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Flocculation of algal cells by amphoteric chitosan-based flocculant
Bioresource Technology 170 (2014) 239–247
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Flocculation of algal cells by amphoteric chitosan-based flocculant Changlong Dong a, Wei Chen a,b,⇑, Cheng Liu a,b a b
College of Environment, Hohai University, Nanjing 210098, PR China Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, PR China
h i g h l i g h t s An amphoteric chitosan-based flocculant (QCMC) was prepared. QCMC presented significant improvement of water solubility in the whole pH range. QCMC presented lower optimal dosages and higher algal turbidity removal efficiencies. QCMC flocs grew faster to produce larger and more compact flocs.
a r t i c l e
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Article history: Received 25 June 2014 Received in revised form 23 July 2014 Accepted 25 July 2014 Available online 4 August 2014 Keywords: Algae Chitosan Flocculation Floc properties
a b s t r a c t A kind of amphoteric chitosan-based flocculant (quaternized carboxymethyl chitosan, denoted as QCMC) has been prepared. QCMC presented significant improvement of water solubility in the whole pH range. The effects of pH, dosage, temperature and original turbidity of algal water on the flocculation performance were investigated. The optimal dosages of QCMC at pH 5, 9 and 12 with original turbidity of 20 NTU at 20 °C were 0.1, 0.6 and 2.0 mg/L, respectively, which were much less than that of chitosan, PAM, Al2(SO4)3 and FeCl3. The floc properties during grow, breakage and regrow period were also evaluated at different pH values in terms of floc size, strength and density. It was demonstrated that QCMC produced larger, stronger and denser flocs than Al2(SO4)3. There is every indication that QCMC is more suitable for algal harvesting than other traditional coagulants or flocculants. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The presence of seasonal algal blooms in water sources such as lakes and reservoirs has posed a serious water safety problem to local water industry. Many lakes are subjected to eutrophication which usually results in seasonal algae blooms (Qu et al., 2012). As of 2008, more than 75% of natural and artificial lakes in China have been eutrophicated in different degrees and suffered from harmful algal blooms (Wu et al., 2011). For instance, the eutrophication in Dian Lake was at high level, while Tai Lake and Chao Lake were at mid-eutrophication level in 2008 (MEP, 2009). Typically, the algal bloom occurred in Tai Lake in Jiangsu province at the end of May 2007 contaminated the main supply of drinking water for millions of people (Shen et al., 2011). Algae and its metabolites can cause serious damage to drinking water quality, such as the production of unpleasant tastes and odors, formation of ⇑ Corresponding author at: Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, PR China. Tel.: +86 15195959921. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.biortech.2014.07.108 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
disinfection byproducts and toxins from cyanobacteria, for example Microcystis aeruginosa which is one of the dominant species in eutrophic surface water (Pan et al., 2006). Moreover, algae is known to disrupt drinking water treatment process, resulting in short filter runs, increases in coagulant/flocculant demand and microbial regrowth in distribution systems (Chen et al., 2009; Plummer and Edzwald, 2001). The general methods used to remove or control algal growth include physical processes such as media filtration (Naghavi and Malone, 1986) and membrane separation (Sun et al., 2013), chemical processes such as coagulation (Pei et al., 2014), flocculation (Beach et al., 2012) and chlorination (Ma et al., 2012), and the electromagnetic radiation such as ultraviolet light (Sakai et al., 2007). Among these methods, Ultraviolet light process has main limitations of high capital and maintenance costs (Fast et al., 2014). Media filtration and membrane separation are cost- and energyintensive while use of oxidizing agents like chlorine may cause physiological stress or cell walls damage resulting in release of taste and odor causing compounds or intracellular organic matter (Plummer and Edzwald, 2001). Thus the optimal strategy for algae control in drinking water treatment would be the intact removal of
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algal cells without cell rupture. It has been concluded that chemical coagulation-flocculation is the safest and most economical step in the water treatment process for algae removal (Chen et al., 2009; Wu et al., 2011). Since algal cells possess negative surface charge, the surface charge can be neutralized by introducing cationic ions, namely aluminum ions, ferric ions and synthetic polymers such as polyacrylamide. These coagulants and flocculants can easily flocculate algal cells and form flocs. However, using these chemicals may have several environmental consequences (Renault et al., 2009): (i) an increase in metal concentration in water which may have human health implications; (ii) production of large volumes of toxic sludge; (iii) dispersion of acrylamide oligomers which may also be a health hazard. For these reasons, alternative natural biopolymers have been considered for environmental applications since they are natural low-cost products and characterized by their environmentally friendly behavior (Vandamme et al., 2010). Among these biopolymers, chitosan is considered as one of the most promising flocculant materials. Chitosan is a linear poly-amino-saccharide produced by deacetylation of chitin which is the second most abundant biopolymer in the world after cellulose. Chitosan has unique properties among biopolymers especially due to the presence of primary amino groups and it is a commercially interesting compound because of its high nitrogen content in comparison to cellulose (Renault et al., 2009). It has characteristics of both coagulants and flocculants, i.e., high cationic charge density, long polymer chains, bridging of aggregates and precipitation (in neutral and alkaline conditions). The main reason for the success of chitosan in water treatment as a flocculant is that chitosan has the advantage of being non-toxic, non-corrosive and safe to handle (Bolto and Gregory, 2007). Apart from that, chitosan is efficient in cold water and at much lower dosages than metal salts, and it does not leave residual metals that can cause second contamination problems. There are, of course, disadvantages which must be balanced against the benefits (Rinaudo, 2006). Chitosan is only efficient over a limited pH range and only soluble in dilute organic acids, which greatly limits the application of chitosan in flocculation process. Therefore, the modification of chitosan aiming at algal harvesting using coagulation-flocculation process is essential. In recent years, extensive attention has been given to research and application of amphoteric chitosan-based flocculants (Cai et al., 2007; Yang et al., 2011). The reason is that amphoteric chitosan is more effective due to its improved flocculation performance and wider range of solubility due to the dual characteristics of both cationic and anionic groups. But without ignorance, the modification reagent (monochloroacetic acid) which was adopted in many related literatures is highly toxic, and the flocculation properties of amphoteric chitosan for algal turbid water has never been evaluated. Under the above background, the amphoteric chitosan flocculant (quaternized carboxymethyl chitosan, denoted as QCMC) has been prepared using safer modification reagents in the current work. The aim of this paper is to determine the flocculation properties and mechanisms of QCMC for algal turbid water and examine the formation, breakage and regrowth of algal flocs using QCMC in comparison with aluminum sulfate (Al2(SO4)3).
2. Methods 2.1. Materials Chitosan powders 80–95% deacetylated supplied by Sinopharm Chemical Reagent Co., Ltd had a molecular weight ranging from 535,000 to 620,000 g/mol. Glyoxylic acid monohydrate (GA) from Aladdin Chemistry Co., Ltd and (3-Chloro-2-hydroxypropyl)-trime-
thylammonium Chloride (CTA) from Tokyo Chemical Industry Co., Ltd were both used without further purification. All other chemicals were purchased from Nanjing Chemical Reagent Co., Ltd. 2.2. Preparation of QCMC QCMC flocculant was designed and synthesized by two steps as shown in Fig. S1. The carboxymethyl chitosan (CMC) was firstly prepared by using GA according to Schiff’s base reaction principle. The toxicity of GA is quite lower than monochloroacetic acid. Final QCMC was obtained on the basis of CMC by quaternized modification using CTA. The preparation process of QCMC was described in brief as follows. A desired amount of chitosan was dispersed in ultrapure water ensuring the mass concentration of chitosan suspension is roughly 2%. After stirring for 30 min at room temperature, a certain amount of GA (n(chitosan):n(GA) = 0.8) was added into the chitosan suspension and the reaction time was controlled at 45 min. After that, the pH of the mixture was adjusted to pH 12 with 2 mol/L NaOH aqueous solution and a certain amount of NaBH3CN (n(GA):n(NaBH3CN) = 0.8, dissolved in 20 mL of ultrapure water) was then added into the reaction system dropwise with stirring for 45 min. The resultant products were precipitated by absolute ethanol and rinsed in 90% ethanol to desalt and dewater, then extracted using acetone as solvent in the Soxhlet apparatus for 24 h, and finally dried fully in a vacuum oven at room temperature. The products were CMC. Then, a desired amount of dried CMC was added into a mixture of isopropanol and NaOH aqueous solution to swell and alkalize at 45 °C for 1 h. A certain amount of CTA was added into the reaction mixture dropwise for 20 min and stirred for 10 h at 60 °C. The pH of the mixture was then adjusted to pH 7 with 0.1 mol/L HCl aqueous solution. The resultant solid was washed and dried using the same method as mentioned in the first step. The final products of QCMC were prepared. 2.3. Characterization FTIR spectra were recorded using Tensor 27 spectrometer (Bruker, Germany). The interval of tested wavenumber was 500–4000 cm1. The XRD patterns were recorded on D8 Advance X-ray diffractometer (Bruker, Germany). The surface morphologies were observed by S4800 SEM (Hitachi, Japan). Zeta potential (ZP) measurement was carried out using Nano ZS90 Zetasizer (Malvern, UK). Water solubility was evaluated based on the method reported by Chen and Park (Chen and Park, 2003). The samples were dissolved in deionized water, and the concentrations of the samples were all 5 mg/mL. The various pH values of solutions were adjusted with 0.1 mol/L HCl or NaOH. Hydrodynamic radius (Rh) of QCMC with different pH conditions were determined by BI200SM dynamic light scattering apparatus (Brookheaven, USA). 2.4. Algae culture and algal turbid waters M. aeruginosa which is a common species in eutrophic surface water was selected for this study. It was purchased from Institute of Hydrobiology, Chinese Academy of Sciences. Axenic cultures were carried out in batch mode in 1 L conical flasks with BG11 medium. The conical flasks were placed in an incubator and the algal cells were cultured at the temperature of 25 °C with illumination of 5000 lx provided for 14 h every day. The live algal suspensions were harvested with culture time between 15 and 28 days and diluted by ultrapure water to prepare stable algal turbid waters with original turbidity of 10–40 NTU.
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2.5. Flocculation experiments 2.5.1. Flocculation procedure Flocculation experiments were carried out using ZR4–6 (Zhongrun, China) Jar Tester with 50 40 mm flat paddle impellers using cylindrical jars containing 1 L samples of algal turbid water. Initially, the algal turbid water was rapidly mixed at 200 revolutions per minute (rpm) for 2 min (G = 105.5 s1) while pH was adjusted with 0.1 mol/L HCl or NaOH and coagulants/flocculants were added. Then, a slow stir phase was conducted at 30 rpm (G = 8.1 s1) for 25 min to promote the collision of particles and hence floc growth. Subsequently, a 30 min settling period was applied. The supernatant was analyzed for residual turbidity (HACH 2100P Portable Turbidimeter, USA) and zeta potential to ascertain dosage response curves and thereby establish optimal dosages (OD, mg/L) under which to undertake subsequent floc property experiments. The OD of various coagulants and flocculants like chitosan, polyacrylamide (PAM), ferric chloride (FeCl3) and Al2(SO4)3 under different pH conditions were determined using the same flocculation procedure as described above. The turbidity removal efficiency (TRE,%) is expressed as
TRE ð%Þ ¼
T raw T treated 100% T raw
ð1Þ
Where Traw and Ttreated are the turbidites of raw and treated water, respectively. 2.5.2. Floc properties To establish floc growth and breakage profiles, flocculation experiments were conducted using the established conditions as discussed in Section 2.5.1. However, after the slow stirring phase for floc growth, rather than applying a 30 min of sedimentation, the impact of increasing shear on floc size was studied by varying the speed of the jar test paddles to 50 (G = 16.1 s1), 100 (G = 41.2 s1) and 200 rpm for 15 min, after which the paddle speed was finally controlled to 30 rpm for further 20 min to examine the potential for floc regrowth. Floc size was analyzed by Mastersizer 2000 (Malvern, UK) using the results of median average equivalent diameter (d50) of the flocs to generate floc growth curves under each condition.
Strength factor ð%Þ ¼
d2 100% d1
ð3Þ
Recovery factor ð%Þ ¼
d3 d2 100% d1 d2
ð4Þ
Where d1 is the steady-state floc size before breakage, d2 is the floc size after breakage and d3 is the floc size on regrowth. A higher strength factor value is indicative of a strong floc, while an increase in recovery factor demonstrates increased potential for regrowth. 2.5.4. Determination of fractal dimension Fractal dimension (DF) was measured by light scattering method. The light scattering technique involves measurement of light intensity I as a function of the scatter vector Q. Q is given by the following equation (Guan et al., 1998):
Q¼
4pn sinðh=2Þ k
ð5Þ
Where n, h and k are the refractive index of the media, the scattered angle and the wavelength of radiation in vacuum, respectively. The relationship among I, Q and DF can be represented by Eq. (6).
I / Q DF
ð6Þ
DF can be determined from the negative slope of log–log plot of I and Q. 2.6. Settling rate The settling rate experiments were conducted with QCMC and Al2(SO4)3 using algal turbid waters with original turbidity of 20 NTU at 20 °C. After the flocculation process as mentioned in Section 2.5.1, the flocculated algal turbid water was transferred into a 100 mL stopped graduated cylinder. Then, the height of the interval between the surface of the supernatant and the settling solid bed was recorded over time. 3. Results and discussion 3.1. Characterization of chitosan-based flocculants
2.5.3. Characteristic parameters of flocs During the flocculation process under different conditions, the growth rate, initial steady-state size, size on exposure to increased shear and regrowth potential can be evaluated from the floc growth curves. Specifically, growth rate was calculated as the slope obtained from plotting d50 during the initial growth period (2–7 min). Steady-state floc size was calculated by taking the average of all floc size after the steady state was achieved but before the breakage period (10–27 min). Floc size after breakage was calculated as the average of the floc size reached after the flocs were exposed to an increase of shear (32–40 min). The floc size on regrowth was obtained by calculating the average of the d50 values during the regrowth phase of the flocs (50–60 min). The floc strength coefficient (log C) and the floc strength constant (c) were obtained according to the empirical expression shown in Eq. (2) (Jarvis et al., 2005).
log d ¼ log C c log G
ð2Þ
Where d is the floc diameter (d50), log C is the intercept, c is the slope and G is the velocity gradient. It has been proposed that the higher the value of log C at a fixed shear, the stronger the floc, and the larger the value of c, the more prone the flocs are to breakage when increasing the shear rate. The floc strength and recovery factor were calculated using the following empirical equations (Jarvis et al., 2005):
The FTIR spectra of chitosan, CMC and QCMC are presented in Fig. S2. The adsorption bands at 3443 (OAH), 1596 (NAH), 1080 (C3AOH), 1030 cm1 (C6AOH) in Fig. S2a were the characteristic FTIR peaks of chitosan (Dong et al., 2014). In Fig. S2b, a new peak appeared at 1563 cm1 for the COO- group, which conformed that the negative carboxymethyl was successfully introduced to chitosan backbone (Chen and Park, 2003). Meanwhile, the NAH peak at 1596 cm1 was dramatically weakened while the C3AOH (1076 cm1) and C6AOH (1030 cm1) peaks have remained nearly unchanged. These indicated that the carboxymethylation was mainly occurred on ANH2 groups of chitosan. The intense peak at 1479 cm1 corresponding to the methyl of the quaternary ammonium group (Cai et al., 2007) in Fig. S2c also indicated that CTA were successfully grafted onto CMC. It is worth noting that the NAH peak disappeared and the C6-OH peak clearly shifted from 1030 to 1023 cm1 after quaternized modification, which proved that the CTA mostly grafted to C6AOH groups and residual ANH2 groups on CMC. In addition, the OAH peak gradually reduced to low band, demonstrating that the intramolecular and intermolecular hydrogen bonds were broken during the preparation of QCMC. The changes of FTIR spectra can be well explained by the synthetic route diagram as shown in Fig. S1. The XRD patterns of chitosan, CMC and QCMC are exhibited in Fig. S3 to investigate the structure of the final products further.
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The XRD pattern of chitosan showed two diffraction peaks at 2h = 10.4° and 20.1°. The diffraction peak at 2h = 10.4° was attributed to the hydrated crystals of low crystallinity and corresponded to the form I, while the diffraction peak appeared at 2h = 20.1° was identified as representative of the crystallinity of form II (Wu et al., 2005). For CMC and QCMC, the peak at 2h = 10.4° disappeared, which suggested that the hydrogen bonding ability of chitosan was reduced after the modification. Moreover, along with the process of modification, the peak at 2h = 20.1° has become relatively weaker for the decrease of ordered structure, indicating that the structure of the amphoteric grafting products became amorphous. Therefore, it can be concluded from the XRD patterns that the ordered structure of chitosan has been notably destroyed by graft chain. To give direct observation of the amorphous features of QCMC, SEM images have been taken and shown in Fig. S4. Significant differences can be observed among chitosan, CMC and QCMC. Chitosan had relatively regular structure. By contrast, the surface of CMC became rough due to the introduction of carboxymethyl groups. After CTA has been grafted onto CMC, the morphologies of QCMC changed more drastically: more porous and looser structures were observed. The porous morphologies of QCMC are expected to exert positive effect on flocculation efficiency, since more pores are favored for flocculating pollutants in water (Yang et al., 2013). It is well known that the charge properties of flocculants wield great influence on flocculation performance. Therefore, the pH independence of ZP of various sample solutions were also measured and shown in Fig. 1. It is found that algal cells were negatively charged over the whole pH range. Chitosan showed significantly positive ZP before pH 6.6 due to protonation of amino groups, whereas the ZP near zero was depicted after pH 7.0. The isoelectric point (IEP) of CMC was reduced to pH 5.5 because of the introduction of negative carboxymethyl groups, while QCMC had a much higher IEP around pH 9.2 due to the effective grafting of positive CTA. This improves the applicability of QCMC for alkaline algal turbid water in a wide pH range according to charge neutralization. 3.2. Water solubility of chitosan-based flocculants Water solubility is one of the most basic characteristics in usage of flocculants in water treatment. Therefore, solubility in a wide pH range is beneficial for flocculants applied in various situations. The solubility of chitosan, CMC and QCMC in aqueous solutions with various pH values are shown in Table S1. Chitosan had the narrowest soluble pH region (pH 6 5), indicating its poor solubility. Although CMC had better solubility than chitosan due to the fact
Fig. 1. Zeta potential-pH profiles of chitosan, CMC, QCMC and algal turbid water.
that the ordered structure of chitosan has been destroyed by carboxymethylation, it was still insoluble or partially soluble in lowacid water (pH = 4–7). Compared with chitosan and CMC, QCMC demonstrated remarkably improved solubility property. It was soluble almost in the whole pH range and only showed partial insolubility near its IEP at around pH 9.2. The excellent solubility of QCMC is mainly due to the amphoteric feature of having both cationic and anionic groups, which can adapt itself to various external conditions and maintain its stretched conformation of polymer chains (Yang et al., 2011). 3.3. Flocculation performance 3.3.1. Effects of pH and dosage A variety of water quality parameters can affect flocculation efficiency of flocculants, of which pH and dosage are the two most important ones. Hence, the flocculation performance of QCMC was systematically studied at different pH values and dosages (Fig. 2) and compared with chitosan and other conventional coagulants/ flocculants (Table 1). From the TRE-dosage profiles at pH 5, 9 and 12, the OD of QCMC were 0.1, 0.6 and 2.0 mg/L, respectively. With the increasing pH, the best residual turbidity corresponding to every OD also increased. However, the flocculation windows under three different flocculation conditions were not narrowed significantly and all of the residual turbidities remained below 1.0 NTU after 30 min of sedimentation. Although the re-stabilization of cell suspensions existed, manifesting as decrease of TRE, the weakening of flocculation performance was not evident especially at higher pH values. All of the TRE remained above 90% when QCMC was overdosed. Since the conformation of QCMC molecule in water has great impacts on flocculation performance, the pH dependence of Rh was measured and shown in Fig. S5. At pH 5, the positive charges of QCMC leaded to high Rh due to the effect of intramolecular electrostatic repulsion. Both outstretched conformation and positive charges of QCMC at lower pH values were beneficial for flocculation efficiency and resulted in lower OD requirements (Yang et al., 2012). Meanwhile, the ZP of supernatant was approximate to zero when OD were reached and became the opposite when QCMC was overdosed. This demonstrates that charge neutralization is dominant during flocculation process using QCMC in acidic algal water (Yang et al., 2014). Negatively charged algal cells were firstly attracted, neutralized and coated by positively stretched QCMC molecules and when the cells were thoroughly covered by these molecules, the destabilized algal cells with near to zero surface charge aggregated continually and form large flocs. At pH 9 near the IEP of QCMC, large decrease of Rh indicated that the polymer chains have become curly and short owing to the loss of net charge. But residual positive charges that were not neutralized by polymer chains drove the deformed chains attached on the negative surface of algal cells. The coverage of QCMC molecules on the surfaces of algal cells was neither full nor uniform and different small regions with opposite charges existed on the cells surfaces resulting in further cells aggregation through electrostatic interactions (Yang et al., 2014). On the other hand, the ZP of supernatant was far away from zero at OD, but went across zero and became the opposite when QCMC were overdosed, indicating that electrostatic patching was the main flocculation mechanism in this case. Despite all this, the loss of net charge reduced the number of active flocculation sites and leaded to higher OD. At pH 12, negative charges on QCMC increased, therefore the polymer chain with high Rh again became stretched. However, negative charges also resulted in greater electrostatic repulsion between QCMC molecules and suspended algal cells, finally leading to poorer flocculation performance, namely much higher OD and
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relatively lower TRE. It can be clearly observed from Fig. 2c that ZP of supernatant after flocculation at pH 12 was less than zero in spite of variation of dosage, owing to the shortage of positive charges. However, the flocculation efficiency in this case was still well performed. This can be explained by the bridging mechanism. The negative algal cells was adsorbed onto QCMC molecules by Van der Waals’ force and hydrogen bond under hydraulic agitation, and the electrostatic repulsion was unlikely to destroy these adsorption forces. The surface charges of algal cells were partially screened by QCMC molecules resulting in increasing ZP but not beyond zero. The flocculation performance of chitosan and several conventional coagulants/flocculants, such as Al2(SO4)3, FeCl3 and PAM, were investigated at different pH values and shown in Fig. S6 for comparison. Based on Fig. S6, the OD and corresponding TRE of different coagulants/flocculants were summarized in Table 1. Based on Table 1, QCMC consistently revealed the best flocculation performance amongst all five coagulants/flocculants, and showed both the lowest OD and highest TRE at pH 5, 9 and 12. Chitosan had a better performance than other conventional coagulants/flocculants in acidic water, but this advantage did not exist in neutral and basic solutions, mainly owing to the insolubility of chitosan at pH > 5. It cannot be ignored that the OD of inorganic coagulants like Al2(SO4)3 and FeCl3 were over 200 times more than that of QCMC, which increased sludge production and cost of water treatment. Flocculant cost is an important limiting factor for the wide use. A detailed cost comparison among chitosan, QCMC, PAM, Al2(SO4)3 and FeCl3 can be found in Table 1. It is obvious that the unit price of QCMC is significantly higher than that of other organic or inorganic flocculants mainly because of the expensive modification chemicals. However, when using these flocculants for algal harvesting under different conditions, QCMC presented both performance and cost advantages at pH 6 9, owing to its considerably lower OD. At pH 12, the cost of QCMC become the highest but considering the short period of seasonal algal blooms, this high expense can be affordable.
Fig. 2. Turbidity removal efficiency and zeta potential of the supernatant as a function of QCMC dosage at pH 5 (a), 9 (b) and 12 (c) with original turbidity of 20 NTU at 20 °C.
3.3.2. Effects of temperature and original turbidity In actual water treatment, temperature and original turbidity of water is different as the season changes, which evidently influences OD of flocculants. Based on Figs. S7 and S8, the OD as a function of temperature and original turbidity of algal turbid water are summarized in Fig. 3. It can be concluded from Fig. 3 that algal turbid water at higher temperature required less flocculants, which can be attributed to two reasons. Firstly, the viscosity of algal turbid water decreased with the increasing temperature, which resulted in enhancement of molecular movements and collision to form large particles. Secondly, the more unfolded QCMC molecules due to the improved solubility were also conducive to flocculation (Yang et al., 2011). It can also be obtained from Fig. 3 that the OD increased with the increase in original turbidity of algal turbid water. Data analysis in Fig. 3 further shows that, for algal turbid waters with original turbidity in the range of 10–40 NTU, the OD
Table 1 The cost, optimal dosages and corresponding algal turbidity removal efficiencies of various flocculants/coagulants. Flocculant/Coagulant
Chitosan QCMC PAM Al2(SO4)3 FeCl3
Unit price (US$/kg)
51.2 368.2 22.1 5.8 3.7
pH = 5
pH = 9
pH = 12
OD (mg/L)
TRE (%)
Cost (US$/m3)
OD (mg/L)
TRE (%)
Cost (US$/m3)
OD (mg/L)
TRE (%)
Cost (US$/m3)
0.4 0.1 3.0 20.0 15.0
94.7 96.3 94.1 89.2 93.4
0.02 0.04 0.07 0.12 0.06
5.5 0.6 8.0 35.0 25.0
85.3 95.8 86.5 88.2 91.2
0.28 0.22 0.18 0.20 0.09
11.5 2.0 10.0 45.0 30.0
81.6 95.0 83.7 89.4 93.5
0.59 0.74 0.22 0.26 0.11
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Fig. 3. Optimal dosage of QCMC for algal turbid water at pH 9 as a function of original turbidity at 20 °C and as a function of water temperature with original turbidity of 20 NTU.
increased linearly with the initial turbidity with an intercept of zero (R2 = 0.928). This stoichiometric linear relation has directed significance to real water production.
3.4. Floc growth profiles The floc growth profiles using QCMC were fully compared with Al2(SO4)3 at different pH levels and OD (Fig. 4). According to previous studies (Jefferson et al., 2005), when using Al2(SO4)3 as coagulant, charge neutralization tends to be influential at pH 5 and relatively low dosages, while sweep flocculation is the dominant mechanism at alkaline pH values and higher dosages.
3.4.1. Initial floc size and growth rate The initial steady-state floc size was consistently larger for QCMC than Al2(SO4)3 (Fig. 4). For example, the floc size achieved using Al2(SO4)3 ranged between 384 and 458 lm while the average floc size of QCMC ranged between 734 and 953 lm (Table 2). This large gap in floc size demonstrates that the stem grafting chain structure of QCMC can lead to much larger flocs through electrostatic attraction or bridging. At the same time, the steady-state floc size for QCMC and Al2(SO4)3 at lower pH values were apparently smaller than that at higher pH levels, indicating that the flocs formed by charge neutralization tended to shape diminutive and frizzy flocs because one negative algal cell may be simultaneously attracted by multiple positive charges on the same polymer chain of QCMC. The growth rate observed from Table 2 for Al2(SO4)3 ranged from 41 to 87 lm/min depending on the conditions applied, whilst for QCMC this was increased to 155–330 lm/min such that steadystate floc size was achieved between 6 and 14 min as opposed to between 4 and 6 min for flocs of Al2(SO4)3 and QCMC respectively (Fig. 4). This may be because the relatively large molecular weight of QCMC results in more chance of contact between QCMC molecules and algal cells. Meanwhile, growth rate also increased with the increasing pH. In other words, the floc growth rate under sweep and bridging flocculation conditions was faster than under charge neutralization and patching conditions. This could be attributed to hydroxide precipitates and outstretched polymer chains having a large collision profile giving a large effective volume concentration and large particle diameter, thus increasing the chance of collision and capturing other algal cells, leading to faster flocculation (Kuster et al., 1997).
3.4.2. Floc strength Fig. 4 illustrated that flocs for both Al2(SO4)3 and QCMC tended to resist breakage under relatively low shear rate as shown by only minor reductions in floc size on increasing the shear rate to 50 rpm (7.3–22.7%). However, when the shear rate was increased to 200 rpm, a reduction of floc size between 45.8% and 77.0% was observed. Similarly, floc strength factors at 200 rpm typically ranged from 25 to 56% for QCMC and 23–42% for Al2(SO4)3. Distinctly, the floc strength for QCMC was stronger than that for Al2(SO4)3 under each flocculation condition, possibly because QCMC has abundant positive charges and chemical groups such as ANH2, AOH and ACOOH with good adsorptive property, which adsorb algal cells onto QCMC molecule chains with much stronger physical or chemical forces. For both QCMC and Al2(SO4)3, flocs formed under sweep and bridging dominant conditions were much stronger than under charge neutralization or electrostatic patching conditions, indicating that physical forces like Van der Waals’ force and hydrogen bond would not be easily broken under relatively high shear rate. Apart from that, the reticular structure of flocs improved flocs’ structural stability against high hydraulic shear force. The change in floc size with increasing shear rate was evaluated to generate the floc strength coefficient (log C) and floc strength constant (c) using Eq. (2), and the specific values were reported in Table 2. It was evident that QCMC flocs had higher log C and lower c values relative to Al2(SO4)3 flocs. These further validated that QCMC flocs were much stronger. It can also be found that the c values at pH 5 were larger than that at pH 9 and 12 for both QCMC and Al2(SO4)3 flocs, which confirmed that flocs formed by charge neutralization were prone to collapse. 3.4.3. Regrowth potential Flocs exhibited varying capacity to regrow to their original size and this was dependent on flocculation conditions (Fig. 4). When applying QCMC as the flocculant, the potential for regrowth was far greater when undertaking flocculation under charge neutralization condition, where the recovery factor was 64% at pH 5 and compared with 23% and 8% under patching and bridging conditions at pH 9 and 12, respectively (Table 2). The recovery potential of flocs formed under bridging was quite inadequate, which suggests that the forces such as Van der Waals’ force, hydrogen bond and chemical bond formed under bridging condition were broken irreversibly during floc disruption (Yu et al., 2010). On the contrary, under charge neutralization condition, aggregates interacted via electrostatic attraction and therefore tended to reform after breakage (Yukselen and Gregory, 2004). For Al2(SO4)3 flocs, it can also be concluded that regrowth was quite difficult for the broken flocs formed under sweep conditions at pH 9 and 12. Even though Al2(SO4)3 flocs formed under charge neutralization had relatively great recovery potential, there was a considerable gap between the recovery factors of QCMC and Al2(SO4)3 flocs. It has been surmised that broken flocs are not generally able to reform to their original size because they tend to lose bonding capacity resulting in fewer active bonding sites being available for reattachment (Jarvis et al., 2004). 3.5. Floc breakage profiles Floc breakage progressed with time in three different ways: Profile (1) large-scale fragmentation emerged at 1 min and 6 min after breakage followed by erosion later, as observed for Al2(SO4)3 and QCMC flocs formed at pH 5 under charge neutralization condition (Fig. 4a and 4b); Profile (2) tiny variation initially at 1 min after breakage but large-scale fragmentation presented from 6 min after breakage followed by minor erosion, as observed for QCMC flocs formed under patching condition at pH 9
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Fig. 4. Floc growth profiles under optimum conditions using Al2(SO4)3 at pH 5 (a), QCMC at pH 5 (b), Al2(SO4)3 at pH 9 (c), QCMC at pH 9 (d), Al2(SO4)3 at pH 12 (e) and QCMC at pH 12 (f) with original turbidity of 20 NTU at 20 °C.
Table 2 Floc properties under different coagulation-flocculation conditions using QCMC and comparison with using Al2(SO4)3.
a
Flocculant/ coagulant
pH OD (mg/L)
Growth rate (lm/min)
Steady-state floc size (lm)
Strength factora (%)
Floc strength coefficient (log C)
Floc strength constant (c)
Steady-state floc size after re-growtha (lm)
Recovery factora (%)
QCMC
5 9 12
0.1 0.6 2.0
155 285 330
734 871 953
25 41 56
3.42 3.30 3.23
0.53 0.36 0.25
536 473 565
64 23 8
Al2(SO4)3
5 9 12
20.0 35.0 45.0
41 64 87
384 454 458
23 35 42
3.18 3.04 3.06
0.60 0.41 0.37
192 187 215
35 10 9
For a shear rate of 200 rpm.
(Fig. 4d); Profile (3) progressive erosion, as observed for Al2(SO4)3 flocs formed under sweep condition at pH 9 and 12 (Fig. 4c and 4e) and QCMC flocs formed under bridging condition at pH 12 (Fig. 4f). Breakage profiles are closely related to flocculation mechanisms and depends on the nature of the interaction between particles, strength of the bonds and average number of bonds per particle (Henderson et al., 2006).
3.6. Floc compactness DF was measured during growth, breakage and regrowth period as a presentation of floc compactness. It was variable between different coagulation/flocculation mechanisms and coagulants/flocculants (Table 3). DF consistently increased during the initial steady-state phase (10–27 min) indicating that flocs became
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Table 3 Fractal dimension (DF) under different coagulation-flocculation conditions using QCMC during growth, breakage and regrowth at a breakage speed of 200 rpm. Flocculant/Coagulant
pH
Fractal dimension Before breakage
After breakage
After regrowth
10 min
27 min
28 min
33 min
42 min
45 min
QCMC
5 9 12
2.21 2.13 2.08
2.24 2.18 2.17
2.27 2.21 2.19
2.33 2.30 2.26
2.56 2.44 2.37
2.48 2.35 2.27
Al2(SO4)3
5 9 12
1.94 1.93 2.05
2.01 2.16 2.10
2.07 2.22 2.18
1.97 2.18 2.16
1.88 2.10 2.09
1.81 2.03 2.02
4. Conclusions An amphoteric chitosan-based flocculant (QCMC) was successfully prepared. QCMC demonstrated improved water solubility and excellent flocculation performance over a wide pH range for algal harvesting. Compared with chitosan, PAM, Al2(SO4)3 and FeCl3, QCMC presented much lower optimal dosages and higher algal turbidity removal efficiencies at various pH values. Charge neutralization, electrostatic patching and bridging were the predominant mechanisms at pH 5, 9 and 12 respectively. In terms of floc growth, breakage and regrowth, flocs produced using QCMC grew faster, and were consistently larger and more compact on breakage and regrowth than those produced using Al2(SO4)3. Acknowledgements Fig. 5. Comparison of settling rate of the flocs flocculated by QCMC and Al2(SO4)3 at pH 5, 9 and 12.
increasingly compact. On breakage at 28 min, an increase in DF was consistently observed, possibly because there was a break at the weak points followed by rearrangement into more stable and compact structure (Spicer and Pratsini, 1996). With increasing time under high shear condition, the DF values reduced for flocs formed using Al2(SO4)3 but, in contrast, increased for flocs formed using QCMC, reaching a maximum of 2.56 at pH 5 under charge neutralization dominant condition, indicating QCMC flocs became increasingly compact during the whole breakage period. On regrowth, the DF reduced by about 4% in all cases, indicating a relaxation of the floc structure on reduction of shear rate. Again, QCMC flocs tended to form more compact flocs than Al2(SO4)3 flocs during the regrowth period. The DF values obtained for QCMC was between 2.27 and 2.48 while that obtained for Al2(SO4)3 ranged from 1.81 to 2.03. Moreover, QCMC flocculation under charge neutralization and patching dominant conditions produced flocs with higher DF on breakage than for bridging. By contrast, for Al2(SO4)3 flocs there was little obvious trend. 3.7. Settling rate of flocs The settling processes of flocs over time flocculated by QCMC and Al2(SO4)3 at different pH levels are shown in Fig. 5. Under identical settling conditions, the flocs formed by QCMC settled more rapidly. This might be because QCMC molecules was in favor of linking among the suspended algal colloids and thus the formation of denser and larger flocs. Under different pH conditions during flocculation, the flocs formed by bridging and sweep settled faster than charge neutralization and electrostatic patching, and the settling solid bed was much thinner for the flocs formed at pH 12, probably because denser flocs generally have higher specific gravity. All these show that QCMC flocculant is more specifically suited for algal harvesting in natural waters.
This research was supported by Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education (Hohai University), China. Also, this research was supported by the National Natural Science Foundation of China (51178159)-China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 07.108. References Beach, E.S., Eckelman, M.J., Cui, Z., Brentner, L., Zimmerman, J.B., 2012. Preferential technological and life cycle environmental performance of chitosan flocculation for harvesting of the green algae Neochloris oleoabundans. Bioresour. Technol. 121, 445–449. Bolto, B., Gregory, J., 2007. Organic polyelectrolytes in water treatment. Water Res. 41, 2301–2324. Cai, Z., Song, Z., Shang, S., Yang, C., 2007. Study on the flocculating properties of quaternized carboxymethyl chitosan. Polym. Bull. 59, 655–665. Chen, J.J., Yeh, H.H., Tseng, I., 2009. Effect of ozone and permanganate on algae coagulation removal-pilot and bench scale tests. Chemosphere 74, 840–846. Chen, X.G., Park, H.J., 2003. Chemical characteristics of O-carboxymethyl chitosans related to the preparation conditions. Carbohydr. Polym. 53, 355–359. Dong, C., Chen, W., Liu, C., 2014. Preparation of novel magnetic chitosan nanoparticles and its application for removal of humic acid from aqueous solution. Appl. Surf. Sci. 292, 1067–1076. Fast, S.A., Kokabian, B., Gude, V.G., 2014. Chitosan enhanced coagulation of algal turbid waters – comparison between rapid mix and ultrasound coagulation methods. Chem. Eng. J. 244, 403–410. Guan, J., Waite, T.D., Amal, R., 1998. Rapid structure characterization of bacterial aggregates. Environ. Sci. Technol. 32, 3735–3742. Henderson, R., Sharp, E., Jarvis, P., Jefferson, B., 2006. Identifying the linkage between particle characteristics and understanding coagulation performance. Water Sci. Technol. 6, 31–38. Jarvis, P., Jefferson, B., Parsons, S., 2004. The duplicity of floc strength. Water Sci. Technol. 50, 63–70. Jarvis, P., Jefferson, B., Parsons, S., 2005. Breakage, regrowth and fractal nature of natural organic matter flocs. Environ. Sci. Technol. 39, 2307–2314. Jefferson, B., Sharp, E., Goslan, E., Henderson, R., 2005. Application of charge measurement to water treatment processes. Water Sci. Technol. 4, 49–56.
C. Dong et al. / Bioresource Technology 170 (2014) 239–247 Kuster, K., Wijers, J., Thoenes, D., 1997. Aggregation kinetics of small particles in agitated vessels. Chem. Eng. J. 52, 107–121. Ma, M., Liu, R., Liu, H., Qu, J., 2012. Chlorination of Microcystis aeruginosa suspension: cell lysis, toxin release and degradation. J. Hazard. Mater. 217– 218, 279–285. MEP, 2009. 2008 Report on the State of Environment in China. Ministry of Environmental Protection of the People’s Republic of China. Naghavi, B., Malone, R.F., 1986. Algae removal by fine sand/silt filtration. Water Res. 20, 377–383. Pan, G., Zhang, M.M., Chen, H., Zou, H., Yan, H., 2006. Removal of cyanobacterial blooms in Taihu Lake using local soils. I. Equilibrium and kinetic screening on the flocculation of Microcystis aeruginosa using commercially available clays and minerals. Environ. Pollut. 141, 195–200. Pei, H., Ma, C., Hu, W., Sun, F., 2014. The behaviors of Microcystis aeruginosa cells and extracellular microcystins during chitosan flocculation and flocs storage processes. Bioresour. Technol. 151, 314–322. Plummer, J.D., Edzwald, J.K., 2001. Effect of ozone on algae as precursors for trihalomethane and haloacetic acid production. Environ. Sci. Technol. 35, 3661– 3668. Qu, F., Liang, H., Tian, J., Yu, H., Chen, Z., Li, G., 2012. Ultrafiltration (UF) membrane fouling caused by cyanobacteria: fouling effects of cells and extracellular organics matter (EOM). Desalination 293, 30–37. Renault, F., Sancey, B., Badot, P.M., Crini, G., 2009. Chitosan for coagulation/ flocculation processes-an eco-friendly approach. Eur. Polym. J. 45, 1337–1348. Rinaudo, M., 2006. Chitin and chitosan: properties and applications. Prog. Polym. Sci. 31, 603–632. Sakai, H., Oguma, K., Katayama, H., Ohgaki, S., 2007. Effects of low- or mediumpressure ultraviolet lamp irradiation on Microcystis aeruginosa and Anabaena variabilis. Water Res. 41, 11–18. Shen, Q., Zhu, J., Cheng, L., Zhang, J., Zhang, Z., Xu, X., 2011. Enhanced algae removal by drinking water treatment of chlorination coupled with coagulation. Desalination 271, 236–240.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Flocculation of algal cells by amphoteric chitosan-based flocculant Changlong Dong a, Wei Chen a,b,⇑, Cheng Liu a,b a b
College of Environment, Hohai University, Nanjing 210098, PR China Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, PR China
h i g h l i g h t s An amphoteric chitosan-based flocculant (QCMC) was prepared. QCMC presented significant improvement of water solubility in the whole pH range. QCMC presented lower optimal dosages and higher algal turbidity removal efficiencies. QCMC flocs grew faster to produce larger and more compact flocs.
a r t i c l e
i n f o
Article history: Received 25 June 2014 Received in revised form 23 July 2014 Accepted 25 July 2014 Available online 4 August 2014 Keywords: Algae Chitosan Flocculation Floc properties
a b s t r a c t A kind of amphoteric chitosan-based flocculant (quaternized carboxymethyl chitosan, denoted as QCMC) has been prepared. QCMC presented significant improvement of water solubility in the whole pH range. The effects of pH, dosage, temperature and original turbidity of algal water on the flocculation performance were investigated. The optimal dosages of QCMC at pH 5, 9 and 12 with original turbidity of 20 NTU at 20 °C were 0.1, 0.6 and 2.0 mg/L, respectively, which were much less than that of chitosan, PAM, Al2(SO4)3 and FeCl3. The floc properties during grow, breakage and regrow period were also evaluated at different pH values in terms of floc size, strength and density. It was demonstrated that QCMC produced larger, stronger and denser flocs than Al2(SO4)3. There is every indication that QCMC is more suitable for algal harvesting than other traditional coagulants or flocculants. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The presence of seasonal algal blooms in water sources such as lakes and reservoirs has posed a serious water safety problem to local water industry. Many lakes are subjected to eutrophication which usually results in seasonal algae blooms (Qu et al., 2012). As of 2008, more than 75% of natural and artificial lakes in China have been eutrophicated in different degrees and suffered from harmful algal blooms (Wu et al., 2011). For instance, the eutrophication in Dian Lake was at high level, while Tai Lake and Chao Lake were at mid-eutrophication level in 2008 (MEP, 2009). Typically, the algal bloom occurred in Tai Lake in Jiangsu province at the end of May 2007 contaminated the main supply of drinking water for millions of people (Shen et al., 2011). Algae and its metabolites can cause serious damage to drinking water quality, such as the production of unpleasant tastes and odors, formation of ⇑ Corresponding author at: Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, PR China. Tel.: +86 15195959921. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.biortech.2014.07.108 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
disinfection byproducts and toxins from cyanobacteria, for example Microcystis aeruginosa which is one of the dominant species in eutrophic surface water (Pan et al., 2006). Moreover, algae is known to disrupt drinking water treatment process, resulting in short filter runs, increases in coagulant/flocculant demand and microbial regrowth in distribution systems (Chen et al., 2009; Plummer and Edzwald, 2001). The general methods used to remove or control algal growth include physical processes such as media filtration (Naghavi and Malone, 1986) and membrane separation (Sun et al., 2013), chemical processes such as coagulation (Pei et al., 2014), flocculation (Beach et al., 2012) and chlorination (Ma et al., 2012), and the electromagnetic radiation such as ultraviolet light (Sakai et al., 2007). Among these methods, Ultraviolet light process has main limitations of high capital and maintenance costs (Fast et al., 2014). Media filtration and membrane separation are cost- and energyintensive while use of oxidizing agents like chlorine may cause physiological stress or cell walls damage resulting in release of taste and odor causing compounds or intracellular organic matter (Plummer and Edzwald, 2001). Thus the optimal strategy for algae control in drinking water treatment would be the intact removal of
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algal cells without cell rupture. It has been concluded that chemical coagulation-flocculation is the safest and most economical step in the water treatment process for algae removal (Chen et al., 2009; Wu et al., 2011). Since algal cells possess negative surface charge, the surface charge can be neutralized by introducing cationic ions, namely aluminum ions, ferric ions and synthetic polymers such as polyacrylamide. These coagulants and flocculants can easily flocculate algal cells and form flocs. However, using these chemicals may have several environmental consequences (Renault et al., 2009): (i) an increase in metal concentration in water which may have human health implications; (ii) production of large volumes of toxic sludge; (iii) dispersion of acrylamide oligomers which may also be a health hazard. For these reasons, alternative natural biopolymers have been considered for environmental applications since they are natural low-cost products and characterized by their environmentally friendly behavior (Vandamme et al., 2010). Among these biopolymers, chitosan is considered as one of the most promising flocculant materials. Chitosan is a linear poly-amino-saccharide produced by deacetylation of chitin which is the second most abundant biopolymer in the world after cellulose. Chitosan has unique properties among biopolymers especially due to the presence of primary amino groups and it is a commercially interesting compound because of its high nitrogen content in comparison to cellulose (Renault et al., 2009). It has characteristics of both coagulants and flocculants, i.e., high cationic charge density, long polymer chains, bridging of aggregates and precipitation (in neutral and alkaline conditions). The main reason for the success of chitosan in water treatment as a flocculant is that chitosan has the advantage of being non-toxic, non-corrosive and safe to handle (Bolto and Gregory, 2007). Apart from that, chitosan is efficient in cold water and at much lower dosages than metal salts, and it does not leave residual metals that can cause second contamination problems. There are, of course, disadvantages which must be balanced against the benefits (Rinaudo, 2006). Chitosan is only efficient over a limited pH range and only soluble in dilute organic acids, which greatly limits the application of chitosan in flocculation process. Therefore, the modification of chitosan aiming at algal harvesting using coagulation-flocculation process is essential. In recent years, extensive attention has been given to research and application of amphoteric chitosan-based flocculants (Cai et al., 2007; Yang et al., 2011). The reason is that amphoteric chitosan is more effective due to its improved flocculation performance and wider range of solubility due to the dual characteristics of both cationic and anionic groups. But without ignorance, the modification reagent (monochloroacetic acid) which was adopted in many related literatures is highly toxic, and the flocculation properties of amphoteric chitosan for algal turbid water has never been evaluated. Under the above background, the amphoteric chitosan flocculant (quaternized carboxymethyl chitosan, denoted as QCMC) has been prepared using safer modification reagents in the current work. The aim of this paper is to determine the flocculation properties and mechanisms of QCMC for algal turbid water and examine the formation, breakage and regrowth of algal flocs using QCMC in comparison with aluminum sulfate (Al2(SO4)3).
2. Methods 2.1. Materials Chitosan powders 80–95% deacetylated supplied by Sinopharm Chemical Reagent Co., Ltd had a molecular weight ranging from 535,000 to 620,000 g/mol. Glyoxylic acid monohydrate (GA) from Aladdin Chemistry Co., Ltd and (3-Chloro-2-hydroxypropyl)-trime-
thylammonium Chloride (CTA) from Tokyo Chemical Industry Co., Ltd were both used without further purification. All other chemicals were purchased from Nanjing Chemical Reagent Co., Ltd. 2.2. Preparation of QCMC QCMC flocculant was designed and synthesized by two steps as shown in Fig. S1. The carboxymethyl chitosan (CMC) was firstly prepared by using GA according to Schiff’s base reaction principle. The toxicity of GA is quite lower than monochloroacetic acid. Final QCMC was obtained on the basis of CMC by quaternized modification using CTA. The preparation process of QCMC was described in brief as follows. A desired amount of chitosan was dispersed in ultrapure water ensuring the mass concentration of chitosan suspension is roughly 2%. After stirring for 30 min at room temperature, a certain amount of GA (n(chitosan):n(GA) = 0.8) was added into the chitosan suspension and the reaction time was controlled at 45 min. After that, the pH of the mixture was adjusted to pH 12 with 2 mol/L NaOH aqueous solution and a certain amount of NaBH3CN (n(GA):n(NaBH3CN) = 0.8, dissolved in 20 mL of ultrapure water) was then added into the reaction system dropwise with stirring for 45 min. The resultant products were precipitated by absolute ethanol and rinsed in 90% ethanol to desalt and dewater, then extracted using acetone as solvent in the Soxhlet apparatus for 24 h, and finally dried fully in a vacuum oven at room temperature. The products were CMC. Then, a desired amount of dried CMC was added into a mixture of isopropanol and NaOH aqueous solution to swell and alkalize at 45 °C for 1 h. A certain amount of CTA was added into the reaction mixture dropwise for 20 min and stirred for 10 h at 60 °C. The pH of the mixture was then adjusted to pH 7 with 0.1 mol/L HCl aqueous solution. The resultant solid was washed and dried using the same method as mentioned in the first step. The final products of QCMC were prepared. 2.3. Characterization FTIR spectra were recorded using Tensor 27 spectrometer (Bruker, Germany). The interval of tested wavenumber was 500–4000 cm1. The XRD patterns were recorded on D8 Advance X-ray diffractometer (Bruker, Germany). The surface morphologies were observed by S4800 SEM (Hitachi, Japan). Zeta potential (ZP) measurement was carried out using Nano ZS90 Zetasizer (Malvern, UK). Water solubility was evaluated based on the method reported by Chen and Park (Chen and Park, 2003). The samples were dissolved in deionized water, and the concentrations of the samples were all 5 mg/mL. The various pH values of solutions were adjusted with 0.1 mol/L HCl or NaOH. Hydrodynamic radius (Rh) of QCMC with different pH conditions were determined by BI200SM dynamic light scattering apparatus (Brookheaven, USA). 2.4. Algae culture and algal turbid waters M. aeruginosa which is a common species in eutrophic surface water was selected for this study. It was purchased from Institute of Hydrobiology, Chinese Academy of Sciences. Axenic cultures were carried out in batch mode in 1 L conical flasks with BG11 medium. The conical flasks were placed in an incubator and the algal cells were cultured at the temperature of 25 °C with illumination of 5000 lx provided for 14 h every day. The live algal suspensions were harvested with culture time between 15 and 28 days and diluted by ultrapure water to prepare stable algal turbid waters with original turbidity of 10–40 NTU.
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2.5. Flocculation experiments 2.5.1. Flocculation procedure Flocculation experiments were carried out using ZR4–6 (Zhongrun, China) Jar Tester with 50 40 mm flat paddle impellers using cylindrical jars containing 1 L samples of algal turbid water. Initially, the algal turbid water was rapidly mixed at 200 revolutions per minute (rpm) for 2 min (G = 105.5 s1) while pH was adjusted with 0.1 mol/L HCl or NaOH and coagulants/flocculants were added. Then, a slow stir phase was conducted at 30 rpm (G = 8.1 s1) for 25 min to promote the collision of particles and hence floc growth. Subsequently, a 30 min settling period was applied. The supernatant was analyzed for residual turbidity (HACH 2100P Portable Turbidimeter, USA) and zeta potential to ascertain dosage response curves and thereby establish optimal dosages (OD, mg/L) under which to undertake subsequent floc property experiments. The OD of various coagulants and flocculants like chitosan, polyacrylamide (PAM), ferric chloride (FeCl3) and Al2(SO4)3 under different pH conditions were determined using the same flocculation procedure as described above. The turbidity removal efficiency (TRE,%) is expressed as
TRE ð%Þ ¼
T raw T treated 100% T raw
ð1Þ
Where Traw and Ttreated are the turbidites of raw and treated water, respectively. 2.5.2. Floc properties To establish floc growth and breakage profiles, flocculation experiments were conducted using the established conditions as discussed in Section 2.5.1. However, after the slow stirring phase for floc growth, rather than applying a 30 min of sedimentation, the impact of increasing shear on floc size was studied by varying the speed of the jar test paddles to 50 (G = 16.1 s1), 100 (G = 41.2 s1) and 200 rpm for 15 min, after which the paddle speed was finally controlled to 30 rpm for further 20 min to examine the potential for floc regrowth. Floc size was analyzed by Mastersizer 2000 (Malvern, UK) using the results of median average equivalent diameter (d50) of the flocs to generate floc growth curves under each condition.
Strength factor ð%Þ ¼
d2 100% d1
ð3Þ
Recovery factor ð%Þ ¼
d3 d2 100% d1 d2
ð4Þ
Where d1 is the steady-state floc size before breakage, d2 is the floc size after breakage and d3 is the floc size on regrowth. A higher strength factor value is indicative of a strong floc, while an increase in recovery factor demonstrates increased potential for regrowth. 2.5.4. Determination of fractal dimension Fractal dimension (DF) was measured by light scattering method. The light scattering technique involves measurement of light intensity I as a function of the scatter vector Q. Q is given by the following equation (Guan et al., 1998):
Q¼
4pn sinðh=2Þ k
ð5Þ
Where n, h and k are the refractive index of the media, the scattered angle and the wavelength of radiation in vacuum, respectively. The relationship among I, Q and DF can be represented by Eq. (6).
I / Q DF
ð6Þ
DF can be determined from the negative slope of log–log plot of I and Q. 2.6. Settling rate The settling rate experiments were conducted with QCMC and Al2(SO4)3 using algal turbid waters with original turbidity of 20 NTU at 20 °C. After the flocculation process as mentioned in Section 2.5.1, the flocculated algal turbid water was transferred into a 100 mL stopped graduated cylinder. Then, the height of the interval between the surface of the supernatant and the settling solid bed was recorded over time. 3. Results and discussion 3.1. Characterization of chitosan-based flocculants
2.5.3. Characteristic parameters of flocs During the flocculation process under different conditions, the growth rate, initial steady-state size, size on exposure to increased shear and regrowth potential can be evaluated from the floc growth curves. Specifically, growth rate was calculated as the slope obtained from plotting d50 during the initial growth period (2–7 min). Steady-state floc size was calculated by taking the average of all floc size after the steady state was achieved but before the breakage period (10–27 min). Floc size after breakage was calculated as the average of the floc size reached after the flocs were exposed to an increase of shear (32–40 min). The floc size on regrowth was obtained by calculating the average of the d50 values during the regrowth phase of the flocs (50–60 min). The floc strength coefficient (log C) and the floc strength constant (c) were obtained according to the empirical expression shown in Eq. (2) (Jarvis et al., 2005).
log d ¼ log C c log G
ð2Þ
Where d is the floc diameter (d50), log C is the intercept, c is the slope and G is the velocity gradient. It has been proposed that the higher the value of log C at a fixed shear, the stronger the floc, and the larger the value of c, the more prone the flocs are to breakage when increasing the shear rate. The floc strength and recovery factor were calculated using the following empirical equations (Jarvis et al., 2005):
The FTIR spectra of chitosan, CMC and QCMC are presented in Fig. S2. The adsorption bands at 3443 (OAH), 1596 (NAH), 1080 (C3AOH), 1030 cm1 (C6AOH) in Fig. S2a were the characteristic FTIR peaks of chitosan (Dong et al., 2014). In Fig. S2b, a new peak appeared at 1563 cm1 for the COO- group, which conformed that the negative carboxymethyl was successfully introduced to chitosan backbone (Chen and Park, 2003). Meanwhile, the NAH peak at 1596 cm1 was dramatically weakened while the C3AOH (1076 cm1) and C6AOH (1030 cm1) peaks have remained nearly unchanged. These indicated that the carboxymethylation was mainly occurred on ANH2 groups of chitosan. The intense peak at 1479 cm1 corresponding to the methyl of the quaternary ammonium group (Cai et al., 2007) in Fig. S2c also indicated that CTA were successfully grafted onto CMC. It is worth noting that the NAH peak disappeared and the C6-OH peak clearly shifted from 1030 to 1023 cm1 after quaternized modification, which proved that the CTA mostly grafted to C6AOH groups and residual ANH2 groups on CMC. In addition, the OAH peak gradually reduced to low band, demonstrating that the intramolecular and intermolecular hydrogen bonds were broken during the preparation of QCMC. The changes of FTIR spectra can be well explained by the synthetic route diagram as shown in Fig. S1. The XRD patterns of chitosan, CMC and QCMC are exhibited in Fig. S3 to investigate the structure of the final products further.
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The XRD pattern of chitosan showed two diffraction peaks at 2h = 10.4° and 20.1°. The diffraction peak at 2h = 10.4° was attributed to the hydrated crystals of low crystallinity and corresponded to the form I, while the diffraction peak appeared at 2h = 20.1° was identified as representative of the crystallinity of form II (Wu et al., 2005). For CMC and QCMC, the peak at 2h = 10.4° disappeared, which suggested that the hydrogen bonding ability of chitosan was reduced after the modification. Moreover, along with the process of modification, the peak at 2h = 20.1° has become relatively weaker for the decrease of ordered structure, indicating that the structure of the amphoteric grafting products became amorphous. Therefore, it can be concluded from the XRD patterns that the ordered structure of chitosan has been notably destroyed by graft chain. To give direct observation of the amorphous features of QCMC, SEM images have been taken and shown in Fig. S4. Significant differences can be observed among chitosan, CMC and QCMC. Chitosan had relatively regular structure. By contrast, the surface of CMC became rough due to the introduction of carboxymethyl groups. After CTA has been grafted onto CMC, the morphologies of QCMC changed more drastically: more porous and looser structures were observed. The porous morphologies of QCMC are expected to exert positive effect on flocculation efficiency, since more pores are favored for flocculating pollutants in water (Yang et al., 2013). It is well known that the charge properties of flocculants wield great influence on flocculation performance. Therefore, the pH independence of ZP of various sample solutions were also measured and shown in Fig. 1. It is found that algal cells were negatively charged over the whole pH range. Chitosan showed significantly positive ZP before pH 6.6 due to protonation of amino groups, whereas the ZP near zero was depicted after pH 7.0. The isoelectric point (IEP) of CMC was reduced to pH 5.5 because of the introduction of negative carboxymethyl groups, while QCMC had a much higher IEP around pH 9.2 due to the effective grafting of positive CTA. This improves the applicability of QCMC for alkaline algal turbid water in a wide pH range according to charge neutralization. 3.2. Water solubility of chitosan-based flocculants Water solubility is one of the most basic characteristics in usage of flocculants in water treatment. Therefore, solubility in a wide pH range is beneficial for flocculants applied in various situations. The solubility of chitosan, CMC and QCMC in aqueous solutions with various pH values are shown in Table S1. Chitosan had the narrowest soluble pH region (pH 6 5), indicating its poor solubility. Although CMC had better solubility than chitosan due to the fact
Fig. 1. Zeta potential-pH profiles of chitosan, CMC, QCMC and algal turbid water.
that the ordered structure of chitosan has been destroyed by carboxymethylation, it was still insoluble or partially soluble in lowacid water (pH = 4–7). Compared with chitosan and CMC, QCMC demonstrated remarkably improved solubility property. It was soluble almost in the whole pH range and only showed partial insolubility near its IEP at around pH 9.2. The excellent solubility of QCMC is mainly due to the amphoteric feature of having both cationic and anionic groups, which can adapt itself to various external conditions and maintain its stretched conformation of polymer chains (Yang et al., 2011). 3.3. Flocculation performance 3.3.1. Effects of pH and dosage A variety of water quality parameters can affect flocculation efficiency of flocculants, of which pH and dosage are the two most important ones. Hence, the flocculation performance of QCMC was systematically studied at different pH values and dosages (Fig. 2) and compared with chitosan and other conventional coagulants/ flocculants (Table 1). From the TRE-dosage profiles at pH 5, 9 and 12, the OD of QCMC were 0.1, 0.6 and 2.0 mg/L, respectively. With the increasing pH, the best residual turbidity corresponding to every OD also increased. However, the flocculation windows under three different flocculation conditions were not narrowed significantly and all of the residual turbidities remained below 1.0 NTU after 30 min of sedimentation. Although the re-stabilization of cell suspensions existed, manifesting as decrease of TRE, the weakening of flocculation performance was not evident especially at higher pH values. All of the TRE remained above 90% when QCMC was overdosed. Since the conformation of QCMC molecule in water has great impacts on flocculation performance, the pH dependence of Rh was measured and shown in Fig. S5. At pH 5, the positive charges of QCMC leaded to high Rh due to the effect of intramolecular electrostatic repulsion. Both outstretched conformation and positive charges of QCMC at lower pH values were beneficial for flocculation efficiency and resulted in lower OD requirements (Yang et al., 2012). Meanwhile, the ZP of supernatant was approximate to zero when OD were reached and became the opposite when QCMC was overdosed. This demonstrates that charge neutralization is dominant during flocculation process using QCMC in acidic algal water (Yang et al., 2014). Negatively charged algal cells were firstly attracted, neutralized and coated by positively stretched QCMC molecules and when the cells were thoroughly covered by these molecules, the destabilized algal cells with near to zero surface charge aggregated continually and form large flocs. At pH 9 near the IEP of QCMC, large decrease of Rh indicated that the polymer chains have become curly and short owing to the loss of net charge. But residual positive charges that were not neutralized by polymer chains drove the deformed chains attached on the negative surface of algal cells. The coverage of QCMC molecules on the surfaces of algal cells was neither full nor uniform and different small regions with opposite charges existed on the cells surfaces resulting in further cells aggregation through electrostatic interactions (Yang et al., 2014). On the other hand, the ZP of supernatant was far away from zero at OD, but went across zero and became the opposite when QCMC were overdosed, indicating that electrostatic patching was the main flocculation mechanism in this case. Despite all this, the loss of net charge reduced the number of active flocculation sites and leaded to higher OD. At pH 12, negative charges on QCMC increased, therefore the polymer chain with high Rh again became stretched. However, negative charges also resulted in greater electrostatic repulsion between QCMC molecules and suspended algal cells, finally leading to poorer flocculation performance, namely much higher OD and
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relatively lower TRE. It can be clearly observed from Fig. 2c that ZP of supernatant after flocculation at pH 12 was less than zero in spite of variation of dosage, owing to the shortage of positive charges. However, the flocculation efficiency in this case was still well performed. This can be explained by the bridging mechanism. The negative algal cells was adsorbed onto QCMC molecules by Van der Waals’ force and hydrogen bond under hydraulic agitation, and the electrostatic repulsion was unlikely to destroy these adsorption forces. The surface charges of algal cells were partially screened by QCMC molecules resulting in increasing ZP but not beyond zero. The flocculation performance of chitosan and several conventional coagulants/flocculants, such as Al2(SO4)3, FeCl3 and PAM, were investigated at different pH values and shown in Fig. S6 for comparison. Based on Fig. S6, the OD and corresponding TRE of different coagulants/flocculants were summarized in Table 1. Based on Table 1, QCMC consistently revealed the best flocculation performance amongst all five coagulants/flocculants, and showed both the lowest OD and highest TRE at pH 5, 9 and 12. Chitosan had a better performance than other conventional coagulants/flocculants in acidic water, but this advantage did not exist in neutral and basic solutions, mainly owing to the insolubility of chitosan at pH > 5. It cannot be ignored that the OD of inorganic coagulants like Al2(SO4)3 and FeCl3 were over 200 times more than that of QCMC, which increased sludge production and cost of water treatment. Flocculant cost is an important limiting factor for the wide use. A detailed cost comparison among chitosan, QCMC, PAM, Al2(SO4)3 and FeCl3 can be found in Table 1. It is obvious that the unit price of QCMC is significantly higher than that of other organic or inorganic flocculants mainly because of the expensive modification chemicals. However, when using these flocculants for algal harvesting under different conditions, QCMC presented both performance and cost advantages at pH 6 9, owing to its considerably lower OD. At pH 12, the cost of QCMC become the highest but considering the short period of seasonal algal blooms, this high expense can be affordable.
Fig. 2. Turbidity removal efficiency and zeta potential of the supernatant as a function of QCMC dosage at pH 5 (a), 9 (b) and 12 (c) with original turbidity of 20 NTU at 20 °C.
3.3.2. Effects of temperature and original turbidity In actual water treatment, temperature and original turbidity of water is different as the season changes, which evidently influences OD of flocculants. Based on Figs. S7 and S8, the OD as a function of temperature and original turbidity of algal turbid water are summarized in Fig. 3. It can be concluded from Fig. 3 that algal turbid water at higher temperature required less flocculants, which can be attributed to two reasons. Firstly, the viscosity of algal turbid water decreased with the increasing temperature, which resulted in enhancement of molecular movements and collision to form large particles. Secondly, the more unfolded QCMC molecules due to the improved solubility were also conducive to flocculation (Yang et al., 2011). It can also be obtained from Fig. 3 that the OD increased with the increase in original turbidity of algal turbid water. Data analysis in Fig. 3 further shows that, for algal turbid waters with original turbidity in the range of 10–40 NTU, the OD
Table 1 The cost, optimal dosages and corresponding algal turbidity removal efficiencies of various flocculants/coagulants. Flocculant/Coagulant
Chitosan QCMC PAM Al2(SO4)3 FeCl3
Unit price (US$/kg)
51.2 368.2 22.1 5.8 3.7
pH = 5
pH = 9
pH = 12
OD (mg/L)
TRE (%)
Cost (US$/m3)
OD (mg/L)
TRE (%)
Cost (US$/m3)
OD (mg/L)
TRE (%)
Cost (US$/m3)
0.4 0.1 3.0 20.0 15.0
94.7 96.3 94.1 89.2 93.4
0.02 0.04 0.07 0.12 0.06
5.5 0.6 8.0 35.0 25.0
85.3 95.8 86.5 88.2 91.2
0.28 0.22 0.18 0.20 0.09
11.5 2.0 10.0 45.0 30.0
81.6 95.0 83.7 89.4 93.5
0.59 0.74 0.22 0.26 0.11
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Fig. 3. Optimal dosage of QCMC for algal turbid water at pH 9 as a function of original turbidity at 20 °C and as a function of water temperature with original turbidity of 20 NTU.
increased linearly with the initial turbidity with an intercept of zero (R2 = 0.928). This stoichiometric linear relation has directed significance to real water production.
3.4. Floc growth profiles The floc growth profiles using QCMC were fully compared with Al2(SO4)3 at different pH levels and OD (Fig. 4). According to previous studies (Jefferson et al., 2005), when using Al2(SO4)3 as coagulant, charge neutralization tends to be influential at pH 5 and relatively low dosages, while sweep flocculation is the dominant mechanism at alkaline pH values and higher dosages.
3.4.1. Initial floc size and growth rate The initial steady-state floc size was consistently larger for QCMC than Al2(SO4)3 (Fig. 4). For example, the floc size achieved using Al2(SO4)3 ranged between 384 and 458 lm while the average floc size of QCMC ranged between 734 and 953 lm (Table 2). This large gap in floc size demonstrates that the stem grafting chain structure of QCMC can lead to much larger flocs through electrostatic attraction or bridging. At the same time, the steady-state floc size for QCMC and Al2(SO4)3 at lower pH values were apparently smaller than that at higher pH levels, indicating that the flocs formed by charge neutralization tended to shape diminutive and frizzy flocs because one negative algal cell may be simultaneously attracted by multiple positive charges on the same polymer chain of QCMC. The growth rate observed from Table 2 for Al2(SO4)3 ranged from 41 to 87 lm/min depending on the conditions applied, whilst for QCMC this was increased to 155–330 lm/min such that steadystate floc size was achieved between 6 and 14 min as opposed to between 4 and 6 min for flocs of Al2(SO4)3 and QCMC respectively (Fig. 4). This may be because the relatively large molecular weight of QCMC results in more chance of contact between QCMC molecules and algal cells. Meanwhile, growth rate also increased with the increasing pH. In other words, the floc growth rate under sweep and bridging flocculation conditions was faster than under charge neutralization and patching conditions. This could be attributed to hydroxide precipitates and outstretched polymer chains having a large collision profile giving a large effective volume concentration and large particle diameter, thus increasing the chance of collision and capturing other algal cells, leading to faster flocculation (Kuster et al., 1997).
3.4.2. Floc strength Fig. 4 illustrated that flocs for both Al2(SO4)3 and QCMC tended to resist breakage under relatively low shear rate as shown by only minor reductions in floc size on increasing the shear rate to 50 rpm (7.3–22.7%). However, when the shear rate was increased to 200 rpm, a reduction of floc size between 45.8% and 77.0% was observed. Similarly, floc strength factors at 200 rpm typically ranged from 25 to 56% for QCMC and 23–42% for Al2(SO4)3. Distinctly, the floc strength for QCMC was stronger than that for Al2(SO4)3 under each flocculation condition, possibly because QCMC has abundant positive charges and chemical groups such as ANH2, AOH and ACOOH with good adsorptive property, which adsorb algal cells onto QCMC molecule chains with much stronger physical or chemical forces. For both QCMC and Al2(SO4)3, flocs formed under sweep and bridging dominant conditions were much stronger than under charge neutralization or electrostatic patching conditions, indicating that physical forces like Van der Waals’ force and hydrogen bond would not be easily broken under relatively high shear rate. Apart from that, the reticular structure of flocs improved flocs’ structural stability against high hydraulic shear force. The change in floc size with increasing shear rate was evaluated to generate the floc strength coefficient (log C) and floc strength constant (c) using Eq. (2), and the specific values were reported in Table 2. It was evident that QCMC flocs had higher log C and lower c values relative to Al2(SO4)3 flocs. These further validated that QCMC flocs were much stronger. It can also be found that the c values at pH 5 were larger than that at pH 9 and 12 for both QCMC and Al2(SO4)3 flocs, which confirmed that flocs formed by charge neutralization were prone to collapse. 3.4.3. Regrowth potential Flocs exhibited varying capacity to regrow to their original size and this was dependent on flocculation conditions (Fig. 4). When applying QCMC as the flocculant, the potential for regrowth was far greater when undertaking flocculation under charge neutralization condition, where the recovery factor was 64% at pH 5 and compared with 23% and 8% under patching and bridging conditions at pH 9 and 12, respectively (Table 2). The recovery potential of flocs formed under bridging was quite inadequate, which suggests that the forces such as Van der Waals’ force, hydrogen bond and chemical bond formed under bridging condition were broken irreversibly during floc disruption (Yu et al., 2010). On the contrary, under charge neutralization condition, aggregates interacted via electrostatic attraction and therefore tended to reform after breakage (Yukselen and Gregory, 2004). For Al2(SO4)3 flocs, it can also be concluded that regrowth was quite difficult for the broken flocs formed under sweep conditions at pH 9 and 12. Even though Al2(SO4)3 flocs formed under charge neutralization had relatively great recovery potential, there was a considerable gap between the recovery factors of QCMC and Al2(SO4)3 flocs. It has been surmised that broken flocs are not generally able to reform to their original size because they tend to lose bonding capacity resulting in fewer active bonding sites being available for reattachment (Jarvis et al., 2004). 3.5. Floc breakage profiles Floc breakage progressed with time in three different ways: Profile (1) large-scale fragmentation emerged at 1 min and 6 min after breakage followed by erosion later, as observed for Al2(SO4)3 and QCMC flocs formed at pH 5 under charge neutralization condition (Fig. 4a and 4b); Profile (2) tiny variation initially at 1 min after breakage but large-scale fragmentation presented from 6 min after breakage followed by minor erosion, as observed for QCMC flocs formed under patching condition at pH 9
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Fig. 4. Floc growth profiles under optimum conditions using Al2(SO4)3 at pH 5 (a), QCMC at pH 5 (b), Al2(SO4)3 at pH 9 (c), QCMC at pH 9 (d), Al2(SO4)3 at pH 12 (e) and QCMC at pH 12 (f) with original turbidity of 20 NTU at 20 °C.
Table 2 Floc properties under different coagulation-flocculation conditions using QCMC and comparison with using Al2(SO4)3.
a
Flocculant/ coagulant
pH OD (mg/L)
Growth rate (lm/min)
Steady-state floc size (lm)
Strength factora (%)
Floc strength coefficient (log C)
Floc strength constant (c)
Steady-state floc size after re-growtha (lm)
Recovery factora (%)
QCMC
5 9 12
0.1 0.6 2.0
155 285 330
734 871 953
25 41 56
3.42 3.30 3.23
0.53 0.36 0.25
536 473 565
64 23 8
Al2(SO4)3
5 9 12
20.0 35.0 45.0
41 64 87
384 454 458
23 35 42
3.18 3.04 3.06
0.60 0.41 0.37
192 187 215
35 10 9
For a shear rate of 200 rpm.
(Fig. 4d); Profile (3) progressive erosion, as observed for Al2(SO4)3 flocs formed under sweep condition at pH 9 and 12 (Fig. 4c and 4e) and QCMC flocs formed under bridging condition at pH 12 (Fig. 4f). Breakage profiles are closely related to flocculation mechanisms and depends on the nature of the interaction between particles, strength of the bonds and average number of bonds per particle (Henderson et al., 2006).
3.6. Floc compactness DF was measured during growth, breakage and regrowth period as a presentation of floc compactness. It was variable between different coagulation/flocculation mechanisms and coagulants/flocculants (Table 3). DF consistently increased during the initial steady-state phase (10–27 min) indicating that flocs became
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Table 3 Fractal dimension (DF) under different coagulation-flocculation conditions using QCMC during growth, breakage and regrowth at a breakage speed of 200 rpm. Flocculant/Coagulant
pH
Fractal dimension Before breakage
After breakage
After regrowth
10 min
27 min
28 min
33 min
42 min
45 min
QCMC
5 9 12
2.21 2.13 2.08
2.24 2.18 2.17
2.27 2.21 2.19
2.33 2.30 2.26
2.56 2.44 2.37
2.48 2.35 2.27
Al2(SO4)3
5 9 12
1.94 1.93 2.05
2.01 2.16 2.10
2.07 2.22 2.18
1.97 2.18 2.16
1.88 2.10 2.09
1.81 2.03 2.02
4. Conclusions An amphoteric chitosan-based flocculant (QCMC) was successfully prepared. QCMC demonstrated improved water solubility and excellent flocculation performance over a wide pH range for algal harvesting. Compared with chitosan, PAM, Al2(SO4)3 and FeCl3, QCMC presented much lower optimal dosages and higher algal turbidity removal efficiencies at various pH values. Charge neutralization, electrostatic patching and bridging were the predominant mechanisms at pH 5, 9 and 12 respectively. In terms of floc growth, breakage and regrowth, flocs produced using QCMC grew faster, and were consistently larger and more compact on breakage and regrowth than those produced using Al2(SO4)3. Acknowledgements Fig. 5. Comparison of settling rate of the flocs flocculated by QCMC and Al2(SO4)3 at pH 5, 9 and 12.
increasingly compact. On breakage at 28 min, an increase in DF was consistently observed, possibly because there was a break at the weak points followed by rearrangement into more stable and compact structure (Spicer and Pratsini, 1996). With increasing time under high shear condition, the DF values reduced for flocs formed using Al2(SO4)3 but, in contrast, increased for flocs formed using QCMC, reaching a maximum of 2.56 at pH 5 under charge neutralization dominant condition, indicating QCMC flocs became increasingly compact during the whole breakage period. On regrowth, the DF reduced by about 4% in all cases, indicating a relaxation of the floc structure on reduction of shear rate. Again, QCMC flocs tended to form more compact flocs than Al2(SO4)3 flocs during the regrowth period. The DF values obtained for QCMC was between 2.27 and 2.48 while that obtained for Al2(SO4)3 ranged from 1.81 to 2.03. Moreover, QCMC flocculation under charge neutralization and patching dominant conditions produced flocs with higher DF on breakage than for bridging. By contrast, for Al2(SO4)3 flocs there was little obvious trend. 3.7. Settling rate of flocs The settling processes of flocs over time flocculated by QCMC and Al2(SO4)3 at different pH levels are shown in Fig. 5. Under identical settling conditions, the flocs formed by QCMC settled more rapidly. This might be because QCMC molecules was in favor of linking among the suspended algal colloids and thus the formation of denser and larger flocs. Under different pH conditions during flocculation, the flocs formed by bridging and sweep settled faster than charge neutralization and electrostatic patching, and the settling solid bed was much thinner for the flocs formed at pH 12, probably because denser flocs generally have higher specific gravity. All these show that QCMC flocculant is more specifically suited for algal harvesting in natural waters.
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