ¼ ε∞ þ
Dε 1 þ ðjutε Þa
(2)
u: is the pulsation of the electric field (rad$s1), Dε ¼ εs ε∞: is the dielectric strength, t: is the relaxation time (s), a: is the parameter associated with the distribution of the relaxation time. The low frequency dispersion (LFD) response could be described very well by a power law.
εLFD *ðuÞ ¼
sho
(3)
ε0 ðiuÞb
b: is the exponent parameter, 0 < b 1, and sho is the hopping conductivity. If b ¼ 1, the complex permittivity becomes: ε*ðuÞLFD ¼
sho ε0 ðiuÞ1
In this case, the real and the imaginary parts of the complex permittivity are: 00
ε0 ðuÞ ¼ 0 and ε ðuÞ ¼
sho
ε0 ðuÞ
:
The hopping conductivity becomes similar to direct current conductivity (sho ¼ sdc).
5
Therefore, the evolution of the complex permittivity could be described by the combination of the two expressions given in Eqs. (2) and (3):
ε* ðuÞ ¼ ε∞ þ
Dε sho þ 1 þ ðjutε Þa ε0 ðjuÞb
(4)
An equivalent circuit model was built in order to develop and to find the analytical expression given in Eq. (4) is shown in Fig. 6. This circuit contains two blocks in parallel to describe both two processes: - Block 1: Describe the behavior of the Cole-Cole relaxation at high frequency (H.F.) - Block 2: Describe the behavior of low frequency dispersion (LFD) at low frequency (L.F.). The impedance of the constant phase element (CPE) is given by the following equation:
ZCPE ðuÞ ¼
1 TðjuÞp
where T is the pseudo capacitance and p is the exponent between 0 and 1. For p ¼ 1, the impedance is purely capacitive. For p ¼ 0, the impedance is purely resistive. The complex impedance of the capacitance (C) is given by Z * ðuÞ ¼ jC11 u. The complex permittivity (ε*) can be obtained from the complex admittance (y*) using the following relationship [7]:
Y * ðuÞ ¼ juC0 ε* ðuÞ
(5)
The global complex admittance is given by the summation of the admittance of both sub-circuit (1) and 2: Y*(u) ¼ Y*1(u) þ Y*2(u). The sub-circuit (1) contains the capacitance (C1) in series with the constant phase element (CPE1) to parallel to the capacitance (C2). Therefore, the complex admittance of the sub-circuit (1) can be writing as:
Y * 1 ðuÞ ¼ juC2 þ
¼ juC2 þ
juC1 *T1 ðjuÞp1 juC1 ¼ juC2 þ juC1 þ T1 ðjuÞp1 C1 1 þ T1 ðjuÞ1p1 juC1 1 þ ðjutε Þð1p1 Þ
where tε ð1p1 Þ ¼ CT11 . On the other hand, the complex admittance of the sub-circuit (2) is: Y * 2 ðuÞ ¼ T2 ðjuÞp2 . The complex admittance of the equivalent circuit can be writing as follow:
Y * ðuÞ ¼ juC2 þ
juC1 1 þ ðjutε Þð1p1 Þ
þ T2 ðjuÞp2
where: C2, C1 are the high and low-frequency capacitances respectively. The capacitances C1 and C2 can be writing as a function of the dielectric constant at high and low-frequency and the empty capacitances C0.
S C1 ¼ ðεs ε∞ ÞC0 ; C2 ¼ ε∞ C0 ; and C0 ¼ ε0 e
Fig. 6. Equivalent circuit to describe the relaxation and hopping conduction processes.
where ε0 ¼ 8.85 1012 F/m1 is the vacuum, S is the surface of electrode and e is the distance between the electrode.
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Fig. 7. (a) Imaginary part versus frequency for the raw sample. De-convolution of the two process by simulation: conduction by LFD (blue color), relaxation (red color). (b) Residual error.
Consequently, the global admittance can be written as follow:
"
" *
Y ðuÞ ¼ juC0
εs ε∞
T þ 2 ðjuÞp2 1 ε∞ þ ð1p1 Þ C 0 1 þ ðjutε Þ
#
Therefore, the expression of the global complex permittivity can be obtained according to the relation given in Eq. (5).
εs ε∞
T ε ðuÞ ¼ ε∞ þ þ 2 ðjuÞp2 1 ð1p1 Þ C 0 1 þ ðjutε Þ *
" ε* ðuÞ ¼ ε∞ þ
εs ε∞ 1 þ ðjutε Þð1p1
þ Þ
#
T2 ,e ðjuÞp2 1 ε0 S
Fig. 8. (a, b and c): Bode diagram of complex permittivity versus frequency for selected concentrations 0; 10 and 25 mg/l.
#
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
Table 1 Dielectric and electrical parameters of electrical circuit: Concentrations Dielectric strength (Dε106) 0 2.5 5 10 15 20 25
Relaxation times t (ms)
0.18 0.52 0.88 1.18 1.04 0.88 0.85
Hopping conductivity sho (ms/cm)
0.14 0.47 0.74 0.92 0.80 0.58 0.45
1.45 1.10 0.90 0.80 1.01 1.07 1.09
The values of bold correspond to the values of deduced parameters from the electrical.
" ε* ðuÞ ¼ ε∞ þ
εs ε∞ 1 þ ðjutε Þð1p1
þ Þ
sho ε0 ðjuÞ1p2
#
where tε ð1p1 Þ ¼ CT11 Dε ¼ ðεs ε∞ Þ ¼ CC10 sho ¼ T2S,e. As it can be seen, the expression of the complex permittivity obtained from the equivalent electrical circuit is similar to the analytical expression given in Eq. (4), if the exponent a and b are attributed to the (1 p1) and (1 p2) respectively. Using the equivalent circuit given in Fig. 6 to which a constantphase element was added to model the LFD it is possible to separate the contribution of these two processes of the relaxation and the hopping conductivity. A separation and de-convolution of both processes were carried out in order to better analyze and examine the relaxation and the diffusion processes. The relaxation process located at high
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frequency (H.F.) was simulated using only the sub-circuit of the block 1, while the low frequency dispersion (LFD) process located at low frequency (L.F.) using the sub-circuit of the block 2. This separation clearly shows the existence of these two processes [7]. Fig. 7 shows the simulation results obtained from the equivalent electrical circuit for one typical composition 0 mg/l. For the low-frequency dispersion (LFD a linear decreases in this region of frequencies exhibited a slope less than (1). This suggests that the (LFD) could be described by the power low, indicating the dominance of a hopping conduction mechanism (sho) instead of direct current conduction (sdc). The contribution related to the relaxation process and that related to the conductivity appear very distinctly. The blue curve clearly shows a relaxation process while the green curve showed the hopping conductivity and the read curve showed the fit. Thus, this procedure of the separation could confirm again very well the existence of the two processes. The first corresponds to the relaxation at the higher frequency, while the second corresponds to the diffusion. In addition, the residuals (relative errors) were extracted based on the work carried out by P. Buschel et al. [26]. The evolution of the residual E over all frequency range is distributed uniformly around the frequency axis showing no systematic deviation as it is shown in Fig. 7b corresponding to the concentration 0 mg/l. For the concentration of 10 and 25 mg/l, the most part of the spectrum showed that the relative errors lie below an absolute value of 0.85%, except the data points at the high frequency limit due to the small inductive artifacts. Based the above consideration, the equivalent circuit given in Fig. 6 was used to analyze and to fit the evolution of the dielectric properties versus frequency at different concentration of cationic
Fig. 9. (a; b and c): Dielectric strength (a); relaxation time (t) and hopping conductivity (sho) at different concentrations of polymer.
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polymer, as it is shown in Fig. 8a, b and c. The solid line corresponds to the fit with the equivalent circuit given good correlation coefficients R2 (0.97e0.99). It can be concluded that the analysis and the fit of the complex permittivity by the equivalent electric circuit showed a good correlation between the experimental data and the different component of the equivalent circuit. The relaxation times (t), dielectric strength (Dε) and hopping conductivity (sho) of the wastewater with the addition of the cationic polymer concentrations were calculated using the relationship between the electrical extracted from the equivalent circuit. tε ð1p1 Þ ¼ CT11 Dε ¼ ðεs ε∞ Þ ¼ CC10 sho ¼ T2S,e The values of these dielectric and electrical properties are given in Table 1. 3.6. Effect of cationic polymer concentrations on dielectric parameters During the wastewater treatment several mechanisms of particle removal by coagulation (complexations between soluble hydrolyzed forms of metals and organic or colloidal matter, reduction of the double layer, adsorption on flocs, neutralization of charges, trapping of particles in available sites and co-precipitation) are generally the most process that could be occurred during the flocculation process. The electrical circuit parameters at different concentrations of the cationic polymer were reported in histograms as it is shown in Fig. 9a, b and c. The evolution of dielectric strength (Dε) and relaxation times (t) show the existence of two regions in which the latter two parameters show an increase in the first region followed by a decrease in the second region. Moreover, the hopping conductivity (sho) showed also the existence of two regions. However, the hopping conductivity decreases the first region and started an increase in the second region. It is important to note that a net transition was showed at 10 mg/l indication the existence of two regions. The first region corresponds to the range of concentration from 0 to 10 mg/l. and the second corresponds to the concentration range from 10 to 25 mg/l. Evolution of these parameters can be directly related to the flocculation-coagulation process. The increase in relaxation time and the amplitude of the dielectric dispersion in the first concentration range as shown in Fig. 9a, b and c can be attributed to an increase in the amount of charges trapped in the flocs. This could suggest an aggregation of the dispersed particles due to the neutralization process. Since the addition of cationic polymer in wastewater generates available sites of positive charges; these are the most interesting for the neutralization and destabilization of colloids generally negatively charged [27]. In this case, the van der Waals interactions are predominant. This allows aggregation of suspended fines and subsequent flocculation/coagulation. In the second concentration range, the two parameters decrease as a function of the concentration. This could suggest a deaggregation of the particles due to the increase of the repulsion forces. This interpretation is confirmed by the evolution of conductivity [28]. The result showed that the relaxation time and the amplitude of the dielectric dispersion increase when aggregation of the particles occurred in the first region and decreased again during de-aggregation. These parameters could be as an indicator in the flocculation state of dispersed particles [29]. 4. Conclusion In this work, the analysis of the complex permittivity, electrical complex modulus, and the hopping conductivity have been employed to investigate and to monitor the flocculation process
during sludge treatment using cationic polymer. The wastewater can be considered as a heterogeneous material composed of pollutants with different dielectric properties. Two dielectric processes of relaxation and diffusion were identified in the raw wastewater at different concentration of cationic polymer and were able to relate each one of them to a known physical process. The relaxation process was associated with the condensed charge and the low frequency dispersed was associated with the free charge. Results showed that the hopping conductivity; the dielectric strength and the relaxation time extracted from the analysis of the relaxation and diffusion process showed a net transition at (10 mg/l) of cationic polymer concentration between the two regions. The first region indicated the formation and the increase of the size of the aggregations; while the second suggested the destruction of the aggregations. 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Broadhead.Dielectric properties of residual water in amorphous lyophilized mixtures of sugar and drug.J. Phys. D. Appl. Phys.. 36 (2003) 330e335. DOI: https://doi. org/10.1088/0022-3727/36/4/302. [9] L.C. Costa, F. Henry, M.A. Valente, S.K. Mendiratta, A.S. Sombra, Electrical and dielectrical properties of the percolating system polystyrene/polypyrrole particles, Eur. Polym. J. 38 (2002) 1495e1499, https://doi.org/10.1016/S00143057(02)00044-7. [10] W. He.. Z. Xie.. W. Lu and M.H.J. Ma. Comparative analysis on floc growth behaviors during ballasted flocculation by using aluminum sulphate (AS) and polyaluminum chloride (PACl) as coagulants. Sep. Purif. Technol. 213(2019) 176e185. Doi.org/https://doi.org/10.1016/j.seppur.2018.12.043. -Fauvel, H. Van Damme, J.C. Baudez, Improving rheological sludge [11] E. Dieude characterization with electrical measurements, Chem. Eng. Res. Des. 87 (2009) 982e986, https://doi.org/10.1016/j.cherd.2008.10.010. [12] L.P. Teo, M.H. Buraidah, A.F.M. Nor, S.R. Majid, Conductivity and dielectric studies of Li2SnO3, Ionics 18 (2012) 655e665, https://doi.org/10.1007/s11581012-0667-2. [13] Y. Mamunya. A. Kanapitsas. P. Pissis. G. Boiteux. E. Lebedev. Water sorption and electrical/dielectric properties of organic-inorganic polymer blends. Macromol. Symp..198(2003) 449e460. https://doi.org/10.1002/masy.200350838. [14] Mortadi A. El Melouky. A. Chahid. E. Nasrellah. H. Bakasse. M. Zradba. A. Cherkaoui. O and El Moznine. R. Rheological and electrical properties to investigate the coagulation process during sludge treatment (2017). Water Sci. Technol..WST-EM161586. Doi: https://doi.org/10.2166/wst.2017.536. [15] M.E. Belghiti, S. Bouazama, S. Echihi, A. Mahsoune, A. Elmelouky, A. Dafali, K.M. Emran, B. Hammouti, M. Tabyaoui, Understanding the adsorption of newly Benzylidene-aniline derivatives as a corrosion inhibitor for carbon steel in hydrochloric acid solution: experimental. DFT and molecular dynamic simulation studies, Arab. J. Chem. (2017), https://doi.org/10.1016/ j.arabjc.(2017).12.003. [16] M.E. Belghiti, Y. Karzazi, A. Dafali, I.B. Obot, E.E. Ebenso, K.M. Emran, I. Bahadur, B. Hammouti, F. Bentiss, Anti-corrosive properties of 4-amino-3.5bis(disubstituted)-1.2.4-triazole derivatives on mild steel corrosion in 2 M H3PO4 solution: experimental and theoretical studies, J. Mol. Liq. 216 (2016) 874e886, https://doi.org/10.1016/j.molliq.2015.12.093. [17] K. G. Shailendra.. P. Sowjanya and G. Ashish. Impedance spectroscopy on degradation analysis of polymer/fullerene solar cells. Sol. Energy 178(2019) 133e141. https://doi.org/10.1016/j.solener.2018.12.024.
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437 [18] A. Elmelouky.. A. Mortadi.. El. Chahid and R. Elmoznine. Impedance spectroscopy as a tool to monitor the adsorption and removal of nitrate ions from aqueous solution using zinc aluminum chloride anionic clay. Heliyon 4 (2018) e00536. https://doi.org/10.1016/j.heliyon.2018.e00536. [19] C. Roldan-Cruza. J. Carmona-Ascencio. E.J. Vernon-Carter and J. AlvarezRamirez. Electrical impedance spectroscopy for monitoring the gum Arabicechitosan complexation process in bulk solution C. Colloids Surf. A Physicochem. Eng. Asp. 495 (2016) 125e135. Doi.org/10.1016/j.colsurfa.2016. 02.004. [20] S. Hiroyuki, N. Misato, Dielectric relaxation due to interfacial polarization for heat-treated wood, Carbon 42 (2004) 211e221, https://doi.org/10.1016/ j.carbon.2003.10.013. [21] M.S. Phe, S. Geoff, Apercolation cluster model of the temperature dependent dielectric properties of hydrated proteins, J. Phys. D. Appl. Phys. 36 (2003) 336e342, https://doi.org/10.1088/0022-3727/36/4/303. [22] L.A. Dissado, R. Hill, Constant-phase-angle and power-law regimes in the frequency response of a general determinate fractal circuit, Phys. Rev. B 37 (1988) 3434e3439, https://doi.org/10.1103/PhysRevB.37.3434. [23] C.V. Peter, Online Detection of Aggregation Processes Using Dielectric Spectroscopy Uniprint, Aalborg University, 2009. June.
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[24] F. Bordi.. C. Cametti and R. H. Colby. Dielectric spectroscopy and conductivity of polyelectrolyte solutions. J. Phys. Condens. Matter 16 (2004)1423e1463. https://doi.org/10.1088/0953-8984/16/49/R01. [25] R.K. Rowe, J.Q. Shang, Y. Xie, Effect of permeating solutions on complex permittivity of compacted clay, Can. Geotech. J. 39 (2002) 1016e1025, https:// doi.org/10.1139/t02-051. [26] P. Buschel, T. Gunther, O. Kanoun, Distribution of Relaxation Times for Effect Identification and Modeling of Impedance Spectra, 978-1-4673-6386-0/14/ $31.00 ©, IEEE, 2014. [27] M.I. Aguilar J. Saez M. Llorens. A. Soler. J.F. Ortuno. V. Meseguer. A. Fuentes Improvement of coagulationeflocculation process using anionic polyacrylamide as coagulant aid. 58(2004) 47e56. DOI:https://doi.org/10.1016/j. chemosphere.2004.09.008. [28] C.L. Yang, EC and EF process for the traitement of alcohol distillery wastewater, separationand purification technology 53 (2007) 135e140, https:// doi.org/10.1016/j.seppur.2006.08.022. [29] E. Chahid.. R. El moznine.. A. Zradba.. A. Dani.. A. El melouky.. L. Idrissi.. O. Cherkaoui.. E. Choukri.. D. Mezzane and A. Belboukhari. Analysis of relaxation processes and low frequency dispersion in waste water. J. Optoelectron. Adv. Mater.. 15(2013)1209 e 1216.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Flocculation monitoring of wastewater by using impedance spectroscopy A. Mortadi a, *, A. Elmelouky a, M. Chahbi a, N. EL Ghyati a, S. Zaim a, O. Cherkaoui b, R. El Moznine a a b
Laboratory Physics of Condensed Matter (LPMC), University Chouaib Doukkali, El-Jadida, Morocco Higher School of Textile and Clothing Industries, Laboratory REMTEX, Casablanca, Morocco
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2019 Received in revised form 26 July 2019 Accepted 28 July 2019 Available online 29 July 2019
The aim of the present work is to monitor the flocculation process using the analysis of the electric and dielectric properties. Therefore the dielectric and electrical characteristics of wastewater with different cationic polymer concentrations were investigated via the impedance spectroscopy (IS) method. Impedance measurements were carried at different concentration of cationic polymer in the frequency range from 0.1 Hz to 100 kHz. The analysis of complex permittivity spectra was described by the superposition of a power law at a low frequency related to the diffusion process and Cole-Cole relaxation behavior at high frequency. Moreover, an equivalent circuit model was developed in order to analyze the experimental data and to further investigate both processes. The variation of the parameters extracted from the equivalent circuit with the increase of cationic polymer concentrations has shown a net transition at 10 mg/l. This behavior could reflect the flocculation of dispersed particles at 10 mg/l. The findings in this work could draw new attention toward the monitoring of the coagulation-flocculation process using impedance spectroscopy and could be extended to other kinds. © 2019 Elsevier B.V. All rights reserved.
Keywords: Wastewater treatment Cationic polymer Dielectric properties Hopping conductivity Flocculation process
1. Introduction Contamination of water with pollutants of various origins has become a global problem [1]. Wastewater is one of the most important sources of pollution of surface waters and groundwater, especially for agricultural soil. The textile industry is one of the industries the most water-consuming and generates a waste consisting of organic and inorganic molecules, generally presenting color problems, high concentrations of BOD5, COD, suspended solids, high toxicity and high conductivity [2]. The waste of the textile industry represents a great hazard for human health and the environment [2,3].In fact, the various dyes used in the different steps of clothing manufacturing cause problems in the quality of wastewater because of their stability and low biodegradability [4e6]. Thus, it is necessary to treat this waste before it gets discharged into the remediation system. These effluents to be treated contain non-biodegradable substances, inhibitory or toxic for most living microorganisms [7]. Moreover, the heterogeneity of their composition makes it difficult or almost impossible to obtain pollution thresholds lower than or equal to those imposed by environmental standards, after treatment with traditional techniques [8]. * Corresponding author. E-mail address: [email protected] (A. Mortadi). https://doi.org/10.1016/j.saa.2019.117437 1386-1425/© 2019 Elsevier B.V. All rights reserved.
Indeed, in order to improve the liquid-solid phase separation, flocculating agents are commonly used in various studies [8e10]. These flocculating agents react with wastewater components to induce flocculation which expedites the liquid-solid phase separation. The success of wastewater treatment using coagulationflocculation process depends mainly on this last step. Thus, in order to understand the mechanism of this process and to improve liquid-solid separation, many researchers have tried to monitor and explain this process following the addition of the flocculent agent, which is usually a high molecular weight cationic polymer [11,12]. Because coagulation-flocculation processes are usually based on electrostatic interaction and because of the strength of the aggregations formed during this process is believed to play a significant role in liquid-solid separation, therefore impedance spectroscopy has been assumed to be the appropriate method to monitor and to investigate the electrical and dielectric properties during treatment wastewater [13,14]. In our previous work [7], the coagulation process with the addition of the calcium oxide (CaO) was investigated using impedance spectroscopy. It was found that the dielectric properties are a good tool to control and to monitor the sludge coagulation process. This technique is also useful in several areas such as, on the corrosion inhibitors [15,16] solar cells [17] and to monitor the adsorption in the anionic clay [18].
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The main objective of the current work is to investigate the effect and the impact of the cationic polymer on the flocculation process. Impedance measurements were investigated based on the analysis of the complex permittivity and the electrical complex modulus M*(u) to further analyze the nature and the origin of relaxation observed in the evolution of the imaginary part of complex permittivity. 2. Material and methods 2.1. Samples The sample of wastewater was collected from a heterogeneous common effluent treatment plant located in Berrechid, Morocco. This wastewater contains effluents from textile industries. A cationic polymer was used as a flocculent. Seven samples at different concentrations (0, 2.5, 5,10,15, 20 and 25 mg/l) of cationic polymer were prepared and analyzed using impedance measurement. For each concentration, the samples were stirred at 20 C during 3 min with a suitable speed. The mixture was then stirred gently during 20 min in order to avoid contact between particles before the experiment. 2.2. Electrical measurement The electrical measurement was carried out, using (Voltalab 10, PGZ 301, Radiometer Analytical Instruments). The impedance data were measured using three electrodes, a saturated calomel electrode Ag/AgCl/KCl saturated, used as a reference electrode against a stainless steel electrode and a working electrode in glacier atoms. The measurements were carried out at room temperature in the frequency range from 10 mHz to 100 kHz and using AC potential amplitude of 10 mV. The experimental data were analyzed and investigated using the Z.View 2.2 software. 3. Results and discussion 3.1. Analysis of the complex permittivity The frequency dependence of dielectric measurement at different concentration of cationic polymer is shown in Fig. 1a, b. Fig. 1a and b depicts two representations of dielectric measurement using Bode and Nyquist diagrams respectively [19,20]. All compositions exhibited similar behavior. As can be seen from Bode representation (Fig. 1a), the imaginary part showed high values at low frequencies shown a linear decrease when the frequency increases. This behavior could reflect the
diffusion process due to the free charge carriers. In addition, Fig. 1b showed also high values at low frequencies. This behavior could be due to the accumulation of the charge carriers at the sample/ electrode interface forming the charged double layer or/and move from one site to another site causing an increase in the conduction behavior, what is usually called the low frequencies dispersion (LFD). On the other hand, when the frequency increases the imaginary part showed a maximum. It is well known that wastewater can be considered as heterogeneous materials. The complexation between the particles of wastewater (negative charges) and the cationic polymer (positive charges) lead to the formation of double-layer at the interface. This fact suggests that the induced dipole due to the accumulation of negative and positive sites can generate an interfacial polarization. The local and fast movements of the induced dipole lead to a dynamic behavior at high frequency (H.F.) resulting in the relaxation process. This kind of relaxation was revealed by Hiroyuki Sugimoto et al. [20] in their study on dielectric relaxation due to interfacial polarization for heat-treated wood Carbon. This similar behavior was also revealed by Roldan-Cruza et al. [19] to investigate the complication process between the gum Arabic as an anionic polymer and chitosan as cationic polymer. The variation of ε00 (u) at higher frequencies showed a relaxation peak as it is observed in both Bode and Nyquist diagrams. It is important to note that, in the Bode representation (Fig. 1a), the frequency corresponding to the relaxation peak in the imaginary part shifted to the low frequency side when the concentration increased from 0 to 10 mg/l. In contrast, this relaxation peak shifted to the high frequency side when the concentration increased from 10 to 25 mg/l. Therefore, the evolution of dielectric properties shown in Fig. 1 was reported in two graphs in order to better illustrate the effect and the impact of cationic polymer concentration on the dielectric properties as it is shown in Figs. 2aeb and 3aeb. Figs. 2ae3a represent the evolution of dielectric properties corresponding to the concentration range from 0 to 10 mg/l. In contrast, Figs. 2be3b represent the evolution of dielectric properties corresponding to the concentration range from 10 to 25 mg/l. consequently; the transition at 10 mg/l could be seen very clearly indicating the existence of the two regions. The first corresponds to the concentration below 10 mg/l. the second region corresponds to the concentration above 10 mg/l. In addition, this separated in two graphs could provide a significant change in the dielectric strength and relaxation time which has occurred with the increase of the cationic polymer concentration. The analysis of the dielectric permittivity data at different concentrations revealed the existence the relaxation process that
Fig. 1. (a) Bode plot and (b) ColeeCole plot at different cationic polymer concentration.
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
3
Fig. 2. (aeb): Imaginary part vs. frequency at different polymer concentration. (a): Concentration below 10 mg/L. (b): Concentration beyond 10 mg/l.
manifests itself at high frequency (H.F.), wheel the diffusion process that manifests itself at low frequency (L.F.). However, it is wellknown that ‘electrode polarization’ effect, mainly due to the spatial charge that accumulates near the electrodes, represents a serious obstacle in the dielectric and electrical measurement of conductive solutions [21]. Therefore, more attention has been made in the analysis of dielectric properties to further investigate the origin of the relaxation peak observed in the imaginary part of the permittivity. Consequently, an alternative approach was used to overcome this situation which consists of the analysis of the electrical complex modulus M*(u). In our previous work [7], this approach was employed to further analysis and examine of the evolution of both complex modulus M*(u) and permittivity ε*(u) to further analysis the ‘electrode polarization’ effect. 3.2. Analysis complex modulus M * (u) The imaginary part of the complex permittivity as a function of frequency showed a very high value at low frequencies due to the low frequencies dispersion. This suggests that any relaxation process could be masked or hidden in the evolution of the imaginary part of the complex permittivity at low frequency. In this case; the relaxation process could be seen very clearly in the evolution of the imaginary part of the complex modulus M*(u) since this later M*(u) is defined as the inverse of the complex permittivity ε*(u) [8].
M*ðuÞ ¼
00 1 1 ¼ ¼ M 0 ðuÞ þ jM ðuÞ 00 ε*ðuÞ ε0 ðuÞ jε ðuÞ
(1)
versus frequency and the permittivity versus frequency is shown in Fig. 4a, b, c for three typical concentrations 0, 10 and 25 mg/l. The first concentration corresponds to the raw sample 0 mg/l taken from the range 0 to 10 mg/l in the first region. The second corresponds to the critical concentration (10 mg/l) where the transition has occurred. The third concentration was selected from the range of concentration (10-25 mg/l) in the second region. From Fig. 4, it is important to note that the plots of M00 (u) vs. log f do not have any significant relaxation peak over the investigated frequency (102e105 Hz) range. However, the spectra show a narrow peak which is located between 101 and 10 Hz. Since this later narrow peak appeared in the very low-frequency value, it appears difficult to ascribe this peak to bulk motions. Therefore, the relaxation peaks revealed by the M(u) indicated the occurrence of electrode polarization (EP) processes. The electrode polarization (EP) process was shown to be a participating process below 1 Hz due to the accumulated charges between the electrodes/sample [21]. The law magnitude of this peak could indicate that aggregation during the flocculation process. It is therefore suggested that the relaxation peak could reflect the phenomenon occurs inside the aggregation during the coagulation process. This relaxation behavior could be attributed to the localized relaxation process due to the short-range hopping motion of ions and water proton trapped inside the aggregations. From electrical/dielectric measurements and considering different parameters (permittivity and electrical modules) and different representations, we have been able to highlight the existence of two phenomena: relaxation phenomena and low frequency dispersion phenomena (H.F.).
The evolution of the imaginary parts of the electric module
Fig. 3. (aeb): Imaginary part vs. real part at different concentration. (a): Concentration below 10 mg/l. (b): Concentration beyond 10 mg/l.
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Fig. 4. (a, b and c): Bode diagram of M00 and ε00 versus frequency for the selected concentrations 0, 10 and 25 mg/l.
There is a small amount of the free ions which had moved up to the electrode. In contact; the relaxation peaks observed in the evolution of the imaginary part of the complex permittivity was attributed to the phenomenon occurs inside the cluster. 3.3. Origin of low frequency dispersion (LFD) According to Jonscher, an important feature of this lowfrequency dispersion situation is that of dipole systems that will show peaks in the imaginary part of the complex permittivity, while bearer-dominated systems will exhibit a low-frequency dispersion (LFD) response [14,19]. It is well known that the process of coagulation-flocculation by the addition of cationic polymer promotes aggregations of charged particles. These aggregates can be considered as clusters of different sizes under an applied electrical field, the ions in the clusters can move inside the clusters (intra-cluster movement) or jump outside the clusters (inter-cluster movement). Indeed, if the distance traveled by the ions is less than the size of the cluster, these ions remain in the cluster and leading to an intra-cluster movement. If the distance traveled by the ions is greater than the size of the cluster, the displacement of the loads takes place between the clusters and therefore leading to an inter-cluster movement. In this case, there are potentially mobile charges that contribute to the conduction process and also the low-frequency dispersion (LFD) [20].
relaxation process that manifests itself at high frequency (H.F.): This process can be attributed to the relaxation of the condensed or adsorbed ions on the polymer chains forming clusters. Therefore, if the distance traveled by the charges, is less than the size of the cluster; these charges remain in the cluster and we have intracluster movement. In this case, the clusters can be considered as ‘linked dipoles’. The condensation of the ions generates induced dipole moments which are translated by local and fast movements and consequently their dynamic behavior manifested at high frequency (H.F.). This kind of relaxation is known as MaxwelleWagnerseSillars (MWS) relaxation [22]. A schematic is shown in Fig. 5 to illustrate the flocculation process during wastewater treatment showing the aggregates resulting from the condensed of counter-ions and free ions [23,24].
3.4. Origin of the relaxation process As shown before, the detailed analysis of the dielectric permittivity data at different concentrations revealed the existence of a
Fig. 5. Illustration of suspensions in wastewater and resulting aggregates [18].
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
It is therefore suggested that the relaxation peak could reflect the phenomenon that has occurred inside the aggregation during the flocculation process. This relaxation behavior could be attributed to the localized relaxation process due to the short-range hopping motion of ions and water proton trapped inside the aggregations.
3.5. Modelisation of the dielectric responses using equivalent circuit Based on the above analysis of the dielectric responses of the wastewater samples at different concentrations of the cationic polymer showed: - The presence of a peak between 300 Hz and 105 Hz in the spectrum of the complex permittivity ε*(u) indicates the appearance of the process of high frequency relaxation (H.F.). - Low frequency dispersion process (LFD), this process has appeared in spectra of complex permittivity with the simultaneous increase of the real and imaginary part. - The presence of a peak between 0.1 Hz to 200 Hz with low amplitude in the evolution of the imaginary part of complex modulus M*(u) indicating the onset of the electrode polarization process (E.P.). Since the magnitude of this later relaxation related to the electrode polarization (E.P.) process was very small as it is shown in Fig. 4aec; the contribution of the relaxation corresponding to the EP was neglected. The use of the Cole-Cole relaxation function is a useful model to describe MaxwelleWagnerseSillars (MWS) relaxation process in the complex permittivity ε*(u) [25].
ε* ðuÞ ¼ ε∞ þ
Dε 1 þ ðjutε Þa
(2)
u: is the pulsation of the electric field (rad$s1), Dε ¼ εs ε∞: is the dielectric strength, t: is the relaxation time (s), a: is the parameter associated with the distribution of the relaxation time. The low frequency dispersion (LFD) response could be described very well by a power law.
εLFD *ðuÞ ¼
sho
(3)
ε0 ðiuÞb
b: is the exponent parameter, 0 < b 1, and sho is the hopping conductivity. If b ¼ 1, the complex permittivity becomes: ε*ðuÞLFD ¼
sho ε0 ðiuÞ1
In this case, the real and the imaginary parts of the complex permittivity are: 00
ε0 ðuÞ ¼ 0 and ε ðuÞ ¼
sho
ε0 ðuÞ
:
The hopping conductivity becomes similar to direct current conductivity (sho ¼ sdc).
5
Therefore, the evolution of the complex permittivity could be described by the combination of the two expressions given in Eqs. (2) and (3):
ε* ðuÞ ¼ ε∞ þ
Dε sho þ 1 þ ðjutε Þa ε0 ðjuÞb
(4)
An equivalent circuit model was built in order to develop and to find the analytical expression given in Eq. (4) is shown in Fig. 6. This circuit contains two blocks in parallel to describe both two processes: - Block 1: Describe the behavior of the Cole-Cole relaxation at high frequency (H.F.) - Block 2: Describe the behavior of low frequency dispersion (LFD) at low frequency (L.F.). The impedance of the constant phase element (CPE) is given by the following equation:
ZCPE ðuÞ ¼
1 TðjuÞp
where T is the pseudo capacitance and p is the exponent between 0 and 1. For p ¼ 1, the impedance is purely capacitive. For p ¼ 0, the impedance is purely resistive. The complex impedance of the capacitance (C) is given by Z * ðuÞ ¼ jC11 u. The complex permittivity (ε*) can be obtained from the complex admittance (y*) using the following relationship [7]:
Y * ðuÞ ¼ juC0 ε* ðuÞ
(5)
The global complex admittance is given by the summation of the admittance of both sub-circuit (1) and 2: Y*(u) ¼ Y*1(u) þ Y*2(u). The sub-circuit (1) contains the capacitance (C1) in series with the constant phase element (CPE1) to parallel to the capacitance (C2). Therefore, the complex admittance of the sub-circuit (1) can be writing as:
Y * 1 ðuÞ ¼ juC2 þ
¼ juC2 þ
juC1 *T1 ðjuÞp1 juC1 ¼ juC2 þ juC1 þ T1 ðjuÞp1 C1 1 þ T1 ðjuÞ1p1 juC1 1 þ ðjutε Þð1p1 Þ
where tε ð1p1 Þ ¼ CT11 . On the other hand, the complex admittance of the sub-circuit (2) is: Y * 2 ðuÞ ¼ T2 ðjuÞp2 . The complex admittance of the equivalent circuit can be writing as follow:
Y * ðuÞ ¼ juC2 þ
juC1 1 þ ðjutε Þð1p1 Þ
þ T2 ðjuÞp2
where: C2, C1 are the high and low-frequency capacitances respectively. The capacitances C1 and C2 can be writing as a function of the dielectric constant at high and low-frequency and the empty capacitances C0.
S C1 ¼ ðεs ε∞ ÞC0 ; C2 ¼ ε∞ C0 ; and C0 ¼ ε0 e
Fig. 6. Equivalent circuit to describe the relaxation and hopping conduction processes.
where ε0 ¼ 8.85 1012 F/m1 is the vacuum, S is the surface of electrode and e is the distance between the electrode.
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Fig. 7. (a) Imaginary part versus frequency for the raw sample. De-convolution of the two process by simulation: conduction by LFD (blue color), relaxation (red color). (b) Residual error.
Consequently, the global admittance can be written as follow:
"
" *
Y ðuÞ ¼ juC0
εs ε∞
T þ 2 ðjuÞp2 1 ε∞ þ ð1p1 Þ C 0 1 þ ðjutε Þ
#
Therefore, the expression of the global complex permittivity can be obtained according to the relation given in Eq. (5).
εs ε∞
T ε ðuÞ ¼ ε∞ þ þ 2 ðjuÞp2 1 ð1p1 Þ C 0 1 þ ðjutε Þ *
" ε* ðuÞ ¼ ε∞ þ
εs ε∞ 1 þ ðjutε Þð1p1
þ Þ
#
T2 ,e ðjuÞp2 1 ε0 S
Fig. 8. (a, b and c): Bode diagram of complex permittivity versus frequency for selected concentrations 0; 10 and 25 mg/l.
#
A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
Table 1 Dielectric and electrical parameters of electrical circuit: Concentrations Dielectric strength (Dε106) 0 2.5 5 10 15 20 25
Relaxation times t (ms)
0.18 0.52 0.88 1.18 1.04 0.88 0.85
Hopping conductivity sho (ms/cm)
0.14 0.47 0.74 0.92 0.80 0.58 0.45
1.45 1.10 0.90 0.80 1.01 1.07 1.09
The values of bold correspond to the values of deduced parameters from the electrical.
" ε* ðuÞ ¼ ε∞ þ
εs ε∞ 1 þ ðjutε Þð1p1
þ Þ
sho ε0 ðjuÞ1p2
#
where tε ð1p1 Þ ¼ CT11 Dε ¼ ðεs ε∞ Þ ¼ CC10 sho ¼ T2S,e. As it can be seen, the expression of the complex permittivity obtained from the equivalent electrical circuit is similar to the analytical expression given in Eq. (4), if the exponent a and b are attributed to the (1 p1) and (1 p2) respectively. Using the equivalent circuit given in Fig. 6 to which a constantphase element was added to model the LFD it is possible to separate the contribution of these two processes of the relaxation and the hopping conductivity. A separation and de-convolution of both processes were carried out in order to better analyze and examine the relaxation and the diffusion processes. The relaxation process located at high
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frequency (H.F.) was simulated using only the sub-circuit of the block 1, while the low frequency dispersion (LFD) process located at low frequency (L.F.) using the sub-circuit of the block 2. This separation clearly shows the existence of these two processes [7]. Fig. 7 shows the simulation results obtained from the equivalent electrical circuit for one typical composition 0 mg/l. For the low-frequency dispersion (LFD a linear decreases in this region of frequencies exhibited a slope less than (1). This suggests that the (LFD) could be described by the power low, indicating the dominance of a hopping conduction mechanism (sho) instead of direct current conduction (sdc). The contribution related to the relaxation process and that related to the conductivity appear very distinctly. The blue curve clearly shows a relaxation process while the green curve showed the hopping conductivity and the read curve showed the fit. Thus, this procedure of the separation could confirm again very well the existence of the two processes. The first corresponds to the relaxation at the higher frequency, while the second corresponds to the diffusion. In addition, the residuals (relative errors) were extracted based on the work carried out by P. Buschel et al. [26]. The evolution of the residual E over all frequency range is distributed uniformly around the frequency axis showing no systematic deviation as it is shown in Fig. 7b corresponding to the concentration 0 mg/l. For the concentration of 10 and 25 mg/l, the most part of the spectrum showed that the relative errors lie below an absolute value of 0.85%, except the data points at the high frequency limit due to the small inductive artifacts. Based the above consideration, the equivalent circuit given in Fig. 6 was used to analyze and to fit the evolution of the dielectric properties versus frequency at different concentration of cationic
Fig. 9. (a; b and c): Dielectric strength (a); relaxation time (t) and hopping conductivity (sho) at different concentrations of polymer.
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A. Mortadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117437
polymer, as it is shown in Fig. 8a, b and c. The solid line corresponds to the fit with the equivalent circuit given good correlation coefficients R2 (0.97e0.99). It can be concluded that the analysis and the fit of the complex permittivity by the equivalent electric circuit showed a good correlation between the experimental data and the different component of the equivalent circuit. The relaxation times (t), dielectric strength (Dε) and hopping conductivity (sho) of the wastewater with the addition of the cationic polymer concentrations were calculated using the relationship between the electrical extracted from the equivalent circuit. tε ð1p1 Þ ¼ CT11 Dε ¼ ðεs ε∞ Þ ¼ CC10 sho ¼ T2S,e The values of these dielectric and electrical properties are given in Table 1. 3.6. Effect of cationic polymer concentrations on dielectric parameters During the wastewater treatment several mechanisms of particle removal by coagulation (complexations between soluble hydrolyzed forms of metals and organic or colloidal matter, reduction of the double layer, adsorption on flocs, neutralization of charges, trapping of particles in available sites and co-precipitation) are generally the most process that could be occurred during the flocculation process. The electrical circuit parameters at different concentrations of the cationic polymer were reported in histograms as it is shown in Fig. 9a, b and c. The evolution of dielectric strength (Dε) and relaxation times (t) show the existence of two regions in which the latter two parameters show an increase in the first region followed by a decrease in the second region. Moreover, the hopping conductivity (sho) showed also the existence of two regions. However, the hopping conductivity decreases the first region and started an increase in the second region. It is important to note that a net transition was showed at 10 mg/l indication the existence of two regions. The first region corresponds to the range of concentration from 0 to 10 mg/l. and the second corresponds to the concentration range from 10 to 25 mg/l. Evolution of these parameters can be directly related to the flocculation-coagulation process. The increase in relaxation time and the amplitude of the dielectric dispersion in the first concentration range as shown in Fig. 9a, b and c can be attributed to an increase in the amount of charges trapped in the flocs. This could suggest an aggregation of the dispersed particles due to the neutralization process. Since the addition of cationic polymer in wastewater generates available sites of positive charges; these are the most interesting for the neutralization and destabilization of colloids generally negatively charged [27]. In this case, the van der Waals interactions are predominant. This allows aggregation of suspended fines and subsequent flocculation/coagulation. In the second concentration range, the two parameters decrease as a function of the concentration. This could suggest a deaggregation of the particles due to the increase of the repulsion forces. This interpretation is confirmed by the evolution of conductivity [28]. The result showed that the relaxation time and the amplitude of the dielectric dispersion increase when aggregation of the particles occurred in the first region and decreased again during de-aggregation. These parameters could be as an indicator in the flocculation state of dispersed particles [29]. 4. Conclusion In this work, the analysis of the complex permittivity, electrical complex modulus, and the hopping conductivity have been employed to investigate and to monitor the flocculation process
during sludge treatment using cationic polymer. The wastewater can be considered as a heterogeneous material composed of pollutants with different dielectric properties. Two dielectric processes of relaxation and diffusion were identified in the raw wastewater at different concentration of cationic polymer and were able to relate each one of them to a known physical process. The relaxation process was associated with the condensed charge and the low frequency dispersed was associated with the free charge. Results showed that the hopping conductivity; the dielectric strength and the relaxation time extracted from the analysis of the relaxation and diffusion process showed a net transition at (10 mg/l) of cationic polymer concentration between the two regions. The first region indicated the formation and the increase of the size of the aggregations; while the second suggested the destruction of the aggregations. 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