Article pubs.acs.org/IECR
Novel Magnetic Fly Ash/Poly(acrylic acid) Composite Microgel for Selective Adsorption of Pb(II) Ion: Synthesis and Evaluation Liping Jiang†,‡ and Peng Liu*,† †
State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ Chemical Department, Gansu Lianhe University, Lanzhou 730000, China S Supporting Information *
ABSTRACT: Novel magnetic fly ash/poly(acrylic acid) (FA/PAA) composite microgels were designed as a selective adsorbent toward Pb2+ ion based on the “treating wastewater with wastes” strategy. Magnetic fly ash (FA) containing magnetic iron oxides was selected as an inorganic cross-linker to produce magnetic FA/PAA composite microgels via an inverse suspension polymerization process, after being surface modified with polymerizable groups. The monomer, acrylic acid (AA), had been successfully grafted onto the magnetic fly ash to form the three-dimensional cross-linking skeleton of the beadlike magnetic FA/ PAA composite microgels without any foreign organic cross-linker added. The microgels possessed high adsorption capacity and good adsorption selectivity to Pb2+ owing to the carboxyl groups from poly(acrylic acid) (PAA). The introduction of the magnetic fly ash could not only improve the strength stability of the FA/PAA composite microgels, but also provide them the ability of magnetic separation. Furthermore, the adsorbed Pb2+ could be easily desorbed, indicating the reusability of the magnetic fly composite microgels.
1. INTRODUCTION Toxic heavy metals have been discharged to the environment as industrial wastes, causing serious water pollution. In recent years, more and more attention has been focused on various adsorbents which have metal-binding capacities and are able to remove unwanted heavy metals from contaminated water at low cost.1 The “treating wastewater with wastes” strategy has rendered a new idea for the treatment of the contaminated water, such as with agricultural waste materials.2 Fly ash (FA) is a kind of tremendous industrial waste from coal-fired power generating plants. With the development and improvement of coal-burning power plants, the output of FA has been increasing steadily and causing many ever-growing pollution problems and great concern.3 Therefore, the utilization of FA is critically important. Ten years ago, nine possible applications of FA could be identified and grouped into four main categories: construction materials (cement, concrete, ceramics, glass, and glass−ceramics), geotechnical applications (road pavement, embankments), “agriculture” (soil amendment), and miscellaneous (sorbent, sludge conditioning).4 Recently, it has also been used for polymer-based composites.5 In our previous work, magnetic cross-linked composites were synthesized via the facile “one-pot” in situ radical bulk polymerization with the functionalized FA as the sole cross-linker.6 As an adsorbent for contaminated water, the oxide composition and surface polarity,7 size fractions,8 and carbon fraction9 of FA have been reported to play an important role in its adsorption properties. Various methods such as washing with alkaline10 or acidic solution,11 chlorination and thermal treatment,12 or mechanochemical activation13 approaches had also been reported to improve its adsorption properties. Only in the most recent years, chitosan-based composite adsorbents © 2014 American Chemical Society
containing FA have been reported for the treatment of the heavy metal or organic compound contaminated water.14−18 In the present work, magnetic fly ash was separated as an inorganic cross-linker for preparing magnetic FA/poly(acrylic acid) (PAA) composite microgels by a facile inverse suspension polymerization process. The preparation was optimized with a designed L9(34) orthogonal experiment. The adsorption and desorption properties of the FA/PAA composite microgels were investigated for the treatment of the heavy metal contaminated water.
2. EXPERIMENTAL SECTION 2.1. Materials. FA (main constituents SiO2 58.64%; Al2O3 21.32%; CaO 5.02%; Fe2O3 7.20%; MgO 1.58%; MnO 2.23%) with a diameter of 0.5−10 μm was obtained from Datang Gansu Power Generation Co., Ltd., China. The FA powder was treated with 0.10 mol/L HCl aqueous solution for 6 h to release its surface hydroxyl groups,19 then washed with distilled water until it was neutral, filtered with an air pump, and followed by baking at 120 °C for 2 h. The activated fly ash was further ground in a mortar and filtered with an 800-mesh screen, followed by magnetic selection to obtain activated magnetic fly ash for use. Acrylic acid (AA, analytical grade) was obtained from Tianjin Kaixin Chemical Industry Co. Ltd., China. Span-80 (analytical grade) and liquid paraffin (chemical pure) were provided by Tianjin Guangfu Fine Chemical Research Institute, China. γMethacryloxy propyl trimethoxy silane (KH-570) (industrial Received: Revised: Accepted: Published: 2924
December 6, 2013 January 25, 2014 January 28, 2014 January 28, 2014 dx.doi.org/10.1021/ie4041274 | Ind. Eng. Chem. Res. 2014, 53, 2924−2931
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Article
absorption ratios (Wa) of the samples were expressed by the ratio of Ms to M0 (Wa = Ms/M0·100%). 2.5. Adsorption Selectivity. At high pH values, the concentration of the free surface −COO− increases and provides a higher potential of binding cations.21,22 In order to get a relatively higher adsorption capacity to metal ions, and avoid their hydrolysis at the same time, the adsorption of metal ions was carried out in aqueous HCl of pH 5. A solution containing Ni2+, Cu2+, Cd2+, Zn2+, and Pb2+ (each of 100 mg/L) was prepared with aqueous HCl of pH 5. The original concentrations of the five ions in the solution were represented as C0(M2+). To 50 mL of the above-prepared solution, 0.1 g of accurately weighed FA/PAA composite microgels (sample 2 to sample 9) was added and stirred for 3 h, respectively. The mass of the dried FA/PAA microgels was represented as W(M2+). The remaining ions in these eight solutions, after being treated with the FA/PAA composite microgel sample 2 to sample 9, respectively, were measured by flame atomic absorption spectrometry (FAAS; AA 240 atomic absorption spectrometer (AAS; Varian, USA)) and represented as Ca(M2+). For all the FAAS detections, the data were presented as the averages of three time measurements with relative standard deviations less than 5%. The adsorption capacities of the FA/PAA composite microgels to these five heavy metal ions, represented as A(M2+), were calculated with the following formula:
grade) was provided by Jiangsu Chenguang Silane Co., Ltd., China. Ammonium persulfate (APS), hydrochloric acid (HCl), and other reagents were of analytical grade and used without any further purification. Double distilled water was used throughout. 2.2. Preparation of Beadlike Magnetic FA/PAA Composite Microgels. The pretreated magnetic FA was mixed with a certain amount of water and dispersed in liquid paraffin with Span-80 as emulsifier, followed by addition of KH570 at one-third mass of the magnetic FA. The mixture was stirred at room temperature for 0.5 h and ultrasonically dispersed by an ultrasonic oscillator for 2 h, and then stirred at 40 °C for 6 h to obtain the KH-570 modified fly ash. AA was dispersed in liquid paraffin with Span-80 as emulsifier, pre-emulsified for about 1 h. The obtained preemulsion was added into the suspension of the KH-570 modified fly ash and stirred at room temperature for 0.5 h, followed by addition of APS. The mixture was stirred at room temperature for 0.5 h and then at 60 °C for 2 h followed by 80 °C for 3 h under N2 atmosphere. The obtained mixture was filtered with an air pump to get the beadlike magnetic FA/PAA composite microgels. The microgel beads were washed with ether and distilled water in sequence, extracted in water for 48 h, and then dried. A series of L9(34) orthogonal experiments was designed, as shown in Table 1, to assess the effects on the properties of the
A(M2 +) = 0.05(C0 − Ca)/W
Table 1. Array of the Designed L9(34) Orthogonal Experiments
2.6. Adsorption and Desorption of the FA/PAA sample 5 for Pb2+. A blank solution containing 100 mg/L (C0(Pb2+)) Pb2+ was prepared with aqueous HCl with a pH value of 1.0, 2.0, 3.0, 4.0, 5.0, or 6.0, respectively. To 50 mL of the Pb2+ solution with different pH values, respectively, 0.1 g of accurately weighed FA/PAA sample 5 was added. Ca(Pb2+) was measured by FAAS for each sample at different times of 1, 2, 3, 4, 5, 7, 9, 12, 18, 24, and 48 h, respectively. A(Pb2+) of the FA/ PAA sample 5 at different conditions were calculated as described above. The FA/PAA sample 5 saturated with Pb2+ in 50 mL of the above-formulated Pb2+ solution at pH 5 for 24 h was filtered and washed, and the residual water on the surface was wiped off; then the sample was added to 50 mL of aqueous HCl of 0.10, 0.20, 0.30, 0.50, 0.75, 1, or 1.25 mol/L and stirred for 0.5, 1.0, 1.5, 2.0, 2.5, or 3.0 h, respectively. The concentrations of the eluted ion (Ce) were measured by FAAS. The desorption ratio (Rd) was expressed by the following formula:
variables no.
fly ash−AA mass ratio
Span-80%
APS%
oil−water mass ratio
1 2 3 4 5 6 7 8 9
1:2 1:2 1:2 1:4 1:4 1:4 1:6 1:6 1:6
1.0 3.0 5.0 1.0 3.0 5.0 1.0 3.0 5.0
0.5 1.0 1.5 1.0 1.5 0.5 1.5 0.5 1.0
2:1 3:1 4:1 4:1 2:1 3:1 3:1 4:1 2:1
FA/PAA microgels from the mass ratio of fly ash to AA, the mass percentage of Span-80 in the total mass of water phase (Span-80%), the mass percentage of APS in the total mass of AA (APS%), and the oil−water mass ratio. The obtained formulas were characterized and compared with each other, and the above four variables were optimized to get the best formulation.20 2.3. Mechanical Stability. The FA/PAA composite microgels were immersed in distilled water until they were fully swollen. The water-saturated composite microgels were dispersed in water and then stirred in a high-speed agitator at 2000 rpm for 2 h. The damage rate of the FA/PAA composite microgels was used to evaluate their antiagitation ability. Four water-saturated microgel beads were put under a glass board with 3 kg loading on it, and the damage rate was used to estimate their pressure-resistance property. 2.4. Water Absorption Ratio. Dried FA, KH-570 modified FA, and FA/PAA composite microgels were weighed accurately, represented as M0; then they were immersed into distilled water for 24 h, wiped with a filter paper to remove the water on the surface and weighed, represented as Ms. The water
R d = [Ce/(C0 − Ca)] × 100%
2.7. Analysis and Characterization. Fourier transform infrared (FT-IR) spectra of the FA, the KH-570 modified fly ash, and the FA/PAA sample 5 were compared with the use of an Avatar 360 FT-IR instrument (Nicolet, USA) in the range 400−4000 cm−1 with a resolution of 4 cm−1 by the KBr pellet technique. The FA/PAA sample 5 was ground into powder and thoroughly dried and then characterized with a thermogravimetric analyzer (TGA 2050, TA Instruments, USA) in the range 30−800 °C with a heating rate 20 °C/min in N2 atmosphere. After being immersed in distilled water for 24 h to swell thoroughly, the swollen FA/PAA sample 5 was then dried in a freeze-dryer. The freeze-dried sample was then cut into halves. A section of the sample was observed by scanning electron 2925
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microscopy (SEM; JSM-6380, JEOL Ltd., Japan) to study its micromorphology. Magnetic hysteresis loops, saturation magnetization, and coercivity measurements were conducted with the use of a vibrating sample magnetometer (VSM; Lakeshore 7400, USA) system at room temperature.
3. RESULTS AND DISCUSSION 3.1. Preparation of the Beadlike Magnetic FA/PAA Composite Microgels. In the designed L9(34) orthogonal experiments, all formulas except the first one could produce the beadlike FA/PAA microgels, implying that the concentration of AA might be the most determining factor for the beadlike FA/ PAA composite microgels. For the first formula, both concentrations of AA and APS were low, so it could produce fewer PAA chains to form the beadlike composite microgels. The probability for the beadlike FA/PAA microgels increased with increasing concentrations of AA and APS. This is because the increase of the concentrations of AA and APS in a certain range accelerates the formation of PAA at the beginning of the reaction, which slows down the rate of droplet breakup and promotes the drop coalescence in the inverse suspension polymerization process,23 subsequently producing the beadlike magnetic FA/PAA composite microgels. Furthermore, all the obtained beadlike samples can sediment from liquid paraffin immediately once the stirring is stopped, indicating that the beadlike FA/PAA composite microgels via the proposed inverse suspension polymerization process are easily separated. After the microgel beads were washed with ether and distilled water in sequence, and extracted in water for 48 h to wash off the possible ungrafted polymer or remaining monomer, the final FA/PAA composite microgels kept the uniform beadlike morphology (Supporting Information, Figure 1S). Therefore, it could be concluded that the three-dimensional cross-linking network must be formed in the inverse suspension polymerization process. The KH-570 modified FA particles containing more than one polymerizable vinyl group may act as the crosslinker in the polymerization. Their surface vinyl groups could copolymerize with the monomer AA. After the polymerization was initiated by APS, the PAA propagating chain radicals reacted with one of the surface vinyl groups on the KH-570 modified FA particles to form the PAA brushes on the particles. The PAA propagating chain radicals could also react with one of the surface vinyl groups from other KH-570 modified FA particles; therefore, the three-dimensional cross-linking network formed, as described previously.6 Thus the PAA chains that linked more than one FA particle exhibited the threedimensional cross-linking network, while those grafted from only one FA particle presented as the polymer brushes in the beadlike FA/PAA microgels. 3.2. Water Absorption Ratios of Beadlike Magnetic FA/PAA Composite Microgels. The water absorption ratios (Wa) of the beadlike magnetic FA/PAA composite microgel samples were determined and compared with the Wa of FA (sample 0) and the KH-570 modified FA (sample 0′). The results (Figure 1) showed that the Wa of FA was a little higher than that of the KH-570 modified FA and much lower than those of the FA/PAA composite microgel samples, implying that KH-570 only acted as a bridge in the polymerization and had no positive effect on the Wa; the hydrophilic carboxyl groups in PAA, however, improved the Wa significantly. Figure 1 also shows that the Wa of the FA/PAA composite microgel samples increased with increasing dosage of AA,
Figure 1. Water absorption ratios of FA (sample 0), KH-570 modified FA (sample 0′), and beadlike magnetic FA/PAA microgels (samples 2−9).
reached the maximum at the fifth sample (the FA/PAA sample 5), and then decreased and reached a constant value. These results indicated that the Wa was associated not only with the amount of hydrophilic carboxyl group in the grafted PAA,24 but also with the cross-linking density of the three-dimensional network structure of the samples.25 With the increase of the concentration of AA, the feeding mass ratio of FA to AA decreased. Therefore, the amount of PAA grafted from the FA particles increased. As a result, the number of hydrophilic carboxyl groups increased and led to the increase of the water absorption ratios. On the other hand, the hydrophilic carboxyl groups on the surface of the beadlike FA/ PAA composite microgels hydrate with water molecules around. Thus, the surface network chain expands and the concentration of the hydrophilic carboxyl groups on the surface decreases. The osmotic pressure resulting from the concentration difference between the inside and surface carboxyl groups promotes water molecules to permeate into the network and hydrate the carboxyl groups inside, leading to the further expansion of the network. Therefore, the Wa of the samples increased when the dosage of AA increased in a certain range. However, the further increase of the AA concentration increases the cross-linking density of the FA/PAA composite microgels and the grid in the FA/PAA network becomes smaller, which hinders the permeation of water molecules. As a result, the Wa reaches the maximum at the fifth sample, and drastically decreases after that. The expansion and shrinkage of the grid in the FA/PAA network reaches a balance when the AA concentration reaches the limit, which leads to a constant Wa value from sample 6 to sample 9, as shown in Figure 2. In addition, the influence of pH on the water absorption ratio of the beadlike magnetic FA/PAA microgels (samples 2−9) were measured as shown in Figure 2. The water absorption of the FA/PAA microgels depends on the hydration of carboxyl groups; as a result, the concentration of H+ has no obvious effect on the water absorption ratios of the FA/PAA samples, as shown in Figure 2. 3.3. Adsorption Selectivity of Beadlike Magnetic FA/ PAA Composite Microgels. The FA/PAA composite microgel samples were stirred in the mixed-ion solution for 3 h to investigate their adsorption selectivity. The competitive adsorption results are presented in Figure 3. It was found that A(Pb2+) was much higher than the other four (A(Cu2+), A(Cd2+), A(Ni2+), and A(Zn2+)) for all the FA/PAA composite microgel samples, indicating that the FA/PAA composite 2926
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Figure 2. Water absorption ratios of FA/PAA samples at different pHs.
Figure 4. FT-IR spectra of FA, KH-570 modified FA, and FA/PAA sample 5.
sample 5, the peak in the range 2800−2900 cm−1 representing the absorbance of the methylene groups was more pronounced than that in the KH-570 modified FA. In addition, a strong peak appeared at 1712 cm−1 which was not found in the spectrum of the KH-570 modified FA, representing the carbonyl groups in PAA.31 All the signals indicated that AA was successfully grafted from the FA particles. The TGA curves of the FA, the KH-570 modified FA, and the FA/PAA sample 5 are presented in Figure 5. The KH-570
Figure 3. Adsorption selectivity of FA/PAA composite microgels to Ni2+, Cu2+, Cd2+, Zn2+, and Pb2+.
microgels exhibited obvious adsorption selectivity to Pb2+. It could be found that 30−60% Pb2+ had been removal from the mixed-ion solution, while only less than 25% of the other four heavy metal ions had been adsorbed. The adsorption selectivity might be explained by the difference in the metal ion properties such as ionic radius, electronegativity, and ionization potential.26 The adsorption capacity for Pb2+ increased slowly at first, then reached a maximum at the fifth sample (the FA/ PAA sample 5), and decreased after that. The changing trend was roughly in accordance with the change of the water absorption ratio (Figure 2). As mentioned above, the balance between the expansion and shrinkage of the grid in the FA/PAA composite network resulted in a maximum of absorption capacity. However, different from the water absorption, the interaction between the carboxyl groups in the composite microgels and the heavy metal ions is associated not only with the amount of hydrophilic carboxyl groups and crosslinking density of the samples, but also with the variety and concentration of heavy metal ions, as well as the adsorption equilibrium established between FA/PAA and the heavy metal ions.27−30 The combined effects from all those factors make the fifth sample the best candidate. Therefore, the FA/PAA sample 5 was selected for further study. 3.4. Characterization of the Beadlike FA/PAA Composite Microgel Sample 5. The FA, the KH-570 modified FA, and the beadlike FA/PAA sample 5 were characterized and compared by FT-IR spectroscopy (Figure 4). A very weak peak in the range 2800−2900 cm−1 in the spectrum of the KH-570 modified FA represented the C−H stretching bands, implying KH-570 had reacted on the FA particles. As for the FA/PAA
Figure 5. TGA curves of FA, KH-570 modified FA, and FA/PAA sample 5.
modified FA had a weight loss of about 20%, which is close to the theoretical value of 25%. This result verifies that most of the KH-570 had modified the FA particles with this new approach. The increased utilization of KH-570 could lower the material cost significantly. In addition, the FA/PAA sample 5 had a weight loss of about 74%, implying that the weight percent of PAA is about 69% after the deduction of the weight loss resulting from KH-570. Compared to its theoretical values of 75% (FA:KH-570:AA = 1:0.33:4), it could be calculated that about 92% AA had been grafted from the KH-570 modified FA. The high utilization ratio of AA in the present work is a promising value for industrial applications. Figure 6 reveals that values of the specific saturation magnetization of the FA, the KH-570 modified FA, and the FA/PAA sample 5 were 3.25, 3.03, and 1.44 emu/g, respectively, and their coercivity was about 130 Oe, indicating that the specific saturation magnetization decreased with the decrease of the FA content, but the coercivity remained almost unchanged. In addition, sample 5 had a certain magnetic induction and was easy to demagnetize, which provided a 2927
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Figure 6. Magnetization curves of FA, KH-570 modified FA, and FA/ PAA sample 5. Figure 8. Adsorption capacities of sample 5 to Pb2+ at different pHs.
convenient approach for separation and removal by applying external magnetic fields.32 The SEM micrograph of the fracture surface of the freezedried sample 5 is presented in Figure 7. It reveals that the FA/
electrostatic attraction between −COO− and Pb2+. The adsorption capacity of the sample 5 to Pb2+ was therefore zero at pH 1 and increased very slightly between pH 1 and 2. The concentration of H+ decreases with the rise of pH value, causing the equilibrium to change, and the amount of the −COO− increases, leading to a sharp increase of the adsorption capacity of the sample 5 to Pb2+ after pH was higher than 2. The increase of the adsorption capacity slowed down when the concentrations of −COOH and −COO− were close to equilibrium, and then reached a plateau at about 27 mg of Pb2+/g when the pH was higher than 5 after 8 h of adsorption at room temperature. To avoid the hydrolysis of Pb2+ in the neutral medium, the optimum pH value for the adsorption of sample 5 to Pb2+ was selected as 5.0. Figure 9 illustrates that the adsorption capacity of sample 5 to Pb2+ increased obviously in the first 5 h, implying that Pb2+
Figure 7. SEM image of the fracture surface of FA/PAA sample 5.
PAA sample 5 is porous and the size of holes is at the micrometer level, and the FA particles distribute separately in the FA/PAA microgels. This result implied that AA had been successfully grafted onto the FA particles, and Pb2+ can easily immigrate into the microgels and interact with the abundant −COO− groups in the FA/PAA microgels. Then, the mechanical stability of the FA/PAA sample 5 was evaluated by its pressure resistance (Supporting Information, Figure 2S) and antishearing ability (Supporting Information, Figure 3S). Due to the three-dimensional cross-linking structure established with PAA as a soft block and the FA particles as a rigid inorganic skeleton, the FA/PAA sample 5 did not break after the antipressure (Supporting Information, Figure 2S) and antishearing tests (Supporting Information, Figure 3S), implying that the FA/PAA sample 5 possesses a long service lifetime and thus further reduces the overall cost. 3.5. Adsorption Property of FA/PAA Sample 5 toward Pb2+. The adsorption capacity of the FA/PAA sample 5 toward Pb2+ as a function of pH value is presented in Figure 8. As shown, the pH value has an obvious effect on the adsorption capacity of the sample 5 to Pb2+. At pH
