Trang 270 Adsorption of As ( V ) and As ( III ) from aqueous solution by lepidocrocite ( γ-FeOOH ) nanoparticle

γ-FeOOH nanorods an adsorbent for As(V) and As(III) removal was prepared by a chemical co-precipitation method. The maximum adsorption capacities at pH6 for As(V) and As(III) were 63.75 and 88.99 mg/g, respectively, higher than those of Fe2O3, Fe3O4... The adsorption data accorded with Freundlich isotherms. At the study pH, for arsen, the adsorption equilibrium was gained after 90 min. Kinetic data fitted well to the pseudo-secondorder reaction model. The adsorption of γFeOOH for As (V) and As(III) could be competed by some other ion such as sulfate, ammonium and chloride. The high adsorption capability and good performance on other aspects make the γFeOOH nanorod a promissing adsorbent for the removal of As (V) and As(III) from the groundwater.


INTRODUCTION
Geogenic arsen (As) contamination in the groundwater is a major health problem that has been recognized in several regions of the world, especially in Bangladesh, West Bengal [1,2], Vietnam [3][4][5], Cambodia [6,7], Myanmar [8], and Mexico, where a large proportion of groundwater is contaminated with arsen at levels from 100 to 2000 μg L -1 [9].
Among the possible treatment processes, the adsorption is considered to be less expensive than the membrane filtration, easier and safer to handle as compared to the contaminated sludge produced by precipitation, and more versatile than the ion exchange [22].Adsorption process is considered to be one of the most promising technologies because the system can be simple to operate and low cost [23].
Among a variety of adsorbents for arsen removal, iron (hydro)oxides including amorphous hydrous ferric oxide, poorly crystalline hydrous ferric oxide (ferrihydrite) [24], goethite [25] and akaganeite [19] are well-known for their ability to removal inorganic arsen from aqueous system Trang 271 with low cost.Among these adsorbents the As(III) adsorption is normally less effective than the As(V) adsorption [15,17,18].FeOOH has high adsorption capacity on arsenic [26], but it could not effectively remove both As(V) and As(III) simultaneously.
In the present study, a γ-FeOOH nanoparticle adsorbent was prepared by a chemical coprecipitation method, which was easy to operate and economic.The adsorbent was characterized and evaluated for its adsorption behavior of arsen.It exhibited high adsorption capacity for both As(V) and As(III).

Materials
Stock solutions of As(V) and As(III) 1000 mg/L (Merk).The working solutions were freshly prepared by diluting Na 2 HAsO 4 •7H2O and As 2 O 3 with bidistilled water.HNO 3 (0.1-0.5 N) and NaOH (0.1-0.5 N) were used for adjusting the pH of the arsenic solution as necessary.
All adsorption data were analysed by the Originlab 8.5.1 software.

Preparation of γ-FeOOH
The γ-FeOOH adsorbent was prepared according to the following procedure [33]: Dissolve 12 g FeCl 2 4H 2 O in 300 mL distilled water with vigorous stirring.The beaker should be equipped with a glass electrode connected to a pH meter, a gas inlet connecting an air or oxygen cylinder and a dropping funnel containing 125 mL 1M NaOH.Adjust the pH of the system to 6.5-6.8 by adding NaOH dropwise, then open the gas cylinder and aerate the air blowing rate 2 L/min.The initial greenish black precipitate becomes orange after 20 min.Throughout the reaction, the pH of the suspension must be maintained at 6.5-6.8, by adding NaOH from the dropping funnel as needed, centrifuge, wash and dry.The dried material was stored in a desiccator for use.

Batch sorption tests
To determine the amount of adsorbed arsen (As(V) or As(III)) under the given conditions, approximately 0.1 g of adsorbent was weighed and placed in a 250-mL Erlenmeyer flask.One hundred millilitres of As(V) or As(III) solution was added into the flask.The concentration of the As(V) or As(III) solution ranged from 40 to 1000 mg/L depending on the type of experiment.Ionic strength was not adjusted during the absorption.The flask was capped and shaken at 180 rpm on an orbital shaker for 24 h to ensure the approximate equilibrium.All batch experiments were conducted at room temperature (20 °C) unless stated otherwise.The pH was manually maintained at a designated value pH= 6.0 in such a way: pH was initially adjusted to a defined value with 0.01 N HNO 3 and NaOH and then measured and adjusted at an interval of 2 h.After 24 h of the period reaction, all samples were centrifuged at 10.000 rpm for 5 minutes and filtered through a 0.45-µm membrane filter and the filtrate was analyzed for arsen.This procedure was used in all adsorption experiments for evaluating isotherms and interferences of competing ions, except for kinetic experiments.The quantity of adsorbed arsen was calculated by the difference of the initial and residual amounts of arsen in the solution divided by the weight of the adsorbent.
The amount of adsorbed metal was calculated from the following expression: Where q is the metal uptake or sorption capacity of adsorbent (in mg/g of adsorbent); C i and C e are the metal concentrations before and after adsorption, respectively, B is the mass of adsorbent used and V the solution volume used.
The pseudo-first-order adsorption and pseudo-second-order adsorption were used to test the adsorption kinetics data.The pseudo-firstorder rate expression of Lagergern is given as [27].log (q e -q t ) = log q e -k 1

t (1) or
Where q e and q t are the amount of arsenic adsorbed on adsorbent (mg/g) at equilibrium and time, and k 1 is the rate constant of pseudo-firstorder adsorption.The pseudo-second-order rate model is expressed as [28]: Where k 2 is the constant of pseudo-secondorder rate (g/mg•min).The experimental data of q e , q t and k 2 can be determined from the slope and the intercept of the plot of t/q t against t.

Studies of adsorption isotherm effect
Experiments for studying the arsenic adsorption isotherm were conducted at 20 °C and pH= 6.0 by following the batch adsorption procedure.A series of different initial concentrations of As(V) or As(III) solutions (40-1000 mg/L) at pH= 6.0 were used.For estimating the thermodynamic parameters of arsenic adsorption, the isotherm experiments were also conducted at 20 °C.

Studies of adsorption time effect
The effects of time on arsenic adsorption were examined in a series of batch sorption experiments that used the same initial As(V) or As(III) concentration (100 mg/L) while maintaining the time at different values from 0 to 180 minutes.

Adsorption kinetics studies
Arsenic adsorption kinetics was evaluated at 20 °C and pH= 6.0.The initial As(V) or As(III) solution concentrations were 100 mg As/L.The kinetic experiments were conducted in a 250-mL flask.The flask was shaken at 180 rpm.With this experimental setup the temperature of the solution inside the flask was well maintained at 20 °C, pH was maintained at around pH= 6.0.

Arsenic adsorption with competing other ions
The interference of ammonium (NH 4 + ), chloride (Cl -) and sulfate (SO 4 2-) on As(V) or As(III) adsorption was evaluated in batch experiments, respectively.The experimental method was similar to the batch adsorption method described previously.The difference was that the arsenic working solutions for these competing adsorption experiments were prepared with the separate addition of ammonium, chloride and sulfate solutions into the arsen solution.The initial addition of arsen was 100 mg/g adsorbent using an arsenic solution in 100 mg As/L and the pH was maintained at approximately pH= 6.0.The concentrations of the competing anions used in the experiments were from 1 to 120 mg/L for ammonium, chloride and sulfate.
q =V (C i -C e )/B e t e 1 ln(q q ) ln q k t   

Characterization of γ-FeOOH adsorbent
Lepidocrocite nanoparticles applied in this work consisted mainly of γ-FeOOH, characterized by the basic reflection appearing at 2θ of 44.9•, as shown in the XRD diagram in (Fig. 1A) and (Fig. 1B) This is evident from the XRD diagram in Fig. 1B, where the oxide appears in the form of lepidocrocite (γ -FeOOH) and hematite (α-Fe 2 O 3 ) (Fig. 1B).The α-Fe 2 O 3 percentage is very low.It is a by product of the synthesis process, and thus the corresponding peaks might be of γ -FeOOH (Fig. 1A).
A typical SEM image of the prepared sample was shown in Fig. 2, which reveals that the Lepidocrocite was a nanorod with the diameter of 20 nm and the length of 100 nm. A.

Adsorption isotherm of γ-FeOOH adsorbent
The adsorbents were tested for adsorption of As(V) and As(III), as shown in Fig. 3A and Fig. 3B.The sorption capacity of As(III) by γ-FeOOH was higher than As(V) and the sorption capacity of γ-FeOOH was high compared to goethite (72.4 mg/g) [29].

The Langmuir expression was:
Where q is the amount of As(V) adsorbed, mg/g; q m the maximum As(V) and As(III) uptake value corresponding to sites saturation, mg/g; Ce the equilibrium As(V) and As(III) concentration in solution, mg/L; and b is the ratio of adsorption/desorption rate.The result sorption of As(V) and As(III) by γ-FeOOH was shown in the Table 1.
The Freundlich expression was:  q m bC e 1+ bC e The adsorption isotherm of γ-FeOOH for As(V) and As(III) were presented in Fig. 3A and Fig. 3B.In this study, both Langmuir and Freundlich isotherms were used to describe the adsorption isotherms.The adsorption constants obtained from the isotherms are listed in Table 1.The correlation coefficient (R 2 ) values of the Langmuir isotherms for As(V) was 0.92 and for As(III) was 0.89, while that of the Freundlich isotherms for As(V) was 0.93 and for As(III) was 0.98.High regression coefficients suggested that the Freundlich model was suitable for describing the adsorption behavior of As(V) and As(III) by γ-FeOOH.
The coefficient value (R 2 ) of the Freundlich isotherms for As(III) was higher than (R 2 ) value of Langmuir isotherms, in the sorption system due to a part of As(III) was transferred to As(V) [12].

Adsorption kinetics of As(V) by γ-FeOOH adsorbent
The kinetics of adsorption is one of the important characteristics that define the adsorption efficiency.Hence, in the present study, the kinetics of arsenic adsorption was analyzed to understand the adsorption behavior of γ-FeOOH.Fig. 4 shows the adsorption data of As(V) and As(III) by γ-FeOOH at different time intervals.The adsorptions of both As(V) and As(III) by γ-FeOOH were found to be time dependent.The adsorption of As(V) and As(III) was rapid for the first 45 min, when the removal rate reached 87 % for As(V), and 76 % for As(III), the removal reach was 97 %, after 90 min and the adsorption equilibrium was approached for both of As(V) and As(III).Table 2, lists of the results of rate constant studies for As(V) and As(III) by pseudo-firstorder and pseudo-second-order.The values of correlation coefficient R 2 for the pseudo-firstorder adsorption model are 0.97 for As(V) and 0.96 for As(III), (Fig. 5) and the adsorption capacities calculated by the model are different to those determined by experiments.The values of R 2 for the pseudo-second-order are extremely high (>0.99)(Fig. 6), for both As(V) and As(III), the experimental data fitted the pseudo-secondorder model better than the pseudo-first-order model.Therefore, it can be concluded that the pseudo-second-order model is more suitable to describe the adsorption kinetics of arsenic on γ-FeOOH.The rate of adsorption depends on the driving force and concentration gradient.In case of pseudo-first-order the rate is proportional to the concentration (△C) and in pseudo-secondorder, it is proportional to the square of concentration gradient (△C 2 ) which refers to both external as well as internal mass transfer [30].This evidence shows that both the external and internal mass transfer is taken place.The As(V) ions existe as negative ions [31] in the experimental conditions.Therefore, As(V) ions may be easy to diffuse into the external and internal adsorption sites of adsorbent.So there was fast removal rate percentage of As(V) before 45 min.This was not contradictory with the adsorption isotherms.

Effect of other constituents on arsenic removal
In natural groundwater or waste water several component might exist, which could compete with arsen for the available adsorption sites or interact with arsenic itself.In this study we select some ions (Cl -, SO 4 2-, and NH 4 + ) to test the effect of co-existing constituents.Fig. 7A and Fig. 7B show the results of the effect of coexisting ions.Cl -had little or no effect on the arsen removal performance of the adsorbent.This is probably that the ions do not compete with the arsen ions.The oxyanions SO 4 2-were selected to assess the effects of co-existing anions on As(V) and As(III) removal.At pH 6, the effects of those oxyanions increased the concentration level were illustrated in( Fig. 7A and 7B).The NH 4 + also had little effect on the arsenate removal performance of the adsorbent.This result is in agreement with previous studies [32].

CONCLUSION
A novel γ-FeOOH nanorod an effective adsorbent for As(V) and As(III) removal, has been prepared by a chemical co-precipitation method.At pH 6 the maximum adsorption capacities for As(V) and As(III) were 63.75 and 88.99 mg/g respectively.At this pH, for arsen, the removal rate reached 95 % after 90 min.In order to reveal useful informations for the sorption mechanism, typical adsorption isotherms (Langmuir and Freundlich) were determined and X-ray photoelectron spectroscopy analysis was used.The mechanism of the removal seemed rather to be a chemisorption, based on the kinetics sorption.
The sulfate was a competitor with arsenic for adsorptive sites on the adsorbent.These results indicated that the γ-FeOOH nanorod was an attractive adsorbent for the removal of arsenic from aqueous solutions.

Figure 4 .
Figure 4. Time effect sorption of As (V) and As(III) by γ-FeOOH

Figure 5 .Figure 6 .
Figure 5. Pseudo-first-order model adsorption of As(V) and As(III) by γ-FeOOH A) Other constituents effect sorption of As(V) by γ-FeOOH; B) Other constituents effect sorption of As(III) by γ-FeOOH

Table 1 .
Langmuir and Freundlich isotherm parameters for As(V) and As(III) adsorption on γ-FeOOH adsorbent at pH= 6.0