Sustainable Cationic Cellulose for Highly Ecient Flocculation of Suspended Particles

Sustainable, cationic cellulose bio-occulants with various amino group contents were successfully prepared by a feasible chemical crosslinking with polyethyleneimine (PEI). The occulation performances of diverse PEI-grafting cellulose (CE-PEI)were evaluated topurify sewage sludge treatment. Further, the preparation conditions and occulation mechanismof CE-PEI were investigated. Beneting from the high surface positive charges and the supramolecular structure of PEI, the results indicate that CE-PEI could remove the turbid Kaolin suspension effectively. The eciency of CE-PEI for removing the turbid in Kaolin suspension is 98.2%.Flocculation kinetic results indicated that charge neutralization was the dominant mechanism for the occulation process. And then, the small Kaolin particles agglomerate together to form large ocs by the function of adsorbing, gathering, and enwrapping. Thus, this work not only exploits a promising application of cellulose as a bio-occulantbut also provides a feasible approach to eciently purify high turbidity wastewater. 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.


Introduction
With the accelerated development of industrialization and urbanization, an enormous quantity of high turbidity wastewater was produced by the paper-making, printing and dyeing, leather,livestock, agriculture, building industries, which aggravates the process of water puri cation.To address the problem and create a sustainable future, a variety of technologies, such as membrane ltration (Jamshidifard et  occulationis deemed to be the most favorable method for wastewater treatment becauseof its advantages of low cost, easy operation, and high e ciency for suspended colloid removal (Carvalho et al. 2018;Essandoh et al. 2019). However, severe problems of massive dosage, chemical sludge recycling, corrosively restrict the application of inorganic metal-based occulants such as iron, aluminum, and titanium salts in turbid water puri cation (Zou et al. 2017; Wang et al. 2019). Whereas in fact, effective organic occulants, for example,polyacrylamide (PAM) and its derivatives were exploited to reduce water turbidity (Chen et al. 2019;). However, the high occulation performance of PAM can be achieved by the addition of coagulant aids (e.g. CaCl 2 ). Meanwhile, the coagulant aids resulting in a large number of loose-sludges. Moreover, organic PAM is synthesized from petrochemical resources, and cannot be biodegraded afterenvironmental restoration, resulting insecondary pollution. Thus, the design and development of sustainable and degradable bio-occulants with high occulation performancesisvitalfor wastewater puri cation.
Cellulose, as the most abundant natural bioresources, has great potential for the removal of various pollutants due to its marvelous properties of biodegradability,nontoxicity, cheap and widely available (Salehizadeh et al. 2018;Shak et al. 2018;Noor et al. 2020).It is well known that functional modi cation of cellulose can be easilyobtained, due to itssu cient hydroxyl groups on the surface.Thereby, a series of anionic groups such as carboxyl groups, phosphategroups, andsulfo groups, were introduced to improve the removal ability of cellulose for cationic dyes or heavy metal ions (Rodríguez et al. 2011;Fu et al. 2017;Wang et al. 2020). For example, Li et al. )successfully prepared carboxylatedcellulose lters by TEMPO-mediated oxidation and presented excellent adsorption capacity for cationic dyes and heavy metal ions.Ge et al. (Ge et al. 2016)andZhang et al. (Zhang et al. 2016) prepared cationic cellulose ber by introducing polyethyleneimine (PEI) for the removal of heavy metal ions.Besides,Dicarboxyl cellulose occulant was manufactured by periodate oxidation for the removal of kaolin suspension assisted with CaCl 2 as a coagulant .However, to the best of our knowledge, the employment of cationic cellulose as bio-occulantfor the directremoval of suspended particles from wastewater has been rarely reported.
Hence,this work is to exploit the potential of cellulose bio-occulant for e ciently removing suspended particles and reducing the turbidity of water. On the basis of abundant surface hydroxyl groups of cellulose, amine-rich cellulose bio-occulantshave been fabricated by a feasible chemical crosslinking withbranched polyethyleneimine (PEI). The obtained cellulose bio-occulantshave a supermolecular structure and large amounts of positively charged amine groups, exhibiting outstanding occulation performance for suspended kaolin particles without any coagulant aid.Further, generated ocs, zeta potential, and occulation kinetics were evaluated for understanding the occulation mechanism.

Materials And Methods
Materials.

Fabrication of cellulose bio-occulant.
Amine-rich cellulose bio-occulantswere fabricated by chemical crosslinking with branched polyethyleneimine (PEI) in a heterogeneous system, according to our previous report (Chen et al. 2018).Brie y, Cellulose (1.0 g) was dispersedindistilled water (100 mL) to swell for 12 h. Desired amounts of PEI weredoped into cellulose suspension under mechanical stirring. Crosslinking reaction was started by introducing glutaraldehyde as a cross-linking agent, under vigorous stirring at different temperatures for 3h. Finally, PEI-grafted cellulose (CE-PEI) was extracted by suction ltration and adequately washed. The surface charge density of CE-PEI was adjusted by varying the reaction conditions, correspondingly the products are denoted as CE-PEI 1, CE-PEI 2, CE-PEI3, and CE-PEI 4. The sample codes of the CE-PEI and relevant synthesis conditions were list in Table 1. Table 1 The codes and synthesis conditions of CE-PEI and their amino contents and ζ-potentials Glutaric dialdehyde (g) Amino group contents (mmol/L) ζ-potentials (mV) CE-PEI Characterization.
The surfacetopography of samples wasinvestigated by using Field Emission Scanning Electron Microscopy (FESEM, vltra55, German). Chemical crosslinking reaction was demonstrated using Fourier Transform Infrared Spectroscopy (FTIR, Nicolet 5700, Thermo, USA). Zeta potential of samples was measured using NanoSizer Nano-ZS90 (Malvern, UK).The contents of amino groups of samples were where C i and C e are initial concentration and equilibrium concentration of HCl solution (mol/L), respectively.
Flocculation performance for suspended particles The occulation performance of CE-PEI for suspended particles from wastewater was assessed, by using Kaolin standard suspension with 500 mg/L as a model. Brie y, CE-PEI was added into 40mL of Kaolin suspension and stirred at 200 rpm for 1 min. Finally, a slow stirring at 50 rpm for 5 min was used to oc aggregation. Subsequently, setting for sedimentation without stirring.After sedimentation,the change of turbidity and zeta potential of the system was tested using Turb 550 turbiditor and NanoSizer Nano-ZS90.
The ocs size was determined using Mastersizer 2000 (Malvern). The setting height was measured by adding 100 mL Kaolin suspension into abeaker(1 L) cylinder with optimal dosage of cellulose bioocculants at 5 min intervals. The effects of CE-PEI dosages (0-10 mg), suspension pH values (3)(4)(5)(6)(7)(8)(9)(10)(11), and settling time (0-30 min) on occulation behavior were investigated in detail. Each test was performed three times.  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. 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,   where N t is the concentration of kaolin particles at t (s). N 0 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 ). k 1 (s -1 ) and k 2 (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 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

Conclusion
In this work, a series of sustainable cationic cellulose bio-occulants with various amino group contents were successfully prepared by a feasible chemical crosslinking with PEI and glutaraldehyde. The occulation performances of diverse CE-PEI bio-occulants were evaluated to purify turbid Kaolin suspension. Further, the occulation kinetics and occulation mechanism were investigated. Bene ting from the high surface positive charges and supramolecular structure, CE-PEI bio-occulants with amino group contents of 17.5 mmol/g displayed the best turbidity removal e ciency with the dosage of 0.15 mg/mL, sedimentation time of 30 min at pH 7.0. The residual turbidity of Kaolin suspension decreased from the initial 480 NTU to 8.6 NTU, a 98.2% reduction. Flocculation kinetic results illustratedthat the interaction of aggregation and collision between CE-PEI bio-occulants and Kaolin particles was su cient for the occulation process at the optimal CE-PEI dosage. Moreover, charge neutralization was the dominant mechanism for the occulation of CE-PEI on Kaolin. Thus, this work not only exploits a Page 9/15 promising application of cellulose as a bio-occulantbut also provides a feasible approach to e ciently purify high turbidity wastewater. Effect of pH on the turbidity (a)and Zeta potential (b) of kaolin solution  Proposed occulation mechanism of CE-PEI for Kaolin particles