Enhancement of the Stability of Biosorbents for Metal-Ion Adsorption

May 25, 2015 - Biosorbents have demonstrated great potential in the treatment of metal-containing wastewater. However, one of the bottleneck issues of...
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Enhancement of the Stability of Biosorbents for Metal-Ion Adsorption Hongxiang Ou,†,‡ Weihui Tan,† Catherine Hui Niu,*,† and Renfei Feng§ †

Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A9 ‡ School of Environment and Safety Engineering, Changzhou University, No. 1 Gehu Road, Changzhou, Jiangsu, China 213164 § Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 2V3 S Supporting Information *

ABSTRACT: Biosorbents have demonstrated great potential in the treatment of metal-containing wastewater. However, one of the bottleneck issues of using biosorbents is that amounts of organic carbon release from biosorbents into water. This seriously limits the application of biosorption technology in treating wastewater. In this work, a novel methodology was developed to greatly reduce the organic carbon release and enhance the stability of biosorbents by using barley straw as the model biosorbent material and nickel as the model metal ion. The raw barley straw was first made into cylindrical pellets, which were coated with a sodium alginate (SA) and poly(vinyl alcohol) (PVA) membrane. The coating conditions including the ratio of SA to PVA, glutaraldehyde (GA) dose, concentration of CaCl2 solution, and cross-linking time were optimized by L9(34) orthogonal array design. The pellets coated at the optimal conditions (1:1 mass ratio of SA to PVA, 1.0 mL of GA, 8% CaCl2, and 20 min of crosslinking time) were then applied for nickel adsorption. The effects of the solution pH and ionic strength on the adsorption equilibrium and desorption of adsorbed nickel ions were investigated. Scanning electron microscopy and synchrotron X-ray fluorescence spectroscopy were used to locate the adsorption sites on the coated pellets. The results demonstrated that organic carbon release of the coated pellets was significantly reduced to 3.8−9.7 mg/g of dry barley straw pellets in the nickel adsorption process, while that of the raw barley straw particles was 44 mg/g. The nickel uptake increased to 25.6 mg/g, higher than that of the raw barley straw particles.

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INTRODUCTION Nickel-containing wastewater discharged from industries including electroplating, battery manufacturing, and metal finishing has been one of the major environmental concerns1,2 because nickel is one of the priority pollutants. A number of nickel compounds have been implicated as potential carcinogens.3 Oral exposure to nickel may result in developmental/ reproductive toxicity effects. Current methods for nickel removal include precipitation,4 ion exchange,5 evaporation,6 reverse osmosis,6 and electrodialysis.7 Many of these methods are expensive and complex. There is great interest in developing cost-effective methods to treat nickel-containing wastewater. Adsorption using biomaterials including byproducts of agricultural, food, and wood industries makes it an inexpensive alternative method. Some investigation has been done on the adsorption of nickel from aqueous solutions by biomaterials.2,8−14 Advantages of the adsorption of heavy metals using biomaterials include low cost, high efficiency, reusability of adsorbents, and metal recovery.15,16 However, biomaterials usually contain a high amount of organic substances such as cellulose, hemicellulose, lignin, protein, lipids, and simple sugars, which are prone to release into the aqueous solution during the adsorption operation. Kratochvil and Volesky16 found that the total organic carbon (TOC) of the effluent from the Sargassum sp. packed column was ∼30 mg/L, and Chen and Yang17 reported that the raw biosorbents derived from raw seaweed of Sargassum sp. had TOC release values of 110.9 and 186.3 © 2015 American Chemical Society

mg/L at pH 5.0 and 2.0, respectively. Organic material release from the biomaterials will result in a secondary pollution and adversely affect the application of biomaterial-based adsorbents to industrial metal-containing wastewater treatment. Therefore, it is of great importance to prevent organic materials from leaching before using the biomaterials. One of the methods is encapsulation, which uses such polymers as poly(vinyl alcohol) (PVA), alginate, and chitosan to encapsulate the biomaterials.18−20 However, challenges still exist in this area of research. Barley straw is abundantly generated as a byproduct in the agriculture industry and is developed to produce sugar,21 fossil fuels,22,23 porous carbon materials,24 etc. Besides, barley straw possesses over 90% of organic substances such as cellulose, hemicellulose, and lignin, which attributes to adsorption of metals such as nickel,25 copper,26,27 cadmium,28 lead,26 and cobalt,29 and methylene blue dye,30 as well as spilled oil cleanup.31 However, because of organic substance release from raw barley straw, it is usually observed that the solution becomes yellowish after adsorption. In addition, it is difficult to collect the powder adsorbents after the adsorbing process. Among the hydrophilic polysaccharide polymers, sodium alginate (SA) is a naturally occurring nontoxic polysaccharide obtained mainly from marine algae containing β-D-mannuronic Received: Revised: Accepted: Published: 6100

February 5, 2015 May 22, 2015 May 25, 2015 May 25, 2015 DOI: 10.1021/acs.iecr.5b00518 Ind. Eng. Chem. Res. 2015, 54, 6100−6107

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Industrial & Engineering Chemistry Research acid and α-L-guluronic acid residues and has gained widespread application for its good film-forming properties and high activity with carbonyl and hydrogen groups on its chain.32−34 However, the main drawbacks of SA are excessive swelling, relative rigidity, and fragility.35,36 Blending SA compatibly with PVA may be an appropriate choice to enhance the durability, flexibility, and mechanical strength of the SA membrane because PVA has excellent hydrophilicity, compact molecular packing, and a high degree of crystallinity.37,38 Moreover, SA and PVA are both the most commonly used polymers with good film-forming ability, which is advantageous for industrial applications.35,39 The objectives of this study were to develop a method to first make cylindrical barley straw pellets, which were then coated with SA/PVA, and to optimize the adsorption capability of the coated barley straw using nickel as the model metal ion. The major advantages of coated barley straw pellets for the adsorption process were the reduction of TOC release of the barley straw and easy separation of barley straw pellets from solutions. The L9(34) orthogonal array design (OAD) was applied to optimize the coating conditions of barley straw pellets. The effects of the solution pH, ionic strength, and nickel concentration on the nickel adsorption equilibrium were investigated. The feasibility of desorption of adsorbed nickel was also discussed.

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MATERIALS AND METHODS Adsorbent Preparation and Optimization. Raw Barley Straw. The raw barley straw was obtained from the Poultry Centre of the University of Saskatchewan. The barley straw was sun-dried and ground into 1.7 mm (10 Hamer Mill, Glen Mills, Saskatchewan, Canada). The moisture content of the straw was 5.6 ± 0.3%. Preparation of Barley Straw Pellets. The method of making barley straw cylindrical pellets was adopted from that reported by Adapa et al.40 To make the pellets, the above ground barley straw particles were used. Deionized water (DW) was first sprayed on the particles to maintain a 13−15% water content of the barley straw particles. Then ligonsulfonate powder (commercial grade, Borregaard LignoTech, Sarpsborg, Norway) was used as a binder to mix with the barley straw particles at a ratio of 2.5 wt % with the aid of a mixer (21-8 Stokes Pharmaceutical and Chemical Equipment, Peusalt Chemicals Corp., Philadelphia, PA). Then the mixture was fed into a pelletizing mill with a die diameter of 4.7 mm (CL5 Laboratory Pellet Mill, Glen Mills, Saskatchewan, Canada). The lengths of the pellets were 5−7 mm. Finally, the generated 4.7 mm pellets were air-dried at room temperature. Optimization of the Coating Conditions. The barley stray pellets prepared from the above steps were coated by sodium alginate (SA) and poly(vinyl alcohol) (PVA). The coating process is shown in Figure 1. Four important factors that affected the stability of the coated pellets were investigated, including the mass ratio of SA to PVA (factor A), glutaraldehyde (GA) dose (factor B), concentration of the CaCl2 solution (factor C), and cross-linking time (factor D). To evaluate the stability of the coated pellets, the following quantities were measured: successful coating ratio (SCR, %), stable ratio (SR, %), and TOC release (TOC-R, mg/g) of the coated pellets. SCR was defined as the ratio of the number of successfully coated (nonbroken) pellets to the total number of raw pellets used for coating at each batch. SR is the ratio of the number of stable pellets to the total number of pellets initially

Figure 1. Processing steps of the coated barley straw pellets’ preparation.

added into DW mixed at 250 rpm for 24 h. TOC-R is the amount of TOC per unit mass of dry coated pellets or dry raw pellets released into DW after 24 h mixing. The L9(34) OAD was applied to the present experiments to optimize the pellet coating conditions to obtain the maximum SCR and SR and minimum TOC-R. SCR (%), SR (%), and TOC-R (mg/g) were calculated according to eqs 1−3, respectively.

SCR = SR =

Nw × 100% Ntc

Ns × 100% Nt

TOC‐R =

C TOCVTOC (mg/g) mp

(1)

(2)

(3)

where Nw and Ntc are the number of successfully coated pellets and the total number of raw pellets in the coating process, respectively, and Ns and Nt are the number of stable pellets after 24 h of shaking in DW at 250 rpm and the total number of pellets added to the water, respectively. CTOC (mg/L) is the TOC concentration after 24 h of mixing, VTOC (L) is the volume of DW used for mixing, and mp (g) is the total mass of the dry coated pellets or dry raw pellets added to the water. The four factors and their levels are listed in Table 1. There were nine experiments in the table, each of the nine experiments were performed in duplicate. The average values were reported. Only the pellets coated under the optimal conditions and being stable after 24 h mixing with DW were air-dried and used for nickel adsorption experiments. Solution Preparation. Coating Solution. The composition of the coating solution was made as per Table 1. First, PVA (95%, Ward’s Science) was dissolved in DW at a ratio of either 6101

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Industrial & Engineering Chemistry Research Table 1. Factors and Levels in OAD Experiments of L9(34)

nickel and TOC analyses and the experiments continued until the system reached equilibrium. It was demonstrated that 6 h was required to reach adsorption equilibrium in the systems containing a 300 ± 1.0 mL nickel solution, 3.05 ± 0.04 g coated barley straw pellets, and an initial nickel concentration up to 1000 mg/L, which was the highest nickel concentration tested in this work. Thus, all adsorption experiments were run for 24 h, sufficient for reaching equilibrium, and all of the experiments were carried out in duplicate or triplicate. Experiments of a blank nickel solution (without the addition of adsorbents) at the tested pH range (98.9%, Fisher Scientific) was dissolved in DW to prepare the stock solution of 1000 mg Ni/L, which was then diluted to make nickel solutions of lower concentration. The solution pH was adjusted by 0.1 M sulfuric acid (H2SO4) and 0.1 M sodium hydroxide (NaOH). By adding sodium sulfate to the nickel ion solution, the effect of the ionic strength was investigated. All reagents used were of reagent grade. Determination of the Point of Zero Net Charge (PZNC). The charge state of the adsorbents has important effects on the metal-ion binding. The PZNC of coated barley straw pellets will provide information on the charge state of the coated pellets. The PZNC of coated pellets was determined by the salt titration method.41,42 In this study, the titration was carried out by measuring the pH change in solution upon the addition of H2SO4 (0.1 M) or NaOH (0.1 M) to each sample containing either 40.0 ± 0.2 mL DW or a 0.1 M sodium sulfate solution and three particles of coated pellets (0.441 ± 0.024 g) and magnetically stirring for 24 h at room temperature. The contact time was confirmed to be sufficient for achieving equilibrium at room temperature. The adsorbent is neutral at the PZNC, at which there is no difference between the solution pH of the suspension containing water and the adsorbent with low ionic strength and that at elevated ionic strength. It is represented by the intersection of the aforementioned titration curves. Adsorption Experiments. Adsorption experiments were carried out with a titrator system (Schott titronic universal, SI Analytics GmbH, Mainz, Germany). A total of 300 mL of aqueous nickel solution initially containing 20−1000 mg Ni/L and 3.0 ± 0.2 g of coated pellets were added to a 500 mL vessel at room temperature. The suspensions were vigorously mixed by magnetic stirring at 250 rpm. The solution pH before and during adsorption was monitored and controlled by the titration system using 0.1 M H2SO4 or 0.1 M NaOH solutions. Samples of 0.5−1.0 mL were taken out at periodic intervals for

Q=

(C 0 − C )V (mg/g) m

(4)

where Q (mg/g) is the amount of nickel adsorbed per gram of sorbent, C0 and C (mg/L) are the nickel-ion concentration initially and at time t in the solution, respectively, V (L) is the volume of the solution, and m (g) is the dry mass of the adsorbent. TOC in the solution was measured by a TOC analyzer (TOC-V csh/csn series SSM-5000A, Shimadzu). Characterization. The morphologies of the raw barley straw pellets and coated pellets before and after nickel adsorption were observed by scanning electron microscopy (SEM; JSM 6010LV, JEOL). The nickel localization/ distribution analysis was performed using synchrotron X-ray fluorescence (XRF) spectroscopy on the VESPERS (Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron) beamline at the Canadian Light Source in Saskatoon, Saskatchewan, Canada.43 The polychromatic X-ray beam (pink beam) with an incident X-ray energy of 5−30 keV was employed to excite the sample. The emitted XRF spectrum was recorded by a Vortex@Silicon Drift Detector, which contains mainly Ni Kα and Kβ characteristic emission lines. The sample was mounted on a motorized XYZ stage for scanning. The beam size of ∼4 μm × 4 μm and the scanning step sizes of 50 and 10 μm were used for one- and twodimensional scans, respectively. The scan was executed along the diameter of the samples. Desorption Experiments. Desorption of nickel adsorbed on the coated barley straw pellets was investigated by first adsorbing nickel ions on the coated pellets by mixing with 500 and 1000 mg Ni/L solutions at pH 6.0 ± 0.2. The achieved equilibrium nickel uptakes were 12.1 ± 2.1 and 25.6 ± 2.8 mg/ g, respectively. Then they were separated from the nickel solution and mixed with DW at pH 2.0 ± 0.2 for desorption, which was adjusted by 0.1 M H2SO4. Because the nickel loaded on coated barley straw pellets was from a nickel sulfate solution, H2SO4 was selected to avoid the introduction of other anions into the desorption system. The ratio of solid (biosorbent) to liquid (elution solution) was 45 ± 1 g/L. Desorption was carried out for 6 h at room temperature because the desorption ratio decreased after 6 h. This may be because of partial dissolution of the pellet coating after 6 h, which may result in adsorption again. The desorption ratio was determined by the ratio of the amount of nickel released from the adsorbent at the end of desorption to the initially loaded nickel.25 6102

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RESULTS AND DISCUSSION

Coating Barley Straw Pellets. With the aim of investigating the effects of coating conditions on the stability of coated barley straw pellets, the OAD, also known as the Taguchi design, was applied. This array allows a consistent and balanced design and arranges different factors for the effective optimization of experimental conditions.44,45 The results of the OAD experiment can be statistically treated by the range analysis, which indicates the effect or importance of a given factor so as to determine the optimal coating conditions.46−48 In the present study, there are four factors: the ratio of SA/ PVA, GA dose, CaCl2 concentration, and cross-linking time. Each factor was considered at three levels. The experimental layout for barley straw pellets’ coating using L9(34) is presented in Table 1S in the Supporting Information (SI). The experiments’ sequence was randomly carried out to avoid personal or subjective errors. Each experiment was run in duplicate or triplicate. The average values of SCR, SR, and TOC-R of each experiment were presented in Table 1S in the SI. In the range analysis of OAD, the average (k) values of SCR, SR, and TOC-R at each level of a factor were calculated from the data listed in Table 1S in the SI and then presented in Table 2S and Figure 1S in the SI. The subscript of k represents a level of the factor. The range value of each factor was determined by the maximum value of k minus the minimum value. The higher the range value, the more important the factor is. The range analysis results of SCR indicated that the factor of the CaCl2 concentration had a significant effect on SCR of coated BS pellets because the range value of this factor was the highest. Similarly, the factor of the mass ratio of SA/ PVA was the most important factor for SR and TOC-R. In light of the results of the range analysis, the optimal coating conditions were proposed as follows: 1:1 mass ratio of SA/ PVA; 1.0 mL of GA, 8% CaCl2, and 20 min of cross-linking time, that is A1B2C3D1, which is not one of the nine experiments listed in Table 1S in the SI. Then the verification experiment was carried out to confirm the results of the optimal coating conditions, and the SCR, SR, and TOC-R results under A1B2C3D1 were 82.9 ± 5.3%, 88.46 ± 7.6%, and 17.1 ± 0.8 mg/g, respectively. The results confirmed that pellets coated under the optimal conditions obtained higher SCR and SR than those obtained at the conditions listed in Table 1S in the SI. In addition, TOC-R was significantly reduced compared with that of raw barley straw pellets, which was 44.6 ± 0.1 mg/g in DW. During the nickel adsorption process, TOC-R of coated barley straw pellets varied from 3.8 to 9.7 mg/g at different nickel equilibrium concentrations, which is even lower than that in a pure water system. Then pellets coated under the optimal conditions were used for the following nickel adsorption experiments. PZNC. Understanding the charge state of the adsorbents is important for metal-ion adsorption. The barley straw pellets coated by the SA/PVA membrane contain carboxyl, hydrogen, and amide groups,25,32 which are able to adsorb metal ions depending on their charge states. The experimental results of PZNC are shown in Figure 2; the intersection of the two curves represents the PZNC,41 which was around 4.5 for the coated barley straw pellets in this work. At this point, the pH did not change with the addition of 0.1 M Na2SO4. As a result, the solution pH has to be higher than the PZNC in order for the coated barley straw pellets to have sufficient negative charge to effectively bind positive nickel ions. This was also confirmed by

Figure 2. Determination of the PZNC of coated barley straw pellets: 40 ± 0.2 mL of DW or a 0.1 M Na2SO4 solution; 0.442 ± 0.024 g of pellets; shaking for 24 h at room temperature.

the results that the nickel uptake was optimized at about pH 6.0, as presented in the following section. The PZNC (4.5) of SA/PVA-coated barley straw pellets is similar to that of raw barley straw particles and acid-washed barley straw particles (4.0−4.5).1,25 Effect of the Solution pH on the Nickel Uptake. The solution pH is an important factor during metal adsorption because it affects the adsorbents’ charge state and speciation of the metal ions in solution. It was reported49,50 that, at pH < 7, nickel ions exist in the form of Ni2+. When the solution pH is higher than 7, hydroxide ions (OH− ions) in the solution can bind with NiII ions to form hydroxide complexes such as nickel hydroxide. In this work, the effect of the solution pH on nickel adsorption was investigated with a pH ranging from 2.0 ± 0.2 to 7.0 ± 0.2. Experiments of a blank nickel solution (without addition of the adsorbents) at the tested pH range were done to confirm that no nickel precipitates were formed. Each sample has the same initial nickel concentration of 250 mg/L, coated barley straw pellets’ weight of 10 ± 0.1 g/L, and solution volume of 40 ± 0.2 mL. The samples were shaken for 24 h at room temperature to ensure that the adsorption process would reach equilibrium (data not shown here). The results shown in Figure 3 indicated that, with increasing pH from 2.0 to 6.0, the

Figure 3. Effects of the pH on the nickel uptake: 40 ± 0.2 mL of a nickel solution; initial concentration of 250 mg/L; 0.402 ± 0.004 g of coated barley straw pellets; shaking for 24 h at room temperature.

nickel uptake increased, while the nickel uptake decreased from pH 6.0 to 7.0. Then pH 6.0 ± 0.2 was selected as the optimal value for nickel adsorption. The PZNC of the coated barley straw pellets was around 4.5. When the solution pH was higher than the PZNC, the coated barley straw pellets had more negatively charged sites and were able to bind nickel ions, while when the pH was increased to 7.0, the nickel uptake reduced, which is in agreement with that reported for nickel adsorption 6103

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Industrial & Engineering Chemistry Research by a husk of Lathyrus sativus.12 At pH 7, nickel may form a complex with hydroxide ions in the solution instead of effectively interacting with the negatively charged groups on the biosorbent. In addition, there may be dissolved carbon dioxide in the nickel sulfate solution. Around pH 7.0, the predominant form of the dissolved carbon dioxide is bicarbonate (HCO3−),44 which will react with nickel ions and prevent nickel ions from effectively accessing the negatively charged groups on the coated barley straw pellets. As a result, the optimal solution pH for nickel adsorption was selected as 6.0 ± 0.2 among the tested pH values. Effect of the Ionic Strength (IS). The increase in the solution IS can influence adsorption in two ways: (1) decreases the activity of the metal and (2) increases the concentration of competing ions.50 The effect of the IS on the nickel uptake was investigated with the addition of sodium sulfate to nickel-ion solutions. The solution pH was controlled to 6.0 ± 0.2. Sodium sulfate was selected as the background electrolyte because sodium is the alkaline light metal that has very low affinity for organic compounds and sulfate is the anion in the nickel solution.25 Then it is 50% possible to study the effect of the IS by decreasing the disturbance resulting from the background cations and anions. The results of IS effects on the nickel uptake are shown in Figure 4. At conditions where the initial

Figure 5. Nickel adsorption isotherms: 300 ± 1.0 mL nickel solution, pH 6.0 ± 0.2, 3.048 ± 0.037 g coated barley straw pellets, shaking for 24 h, and room temperature.

log Q e = log KF +

1 log Ce n

where Qe is the equilibrium uptake of the sorbate (mg/g), Qm is the maximum binding capacity (mg/g), Ce is the equilibrium concentration of the sorbate in the solution (mg/L), KL is the adsorption equilibrium constant of the Langmuir equation (L/ mg), KF is the adsorption coefficients, and n is the Freundlich constant. The simulation results of the Langmuir and Freundlich parameters were summarized in Table 3S in the SI. The correlation coefficient of the Freundlich model was 0.987, higher than that of the Langmuir model, 0.621. The Freundlich equation is applicable for a heterogeneous surface. The adsorption surface of the coated barley straw pellets may be involved in the surface of barley straw pellets and the surface of the SA/PVA coating membrane. However, the adsorption mechanism needs to be further investigated. The result will be of practical importance for engineering design and scale-up. Table 2 summarized a wide range of biosorbents reported in Table 2. Nickel-Ion Adsorption Uptake of Some Sorbents Reported in the Literature

Figure 4. Effect of the IS on the nickel uptake: 100 ± 0.5 mL of a 250 or 500 mg/L initial nickel solution; pH 6.0 ± 0.2; 1.048 ± 0.021 g of coated barley straw pellets; shaking for 3 h at room temperature.

adsorbent L. chinensis seeds C. reticulata Cyanophyceae Lyngbya taylorii husk of L. sativus sphagnum moss peat sugar cane bagasse cork powder chitosan-encapsulated Sargassum sp. barley straw acid-washed barley straw coated barley straw pellets

nickel concentrations were both 250 and 500 mg/L, the nickel uptake significantly reduced when the IS was increased from less than 0.02 M (without IS control) to 0.6 ± 0.05 M. Especially when the initial nickel concentration was 500 mg/L, the uptake reduced 52% from 12.15 ± 2.1 to 6.3 ± 1.4 mg/g. The results are in agreement with those previously reported for nickel adsorption.25,51 Adsorption Isotherms. The nickel adsorption isotherm by the coated barley straw pellets at the equilibrium pH of 6.0 ± 0.2 is shown in Figure 5. The IS of the solutions was less than 0.02 M. With increasing equilibrium nickel concentration, the nickel uptake increased. The equilibrium data were analyzed using the Langmuir equation (5) and Freundlich equation (6), which are two wellknown isotherm models used to describe the adsorption isotherms. Equations (5) and (6) are the linear expressions of the aforementioned two models: Ce C 1 = + e Qe Q mKL Qm

(6)

uptake (mg/g) 66.6 80−158 38.1 15.9 9.2 26.3 10.0 29.9 23.4 ≈9.00 25.6

reference Flores-Garnica et al., 201353 Ajmal et al., 200054 Klimmek et al., 200155 Panda et al., 200712 Ho et al., 199514 Garg et al., 20082 Chubar et al., 200352 Yang et al., 201118 Thevannan et al., 201025 Thevannan et al., 20111 this work

the literature used for nickel removal and adsorption. The nickel uptake ability of the coated barley straw pellets is slightly improved over that of the raw barley straw particles reported by Thevannan et al.,25 which confirms that the coating process has positive effects on the nickel adsorption. In addition, the coated barley straw has a nickel uptake higher than that of a husk of L. sativus,12 sphagnum moss peat,14 and cork powder,52 similar to sugar cane bagasse2 and chitosan-encapsulated Sargassum sp.18

(5) 6104

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Industrial & Engineering Chemistry Research but lower than Litchi chinensis seeds53 and Citrus reticulata,54 as shown in Table 2. Characterization. The coating treatment increased the surface area, which may result from the higher surface area of the coating materials SA and PVA. Further research is necessary to elucidate the mechanism of this aspect. The morphologies of the freeze-dried coated pellets with and without nickel uptake were examined with SEM, as shown in Figure 6. The image of a cross-sectional area along the diameter

The results are shown in Figure 7. The horizontal axis represents the position among the diameter of the cross-

Figure 7. XRF analysis of nickel disturbutions: (a) along the diameter direction on the pellet samples of raw barley straw (RBS), blank coated barley straw (CBS), and nickel-loaded CBS (25.6 ± 2.8 mg of Ni/g), the intensities of the RBS and blank CBS pellets magnified 100 times; (b) the nickel intensity distribution on the edge of the nickelloaded CBS (0.6 mm × 0.1 mm); the rectangular area in part a.

sectional area against the length of the cylindrical pellets. Position 32 represents one edge of the diameter, and position 40 represents the other edge. The results showed that the amounts of nickel in the blank coated barley straw pellet before nickel adsorption and the raw barley straw pellet without coating are both negligible. However, after adsorption, the nickel intensity became much higher (60−1200 times) in the pellet, which indicated an effective nickel adsorption. It is also discovered that the nickel intensity within the coating membrane was much higher (5−6 times) than that in the interior barley straw pellet. The results of the nickel intensity localization proved that (1) the coating membrane exhibits a better capability for nickel adsorption and (2) during the adsorption process, the nickel ions were first adsorbed by the coating membraneand then gradually and uniformly adsorbed by the interior barley straw material in the pellets. The SEM images (Figure 6d,e) showing particle aggregates in the coating membrane and the interior barley straw could be another indication. The nickel intensity within the barley straw pellets was lower than that on the coating membrane because, after the adsorption of nickel ions on the coating material, the mass transportation force between the nickel ions and barley straw reduced. Although the membrane materials (SA and PVA) have higher nickel uptake, they are more expensive. Thus, using them for coating not only can enhance the uptake and stability but also is more economical. Desorption of Nickel Loaded on the Coated Barley Straw Pellets. Desorption of nickel loaded on the coated barley straw pellets (12.1 ± 2.1 and 25.6 ± 2.8 mg of Ni/g) was investigated in DW of pH 2.0 ± 0.2, which was adjusted by a 0.1 M H2SO4 solution. The results indicated that, after 6 h of elution, the desorption ratio of nickel from the barley straw pellets initially containing 12.1 ± 2.1 mg of Ni/g was 26.1%, while that from the one initially containing 25.6 ± 2.8 mg of

Figure 6. SEM images of coated barley straw pellets: (a) cutting surface; (b and c) a coated pellet without nickel adsorption; (d and e) a nickel-loaded coated pellet (25.6 ± 2.8 mg of Ni/g).

of a blank pellet (Figure 6a) demonstrated that the barley straw particles were successfully coated by the SA/PVA membrane, and the thickness of the membrane was around 80 μm. Figure 6b shows that there were pore structures in the membrane and the interior barley straw particles. The surface of the coated membrane was rough, which is shown in Figure 6c. After the nickel adsorption process, particles (around 3−8 μm) appeared on the coated membrane and the interior barley straw material (Figure 6d,e), which are suspected of being nickel clusters. The pore structure of the membrane and the barley straw material may contribute to the nickel ion transporting through the coated membrane to the interior barley straw material. Because the nickel-loaded pellets were freeze-dried, the nickel ions were dehydrated and aggregated on the membrane and interior pellets. In order to confirm whether nickel was adsorbed on the pellets and investigate the distribution of the adsorbed nickel in the coated pellets, XRF analysis was performed along the diameter of a cross-sectional area of a pellet with a nickel uptake (25.6 ± 2.8 mg/g) based on the mass balance calculation of nickel in the solution before and after adsorption. In order for comparison, same XRF analyses were also carried out on a blank coated pellet and a raw pellet without coating. 6105

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Qm = maximum binding capacity, mg/g V = volume of solution, L VTOC = volume of DW used for mixing water and pellets to determine TOC release, L

Ni/g was 62.5%. The results demonstrated that nickel ions adsorbed on the coated barley straw pellets were able to be desorbed. However, the desorption ratio is still low. This may result from the equilibrium established between the nickel concentration on the coated barley straw pellets and in the solution. The results also indicated that adsorption of nickel on coated barley straw pellets might involve mechanisms including chelation and electrostatic attraction because the binding of nickel on coated barley straw pellets was stronger than that of nickel on chitosan-encapsulated Sargassum sp. biosorbents mainly by ion exchange.18 More work needs to be done on the systematic study of desorption.

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Abbreviations

ASSOCIATED CONTENT

* Supporting Information S

L9(34) orthogonal array and experimental results, range analysis results, and Langmuir and Freundlich parameters. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00518.

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AUTHOR INFORMATION

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Corresponding Author

*Tel.: 1-306-966-2174. Fax: 1-306-966 4777. E-mail: catherine. [email protected].

CBS = coated barley straw DW = deionized water GA = glutaraldehyde PVA = poly(vinyl alcohol) RBS = raw barley straw SA = sodium alginate SCR = ratio of the number of successfully coated (non broken) pellets to the total number of raw pellets used for coating at each batch SR = ratio of the number of stable pellets to the total number of pellets initially added into DW mixed at 250 rpm for 24 h TOC = total organic carbon TOC-R = total organic carbon release into the solution, mg/ g of dry fresh adsorbent XRF = X-ray fluorescence SEM = scanning electron microscopy

REFERENCES

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Natural Science and Engineering Research Council of Canada Discovery Grant, Canada Foundation for Innovation Leaders Opportunity Fund, and Jiangsu Government Scholarship for Overseas Studies (Grant JS-2012-245). The synchrotron XRF measurements were performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, National Research Council Canada, Canadian Institutes of Health Research, Province of Saskatchewan, Western Economic Diversification Canada, and University of Saskatchewan. We sincerely thank Blondin Richard of the Department of Chemical and Biological Engineering at the University of Saskatchewan for his help in AAS analysis.

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NOMENCLATURE C = nickel-ion concentration at time t in the solution, mg/L C0 = initial nickel-ion concentration, mg/L Ce = equilibrium concentration of the sorbate in the solution, mg/L CTOC = TOC concentration after 24 h of mixing, mg/L KF = adsorption coefficients in the Freundlich model KL = adsorption equilibrium constant of the Langmuir equation, L/mg M = dry mass of fresh adsorbent, g mp = total mass of the dry coated pellets or dry raw pellets added to the water, g Ns = number of stable pellets after 24 h of shaking in DW at 250 rpm Nt = total number of pellets added to the water Ntc = total number of raw pellets in the coating process Nw = number of successfully coated pellets n = Freundlich constant Q = amount of nickel adsorbed per gram of sorbent, mg/g Qe = equilibrium uptake of the sorbate, mg/g 6106

DOI: 10.1021/acs.iecr.5b00518 Ind. Eng. Chem. Res. 2015, 54, 6100−6107

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