Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption

Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption

Journal Pre-proof Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption Tao Wang, Xiaomin Tang, Shixin Zhang...

3MB Sizes 0 Downloads 7 Views

Journal Pre-proof Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption Tao Wang, Xiaomin Tang, Shixin Zhang, Jie Zheng, Huaili Zheng, Ling Fang

PII:

S0304-3894(19)31460-8

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121506

Reference:

HAZMAT 121506

To appear in:

Journal of Hazardous Materials

Received Date:

24 July 2019

Revised Date:

23 September 2019

Accepted Date:

18 October 2019

Please cite this article as: Wang T, Tang X, Zhang S, Zheng J, Zheng H, Fang L, Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121506

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Roles of functional microbial flocculant in dyeing wastewater treatment: Bridging and adsorption Tao Wang1, Xiaomin Tang1, 2*, Shixin Zhang2, Jie Zheng1, Huaili Zheng2, Ling Fang1 1

Chongqing Key Laboratory of Catalysis & Functional Organic Molecules, College

of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, P.R. China Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State

ro of

2

Ministry of Education, Chongqing University, Chongqing 400045, P.R. China

-p

*Corresponding author: College of Environment and Resources, Chongqing

re

Technology and Business University, Chongqing 400067, P.R. China. Tel &Fax: +86

Jo

ur

na

Graphical abstract

lP

02388326467, Email: [email protected]

1

Highlights



► Grafting copolymerization occurred at the amino group of MBF was confirmed.



► PSA and MBF-g-P(AM-DAC) played different roles in treatment of dyeing wastewater. ► MBF-g-P(AM-DAC) improved settlement efficiency for bridging and adsorption.



► Flocs possessed surface area of 24.8201 m2/g and adsorption capacity of 140 mg/g.

ro of



Abstract:

-p

Congo red (CR) is a typical and widely used azo dye in industries. It possesses the

serious threat to ecosystem and public for its indiscriminate discharge. Microbial

re

flocculant (MBF) with various functional groups is a potential flocculant applied in

lP

dyeing wastewater treatment, and it has the advantages of high treatment efficiency, biodegradability and non-toxicity. In this study, the functional groups, amino group,

na

ammonium group and acyloxy group, were grafted onto MBF to further improve its thermal stability, solubility and performance. Grafting copolymerization occurred at

ur

the amino group of MBF was confirmed by XPS. Polyaluminum silicate (PSA) and

Jo

self-prepared functional microbial flocculant, MBF-g-P(AM-DAC), played different roles in CR wastewater treatment. PSA contributed to charge neutralization, but its yielded flocs were small. On the contrary, MBF-g-P(AM-DAC) possessed weak charge neutralization but big flocs. Its settlement efficiency has significantly improved. The unsaturated active sites on MBF-g-P(AM-DAC) and its flocs contributed to the adsorption of CR in terms of high surface area and adsorption 2

capacity of the flocs. Physical adsorption and chemical adsorption were both discovered in the treatment.

Keywords: Azo dye; functional microbial flocculant; polysilicate aluminium; floc

Abbreviation: Polyaluminum silicate (PSA) Congo red (CR)

-p

Acryloyloxyethyl trimethylammonium chloride (DAC)

lP

Microbial flocculant (MBF)

re

Acrylamide (AM)

Removal efficiency (RE)

ro of

characteristic; removal mechanism

na

Scanning electron microscopy (SEM)

Brunauer-Deming-Deming-Teller (BDDT)

ur

Binding energies (BE)

X-ray photoelectron spectroscopy (XPS)

Jo

Fourier transform infrared spectroscopy (FTIR) Thermogravimetric analysis (TGA) Differential thermal gravity (DTG) The median equivalent volumetric diameter (d50) Initial concentration (C0) 3

Adsorption capacity (qe) The theoretical maximum adsorption capacity (qm)

1. Introduction Dyeing wastewater deriving from the textile dyeing industry possesses the serious threat to public and ecosystem due to nonbiodegradable synthetic dyes, heavy metals

ro of

and other chemicals in wastewater [1, 2]. Azo dyes, as one kind of the most harmful dyes, account for more than half of the total usage of dyes in the printing and dyeing

industry [3]. Congo red (CR) as a typical azo dye is used in textile, paper, plastic and

-p

cosmetics industries due to its excellent surface adhesion and low-cost, and it is a

re

well-known toxic and carcinogenic pollutant [4]. It also influences the photosynthetic activity of aquatic plants and threatens aquatic animals in ecosystem [5]. It has high

lP

optical, thermal and physiochemical stability due to presence of aromatic structure[6].

na

In azo dyeing wastewater treatment, it mainly focuses on the decolorization of wastewater. And the chemical and biological methods, including chemical oxidation,

ur

photocatalytic degradation and biodegradation, have been investigated to degrade dyes. Chemical oxidation and Photocatalytic degradation are simple and efficient [7,

Jo

8]. However, their cost is high and some degradation products are still toxic [9]. Their efficiencies are influenced by other inorganic and organic matters as well [10]. Although the cost of biodegradation is low, some tough dyeing wastewater is hardly treated by biological method, and its treatment efficiency is low for its long treatment period [11]. Coagulation-flocculation, an indispensable preliminary treatment that 4

basically separates colloidal particles from water, is to mainly improve the degradation efficiency of chemical and biological methods in dyeing wastewater plant [12]. However, dyes are hardly removed by coagulation-flocculation from wastewater in most cases since dyes are almost dissolved in wastewater [13]. It is a challenging task to increase the removal rate of dissolved dyes via preparing an efficient flocculant since flocculant is the most essential factor in coagulation-flocculation

ro of

treatment [14].

Although conventional flocculants, aluminum salt flocculants and ferric salt

flocculants, not well perform in dyeing wastewater treatment, the treatment efficiency

-p

is able to be improved through modifying flocculants. Functional groups of acyloxy

re

grafted onto dextran [15] and carboxyl groups grafted onto chitosan are reported in our previous studies [16]. Microbial flocculant (MBF) produced by microorganism is

lP

mostly polysaccharide-like substances. It owning a large number of various functional

na

groups is applied in the treatment of refractory pollutants, such as heavy metals [17]. MBF has the advantages of high removal rate of pollutants, biodegradability and

ur

safety [18]. But it is rarely used in dyeing wastewater treatment. In this study, amino group, ammonium group and acyloxy group were introduced onto

Jo

molecular chain of MBF via grafting copolymerization of MBF, acryloyloxyethyl trimethylammonium chloride (DAC) and acrylamide (AM). The optimal synthesis conditions and characteristics of flocculants were investigated to achieve the best performance in CR wastewater treatment. Grafting copolymerization mechanism was discussed via characterization analysis that also represented the relationship between 5

the structure of MBF-g-P(AM-DAC) and its performance in the treatment. The role of MBF-g-P(AM-DAC) in CR wastewater treatment was emphasized through assessing its capacities of bridging and adsorption. The removal mechanisms of CR were described according to the results.

ro of

2. Materials and methods 2.1 Materials

Microbial flocculant (Qingdao Tung Biological Engineering Co. Ltd., China) mainly

-p

containing polysaccharide was harvested from the fermentation liquor of Klebsiella

re

peneumoniae. Polysilicate aluminum (PSA) was prepared via previous method [19]. Monomer, initiator and other chemicals used in this experiment were all technical

lP

grade (Text S1 in Supplementary Information). All aqueous and standard solutions

na

were prepared with deionized water.

ur

2.2 Synthesis of MBF-g-P(AM-DAC) MBF-g-P(AM-DAC) was prepared via grafting monomers of AM and DAC onto the

Jo

molecular chain of MBF. 12% AM and 6% DAC were prepared in deionized water and the solution was stirred until monomers were dissolved. 3% MBF was added into the reaction system with intensely stirring until emulsion was produced. Graft copolymerization was carried out step-by-step: adjusting pH to 3, adding 0.025 mol/L potassium persulfate and 0.025 mol/L sodium bisulfate as redox initiator, 6

deoxygenating by bubbling with pure N2 for 30 min and heating in water bath at temperature of 60 ℃. MBF-g-P(AM-DAC) was purified in acetone and ethyl alcohol after heating for 3 h and aging for 12 h. The solid MBF-g-P(AM-DAC) was, finally, obtained by drying in a vacuum freeze drier and stored in a dry condition.

ro of

2.3 Characterization of MBF-g-P(AM-DAC)

Intrinsic viscosity, molecular weight and cation degree of MBF-g-P(AM-DAC) were measured using the previous methods [20, 21]. The functional groups and

-p

characteristics of MBF-g-P(AM-DAC) were analyzed by Fourier transform infrared

re

spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and differential thermal gravity (DTG). Their analyses provided clues

lP

for the study of the synthesis mechanism of MBF-g-P (AM-DAC). FTIR and XPS

na

spectra of MBF-g-P(AM-DAC) were recorded by a Nicolet IS10 infrared spectrometer (Thermo Fisher Scientific, USA) covering the wave numbers from 4000

ur

to 400 cm-1 and an ESCALAB 250 Xi XPS spectrometer (Thermo Fisher Scientific, USA) starting with a survey scan from 0 to 1200 eV with steps of 1 eV. TGA and

Jo

DTG curves of MBF-g-P(AM-DAC) were measured by Pyris Diamond 1 thermogravimetric analyzer (PerkinElmer, USA) heating from 30 ℃ to 800 ℃ at a 20 K/min ramp rate in a nitrogen protective atmosphere. SEM analysis was performed on a MIRA 3 LMU SEM system (TES-CAN Company, Czech Republic) after spraying gold on the samples. 7

2.4 Jar tests CR as a typical azo dye was dissolved in deionized water to prepare simulated wastewater with an initial concentration of 40 mg/L. The simulated dyeing wastewater was freshly prepared before treating. All stirring processes in tests were conducted by a MY3000-6B six-place programmed paddle mixer (Wuhan Meiyu

ro of

Instrument Co., Ltd., China). A series of dosages of PSA was added into CR wastewater, and the mixture solution was vigorously stirred at the speed of 300 rpm

for 2 min, followed by slow mixing at the speed of 40 rpm for 20 s. And then, A series

-p

of dosages of MBF-g-P(AM-DAC) was added into the wastewater. A new stirring was

re

carried out including the rapid stirring at 300 rpm for 2 min and the slow stirring at 40 rpm for 10 min. The treated wastewater was allowed to settle for 5 min. Water

lP

samples were taken at a depth of 2 cm below the surface of supernatant to determine

na

the concentrations of CR in treated water using an UV-1900 spectrophotometer (Shanghai aoyi Instrument Co., Ltd., China) at the wavelength of 500 nm. All tests

ur

were run in triplicate, and removal efficiency (RE) of CR from wastewater was calculated according to the following equation: (1)

Jo

RE (%)=(C0-C1)/C0×100%

where C0 and C1 are the initial and the final concentration of CR in water. Zeta potential of treated wastewater was measured by a ZS90 Malvern potential analyzer (Malvern Instruments Ltd., UK). Floc size distribution and floc characteristic was also investigated using a Mastersizer2000 laser diffraction instrument (Malvern 8

Instruments Ltd., UK) and scanning electron microscope (SEM). These results contributed to the discussion of flocculation mechanisms of bridging and adsorption of MBF-g-P(AM-DAC) in the treatment. The flocs formed by PSA and MBF-g-P(AM-DAC) in deionized water were collected and dried in a vacuum freeze drier.

Their

surface

areas

and

pore

volume

were

estimated

via

Brunauer-Emmett-Teller method (BET, Micromeritics ASAP 2020). Adsorption

ro of

thermodynamics was carried out with different initial concentration of CR under the

optimal flocculants dosages and reaction time. The parameters were fitted using the typical Langmuir, Freundlich, Dubinin-Radushkevich adsorption isotherm models

re

-p

[22].

3. Results and discussion

lP

3.1 Synthesis and characteristics of MBF-g-P(AM-DAC)

na

Synthesis conditions of MBF-g-P(AM-DAC) in terms of initiator dosage, mass ratio of MBF and AM, DAC content, reaction temperature, reaction time and pH are

ur

optimized in order to improve its molecular weight and graft copolymerization rate that are positively correlated with its performance in wastewater treatment. Initiator

Jo

dosage, pH and mass ratio of MBF and AM, play the essential roles in the synthesis. And the significant decreases of molecular weight and graft copolymerization rate are found when their values are out of optimum range (Text S2-S7, Fig. S1-S6). Thermal stability of MBF-g-P(AM-DAC) is assessed through DTA-TGA analysis (Fig. S7(a)). Three main stages of thermal decomposition are observed corresponding 9

to the weight loss of MBF-g-P(AM-DAC) in TGA curve. At the first stage (80-150 ℃), the weight loss rate of MBF-g-P(AM-DAC) is about 5% that results from the evaporation of adsorbed and bound water. At the second stage (200-300 ℃), MBF-g-P(AM-DAC) undergoes chemical decomposition and the weight loss rate of it is about 30% since the side chain of the AM monomer, known as -CO-NH2-, reacts with the imide of the amide group. And the methyl group on quaternary ammonium

ro of

groups is detached with the decomposition of hydrogen chloride [23]. The third stage is found when temperature rises above 300 ℃ . The main chain of MBF-g-P(AM-DAC) is broken, which leads to the weight loss rate is highest to 50%.

-p

The weight loss corresponds to the endothermic peak in DSC curves. Three

re

endothermic peaks, 80 ℃, 280 ℃ and 330 ℃, are discovered in three stages in MBF-g-P(AM-DAC). Three endothermic peaks in the DSC curves of MBF occur at

lP

77.4 ℃, 94 ℃ and 307 ℃, respectively (Fig. S7(b)). It is confirmed that the

na

thermal stability of MBF is markedly improved after graft copolymerization.

ur

3.2 Grafting copolymerization mechanism of MBF-g-P(AM-DAC) FTIR spectra of MBF-g-P(AM-DAC) prove DAC grafting onto MBF with the

Jo

assistance of AM and present functional groups in MBF-g-P(AM-DAC). They contribute to the analysis of synthesis mechanisms and flocculation mechanisms as well. The broad and strong absorption peak appearing around 3447 cm-1 is the characteristic absorption band of amide groups (-CONH2) in chain unit of AM (Fig. 1(b) and Fig. 1(d)). The absorption peak at 1668 cm-1 results from the stretching 10

vibration of the -C=O- in the AM unit [24]. The peak at 1451 cm−1 is attributed to -CH2-N+(CH3)3 methylene groups in DAC unit (Fig. 1(c) and Fig. 1(d)) [25]. The vibration absorption peak of quaternary ammonium appears at 952 cm−1 (Fig. 1(d)) [26]. The peak at 1170 cm-1 is assigned to the asymmetric stretching vibration absorption peak of C-O-C groups in -COOCH2-. The absorption peak of primary alcohol (C-OH) in MBF is 1124 cm−1. The double bond absorption peak disappears at

ro of

1640 cm−1, indicating that the double bond is broken and forms free radical. The free radical polymerizes with other free radical to generate the polymer/copolymer. There

is a strong C-H absorption peak at 2960 cm−1 and 1451 cm−1 indicating the presence

-p

of a longer carbon chain in MBF-g-P(AM-DAC). The characteristic peaks of amino

re

group, ammonium group and acyloxy group are observed in infrared spectra of

lP

MBF-g-P(AM-DAC).

na

Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC)

ur

XPS spectra represent the main elementary composition and functional groups of polymer. They are the clues to speculate preparation mechanisms. C, N, O and Cl

Jo

elements mainly exist in MBF-g-P (AM-DAC) (Fig. 2(a)). The O 1s spectrum of MBF is curve-fitted by two peaks at about 531.3 and 532.7 eV, associated with the C=O, and O-H species, respectively Fig. S8(b) [27]. A new peak at 530.8 eV corresponding to C-N is detected for grafting AM and DAC onto MBF (Fig. 2(b)). And the peak areas of C=O and C-N are significantly increased as the same reason. 11

The C 1s spectrum of MBF-g-P (AM-DAC) is dissected into four peaks with binding energies (BE) of about 284.46, 285.95, 287.51 and 288.55 eV, corresponding to the C-H/C-C, C-O, O-C-O and O-C=O species, respectively (Fig. 2(c)). The peaks shift to lower BE, compared with that in the C 1s spectra of MBF, which is attributed to grafting copolymerization (Fig. S8(c)) [28]. The N 1s spectrum of MBF are dissected into two peaks with binding energy (BE) at 399.9 and 401.7 eV, corresponding to

ro of

-NH2 and C-N species, respectively (Fig. S8(d)) [29, 30]. A new peak occurs at 399.2

eV corresponding to –NH, and the peak area of -NH2 markedly reduces (Fig. 2(d)). It

-p

implies that grafting copolymerization is occurred at -NH2 of MBF (Fig. 3).

re

Fig. 2 (a) the fully scanned spectrum of XPS, (b) O1s spectrum, (c) C1s spectrum and

lP

(d) N1s spectrum of MBF-g-P (AM-DAC)

na

Grafting cationic monomer onto the molecular chain of MBF is able to improve water solubility and flocculation efficiency of MBF. However, cationic monomer, such as

ur

DAC, often hardly copolymerizes with MBF directly using redox-based initiator since initial free radicals formed by DAC do not possess enough energy to induce MBF,

Jo

macromolecular organic matter, to generate new free radicals. It impedes copolymerization among cationic monomers and MBF. This evidence has also been found in the cationic modification of chitosan [16, 31]. AM, as the high-activity monomer that is more effectively attack MBF, plays the bridge role in the graft copolymerization of DAC and MBF. Although synthesis mechanisms including chain 12

initiation, chain growth and chain termination are similar with the copolymerization of multi-monomer, free radicals in MBF are formed by the dehydrogenation of amido, which is confirmed by the results of XPS (Fig. 2). And chain growth is occurred on the side chain of MBF (Fig. 3).

ro of

Fig. 3 Grafting polymerization scheme of MBF-g-P(AM-DAC)

3.3 Roles of MBF-g-P(AM-DAC) in dyeing wastewater treatment 3.3.1 Bridging of MBF-g-P(AM-DAC)

-p

Bridging of MBF-g-P(AM-DAC) plays an essential role in CR wastewater treatment.

re

It improves removal efficiency of pollutants in terms of the high remove rate, rapid settlement rate and short settlement time (residence time for actual wastewater processes)

due

the

big

and

compacted

flocs

formed

by

na

MBF-g-P(AM-DAC).

to

lP

treatment

In CR wastewater treatment, PSA is firstly added into the wastewater. It undergoes

ur

coordination hydrolysis to form a positively charged polyhydroxy cation complex and plays the role of charge neutralization that results in the destabilization, encounter and

Jo

gather of CR. Although the removal rate of only PSA is acceptable, its removal efficiency is low. Flocs formed by it are small and slowly goes down the bottom of treater (Fig. 4(a) and Fig. 4(b)). Its removal efficiency is enhanced through combining MBF-g-P(AM-DAC) in the treatment. The removal rate is increased and the settlement time is shortened about a half (Fig. 4(b)). PSA is combined with PAM and 13

P(AM-DAC) as the comparison. Their performance is worse since PAM is nonionic organic flocculant and P(AM-DAC) is cationic organic flocculant (Fig.4(a)). MBF-g-P(AM-DAC) is an amphoteric organic flocculant that integrates anionic groups of MBF and cationic groups in DAC. These functional groups are able to react with PSA and CR for charge neutralization and charge attract. It conduces to solid-liquid separation and performance in the treatment. Besides, most of flocs

ro of

formed by CR and flocculants suspend in the treated water at the initial phase of

settlement, which increases the partial concentration for the large and compacted flocs.

-p

Thus, the removal efficiency is negative at the beginning of settlement (Fig. 4(b)).

re

Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with

lP

different settlement time

na

In flocculation process, the characteristics of flocs have great influences on their settlement performance [32]. Large and compacted flocs are quickly separated from

ur

wastewater, which is confirmed by aforementioned results and previous study [33]. Flocs formed by PSA+MBF-g-P(AM-DAC) show a compact and highly aggregated

Jo

structure. However, flocs yielded by PSA are loose and small. Floc size distribution of only PSA and PSA+MBF-g-P(AM-DAC) are significantly different. The median equivalent volumetric diameter (d50) of floc size distribution is only 100-220 μm in case of PSA (Fig. 5(b)), while the value is able to reach 776-1459 μm in case of PSA+MBF-g-P(AM-DAC)

(Fig.

5(a)). 14

It

attributes

to

the

bridging

of

MBF-g-P(AM-DAC) that ‘links’ small particles, CR and PSA. Floc size and surface morphology are described via SEM (Fig. S9 (a)-(d)). Flocs generated by only PSA are scattered. Flocs are bigger after adding MBF-g-P(AM-DAC), and their surface are rough. CR is intertwined by MBF-g-P(AM-DAC) for its functional groups. Flocs with three-dimensional network structure are discovered. Moreover, the active sites, the functional groups on MBF-g-P(AM-DAC), play role of adsorption centers that also

ro of

improve the formation of large and compacted flocs. In the cases of PSA+PAM and

PSA+P(AM-DAC), d50 of the floc size distribution are 301-566 μm and 352-776 μm, separately (Fig. 5(c) and 5(d)). Although their floc size are increased compared with

-p

PSA, they are smaller than that of PSA+MBF-g-P(AM-DAC). The similar results are

re

found in Fig. 4.

lP

Fig. 5 Particle size distribution of flocs formed by (a) PSA+MBF-g-P(AM-DAC) (b)

na

PSA (c) PSA+PAM and (d) PSA+P(AM-DAC)

ur

3.3.2 Adsorption of flocculants

Compound flocculants, PSA+MBF-g-P(AM-DAC), are added into distilled water, and

Jo

solution is stirred to obtain flocs. Flocs are freeze dried in vacuum and characterized by BET. The adsorption isotherm of flocs does not coincide with their desorption isotherm. The adsorption-desorption curve has a hysteresis loop (Fig. 6(a)). According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the curve is a typical type of IV isotherms. Hysteresis loops are type of H3. It indicates that flocs contain 15

mesopore structure [34]. The BET surface area of flocs is 24.8201 m2/g. Pore volume is 0.0389 cm3/g. According to the BJH pore size distribution plot (Fig. 6(b)), the largest porous pore size of flocs is 1.9 nm. The average pore diameter was 9.4723 nm.

Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution

ro of

plot

Adsorption thermodynamics is usually used to describe adsorption capacity of sorbent

and adsorption mechanism [35]. In the study, adsorption mechanism of flocculant is

-p

complicated. Three typical adsorption isothermal models are used to determine the

re

parameters of models, adsorption capacity and adsorption mechanism of PSA+MBF-g-P(AM-DAC) in CR wastewater treatment. Initial concentration (C0) of

lP

CR in wastewater influences adsorption capacity (qe) of compound flocculants at

na

reaction equilibrium (Fig.S10(a)). The qe value increases with increasing C0. After the Langmuir adsorption isotherm model was fitted, the correlation coefficient (R2) was

ur

close to 1. Langmuir model is an excellent description of the adsorption process of compound flocculants in the treatment. It indicates that there is a homogeneous single

Jo

layer adsorption, and it is an exothermic reaction. The theoretical maximum adsorption capacity (qm) is 140mg/g. It is also found that Freundlich adsorption isotherm model well describes the adsorption process (Fig. S10(b)). It implies that it is a multimolecular adsorption process [36]. Adsorption capacity is rarely affected by the concentration of CR in the wastewater. It is a chemical adsorption [37]. The 16

equilibrium adsorption energy E value is used to judge the adsorption mechanism in Dubinin-Radushkevich adsorption isotherm model. It indicates that adsorption reaction is mainly a physical adsorption when E value is less than 8 kJ/mol. On the contrary, it is mainly an ion exchange when E value is between 8 kJ/mol and 16 k J/mol. Dubinin-Radushkevich adsorption isotherm model also well fits the relation curve of C0 and qe in CR wastewater treatment by compound flocculants. And it

ro of

indicates that it is chemical adsorption caused by ion exchange (Fig. S10(c)). Dates of linear fit of adsorption isotherms are represented in the Supplementary Information

-p

(Table S1).

re

3.4 Treatment mechanisms of dyeing wastewater

Treatment efficiency and mechanisms in wastewater treatment are determined by the

lP

characteristics of pollutants and flocculants. Zeta potential is often used to describe

na

charge neutralization of flocculants. The initial zeta potential of CR wastewater was -15.6 ± 0.5 mV. Although zeta potential of treated water increases with the increase of

ur

flocculant dosage, the increase is higher in case of PSA than that in case of PSA+MBF-g-P (AM-DAC), PSA+PAM and PSA+P(AM-DAC). It implies that the

Jo

capacity of charge neutralization of PSA is the strongest. And the second one is MBF-g-P (AM-DAC). The optimal removal rate is obtained near the isoelectric point of CR wastewater treated by PSA. It indicates that charge neutralization is dominant in the treatment using PSA. However, the optimal removal rate is achieved further above the isoelectric point in case of PAS+MBF-g-P (AM-DAC). Charge 17

neutralization is not the dominant treatment mechanism. Other mechanisms play their role in the case.

Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants

In the treatment using PSA+MBF-g-P(AM-DAC), the hydroxyl aluminum generated

ro of

by the hydrolysis of PSA reduces charge repulsion and chelates with CR, which is

deduced to be flocculation center. The -OH, -NH2 and N+ carried by MBF-g-P (AM-DAC) are able to bond with H+ on the CR. The -COOH of MBF-g-P(AM-DAC)

-p

are hydrolyzed, and the produced negatively charged -COO- combines with positively

re

charged hydroxyl aluminum by van der Waals force. These reactions enforce the bridging among CR, flocs, PSA and MBF-g-P(AM-DAC). It promotes the further

lP

growth of flocs. The big flcos are found in this stage. Furthermore, the unsaturated

na

active sites on flocculants and flocs contribute to the adsorption of CR. Some CR is

ur

enmeshed by flocs at the settlement period.

Jo

Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants

4. Conclusion

A functional microbial flocculant, MBF-g-P(AM-DAC), was prepared via grafting functional groups, amino group, ammonium group and acyloxy group, onto molecular chain of MBF. Preparation condition was optimized to achieve the high molecular 18

weight and performance in the treatment of azo dyeing wastewater. Thermal stability and solubility of MBF was improved via modification. It was confirmed that grafting copolymerization was occurred at the amino group of MBF. AM as a bridge links MBF and DAC in preparation. PSA and MBF-g-P(AM-DAC) played different roles in CR wastewater treatment. PSA had the strong charge neutralization confirmed by zeta potential. It resulted in destabilization, encounter and gather of CR, but the yielded

ro of

flocs were small. MBF-g-P(AM-DAC) possessed the strong bridging since its large amount of functional groups reacted and linked with CR, flocs and PSA. Big flcos and

high settlement efficiency were discovered after introducing MBF-g-P(AM-DAC).

-p

The unsaturated active sites on flocculants and flocs contributed to the adsorption of

re

CR. The BET surface area of flocs formed by flocculants in the deionized water was 24.8201 m2/g, and pore volume was 0.0389 cm3/g. The average pore diameter was

lP

9.4723 nm. The theoretical maximum adsorption capacity was 140 mg/g. Physical

na

adsorption and chemical adsorption were both discovered in the treatment.

ur

ACKNOWLEDGEMENTS

We are grateful for t he financial support provided by the National Natural Science

Jo

Foundation of China (Project No. 51608078), the Natural Science Foundation Project of CQ CSTC (Project No. cstc2016jcyjA0197) and the China Postdoctoral Science Foundation funded project (Project No. 2017M622970).

19

References [1] M. Vakili, M. Rafatullah, B. Salamatinia, A.Z. Abdullah, M.H. Ibrahim, K.B. Tan, Z. Gholami, P. Amouzgar, Application of chitosan and its derivatives as adsorbents for dye removal from water and wastewater: A review, Carbohyd Polym, 113 (2014) 115-130. [2] Z. Al-Qodah, W.K. Lafi, Z. Al-Anber, M. Al-Shannag, A. Harahsheh, Adsorption

ro of

of methylene blue by acid and heat treated diatomaceous silica, Desalination, 217 (2007) 212-224.

[3] J.L. Wang, X.U. LE JIN, Advanced Oxidation Processes for Wastewater Treatment:

-p

Formation of Hydroxyl Radical and Application, Crit Rev Env Sci Tec, 42 (2012)

re

251-325.

[4] J. Yu, X. Wang, P.L. Yue, Optimal Decolorization and Kinetic Modeling of

lP

Synthetic Dyes by Pseudomonas Strains, Water Res, 35 (2001) 3579-3586.

na

[5] A.K. Sarkar, A. Pal, S. Ghorai, N.R. Mandre, S. Pal, Efficient removal of malachite green dye using biodegradable graft copolymer derived from amylopectin

ur

and poly(acrylic acid), Carbohyd Polym, 111 (2014) 108-115. [6] T. Kamal, S.B. Khan, A.M. Asiri, Synthesis of zero-valent Cu nanoparticles in the

Jo

chitosan coating layer on cellulose microfibers: evaluation of azo dyes catalytic reduction, Cellulose, 23 (2016) 1911-1923. [7] S. Erdemoğlu, S.K. Aksu, F. Sayılkan, B. İzgi, M. Asiltürk, H. Sayılkan, F. Frimmel, Ş. Güçer, Photocatalytic degradation of Congo Red by hydrothermally synthesized nanocrystalline TiO2 and identification of degradation products by LC– 20

MS, J Hazard Mater, 155 (2008) 469-476. [8] C. Chen, T. Ma, Y. Shang, B. Gao, B. Jin, H. Dan, Q. Li, Q. Yue, Y. Li, Y. Wang, X. Xu, In-situ pyrolysis of Enteromorpha as carbocatalyst for catalytic removal of organic contaminants: Considering the intrinsic N/Fe in Enteromorpha and non-radical reaction, Appl Catal B-Environ, 250 (2019) 382-395. [9] I. Oller, S. Malato, J.A. Sánchez-Pérez, Combination of Advanced Oxidation

ro of

Processes and biological treatments for wastewater decontamination—A review, Sci Total Environ, 409 (2011) 4141-4166.

[10] R. Liang, F. Jing, L. Shen, N. Qin, L. Wu, MIL-53(Fe) as a highly efficient

re

dyes, J Hazard Mater, 287 (2015) 364-372.

-p

bifunctional photocatalyst for the simultaneous reduction of Cr(VI) and oxidation of

[11] M. Punzi, F. Nilsson, A. Anbalagan, B.-M. Svensson, K. Jönsson, B. Mattiasson,

lP

M. Jonstrup, Combined anaerobic–ozonation process for treatment of textile

na

wastewater: Removal of acute toxicity and mutagenicity, J Hazard Mater, 292 (2015) 52-60.

ur

[12] Z. Su, T. Liu, W. Yu, X. Li, N.J.D. Graham, Coagulation of surface water: Observations on the significance of biopolymers, Water Res, 126 (2017) 144-152.

Jo

[13] P.A. Moussas, A.I. Zouboulis, A new inorganic–organic composite coagulant, consisting of Polyferric Sulphate (PFS) and Polyacrylamide (PAA), Water Res, 43 (2009) 3511-3524. [14] S. Pal, S. Ghorai, M.K. Dash, S. Ghosh, G. Udayabhanu, Flocculation properties of polyacrylamide grafted carboxymethyl guar gum (CMG-g-PAM) synthesised by 21

conventional and microwave assisted method, J Hazard Mater, 192 (2011) 1580-1588. [15] C. Zhao, H. Zheng, Y. Sun, S. Zhang, J. Liang, Y. Liu, Y. An, Evaluation of a novel dextran-based flocculant on treatment of dye wastewater: Effect of kaolin particles, Sci Total Environ, 640-641 (2018) 243-254. [16] B. Liu, H. Zheng, Y. Wang, X. Chen, C. Zhao, Y. An, X. Tang, A novel carboxyl-rich chitosan-based polymer and its application for clay flocculation and

ro of

cationic dye removal, Sci Total Environ, 640-641 (2018) 107-115.

[17] W.-W. Li, H.-Q. Yu, Insight into the roles of microbial extracellular polymer substances in metal biosorption, Bioresource Technol, 160 (2014) 15-23.

-p

[18] A.H.R. Aljuboori, A. Idris, N. Abdullah, R. Mohamad, Production and

Technol, 127 (2013) 489-493.

re

characterization of a bioflocculant produced by Aspergillus flavus, Bioresource

lP

[19] Z.-m. Qiu, W.-t. Jiang, Z.-j. He, Post-treatment of banknote printing wastewater

740-745.

na

using polysilicate ferro-aluminum sulfate (PSFA), J Hazard Mater, 166 (2009)

ur

[20] K. Guo, B. Gao, W. Wang, Q. Yue, X. Xu, Evaluation of molecular weight, chain architectures and charge densities of various lignin-based flocculants for dye

Jo

wastewater treatment, Chemosphere, 215 (2019) 214-226. [21] H. Zhou, L. Zhou, X. Yang, Optimization of preparing a high yield and high cationic degree starch graft copolymer as environmentally friendly flocculant: Through response surface methodology, Int J Biol Macromol, 118 (2018) 1431-1437. [22] M. Hallajiqomi, H. Eisazadeh, Adsorption of manganese ion using polyaniline 22

and it’s nanocomposite: Kinetics and isotherm studies, J Ind Eng Chem, 55 (2017) 191-197. [23] M.V. Kök, M.A. Varfolomeev, D.K. Nurgaliev, Crude oil characterization using TGA-DTA, TGA-FTIR and TGA-MS techniques, J Petrol Sci Eng, 154 (2017) 537-542. [24] H. Zheng, Y. Sun, J.-S. Guo, F. Li, W. Fan, Y. Liao, G. Qingqing,

ro of

Characterization and Evaluation of Dewatering Properties of PADB, a Highly Efficient Cationic Flocculant, Ind Eng Chem Res, 53 (2014) 2572–2582.

[25] H. Zheng, Y. Sun, C. Zhu, J. Guo, C. Zhao, Y. Liao, Q. Guan, UV-initiated

-p

polymerization of hydrophobically associating cationic flocculants: Synthesis,

re

characterization, and dewatering properties, Chem Eng J, 234 (2013) 318-326. [26] A. Srivastava, D.K. Mishra, K. Behari, Graft copolymerization of

lP

N-vinyl-2-pyrrolidone onto chitosan: Synthesis, characterization and study of

na

physicochemical properties, Carbohyd Polym, 80 (2010) 790-798. [27] U. Janciauskaite, V. Rakutyte, J. Miskinis, R. Makuska, Synthesis and properties

ur

of chitosan-N-dextran graft copolymers, React Funct Polym, 68 (2008) 787-796. [28] Z.-H. Wang, Y. Zhu, M.-Y. Chai, W.-T. Yang, F.-J. Xu, Biocleavable

Jo

comb-shaped gene carriers from dextran backbones with bioreducible ATRP initiation sites, Biomaterials, 33 (2012) 1873-1883. [29] S.-X. Teng, S.-G. Wang, X.-W. Liu, W.-X. Gong, X.-F. Sun, J.-J. Cui, B.-Y. Gao, Interaction between congo red and copper in a binary adsorption system: Spectroscopic and kinetic studies, Colloid Surface A, 340 (2009) 86-92. 23

[30] D. M, B. S, Synthesis, characterization and application of acryloyl chitosan anchored copolymer towards algae flocculation, Carbohyd Polym, 152 (2016) 459-467. [31] d.L.M. Salzano, R. Castaldo, R. Altobelli, L. Gioiella, G. Filippone, G. Gentile, V. Ambrogi, Chitosan hydrogels embedding hyper-crosslinked polymer particles as reusable broad-spectrum adsorbents for dye removal, Carbohyd Polym, 177 (2017)

ro of

347.

[32] A. Suresh, E. Grygolowicz-Pawlak, S. Pathak, L.S. Poh, M.b. Abdul Majid, D. Dominiak, T.V. Bugge, X. Gao, W.J. Ng, Understanding and optimization of the

-p

flocculation process in biological wastewater treatment processes: A review,

re

Chemosphere, 210 (2018) 401-416.

[33] L. Feng, H. Zheng, W. Zhou, C. Zhao, S. Zhang, N. Chen, Enhancement of

lP

textile-dyeing sludge dewaterability using a novel cationic polyacrylamide: role of

na

cationic block structures, Rsc Adv., 7 (2017) 11626-11635. [34] J.-L. Liu, R.-B. Lin, Structural properties and reactivities of amino-modified

ur

silica fume solid sorbents for low-temperature CO2 capture, Powder Technol, 241 (2013) 188-195.

Jo

[35] M. Dalaran, S. Emik, G. Güçlü, T.B. İyim, S. Özgümüş, Study on a novel polyampholyte nanocomposite superabsorbent hydrogels: Synthesis, characterization and investigation of removal of indigo carmine from aqueous solution, Desalination, 279 (2011) 170-182. [36] A. Mittal, L. Kurup, J. Mittal, Freundlich and Langmuir adsorption isotherms and 24

kinetics for the removal of Tartrazine from aqueous solutions using hen feathers, J Hazard Mater, 146 (2007) 243-248. [37] E. Nakkeeran, N. Selvaraju, Biosorption of Chromium(VI) in aqueous solutions by chemically modified Strychnine tree fruit shell, Int J Phytoremediat, 19 (2017)

Jo

ur

na

lP

re

-p

ro of

1065.

25

Figure Legends Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC) Fig. 2 (a) the fully scanned spectrum of XPS, (b) O1s spectrum, (c) C1s spectrum and (d) N1s spectrum of MBF-g-P (AM-DAC) Fig. 3 Grafting polymerization scheme of MBF-g-P(AM-DAC) Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with

ro of

different settlement time

Fig. 5 Particle size distribution of flocs formed by (a) PSA+MBF-g-P(AM-DAC) (b) PSA (c) PSA+PAM and (d) PSA+P(AM-DAC)

-p

Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution

re

plot

Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants

Jo

ur

na

lP

Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants

26

ro of

Jo

ur

na

lP

re

-p

Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC)

27

28

ro of

-p

re

lP

na

ur

Jo

ro of -p re lP na ur

Jo

Fig. 2 (a) the fully scanned spectrum of XPS, (b) O1s spectrum, (c) C1s spectrum and (d) N1s spectrum of MBF-g-P (AM-DAC)

29

ro of -p

Jo

ur

na

lP

re

Fig. 3 Grafting polymerization scheme of MBF-g-P(AM-DAC)

30

ro of -p re lP na ur

Jo

Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with different settlement time

31

32

ro of

-p

re

lP

na

ur

Jo

ro of -p re lP na ur

Jo

Fig. 5 Particle size distribution of flocs formed by (a) PSA+MBF-g-P(AM-DAC) (b) PSA (c) PSA+PAM and (d) PSA+P(AM-DAC)

33

ro of -p re lP na ur Jo

Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution plot

34

ro of

Jo

ur

na

lP

re

-p

Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants

35

Jo

ur

na

lP

re

-p

ro of

Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants

36