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Surface Modification of Basalt Fiber with Organic/Inorganic Composites for Biofilm Carrier Used in Wastewater Treatment Xiaoying Zhang, Xiangtong Zhou, Huicheng Ni, Xinshan Rong, Qian Zhang, Xiang Xiao, Huan Huan, Jun Feng Liu, and Zhiren Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04089 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Sustainable Chemistry & Engineering

Surface Modification of Basalt Fiber with Organic/Inorganic Composites for Biofilm Carrier Used in Wastewater Treatment Xiaoying Zhanga, Xiangtong Zhoua*, Huicheng Nia, Xinshan Ronga, Qian Zhanga, Xiang Xiaoa, Huan Huanb, Jun Feng Liuc, Zhiren Wua* a

School of the Environment and Safety Engineering, Jiangsu University, No. 301 Xuefu Road, Jingkou District, Zhenjiang 212013, China b

State Environmental Protection Key Laboratory of Simulation and Control of

Groundwater Pollution, Chinese Research Academy of Environmental Sciences, No.8 Dayangfang, Anwai Beiyuan, Chaoyang District, Beijing 100012, China c

School of Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, China

*Corresponding authors: E-mail: [email protected] (Zhiren Wu); [email protected] (Xiangtong Zhou)

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ABSTRACT: Support carrier constitutes the technical core of biofilm processes in wastewater treatment. In this study, basalt fiber (BF) was modified by grafting an organic/inorganic composite, attached to which rich microorganisms were supposed to form biofilm for wastewater treatment. The modified BF (MBF) used as biofilm carriers were investigated in terms of hydrophilicity and surface roughness, which determined their bio-affinity. Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) were conducted to study the chemical components. Scanning electron microscopy (SEM) was carried out to observe BF surface morphology. The bio-affinity of BF and MBF was compared in terms of the rate of bacterial adhesion and the ratio of immobilization onto basalt fiber. The bio-affinity of MBF was significantly improved due to introduction of many hydrophilic groups onto BF surfaces, which were subsequently proved to facilitate biofilm formation. The results showed that the adhesion rate, immobilization ratio and biomass of MBF were higher than that of BF. Thus, the polyacrylamide/epoxy/nano-SiO2 coating modification technology holds a promising future for BF application in wastewater treatment. KEYWORDS: Surface modification technology; Basalt fiber; Biofilm carrier; Bacterial immobilization; Wastewater treatment INTRODUCTION Great consumption of water resources is inevitable with the increasing industrialization and urbanization, whether part of them serving as wastewater were


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directly discharged into the environment without any pretreatment 1. Subsequently, a series of the environmental problems are brought about, posing serious risks to human health and ecological security

2-4

. Therefore, wastewater must be treated before

entering environment 5. Considering the fact that large space is required for the construction of conventional wastewater treatment plants, biochemical treatment technologies have gained more attention. Among them, biofilm process is a preferential solution due to their advantages in space requirement, aeration cost, biomass

retention,

and

synergistic

function

of

microbial

community

6

.

Microorganisms tend to adhere to carrier surface, forming structures known as biofilms 7. Thus, selection of the desired carrier plays an important role in biofilm formation, which in turn determines efficacy of biofilm process in wastewater treatment. An ideal biofilm carrier should possess low cost, good mechanical strength, large surface area, and high bio-affinity. Presently, bio-carriers used in wastewater treatment are generally activated carbon 8, ceramic9, carbon fiber 10, polyester (PES) fiber 11, polypropylene (PP) fiber

12

and preoxidized polyacrylonitrile (PAN) fiber 13.

Among them, granular activated carbon and ceramic carriers have advantages in cost and mechanical strength, but have disadvantages in mass transport caused by low porosity and easy blockage. For organic fiber materials, they have large specific surface area, but have low bio-affinity because of the smooth surface, which readily cause biofilm detachment. Furthermore, they often cause pollution during production 14

. In such case, exploitation of a novel biofilm carrier is imperative.



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BF is a high performance inorganic silicate fiber, and generally fabricated by heating the basalt rocks and extruding molten liquid through a die in the form of fibers 15. Basalt rocks, suitable for BF production, are inexpensive and easy available around the world 16. Despite the forms changing, these properties are still maintained in BF

17

. Although BF has been widely used in a variety of industries

18, 19

, few

researchers pay attention to its potential applications as biofilm carrier. BF could immobilize microorganisms, allowing biofilm formation because of its mechanical strength as well as large specific surface area. Even so, BF should be further modified from surface properties (e.g., hydrophilicity and roughness) before realizing real application. Carrier surface properties are considered to be a determining factor affecting bacteria adhesion and biofilm formation

20

. Fernández et al reported

bacterial viability assays using confocal laser scanning fluorescence microscopy, indicating that the inclusion of hydrophilic groups on a carrier surface significantly improved both cell adhesion and viability

21

. Moreover, Chavant et al showed that

biofilm formation was faster on the hydrophilic carrier 22. These results indicated that enhancing hydrophilicity as a technical approach to improve biofilm formation was possible. Therefore, the purpose of this study was to modify BF surface by the introduction of hydrophilic groups and improvement of surface roughness. The fiber surface modification technology mainly consists of oxidation, plasma and coating modifications. Among them, coating modification is preferentially selected since no influence will be exerted on fiber’s body structure

23

. The

organic/inorganic coating is a newly developed technology and could combine


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organic and inorganic components in a single material, which provides possibility of designing a special BF coating depending on its own structure and composition. In this study, a polyacrylamide/epoxy/nano-SiO2 coating was prepared and grafted onto BF surface to obtain the desired carrier media, which could improve surface hydrophilicity and roughness. The hydrophilicity property of basalt fiber was investigated via static water contact angle. Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) were conducted to investigate the chemical components, providing evidence for the introduction of hydrophilic group. Scanning electron microscopy (SEM) was used to observe the surface morphology and roughness of basalt fiber. Furthermore, the feasibility of using basalt fiber as biofilm carrier was also evaluated through bacterial adhesion and immobilization tests. The results showed that MBF presented an alternative to conventional biofilm carriers in wastewater treatment. EXPERIMENTAL DETAILS Materials BF (CBF9-800) was supplied by Jiangsu GMV new material science T&D Co., Ltd., Jiangsu, China. Epoxy emulsion and Hexadecyl trimethyl ammonium chloride (HTMAC) were purchased from usolf Chemical Technology Co., Ltd., Shandong, China. The other chemicals, such as tetraethyl orthosilicate (TEOS), ethanol (C2H5OH), hydrochloric acid (HCl), sulphuric acid (98%, H2SO4), hydrogen peroxide (30%, H2O2), potassium hydroxide (KOH), silane coupling agent (γ-Aminopropyl triethoxysilane, KH-550) and polyacrylamide (PAM) were purchased from Sinopharm



Chemical Reagent Co., Ltd., Shanghai, China. Potassium persulfate (KPS) was supplied by Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Preparation of polyacrylamide/epoxy/nano-SiO2 coating The polyacrylamide/epoxy/nano-SiO2 coating was prepared by a two-step reaction process (See Figure S1). First, nano-SiO2 was obtained by hydrolyzing TEOS in acidic condition 24. Then, a certain amount of nano-SiO2, KH-550, PAM, HTMAC and part of epoxy solution were added into a three-neck round bottle flask and stirred at 45 ºC for 30 min. Subsequently, KPS aqueous solution and the rest of epoxy solution were added stepwise into the flask at 75 ºC and the pH value was adjusted to about 13.0. Then, the whole system was stirred for 5 h. Finally, the emulsion was cooled to room temperature and pH value was adjusted to 7.0, with the polyacrylamide/epoxy/nano-SiO2 coating successfully prepared. The recipe for preparation of polyacrylamide/epoxy/nano-SiO2 coating is presented in Table S1. Surface modification of BF Prior to grafting an organic/inorganic coating (polyacrylamide/epoxy/nano-SiO2 coating), raw basalt fibers were washed with acetone and dried under vacuum for 1 h to eliminate the agent on the surface and then treated with piranha solution (H2SO4:H2O2 (v/v) =7:3) at 90 ºC for one hour to produce more active silanol groups on the BF surface. Subsequently, basalt fibers were engraved by 1M HCl solution at 40 ºC for another hour, and washed with deionized water and dried in an oven. The dry basalt fibers were immersed in polyacrylamide/epoxy/nano-SiO2 coating at 45 ºC for a predetermined time to ensure grafting reaction completely. Finally, basalt fibers


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modified with polyacrylamide/epoxy/nano-SiO2 coating (MBF) were dried to a constant weight in an oven at 80 ºC. Scheme for surface modification of basalt fiber is shown in Figure 1. Characterization The degree of grafting polyacrylamide/epoxy/nano-SiO2 coating was calculated as follows: dg c =

mMBF − mBF × 100 mBF

(1)

The acid/alkali-resistance of basalt fiber was evaluated by the procedures as follows: BF and MBF samples were weighted (recorded as m1), and then immersed in H2SO4 solution (pH = 5.0) or NaOH solution (pH = 9.0) at 25 ºC for a predetermined time. Subsequently, basalt fibers were taken out from H2SO4 or NaOH solution and solution residues on the basalt fiber surface were sucked with paper tissues. Basalt fibers were dried until weight reached a constant (m2). The weight loss, ω, was calculated as follows:

ω=

m1 − m2 × 100 m1

(2)

The hydrophilicity of BF monofilament was measured with a SDC-200 optical contact angle measuring device (Dongguan Shengding Precision Instrument Co., Ltd., China). The morphology of BF was observed with an S-4800 field emission scanning electron microscope (Hitachi Corp., Japan) at a voltage of 15.0 kV and EDS was collected on the S-4800 equipped with an EDAX detector. Fourier transform infrared (FTIR) spectra were performed on an AVATAR 360 spectrometer (Madison and



Nicolet, USA) in the range from 4000 to 400 cm-1. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALABTM 250Xi spectrometer (Thermo Fisher Scienticfic, USA) with a monochromated Al Kα radiation.

Evaluation of bacterial adhesion onto basalt fiber biofilm carrier Escherichia coli K-12 was aerobically cultured for 12 h at 37 ºC, harvested at exponential phase by centrifugation (5478 g, 5 min) and resuspended in phosphate buffered saline (pH 7.0). 50 mL of E. coli cell suspension was added to an Erlenmeyer flask. The concentration of the cell suspension was adjusted at an OD600 of 0.100. BF and MBF samples were cut into segments of 5 cm in length, and each sample was weighted to have a specific area of 2×10-2 m2. Then, BF and MBF samples were immersed in the Erlenmeyer flasks containing cell suspension, which were incubated with shaking at 200 rpm at 37 ºC. The cell suspension was sampled over time. The adhesion rate of pure bacterial cells to each carrier was calculated from the decrease of each cell suspension at OD600. The adhesion rate constant (k) was defined as follows 25:

V

dc = − kAC dt

(3)

Then,

 V 1  c  k = −     ln  t   A   t   c0 

(4)

where k is the adhesion rate constant (mL/cell·h); V is the volume of the cell suspension (mL); A is the surface area of biofilm carrier (m2); t is the contact time (h);

Ct is the concentration at time t (cell/mL); C0, is the initial concentration (cell/mL).


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The bacterial adhesion test was conducted in triplicate and the results were presented as mean±standard deviation.

Microorganism immobilization test Activated sludge was collected from the Jingkou sewage treatment plant (Zhenjiang, China). The microorganisms were cultured and enriched in a temperature controlled reactor (2 L, 30 ºC) under aeration conditions. The concentration of mixed liquor suspended solids (MLSS) was maintained at about 4000 mg/L, and the pH value was adjusted to 7.8 with Na2CO3. A certain amount of BF and MBF samples (m1, about 1 g) were immersed in activated sludge suspension. BF and MBF samples were taken out from the reactor after 24 h and gently washed with sterile water. These samples were dried until constant weight (m2) was achieved, and then moved to a desiccator. The immobilization ratio of microorganisms (IRM) indicated the capacity of microorganism immobilization, and was calculated as follows:

IRM =

m2 ×100 m1

(5)

Microorganism immobilization test was carried out in triplicate, with the results expressed as mean±standard deviation. Furthermore, microorganism adhesion on the BF or MBF materials was qualitatively observed by a Leica DM2500 optical microscope (Leica Microsystems, Germany). After immobilization test, BF and MBF samples with a certain length were triple washed with PBS solution and mounted on a glass slide. Then the samples were captured by optical microscope.

RESULTS AND DISCUSSION Properties of basalt fibers



Page 10 of 42

Considering the potential damage of acidic or alkaline wastewater to BF carrier, BF resistance to acid and alkaline was investigated. Figure 2 showed the weight loss of BF and MBF in H2SO4 and NaOH solution. The weight loss of MBF was 14.23% in H2SO4 solution and 15.02% in NaOH solution and became stable after 25 days, comparable to that of BF in H2SO4 (12.01%) and NaOH (14.21%) solutions. Therefore, MBF didn’t significantly alter basalt fiber structure, suggesting that the acid and alkali resistance of original basalt fiber was maintained. In this study, the hydrophilicity of basalt monofilament fiber was examined via water contact angle (Figure 3). As can be seen from Figure 3, basalt fiber was a hydrophobic material, with contact angle maintaining at about 155º within 150 s. Compared with BF, MBF had a relatively small contact angle, which could be attributed to the introduction of hydrophilic groups (See Table S2). This result also proved indirectly polyacrylamide/epoxy/nano-SiO2 coating was successfully grafted onto BF surface, making BF hydrophilicity significantly improved.

Surface morphology analysis The fiber morphology and elemental composition were analyzed by SEM-EDS. As can be seen from Figure 4, basalt fibers had a smooth surface, whereas the surface of

MBF

became

rough,

which

was

mainly

due

to

the

grafting

of

polyacrylamide/epoxy/nano-SiO2 coating on MBF surface. The degree of grafting reached 20.82% (Figure 4B), as opposed to basalt fiber without coating (Figure 4A). EDS analysis showed composition of both samples in percentage of elements. The control revealed similar elemental composition with that of the MBF, whereas two


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other elements (C and N) were detected on the MBF surface, which accounted for about 38 % of the total elements (Figure 4). The presence of C and N peaks in the EDS spectrum was due to the introduction of polyacrylamide/epoxy/nano-SiO2 coating on the modified basalt fiber surface. Furthermore, most all elements originally existing on the control were still detected on the MBF sample, indicating that the components on the control didn’t change after modification.

FTIR analysis BF and MBF samples were characterized by FTIR analysis (Figure 5). From spectrum of BF, the characteristic peaks of Si-O-Si at about 1019 cm-1 and 456 cm-1 were observed 26. By comparison, new peaks were found on the MBF spectrum. The absorption peak at about 2964 cm-1 might be ascribed to -CH2 groups, while the bending vibration peaks at 1660 cm-1 and 1450 cm-1 might contribute to amide C=O (CONH2) and C-O stretching vibrations 27. Besides, the characteristic stretching peaks of C-H (CH2) at 2866 and 1608 cm-1 were due to hydrogen of aromatic ring. Furthermore, there was a wider FTIR absorption at 950-1300 cm-1 attributed to overlapping peaks of the Si-O-Si and C-O groups 28. Another wide absorption peak at 3500-3100 cm-1 might be the overlapping peaks of N-H and -OH groups. The results demonstrated that the polyacrylamide/epoxy/nano-SiO2 coating had been successfully grafted onto BF surface.

XPS analysis XPS spectra of BF and MBF samples were shown in Figure 6. As can be seen from Figure 6A, basalt fiber was mainly composed of Si, O and other metal elements,



which well agreed with the EDS results. From Figure 6A, it could be easily found that a characteristic peak of silicates was at about 102.6 eV and a wide valence band region of O 1s was at approximately 535.0 eV, which was due to the silicon dioxide and other metal oxides in basalt fibers. Besides, the binding energy of C 1s at around 284.0 eV appeared in both spectra. However, C 1s of BF sample was attributed to adventitious contamination

29

. Compared with XPS spectrum of BF, C 1s peak of

MBF was more evident and small concentration of nitrogen could be found. Furthermore, the high-resolution spectra of Si, C and N elements were investigated for a detailed chemical analysis. The fitted high-resolution spectra of N 1s, C 1s and Si 2p peaks were as depicted in Figure 6 (B-F). For XPS of N1s (Figure 6B), the peak was at about 399.2 eV, which probably belonged to amino or amide groups. Figure 6D showed the high-resolution measurements of the C 1s and valence band regions, revealing the presence of three different types of carbon atoms. The fitted peak situated at 284.6 eV represented C-C/C-H groups, and the two other peaks located at 285.4 eV and 286.7 eV belonged to C-N and -C=O groups, respectively 30. A characteristic peak of silicates at 102.5 eV could be observed in BF sample (Figure 6E). The peaks of Si 2p were divided into three peaks at 101.7 eV (Si-O), 102.5 eV (Si-OH) and 103.4 eV (Si-O-Si), respectively 31. Similarly, the peaks of Si 2p could also be found in MBF sample (Figure 6F). Apparently, polyacrylamide/epoxy/nano-SiO2 coating resulted in carbon content increase.

Adhesion and immobilization tests As shown in Table 1, the results indicated that the surface modification of BF


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had a positive influence on the adhesion rate of E. coli. The rate of the E. coli adhesion onto MBF surface was 1.33 mL/cell·h, which was almost 1.44 times higher than BF samples. This was due to the hydrophilic groups on MBF surfaces successfully developed. As can be seen from Table 1, IRM of MBF was 218.40%, increasing by 46.86% compared to that of BF samples. Furthermore, the pictures of BF and MBF before and after microorganism immobilization test were showed in Figure S2. Compared with BF samples, a large amount of biomass attached to the MBF surface. Figure 7 showed that biofilm grown on the BF sample (Figure 7A) was sparse and less dense, as opposed to that on the MBF surface (Figure 7B-D), which was relatively dense and homogeneous. In addition, SEM images showed that MBF was surrounded by EPS (Figure 8A), whereas EPS around BF surface was relatively little (Figure 8B). The introduction of hydrophilic groups and the increase of surface roughness could promote the initial adhesion of bacteria and accelerate the adsorption of EPS onto the MBF surface during the biofilm formation process, providing better environment for microorganism growth

32

. Generally, most of bacteria in the biofilm are typically

rod-shaped or round-shaped, and about 1-4 µm in length and 0.5-2 µm in diameter, making it difficult to contact BF monofilament (13 µm in diameter)

33, 34

. Therefore,

there is little probability for microorganisms to firmly attach to fiber surface. Considering this situation, we presented a hypothesis that hydrophilic substances like extracellular polymeric substances (EPS) preferentially interlock basalt fibers, allowing microorganisms densely distributed (Figure 9). Thus, surface modification of



Page 14 of 42

BF in terms of roughness and hydrophilicity could provide alternative to conventional biofilm carriers used in wastewater treatment.

CONCLUSION In this study, polyacrylamide/epoxy/nano-SiO2 coating was successfully grafted on basalt fiber surface. The modified basalt fiber was used to be biofilm carrier (MBF). Based on SEM analysis, MBF sample presented rough surface, while based on XPS and FTIR analysis, it demonstrated that -CONH- and -OH groups were successfully introduced onto basalt fibers. The E. coli K12 adhesion and sludge immobilization tests showed that the adhesion rate constant (k) of MBF carriers was about 1.44 times higher than that of BF carriers. Correspondingly, IRM increased by 46.86%, compared to that of BF samples. The optical microscope and SEM images showed that MBF samples were densely surrounded by EPS and microorganisms, showing that microorganisms preferred to adhesion onto a hydrophilic and rough carrier

surface.

Therefore,

surface

modification

of

BF

through

polyacrylamide/epoxy/nano-SiO2 coating could present an alternative to conventional biofilm carrier.

ASSOCIATED CONTENT Supporting Information Synthetic route of polyacrylamide/epoxy/nano-SiO2 coating; The recipe for the preparation of polyacrylamide/epoxy/nano-SiO2 coating; Water contact angle data of BF and MBF samples; Pictures of BF and MBF after microorganism immobilization tests.


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AUTHOR’S INFORMATION Corresponding Author: Prof. Zhiren Wu E-mail: [email protected] (Zhiren Wu); [email protected] (Xiangtong Zhou)

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the National Key R&D Program of China (Grant No. 2016YFE0126400),

China

Postdoctoral

Science

Foundation

(Grant

No.

2016M600377), Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No. 1701057B), National Natural Science Foundation of China (Grant No. 41602260) and Beijing Natural Science Foundation (Grant No. 8164066).

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TABLES Table 1 k and IRM data of BF and MBF samples Samples

k (×10-9 mL/cell·h)

IRM (%)

BF

0.92±0.01

148.71±1.81

MBF

1.33±0.01

218.40±7.02



Figure captions Figure 1 Scheme for surface modification of BF Figure 2 Weight loss of BF and MBF samples in H2SO4 or NaOH solution Figure 3 Water contact angle change images of BF and MBF samples Figure 4 EDS spectra and SEM images of BF (A) and MBF (B) samples Figure 5 FTIR spectra of BF and MBF samples Figure 6 XPS survey and high-resolution spectra obtained from BF and MBF samples: (A) survey spectra of BF and MBF, (B) N 1s spectra of MBF, (C) C 1s spectra of BF, (D) C 1s spectra of MBF, (E) Si 2p spectra of BF and (F) Si 2p spectra of MBF Figure 7 The microscope images of BF (A) and MBF (B-D) after microorganism immobilization test Figure 8 SEM images of BF (A) and MBF (B) after microorganism immobilization test Figure 9 A hypothesized model of biofilm formation on the MBF surface


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Figure 1 Scheme for surface modification of BF



Figure 2 Weight loss of BF and MBF samples in H2SO4 or NaOH solution


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Figure 3 Water contact angle change images of BF and MBF samples



Figure 4 EDS spectra and SEM images of BF (A) and MBF (B) samples


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Figure 5 FTIR spectra of BF and MBF samples



Figure 6 XPS survey and high-resolution spectra obtained from BF and MBF samples: (A) survey spectra of BF and MBF, (B) N 1s spectra of MBF, (C) C 1s spectra of BF, (D) C 1s spectra of MBF, (E) Si 2p spectra of BF and (F) Si 2p spectra of MBF


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Figure 7 The microscope images of BF (A) and MBF (B-D) after microorganism immobilization test



Figure 8 SEM images of BF (A) and MBF (B) after microorganism immobilization test


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Figure 9 A hypothesized model of biofilm formation on the MBF surface



For Table of Contents Use Only

Abstract Graphic Synopsis: Environmentally friendly inorganic basalt fibers were surface-modified by introducing hydrophilic groups to facilitate biofilm formation in wastewater treatment.


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Figure 1 Scheme for surface modification of BF 120x98mm (220 x 220 DPI)



Weight loss of BF and MBF samples in H2SO4 or NaOH solution 106x156mm (220 x 220 DPI)


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Water contact angle change images of BF and MBF samples 146x53mm (220 x 220 DPI)



Figure 4 EDS spectra and SEM images of BF (A) and MBF (B) samples 146x210mm (220 x 220 DPI)


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Figure 5 FTIR spectra of BF and MBF samples 274x234mm (96 x 96 DPI)



Figure 6 XPS survey and high-resolution spectra obtained from BF and MBF samples: (A) survey spectra of BF and MBF, (B) N 1s spectra of MBF, (C) C 1s spectra of BF, (D) C 1s spectra of MBF, (E) Si 2p spectra of BF and (F) Si 2p spectra of MBF 146x156mm (220 x 220 DPI)


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Figure 7 The microscope images of BF (A) and MBF (B-D) after microorganism immobilization test 146x120mm (220 x 220 DPI)



Figure 8 SEM images of BF (A) and MBF (B) after microorganism immobilization test

146x54mm (220 x 220 DPI)


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Figure 9 A hypothesized model of biofilm formation on the MBF surface 146x58mm (220 x 220 DPI)



The bio-affinity of MBF was significantly improved due to introduction of many hydrophilic groups onto BF surfaces, which were subsequently proved to facilitate biofilm formation. 119x122mm (220 x 220 DPI)


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Inorganic

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