Chitosan Nanocomposite Beads as

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Materials and Interfaces

Layered Double Hydroxide/Chitosan Nanocomposite Beads as Sorbents for Selenium Oxoanions Man Li, Andrew Dopilka, Andrea N Kraetz, Hangkun Jing, and Candace K. Chan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00466 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Industrial & Engineering Chemistry Research

Layered Double Hydroxide/Chitosan Nanocomposite Beads as Sorbents for Selenium Oxoanions

Man Li1†, Andrew Dopilka1†, Andrea N. Kraetz2, Hangkun Jing1, and Candace K. Chan1*

1. Materials Science and Engineering, 2. Chemical Engineering; School for Engineering of Matter, Transport and Energy; Nanosystems Engineering Research Center for Nanotechnology – Enabled Water Treatment Arizona State University, Tempe, AZ 85287, United States

†

These authors contributed equally to this work

*Corresponding author: [email protected], (480) 727-8614

KEYWORDS: Layered double hydroxide, chitosan, selenium, adsorption, nanocomposite, beads

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Layered double hydroxides (LDHs) nanoparticles are effective sorbents for selenium oxoanions, but must be fabricated in a suitable fashion for implementation in water treatment applications using packed columns. In this work, we demonstrate the preparation of nanocomposite beads prepared from Mg-Al-CO3 LDH nanoparticles and chitosan, a sustainable and bio-degradable biopolymer. The synthesis of the nanocomposite beads is achieved by direct mixing or in-situ synthesis of the LDH nanoparticles into the chitosan matrix. The effect of the preparation route on the nanocomposite structure, maximum loading of LDH in the composite, removal kinetics, and the maximum sorption capabilities for selenate and selenite oxoanions are studied and compared to LDH nanopowders and granular media. The results indicate that the insitu synthesis of LDH inside the beads leads to several favorable characteristics, such as a higher mass loading of LDH and better dispersion of the nanoparticles, while displaying good selenium removal over a wide pH range, superior sorption capacities to the nanopowder, and similar sorption kinetics to the granulated media. The maximum adsorption capacities for the nanocomposite beads from Langmuir isotherms were 17 mg/g for Se(IV) and ~12 mg/g for Se(VI) with respect to the mass of LDH, which is higher than reported capacities obtained in chitosan beads embedded with other nanocrystalline metal oxide fillers. These results show that the LDH/chitosan nanocomposite beads are promising alternatives to granulated media for selenium removal and sheds light on how best to design and fabricate high performance and sustainable nano-enabled sorbents for water treatment applications.


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Introduction Although an essential element, excessive levels of selenium can lead to toxicity in humans and wildlife in aquatic environments.1,2 Selenium is found at trace levels in groundwater and can also enter the environment by contamination from agricultural drainage and industrial (e.g. oil refining, mining)3,4 wastewaters. The U.S. Environmental Protection Agency (EPA) mandated maximum contaminant level for selenium in drinking water of 50 ppb,5 while the World Health Organization (WHO) has a lower guideline value of 10 ppb.6 Several chemical, biological, and physical methods for selenium removal have been investigated,4,7,8 but selenium removal via sorption onto solid-phase extraction materials is attractive because it is a simple and low-cost process. Since selenium is typically found as the oxoanions biselenite (HSeO3-) or selenate (SeO42-) in natural waters,9,10 sorbents with positively charged surfaces, such as metal oxides with high point-of-zero charge (PZC), must be employed to facilitate binding of the negatively charged selenium species. For this reason, one of the most effective sorbents for selenium oxoanions are layered double hydroxides (LDHs), which due to the higher PZC in the range of ~8 – 12.5,11 can display better selenium removal capacities at pH > 7 compared to other common metal oxide sorbents such as TiO2,12,13,14 Al2O3,15,16 and iron oxides.17,18 LDHs are anionic clays that have been investigated for a wide number of uses, including as photo- and electrocatalysts,19,20 gas adsorption materials,21,22 catalyst supports,23 flame retardants,24 and in water treatment11,25 and biomedical/drug-delivery applications.26,27 LDHs contain positively charged layers of brucitelike octahedral hydroxide sheets separated by compensating anions and water molecules that can x be exchanged with other anions28,29 and can adopt the formula [ (OH)2]x+[ ⁄ ] •



mH2O, where MII is a divalent metal, MIII is a trivalent metal, An- is an exchangeable anion with a valence of n, and the x value is in the range of 0 – 0.33.30 LDHs have been studied as sorbents and ion-exchange materials for the removal of a number of anions.11,31,32 In particular, the Mg-Al-CO3 LDH (i.e., MII = Mg2+, MIII = Al3+, A = CO32-) has shown high sorption capacities for both selenite, Se(IV), and selenate, Se(VI),31,33,34,35 which is extremely attractive since sorbents generally tend to show higher selectivity for selenite, Se(IV), over selenate, Se(VI).36 We recently demonstrated that a commercially available form of this LDH (granules comprised of aggregated LDH nanosheets ~ 200 nm in size) is effective in packed beds for the removal of selenium from groundwater samples containing naturally occurring levels of trace selenium (< 2 ppb) and sulfate concentration around 50 ppm, a difference in concentration of 20,000.37 One drawback of granular media in general, however, is that they are susceptible to attrition or particle breakdown, which could lead to media loss and contamination of treated water with the nanopowder sorbents.38,39 This may necessitate energy intensive filtration to separate the adsorbent from solution post-treatment, thus increasing the overall cost of use and regeneration.40,41 To overcome this barrier, engineered composite adsorbents comprising nanopowders embedded in various substrates have been developed. Such “nano-enabled sorbents” have the potential to display higher sorption capacities and faster kinetics compared to conventional sorbents due to their high specific surface area, short intraparticle diffusion distances, and tunable pore size and surface chemistry, while also maintaining compatibility with existing treatment processes (e.g., slurry reactors, fixed bed).42,43,44 Polymer-clay composites have attracted a great deal of attention for the removal of non-ionic and anionic pollutants45 and


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organic pollutants.46 Among them, chitosan-clay nanocomposites have been well investigated for the adsorption of anionic pollutants.47,48 Chitosan, a polyaminosaccharide, can be isolated from the several million tons of shellfish waste generated globally per year and has the potential of being an inexpensive, sustainable, biodegradable, and non-toxic biosorbent.49,50,51 Chitosan is effective for removal of metals including chromium, cadmium, mercury, and copper, and can be formulated into films and beads.49,52,53 Previous work by Zimmerman’s group 54,55-56 successfully demonstrated that chitosan beads impregnated with metal oxide nanopowders (e.g. TiO2, Al2O3) could remove numerous cations and oxoanions simultaneously from water. Further, Bleiman et al.57 designed composites of chitosan with clays (i.e. montmorillonite) that reported Se(VI) capacities of 18.4 mg/g. To our knowledge, LDH/chitosan nanocomposite beads for selenium removal have not been reported yet, although nanocomposite beads of LDH with carboxymethyl cellulose58 and chitosan59 for drug delivery applications, and LDH with polyvinyl alcohol/alginate hydrogel beads for phosphate removal60 have been reported. Herein, we investigate the combination of LDH with chitosan using two different preparation methods. The effect of the preparation route on the nanocomposite structure, maximum loading of LDH in the composite, and the removal kinetics and maximum sorption capabilities for selenium oxoanions are studied and compared to LDH nanopowders and granular media. From these studies, we aim to understand how the sorption properties of the LDH are affected when it is utilized in different form factors (i.e., as a nanopowder sorbent, granular media, or nanocomposite bead) to shed light on how best to design and prepare high performance nano-enabled sorbents for water treatment applications.



Experimental Methods Detailed descriptions of the materials, reagents, water samples, and characterization/analysis are provided in the Supplementary Information. Briefly, chitosan flakes were dissolved in HCl to form a gel, which was then dropped into a solution of NaOH with a syringe to undergo precipitation and form beads.61,62 Cross-linking, which improves the stability of chitosan beads in acidic and basic solutions,63 was not implemented because it can lead to a decrease in contaminant sorption capacity;64 moreover, selenium oxoanions can be sorbed by LDH materials at pH 7 – 8 31,34 making acidic or basic testing solutions unnecessary. Granular LDH was obtained from Sasol Germany GmbH (PURALOX MG 63 HT – Granulate) with a reported median particle diameter of 1.46 mm. The granular media were gently hand ground with a mortar and pestle to form LDH nanoparticles, as confirmed using scanning electron microscopy (SEM) imaging (Figure S1). This nanopowder was used in the preparation of the nanocomposite beads obtained through direct mixing of LDH into the chitosan gel. Beads formed using this approach are designated DM-X, where X indicates the wt% of LDH nanopowder directly mixed (DM) into the gel. The sorption properties of the as-obtained granular LDH were also evaluated for comparison. In the second nanocomposite preparation approach, the LDH was prepared in-situ (IS) by adding the precursors for LDH (MgCl2 and AlCl3·6H2O) into the chitosan gel. The wt% of the precursors added to the chitosan gel was varied from 20 – 70% and composite beads formed using this approach are designated IS-X’, where X’ indicates the wt% of LDH precursor added to the gel. For comparison, the LDH precursors were also added into the NaOH solution without chitosan to obtain LDH nanopowder (hereafter referred to as “in-situ LDH”). Chitosan beads were also prepared without containing LDH to serve as control samples.


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Results and Discussion Materials Characterization As illustrated in Scheme 1, two preparation methods were studied for obtaining the LDH/chitosan nanocomposite beads: one that embeds the LDH into the beads using direct mixing of nanopowder into the chitosan gel, and one that synthesizes the LDH “in-situ” within the chitosan matrix. The chitosan bead formation process involves the dissolution of chitosan flakes into an acid, followed by neutralization and precipitation when the chitosan droplet is added to an alkaline solution in which chitosan is insoluble.61 In general, chitosan nanocomposite beads can be prepared by mixing pre-formed nanopowders into the chitosan precursor gel,55,56 provided the nanopowders are stable in both the acid and base environments used in the bead formation process. LDHs generally show good stability over the pH range of 4 – 10,65 but an additional feature that can be exploited for preparing the nanocomposite beads is that LDHs can be prepared by co-precipitation of the metal salts in alkaline solutions. In a typical coprecipitation synthesis of LDH, the formation mechanism is a two-stage process starting with the precipitation of Al(III) hydrous oxide at lower pH, followed by its transformation to LDH in the presence of Mg2+ at ~ pH 8.66,67,68 Since the second step involves replacement of Al3+ by Mg2+ in a solid substitution-filling model rather than a simple precipitation from solution,66,68 there is a possibility that the presence of the chitosan matrix can affect the nucleation and growth of the LDHs. For this reason, a systematic study was undertaken to compare the two preparation methods for the LDH/chitosan nanocomposite beads.



Scheme 1. Process of making Mg-Al-CO3 LDH/chitosan beads with two different methods: (1) Direct mixing of LDH nanopowder into the chitosan gel (DM-X beads, where X = wt% of LDH mixed into the gel), or (2) in-situ synthesis of LDH into the chitosan gel (IS-X’ beads, where X’ = wt% of LDH precursor added to the gel), followed by composite bead formation. The boxed text indicates samples that were tested in the selenium removal studies.

Directly mixing the LDH nanopowders into the chitosan precursor solution was found to lead to formation of a sticky gel, likely due to the basic properties of the LDH.69 When attempting to add more than 30 wt% of LDH nanopowder to the gel, it was observed that many of the particles could not be mixed well and it became difficult to push the gel through the syringe. Hence, from these studies, 30 wt% of LDH was determined to be the optimum ratio for the mixed LDH/chitosan nanocomposite beads and further studies were focused on this composition (referred to as sample DM-30).


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On the other hand, for the nanocomposite beads prepared with LDH synthesized in-situ, the precursors were effectively dissolved into the chitosan solution to make a uniform gel. Upon dropping this gel into the NaOH solution, both the LDH and chitosan could be co-precipitated into the final form simultaneously. Good bead formation was observed until the amount of LDH precursor added to the chitosan solution was increased to 60 wt%. In this case, LDH particles were observed suspended in the NaOH solution during the bead formation (i.e. not embedded in the beads), and as shown in Figure S2A, most of the beads that were formed were misshapen. Further increasing the wt% of the LDH precursor to 70% did not lead to bead formation at all, likely due to the insufficient amount of chitosan to serve as matrix (Figure S2B). From these results, 50 wt% precursor was determined to be the optimum ratio for the nanocomposite beads prepared using the in-situ method and further detailed studies were focused on this composition (referred to as sample IS-50). Figure 1A shows a photograph comparing the size of the pure chitosan beads and the IS-50 LDH/chitosan composite beads.



Figure 1. (A) Photograph of pure chitosan beads and IS-50 nanocomposite beads; SEM images of (B) pure chitosan beads, (C) DM-30 nanocomposite beads, (D)-(E) IS-50 nanocomposite beads, (F) in-situ LDH nanopowder.

The surface morphology of the beads was characterized using scanning electron microcopy (SEM). Whereas the chitosan beads without LDH had very smooth surfaces (Figure 1B), the nanocomposite beads had very different surface morphologies with much rougher surfaces. Some platelet-shaped particles were observed on the surface of the DM-30 nanocomposite beads (Figure 1C), which were similar in size and shape as the LDH nanopowders (Figure S1). On the other hand, the IS-50 composite beads had very wrinkled surface topology and rough surfaces containing small, well-dispersed particles (Figure 1D-E). SEM imaging revealed that the LDH material synthesized in the absence of chitosan (in-situ LDH) was composed of nanoparticles (Figure 1F). 10 ACS Paragon Plus Environment

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To evaluate the structure and composition of the beads, Fourier transform infrared spectroscopy (FTIR) was employed (Figure 2). The FTIR spectra of the chitosan flakes (Figure 2A) and beads (Figure 2B), both showed the characteristic chitosan bands (Supporting Information)70,71 suggesting little chemical structure change in the chitosan after bead formation. The FTIR spectrum of the in-situ LDH (Figure 2C) showed characteristic bands associated with LDH (Supporting Information). In the FTIR spectrum of the IS-50 composite beads, both chitosan and LDH characteristic bands were observed, demonstrating that the preparation method was effective for preparing the nanocomposite from both materials.

Figure 2. FTIR characterization (A) as-received chitosan flakes; (B) synthesized chitosan beads without LDH; (C) in-situ synthesized LDH nanopowder; (D) IS-50 composite beads. Features



from LDH and chitosan are labelled with red and black text, respectively. Assignments of the characteristic bands are described in the Supporting Information. X-ray diffraction (XRD) was used to further characterize the structure of the materials. The XRD pattern of the as-obtained chitosan flakes (Figure 3A) and the prepared chitosan beads (Figure 3B) displayed only a broad reflection at around 2θ = 20o. These results show that the chitosan structure was amorphous72 and was not altered during the bead formation process. The XRD pattern of the in-situ LDH powder (Figure 3C) matched the characteristic reflections for the layered form of LDH (PDF 01-089-0460),73 indicating that the LDH could successfully crystallize from the precursors in the NaOH solution using this procedure.


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Figure 3. XRD patterns of (A) as-received chitosan flakes; (B) synthesized chitosan beads; (C) in-situ LDH nanopowder; (D) IS-60 beads; (E) IS-50; (F) IS-30 beads; (G) DM-30; reference pattern of Mg0.667Al0.333(OH)2(CO3)0.167(H2O)0.5 from PDF 01-089-0460 (bottom).

For the nanocomposite beads (Figure 3D-G), the XRD patterns showed reflections both from the chitosan peaks and the LDH characteristic peaks, which matched the LDH reference pattern, confirming that the LDH/chitosan composite beads could be successfully synthesized using either the direct mixing method or the in-situ synthesis. As the amount of nominal LDH precursor increased from 30 to 50 wt%, the LDH reflections in the beads increased in intensity compared to the broad reflection from the chitosan. However, the LDH reflection decreased in intensity for the IS-60 beads, suggesting less LDH incorporation into the beads. This is consistent with the observations that LDH had precipitated outside of the beads in this preparation process, likely because there was insufficient chitosan available in the mixture to bind the LDH. To better understand the amount of LDH incorporated into the chitosan composite beads, thermogravimetric analysis (TGA) was performed for pure chitosan beads, several LDH/chitosan composite beads, and the in-situ LDH nanopowder (Figure 4). The TGA curve of the chitosan peads and in-situ LDH nanopowder was consistent with previous reports (Supporting Information). Taking into account the mass from the residual chitosan and weight loss due to the decomposition of LDH, the actual mass of LDH in the composite beads was derived from the mass remaining at 1000 oC in the TGA measurement (Supporting Information).



Figure 4. TGA results for pure chitosan beads, in-situ LDH nanopowder, and LDH/composite beads generated with a scan rate of 5 oC/min in air.

As shown in Table 1, the results show that there was a close correspondence between the expected and measured amounts of LDH in the beads. For IS-30 and IS-50, the difference was within experimental error, showing that the in-situ method to prepare the composite beads is very effective. On the other hand, DM-30 had roughly 5% less LDH than expected, indicating that the process is not as efficient and some LDH particles were not incorporated into the beads. The IS60 beads showed a larger mass than expected; this can be explained by the insufficient amount of chitosan matrix leading to poorly formed beads, with some of the LDH precipitated outside of the beads. As a result, the sample was inhomogeneous and the assumptions used to determine the expected wt% of LDH in the IS-60 bead are likely not valid. 14 ACS Paragon Plus Environment

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Table 1. Comparison of expected and measured amount of LDH in the nanocomposite beads Sorbent

Expected wt% of LDH in bead

Measured wt% of LDH in bead

% Difference

IS-30

19.41

19.50

0.46%

IS-50

35.99

36.46

1.31%

IS-60

45.75

51.54

12.66%

DM-30

30.00

28.67

-4.64%

The good correspondence between the expected values of LDH in the IS-30 and IS-50 beads may be explained by the sorption of the Mg2+ and Al3+ by the chitosan functional groups in the gel. Although previous studies have shown that native chitosan shows more affinity for transition metals compared to alkali or alkaline earth metals,74 the sorption of Mg2+ onto chitosan has been reported.75 In the case of aluminum, it is effectively sorbed at pH 4 or below, where it is present as predominately Al3+ and can be sorbed on the amine groups of chitosan.76,77

Selenate Removal Characteristics To evaluate the selenium removal efficacy of the materials, jar tests were performed in de-ionized (DI) water spiked with 1 ppm initial concentration of selenate, Se(VI). Figure 5A shows the Se(VI) remaining after 48 h of exposure. The chitosan alone showed very little efficacy for Se(VI) removal, different from previous studies showing that chitosan was an effective sorbent for Se(VI).41,78 This might be because the chitosan had a different biological origin, degree of deacetylation and/or crystallinity from the other types of chitosan studied, all characteristics which can affect the sorption properties of chitosan.79 15 ACS Paragon Plus Environment


Figure 5. (A) Comparison of sorbent materials for removal of 1 ppm selenate from spiked DI at 48 h of exposure time. Dosages: 1 g/L chitosan, 0.42 g/L granular LDH, 0.42 g/L in-situ LDH nanopowder; all nanocomposite beads were used at a 1 g/L dosage with respect to the amount of chitosan in the bead. (B) pH dependence of Se(VI) removal on IS-50 beads. The final pH was measured after 48 h of equilibration (each point represents an average of duplicates, with error bars showing the range).

When using the DM-30 beads (used at 1 g/L with respect to the amount of chitosan in the bead) as sorbent, 57% of the selenate could be removed after 48 h of exposure, confirming that the incorporation of LDH could add selenium sorption properties to the chitosan beads. Due to the LDH only comprising ~30 wt% of the DM-30 beads, the effective dosage of just the LDH domains within the nanocomposite was 0.42 g/L. For comparison, the Se(VI) removal characteristics of the in-situ LDH nanopowder and granular LDH were also evaluated using a dosage of 0.42 g/L. As shown in Figure 5A, both types of the LDH alone could remove ~80% of the Se(VI). For the IS-X’ nanocomposite beads, increased Se(VI) removal was observed as the 16 ACS Paragon Plus Environment

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wt% of LDH precursor increased. The IS-50 beads could remove more than 80% of the selenate after 48 h. This indicates that the Se(VI) removal is attributed to the synthesized LDH domains inside the composite beads, since native chitosan used in this study did not display good selenium removal properties. This was also confirmed by studying the pH dependence of the Se(VI) removal. As shown in Figure 5B, the pH of the solution increased after the Se(VI) sorption tests using the IS50 beads. This is due to the buffering properties of LDH, which can exchange hydroxide and carbonate anions with the selenate.11 Similar to other reports, the equilibrium pH remained at around 8 and the Se(VI) removal properties were not affected until the initial pH was increased past 9, at which point hydroxide sorption can compete with Se(VI) removal.31,30 On the other hand, studies using chitosan to remove Se(VI) reported decreased sorption above pH 6 due to the deprotonation of the amine groups, which serve as the electrostatic binding sites for selenium oxoanions.41,57,78

Kinetic Behavior of Nanocomposite Beads, Granular LDH, and LDH nanopowder The kinetics of Se(IV) and Se(VI) adsorption on the IS-50 beads, granular LDH, and insitu LDH nanopowder were studied using initial oxoanion concentrations of 1 and 5 ppm (Figure 6, Figure S3). Equilibrium was reached within 10 h using all of the samples, and was reached in as little as 2 h for the granular LDH (Figure S3) and IS-50 beads. In comparison, chitosan beads impregnated with Al2O3 nanopowders required 24 h to reach equilibrium for sorption of both types of selenium oxoanions.41



Figure 6. Kinetics of selenite, Se(IV), and selenate, Se(VI) sorption on in-situ LDH nanopowder (left) and IS-50 nanocomposite beads (right) using initial concentrations of 1 and 5 ppm. (A) qt: amount of selenium sorbed at a particular time (mg/g); data were fit to a pseudo-second-order model (fitting shown as solid lines). (B) Percentage of initial selenium concentration that was removed. For the in-situ LDH, the sorbent dose was 1 g/L; for the IS-50 beads, the dosage was 2 g/L (total mass of sorbent) or 0.73 g/L (considering only the mass of LDH). Each point represents an average of duplicates, with error bars showing the range. 18 ACS Paragon Plus Environment

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The experimental data were analyzed using a pseudo-second order model80,81 (Supporting Information, Table S2) and displayed a good fit, similar to other studies,34,82 with correlation coefficient (r2) values all equal to about 0.999. For the IS-50 beads, the fitting was performed considering the total sorbent mass and also the mass of LDH only. According to the results, the in-situ LDH nanopowder displayed the largest rate constant. Since it also had the highest surface area (144 m2/g vs. 116 m2/g for the granular LDH, Table S1), this suggests that the higher surface area of the in-situ LDH nanopowder could have enabled sorption of a larger amount of selenium more quickly compared to the other sorbents. After 15 min of exposure, the in-situ LDH removed 65 – 75% of the initial Se concentration, compared to only around 20 – 40% for the granular LDH and IS-50 beads. However, after the first 15 min, some desorption of selenium was observed, followed by a slow increase in sorption to reach qe. The fast uptake followed by slow sorption was also observed in studies on phosphate removal using LDH and is attributed to a fast initial adsorption on the surface followed by anion exchange, which relies on diffusion and is a slower process.83,84 Applying an intraparticle diffusion model85 to the in-situ LDH selenium sorption vs. time plots (Figure S4) showed a linear fit of qt vs. t1/2 between 1 – 4 h of contact time, indicating that intraparticle diffusion is the limiting step during this time period and confirming the multi-step sorption process.86 On the other hand, this behavior was not observed in the granular LDH or the nanocomposite beads. Notably, the values for k2 were comparable for these samples, suggesting the sorption processes were similar for the granular LDH and nanocomposite beads. This could be because the hydrogel properties of chitosan87 enable the beads to be swollen with water and maintain solution access to the LDH despite the embedding of the nanoparticles inside the beads.



Evaluation of the qe values showed that the granular LDH has the highest loading capacity of selenium and could remove close to 100% of both types of oxoanions at both initial concentrations. Interestingly, the in-situ LDH showed lower qe values and less than 100% removal efficacy. Our previous characterization of this granular LDH37 suggested that the LDH in this media had been calcined, which typically leads to higher sorption capacities than noncalcined LDHs.34,88 This could be one explanation for why the in-situ LDH displayed lower sorption capacities than the granular LDH. Further, N2 adsorption measurements showed that the pore volume and pore sizes were smaller for the in-situ LDH compared to the granular LDH (Table S1). As a result, the observed sorption behavior can also be explained by the lower porosity and smaller pore size in the in-situ LDH nanopowders. Although the in-situ LDH had a higher surface area, allowing for faster initial sorption, the smaller pores can become blocked, rendering the inner pores less accessible to additional selenium ions. These smaller pores may also limit the intraparticle diffusion, as indicated by the kinetic results. As illustrated in Figure 7 and Table S2, the IS-50 beads had the lowest values for qe when normalizing to the total sorbent mass since the chitosan did not sorb selenium. However, when considering only the mass of LDH, as determined from the TGA analysis (Table 1), the qe was comparable to that for the granular LDH for the sorption of 5 ppm Se(IV). The difference in sorption capacity for Se(VI) vs. Se(IV), while noticeable in the in-situ LDH, is more evident in the IS-50 beads, but negligible in the granular LDH. The insensitivity of the granular LDH to the selenium speciation is likely because the calcined LDH within the media has a different removal mechanism; after calcination, the LDH has a non-layered structure, which reconstructs into the layered structure upon rehydration indiscriminately around any anions present in the solution.11,30 On the other hand, Se(IV) and Se(VI) have different removal mechanisms on the layer-structured


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LDHs, as reported by Chubar.89 It was shown through detailed X-ray absorption and FTIR studies that Se(IV) removal is mostly through chemisorption in an inner-sphere process, while Se(VI) removal is solely through an outer-sphere, ion-exchange process with surface hydroxide and interlayer carbonate anions.89 The higher rate constants for removal of Se(VI) compared to Se(IV) is also due to the difference in oxoanion binding mechanisms, since the chemisorption of Se(IV) is slower than the physisorption of Se(VI).82 Additionally, it is interesting to note that the IS-50 beads could remove more of the 1 ppm Se(IV) solution compared to the in-situ LDH despite the lower loadings (0.73 g/L considering only the mass of LDH in the beads, vs. 1 g/L of in-situ LDH). This suggests that the nanocomposite beads enable better LDH sorption of selenium, particularly Se(IV), as compared to LDH in suspended nanopowder form. This could be due to the well dispersed nature of the LDH within the beads and the hydrogel properties of chitosan enabling sufficient access of selenium into the LDH within the polymer network, while aggregation or agglomeration of the in-situ LDH nanoparticles may block access to the chemisorption sites on the LDH surface.



Figure 7. Adsorption kinetics results and pseudo second-order fitting parameters obtained using different sorbents (k2: rate constant, qe: anion removal capacity at equilibrium) using initial Se concentrations of (A) 1 ppm, (B) 5 ppm. Results for IS-50 beads were considered using the total sorbent mass and also the mass of only the LDH component.


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Selenate and Selenite Adsorption Isotherms on Nanocomposite Beads To better understand the differences between sorption of Se(VI) and Se(IV) onto the IS50 beads, adsorption isotherms were performed (Figure 8). Both Langmuir and Freundlich models were applied to describe the adsorption behavior at equilibrium. For Se(VI), a higher correlation coefficient using the Langmuir model indicated a better fit (Table 2). However, for selenite adsorption isotherms, fitting to the Freundlich model showed a larger correlation coefficient value, indicating a better fit than the Langmuir model. This is consistent with a previous report by Yang et al.34, although another study showed a Langmuir fit for selenite adsorption onto LDH.82

Figure 8. Selenate and selenite adsorption isotherms using IS-50 nanocomposite beads at 2 g/L for 48 h exposure time (each point represents an average of duplicates, with error bars showing the range).



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Table 2. Determined parameters from fitting selenate and selenite adsorption isotherms to the Langmuir and Freundlich models. qmax (mg/g) is the maximum adsorption capacity for loading selenium onto the sorbent; b (L/mg) is the Langmuir equilibrium constant; Kf (mg/g) is Freundlich isotherm constant; n is adsorption intensity; r2 is the correlation coefficient. Errors for the Freundlich fitting were < |0.005|.

Langmuir

b (L/mg)

qmax (mg/g)

r2

Selenite

0.14 ± 0.07

6.21 ± 0.07

0.9851

Selenate

0.72 ± 0.05

4.48 ± 0.19

0.9989

Freundlich

Kf

n

r2

Selenite

1.03

1.86

0.9955

Selenate

1.53

1.80

0.9642

To place these adsorption isotherm results in context, the obtained qmax values are compared to those obtained by Yamani et al. on chitosan beads impregnated with 8 wt% Al2O3 or 12.4 wt% TiO2 nanoparticles.54 Although the chitosan used in that work displayed efficacy for removing selenate, the reported qmax values were normalized only to the mass of the metal oxide nanoparticle, which artificially inflates the qmax since it excludes the mass of chitosan. Since the chitosan used in our study was not effective for removing Se(VI), we have compared the qmax on an absolute basis (i.e., per total sorbent mass). As shown in Table S3, the qmax for the IS-50 beads are more than 2.5 and 7 times higher for Se(VI) and Se(IV) sorption, respectively than those for Al2O3 and TiO2 impregnated chitosan beads. This can be attributed to the higher 24 ACS Paragon Plus Environment

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loading of LDH into the beads, but can also be due to the higher efficacy for LDH to sorb selenium compared to Al2O3 and TiO2. The latter statement is confirmed by taking our absolute qmax values and normalizing to the mass of LDH in the nanocomposite to obtain 17 mg Se(IV)/g LDH, which is higher than what was reported for the Al2O3 and TiO2 nanocomposite beads (11.08 and 5.29 mg/g, respectively) (Table S3). For Se(VI), we obtain a value of 12.29 mg/g LDH, which cannot be directly compared to the results for Al2O3 and TiO2 due to the synergistic sorption of Se(VI) by chitosan in that work. However, this adsorption capacity is comparable to those reported for LDH sorbents prepared using alkoxide sol-gel (4 mg/g) and hydrothermal precipitation (18 mg/g) methods, although still much lower than the best performing LDH sorbent prepared using an alkoxide-free sol-gel approach (45 mg/g), which has a modified structure more effective for oxoanion removal.33

Conclusions We confirm that selenium removal via sorption onto nanocomposite beads prepared through impregnation of nanoparticles into chitosan, first demonstrated for Al2O3 and TiO2 nanoparticles by Yamani et al.,54 is also effective when using LDH as the nanoparticle fillers. Insitu synthesis of the LDH into the beads, which is enabled by the ability of both the LDH and the chitosan to be dissolved in acid and co-precipitated in basic solutions, leads to higher mass loadings of LDH embedded into the beads. Further, this in-situ synthesis method leads to more homogenous dispersion of the LDH particles within the chitosan and closer correspondence between the actual and expected wt% of LDH incorporated into the beads compared to directly mixing the LDH nanoparticles into the chitosan gel. Kinetic sorption studies reveal that while non-immobilized LDH nanoparticles can display fast initial surface sorption of selenium



oxoanions, there is a slower intraparticle diffusion process that serves as the rate-limiting step. Further, there is actually less selenium removed when the nanoparticles are used alone as compared to when the LDH is embedded in the chitosan beads (considering mg Se/g LDH loadings). For the nanocomposite beads, the second order rate constants are similar to those found for granulated LDH media, indicating that selenium oxonions could still diffuse through the chitosan due to its hydrogel properties. Further, the maximum sorption capacities for the nanocomposite beads (17 mg/g for Se(IV) and ~12 mg/g for Se(VI), when normalized to the mass of LDH), indicate good accessibility of selenium to the binding sites on LDH. The LDH/chitosan nanocomposite beads display good selenium removal over a wider pH range than other metal oxide sorbents and chitosan alone and may be a promising alternative to granulated media.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.XXXXXXX. Detailed description of materials and reagents; detailed procedures for materials synthesis, materials characterization, analysis of Se concentrations in water, determination of expected and actual wt% of LDH inside the composite beads, selenium sorption tests. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. 26 ACS Paragon Plus Environment

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Acknowledgments This work was supported by the NSF Nanosystems Engineering Research Center (EEC1449500) and funding from the Salt River Project (SRP)-ASU Joint Research Program. A. Dopilka acknowledges support from an ASU Fulton Schools of Engineering Dean’s Fellowship. We thank J. Zimmerman, L. Pincus, P. Westerhoff, and K. Hristovski for helpful discussions and J. Van Ornum for providing the chitosan flakes. We also gratefully acknowledge the use of facilities within the LeRoy Center for Solid State Science and Goldwater Environmental Laboratory at ASU.

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TOC/Abstract

Synopsis Nanocomposite beads are prepared from layered double hydroxide nanoparticles and chitosan, a sustainable biopolymer, and studied as sorbents for removal of selenium oxoanions from water.


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Chitosan Nanocomposite Beads as

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