Characterization of cellulosebio-flocculant
On the basis of the abundant surface hydroxyl groups of cellulose, cellulosebio-flocculantcan be easily fabricated by crosslinking reaction with PEI and glutaraldehyde (Scheme 1). The grafting reaction is verified by FTIR spectra in Fig 1a.After grafting, the characteristic peak of cellulosefrom3349 cm-1shifts to 3417 cm-1 in CE-PEI associated with the overlapping of O-H and N-H stretching vibration (Cheng et al. 2014). Three new peaks at 1656, 1579 and 1430 cm-1appear, corresponding to bending vibration of N-H, stretching vibration of C=N and C-N, respectively (Park et al. 2014). This indicated the successful Schiff base reaction between aldehyde group (-COH) of glutaraldehyde and-NH2 of PEI. Moreover, the typical C-O-C stretching vibrations of cellulose polysaccharide at 1057-1160 cm-1 shift to a largerwavenumber,indicating thatthe formation of new ether bondbetweenglutaraldehyde and cellulose. In addition, the occurrence of -CH2- stretching vibrations at 2923 and 2848 cm-1 and C-H bending vibration at 771 cm-1 also confirms the successful grafting of PEI.The content of amino groups and the surface zeta potential of CE-PEI can be controlled by adjusting the reaction conditions, including the ratio of cellulose and PEI, reaction temperature, and glutaraldehyde amount. Fig 1band Table 1 showthat the amino group content and the zeta potential of CE-PEI 1 presented the maximum value of 17.5 mmol/g and 51.4 mV, revealing a positively charged surface due to the existence of multiple amino groups. The morphology of cellulose and CE-PEI was observed using FESEM and displayed an irregular shape (Fig 1c and d). The surface of cellulose is smooth. After PEI modification, many micro-crackswere found along withthe cellulose microfibers, whichcan facilitate the adsorption of pollutants.
Flocculation performanceof CE-PEI for Kaolin particles
To understand the relationship of surface property-activity, four bio-flocculants with different amino group content and zeta potential were employed to assess the removal of Kaolin particles.Here, the effects of CE-PEI dosage, pH value, setting time on flocculation performances were investigated in detail. Variation in turbidity, zeta potential, floc size, interface height of system was determined.
Effect of CE-PEI dosage
The effect of CE-PEI dosage on the residual turbidity and average floc size of the Kaolin suspension was studied and shown in Fig 2. In Fig 2a, the turbidity of the suspension all declined sharply with the increase of CE-PEI dosages until minimum values at corresponding optimal dosage, subsequently a tiny increase. Compared with CE-PEI 4, the turbidity removal efficiencies of CE-PEI 1, CE-PEI 2, and CE-PEI 3 are higher because of active sites increases with the amount of amino group. Theturbidity of Kaolin suspension significantlyreduced from initial to 480 NTU to 8.7 NTU for CE-PEI 1 with 6 mg. The removal efficiency approached a maximum value of 98.2%, higher than that of CE-PEI 2 (95.7%) and CE-PEI 3 (97.1%). However, the turbidity tiny increased when the dosage was 10 mg, which revealed that charge neutralization is dominant during the flocculation process. Positively charged CE-PEI neutralizes the surface negative charge in Kaolin particles to generate insoluble floc. Whereafter, flocs further aggregate, grow, and settle down, consequently resulting in turbidity reduction. Nonetheless, excessive flocculants lead to the destabilization of flocs due to the electrostatic repulsion betweenthe initial flocs, which isfurther demonstrated by the average floc size.
Fig 2b shows the change of the average floc size as the increase of CE-PEI dosages.Increasing CE-PEI dosages from 2 to 8 mg, the average floc size remarkably increased, subsequently decreased when the dosage reaches 10 mg. Maximum floc sizes produced by CE-PEI 1, CE-PEI 2, CE-PEI 3, and CE-PEI 4 reached 48.6±0.9, 45.0±1.4, 38.7±2.6, and 36.4±1.9 μm, respectively, which gradually enlargedwith the increase of amino group contents of four CE-PEI.Adding CE-PEI, positively charged flocculants have been transferred from the solution phase to the surface of Kaolin particles to generate insolubleflocculant-kaolin complexesthrough charge neutralization. The flocculant-kaolin complexes still have vacant active sites for absorbing other Kaolin particles which can produce bridging actions between Kaolin particles, consequently, forming larger flocs with a 3D network structure.Simultaneously,the flocs with a large size further capture small flocs and residual kaolin particles in the system via the capture effect and sweeping(Li et al. 2015). All these resulted in the optimal rate value for the precipitation of particles.Excessive flocculants might impart a positive electric charge to Kaolin particles to cause electrostatic repulsion. Additionally, at higher flocculants dosages could cover most of the available sites on the suspended particles, leading to ignorable bridging action. Thus, the floc size decreased.
Effect of pH
Fig 3shows the variation in turbidity and zeta potential of the suspension as a function of pH values. Introducing CE-PEI into suspension, residual turbidity of the system was declined at pH 3-7 butincreased when pHincreased from 8 to 12(Fig 3a). Moreover, CE-PEI 1 displayed the best Kaolin removal of CE-PEI can reach94.9%at pH 7.0.Notably, Kaolin standard suspensions are negatively charged within the entire pH range (Hosseinpour et al. 2020). The zeta potential of the system became increases and then decreases with increasing pH values(Fig 3b). Increased zeta potential could be because that the electronegative charges on the Kaolin particle surface were neutralized by the electropositive CE-PEI. The decrease might be due to the deprotonation of amino groups, where the negative surface charges on the CE-PEI increased at alkaline conditions. The maximum zeta potential of the system was found at pH 5.0. Moreover, it is clearly that the zeta potential of the system approached zero at pH 6.4-7.0 with different CE-PEI flocculants, revealing that thecharge neutralization effectperformed a dominant role in the flocculation process. By contrast, CE-PEI 4 showed higher turbidity reduction and zeta potential under an alkaline condition which revealed that contrasted with charge neutralization, bridging action and/or adsorption played a role in the flocculation process.
Effect of settling time
The time dependence of settling property was investigated turbidity and settling height of Kaolin suspension. As shown in Fig 4a,the turbidity of Kaolin suspension sharply decreased within the first 5 min, the reduction slightly fluctuated in the range of 78.3-85.0% corresponding to four CE-PEI flocculants with different surfaces properties. Fig 4b further displayed a rapid sedimentation process in view of the remarkable decline in interface height. Moreover, higher amino group content, faster settling velocity. These results showed that the sedimentation of most flocs was accomplished in a short time. With anincreasing settling time from 5 to 30 min, remnant turbidity decreased slowlybecause thegrowth of small flocs is slowly adversed to settlement. The best removal efficiency of the Kaolin turbidity of CE-PEI reached 98.2% in 30 min.
Flocculation kinetics and mechanism
Table 2 The rateconstants and correlation coefficients (R2) for flocculation of Kaolin by CE-PEI at different initial concentrations
|Flocculant doses (ppm)
||Kinetics of aggregation of particles
||Frequency of collisions of particles
|k1 (×10-14 count-1 s1)
||k2 (×10-3 s1)
||k (×10-14 s-1)
To further explain the flocculationmechanism, flocculation kinetics of representative CE-PEI 1 was investigated through blending flocculants (2-10 mg) into 40 mL of Kaolin suspension with rapid stirring (200 rpm) and then settling for 30 min. The supernatant was collected for kinetic analysis. The flocculation process is a mainly bimolecular reaction, thereby, the aggregation and collision of particles were determined according to the flocculation kinetic models, expressed as Eq(2) and Eq(3), respectively.
where Nt is the concentration of kaolin particles at t (s). N0 is the initial concentration of kaolin particles, which can be calculated by considering the particle diameter (1.2 μm) and density of kaolin (2.6 g cm-3). k1(s-1) and k2(s-1) is the kinetic constant for the particle aggregation and aggregate breakage, respectively. k (s-1)is the rate constant for particle collisions.
The theoretically simulated curves were fitted and the results are shown in Fig 5 and Table 2.The concentration of Kaolin particles was shown a remarkable reduction at the initial 60 s when CE-PEI 1 was added to the system, then gradually declined or remained stable with increased flocculation time, which revealed that the excellent adsorption-flocculation-sedimentation of CE-PEI for Kaolin, as show in Fig 5a. The concentration of Kaolin particles at a lower dosage (50-100 mg/L) and a higher dosage (250 mg/L) were higher than those at the intermediate dosage (150-200 mg/L). It's due to the number of positive charges on CE-PEI 1 was not enough to completely neutralize the negative charges on the surface of Kaolin particles at the low dosage. Andexcessive dosage led toelectrostatic repulsion and cage effect caused by CE-PEI at higher dosages covering most of the available sites on each particle. Thus, the rate for aggregation of Kaolin particles decreased. As shown in Table 2, the rate constant k1andk2 for particleaggregation and aggregate breakage increased with increasing flocculant dosages from 50 to 200 mg/Lexcept for k2 at 150 mg/L.k1 andk2 obtained with 250 mg/Lwere 123.26×10-14 s-1 and 85.15×10-3 s-1, respectively. This shows that the floc generated at high dosage was harder than floc formed at low dosage due to the steric and electrostatic repulsion forces among particles. Moreover, the minimum k2 of 4.08×10-3 s-1 was found at the dosage of 150 mg/L, which indicated a moderate CE-PEI could improve the floc strength and stability. Further, the maximum kof 10.51×10-14 s-1 was also found at 150 mg/L whichindicated that collision between particles was effective at the optimal dosage,and increase or reduction dosages will cause the decrease in the value of k(Peng et al. 2010). At a low dosage, the interaction including charge neutralization and bridging action between CE-PEI and Kaolin particles was very weak,it's due to the low-density positive charge in the system, leading tolower k. There were significant positive correlations between the value of k and the dosage of CE-PEI resulting from more positive charges and adsorption sites on the flocculants, which is beneficial for accelerating the molecular collisions. However, an excess of CE-PEI caused steric and static repulsion and reduced the number of effective junction points causing decreased k.
Therefore, according to the analysis on the flocculation performances and flocculation kinetics, the assumed flocculation mechanism of CE-PEI for Kaolin particles was proposed and illustrated in Fig. 6.The stabilization of Kaolin suspension was destroyed by the addition of positively charged CE-PEI via charge neutralization, and numerous small aggregates generated. These aggregates collided and further agglomerated with regional electrostatic attraction and supramolecular structure of CE-PEI through bridging action. These larger netlike aggregates could further capture the suspended Kaolin particles by capture and sweeping effect, forming heavier and denser flocs, finally sedimentation.