- Email: [email protected]
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, 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
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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
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2
Ministry of Education, Chongqing University, Chongqing 400045, P.R. China
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*Corresponding author: College of Environment and Resources, Chongqing
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Technology and Business University, Chongqing 400067, P.R. China. Tel ＆Fax: +86
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Graphical abstract
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02388326467, Email: [email protected]
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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.
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Abstract:
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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
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flocculant (MBF) with various functional groups is a potential flocculant applied in
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dyeing wastewater treatment, and it has the advantages of high treatment efficiency, biodegradability and non-toxicity. In this study, the functional groups, amino group,
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ammonium group and acyloxy group, were grafted onto MBF to further improve its thermal stability, solubility and performance. Grafting copolymerization occurred at
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the amino group of MBF was confirmed by XPS. Polyaluminum silicate (PSA) and
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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)
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Acryloyloxyethyl trimethylammonium chloride (DAC)
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Microbial flocculant (MBF)
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Acrylamide (AM)
Removal efficiency (RE)
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characteristic; removal mechanism
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Scanning electron microscopy (SEM)
Brunauer-Deming-Deming-Teller (BDDT)
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Binding energies (BE)
X-ray photoelectron spectroscopy (XPS)
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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
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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
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cosmetics industries due to its excellent surface adhesion and low-cost, and it is a
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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
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optical, thermal and physiochemical stability due to presence of aromatic structure[6].
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In azo dyeing wastewater treatment, it mainly focuses on the decolorization of wastewater. And the chemical and biological methods, including chemical oxidation,
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photocatalytic degradation and biodegradation, have been investigated to degrade dyes. Chemical oxidation and Photocatalytic degradation are simple and efficient [7,
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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
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treatment [14].
Although conventional flocculants, aluminum salt flocculants and ferric salt
flocculants, not well perform in dyeing wastewater treatment, the treatment efficiency
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is able to be improved through modifying flocculants. Functional groups of acyloxy
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grafted onto dextran [15] and carboxyl groups grafted onto chitosan are reported in our previous studies [16]. Microbial flocculant (MBF) produced by microorganism is
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mostly polysaccharide-like substances. It owning a large number of various functional
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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
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safety [18]. But it is rarely used in dyeing wastewater treatment. In this study, amino group, ammonium group and acyloxy group were introduced onto
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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.
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2. Materials and methods 2.1 Materials
Microbial flocculant (Qingdao Tung Biological Engineering Co. Ltd., China) mainly
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containing polysaccharide was harvested from the fermentation liquor of Klebsiella
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peneumoniae. Polysilicate aluminum (PSA) was prepared via previous method [19]. Monomer, initiator and other chemicals used in this experiment were all technical
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grade (Text S1 in Supplementary Information). All aqueous and standard solutions
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were prepared with deionized water.
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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
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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.
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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
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characteristics of MBF-g-P(AM-DAC) were analyzed by Fourier transform infrared
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spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and differential thermal gravity (DTG). Their analyses provided clues
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for the study of the synthesis mechanism of MBF-g-P (AM-DAC). FTIR and XPS
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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
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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
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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
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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
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of dosages of MBF-g-P(AM-DAC) was added into the wastewater. A new stirring was
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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
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samples were taken at a depth of 2 cm below the surface of supernatant to determine
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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
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were run in triplicate, and removal efficiency (RE) of CR from wastewater was calculated according to the following equation: (1)
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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
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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
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[22].
3. Results and discussion
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3.1 Synthesis and characteristics of MBF-g-P(AM-DAC)
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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
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optimized in order to improve its molecular weight and graft copolymerization rate that are positively correlated with its performance in wastewater treatment. Initiator
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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
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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%.
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The weight loss corresponds to the endothermic peak in DSC curves. Three
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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
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77.4 ℃, 94 ℃ and 307 ℃, respectively (Fig. S7(b)). It is confirmed that the
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thermal stability of MBF is markedly improved after graft copolymerization.
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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
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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
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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
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of a longer carbon chain in MBF-g-P(AM-DAC). The characteristic peaks of amino
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group, ammonium group and acyloxy group are observed in infrared spectra of
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MBF-g-P(AM-DAC).
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Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC)
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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
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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
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-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
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implies that grafting copolymerization is occurred at -NH2 of MBF (Fig. 3).
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Fig. 2 (a) the fully scanned spectrum of XPS, (b) O1s spectrum, (c) C1s spectrum and
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(d) N1s spectrum of MBF-g-P (AM-DAC)
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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
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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,
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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).
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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)
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Bridging of MBF-g-P(AM-DAC) plays an essential role in CR wastewater treatment.
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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
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MBF-g-P(AM-DAC).
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treatment
In CR wastewater treatment, PSA is firstly added into the wastewater. It undergoes
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coordination hydrolysis to form a positively charged polyhydroxy cation complex and plays the role of charge neutralization that results in the destabilization, encounter and
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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
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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.
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Thus, the removal efficiency is negative at the beginning of settlement (Fig. 4(b)).
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Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with
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different settlement time
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In flocculation process, the characteristics of flocs have great influences on their settlement performance [32]. Large and compacted flocs are quickly separated from
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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
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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
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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
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PSA, they are smaller than that of PSA+MBF-g-P(AM-DAC). The similar results are
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found in Fig. 4.
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Fig. 5 Particle size distribution of flocs formed by (a) PSA+MBF-g-P(AM-DAC) (b)
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PSA (c) PSA+PAM and (d) PSA+P(AM-DAC)
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3.3.2 Adsorption of flocculants
Compound flocculants, PSA+MBF-g-P(AM-DAC), are added into distilled water, and
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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
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plot
Adsorption thermodynamics is usually used to describe adsorption capacity of sorbent
and adsorption mechanism [35]. In the study, adsorption mechanism of flocculant is
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complicated. Three typical adsorption isothermal models are used to determine the
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parameters of models, adsorption capacity and adsorption mechanism of PSA+MBF-g-P(AM-DAC) in CR wastewater treatment. Initial concentration (C0) of
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CR in wastewater influences adsorption capacity (qe) of compound flocculants at
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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
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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
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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
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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
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(Table S1).
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3.4 Treatment mechanisms of dyeing wastewater
Treatment efficiency and mechanisms in wastewater treatment are determined by the
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characteristics of pollutants and flocculants. Zeta potential is often used to describe
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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
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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
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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
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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)
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are hydrolyzed, and the produced negatively charged -COO- combines with positively
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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
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growth of flocs. The big flcos are found in this stage. Furthermore, the unsaturated
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active sites on flocculants and flocs contribute to the adsorption of CR. Some CR is
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enmeshed by flocs at the settlement period.
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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
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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).
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The unsaturated active sites on flocculants and flocs contributed to the adsorption of
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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
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9.4723 nm. The theoretical maximum adsorption capacity was 140 mg/g. Physical
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adsorption and chemical adsorption were both discovered in the treatment.
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ACKNOWLEDGEMENTS
We are grateful for t he financial support provided by the National Natural Science
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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).
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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
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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)
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Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution
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plot
Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants
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Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants
26
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na
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Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC)
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28
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na
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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)
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na
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Fig. 3 Grafting polymerization scheme of MBF-g-P(AM-DAC)
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Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with different settlement time
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na
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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)
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Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution plot
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Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants
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Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants
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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
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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
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2
Ministry of Education, Chongqing University, Chongqing 400045, P.R. China
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*Corresponding author: College of Environment and Resources, Chongqing
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Technology and Business University, Chongqing 400067, P.R. China. Tel ＆Fax: +86
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Graphical abstract
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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.
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Abstract:
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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
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flocculant (MBF) with various functional groups is a potential flocculant applied in
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dyeing wastewater treatment, and it has the advantages of high treatment efficiency, biodegradability and non-toxicity. In this study, the functional groups, amino group,
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ammonium group and acyloxy group, were grafted onto MBF to further improve its thermal stability, solubility and performance. Grafting copolymerization occurred at
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the amino group of MBF was confirmed by XPS. Polyaluminum silicate (PSA) and
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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)
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Acryloyloxyethyl trimethylammonium chloride (DAC)
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Microbial flocculant (MBF)
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Acrylamide (AM)
Removal efficiency (RE)
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characteristic; removal mechanism
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Scanning electron microscopy (SEM)
Brunauer-Deming-Deming-Teller (BDDT)
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Binding energies (BE)
X-ray photoelectron spectroscopy (XPS)
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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
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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
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cosmetics industries due to its excellent surface adhesion and low-cost, and it is a
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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
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optical, thermal and physiochemical stability due to presence of aromatic structure[6].
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In azo dyeing wastewater treatment, it mainly focuses on the decolorization of wastewater. And the chemical and biological methods, including chemical oxidation,
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photocatalytic degradation and biodegradation, have been investigated to degrade dyes. Chemical oxidation and Photocatalytic degradation are simple and efficient [7,
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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
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treatment [14].
Although conventional flocculants, aluminum salt flocculants and ferric salt
flocculants, not well perform in dyeing wastewater treatment, the treatment efficiency
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is able to be improved through modifying flocculants. Functional groups of acyloxy
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grafted onto dextran [15] and carboxyl groups grafted onto chitosan are reported in our previous studies [16]. Microbial flocculant (MBF) produced by microorganism is
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mostly polysaccharide-like substances. It owning a large number of various functional
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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
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safety [18]. But it is rarely used in dyeing wastewater treatment. In this study, amino group, ammonium group and acyloxy group were introduced onto
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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.
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2. Materials and methods 2.1 Materials
Microbial flocculant (Qingdao Tung Biological Engineering Co. Ltd., China) mainly
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containing polysaccharide was harvested from the fermentation liquor of Klebsiella
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peneumoniae. Polysilicate aluminum (PSA) was prepared via previous method [19]. Monomer, initiator and other chemicals used in this experiment were all technical
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grade (Text S1 in Supplementary Information). All aqueous and standard solutions
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were prepared with deionized water.
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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
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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.
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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
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characteristics of MBF-g-P(AM-DAC) were analyzed by Fourier transform infrared
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spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and differential thermal gravity (DTG). Their analyses provided clues
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for the study of the synthesis mechanism of MBF-g-P (AM-DAC). FTIR and XPS
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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
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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
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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
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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
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of dosages of MBF-g-P(AM-DAC) was added into the wastewater. A new stirring was
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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
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samples were taken at a depth of 2 cm below the surface of supernatant to determine
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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
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were run in triplicate, and removal efficiency (RE) of CR from wastewater was calculated according to the following equation: (1)
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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
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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
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[22].
3. Results and discussion
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3.1 Synthesis and characteristics of MBF-g-P(AM-DAC)
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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
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optimized in order to improve its molecular weight and graft copolymerization rate that are positively correlated with its performance in wastewater treatment. Initiator
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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
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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%.
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The weight loss corresponds to the endothermic peak in DSC curves. Three
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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
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77.4 ℃, 94 ℃ and 307 ℃, respectively (Fig. S7(b)). It is confirmed that the
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thermal stability of MBF is markedly improved after graft copolymerization.
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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
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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
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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
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of a longer carbon chain in MBF-g-P(AM-DAC). The characteristic peaks of amino
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group, ammonium group and acyloxy group are observed in infrared spectra of
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MBF-g-P(AM-DAC).
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Fig. 1 FTIR spectra of (a) MBF, (b) AM, (c) DAC and (d) MBF-g-P(AM-DAC)
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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
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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
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-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
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implies that grafting copolymerization is occurred at -NH2 of MBF (Fig. 3).
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Fig. 2 (a) the fully scanned spectrum of XPS, (b) O1s spectrum, (c) C1s spectrum and
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(d) N1s spectrum of MBF-g-P (AM-DAC)
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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
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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,
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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).
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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)
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Bridging of MBF-g-P(AM-DAC) plays an essential role in CR wastewater treatment.
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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
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MBF-g-P(AM-DAC).
to
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treatment
In CR wastewater treatment, PSA is firstly added into the wastewater. It undergoes
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coordination hydrolysis to form a positively charged polyhydroxy cation complex and plays the role of charge neutralization that results in the destabilization, encounter and
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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
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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.
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Thus, the removal efficiency is negative at the beginning of settlement (Fig. 4(b)).
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Fig. 4 Removal efficiency of flocculants: (a) with different dosage and (b) with
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different settlement time
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In flocculation process, the characteristics of flocs have great influences on their settlement performance [32]. Large and compacted flocs are quickly separated from
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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
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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
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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
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PSA, they are smaller than that of PSA+MBF-g-P(AM-DAC). The similar results are
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found in Fig. 4.
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Fig. 5 Particle size distribution of flocs formed by (a) PSA+MBF-g-P(AM-DAC) (b)
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PSA (c) PSA+PAM and (d) PSA+P(AM-DAC)
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3.3.2 Adsorption of flocculants
Compound flocculants, PSA+MBF-g-P(AM-DAC), are added into distilled water, and
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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
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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
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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
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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
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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
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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
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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
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(Table S1).
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3.4 Treatment mechanisms of dyeing wastewater
Treatment efficiency and mechanisms in wastewater treatment are determined by the
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characteristics of pollutants and flocculants. Zeta potential is often used to describe
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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
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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
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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
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growth of flocs. The big flcos are found in this stage. Furthermore, the unsaturated
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active sites on flocculants and flocs contribute to the adsorption of CR. Some CR is
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enmeshed by flocs at the settlement period.
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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
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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
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adsorption and chemical adsorption were both discovered in the treatment.
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ACKNOWLEDGEMENTS
We are grateful for t he financial support provided by the National Natural Science
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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
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na
lP
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-p
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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
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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)
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Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution
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plot
Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants
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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
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-p
re
lP
na
ur
Jo
ro of -p re lP na ur
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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
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Fig. 3 Grafting polymerization scheme of MBF-g-P(AM-DAC)
30
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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
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ro of -p re lP na ur
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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
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Fig. 6 (a) N2 adsorption-desorption curve of flocs and (b) BJH pore size distribution plot
34
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ur
na
lP
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-p
Fig. 7 Zeta potential of supernatant of CR wastewater treated by different flocculants
35
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na
lP
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-p
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Fig. 8 Removal mechanisms of CR in wastewater treatment using flocculants
36