Natural-based polyelectrolytes (PELs), with all the advantages coming from being produced from renewable and biodegradable sources, are a potential solution for the removal of dyes from wastewater. In this work, surplus Eucalyptus bleached cellulose fibres from a paper mill were modified to increase the charge and solubility of cellulose. First, reactive aldehyde groups were introduced in the cellulose backbone by periodate oxidation of cellulose. Further modification with alkylammonium produced positively charged cellulose-based PELs. The final products were characterized by several analytical techniques. The PEL with the highest substitution degree of cationic groups was evaluated for its performance in decolouration processes, bentonite being used as aid. This was found to be effective for colour removal of either anionic or cationic dyes. Bio-PELs can thus be considered as very favourable eco-friendly flocculation agents for decolouration of harsh effluents from several industries, considering their biodegradable nature and thus the ability to produce less sludge.

INTRODUCTION

The availability of drinking water has decreased over the past few decades due to several influences, such as poor land use, population growth, urbanization, climate change or even agriculture (Ongley 1996; Ritter et al. 2002). Of all the different possibilities of polluting water, coloured pollutants are one of those raising more concern. Due to the great amount of synthetic organic compounds as well as of their modifications, an unnumbered amount of shades and colours is available, which can meet all the application needs of several industries, e.g., textile, paper, cosmetics, pharmaceuticals, etc. (Zollinger 1987; Carneiro et al. 2007). This constitutes another problem when dealing with the treatment of such effluents, due to the variety of molecules involved.

The textile industry is one of the biggest industries producing the largest amount of coloured wastewaters. The whole process (fibre dyeing and washing) releases effluents that contain 10–15% of colourant, which need to be treated in order to discharge them to the environment, namely rivers. Besides presenting a high value of biochemical oxygen demand, which increases with the dye contamination, the coloured effluents can also contain toxicants, pH altering agents, sulfur or salts, and be considered as toxic, carcinogenic or mutagenic (Dos Santos et al. 2007).

Due to the great growth of the colouring industry and hard environmental regulations, there is a strong need to develop inexpensive, effective and environmentally friendly methods of decolouration and wastewater purification (Robinson et al. 2001).

Dyes may be classified according to their chemistry, by their end-use, or application class, but it is the first criterion that is employed by the Society of Dyers and Colourists. Accordingly, the basic dyes, often called cationic, possess positively charged groups in the molecule; the acid dyes, on the other hand, have a negative charge, typically due to the presence of sulfonate (SO3) groups. The direct dyes have also negatively charged groups (mainly SO3) as well as the reactive dyes, but these are longer molecules and have different constituent groups. Other types of dyes include disperse dyes, sulfur and naphthol dyes, which are based on neutral chromophores. The first four types (basic, acid, direct and reactive) are water-soluble, while the remaining types are typically insoluble in water (Hubbe et al. 2012). In the present work, only cationic and anionic dyes are considered for the performance tests.

Dye decolourization processes in wastewaters have been compiled by several authors and may include adsorption onto activated sludge biomass, where bacteria are used for the biological organics oxidation, chemical coagulation (which is often less effective for the highly water-soluble dyes), electrochemical processes, oxidation processes including advanced oxidation processes involving the generation of highly reactive free radicals, membrane processes, enzymatic degradation of dyes, and adsorption of dyes onto various substrates, including activated carbon and lignocellulosic sorbents (Hubbe et al. 2012). However, although very interesting approaches have been proposed, it is difficult to find a solution that works for the wide range of existing dyes and that is effective for the complex mixtures of some dye-contaminated effluents. Besides, several factors (constraints) must be taken into account, such as the type and concentration of dyes present, the eventual presence of other interfering substances, pH and temperature of operation (Hubbe et al. 2012).

In this context, the use of cellulose-based polyelectrolytes (PELs) (natural-based) of high ionic charge, obtained from cellulosic wastes from paper mills or other wood wastes from various industries, for the treatment of dye-containing wastewaters could be a promising solution to consider and to possibly implement in the future. It was recently reported in the literature that treatment of cellulose substrate with (cationic) chitosan enhanced the uptake of anionic dyes (Ngah et al. 2011).

Cellulose-based polyelectrolytes are thus alternative flocculants for wastewater treatment. The modification of cellulose to produce, ideally, water-soluble, cationic, anionic or amphoteric products has long been pursued and, depending on the target type of modification, different approaches can be applied. The cellulose cationization can be carried out by two different methods. One is based on the reaction of cellulose (or a cellulose-rich product) with 2,3-epoxypropyltrimethylammonium chloride (EPTAC) or its precursor 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) (Hasani et al. 2008; Song et al. 2008; Kono & Kusumoto 2014; Li et al. 2015; Moral et al. 2015). This reaction requires highly alkaline conditions, being conducted via a nucleophilic addition of alkali-activated cellulose hydroxyl groups to the epoxy moiety of EPTAC, where the etherification of cellulose with concomitant introduction of alkylammonium groups in the anhydroglucose units (AGU) occurs. The main drawback of this reaction is that it usually provides products with low degree of substitution (DS), difficult to solubilize in water, which, from the point of view of an application of the final product as a PEL in wastewater treatment, is not favourable. In fact, DS typically lower than 0.6 is obtained. As well, it usually requires an additional component (e.g., urea) to improve accessibility of the etherifying agent to cellulose (due to the strong intra- and inter-molecular hydrogen bonding that held together the cellulose chains and hampers the reaction). A second strategy to cationize cellulose is based on two main steps: the first step is an oxidation with sodium periodate of the vicinal hydroxyl groups at the C2 and C3 positions of the AGU to two aldehyde groups, with simultaneous breaking of the C2-C3 bond; this reaction provides high degrees of substitution, in which almost 100% of the AGU can be converted to the dialdehyde cellulose (DAC). In the second step, the dialdehyde reacts with Girard's reagent T (2-hydrazinyl-2-oxoethyl)-trimethylazanium chloride, forming an imine bond and providing the introduction of quaternary ammonium groups into the cellulose backbone (Liimatainen et al. 2011; Sirviö et al. 2011a, 2011b). In comparison to the former approach of cationization based on the reaction with EPTAC, the latter provides much higher cationicity and degrees of substitution in the cellulose backbone (note that per each AGU two cationic groups are introduced simultaneously in the C2 and C3 positions). Also, the cationicity level can be controlled, in order to obtain water-soluble products or less soluble ones, based on the selection of appropriate conditions of temperature, time of reaction, periodate/cellulose and (2-hydrazinyl-2-oxoethyl)-trimethylazanium chloride (GT)/aldehyde ratios. The intermediate DAC can also be used as the basis for the introduction of anionic groups such as sulfonates into the cellulose backbone by subsequent reaction, for instance, with sodium metabisulfite. Similarly, high degrees of substitution and charge density (anionic groups content higher than 3 mmol/g) on the cellulose backbone are obtained for water-soluble products (Liimatainen et al. 2012). Amphoteric cellulose-based products have also been obtained, such as carboxymethyl cellulose functionalized with quaternary ammonium groups (Kono & Kusumoto 2014).

In the present work, the cellulose from an alkali-extracted bleached Eucalyptus pulp was cationized by a two-step reaction (with sodium periodate and Girard's reagent T, respectively) and the resultant cellulose-based cationic PEL evaluated as a natural flocculant in the treatment of several model coloured wastewaters, bearing in mind that it is a more environmentally friendly PEL than the oil-based ones, also due to its biodegradable nature. The treatment procedures were tuned, and the results compared with those obtained using the most popular commercial synthetic flocculant, polyacrylamide (PAA).

MATERIALS AND METHODS

Alkaline extraction of eucalyptus bleached fibres

Disintegration

A mass of 15 g of Eucalyptus bleached kraft pulp was placed in a 2 L beaker. The disintegration was carried out with distilled water at 1% consistency, using a magnetic stirrer at room temperature, overnight. The pulp slurry was filtered in a large Buchner funnel.

Alkaline extraction

Alkaline extraction of Eucalyptus bleached pulp after disintegration was carried out at room temperature for 2 h with 1 M NaOH at 5% consistency, using mechanical stirring. After alkaline extraction, the pulp slurry was filtered in a large Buchner funnel, washed with a large amount of distilled water and air-dried. Sugars analysis by high pressure liquid chromatography showed the extracted pulp to be composed of ca. 93 wt% of cellulose and 6 wt% of hemicellulose (xylan) (on a dry basis).

Synthesis and characterization of cellulose-based flocculant

Preparation of DAC by periodate oxidation of cellulose

Highly oxidized cellulose was first produced by weighing 100 g of cellulose suspension with a consistency of 4% into a 500 mL flask and adding 300 mL of distilled water containing 7.2 g of LiCl and 8.2 g of NaIO4. The reaction vessel was covered with aluminium foil to prevent the photo-induced decomposition of periodate and placed in an oil bath. The reaction mixture was stirred with a magnetic stirrer at 70 °C. After 3 h, the product was filtered and washed several times with distilled water to remove iodine-containing compounds.

Determination of the aldehyde content of DAC

The aldehyde content of DAC was determined based on the oxime reaction between aldehyde groups and NH2OH·HCl. The never-dried periodate oxidized cellulose (0.1 g on a dry basis) was placed in a 250 mL beaker containing 1.39 g of NH2OH·HCl dissolved in 100 mL of 0.1 M acetate buffer (pH = 4.5). The beaker was covered with a thin rubber foil and the mixture was stirred for 48 h at room temperature with a magnetic stirrer. The product was filtrated and washed with 600 mL of distilled water, after which it was dried in a freeze-dryer. Since 1 mol of aldehyde reacts with 1 mol of NH2OH·HCl giving 1 mol of the oxime product, the aldehyde content in DAC can be calculated directly from the nitrogen content of the product.

Preparation of water-soluble cationic cellulose by cationization of DAC

Non-dried DAC (0.8 g on a dry basis) was weighed into a 100 mL flask and 80 mL of distilled water and GT was added. The pH of the reaction mixture was adjusted to 4.5 with dilute HCl and the mixture was stirred for 1 h at 70 °C. After cooling to room temperature, the mixture was transferred into centrifuge tubes and isopropanol was added to each tube to precipitate the soluble products. The mixtures were then centrifuged for 30 min at 4,500 rpm, after which the supernatants were removed. The product was washed with a water/isopropanol solution (1/9 v/v) and the centrifugation was repeated twice more. Removal of the GT was monitored by adding a small amount of AgNO3 to the supernatant. When no AgCl precipitate formation was observed the washing was considered complete. Finally, the cationic product was oven-dried at 60 °C and then stored in a desiccator. The GT/aldehyde content was varied from 0.78 to 3.9, in order to obtain cationic celluloses with different degrees of cationization.

The final cationic celluloses were characterized by Fourier transform infrared spectroscopy (FTIR) and 1H NMR spectroscopy, elemental analysis, size and ζ-potential measurements. FTIR-ATR spectra were obtained on a Bruker Tensor 27 spectrometer, using 128 scans and a resolution of 4 cm−1, in the range of 600–4,000 cm−1. 1H NMR spectra of the cationic cellulose samples dissolved in D2O (10 mg/mL) were collected in a Bruker Avance III 400 MHz NMR spectrometer using a Bruker standard pulse programme. C, H and N elemental analyses were performed using an element analyser EA 1108 CHNS-O from Fisons. 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene was used as standard. The nitrogen content was used to obtain the corresponding degree of cationization of cellulose.

Hydrodynamic diameter and zeta potential of the cellulose-based PELs were determined by dynamic light scattering and electrophoretic light scattering (ELS), in a Zetasizer NanoZS, ZEN3600, from Malvern Instruments, with backscatter detection at a 173 ° angle with temperature set up to 25 °C. For the hydrodynamic diameter, a stock solution in Milli-Q water of 0.2 g/L was prepared, stirred for 1 h, and then sonicated for 2 min. After that process, the cationic cellulose solution was passed through a 1 μm syringe filter directly to the glass cell. Zeta potential measurements were performed using a 0.4 g/L stock solution prepared in Milli-Q water. With the syringe, 1 mL of sample for analysis was carefully injected directly into the disposable plastic capillary cell.

Selected dyes and dye characterization

The dyes considered in the present study to simulate coloured effluents were: Methylene Blue (Fluka), Crystal Violet (Feldkirch Inc.), Duasyn Direct Red 8BLP (Feldkirch Inc.), Orange 2 (Roth), Basic Green 1 (Alfa Aesar) and Acid Black 2 (Roth). These were characterized for their UV-VIS spectra and absorption maxima. Conductivity of their aqueous solutions (1%) was also measured as well as the zeta potential, using ELS in a Zetasizer NZS (Malvern Instruments, UK). UV-VIS spectra were measured on a JASCO V550 spectrophotometer, in the 800–200 nm wavelength range, with a scanning speed of 200 nm/min.

Coagulation-flocculation experiments

The jar-test was applied in order to evaluate the flocculation performance of the water-soluble cationic cellulose that possessed the highest degree of cationization. The cationic cellulose solution for the flocculation test was prepared by dissolving the cationic polymer at 0.04% concentration in distilled water and stirring at 500 rpm for 30 min. Model effluent was prepared by adding an amount of dye (the amount for each dye was the one leading to saturation and depended on the type of dye) to distilled water and stirring at 500 rpm; for the flocculation experiment, 150 mL of the dye solution was placed in a beaker, and, if required, the pH adjusted to the target value by adding NaOH 10% or H2SO4 10%. A suitable dosage of flocculant was then added dropwise, while mixing slowly for 20 s; for most of the experiments, bentonite was also added before the cationic flocculant addition. A water sample of approximately 2 mL was pipetted from the supernatant water, at the surface from the centre of the beaker, for determination of colour removal over time (10 min, 1 h and 24 h). The decolouration was calculated by measuring absorbance at specific wavelength (typically at the visible absorption maximum of each dye). Comparison was made with a commercial cationic PAA.

RESULTS AND DISCUSSION

Synthesis and characterization of DAC and cationic cellulose

Cationic cellulose was obtained from alkali-extracted cellulose in a two-step reaction: DAC was synthesized by oxidation of cellulose with sodium periodate and then the resultant DAC was reacted with Girard's reagent T yielding the cationic derivative (Figure 1). It is important to mention here that cellulose from bleached Eucalyptus pulp was previously alkali-extracted in order to obtain a cellulosic material richer in cellulose (with a smaller amount of hemicelluloses). Bleached birch cellulosic pulp, which contained a hemicellulose amount of ca. 25%, has been cationized by this two-step reaction (Sirviö et al. 2011b). This is an important issue to consider in the context of the present work, since the chemical composition of the original cellulose source (namely, the relative content of cellulose and hemicelluloses), and the molecular weight and size of the polysaccharide molecules, may certainly influence not only the characteristics of the final cationic product but also its performance in flocculation-related studies. It was decided to reduce the content of hemicellulose(s) in the initial bleached pulp by an alkaline extraction in order to have more uniformity in the distribution of the molecular size of the polymers.
Figure 1

Reaction scheme showing the two-step reaction used to produce cationic cellulose.

Figure 1

Reaction scheme showing the two-step reaction used to produce cationic cellulose.

In order to obtain cationic cellulose with a high DS, which would be ideally water-soluble, first, a DAC with a high aldehyde content was produced; the produced DAC possessed ca. 10 mmol/g of aldehyde groups (determined based on the nitrogen content of the oxime derivative produced by reaction with NH2OH·HCl, as explained in the experimental section), which corresponds to a DS of 1.60 (Table 1). Several experiments were then carried out for the synthesis of the cationic cellulose derivative from DAC (second reaction step) by changing the reaction conditions, namely, the GT/aldehyde molar ratio. This ratio influenced the degree of cationization of the final product, cationicity values ranging from 3.0 to 3.8 mmol/g having been obtained (Table 1). Typically, the increase of the GT/aldehyde molar ratio yields higher cationicity; however, for a GT/aldehyde ratio of 0.78 (Table 1, sample CC4), a still high level of cationization is achieved (3.0 mmol/g). The present results are in satisfactory agreement with those previously published with other cellulosic substrates (Sirviö et al. 2011a, 2011b). Considering the requirement of a high content of charged cationic groups in the cellulose derivative, for possible application of the latter in dye removal, the cellulose product with a higher cationicity (Table 1, sample CC1) was preferred over the others, and further used in the experiments for dye removal.

Table 1

Some reaction conditions used for the production of DAC and cationic cellulose, and corresponding degrees of substitution and cationicity, zeta potential and hydrodynamic diameter

  Temp (°C)Time (h)Aldehyde content (mmol/g)DSaGT/aldehyde (molar ratio)Cationicity index (mmol/g)bZeta potential (mV)Z-average diameter (d.nm)
DAC  71 3.5 9.97 1.60 – –   
 CC1 70 – – 3.9 3.85 69 245 
 CC2 70 – – 1.95 3.33 68 185 
 CC3 70 – – 0.975 3.26 55 137 
 CC4 70 – – 0.78 3.04 54 138 
  Temp (°C)Time (h)Aldehyde content (mmol/g)DSaGT/aldehyde (molar ratio)Cationicity index (mmol/g)bZeta potential (mV)Z-average diameter (d.nm)
DAC  71 3.5 9.97 1.60 – –   
 CC1 70 – – 3.9 3.85 69 245 
 CC2 70 – – 1.95 3.33 68 185 
 CC3 70 – – 0.975 3.26 55 137 
 CC4 70 – – 0.78 3.04 54 138 

aDS = degree of substitution of aldehyde groups.

bCationicity determined as the amount of alkylammonium groups (mmol) per g (dry weight) of cationic cellulose sample.

The presence of substituent aldehyde groups (at the C2/C3 position) in the DAC product was checked by FTIR spectroscopy. As mentioned, a DAC with an aldehyde content of 9.97 mmol/g was obtained, which indicates a high DS (Sirviö et al. 2011a, 2011b; Liimatainen et al. 2011, 2012). FTIR spectroscopy (Figure 2) showed several differences between the DAC prepared from the cellulosic pulp and the initial material. In particular, a new band appeared in the spectrum of DAC at 1,730 cm−1 due to the C = O stretching of the incorporated aldehyde groups. In addition, the characteristic C1-H bending band at 897 cm−1 of cellulose was shifted to 882 cm−1 in DAC. In the region between 1,000 and 1,200 cm−1, several differences could also be noted. For instance, the band at 1,162 cm−1, due to the asymmetric C-O-C stretching of the glycosidic bond of cellulose, is not well resolved in the spectra of DAC. These new features are certainly the result of the ring opening and oxidation of OH groups at C2/C3 positions; new hemiacetal linkages can also be established between the aldehyde groups and the alcohol groups of different chains, which should be held responsible for the observed spectroscopic changes (Sirviö et al. 2011b).
Figure 2

FTIR spectra of cellulose, dialdehyde cellulose (DAC) and cationic cellulose from DAC (CDAC).

Figure 2

FTIR spectra of cellulose, dialdehyde cellulose (DAC) and cationic cellulose from DAC (CDAC).

The DAC was further derivatized to imine-containing alkylammonium groups (Figure 1) and the products characterized by elemental analysis, FTIR and 1H NMR spectroscopy, size and zeta potential. The 1H NMR spectra (illustrative spectrum of sample CC1 in Figure 3) showed the presence of high-intensity signals at 3.23–3.29 ppm, which can be attributed to methyl protons (H3C) of alkylammonium moieties, protons linked to the carbon between the alkylammonium group and the C = O(NH) amide group, and protons linked to carbon in the HC = N- imine bond. A sharp signal at 3.97 ppm was also observed, probably due to protons linked to carbon 6. The FTIR spectrum of cationic cellulose (Figure 2) showed several new bands not present in the spectrum of original cellulose and DAC; namely, a very intense band appeared at 1,687 cm−1, which is due to the carbonyl stretching of amide bond (amide 1 band). This was accompanied by the less intense amide 2 band at 1,562 cm−1. As well, bands appeared at 1,475 and 1,415 cm−1, from the asymmetric and symmetric bending of methyl groups. An intense band also appeared at 925 cm−1, which can be attributed to the asymmetric C4-N stretching of the alkylammonium groups. The relative intensity of the bands due to the presence of alkylammonium groups (at 1,475, 1,415 and 925 cm−1) of the different samples confirmed the results obtained by nitrogen elemental analysis, increasing with increasing cationicity (Figure 4), as would be expected. Thus, 1H NMR and FTIR spectroscopy gave clear evidence that cationization of cellulose through the formation of imine bonds by reaction of DAC with GT reagent occurred. According to the nitrogen elemental analysis (16.2 wt%), the higher cationicity index obtained was 3.85 mmol/g.
Figure 3

1H NMR spectrum of cationic cellulose CC1.

Figure 3

1H NMR spectrum of cationic cellulose CC1.

Figure 4

FTIR spectra of cationic cellulose samples with different cationicity (3.85, 3.33 and 3.04 mmol/g for CC1, CC2 and CC4, respectively).

Figure 4

FTIR spectra of cationic cellulose samples with different cationicity (3.85, 3.33 and 3.04 mmol/g for CC1, CC2 and CC4, respectively).

Zeta potential of cellulose-based, water-soluble, cationic PEL varied between 54 and 69 mV, which confirmed the success of the cationization process and the production of positively charged molecules. The variations in zeta potential agree with the different degrees of substitution achieved (Table 1). Moreover, the moderate dispersion of size (polydispersity index 0.3–0.4) of the modified natural-based polymer, with average hydrodynamic diameter between 138 and 245 nm, proved that during the periodate oxidation the cellulose backbone was not destroyed to individual molecules, and the reaction allowed obtaining DAC, which was afterwards submitted to further modification with an alkylammonium compound, leading to a polydisperse branched type of cellulose with positive charges. The lower sizes, associated with the lower DS, can be related to the more coiled conformation in solution of the less charged molecules (lower electrostatic repulsion between charged sites).

Dye characterization

Some relevant characteristics of the used dyes are summarized in Table 2. As known from their chemical structures (PubChem Crystal Violet; Methylene Blue; Orange 2 and Dye/World dye variety Acid Black 2; Basic Green 1; Direct Red), some of the dyes have positively charged organic chromophores while others have negatively charged ones. The ionic conductivity of the 1% dye aqueous solutions was the highest for Duasyn Direct Red. This dye possesses two ionized sulfonic acid groups per molecule, contributing thus to its higher ionic mobility and the corresponding higher ionic conductivity. Orange 2, possessing only one ionized sulfonic acid group per molecule, showed a lower conductivity. Table 2 presents also the values of zeta potential for all the dyes tested. The pHs of the dyes' aqueous solutions were reasonably similar, except for Basic Green 1 dye. For the latter, the pH was around 2.5, due to the hydrolysis of hydrogenosulfate ions (acting as counter-ions of the organic counterpart). Finally, the different wavelength for the visible absorption maximum of each dye reflects the different chemical structure of the organic chromophores. The UV-VIS spectra of the dyes considered in the present work are shown in Figure 5.
Table 2

Some useful characteristics of the dyes used to model wastewaters

IdentificationChargeaZeta potential (mV)Conductivity (μs/cm, 25 °C)bpHbWavelength (nm)c
Methylene Blue (+) +3 11.4 7.0 663 
Duasyn Direct Red (−) −35 30.5 5.4 511 
Acid Black 2 (−) −20 9.5 5.5 574 
Crystal Violet (+) +46 – 6.8 589 
Orange 2 (−) −26 19.3 6.5 485 
Basic Green 1 (+) +6 14.3 2.6 624 
IdentificationChargeaZeta potential (mV)Conductivity (μs/cm, 25 °C)bpHbWavelength (nm)c
Methylene Blue (+) +3 11.4 7.0 663 
Duasyn Direct Red (−) −35 30.5 5.4 511 
Acid Black 2 (−) −20 9.5 5.5 574 
Crystal Violet (+) +46 – 6.8 589 
Orange 2 (−) −26 19.3 6.5 485 
Basic Green 1 (+) +6 14.3 2.6 624 

aRefers to the organic dye charge.

b1% aqueous suspensions.

cWavelength of the visible absorption maximum.

Figure 5

UV-VIS spectra of the dyes used in the present study.

Figure 5

UV-VIS spectra of the dyes used in the present study.

Decolouration studies (dye removal)

The results of the decolouration tests for the six dyes tested are presented here and discussed individually for each dye.

Table 3 summarizes the results obtained for Methylene Blue. To obtain high percentages of colour removal required the addition of bentonite (dual system) and changing of pH to acidic conditions. Under these conditions, more than 95% of colour removal could be attained just after 10 min of treatment. At high pH, even with a higher dosage of cationic cellulose and in the presence of bentonite, decolouration was always lower than 80% after 24 h. Without bentonite it was possible to remove colour but the process was slow (88% removal after 24 h and 24% after 10 min). Worse results were obtained with only PAA and also when PAA was used with bentonite, when compared with the use of CC1 in the same conditions. Overall, it seems that the cationic cellulose, in the presence of bentonite, is efficient for the decolouration of Methylene Blue.

Table 3

Decolouration test results for Methylene Blue

pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
10 min1 h24 h
7.0 – – – 
7.0 0.3 – – 18 34 48 
7.0 – – 2.67 24 47 88 
7.0 – 2.67 – 17 77 82 
10.2 0.3 – 10.67 48 62 77 
7.0 0.3 – 2.67 47 80 87 
7.0 0.3 2.67 – 70 79 83 
5.6 0.3 2.67 – turb 48 61 
5.6 0.2  2.67 98 99 99 
5.6 0.2 – 1.34 88 88 98 
5.6 0.2 – 2.13 97 98 98 
2.3 0.07 2.67 – 96 96 96 
2.3 0.07 – 2.67 96 96 97 
4.0 0.07  2.67 97 98 99 
7.0 0.07  2.13 97 99 99 
pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
10 min1 h24 h
7.0 – – – 
7.0 0.3 – – 18 34 48 
7.0 – – 2.67 24 47 88 
7.0 – 2.67 – 17 77 82 
10.2 0.3 – 10.67 48 62 77 
7.0 0.3 – 2.67 47 80 87 
7.0 0.3 2.67 – 70 79 83 
5.6 0.3 2.67 – turb 48 61 
5.6 0.2  2.67 98 99 99 
5.6 0.2 – 1.34 88 88 98 
5.6 0.2 – 2.13 97 98 98 
2.3 0.07 2.67 – 96 96 96 
2.3 0.07 – 2.67 96 96 97 
4.0 0.07  2.67 97 98 99 
7.0 0.07  2.13 97 99 99 

Table 4 summarizes the results for the Duasyn Direct Red dye. Once more, very good results (95% removal for 10 min) were obtained in acidic conditions (pH around 2) with the dual system bentonite/cationic cellulose. Without bentonite the results were poorer and no more than 50% of decolouration was obtained after 24 h. Increasing the pH, still under acidic conditions, also decreased the efficiency of colour removal. The results with cationic cellulose were comparable to those obtained by substituting it with PAA.

Table 4

Decolouration test results for Duasyn Direct Red

pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
10 min1 h24 h
5.4 – – – 
5.4 0.3 – – turb turb turb 
5.4 – 2.67 – 41 42 47 
5.4 – – 2.67 33 38 47 
5.4 0.3 – 1.76 turb turb turb 
2.9 0.3 – 1.76 turb 94 93 
4.4 0.3 – 1.74 79 87 86 
2.1 0.5 2.67 – 96 96 97 
2.1 0.5  2.67 95 95 95 
pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
10 min1 h24 h
5.4 – – – 
5.4 0.3 – – turb turb turb 
5.4 – 2.67 – 41 42 47 
5.4 – – 2.67 33 38 47 
5.4 0.3 – 1.76 turb turb turb 
2.9 0.3 – 1.76 turb 94 93 
4.4 0.3 – 1.74 79 87 86 
2.1 0.5 2.67 – 96 96 97 
2.1 0.5  2.67 95 95 95 

For Acid Black 2 (Table 5), more time was typically required to achieve a high decolouration level (at least 1 h), with either CC1 or PAA, under acidic conditions and requiring always the use of a dual system with bentonite. For pHs of 4.1 and 5.8 and using a dual system with bentonite, 95–99% of colour removal could be achieved after 1 h. Also, after 24 h of CC1 addition, colour removal was always very high. Under basic conditions colour removal was low. Overall, results were better than those obtained with PAA under the same tested conditions.

Table 5

Decolouration test results for Acid Black 2

pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
5.5 – – – 
5.5 0.3 – – turb turb 26 
2.2 0.3 – 2.67 turb 85 97 
2.2 0.3 2.67 – turb 86 97 
1.9 0.1 – 2.67 turb 84 98 
1.9 0.1 2.67 – turb turb 90 
4.1 0.6  1.75 turb 99 100 
5.8 0.6  1.74 turb 95 100 
11.4 0.3 – 2.67 turb 30 
11.4 0.3 2.67 – turb turb 
1.8 0.2 – 1.34 turb turb 97 
1.8 0.2 1.34 – turb turb 65 
pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
5.5 – – – 
5.5 0.3 – – turb turb 26 
2.2 0.3 – 2.67 turb 85 97 
2.2 0.3 2.67 – turb 86 97 
1.9 0.1 – 2.67 turb 84 98 
1.9 0.1 2.67 – turb turb 90 
4.1 0.6  1.75 turb 99 100 
5.8 0.6  1.74 turb 95 100 
11.4 0.3 – 2.67 turb 30 
11.4 0.3 2.67 – turb turb 
1.8 0.2 – 1.34 turb turb 97 
1.8 0.2 1.34 – turb turb 65 

The results obtained for Crystal Violet were always very good (Table 6). The dual system was very efficient in the colour removal of this dye, colour removal efficiencies close to 100% having been obtained after just 5 min, even under neutral pH conditions. For this dye, bentonite alone led to a reasonable colour removal of 73% after 5 min (and 87% after 24 h). The same happened with PAA except for neutral pH conditions.

Table 6

Decolouration test results for Crystal Violet

pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
6.8 – – – 
6.8 0.11 – – 73 82 87 
6.8 0.11 – 0.51 96 96 98 
6.8 0.3 – 1.34 65 75 89 
6.8 0.3 1.34 – turb 37 75 
6.8 0.06 – 2.67 98 98 98 
6.8 0.06 2.67 – 98 98 99 
1.8 0.11  0.51 97 97 100 
10.2 0.57 – 0.51 86 96 95 
3.5 0.06 – 1.34 98 99 99 
3.5 0.06 1.34 – 97 98 99 
10.1 0.06 – 2.67 95 98 99 
10.1 0.06 2.67 – 98 98 99 
pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
6.8 – – – 
6.8 0.11 – – 73 82 87 
6.8 0.11 – 0.51 96 96 98 
6.8 0.3 – 1.34 65 75 89 
6.8 0.3 1.34 – turb 37 75 
6.8 0.06 – 2.67 98 98 98 
6.8 0.06 2.67 – 98 98 99 
1.8 0.11  0.51 97 97 100 
10.2 0.57 – 0.51 86 96 95 
3.5 0.06 – 1.34 98 99 99 
3.5 0.06 1.34 – 97 98 99 
10.1 0.06 – 2.67 95 98 99 
10.1 0.06 2.67 – 98 98 99 

The results obtained for tests on colour removal of Orange 2 were very poor and thus are not shown here in detail. The best result obtained was a decolouration of 18% after 24 h of settling with the addition of 2.67 ppm of cationic cellulose (only), leading to the conclusion that this procedure cannot be considered adequate for this coloured wastewater treatment. The same inability was also observed with PAA.

As for Basic Green 1 (Table 7), CC1 was also found to be effective in the colour removal of this dye, if adequate conditions are used. Decolourations higher than 85% after 5 min of treatment were achieved for certain experimental conditions. For pHs of 7 and higher, dye removal achieved modest values (typically less than 60% after 5 min). The best results were obtained for a pH of 6.4 and also under acidic conditions using the dual system. In general, results were better with CC1 than those obtained with PAA for the same conditions.

Table 7

Decolouration test results for Basic Green 1

pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
2.6 – – – 
7.2 0.3 – – turb 43 
8.2 0.3 – 2.67 42 54 52 
7.8 0.3 2.67 – 37 55 52 
6.4 0.3  2.67 98 99 99 
6.4 0.3 2.67 – 86 95 93 
7.2 0.3 – 1.34 59 67 69 
7.2 0.3 1.34 – 24 25 33 
7.2 0.3 – 2.67 56 62 64 
7.2 0.3 2.67 – 28 31 46 
3.4 0.1 2.67 – 85 96 96 
3.3 0.1  2.67 86 99 99 
pHBentonite (%)PAA (ppm)CC1 (ppm)Colour removal (%)
5 min1 h24 h
2.6 – – – 
7.2 0.3 – – turb 43 
8.2 0.3 – 2.67 42 54 52 
7.8 0.3 2.67 – 37 55 52 
6.4 0.3  2.67 98 99 99 
6.4 0.3 2.67 – 86 95 93 
7.2 0.3 – 1.34 59 67 69 
7.2 0.3 1.34 – 24 25 33 
7.2 0.3 – 2.67 56 62 64 
7.2 0.3 2.67 – 28 31 46 
3.4 0.1 2.67 – 85 96 96 
3.3 0.1  2.67 86 99 99 

The prevailing flocculation mechanism in all the tests conducted was complexation with bentonite and bridging between the polymer chains. In the case of the positive dyes, bentonite is first electrostatically adsorbed on the surface of the dyes, and then forms complexes with the cationic cellulose. The opposite occurs with the negative dyes. Still, bentonite alone could never achieve reasonable colour removal for all the dyes tested.

This mechanism may justify the better removal of the positive dyes. Also, Crystal Violet is the dye exhibiting a higher removal, namely for low contact times, which agrees with the mechanism described, since it is the dye with the highest charge and thus conductivity. The opposite occurs for Basic Green 1, which is the positive dye with the lowest charge.

Looking at the results for the negative dyes, Duasyn Direct Red was the dye for which a higher colour removal, for low contact time, was achieved, which can again be related to its higher charge. On the contrary, Acid Black presented the lowest colour removal for low contact time, which agrees with its lower charge.

One possible explanation for the poor performance of the dual system tested on the removal of Orange 2 may be related to the much smaller size of this dye molecule (PubChem and Dye/World dye variety) in comparison with all the other negative dyes tested. In the case of the negative dyes it may be interesting to test the effect of the order of addition of the components of the dual system. Considering that bentonite is also negative, altering the order of addition may favour the adsorption of CDAC.

It is worth stressing that for all the dyes tested, except for Orange 2, removal was always achieved with a very low amount of CDAC and also with a low amount of bentonite. Moreover, in general, CDAC exhibited better performance than a standard PAA quite common in colour removal processes.

CONCLUSIONS

In this work, water-soluble cationic cellulose synthesized from bleached Eucalyptus kraft pulp fibres in a two-step reaction was used as flocculant for dye removal. A dual system with bentonite was found to be efficient for the colour removal of five different types of dyes: Methylene Blue, Duasyn Direct Red, Acid Black 2, Crystal Violet and Basic Green 1. Among all the tested dyes, only with Orange 2 dye, which corresponds to a very small molecule, were poor results obtained. Typically, better results were obtained while using the natural-based flocculant than with a commercial PAA normally used for this purpose (for the same pH and flocculant dose). A new type of system can thus be proposed for the colour removal of wastewaters from industries producing coloured effluents, namely, the textile industry. This strategy can be included in the group of environmentally friendly strategies, since the polymer used is biodegradable, thus generating limited sludge, and, additionally, is based on the valorization of a natural waste (surplus of Eucalyptus pulp).

ACKNOWLEDGEMENTS

The authors are grateful for the financial support from Marie Curie Initial Training Networks (ITN) – European Industrial Doctorate (EID), through Grant agreement FP7-PEOPLE-2013-ITN-604825 and from FCT/MCTES (PIDDAC), co-financed by the European Regional Development Fund (ERDF) through the program COMPETE (POFC).

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