Mono-, Di-, and Tricarboxylic Acid Facilitated Lanthanum-Based

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Mono-, di- and tri-carboxylic acid facilitated lanthanum-based organic frameworks: Insights into the structural stability and mechanistic approach for superior adsorption of arsenate from water Subbaiah Muthu Prabhu, Shunsuke Imamura, and Keiko Sasaki ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06489 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Sustainable Chemistry & Engineering

Mono-, di- and tri-carboxylic acid facilitated lanthanum-based organic frameworks: Insights into the structural stability and mechanistic approach for superior adsorption of arsenate from water

Subbaiah Muthu Prabhu, Shunsuke Imamura, Keiko Sasaki*

Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan

*Corresponding Author:

Prof. Keiko Sasaki Tel & Fax: +8192 802 3338 Email: [email protected]

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ABSTRACT

In this study, we tried to synthesize lanthanum organic frameworks (MOFs) with the linkers of benzoic acid (BA), 1,4 – benzene dicarboxylic acid and 1,3,5 - benzenetricarboxylic acid (BTC), abbreviated as La-BA, La-BDC and La-BTC, respectively. Interestingly, the BA linker approached La metal to form lanthanum methanoate (La(HCOO)3 instead of the La-BA MOF through an acid catalyst amide-hydrolysis mechanism, whereas La-BDC and La-BTC act as MOFs, confirmed by PXRD patterns. Various sophisticated instrumentation techniques like FTIR, PXRD, XPS, BET, and TGA were utilized to understand the formation of MOF. This is the first report to investigate AsO43- adsorption and the dissolution behavior of La-BA, LaBDC and La-BTC in detail using different spectroscopic methods. The maximum AsO43adsorption densities obtained from Langmuir isotherm model were found to be 2.623, 3.891 and 0.280 mmol/g for La-BA, La-BDC and La-BTC, respectively, where the dose ratio of 1 g/L with the speed of 100 rpm at room temperature. The value for La-BDC was significantly superior to the previously reported adsorbents for AsO43- to date. The presence of AsO43- on both La-BA and La-BDC was confirmed by FTIR and XPS As 3d. After adsorption of 2.4 mM AsO43-, the precipitation mechanism controls the adsorption capacities on La-BA and ligand exchange mechanism on La-BDC confirmed by solution as well as solid analyses. Sorption kinetic data of AsO43- followed a pseudo-second-order model, which is consistent with 2

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chemisorption involving the possible coordination of AsO43- on La-BA and La-BDC. These results suggested that the MOF materials can be developed to immobilize arsenic-rich wastewater.

Keywords: Metal organic frameworks; Lanthanum; Arsenate; Adsorption; Ligand exchange. Introduction In natural water, arsenic (As) predominately occurs in two forms i.e., arsenite (AsO33-) and arsenate (AsO43-), in which AsO33- is highly dangerous due to its mobility and toxicity. As concentration in drinking water is considered to be a serious global environmental issue due to its toxicity and carcinogenicity.1 The acceptable concentration of As in drinking water is 10 µg/L set by World Health Organization (WHO).2 Of all the existing techniques, adsorption is shown to be more attractive and is traditionally used because of its greater efficiency, inexpensive expenditure and simplicity.3-6 A new kind of material namely, metal organic framework (MOF) and it based composites were highly exposed in the fields of energy and environmental science, as they are potentially porous compounds formed by the linking of inorganic and organic units through coordinative bonding.7 Recently, Zr-based MOFs/composites were shown good performance towards the removal of toxic ions from water. Wang et al. proposed UiO-66 as the superior adsorbent for AsO43- adsorption (303 mg/g).8 Fe-BTC MOF was synthesized via a simple solvothermal 3



method which exhibited a high uptake capacity for AsO43- of 12.287 mg/g.9 Zeolitic imidaloate framework-8 for the removal of trace amounts of AsO43- from water was described by Li et al., showing a maximum AsO43- uptake capacity of 76.5 mg/g.10 Followed by Fe-BTC, MIL-53(Fe) (Fe-BDC) was

synthesized and it shown higher uptake performance toward AsO43- (21.27 mg/L)

than the Fe-BTC.11 Though, in the literature there are lot of studies of toxic AsO43-/AsO33removal using iron-based materials, La-based materials have shown higher adsorption density than Fe(III)-based materials due to the affinity of lanthanum, which is much higher than iron. Also, Fe(III)-arsenate showed less stable and higher solubility of the product (Ksp) (5.7 x 10-21) than La-arsenate, (1.07 x10-21). This means that the iron is easily released from the structure after adsorption of AsO43- and is less suitable for drinking water and also it needs substantial methods to stabilize the materials, which in turn increase the cost-factor. However, a limited number of studies were performed with La/La-based composites for the removal of toxic As. Moreover, La3+-encapsulated materials have been demonstrated to be the most effective materials for toxic ion removal in the literature owing to its specific affinity.12,13 In addition, La3+ is considered as an inexpensive abundant rare earth element with excellent biocompatibility as well as environmental benign material.14 Nevertheless, until now, La-based MOFs are hardly reported in the literature. Recently, the calcined form of La (1,3,5-benzene dicarboxylic acid) was utilized for phosphate adsorption from water by Zhang et al. The

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maximum adsorption capacity of phosphate on La(1,3,5-BDC) was 10 mg/g.15 Until now, there has been no report for how the metal-organic linker interaction is formed and the role of DMF in structure elucidation. The main aim of this present work is to prepare La-based MOFs like La-BA, La-BDC and La-BTC MOFs for the immobilization of AsO43-. The synthesized La-based materials were characterized well by FTIR, PXRD, TGA, BET, SEM and BET to understand the mechanism of formation of MOF/MOF-like materials and to examine high concentration of AsO43removal by batch experiments. The influence of time, pH, coexisting anions and different initial concentration studies on the adsorption capacity were also investigated in detail. The synthesized materials exhibited high adsorption densities and were applicable for wide pH ranges, making them for an excellent material for the removal of toxic AsO43- removal from water. Experimental Section Materials Benzoic acid (BA), 1,4-benzenedicarboxylic acid (BDC), 1,3,5-benzene tricarboxylic acid (BTC), lanthanum nitrate hexahydrate (La(NO3)3.6H2O, potassium arsenate (KH2AsO4), dimethyl formamide (DMF), and ethanol, were purchased from Wako Chemicals, Japan. All chemicals were used as received without further purification. Milli-Q water (18.2 MΩ) was

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used for solution preparation and synthesis, and 2.3 mM AsO43- stock solution was prepared by dissolving the proper amount of KH2AsO4 in Milli-Q water. Synthesis of La-BA, La-BDC and La-BTC The La-BDC MOF was synthesized by the solvothermal method based on the previous studies. 8,16 743 mg (2 mmol) of La(NO3)3.6H2O and 250 mg (2 mmol) of BDC were dissolved in DMF (70 mL) and sonicated for 5 min for complete dissolution. The reaction was continued stirring for 30 min magnetically, and then the mixture was placed in a 100 mL Teflon-lined stainless steel autoclave at 120 °C for 24 h. After 24 h reaction, the autoclave was cooled down to room temperature and the powdery product was collected. The MOF materials were washed with reaction solvent first to remove any unreacted BDC moieties. Successively, the reaction product was washed with ethanol 3-4 times to replace the reaction solvent. The obtained slurry was vacuum dried for 24 h at 80 °C. Then, the product was water washed to remove any unreacted lanthanum species and freeze-dried. The similar procedure has been followed for the synthesize of La-BA and La-BTC with BA and BTC linkers, respectively. Adsorption experiments For the batch adsorption experiment, 40 mg the adsorbent was added to 40 mL of 2.4 mM KH2AsO4 and the mixture was shaken thoroughly at 100 rpm at 25 ºC. After the reaction the adsorbent was separated by filtration and the equilibrium concentration of AsO43- was analyzed

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by ICP-OES (Optima 8300, PerkinElmer Co., Ltd, Yokohama, Japan). The influence of pH, and coexisting anions and natural organic matters on the adsorption of AsO43- were investigated in detail. The kinetics of AsO43- adsorption were examined under similar conditions by withdrawing samples at time intervals. The solution was later filtered to determine the residual arsenic and lanthanum concentrations using ICP-OES. All adsorption results were expressed as the average of duplicate measurements.

Results and Discussion Adsorbents characterization The structural integrity after the introduction of organic ligands on La metal was examined by PXRD patterns. When the ligand BA was complexed with La, it did not form a La-BA MOF but instead formed lanthanum methanoate [La(HCOO)3] (JCPDS 00-018-0674) through acidcatalyst amide hydrolysis approach, as shown in reaction Scheme 1.17 The BA has acted an important role for the development of [La(HCOO)3] as well as La(OH)3. First, in this reaction, the BA ligand acts as a Lewis acid, which protonates the amide carbonyl of DMF to be more electrophilic when DMF is used as a solvent. As we utilized the purchased DMF without any purification, a small amount of water is present in the DMF. The nucleophile oxygen from water attacks the electrophilic carbon in the carbonyl group and the electrons ae moving toward

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the oxonium ions. As a result, it forms a tetrahedral intermediate (1a). The next step is to neutralize the charge by deprotonation of the oxygen from water molecule. To make it a good leaving group of an amine moiety, it should become protonated first using BA and the final product of dimethylamine is formed as a by-product (1b) and the deprotonation of the oxonium ions to carboxylic acid to formic acid as the main product (1c). Second, the BA considerably influenced the growth of La(OH)3 crystals by chelation, as it has one -COOH group. The carboxylic groups control the growth and particle distribution of the metals and as a result rodlike crystal morphology of the product was obtained. Likewise, the different organic acids not only influence the product shapes but also their crystallinities. The La(OH)3 crystals were synthesized via citric-acid assisted hydrothermal method by Tang et al group18, where the citric acid was acted as protective agent to control the growth rate of crystals in different directions as follows. The formation of the stable lanthanum-citrate complex was further hydrolysis to produce colloidal La(OH)3 nanoparticles capped with citric acid and it controls the growth rate of the La(OH)3 nanoparticles. The similar phenomenon was applied here instead of citric acid; BA was utilized to control the growth rate of La(OH)3. Further, the BA capped La(OH)3 nanorods were reacted with formic acid to form a lanthanum methanoate (1d) as the final product. In the case of BDC and BTC ligands with La, the highly crystalline MOFs, La(1,4BDC) (JCPDS: 00-034-1984)19 and La(1,3,5-BTC) (JCPDS: 00-023-1746),20 were obtained.

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As the BDC and BTC ligands have two and three carboxylate groups, respectively, they could act as not only the ligands with La but also linkers to extend the clusters of the MOF (Figure 1). H O H

O N

CH3 H

H

N

CH3

H C

CH3

Dimethylforamide

H

H2O

N

CH3

H

O

CH3

H C H2O

O

H N H

H

O

Dimethylamine 1b

HN

La(OH)3

O H C

H C

CH3

CH3 CH3

H

O

CH3

HN

H C OH

O

H

1a

O

Formic acid 1c

O

CH3

H

O

H H CH3 N H

CH3

CH3 CH3

O C O La O C O H

O

H

C H O Lanthanum methanoate

1d

Scheme 1. Possible pathway for the formation of lanthanum-methanoate in La-BA.

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110

JCPDS: 00-023-1746

La-BTC 211

Intensity (a.u)

JCPDS: 00-034-1984

10

20

30 40 50 60 2-Theta (deg)

70

303 342

La-BA 440

012

JCPDS: 00-018-0674

202 122 051 312 600 232 431

131

021

La-BDC

101

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

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80

Figure 1. PXRD patterns of the synthesized adsorbents

The formation of La-BA, La-BDC and La-BTC with their respective ligands was confirmed by FTIR spectra (Figure 2). In the case of La-BA, the –OH stretching vibration mode occurred at 3423 cm-1. The peaks at 2920 cm-1 and 1426 cm-1 are assigned to -CH stretching and it’s corresponding bending vibration mode, respectively, confirming the presence of methanoate structure of La(OH)3 facets and a sharp peak at 778 cm-1 suggesting the presence of benzene ring C-H bending vibration mode of BA, in which a small amount of BA was still present in the structure. The narrow intense peak at 1426 cm-1 attributed to –C-O-C-O. A small peak positioned at 432 cm-1 is due to the stretching vibration mode of the La-O bond.21 The La-OH

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moiety also revealed that the peaks present at the wavenumbers of 660 and 512 cm-1. Although La-BA was confirmed as La(HCOO)3 by PXRD, the presence of the –CH stretching vibration mode confirms the presence of a few benzoic acid and/or dimethyl amine groups, which are

La-BTC

1663 1582 1622 1536 1536 1446 1400 1376 1310 1426 1153 1113 1019 932 828 766 756 778 725 529 444 512 436

2922

3445

selectively bound to specific crystallographic facets of the La-BA surface.

880

La-BA

1596

2920

3423

La-BDC

3423

Transmittance (a.u)

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


1356

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavenumber (cm ) Figure 2. FTIR spectra of La-BA, La-BDC and La-BTC.

The La-BDC spectrum indicated the peak at 3423 cm-1 is ascribed to –OH stretching vibration mode of the BDC linker, and the peaks at 1588, 1536, and 1400 cm-1 correspond to the vibration 11



modes of the aromatic carbons.22 The C-H bending vibration mode appeared at 1310 cm-1. The peaks at 1153 and 1019 cm-1 were identified as C-N and C-O stretching vibration modes from DMF, suggesting that the presence of a small amount of DMF molecules occupied the surface of the La-BDC and this statement is supported by previous studies.9 The sharp peaks at 756 and 512 cm-1 suggesting the modes of benzene ring C-H bending vibration and La-O bending vibrations, respectively. After the formation of the La-BTC MOF, no other significant peaks were obtained because the similar functional groups were present in BTC. Figure 3 shows the TGA profiles of the synthesized products along with the standard La(OH)3 as reference. The reference La(OH)3 showed a very small degradation at 100-200 °C, is due to the water evaporation confirming that there is no moisture in the products. The next decomposition starts at 310 ºC due to the transformation of La(OH)3 to La2O3 through La2O2CO3 over 480 °C and this statement is confirmed from our previous report.23 For LaBDC, in contrast, the first degradation occurred between 150 and 230 ºC is due to the trace amount of DMF evaporation from the structure. The next degradation interval on La-BDC at 330 ºC is the transformation of its carbonates, and the third degradation interval at 470 ºC is due to the formation of La2O3. The final degradation at higher than 645 ºC is due to the decomposition of ligands which are coordinated to metal ions. The TGA results indicate that La-BDC possesses significantly higher thermal stability compared with the other La-BA

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clusters, indicating that the BA ligand was not coordinated to the La atoms. In the case of LaBTC, the main degradation started at 350 ºC due to the BTC molecules,24 and the final degradation at 645 ºC was due to the coordinated bonding between the lanthanum and the carboxylate groups in a similar manner to La-BA and La-BDC.

100

std.La(OH)3

90 80

Weight (%)

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


70 60

La-BA

50

La-BDC

40 30

La-BTC 0 100 200 300 400 500 600 700 800 900 1000 o

Temperature ( C) Figure 3. TGA profiles of the synthesized La-based products with std. La(OH)3 as reference. (conditions: Temp. 25 to 1000 oC with the heating rate of 10 ºC/min).

Figure 4 shows the SEM images of as-prepared La-BA and La-BDC products, showing that a large quantity of nanorods were successfully formed in the case of La-BA. Also, the BA moiety acted as capping agent as well as protective layer to control the growth of La(OH)3 particles, which results in the nanorods morphology of La-BA being obtained. In addition, BA is also known as a modulator for the synthesis of UiO-66-based materials. The small amount of dimethyl amine as a minor product also disturbs the formation of the uniform structure of 13



La-BA nanorods. The SEM image of La-BDC MOF shows that this bulk solid interconnected structure is composed of clustered lanthanum and BDC-linkers. This is totally different from La-BA, suggesting the formation of a MOF but not any oxides/carbonates, which has a different morphology than is shown in the literature.25 However, the La-BTC MOF also shows a rod-like structure similar to aggregated La-BA. Similarly, Shang et al. reported the formation of a needle-like La(1,3,5-BTC) MOF using an ionic liquid-assisted microemulsion technique.26

Figure 4. SEM images of La-BA, La-BDC and La-BTC products. Scale bars at 2 µm. The BET specific surface areas (SSAs) were approximately 3.50, 5.30, and 12.6 m2/g for LaBA, La-BDC, and La-BTC, respectively, and their average pore sizes are as shown in Table 1. The final products were dissolved in 1 M HNO3 to calculate the atomic molar ratio of La-BA and La-BDC. The presence of C, N and H and their molar ratios were calculated using CHN analyses as shown in Table 1. The presence of a trace amount of N from DMF also participated in the adsorption of AsO43- via coordination for the La-BDC MOF. However, La-BA does not

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have any DMF molecules, clearly depicts the formation of formic acid which further converted into lanthanum methanoate with La(OH)3.

Table 1 – Powdery properties and elemental compositions of synthesized MOFs Adsorbent

BET SSA (m2/g)

Avg. pore size (nm)

Atomic ratio C

N

La

C/La

As/La

La-BA

3.50

29.02

10.975

0.033

3.731

2.94

0.70

La-BDC

5.30

21.42

23.950

0.433

1.936

12.37

1.98

La-BTC

12.6

9.16

23.008

1.317

1.734

13.26

0.16

Figure 5a shows the wide XP spectrum of La-BA. The high-resolution C 1s spectrum of LaBA, is further divided into two main components with respect to formic acid structure of CC/C-H, and O=C-O. A peak at EB[C 1s] = 284.6 eV are attributed to the C-C from the BA moiety; the O=C-O peak detected at EB[C 1s] = 288.2 eV is primarily contributed by the methanoate groups, as they have only C-C/C-H and COOH groups in the structure. Additionally, the La-BA sample shows two components peaks at 529.4 and 531.6 eV for EB[O 1s] which can be assigned to oxygen in the La-O and the C-O groups, respectively, as it has no free -OH groups in the structure. The peak at EB[O 1s]=531.6 eV on the surface of the material might be from the methanoate rather than the BA material (Figure 5c). The La 3d peaks further divided into the doublets of La 3d3/2 and La 3d5/2, at 837.0 and 853.3 eV 15



respectively.27,28 The difference between the La-doublets is 16.5, which is close to the theoretical value 16.4 eV of the standard La(OH)3 pattern.

Figure 5. XP-spectra of the La-BA: (a) wide scan, (b) C 1s, (c) O 1s, (d) La 3d regions.

For, La-BDC, the wide spectrum shows the photoelectron spectrum lines at EB=862, 556, and 310 eV, corresponding to C 1s, O 1s and La 3d, respectively (Figure 6a). The C 1s of LaBDC consists of three different chemically shifted components observed by a deconvolution as –C-C-, C=O, and -COO-, with their corresponding EB[C 1s]= 284.6, 286.4 and 290.3 eV, 16


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respectively (Figure 6b).29 The ratios of carbon in carboxyl, aromatic alkene and aromatic alkane groups from the BDC group were found to be 1:2:1, as shown in the structure in the mechanism part later. As expected, the area ratio of the above groups is found to be the same. The peaks at EB[O 1s] =530.2, 531.6 and 533.1 eV is due to the La-O bond, free –OH and C=O groups (Figure 6c)on the sample surfaces, respectively.30 In this case, the carboxylate concentration is relatively higher than hydroxyl groups, and hence, the intensity of carboxylate oxygen is higher than that of hydroxyl oxygen. In the 3d core level photoelectron spectra for La in La-BDC, the peaks observed at 838.0 and 854.4 eV correspond to the La 3d5/2 and La 3d3/2 orbitals, respectively, which are similar to the La-BA (Figure 6d). The difference in EB between La 3d5/2 and La 3d3/2 was fixed to 16.4 eV. For the La species in La-BA and La-BDC, the latter part has higher binding energy than La-BA due to the presence of the bulky BDC group, whereas the La-BA part has a smaller methanoate group with La.

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Figure 6. XP-spectra of the La-BDC: (a) wide scan, (b) C 1s, (c) O 1s, (d) La 3d regions.

Effect of pH La-BA and La-BDC MOF can be applied in practical AsO43- removal from water. It was found that the adsorption capacity of AsO43- on both La-BA and La-BDC did not have obvious differences in the wide pH from 3 to 10, as shown in Figure 7a. The AsO43- adsorption capacity at pH 2 was very low due to the partial dissolution of La species from La-BA and La-BDC MOFs. Also, at pH 2, the dominant arsenate species is present in the molecular form (H3AsO4) and delivers no electrostatic attraction. The adsorption density of AsO43- is very low at pH 12

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is due to the presence of a very high concentration of hydroxyl ions. After the adsorption of 2.3 mM AsO43- on La-BA, the methanoate molecules were replaced by arsenate ions. This formed lanthanum arsenate as a stable final product, confirmed by the PXRD pattern (JCPDS: 01-076-4590). The formic acid is categorized as a good leaving group as it has a pKa of 3.77, compared to AsO43-, which has a value of 9.2. It is worth mentioning that the release of the formic acid group is reasonable for lowering the equilibrium pH of the solution, which favors enhancing the adsorption of AsO43- over the pH range. To confirm the formation of lanthanum arsenate on lanthanum, we carried out the experiment with standard La(OH)3 with 2.3 mM AsO43- and lanthanum arsenate as a final product, as shown in later parts. After the pH experiments, the supernatant was analyzed to determine the lanthanum concentration in ICPOES. The results indicated that at pH 2, the lanthanum release was high due to the dissolution of lanthanum, which is represented in Figure 7b.

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Figure 7. (a) Effect of pH. Experimental conditions: [AsO43-]0 - 2.4 mM; dose ratio - 1 g/L; agitation speed - 100 rpm; and temp. - 25 °C and (b) residual ion concentration on AsO43adsorption. Adsorption kinetics To investigate the adsorption behavior of 2.3 mM AsO43- on La-BA, La-BDC and La-BTC MOFs, the kinetic study was carried out under the fixed adsorbent dose of 1 g/L. As time increases, the adsorption densities were increased and reached equilibrium within 30 min with La-BA and 360 min with La-BDC MOF. However, the La-BTC MOF showed the lowest adsorption capacity of the adsorbents due to less availability of the active sites, which is supported by previous result by Zhang et al.15 for phosphate adsorption. The maximum adsorption capacities of AsO43- on La-BA, La-BDC and La-BTC were 2.068, 2.234 and 0.280 mmol/g, respectively, as shown in Figure 8a. The As/La atomic ratio of As/La of La-BA, LaBDC and La-BTC were different from each other as shown in Figure 8b, where the La concentration was analyzed by ICP-OES by acid decomposition. The La-BDC had a higher As/La atomic ratio of 1.98 than La-BA (0.7) and La-BTC (0.16) in which the La-BDC has a higher amount of active functional groups responsible for the adsorption of arsenate is shown in Figure 8b. To identify the rate of AsO43- adsorption onto the La-BA and La-BDC MOFs, the pseudo-second-order kinetic model31 was applied (Figure 8c). The experimental AsO43adsorption capacity values of (qeexp) 2.068 and 2.234 mmol/g and the calculated adsorption 20


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capacity values (qecal) of 2.067 and 2.233 mmol/g were reasonably consistent with the observed ones for La-BA and La-BDC MOF, indicating the suitability of the pseudo-second-order kinetics model. The higher value of the rate constants (k2=2.4608 g mg-1 min-1) than the other rate constants suggested that the adsorption of AsO43- was notably faster for La-BA than the other two MOFs as shown in Table 2.

Table 2 – Calculated pseudo-second-order kinetic parameters for the adsorption of AsO43Materials

Qexp (mmol/g)

Qcal (mmol/g)

k2 (g/mmol.min)

R2

La-BA

2.0676

2.0683

2.4608

1.000

La-BDC

2.2338

2.2447

0.0816

0.9957

La-BTC

0.2810

0.2885

0.7269

1.0000

21



Figure 8. (a) Effect of contact time, (b) As/La atomic ratio for La-BA, La-BDC and La-BTC and (c) pseudo-second-order kinetic model. Experimental conditions: [AsO43-]0 -2.4 mM; dose ratio -1 g/L; agitation speed - 100 rpm; and temp. - 25 oC. Influence of foreign ions and NOM on the removal of AsO43Naturally, ground waters and industrial effluents contain major anions viz., chloride, sulfate, carbonate, nitrate, phosphate, silicate and bicarbonate, which can interfere with the uptake of 22


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AsO43- by competing for surface binding sites available on lanthanum. The adsorption density of 2.3 mM AsO43- on both La-BA and La-BDC was not influenced by any other anions, except phosphate and silicate, although their initial concentration was 1 mM (Figure 9). In addition, the presence of other co-existing natural organic matters (NOM), such as humic acid (HumA), fulvic acid (FulvA) and oxalic acid (OxA). The effect of NOM interaction with AsO43- is stronger than the inorganic coexisting anions. The HumA has influenced much on adsorption of AsO43- than Fluv A due to their particle size is closer to the oxide surface since, which leads to stronger repulsion. Similar behavior has also been observed by Weng et al.32,33 In addition, as discussed by our previous study34 that Oxa would tend to form a complex with lanthanum leads to form a La-oxalate which blocks the active sites for adsorption of AsO43-. Though the interference is high in the case of NOM for AsO43- adsorption, still the La-BDC MOF maintained its adsorption density towards AsO43-. Hence, it can be concluded that these adsorbents could be utilized in real water systems.

23


2.4

La-BA

Page 24 of 43

La-BDC

2.1 1.8 1.5 1.2 0.9 0.6

HumA

OxA

FulvA

3-

PO4

2-

SiO3

2-

CO3

-

NO3

2-

SO4

-

Cl

0.0

-

0.3

HCO3

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

Arsenate adsorption density (mmol/g)


Figure 9. Effect of foreign ions on AsO43- adsorption. Experimental conditions: [AsO43-]0 - 2.4 mM; co-anion - 1 mM; dose ratio - 1 g/L; agitation speed - 100 rpm; and temp. - 25 °C. Langmuir isotherm: The Langmuir adsorption isotherm model35 was used to identify the relationship between the equilibrium concentration (Ce) and the amount of AsO43- adsorbed on La-BA and La-BDC at contact temperature, 25 oC is shown in Figure 10. The calculated maximum adsorption density (qm) from the Langmuir model has a higher value than that of the other material, which corresponds to the observed data. The applicability of the Langmuir isotherm model suggests that the adsorption of AsO43- on the surface of the material is probably due to the monolayer surface complexation with La-OH moieties. The obtained qm values were also compared with the existing reports in the literature and found that the La-BDC adsorbent

24


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showed superior performance towards the removal of AsO43-, which indicates that the La-BDC adsorbent can effectively remove AsO43- from water. It is noteworthy to mention that La-BDC adsorbent is the one of the highest AsO43- uptake performance materials in the literature.

Figure 10. Langmuir adsorption isotherm of AsO43- on La-BA (red color) and La-BDC MOF (blue color). Experimental conditions: dose ratio -1 g/L; agitation -100 rpm; and temp. -25 oC.

Reusability of the adsorbents The reusability of the AsO43- adsorbed La-BDC MOF was carried out to identify the costeffectiveness and the stability of the material by successive adsorption – desorption cycles. After regeneration of each cycle with the initial concentration of 2 mM at pH 6.3 for 24 h, the 25



adsorbent was separated, washed with 0.1 M NaOH, freeze-dried and the freshly prepared 2 mM AsO43- was added before each run. When increase the reuse cycle, the AsO43- adsorption densities become weak. Four adsorption and desorption cycle was carried out totally and interestingly, the AsO43- adsorption densities were retained up to three cycles with the difference in negligible amount of material loss (Figure S1). After fourth cycles, the interaction between AsO43- and MOF became weak due to less active sites for adsorption as well as material loss. Also, the PXRD patterns after each cycle was carried out and it supports the regeneration studies as the fourth cycle was complete precipitation as shown in Figure S2. These results indicated that it could be easily recovered with 0.1 M NaOH and also showed that La-BDC MOF adsorbent has sufficient chemical stability, further exhibited favorable reuse performance over four repeated cycles.

Mechanism of AsO43- adsorption The mechanism of AsO43- adsorption on La-BA, La-BDC and La-BTC is proposed as follows: the ligand of BA approached La metal, forming lanthanum methanoate instead of LaBA MOF through an acid catalyst amide-hydrolysis mechanism, confirmed by the PXRD patterns. In the La-BA structure, the formic acid acted as a good leaving group during the of adsorption of AsO43-. The PXRD pattern of AsO43- adsorbed on La-BA revealed the formation

26


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of lanthanum arsenate structure, supported with the standard LaAsO4 PXRD pattern (JCPDS file no: 01-076-4590). From the adsorption studies, it could be concluded that the pristine As/La molar ratio was 1:0.7, which means that the remaining formate molecules were dissolved from the structure and re-precipitated with arsenate, which was also confirmed by TOC analysis. The adsorption mechanism is mainly followed by ligand exchange and the precipitation reaction. The ratio of La:As is 1:0.7, and the equilibrium pH becomes acidic, confirming the formation of the lanthanum arsenate as the final precipitated product, as shown in Scheme 2. La(NO3)3. 6H2O Benzoic acid O

DMF, 120 oC

H

24 h nd ga Li

O

C

O La O

C

H

O

H

O

C O La O As OH

-

C OH O

C O

H

Lanthanum arsenate

O4 As 2

2H

O

O

e ng ha c ex

H

OH

Formic acid La:As = 1:0.7

Pa rti al C H dis so O lut ion Lanthanum methanoate

HCOOH

HCOO-

H+ pH drop

La3+

AsO43LaAsO4

Reprecipitation

Scheme 2. Possible mechanism of AsO43- adsorption using La-BA.

In the case of La-BDC, the central metal La atom has nine-fold coordination in which three oxygen atoms from hydroxyl groups and six oxygen from BDC moiety to form a distorted 27



dodecahedron.36 According to Zhang et al.,15 the BDC prefers to complex with La through three BDC anions through the possible linkage of a La-O-La bridge and a C-O-La interaction between BDC linker and La metal10 and the free hydroxyl groups are the responsible for the AsO43adsorption. The arsenate species such as H3AsO4 is categorized as hard acid which tend to form a complex with hydroxyl groups of La-cluster. The release of H+ ions and the hydroxyl ions formed water to maintain a charge balance in the solution. Another possibly for the existing adsorption site is the La-O-C complex between -COOH of BDC and La, where the adsorption takes place by replacing of BDC ligand by AsO43- through ligand exchange mechanism in which La-BDC framework partially disintegrate the crystal structure of the La-BDC adsorbent. The partial collapse of the La-BDC structure was due to the fact that arsenate interacted with the lanthanum to form the LaAsO4 precipitate. The molar ratio of La:As was 1:2, the La-BDC would tend to follow the ligand exchange rather than partial dissolution, and their chemical equilibria are different (Scheme 2). A small amount of BDC tends to be released from the structure to form lanthanum arsenate. The lanthanum-ligand interaction became weakened when AsO43- approach metal center which in turn collapse of the MOF structure by releasing the BDC groups that were dissolved and re-precipitated using the H+ ions in the water. LaAsO4 was confirmed by the corresponding PXRD diffractograms. Recently, Jian et al. demonstrated ZIF – 8 (Zeolitic imidazolate framework – 8) nanosorbent for AsO43- adsorption and the

28


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adsorbent was not stable at acidic pH conditions, in which the metal, Zn2+ ions were released from the structure. However, the AsO43- adsorption density on La-BDC did not affect much on neutral and basic conditions as like ZIF -8.37 On the other hand, the La-BTC molecule has shown a considerably lower AsO43- adsorption capacity of 0.2810 mmol/g. In the La-BTC MOF, the La sites were completely occupied by the three carboxylic groups of BTC through a complex reaction. Hence, no active sites were available for AsO43- binding. In addition, it is very difficult to replace the BTC groups in La-BDC MOF with AsO43- by ligand exchange mechanism/dissolution re-precipitation, as it is a highly stable moiety as well as being most likely linked in a bidentate manner.

29



Page 30 of 43

La(NO3)3. 6H2O 1,4-Benzene dicarboxylic acid DMF, 120 oC 24 h OH O

-

O HO H2O HO

O4 As

C

H2

O OH2 O O C La OH O

OH2

C

O

d an Lig Di ss olu tio n

O As O HO H2O O HO As O ge OH an h c ex

eq uil ibr

O OH2 O La

O C

O C OH2 OH O As O O

OH

La:As = 1:2

ium

RCOO-

HOOC-R-COOH

H+

pH drop La3+

La-BDC

C

AsO43LaAsO4

Reprecipitation

where R =

Scheme 3. Possible mechanism of AsO43- adsorption using La-BDC MOF. Characterization of solid residues after the adsorption of AsO43After the adsorption of 2.4 mM AsO43- on La-BDC (Figure 11a), the FTIR peak at 3439 cm1

is assigned to the –OH stretching vibration mode. The peaks at 2972 and 2824 cm-1 were

attributed to the -CH and –CH2 stretching vibration mode. Also, in the green-colored La-BDC MOF, a few miscellaneous weak peaks were present. This once again confirms the presence of the BDC moiety over LaAsO4. Wang et al.8 have already reported the release of BDC molecules during the interaction of AsO43- ions in aqueous solution. The asymmetric and symmetric 30


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vibration mode of C-O at the peak position of 1538 and 1418 cm-1 confirming the carboxylic groups present from the BDC moiety. The peak at 729 cm-1 corresponds to the C-H bending vibrations of benzene. After adsorption, a new wide peak positioned at 854 cm-1 is assigned to the stretching vibration mode of As-O-La groups, indicating that AsO43- is coordinated with La-BDC MOF. A strong band appears at approximately 447 cm-1, which is due to the stretching vibration mode of La-O. Also, after the adsorption of 2.4 mM AsO43- on lanthanum-methanoate, the strong and wide peak at 854 cm-1 was attributed to the presence of La-O-As, where the methanoate molecules were replaced by AsO43- ions, and it formed lanthanum arsenate as a stable final product, confirmed by the PXRD pattern (JCPDS: 01-076-4590). In addition, it has been confirmed that only partial dissolution of formic acid and the BDC moiety by AsO43occurs. The release of BDC by re-precipitating over LaAsO4 was noted (Figure 11b), which showed that the less stable structure of La-BA and the MOF structure of La-BDC withstood on the same reaction. The SEM images after adsorption of AsO43- on La-BA and La-BDC are shown in Figures 11c and 11d, respectively. It is worth noting that AsO43- occupied the edges on La-BA and formed an urchin-like structure of lanthanum arsenate. The AsO43-sorbed LaBDC forms a thread-like continuous structure. When compared with the structure before adsorption, the structure was not collapsed much, which confirms the stability of the MOF.

31



To understand the AsO43- adsorption mechanism on La-BA and La-BDC, surface compositions and surface molar ratio were further investigated by XPS. The narrow scan for the As 3d spectrum of both AsO43- adsorbed La-BA and La-BDC showed the respective characteristic peaks for EB[As 3d] at 44.3 and 44.9 eV (Figure 11e-f).38 This confirmed the formation of the AsO43- complex with the La-BA and La-BDC MOF. The intensity of the As 3d peak in La-BA is much higher than La-BDC, which clearly indicated the precipitation of LaAsO4, where EB[As 3d] is close to KH2AsO4. On the other hand, As 3d mostly follows ligand exchange rather than chemical precipitation; hence, the intensity is very low. Further, the surface molar ratio of nAs/nLa on La-BA and La-BDC MOF after adsorption of 2.3 mM AsO43was calculated by XPS relative surface sensitivity factor and the results of nAs/nLa are 0.612 and 1.894 for La-BA and La-BDC, respectively, which are close to 0.70 and 1.98 obtained by experimentally. The wide spectrum and the other spectrum of C 1s, O 1s and La 3d for La-BA and La-BDC after adsorption of 2.4 mM AsO43- are shown in Figures S3 and S4.

32


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33



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Figure 11. (a) FTIR spectra, (b) PXRD patterns (c-d) SEM images and (e-f) XP spectra of La-BA and La-BDC after adsorption of 2.4 mM AsO43Conclusions Three types of carboxylate functionalized La-based products, namely, La-BA, La-(1,4-BDC) MOF and La(1,3,5-BTC) MOF were successfully synthesized and characterized well before and after adsorption of high concentration (2.4 mM) of AsO43- from aqueous solution. We have successfully derived the mechanism for the formation of La-BA as lanthanum-methanoate, and La-BDC and La-BTC were identified as MOFs using standard PXRD patterns. This is the first report of an investigation of the formation of La(HCOO)3 and the associated AsO43- uptake performance. The maximum adsorption capacities of AsO43- on La-BA and La-BDC were 0.691 and 1.936 mmol/mmol-La, respectively, which is one of the highest AsO43- uptake capacities compared to many previously reported materials to date. The adsorption density of AsO43- on both La-BA and La-BDC was not influenced by any other anions, except phosphate and silicate although the initial


34

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concentrations of both AsO43- and interfering anions are equimolar. The AsO43- adsorption capacity on both La-BA and La-BDC did not have obvious differences over the wide pH range of 3-11 and remained constant. The FTIR spectra suggest the formation of the As-O bond with peaks at 854 cm-1 for La-BA and La-BDC, and PXRD patterns confirm the formation of LaAsO4 along with the MOF moieties after adsorption. The XP-spectra also confirmed that the characteristic EB peaks at 44.3 and 44.9 eV were corresponding to Asv-O on La-BA and La-BDC, respectively. The mechanism of AsO43- adsorption by La-BA mainly occurs by ligand-exchange followed by chemical precipitation. On the other hand, La-BDC mainly features ligand-exchange. The adsorption of AsO43- onto carboxylate-mediated La-based adsorbents follows pseudo-secondorder kinetics, suggesting that the adsorption of AsO43- on the adsorbents might involve chemisorption. These results suggest that La-BA and La-BDC MOF are superior materials to remove AsO43- from waters.


35


Page 36 of 43

Associated content: Supporting information XP-spectra of the AsO43- adsorbed La-BA and La-BDC: (a) Wide scan (b) C 1s spectrum, (c) O 1s spectrum, and (d) La 3d spectrum (Figure S1 and Figure S2), regeneration study (Figure S3), PXRD patterns of the La-BDC after each regeneration cycle (Figure S4), a comparison table with the reported materials in the literature (Table S1), adsorption characterization techniques, and the equations of pseudo-second-order kinetic model and Langmuir adsorption isotherm. Corresponding Author * Prof. Keiko Sasaki, Email: [email protected], Tel: +81 92 802 3338; Fax: +81 92 802 3338. ORCID ID Subbaiah Muthu Prabhu – 0000 0003 1553 3136 Keiko Sasaki – 0000 0002 2882 0700

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Financial support was provided to KS by the Japan Society for the Promotion of Science (JSPS) - Japan through research grants (JP16F16082) and to SMP (P16082) by the JSPS Postdoctoral Fellowship for Foreign Researchers.


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ACS

Sustainable

Chem.

Eng.

2017,

5(5),

4371-4380,

DOI:

10.1021/acssuschemeng.7b00418.


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TABLE OF CONTENT (TOC)

Mono-, di-, tri-carboxylic acid facilitated La-based organic frameworks were prepared and utilized for effective adsorption of arsenate from water.


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Mono-, Di-, and Tricarboxylic Acid Facilitated Lanthanum-Based

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