FlocculationâDewatering Behavior of Microalgae at Different Growth

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Flocculation-dewatering behavior of microalgae at different growth stages under inorganic polymeric flocculant treatment: the relationships between algal organic matters (AOM) and floc dewaterability Weijun Zhang, Qingwei Cao, Guangling Xu, and Dongsheng Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02551 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Sustainable Chemistry & Engineering

Flocculation-dewatering behavior of microalgae at different growth stages under inorganic polymeric flocculant treatment: the relationships between algal organic matters (AOM) and floc dewaterability Weijun Zhanga,*, Qingwei Caoa, Guangling Xua, Dongsheng Wangb,c

a

School of Environmental Studies, China University of Geosciences, Wuhan 430074, Hubei, China

b

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, Hubei, China

c

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

*

Corresponding Author

Email address:

[email protected]

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Algal bloom is a naturally occurring phenomenon in freshwaters due to discharge of wastewater containing nitrogen and phosphate. Coagulation-flocculation was widely used in removal of cyanobacteria and algae organic matters (AOM) from water body. Microalgae and AOM differ greatly in physiochemical properties at different growth stages, which are likely to have important effects on their coagulation behavior. In this study, the interacting mechanisms between polymeric aluminum chloride (PACl) and microalgae cells and AOM at the different growth stages were investigated by characterizing morphology and AOM properties of microalgae flocs formed from PACl treatment. The results showed that PACl coagulation exhibited better removal efficiency for the microalgae cells and AOM in exponential and stationary phase than that in decline phase. The protein and humic acid content in loose bound AOM (LB-AOM) were found to be key constituents affecting floc dewatering behavior, and complexation adsorption of hydrolyzed products for proteins and humic substances in LB-AOM was responsible for microalgae floc dewatering improvement. These results provide a novel insight into optimization of flocculation and dewatering of cyanobacteria at different growth stages with PACl treatment.

Key words: microalgae; growth stages; aluminum salts coagulants; algae organic matter (AOM); floc dewaterability


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Introduction The blooms of microalgae is increasing globally in both frequency and distribution. 1-3

The blooms of microalgae generally produce large amounts of algal organic

matters and toxins into water, reducing dissolved oxygen, resulting in unpleasant odor and tastes. It gives a challenge to the local water supply industry and aquatic system.1, 2, 4, 5

On the other hand, biodiesel production and other applications of microalgae is

considered to be of great potential and economic value.6-8 Coagulation/flocculation has been widely used in treatment of water containing microalgae and microalgae production.6, 9 Coagulation-flocculation is a cost-effective method which can quickly resulted in fast algae-water separation and improve algae harvesting efficiency.10,

11

Algal organic matters (AOM) was considered to be an

important factor affecting microalgae coagulation performance.12,

13

AOM helped

maintain the colloidal state of the microalgae system. In coagulation process, the hydrolyzed products of metal salts are able to remove the hydrophilic AOM through complexation adsorption. Since the AOM is generally adverse to microalgae dewatering, high AOM removal efficiency can facilitate the separation of microalgae from water. Both natural and cultured cyanobacteria showed obviously distinct physicochemical properties at different growth stages,14 and AOM produced at different growth stages are also different. Hence, understanding the changes in physicochemical characteristics of AOM at different growth stages is essential for designing appropriate water treatment and microalgae production.



Many works so far have focused on the relationships between AOM and PACl coagulation performances. AOM can form complexes with cations in inorganic coagulants, which deteriorates the coagulation ability of the coagulant and resulted in increase of coagulant demand.15 The protein of AOM interacted with PACl by complexation reactions such that it was unavailable for charge neutralization.12, 13 Moreover, AOM is considered highly hydrophilic due to the presence of proteins, polysaccharides and hydroxy acids.16 Coagulation is a preparatory step prior to other algae harvesting processes such as filtration, flotation or gravity sedimentation in many studies.12, 17-19 few of them have comprehensively investigated the properties of AOM at different growth stages in relation to coagulation-flotation performance. In addition, moisture content is often as high as 99.9% in fresh microalgae effluent,6 which brings high economic costs for microalgae dewatering process.20 Coagulation can help microalgae cell aggregation and decrease the moisture content from more 99.9% to 97%-98% after sedimentation-thickening, consequently reducing its volume to be further disposed significantly.9 Therefore, chemical coagulation of microalgae prior to mechanical separation is often necessary to enhance the operating efficiency of solid-water separation devices. Few studies paid attention to dewatering of algae flocs at different growth stages, which is of great importance to subsequent disposal and recycle of microalgae flocs. The extracellular polymeric substances (EPS) of microorganisms can be regarded as key constituents in dewatering process of microbial aggregates.21 EPS generally has


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double layers including tightly bound EPS (TB-EPS) surrounding the cell surface and the loosely bound EPS (LB-EPS) diffused from TB-EPS. According to previous studies,22, 23 AOM is consisted of three components: soluble AOM, which is dissolved in the water in the form of colloidal and negatively charged system with cells,12, 18 loosely bound AOM (LB-AOM) and tight bound AOM (TB-AOM). However, the changes in AOM distribution and composition in relation to floc dewaterability are still not clear. In this work, the changes in morphological properties, surface charge, AOM composition (protein, polysaccharide, humic acid) and solution chemistry of microalgae at three growth stages were comprehensively investigated under PACl coagulation. And the relationships between dewatering behavior and physicochemical properties of microalgae flocs were also considered. The main findings of this study are expected to provide a guide for coagulation and dewatering of microalgae at different growth stages.

Materials and methods Algae cultivation The microalgae used in this study belongs to Microcystis aeruginosa (micro, spherical cyanobacteria) which is the most common algae causing water bloom. The microalgae, Microcystis aeruginosa, was purchased from the Wuhan Institute of Hydrobiology, Chinese Academy of Sciences. The algae cells were cultured in cell culture flasks with BG-11 medium prepared by mixing the following : NaNO3 (1.5



g/L), Na2CO3 (20 mg/L), K2HPO4 (40 mg/L), MgSO4·7H2O (75 mg/L), CaCl2·2H2O (36 mg/L), Citric acid (6 mg/L), Ferric ammonium citrate (6 mg/L), EDTA (1 mg/L), A5 solution (H3BO3 (2.86 mg/L), MnCl2·4H2O (1.81 mg/L), ZnSO4·7H2O (0.22 mg/L), Na2MoO4·2H2O (0.39 mg/L), CuSO4·5H2O (0.079 mg/L), CoNO3·6H2O (0.049 mg/L)).24 BG-11 medium put into the glass conical flasks under the temperature of 120-125 oC for 30 min. After being cooled down to the room temperature, the medium was inoculated with Microcystis aeruginosa and then placed in the artificial climate box (LRH-250-G, Guangdong Medical Apparatus Co. Ltd., China) for culturing. The culture conditions were as follows: temperature=25 oC, light intensity=2500 lux, light to dark ratio (L:D) = 14 h:10 h, ventilation ratio=0.15 L/min. PACl preparation The aluminum salts coagulants were prepared according to the method described by Xu et al (2010). The reagents used in preparing the coagulants were of analytical grade and deionized water was used in all solutions. PACl with a basicity (B, OH/Al molar ratio) was synthesized by adding pre-determined amount of Na2CO3 slowly into 1 mol/L AlCl3 solution under intense agitation at rotational speed of 300 rpm. Then, deionized water was added to dilute total aluminum to 0.5 mol/L under 2 h agitation. The concentration and basicity of aluminum salt coagulant were 0.5 molAl/L and 1.5 respectively. Experimental procedure Growth curve establishment


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OD680 and dry weight were measured daily to establish the time curve of algae growth so that the growth period of algae could be obtained. Coagulation procedure Algae samples were reacted with PACl by using magnetic stirrer (MY3000-6F, Wuhan Meiyu, Ltd., China). 500 mL algae suspension was put into 6 beakers. The coagulation experiments were performed according to the following procedures: a rapid mixing period for 2 min 30 s at 200 rpm followed by a slow-stir phase at 40 rpm for 10 min. At the end of mixing, floc dewaterability of microalgae suspension was measured. After settling time is 30 min for exponential and stationary phase, 60 min for decline phase, the supernatant and flocs were carefully separated. Microalgae flocs were used for AOM and SEM analysis. Analytical method Floc properties analysis Floc dewatering performance Capillary suction time (CST) test was used to assess floc dewaterability. CST instrument (model 319, Triton, UK) was equipped with an 18-mm diameter funnel and Whatman No.17 chromatography-grade paper. The samples used here were obtained from the precipitates obtained from the coagulation procedure. Zeta potential and particle size Zeta potential and floc size were measured by Zetasizer 2000 (Malvern,UK). SEM



Scanning electron microscopy (SEM, Leica, Steroscan 420) was used to observe microstructure of algae at different growth phases. The flocs were freeze-dried at temperature of -50 oC for 72 h. The samples of algae cell and floc were fixed on the metal plate by conductive adhesive. After scanning for 300 s, SEM images were obtained using a scanning electron microscope at 5 kV. AOM extraction and analysis AOM extraction The AOM extraction was performed according to modified procedure described by Henderson et al (2008). After coagulation and standing, the supernatant and bottom flocs were separated. The supernatant was considered as soluble AOM (SAOM). After that, 50 mL of algae floc was centrifugalized at 2,000 g for 15 min and subsequently supernatant was obtained as LB-AOM. The residual sediments were resuspended in 50 mL of buffer (Na3PO4 (327.88 mg/L), NaH2PO4 (479.92 mg/L), NaCl (525.60 mg/L), KCl (74.56 mg/L)). After being centrifuged at 10,000 g for 15min, the supernatant was separated as TB-AOM. UV-vis Analysis UV-vis spectra were obtained by UV spectrophotometer (UV-8500.Tianmei, China). Soluble AOM samples were scanned at full wavelength from 200 nm to 700 nm. DOC Analysis Dissolved organic carbon (DOC) was measured using Total Organic Carbon (TOC) analyzer (Teledyne Tekmer, USA), and the particulates present in water samples were removed through a 0.45 µm membrane.


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EEM Analysis EEM spectra were measured on a Varian Eclipse fluorescence spectrophotometer (Hitachi F-4500,Japan) in scan mode with an excitation range from 200 nm to 400 nm at 5nm sampling intervals and a emission range from 220 nm to 550nm at 5nm sampling intervals. The spectra were recorded at a scan rate of 12,000 nm/min, using excitation and emission slit bandwidths of 5 nm. Each scan had 68 emission and 42 excitation wavelengths. All the samples were filtered through 0.45 µm members before being measured. Polysaccharides, humic acids and proteins measurement The organic compounds in AOM are mainly polysaccharide, protein and humic acid. The polysaccharide was determined using the phenol-sulphuric acid method.25 Humic acids and proteins were measured using the modified Lowry’s method.26 Other indicators Turbidity measurements were tested by turbidity HACH2100N turbidity meter. pH was measured by pH meter (pHS-3C, Shanghai, China).

Results and discussion Characteristics of M. aeruginosa in three growth stages. Table 1 Basic Properties Of Microalgae Suspensions At Three Growth Stages. Phase

OD680 (Abs)

pH

Turbidity(NTU)

CST (s)

TSS (%)

Exponential

1.086

9.02

466

12.9

0.213

Stationary

2.794

9.90

1125

23.5

0.236

Decline

2.062

8.19

668

35.3

0.293

*TSS

：Total suspended solids



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The basic properties of algae in three growth stages are shown in Table 1. Turbidity and OD680 increased firstly and then decreased in microalgae growth. Photosynthesis in the exponential and stationary phase is strong, bicarbonate was consumed and hydroxyl ions were generated (Eq. (1)),27 resulting in an increase of pH (initial media pH = 7). 6

aq + 6

→

+6

+6

1

Respiration is dominant in decline phase, producing a large amount of CO2 and caused decrease of pH. CST values indicate that algae dewaterability was gradually deteriorated with microalgae growth. The microalgae in later period of exponential and decline phases and early stationary phase were selected in coagulation experiments to keep a constant TSS concentration and OD680. SEM images of algae cells in different growth stages were shown in Figure S1 of supporting information (SI). Algae cells of exponential phase showed smooth sphericity with diameters of 1-2 µm. Compared with the exponential phase, microalgae cells at stationary phase became larger and cell surface turned to be rough, shown as near spherical shape with diameter of 3-4 µm. In decline phase, the cells become larger, growing into a long narrow spindle with diameters of 2-3 µm and lengths of 6-7 µm, and the cell surface became rougher. Microalgae with intact cell structure were reduced and intracellular matters were released at the same time.


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Fig. 1 Concentration of SAOM, LB-AOM and TB-AOM at different growth stages Fig. 1 showed the changes in distribution and composition of SAOM, LB-AOM, and TB-AOM at different growth stages. Fig. 1 (a) showed that the composition of SAOM was similar in all growth stages, in which protein was the major component (51% - 53%), which was similar to EPS composition of activated sludge.28 The SAOM of microalgae at stationary phase reached the maximum, since a large amount of metabolites were released.29 Fig. 1 (b) showed that the proportion of polysaccharide increased from 22% - 27% in SAOM to 37% - 61% in LB-AOM in all growth stages, vindicating that more polysaccharide were produced on the cell surface, which was consistent with previous studies.30 The polysaccharide content decreased from 61% at exponential phase to 37% at decline phase, indicating a decrease in cell activity. In contrast, the proportion of humic acid increased from 5% at exponential



phase to 23% at decline phase. The concentration of LB-AOM reached the maximum at decline stage, which indicated that a large amount of organic substance in cells was released and attached onto cell surface. Fig. 1 (c) showed that the content of TB-AOM in exponential and decline phase was much larger than the stationary phase. This was because exponential phase cells were more active and had more plentiful functional organic groups on surface. The cell membrane became weaker in the decline phase. As a result of autolysis, cell surface materials and certain intracellular organic matter were released.16 Fig.1 (d) presented the total concentration of AOM at different stages. The AOM concentration at stationary phase was slightly higher than that at exponential phase, and the AOM concentration at decline phase was the highest. SAOM and LB-AOM accounted for 90% - 96% of AOM. LB-AOM showed the highest proportion (38%) in the decline phase and reached the minimum at 25% in exponential phase. Therefore, it was clear that there are significant differences in AOM at different growth stages, which might have important effects on coagulation process.


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Fig. 2 Excitation-emission matrix (EEM) spectra of AOMs in different growth stages. All samples were diluted by 100 times. Excitation-emission matrix (EEM) is an advanced analytical tool with high sensitivity and selectivity.31 It has been widely used in characterization of natural organic matters (NOMs) containing fluorescent groups, such as proteins and humic substances. Fig. 2 showed EEM profile of AOMs in different growth stages. It was obvious that two fluorescent peaks were detected in all examples: peak A (λex/em=280/335) – tryptophan-like protein, peak B (λex/em=230/330) – aromatic-like protein. SAOM and LB-AOM fraction had similar chemical components which were mainly composed by tryptophan-like and aromatic-like substances. And TB-AOM was mainly composed of aromatic-like substances. In addition, fluorescent intensity of humic substances (peak: λex/em=330/410,



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λex/em=275/425) was greatly enhanced at decline phase in Fig. 2 c1, c2, indicating that significant humification presented at decline phase. Table 2 EEM Profiles Of AOMs At Different Growth Stages. Phase

SAOM

LB-AOM

TB-AOM

TP

AP

HA

FA

TP

AP

HA

FA

TP

AP

HA

FA

λex/em

280/335

225/340

330/410

275/425

280/335

225/340

330/410

275/425

280/335

225/340

330/410

275/425

Exponential

69.11

110.2

2.39

5.32

15.17

20.66

1.76

3.94

1.81

4.758

0

0.03

Stationary

220.7

364.9

4.03

11.93

32.58

53.9

2.37

3.53

3.48

4.578

0.75

0.79

Decline

331

766.2

16.77

32.24

176.6

392.6

18.42

30.42

7.53

18.66

0.24

0.41

：

，：

，

：

* TP Tryptophan protein AP Aromatic Protein HA: Humic Acid, FA Fulvic acid. All samples were diluted by 100 times.

Table 2 listed the intensities of peak A (λex/em=280/335) - tryptophan-like protein, peak B (λex/em=230/330) - aromatic-like protein, peak C (λex/em=275/455) - fulvic acid, and peak D (λex/em=230/330) - humic acid in different growth stages. Minor differences in AOM at different growth stages were shown on the table. In general, intensities of all four fluorescent substances were increased with time. In SAOM, fluorescence intensities of humic and fulvic acid in exponential phase were almost undetectable and then increased significantly at decline phase. In LB-AOM, fluorescent intensities of all substances were significantly lower than that of SAOM in the same phase, while the intensities of fulvic and humic acid were the same as that of SAOM in decline phase, and the fluorescent intensities of proteins were also much higher than that at the other phases. This observation revealed that a large amount of LB-AOM composed of the same high concentration of humic/fulvic acid as SAOM, was attached onto cell surface at decline phase. In decline phase, fulvic and humic acid were much higher than other phases and reached 16.8 and 32.2, while they were


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2.4 and 5.3 in exponential phase and 4.0 and 11.0 in stationary phase. It was noted that only the proteins were presented in TB-AOM fraction in all growth stages. Effects of PACl coagulation on microalgae removal efficiency in different growth stages

Fig. 3 Effects of PACl dosage on turbidity (a) and DOC removal efficiency (b) at different growth stages Fig.3 (a) presented the effects of PACl dosage on turbidity removal of microalgae at different growth stages. The removal efficiency of turbidity reached the maximum at 99.3%, 99.7% and 68.1% at the PACl dosage of 50 mg/L, 75mg/L and 75 mg/L for microalgae at exponential, stationary and decline phase. Although initial turbidity at decline phase was lower than that at stationary phase, it was more difficult to be



removed through coagulation. In addition, it was observed that the flocs formed by microalgae at decline phase showed poor settleability. The coagulant dosage required for microalgae removal is 23.6 mg/g, 31.9 mg/g and 37.6 mg/g in exponential, stationary and decline phase, which is likely to be related to the SAOM content at different phases. Fig. 3 (b) showed the effects of different dosages of PACl on the DOC removal of microalgae at different growth stages. In PACl coagulation treatment, change in DOC was similar with that of turbidity. The removal efficiency of DOC reached the maximum at 65.6%, 78.0% and 58.0% at the PACl dosage of 50mg/L, 150 mg/L and 125 mg/L for microalgae at exponential, stationary and decline phase, 184.8mg/L, 323.0 mg/L and 433 mg/L DOC were removed respectively. SAOM containing OH and COOH groups can form complexes with coagulant, resulting in an increase in dosage of coagulant required and decrease in coagulation efficiency.32 Bernhardt (1989) suggested that SAOM could be removed through complexation adsorption of hydrolyzed products of metal salts, which was always enhanced by increasing PACl dosage. But low molecular weight SAOM could form complexes that were soluble or in colloidal forms which could pass through the filter and were difficult to be removed through coagulation-precipitation.33 The concentration of these complexes which is hard to be removed was reflected in Fig.3 (b) as the residual SAOM. For SAOM in decline phase, compared to stationary phase, the proportion of large molecular weight organics was decreased, while the low molecular weight organics increased significantly. Such changes in hydrophilic SAOM are particularly noticeable.34 Therefore, coagulation removal efficiency of


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SAOM in the decline phase was greatly reduced due to formation of soluble and colloidal state of the complexes. Interaction mechanism between microalgae at different growth stages and PACl

Fig.4 Changes in pH (a), zeta potential (b) and microalgae floc particle size (c) with different PACl dosages at different growth stages.



Generally, microalgae cells are negatively charged due to ionization of anionic functional groups, which can prevent them from aggregation in suspension.35 The surface charge can be reduced or neutralized by adding cationic flocculants. As shown in Fig. 4, zeta potential values of algal suspension were -29.4 mV, -50.3 mV and -40.6 mV at exponential, stationary and decline phase respectively. The slope of zeta potential and coagulant dosage reflected the neutralization ability of PACl. In exponential phase, zeta potential of the algae increased and reached zero when PACl dosage raised to 50 mgAl/L. Excessive PACl addition caused charge reversal and restabilization of algae system, which was consistent with turbidity and DOC analysis. As for the microalgae at stationary phase, the zeta potential reached zero point at optimal PACl dosage of 150 mgAl/L, which was much higher than that of microalgae at exponential phase. This observation demonstrated that charge neutralization was dominant in removal of microalgae at exponential and stationary phases. Algae suspension pH levels at exponential and stationary phases were decreased with increasing PACl dosage due to hydrolysis of aluminum salts. For the microalgae at decline phase, there were no evident changes in zeta potential and pH of microalgae under coagulation, indicating the main mechanism was not charge neutralization. In addition, microalgae suspension at decline phase exhibited higher pH buffering ability due to production of more bicarbonate ions. The coagulants added into the high pH of cyanobacteria system mainly generated hydroxide precipitates for sweep coagulation and complexation adsorption rather than charge neutralization.36 Previous studies have demonstrated that flocs formed by


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charge neutralization coagulation mechanism were more compact and showed better settleability, while that formed by sweep coagulation mechanism were generally loose and difficult to settle.37 Fig. 4 (c) presented the changes of average floc particles size with varied PACl dosage. The initial average particle size of flocs at exponential and stationary phase was similar, while that at decline phase was much smaller (0.45µm). This was likely that microalgae at decline phase processed autolysis and cellular substances were released, which were confirmed by SEM and DOC analysis. The microalgae floc reached the maximum particle size at 27.12 µm, 56.10 µm and 10.33 µm when PACl dosage were 50 mgAl/L, 125 mgAl/L and 125 mgAl/L at exponential, stationary and decline phase. Particles flocculation was influenced by solid concentration and organic matters. Since higher solid content was always conductive to floc growth, so microalgae floc size at stationary phase was larger than that at exponential stage.37 However, it was evident that PACl showed limited flocculation ability for microalgae at decline phase. This phenomenon was likely related to AOM composition, which was investigated in the following section. It was interesting to note that both floc size and zeta potential of microalgae at stationary and exponential phases reached the maximum at the same PACl dosages. However, this phenomenon was not observed for microalgae at decline phase, confirming that distinct mechanisms were dominant in microalgae flocculation with PACl at different growth stages.



Floc dewatering behavior

Fig. 5 Effect of PACl dosage on floc dewatering performance at different growth stages. Capillary suction time (CST) test is a commonly used method to assess floc dewaterability with high moisture content. Fig. 5 showed CST changes of microalgae flocs coagulated with PACl at different growth stages. CSTn of microalgae were reduced from 6.03 s·L/g, 9.96s·L/g and 12.04 s·L/g to 2.23 s·L/g, 2.16 s·L/g and 8.95 s·L/g by raising PACl dosage, respectively. PACl coagulation significantly improved dewaterability of microalgae flocs at exponential and stationary phases, while it had limited effects on that at decline phase. Since AOM fraction played an important role in aggregates dewatering,12,

35

high AOM concentration could cause blockage of

filtration medium and increase resistance to filtration. PACl was not effective in removing AOM of microalgae at decline stage, the floc dewatering behavior was poorer than that at exponential and stationary phases. In addition, as mentioned above, charge neutralization generally produced more compact floc structure, while the flocs formed by sweeping were looser. The more compact floc structure was associated


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with lower microalgae cake compressibility and consequently better dewatering performance.38 Floc morphology

Fig. 6 SEM images of flocs under PACl coagulation at different growth stages. The magnification was 2.5k in the exponential phase while the other phases were 5k.

Fig. 6 showed SEM images of flocs coagulated with PACl at 25 mgAl/L, 75mgAl/L and 150 mgAl/L dosage at three growth stages. It was obvious that hydrolyzed products of PACl covered onto the surface of microalgae cells and caused aggregation of negatively charged algal surface.39 At exponential phase, excellent cell aggregation efficiency was achieved at PACl dosage of 25 mgAl/L. In stationary phase, the microalgae floc structure was enhanced by raising PACl dosage. There was no significant change in microalgae floc at 25 mgAl/L PACl dosage at decline phase.



When PACl dosage was increased to 75 mgAl/L, flocs were presented as smaller fragments which aggregated further with increasing PACl dosage. It was evident that floc structure was more regular and compact in exponential and stationary phase than that in decline stage. AOM changes with PACl coagulation

Fig. 7 Effects of PACl dosage on concentration and composition of SAOM at different growth stages The contents of protein, humic acid and polysaccharide in SAOM with different PACl dosages in three growth stages were given in Fig.7. In exponential and stationary phases, PACl coagulation showed a good removal efficiency for proteins and which were always negatively charged and could form complexes with cations.15 The PAC treatment was less effective in polysaccharide removal, which may be


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caused by higher complexity of polysaccharide composition with high biological activity. In addition, some polysaccharides is not charged such as those formed by glucose,30 thus it can’t be removed by charge neutralization. In decline phase, there were more low molecular weight AOM as a result of biodegradation. Therefore, coagulation treatability of proteins, humic substances and polysaccharide was greatly reduced in comparison to that at exponential and stationary phases.

Fig. 8 Effects of PACl dosage on concentration of LB-AOM and TB-AOM at different growth stages. As shown in Fig. 8 (a), at the exponential phase, concentrations of humic acid and protein reached the minimum at PACl dosage of 75 mgAl/L then increased with further addition of PACl. Figure 8 showed the changes in content of LB-AOM and



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TB-AOM in flocs. TB-AOM concentration increased significantly under coagulation, while that of LB-AOM and SAOM were reduced at the same time, indicating the AOM transformations occurred. It was likely that a portion of SAOM was converted into LB-EPS at low PACl dosage, and it was transferred into TB-AOM fraction as PACl dosage increased.22, 23 The minimum LB-AOM content may indicate the most cost-effective coagulation process. In Table 3, the contents of protein and humic acid in LB-AOM showed a high correlation with CST in stationary and decline phase, which indicated that the composition of LB-AOM fraction was a key constituent affecting floc dewatering performance. As can be seen in Figure 8 (b), TB-AOM concentration was increased initially and reached the maximum at 303.7 and 187.7 mgDOC/L respectively by raising PACl dosage to 125 mgAl/L at exponential and decline stages. In conjunction with results of LB-AOM increased at high PACl dosages, it was demonstrated that a portion of TB-AOM was converted into LB-AOM at high PACl dosages. In general, SAOM and BL-SAOM tended to be TB-SAOM in coagulation, meaning the increase of microalgae coagulation efficiency. Table 3 Correlation Analysis of CST with LB-AOM CST Stationary phase

LB-AOM Protein r = 0.935 p < 0.01

LB-AOM Humic Acid r = 0.936 p < 0.01

Decline phase

r = 0.827

r = 0.862

p < 0.05

*r: Pearson correlation coefficient, p: Significant coefficient


p < 0.05

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EEM analysis

Fig. 9 Influence of PACl coagulation on EEM profiles of AOM at different growth stages. All samples were diluted by 100 times.

Fig. 9 presented the changes of four fluorescent peaks including tryptophan-like protein, aromatic-like protein, fulvic acid and humic acid in all growth stages. It can be seen that all peaks in SAOM and LB-AOM fractions decreased, while that in TB-AOM were enhanced, confirming that SAOM and LB-AOM fractions were converted into TB-AOM under coagulation. At optimal dosage of PACs at exponential and stationary phase, tryptophan-like and aromatic-like proteins in SAOM showed good coagulation treatability, the fluorescent peaks of tryptophan-like protein decreased by 98% and aromatic-like protein was almost undetectable. Fulvic acid and humic acid decreased by 34% and 64% in



exponential phase and 58% and 82% in stationary phase respectively. In decline phase, tryptophan-like protein, aromatic-like protein, fulvic acid and humic acid were decreased by 83%, 84%, 4% and 21%. It is obvious that fulvic and humic acid was more difficult to be removed in decline phase. Therefore higher LB-AOM concentration was related to higher optimal coagulant dosage and poorer floc dewaterability under PAC coagulation. In addition, PACl coagulation promoted conversion of biopolymers in SAOM and LB-AOM fractions into TB-AOM, consequently producing more compact structure and better dewatering property of microalgae flocs. As shown in Fig. 9, it was noted that soluble AOM of microalgae at decline stage was rather difficult to be removed.34 It inspires us that the microalgae are better to be flocculated and harvested at stationary stages (eg. algae bloom and algae cultivation with wastewater), since coagulation showed good performance in removing soluble AOM and improving microalgae floc filtration dewaterability at this time.

Conclusion In this study, the effects of PACI of different dosages on dewatering performance of microalgae flocs in different growth stages were evaluated, the interaction mechanisms between aluminum salt coagulants and microalgae at different growth stages were unraveled by characterizing the changes in morphology and AOM properties.


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1. There were significant differences in morphological properties of algae and chemical composition of AOM at three microalgae growth stages. Charge neutralization was responsible for flocculation of microalgae at the exponential and stationary phases, while sweeping and complexation adsorption were dominant in microalgae removal at decline stage under PACl treatment. 2. The protein and humic acid content of LB-AOM is a key constituent that determines the dewatering performance of flocs. The minimum LB-AOM content also indicates the most cost-effective coagulation process. PACl coagulation significantly improved dewaterability of microalgae flocs at exponential and stationary phases, while it had limited effects on that at decline phase. 3. AOMs coagulation treatability is affected by algae growth stage. PACl treatment showed better performance in removal of AOM at exponential and stationary phases than that at decline phase. 4. PACl coagulation promoted conversion of biopolymers in SAOM and LB-AOM fractions into TB-AOM, consequently producing more compact structure and better dewatering property of microalgae flocs.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.



SEM images of microalgae cells at different growth stages, Effects of PACl dosage on concentration and composition of LB-AOM and TB-AOM at different growth stages.


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Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (41630318 and 51678546), National Students Innovation Training Program of China (201710491155) and Chinese Universities Scientific Fund for Cradle plan of China University of Geosciences (Wuhan).

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TOC Graphic:

Synopsis The flocculation-dewatering behavior of microalgae at different growth stages was investigated, and the relationships between AOM and floc dewaterability were established.


FlocculationâDewatering Behavior of Microalgae at Different Growth

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FlocculationâDewatering Behavior of Microalgae at Different Growth