A Mechanistic Approach for the Synthesis of Carboxylate-Rich

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A mechanistic approach for the synthesis of carboxylate-rich carbonaceous biomass-doped lanthanum-oxalate nanocomplex for arsenate adsorption Subbaiah Muthu Prabhu, Chitiphon Chuaicham, and Keiko Sasaki ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04678 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Sustainable Chemistry & Engineering

A mechanistic approach for the synthesis of carboxylate-rich carbonaceous biomass-doped lanthanum-oxalate nanocomplex for arsenate adsorption Subbaiah Muthu Prabhu, Chitiphon Chuaicham, Keiko Sasaki* Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan *Corresponding Author: Prof. Keiko Sasaki Tel: +81 92 802 3338; Fax: +81 92 802 3338. E-mail addresses: [email protected] (K. Sasaki) [email protected] (S. Muthu Prabhu) [email protected] (C. Chuaichum)

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ABSTRACT

Considering the protocol of ‘zero-alkaline waste disposal’ for green and arsenic-free environment, lanthanum (La3+)-based MOF-like complex materials were designed, and the complex materials have been employed as adsorbent for AsO43- adsorption from water. The sucrose-derived porous carbon (SPC)@La-oxalate complex was prepared by a simple one-pot co-precipitation method at room temperature, where oxalate has been used as an organic ligand, and the carbonaceous biomass has been used as a doping material that is naturally carboxylaterich functional groups derived from sucrose biomass. In addition to SPC@La-oxalate, bare-SPC, La(OH)3, and SPC@La(OH)3 were also prepared via simple base-addition conventional methods, and their performances in AsO43- removal were compared. The FTIR peak at 848 cm-1 confirmed the presence of AsO43- on the SPC@La-oxalate complex after adsorption of 1 mM AsO43-. The high resolution X-ray photoelectron spectrum for the AsO43- adsorbed SPC@La-oxalate showed a peak at EB[As 3d] = 45.2 eV, which could be attributed to As5+. The EXAFS of As- K edge revealed that there are two distinct atomic shells, As-O with the distance of 1.68 Å and As-La with the distance of 3.32 Å, indicating the formation of monodentate complex of La with AsO43-. Additionally, an electrostatic interaction and hydrogen bonding are also possible adsorption mechanism in acidic conditions. The SPC@La-oxalate complex adsorbent showed excellent dearsenate behavior of 1.093 mmol/g and the maximum AsO43- removal was maintained in a wide pH range from 3 to 8. Sorption kinetic data were the best expressed by a pseudo-secondorder rate equation, and the maximum adsorption capacity was 1.858 mmol/g based on Langmuir monolayer adsorption. Compared with previous reports, SPC@La-oxalate adsorbent could be easily prepared, and the uptake amounts for AsO43- were enriched. Reusability of the material


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after 6 cycles is yet another advantage to the present adsorbent. This work will help to facilitate the research on novel complex adsorbents for the removal of AsO43- from water.

KEYWORDS: Sucrose-carbon biomass; Lanthanum; Oxalic acid; Arsenate adsorption; MOFlike complex.

INTRODUCTION The quality of drinking water is very important for the quality of life and public safety. The arsenic (As) contamination in drinking water is a major issues in several countries, e.g., China, India and Bangladesh.1 This contamination is caused by raised levels of inorganic arsenic compounds in the groundwater, which leads to the adverse health effects, specifically skin lesions and cancer. World Health Organization (WHO) has set the permissible concentration for As in drinking water to be up to 10 µg/L.2 The U.S. Environmental Protection Agency and the European Commission have also revised the As concentration in drinking water from 50 to 10 µg/L as maximum contaminant level (MCL).3 When prolonged exposure of higher dose of As may lead to skin, bladder and kidney cancers.4,5 Various techniques were adopted for arsenic adsorption including membrane, precipitation-coagulation, adsorption, bioremediation, ionexchange and solvent extraction. Among these techniques, adsorption is a good alternative method due to its simplicity in design, operational cost, and regeneration efficiency. Lanthanum (La) is a soft metal and has no biological role in the human body but is essential to bacterial and some antimicrobial activity.6 La3+ is classified as Lewis hard acid by Pearson’s HSAB principle, and is also known to be nontoxic and environmentally friendly.7 La3+ entrapped adsorbents have been addressed as the potential materials for toxic ions removal because of their 3 ACS Paragon Plus Environment


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specific affinity of La3+ sites. But, a few studies were given attention to develop efficient adsorbents with lanthanum for AsO43- adsorption. Notably, lanthanum impregnated activated alumina has been found to be one of the promising adsorbents for arsenic ions removal because it is more selective and efficient with 2-3 times the sorption capacity of activated alumina.8 Jais et al.9 proposed a lanthanum and nanomagnetite incorporated active carbon derived from palm shell waste for removing arsenate and demonstrated a sorption capacity of 227.6 mg/g. Lanthanum functionalized mesoporous silica SBA-15 and lanthanum-loaded zeolite adsorbent for AsO43- by Jang et al10 and Pu et al11, respectively. Guo et al. reported that a La-containing layered double hydroxide was efﬁcient in removal of AsO43- from aqueous solutions.12 La(OH)3 is classified as non-toxic and is good adsorption capacity of AsO43- over a wide pH range, but the particles are very small, which enables them to pass through filters easily. Hence, to overcome this problem, La(OH)3 may be embedded in a composite; therefore, sucrose-derived porous carbon (SPC) has been selected because it is environmentally benign highly porous substrate, with a high mechanical strength. Conversely, oxalate has been widely used as an organic ligand for metal organic framework (MOF), but less attention has been paid to the adsorbents for AsO43- with these ligand functionalized materials. Moreover, carboxyl group from an organic acid, such as oxalate, could coordinate to La during the synthesis, which could potentially control the growth and the final particle size distribution. In this study, the protocol is designed towards ‘zero-alkaline waste disposal’ to an aquatic environment. Thus, the SPC-doped MOF-like La-oxalate nanocomplex has been prepared as an adsorbent by a simple, economic and environmentally benign route via a one-pot co-precipitation method at room temperature, where oxalate has been chosen as an organic ligand. In addition to SPC@La-oxalate, bare-SPC, La(OH)3, and SPC@La(OH)3 were also prepared via simple base4 ACS Paragon Plus Environment

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addition conventional methods, and their performances in AsO43- removal were compared. Meanwhile, the influence of AsO43- concentration, coexisting anions, adsorption time, pH and the adsorption isotherms/kinetics were investigated in detail. The adsorption mechanisms were derived using several spectroscopy methods. The excellent adsorption ability of the synthesized complex adsorbent demonstrates that it is a promising potential candidate for environmental applications.

MATERIALS AND METHODS Materials Sucrose (C12H22O11), lanthanum nitrate hexahydrate (La(NO3)3·6H2O), sodium hydroxide (NaOH), potassium dihydrogen arsenate (KH2AsO4), and oxalic acid (C2H2O4) were purchased from Wako Chemicals, Japan. The chemicals were directly used as received without further purification. Milli-Q water was used for synthesis, solution preparation and the stock solution of AsO43- was prepared by dissolving the proper amount of KH2AsO4 in Milli-Q water.

Synthesis of SPC@La-oxalate The SPC@La-oxalate was synthesized through a simple co-precipitation method. The carboxylate-rich porous carbon was prepared according to a previously reported method with slight modification and the BET surface area was 374.1 m2/g.13 The typical synthesis of SPCdoped La-oxalate was performed as follows: 30 mg of SPC and 0.2 M La(NO3)3·6H2O were combined and stirred for 20 min, and the resulting solution was sonicated for 5 min. Freshly prepared 0.2 M oxalate was slowly added with vigorous stirring until precipitation was complete at pH 1.5-1.8 and then stirring was continued for 2 h. The blackish color SPC@La-oxalate was



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washed with ultrapure water until it reached neutral pH, and the final material was freeze-dried overnight. The same procedure was followed for the synthesis of La-oxalate without SPC. The schematic representation for the preparation of SPC@La-oxalate complex is shown in Scheme 1. The other adsorbents, namely La(OH)3 and SPC@La(OH)3, were prepared using a simple conventional co-precipitation as described in the previous report.13

Adsorption experiments In the adsorption process, a fixed dose of adsorbent (20 mg) in 20 mL with an initial concentration 1.2 mM KH2AsO4 solution, was added separately. The mixture was shaken thoroughly at 100 rpm and 25 ºC and the filtrate was supplied for determination of AsO43concentration using inductively coupled plasma-optimal emission spectroscopy (ICP-OES, Optima 8300, PerkinElmer Co, Ltd, Yokohama, Japan). The effective of contact time, pH and coexisting anions on AsO43- adsorption using the synthesized materials were investigated. The influence of the initial AsO43- concentration was studied in the range of 0.25, 0.5, 0.75, 1.0, 1.5 and 2.0 mM. The kinetic studies were conducted at different time intervals at 25 ºC with the material to volume ratio as 1 g/L, then the solution was filtered and the remaining AsO43concentration were analyzed. The amount of lanthanum released during the AsO43- adsorption was measured by ICP-OES.

Adsorbent characterization techniques The BET specific surface area (SSA) and pore size distribution of La-oxalate and SPC@Laoxalate samples were determined. The samples were pre-treated in two steps: vacuum degassing for 90 min at 150 °C and a final vacuum pretreatment for 15 h at 150 °C. The measurement was


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conducted by N2 (99.99%) adsorption using a BELSORPmax porosimeter (JAPAN BELL, Osaka, Japan). The functional groups of the as-synthesized materials before and after adsorption of AsO43- were characterized by FTIR (JASCO FTIR-670 Plus, Tokyo, Japan) under transmission mode. Powdered XRD (PXRD) patterns were carried out on Ultima IV diffractometer (RIGAKU, Akishima, Japan). The morphology of the as-synthesized adsorbents and after treatment with AsO43- was investigated by the scanning electron microscope (SEM, VE-9800 model, Keyence, Osaka, Japan). The pH of the suspension was adjusted to analyze the Zeta potentials by a ZETA SIZER NANO-Z (Malvern, Kobe, Japan). X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCA 5800 (ULVAC-PHI, Inc, Kanagawa, Japan) using a monochromated Al Kα X-ray source at 200 W for La 3d, As 3d, C 1s and O 1s, and Casa XPS software (Version 2.3.12.8) was used to analyze. EB[C 1s] = 284.6 eV for adventitious carbon was used to calibrated the binding energies (EB). EXAFS spectra of the La LIII-edge and the As K-edge for the solid residues before/after adsorption of AsO43- on SPC@La-oxalate were obtained on a BL15 at the Kyushu Synchrotron Light Research Center (SAGA-LS; Tosu, Japan). The spectra of samples were measured using an ionization chamber in fluorescence mode for the La L-edge and As K-edge using a silicon drift detector (SDD). The photon energy was scanned in the range of 11.7 to 13.4 keV for the As K-edge. Standard substances such as KH2AsO4 and synthesized materials after AsO43adsorption, such as La-oxalate and SPC@La-oxalate, were analyzed. The Fourier transformation of EXAFS spectra were obtained using the radial distribution function around the central As atom. The peaks obtained from Fourier transform were related to shells of back scattering atoms, which was characterized by a specific atom, number of atom in the shell (N), the absorber-scatter distance (R), and a Debye-Waller factor (σ2). The amplitude and the phase shift from a



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theoretical model were obtained for this study. The EXAFS data analysis was performed using the ATHENA and ARTEMIS programs in the Demeter computer package.14, 15

RESULTS AND DISCUSSION The specific surface area (SSA) of La-oxalate and SPC@La-oxalate was investigated by BET gas sorption measurements and the results are shown in Table 1. From the BET measurements (Figure 1), it can be seen that the SPC@La-oxalate adsorbent has a higher surface area (35.79 m2/g) than La-oxalate (30.40 m2/g). The pore size and pore volume are also higher than Laoxalate, which means that the SPC material would provide their surface area for the La-oxalate materials to grow through a chelation mechanism. In fact, the SPC material itself has higher surface area of 374.1 m2/g and is of a porous nature. Figure 2 shows the FTIR spectra for freshly prepared SPC, La(OH)3, SPC@La(OH)3, Laoxalate and SPC@La-oxalate complexes. The SPC shown their characteristic peaks of –OH stretching vibration at 3443 cm-1 and its corresponding to bending vibration at 1697 cm-1, -CH stretching and bending vibrations at 2924 cm-1 and 1405 cm-1, respectively. The C-O stretching vibration was observed at 1089 cm-1 confirms the presence of –COOH groups in SPC groups. The FTIR spectrum for La(OH)3, a sharp peak at 3610 cm-1 is assigned to the –OH stretching vibration mode.16 The broad peak at 3414 cm-1 is ascribed to the stretching vibration of –OH. Sharp peaks at approximately 492, 501 and 516 cm-1 can be ascribed to the stretching vibration mode of the La-O bond. After formation of the SPC@La(OH)3 composite, in addition to the aforementioned peaks derived from La(OH)3, two characteristic peaks appeared at 1626 cm-1 and 1421 cm-1 assigned to C=O and C=C bonds, respectively, derived from sucrose carbon in the composite.17,18 In the FTIR spectrum for La-oxalate, the absence of the peak around 3620 cm-1


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confirms that the oxalate had formed a complex with La and the peak at 1620 cm-1 corresponds to the stretching vibration mode of the carbonyl group is present in the oxalate moiety. The absence of the peak at 3620 cm-1 also indicates that no free La-OH groups are present, and the oxalate moiety might have coordinated to La to form a complex. Moreover, the sharp and intense peak at 613 cm-1 is assigned to the La-O bond, which is shifted to a slightly higher wavenumbers due to overlapping with the signals of La-oxalate complex. There is no significant difference after SPC material was used as template-like material, except for appearance of weak peak at 2920 cm-1 attributed to -CH stretching vibration derived from sucrose carbon. A typical PXRD pattern of La(OH)3 nanomaterial and all the reflections could be readily indexed to the hexagonal phase of La(OH)3 (JCPDS 36-1481) as shown in Figure 3. The PXRD pattern for SPC@La(OH)3 is consistent with the original La(OH)3, suggesting that the sucrose carbon did not affect the crystal phases in SPC@La(OH)3. Mostly, sucrose carbon is less crystalline, assisting the crystal growth of La(OH)3 by providing the functional groups on the surface. Further, when La(OH)3 was modified with oxalate, it developed a less pronounced crystalline structure in the complex of La-oxalate. This conversion of very crystalline phase to amorphous phase is due to complex behavior. The oxalate might have coordinated to active LaOH groups through self-assembly as an intermediate and further it decreased the planer size of the crystalline La(OH)3 layers arrangement. A similar phenomenon was also observed in the SPC@La-oxalate nanocomplex and this conversion has been explained well in the section of the mechanism. Figure 4A shows the wide scan of XPS spectrum for the SPC@La-oxalate where the distinct photoelectron peaks at EB=284.0, 530.0 and 711.0 eV are attributed to C 1s, O 1s, and La 3d, respectively. The high-resolution C 1s spectrum can be separated into four components as shown



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in Figure 4B. A peak at EB[C 1s] = 284.6 eV is assigned to the C-C from the hydrocarbon surface of the sucrose carbon moiety. Two components at EB[C 1s] =286.5 and 289 eV are assigned to C-O and O-C=O bonds in oxalate and sucrose carbon, respectively.19 The peak component at EB[C 1s] = 282.4 eV corresponds to sp3-C in sucrose carbon. Most carbon atoms in SPC are in the sp3 hybridized state.16 Additionally, it can be seen that the SPC@La-oxalate shows two O 1s peaks at EB[O 1s] =529.7 and 531.9 eV, which can be assigned to La-O18 bond and C=O20 groups on the surface of samples, respectively. The spectrum confirms that the La3+ have fully occupied free –COO and –OH groups of sucrose carbon and there are no more free –OH groups in the sucrose carbon moiety. The peak at EB[O 1s]=531.9 eV on the surface of the material might be from oxalate, rather than sucrose carbon (Figure 4C). XP-spectrum in the range of 830 to 860 eV can be separated into two pairs of La 3d peaks suggesting two La compounds. That is, oxides and oxalate21,22 are at EB[La 3d3/2] = 835.7 and 838.7 eV and EB[La 3d5/2] = 852.2 and 855.4 eV. Generally, the XP-spectra of La 3d accompany the strong satellite peaks, which suggested that the presence of both bonding and antibonding states of La. The presence of these peaks from multiplet splitting adds to the complexity of the spectra.21 SEM image of the prepared SPC material was observed to be porous as shown in Figure 5A. The base addition to synthesize La(OH)3 lead a microstructure that typically exhibits a flake-like and disordered morphology (Figure 5B). Moreover, after making of the SPC@La(OH)3 composite, the high porosity of SPC has adopted La(OH)3 particles as in a tail-thread structure. The morphology of both SPC and La(OH)3 did not change when both were mixed. It also shows that the La(OH)3 particles are randomly decorated on SPC surface (Figure 5C). The formation of La-oxalate was synthesized independently as nano-sized particles and produced higher surface area than the pristine La(OH)3, as shown in Figure 5D. The agglomeration of La-oxalate was 10 ACS Paragon Plus Environment

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avoided due to the high surface area of La-oxalate particles that occur during preparation on the sucrose carbon surface. The surface of SPC@La-oxalate was altered to be a uniform nanorodlike structure, suggesting that the SPC material indirectly supports the function of La-oxalate with smaller particle size than the SPC, as shown in Figure 5D and 5E. The adsorbents La-oxalate and SPC@La-oxalate were further characterized by thermogravimetric analysis in N2 atmosphere, where oxalic acid and La(OH)3 were used for standard references as shown in Figure S1A. The TG-DTA curve of La(OH)3 shows the conversion of hydroxide to oxide (La2O3) at 410 ºC. The TG-DTA curve of the La-oxalate showed very small mass loss due to water evaporation at 100-150 ºC. The second weight loss started at 400 ºC due to conversion of La-oxalate into La2O2CO3 and finally it was converted into La2O3 at higher than 680 ºC, and these results are in good agreement with the previous studies.23,24 The crystal phase of final product was confirmed to be La2O3 by PXRD as shown in Figure S1B. For SPC@Laoxalate, the first weight loss occurred between 100 to 130 ºC due to evaporation of trace amount of water. The second weight loss at 330 ºC is caused by the conversion of both SPC and the Laoxalate moieties to their corresponding carbonates and the third weight loss at 690 ºC was due to the dehydroxylation accompanied by the decomposition of interlayer carbonate anions in SPC and oxalate moieties to the formation of La2O3. The SPC@La-oxalate showed higher weight loss percentage than the La-oxalate complex due to the presence of sucrose carbon. The La content in both La-oxalate and SPC@La-oxalate adsorbents was measured to be 3.0805 and 2.3362 mmol/g using ICP-OES. They are matched with TGA results of 3.0690 mmol/g for La-oxalate and 2.3020 mmol/g for SPC@La-oxalate. Based on TGA and CHN analysis, the percentage of Laoxalate is 62.4 % in SPC@La-oxalate and is lower than the original La-oxalate (83.2 %).



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The adsorption capacity of AsO43- on as-synthesized adsorbents, SPC, La(OH)3, SPC@La(OH)3, La-oxalate and SPC@La-oxalate complexes were kinetically determined over time using 1 g/L of adsorbent to volume ratio with 1.2 mM AsO43- of the initial concentration at ambient temperature with solution pH, as demonstrated in Figure 6A. The AsO43- removal efficiency increased with time and reached a plateau after 60 min with the SPC and La(OH)3. The maximum adsorption capacities of the SPC@La(OH)3, La-oxalate and SPC@La-oxalate complexes, on the other hand, were reached within 30 min. The greater maximum adsorption capacity was obtained with the SPC@La-oxalate than with La-oxalate, because of the synergistic effect of both SPC and La-oxalate materials, where the SPC moiety may provide the adsorption sites of both carboxyl and hydroxyl functional groups to enable the formation of the La-oxalate, which in turn the AsO43- adsorption capacity was increased significantly. The maximum AsO43adsorption capacities for SPC, La(OH)3, SPC@La(OH)3, La-oxalate and SPC@La-oxalate complexes were 0.236, 0.924, 1.037, 0.812, and 1.193 mM, respectively. The synergistic adsorption performance of both SPC and La(OH)3 (SPC@La(OH)3) had almost similar adsorption capacity to La-oxalate; nevertheless, their preparation is via a strongly alkaline medium and it needs substantial washing to remove excess alkali, which in turn pollutes the aquatic environment. However, the La-oxalate and SPC@La-oxalate does not require such harsh reaction but by simple addition of oxalic acid to La material which is doped with SPC material possess adsorption behavior is much higher than SPC@La(OH)3. Figure 6B indicates the molar ratio of As/La for adsorption of AsO43- is higher with SPC@La-oxalate than with the other forms, and the results are also summarized in Table 1. The AsO43- adsorption densities are strongly influenced by the solution pH. The adsorption capacity is higher at lower pH and maintained their removal performance with increasing pH in


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the range of 3 to 8 as shown in Figure 6C. Above pH 8, the adsorption efficiency for AsO43- is reduced due to hydroxyl interference. At pH 2, the dissolution of La species happened leading that the adsorption of AsO43- was relatively low. The mechanism of AsO43- adsorption could be elucidated by the pH effect. Mostly, the surface complexation reaction occurs at pH 5-8, where the arsenic species are readily complexed with -COOH and La-O formation by replacing the -OH groups through ligand exchange. Additionally, in acidic conditions, the hydronium ions allow larger amounts of AsO43- to be adsorbed via electrostatic interactions or by hydrogen bonding. Moreover, the equilibrium pH showed no significant differences, as shown in Figure 6C. In Figure 6D, the dissolved La3+concentrations was determined at different pH values. At pH 2, the dissolution rate of La3+ was remarkable and at higher than pH 2 the dissolution was negligibly observed for both La-oxalate and SPC@La-oxalate. In addition, the zeta potential for SPC, Laoxalate and SPC@La-oxalate were observed to decrease with increasing pH, which suggested that the surface charge of adsorbents were pH dependent. At pH3.5, which is due to the presence of large number of carboxyl groups were present in the carbon material. It can be concluded that the La-oxalate and SPC@La-oxalate complex can be applied for wide pH range for the removal of AsO43- in practical wastewaters. The pHzpc values for SPC, La-oxalate and SPC@La-oxalate were found to be 3.5, 9.70 and 9.30, respectively. In natural water systems, other anions, such as chloride, nitrate, sulfate, carbonate and bicarbonate, always coexist and form surface complexes with La(OH)3. In this study, the 13 ACS Paragon Plus Environment


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coexisting ions concentration and AsO43- concentration were kept as 1 mM for each at room temperature. As shown in Figure 6F, there is no alteration in the AsO43- adsorption efficiency with coexisting anions, except for carbonate and bicarbonate. Because carbonate and bicarbonate anions in solution increased the solution pH (~10.4) which affect the AsO43- adsorption and also it is presumed that the adsorption density decreases with increased valence of the particular anion. The formation of the SPC@La-oxalate complex was formed at pH 1.8~2.0. The oxalate complexed with hydroxyl groups of La(OH)3 in aqueous solution. In order to avoid the high surface potentials, the surface coordination complexation required to be maintained close to electroneutrality.25 During the surface complexation, the chelate ring is formed by the deprotonated oxalic acid, ligand and the La3+ ion. The dissolved oxalate species in the reaction is a monoanion, which is predominant at acidic pH. The intermediate step of the formation of the five membered chelate ring or, in other words, a monodentate oxalate surface complex is formed. Oxalate is a strong Lewis base (electron donor) that produces a trans-labile effect and as increased the electron density of La, the rate of the oxo-bond breaking is improved due to inductive effect. The oxygen atom contains the unpaired electrons in La-oxalate complexes can attract arsenic atom (As) to make more positive, which can, in turn, attack through the surface complexation as below Scheme 2. Hence, the oxalate ion could complex with La3+ metal via a bidentate rather than monodentate complex. A possible AsO43- adsorption mechanism can be drawn with all microscopic evidence as shown in Scheme 2. During the hydrolysis of La(NO3)3, La(OH)3 is formed which adsorbs on the SPC surface through coordination using -COOH groups and -OH groups and as a result, C-O-La bond is formed. When oxalate was added to form the La-oxalate complex, in which La3+ is


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coordinated with -COOH of oxalate. According to labile theory, the three -OH groups in La3+ get coordinated with oxalate. The oxalate tends to form self-assembled five membered coordination and/or monodentate, as described by Rodenas et al.25 The other –OH group may tend to polymerize through oxalate and/or the available free active sites as shown in Scheme 2. In addition, the role of oxalic acid is to avoid the aggregation due to stereic effects and electrostatic repulsion between the particles and as a result the La-oxalate complex showed high BET surface area than the other pristine materials. The small La-oxalate moiety was attached physically or locally in the carbonaceous surface, which contribute to the increase of the AsO43- adsorption capacity. The final step for adsorption of AsO43- is the directional linkage with the lanthanum groups of freely available non-oxalate –OH groups. Clearly, the AsO43- adsorption may also be the result of coordination with both –C=O groups of -oxalate and oxygen atom contains a free lone pair of electrons through coordinate covalent bonds. Additionally, an electrostatic interaction with six H3O+ ions and/or hydrogen bonding26 is possible in acidic conditions. When a small amount of dopant SPC was added to La-oxalate, the adsorption capacity of AsO43- was maintained, where the SPC moiety reactivates the adsorption sites by providing their functional groups of both carboxyl and hydroxyl groups to La-oxalate to facilitate the formation of a complex. As a result, the synergistic effect of both SPC and La-oxalate would significantly enhanced the adsorption capacity of AsO43- from aqueous solution. To reveal the mechanism of AsO43- adsorption, the surface compositions and the variation of EB[As 3d] of SPC@La-oxalate after 1 mM AsO43- adsorption were further investigated by XPS. The wide scan for AsO43-+SPC@La-oxalate complex showed the characteristic As 3d peak at EB[As 3d] = 45.0 eV, confirming the presence of AsO43- in the complex when compared to before adsorption (Figure 4A) as illustrated in Figure 7A. In addition, the narrow scan for the As



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3d spectrum of SPC@La-oxalate after adsorption of AsO43- showed characteristic peaks located at 45.2 and 45.9 eV, which were attributed to EB[As 3d] for AsO427 (Figure 7B). The other regions for C 1s, O 1s and La 3d are shown in Figure S1. The oxygen charge of the AsO43+SPC@La-oxalate complex is less negative than that of SPC@La-oxalate complex, which indicated that during the chemical interaction of AsO43- on SPC@La-oxalate, an electron transfer occurred to adsorbate via coordination or ligand exchange reactions. In another way, the SPC@La-oxalate complex act as an electron donor (Lewis base) and AsO43- as an electron acceptor (Lewis acid). The chemical shift of the EB[O 1s] indicates the surface strength of the oxyanion interaction without giving explicit information of the nature of this interaction either by coordination or ligand exchange. The other elements, such as C 1s and La 3d, do not vary considerably after adsorption of 1 mM AsO43- on the SPC@La-oxalate complex, as shown in Figure S2. The As K-edge XANES results for AsO43- adsorbed adsorbents are shown in Figure 7C. The XANES spectra of AsO43- adsorbed La-oxalate and SPC@La-oxalate adsorbents exhibited the adsorption edge at 11875.5 eV, assigned to AsO43-. They are slightly higher adsorption edge than the standard KH2AsO4, which normally shows the adsorption edge at 11875.1 eV. The adsorption edge shift can be depicted in that the white-line intensity was increased compared with the standard, due to an increase in the characteristics of covalent bonds between As-O-La as La is in the second shell. Notably, the peak after white-line is known as oscillation peak in the standard KH2AsO4. However, in the case of La-oxalate and SPC@La-oxalate, there were different oscillation patterns due to the arrangement of neighboring atoms. For both cases, the neighboring bulky environment is the oxalate moiety, which may result in different oscillation patterns from the standard KH2AsO4 as shown in Figure 7C. EXAFS analysis was applied to 16 ACS Paragon Plus Environment

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identify the local coordination of AsO43- on La-oxalate and SPC@La-oxalate as shown in Figure 7D (i-iii). In Figure 7D(i), we have demonstrated the fitting of k-space with standard KH2AsO4 of La-oxalate and SPC@La-oxalate complexes. The EXAFS spectra of As K-edge of AsO43adsorbed La-oxalate and SPC@La-oxalate showed two separated shells in the radial structure function (RSF) (ii) compared with the standard KH2AsO4, which does not provide any information regarding the second shell (Figure 7D and Table 2). The four oxygen atoms are located at a distance of 1.68 Å at the first coordination shell to As, which is consistent for all the materials investigated and were well-matched with the previous studies.8, 28-30 In addition, the second shell of As with the distance of 3.33 Å with La atom for La-oxalate and SPC@La-oxalate after adsorption of AsO43-, suggests the formation of a monodentate surface complexation of SPC@La-oxalate, which were well supported by real parts of the Fourier transform spectra for La-oxalate and SPC@La-oxalate after adsorption of AsO43- as shown in Figure 7D (iii). The FTIR spectrum for SPC@La-oxalate after adsorption of 1 mM AsO43- is shown in Figure 7A. The peaks were observed at 3401 cm-1 related to -OH stretching vibration mode and 1643 cm-1 for its corresponding bending vibration mode. The CH stretching vibration at 2927 cm-1 and its symmetric bending vibration mode at 1403 cm-1 were obtained for AsO43- adsorbed SPC@Laoxalate. The peak at 1114 cm-1 for C-O stretching vibration mode in both oxalate and SPC groups, as shown in Figure 8A. The peaks at 848 cm-1 after adsorption of AsO43- corresponds to stretching vibration mode of As-O in the H2AsO4- group.31 Moreover, a peak at 617 cm-1 is due to the symmetric stretching mode of As-O bond in AsO43-32 which confirms the presence of AsO43-. The peak at 462 cm-1 can be assigned to the La-O vibration mode. No obvious changes were observed in the PXRD pattern of AsO43-+SPC@La-oxalate, suggesting that their crystallites were very small, as shown in Figure 8B. After adsorption of AsO43- on SPC@La17 ACS Paragon Plus Environment


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oxalate complex, the surface of the material was modified to a nanorod-like fibrous network due to the presence of AsO43- on the surface of the SPC@La-oxalate complex as shown in Figure 8C. The pseudo-second-order kinetic model was used to determine the adsorption rate of AsO43- on SPC@La-oxalate and the linear form of Ho’s pseudo-second-order model33 is as follows:

=

( )

+

(1)

where qe (mmol/g) is the adsorption amount at equilibrium, qt (mmol/g) is the adsorption amount at time t (min) and k2 (g/mmol.min) is the kinetic constant. The slope and intercept values obtained from the plot t/qt vs t provides a straight line and the values are indicated the applicability of pseudo-second-order kinetic model to a solution with 1 mM AsO43- at 25 °C, as shown in Figure 8A. The correlation coefficient values (R2) and the kinetic parameter values suggested that the experimental data were fitted well to pseudo-second-order kinetic model. The AsO43- adsorption capacity obtained from both experimental data and calculation from the pseudo-second-order model were consistent for the SPC@La-oxalate material, also suggesting the suitability of the model. The adsorption isotherms of AsO43- were obtained with the initial AsO43- concentrations to 0.5, 1.0, 1.5, 2.0 and 2.5 mM with SPC@La-oxalate adsorbent ratio of 1g/L at 25 ºC. The Langmuir isotherm model34 was used to determine the maximum adsorption capacity and also express the relationship between the equilibrium concentration (Ce) and the amount of AsO43- (qe) on SPC@La-oxalate at 25 °C, as shown in Figure 8B.

=

+

(2)


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where qe (mmol/g) is the amount of AsO43- adsorbed on per gram of SPC@La-oxalate at equilibrium and Ce (mmol/g) represent the equilibrium concentration of AsO43-. The calculated maximum adsorption density (qm,, mmol/g) qm and the Langmuir constant b (L/mmol) were obtained from the linear method by plotting a graph Ce/qe vs Ce. From the Figure 8B, the SPC@La-oxalate showed the higher value of calculated maximum adsorption capacity than the pristine materials, SPC and La-oxalate (Table 4). The obtained maximum adsorption capacities of AsO43- on SPC@La-oxalate were compared with previously published materials and found that the SPC@La-oxalate adsorbent is superior to those reported materials as shown in Table S135-54. These results indicated that the SPC@La-oxalate adsorbent has higher affinity with AsO43-. Hence, the SPC@La-oxalate can be used as an effective and efficient material for AsO43from polluted aqueous solutions.

Reusability test Reusability of the AsO43- adsorbed SPC@La-oxalate adsorbent was carried out to identify the cost-effectiveness of the adsorbent for AsO43- removal in the successive sorption-desorption cycles. The initial concentration of AsO43- was 1 mM at pH 4.8 for 1 h in room temperature for the each regeneration cycle. After regeneration of each cycle, the adsorbent was separated, washed with 0.1 M NaOH as eluent, freeze-dried and the freshly prepared 1 mM AsO43- was added before each run. Totally, six adsorption-desorption cycles were examined (Figure S3) and notably, the adsorption capacities for AsO43- were maintained for the first four cycles with negligible amount of dearsenate capacities. After that the AsO43- adsorption density was decreased to 38 % at fifth cycles and then significantly decreased to 28 % at sixth cycle. Because the interaction between SPC@La-oxalate and AsO43- became weak due to less available active 19 ACS Paragon Plus Environment


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sites for AsO43- adsorption. These results showed that SPC@La-oxalate adsorbent has sufficient chemical stability over six sorption-desorption repetitions.

CONCLUSIONS In this study, we report the sucrose-derived carboxylate rich porous carbon doped lanthanumoxalate complex that has been prepared via a co-precipitation method, and the complex has been used as a potential adsorbent for AsO43- from water. The organic acid, such as oxalate, could coordinate with the lanthanum surface during the synthesis and the final particle size distribution of dispersed lanthanum particle’s growth was controlled. Also, to avoid alkali disposal in the environment, we designed the material using oxalate which has high affinity with AsO43-. The SPC material would enable to increase porous surfaces to grow in the La-oxalate complex. As a result, the SPC@La-oxalate complex has a higher surface area than bare La-oxalate. Adsorption equilibrium was reached within 30 min with high Langmuir adsorption capacity (Qm) of 1.858 mmol/g at 25 °C, which is a superior uptake capacity of AsO43- to the previously reported adsorbents. The mechanism of AsO43- uptake was explained in detail with spectroscopic evidence, as well as batch experimental results. For the mechanism, ligand-exchange, chelation and electrostatic interaction occur between AsO43- and SPC@La-oxalate. The presence of other ions did not significantly affect the adsorption of AsO43-, which indicates the high selectivity. The kinetic data were fitted well to a pseudo-second-order kinetic model. Reusability of the material after 6 cycles are yet another advantage to this novel adsorbent. It can be concluded that the results obtained from this study demonstrated that the novel SPC@La-oxalate adsorbent exhibited a novel opportunity for adsorption of AsO43- from aqueous solution in an efficient and environmentally friendly way.


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ASSOCIATED CONTENT Supporting Information TGA analysis of synthesized adsorbents along with PXRD pattern, XPS analysis of SPC@Laoxalate after AsO43- adsorption, reusability test and the comparison table are provided in the supporting information.

AUTHOR INFORMATION Corresponding Author *Prof. Dr. Keiko Sasaki, Department of Earth Resources Engineering Faculty of Engineering, Kyushu University 744, Motooka, Fukuoka 819-0395, Japan Tel./Fax. +81 92 802 3338. E-mail: [email protected] E-mail addresses: [email protected], [email protected] (S. Muthu Prabhu), [email protected] (C. Chuaichum) ORCID Keiko Sasaki: 0000-0002-2882-0700 Subbaiah Muthu Prabhu: 0000-0003-1553-3136 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS



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Financial supports were provided to KS by the Japan Society for the Promotion of Science (JSPS) through research grants (JP16H02435 and JP16F16082) and to SMP (P16082) by the JSPS Postdoctoral Fellowship for Foreign Researchers. The EXAFS experiments were performed at Kyushu University Beamline (SAGA-LS/BL06) with the proposal No. 2017IK006.


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Scheme 1. Synthesis of the SPC@La-oxalate complex



120 Volume adsorbed (cm /g) STP

100

3

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 60 La-Oxalate SPC@La-Oxalate

40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po) Figure 1. Nitrogen adsorption-desorption isotherms of La-oxalate and SPC@La-oxalate complexes

32

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SPC@ La-oxalate

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


La-oxalate

SPC@La(OH)3

La(OH)3 SPC

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavenumber (cm ) Figure 2. FTIR spectra of SPC, La(OH)3, SPC@La(OH)3, La-oxalate and SPC@La-oxalate

33



SPC@La-oxalate

Intensity (a.u)

La-oxalate

10

20

30

40

50

60

La(OH)3 400

220 310

211

JCPDS 36-1481

210

200

201

101

SPC@La(OH)3

100

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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70

80

2-Theta (deg) Figure 3. PXRD patterns of La(OH)3, SPC@La(OH)3, La-oxalate and SPC@La-oxalate

34


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Figure 4. XPS spectra of the SPC@La-oxalate complex: (A) wide scan, (B) C 1s spectrum, (C) O 1s spectrum, (D) La 3d spectrum.

35


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Figure 5. SEM images of (A) SPC, (B) La(OH)3, (C) SPC@La(OH)3, (D) La-oxalate (E) SPC@La-oxalate (Scale bars indicates 2 µm) and (E) Magnified view of (D) at 1 µm. 36


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Figure 6. Effect of (A) contact time (B) As/La ratio, (C) pH, (D) residual La3+ ion concentration (E) zeta potential and (F) coexisting anions on AsO43- adsorption. Experimental conditions: for the system, [AsO43-]0 = 1.2 mM; dose ratio = 1 g/L, agitation = 100 rpm, and temp = 25 °C. 37



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Scheme 2. Possible mechanism of adsorption of arsenate on SPC@La-oxalate

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Figure 7. Characterization of the solid residue after adsorption of 1 mM AsO43- by SPC@La-oxalate (A) XPS wide scan (B) As 3d XPS narrow scan (C) As K-edge of EXAFS spectra of (i) Normalized k3-weighted experimental (lines) and simulated (dotted) spectra of AsO43- adsorbed La-oxalate and SPC@La-oxalate, (ii) the corresponding Fourier transformed magnitude and (iii) real parts of the Fourier

transform. 39



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Figure 8. Characterization of the solid residue after adsorption of 1 mM AsO43- by SPC@La-oxalate (A) FTIR spectrum, (B) PXRD pattern, and (C) SEM image at 2 µm magnification.

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Figure 9. (A) Kinetics and (B) Langmuir fit of AsO43- adsorption by SPC@La-oxalate. Experimental conditions: dose ratio = 1 g/L, agitation = 100 rpm, temp = 25 °C.



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Table 1 - Physicochemical analysis of SPC, La-oxalate and SPC@La-oxalate-based materials Adsorbents

BET

SSA Pore volume

(m2/g)

(cm3/g)

Avg. Pore size La/As (nm)

ratio

SPC

374.1

0.604

--

--

La-oxalate

30.40

0.070

9.227

0.298

SPC@La-oxalate

35.79

0.175

19.541

0.427

mole

Table 2 - Structural parameters derived from AsO43- K-edge EXAFS analysis Adsorbents

Path

CNa

R (Å) b

σ2 (Å2)c

∆E0 (eV)d

∆R(Å)e

La-oxalate

As-O

4.58

1.68

0.003

3.6

-0.12

As-La

1.14

3.32

0.003

3.6

0.03

As-O

4.49

1.68

0.003

3.3

-0.02

As-La

1.13

3.32

0.003

3.3

0.02

SPC@Laoxalate a

coordination number, binteratomic distance, cDebye-Waller factor, dthreshold energy shift.

e

interatomic distance shift.


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Table 3 - Kinetics parameters calculated for AsO43- adsorption Materials

Qexp (mg/g)

Qcal (mg/g)

K2 (g mg-1 min-1)

R2

SPC@La-oxalate

1.0934

1.0930

11.8149

1.000

Table 4 - Langmuir parameters calculated for AsO43- adsorption Materials

Qe (mmol/g)

Qm (mmol/g)

RL (L/mmol)

R2

SPC@La-oxalate

1.858

1.931

14.474

0.995



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TABLE OF CONTENTS (TOC) Synopsis: An Alkali-free, SPC@La-oxalate MOF-like nanocomplex has been synthesized at room temperature for effective adsorption of arsenate from water.


A Mechanistic Approach for the Synthesis of Carboxylate-Rich

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