Al2O3 Adsorbent and Its Application for

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Facile Fabrication of Biochar/Al2O3 Adsorbent and Its Application for Fluoride Removal from Aqueous Solution Xiancai Jiang,* Xiaotong Xiang, Haifeng Hu, Xiangchao Meng, and Linxi Hou School of Chemical Engineering, Fuzhou University, Fuzhou 350118, People’s Republic of China

J. Chem. Eng. Data Downloaded from pubs.acs.org by TULANE UNIV on 12/05/18. For personal use only.

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ABSTRACT: In this research, pristine and boron-doped biochar/Al2O3 adsorbents for fluoride removal from aqueous solutions were prepared by employing chitosan (CS), poly(vinyl alcohol) (PVA), and AlCl3·6H2O as the raw materials. CS was dissolved in AlCl3·6H2O aqueous solution, and the CS/PVA aerogel was prepared by a freezing process. The biochar/Al2O3 was obtained by the carbonization process at high temperature. AlCl3·6H2O was used as both the solvent for CS and the source for Al2O3. The structure of biochar/Al2O3 was characterized by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The adsorption performance of biochar/Al2O3 for fluoride was evaluated by the batch adsorption experiment. Furthermore, the effect of HBO3 cross-linking on the adsorption property of biochar/Al2O3 was studied. The results showed that the HBO3 cross-linking process would promote the adsorption performance of biochar/Al2O3. The adsorption kinetics and equilibrium properties are well-described with a pseudo-second-order equation and Freundlich model, respectively. The Qe for biochar/Al2O3 could reach 196.1 mg/g. The high adsorption capacity would enable the biochar/Al2O3 to be useful for real contaminated water treatment. This paper provides a valuable and economic solution for the utilization of CS.

1. INTRODUCTION The dissolution of fluoride-bearing minerals and anthropogenic activities would lead to high fluoride concentration of underground drinking water. Although fluoride is an essential constituent for both humans and animals, excess discharge of fluoride into underground water could cause detrimental effects on ecosystems and public health. The concentration of fluoride in drinking water must be within certain limits. To protect human health, the maximum acceptable concentration of fluoride is 1.5 mg/L set by the World Health Organization.1 However, the concentrations of fluoride in groundwater in many places in the world, such as China, India, USA, and South America, exceed these values, threatening the health of billions of people.2,3 As a consequence, the removal of excess fluoride from water has to be introduced into water treatment.4 Many researchers have developed different methods to remove fluoride from water, namely, electro-coagulation, chemical precipitation, ion exchange, membrane techniques, reverse osmosis, Donnan dialysis,5 electrodialysis, and adsorption. One can adopt a technology according to the real situation such as the amount of water to be treated, the © XXXX American Chemical Society

electricity supply, and funds. The adsorption technique has been recognized as the most extensively adopted method due to its strong operability, low cost, high effectiveness, and environmentally friendly property.6 Therefore, a high-quality adsorbent with various excellent characteristics is in keen demand and critical. Adsorbents used for the removal of fluoride from water are activated alumina,7−11 alumina impregnated with lanthanum hydroxide (La(OH)3),11 rare earth oxides,9 activated clay,9,12 carbonaceous materials,9,11,14 solid industrial wastes such as red mud, spent catalysts and fly ash,9 zeolites,9,15 biosorbents,11,16,17 alum-impregnated activated alumina,9,11 aluminum-hydroxide-coated activated carbon,18 as well as activated rice straw and fishbone charcoal.9,11 Among these reported adsorbents used for fluoride adsorption, activated carbon is the mostly commonly used adsorbent owing to its high porosity, large surface area, versatile surface chemistry, and acceptable Received: June 29, 2018 Accepted: November 22, 2018

A

DOI: 10.1021/acs.jced.8b00556 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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hydraulic conductivity for packing into columns.13 However, the use of activated carbon in different fields is limited as it has limited ability to adsorb fluoride. Moreover, the interest is growing for preparing biochar from low-cost biomass. Thus, the preparation of functionalized biochar with high adsorption efficiency for fluoride is important. Meanwhile, activated alumina is one of the best options as a fluoride adsorbent due to its high adsorption capacity and affinity for fluoride. Thus, the combination of biochar and activated alumina has aroused more and more attention. Chitosan (CS) is the most important derivative of chitin, which is the second largest biopolymer on the earth. More importantly, our previous research has proven that CS can be well-dissolved in AlCl3·6H2O aqueous solution.19 Thus, it could provide feasibility for the preparation of biochar/Al2O3. Thus, the aim of the present work is to prepare biochar/Al2O3 with CS and AlCl3·6H2O as the raw materials and to study different adsorption models to understand the adsorbent pattern of fluoride using biochar/Al2O3 as the adsorbent.

(MP519, Shanghai San-xin Instrumentation). The adsorption capacity, qe (mg/g), was calculated as follows: qe =

(ci − ce)V m

(1)

where Ci (mg/L) is the initial fluoride concentration, Ce (mg/ L) the fluoride equilibrium concentration, V (L) the volume of the fluoride solutionm, and m (g) the mass of adsorbent.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characteristics of Biochar/ Al2O3. Figure 1 shows the Raman spectra of CPA and

2. EXPERIMENTAL SECTION 2.1. Materials. CS (degree of deacetylation ≥95%, viscosity 100−200 mPa·s), sodium fluoride (NaF), and boric acid (HBO3) were provided by Aladdin Chemical Reagent (China) and used without further treatment. AlCl3·6H2O was purchased from Kelong Chemical Co. (Chengdu, China). Distilled water was used throughout the experiment. 2.2. Preparation and Characterization of Biochar/ Al2O3. First, 3.0 g of PVA was dissolved in 15 mL of distilled water at 95 °C. Next, 3.0 g of CS and 10.0 g of AlCl3·6H2O were added into 90 mL of distilled water, and the CS aqueous solution was obtained after heating at 90 °C for 30 min under stirring. Then, the PVA and CS aqueous solutions were mixed together. Twenty milliliters of HBO3 aqueous solution (5 wt %) was added into the mixed solution and kept at 60 °C for 1 h to form the hydrogel.20 The mixed solution was freeze-dried to the aerogel. The aerogel was sintered at 600 °C for 1 h to remove the volatile organics and to carbonize the solid residues. The black powder was obtained after being cooled to room temperature and was named CPAB. For comparison, the CPA adsorbent was prepared with the same process without the addition of HBO3 aqueous solution. 2.3. Characterization. The microstructure and surface morphology of biochar/Al2O3 adsorbents were observed using a scanning electron microscope (SEM) instrument (Nova NanoSEM 230, USA). Raman spectrocopy analysis was performed with a microscope (λ = 532 nm) using Invia Reflex from the U.K. The crystallographic character of the biochar/ Al2O3 adsorbents was observed by powder X-ray diffractometry (XRD, Rigaku D/Max 2500PC) with Cu Kα radiation in the region of 2θ from 5 to 80°. 2.4. Adsorption Experiments. Adsorption experiments were performed using a batch equilibration technique. Stock solutions with different concentrations of fluoride were prepared by dissolving NaF in ultrapure water and then added into a 250 mL stopper polyethylene bottle. Two milligrams of CPA and CPAB adsorbent was added to a flask and shaken in a thermostat shaker with 200 mL of NaF aqueous solution under the stirring speed of 150 rpm at 298 ± 1 K. The adsorbent dose was kept as 0.10 g/L for all of the experiments. A 0.1 M aqueous solution of HCl or NaOH was used to adjust the pH. The fluoride concentration in the solution was analyzed using a fluorine ion concentration meter

Figure 1. Raman spectra of the CPA and CPAB adsorbents.

CPAB adsorbents. Two characteristic peaks for amorphous carbon materials, one peak at 1341 cm−1 (D-peak) and the other at 1595 cm−1 (G-peak), are apparent on both spectra of CPA and CPAB. This showed that both the CPA and CPAB had amorphous structure. Previous studies have shown that the ratio of ID/IG could correspond to the degree of graphitization. The degree of graphitization could be indicated from the low ID/IG value. The ID/IG calculated from the Raman spectra was 0.843 and 0.849 for CPA and CPAB, respectively. This indicated that the degree of graphitization of CPAB was a little lower than that of CPA. The low degree of graphitization would be beneficial for the adsorption performance of the carbon materials. The XRD patterns of CPA and CPAB were also obtained and are shown in Figure 2. It could be seen that there was a diffraction peak at 2θ = 24°, which was indexed as the amorphous nature of carbon. These further showed that both the CPA and CPAB were amorphous. The interlayer spacing was calculated to be 0.372 and 0.369 nm for CPA and CPAB, respectively. This also indicated that CPAB had a degree of graphitization lower than that of CPA. The morphologies of CPA and CPAB adsorbents are shown in Figure 3. From Figure 3, it can be seen that CPAB showed rougher structure. The rough structure would provide more adsorbing sites during the adsorption process. This also indicated that CPAB was a more suitable adsorbent for the removal of fluoride from drinking water. 3.2. Adsorption Tests. 3.2.1. pH Effect on Fluoride Adsorption. The initial solution pH value would change the surface properties of the adsorbent and the degree of ionization/dissociation of the adsorbent. Therefore, the initial B

DOI: 10.1021/acs.jced.8b00556 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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addition, it also could be found that CPAB showed an adsorption capacity higher than that of CPA. This proved that the HBO3 cross-linking process was effective for improving the adsorption performance of biochar/Al2O3. The ζ-potential of the CPA and CPAB at different pH values is shown in Figure 4. From Figure 4, it can be seen that the ζ-potential values decreased with pH. When the pH value was 5−8, the ζpotential values were close to zero, resulting in the stabilization of the fluoride adsorption capacity. When the pH value was less than 5, the CPA and CPAB were positively charged, resulting in the increase of fluoride removal. When the pH value was above 8, the CPA and CPAB were negatively charged, leading to the decrease of fluoride removal. Thus, we can conclude that an electrostatic interaction between Al2O3 and fluoride was involved in the fluoride adsorption. 3.2.2. Adsorption Kinetics. The adsorption kinetics for fluoride were studied with CPA and CPAB. Three kinetic models such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were employed to the experimental data. The pseudo-first-order is one of the most widely used kinetic equations, and this model is expressed in Table 1, where K1 is the pseudo-first-order rate constant (min−1) of adsorption, and qe and qt (mg/g) are the amount of fluoride adsorbed at equilibrium and at time t (min), respectively. Meanwhile, the linear equation of the pseudo-second-order model is expressed in Table 1,21 where K2 (g mg−1 min−1) is the rate constant of the pseudo-second-order model. The intraparticle diffusion rate model determines the ratecontrolling step as that of pore diffusion in an adsorption mechanism and is described in Table 1, where kp (mg/g min0.5) is the intraparticle diffusion rate constant and C is the intercept. Value of C (mg/g) is a constant, indicating the boundary layer effect. The higher C values are indicative of the larger boundary layer effect. According to this model, the plot of uptake should be linear if intraparticle diffusion is involved in the adsorption process. Moreover, when the intraparticle diffusion is the rate-controlling step, the fitted lines would pass through the origin. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control, and this further indicates that the intraparticle diffusion is not the only rate-limiting step.22 The kinetic parameters for the pseudo-first-order, pseudosecond-order, and intraparticle diffusion models are determined from the linear plots of log(qe − qt) versus t (Figure S1a,b), (t/qt) versus t (Figure S1c,d), and qt versus t0.5 (Figure S1e,f), respectively, and the values of these constants are shown in Table 1. The linear correlation coefficients (R2) of the three models listed in Table 1 showed that the fitting of the pseudo-second-order model was the best-fit model in describing the fluoride uptake from aqueous solution of different initial concentrations, with the highest R2 value. Moreover, the calculated qe values of the pseudo-second-order model are nearly consistent with the experimental data, also indicating that the pseudo-second-order model was more suitable for describing the adsorption process. The pseudosecond-order model is based on the assumption that the ratedetermining step is the a chemical sorption involving valence forces through sharing or exchange of electrons between adsorbent and sorbate.23 This indicated that the rate of fluoride adsorption was mainly governed by chemical reaction, including ion sharing and transferring. Furthermore, the value of K2 decreases with increasing fluoride concentration owing to

Figure 2. XRD patterns of the CPA and CPAB adsorbents.

Figure 3. SEM images of the surface of (a) CPA and (b) CPAB adsorbents.

solution pH value is one of the critical controlling factors that would affect the adsorption process. The effect of initial solution pH on the adsorption of fluoride onto CPA and CPAB is shown in Figure 4. From Figure 4, it could be seen

Figure 4. Effect of initial pH on the fluoride adsorption and ζpotential of CPA and CPAB at 298 K (with the initial concentration of 20 mg/L).

that the adsorption capacity of CPA and CPAB decreased with an increase of the initial solution pH value. On the whole, CPA and CPAB showed higher adsorption capacity with the acidic solution than with the basic solution. This also proved that Al2O3 would play an important role in the adsorption process. In the acidic solution, Al2O3 would be more likely to transfer into Al3+, and Al3+ would attract fluoride to form AlF3. In C

DOI: 10.1021/acs.jced.8b00556 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Parameters of the Kinetics Study for Fluoride Adsorption onto CPA and CPAB

adsorbent CPA

CPAB

C0 (mg/L) Qe,exp (mg/g) 10 20 40 10 20 40

pseudo-first-order kinetic model

pseudo-second-order kinetic model

intraparticle diffusion model

log (qe − qt) = log qe − (K1/2.303)t

(t/qt) = (t/K2qe2) + (1/qe)t

qt = kpt0.5 + C

K1 (min−1)

qe,calcd (mg g−1)

R12

K2 (g mg−1 min−1)

qe,calcd (mg g−1)

R22

KP

R32

C

0.01453 0.01783 0.00811 0.00532 0.00615 0.01670

12.24 17.60 14.38 14.74 17.91 20.45

0.924 0.950 0.768 0.847 0.922 0.774

0.00249 0.00069 0.00018 0.00221 0.00042 0.00015

20.04 38.12 73.53 21.28 49.07 80.45

0.994 0.998 0.998 0.996 0.988 0.999

0.856 1.207 1.876 0.894 1.365 1.258

0.699 0.821 0.853 0.907 0.892 0.838

8.442 29.947 54.326 6.313 18.092 54.981

19.19 39.70 73.80 20.47 53.96 79.71

Table 2. Isotherm Models of CPA Adsorbents Langmuir isotherm

adsorbent CPA

CPAB

2

Ce qe

=

Ce qmax

+

Freundlich isotherm

1 qmax KL

log qe = log KF + 2

1 n

Tempkin isotherm

log Ce

qe = B ln KT + B ln Ce

T (K)

R1

QL (mg/g)

KL (L/mg)

R2

n

KF

KT (mg/L)

B

R32

298 308 318 298 308 318

0.996 0.996 0.991 0.993 0.998 0.990

151.3 161.8 199.6 196.1 204.9 456.6

0.0124 0.0156 0.0129 0.0128 0.0161 0.0101

0.973 0.953 0.946 0.942 0.983 0.987

1.1082 1.1113 1.2826 1.1684 1.0868 1.1994

1.7050 2.1727 4.2633 2.3161 3.2788 6.1043

0.2653 0.2849 0.2831 0.3449 0.4108 0.3829

22.436 26.384 28.915 25.848 29.024 49.088

0.942 0.953 0.941 0.928 0.932 0.898

Isotherm data were tested with Freundlich isotherm. The Freundlich isotherm model assumes that the adsorption occurred on a heterogeneous surface through a multilayer adsorption system.22 The Freundlich model equation is given in Table 2, where KF and n are Freundlich constants that refers to the adsorption capacity (mg/g) and adsorption intensity of the solute on the adsorbent, respectively.29 Both KF and n could be acquired from the slope and intercept of the linear plot of log qe versus log Ce. Furthermore, the Temkin adsorption isotherm model was used to study the adsorption process. The Temkin adsorption isotherm model equation can be found in Table 2, where Ce and qe are defined the same as with the Langmuir and Freundlich model. KT (mg/L) is the equilibrium binding constant related to the maximum binding energy, and B is the Temkin constant related to the heat of adsorption. KT and B could be determined from the slope and intercept of the plot of qe versus ln Ce, respectively. The Temkin isotherm model assumes that the heat of adsorption of all the molecules in a layer decreases linearly rather than logarithmically due to the adsorbent−adsorbate interactions.30 The isotherm parameters for Langmuir, Freundlich, and Temkin isotherms are obtained from the linear plots of Ce/qe versus Ce, log qe versus log Ce, and qe versus ln Ce, respectively (Figure S2a−f). The values of KL, KF, KT, R12, R22, R32, qmax, n, and B are recorded in Table 2. According to the data in Table 2, the Langmuir isotherm model with the correlation coefficients between 0.991 and 0.998 is more appropriate to describe the adsorption of fluoride onto biochar/Al2O3 than the Freundlich and Temkin isotherm model. This implied that the adsorption of fluoride on the biochar/Al2O3 is a monolayer and homogeneous, with uniform surface properties for the adsorption process. Similar results also have been found for other adsorbents reported in previous research, such as Mg−Al layered double hydroxides.31 According to the Langmuir isotherm, the maximum uptake values (qmax) for fluoride were 151.3, 161.8, and 199.6 mg/g and 196.1, 204.9, and 456.6 mg/g for unmodified and HBO3

the weaker competition of the adsorption surface sites at the lower concentration.24 The previous studies of Chang et al.25 show that the dynamics of adsorption takes place in four definable steps, namely, bulk solution transport, film diffusion transport, pore transport, and adsorption. It was also mentioned that the first and fourth steps do not belong to the rate-controlling steps because the former is not involved with the adsorbent and the latter is a very rapid process. To study the influence of intraparticle diffusion during the adsorption process, the intraparticle diffusion model was used to study the adsorption process. It was shown that the linear portion of the plot did not pass through the origin in Figure S1e,f, suggesting that the mechanism of fluoride adsorption on biochar/Al2O3 is complex, and both surface adsorption and intraparticle diffusion contribute to the rate-determining step. However, the lower value of R32 (