Modular Cross-Linked Chitosan Beads with Calcium Doping for

Article pubs.acs.org/IECR

Modular Cross-Linked Chitosan Beads with Calcium Doping for Enhanced Adsorptive Uptake of Organophosphate Anions Mohammad H. Mahaninia and Lee D. Wilson* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada S Supporting Information *

ABSTRACT: Chitosan beads were cross-linked at variable composition with glutaraldehyde (GA) and epichlorohydrin (EP), respectively. The beads were post-treated by impregnation with a CaCl2 solution and characterized to evaluate the structure and physicochemical effect of calcium doping. The bead adsorption properties were studied at pH 8.5 with pnitrophenyl phosphate (PNPP), where beads cross-linked with GA showed higher uptake relative to beads cross-linked with EP. Calcium doping of GA beads showed a 4-fold greater uptake (0.97 mmol g−1) over non-cross-linked (NCL) beads (0.23 mmol g−1). By comparison, EP-based beads with calcium doping showed a 2-fold enhancement for the uptake of PNPP (0.90 mmol g−1) over NCL beads. This work illustrates the utility of cross-linking and calcium doping as modular strategies for tuning the adsorption behavior of chitosan-based beads. Calcium-doped beads cross-linked with glutaraldehyde showed favorable adsorption−desorption properties where the uptake capacity of PNPP remained relatively constant (19.6−17.5%) over several regeneration cycles. The results of this work contribute significantly to the development of advanced materials for the controlled uptake and treatment of waterborne phosphate species.

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INTRODUCTION Organophosphates (OPs) are esters of phosphoric acid that are used for the synthesis of many pesticides, insecticides, herbicides, and fertilizers. OPs are listed as acutely toxic to humans and the environment, as outlined by the United States Environmental Protection Agency.1 By contrast, OPs are constituent components in biochemical systems of living cells and biomolecular components in DNA and RNA. Also, OPs can inhibit enzyme activity and have adverse effects on neuronal activity.2,3 To address the adverse effects of OPs in aquatic systems, the development of effective removal strategies has become a vital research inquiry. Several methods have been reported for the removal of OPs and include enzymatic biodegradation,4 electrochemical,5 catalytic oxidation,6 atmospheric pressure plasma,7 photolytic,8 and hydrolysis methods.9−11 More recently, adsorption-based methods offer an alternative means for the removal or elimination of OPs using clay,12 soil components,13 and zeolites.14,15 Chitosan is an abundant and biodegradable biopolymer16 with variable structural forms that include powders, flakes, and beads. Chitosan materials have been examined for their removal efficacy of heavy metals,16 dyes,17 and inorganic ions.18 Synthetic modification of the hydroxyl and/or amino groups of chitosan can yield materials with tunable physicochemical properties.17 Modification of materials via cross-linking yields products with differing acid stabilities, mechanical strengths, pore-size distributions, and hydrophobicities. In general, crosslinking often increases the adsorption capacity of sorbents due © 2016 American Chemical Society

to changes in the surface area (SA), pore structure properties, and surface functionality of materials.18−20 Covalent crosslinking of chitosan with bifunctional units using epichlorohydrin or glutaraldehyde, along with divalent metal-ion coordination, contributes to variation of the structure and chemical properties, along with the creation of favorable adsorption sites. Modification of chitosan by this approach has been reported as a convenient strategy for targeting the uptake of anion species.20−24 The application of cross-linked chitosan bead materials for the uptake of inorganic phosphate (Pi) was reported previously,25 where variable cross-linking was reported to affect the adsorption properties with Pi and a phenolic dye in an incremental manner. In another study, the factors affecting the phosphate adsorption properties of chitosan beads were studied using several complementary techniques.26 Synthetic modification of chitosan beads via covalent cross-linking resulted in materials with enhanced adsorption properties toward Pi due to variation in the hydrophile−lipophile balance (HLB) of the chitosan bead systems. The HLB effect was related to the nature of the cross-linkers (epichlorohydrin and glutaraldehyde) due to differences in their relative apolar character.25 By contrast, the surface charge of chitosan may be varied Received: Revised: Accepted: Published: 11706

July 26, 2016 September 14, 2016 October 24, 2016 October 24, 2016 DOI: 10.1021/acs.iecr.6b02814 Ind. Eng. Chem. Res. 2016, 55, 11706−11715

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Industrial & Engineering Chemistry Research incrementally by incorporating calcium ions through coordinate cross-linking. In turn, the creation of Lewis acid sites is posited to favor electrostatic binding of such modified chitosan beads with Pi species.27 The uptake of phosphate anions at pH 8.5 using chitosan represents a challenge because the surface charge of chitosan is close to neutral for such conditions. In the case of cross-linked beads described above, the incorporation of Ca2+ onto the surface of the chitosan affords Lewis acid binding sites for suitable anions over a broad range of pH conditions, in contrast to beads without metal-ion incorporation. The chelation of chitosan with metal species such as zirconium,21 iron,22 lanthanum,23 and copper24,28 provide supporting evidence that this strategy favors the uptake properties of anions. The use of such metals and the potential role of leaching may pose challenges to human and environmental health because of their potential toxicity. By contrast, calcium is a relatively benign alternative that offers modular Lewis acid binding sites for chitosan materials to enhance the anion adsorption properties. There are sparse studies related to the adsorption properties of materials doped with calcium. In a report by Kumar et al.,29 calcium-impregnated activated charcoal from Jatropha seeds was reported to have favorable uptake properties toward AsIII species in aqueous solution. As well, Mondal et al.30 investigated the adsorptive removal of arsenic species from groundwater using granular activated carbon (GAC) doped with Ca2+. They report that the GAC/Ca2+ sorbent remains positively charged over a relatively wide pH range according to the enhanced adsorption of AsIII. Herein, we report the preparation and characterization of cross-linked chitosan beads with glutaraldehyde and epichlorohydrin at variable levels, along with divalent coordination of calcium ions. The equilibrium adsorption properties of these materials with a beadlike morphology were studied in aqueous solution with a model OP (p-nitrophenyl phosphate; PNPP) at pH 8.5 to assess the role of structural modification on adsorption. The bead materials were characterized using thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), confocal microscopy, and solid-state NMR/IR spectroscopy. PNPP was chosen as a model OP herein because it can be used as a surrogate model of orthophosphate and other typical OP pesticides listed in Table 1. A study of the adsorption properties of PNPP may provide a greater understanding of the molecular-level processes relevant to the uptake of other phosphate species. PNPP is a suitable chromogenic compound for direct UV−vis detection, and it bears a structural resemblance to other related OPs such as paraxon and parathion in Table 1. The results of this study will contribute to the development of advanced materials for the efficient removal of Pi and OP species.

Table 1. Comparison of PNPP and Related OP Pesticides

glacial acetic acid solution (2.0%) and then added dropwise to a 0.5 M NaOH aqueous solution using a volumetric buret. The resulting spherical beads were left in aqueous NaOH for a minimum of 16 h to neutralize the residual acid. After removal from aqueous NaOH, the beads were washed with Millipore water until the washings yielded a neutral pH. After washing, the wet beads were cross-linked using 2.5 and 5.0 wt % GA (or EP) by steady agitation for 48 h. GA cross-linking employed pH 7 and 295 K, while EP was cross-linked at pH 14 and 60 °C. Subsequently, unreacted cross-linker was removed from the beads after immersion in water for 1 day with repeated washing using Millipore water. Thereafter, the beads were imbibed with excess calcium using a 0.1 M solution. The required calcium to react with chitosan was calculated based on the number of chitosan monomer units according to the number of available amine groups of chitosan. The calcium solution was prepared by dissolving CaCl2 (0.1 M) in Millipore water using a 10-fold excess to achieve sufficient calcium doping. The sample IDs and composition of the bead systems are summarized in Table 2. Characterization. TGA curves were recorded on a Q50 TA Instrument analyzer with the aluminum sample pans. Samples were heated to 308 °C and allowed to equilibrate for 5 min prior to heating at 58 °C min−1 up to 500 °C in a nitrogen atmosphere. IR spectra were obtained in reflectance mode using a Bio-Rad FTS-40 spectrophotometer to yield diffusereflectance infrared Fourier transform (DRIFT) spectra. Samples weighing ca. 6 mg were mixed with 60 mg of spectroscopic-grade KBr and corrected relative to a background spectrum of KBr in reflectance mode. The spectral resolution was 4 cm−1 over the 400−4000 cm−1 region. The concentration of PNPP in aqueous solution was obtained using a Varian Cary100 Scan UV−vis spectrophotometer at λmax = 405 nm. Solid-state 13C NMR spectra were obtained with a wide-bore (89 mm) 8.6 T Oxford superconducting magnet system equipped with a 4 mm CP-MAS (cross-polarization with magic angle spinning) solids probe. An Avance DRX360 console and workstation running TopSpin 1.3 (Bruker Bio Spin Corp.; Billerica, MA) was used to control the acquisition parameters using standard pulse programs. The samples were

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MATERIALS AND METHODS Materials. CaCl2, glutaraldehyde (GA), epichlorohydrin (EP), low-molecular-weight chitosan (75−85% deacetylation with a molecular weight range of 50000−190000 kDa), and pnitrophenyl phosphate (PNPP) were purchased from SigmaAldrich Canada (Oakville, Ontario, Canada). All chemicals were used without further treatment unless specified otherwise. Synthesis of Cross-Linked Chitosan Beads. Figure 1 shows the cross-linking process for chitosan beads with two types of cross-linkers (EP and GA) followed by calcium doping. The procedure for the preparation of the beads is briefly outlined. Chitosan flakes (5 g) were dissolved in a 250 mL 11707

DOI: 10.1021/acs.iecr.6b02814 Ind. Eng. Chem. Res. 2016, 55, 11706−11715

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Industrial & Engineering Chemistry Research

Figure 1. Illustration of covalent cross-linking and calcium doping of chitosan beads. Two coordination models for calcium-doped chitosan are shown by the bridge model (interior of blue rectangle) and the pendant model on the chitosan surface.

collected using a personal computer and analyzed by ZEISS LSM software, version 4.2. Diffraction spectra of the bead materials were obtained using a PANalytical Empyrean powder X-ray diffractometer. Monochromatic Co Kα1 radiation was used where the applied voltage and current were set to 40 kV and 45 mA, respectively. The samples were mounted in a horizontal configuration after evaporation of methanol films. The PXRD patterns were measured in continuous mode over a 2θ range of 7−50° with a scan rate of 3.2° min−1. The crystallinity index (CrI) was obtained by calculation of the height ratio between the intensity of the crystalline peak (Icr) and that of the amorphous peak (IAM). Many studies are limited by evaluation of CrI, based on the ratio of the peak heights. The equation CrI = (Icr − IAM)/Icr estimates the value of CrI by showing the comparative fraction of the crystallinity of samples to a greater or lesser extent.31 To estimate the calcium leaching from the doped chitosan beads, approximately 0.1 g of beads was placed in a beaker after the imbibing step. The volume of Millipore water (0.5 L), where the Ca2+ level was measured at variable time intervals, used a hydride generation atomic absorption spectrophotometer (HGAA, novAA 300, Analytik Jena Group) at λmax = 422.7 nm. Adsorption Studies. PNPP Adsorption Isotherms. Fixed amounts (∼10 mg) of the beads were mixed with 10 mL of PNPP at variable concentration (5−70 mM). Samples were equilibrated on a horizontal shaker table (SCILOGEX model SK-O330-Pro) at 295 K for 24 h and 250 rpm. The concentration was determined before adsorption (C0) and after adsorption (Ce) in triplicate in a sodium bicarbonate buffer (0.01 M) at pH 8.5. The relative uptake of the adsorbate was determined by the difference between the initial blank (C0) and residual PNPP (Ce) concentrations in solution after the adsorption process using eq 1. The error bars for the isotherm results denote the standard error determination. Regeneration Study. To investigate the regeneration properties of a selected PNPP/bead system at equilibrium conditions, four cycles of adsorption−desorption were

Table 2. Chitosan Bead Materials with Variable Composition and Cross-Linker Units bead material (sample ID) NCL EP 2.5 GA 2.5 EP 5 GA 5 NCLCA EP2.5CA GA2.5CA EP5-CA GA5-CA

chitosan mass (mg)

crosslinker typea,b

crosslinker wt %c

crosslinker/ chitosan mole ratiod

160 160 160 160 160 160

EPI GLU EPI GLU

2.5 2.5 5.0 5.0

1.7 1.6 3.4 3.2

0 0 0 0 0 10

14 7 14 7 7

160

EPI

2.5

1.7

10

14

160

GLU

2.5

1.6

10

7

160 160

EPI GLU

5.0 5.0

3.4 3.2

10 10

14 7

calcium/ chitosan monomer mole ratio

pH of the crosslinking reaction

a

Glutaraldehyde is denoted as GLU. bEpichlorohydrin is denoted as EPI. cThe weight percent is based on the cross-linker weight content in solution used for cross-linking. dChitosan is assumed to be fully deacetylated, where all monomers are available for cross-linking.

packed in 4-mm-outer-diameter zirconium oxide rotors capped with Teflon MAS rotor caps. Acquisition was carried out with MAS at 5 kHz along with a 2 s recycle delay and 750 μs crosspolarization. Confocal microscopy (Bio-Rad MRC-1024) images were obtained to investigate the morphology and accessibility of dye species on the adsorption sites of the bead materials. The microscope was attached to a Nikon Diaphot inverted microscope equipped with a 15 mW krypton/argon laser and fluorescein isothiocyanate (excitation filter 470−490) filter sets. Individual beads were analyzed by horizontal scanning (section scanning) before and after dye adsorption. The background fluorescence was negligible, and the bead samples were viewed using a 10×, 0.30 plan fluor objective. Digital images were 11708


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Figure 2. IR and TGA results for chitosan materials: (A) FTIR spectra and (B) TGA curves for (a) NCL-CA, (b) EP2.5-CA, (c) GA2.5-CA, (d) EP5-CA, and (e) GA5-CA.

KL is the Langmuir adsorption constant, and Ce and Qm are defined as above.

performed using GA5-CA as a model system among those investigated herein. Aqueous NaCl (50 mM) was employed as an eluent to wash the beads after each PNPP adsorption step. Because the adsorption of PNPP onto the chitosan beads is primarily physical in nature, saturation of the binding sites on the beads using the salt solution results in desorption of the adsorbed PNPP due to competitive ion binding effects. The total and residual concentrations of PNPP were measured using a Varian Cary-100 Scan UV−vis spectrophotometer at λmax = 405 nm. Equations and Models. The equilibrium uptake (Qe, mmol g−1 or mg g−1) of the adsorbate species by the adsorbent phase from aqueous solution was plotted against the residual equilibrium concentration of unbound adsorbate species (Ce, mM) according to eq 1.32 Qe =

(C0 − Ce)V m

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RESULTS AND DISCUSSION Characterization. Figure 2A illustrates the Fourier transform infrared (FTIR) spectra of calcium-doped chitosan beads, where coordination of Ca2+ by chitosan occurs by the “‘bridge’” model (cf. Figure 1). The formation of Ca2+ complexes concurs with the enhancement of the CO band at around 1150 cm−1, and the N−H bending of amine groups shows a spectral shift from 1635 to 1591 cm−1 for Ca2+-doped samples (cf. Figure 3 in ref 25). The characteristic N−H bands for cross-linked beads with Ca2+ doping are not observed, and this suggests that fewer amine groups are available on the surface compared with crosslinked materials before Ca2+ doping. The bands above 3500 cm−1 for chitosan beads are not shown for samples without Ca2+ doping. The TGA results shown in Figure 2B for Ca2+-doped samples appear at lower temperature relative to those for undoped chitosan materials. The temperature shift for these thermal events relates to the effects of doping because metal-ion coordination likely reduces the effects of intra- and interchain hydrogen bonding of chitosan. Beads doped with Ca2+ also show thermal events over the 200−300 °C range (cf. Figure 2B), indicating that calcium is bound at multiple coordination sites.35 Thermal events in the 300−500 °C range relate to the effects of GA and EP cross-linking, according to the greater thermal stability reported for such cross-linked polymers.36 The reduced temperature stability of cross-linked chitosan with Ca2+ relates to the formation of chitosan/metal-ion complexes according to the anticipated reduction in the activation energy for the thermal degradation process.37 Characterization of the bead materials in solution versus the solid state has limitations because of the potential exchange of calcium ions with water on the surface of chitosan, resulting in structural alteration or dissolution effects. Dissolution of chitosan materials in aqueous solvents typically requires mild acids, ionic liquids, or harsh solvents such as aqueous urea/ NaOH. To overcome the limited solubility of chitosan materials and the use of harsh solvents, solid-state 13C NMR spectra were obtained (cf. Figure 3). NMR spectroscopy

(1)

C0 is the initial adsorbate concentration, V is the volume of the solution, and m is the mass of the adsorbent material.32 The equilibrium uptake of PNPP by the bead systems was analyzed by various isotherm models, including the Sips model, defined by eq 2.33 Qe = Qm

(K sCe)ns 1 + (K sCe)ns

(2)

Ks is the adsorption constant, ns represents the heterogeneity parameter of the sorbent surface, Ce is the residual adsorbate concentration, and Qm is the monolayer adsorption capacity of the sorbent. Equation 2 provides a measure of the heterogeneity of the adsorption process according to the value of ns, where Langmuir behavior is described when ns ≈ 1. In cases where ns deviates from unity, this model accounts for surface heterogeneities.33 Thus, the Sips isotherm accounts for heterogeneous sorption behavior according to the value of ns. The Langmuir model accounts for monolayer adsorption behavior of equivalent adsorption sites by eq 3.34 Qe = Qm

KLCe 1 + KLCe

(3) 11709


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Figure 3. Solid-state CP-MAS 13C NMR spectra of bead materials at 295 K: (a) before doping with calcium; (b) after calcium doping. Spectral acquisition at a MAS rotational speed of 5 kHz, 2 s recycle delay, and 750 μs cross-polarization time.

Figure 4. Confocal microscopy images of GA5 chitosan beads with FL (1 mM) at variable exposure times with 10× image magnification: (a) 2 min; (b) 10 min; and (c) 30 min.

The −CH3 band (23.9 ppm) of the acetyl group has a notable line intensity, in accordance with its shorter T1 value relative to the carbonyl group described above. An increase in the intensity of the C6 group (60.8 ppm) indicates that crosslinking occurs with EP, while CN bond formation between chitosan and GA is evidenced by a CN signature at 149 ppm. GA2.5 and GA5 reveal the greatest variation in the line broadening relative to NCL, especially for calcium-doped beads. The cross-linker signature lies in the 20−50 ppm region, where a new band appears at ca. 45 ppm for GA2.5-CA and GA5-CA beads. The effect of cross-linking due to calcium coordination appears greater in the case of chitosan with GA cross-linking, as evidenced by line-broadening effects. Coordination of Ca2+ with chitosan results in greater structural rigidity, which enhances the spin−spin coupling, in accordance with the observed line broadening. The 13C spectral signatures for Ca2+doped materials occur over a large spectral bandwidth, resulting in an apparent decrease in the resolution and peak amplitude due to spin−spin coupling with longer T2 values.40 Confocal microscopy in conjunction with dye staining provides useful structural information for materials with

provides useful information on the structure and short-range interactions in biopolymers, especially amorphous materials such as chitosan. In Figure 3, the 13C NMR spectra for the cross-linked materials reveal greater line broadening compared with the chitosan powder or NCL. The observed broadening relates to the reduced motional chain dynamics (longer correlation times) due to cross-linking effects, as reported for chitosan with EP and GA.38,39 In general, the spectral features for chitosan beads with and without Ca2+ doping share similar features aside from variable line broadening of the biopolymer framework (ca. 55−110 ppm). The spectral signatures of chitosan (NCL) were assigned as follows; C1 (105.3 ppm), C4 (83.6 ppm), C3,5 (57.6 ppm), C6 (60.8 ppm), and C2 (57.7 ppm). The carbonyl group (C7) at 173.0 ppm has low intensity and relates to the low level of acetylation and/or longer relaxation time (T1) of the CO group in NCL and EP beads. By comparison, the greater line intensity for C7 in GA crosslinked beads may be due to the open-chain form of GA that contains an unreacted CO group or variation in the hydrate content of the biopolymer, which affects the cross-polarization transfer efficiency. 11710


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Industrial & Engineering Chemistry Research variable morphology, especially when dyes are localized in specific chemical environments such as hydrophilic versus hydrophobic binding domains.41 Previously, we illustrated the utility of a dye-based method using phenolphthalein (Phth) to estimate the surface accessibility of hydroxyl functional groups of polysaccharides.42,43 Herein, fluorescein (FL) was used as a dye probe to map the structure and morphology of chitosan beads due to their similar chemical structures to Phth. Figure 4 shows several representative confocal images for the GA5 bead system in the presence of FL at defined exposure intervals (2, 10, and 30 min). The variable fluorescence emission profile along the diameter of the bead (30 μm) reveals the diffusion path of the dye into the bead microstructure after exposure to FL in solution. The initial FL concentration (C0 = 1 mM) was fixed, and the progression of dye diffusion from the outer bead surface (bulk solution) to the interior adsorption sites is observed in Figure 4. The continual migration of dye over this interval indicates that cross-linking does not impede dye diffusion. Likewise, it can be inferred that anion species such as PNPP may access the inner micropore structure of the chitosan beads. Thus, adsorbates are able to diffuse through the biopolymer network and cross-linked domains of the bead over relatively short intervals (30 min), even for beads at the higher cross-linker ratios employed herein. The long-range structural features of chitosan after crosslinking and doping with calcium were investigated using PXRD, as shown in Figure S1. The X-ray diffraction (XRD) patterns of NCL show two diffraction peaks at 2θ = 10° and 20° with greater intensity and sharper features relative to the cross-linked bead materials, in the presence and absence of calcium doping. The greater amorphous character of modified beads indicates disruption of the intra- and intermolecular hydrogen-bonding network of NCL due to inefficient packing of the biopolymer chains. Greater line broadening occurs for bead materials crosslinked with GA relative to beads cross-linked with EP, where the effect is more pronounced for calcium-doped beads. The signature at 2θ = 10° is lost for GA5-CA and EP5-CA, and the appearance of a broad signature occurs at 2θ = 15°. Similar XRD patterns are observed for NCL and NCL-CA, which indicates that low uptake of calcium occurs, in contrast to the cross-linked bead systems. The leaching of calcium was examined (cf. Figure S2), where the level of leaching relates to the relative stability of the calcium complex for the bead system. In general, the greatest leaching is observed for NCL, while reduced leaching occurs for cross-linked beads. The leaching results find indirect support from the PXRD results, where it can be inferred that crosslinked beads containing GA and EP have greater calcium content because of their greater binding affinity, compared with NCL beads. Isotherm Studies of PNPP. In order to provide a detailed understanding for the adsorption capacity and affinity of PNPP with the various bead systems, adsorption isotherms were obtained at pH 8.5 and 295 K (cf. Figure 6). In Table 3, the monolayer adsorption capacity (Qm) provides a measure of the uptake of PNPP along with the best-fit results according to the Langmuir isotherm model. A comparison of the Qm values for the bead materials indicate that GA5 displays the highest uptake relative to the other bead systems. The cross-linker content of GA5 concurs with the CHN analysis.25 The relationship between the crosslinker content and the adsorption capacity seems to follow a reverse or proportional relationship, depending on the polarity

Table 3. Langmuir Isotherm Parameters for Various Bead Materials Langmuir isotherm model bead material (sample ID)

Qm (mmol g−1)

KL (L mmol−1)

R2

NCL EP 2.5 GA 2.5 EP 5 GA 5 NCL-CA EP2.5-CA GA2.5-CA EP5-CA GA5-CA

0.23 0.53 0.57 0.54 0.62 0.40 0.67 0.78 0.90 0.97

0.14 0.09 0.12 0.07 0.08 0.12 0.07 0.12 0.05 0.07

0.98 0.98 0.99 0.99 0.98 0.98 0.99 0.98 0.98 0.99

of the adsorbate.25,44 In a previous report, the adsorption capacity of Pi was found to be inversely proportional to the level of EP cross-linking. The trend in uptake of the Pi dianion relates to several factors: (i) adsorption site accessibility; (ii) HLB properties of the chitosan bead; (iii) SA and micropore domains of beads (cf. Figure 6 in ref 25). As mentioned above, the relative polarity of the OP species follows a trend in uptake related to the role of hydrophobic effects and dipolar interactions for various bead systems. In Figure 5, greater

Figure 5. Effect of cross-linking and doping on the KL values (cf. eq 3).

uptake of PNPP (or decreasing KL) occurs as the GA content of the bead increases. The uptake of PNPP parallels the uptake of p-nitrophenolate (PNP) anions,25 according to the lipophilic binding contributions of the apolar cross-linker domains of GA with PNP. Turning to Figure 5, KL decreases ca. 10−20% because of cross-linking and further decreases after calcium doping. The greatest effect is evident for NCL and NCL-CA, which relate 11711


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Figure 6. Adsorption isotherms of various bead materials at pH 8.5: (a) beads without calcium doping; (b) beads after calcium doping. The best-fit line through the experimental data represents the best fit according to the Langmuir model.

Figure 7. Variable binding of PNPP with different sites on the surface of chitosan beads.

favorable adsorption sites for PNPP because of the greater presence of Lewis acid sites. The cross-linking and formation of bead/Ca2+ complexes are inferred to result in greater binding site accessibility because of the pillaring ef fects of chitosan beads (cf. Scheme 1 in ref 45). Favorable R2 values provide suitable best-fit criteria between the experimental and calculated results. However, the Langmuir model results were chosen for the present discussion based on the comparable best-fit results between the Sips and Langmuir models (cf. Tables 3 and S1). As the cross-linker ratio increases, the Qm value increases and reveals that the adsorption site for PNPP binding is favored because of the enhanced SA and pillaring effects.45 Also, GA cross-linked beads may present secondary binding sites20 because of the formation of imine linkages and the variable structure of GA in open-chain and cyclic forms.46 The cross-linking of the open-chain form of GA was reported as a potential sorption site for phenolic compounds.47 The presence of unreacted functional groups such as hydroxyl, amine, and incomplete cross-linking48 may contribute to the heterogeneity of the chitosan adsorption properties because of their variable binding contributions outlined above. Evidence of heterogeneities relate to the ns values of cross-linked beads (cf. Table S1). In the case of

the importance of amine and hydroxyl groups as the active adsorption sites. Although cross-linking alters the number of functional groups on the surface of the chitosan beads, there are changes in the bead SA and adsorption site accessibility upon cross-linking. Thus, cross-linked bead materials show higher Qm values with PNPP despite the decreasing KL values. This can be understood on the basis of variation in the textural properties with cross-linking due to pillaring ef fects,45 as described above. According to Figure 6A, calcium doping has a pronounced effect on the adsorption capacity of beads as the cross-linker content increases, where GA beads have a greater effect on EP cross-linked beads. This observation can be understood because GA-containing beads possess more favorable binding sites for PNPP along with chelation sites for calcium. By comparison, NCL and EP cross-linked beads display lower uptake of PNPP and reduced calcium doping, in agreement with the reduced binding sites and lower uptake of calcium ions. Figure 6B illustrates the PNPP adsorption results for calcium-doped beads at pH 8.5 in aqueous solution. The adsorption results for doped beads share trends similar to those seen in Figure 6A. However, the doping of beads with calcium enhances the uptake of PNPP, especially for GA2.5-CA and GA5-CA. The formation of bead/Ca2+ complexes provides 11712


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Industrial & Engineering Chemistry Research calcium-doped beads, the value of ns ≈ 1 indicates the prominent role of the calcium binding sites for the uptake of PNPP and the binding site variability of doped versus undoped beads. The GA cross-linker domains favor the binding of PNPP through apolar binding contributions with the linker sites. By comparison, the cross-linker domains of EP are more hydrophilic in nature, in agreement with the favorable uptake of Pi dianion species.25 The model parameters indicate that beads with greater cross-linking have greater SA and enhanced adsorptive properties. By contrast, calcium-ion coordination enhance pillaring effects,45 especially in the case of GA-based chitosan beads. On the basis of the structural similarity of PNP and PNPP, it may be inferred that the sorption processes for PNPP and PNP are similar at pH 8.5. PNPP is more lipophilic in nature relative to Pi species because of the phenyl moiety of the adsorbate. The apolar character of PNPP (and PNP) displays preferred adsorption of the phenyl group onto the apolar domains of the bead, especially at the GA cross-linker domains. Greater uptake of Pi occurs with beads cross-linked with EP relative to GA because of the greater hydrophilic character of EP-containing beads. The pKa values for PNPP (pKa1 = 4.63 and pKa2 = 5.20)49 are variable, where PNPP exists as a dianion species at pH 8.5 because the pKa values lie below the pH of the solution at this condition.49 The phosphate group of PNPP is hydrophilic and contributes favorable interactions with the bead via hydrogen bonding and electrostatic forces, especially for beads containing Ca2+ binding sites (cf. Figure 7). The concentration of bound PNPP with the GA2.5-CA bead system is shown in Figure 8 for four cycles of adsorption−

bead systems, in contrast to sorbent materials with a powder morphology. The sorptive uptake capacities of different species that are structurally related to OPs are described in Table 4. The uptake Table 4. Comparison of the Monolayer Sorption Capacities (Qm) of OPs and Related Adsorbates with Various Adsorbents adsorbent mango kernels powder soil powder soil powder GA cross-linked chitosan powder GA5 bead (no Ca2+) GA5-CA bead (with Ca2+)

isotherm model

Qm (mmol g−1)

pH

Langmuir

0.42

6.0

50

Langmuir

0.33

7.0

51

Langmuir

0.32

7.0

51

Langmuir

0.98

7.0

48

PNPP

Langmuir

0.62

8.5

PNPP

Langmuir

0.97

8.5

this study this study

adsorbate methyl parathion methyl parathion ethyl parathion roxarsone

ref

capacities of the beads (GA5 and GA5-CA) for PNPP are significantly higher than other reported values for OPs. The results reported herein for PNPP are in good agreement with the sorptive uptake of roxarsone (4-hydroxy-3-nitrobenzylarsonic acid), which is structurally similar to that of PNPP in this study.

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CONCLUSION This study demonstrates that chitosan bead materials crosslinked with GA and EP serve as versatile adsorbents for the removal of PNPP. The modular design employing calcium doping represents a facile approach for tuning the sorption capacity especially for beads cross-linked with GA. The formation of bead/Ca2+ complexes enhances PNPP uptake by introducing Lewis acid sites onto the pillared bead structure, as evidenced by a 4-fold enhancement in uptake. This approach widens the range of pH conditions for the efficient uptake of PNPP and other anion-based species such as orthophosphate. Synergistic effects related to cross-linking enhance the sorption capacity of chitosan beads. The variable hydrophile−lipophile characteristics of the bead surface of GA-based beads display optimal adsorption properties toward PNPP. The improved adsorption properties of chitosan bead materials reported herein for the removal of OPs and related waterborne pesticide residues42 will contribute to further developments of advanced materials for water and wastewater treatment processes.

Figure 8. Adsorption−desorption cycle of a chitosan bead (GA2.5CA)/PNPP system at 293 K. The error bars shown denote the standard error of triplicate measurements.

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ASSOCIATED CONTENT

S Supporting Information *

desorption at steady-state conditions. The desorption of PNPP approaches equilibrium within ca. 30 min, while approximately 95% of the bound PNPP molecules are desorbed within the first 15 min of the washing process. For each desorption cycle, the GA2.5-CA beads were washed with Millipore water and subjected to additional cycles of adsorption−desorption. In Figure 8, the process was repeated for four cycles, where the uptake capacity of PNPP was relatively constant (19.6−17.5%), indicating the good efficiency of the beads as reusable adsorbents. The use of NaCl(aq) for the elution process offers an efficient and low-cost method for the removal of adsorbed phosphate for each cycle because it eliminates intermediate drying steps between adsorption−desorption cycles for such

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02814. Details on the XRD spectra, leaching results, and isotherm adsorption parameters for chitosan bead systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +1 306 966 4730. E-mail: [email protected] (L.D.W.). Notes

The authors declare no competing financial interest. 11713


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(19) Liu, X.; Zhang, L. Removal of phosphate anions using the modified chitosan beads: Adsorption kinetic, isotherm and mechanism studies. Powder Technol. 2015, 277, 112−119. (20) Pratt, D. Y.; Wilson, L. D.; Kozinski, J. A. Preparation and sorption studies of glutaraldehyde cross-linked chitosan copolymers. J. Colloid Interface Sci. 2013, 395 (1), 205−211. (21) Sowmya, A.; Meenakshi, S. A novel quaternized chitosanmelamine-glutaraldehyde resin for the removal of nitrate and phosphate anions. Int. J. Biol. Macromol. 2014, 64, 224−232. (22) Fagundes, T.; Bernardi, E. L.; Rodrigues, C. A. Phosphate adsorption on chitosan-FeIII-crosslinking: Batch and column studies. J. Liq. Chromatogr. Relat. Technol. 2001, 24 (8), 1189−1198. (23) Sowmya, A.; Meenakshi, S. Phosphate uptake studies on different types of lanthanum-loaded polymeric materials. Environ. Prog. Sustainable Energy 2015, 34 (1), 146−154. (24) Wilson, L. D.; Xue, C. Macromolecular sorbent materials for urea capture. J. Appl. Polym. Sci. 2013, 128 (1), 667−675. (25) Mahaninia, M. H.; Wilson, L. D. Cross-linked chitosan beads for phosphate removal from aqueous solution. J. Appl. Polym. Sci. 2016, 133 (5), 42949−42959. (26) Mahaninia, M. H.; Wilson, L. D. Phosphate uptake studies of cross-linked chitosan bead materials. J. Colloid Interface Sci. 2017, 485, 201. (27) Rietra, R. P. J. J.; Hiemstra, T.; van Riemsdijk, W. H. Interaction between Calcium and Phosphate Adsorption on Goethite. Environ. Sci. Technol. 2001, 35 (16), 3369−3374. (28) Dai, J.; Yang, H.; Yan, H.; Shangguan, Y.; Zheng, Q.; Cheng, R. Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II). Chem. Eng. J. 2011, 166 (3), 970−977. (29) Kumar, P. V.; Neeraj, A.; Masihur, R.; Naseema, K. Applications of calcium impregnated activated charcoal prepared from Jatropha seed residue for removal of arsenic (III) from water. J. Environ. Res. Dev. 2013, 8 (2), 196−205. (30) Mondal, P.; Mohanty, B.; Balomajumder, C. Treatment of Arsenic Contaminated Groundwater Using Calcium Impregnated Granular Activated Carbon in a Batch Reactor: Optimization of Process Parameters. Clean: Soil, Air, Water 2010, 38 (2), 129−139. (31) Ioelovich, M. Crystallinity and Hydrophility of Chitin and Chitosan. Res. Rev.: J. Chem. 2014, 3 (3), 7−14. (32) Wilson, L. D.; Mohamed, M. H.; Headley, J. V. Surface area and pore structure properties of urethane-based copolymers containing βcyclodextrin. J. Colloid Interface Sci. 2011, 357 (1), 215−222. (33) Sips, R. On the Structure of a Catalyst Surface. II. J. Chem. Phys. 1950, 18 (1948), 1024−1026. (34) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40 (9), 1361−1403. (35) Ng, J. C. Y.; Cheung, W. H.; McKay, G. Equilibrium Studies of the Sorption of Cu(II) Ions onto Chitosan. J. Colloid Interface Sci. 2002, 255 (1), 64−74. (36) Cestari, A. R.; Vieira, E. F. S.; Matos, J. D. S.; dos Anjos, D. S. C. Determination of kinetic parameters of Cu(II) interaction with chemically modified thin chitosan membranes. J. Colloid Interface Sci. 2005, 285 (1), 288−295. (37) Li, S.-D.; Zhang, C.-H.; Dong, J.-J.; Ou, C.-Y.; Quan, W.-Y.; Yang, L.; She, X.-D. Effect of cupric ion on thermal degradation of quaternized chitosan. Carbohydr. Polym. 2010, 81 (2), 182−187. (38) Dehabadi, L.; Wilson, L. D. Polysaccharide-based materials and their adsorption properties in aqueous solution. Carbohydr. Polym. 2014, 113, 471−479. (39) Webster, A.; Halling, M. D.; Grant, D. M. Metal complexation of chitosan and its glutaraldehyde cross-linked derivative. Carbohydr. Res. 2007, 342 (9), 1189−1201. (40) Gartner, C.; Löpez, B. L.; Sierra, L.; Graf, R.; Spiess, H. W.; Gaborieau, M. Interplay between structure and dynamics in chitosan films investigated with solid-state NMR, dynamic mechanical analysis, and X-ray diffraction. Biomacromolecules 2011, 12 (4), 1380−1386.

ACKNOWLEDGMENTS The authors are grateful to the Government of Saskatchewan through the generous support of the Agriculture Development Fund (Project 20110162). The University of Saskatchewan is acknowledged for its support of this research.

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Modular Cross-Linked Chitosan Beads with Calcium Doping for

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