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Facile fabrication of inorganic polymer microspheres as adsorbents for removing heavy metal ions
Materials Research Bulletin 113 (2019) 202–208
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Facile fabrication of inorganic polymer microspheres as adsorbents for removing heavy metal ions
T
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Qing Tanga, Kaituo Wangb, , Jianfeng Sua, Yingqian Shena, Sijie Yanga, Yuanyuan Gea, ⁎ Xuemin Cuia, a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Guangxi, Nanning, 530004, China b School of Resources, Environment and Materials, Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University, Guangxi, Nanning, 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inorganic polymer Microsphere Suspension dispersion solidification Heavy metal ions
This work reports a simple preparation process of slag-based inorganic polymer microspheres (IPMs) using the suspension dispersion solidification (SDS) method. The SEM results showed that the microspheres exhibit good spherical shape with controllable diameter range within 40–1000 μm. The size of the microspheres is closely related to the temperature and dispersion rate of the suspension. Based on the results obtained by varying the temperature and dispersion rate of the suspension, it was found that the best spherical shape and the highest yield are obtained at 80°C and 800 r/min. The H2O/Na2O molar ratio of the geopolymer paste affected the BET surface area of the microspheres. The microspheres exhibited outstanding adsorption capacity towards the removal of Pb2+ (310.84 mg/g), Cu2+ (47.71 mg/g) and Cd2+ (36.26 mg/g), with their high adsorption rate attributed to their large surface area. Selectivity adsorption experiments show that the IPMs have high selectivity for Pb2+.
1. Introduction Geopolymer (also called inorganic polymer) is a type of a 3-dimensional structure gel material with SiO4 and AlO4 tetrahedra [1–4]. Due to rapid hardening, acid and alkali resistance, low toxicity and low cost, the inorganic polymer material has been usually used in construction cement and as refractory material [5,6]. In addition, since the inorganic polymer possesses a zeolite-like porous structure [7] and remain stable in water with good mechanical strength, it has attracted much research attention for the investigation of its adsorbent properties and hazardous ion solidification. Commercial adsorbents require properties such as high selectivity, large surface area, and low cost [8]. The currently most widely used adsorbent, active carbon, has disadvantages such as limited adsorption capacity and high cost [8,9]. Researchers are now seeking low-cost adsorbents with high adsorption capacity and have mostly focused on spherical adsorbents [10,11]. Currently, most studies of geopolymer adsorbents have focused on powders, not spherical particles, and since these geopolymer powders cannot be directly used in packed beds or be easily recycled for the removal of heavy metal ions from aqueous solutions, spherical geopolymer adsorbents are more important than
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powdery adsorbents. More importantly, the spherical adsorbents can be used in filtration columns for the continuous treatment of industrial wastewater [12–16]. Currently, spherical adsorbent preparation can be divided into physical and chemical methods. The main physical methods are the agitation method and the spray granulation method, whereas suspension polymerization is the main chemical method [17]. However, there are few reports in the literature on spherical geopolymers, which is mainly because the geopolymer spheres with small diameter (less than 1 cm) are not suitable for the preparation of common injection molding. In fact, geopolymer millimeter-sized spheres with the diameter of 2–3 mm have been reported by Ge [18] and were used in Cu2+ removal in continuous water treatment, showing an adsorption capacity of 52.63 mg/g. While a smaller sphere size can enhance the adsorption capacity, microsphere preparation is a rather difficult task. The use of a one-step foaming method with controllable diameter could solve this problem. Tang [19] prepared geopolymer spheres in polyethylene glycol (PEG) and transformed them into zeolite P by hydrothermal treatment. In another work, Tang [20] prepared inorganic polymer millimeter-sized spheres with high compressive strength and diameter of 2–3 mm using the suspension solidification method (SSM), which
Corresponding authors. E-mail addresses: [email protected] (K. Wang), [email protected] (X. Cui).
https://doi.org/10.1016/j.materresbull.2019.02.009 Received 14 May 2018; Received in revised form 29 January 2019; Accepted 7 February 2019 Available online 08 February 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 113 (2019) 202–208
Q. Tang, et al.
2.3. Adsorption experiments
could be used as substrate in the oil cracking industry. The suspension dispersion solidification (SDS) method has usually been used to prepare microspheres with a diameter of 10–5000 μm, such as polymer beads and styrene spheres [17,21,22]. The diameter could be controlled by adjusting process parameters such as stirring speed, viscosity of continuous phase and height of agitator blade [23]. To date, there have been no reports in the literature on the preparation of microspheres using inorganic polymer via the SDS approach. Thus, herein we report the facile synthesis of inorganic polymer prepared via SDS for the adsorptive removal of heavy metal ions. An inorganic polymer slurry was crushed into small droplets by an agitator in silica oil. Through a dissolution depolymerization polymerization reaction process [24,25], the droplets solidified rapidly at high temperature, inorganic polymer microspheres (IPMs) were obtained and special arrangements were made in the equipment to prepare microspheres with different diameters. The obtained microspheres showed a good spherical structure, a large BET surface area and a good adsorption capacity of heavy metal ions.
0.07 g of microspheres adsorbents (prepared by 1.7 M waterglass, at 80 °C and 800 r/min) were separately added into four Erlenmeyer flasks with CPb2+ = 200 mg/L, CCu2+ = 200 mg/L, CCd2+ = 200 mg/L and mixed solution (containing CPb2+ = 200 mg/L, CCu2+ = 200 mg/L, and CCd2+ = 200 mg/L) at pH = 5. The Erlenmeyer flasks were placed into a shaker at 25 °C with the shaking speed of 250 shakes/min. After 24 h, the samples were analyzed for Pb2+, Cu2+ and Cd2+ concentration using ICP. 3. Results and discussion 3.1. Microsphere fabrication mechanism The crushing of the geopolymer slurry in silicon oil into droplets was mainly affected by the agitator blade and viscous shear force. As shown in Fig. 2a, the geopolymer slurry collided with the agitator blade to break the slurry. The slurry was injected into the silicone oil and drawn into the vortex with the silicone oil movement. The geopolymer slurry collided with the high-speed rotating agitator blades and was broken into droplets of different sizes. The droplets then moved out of the dispersed center area, moving around and into the settlement area as the silicon oil moved. As shown in Fig. 2b, the geopolymer slurry was broken by the viscous shearing force produced by the silicone oil. Because of the speed gradient of the silicone oil in the radial direction of the agitator blade, the viscous shear force was produced with the flow of silicon oil, and the large-volume slurry was elongated and broken into small droplets. Thus, in the preparation of IPMs via the SDS approach, the dispersion rate of the agitator blades and the temperature of the silicon oil strongly influence the particle size, distribution and degree of sphericity of the IPMs.
2. Materials and methods 2.1. Materials and characterization In this study, the slag was supplied by Chengde Company, Beihai of China. The content of the CaO, MgO, SiO2 and Al2O3 chemical components were 36.91 mass%, 9.27 mass%, 33.98 mass%, and 15.22 mass %, respectively. Water glass was supplied by Guangxi Chunxu chemical company, China. The module (M = nSiO2/nNa2O) of water glass (industrial grade) was 3.31, and the solid content was 38.7 wt.%. Analytical grade sodium hydroxide, copper nitrate, cadmium nitrate and lead nitrate were used. Dimethyl silicone oil was of industrial grade with the kinematic viscosity of 2000 mm2/s. The geopolymer morphology was observed using a Hitachi S-3400 N scanning electron microscope (SEM). The elemental content was analyzed using energy dispersive analysis (EDS, EDAX PV8200) while crystalline phases were identified using a Rigaku MiniFlex 600 X-ray diffractometer (XRD) with a CuKα source operating at 40 kV and 15 mA. The scanning range 2θ was from 10° to 70°, with a step size of 0.02° and a rate of 10°/min. The specific surface area (SSA) was analyzed using a Micromeritic Gemini 2390 surface area and porosity analyzer. The roundness and sphericity of the microspheres were observed using an Olympus SZ2-ILST light microscope. Inductively coupled plasma optical emission spectrometer (ICP, Thermo Fisher ICAP 6300) was used to detect the concentration of heavy metal ions in solution, with a radiofrequency power of 1150 W, carrier gas flux of 0.5 L/min, peristaltic pump of 50 rpm and auxiliary air flux of 0.5 L/min. The shaker (QYC200) was supplied by Shanghai Fuma experimental facilities company.
3.2. Effect of stir rate The geopolymer slurry is dispersed into droplets using the agitator blade, and the particle size distribution is directly related to the stirring speed. It can be seen from the experimental results presented in Fig. 3 that the proportion of IPM (280˜800 μm) is increased with the increase in the stirring speed from 600 r/min to 900 r/min. When the stirring speed was 600 r/min and 700 r/min, the IPM (1000˜1600 μm) accounted for 67.15% and 56.55%, respectively. At the stirring speed of 900 r/min, the IPM (280˜450 μm) fraction reached 47.44%, and the IPM (450˜800 μm) accounted for 42.11%. The target IPM (450˜800 μm) had the highest proportion (45.57%) at the stirring speed of 800 r/min. Comparison of the different stirring speeds from 600 r/min to 900 r/ min showed that the particle size distribution was bimodal [26], namely, the fractions of IPM (280˜800 μm) and IPM (1000˜1600 μm) were greater and that of IPM (800˜1000 μm) was smaller. This result was because in this experiment, the viscosity of the silicon oil is high, and it is easy to produce uneven ruptures [27]. Since the aim of this experiment was to prepare an IPM particle size distribution of 450˜800 μm, based on the obtained particle size distribution and sphericity analysis results, it is most appropriate to choose the stirring speed of 800 r/min.
2.2. Preparation of microsphere adsorbents Slag and water glass were uniformly mixed according to mass ratio (slag:water glass = 2.3, modulus of water glass (SiO2/Na2O): 1.5 M, 1.7 M, and 1.9 M) to form a slurry that was poured into a syringe. Silica oil used as the dispersion medium was heated to 60–80 °C and stirred using a dispersion machine. The slurry was injected into silica oil and broken into small droplets under the sheer force of silica oil. The agitation speed was controlled at 600–900 r/min to obtain droplets with different diameters, which solidified rapidly at high temperature. Fully solidified droplets were collected, and silica oil on the surface was washed followed by drying. Finally, the spheres were sintered at 450 °C for 6 h to clean off any remaining silica oil on the surface and in the pores of the spheres. The preparation process is depicted in Fig. 1. Prior to adsorption, the spheres were washed to be neutral using distilled water.
3.3. Effect of temperature Alkali activation of slag is divided into two processes of depolymerization and polycondensation, and the temperature influence on the slag-based geopolymer reaction is extremely obvious [28]. When the silicone oil temperature is lower, the slurry droplets are still not fully cured in the settling to the bottom of the reactor. At this time, the slurry droplets are not solidified and extruded, and agglomerate at the bottom of the reactor. As the silicone oil temperature increases, the slurry 203
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Fig. 1. Preparation scheme of porous inorganic spheres with the SDS method.
Fig. 2. Crushing schematic of droplets.
Fig. 3. Influence of stir speed on particle size distribution.
Fig. 4. Influence of temperature on particle size distribution.
droplets can be rapidly solidified before falling to the bottom of the reactor and maintain a good degree of sphericity. It can be seen from Fig. 4 that the IPM content of the small size rises with the increase in the silicone oil temperature, and the IPM (450˜800 μm) content was 45.57% at 80 °C. Fig. 5 shows the sphericity of IPMs at different silicone oil temperatures. At 60 °C, the sphericity of the IPMs was poor, and bonding was observed. At 70 °C, the sphericity of the IPMs was obviously improved, but the bonding phenomenon was still present. At
80 °C, a good sphericity of the IPMs could be obtained, and there was little adhesion because of the slag-based geopolymer fast setting [28]. Although a high temperature is beneficial for the reduction of the bonding of IPM, the use of excessively high temperature is not suitable. It is well known that alkali activation of slag can lead to simultaneously occurring depolymerization and polycondensation under high temperature [28]. At excessively high temperature, the dissolved substance quickly wrapped the slag particles, inhibiting the further reaction of the slag particles. Therefore, the elevated reaction temperature does not 204
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Fig. 5. Influence of temperature on the sphericity of IPMs (a: 60 °C, b: 70 °C, c: 80 °C).
dissolution-depolymerization-polymerization reaction with rapid solidification. A wide dispersion peak is observed near 20˜40° for unsintered spheres, identifying the amorphous structure without any crystallinity [31]. Compared to the unsintered spheres, the amorphous phase in the sintered spheres did not change much after sintering at 450 °C, indicating the feasibility of removing redundant silica oil by sintering. As shown in Fig. 7, the microspheres were good spherical. No change in the shape and sphericity was observed after sintering at 450 °C. Silicone oil is difficult to clean because of its high viscosity. Comparing Fig. 7(A) and (B), it can be seen that the surface of the IPM is wrapped in silicone oil that has not been cleaned, which affects the adsorption properties of the IPMs. Meanwhile, after sintering at 450 °C, it is obvious that the surface of the IPM becomes more smooth, which is beneficial for decreasing the diffusion resistance of the ions in the adsorbent and increasing the adsorption rate [32]. Fig. 6. XRD patterns of slag, unsintered and sintered IPM.
3.5. Surface area and pore structure analysis
favor the complete reaction of raw material and cannot fully display the intensity [29,30]. This study adopts 80 °C as the best preparation temperature.
Surface area and pore structure are closely related to the adsorption effect of the adsorbent because a large surface area provides more adsorption active sites for heavy metal ion interaction with the adsorbent [33]. Furthermore, a porous structure helps to decrease the diffusion resistance for heavy metal ions. The BET surface area results for IPM are shown in Fig. 8(A). Upon increasing the modulus of water glass (SiO2/ Na2O), the surface area initially increased and then decreased. The surface area increased to the maximum value of 87.74 m2/g for the SiO2/Na2O molar ratio of 1.7 for IPM. This effect improved the adsorption rate, while other geopolymer adsorbents presented lower
3.4. XRD and SEM analysis The XRD pattern of the slag presented in Fig. 6 shows a wide hump near 30°, demonstrating the amorphous structure of the slag along with a few stable crystal structures. The amorphous structure of the slag demonstrates that it could be easily activated and could follow the
Fig. 7. Image of an IPM (A: unsintered, B: sintered). 205
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Fig. 8. (A) Influence of modulus of water glass on the surface area of IPMs (at 80 °C and 800 r/min); (B) Pore size distribution of the IPMs (modulus of water glass = 1.7).
specific surface areas of 40˜66 m2/g [16,18,34]. The pore diameter of the IPMs was approximately 8.9 nm (Fig. 8(B)), and showed comparatively narrow unimodal distribution with uniform pore size. As can be seen from Table 1, the relationship between the particle size and the surface area decreased gradually with the increase of particle size, but the change trend was small.
Table 1 The surface area data of microspheres with different particle sizes. Particle size
Surface area (m2/g)
280-450 μm 450-800 μm 800-1000 μm 1000-1600 μm
93.17 87.74 86.19 85.62
3.6. Adsorption studies To study the adsorption for different heavy metal ions, the adsorption capacities for the separate adsorption of Pb2+, Cu2+ and Cd2+ were determined and are given in Table 2. The adsorption capacities for Pb, Cu and Cd are 310.84 mg/ g, 47.71 mg/g and 36.26 mg/g, respectively. This result demonstrated that the adsorption ability of porous inorganic polymer microspheres is very different for different ions (Fig. 9). To study the selectivity adsorption performance of porous inorganic polymer microspheres, adsorption experiments were performed at the mixture solution containing Pb2+, Cu2+, and Cd2+. The results showed that the adsorption amount of the porous inorganic polymer microspheres for Pb2+, Cu2+, and Cd2+ are 58.57 mg/g, 20.00 mg/g, and 10.00 mg/g, respectively. The competitive coefficient (a) is used to represent the selective ability and is calculated as
Table 2 Adsorption capacity comparison for Pb2+, Cu2+ and Cd2+. Ion
Pb2+
Cu2+
Cd2+
Adsorption capacity (mg/g)
310.84
47.71
36.26
Competitive coefficient ai =
Table 3 Adsorption amounts and competitive coefficients of ions in mixed solutions. Pb2+
Cu2+
Cd2+
Adsorption amount Competitive coefficient ai
58.57 0.6463
20.00 0.2772
10.00 0.0765
(1)
where, Xi—adsorption amount for component i, mg; C0i—initial concentration of component i, mg/L The competitive coefficients are calculated as shown in Table 3: By comparing the competitive coefficients of the three ions, it can be seen that the competitive adsorption performance is Pb2+ > Cu2+ > Cd2+. This result demonstrated that the adsorption ability of porous inorganic polymer microspheres is very different for different ions as seen in Table 3 and is consistent with literature data [35,36]. The adsorption ability for different heavy metal ions is related to the radius of the hydrate ions, free energy of hydration and activities of the ions [16]. Ions with a smaller radius of hydration, lower free energy of hydration and higher activity are adsorbed more easily. For these three ions, the order of radius of hydration is Cd2+ > Cu2+ > Pb2+. The order of the free energy of hydration values is Cu2+ > Cd2+ > Pb2+. The activities are in the order of Pb2+ > Cu2+ > Cd2+. Thus, the selectivity adsorption performance of porous inorganic polymer microspheres for these three ions is in the order of Pb2+ > Cu2+ > Cd2+. To investigate the diffusion of heavy metal ions in the IPM, the distribution of the heavy metal elements in the IPMs were analyzed by
Fig. 9. Comparison of IPM in adsorption performance (CCu2+ = 200 mg/L, CCd2+ = 200 mg/L, and CPb2+ = 200 mg/L, 0.07 g/100 mL, pH = 5, 25 °C, 24 h).
Ion
Xi / C0i n ∑ j = 1 Xj / C0j
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Fig. 10. Distribution of heavy metal elements on the internal cross section.
References
EDS. As shown in Fig. 10, a large amount of heavy metal ions spread to the interior of the IPM. Because the IPM exposes more active sites and surface area, the IPM internal diffusion resistance is small so that the heavy metal ions can easily diffuse and be adsorbed.
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4. Conclusions In this work, slag and water glass are used as the main raw material. IPMs were prepared by using the suspension dispersion solidification (SDS) method. The diameter and sphericity of the IPMs were controlled well. This control was achieved by adjusting the modulus of water glass to obtain a greater surface area and a large amount of pore structures. The microstructure and phase of the IPM were analyzed and characterized by the SEM/EDS, BET and XRD methods. The adsorption properties of the IPMs for Pb2+, Cu2+ and Cd2+ were studied, and it was found that the adsorption ability of IPMs is very different for different ions. Thus, the selectivity adsorption performance of IPMs is best for Pb2+. EDS analysis shows that heavy metal ions diffused easily through the pores and reached the internal region of the adsorbents, with the adsorption active sites fully utilized. The current approach, based on a cost-effective, novel and facile synthesis procedure for an inorganic polymer adsorbent, can be effectively applied for the removal of Pb2+ ions from wastewater at industrial level.
Acknowledgements This work was supported by the Chinese Natural Science Fund (grant: 51772055 and 51561135012), the Guangxi Natural Science Fund (grant: 2018GXNSFBA281064 and 2016GXNSFGA380003) and Innovation Project of Guangxi Graduate Education (No. YCBZ2017007)
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208
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Facile fabrication of inorganic polymer microspheres as adsorbents for removing heavy metal ions
T
⁎
Qing Tanga, Kaituo Wangb, , Jianfeng Sua, Yingqian Shena, Sijie Yanga, Yuanyuan Gea, ⁎ Xuemin Cuia, a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Guangxi, Nanning, 530004, China b School of Resources, Environment and Materials, Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University, Guangxi, Nanning, 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inorganic polymer Microsphere Suspension dispersion solidification Heavy metal ions
This work reports a simple preparation process of slag-based inorganic polymer microspheres (IPMs) using the suspension dispersion solidification (SDS) method. The SEM results showed that the microspheres exhibit good spherical shape with controllable diameter range within 40–1000 μm. The size of the microspheres is closely related to the temperature and dispersion rate of the suspension. Based on the results obtained by varying the temperature and dispersion rate of the suspension, it was found that the best spherical shape and the highest yield are obtained at 80°C and 800 r/min. The H2O/Na2O molar ratio of the geopolymer paste affected the BET surface area of the microspheres. The microspheres exhibited outstanding adsorption capacity towards the removal of Pb2+ (310.84 mg/g), Cu2+ (47.71 mg/g) and Cd2+ (36.26 mg/g), with their high adsorption rate attributed to their large surface area. Selectivity adsorption experiments show that the IPMs have high selectivity for Pb2+.
1. Introduction Geopolymer (also called inorganic polymer) is a type of a 3-dimensional structure gel material with SiO4 and AlO4 tetrahedra [1–4]. Due to rapid hardening, acid and alkali resistance, low toxicity and low cost, the inorganic polymer material has been usually used in construction cement and as refractory material [5,6]. In addition, since the inorganic polymer possesses a zeolite-like porous structure [7] and remain stable in water with good mechanical strength, it has attracted much research attention for the investigation of its adsorbent properties and hazardous ion solidification. Commercial adsorbents require properties such as high selectivity, large surface area, and low cost [8]. The currently most widely used adsorbent, active carbon, has disadvantages such as limited adsorption capacity and high cost [8,9]. Researchers are now seeking low-cost adsorbents with high adsorption capacity and have mostly focused on spherical adsorbents [10,11]. Currently, most studies of geopolymer adsorbents have focused on powders, not spherical particles, and since these geopolymer powders cannot be directly used in packed beds or be easily recycled for the removal of heavy metal ions from aqueous solutions, spherical geopolymer adsorbents are more important than
⁎
powdery adsorbents. More importantly, the spherical adsorbents can be used in filtration columns for the continuous treatment of industrial wastewater [12–16]. Currently, spherical adsorbent preparation can be divided into physical and chemical methods. The main physical methods are the agitation method and the spray granulation method, whereas suspension polymerization is the main chemical method [17]. However, there are few reports in the literature on spherical geopolymers, which is mainly because the geopolymer spheres with small diameter (less than 1 cm) are not suitable for the preparation of common injection molding. In fact, geopolymer millimeter-sized spheres with the diameter of 2–3 mm have been reported by Ge [18] and were used in Cu2+ removal in continuous water treatment, showing an adsorption capacity of 52.63 mg/g. While a smaller sphere size can enhance the adsorption capacity, microsphere preparation is a rather difficult task. The use of a one-step foaming method with controllable diameter could solve this problem. Tang [19] prepared geopolymer spheres in polyethylene glycol (PEG) and transformed them into zeolite P by hydrothermal treatment. In another work, Tang [20] prepared inorganic polymer millimeter-sized spheres with high compressive strength and diameter of 2–3 mm using the suspension solidification method (SSM), which
Corresponding authors. E-mail addresses: [email protected] (K. Wang), [email protected] (X. Cui).
https://doi.org/10.1016/j.materresbull.2019.02.009 Received 14 May 2018; Received in revised form 29 January 2019; Accepted 7 February 2019 Available online 08 February 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 113 (2019) 202–208
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2.3. Adsorption experiments
could be used as substrate in the oil cracking industry. The suspension dispersion solidification (SDS) method has usually been used to prepare microspheres with a diameter of 10–5000 μm, such as polymer beads and styrene spheres [17,21,22]. The diameter could be controlled by adjusting process parameters such as stirring speed, viscosity of continuous phase and height of agitator blade [23]. To date, there have been no reports in the literature on the preparation of microspheres using inorganic polymer via the SDS approach. Thus, herein we report the facile synthesis of inorganic polymer prepared via SDS for the adsorptive removal of heavy metal ions. An inorganic polymer slurry was crushed into small droplets by an agitator in silica oil. Through a dissolution depolymerization polymerization reaction process [24,25], the droplets solidified rapidly at high temperature, inorganic polymer microspheres (IPMs) were obtained and special arrangements were made in the equipment to prepare microspheres with different diameters. The obtained microspheres showed a good spherical structure, a large BET surface area and a good adsorption capacity of heavy metal ions.
0.07 g of microspheres adsorbents (prepared by 1.7 M waterglass, at 80 °C and 800 r/min) were separately added into four Erlenmeyer flasks with CPb2+ = 200 mg/L, CCu2+ = 200 mg/L, CCd2+ = 200 mg/L and mixed solution (containing CPb2+ = 200 mg/L, CCu2+ = 200 mg/L, and CCd2+ = 200 mg/L) at pH = 5. The Erlenmeyer flasks were placed into a shaker at 25 °C with the shaking speed of 250 shakes/min. After 24 h, the samples were analyzed for Pb2+, Cu2+ and Cd2+ concentration using ICP. 3. Results and discussion 3.1. Microsphere fabrication mechanism The crushing of the geopolymer slurry in silicon oil into droplets was mainly affected by the agitator blade and viscous shear force. As shown in Fig. 2a, the geopolymer slurry collided with the agitator blade to break the slurry. The slurry was injected into the silicone oil and drawn into the vortex with the silicone oil movement. The geopolymer slurry collided with the high-speed rotating agitator blades and was broken into droplets of different sizes. The droplets then moved out of the dispersed center area, moving around and into the settlement area as the silicon oil moved. As shown in Fig. 2b, the geopolymer slurry was broken by the viscous shearing force produced by the silicone oil. Because of the speed gradient of the silicone oil in the radial direction of the agitator blade, the viscous shear force was produced with the flow of silicon oil, and the large-volume slurry was elongated and broken into small droplets. Thus, in the preparation of IPMs via the SDS approach, the dispersion rate of the agitator blades and the temperature of the silicon oil strongly influence the particle size, distribution and degree of sphericity of the IPMs.
2. Materials and methods 2.1. Materials and characterization In this study, the slag was supplied by Chengde Company, Beihai of China. The content of the CaO, MgO, SiO2 and Al2O3 chemical components were 36.91 mass%, 9.27 mass%, 33.98 mass%, and 15.22 mass %, respectively. Water glass was supplied by Guangxi Chunxu chemical company, China. The module (M = nSiO2/nNa2O) of water glass (industrial grade) was 3.31, and the solid content was 38.7 wt.%. Analytical grade sodium hydroxide, copper nitrate, cadmium nitrate and lead nitrate were used. Dimethyl silicone oil was of industrial grade with the kinematic viscosity of 2000 mm2/s. The geopolymer morphology was observed using a Hitachi S-3400 N scanning electron microscope (SEM). The elemental content was analyzed using energy dispersive analysis (EDS, EDAX PV8200) while crystalline phases were identified using a Rigaku MiniFlex 600 X-ray diffractometer (XRD) with a CuKα source operating at 40 kV and 15 mA. The scanning range 2θ was from 10° to 70°, with a step size of 0.02° and a rate of 10°/min. The specific surface area (SSA) was analyzed using a Micromeritic Gemini 2390 surface area and porosity analyzer. The roundness and sphericity of the microspheres were observed using an Olympus SZ2-ILST light microscope. Inductively coupled plasma optical emission spectrometer (ICP, Thermo Fisher ICAP 6300) was used to detect the concentration of heavy metal ions in solution, with a radiofrequency power of 1150 W, carrier gas flux of 0.5 L/min, peristaltic pump of 50 rpm and auxiliary air flux of 0.5 L/min. The shaker (QYC200) was supplied by Shanghai Fuma experimental facilities company.
3.2. Effect of stir rate The geopolymer slurry is dispersed into droplets using the agitator blade, and the particle size distribution is directly related to the stirring speed. It can be seen from the experimental results presented in Fig. 3 that the proportion of IPM (280˜800 μm) is increased with the increase in the stirring speed from 600 r/min to 900 r/min. When the stirring speed was 600 r/min and 700 r/min, the IPM (1000˜1600 μm) accounted for 67.15% and 56.55%, respectively. At the stirring speed of 900 r/min, the IPM (280˜450 μm) fraction reached 47.44%, and the IPM (450˜800 μm) accounted for 42.11%. The target IPM (450˜800 μm) had the highest proportion (45.57%) at the stirring speed of 800 r/min. Comparison of the different stirring speeds from 600 r/min to 900 r/ min showed that the particle size distribution was bimodal [26], namely, the fractions of IPM (280˜800 μm) and IPM (1000˜1600 μm) were greater and that of IPM (800˜1000 μm) was smaller. This result was because in this experiment, the viscosity of the silicon oil is high, and it is easy to produce uneven ruptures [27]. Since the aim of this experiment was to prepare an IPM particle size distribution of 450˜800 μm, based on the obtained particle size distribution and sphericity analysis results, it is most appropriate to choose the stirring speed of 800 r/min.
2.2. Preparation of microsphere adsorbents Slag and water glass were uniformly mixed according to mass ratio (slag:water glass = 2.3, modulus of water glass (SiO2/Na2O): 1.5 M, 1.7 M, and 1.9 M) to form a slurry that was poured into a syringe. Silica oil used as the dispersion medium was heated to 60–80 °C and stirred using a dispersion machine. The slurry was injected into silica oil and broken into small droplets under the sheer force of silica oil. The agitation speed was controlled at 600–900 r/min to obtain droplets with different diameters, which solidified rapidly at high temperature. Fully solidified droplets were collected, and silica oil on the surface was washed followed by drying. Finally, the spheres were sintered at 450 °C for 6 h to clean off any remaining silica oil on the surface and in the pores of the spheres. The preparation process is depicted in Fig. 1. Prior to adsorption, the spheres were washed to be neutral using distilled water.
3.3. Effect of temperature Alkali activation of slag is divided into two processes of depolymerization and polycondensation, and the temperature influence on the slag-based geopolymer reaction is extremely obvious [28]. When the silicone oil temperature is lower, the slurry droplets are still not fully cured in the settling to the bottom of the reactor. At this time, the slurry droplets are not solidified and extruded, and agglomerate at the bottom of the reactor. As the silicone oil temperature increases, the slurry 203
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Fig. 1. Preparation scheme of porous inorganic spheres with the SDS method.
Fig. 2. Crushing schematic of droplets.
Fig. 3. Influence of stir speed on particle size distribution.
Fig. 4. Influence of temperature on particle size distribution.
droplets can be rapidly solidified before falling to the bottom of the reactor and maintain a good degree of sphericity. It can be seen from Fig. 4 that the IPM content of the small size rises with the increase in the silicone oil temperature, and the IPM (450˜800 μm) content was 45.57% at 80 °C. Fig. 5 shows the sphericity of IPMs at different silicone oil temperatures. At 60 °C, the sphericity of the IPMs was poor, and bonding was observed. At 70 °C, the sphericity of the IPMs was obviously improved, but the bonding phenomenon was still present. At
80 °C, a good sphericity of the IPMs could be obtained, and there was little adhesion because of the slag-based geopolymer fast setting [28]. Although a high temperature is beneficial for the reduction of the bonding of IPM, the use of excessively high temperature is not suitable. It is well known that alkali activation of slag can lead to simultaneously occurring depolymerization and polycondensation under high temperature [28]. At excessively high temperature, the dissolved substance quickly wrapped the slag particles, inhibiting the further reaction of the slag particles. Therefore, the elevated reaction temperature does not 204
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Fig. 5. Influence of temperature on the sphericity of IPMs (a: 60 °C, b: 70 °C, c: 80 °C).
dissolution-depolymerization-polymerization reaction with rapid solidification. A wide dispersion peak is observed near 20˜40° for unsintered spheres, identifying the amorphous structure without any crystallinity [31]. Compared to the unsintered spheres, the amorphous phase in the sintered spheres did not change much after sintering at 450 °C, indicating the feasibility of removing redundant silica oil by sintering. As shown in Fig. 7, the microspheres were good spherical. No change in the shape and sphericity was observed after sintering at 450 °C. Silicone oil is difficult to clean because of its high viscosity. Comparing Fig. 7(A) and (B), it can be seen that the surface of the IPM is wrapped in silicone oil that has not been cleaned, which affects the adsorption properties of the IPMs. Meanwhile, after sintering at 450 °C, it is obvious that the surface of the IPM becomes more smooth, which is beneficial for decreasing the diffusion resistance of the ions in the adsorbent and increasing the adsorption rate [32]. Fig. 6. XRD patterns of slag, unsintered and sintered IPM.
3.5. Surface area and pore structure analysis
favor the complete reaction of raw material and cannot fully display the intensity [29,30]. This study adopts 80 °C as the best preparation temperature.
Surface area and pore structure are closely related to the adsorption effect of the adsorbent because a large surface area provides more adsorption active sites for heavy metal ion interaction with the adsorbent [33]. Furthermore, a porous structure helps to decrease the diffusion resistance for heavy metal ions. The BET surface area results for IPM are shown in Fig. 8(A). Upon increasing the modulus of water glass (SiO2/ Na2O), the surface area initially increased and then decreased. The surface area increased to the maximum value of 87.74 m2/g for the SiO2/Na2O molar ratio of 1.7 for IPM. This effect improved the adsorption rate, while other geopolymer adsorbents presented lower
3.4. XRD and SEM analysis The XRD pattern of the slag presented in Fig. 6 shows a wide hump near 30°, demonstrating the amorphous structure of the slag along with a few stable crystal structures. The amorphous structure of the slag demonstrates that it could be easily activated and could follow the
Fig. 7. Image of an IPM (A: unsintered, B: sintered). 205
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Fig. 8. (A) Influence of modulus of water glass on the surface area of IPMs (at 80 °C and 800 r/min); (B) Pore size distribution of the IPMs (modulus of water glass = 1.7).
specific surface areas of 40˜66 m2/g [16,18,34]. The pore diameter of the IPMs was approximately 8.9 nm (Fig. 8(B)), and showed comparatively narrow unimodal distribution with uniform pore size. As can be seen from Table 1, the relationship between the particle size and the surface area decreased gradually with the increase of particle size, but the change trend was small.
Table 1 The surface area data of microspheres with different particle sizes. Particle size
Surface area (m2/g)
280-450 μm 450-800 μm 800-1000 μm 1000-1600 μm
93.17 87.74 86.19 85.62
3.6. Adsorption studies To study the adsorption for different heavy metal ions, the adsorption capacities for the separate adsorption of Pb2+, Cu2+ and Cd2+ were determined and are given in Table 2. The adsorption capacities for Pb, Cu and Cd are 310.84 mg/ g, 47.71 mg/g and 36.26 mg/g, respectively. This result demonstrated that the adsorption ability of porous inorganic polymer microspheres is very different for different ions (Fig. 9). To study the selectivity adsorption performance of porous inorganic polymer microspheres, adsorption experiments were performed at the mixture solution containing Pb2+, Cu2+, and Cd2+. The results showed that the adsorption amount of the porous inorganic polymer microspheres for Pb2+, Cu2+, and Cd2+ are 58.57 mg/g, 20.00 mg/g, and 10.00 mg/g, respectively. The competitive coefficient (a) is used to represent the selective ability and is calculated as
Table 2 Adsorption capacity comparison for Pb2+, Cu2+ and Cd2+. Ion
Pb2+
Cu2+
Cd2+
Adsorption capacity (mg/g)
310.84
47.71
36.26
Competitive coefficient ai =
Table 3 Adsorption amounts and competitive coefficients of ions in mixed solutions. Pb2+
Cu2+
Cd2+
Adsorption amount Competitive coefficient ai
58.57 0.6463
20.00 0.2772
10.00 0.0765
(1)
where, Xi—adsorption amount for component i, mg; C0i—initial concentration of component i, mg/L The competitive coefficients are calculated as shown in Table 3: By comparing the competitive coefficients of the three ions, it can be seen that the competitive adsorption performance is Pb2+ > Cu2+ > Cd2+. This result demonstrated that the adsorption ability of porous inorganic polymer microspheres is very different for different ions as seen in Table 3 and is consistent with literature data [35,36]. The adsorption ability for different heavy metal ions is related to the radius of the hydrate ions, free energy of hydration and activities of the ions [16]. Ions with a smaller radius of hydration, lower free energy of hydration and higher activity are adsorbed more easily. For these three ions, the order of radius of hydration is Cd2+ > Cu2+ > Pb2+. The order of the free energy of hydration values is Cu2+ > Cd2+ > Pb2+. The activities are in the order of Pb2+ > Cu2+ > Cd2+. Thus, the selectivity adsorption performance of porous inorganic polymer microspheres for these three ions is in the order of Pb2+ > Cu2+ > Cd2+. To investigate the diffusion of heavy metal ions in the IPM, the distribution of the heavy metal elements in the IPMs were analyzed by
Fig. 9. Comparison of IPM in adsorption performance (CCu2+ = 200 mg/L, CCd2+ = 200 mg/L, and CPb2+ = 200 mg/L, 0.07 g/100 mL, pH = 5, 25 °C, 24 h).
Ion
Xi / C0i n ∑ j = 1 Xj / C0j
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Fig. 10. Distribution of heavy metal elements on the internal cross section.
References
EDS. As shown in Fig. 10, a large amount of heavy metal ions spread to the interior of the IPM. Because the IPM exposes more active sites and surface area, the IPM internal diffusion resistance is small so that the heavy metal ions can easily diffuse and be adsorbed.
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4. Conclusions In this work, slag and water glass are used as the main raw material. IPMs were prepared by using the suspension dispersion solidification (SDS) method. The diameter and sphericity of the IPMs were controlled well. This control was achieved by adjusting the modulus of water glass to obtain a greater surface area and a large amount of pore structures. The microstructure and phase of the IPM were analyzed and characterized by the SEM/EDS, BET and XRD methods. The adsorption properties of the IPMs for Pb2+, Cu2+ and Cd2+ were studied, and it was found that the adsorption ability of IPMs is very different for different ions. Thus, the selectivity adsorption performance of IPMs is best for Pb2+. EDS analysis shows that heavy metal ions diffused easily through the pores and reached the internal region of the adsorbents, with the adsorption active sites fully utilized. The current approach, based on a cost-effective, novel and facile synthesis procedure for an inorganic polymer adsorbent, can be effectively applied for the removal of Pb2+ ions from wastewater at industrial level.
Acknowledgements This work was supported by the Chinese Natural Science Fund (grant: 51772055 and 51561135012), the Guangxi Natural Science Fund (grant: 2018GXNSFBA281064 and 2016GXNSFGA380003) and Innovation Project of Guangxi Graduate Education (No. YCBZ2017007)
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