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Enhancement of flocculation and shear resistivity of bentonite suspension using a hybrid system of organic coagulants and anionic polyelectrolytes
Separation and Purification Technology 237 (2020) 116462
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Enhancement of flocculation and shear resistivity of bentonite suspension using a hybrid system of organic coagulants and anionic polyelectrolytes
T
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Mohamed Shamlooha, Abrar Rimehb, Mustafa S. Nassera, , Mohammad A. Al-Ghoutib, Muftah H. El-Naasa, Hazim Qiblaweyc a
Gas Processing Center, College of Engineering, Qatar University, Doha, Qatar Department of Biological and Environmental Sciences, College of Arts and Sciences, Qatar University, Doha, Qatar c Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b
A R T I C LE I N FO
A B S T R A C T
Keywords: Bentonite Colloidal suspension Coagulation Flocculation Hybrid Rheology
In this study, the influence of the hybrid coagulation-flocculation system on the flocculation and shear resistivity of bentonite suspension has been investigated. Two short-chained coagulants, polyamine and polyDADMAC, accompanied with low and high Molecular Weight (Mw) long-chained anionic Polyacrylamide (PAM) flocculants have been used to enhance the size and mechanical properties of the produced flocs. Four characterization techniques were used as evaluation criteria of degree of flocculation including turbidity, zeta potential (ζ), floc size analysis and rheology. Optimum dosages between 5 and 10 mg/L were obtained for flocculants and between 20 and 30 mg/L for coagulants in hybrid systems. Compared to single systems, hybrid systems performed exceptionally well. Removal efficiency of 99% was achieved using hybrid systems producing water with turbidity as low as 1.05 NTU. Although differences were insignificant in the treatment quality, significant improvement was achieved in floc size and rheological behavior. Hybrid systems were able to produce large flocs of more than 400 µm, five times bigger than flocs from the single cationic coagulant systems. Most importantly, rheological testing of produced slurries revealed the remarkable advancement achieved by hybrid systems. Hybrid systems were able to increase the elastic modulus (G′) by more than ten times resulting from the compacted flocs present in the system. Furthermore, resistivity toward shear and oscillations were improved. Ultimately, hybrid systems have surpassed the single cationic systems by achieving slightly better treatment besides producing larger, denser and more resistive flocs. Generally, polyDADMAC performed better than polyamine in hybrid systems. The combination of polyDADMAC with high Mw PAM gave the best separation as well as the largest flocs, while the combination with low Mw PAM resulted in the best rheological properties.
1. Introduction
Bentonite is a clay that has a platelets-like structure of alternating tetrahedral and octahedral sheets carrying an overall negative charge [9]. The amphoteric nature of bentonite makes it form a stable colloidal system when dispersed in water because of its electrostatic and physical properties [10]. Two factors affect the stability of clay particles in water, mainly particle size and charge density, where the size of bentonite particles (mean size of < 10 µm) and the negative charge (Zeta Potential of < −35 mV) keep it suspended with the help of Brownian motion [11]. Several treatment techniques have been reported in the literature for bentonite separation such as electrocoagulation, crossflow membranes, electro-osmosis and thermal-mechanical dewatering which all are targeting either the charge, particle size or both [12–15]. coagulation-flocculation is the most common process to separate the colloidal suspensions as minimum energy is needed to be operated [16].
Clay minerals have attracted increasing industrial attention in the past few years leading to the accumulation of such material in wastewater produced from different industries [1]. Bentonite has been widely adopted in the field of engineering such as in drilling activities [2], landfill application [3], handling of radioactive wastes [4], soil treatment [5] as well as in other fields such as pharmacological [6] and papermaking [7] industries. Bentonite has also been recently introduced to new developing technologies such as nanomaterials and fine chemicals [8]. The relatively low, price as well as the physical properties of bentonite, gave it popularity to have this wide variety of applications leading to the generation of considerable amounts of claycontaminated wastewater.
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Corresponding author. E-mail address: [email protected] (M.S. Nasser).
https://doi.org/10.1016/j.seppur.2019.116462 Received 28 August 2019; Received in revised form 22 November 2019; Accepted 20 December 2019 Available online 23 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 237 (2020) 116462
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Table 1 Properties of used coagulants and flocculants. Commercial name
Polymer name (type)
Molecular weight (g/mol)
Charge density (%)
AN 956 (F1)
Polyacrylamide (anionic)
6–10 million
50
AN 956 SH (F2)
Polyacrylamide (anionic)
greater than 15 million
50
FL 2949 (C1)
Polyamine (cationic)
60,000
Very high
FL 4440 (C2)
PolyDADMAC (cationic)
100,000
Very high
Structure
investigation on kaolinite flocculation using anionic PAM that the presence of divalent cations (Ca2+ and Mg2+) provides a better flocs stability with higher density. The presence of cations was explained to have a double effect of polymer and particle binding leads to the production of denser flocs. In spite of, Witham et al. [34] observed that under continuous shear flocculation process, the effect of divalent cations tend to be different as the developed interactions fail to withstand the continuous shear. In the current study, one way to overcome this problem is addressed by having a hybrid system of short-chained cationic organic coagulant and long-chained anionic flocculant. The proposed system is to produce multiple alternating layers of positively and negatively charged polymers on the charged surface of clay particles. The existence of a cationic coagulant will not only reduce the optimum dosage of anionic PAM but will also help in producing larger and denser flocs. Hybrid systems for coagulation-flocculation processes are mildly studied in the literature [35,36]. However, no study to date has examined the rheological behavior of such systems. This study will evaluate four types of hybrid systems, including two types of coagulants, polyamine and polyDADMAC and two different molecular weight (MW) of anionic PAM flocculant. Both polyamine, and polyDADMAC were widely investigated individually; both coagulants achieved high separation performance [37,38]. In the case of anionic PAMs, the MW PAM was found to have a minor effect [39]; however, this may not be the case for hybrid systems. The objectives of this study are: (i) asses the separation performance of the proposed hybrid systems using turbidity (NTU) and ζ (mV) measurements, (ii) evaluate the size of the produced flocs resulting from applying hybrid systems; and (iii) investigate the rheological behavior of the dense phase resulting from hybrid and single systems, and compare the performance of hybrid systems to single systems.
This field is maturing, with a wealth of well-understood methods and procedures. Coagulation aims primarily to destabilize a colloidal system by inducing flocs formation, where charge neutralization is usually reported as the mechanism to provide such effect. Flocculation enhances the separation process then by aggregation resulting from polymer bridging making the flocs bigger and settle faster. Several researchers have reported the presence of a strong affinity between the negatively charged surface of bentonite and polymers with different functional groups. This came in alignment with the success of organic polymers to achieve high degrees of separation, not only for clay particles but for other wastes such as activated sludge [17]. Properties of the investigated clay suspension such as pH [18], morphology [19], particle size [20], salinity [21] and zeta potential [22] affect highly the performance of a coagulation-flocculation process. At the same time, the choice of flocculant has also a major influence on factors including the type of polymer [1], iconicity [23], charge density [24] and molecular weight (Mw) [25]. This broadness gave researchers plenty of room to study the different factors. Many polymers such as polyacrylamide (PAM), dextran, chitosan, guar, and lignin have been reported to achieve high treatment performances under a variety of conditions [20,21,25–27]. Nevertheless, the spotlight was usually directed toward the quality of treated water while the rigidity of the produced flocs was rarely considered. Practically, the presence of loose flocs in a continuous flow system may result in the resuspension of some particles and poor performance of the overall process. Several studies reported the shear sensitivity of flocs produced by polymer bridging [28,29]. This implies that purification performance and size of produced flocs are not the only parameters to consider in evaluating a coagulation-flocculation system but also rheological properties of produced sludge matters. Moreover, Lee et al. [16] in their review about flocculation processes have concluded that more research should be further conducted to meet the actual needs of wastewater industries where the rigidity of the produced flocs plays a major role. Although anionic PAM develops a repulsive electrostatic force with the negatively charged clay surfaces, it was remarked that the adsorption tendency of PAM into clays surpasses this effect and was able to flocculate particles [30,31]. Nasser and James [32] elaborated that anionic flocculated clay particles are loose as they failed to have high yield stress, compared to cationic polymers. This effect was found to decrease under saline conditions, Lee et al. [33] explained in their
2. Materials and methods All flocculants and coagulants were supplied by SNF Floerger, France, and are summarized in Table 1. F1 and F2 refer to the flocculants of low and high Mw, respectively. C1 and C2 refer to coagulants, polyamine, and polyDADMAC, respectively. Bentonite was purchased from Sigma-Aldrich Ltd, Germany. All chemicals were used as received without further modification. All experiments conducted in this study, the experiments run in duplicates with an average error of less than 5%. 2
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Fig. 1. Turbidity and zeta potential of treated water at different dosages of flocculants.
2.1. Jar test
2.4. Rheological testing
Coagulants and flocculants were prepared a day prior to conducting experiments, left on stirring throughout the day to ensure the homogeneity. Bentonite suspension was prepared at a concentration of 1.5 g/ L by adding a calculated amount of bentonite to water in a high-shear homogenizer. Beakers were then placed in the jar test (Stuart Flocculator SW6, UK); a predetermined amount of coagulant is added during rapid mixing with a rate of 180 rpm (G = 350 s−1) for 2 min. Followed by 2 min of another rapid mixing to add the flocculant. Afterward, slow mixing with a rate of 50 rpm (G = 52 s−1) for 20 min is allowed to mix and flocs to form. Systems are then left to settle for 5 min, a sample of the treated water is then withdrawn for turbidity (NTU) and ζ (mV) measurements. In this study, individual systems using only the two coagulants and the two flocculants were first studied to determine the optimum dosages as well as to be used as a benchmark to assess hybrid systems. Then all possible combinations of coagulants with flocculants were studied resulting in four hybrid systems and eight systems in total (C1, C2, F1, F2, C1&F1, C1&F2, C2&F1, and C2&F2). In hybrid systems, the optimum dose of coagulant was fixed from the single system study, while the amount of flocculant was varied.
Assessing the flowability and viscoelastic properties of the dense phase resulting from the treated water were done through rheology. Bentonite with a concentration of 6 g/L was prepared for these tests, optimum doses from the first part were used and regular jar test procedure was followed. A settling time of 15 min was allowed, treated water was then removed carefully ensuring to keep the flocs unaffected. Anton Paar MCR 302 Rheometer with cup and bob geometry was used to perform the tests. Two tests were conducted for each of the eight systems at their optimum dosages. Frequency sweep was done in the range between 0.1 and 500 rad/s at a constant 0.3% shear strain. Moreover, shear strain was performed in the range between 0.01 and 1000 1/s. An algorithm from RheoPlus software was used to fit viscosity data on Bingham model. All tests were performed at a controlled temperature of 25 ± 0.1 °C. 3. Results and discussion 3.1. Dosage optimization The optimum dosage of a flocculant or a coagulant reflects the amount that achieves the maximum treatment with minimum amount of chemicals. This ensures the best performance with the lowest possible cost. Ideally, a turbid system tends to decrease in turbidity upon addition of coagulant dosages at the beginning, until the optimum dosage is reached. Further addition of more coagulants may reverse the effect and re-increase the turbidity due to the presence of an excessive amount of charged coagulants. Thus, the optimum dose must be determined precisely. Theoretically, optimum dose exists at the point where the charge in the system is neutralized, since repulsion forces are minimum, consequently, minimum turbidity is usually associated with a zeta potential value of near zero. However, this is not always the case as charge neutralization is not the only mechanism participating in producing flocs. Other mechanisms are reported in the literature such as polymer bridging and sweep flocculation [40,41].
2.2. Turbidity and zeta potential Optimum dosages were obtained based on the results from turbidity and zeta potential. Ideally, the minimum amount of chemical that will lead to maximum flocculation is the point where a crossover will take place in zeta potential (ζ-potential = 0 mV). Hach 2100 N turbidimeter was used for turbidity measurements and Zetasizer ZEN3600 (Malvern Instruments Ltd., UK) for zeta potential tests. Both devices were operated at ambient conditions. Samples were filtered through 0.45 µm syringe filters before conducting zeta potential tests.
2.3. Floc size analysis Knowledge of floc size in a coagulated/flocculated system is essential to predict the settling behavior. To prepare for such tests, bentonite suspension with a concentration of 3.5 mg/L was prepared. The standard jar test procedure was followed using optimum sizes obtained from the first part. Measurements were conducted in Mastersizer 3000 (Malvern Instruments) at room temperature.
3.1.1. Single anionic flocculant systems Although both bentonite and anionic PAM carry negative charge where repulsion static force develops. The adsorption tendency of PAM on bentonite surface through the PAM’s amide group can exceed that effect producing big flocs. Fig. 1 shows the behavior of bentonite suspension on the occasion of adding different dosages of anionic PAM. 3
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Fig. 2. Turbidity and zeta potential of treated water at different dosages of coagulants.
anionic PAM leads to a high increase in turbidity due to the high density of repulsive forces. Fig. 4 displays the removal performance of all investigated systems. All systems, except for single anionic flocculants, succeeded to provide high turbidity removals. The combination of PolyDADMAC and high Mw PAM has performance reaching a removal percentage of around 99% similar to single polyDADMAC. Although differences are insignificant here, major differences are to be observed in floc size and rheological behavior.
Both high and low Mw PAMs were able to decrease the turbidity at small dosages which indicates the superiority of bentonite-PAM bonds in that region, similar behavior was observed by Nasser and James [39]. Further addition of anionic PAM caused the turbidity to increase again as a result of excessive repulsive forces between the negatively charged particles and polymer chains. Zeta Potential was almost steady at the negative region flocculating between −40 and −30 mV which verifies the poor attempt to destabilize the colloidal system using anionic PAM. Overall, anionic PAM alone failed to treat the turbid water significantly but was able to produce some flocs.
3.2. Floc size distribution 3.1.2. Single cationic coagulant systems Polyamine and polyDADMAC are well-known coagulants that are used commercially. Determining the optimum dosage for these coagulants is the first step toward establishing a hybrid system that produces shear-resistive flocs. Both polymers achieved good treatment performances (Fig. 2), minimum turbidity of 1.98 and 1.56 NTU at concentrations of 20 and 30 mg/L were achieved by polyamine and polyDADMAC, respectively. Predictably, further addition of coagulant beyond optimum concentration raises the turbidity due to the emergence of repulsive forces between the positive charges on the polymer in the crowded system. Zeta potential has verified this behavior as it rose from −38 mV before polymer addition to more than 20 mV at a concentration of 50 mg/L for both systems.
The first step toward observing a substantial difference between single cationic coagulant systems and hybrid systems is through measuring sizes of the flocs produced after the solution is allowed to settle. While anionic flocculants showed weak treatment performance with a maximum of 27% turbidity removal were achieved (Fig. 5), they succeeded in producing bigger flocs as high Mw PAM has produced the largest floc size among the single systems. However, hybrid systems have combined the advantages of the two by achieving high turbidity removal and producing even bigger flocs. Fig. 5 demonstrates the size distribution of all investigated systems; untreated bentonite is symbolled with black crosses, single systems with squares and hybrid systems with circles. A shift toward the right signifying bigger flocs is clear for hybrid systems. Number (Dn) and weight (Dw) average of floc sizes are represented in Fig. 6; hybrid systems prospered to increase floc sizes to more than 500%. Largest flocs were achieved by (C2 + F2) system with a number average of 286.7 µm and a weighted average of 411.0 µm. Thus, besides achieving the best performance in turbidity removal, the same system performed the best in producing the largest flocs.
3.1.3. Hybrid cationic-anionic systems Hybrid systems consisting of cationic coagulant and anionic flocculant are the main objective of this research. Thus, the first step is testing their ability to treat bentonite turbid water. Optimum dosages of cationic coagulants were fixed at values retrieved from Fig. 2, while the concentration of anionic flocculants was varied. Fig. 3 provides that all four combinations between the two studied coagulants and two flocculants were able to achieve high removal percentages. While the first flocculant resulted in slight increase in the turbidity with an optimum of 3.30 and 2.78 NTU in combination with C1 and C2 respectively (Fig. 3a), the second flocculant provided a better performance than single systems reaching as low as 1.40 and 1.05 NTU for systems with C1 and C2 respectively (Fig. 3b). Although zeta potential values have increased significantly from −38 mV to around −20 mV, it did not cross the zero in all systems indicating a negative net charge in the treated systems. While zeta potential readings reflect the charge of the outer surface, it is expected to achieve negative values proposing that the outer surface is covered with anionic polymer. Excessive addition of
3.3. Rheological behavior of dense phase Although the ratio between number average and weight average sizes can give an idea about the rigidity of produced flocs, these tests cannot predict precisely how loose the produced flocs are. Thus, testing the rheological behavior of the flocs is essential to predict the shear resistivity of the flocs. Fig. 7 shows the non-Newtonian behavior of the coagulated/flocculated bentonite dispersion. according to this figure, for all tested samples, viscosity is decreasing as the shear rate increases. Shear stress was correlated to shear rate using and Bingham plastic model (Eqn. (1)) 4
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Fig. 3. Turbidity and zeta potential of treated water in hybrid systems by fixing coagulants concentration and varying (a) polyamine (b) PolytDADMAC concentrations.
Fig. 4. Turbidity removal percentages at optimum dosages. 5
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Fig. 5. Comparison of size distribution for flocs produced in single and hybrid systems.
Significant differences in yield stress values between hybrid and single system, which is another indicator of the increase in flocs density provided by hybrid systems. In addition to shear flow behavior, the oscillatory measurement is an approach used to measure the strength of the flocculated dispersion through the elastic properties described by the elastic modulus (G′, ratio of elastic stress over strain) and viscous modulus (G″, ratio of viscous stress over strain) are corresponding to the amount of energy stored and dissipated during deformation. These results of oscillatory tests displayed in Fig. 9; elastic modulus (G′) exceeds the viscous modulus (G″) in all cases excluding slurries produced from the anionic PAM. The greater value of G′ compare to G″ proves the viscoelastic solid-like behavior of flocculated bentonite and indicates a high structural gel-like strength of the produced flocs. Comparing hybrid and single systems, Values of elastic modulus (G′) are more by at least one order of magnitude in hybrid systems. Equally important, flocs from the hybrid system are found to be more resistive
model.
τ = τo + kγ̇
(1)
where τo and k are known as the Bingham yield stress (Pa) and Bingham plastic viscosity (Pa s), respectively. Results from Fig. 7 reveals that all slurries, except for F1 and F2, can be attributed as plastic fluids. A plastic fluid refers to a material that behaves as a solid in low shear where the elastic forces are stronger, while liquid-like behavior is exhibited at high shear giving that viscous forces are surpassing. Observations from Fig. 7 divulge the differences between the dense phases resulted from hybrid systems (circles) and single systems (squares). Very high viscosities are detected at low shear for hybrid systems designating that flocs are more compacted compared to them in single systems. Flocs from single anionic PAM showed a liquid-like behavior even in very low shear reflecting that the flocs are very loose and sensitive. Viscosity data from Fig. 7 are fitted using Bingham plastic model; calculated parameters are illustrated in Fig. 8.
Fig. 6. Number and weight average of floc size at optimum dosages. 6
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Fig. 7. Viscoelastic behavior of slurries under varying shear rate.
and produce bigger flocs [Section 3.2] that are stronger and more resistive toward shearing and oscillations [Section 3.3]. Fig. 10 illustrates schematically the working mechanism in treating suspended bentonite particles. A cationic coagulant does the major role of agglomerating and collecting bentonite particles producing micro-flocs held through electrostatic force as well as Van der Walls attraction forces. Anionic PAM then attracts and gather the micro-flocs producing larger flocs (polymer bridging); the electrostatic force between the negative charges on PAM and positive charges on coagulant provides stability to the flocs making it more shear resistive. This mechanism is verified by zeta potential results from water treated with hybrid systems (Fig. 3). The values remained negative regardless of the treatment percentage; which reflects the presence of negative charges at the outer surface of floating components pointing toward the anionic PAM. Moreover, Fig. 10 explains the notable increase in the size of flocs where the long anionic electrolytes gathers the micro-flocs formed from the first step. Lastly, the strong affinity between the cationic coagulants and the anionic polyelectrolytes in the outer layer provides more stability to the flocs; making them more resistive toward any external force. Similar behavior of treatability
compared to them in single systems. Fig. 9a reveals that values of storage modulus in the hybrid system are steady under oscillations with frequencies higher than 100 rad/s, while flocs from single cationic coagulants break at frequencies of less than 40 rad/s. All hybrid systems behaved in the same manner under varied shear or frequency, nonetheless, polyDADMAC with low Mw PAM (C2 + F1) exhibited the most solid-like behavior (Fig. 9a) suggesting that it has the most compacted flocs. Again, flocs from single anionic PAM breaks with very low oscillations of around 5 rad/s indicating the weak unresistant flocs. Additionally, Fig. 9b shows that the liquid-like behavior of all systems is the same at high frequencies. 3.4. Single systems vs hybrid systems While single coagulants are well known to provide good separation for clays and other suspended matters, hybrid systems have provided better characteristics that enhance the performance and ensures the smoothness of the overall process. Hybrid systems proved to have positively exceeded all tested properties in this research compared to single systems. They provide good purification quality [Section 3.1]
Fig. 8. Bingham parameters for viscoelastic behavior in slurries. 7
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Fig. 9. Rheological behavior of produced slurries under varying frequencies (a) Storage Modulus (b) Loss Modulus.
enhancement was reported in the presence of inorganic cations with anionic PAM [30,33]. While, William et al. [34] proved the instability of flocs from such systems, the hybrid system on organic anionic-cationic polymers proved to produce stable rigid flocs.
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4. Conclusions
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This paper investigated the potential of applying a hybrid system of anionic PAM and cationic coagulant to enhance the process efficiency in removing suspended clay particles. Conventionally used cationic coagulants, polyamine, and polyDADMAC were studied as part of hybrid systems as well as single systems to benchmark and assess the performance of the proposed hybrid systems. The main conclusions from this investigation are as follow:
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that flocs from hybrid systems are more stable, resistive and compacted compared to the single systems with yield stresses of ten times more. All slurries from hybrid systems behaved as plastic fluids, reflecting that transferring and pumping such solutions with high shear rates will not be a problem. Among all investigated systems, polyDADMAC combined with High Mw PAM provided the best treatment efficiency with the biggest flocs, while flocs from polyDADMAC with low Mw PAM produced the most compacted flocs. Hybrid systems work in two steps: cationic coagulants form microflocs then anionic flocculants attract and gather micro-flocs together, essentially, providing better treatment efficiency with bigger and denser flocs.
• Hybrid systems succeeded in significantly increasing the size of flocs
Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
produced with achieving the same degree of separation as single systems. Flocs from hybrid systems were more than five times bigger compared to those from single cationic systems. Rheological testing of studied systems at optimum dosages revealed 8
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Fig. 10. Schematic representation of the coagulation-flocculation process in hybrid systems.
Acknowledgment
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The authors would like to acknowledge the support from SNF Floerger, France by providing coagulants and flocculants samples. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116462. References [1] S.M.R. Shaikh, M.S. Nasser, I. Hussein, A. Benamor, S.A. Onaizi, H. Qiblawey, Influence of polyelectrolytes and other polymer complexes on the flocculation and rheological behaviors of clay minerals: a comprehensive review, Sep. Purif. Technol. 187 (2017) 137–161, https://doi.org/10.1016/j.seppur.2017.06.050. [2] M. Magzoub, M. Mahmoud, M. Nasser, I. Hussein, S. Elkatatny, A. Sultan, Thermochemical upgrading of calcium bentonite for drilling fluid applications, J. Energy Resour. Technol. 141 (2018) 042902, , https://doi.org/10.1115/1. 4041843. [3] Q. Tang, T. Katsumi, T. Inui, Z. Li, Membrane behavior of bentonite-amended compacted clay, Soils Found. 54 (2014) 329–344, https://doi.org/10.1016/j.sandf. 2014.04.019. [4] M. Lopez-Fernandez, R. Vilchez-Vargas, F. Jroundi, N. Boon, D. Pieper, M.L. Merroun, Microbial community changes induced by uranyl nitrate in bentonite clay microcosms, Appl. Clay Sci. 160 (2018) 206–216, https://doi.org/10.1016/j. clay.2017.12.034. [5] Y. Sun, Y. Li, Y. Xu, X. Liang, L. Wang, In situ stabilization remediation of cadmium (Cd) and lead (Pb) co-contaminated paddy soil using bentonite, Appl. Clay Sci. 105–106 (2015) 200–206, https://doi.org/10.1016/j.clay.2014.12.031. [6] J. Nones, H.G. Riella, A.G. Trentin, J. Nones, Effects of bentonite on different cell types: a brief review, Appl. Clay Sci. 105–106 (2015) 225–230, https://doi.org/10. 1016/j.clay.2014.12.036. [7] P. Bajpai, Papermaking Chemistry, Biermann’s Handb. Pulp Pap. (2018) 207–236. doi:10.1016/b978-0-12-814238-7.00010-6. [8] Z. Gong, L. Liao, G. Lv, X. Wang, Applied Clay Science A simple method for physical purification of bentonite, Appl. Clay Sci. 119 (2016) 294–300, https://doi.org/10. 1016/j.clay.2015.10.031. [9] B. Jönsson, T. Åkesson, B. Jönsson, S. Meehdi, J. Janiak, R. Wallenberg, Structure and forces in bentonite MX-80, 2009. [10] K. Song, Q. Wu, M. Li, S. Ren, L. Dong, X. Zhang, T. Lei, Y. Kojima, Water-based bentonite drilling fluids modified by novel biopolymer for minimizing fluid loss and formation damage, Colloids Surfaces A Physicochem. Eng. Asp. (2016), https://doi. org/10.1016/j.colsurfa.2016.07.092. [11] G. Tchobanoglous, F.L. Burton, H.D. Stensel, Metcalf & Eddy wastewater engineering: treatment and reuse, 2003. [12] V. Kuokkanen, T. Kuokkanen, J. Rämö, U. Lassi, Recent applications of electrocoagulation in treatment of water and wastewater—a review, Green Sustain. Chem. 03 (2013) 89–121, https://doi.org/10.4236/gsc.2013.32013. [13] N. Hilal, O.O. Ogunbiyi, N.J. Miles, Experimental investigation on the separation of bentonite using ceramic membranes: effect of turbulence promoters, Sep. Sci.
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Enhancement of flocculation and shear resistivity of bentonite suspension using a hybrid system of organic coagulants and anionic polyelectrolytes
T
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Mohamed Shamlooha, Abrar Rimehb, Mustafa S. Nassera, , Mohammad A. Al-Ghoutib, Muftah H. El-Naasa, Hazim Qiblaweyc a
Gas Processing Center, College of Engineering, Qatar University, Doha, Qatar Department of Biological and Environmental Sciences, College of Arts and Sciences, Qatar University, Doha, Qatar c Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b
A R T I C LE I N FO
A B S T R A C T
Keywords: Bentonite Colloidal suspension Coagulation Flocculation Hybrid Rheology
In this study, the influence of the hybrid coagulation-flocculation system on the flocculation and shear resistivity of bentonite suspension has been investigated. Two short-chained coagulants, polyamine and polyDADMAC, accompanied with low and high Molecular Weight (Mw) long-chained anionic Polyacrylamide (PAM) flocculants have been used to enhance the size and mechanical properties of the produced flocs. Four characterization techniques were used as evaluation criteria of degree of flocculation including turbidity, zeta potential (ζ), floc size analysis and rheology. Optimum dosages between 5 and 10 mg/L were obtained for flocculants and between 20 and 30 mg/L for coagulants in hybrid systems. Compared to single systems, hybrid systems performed exceptionally well. Removal efficiency of 99% was achieved using hybrid systems producing water with turbidity as low as 1.05 NTU. Although differences were insignificant in the treatment quality, significant improvement was achieved in floc size and rheological behavior. Hybrid systems were able to produce large flocs of more than 400 µm, five times bigger than flocs from the single cationic coagulant systems. Most importantly, rheological testing of produced slurries revealed the remarkable advancement achieved by hybrid systems. Hybrid systems were able to increase the elastic modulus (G′) by more than ten times resulting from the compacted flocs present in the system. Furthermore, resistivity toward shear and oscillations were improved. Ultimately, hybrid systems have surpassed the single cationic systems by achieving slightly better treatment besides producing larger, denser and more resistive flocs. Generally, polyDADMAC performed better than polyamine in hybrid systems. The combination of polyDADMAC with high Mw PAM gave the best separation as well as the largest flocs, while the combination with low Mw PAM resulted in the best rheological properties.
1. Introduction
Bentonite is a clay that has a platelets-like structure of alternating tetrahedral and octahedral sheets carrying an overall negative charge [9]. The amphoteric nature of bentonite makes it form a stable colloidal system when dispersed in water because of its electrostatic and physical properties [10]. Two factors affect the stability of clay particles in water, mainly particle size and charge density, where the size of bentonite particles (mean size of < 10 µm) and the negative charge (Zeta Potential of < −35 mV) keep it suspended with the help of Brownian motion [11]. Several treatment techniques have been reported in the literature for bentonite separation such as electrocoagulation, crossflow membranes, electro-osmosis and thermal-mechanical dewatering which all are targeting either the charge, particle size or both [12–15]. coagulation-flocculation is the most common process to separate the colloidal suspensions as minimum energy is needed to be operated [16].
Clay minerals have attracted increasing industrial attention in the past few years leading to the accumulation of such material in wastewater produced from different industries [1]. Bentonite has been widely adopted in the field of engineering such as in drilling activities [2], landfill application [3], handling of radioactive wastes [4], soil treatment [5] as well as in other fields such as pharmacological [6] and papermaking [7] industries. Bentonite has also been recently introduced to new developing technologies such as nanomaterials and fine chemicals [8]. The relatively low, price as well as the physical properties of bentonite, gave it popularity to have this wide variety of applications leading to the generation of considerable amounts of claycontaminated wastewater.
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Corresponding author. E-mail address: [email protected] (M.S. Nasser).
https://doi.org/10.1016/j.seppur.2019.116462 Received 28 August 2019; Received in revised form 22 November 2019; Accepted 20 December 2019 Available online 23 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Properties of used coagulants and flocculants. Commercial name
Polymer name (type)
Molecular weight (g/mol)
Charge density (%)
AN 956 (F1)
Polyacrylamide (anionic)
6–10 million
50
AN 956 SH (F2)
Polyacrylamide (anionic)
greater than 15 million
50
FL 2949 (C1)
Polyamine (cationic)
60,000
Very high
FL 4440 (C2)
PolyDADMAC (cationic)
100,000
Very high
Structure
investigation on kaolinite flocculation using anionic PAM that the presence of divalent cations (Ca2+ and Mg2+) provides a better flocs stability with higher density. The presence of cations was explained to have a double effect of polymer and particle binding leads to the production of denser flocs. In spite of, Witham et al. [34] observed that under continuous shear flocculation process, the effect of divalent cations tend to be different as the developed interactions fail to withstand the continuous shear. In the current study, one way to overcome this problem is addressed by having a hybrid system of short-chained cationic organic coagulant and long-chained anionic flocculant. The proposed system is to produce multiple alternating layers of positively and negatively charged polymers on the charged surface of clay particles. The existence of a cationic coagulant will not only reduce the optimum dosage of anionic PAM but will also help in producing larger and denser flocs. Hybrid systems for coagulation-flocculation processes are mildly studied in the literature [35,36]. However, no study to date has examined the rheological behavior of such systems. This study will evaluate four types of hybrid systems, including two types of coagulants, polyamine and polyDADMAC and two different molecular weight (MW) of anionic PAM flocculant. Both polyamine, and polyDADMAC were widely investigated individually; both coagulants achieved high separation performance [37,38]. In the case of anionic PAMs, the MW PAM was found to have a minor effect [39]; however, this may not be the case for hybrid systems. The objectives of this study are: (i) asses the separation performance of the proposed hybrid systems using turbidity (NTU) and ζ (mV) measurements, (ii) evaluate the size of the produced flocs resulting from applying hybrid systems; and (iii) investigate the rheological behavior of the dense phase resulting from hybrid and single systems, and compare the performance of hybrid systems to single systems.
This field is maturing, with a wealth of well-understood methods and procedures. Coagulation aims primarily to destabilize a colloidal system by inducing flocs formation, where charge neutralization is usually reported as the mechanism to provide such effect. Flocculation enhances the separation process then by aggregation resulting from polymer bridging making the flocs bigger and settle faster. Several researchers have reported the presence of a strong affinity between the negatively charged surface of bentonite and polymers with different functional groups. This came in alignment with the success of organic polymers to achieve high degrees of separation, not only for clay particles but for other wastes such as activated sludge [17]. Properties of the investigated clay suspension such as pH [18], morphology [19], particle size [20], salinity [21] and zeta potential [22] affect highly the performance of a coagulation-flocculation process. At the same time, the choice of flocculant has also a major influence on factors including the type of polymer [1], iconicity [23], charge density [24] and molecular weight (Mw) [25]. This broadness gave researchers plenty of room to study the different factors. Many polymers such as polyacrylamide (PAM), dextran, chitosan, guar, and lignin have been reported to achieve high treatment performances under a variety of conditions [20,21,25–27]. Nevertheless, the spotlight was usually directed toward the quality of treated water while the rigidity of the produced flocs was rarely considered. Practically, the presence of loose flocs in a continuous flow system may result in the resuspension of some particles and poor performance of the overall process. Several studies reported the shear sensitivity of flocs produced by polymer bridging [28,29]. This implies that purification performance and size of produced flocs are not the only parameters to consider in evaluating a coagulation-flocculation system but also rheological properties of produced sludge matters. Moreover, Lee et al. [16] in their review about flocculation processes have concluded that more research should be further conducted to meet the actual needs of wastewater industries where the rigidity of the produced flocs plays a major role. Although anionic PAM develops a repulsive electrostatic force with the negatively charged clay surfaces, it was remarked that the adsorption tendency of PAM into clays surpasses this effect and was able to flocculate particles [30,31]. Nasser and James [32] elaborated that anionic flocculated clay particles are loose as they failed to have high yield stress, compared to cationic polymers. This effect was found to decrease under saline conditions, Lee et al. [33] explained in their
2. Materials and methods All flocculants and coagulants were supplied by SNF Floerger, France, and are summarized in Table 1. F1 and F2 refer to the flocculants of low and high Mw, respectively. C1 and C2 refer to coagulants, polyamine, and polyDADMAC, respectively. Bentonite was purchased from Sigma-Aldrich Ltd, Germany. All chemicals were used as received without further modification. All experiments conducted in this study, the experiments run in duplicates with an average error of less than 5%. 2
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Fig. 1. Turbidity and zeta potential of treated water at different dosages of flocculants.
2.1. Jar test
2.4. Rheological testing
Coagulants and flocculants were prepared a day prior to conducting experiments, left on stirring throughout the day to ensure the homogeneity. Bentonite suspension was prepared at a concentration of 1.5 g/ L by adding a calculated amount of bentonite to water in a high-shear homogenizer. Beakers were then placed in the jar test (Stuart Flocculator SW6, UK); a predetermined amount of coagulant is added during rapid mixing with a rate of 180 rpm (G = 350 s−1) for 2 min. Followed by 2 min of another rapid mixing to add the flocculant. Afterward, slow mixing with a rate of 50 rpm (G = 52 s−1) for 20 min is allowed to mix and flocs to form. Systems are then left to settle for 5 min, a sample of the treated water is then withdrawn for turbidity (NTU) and ζ (mV) measurements. In this study, individual systems using only the two coagulants and the two flocculants were first studied to determine the optimum dosages as well as to be used as a benchmark to assess hybrid systems. Then all possible combinations of coagulants with flocculants were studied resulting in four hybrid systems and eight systems in total (C1, C2, F1, F2, C1&F1, C1&F2, C2&F1, and C2&F2). In hybrid systems, the optimum dose of coagulant was fixed from the single system study, while the amount of flocculant was varied.
Assessing the flowability and viscoelastic properties of the dense phase resulting from the treated water were done through rheology. Bentonite with a concentration of 6 g/L was prepared for these tests, optimum doses from the first part were used and regular jar test procedure was followed. A settling time of 15 min was allowed, treated water was then removed carefully ensuring to keep the flocs unaffected. Anton Paar MCR 302 Rheometer with cup and bob geometry was used to perform the tests. Two tests were conducted for each of the eight systems at their optimum dosages. Frequency sweep was done in the range between 0.1 and 500 rad/s at a constant 0.3% shear strain. Moreover, shear strain was performed in the range between 0.01 and 1000 1/s. An algorithm from RheoPlus software was used to fit viscosity data on Bingham model. All tests were performed at a controlled temperature of 25 ± 0.1 °C. 3. Results and discussion 3.1. Dosage optimization The optimum dosage of a flocculant or a coagulant reflects the amount that achieves the maximum treatment with minimum amount of chemicals. This ensures the best performance with the lowest possible cost. Ideally, a turbid system tends to decrease in turbidity upon addition of coagulant dosages at the beginning, until the optimum dosage is reached. Further addition of more coagulants may reverse the effect and re-increase the turbidity due to the presence of an excessive amount of charged coagulants. Thus, the optimum dose must be determined precisely. Theoretically, optimum dose exists at the point where the charge in the system is neutralized, since repulsion forces are minimum, consequently, minimum turbidity is usually associated with a zeta potential value of near zero. However, this is not always the case as charge neutralization is not the only mechanism participating in producing flocs. Other mechanisms are reported in the literature such as polymer bridging and sweep flocculation [40,41].
2.2. Turbidity and zeta potential Optimum dosages were obtained based on the results from turbidity and zeta potential. Ideally, the minimum amount of chemical that will lead to maximum flocculation is the point where a crossover will take place in zeta potential (ζ-potential = 0 mV). Hach 2100 N turbidimeter was used for turbidity measurements and Zetasizer ZEN3600 (Malvern Instruments Ltd., UK) for zeta potential tests. Both devices were operated at ambient conditions. Samples were filtered through 0.45 µm syringe filters before conducting zeta potential tests.
2.3. Floc size analysis Knowledge of floc size in a coagulated/flocculated system is essential to predict the settling behavior. To prepare for such tests, bentonite suspension with a concentration of 3.5 mg/L was prepared. The standard jar test procedure was followed using optimum sizes obtained from the first part. Measurements were conducted in Mastersizer 3000 (Malvern Instruments) at room temperature.
3.1.1. Single anionic flocculant systems Although both bentonite and anionic PAM carry negative charge where repulsion static force develops. The adsorption tendency of PAM on bentonite surface through the PAM’s amide group can exceed that effect producing big flocs. Fig. 1 shows the behavior of bentonite suspension on the occasion of adding different dosages of anionic PAM. 3
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Fig. 2. Turbidity and zeta potential of treated water at different dosages of coagulants.
anionic PAM leads to a high increase in turbidity due to the high density of repulsive forces. Fig. 4 displays the removal performance of all investigated systems. All systems, except for single anionic flocculants, succeeded to provide high turbidity removals. The combination of PolyDADMAC and high Mw PAM has performance reaching a removal percentage of around 99% similar to single polyDADMAC. Although differences are insignificant here, major differences are to be observed in floc size and rheological behavior.
Both high and low Mw PAMs were able to decrease the turbidity at small dosages which indicates the superiority of bentonite-PAM bonds in that region, similar behavior was observed by Nasser and James [39]. Further addition of anionic PAM caused the turbidity to increase again as a result of excessive repulsive forces between the negatively charged particles and polymer chains. Zeta Potential was almost steady at the negative region flocculating between −40 and −30 mV which verifies the poor attempt to destabilize the colloidal system using anionic PAM. Overall, anionic PAM alone failed to treat the turbid water significantly but was able to produce some flocs.
3.2. Floc size distribution 3.1.2. Single cationic coagulant systems Polyamine and polyDADMAC are well-known coagulants that are used commercially. Determining the optimum dosage for these coagulants is the first step toward establishing a hybrid system that produces shear-resistive flocs. Both polymers achieved good treatment performances (Fig. 2), minimum turbidity of 1.98 and 1.56 NTU at concentrations of 20 and 30 mg/L were achieved by polyamine and polyDADMAC, respectively. Predictably, further addition of coagulant beyond optimum concentration raises the turbidity due to the emergence of repulsive forces between the positive charges on the polymer in the crowded system. Zeta potential has verified this behavior as it rose from −38 mV before polymer addition to more than 20 mV at a concentration of 50 mg/L for both systems.
The first step toward observing a substantial difference between single cationic coagulant systems and hybrid systems is through measuring sizes of the flocs produced after the solution is allowed to settle. While anionic flocculants showed weak treatment performance with a maximum of 27% turbidity removal were achieved (Fig. 5), they succeeded in producing bigger flocs as high Mw PAM has produced the largest floc size among the single systems. However, hybrid systems have combined the advantages of the two by achieving high turbidity removal and producing even bigger flocs. Fig. 5 demonstrates the size distribution of all investigated systems; untreated bentonite is symbolled with black crosses, single systems with squares and hybrid systems with circles. A shift toward the right signifying bigger flocs is clear for hybrid systems. Number (Dn) and weight (Dw) average of floc sizes are represented in Fig. 6; hybrid systems prospered to increase floc sizes to more than 500%. Largest flocs were achieved by (C2 + F2) system with a number average of 286.7 µm and a weighted average of 411.0 µm. Thus, besides achieving the best performance in turbidity removal, the same system performed the best in producing the largest flocs.
3.1.3. Hybrid cationic-anionic systems Hybrid systems consisting of cationic coagulant and anionic flocculant are the main objective of this research. Thus, the first step is testing their ability to treat bentonite turbid water. Optimum dosages of cationic coagulants were fixed at values retrieved from Fig. 2, while the concentration of anionic flocculants was varied. Fig. 3 provides that all four combinations between the two studied coagulants and two flocculants were able to achieve high removal percentages. While the first flocculant resulted in slight increase in the turbidity with an optimum of 3.30 and 2.78 NTU in combination with C1 and C2 respectively (Fig. 3a), the second flocculant provided a better performance than single systems reaching as low as 1.40 and 1.05 NTU for systems with C1 and C2 respectively (Fig. 3b). Although zeta potential values have increased significantly from −38 mV to around −20 mV, it did not cross the zero in all systems indicating a negative net charge in the treated systems. While zeta potential readings reflect the charge of the outer surface, it is expected to achieve negative values proposing that the outer surface is covered with anionic polymer. Excessive addition of
3.3. Rheological behavior of dense phase Although the ratio between number average and weight average sizes can give an idea about the rigidity of produced flocs, these tests cannot predict precisely how loose the produced flocs are. Thus, testing the rheological behavior of the flocs is essential to predict the shear resistivity of the flocs. Fig. 7 shows the non-Newtonian behavior of the coagulated/flocculated bentonite dispersion. according to this figure, for all tested samples, viscosity is decreasing as the shear rate increases. Shear stress was correlated to shear rate using and Bingham plastic model (Eqn. (1)) 4
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Fig. 3. Turbidity and zeta potential of treated water in hybrid systems by fixing coagulants concentration and varying (a) polyamine (b) PolytDADMAC concentrations.
Fig. 4. Turbidity removal percentages at optimum dosages. 5
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Fig. 5. Comparison of size distribution for flocs produced in single and hybrid systems.
Significant differences in yield stress values between hybrid and single system, which is another indicator of the increase in flocs density provided by hybrid systems. In addition to shear flow behavior, the oscillatory measurement is an approach used to measure the strength of the flocculated dispersion through the elastic properties described by the elastic modulus (G′, ratio of elastic stress over strain) and viscous modulus (G″, ratio of viscous stress over strain) are corresponding to the amount of energy stored and dissipated during deformation. These results of oscillatory tests displayed in Fig. 9; elastic modulus (G′) exceeds the viscous modulus (G″) in all cases excluding slurries produced from the anionic PAM. The greater value of G′ compare to G″ proves the viscoelastic solid-like behavior of flocculated bentonite and indicates a high structural gel-like strength of the produced flocs. Comparing hybrid and single systems, Values of elastic modulus (G′) are more by at least one order of magnitude in hybrid systems. Equally important, flocs from the hybrid system are found to be more resistive
model.
τ = τo + kγ̇
(1)
where τo and k are known as the Bingham yield stress (Pa) and Bingham plastic viscosity (Pa s), respectively. Results from Fig. 7 reveals that all slurries, except for F1 and F2, can be attributed as plastic fluids. A plastic fluid refers to a material that behaves as a solid in low shear where the elastic forces are stronger, while liquid-like behavior is exhibited at high shear giving that viscous forces are surpassing. Observations from Fig. 7 divulge the differences between the dense phases resulted from hybrid systems (circles) and single systems (squares). Very high viscosities are detected at low shear for hybrid systems designating that flocs are more compacted compared to them in single systems. Flocs from single anionic PAM showed a liquid-like behavior even in very low shear reflecting that the flocs are very loose and sensitive. Viscosity data from Fig. 7 are fitted using Bingham plastic model; calculated parameters are illustrated in Fig. 8.
Fig. 6. Number and weight average of floc size at optimum dosages. 6
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Fig. 7. Viscoelastic behavior of slurries under varying shear rate.
and produce bigger flocs [Section 3.2] that are stronger and more resistive toward shearing and oscillations [Section 3.3]. Fig. 10 illustrates schematically the working mechanism in treating suspended bentonite particles. A cationic coagulant does the major role of agglomerating and collecting bentonite particles producing micro-flocs held through electrostatic force as well as Van der Walls attraction forces. Anionic PAM then attracts and gather the micro-flocs producing larger flocs (polymer bridging); the electrostatic force between the negative charges on PAM and positive charges on coagulant provides stability to the flocs making it more shear resistive. This mechanism is verified by zeta potential results from water treated with hybrid systems (Fig. 3). The values remained negative regardless of the treatment percentage; which reflects the presence of negative charges at the outer surface of floating components pointing toward the anionic PAM. Moreover, Fig. 10 explains the notable increase in the size of flocs where the long anionic electrolytes gathers the micro-flocs formed from the first step. Lastly, the strong affinity between the cationic coagulants and the anionic polyelectrolytes in the outer layer provides more stability to the flocs; making them more resistive toward any external force. Similar behavior of treatability
compared to them in single systems. Fig. 9a reveals that values of storage modulus in the hybrid system are steady under oscillations with frequencies higher than 100 rad/s, while flocs from single cationic coagulants break at frequencies of less than 40 rad/s. All hybrid systems behaved in the same manner under varied shear or frequency, nonetheless, polyDADMAC with low Mw PAM (C2 + F1) exhibited the most solid-like behavior (Fig. 9a) suggesting that it has the most compacted flocs. Again, flocs from single anionic PAM breaks with very low oscillations of around 5 rad/s indicating the weak unresistant flocs. Additionally, Fig. 9b shows that the liquid-like behavior of all systems is the same at high frequencies. 3.4. Single systems vs hybrid systems While single coagulants are well known to provide good separation for clays and other suspended matters, hybrid systems have provided better characteristics that enhance the performance and ensures the smoothness of the overall process. Hybrid systems proved to have positively exceeded all tested properties in this research compared to single systems. They provide good purification quality [Section 3.1]
Fig. 8. Bingham parameters for viscoelastic behavior in slurries. 7
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Fig. 9. Rheological behavior of produced slurries under varying frequencies (a) Storage Modulus (b) Loss Modulus.
enhancement was reported in the presence of inorganic cations with anionic PAM [30,33]. While, William et al. [34] proved the instability of flocs from such systems, the hybrid system on organic anionic-cationic polymers proved to produce stable rigid flocs.
•
4. Conclusions
•
This paper investigated the potential of applying a hybrid system of anionic PAM and cationic coagulant to enhance the process efficiency in removing suspended clay particles. Conventionally used cationic coagulants, polyamine, and polyDADMAC were studied as part of hybrid systems as well as single systems to benchmark and assess the performance of the proposed hybrid systems. The main conclusions from this investigation are as follow:
•
that flocs from hybrid systems are more stable, resistive and compacted compared to the single systems with yield stresses of ten times more. All slurries from hybrid systems behaved as plastic fluids, reflecting that transferring and pumping such solutions with high shear rates will not be a problem. Among all investigated systems, polyDADMAC combined with High Mw PAM provided the best treatment efficiency with the biggest flocs, while flocs from polyDADMAC with low Mw PAM produced the most compacted flocs. Hybrid systems work in two steps: cationic coagulants form microflocs then anionic flocculants attract and gather micro-flocs together, essentially, providing better treatment efficiency with bigger and denser flocs.
• Hybrid systems succeeded in significantly increasing the size of flocs
Declaration of Competing Interest
•
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
produced with achieving the same degree of separation as single systems. Flocs from hybrid systems were more than five times bigger compared to those from single cationic systems. Rheological testing of studied systems at optimum dosages revealed 8
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Fig. 10. Schematic representation of the coagulation-flocculation process in hybrid systems.
Acknowledgment
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