Synthesis and arsenic adsorption performances of ferric-based layered double hydroxide with α-alanine intercalation

Accepted Manuscript Synthesis and arsenic adsorption performances of ferric-based layered double hydroxide with α-alanine intercalation Hong Jun, Zhu Zhiliang, Lu Hongtao, Qiu Yanling PII: DOI: Reference:

S1385-8947(14)00583-X http://dx.doi.org/10.1016/j.cej.2014.05.019 CEJ 12113

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

18 March 2014 4 May 2014 5 May 2014

Please cite this article as: H. Jun, Z. Zhiliang, L. Hongtao, Q. Yanling, Synthesis and arsenic adsorption performances of ferric-based layered double hydroxide with α-alanine intercalation, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.05.019

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Synthesis and arsenic adsorption performances of ferric-based layered double hydroxide with α-alanine intercalation Hong Jun1, Zhu Zhiliang1*, Lu Hongtao1, Qiu Yanling2 1

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai

200092, China； 2

Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University,

Shanghai 200092, China； *Corresponding author, E-mail: [email protected], Phone: +86-21-6598 2426, Fax: +86-21-6598 4626

Abstract: In order to obtain a highly efficient functional material for As(III) removal, a ferric-based layered double hydroxide intercalated with α-alanine (denoted as Mg-Fe-Ala-LDH) was synthesized by a co-precipitation method. The adsorption performances for arsenite and arsenate species in aqueous solutions were studied with the obtained Mg-Fe-Ala-LDH material by batch experiments. The effects of various factors, such as solution pH, adsorbent dosage, competing anions, contact time and initial arsenic concentrations, have been investigated. The results showed that the adsorption isotherm of both As(III) and As(V) onto this Mg-Fe-Ala-LDH material can be well described by the Langmuir model, and the adsorption process followed the pseudo-second-order model. It was found that the maximum adsorption capacity for As(III) was 49.8 mg·g1 , which is significantly higher than that for As(V). Desorption experiments also indicated that arsenite species showed stronger affinity to this Mg-Fe-Ala-LDH than arsenate. This interesting result is closely related to the characteristics of the intercalation species, such as α-alanine in layered double hydroxide materials. It was demonstrated that the synthesized Mg-Fe-Ala-LDH material might be a potential efficient adsorbent for As(III) removal in water. Keyword: Layered double hydroxides; Alanine; Arsenic; Adsorption; Water pollution 1

1. Introduction The presence of arsenic with elevated concentrations in natural environment, especially in groundwater, has been considered as one of the most serious global environmental threats. A long-term drinking water exposure to arsenic may cause severe human health hazards such as skin lesions, peripheral vascular disease, hypertension, black-foot disease, and high risk of cancers. [1-6] Most environmental arsenic problems are related to the mobilization of arsenic species under natural conditions. Besides, anthropogenic sources like mining activities, combustion of fossil fuels, and use of arsenic pesticides produce additional negative impact. Arsenic pollution, as a worldwide problem, has been reported in a lot of countries and regions including the USA [7], Canada [8], Mexico [9], Argentina [10], Australia [11], Japan [12], China [13], India [1], Bangladesh [14] and so on [15]. Accumulation of evidences for chronic toxicological effects of arsenic in drinking water leads the World Health Organization [16], European Union [17], and the U.S. Environmental Protection Agency [18] to lower the regulatory limits and the maximum contaminant level of arsenic in drinking water from 50 to 10 μg·L1. Technologies to efficiently remove arsenic from drinking water sources are therefore urgently and universally required. Moreover, arsenic toxicity and mobility are highly related to the oxidation state of arsenic. In natural water environments, arsenic is commonly present as arsenite (As(III)) or arsenate (As(V)) [19]. Pentavalent arsenate predominates and is stable in oxygen rich aerobic environments, while trivalent arsenite predominates in moderately reducing anaerobic environments such as groundwater. Compared to As(V), As(III) is more toxic, more mobile and more difficult to be removed from water. The removal of arsenic from water has received extensive attentions and the techniques have been well investigated [20]. Among various treatment technologies, adsorption technique is considered as one of the most popular, efficient and practical methods, due to its high efficiency, easy operation, low cost and little risk. Owing to 2

the worldwide research efforts, more and more adsorbents have been developed, which include activated carbon [21], activated alumina [22], iron hydroxide and oxide [23], anionic clays [24], zeolites [25], chitosan [26] and others [27, 28]. Recently, layered double hydroxides (LDHs), known as a kind of multifunctional materials, have been reported to serve as adsorbents for anionic contaminants removal due to their large surface area, high anion exchange capacity and good thermal stability [29]. Layered double hydroxides are a class of anionic clays or hydrotalcite-like compounds whose structures are based on brucite-like layers. They are composed of positively charged brucite-like sheets and negatively charged anions in the hydrated interlayer regions. They can be represented by this general formula, [M II1−xMIIIx (OH)2]x+(An-)x/n·mH2O, abbreviated as [MII-MIII-A], where MII are divalent cations like Mg2+, Zn2+, Cu2+, etc., MIII are trivalent cations like Al3+, Cr3+, Fe3+, etc., Andenotes exchangeable interlayer anion with negative charge n, m is the number of interlayer water, and x is defined as the MIII/(MII+MIII) ratio. The intercalation chemistry of LDH hosts is very interesting. A variety of anions can be incorporated into the interlayer region using different methods to dramatically change the chemical, electronic, optical, and magnetic properties of LDH hosts. Besides conventional anions like halides, oxyanion, transition metal complexes, most recently the focus has been shifted to the intercalation of biologically active materials such as porphyrins, vitamins, peptides, and drug molecules [30]. Since iron oxides or hydroxides have a high affinity to arsenic, the iron based LDHs are promising materials for arsenic removal and several researches have been reported [31-35]. The amino acid intercalation in the interlayer region changes the properties of LDHs, and provides the useful information for the effect on drug release from LDHs in the life body. Since the possible similarity of some amino acids and arsenic acids, the interaction between amino acids intercalated LDHs and trace arsenic in water solution is interesting and important for both drug release and decontamination of arsenic polluted water. One of significant applications of LDHs materials is the drug release, where the drug molecules might be combined in LDH structure as intercalation compositions. Amino acids, drugs and trace arsenic species 3

from drinking water may be coexistent together in some biological or natural water systems. To the best of our knowledge, although the removal of arsenic by LDHs has been studied by several researchers using different metal combinations, the interlayer anions in the iron based LDHs are usually inorganic. The arsenic adsorption performance of the LDHs intercalated with small biological molecules like amino acids has not been reported up to now. In this study, in order to obtain an efficient functional material for As(III) removal, we focused on the synthesis of a ferric-based LDH with direct intercalation of α-alanine by a co-precipitation method and application as an adsorbent for As(III) removal. The various effecting factors on the adsorption performance, adsorption isotherms, kinetics and desorption experiment were investigated. The possible mechanism was discussed. The results can help further understand the effect of amino acids intercalation in LDHs on their structure, the interaction with arsenic species, and related adsorption properties.

2. Materials and methods 2.1 Materials Analytic grade chemicals used in the experiment including Mg(NO3)2·6H2O, Fe(NO3)3·9H2O, α-alanine, and NaOH were purchased from Sinopharm Chemical Regent Co. Ltd. The disodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O) and sodium arsenite (NaAsO2) were purchased from Sigma Aldrich with a purity higher than 98%. The stock solution of 1000 mg·L1 As(V) was prepared by dissolving 4.1653 g Na2HAsO4·7H2O in 1L MilliQ water, and working solutions of required concentrations were obtained by diluting the As(V) stock solution with deionized water. The As(III) solutions of required concentrations were all freshly prepared to minimize the possible oxidation of As(III). 2.2 Preparation of Mg-Fe-Ala-LDH The ferric-based layered double hydroxide intercalated with α-alanine was prepared by a modified co-precipitation method at room temperature. Firstly, a solution of 4

NaOH (1 mol·L1) was added drop-wisely into a stirring α-alanine solution (1.78 g α-alanine dissolved in 150 ml de-ionized water) until the pH reached 10. And then a mixed solution (150 ml) containing Mg(NO3)2·6H2O (7.69 g) and Fe(NO3)3·9H2O (4.04 g) was added drop-wisely into the above α-alanine solution. During the reaction process, the solution pH was kept at about 10 by adding appropriate amount of NaOH or HCl solution, while nitrogen was bubbled into the reacting solution to prevent the contamination from atmospheric CO2. The resulting slurry was aged for 2 hours and then centrifuged, washed with de-ionized water until the pH of supernatant reached 7. After that, the solid product was dried at 353 K. The final product with α-alanine intercalation was named as Mg-Fe-Ala-LDH in this article. 2.3 Characterization of Mg-Fe-Ala-LDH The metal contents of the synthesized Mg-Fe-Ala-LDH were determined by the optima 2100DV Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) after the sample was dissolved in concentrated nitric acid. The α-alanine and NO3 content in the Mg-Fe-Ala-LDH was analyzed by Elementar (Vario EL III). The X-ray diffraction pattern was measured on a D-8 Advance X-ray diffractometer (Bruker-AXS, Germany) with Cu Kα radiation operated at a voltage of 40 kV and a current of 40 mA. The Fourier transfer infrared (FTIR) spectrum was recorded by using a Thermo Nicolet 5700 (Thermo Nicolet Corporation, USA) FTIR instrument, in the wavenumber range of 400-4000 cm1, sample was mixed with oven dried spectroscopic grade KBr and pressed into disks. The surface area and pore structure of the material were analyzed by N2 adsorption-desorption at 77 K on a QuantaChrome Autosorb-Iq-Mp analyzer (QuantaChrome Instruments, USA). Thermo analytical measurement was performed using a SDT Q600 simultaneous thermal analyzer, and sample was heated from 323 K to 973 K. Scanning electron microscopy (SEM) images were recorded with a field emission XL-30 SEM, while transmission electron microscopy (TEM) with JEOL2010F. 2.4 Batch adsorption experiments Adsorption experiments were carried out by batch equilibrium technique at the temperature of 298 K and were replicated three times, and the average results were 5

used. The effect of initial solution pH, adsorbent dosage and competing anions on both As(V) and As(III) adsorption, adsorption kinetics and adsorption isotherm were analyzed to evaluate the adsorption performance of As(III) and As(V) species onto the synthesized Mg-Fe-Ala-LDH. All the adsorption experiments were conducted in well capped 250 mL flasks, containing 100 mL As(III) or As(V) solution with required concentrations. After a dosage of 20 mg adsorbent was added, the flask had been shaken in a thermostatic shaker at 150 rpm for 24 hours. After filtration, the residual As(III) or As(Ⅴ) concentration was determined by ICP-OES method. The adsorption amount removed by the adsorbent, Q (mg·g1), was calculated as follows:

Q

(C0  Ce )  V m

(1)

Where C0 and Ce are the initial and equilibrium arsenic solution concentrations, respectively, V is the volume of arsenic solution (L) and m is weight of the sorbent (g). The effect of initial solution pH on As(V) and As(III) removal was tested in a solution of 10 mg·L-1 As(III) or As(V) under pH ranging from 3 to 9. The adsorption isotherms were studied at pH=6 for As(III) or As(V), and the initial concentration of As(III) or As(V) solution was set from 1 mg·L1 to 15 mg·L1. In kinetic study, samples were collected at different intervals in arsenate (5 mg·L1) or arsenite (10 mg·L1) solution at pH=6. The pH values of all solutions were adjusted by a series of solutions of HCl or NaOH with desired concentrations (0.01M, 0.1 M and 1M), and there were not any significant volume changes. 2.5 Desorption experiment The arsenic-loaded Mg-Fe-Ala-LDH was prepared by dispersing a portion of synthesized Mg-Fe-Ala-LDH (20 mg) in 100 mL arsenic (As(V) or As(III)) solutions of 10 mg·L1. After the adsorption was saturated, the mixture was centrifuged and the As-loaded adsorbent was separated. And then, a series of 100 ml desorption solution was used to be mixed with As-loaded adsorbents, and then the mixture was shaken for 24 hours. After that, the suspension was centrifuged and separated. The desorbed 6

arsenic was determined through analyzing the arsenic released into the solution. Desorption agent used in the experiments included NaOH, Na2CO3, and Na2HPO4 (100 mg·L1) and distilled water.

3. Results and discussions 3.1 Characterization 3.1.1 Chemical composition analysis of Mg-Fe-Ala-LDH Based on the chemical analysis, composition of the synthesized Mg-Fe-Ala-LDH was calculated and the result is shown in Table 1. Considering the general formula of LDHs, the chemical composition of synthesized Mg-Fe-Ala-LDH was obtained. It can be found that the molar ratio of Mg/Fe for the material was close to the initial value of the starting salts and α-alanine was successfully intercalated into Mg-Fe-Ala-LDH through the co-precipitation method. As the reaction solution containing competitive NO3, a portion of NO3 was partly co-intercalated in Mg-Fe-Ala-LDH interlayer space.

Table 1 Chemical compositions of the Mg-Fe-Ala-LDH

3.1.2 XRD analysis The XRD pattern of the synthesized Mg-Fe-Ala-LDH sample is shown in Figure 1, which

resembles

previous

reports

[36-38].

The

characteristic

peaks

of

hydrotalcite-like materials with hexagonal crystal system assuming a 3R packing of layers such as (003), (006), (110), (113), (012), (015) and (018) were observed and no other crystalline phases were present. The basal spacing (d003) of synthesized Mg-Fe-Ala-LDH was 0.81nm, which was different from the value of LDHs with carbonate or nitrate intercalation reported by other authors [36-38]. The high intensity of the main reflections indicated that the sample was highly crystalline. The value of the crystallographic parameter α, which corresponds to the cation-cation distance in the brucite-like layer of the LDH sample, has been calculated from the d-spacing of 7

the (110) reflection (α = 2d110 = 0.312 nm). The value of parameter c, which is related to the thickness of the brucite-like layers and the interlayer space, has also been determined from the d-spacing of the (003) reflection (c = 3d003 = 2.43 nm).

Fig. 1 XRD pattern of synthesized Mg-Fe-Ala-LDH

3.1.3 FTIR spectroscopy The FTIR spectra of synthesized Mg-Fe-Ala-LDH and neat alanine were recorded and shown in Figure 2. The strong and broad absorbance peaks around 3450 cm1, which were associated with the stretching vibration of hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water, were typical for hydrotalcite-like materials [39, 40] . And the stretching vibration of amino group also gives an absorbance peak in this region. The peaks assigned to the bending vibration of hydroxyl and amino group were found around 1630 cm1. The residual NO3 intercalated in the interlayer space gives a strong peak at 1380 cm1. The active absorption peaks of ν(CH3), ν(COO), ν(CN) and δ(CH3) arising from α-alanine were observed at 3049 cm1, 2578 cm1, 2079 cm1 and 1415 cm1 respectively. In the low-wavenumber region of the spectrum (<800 cm1), the recorded peaks can be interpreted as lattice vibration modes such as Mg–O–H, Fe–O–H vibration and O–Mg–O, O–Fe–O stretching.

Fig. 2 FTIR spectra of synthesized Mg-Fe-Ala-LDH and alanine

3.1.4 Thermal analysis Thermal analysis of the synthesized Mg-Fe-Ala-LDH was conducted and the result of DTG-DTA analysis is presented in figure 3. The curves exhibited typical thermal decomposition characteristics of LDH materials as described in previous reports [40, 41]. There were two obvious mass loss temperature intervals. The first was at about 423 K, corresponding to the removal of the surface and interlayer water molecules, and there was a corresponding endothermic peaks around 438 K. The second mass 8

loss temperature interval was at about 625 K, which involved the dehydroxylation of brucite layers and loss of interlayer anions (including alanine and NO3) resulting in the formation of mixed oxides, and the corresponding endothermic peak appeared around 640 K.

Fig. 3 DTG-DTA curves of synthesized Mg-Fe-Ala-LDH

3.1.5 Surface area and pore size analysis The N2 adsorption-desorption isotherms of the Mg-Fe-Ala-LDH are shown in Figure 4. The inset shows the pore size distribution determined by the Barrett-Joyner-Halenda

(BJH)

analysis

using

desorption

data.

The

adsorption-desorption isotherms exhibited the characteristics of types IV isotherms (according to the IUPAC classification) with a H3 type hysteresis loop. The H3 type hysteresis loop was related to slit-shaped pores of packing plate-like particles. These results showed that the multilayered physical adsorption of N2 was taking place between the aggregates of platelets particles and indicated the lamellar morphology of the materials. These were also in accordance with other reports [42, 43]. The specific area of Mg-Fe-Ala-LDH calculated by the Brunauer-Emmett-Teller (BET) method was 63.6 m2·g1 and C-value in BET equation was 163.7. The pore size distribution and total volume were determined by BJH method based on the desorption branch. The results showed that the average pore diameter was about 18 nm, and total pore volume was 0.381 cc·g1. No microporosity (by t-plot method) has been found in the sample. These results demonstrated the mesoporous structures of the synthesized Mg-Fe-Ala-LDH.

Fig. 4 N2 adsorption-desorption isotherms and pore size distribution based on BJH analysis of desorption data (inset) of synthesized Mg-Fe-Ala-LDH

3.1.6 SEM and TEM images The particle morphology of the Mg-Fe-Ala-LDH sample can be seen in the SEM 9

and TEM images presented in Figure 5. It can be found that the particles of the sample are aggregates of plate-like particles, similar to previous reports [35, 44].

Fig. 5 SEM and TEM images of the synthesized Mg-Fe-Ala-LDH

3.2 Effect of initial solution pH on adsorption As solution pH can affect both the zeta potential of the adsorbents and arsenic species, the effect of initial solution pH on arsenic adsorption on the synthesized Mg-Fe-Ala-LDH was investigated at different initial pH values ranging from 3.0 to 9.0. The results in figure 6 showed that, under the experimental conditions, the removal efficiency for both As(III) and As(V) decreased with the increment of initial solution pH values. The removal mechanism of arsenic by Mg-Fe-Ala-LDH may be related with two major factors. One is the exchange of intercalation alanine with arsenic, another is the adsorption of arsenic species on the surface of Mg-Fe-Ala-LDH. Since the alanine with the isoelectric point of 6.02 belongs to neutral amino acids, and the As(III) species exists mainly as H3AsO3 in the pH range of 3.0-9.0 [19], the exchange of intercalation alanine with As(III) in neutral states might be the main driving forces. For As(V) species, the dominant species of As(V) are H2AsO4- and HAsO42- in the pH range of 3.0-9.0, the adsorption on the surface of Mg-Fe-Ala-LDH may play a more important role. Therefore, due to an increase in the electrostatic repulsion between negatively charged anion and surface of sorbent, the decrease in the uptake of both As (V) and As (III) with an increase in the pH of medium was found, but the solution pH affected the removal of As (V) more obviously than that for As(III).

Fig. 6 Effect of initial solution pH on arsenic adsorption on Mg-Fe-Ala-LDH Initial arsenic concentration was 5 mg·L1; the dosage of adsorbents was 0.2 g·L1.

10

3.3 Effect of Mg-Fe-Ala-LDH dosage on arsenic adsorption The effect of LDHs dosage on the arsenate adsorption is shown in Figure 7. As expected, the removal efficiency of arsenate and arsenite increased sharply with increasing adsorbent dosage in a low dosage range, and then leveled off with the adsorbent dosage reaching above 0.5 g·L1. Furthermore, when the dosage of the adsorbent was 1.0 g·L1 for arsenite solution or 1.5 g·L1 for arsenate solution, the residual arsenic concentration was lower than 10 μg·L1. It can meet the standard of the arsenic limit for drinking water recommended by World Health Organization, European Union and the U.S. Environmental Protection Agency.

Fig. 7 Effect of Mg-Fe-Ala-LDH dosage on arsenic adsorption Initial arsenic concentration was 5 mg·L1; initial solution pH was 6.

3.4 Effect of competing anions on arsenic adsorption The effect of coexistent anions with excess concentration in arsenic solution has been examined. The anions included prevalent anions in water environment such as hydrogen phosphate, carbonate, sulfate, chloride and nitrate ions, and the ratio of coexistent anion to arsenic was 1:1000. The result is shown in Table 2. It indicated that the adsorption of arsenic was closely related with the type of competing anions. Generally, for both As(III) and As(V), high concentration of HPO42, and CO32 showed an obvious inhibiting effect, while the inhibiting effect of SO42, Cl, and NO3 was not significant. Thus, the order of their inhibiting effect on the arsenic adsorption followed as HPO42-> CO32-> SO42- (Cl-, NO3-).

Table 2 Effect of competing anions on arsenic adsorption

3.5 Adsorption kinetics In order to investigate the adsorption kinetics of arsenic (both As(III) and As(V)) on the synthesized Mg-Fe-Ala-LDH, the adsorption process with respect to contact time was studied and the kinetic fitting curves are shown in Figure 8. The adsorption was 11

rapid in the first 6 hours for both As(III) and As(V), and thereafter it proceeded at a relatively slower rate and finally reached equilibrium after 24 hours. The initial rapid adsorption may be due to the large number of available sites on the material surface. The accumulation of arsenic adsorbed on the surface then led to the decrease in the adsorption rate. In order to further understand the characteristics of the adsorption process, the pseudo-first-order and pseudo-second-order kinetic models were applied to fit the experimental data from batch experiments. These two models can be expressed as follows [45]: ln(qe  qt )  ln qe  k1t

(1)

t 1 t   2 qt k 2 qe qe

(2)

Where qe (mg·g1) and qt (mg·g1) represent the adsorption amount of the adsorbate at equilibrium and at time t, respectively; k1 (min1) and k2 (g·(mg·min)1) are the adsorption rate constant of the pseudo-first-order and pseudo-second-order kinetic models, respectively.

Fig. 8 Effect of contact time on arsenate adsorption onto Mg-Fe-Ala-LDH and kinetic fitting curves Initial arsenic concentration was 5 mg·L1; solution pH was 6; adsorbent dosage was 0.2 g·L1.

The kinetic parameters estimated by nonlinear regression are represented in Table 3. It is evident that the equilibrium data was described better by the pseudo-second-order model for both As(III) and As(V), as the correlation coefficient (R2) was closer to 1 compared to the R2 of other models. This indicated that the adsorption process on the Mg-Fe-Ala-LDH was a second order adsorption kinetics system. It was assumed that the adsorption capacities of these materials were mainly proportional to the number of active sites on their surfaces. Besides, the adsorption rate constant k2 for As(V) was greater than that for As(III), indicating a higher rate for As(V) adsorption. 12

Table 3 Adsorption kinetic parameters for arsenic adsorption onto Mg-Fe-Ala-LDH

3.6 Adsorption isotherms To evaluate the adsorption characteristics of the Mg-Fe-Ala-LDH for As(III) and As(V), three isotherm models including Langmuir, Freundlich, and DubininRadushkevish (D-R) models were used to analyze the adsorption experimental data. These isotherm models can be represented as follows [46]: The Langmuir isotherm model:

Ce Ce 1   qe qm K L qm

(3)

1 The Freundlich isotherm model: ln qe  ln K F  ln Ce n

(4)

The D-R isotherm model: ln qe  ln qm  K D ε 2

(5)

where Ce (mg·L1) is the equilibrium concentration, qm (mg·g1) and qe (mg·g1) are the maximum and equilibrium adsorption capacity, respectively, KL(L·mg1) is the Langmuir adsorption constant; KF (L·mg1) is the Freundlich constant and 1/n is the heterogeneity factor; KD (mol2·kJ2) is the D-R isotherm constant, ε is the Polanyi potential, which is equal to RT ln(1 + 1/Ce), R is the universal gas constant (8.314 J·(mol·K)1), T (K) is the absolute temperature. The mean free energy of adsorption (E, kJ·mol1) in D-R isotherm model can be calculated as: 1

 1 2  E    2K D 

(6)

The adsorption isotherm fitting curves were shown in figure 9 and the acquired parameters of these isotherm models are summarized in Table 4. By comparison of correlation coefficient (R2), it can be found that the isotherm data fitted well to the Langmuir isotherm model, as the correlation coefficients (R2) for Langmuir isotherm model were all larger than 0.99. This indicated that adsorption behavior of both As(III) and As(V) onto Mg-Fe-Ala-LDH could be better described by Langmuir isotherm model than the other two. The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogenous adsorbent surface. Once an adsorbate 13

molecule occupies a site, no further adsorption can take place at that site.

Fig. 9 Adsorption isotherms for arsenic adsorption onto Mg-Fe-Ala-LDH. Initial arsenic concentration range from 1 to 15 mg·L1; solution pH was 6; adsorbent dosage was 0.2 g·L1.

Table 4 Adsorption isotherm parameters for arsenic adsorption onto Mg-Fe-Ala-LDH

It was found that the maximum adsorption capacity for As(III) was 49.8 mg·g1 , which is significantly higher than that for As(V) (23.6 mg·g1). The further desorption experiments also indicated that arsenite species showed stronger affinity to this alanine-intercalated LDH. This interesting result may be related to the following reason. For As(III) species in a water solution, they have less dissociation trend of hydrogen ions than that of As(V) (H3AsO4), and more As(III) species exist in neutral molecular state of H3AsO3. The similarity of α-alanine acid and As(III) species (H3AsO3) in electrical property may promote the exchange of the α-alanine acid in intercalation with As(III) species. Table 5 showed the comparison results of the maximum adsorption capacities of various LDHs adsorbents for arsenic adsorption. It is found that Mg-Fe-Ala-LDH in this study has a relatively higher adsorption capacity for arsenite, which makes it a possible efficient adsorbent for arsenite removal from aqueous solutions.

Table 5 Comparison of adsorption capacity for arsenic onto Mg-Fe-Ala-LDH with other reported LDHs adsorbents

Usually, the layered double hydroxides such as Mg-Fe-LDHs with inorganic anions as intercalation ions showed lower adsorption capacity for As(III) species than that for As(V) [47], therefore, a nonpolar amino acid was applied as intercalation species in this work to improve the adsorption performance for As(III). The results demonstrated that the intercalation of α-alanine into LDHs has successfully improved the adsorption performance of the synthesized LDHs for As(III) species. 14

3.7 Desorption and stability of As-loaded Mg-Fe-Ala-LDH Desorption experiments were conducted to investigate the stability of the Arsenic-loaded Mg-Fe-Ala-LDH. Desorption solutions used in the experiments included distilled water, solutions of NaOH, Na2CO3 or Na2HPO4. The results showed that, when distilled water served as the desorption agent, both As(V) and As(III) could not be detected in the eluent. This suggested that the As-loaded Mg-Fe-Ala-LDH was stable in water and the As(V) or As(III) adsorbed onto Mg-Fe-Ala-LDH are not easily released to the water environment under the normal conditions. When a solution of NaOH, Na2CO3 or Na2HPO4 was used as the desorption agent, the desorption ratio for As(V)-loaded or As(III)-loaded Mg-Fe-Ala-LDH was 38.9%, 33.6%, 29.1% or 16.3%, 14.4%, 12.9%. It can be found that the desorption ratio for As(III)-loaded Mg-Fe-Ala-LDH was much lower than that for As(V)-loaded material. It indicated that arsenite had stronger affinity to Mg-Fe-Ala-LDH than arsenate and was harder to be released from As-loaded Mg-Fe-Ala-LDH materials.

4. Conclusions In this study, through a direct intercalation of α-alanine into layered double hydroxide, a ferric-based LDH material intercalated with α-alanine was successfully synthesized by a co-precipitation method. The synthesized Mg-Fe-Ala-LDHs material has been used to investigate its performances of removing arsenite and arsenate from aqueous solutions. It was found that the maximum adsorption capacity for As(III) was much higher than that for As(V) because of the intercalation of alanine in LDH structure. Desorption experiment also indicated that arsenite had strong affinity to this alanine-intercalated

LDH

material.

It

is

concluded

that

the

synthesized

Mg-Fe-Ala-LDHs material in this work is a potential efficient adsorbent for decontamination of arsenic polluted water.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41372241) 15

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Table captions Table 1 Chemical composition of the Mg-Fe-Ala-LDH. Table 2 Effect of competing anions on arsenic adsorption Table 3 Adsorption kinetic parameters for arsenic adsorption onto Mg-Fe-Ala-LDH Table 4 Adsorption isotherm parameters for arsenic adsorption onto Mg-Fe-Ala-LDH Table 5 Comparison of adsorption capacity for arsenic onto Mg-Fe-Ala-LDH with other reported LDHs adsorbents

Figure captions Fig. 1 XRD pattern of synthesized Mg-Fe-Ala-LDH Fig. 2 FTIR spectra of synthesized Mg-Fe-Ala-LDH and alanine Fig. 3 DTG-DTA curves of synthesized Mg-Fe-Ala-LDH Fig. 4 N2 adsorption-desorption isotherms and pore size distribution based on BJH analysis of desorption data (inset) of synthesized Mg-Fe-Ala-LDH Fig. 5 SEM and TEM images of the synthesized Mg-Fe-Ala-LDH Fig. 6 Effect of initial solution pH on arsenic adsorption on Mg-Fe-Ala-LDH. Initial arsenic concentration was 5 mg·L1; the dosage of adsorbents was 0.2 g·L1. Fig. 7 Effect of Mg-Fe-Ala-LDH dosage on arsenic adsorption. Initial arsenic concentration was 5 mg·L1; initial solution pH was 6. Fig. 8 Effect of contact time on arsenate adsorption onto Mg-Fe-Ala-LDH and kinetic fitting curves. Initial arsenic concentration was 5 mg·L1; solution pH was 6; adsorbent dosage was 0.2 g·L1. Fig. 9 Adsorption isotherms for arsenic adsorption onto Mg-Fe-Ala-LDH. Initial arsenic concentration range from 1 to 15 mg·L1; solution pH was 6; adsorbent dosage was 0.2 g·L1.

22

Table 1 Chemical composition of the Mg-Fe-Ala-LDH M(Mg/Fe)a

m(Mg)b /%

m(Fe)c /%

m(C)d /%

m(N)e /%

Chemical composition

3.17

19.9

14.7

7.13

3.83

[Mg0.76Fe0.24(OH)2]Ala0.18(NO3)0.06•0.39H2O

a

Molar ratio of Mg/Fe. b Mass content of Mg in material. c Mass content of Fe in material. d Mass

content of carbon in material. e Mass content of nitrogen in material.

Table 2 Effect of competing anions on arsenic adsorption Competing anions

As(III) removal efficiency (%)

As(V) removal efficiency (%)

None ClNO3SO42CO32HPO42-

88.5 87.5 87.3 85.1 68.8 30.9

75.2 74.4 74.5 72.1 60.3 29.1

Initial arsenic concentration was 5 mg·L1; initial solution pH was 6; the dosage of adsorbents was 0.2 g·L1; ratio of arsenic to competing anions was 1:1000.

Table 3 Adsorption kinetic parameters for arsenic adsorption onto Mg-Fe-Ala-LDH Arsenic species

Pseudo first-order kinetic model qe(mg·g )

k1(min )

R

As(V) As(III)

18.4 23.5

0.0108 0.00439

0.881 0.929

-1

-1

Pseudo second-order kinetic model

2

qe(mg·g-1)

k2(g·(mg•min)-1)

R2

20.2 26.5

0.000764 0.000214

0.991 0.992

Table 4 Adsorption isotherm parameters for arsenic adsorption onto Mg-Fe-Ala-LDH Arsenic species As(III) As(V)

Langmuir isotherm model qm (mg·g-1)

KL (L·mg-1)

R2

49.8 23.6

0.269 3.35

0.995 0.991

Freundlich isotherm model KF ·L1/n·g-1)

1-1/n

(mg

21.4 14.6

23

D-R isotherm model

1/n

R2

qm (mmol·g-1)

E (kJ·mol-1)

R2

0.542 0.273

0.988 0.951

0.415 0.283

1.11 3.03

0.876 0.812

Table 5 Comparison of adsorption capacity for arsenic onto Mg-Fe-Ala-LDH with other reported LDHs adsorbents Adsorbents

Adsorption capacity (mg·g-1)

Interlayer anions

As(III)

As(V)

References

Mg-Fe-Ala-LDH Mn-Fe-CO3-LDH HT-Fe Zn-Al- SO4-LDH NO3-HT Li/Al-LDH FCHT-LDH

α-alanine carbonate carbonate sulfate nitrate chloride nitrate

49.8 112.4 --2.16 ---

23.6 52.4 24.1 74.9 -27.4 85

This study [48] [49] [50] [33] [51] [52]

Mg/Al-LDH

chloride nitrate carbonate

24.7 32.8 15.6

31.2 37.4 33.4

[53, 54]

24

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Highlights 1. Ferric-based layered double hydroxide intercalated with α-alanine was synthesized. 2. Mg-Fe-Ala-LDH showed much higher adsorption capacity for As(III) than for As(V). 3. Desorption experiments indicated As(III) had stronger affinity to Mg-Fe-Ala-LDH. 4. The obtained Mg-Fe-Ala-LDHs is a potential efficient adsorbent for arsenic removal.