Ultralong Hydroxyapatite Nanowire-Based Filter Paper for High

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Ultralong Hydroxyapatite Nanowire-Based Filter Paper for HighPerformance Water Purification Qiang-Qiang Zhang,†,‡ Ying-Jie Zhu,*,†,‡ Jin Wu,† Yue-Ting Shao,†,‡ An-Yong Cai,†,‡ and Li-Ying Dong*,† †

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/18/19. For personal use only.

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: A new kind of environmentally friendly filter paper based on ultralong hydroxyapatite nanowires (HAPNWs) and cellulose fibers (CFs) with excellent filtration and adsorption properties has been developed for the application in high-performance water purification. The use of polyamidoamine-epichlorohydrin (PAE) resin increases the wet mechanical strength of the as-prepared HAPNW/CF filter paper. The addition of CFs enhances the mechanical strength of the HAPNW/CF filter paper. Owing to the porous structure and superhydrophilicity of the as-prepared HAPNW/CF filter paper, the pure water flux is as high as 287.28 L m−2 h−1 bar−1 under cross-flow conditions, which is about 3200 times higher than that of the cellulose fiber paper with addition of PAE. More importantly, the as-prepared HAPNW/CF filter paper shows superior performance in the removal of TiO2 nanoparticles (>98.61%) and bacteria (up to 100%) in water by the size exclusion and blocking effect. In addition, the HAPNW/CF filter paper also exhibits high adsorption capacities for methyl blue (273.97 mg g−1) and Pb2+ ions (508.16 mg g−1). The adsorption mechanism of the HAPNW/CF filter paper is investigated. The as-prepared environmentally friendly HAPNW/CF filter paper with both excellent filtration and adsorption properties has promising application in high-performance water purification to tackle the worldwide water scarcity problem. KEYWORDS: nanowires, hydroxyapatite, nanostructured materials, cellulose, filter paper, water purification

1. INTRODUCTION

microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, reverse osmosis membrane, and so on. In the past few years, natural plant cellulose fibers (CFs) have shown great potential in water treatment because of advantages such as low cost, excellent biocompatibility and biodegradability, high strength, and sustainability.9 For example, Wu et al. prepared an amino-functionalized cellulose membrane with a tensile strength to adsorb methyl blue (MB) in aqueous solution with a high water flux.10 Heydarifard et al. developed a foam-formed cellulose filter paper for drinking water treatment, and the cellulose foam paper showed a stable wet strength performance and excellent antimicrobial properties.11 In addition, the cellulose fiber paper could effectively reject nanoparticles and oil, along with a specific adsorption capacity on heavy-metal ions in water.12,13 Thus, plant cellulose fibers are ideal materials for the preparation of

Water is the origin of life, and the development of nature and human society is inseparable from water. Although about 72% of the earth surface is covered by water, the available freshwater resources are less than 2.5%.1 Due to the imbalance of spatial distribution of water and the difference in science and technology, over 2 million people die each year from diseases caused by water quality problems.2 In recent years, various advanced water treatment technologies have been developed to meet the enormous demand for clean water, such as seawater desalination,3,4 sewage water treatment,5 and drinking water treatment.6,7 Compared with the traditional water treatment methods, including the biological treatment, coagulation/ flocculation sedimentation, electrochemical oxidation, and activated carbon adsorption, the separation membrane technology has many advantages for water treatment, such as convenient operation, low requirements for processing equipment, high separation efficiency, and low chemical sludge effluent.8 According to the pore diameter and the intercepting ability, the separation membrane can be classified into the © XXXX American Chemical Society

Received: November 25, 2018 Accepted: January 8, 2019

A

DOI: 10.1021/acsami.8b20703 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces environmentally friendly filter paper sheets with low cost, high strength, and high efficiency. Hydroxyapatite (HAP) is the main inorganic component of vertebrate hard tissues such as bones and teeth.14 HAP materials have the advantages of high biocompatibility, good biological activity, nontoxicity, high thermal stability, etc. Therefore, HAP materials are considered to be ideal biomaterials in many biomedical fields, including bone defect repair,15 tissue engineering,16 drug delivery,17 and gene delivery.18 The application of HAP-nanostructured materials in water treatment is derived from the fact that Ca2+ and OH− ions in HAP can exchange with heavy-metal ions (for example, Pb2+ ions) and anions (such as F− ions) and have high adsorption capacities for pollutants. The adsorption mechanisms of HAP materials include electrostatic adsorption, ion exchange, complexion mechanism, partial dissolution, and reprecipitation.19−21 Ultralong hydroxyapatite nanowires (HAPNWs) have advantages such as high flexibility and biocompatibility; thus, they are promising for the application in water treatment. The Zhu group developed the calcium oleate precursor solvothermal method for the synthesis of HAPNWs, and the as-prepared HAPNWs had high performance in the treatment of wastewater containing various organic pollutants. The HAPNWs could be recycled and reused owing to their excellent resistance to high temperature and fire.22 Li et al. reported highly efficient multifunctional Ag3PO4-loaded HAP nanowires for water treatment.23 He et al. adopted HAP nanowires for the treatment of fluoride-contaminated water to remove fluoride ions in water.24,25 Herein, we have designed and fabricated ultralong HAP nanowire/cellulose fiber (HAPNW/CF) filter paper sheets with different HAPNW weight ratios and successfully used them as the filter paper for water treatment. Both hydroxyapatite and cellulose are biodegradable materials and are promising for the application in water treatment. This new kind of HAPNW/CF filter paper is environmentally friendly and has a high biocompatibility, especially suitable for the water purification application. The method reported in this work is capable of the scaled-up production, which is promising for commercialization. The effects of the HAPNW percentage in the HAPNW/CF filter paper on the mechanical strength, pore size distribution, porosity, water flux, and hydrophilicity are investigated. In addition, the performances of the HAPNW/CF filter paper in the removal of nanoparticles, bacteria, dyes, and heavy-metal ions in water are also studied. Owing to the porous structure and superhydrophilicity of the as-prepared HAPNW/CF filter paper, the pure water flux (PWF) can reach as high as 287.28 L m−2 h−1 bar−1 under cross-flow conditions, which is about 3200 times higher than that of the cellulose fiber filter paper with addition of polyamidoamine-epichlorohydrin (PAE). More importantly, the as-prepared HAPNW/CF filter paper shows superior performance in the removal of TiO2 nanoparticles (>98.61%) and bacteria (up to 100%) in water by the size exclusion and blocking effect. In addition, the HAPNW/CF filter paper has high adsorption capacities for methyl blue (273.97 mg g−1) and Pb2+ ions (508.16 mg g−1). The as-prepared environmentally friendly HAPNW/CF filter paper with both excellent filtration and adsorption properties has promising application in practical water treatment.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Calcium chloride (CaCl2), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), copper chloride dihydrate (CuCl2·2H2O), lead chloride (PbCl2), and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium oleate and methyl blue (MB) were obtained from Aladdin Industrial Corporation. Titanium dioxide (TiO2) nanoparticles, silicon dioxide (SiO2) nanoparticles, and bovine serum albumin (BSA) were obtained from Shanghai Titan Scientific Co., Ltd. Polyamidoamineepichlorohydrin (PAE) resin was obtained from Shandong Tiancheng Chemical Co., Ltd. Cellulose fibers (CFs, jute pulp) were commercially available. All chemicals were used as received without further purification. Deionized water was used in all related experiments. 2.2. Synthesis of Ultralong HAP Nanowires (HAPNWs). HAPNWs were synthesized according to a modified sodium oleate precursor solvothermal method previously reported by this research group.26 In brief, sodium oleate (400.0 g) was dissolved in deionized water (4 L) under stirring, and an aqueous solution (1.5 L) containing CaCl2 (44.0 g) was added to the above sodium oleate aqueous solution under stirring. Then, an aqueous solution (1.5 L) containing NaH2PO4·2H2O (56.0 g) was added to the above suspension under stirring. Finally, the reaction mixture was transferred into a stainless steel autoclave with a volume of 10 L, sealed, heated to 200 °C, and maintained at that temperature for 36 h. The product was washed with ethanol and deionized water three times, respectively. The obtained HAPNWs were dispersed in deionized water for further use. 2.3. Preparation of the HAPNW/CF Filter Paper. To fabricate the HAPNW/CF filter paper, different weights of HAPNWs and CFs were added to deionized water (1 L) under mechanical stirring. The HAPNW weight ratios in the HAPNW/CF filter paper were 0, 20, 40, 60, 80, and 100%, and the total weight of the HAPNWs and CFs was 2.000 g. Subsequently, 0.060 g of PAE resin was added into the mixture under stirring for 30 min. The HAPNW/CF filter paper was prepared using the above aqueous suspension by the vacuum-assisted filtration and subsequently dried at 95 °C for 30 min. The resulting HAPNW/CF filter paper was immersed in deionized water for 1 h to remove residual PAE resin and dried at 60 °C for 2 h. The asprepared HAPNW/CF filter paper is labeled as (λ) HAPNWs, where λ stands for the HAPNW weight ratios in the filter HAPNW/CF paper not considering the PAE resin. 2.4. Characterization of the As-Prepared HAPNW/CF Filter Paper. The as-prepared HAPNW/CF filter paper sheets were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan), X-ray diffraction (XRD, Cu Kα radiation, λ = 1.54178 Å, Rigaku D/max 2550V, Japan), Fourier transform infrared (FTIR) spectroscopy (FTIR-7600, Australia), X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi), and thermogravimetric analysis (TGA) (STA 409/PC, NETZSCH, Germany) with a heating rate of 10 °C min−1 in air. A universal testing machine (DRK-101A, China) was used to estimate the mechanical properties of the dry and wet filter paper sheets with a gauge length of 10 mm at a loading rate of 2 mm min−1. The wet HAPNW/CF filter paper was obtained by immersing the dried HAPNW/CF filter paper in deionized water for 1 min, and then excess water was removed using a fixed weight roller. The Brunauer− Emmett−Teller (BET) specific surface area of the HAPNW/CF filter paper was obtained by a surface area and porosity analyzer (TriStar II 3020, Micromeritics). The hydrophilicity of the as-prepared HAPNW/CF filter paper was measured by an optical contact angle system (SL200B). The water uptake measurement of the as-prepared HAPNW/CF filter paper was carried out according to ISO 535:2014: the paper and board determination of water absorptivenessthe Cobb method. The water uptake Wu (weight percentage of water in the wet HAPNW/CF filter paper), porosity P, and bulk B were calculated by the following equations27

Wu = B

Ww − Wd × 100% Ww DOI: 10.1021/acsami.8b20703 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Preparation of the HAPNW/CF Filter Paper

P=

Ww − Wd × 100% ρV

B=

V × 100% Wd

was tested three times. The rejection percentage was calculated by the following formula C p yz ij zz × 100% R n = jjj1 − z j Cf z{ k Np yz ij zz × 100% R b = jjj1 − z j Nf z{ k

where Ww is the weight of the wet HAPNW/CF filter paper; Wd is the weight of the dry HAPNW/CF filter paper; ρ is the density of water at 25 °C (1 g cm−3); and V is the volume of the HAPNW/CF filter paper (cm3). 2.5. Filtration Performance of the HAPNW/CF Filter Paper. Based on the bubble point method, the pore size distribution of the HAPNW/CF filter paper in the wet state was measured with a pore size analyzer (3H-2000PB, China). The pore size distribution of the HAPNW/CF filter paper was also measured using a mercury porosimeter (AutoPore IV 9510, Micromeritics). The filtration performance of the as-prepared HAPNW/CF filter paper was evaluated by a cross-flow low-pressure flat membrane test equipment (TYLG-18, China) with an effective diameter of 7.8 cm. The schematic diagram of the filtering process is shown in Scheme S1 in the Supporting Information. Typically, to reach a steady state, the HAPNW/CF filter paper was prefiltrated for 0.5 h at 2.0 bar before each test. The pure water flux (PWF) was measured at a pressure of 0.5−2.0 bar, which was calculated by the following equation PWF =

where R n and Rb are the rejection percentages of the TiO 2 nanoparticles and bacteria in water, respectively; Cp and Cf are the concentrations (ppm) of the permeation and feeding solutions, respectively; Np and Nf are the total number of colonies (CFU) of the permeation solution and feeding solution, respectively. 2.6. Adsorption Performance of the HAPNW/CF Filter Paper. Methyl blue (MB) and Cu2+ and Pb2+ heavy-metal ions were used to evaluate the adsorption properties of the HAPNW/CF filter paper with 60 wt % HAPNWs. The square-shaped HAPNW/CF filter paper sheets (size 1.5 cm × 1.5 cm) were immersed in MB solutions with different concentrations for 48 h, and the concentrations of the MB solutions after adsorption were measured by UV−vis absorption spectroscopy at a wavelength of 590 nm. Dye adsorption tests were also carried out in the filtration mode at a pressure of 0.5 bar. The MB solution (100 mL) with an initial concentration of 20 ppm was filtered through the HAPNW/CF filter paper with a diameter of 7.8 cm. The rectangular HAPNW/CF filter paper sheets (length 3 cm × width 1.5 cm) were immersed in heavymetal ion solutions with various concentrations (100−500 ppm) for 48 h, and then the residual concentrations of heavy-metal ions in the solutions were analyzed by an inductively coupled plasma (ICP)optical emission spectrometer (JY 2000-2, Horiba Jobin Yvon, France). Aqueous solution (50 mL) of heavy-metal ions with an initial concentration of 20 ppm was filtered through the HAPNW/CF filter paper with a diameter of 7.8 cm at a pressure of 0.1 bar to measure the dynamic adsorption properties. 2.7. Antifouling Performance of the HAPNW/CF Filter Paper. The antifouling performance of the HAPNW/CF filter paper with 60 wt % HAPNWs was evaluated using BSA as a model protein28−30 for the filtration tests at 1.0 bar. First, pure water was filtrated by the HAPNW/CF filter paper in the cross-flow test equipment and the average PWF was recorded as Jw1. Second, the BSA feeding solution with a concentration of 500 ppm was poured into the device. The BSA concentration of the permeation solution

Q A×T

where PWF is the pure water flux (L m−2 h−1), Q is the volume of permeated water (L), A is the effective area of the filter paper (m−2), and T is the filtration time (h). The filtration experiments were conducted under 1.0 bar at room temperature. TiO2 nanoparticles with an average size of 40 nm and unsterilized drinking water were used to test the rejection performance of the HAPNW/CF filter paper. The concentrations of TiO2 nanoparticles in permeation solutions were analyzed by UV−vis absorption spectroscopy (UV-2300II). Bacterial filtration experiments were performed using the untreated municipal tap water directly. Colony-forming unit in water before and after filtration was tested by 3M Petrifilm count plates. The number of bacteria trapped on the surface of the HAPNW/CF filter paper was tested by the LuriaBertani (LB) solid medium. The HAPNW/CF filter paper after filtration was attached to the LB solid medium for 30 s to transfer the surface bacteria to the medium, and each HAPNW/CF filter paper C


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ACS Applied Materials & Interfaces was measured using UV−vis absorption spectroscopy at 280 nm, and the water flux was recorded as Jp. Third, the HAPNW/CF filter paper was washed and immersed in deionized water for 30 min after BSA filtration. Then, the PWF was measured again and recorded as Jw2. The rejection of BSA (RBSA), flux recovery ratio (FRR), reversible fouling ratio (Rr), irreversible fouling ratio (Rir), and the total fouling ratio (Rt) were calculated using the following equations31 ij Cp yzz zz × 100% RBSA = jjjj1 − j Cf zz k {

FRR =

Rr =

R ir =

Jw2 Jw1

× 100%

Jw2 − Jp Jw1

× 100%

Jw1 − Jw2 Jw1

× 100%

Jp zy ji zz × 100% R t = jjjj1 − zz j z J w1 k { where Cp and Cf are the BSA concentrations (ppm) of the permeation solution and feeding solution, respectively.

3. RESULTS AND DISCUSSION Scheme 1 illustrates the process for the preparation of the HAPNW/CF filter paper. First, the HAPNWs and CFs were uniformly mixed to obtain a stable aqueous suspension. Then, the PAE resin was added to increase the dry and wet strengths of the HAPNW/CF filter paper. After suction filtration, HAPNWs and CFs self-assembled into a freestanding HAPNW/CF filter paper. Aging is performed at 95 °C to remove residual water from the HAPNW/CF filter paper and to accelerate the dehydration polymerization of PAE. Digital images of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios are shown in Figure 1a−f. The as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios and with a diameter of 20 cm exhibit a uniform white color. In this study, the HAPNW/ CF filter paper sheets were prepared with different HAPNW weight ratios (0, 20, 40, 60, 80, and 100 wt %). Many filtration membranes reported in the literature were small with diameters of a few centimeters. However, the preparation of large filter paper can broaden its application field and reduce the preparation cost. Encouragingly, we have successfully prepared a large HAPNW/CF filter paper sheet with a length of 44 cm and a width of 31 cm, which has a great potential for the application in water treatment (Figure 1g). Figure 2 shows SEM images of the as-prepared HAPNW/ CF filter paper sheets with different HAPNW weight ratios. The as-prepared HAPNWs have diameters of about 10 nm and lengths of up to hundreds of micrometers, and the HAPNWs self-assemble along the longitudinal direction to form HAP nanowire bundles in many cases (Figure 2f and Figure S1b in the Supporting Information). In the HAPNW/CF filter paper, the HAPNWs and CFs interweave with each other to form a porous network structure, as shown in Figure 2b−e. Through the interweaving of the two kinds of fibers (Figure S1a in the Supporting Information), the loose and porous networked structure of the HAPNW/CF filter paper is formed.

Figure 1. Digital images of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) 0 wt % (100 wt % CFs); (b) 20 wt %; (c) 40 wt %; (d) 60 wt %; (e) 80 wt %; (f) 100 wt %; and (g) a large HAPNW/CF filter paper sheet with 60 wt % HAPNWs.

The FTIR spectra of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios were measured. As shown in Figure 3a, a broad absorption peak at around 3406 cm−1 is derived from the adsorbed water in the filter paper. The absorption peak at around 3567 cm−1 is derived from the hydroxyl group (−OH) in HANPWs, and this peak overlaps with the broad absorption band of the adsorbed water in the HAPNW/CF filter paper. The characteristic absorption peaks at 1097, 1031, and 962 cm−1 correspond to the vibration and stretching modes of P−O, and D


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ACS Applied Materials & Interfaces

The XPS survey spectra of the as-prepared HAPNW/CF filter paper sheets with 60 wt % HAPNWs and 3 wt % PAE or without PAE are shown in Figure 3c. The peaks at 133.01, 285.09, 347.08, and 531.47 eV for both samples correspond to P 2p, C 1s, Ca 2p, and O 1s, mainly originating from the cellulose fibers and ultralong hydroxyapatite nanowires.36 In comparison to the HAPNW/CF filter paper without PAE, a new characteristic peak emerges, i.e., N 1s (400.08 eV), which further confirms the existence of the PAE resin in the asprepared HAPNW/CF filter paper.37 The HAPNW weight ratios in the as-prepared HAPNW/CF filter paper were investigated by TGA (Figure 3d). The weight loss between 300 and 500 °C is derived from the decomposition of the CFs and PAE resin. After heating to 750 °C in air, the residue weights of the as-prepared HAPNW/ CF filter paper sheets are 15.53, 41.60, 56.87, 76.82, and 99.03% for different HAPNW weight ratios of 20, 40, 60, 80, and 100%, respectively. More precisely, the theoretical weight ratios of HAPNWs in the as-prepared HAPNW/CF filter paper sheets including the PAE resin are 19.41, 38.83, 58.25, 77.67, and 97.09%, which are similar to the weight ratios of HAPNWs in the as-prepared HAPNW/CF filter paper sheets measured by TGA. Cellulose fibers are the primary raw material for papermaking, and the wet strength agent is a common industrial additive in paper. Polyamidoamine-epichlorohydrin (PAE) resin is a water-soluble, cationic thermosetting resin, which is an excellent wet strength agent in the paper industry.38,39 Compared to many traditional wet strength agents, PAE has advantages such as nontoxicity, wide application range, small addition amount, and superior reinforcing effect. In addition, PAE is a nonformaldehyde wet strength agent, which may be used in the filter paper, cosmetic paper, food wrapping paper, and medical paper.37 The molecular structure of the PAE resin is shown in Figure S2a in the Supporting Information. There are two main enhancement mechanisms of PAE:40,41 (a) as a cationic resin, PAE can adsorb on the surface of negatively charged cellulose fibers by the electrostatic interaction and then self-polymerize during heating to form a cross-linked network structure on the surface of fibers; (b) PAE can crosslink with the reactive groups on the fiber surface (−COOH, etc.) so that PAE and cellulose fibers can form a network structure together, which will restrict the movement of fibers in the wet state. In addition, due to the formation of a physical adsorption layer, the paper has a specific water resistance, thereby achieving a mechanical strengthening effect in the wet state. The experimental results indicate that the tensile strength of the dried HAPNW/CF filter paper is higher than that of the wet HAPNW/CF filter paper. The effect of the added amount of PAE on the wet tensile strength of the HAPNW/CF filter paper with 60 wt % HAPNWs was investigated. As shown in Figure S2b in the Supporting Information, the highest wet tensile strength of the HAPNW/CF filter paper is observed when the addition amount of PAE is 3 wt %. Therefore, to reduce the experimental variables, the added amount of PAE is fixed at 3 wt % in the subsequent experiments. The tensile strengths of the HAPNW/CF filter paper sheets with different weight ratios of HAPNWs were measured, and the experimental results are shown in Figure 4. As shown in Figure 4a,b, the tensile strength decreases with increasing amount of HAPNWs for both dried and wet HAPNW/CF filter papers. The tensile strengths of the dried HAPNW/CF

Figure 2. SEM images of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) 0 wt % (100 wt % CFs); (b) 20 wt %; (c) 40 wt %; (d) 60 wt %; (e) 80 wt %; and (f) 100 wt %.

the absorption peaks at 603 and 561 cm−1 are attributed to the v4 bending mode of O−P−O in the phosphate group.32−34 In addition, the absorption peaks at 2911 and 898 cm−1 are derived from C−H in CFs, and the absorption peak at 1637 cm−1 corresponds to the −OH group. The absorption peaks at 1457 and 1417 cm−1 are attributed to CO and O−CH3 in lignin, respectively.35 The XRD patterns of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios are shown in Figure 3b. The diffraction peaks at 2θ = 10.8, 21.8, 29.0, 31.8, 32.9, 39.8, 48.6, 50.5, 51.3, and 59.9° correspond to the (100), (200), (210), (211), (300), (310), (320), (321), (410), and (420) crystal planes of hydroxyapatite (JCPDS No. 09-0432), respectively. The XRD pattern of the CF membrane shows broad diffraction peaks at around 15 and 22°. As the proportion of HAPNWs increases, the intensities of diffraction peaks of hydroxyapatite in the XRD pattern increase, and the peaks of CFs decline, indicating the increase of the HAP content in the as-prepared HAPNW/CF filter paper. E


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Figure 3. Characterization of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) FTIR spectra; (b) XRD patterns; (c) XPS survey spectra; and (d) TGA curves.

filter paper sheets are 41.32, 18.14, 10.02, 6.05, and 1.58 MPa, respectively, for the HAPNW weight ratios ranging from 20 to 100 wt % (Figure 4a). Compared to the dried HAPNW/CF filter paper sheets, the tensile strengths of the wet HAPNW/ CF filter paper sheets decrease, which are 18.70, 10.42, 8.05, 2.09, and 0.71 MPa (Figure 4b). The addition of the PAE resin allows the HAPNW/CF filter paper to maintain a relatively high mechanical strength in the wet state, which is a required feature for the water filter paper. Compared to other cellulosic materials, the commercial filter paper, and carbon materials, the as-prepared HAPNW/CF filter paper has a higher tensile strength (Table S1 in the Supporting Information). The wet tensile strength can be maintained to be 80.3% of that of the dried HAPNW/CF filter paper when the HAPNW weight ratio is 60 wt %. The pure water flux (PWF) change as a function of working pressure of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios is shown in Figure 5. Generally, the PWF relates with the porosity of the filter paper. As for the CF filter paper, the PWF is only 4.87 L m−2 h−1 under a pressure of 1 bar (Figure S3 in the Supporting Information). When the PAE resin is added into the CF filter paper, the PWF drops to only 0.09 L m−2 h−1 because of the very low porosity (28.96%, Table 1). When the HAPNW weight ratio increases from 20 to 80 wt %, the PWF of the HAPNW/CF filter paper increases from 18.12 to 287.28 L m−2 h−1 at a pressure of 1 bar. The PWF of the HAPNW/CF filter paper significantly increases with increasing working pressure (Figure 5). The enhanced PWF is mainly ascribed to the

highly porous structure and superhydrophilicity of the HAPNW/CF filter paper. In this work, we measured the porosity and bulk to evaluate the porous structure of the as-prepared HAPNW/CF filter paper. The porosity of the HAPNW/CF filter paper increases from 28.96 to 66.20%, and the bulk increases from 1.27 to 2.44 cm3 g−1 when the weight ratio of HAPNWs increases from 0 to 100% (Figure 6a and Table 1). In addition, while the basis weight reduces from 79.33 to 64.76 g m−2, the thickness of the HAPNW/CF filter paper increases from 101 to 158 μm when the weight ratio of HAPNWs increases from 0 to 100% (Table 1). The nitrogen adsorption/desorption isotherms of the HAPNW/CF filter paper sheets with different HAPNW weight ratios are shown in Figure 6b. Accordingly, the BET specific surface area of the HAPNW/CF filter paper increases significantly from 13.2 m2 g−1 for 20 wt % HAPNWs to 63.7 m2 g−1 for 100 wt % HAPNWs, showing the same trend as the porosity and bulk. Although the PAE resin can increase the hydrophobicity of the CF filter paper (Figure S4 in the Supporting Information), the hydrophilicity and water uptake of the HAPNW/CF filter paper are significantly enhanced as the weight ratio of HAPNWs increases. As shown in Figure 6c, the average water contact angle reduces from 88.40 to 0° when the weight ratio of HAPNWs increases from 0 to 100%. Accordingly, due to the increase in porosity and hydrophilicity, the water uptake of the HAPNW/CF filter paper increases when the weight ratio of HAPNWs increases from 0 to 100%, implying that the water absorption capacity of the HAPNW/CF filter paper is enhanced. As the weight ratio of HAPNWs increases, the F


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Figure 4. Mechanical properties of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) stress− strain curves of the dry HAPNW/CF filter paper sheets and (b) stress−strain curves of the wet HAPNW/CF filter paper sheets.

Figure 6. Properties of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) porosity and bulk vs HAPNW content; (b) nitrogen adsorption/desorption isotherms and BET specific surface area (inset); and (c) contact angle and water uptake vs HAPNW content.

% HAPNWs has a porosity of 50.80%, a bulk of 1.92 cm3 g−1, and a PWF of 205.11 L m−2 h−1 at a working pressure of 1 bar. The pore size distribution of the as-prepared HAPNW/CF filter paper was evaluated by the bubble point method.42,43 As shown in Figure 7a−d, the average pore diameters of the asprepared HAPNW/CF filter paper are 117.5, 135.2, 139.0, and 144.3 nm, respectively, for the HAPNW weight ratios ranging from 20 to 80 wt %. The average pore size of the cellulose fiber filter paper with 3 wt % PAE is only 17.2 nm, as shown in Figure S5 in the Supporting Information. By the addition of HAPNWs, the pore size of the filter paper can be significantly increased. It is worth noting that the pore size distribution of the HAPNW/CF filter paper is relatively narrow. We further verified the pore size distribution of the HAPNW/CF filter paper by the mercury intrusion method. As shown in Figure S6 in the Supporting Information, the most probable pore size of the HAPNW/CF filter paper with 60 wt % HAPNWs is 127.0 nm, which is similar to the result obtained by the bubble point method. Apparently, the average pore size of the HAPNW/CF filter paper can be adjusted by the HAPNW weight ratio. The

Figure 5. Pure water flux vs working pressure of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios.

Table 1. Physical Properties of the As-Prepared HAPNW/ CF Filter Paper Sheets with Different HAPNW Weight Ratios filter paper

basis weight (g m−2)

thickness (μm)

bulk (cm3 g−1)

porosity (%)

CFs 20% HAPNW/CF 40% HAPNW/CF 60% HAPNW/CF 80% HAPNW/CF 100% HAPNWs

79.33 71.07 67.69 66.80 65.60 64.76

101 101 114 128 132 158

1.27 1.42 1.69 1.92 2.01 2.44

28.96 40.62 50.23 50.80 58.08 66.20

porosity and PWF of the HAPNW/CF filter paper increase, and the hydrophilicity of the HAPNW/CF filter paper is obviously improved. The HAPNW/CF filter paper with 60 wt G


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Figure 7. Pore size distribution curves of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) 20 wt %; (b) 40 wt %; (c) 60 wt %; and (d) 80 wt %.

the average pore size of the HAPNW/CF filter paper with 60 wt % HAPNWs is 139.0 nm (Figure 7c), which is smaller than that of the aggregated TiO2 particles. On the other hand, during the filtration process, TiO2 particles form a dense layer on the surface of the HAPNW/CF filter paper, blocking other tiny particles passing through the HAPNW/CF filter paper, which is consistent with the results reported in the literature.12,44 Monodisperse SiO2 nanoparticles with a hydrodynamic size of 185.2 nm (Figure S7c,d in the Supporting Information) were also used for the filtration tests. As shown in Figure 8d, as the weight ratio of HAPNWs increases, the rejection rate of the HAPNW/CF filter paper for SiO2 nanoparticles increases from 75.65 to 98.04%. Since the sizes of SiO2 nanoparticles are close to the pore sizes of the HAPNW/CF filter paper, the rejection rate for SiO2 nanoparticles is relatively low; however, the rejection rate is significantly enhanced at a high concentration of SiO2 nanoparticles owing to the blocking effect. Figure 8e shows the recyclability performance of the HAPNW/CF filter paper for the rejection of TiO2 nanoparticles. The high rejection rate of the HAPNW/CF filter paper for TiO2 nanoparticles can be well preserved during five cycles, showing good recyclability of the HAPNW/CF filter paper. To further investigate the performance of the HAPNW/CF filter paper in the rejection of bacteria in water, we used drinking water for the experiments. As shown in Figure 9a and Figure S8 in the Supporting Information, with the increase of the weight ratio of the HAPNWs in the HAPNW/CF filter paper, the rejection rate of bacteria in water increases from 91.16 to 100%. Furthermore, we applied the HAPNW/CF filter paper to the surface of the LB solid medium for 30 s, and then the LB solid medium was cultured for 24 h. Interestingly, it is found that the rejection rate of the HAPNW/CF filter

interaction between the fibers plays a vital role in determining the pore size distribution. Cellulose fibers are interconnected by a large number of hydrogen bonds. The HAPNWs and CFs intertwine with each other by the van der Waals force and hydrogen bonding at the interface. As the weight ratio of HAPNWs increases, the interaction between the cellulose fibers is reduced, leading to an increase in the average pore size. To evaluate the filtration performance of the as-prepared HAPNW/CF filter paper, TiO2 nanoparticles with an average particle size of about 40 nm were used to simulate the impurity particles in water. The rejection rates of the HAPNW/CF filter paper sheets with different HAPNW weight ratios for TiO2 nanoparticles at a concentration of 250 ppm are higher than 99.86% (Figure 8a). Taking into account the mechanical strength, pure water flow rate, and hydrophilicity, the HAPNW/CF filter paper with 60 wt % HAPNWs was selected for further investigation. As shown in Figure 8b, the rejection rates of the HAPNW/CF filter paper with 60 wt % HAPNWs for TiO2 nanoparticles at different concentrations are higher than 98.61%. To evaluate the water filtration stability of the HAPNW/CF filter paper, the water filtration experiments were carried out for 4 h. Figure 8c shows the water fluxes and rejection rates of TiO2 nanoparticles at a concentration of 250 ppm during a time period of 4 h. It is found that the water flux decreases to some extent, together with a slight reduction of rejection rate for TiO2 nanoparticles. This may be explained by the formation of a layer of TiO2 nanoparticles on the surface of the HAPNW/CF filter paper. On the one hand, TiO2 nanoparticles are unstable in water and agglomerate to form larger particles with a hydrodynamic size of 321 nm (Figure S7a,b in the Supporting Information). As mentioned above, H


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Figure 8. Particle rejection performance of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) rejection rates of TiO2 nanoparticles at a concentration of 250 ppm using the HAPNW/CF filter paper with different HAPNW weight ratios; (b) rejection rates of TiO2 nanoparticles at different concentrations using the HAPNW/CF filter paper with 60 wt % HAPNWs; (c) water fluxes and rejection rates of TiO2 nanoparticles at a concentration of 250 ppm during a time period of 4 h using the HAPNW/CF filter paper with 60 wt % HAPNWs; (d) rejection rates of SiO2 nanoparticles at different concentrations using the HAPNW/CF filter paper with 60 wt % HAPNWs; and (e) recyclability performance of the HAPNW/CF filter paper for rejection of TiO2 nanoparticles. All of the experiments were carried out at a working pressure of 1 bar.

value of practical applications of the HAPNW/CF filter paper. As shown in Figure 9d, the HAPNW/CF filter paper has a porous network structure constructed by interwoven ultralong hydroxyapatite nanowires and cellulose fibers. The HAPNWs fill in the pores formed by cellulose fibers. In addition, since the hydrogen bonding between HAPNWs is weaker than that between CFs, the porosity of the HAPNW/CF filter paper increases with increasing weight ratio of HAPNWs, resulting in a significant increase in pure water flux (Figure 5). Furthermore, owing to the size exclusion and blocking effect, the porous structure formed by interwoven ultralong HAP nanowire is capable of effectively trapping particles and bacteria in water. These properties indicate that the asprepared HAPNW/CF filter paper is promising for the application in high-performance water filtration and water purification.

paper for bacteria is related to the proportion of large pores (>220 nm). From Figure 9b, one can see that the percentages of large pores (>220 nm) measured by the bubble point method are 1.68, 1.30, 0.49, and 0.53%, corresponding to 20, 40, 60, and 80 wt % HAPNWs in the HAPNW/CF filter paper. In addition, the number of colonies trapped on the surface of the HAPNW/CF filter paper are 127, 182, 361, and 229 CFU (Figure 9b and Figure S9 in the Supporting Information). Notably, when the weight ratio of HAPNWs is 80%, the rejection rate of bacteria in water is as high as 100%. In addition to the size exclusion of the surface of the HAPNW/ CF filter paper, the porous structure inside the filter paper also plays an essential role in the interception of bacteria. The recycling performance of the HAPNW/CF filter paper for the rejection of bacteria in drinking water is shown in Figure 9c. In five cycles of experiments, the HAPNW/CF filter paper is able to maintain a high rejection rate of bacteria, indicating the high I


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Figure 9. Bacterium rejection performance of the as-prepared HAPNW/CF filter paper sheets with different HAPNW weight ratios: (a) rejection rate of the HAPNW/CF filter paper sheets with different HAPNW weight ratios for bacteria in drinking water; (b) the number of bacterium colonies trapped on the surface of the HAPNW/CF filter paper after filtration, and the cumulative distribution of pores larger than 220 nm in the HAPNW/CF filter paper measured by the bubble point method; (c) recyclability performance of the HAPNW/CF filter paper for the rejection of bacteria in drinking water; and (d) schematic illustration of the structure and filtration process of the as-prepared HAPNW/CF filter paper. All of the experiments were carried out at a working pressure of 1 bar.

HAPNW/CF filter paper with 60 wt % HAPNWs for the adsorption tests. To evaluate the adsorption performance of the HAPNW/CF filter paper with 60 wt % HAPNWs for the dye, methyl blue was used as the adsorbate. The adsorption isotherms of the HAPNW/CF filter paper for methyl blue are shown in Figure 11a. The adsorption capacity (Qe) of the HAPNW/CF filter paper increases with the increase of equilibrium concentration Ce, and the apparent color change of the HAPNW/CF filter paper shown in the inset also indicates effective adsorption of methyl blue. The Langmuir and Freundlich models were used to describe the adsorption of methyl blue by the HAPNW/CF filter paper in this study. Parameters of fitting lines based on the Langmuir and Freundlich equations are listed in Table S2 in the Supporting Information. The linear regression coefficient (R2) of the Langmuir equation is higher than that of the Freundlich equation, which means that the Langmuir adsorption model provides a better explanation for methyl blue adsorption of the HAPNW/CF filter paper. The maximum adsorption capacity (Qmax) obtained from the Langmuir equation is 273.97 mg g−1 (Figure 11b), which is close to the powder adsorbent materials and activated carbons.45,46 The reason for the high adsorption capacity is that the high specific surface area and porous network structure of the HAPNW/CF filter paper provide more sites for the adsorption of methyl blue. Since Ca2+ ions can exchange with other ions, hydroxyapatite materials were investigated for adsorption of heavymetal ions in water treatment.21,47,48 To evaluate the adsorption performance of the HAPNW/CF filter paper for heavy-metal ions in water, aqueous solutions of Cu2+ and Pb2+ ions with different concentrations were prepared for the

In addition to the filtration efficiency, the adsorption ability of dyes and heavy-metal ions in water is also vital for the filter paper.10,13 As shown in Figure 10, the zeta (ζ) potentials of

Figure 10. ζ potentials of CFs, ultralong HAP nanowires, and HAPNW/CF filter paper sheets with different HAPNW weight ratios under neutral pH conditions.

HAPNWs and CFs are −5.30 and −21.09 mV under neutral pH conditions, respectively, and PAE has a positive charge. The addition of PAE makes the ζ potential of the HAPNW/ CF filter paper more positive, which is more advantageous for adsorbing the negatively charged dye molecules. Furthermore, owing to the hydrogen bonding between the amine groups of methyl blue and hydroxyl groups on the surface of cellulose fibers and hydroxyapatite nanowires, the as-prepared HAPNW/CF filter paper is able to adsorb methyl blue effectively. Considering the mechanical strength, pure water flux, and rejection efficiency discussed above, we selected the J


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Figure 11. Adsorption performance of the as-prepared HAPNW/CF filter paper with 60 wt % HAPNWs: (a) adsorption isotherms of methyl blue; (b) Langmuir model for methyl blue adsorption; (c) adsorption amounts of Cu2+ or Pb2+ ions in the CuCl2 or PbCl2 aqueous solutions with different initial concentrations; (d) XRD patterns of the HAPNW/CF filter paper with 60 wt % HAPNWs after adsorption tests for heavy-metal ions at an initial concentration of 500 ppm for 48 h; (e) absorption spectra of the feeding and filtrate solutions, and digital images of the HAPNW/ CF filter paper with 60 wt % HAPNWs (inset) before and after filtering MB solution with an initial concentration of 20 ppm; and (f) adsorption performance of the HAPNW/CF filter paper with 60 wt % HAPNWs for Cu2+ or Pb2+ ions with a concentration of 20 ppm under the filtration mode.

Figure 11d. The diffraction peaks of pyromorphite (Pb5(PO4)3Cl, JCPDS No. 19-0701) appear after the adsorption of Pb2+ ions in the HAPNW/CF filter paper, indicating the formation of pyromorphite. However, the XRD pattern of the HAPNW/CF filter paper has no obvious change after the adsorption of Cu2+ ions. Ca2+ ion release amounts of the HAPNW/CF filter paper with 60 wt % HAPNWs after adsorbing Cu2+ or Pb2+ ions in CuCl2 or PbCl2 aqueous solutions with different concentrations were measured by ICP (Figure S10 in the Supporting Information). It is found that the adsorption of Pb2+ ions by the HAPNW/CF filter paper is accompanied by the obvious release of Ca2+ ions in the aqueous solution. In addition, the weight changes of the HAPNW/CF filter paper after adsorption were measured. The experimental results indicate that the weight of the HAPNW/ CF filter paper has no obvious change after the adsorption of

experiments. The adsorption amounts of the HAPNW/CF filter paper with 60 wt % HAPNWs for Cu2+ and Pb2+ ions at different initial concentrations are shown in Figure 11c. The adsorption amount of the HAPNW/CF filter paper for Cu2+ ions at an initial concentration of 500 ppm is measured to be 60.42 mg g−1. The adsorption amount of the HAPNW/CF filter paper for Pb2+ ions approximately linearly increases with the initial concentration of Pb2+ ions. The Qmax of the HAPNW/CF filter paper for Pb2+ ions is as high as 508.16 mg g−1 when the initial concentration of Pb2+ ions is 500 ppm. The highest adsorption efficiency of the HAPNW/CF filter paper for Pb2+ ions is 98.9%. To further illustrate the adsorption mechanism for heavymetal ions, the HAPNW/CF filter paper and solution after adsorption were analyzed. The corresponding XRD pattern of the HAPNW/CF filter paper after adsorption is shown in K


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decreases from 206.6 to 119.8 L m−2 h−1 within 1 h, and the rejection rate of BSA is 17.21%. After the fouled HAPNW/CF filter paper is immersed in pure water for 1 h, the pure water flux increases from 119.8 to 159.8 L m−2 h−1. As shown in Figure 12b, the flux recovery ratio (FRR), total flux decline ratio (Rt), reversible fouling ratio (Rr), and irreversible fouling ratio (Rir) of the as-prepared HAPNW/CF filter paper with 60 wt % HAPNWs are 67.19, 42.10, 9.29, and 32.81%, respectively. The flux recovery ratio (FRR) and reversible fouling ratio (Rr) are close to graphene oxide and poly(ether sulfone) membranes reported in the literature.28,30,51 The main reason is that protein molecules are easily trapped and constrained in the pores of the HAPNW/ CF filter paper.

Cu2+ ions. However, the weight of the HAPNW/CF filter paper increases after the adsorption of Pb2+ ions and with increasing initial Pb2+ ion concentration (Figure S11 in the Supporting Information). These experimental results indicate that the adsorption process is dominated by the ion adsorption mechanism in the case of Cu2+ ions. In contrast, the ion exchange occurs to form a stable crystal phase of pyromorphite in the case of Pb2+ ions.20 In the filtration mode, the HAPNW/CF filter paper also shows good adsorption performance for dyes and heavy-metal ions. After 100 mL of MB solution with an initial concentration of 20 ppm is filtered through the HAPNW/ CF filter paper with 60 wt % HAPNWs, the surface of the HAPNW/CF paper exhibits a dark blue color, and the adsorption efficiency for MB reaches 96.39% (Figure 11e). Furthermore, the as-prepared HAPNW/CF filter paper has a good adsorption capacity for the low-concentration solution of Cu2+ or Pb2+ ions under dynamic conditions, and the removal efficiency can reach as high as 100% when the initial ion concentration is 20 ppm (Figure 11f). Typically, the good hydrophilicity, electrical neutrality, and free from hydrogen-bonding donors are prerequisites for the superior antifouling performance of the filter paper.49,50 Although the added PAE increases the hydrophobicity of the HAPNW/CF filter paper, HAPNWs reduce the contact angle to below 5° (Figure 6c and Figure S4 in the Supporting Information). Figure 12a shows the water flux of the HAPNW/CF filter paper with 60 wt % HAPNWs before and after the BSA filtration. When the feeding solution changes from pure water to BSA aqueous solution, the water flux

4. CONCLUSIONS In this work, a new kind of environmentally friendly HAPNW/ CF filter paper based on ultralong hydroxyapatite nanowires has been developed and investigated for the application in high-performance water purification. The as-prepared HAPNW/CF filter paper is composed of ultralong hydroxyapatite nanowires, cellulose fibers, and a small amount of polyamidoamine-epichlorohydrin resin, and the composition and porous structure of the HAPNW/CF filter paper can be controlled. The experimental results indicate that the pure water flux of the HAPNW/CF filter paper significantly increases with increasing HAPNW weight ratio and working pressure, and the pure water flux of the HAPNW/CF filter paper increases from 18.12 to 287.28 L m−2 h−1 when the HAPNW weight ratio increases from 20 to 80 wt % at a pressure of 1 bar. The high pure water flux (287.28 L m−2 h−1 bar−1) of the HAPNW/CF filter paper is about 3200 times higher than that of the cellulose fiber paper with addition of PAE. More importantly, the as-prepared HAPNW/CF filter paper shows superior performance in the removal of TiO2 nanoparticles (>98.61%) and bacteria (up to 100%) in water by the size exclusion and blocking effect. In addition, the HAPNW/CF filter paper has high adsorption capacities for methyl blue (273.97 mg g−1) and Pb2+ ions (508.16 mg g−1). The as-prepared environmentally friendly HAPNW/CF filter paper with both excellent filtration and adsorption properties has promising application in high-performance water purification and clean water regeneration to tackle the worldwide water scarcity problem.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20703. Schematic diagram of the cross-flow low-pressure flat membrane test equipment; SEM images of the asprepared HAPNW/CF filter paper sheets with different weight ratios of HAPNWs; the molecular structure of PAE; the curve of wet tensile strength vs PAE weight percentage of the HAPNW/CF filter paper with 60 wt % HAPNWs; comparison of tensile strengths of different filtration membranes; pure water flux of the cellulose fiber filter paper with and without 3 wt % PAE; water contact angles of the cellulose fiber filter paper without and with 3 wt % PAE; pore size distribution curve of the cellulose fiber filter paper with 3 wt % PAE; pore size distribution curve of the HAPNW/CF filter paper with

Figure 12. Antifouling performance of the HAPNW/CF filter paper with 60 wt % HAPNWs: (a) water flux of the HAPNW/CF filter paper and UV−vis absorption spectra of BSA solution (inset) before and after filtration of the BSA solution (500 ppm); (b) antifouling indexes of the HAPNW/CF filter paper. L


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60 wt % HAPNWs; particle size distributions and TEM images of TiO2 and SiO2 nanoparticles in water; digital images of total number of colonies in the permeation solution filtrated by the HAPNW/CF filter paper sheets with different HAPNW weight ratios; digital images of culturing bacteria on the surface of the HAPNW/CF filter paper sheets with different HAPNW weight ratios using the LB solid medium; fitting parameters based on the Langmuir and Freundlich models; Ca2+ ion release amount of the HAPNW/CF filter paper with 60 wt % HAPNWs after adsorbing Cu2+ or Pb2+ ions in CuCl2 or PbCl2 aqueous solution; weight change of the HAPNW/ CF filter paper with 60 wt % HAPNWs after adsorbing Cu2+ or Pb2+ ions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 0086-21-52412616. Fax: 0086-21-52413122 (Y.-J.Z.). *E-mail: [email protected] (L.-Y.D.). ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51702342 and 21875277) and the Industrialization Innovation Project of Shanghai Institute of Ceramics, Chinese Academy of Sciences (Y71ZCC1C0G), is gratefully acknowledged.

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Ultralong Hydroxyapatite Nanowire-Based Filter Paper for High

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