Evolving a flocculation process for isolating lignosulfonate from solution

Separation and Purification Technology 222 (2019) 254–263

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Evolving a flocculation process for isolating lignosulfonate from solution Zahra Hosseinpour Feizi a b

a,b

, Armin Eraghi Kazzaz

a,b

a,⁎

b,⁎

T

, Fangong Kong , Pedram Fatehi

State Key Laboratory of Paper Science and Technology of Ministry of Education, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China Chemical Engineering Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: Lignosulfonate PDADMAC Purification Flocculation Sedimentation Biorefining

Lignosulfonate is a major component of the spent liquor of the sulfite pulping process. Flocculation has been used worldwide for isolating colloidal substances from solutions and thus can be adapted for separating lignosulfonate from the spent liquor. The adaptation of flocculation process would generate lignosulfonate-based flocs with a potential use as a value-added product and reduce the concentration of lignosulfonate in the spent liquor implying that this process can facilitate spent liquor’s treatment operation. To evaluate the performance of this flocculation process, the interaction of lignosulfonate with polydiallyldimethylammonium chloride (PDADMAC) having various molecular weights was fundamentally studied in simulated systems under stirring and non-stirring environments. Focused beam reflectance measurement (FBRM) and vertical scan analyzer confirmed the fast kinetics of floc formation. A correlation was observed between the flocculation index of complexes and the molecular weight of PDADMAC. Increasing the PDADMAC’s molecular weight led to the production of more flocs with larger sizes in the stirred system. However, larger flocs were weaker than smaller ones. Sedimentation analysis proved that the higher molecular weight PDADMAC generated more flocs with looser structures, but these flocs settled faster than the flocs produced by PDADMAC with smaller sizes. The results confirmed that the non-stirred system was more sensitive than stirring one to the dosage of PDADMAC. The flocculation efficiency was improved in the presence of salt while no change in the flocculation process was observed in the presence of sugar. The outcomes of this study suggest a promising alternative to isolate lignosulfoante from the spent liquor.

1. Introduction The spent liquor of sulfite pulping process contains lignosulfonate. In most cases, the spent liquor is treated in the wastewater facilities of the process and thus its lignosulfonate is decomposed. Alternatively, lignosulfonate can be recovered from the spent liquor and considered as a by-product [16]. Lignosulfonate was reported to be used in various industries, such as food [59], papermaking [43] and wastewater treatment systems as a flocculant [12,57]. Despite a vast production and wide range of applications, the ultimate use of lignosulfonate depends on its extraction operation from pulping spent liquor [34,50,69]. In this context, utilizing lignosulfonate in value-added applications would be beneficial as it would (1) enhance the profitability of the sulfite pulping process and (2) reduce the load of organics in the pulping spent liquor that is treated in wastewater treatment facilities [7]. In opposition to energy intensive and inefficient evaporation processes, flocculation process has shown advantages in concentrating solutions [16,9]. Flocculation of oppositely charged polymers in

⁎

solutions has been recognized to be an effective method for separating polymers, such as lignosulfonate, from solutions [60,46,37]. Since lignosulfonate has a negative charge, the addition of a positively charged polymer can trigger flocculation and thus isolation of lignosulfonate [5,51]. Following the charge neutralization concept, cationically charged polymers can be considered for the flocculation and hence separation of lignosulfonate from pulping spent liquors. Polydiallyldimethylammonium chloride (PDADMAC) is a positively charged polymer with a wide range of applications in industry [62,47]. In this study, PDADMAC was selected to be used as a positively charged flocculant due to several reasons. Firstly, PDADMAC has a relatively high charge density that facilitates the flocculation process [53,35,25,21]. Secondly, the commercially available PDADMAC polymers can be obtained with similar charge densities but different molecular weights, which makes it feasible to track the fundamentals associated with the impact of the molecular weight of PDADMAC on its flocculation efficiency. Thirdly, the toxicity of PDADMAC is considerably lower than other positively charged flocculants, such as polyacrylamide [41], which makes it a safer flocculant in wastewater

Corresponding authors. E-mail address: [email protected] (P. Fatehi).

https://doi.org/10.1016/j.seppur.2019.04.042 Received 24 February 2019; Received in revised form 7 April 2019; Accepted 14 April 2019 Available online 15 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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2.2. Lignosulfonate and PDADMAC properties

purification processes. In one study, the interaction of PDADMAC polymer and lignosulfonate led to the remarkable isolation of lignosulfonate from the spent liquor of neutral sulfite semichemical pulping (NSSC) process [46]. In another study, PDADMAC polymers with altered molecular weights were used as flocculants for isolating lignocelluloses from the thermomechanical pulping spent liquor and the results revealed that the higher the PDADMAC’s molecular weight, the larger organic flocs were formed [61]. Interacting the positively charged PDADMAC with lignosulfonate leads to the formation of complexes. Despite the reports on the flocculation of lignocelluloses and cationic polymers [46,61], the interaction of PDADMAC and lignosulfonate has not been studied in detail. Flocculation mechanism highly depends on the structure, charge density and molecular weight of both flocculant and flocculent [5,58]. As PDADMAC is a promising flocculant for lignosulfonate isolation, the first objective of this work was to study the fundamentals associated with the impact of PDADMAC’s molecular weight on isolating lignosulfonate from solutions. After floc formation, the flocculation process should facilitate the isolation of the generated flocs. The sedimentation velocity and the properties of the formed flocs are crucial factors in flocculation processes [15]. In various flocculation systems, induced flocs are exposed to different stresses, e.g., shear rates, in the suspension before their sedimentation and collection. The floc’s strength is a principal factor in the flocculation process, as the floc breakage and regrowth directly affect the efficiency of the process [40]. Also, flocs’ compactness is another crucial factor in investigating the floc properties, since it directly affects the strength of the flocs and flocs’ dewatering process [28,38,30]. In the present study, the sedimentation kinetics, the size of the PDADMAC/lignosulfonate flocs, the flocs strength and recovery as well as sediment compactness were investigated as the second objective of this work. For the first time, the fundamentals of the interaction between PDADMAC and lignosulfonate as well as the sedimentation performance of their induced flocs were studied in a simulated solution in this work. The main aim of this study was to provide fundamental understandings associated with the interaction of lignosulfonate and a wellknown and industrially used cationic polymer (PDADMAC) in order to develop an attractive flocculation process for generating a new lignosulfonate-based product that would also facilitate the purification of sulfite spent pulping liquor. The impact of salt and sugars on isolating lignosulfonate from solution was also studied to learn how the proposed system would function in the industrially produced spent liquor rather than simulated solution. It should be stated that the isolated lignosulfonate flocs may be used as animal feed pellet binders [45] or in producing composites, stabilizers, and adhesives [64,4]. However, the analysis on the characteristics and application of induced flocs and on the isolation of lignosulfonate from industrially produced spent liquors are outside the scope of this study.

Lignosulfonate was dialyzed with membrane dialysis in distilled water for purification and then dried at 105 °C overnight in an oven. This dialysis process was performed to remove impurities so that the true interaction of lignosulfonate and PDADMAC was identified in simulated solutions. Lignosulfonate solution was then prepared at 5 g/L concentration and pH 7, and it was used in all experiments. All PDADMAC polymers were used at 200 g/L concentration and pH 7. The charge density of lignosulfonate and PDADMAC polymers were determined by a Particle Charge Detector (Mutek, PCD 04, Germany) [17]. Solutions (200 g/L) of PDADMAC polymers and lignosulfonate were prepared and stirred overnight at ambient temperature. The molecular weight of lignosulfonate was determined using a Gel Permeation Chromatography (GPC), Malvern GPCmax VE2001 Module + Viscotek TDA305 equipped with UV-detector, refractive index (RI) and intrinsic viscosity-differential pressure (IV-DP). In this experiment, GPC was set-up as explained in our previous work [17]. In this experiment, dried lignosulfonate was dissolved in 0.1 mol/L sodium nitrate solutions to produce a sample with 5 g/L concentration prior to its molecular weight measurement. The molecular weight analysis of PDADMAC was conducted in a 5 wt% acetic acid solution at 35 °C. The PDADMAC polymers’ and lignosulfonate’s hydrodynamic diameter (dh) was measured by a dynamic light scattering (DLS) instrument (Brookhaven BI200, USA) at a wavelength of 632 nm and a scattering angle of 90°. Samples were freshly prepared (at 0.1 g/L concentration) in a KCl solution (1 mM) at 25 °C temperature, and the average of three measurements was reported in this study. 2.3. Zeta potential analysis The zeta potential of the lignosulfonate solution and PDADMAC/ lignosulfonate suspensions was determined using a zeta potential analyzer, ZetaPALS (Brookhaven Instruments Co., USA) [14,36,61] with the electric field of 8.4 V/cm. In this set of experiments, various dosages of PDADMAC (200 g/L) were added to 20 mL of lignosulfonate solutions (5 g/L). The mixtures were incubated for 30 min at 25 °C in a water bath shaker at 150 rpm. Upon completion, samples were dispersed by ultrasonication, and then 400 µL of them were added to 10 mL of a previously filtered 1 mM KCl solution to produce samples in 1 g/L concentrations. The measurements were conducted three times for each sample, and the average values were reported in this work. 2.4. Flocculation analysis under stirring 2.4.1. Photometric dispersion analysis (PDA) The flocculation of lignosulfonate by PDADMAC polymers was monitored by a photometric dispersion analyzer (PDA 3000, Rank Brothers Ltd). In this set of experiments, lignosulfonate solution (5 g/L, pH 7) was added to the container of a dynamic drainage jar (DDJ) at 300 rpm fitted with a 200-mm mesh screen connected to the PDA via a 3-mm plastic tube. The lignosulfonate solution was recycled with the help of a peristaltic pump at a constant flow rate of 20 mL/min. Afterward, different dosages of PDADMAC were added to the solution to initiate flocculation. The effect of PDADMAC on the direct current (DC) voltage of the suspension in the plastic tube was detected by the PDA. The relative turbidity of the suspensions was determined as explained elsewhere [67]. Also, the ratio of root mean square (RMS) of the fluctuation in the transmitted light to DC voltage (RMS/DC) was determined. This ratio represents the flocculation index in the suspension and considered as an indirect measurement of the floc size [10,67,24]. The above analysis was repeated with PDADMAC having different molecular weights in the lignosulfonate solution at pH 7.

2. Materials and methods 2.1. Materials Sodium lignosulfonate and polydiallyldimethylammonium chloride (PDADMAC) with three different molecular weights were purchased from Sigma-Aldrich and used as received. Potassium chloride (KCl, 99%), sodium nitrate (NaNO3, 99%), acetic acid (CH3CO2H, 99.5%), sodium hydroxide (NaOH, 97%), and sulfuric acid (H2SO4, 98%) were all purchased from Sigma-Aldrich and diluted before use. Potassium polyvinyl sulfate (PVSK) was obtained from Wako Pure Chemical Industries Ltd., Japan and was diluted to 0.005 M before use. Dialysis membrane made of cellulose acetate (molecular weight cut-off of 1000 g/mol) was purchased from Spectrum Labs.

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Generally, this sedimentation occurs at various rates depending on the floc properties. The efficiency of PDADMAC in flocculating lignosulfonate was determined according to the transmission data collected from the top layer and backscattered data collected from the sediment layer at the bottom of the cell. The compactness of the sediments was also determined as the ratio of the mass to volume of settled flocs after 30 min of experiments [32,72]. In this analysis, one sample (1 mL) was collected from the top layer of the samples after mixing lignosulfonate with the polymers and then another sample was collected from the top layer after 30 min of settling. The collected samples were then dried at 105 °C overnight, which helped determine the mass of settled flocs via developing a mass balance. By taking the mass of settled flocs, the cross-section area of the cell and the thickness of the sediment into account (from backscattering data), the compactness of sediments was determined. The settling velocity of flocs was also determined via monitoring the kinetics of sediment growth at the bottom of the cell.

2.4.2. Focused beam reflectance measurement (FBRM) The flocculation performance of lignosulfonate with PDADMAC under stirring was also assessed using FBRM. The chord length, i.e., the distance between the two edges of an aggregated floc, and squareweighted counts of flocs were identified using a focused beam reflectance measurement (FBRM), Particle Track 25 (Mettler-Toledo AutoChem, USA) [5,61]. In this set of experiments, 200 mL of 5 g/L lignosulfonate solution (pH 7) was added to a 400-mL glass beaker, and the probe was submerged 20 mm below the solution surface, while the solution was agitated at 150 rpm. Afterward, various dosages of PDADMAC (200 g/L) were added to the lignosulfonate solution. The scanning experiment was performed for 30 min with 3 s time intervals at ambient temperature to monitor the flocculation process. This analysis was repeated with PDADMAC having different molecular weights. Flocs strength and formation were analyzed at ambient temperature. Flocs start to break when the stress applied to the floc surface is higher than their internal bonds [28]. The following experiment was conducted to calculate the flocs strength. At the beginning, the shear rate of the solution was set to 150 rpm and kept for 3 min for floc formation. Then, the agitation intensity was boosted to 550 rpm for 3 min to break the formed flocs and then the intensity was reduced to 150 rpm for 3 min for flocs regrowth. This trend was repeated until neglected changes were observed in the chord length of the flocs in low and high shear rates. The strength factor of the flocs and their recovery factor were determined according to Eqs. (1) and (2), respectively [28]:

Strength factor =

d2 ×1 d1

(1)

Recovery factor =

d3 − d2 × 100 d1 − d2

(2)

2.7. Lignosulfonate removal analysis The separation efficiency of lignosulfonate by PDADMAC polymers was analyzed using a UV–Vis spectrophotometer (Genesys 10S UV–Vis, Thermo Scientific) at a wavelength of 280 nm with the help of a calibration curve. In this case, PDADMAC samples were added to lignosulfonate solution (5 g/L) at their optimum dosages in glass flasks. Samples were then incubated at 25 °C for 30 min in a water bath shaker at 150 rpm. After 30 min, samples were collected from the top part of the glass flasks. Then, the concentration of lignosulfonate in the solutions were analyzed using the calibration curve. 2.8. Total nitrogen bound analysis (TNb)

where d1 is the floc size before breaking (at 150 rpm), d2 is the floc size after the breakage (at 550 rpm), and d3 is the floc size after the regrowth (at 150 rpm) [61].

In general, TNb analysis determines the level of nitrogen containing materials contributing to the pollution of solutions [41]. Since the nitrogen content of the test solution in this work was originated from PDADMAC [61], this analysis was carried out to determine the amount of PDADMAC remained in the solution after separating the formed flocs. In this study, the nitrogen content of the samples was analyzed by total organic carbon analyzer, Vario cube, Elementar [48]. This test was carried out for PDADMAC1, PDADMAC2 and PDADMAC3 in dosages of 0.68, 1.28 and 1.88 mg/L.

2.5. Hydrodynamic diameter (dh) analysis of the flocs A particle size analyzer (Mastersizer 3000, UK) was used for monitoring the size distribution and hydrodynamic diameter (dh) of the formed flocs after mixing lignosulfonate and PDADMAC. In this set of experiments, altered dosages of PDADMAC (200 g/L, pH 7) were added to 20 mL of lignosulfonate (5 g/L, pH 7), and the samples were incubated for 30 min at 25 °C in a water bath shaker at 150 rpm. Afterward, the samples were analyzed by the instrument, and their dh was measured by laser diffraction with a red and blue laser light sources having the wavelength of 632.8 and 470 nm, respectively. The mean diameter was calculated as the average median diameter of the volumetric distribution of the three parallel measurements.

2.9. Salt and sugar effects on the flocculation of lignosulfonate Industrially produced spent liquor of sulfite pulping process contains up to 59 g/L of lignosulfonate, 32 g/L sugars and 28 g/L salt [55]. Considering the relatively high amount of salt and sugar in the spent liquor, the effect of these elements on the lignosulfonate/PDADMAC flocculation was analyzed to track the efficiency of the proposed flocculation process in more industrially relevant environment. In this analysis, sodium sulfite was used as salt due to its presence in the sulfite pulping process, and xylose was chosen as the main sugar in the spent sulfite liquor [68]. These elements were added separately to the simulated lignosulfonate solution (5 g/L) in different 0.3–2.4 g/L sodium sulfite concentration, and 0.2–3 g/L xylose concentration. The diluted samples were considered in this analysis as the instrument could not function at higher organic/inorganic concentrations. The PDA and vertical scan analyses were repeated for these samples following the experimental procedures stated in Sections 2.4.1 and 2.6, respectively.

2.6. Flocculation analysis under non-stirring The flocculation performance of lignosulfonate and PDADMAC under a non-stirring state was also studied using a vertical scan analyzer, Turbiscan Lab Expert, Formulaction, France. This analysis helps monitor the kinetics of settlement of lignosulfonate before and after adding PDADMAC polymers to the system and floc formation. The affinity of the samples to transmitting and backscattering laser lights were monitored by Turbisoft 2.1 software at the wavelength of 880 nm [31]. In this set of experiments, different dosages of PDADMAC (200 g/ L, pH 7) were added to the solutions of 5 g/L lignosulfonate (pH 7) and stirred at 150 rpm for 2 min. Next, 20 mL of the suspensions were poured into the cylindrical glass cells of the instrument, and the samples were analyzed by the instrument [33]. The interaction of lignosulfonate and PDADMAC polymers created flocs, which were settled at the bottom of the cell as sediments while generating a clear layer at the top portion of the cell in this analysis.

3. Results and discussion 3.1. Properties of lignosulfonate and PDADMAC polymers The properties of PDADMAC and lignosulfonate polymers are presented in Table 1. PDADMACs had similar charge densities 256

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Table 1 Properties of various PDADMAC and lignosulfonate polymers. Label

PDADMAC1

PDADMAC2

PDADMAC3

Lignosulfonate

Charge density, meq/g Mw, g/mol Hydrodynamic diameter (dh), nm

6.7

6.5

6.2

−2

45,000 28

186,000 61

1,045,000 281

51,560 36

(6.2–6.7 meq/g), while they had significantly different molecular weights. Since PDADMAC is a cationic polymer, while lignosulfonate is an anionic polymer, PDADMAC can be considered as a favorable flocculant for aggregating lignosulfonate and thus removing it from solution. Lignosulfonate had a charge density of −2 meq/g and molecular weight of 51,560 g/mol. A wide range of molecular weight has been reported for the lignosulfonate in the literature, e.g., 27, 37 and 70 kg/ mol [49,52,8]. This wide range of molecular weight may be attributed to the alteration used in producing them in industry [70,39]. The dh of the PDADMAC polymers and lignosulfonate were also reported in Table 1. As expected, PDADMAC3 and PDADMAC1 had the largest and smallest dh, respectively, which are well in harmony with their molecular weights [71]. In one study, the hydrodynamic size of PDADMAC with the molecular weight of 5 kg/mol was found to be 4 nm [6]. In another study, the PDADMAC’s hydrodynamic size with the molecular weight of 100 kg/mol was measured to be 38.5 nm [2].

Fig. 2. Flocculation index of lignosulfonate as a function of (a) PDADMAC dosage and (b) charge ratio of PDADMAC/lignosulfonate in the suspension at 25 °C, 300 rpm and pH 7.

3.2. Zeta potential analysis

3.3. Flocculation analysis under stirring state

Fig. 1 shows the zeta potential of the lignosulfonate solution as a function of PDADMAC dosage at pH 7. Analyzing the zeta potential changes in the suspensions leads to anticipating the suspension’s stability, which helps identify the flocculation process [44]. The zeta potential of lignosulfonate solution was reported to be around −40 mV. It is seen that almost 1 mg/L of PDADMAC was sufficient to render the zeta potential of the suspension cationic, regardless of the PDADMAC’s molecular weight. Further increase in the PDADMAC dosage did not affect the zeta potential of the suspensions considerably since the zeta potential reached almost a plateau after the addition of 2.9 mg/L of the polymers. Since the charge density of the PDADMAC polymers is similar, the molecular weight difference in PDADMAC polymers did not show a significant effect on the zeta potential changes. In one study, the zeta potential of lignocelluloses in the spent liquor of thermomechanical pulping process was changed from −25 mV to +20 mV by raising the concentration of PDADMAC polymers from around 25 to 160 mg/L in the solutions. Also, a similar trend was observed for the zeta potential of the thermomechanical pulping spent liquor via adding PDADMAC polymer with different molecular weighs in the system [61].

3.3.1. Photometric dispersion analysis (PDA) The effect of PDADMAC dosage on the flocculation rate of lignosulfonate is demonstrated in Fig. 2a. In this figure, RMS/DC is the flocculation index, which is an indirect indication of the floc size [10]. The addition of PDADMAC improved the flocculation index of lignosulfonate, and it was more rapidly changed for PDADMAC1 with the smallest size, which could be attributed to its slightly higher charge density. As seen, PDADMAC3 generated the largest flocs, while PDADMAC1 created the smallest ones. This may be due to the higher dh of PDADMAC3 which interacts more greatly with the lignosulfonate polymer presented in the simulated solution and produces larger flocs [61]. The flocculation index reached a maximum at the dosages of 0.68, 1.28 and 1.88 mg/L for PDADMAC1, PDADMAC2, and PDADMAC3, respectively. Higher dosages suppressed the flocculation index in all PDADMAC samples. This behavior was observed at the reversed (positive) zeta potential (Fig. 1), and maybe due to the repulsion force developed between the flocs at the higher dosages [46,61]. Thus, 0.68, 1.28 and 1.88 mg/L were chosen as optimum dosages for PDADMAC1, PDADMAC2, and PDADMAC3, respectively, for analyzing the flocculation performance of polymers in their effective dosages in this study. In Fig. 2b, the flocculation index was reported as a function of PDADMAC polymers to lignosulfonate’s charge ratio. As seen, the maximum flocculation occurred at the charge ratio of 0.5, 0.86 and 1.3 meq/meq of PDADMAC1/lignosulfonate, PDADMAC2/lignosulfonate, and PDADMAC3/lignosulfonate, respectively. PDADMAC1 reached its maximum flocculation efficiency at a charge ratio of smaller than 1 meq/meq illustrating that its high charge may have prevented its ideal stoichiometric charge interaction of 1 meq to 1 meq. PDADMAC3 reached a much higher flocculation efficiency and at a charge ratio of greater than 1 meq/meq postulating that its bridging effect compensated for the charge repulsion that it could have developed at this charge ratio (Fig. 2b). This implies that the charge neutralization along with bridging may have played a role in the flocculation of lignosulfonate [61]. Therefore, the molecular weight of PDADMAC will

Fig. 1. Zeta potential of the lignosulfonate solution as a function of PDADMAC dosage conducted under the conditions of pH 7, at 25 °C. 257

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Fig. 3. (a) Chord length and (b) square weighted counts of the formed flocs in the treatment of lignosulfonate with PDADMAC polymers at the dosages of 0.68, 1.28 and 1.88 mg/L for PDADMAC1, PDADMAC2 and PDADMAC3, respectively, and (c) the chord length and (d) square weighted counts of the produced flocs at 5.88 mg/ L of PDADMAC polymers in 30 min at pH 7.

have a significant impact on its flocculation efficiency with lignosulfonate. In one study, PDADMAC polymer with the molecular weight of 50–100 kg/mol was found to produce the largest flocs at the dosage of 16 mg/L in treating wood extract suspension [14]. In another study, PDADMAC with the higher molecular weight of 400–500 kg/mol was reported to perform more efficiently than the low molecular weight ones in flocculating and removing lignocellulosic materials from a pulping spent liquor [61].

complexes immediately after the addition of the PDADMAC polymers. As time elapsed, it was observed that PDADMAC3 could produce more flocs, which would depict its more intense interaction with the soluble lignosulfonate and greater production of insoluble complexes [18]. A larger dosage (5.88 mg/L) of the PDADMAC polymers was also studied in the lignosulfonate solution to track the changes in the produced flocs (Fig. 3c and d). As seen, the formed flocs had smaller chord length in comparison with that in Fig. 3a. The number of the flocs (Fig. 3d) was also greater especially for PDADMAC1 and PDADMAC2, which may stem from the electrostatic repulsion elevation among the polymers [11]. These results are in good correlation with the results presented in Fig. 1a, as PDADMAC3 produced larger flocs than did PDADMAC with lower molecular weight. It is implied from the results of Fig. 3 that flocculation was relatively fast, and it reached equilibrium in 15 min. The results in Figs. 2 and 3 are significant as they confirmed that the system is sensitive to the dosage of PDADMAC, and overdosing will seriously hamper the efficiency of lignosulfonate flocculation and lignosulfonate removal.

3.3.2. Flocculation growth rate by FBRM Fig. 3 shows the chord length and square weighted counts of the formed flocs in 30 min (1) in the different dosages of the 0.68, 1.28 and 1.88 mg/L for PDADMAC1, PDADMAC2, and PDADMAC3, respectively, and (2) 5.88 mg/L dosage for all PDADMAC polymers in the lignosulfonate solution. As seen in Fig. 3a, the chord length of the produced flocs for PDADMAC1 and PDADMAC2 followed a similar trend of slight increase in chord length and reached a chord length of around 115 µm after 20 min. For PDADMAC3, flocs formed more rapidly with the chord length of 170 µm in 17 min. By comparing all three systems, it was observed that the flocculant’s molecular weight could remarkably influence the produced floc’s size as larger flocs were formed by PDADMAC3. This may be attributed to the fact that a higher molecular weight polymer can have more interaction with the environment and can bridge more of lignosulfonates together [61]. Also, the square weighted counts of the produced flocs were monitored to determine the number of the formed flocs in 30 min of the experiment. Since the square weighted counts are sensitive to larger flocs than the unweighted counts, more accurate results could be achieved regarding the fine particles present in the suspension [23]. As seen in Fig. 3b, the number of the flocs produced was boosted in the first 3 min for all samples, which showed a rapid interaction of PDADMAC and lignosulfonate, and the production of insoluble

3.4. Floc strength analysis Fig. 4 shows the strength of the PDADMAC/lignosulfonate flocs at the dosages of 0.68, 1.28 and 1.88 mg/L for PDADMAC1, PDADMAC2, and PDADMAC3, respectively. In this experiment, the polymers were added to form flocs while stirring at 150 rpm. After 3 min, the agitation intensity was increased to 550 rpm leading to floc breakage. Degradation was more pronounced for PDADMAC3 and less detected for PDADMAC1 among all three types of the polymer. This is influenced by the polymer’s molecular weight as PDADMAC3 could form larger flocs, which were more sensitive to shear rate than the smaller ones produced by PDADMAC1 [20,29,61]. The shear rate was reduced to 150 rpm, and the flocs started to regrow. The obtained chord length was used to 258

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Table 3 The hydrodynamic diameter of the PDADMAC/lignosulfonate flocs. Sample

Dosage, mg/L

Hydrodynamic Size (dh), µm

Blank (LS )

–

0.1

PDADMAC1/LS

0.68 5.88

37 23

PDADMAC2/LS

1.28 5.88

54 41

PDADMAC3/LS

1.88 5.88

109 74

*

* LS: lignosulfonate. Fig. 4. Floc strength analysis conducted at different agitation intensities of 150 and 550 rpm at ambient condition and pH 7.

accelerated electrostatic repulsion between the polymers in overdosed condition [33,61]. These results are correlated well with the results presented in Figs. 2 and 3, as PDADMAC3 with the highest molecular weight could form larger flocs.

Table 2 Flocs strength and recovery factor of different PDADMAC polymers with lignosulfonate. Floc properties (%)

PDADMAC1/LS

PDADMAC2/LS

PDADMAC3/LS

Strength factor Recovery factor

57 37

54 35

53 32

3.6. Mass balance for lignosulfonate in spent liquor At their optimum dosages of 0.68, 1.28 and 1.88 mg/L, the lignosulfonate removal efficiency of PDADMAC1, PDADMAC2 and PDADMAC3 were 57, 88 and 92%, respectively. This means that 43, 12 and 8% of lignosulfonate was remained in the solutions when PDADMAC1, PDADMAC2 and PDADMAC3 were used as a flocculant, respectively. As observed, PDADMAC3 with a higher molecular weight could remove more of the lignosulfonate by producing larger flocs through bridging mechanism (Fig. 3a and b), facilitating the settlement of flocs. Although insoluble complexes were formed via using PDADMAC1, the formed flocs were not sufficiently large to settle, leaving more lignosulfonate flocs suspended in the solutions (Table 4, Fig. 3a and b). In another study, around 50% of lignosulfonate was isolated from the sulfite semi‐chemical spent liquor using PDADMAC with the molecular weight of 100–200 kg/mol [46]. In the present work, a significantly higher lignosulfonate removal was observed, which is attributed to the higher molecular weight of PDADMAC used in this study.

LS: lignosulfonate.

evaluate the strength and recovery factor of the flocs (Table 2) [28]. The increment and decrement in the shear rate were repeated for 18 min. The flocculation mechanism was seen to be irreversible after the second breakage as no significant enhancement in the chord length was observed [29]. Generally, in the flocculation processes containing oppositely charged polymers, the charge interaction would generate the driving force for agglomeration. At the optimum dosage, the generated flocs would coagulate and generate larger flocs. This phenomenon is promoted by stirring. The results in Fig. 4 depicts that the stirring intensity is important in this flocculation process, as it can break the generated flocs and hamper the flocculation. Flocs strength is a crucial factor in flocculation processes. Weak flocs may be broken under shear hampering the efficiency of flocculation processes. Table 2 tabulates the strength factor and recovery factor of the PDADMAC/lignosulfonate flocs. As seen, the strength factor was smaller for higher PDADMAC molecular weights as PDADMAC1/lignosulfonate flocs showed around 57% of strength to the breakage and PDADMAC3/lignosulfonate exploited around 53% of strength. This behavior was reported to be attributed to the bridging mechanism of the flocculation, which forms weaker and looser flocs [61]. Also, the larger the flocs, the lower regrowth was observed for them to happen, as PDADMAC1/lignosulfonate flocs could recover by around 4.8% more compared to the PDADMAC3/lignosulfonate flocs, which may be due to the more breakage of the PDADMAC3/lignosulfonate flocs [25,61]. In one study, Sun and coworkers used PDADMAC polymer to flocculate lignocelluloses from the thermomechanical pulping spent liquor and found that flocs produced by using PDADMAC polymer having the low molecular weight of 45 kg/mol showed around 8% higher strength and could recover up to 9% more than the flocs produced using a high molecular weight PDADMAC.

3.7. Flocculation analysis under non-stirring state Flocculation and sedimentation under non-stirring conditions has been practiced in industry for decades [13,66]. Non-stirring flocculation processes benefit from independency from agitation systems, but they may be slower than agitated systems. An efficient flocculant is a polymer that produces sediment fast, leading to a clear and transparent solution [22]. Fig. 5 shows the kinetic transmission of the lignosulfonate solution. In this experiment, the optimum dosages of 0.68, 1.28 and 1.88 mg/L of PDADMAC1, PDADMAC2, and PDADMAC3 in the simulated lignosulfonate solution were applied. Also, the analysis Table 4 Settling velocity and sediment compactness of the flocs after 30 min of treatment at different ratios of PDADMAC to lignosulfonate. Label

3.5. Hydrodynamic diameter (dh) analysis of the flocs The hydrodynamic diameter of the PDADMAC/lignosulfonate flocs is presented in Table 3. As seen, PDADMAC polymers could produce insoluble flocs with the dh of 37, 54 and 109 µm for the system of PDADMAC1, PDADMAC2, and PDADMAC3, respectively. By increasing the dosage of the polymers to 5.88 mg/L, the dh of the produced flocs was reduced to 23, 41 and 74 µm for PDADMAC1, PDADMAC2, and PDADMAC3, respectively. This behavior may be attributed to the

Dosage (mg/ L)

Settling velocity (mm/h)

Lignosulfonate

–

0.05

0.2

PDADMAC1/LS*

0.68 5.88

18 4

22 6

PDADMAC2/LS

1.28 5.88

8 4

26 7

PDADMAC3/LS

1.88 5.88

3 4

32 12

* LS: lignosulfonate. 259

Sediment compactness (g/L)

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PDADMAC induced a faster settling performance for generated PDADMAC/lignosulfonate flocs [1,65]. However, it should be emphasized that this settling velocity is generally slower than that of sludge in sequencing batch reactor (SBR) for treating wastewater [63], which is attributed to the smaller size of flocs generated in this work (Figs. 2 and 3) and that of sludge reported in other studies [63]. These results illustrate that a relatively longer resident time in the sedimentation process for these flocs are needed than other commercial processes, which may make it industrially unattractive. In addition, the TNb analysis revealed that the residual concentration of PDADMAC in the solutions after floc separation was 0.15, 0.38 and 0.61 mg/L when 0.68, 1.28 and 1.88 mg/L of PDADMAC1, PDADMAC2 and PDADMAC3 was used as a flocculant, respectively. These results suggest that less than 35% of added PDADMAC was left in the solution after floc isolation. Also, according to the literature [41], PDADMAC dosage below 30 mg/ mL is not harmful to the body cells. Thus, the remaining PDADMAC in the solution after flocs’ separation may not have adverse effects on ecosystems if released to the environment.

Fig. 5. Transmission of the lignosulfonate/PDADMAC systems as a function of time at pH 7 and 30 °C.

was repeated at a 5.88 mg/L dosage for all samples. The addition of PDADMAC polymers improved the transmission of the top layer of the lignosulfonate samples in the optimum dosages for all the three samples. As expected, PDADMAC3 showed the best clarification performance. These results are in good correlation with the results displayed in Fig. 3 and indicate that, by raising the PDADMAC’s molecular weight, more lignosulfonate polymer would be flocculated (more settling in non-stirring state), leading to system's improved transparency. Furthermore, the larger dosage (5.88 mg/L) of PDADMAC induced a lower transmission in the system, implying a reasonably stable solution (i.e., poor settling), which is due to the repulsion force generated among the flocs. The results are supported by literature reports in which an upsurge in PDADMAC’s molecular weight from 100 to 500 g/ mol accelerated the clearness of solution by 5% when used as a flocculant for wastewater treatment at the dosage of 9 mg/L [27]. As stated earlier, the interaction of oppositely charged polymers would induce small flocs. Overdosing of the system with PDADMAC would yield small flocs with a net charge, as seen as a positive zeta potential in Fig. 1. If these flocs bear a net charge, they can repel each other and stabilize the system due to their small sizes. The small increase in the transmission (Fig. 5) shows the gradual sedimentation of these small flocs and/or slow coagulation of these small flocs to larger ones for sedimentation. Interestingly, the sensitivity of the system to the dosage of PDADMAC was more observed under non-stirring condition (Fig. 5) than stirring condition (Fig. 3). In this case, stirring would disrupt the stability of the system derived by electrostatic repulsion, facilitating the agglomeration of particles. In this work, sediment compactness and settling velocity of the systems were also analyzed, and the results are tabulated in Table 4. The sediment compactness was identified by developing a mass balance for the top part of the samples before and after 30 min of PDADMAC addition to the system. As can be seen, the sediment structure was changed from 0.05 g/L for the lignosulfonate solution to 18, 8 and 3 g/L for PDADMAC1/lignosulfonate, PDADMAC2/lignosulfonate, and PDADMAC3/lignosulfonate, respectively, in their optimum dosages. This implies that the generation of flocs with higher molecular weight PDADMAC leads to the formation of sediment with higher thickness but lower compactness [73,56,72]. In other words, although PDADMAC3 produced more and large flocs, these flocs were more porous and loose in structure compared to the PDADMAC1/lignosulfonate flocs. The obtained results are in good harmony with the literature results [42,19,54]. The reduction in compactness of the sediment at a 5.88 mg/ L dosage of PDADMAC polymers may be due to the elevated repulsive forces between the polymers, as discussed earlier [54]. Also, the settling velocity of the samples was increased from 0.2 mm/h for lignosulfonate to 22 mm/h for lignosulfonate/ PDADMAC1, 26 mm/h for lignosulfonate/PDADMAC2 and 32 mm/h for lignosulfonate/PDADMAC3. These results could be attributed to the impact of the molecular weight of PDADMAC in that the larger

3.8. Fate of salt and sugar on the flocculation performance of lignosulfonate The effect of salt and sugar on the flocculation of PDADMAC and lignosulfonate was studied. In these experiments, 0.68 mg/L of PDADMAC1, 1.28 mg/L of PDADMAC2, and 1.88 mg/L of PDADMAC3 were added to the lignosulfonate solution (5 g/L) and the flocculation processes were tracked in these systems at different salt and sugar concentrations. 3.8.1. Salt effect The impact of sodium sulfite (in the dosage of 0.3–2.4 g/L) on the lignosulfonate flocculation is presented in Fig. 6 under stirring and nonstirring conditions, respectively. As seen, the increase in salt concentration has improved the flocculation by approximately 13% in both stirring and non-stirring environments, and the change was more

Fig. 6. (a) Flocculation index at 300 rpm and (b) transmission of lignosulfonate as a function of sodium sulfite concentration conducted at 25 °C and pH 7. 260

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stirring system, which should be considered for selecting the flocculation process. In short, the use of small amount of PDADMAC with a high molecular weight would be recommended as a promising alternative for extracting lignosulfonate from solution. Addition of the salt to the lignosulfonate solution improved the flocculation system of PDADMAC polymers, while insignificant effect of sugar on lignosulfonate flocculation was observed. However, it is recommended that the use of PDADMAC in the spent liquors of sulfite pulping process be evaluated to further develop PDADMAC based flocculation process for isolating lignosulfonate from pulping spent liquor. Acknowledgment The authors would like to thank NSERC, Canada Foundation for Innovation, Northern Ontario Heritage Fund Corporation, Ontario Research Fund, and Canada Research Chairs programs for supporting this research. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.04.042. References [1] Y. Adachi, Y. Tanaka, Settling velocity of an aluminium-kaolinite floc, Water Res. 31 (3) (1997) 449–454, https://doi.org/10.1016/S0043-1354(96)00274-6. [2] Z. Adamczyk, K. Jamroży, P. Batys, A. Michna, Influence of ionic strength on poly (diallyldimethylammonium chloride) macromolecule conformations in electrolyte solutions, J. Colloid Interf. Sci. 435 (2014) 182–190, https://doi.org/10.1016/j.jcis. 2014.07.037. [3] B. Al-Rudainy, M. Galbe, O. Wallberg, Influence of prefiltration on membrane performance during isolation of lignin-carbohydrate complexes from spent sulfite liquor, Sep. Pur. Technol. 187 (2017) 380–388, https://doi.org/10.1016/j.seppur. 2017.06.031. [4] T. Aro, P. Fatehi, Production and application of lignosulfonates and sulfonated lignin, ChemSusChem 10 (2017) 1861–1877, https://doi.org/10.1002/cssc. 201700082. [5] A. Blanco, E. Fuente, C. Negro, J. Tijero, Flocculation monitoring: focused beam reflectance measurement as a measurement tool, Can. J. Chem. Eng. 80 (4) (2002) 1–7, https://doi.org/10.1002/cjce.5450800403. [6] U. Bohme, U. Scheler, Hydrodynamic size and charge of polyelectrolyte complexes, J. Phys. Chem. B 111 (29) (2007) 8348–8350, https://doi.org/10.1021/jp070611e. [7] G. Cave, P. Fatehi, Separation of lignosulfonate from spent liquor of neutral sulphite semichemical pulping process via surfactant treatment, Sep. Purif. Technol. 151 (2015) 39–46, https://doi.org/10.1016/j.seppur.2015.07.017. [8] F. Chen, J. Li, Aqueous gel permeation chromatographic methods for technical lignins, J. Wood Chem. Technol. 20 (3) (2000) 265–276, https://doi.org/10.1080/ 02773810009349636. [9] J. Chen, A. Eraghi Kazzaz, N. AlipoorMazandarani, Z. Hosseinpour Feizi, P. Fatehi, Production of flocculants, adsorbents, and dispersants from lignin, Molecules 23 (4) (2018) 868–890, https://doi.org/10.3390/molecules23040868. [10] D. Chena, T.G.A. van de Ven, Flocculation kinetics of precipitated calcium carbonate induced byelectrosterically stabilized nanocrystalline cellulose, Colloid. Surf. A: Physicochem. Eng. Asp. 504 (2016) 11–17, https://doi.org/10.1016/j.colsurfa. 2016.05.023. [11] W.P. Cheng, F.H. Chi, C.C. Li, R.F. Yu, A study on the removal of organic substances from low-turbidity and low-alkalinity water with metal-polysilicate coagulants, Colloid. Surf. A: Physicochem. Eng. Asp. 312 (2–3) (2008) 238–244, https://doi. org/10.1016/j.colsurfa.2007.06.060. [12] G. Crini, Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment, Prog. Polym. Sci. 30 (1) (2005) 38–70, https://doi.org/ 10.1016/j.progpolymsci.2004.11.002. [13] H.M. De Paula, M.S. de Oliveria Iiha, L.S. Andrade, Concrete plant wastewater treatment process by coagulation combining aluminum sulfate and Moringa oleifera powder, J. Clean. Pro. 76 (2014) 125–130, https://doi.org/10.1016/j.jclepro.2014. 04.031. [14] G.V. Duarte, B.V. Ramarao, T.E. Amidon, Polymer induced flocculation and separation of particulates from extracts of lignocellulosic materials, Bioresour. Technol. 101 (2010) 8526–8534, https://doi.org/10.1016/j.biortech.2010.05.079. [15] K.R. Dyer, A.J. Manning, Observation of the size, settling velocity and effective density of flocs, and their fractal dimensions, J. Sea Res. 41 (1–2) (1999) 87–95, https://doi.org/10.1016/S1385-1101(98)00036-7. [16] P. Fatehi, Y. Ni, Integrated forest biorefinery, sulfite process, in: J.Y. Zhu, X. Zhang, X.J. Pan (Eds.), ACS Symposium Series, 1067 2011, pp. 409–441 (Chapter 16). [17] P. Fatehi, W. Gao, Y. Sun, M. Dashtban, Acidification of prehydrolysis liquor and spent liquor of neutral sulfite semichemical pulping process, Bioresour. Technol. 218 (2016) 518–525, https://doi.org/10.1016/j.biortech.2016.06.138.

Fig. 7. (a) Flocculation index at 300 rpm and (b) transmission of lignosulfonate as a function of xylose concentration performed at 25 °C and pH 7.

apparent for PDADMAC3. It was reported that salt may shrink lignosulfonate and reduce the repulsion force developed between formed flocs, and thus may generate more compact flocs that can settle easily [45,48]. 3.8.2. Sugar effect Fig. 7 depicts the effect of xylose concentration on the flocculation efficiency of lignosulfonate and PDADMAC. As seen, xylose had a marginal effect on the flocculation efficiency of lignosulfonate. The limited impact of sugar on isolation of lignosulfonate from pulping liquor was also noticed in other studies [46,26,17,3]. Therefore, the presence of xylose does not have any significant effect on the flocculation of lignosulfonate. 4. Conclusions The results depict that a small dosage of PDADMAC (less than 1.88 mg/L) was sufficient for efficient flocculation and thus isolation of lignosulfonate from simulated solutions. Due to the high charge density of PDADMAC, overdosing will render the zeta potential of the lignosulfonate solution cationic. This change in zeta potential may impact downstream processes for treating pulping spent liquor and thus overdosing should be prevented. The kinetic studies revealed that the interaction of PDADMAC and lignosulfonate was fast, which is beneficial for developing a separation process for lignosulfonate via using PDADMAC. The molecular weight of PDADMAC plays a crucial role in flocculation and more importantly on the structure and settlement of formed flocs. The flocs generated via high molecular weight PDADMAC (PDADMAC3) were more porous (contained more water) and had a faster settling performance than other samples. The high water content of the flocs may impose financial burden in the drying process for generating dried PDADMAC/lignosulfonate flocs, but dewatering the flocs may not be technically challenging as the flocs were porous. In addition, the stirring system was less sensitive to dosage than did non261

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