Abstract
Cellulose-based flocculants have shown excellent performance for wastewater flocculation, being low-cost and eco-friendly. However, they are still disturbed by the problems of incomplete biodegradability and unstable chemical structure. In the present study, 2,3,6-tricarboxylate cellulose (TCC) was developed as a novel fully biodegradable flocculant to deal with the preceding problems. The key carboxymethylation of cellulose was first carried out to make the subsequent NaIO4 oxidation occur under homogeneous conditions, which greatly enhanced the carboxylate content of the final TCC products. The chemical structure and solution properties of the TCCs were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer system (XRD), field emission scanning electron microscopy (FESEM), charge density, particle size, and zeta potential. The flocculation performance of the TCCs was evaluated preliminarily by the turbidity removal of kaolin suspension. The positive results showed that all the TCC products had high carboxylate contents (more than 10 mmol/g) and zeta potentials. They exhibited excellent flocculation performance for the kaolin suspensions, in which the residual turbidities decreased from 610 to 14.9 NTU. Considering the degradation of cellulose caused by excessive NaIO4 oxidation, the TCC IV, together with its synthesis technology, could be used for practical applications in wastewater flocculation.
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2,3,6-Tricarboxylate Cellulose as a Fully Biodegradable Flocculant: Efficient Synthesis and Flocculation Performance
Kexin Chen,a Weijun Lv,b,* Wei Chen,a Yifeng Wang,c Yong Zhang,a,* Xiumei Zhang,a and Juming Yao a
Cellulose-based flocculants have shown excellent performance for wastewater flocculation, being low-cost and eco-friendly. However, they are still disturbed by the problems of incomplete biodegradability and unstable chemical structure. In the present study, 2,3,6-tricarboxylate cellulose (TCC) was developed as a novel fully biodegradable flocculant to deal with the preceding problems. The key carboxymethylation of cellulose was first carried out to make the subsequent NaIO4 oxidation occur under homogeneous conditions, which greatly enhanced the carboxylate content of the final TCC products. The chemical structure and solution properties of the TCCs were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer system (XRD), field emission scanning electron microscopy (FESEM), charge density, particle size, and zeta potential. The flocculation performance of the TCCs was evaluated preliminarily by the turbidity removal of kaolin suspension. The positive results showed that all the TCC products had high carboxylate contents (more than 10 mmol/g) and zeta potentials. They exhibited excellent flocculation performance for the kaolin suspensions, in which the residual turbidities decreased from 610 to 14.9 NTU. Considering the degradation of cellulose caused by excessive NaIO4 oxidation, the TCC IV, together with its synthesis technology, could be used for practical applications in wastewater flocculation.
Keywords: Tricarboxylate cellulose; Natural flocculant; Homogeneous oxidation; Carboxylate content; Full-biodegradation
Contact information: a: The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Ministry of Education, College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, China; b: The National Engineering Laboratory for Pulp and Paper Technology, China National Pulp and Paper Research Institute, Beijing 100020, China; c: The Hydrology and Water Resources Monitoring Station in Hangzhou, Hangzhou 310016, China;
* Corresponding author: zhangyong@zstu.edu.cn, lvweijun761005@163.com
INTRODUCTION
Wastewater, generated from various industries and containing toxic chemical contaminations, is discharged into natural water bodies and can cause serious environmental and ecological problems. Flocculation is an economical and effective technique for removal of particle contaminants, such as organic pollutants in chemical and papermaking wastewater (Faust et al. 2014), as well as dyes from dyeing effluent (Yang et al. 2013a). In recent years, synthetic organic polymer flocculants, especially polyacrylamide and poly diallyl dimethyl ammonium chloride, have seen extensive application because of their mature production technology and high flocculation efficiency. However, their poor degradability and hazardous degradation products eventually remain a potential threat to the natural environment and human health (Faris et al. 2015).
Natural polymer-based flocculants, because of their non-toxicity, biodegradability, and regeneration, can meet the increasing demand for eco-friendly and sustainable materials. Recently, they have become a key research focus around the world, e.g., starch-based flocculants (Yang et al. 2014), inulin-based flocculants (Rahul et al. 2014), and cellulose-based flocculants (Vandamme et al. 2015). Among these natural materials, cellulose is the most abundant natural polymer worldwide and can be used as a raw material to develop various eco-friendly materials (Kureli and Doganay 2015). Based on its physical properties and chemical reactivity, cellulose is a suitable alternative for modification and preparation of natural flocculants. In current cellulose-based flocculants, semi-natural flocculants that graft copolymers onto a cellulose backbone have been an important and often-used type of flocculant (Zhu et al. 2015). However, these flocculants have the same environmental problems as synthetic flocculants because of their incomplete biodegradability and unstable chemical structure.
To solve this problem, it is a good way to obtain fully biodegradable flocculants by modifying cellulose directly. One of the core technologies is the introduction of carboxyl groups onto cellulose macromolecules to increase their charge density, such as preparations of dicarboxylic acid nanocellulose (Suopajärvi et al. 2013), carboxymethyl cellulose (Qi et al. 2010), and dicarboxyl cellulose (Zhu et al. 2015). The resultant flocculants are completely biodegradable and do not provide non-secondary contamination to the environment. However, the biggest challenge in this process now is the limited carboxylate content, which leads to lower charge density on the flocculants. This results in a large impact on their flocculation performance. In previous work, C6-carboxyl cellulose was synthesized by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation, and its carboxylate content was 1.69 mmol/g (Okita et al. 2010). To further develop this work, Takaichi and Isogai (2013) adopted the 2-azaadamantane N-oxyl (AZADO) /NaBr/NaOCl system to oxidize softwood pulp for the production of tricarboxylate cellulose. The carboxylate content of this product grew to 9.70 mmol/g, which was contributed by the three carboxyl groups grafted on the cellulose skeleton (Takaichi et al. 2014). However, the reagents used in this process are expensive, substantially increasing production costs. In Qi’s work, carboxylmethylation of cellulose was investigated to obtain carboxymethyl cellulose with a carboxylate content of 1.48 mmol/g (Qi et al. 2010). Although the carboxyl groups were conveniently and mildly formed in this method, the final carboxylate content was still relatively low.
Another method for carboxyl cellulose preparation is NaIO4 oxidation followed by NaClO2oxidation. The C2 and C3 of anhydroglucose units (AGUs) are first oxidized to aldehyde groups through NaIO4 oxidation; they are then continuously oxidized to carboxyl groups by NaClO2. Liimatainen et al. (2012) adopted this process to synthesize dicarboxyl cellulose (DCC) with a carboxylate content of 1.75 mmol/g. The aldehyde content of its precursor (dialdehyde cellulose, DAC) was 1.68 mmol/g. The work of Kim and Kuga (2001) also resulted in a similar carboxylate content of 0.92 mmol/g in DCC and aldehyde content of 0.93 mmol/g in DAC. In our most recent work, a one-step homogeneous process was developed to synthesize DCC. The aldehyde and carboxylate contents were also both in the range of 2 to 3 mmol/g (Zhu et al. 2015). The low aldehyde content severely restricts the formation of carboxyl groups on cellulose macromolecules. A breakthrough in the oxidation process is strongly desired.
Recently, it was found by the authors that conducting carboxymethylation first on cellulose, followed by NaIO4 oxidation, can enhance the aldehyde content of the resultant DAC. To obtain evidence for this, a novel NaIO4 and NaClO2 oxidation route was investigated to synthesize a 2,3,6-tricarboxylate cellulose (TCC) with high carboxylate content in a cost-effective manner, designed to result in fully biodegradable cellulose flocculants based on the biodegradability of cellulose (Klemm et al. 2005; Liu et al. 2008). The chemical structure of the TCC product was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer system (XRD), and field emission scanning electron microscopy (FESEM). The charge density (CD), aldehyde and carboxylate contents, particle size, and zeta potential of the TCC were also analyzed in detail. Meanwhile, a standard kaolin suspension was employed to evaluate the flocculation performance of the TCCs.
EXPERIMENTAL
Materials
α-Cellulose powder (DP 170-200, size 25 μm; Aladdin Chemistry Co. Ltd., China) and kaolin particles (superfine size of 11 μm; Aladdin Chemistry Co. Ltd., China) were used as received. The reagents for dissolution (NaOH, H2NCONH2), oxidation (NaIO4, NaClO2, COHCH2CH2OH), carboxyl and aldehyde content analysis (NH2OH.HCl, NaCl, CH3COOH, CH3COONa.3H2O), poly (dimethyldiallylammonium chloride),and auxiliary process (AlCl3.6H2O, CH3CH2OH) were of analytical grade (Guoyao Chemcial Reagent Co., Ltd., China), and used without further purification. Commercial-grade carboxymethyl cellulose (CMC) and anionic polyacrylamides (APAMs) with the molecular weight of 90 thousand and 12 million were supplied by the Aladdin Chemistry Co. Ltd., China. Deionized water, produced by a Millipore Direct-Q 5 ultrapure water system (EDM Millipore Co., Germany), was used throughout the experiments.
Synthesis of TCC
Carboxymethylation of cellulose
Anionic cellulose (AC) was synthesized through carboxymethylation of cellulose. One gram of α-cellulose powder and 50 mL of urea-NaOH aqueous solution (12 wt% urea and 7 wt% NaOH, precooled at 4 C) were added to a 250-mL flask. The mixture was stirred vigorously to make it well dispersed. Then the suspension was cooled to -12 C for at least 1.5 h to obtain a transparent and viscous cellulose solution (Yang et al. 2013a). Next, 4.3 g of sodium monochloroacetate was added to the cellulose solution and vigorously stirred by a magnetic stirrer in a water bath at 55 C (Qi et al. 2010). After 5 h, the mixture was neutralized with 3 M HCl and extracted by 95% ethanol. After centrifugation, the AC product was dried in a freeze-dryer.
Fig. 1. Proposed synthesis route for the TCC product (cellulose: α-cellulose powder, AC: anionic cellulose, ADAC: anionic dialdehyde cellulose, TCC: 2,3,6-tricarboxylate cellulose)
NaIO4 and NaClO2 oxidations of AC
Anionic dicarboxyl cellulose (ADAC) with various charge densities was synthesized by first oxidizing the hydroxyl groups on C2 and C3 of AGUs of the AC using NaIO4. The aldehyde groups on the obtained ADAC were subsequently oxidized to carboxyl groups by NaClO2 to produce TCC. In brief, 1 g of AC was subjected to oxidation with 0.8 to 2.4 g NaIO4 at 50 C in the dark for 3 h (Liimatainen et al. 2012). The residual NaIO4 was neutralized with excess ethylene glycol. The product was subsequently extracted by 500 mL of ethanol, centrifuged, washed, and further oxidized with NaClO2 in an acetic acid buffer (pH 4.5) for 48 h at room temperature to obtain the TCC. The resultant TCC product was finally extracted and washed by ethanol and water, respectively. The proposed synthesis route of the TCC is depicted in Fig. 1.
Characterization of AC, ADAC, and TCC
Analysis of aldehyde and carboxylate contents
The aldehyde content of ADAC was determined by an oxime reaction as described in the literature (Sirvio et al. 2011). The ADAC was converted to oxime by Schiff’s base reaction using NH2H2O.HCl. First, 0.1 g of ADAC was suspended in 30 mL of deionized water and the pH was adjusted to 4.5 with 0.1 M HCl or NaOH. The ADAC suspension was added to the NH2H2O.HCl solution (0.43 g in 20 mL H2O, pH 4.5). The mixture was stirred for 24 h at room temperature. Then, the released HCl was titrated using 0.01 M NaOH. The aldehyde content of the ADAC was calculated through the consumption of the 0.01 M NaOH. The carboxylate contents of AC and TCC were analyzed with a DDS-307 conductivity meter (Shanghai REX Instrument Factory, China) using the procedure described by Zhu et al. (2015). Briefly, 0.3 g of dried sample was added to a solution consisting of 5 mL of 0.01 M NaCl and 55 mL of H2O. The mixture was sufficiently stirred to obtain a well-dispersed slurry. Then, 0.1 M HCl was added to the slurry to control its pH in the range of 2.5 to 3.0. The slurry was titrated with 0.04 M NaOH at a rate of 0.1 mL/min to pH 12. The carboxylate content was determined based on the conductivity and pH curves.
Characterization of products
The zeta potential and particle size of the AC and TCC were investigated using a Malvern Nano ZS90 (Malvern Instruments Ltd., USA). The solution for zeta potential measurement was at neutral pH in 0.5 mM [Na+] buffer. The charge densities of the AC and TCC solutions (1 wt%) were tested by polyelectrolyte titration with a cationic polyelectrolyte (poly- dimethyldiallylammonium chloride, PDADMAC, 0.0001 M) using a Mütek PCD 03 particle charge detector (BTG Co., Sweden). The FTIR spectra of the cellulose, AC, ADAC, and TCC were recorded using a Thermo Nicolet 5700 FTIR spectrometer (Thermo Fisher Scientific Inc., USA). The crystallinity of the cellulose, AC, ADAC, and TCC (powder samples) was measured by Bruker AXS D8 (Bruker Co., Germany) X-ray diffractometry. The crystallinity index (CI) of samples was calculated through peak fitting and calculating program (Java and UML Developers’ Environment). The morphology of samples was photographed using a Hitachi S4800 FESEM (Hitachi Ltd., Japan).
Evaluation of TCC Coagulation-Flocculation
The flocculation of the TCC products was evaluated by measuring the residual turbidity of kaolin suspension with a concentration of 1 g/L after coagulation-flocculation. The initial pH of the kaolin suspension was 6. The experiments were carried out by adding Al3+ (150 mg/L, optimized dosage, derived from AlCl3.6H2O) as coagulant into 100 mL of kaolin suspension. The mixture was stirred at 200 rpm for 1 min. Then, a set amount of TCC was added with slow stirring at 40 rpm for 3 min. Finally, the suspension was allowed to settle for 10 min and the residual turbidity of the supernatant was measured by Turb 550 turbidity (Wissenschaftlich-Technische Werkstätten GmbH., Germany). The zeta potential of the initial and coagulated kaolin suspension was monitored to assess the influence of pH in the coagulation process. The range of pH in the kaolin suspension increased from 3 to 10, as adjusted by 0.1 M HCl or NaOH solution.
RESULTS AND DISCUSSION
Optimization of ADAC Synthesis
Because of the high crystallinity of natural cellulose, a large amount of NaIO4 and a long oxidation time are required to obtain high aldehyde content in DAC. In the present work, a novel synthesis route was designed to solve this problem. Water-soluble AC was prepared before NaIO4oxidation to reduce the crystallinity of cellulose. Since the excellent water-solubility of AC, efficient NaIO4 oxidation could be carried out under homogeneous conditions to obtain the target ADACs with high aldehyde content. Meanwhile, considering the relatively high cost of NaIO4, a single-factor experiment was conducted to explore an economical and eco-friendly oxidation process. The detailed experimental schedule is presented in Table 1.
Table 1. Optimization of NaIO4 Dosage for ADAC Synthesis
The aldehyde contents of the resultant ADAC (I through V) increased with increasing mass ratio of NaIO4 to AC, especially for ADAC I through III. Furthermore, the aldehyde contents of ADAC III-V all exceeded the theoretical maximum value of 12.34 mmol/g. This was attributed to the fact that in addition to the almost complete transformation of the hydroxyl groups on C2 and C3 of the AC, the residual hydroxyl groups on C6 were also oxidized to aldehyde groups by NaIO4 (Sirviö et al. 2011). As a result, relatively high aldehyde contents of 8.70 to 13.55 mmol/g and low consumption of NaIO4 for ADAC synthesis were achieved in comparison with other works.
Synthesis of TCC
The next transformation from aldehyde group to carboxyl group was conducted by chlorite oxidation to obtain TCC flocculants with high negative zeta potential and charge density. The precursor ADAC I-V were transferred into acetic acid buffer and subsequently oxidized by NaClO2 for 48 h to synthesized the corresponding TCC I-V. Table 2 exhibits the electrochemical characteristics of the TCC products with various oxidation degrees. The maximum carboxylate content was determined to be 14.63 mmol/g of TCC V, which was lower than the theoretical maximum value of 18.52 mmol/g. This value also indicated that the aldehyde groups on ADAC were almost completely transformed to the carboxyl groups of TCC. This fact confirmed that the high carboxylate content was closely related to the oxidation degree of ADAC and the NaIO4oxidation was remarkably efficient under homogeneous conditions. Correspondingly, the zeta potentials of the TCCs increased from -23.3 to -41.8 mV with increasing carboxylate content. The CDs of the TCCs and AC presented high consistency with their carboxylate contents. Moreover, the variation of particle size from AC to TCC indicated that the degradation of cellulose macromolecules occurred during the NaIO4 and NaClO2 oxidation processes. This might affect the performance when the TCC is employed as a flocculant.
Table 2. Carboxylate Content, Particle Size, Charge Density, and Zeta Potential in 0.5 M [NaCl] Buffer of TCC Products from ADACs with Various Oxidation Degrees
Characterization of AC, ADAC, and TCC
Chemical structure
The structural changes of the cellulose and related products were characterized by FTIR. Figure 2 presents the FTIR spectra of the cellulose, AC, ADAC, and TCC. The absorption peak of the AC at 1603 cm-1 indicated that COO– was successfully introduced onto the cellulose skeleton.
Fig. 2. FTIR spectra of the cellulose, AC, ADAC, and TCC
The spectra for the ADAC showed characteristic bands at 1760 and 893 cm-1. They were assigned to carbonyl groups and hemiacetals (Teotia 2012). It was further observed that the band at 1603 cm-1 moved to 1630 cm-1. This was attributed to the formation of aldehyde groups. According to the spectrum of the TCC, the disappearance of aldehyde bands at 1760 cm-1 and generation of an intense carboxyl characteristic peak at 1603 cm-1 demonstrated the important transformation from aldehyde group to carboxyl group on the TCC macromolecules, confirming that the target TCC product was successfully synthesized.
Crystallinity
The influences of carboxymethylation and oxidation on the crystal structure of cellulose are presented in Fig. 3. As shown in Fig. 3a, AC had a crystallinity of 36.4%, which was much lower than that of the original cellulose, at 68.4%. This suggests that the crystalline region of cellulose was damaged extensively by the carboxymethylation of the hydroxyl groups on C6. Meanwhile, the typical peak of cellulose at 22.6° disappeared and a broadened peak at 21.7° was generated with the formation of the crystalline structure on the AC. In Fig. 3b, in the X-ray diffraction pattern of ADAC, disordered amorphous regions were observed. The well-defined AC pattern was obviously diminished during the oxidation. The calculated crystalline index of the ADAC sharply decreased to 17.1%, mostly caused by the ring-opening reaction of AGUs, changes in hydrogen bonding, and degradation of cellulose chains (Teotia 2012). With the subsequent transformation from ADAC to TCC, the crystalline region near 21° was re-established by the formation of new hydrogen bonds from the carboxylate groups of TCC. The CI of the TCC finally returned to 49.8%. The specific crystallinity index, particle Size and L/D of the series products are summarized in Table 3.
Table 3. Crystallinity index, Particle Size, and L/D of the series products
Fig. 3. Transformation of crystal structure of cellulose, AC, ADAC, and TCC: (a) crystallinity index, (b) XRD pattern
Fig. 4. FESEM pictures of (a) original cellulose, (b) AC, (c) ADAC, and (d) TCC
Morphology
Figure 4 exhibits the surface morphologies of cellulose, AC, ADAC, and TCC. It can be seen from Fig. 4a that the surface of the original cellulose was rough and wrinkled with the form of rod-like fibers. As shown in Fig. 4b, the surface of the AC became amorphous and showed a disordered distribution after carboxymethylation. This was primarily due to the destruction of crystalline regions on cellulose, which provided the lower degree of crystallinity and better water-solubility to the AC. In the case of oxidized cellulose, Fig. 4c shows that the morphology and structure of the ADAC was reshaped. The L/D (length-diameter) ratio also became higher than that of the AC. With the following conversion from ADAC to TCC, shown in Fig. 4d, the L/Dratio of the TCC was further increased, which was attributed to the destruction of intermolecular H-bonding on cellulose macromolecules. Moreover, the distribution of the TCC in Fig. 4d was very regular. This is mostly caused by electrostatic repulsions from the newly formed carboxyl groups of the TCC product.
Coagulation-Flocculation of Kaolin Suspension
Optimization of coagulant dosage
To destabilize a kaolin suspension, the process of coagulation is necessary and the dosage of coagulant is a crucial factor in coagulation. Inorganic Al3+ was employed as a coagulant in the subsequent coagulation-flocculation of the kaolin suspension. Figure 5 shows the residual turbidity and zeta potential variations of the kaolin suspension (1 g/L, original turbidity 610 NTU) with increasing coagulant dosage at the initial pH 6. The Al3+ solution (AlCl3.6H2O, 10 g/L) was used during the coagulation process. The residual turbidity of the kaolin suspension decreased substantially before the Al3+ dosage of 150 mg/L. Then, the turbidity decline was mild with increasing Al3+ dosage. The residual turbidity reached the lowest point, 90.5 NTU, when the Al3+dosage was 400 mg/L. Simultaneously, the zeta potential of the kaolin suspension was close to zero at this point. This phenomenon indicated that the charge neutralization mechanism played a major role in this coagulation process (Yang et al. 2013b). Considering the low cost of practical application, the optimal Al3+ dosage of 150 mg/L was adopted for subsequent TCC flocculation.
Fig. 5. Residual turbidity of the kaolin suspension (1 g/L) as a function of coagulant dosage (Al3+) after the coagulation (pH of the kaolin suspension as 7 and settling time as 30 min).
Influence of pH on coagulation
Coagulation with Al3+ was tested as a function of pH to obtain the feasible pH conditions and help to explain its assistant role in the subsequent TCC flocculation. Figure 6a indicates that the coagulation showed better performance in acidic conditions. After the point of pH 6, the residual turbidity of kaolin suspension remained approximately constant with increasing pH. This can be explained by the reaction between Al3+ and OH– in the suspension. With increasing pH, Al3+combined with OH– to form AlO2-, which maintained the pH stability of the suspension (Wei et al. 2015). This was also confirmed in Fig. 6b, in which the final pH reached a plateau after coagulation in the initial pH range from 5 to 10. In Fig. 6c, because of its negative zeta potential, kaolin served as an anionic particle in the suspension.
Fig. 6. pH influence of Al3+ coagulation on the kaolin suspension (1 g/L): (a) coagulation performance of Al3+ at various pH values, (b) pH variation after Al3+ coagulation, (c) zeta potential of the kaolin suspension with and without Al3+ in the pH range from 3 to 10
After the introduction of Al3+, the zeta potential of the suspension was positive when the pH was lower than 6, which was partly caused by the formation of amorphous Al3+ hydroxide (Al(OH)2+or Al(OH)2+). This could be illustrated by the synergistic effect of charge neutralization and sweeping during the coagulation process. With increasing pH, the zeta potential of the suspension became negative, which revealed that this coagulation was primarily based on the charge neutralization mechanism (Li et al. 2006). Considering the economic and mild operation, an initial pH 6 for the kaolin suspension was chosen for subsequent TCC flocculation.
Influence of TCC dosage on coagulation-flocculation
The dosage of flocculant is an important parameter for coagulation-flocculation. Excess or insufficient flocculant will both cause an increase in the residual turbidity of the effluent. Figure 7a presents the flocculation performance of the TTC products with respect to the kaolin suspension (1 g/L, original turbidity 610 NTU) with the assistance of Al3+. The residual turbidities of the kaolin suspensions after the coagulation-flocculation by Al3+ and TCCs declined sharply with increasing dosage of TCC before 80 mg/L, and then changed slightly. All the synthesized TCCs exhibited excellent flocculation performance for the kaolin suspensions, in which the residual turbidities declined from 610 to a range of 69.4 to 14.9 NTU. The corresponding turbidity removal ratios reached 88.6% to 97.6%, respectively. In comparison with the single coagulation of Al3+ shown in Fig. 5, the combined coagulation-flocculation with Al3+and TCCs achieved a better performance with lower residual turbidity and smaller coagulant dosage. Considering flocculation efficiency and economy, a TCC dosage of 80 mg/L was adopted for the following flocculation experiments.
In addition, the flocculation of TCCs I through IV was obviously strengthened with an increase in carboxylate content in TCC. TCC IV achieved the best flocculation performance (residual turbidity declined to 14.9 NTU). However, in the case of TCC V, which had the highest carboxylate content, relatively poor flocculation performance was exhibited in comparison with TCC IV (residual turbidity increased to 25.1 NTU). This was attributed to the size effect of TCC macromolecules. In this study, the NaIO4 oxidation for ADAC was innovatively carried out under homogeneous conditions to obtain the high aldehyde content and fully oxidized cellulose samples. With excessive NaIO4, the ring-opening reaction of AGUs was not dominant. The -1,4-glycosidic bonds between AGUs began to be broken, which directly caused a decrease in the degree of polymerization of the cellulose (Liimatainen et al. 2012). This reduced the size of the TCC product and adversely affected its flocculation performance. Thus, TCC IV, together with its synthesis technology, is suitable for practical application in wastewater flocculation.
With the comparison to commercial CMC and APAM, the results in Fig. 7c show that the TCC IV achieved an excellent flocculation performance.
Fig. 7. Residual turbidity of the kaolin suspension (1 g/L) after coagulation-flocculation as a function of (a) flocculant TCC dosage (coagulant Al3+ dosage of 150 mg/L, pH of the kaolin suspension of 7, and settling time of 10 min) and (b) suspension pH (coagulant Al3+ and flocculant TCC dosages of 150 and 80 mg/L, respectively, settling time of 10 min); (c) comparison of flocculation performance of the TCC IV, commercial CMC and APAM.
The residual turbidity of the kaolin suspension declined to 28.5 NTU at the dosage of 60 mg/L. For reference, the cellulose-based flocculant CMC exhibited the limited flocculation performance (the residual turbidity only declined to 73.2 NTU at the dosage of 80 mg/L). After the treatment of APAM, the lowest turbidity appeared as 23.5 NTU at the dosage of 40 mg/L. But the re-stabilization phenomenon emerged with the increase of dosage. The biodegradability of APAM is also a remaining issue in comparison to natural flocculants.
Influence of pH on coagulation-flocculation
Figure 7b shows the influence of pH on the combined coagulation-flocculation with Al3+ and TCC. The dosages of Al3+ and TCC were determined from the above optimization results to be 150 and 80 mg/L, respectively. The residual turbidities of the kaolin suspensions rapidly increased after the point of pH 6. This was attributed to the coagulation property of the Al3+. As shown in Fig. 6, in acidic conditions, charge neutralization and sweeping have a synergistic effect on the coagulation. This could play a facilitating role in the flocculation of the TCCs. However, in alkaline conditions, charge neutralization plays the dominant role in coagulation. The conversion from Al3+ to AlO2- caused a negative zeta potential, which meant that part of the kaolin particles could not be coagulated by Al3+. This further affected the subsequent flocculation of the TCC. Therefore, the effective pH range of the coagulant had an important influence on the process of coagulation-flocculation. The original pH 6 of the kaolin suspension was chosen for the combined coagulation-flocculation with Al3+ and TCC IV. Under these conditions, a residual turbidity of 28.7 NTU was finally achieved for the kaolin suspension, corresponding to a turbidity removal ratio of 95.3%.
CONCLUSIONS
- A novel 2, 3, 6-tricarboxylate cellulose with high charge density was developed as an efficient and fully biodegradable flocculant for removal of particle contaminations from wastewater. The cellulose was innovatively carboxymethylated before NaIO4 oxidation, which not only introduced carboxyl groups on the C6 of AGUs, but also allowed subsequent NaIO4 oxidation under homogeneous conditions. This provided the preconditions for the final TCC products with remarkably high carboxylate contents of more than 10 mmol/g.
- With the advantages of abundant raw materials, high charge density, and full biodegradation, the TCCs showed excellent performance in the coagulation-flocculation of kaolin suspension. A turbidity removal ratio of 95.3% was achieved for the kaolin suspension by combined coagulation-flocculation with Al3+ and TCC IV. The results demonstrate that these high-carboxylate content materials are good candidates for particle contamination removal in wastewater flocculation.
ACKNOWLEDGMENTS
The authors are grateful for the support of the National Natural Science Foundation of China, Grant. No. 51372226 and 61571002, the Science and Technology Program of Zhejiang Province of China, Grant. No. 2015C32098, the Natural Science Foundation of Zhejiang Province of China, Grant. No. LY14C160004, and the Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Grant. No. YR2011017.
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Article submitted: February 20, 2016; Peer review completed: April 10, 2016; Revised version received and accepted: May 8, 2016; Published: May 16, 2016.
DOI: 10.15376/biores.11.3.5918-5930