Oxyanion-Binding in a Bioinspired Nanoparticle-Assembled Hybrid

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Oxyanion-Binding in a Bio-Inspired Nanoparticle-Assembled Hybrid Microsphere Structure: Effective Removal of Arsenate/Chromate From Water Joydeb Manna, Nagaraju Shilpa, Arun Kumar Bandarapu, and Rohit Kumar Rana ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00003 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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ACS Applied Nano Materials

Oxyanion-Binding in a Bio-Inspired NanoparticleAssembled Hybrid Microsphere Structure: Effective Removal of Arsenate/Chromate From Water Joydeb Manna,†, ‡ Nagaraju Shilpa,† Arun Kumar Bandarapu and Rohit Kumar Rana* Nanomaterials Laboratory, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India. KEYWORDS. biomimetic chemistry, self-assembly, nanostructures, anion-binding, ionimprinting

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ABSTRACT. We demonstrate a bio-inspired assembly, wherein the specific interaction of polyamines with multivalent anions allows assembly of silica nanoparticles to generate hybrid microsphere structures, while this very phenomenon further provides ways for the microspheres to adsorb oxyanions like arsenate and chromate. In a typical method based on the biomineralization of diatomaceous bio-silica structure, thus produced nanoparticle-assembled microspheres with a porous structure and hybrid functionalities exhibit efficient adsorption and separation of these toxic anions from water. The adsorption follows Freundlich isotherm with an inference for stronger interaction between adsorbate and adsorbent with non-uniform distribution of adsorption affinities. The opportunities to tune the composition with respect to the multivalent anion and their interaction with the polyamine, charge ratio, etc. illustrate designing of bioinspired robust structures with efficient oxyanion-binding property and recyclability. The consequence of competing anions shows that the binding selectivity follows the Hofmeister series of counter-ion interaction. Interestingly, in accordance with a molecular imprinting mechanism, the silica nanoparticle-assembled structure stabilizes and preserves the polyamineanion nanostructure creating cavities/voids complementary to the adsorbing ions in shape, size, and functional groups. As a result, the polyamine with phosphate as the multivalent anion exhibits efficient binding and removal of these toxic contaminants, which is better than most of the other reported adsorbents.

INTRODUCTION Despite the controversial conclusions that certain bacteria can grow in the presence of arsenate instead of phosphate, there has been continuous efforts to find biological or bio-inspired solutions for arsenic remediation.1,2 This includes investigations using organisms like Microalgae (phytoplankton), which are known to be the key contributors to arsenic cycling in the


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marine environment.3 Other marine organisms, such as, fish and invertebrates have also been shown to facilitate accumulation of arsenic mainly in the form of organo-arsenicals. This bioaccumulation by a number of marine organisms certainly suggests that they have an affinity for the arsenic containing compounds.4 Although the reason underlying this biological process is still a matter of discussion, mimicking these hybrid structures may provide clues not only for the phenomenon, but also for the fabrication of advanced materials for the removal of these toxic anions from water .5 Therefore, in the present work, our focus is to explore the bio-inspired structures, particularly those based on the Diatoms, which represent a type of microalgae that dominate the phytoplankton blooms. Via bio-silicification processes, these marine species produce shells called frustules composed of silica and long-chain polyamine-containing proteins (silaffin) assembled together generating intricately designed structures. Thus formed hybrid structure provides controlled porosity, large surface area and mechanical protection.6-8 Many efforts have been made not only to understand the bio-silicification process, but also to help develop bio-inspired methods for the synthesis of advanced materials for various desirable applications.9-13 In the context of the ground water contamination, the toxic oxyanions like arsenate and chromate are known to affect millions of people globally.14-16 According to WHO (World Health Organization) guidelines, the maximum tolerable concentrations of arsenic and chromium in drinking water are chromate@MS > sulfate@MS > citrate@MS > tartarate@MS. Interestingly, this trend was found to correlate well with the amount of PAH present in various MSs. As estimated from the TGA analyses, there was an increase in the PAH amount for the MSs prepared with different anions in the following order: citrate < sulfate ≈ tartrate sulfate ≈ tartrate > chromate > phosphate. It implies that the stronger the influence of anion on the PAH-aggregation, lower is the percentage of PAH found in the MS. Therefore, phosphate as the anion results in MSs with more percentage of polyamine, which further bestows the thus formed phosphate@MS the highest oxyanion removal capacity.


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(a)

(b)

(c)

(d)

(e)

(f)

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Figure 2. FE-SEM images and corresponding Energy-Dispersive X-Ray spectra (EDS) along with (inset: scale bar 1 µm) the elemental maps for (a, b) phosphate@MS, (c, d) arsenate@MS and (e, f) chromate@MS, respectively. Inset in Figure (a) depicts the hollow structure of a broken MS. The anion-polyamine interaction is also expected to influence the oxyanion-removal capacity of the MSs as it depends on the ability of these oxyanions (arsenate/chromate) to replace the multivalent anions, which are already present in the MS. As per the Hofmeister series, sulfate, citrate and tartrate anions can have stronger interaction with the polyamine. Therefore, these three anions in the respective MSs (sulfate@MS, citrate@MS and tartrate@MS) can’t be easily replaced with chromate or arsenate, and therefore it results in lower removal


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capacities (Figure 3a). On the other hand, phosphate, being a weaker anion compared to chromate, could be efficiently exchanged with the latter anion enabling highest removal capacity for the phosphate@MS. The results evidently signify the role of polyamines, which are entrapped in the MSs, in the removal of chromate and arsenate anions from water. Apart from the counter-ion interaction, it is also believed that the sizes of the respective anions might play a role in the removal process, wherein anions with similar diameter can replace each other easily (discussed in the later part).

(a)

(b)

(c)

(d)

Figure 3. Adsorption capacities of (a) MSs prepared using different multivalent anions (1suphate, 2-phosphate, 3-chromate, 4-citrate, 5-tratarate, 6-arsenate) at [PAH] = 2 mg/mL and R =11, (b) phosphate@MS with varying R at [PAH]= 2 mg/mL, (c) phosphate@MS with varying [PAH] at R = 11, for the removal of chromate and arsenate at pH 6. (d) Effect of pH on the adsorption capacities of phosphate@MS (prepared at R =11 and [PAH] = 10 mg/mL). The


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adsorption experiments were carried out using 3 mM chromate/arsenate solution and the standard deviation was less than 5%. Keeping the MS prepared with phosphate as an ideal aspirant, we varied both the charge ratio (R) and the PAH concentration during the MS assembly process to obtain an optimized composition for efficient removal of arsenate and chromate (Table S2). For both chromate and arsenate removal, we found that the charge ratio R = 11 was the optimum with removal capacities of 176 mg/g and 201 mg/g, respectively (Figure 3b). Further, at a constant charge ratio (R = 11), the PAH concentration was varied within 5-40 mg/mL for the synthesis of MSs. It showed an enhanced adsorption capacities of 232 mg/g and 218 mg/g for chromate and arsenate removal, respectively, when the MSs were prepared with PAH of 10 mg/mL (Figure 3c). The enhanced removal capacities could be related to the increased PAH content besides the better textural properties of the MSs as characterized by the N 2 -sorption studies (Figure S5 & S6). The removal capacity of the above samples was further analyzed at different concentrations of the contaminant. At a constant pH = 6.0, the removal capacity was seen to increase with the contaminant concentration (Figure S7). Thus obtained adsorption capacities for chromate and arsenate removal could reach 290 mg/g and 289 mg/g at an equilibrium concentration of 2.5 mM and 2.92 mM, respectively. Arsenate and chromate are known to exist in different protonated forms at different pHs. Accordingly, as the pH of the medium was varied within 2.0-8.0, the removal capacity of the MSs increased with increasing acidity up to a pH of 3.0 (Figure 3d). It resulted in a maximum removal capacity of 283 mg/g and 336 mg/g for chromate and arsenate, respectively. Since at pH = 3.0 both chromate and arsenate anions are partially protonated, it is believed that apart from the ionic interaction of these anions with the ammonium groups of PAH, there is a plausibility of binding via hydrogen bonding, which results


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in a optimized removal capacity for the MSs at this acidic pH.40 There have been many adsorbents reported for the removal of arsenate or chromate as given in Table S4. Comparison of their adsorption capacities reveals that the present bio-inspired hybrid system has removal capacity comparable to the best ones. Moreover, to the best of our knowledge, particularly amongst the materials synthesized under mild reaction conditions, the performance of the present material is the most efficient. This activity of the MSs could be attributed to the composition and structure of the hybrid system, wherein the polyamine-anion interaction is the key to the removal processes. We further analyzed the MS samples after their use in the removal of chromate and arsenate by X-ray Photoelectron spectroscopy (XPS) to understand the chemical state of the absorbed anions in the matrix (Figure S8). The XPS data shows that the binding energies of As 3d electrons at 46.0 and 44.8 eV correspond to As(V) state.41 In case of chromate, the Cr 2p binding energies at 588.2 and 578.3 eV represents the Cr(VI) state.42 Therefore, the adsorption of the arsenic or chromium ions in the MS doesn’t affect their oxidation state, which further indicates that the polyamine-anion interaction is the major driving force for their adsorption on the MS. The adsorption isotherms were analyzed using Langmuir and Freundlich isotherms (Figure S9 & S10). The isotherm parameters were obtained through both linear and non-linear fittings of the data (Table 1). It has been reported that the mechanism for the removal of ions from water involves either the exchange of ions or the adsorption-complexation process.43 In case of the Langmuir isotherm, an ion-exchange mechanism is considered to be the main force for removal of the metal ions. The ion-exchange can be via cationic, anionic or amphoteric type of adsorbents, which removes the cations, anions or simultaneously both cations and anions, respectively. On the other hand, the Freundlich isotherm deals with adsorption-complexation


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reactions.43 It involves adsorption of the metal ions via formation of complexes or stronger interaction between the metal and the active sites present on the adsorbent. In our case, as seen from the fitted isotherms, with better Regression Coefficient (R2) and the smaller values of ϰ2, the Freundlich isotherm fits fairly well with our experimental data.44 It suggests that the adsorption in the MS structure is multilayer with non-uniform distribution of the adsorption affinities. The Freundlich constant ‘n’ should be between 1 and 10 for the adsorption to be favorable, while higher values infer stronger interaction between adsorbate and adsorbent.45 In the present case, the fitted data shows n >, 1further justifying the observed capability of the MSs for adsorption of the oxyanions. Table 1. Freundlich and Langmuir isotherm parameters obtained from linear and non-linear fittings for oxyanions adsorption on phosphate@MS. Freundlich parameters

Chromate

Arsenate

a

Langmuir parameters

k [(mg/g) (L/mg)1/n]

n

R2

ϰ2

Linear

28.4

2.3

0.987

n.aa 306.7

0.036

0.961

n.aa

NonLinear

37.3

2.7

0.972

314 182.2

1

n.i.rb

7118

Linear

28.1

2.4

0.927

n.aa 319.4

0.020

0.992

n.aa

NonLinear

39.6

2.9

0.954

352 181.7

1

n.i.rb

9008

Q max b R2 (mg/g) (L/mg)

ϰ2

n.a: not applicable; b n.i.r: not in range The phosphate@MS sample was further examined for their capability in competitive

sorption. The oxyanion-removal experiments were carried out in presence of other anions, which may compete with chromate or arsenate for the adsorption sites (Figure 4). The results showed that the adsorption capacity of phosphate@MS decreased in the following order with respect to


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the competitive anion present in the solution: CO 3 2- > HPO 4 2- > SO 4 2-. While sulfate caused the greatest decrease in arsenate/chromate adsorption amongst the anions, the carbonate ions had only a little effect. This trend again follows the Hofmeister series as the protonated carbonate ions at the experimental pH are less influential than SO 4 2- and HPO 4 2- for the binding with PAH. From the above data, we estimated the selectivity coefficients for arsenate and chromate against the competitive anions to determine the recognition specificity for their adsorption on the microspheres (Table S5). Higher the value of selectivity coefficient, lower the interference of the particular anion in arsenate/chromate removal. So, the higher value for carbonate ion indicates that its interference was the lowest.

a)

b)

Figure 4. Adsorption of (a) chromate and (b) arsenate on phosphate@MS in presence of several competitive anions. Experiments were done with an initial concentration of chromate/arsenate and competitive anion of 3 mM each at pH 6 and the standard deviation was less than 5%. The selectivity data suggests that besides the molecular interaction of the anions with polyamine, the molecular dimension of the anions may play an important role in their adsorption. It can be seen that the MSs prepared with chromate anions (chromate@MS) also exhibited appreciable activity for the chromate removal. This result evidently suggests for a plausible molecular imprinting mechanism, in which cavities or voids complementary to the adsorbing


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ions in shape, size, and functional groups are created to impart specific ion- binding affinity to the material. In order to examine this phenomenon, the chromate ions in chromate@MS were first exchanged with various monovalent and multivalent anions. The initial yellow colored chromate@MS changed gradually to white after the anion-exchange. Thus modified MSs were then examined in the removal of chromate from water. As seen from the results, depending on the different anions used for the exchange process, the adsorption capacities of the modified chromate@MS varied in the following order: SO 4 2- > HPO 4 2- > NO 3 - > OH- > Cl- (Figure S11). This indicates that the sulfate anion having the strongest influence on PAH as per the Hofmeister series, is suited best for efficient removal of the chromate from chromate@MS. Moreover, the sulfate modified chromate@MS could result in an increased removal capacity from that of unmodified chromate@MS. At an optimum concentration of 0.05 M sulfate solution for exchange of chromate, the chromate@MSs had a removal capacity of 273 mg/g for chromate anions, whereas a similar exchange procedure for the arsenate@MSs showed a removal capacity of 261 mg/g for arsenate anions. It is also interesting to note that though the polyamine interaction with multivalent anion is non-specific, the MSs prepared using phosphate show selectivity towards binding / removal of both arsenate and chromate. This could be due to an ionrecognition mechanism based on the similarity in the size, shape and functionality of the anions. Moreover, the results also reveal that the silica nanoparticle-assembled structure, while acts as the support for such molecular cavities or voids, doesn’t have any adverse effect on the functionality of the entrapped polyamines for their interaction with the oxyanions. Recently, it has been shown that the bio-inspired silica coating on peptide building blocks can stabilize and preserve the soft-matter 3D nanostructures even with structural details down to

Oxyanion-Binding in a Bioinspired Nanoparticle-Assembled Hybrid

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