Enhanced Removal of Arsenic Species from Water by Functionalised Mesoporous Hybrid Material

Enhanced Removal of Arsenic Species from Water by Functionalised Mesoporous Hybrid Material

Corresponding author: Dr. Salah Ali Mahgoub Idris, Chemistry Department, Faculty of Science, University of Omar Al-Mukhtar, Tobruk, Libya, P.O. Box 179. Tel: 00218 (0) 91 990 1832; Email: salah.idris@ymail.com

Abstract

In this study, two types of functionalised mesoporous silica, mercaptopropyl-SBA-15 (MP-SBA-15) and aminopro- pyl-SBA-15 (AP-SBA-15) were prepared and the adsorption of As (III) and As(V) species by these adsorbents was investigated. The equilibrium data well fitted the Langmuir sorption isotherms and the maximum adsorption ca- pacity of As (III) and As(V) reached 98.2 mg/g and 97.2 mg/g for MP-SBA-15 towards As(V) extracted and AP- SBA-15 when As(III) extracted, respectively. The kinetics data found to be fitted by pseudo-second-order kinetic model. The adsorption thermodynamic parameters revealed that the uptake reactions of both As(III) and As(V) to adsorbents are spontaneous, endothermic and randomness adsorption behaviour at the solid/solution interface with some structural changes in the adsorbate and adsorbent has observed by the positive values of . Even the adsorption capacity were high for both sorbents and pre-treatment of solution will permit either almost a com- plete removal of As(III) or As(V), a sorbent that can be used to extract a wide range of other PTMs from water. No reduction in performance was observed even when removing As(III) or As(V) from water which contained other potentially toxic metal ions at higher concentrations of other metal ions which suggest that both sorbents could be used for environmental remediation.

Keywords: Arsenic Sorption; Mesoporous Silica; Sorption Kinetics; Sorption Thermodynamic; Equilibrium IsothermsIn

Introduction

Arsenic is a toxic element for humans and that depen- dent on its chemical species [1]. In general, the inor- ganic form of arsenic are highly toxic and more than the organic species, and it was found that the toxicity of As(III) is sixty times than As(V) [2, 3]. The World Health Organization (WHO) guideline for the maximum accep- tance levels of arsenic in drinking water 10 µgL−1 (for daily intake) [4]. Arsenic can be found in environmen- tal waters in two chemical forms: As(III) and As(V). The remediation of arsenic from water samples and the big concern about high concentration of arsenic in ground water should take place. The fast, sensitive, accurate and simple analytical methods for the speciation of in- organic arsenic in natural water and more understand- ing about adsorption of arsenic by sorbents are required for obtaining helpful information as adsorption of toxic species by porous materials. The synthesis and appli- cation of mesoporous silica have received considerable attention for last two decades. Surface modifications of porous materials are greatly enhanced metal exchange capacity as well as selectivity of such produced phases towards metal ion removal [5]. That was according to the structure of the organic compound, nature of the functional group present and the incorporated (O, S, N or P) donor atoms. In this context, we have recently de- veloped some selective modified mesoporous silica sorbents [6-10] to extract some heavy metals from aqueous media.

This work explores the preparation of new mercapto- and amino-functionalised-SBA-15-based mesoporous silicas and their application for As(III) and As(V) removal from water samples. The influence of pH on the adsorption of As(III) and As(V) by both functionalised mesoporous sil- ica was investigated. The obtained adsorption isotherm provided useful information for the mechanism of As(III) and As(V) adsorption by the functionalised mesoporous silica. Second, the equilibrium adsorption capacity pro- vides a measurement of the total amount of material that can be adsorbed by the mesoporous material under spe- cific temperature and concentration conditions. The inter- ference of other coexisting metals ions on the adsorption and determination of As(III) and As(V) were examined under the optimal conditions.

Experimental

Materials and reagent

Pluronic P123, PEO20PPO70PEO20 was supplied from BASF Corporation, tetraethoxysilane (TEOS) 98 %,3-mercap- topropyltrimethoxy-silane (MP-TMS), 3-aminopropyl- trimethoxy-silane (AP-TMS), 99%, toluene (+ 99 %), L-ascorbic acid (99%), ammonium molybdate (99.98%), antimony potassium tartrate (≥99%), H2SO4 (99.99), and the stock solutions (1.000 g L−1) of As(III) and As(V) were prepared by dissolving appropriate amounts of Na3A- sO3.7H2O (99.99%) and Na2HAsO4.7H2O (99.99%) in high

purity deionized water, respectively. Nitric acid (HNO3, 65 wt. %), and hydrochloric acid (36%) were purchased from Sigma-Aldrich. Glassware was soaked in 5 % HNO3 overnight and cleaned with deionised water before use. All products were used as supplied and high purity deion- ised water was used throughout this work.

Mesoporous Silica Preparations and Functionaliza- tion

SBA-15 was prepared using method reported in ref [11]. A surfactant tri-block copolymer solution of 5 g Pluron- ic P123, PEO20PPO70PEO20 was dissolved in 150 cm3 of 2 M hydrochloric acid and 100 cm3 of distilled water in a sealed glass bottle at room temperature, and the mixture was magnetically stirred at 330 rpm. The surfactant solu- tion was then heated to 40 o C and 15 g of tetraethylor- thosilicate (TEOS) was added and left for 48 h at 40 o C. The mixture was then placed in an oven for 7 days at 60

o C. The material was filtered and washed with water and dried overnight at 60 o C before calcination at 550 o C for 24 h.

Surface modification of SBA-15 was carried out by con-

densation using the organosilane of choice with SBA-15 (scheme 1). Briefly, approximately 5 g of SBA-15 was pre-treated at 140oC for 2 h before being immersed in 80 cm3 of toluene and 12 cm3 of MP-TMS, or AP-TMS, in a 250 cm3 flask. The mixture was refluxed for 4 h and the solid produced was filtered, washed with 100 cm3 ethanol, and oven-dried at 80oC for 4 h to produce an MP-SBA-15, or AP- SBA-15 sorbent, respectively.

Scheme 1. Modification of SBA-15 using mercaptopropyltrime- thoxysilane aminopropyltrimethoxysilane.

Characterisation and Analysis

The surface area of the SBA-15 and functionalised-SBA-15 were measured using nitrogen physisorption isotherms on a Micrometrics Gemini 2375 volumetric analyser. Each sample was degassed prior to analysis for 6 h at 200 ºC. The Brumauer–Emmett–Teller (BET) surface areas were calculated using experimental points at a relative pres- sure (P/P0) of 0.05–0.25. The total pore volume was calcu- lated from the N2 amount adsorbed at the P/P0 of 0.99 for each sample and the average pore size distribution of the materials was calculated using the Barrett–Joyner–Halan- da (BJH) model from a 30-point BET surface area plot. All samples exhibited a Type IV adsorption isotherm typical of mesoporous solids. Desorption isotherms were used to calculate the pore diameters. Elemental analysis (EA) was carried out using an Exeter Analytical CE440 elemental function. Colorimetric method (molybdate method) was used to determine As(III). The preparation of these solu- tions follows the standard procedure [12]. Briefly, all sam- ples were acidified to 1% HCl as they cannot be acidified with nitric acid for analysis by this method. The presence of nitrate leads to colour instability [13]. Acidified sam- ples are analysed by adding 5 ml aliquots into 10 ml vials and 0.5 ml of a 2% HCl, then 0.5 ml of colour reagent is added (10.8% L-ascorbic acid C6H8O6 (613 mmol L−1), 3% ammonium molybdate (NH4)6Mo7O24 .4H2O (24 mmol L−1), 0.56% antimony potassium tartrate C8H4K2O12Sb2 .3H2O (8 mmol L−1), and 13.98% H2SO4 (2.5 mol L−1)). The colour reagent is mixed thoroughly with the sample immediately

after addition by shaking and allowed to react for 10 min before measuring the absorbance at 880 nm. Derivatised solutions were measured using double beam Spectropho- tometer (UV1900). The calibration slope was obtained by three replicated analyses at different standard solutions

 

( ) ()

(2)

concentrations of As(III) (10, 50, 200, 400, 1000 µg L−1). Total concentrations of arsenic in water samples were determined by inductivity coupled plasma atomic emis- sion spectrometry (ICP-AES) using a Perkin Elmer Optima

 

( )

(3)

5300DV instrument (Perkin Elmer, UK) at an RF power of 1300 W and with plasma, auxiliary and nebuliser argon gas flows of 15, 0.2 and 0.75 L min-1 respectively, and a pump flow rate of 1.5 mL min-1 , Analytical wavelength was 189.042 nm and analytical precision (RSD) was typi- cally 1-5% for individual aliquots (n=3). As(V) concentra- tion was obtained as the respective differences between total arsenic and As(III) obtained by colorimetric method.

Extraction, Recovery and Regeneration Procedures

The general procedure for extraction of As(III) or As(V) from solution can be summarised as follows. Approxi- mately 25 mg samples of MP-SBA-15 or AP-SBA-15 were suspended in 25 cm3 solutions containing known concen- trations of As(III) or As(V) at various pH values (between 1 and 11). Solutions were stirred (250 rpm) for approx- imately 2 h then filtered (Fisherbrand QL100) under vacuum and the filtrate was analysed using ICP/AES for total arsenic and colrimtric method for As(III). To recov- er As(III) or As(V) ions from loaded sorbents the silica material was washed with 25 cm3 water adjusted to pH 1 using 1M HCl.

Adsorption Isotherms

The Langmuir [14] or Freundlich [15] models were ap- plied to measured data to study adsorption isotherms. Solutions containing initial concentrations of As(III) and As(V) at 10, 50, 100 or 200 µg cm-3 were prepared. To each solution 0.05 g of sorbent was added and the solu- tion was stirred at 250 rpm for 120 min at room tempera- ture, solutions were adjusted to provide a pH of 7 with

    1. mol L−1 HCl and NH .H O. The amounts of As(III) and

where and were the equilibrium concentrations of the As(III) and As(V) ions in the adsorbed and liquid phases in mg/g and mg/L, respectively. (mg/g) and (L/mg) were the Langmuir constants. Whereas the is the maximum monolayer capacity and was the adsorption affinity onto the adsorption. (mg/g) and (L/mg) were the Freundlich constants which are related to the sorption capacity and intensity, respectively. The Langmuir and Freundlich con- stants were calculated from the slope and intercept of the linear plot obtained from eq. 2 and 3 respectively.

For predicting the favorability of an adsorption system, the Langmuir equation can also be expressed in terms of a dimensionless separation factor () by using the Langmuir constant b and the initial concentrations of the As(III) and As(V) (Eq. 4).

(4)

When, >1, =1, 0<<1 and =0, indicates unfavorable, linear, favor- able and irreversible, adsorption isotherms, respectively [18, 19].

Adsorption Kinetics Study

Kinetic studies to determine the rate of As(III) and As(V) removal from water samples were conducted for MP- SBA-15 and AM-SBA-15. Solutions were prepared with the same initial As(III) and As(V) concentration of 10 µg cm-3 and stirred with 0.05 g of each sorbent at 250 rpm. 25 cm3 aliquots of each solution were stirred at 25 º C for 1,

3 2 5, 10, 20, 30 or 40 min. After each time period, solutions

As(V) extracted at equilibrium, (mg/g) were calculated according to Eq. (1):

 

were filtered and analyzed by ICP-AES to determine the concentration of total As and colorimetric method to de- termine the concentration of As(III) in the final solution. The kinetics of As adsorption onto the surface of the sili-

      1. ca nanoparticles were analyzed using pseudo first-order

(equation 5) [20], pseudo second-order (equation 6) [21]

where and (mg/g) were the liquid phase initial and equi- librium concentrations of the As(III) and As(V) respec- tively. was the volume of the solution (cm3), and was the mass of sorbent (g) used [16]. The sorption equilibrium data were analysed according to Langmuir Eq. (2) and Freundlich Eq. (3) isotherm models [17].

kinetic models.

(

) ( )

(5)

 

( )

(6)

(10)

The conformity between experimental data and the mod- el predicted values was expressed by the correlation coef- ficients (R2). A relatively high R2 value (close or equal to 1) was used to indicate best fit to the kinetic model.

Thermodynamic Study

The values of enthalpy change () and entropy change () calculated from the intercept and slope of the plot of ver- sus [22].

The activation energy for As(III) and As(V) adsorption was calculated by the Arrhenius equation:

Thermodynamic studies for As(III) and As(V) removal from water samples were conducted for MP-SBA-15 and AP-SBA-15. Solutions were prepared with the same initial

(11)

As(III) and As(V) concentration of 50 µg cm-3 and stirred with 0.05 g of each sorbent at 250 rpm. 25 cm3 aliquots of each solution were stirred at 25, 35, 45 and 55 º C for 1,

5, 10, 20, 30 or 40 min. After each time period, solutions were filtered and analysed by ICP-AES to determine the concentration of total As and colorimetric method to de- termine the concentration of As(III) in the final solution.

The free energy (ΔG0) of the adsorption reaction is given by the following equation:

where is the activation energy (kJ/mol), is the frequen- cy factor, is the absolute temperature (K), and is the gas constant. From the plot of vs. 1/T, the activation energy for the adsorption of As(III) and As(V) can be calculated.

Discussion

Materials Characterisation

The SBA-15 and Functionalised-SBA-15 materials were characterised using BET to examine their pore size and surface area. The physicochemical properties of each ma-

(7) terial are summarised in Table 1.

Table 1. Physicochemical properties of mesoporous materials used.

Where is the adsorption equilibrium constant, is the gas

constant and is the absolute temperature (K). The ad- sorption equilibrium constant () can be calculated from:

Sorbent BET Surface

Area (m2 g-1)a

Pore Size

(nm)b

Pore Volume

(cm3 g-1)c

SBA-15 1050 7.51 1.186
MP-SBA-15 564.3 6.88 0.881
AP-SBA-15 608.4 7.08 0.917

(8)

Where is the fraction attainment of As(III) and As(V) ad- sorbed at equilibrium time, and is obtained by the expres- sion:

a Calculated by the BJH model from sorption data in a relative pressure range from 0.05–0.25.

b Calculated by the BJH model from the adsorption branches of isotherms.

(9)

c Calculated from N2

amount adsorbed at a relative pressure P/

Where and are the initial and equilibrium concentrations of As(III) and As(V) in solution (mg/L). The value of the adsorption equilibrium constant () for the adsorption of As(III) and As(V) on the adsorbent were calculated at dif- ferent temperature and at equilibrium time using Eqs. (8) and (9). The Gibbs free energy can be represented as fol- lows:

P0 of 0.99.

The N2 sorption isotherms (Fig. 1) were type IV for all samples confirming their mesoporous natures, however different volumes of nitrogen gas adsorbed on the mes- oporous silica surfaces were noted at higher relative pressures for SBA-15 compared with MP-SBA-15, and AP- SBA-15 suggesting large of the surface are occupied by functional groups.

Figure 1. Nitrogen adsorption isotherms for mesoporous silica sorbents.

Elemental analysis was used to estimate the amount of molecules (Lo) attached to functionalised samples from the percentage of nitrogen or sulphur, in the function- alised mesoporous silica [23], using equation (12):

%N or %S

Table 2. Elemental analysis data recorded for the SBA-15.

Silica % C % H % N % S Lo (mm l/g)a
SBA-15 Nil 0.7 Nil Nil …..
MP-SBA-15 10.8 2.1 Nil 9.7 3.03
AP-SBA-15 12.1 2.4 5.3 Nil 3.79

a Functionalisation degree (L0 = millimoles of ligand per gram of

functionalised silica).

Extraction of As (III) and As(V) at Different pH and Sorbent Recovery

MP-SBA-15 AND AP-SBA-15 sorbents (25 mg) were used to extract As(III) and As(V) from spiked aqueous solutions (25 µg cm-3, 25 cm3) at different solution pH values; the results are given in Figure 2. At pH 1 As(III) and As(V) remained in solution with zero % extraction efficiency whilst at pH 7 and above As(III) and As(V) extracted com- pletely from solution by using AP-SBA-15 and MP-SBA-15 respectively with 100 % extraction efficiency. As(III) and As(V) removed from solution with less than 40 % ex-

L0  nitrogen or sulphur atomic weight 10

(12)

traction efficiency by using MP-SBA-15 and AP-SBA-15 in- dicting more adsorption affinities of As(V) to be adsorbed by MP-SBA-15 than AP-SBA-15 and also this phenomenon

The calculated Lo (Table 2) values for MP-SBA-15 and AP- SBA-15 were high indicating the successfully functional- isation for mercaptopropyl and aminopropyl functional groups.

Figure 2. The effect of pH on the extraction of As(III) and As(V) ions.

observed As(III) to be adsorbed by AP-SBA-15 than MP- SBA-15. This permits reuse of the functionalised adsor- bents making their use economically feasible. Note that recovery values for As(V) when using MP-SBA- 15 and As(III) when used AP-SBA-15 were slightly higher when removed from As(V) when using AP-SBA- 15 and As(III) when used MP-SBA-15 with recovery values of approxi- mately 99.8 %. It is suggested that the MP-SBA-15 sor- bent is a most suited sorbent for selective removal and recovery of As(V) and AP-SBA-15 sorbent is a most suited sorbent for selective removal and recovery of As(III) from contaminated water systems.

precipitation of As oxides/hydroxides at high equilibrium concentration and pH has also checked, the best way was by Regeneration of the functionalised SBA-15 material after ad- sorption of As(III) and As(V) from water. A 20 mL solution containing 20 μg mL-1 As(III) or As(V) was added to a conical flask containing 100 mg of functionalised SBA-15. After stir- ring for 2 h at room temperature, the suspension was separat- ed by filtration. The filtrate was measured with ICP-AES to de- termine whether the As(III) or As(V) had been removed from water and trapped by the sorbent. The filter paper containing the recovered sorbent was washing the sorbents using 0.01 M ammonium hydroxide and then immersed into 20 mL of a 1 mol L-1 solution of HCl for 2 h at room temperature. The solu-

tion was filtered and the As(III) or As(V) concentration of the acid filtrate was measured by ICP-AES. The sorbent was then dried at 80 oC for 2 h and the extraction repeated to examine the capacity of the regenerated sorbent. The extraction and recovery values for both As(III) or As(V) by both sorbents in the water samples were calculated and were found more than 95% of As(III) or As(V) were recovered which indicate that the adsorption process was preferable and only may be small amounts of As(III) and As(V) has precipitated.

washing the sorbents using 0.01 M ammonium hydroxide solution and second washing the sorbent by 0.01M HCl

Adsorption Isotherms

The adsorption isotherms were studied to find the rela- tionship between equilibrium adsorption capacity and equilibrium concentration at a certain temperature (Fig 3).

Figure 3. Langmuir and Freundlich isotherms for As(III) an

As(V) adsorption onto functionalised SBA-15 sorbents.

Langmuir and Freundlich isotherms are the most com- monly used isotherms for different adsorbent/adsorbate systems to explain solid-liquid adsorption systems and to predict their equilibrium parameters [24-26].

The relevant parameters for these isotherms are present- ed in table 3. As seen from table 3, the R2 values obtained from the Langmuir model are much closer to one than are those from the Freundlich model, suggesting that the Langmuir model is better than the Freundlich isotherm. Thus the adsorption can be described by the Langmuir isotherm and the Arsenic(III) and (V) adsorption occurs on a homogeneous surface by monolayer sorption with- out interaction between the adsorbed ion [27]. The max- imum adsorption capacity of As(III) 97.2 mg/g with AP- SBA-15 and 44.6 when MP-SBA-15 used. In contrast the As(V) shows better adsorption capacity with MP-SBA-15 (98.2 mg/g) than AP-SBA-15 (24.5 mg/g). This phenom- enon may be due to arsenic(III) is a hard acid and prefer- entially complexes with oxides and nitrogen. Conversely, arsenic(V) behaves like a soft acid, forming complexes with sulphides [28]. That was confirmed by the results of adsorption affinities (b) and dimensionless separation factors () values which were higher when As(III) extracted by AP-SBA-15 and As(V) when extracted by MP-SBA-15.

Table 3. Isotherm parameters for Arsenic (III) and Arsenic (V) sorption on functionalised mesoporous silica.

Adsorbents

As(III)

MP-SBA-15 AP-SBA-15
As(V) As(III) As(V)
Langmuir qm (mg/g) 44.6 98.2 97.2 24.5
b (L/mg) 0.13 1.83 3.53 0.26
RL 0.99 0.82 0.74 0.97
R2 0.9865 0.8992 0.9979 0.9982
Freundlich Kf (mg/g) 8.7 26.1 26.2 6.3
n (L/mg) 4.3 3.1 2.9 4.7
R2 0.9806 0.8339 0.9774 0.9915

Adsorption Kinetics

The experimental kinetic data were fitted using a pseu- do-first-order kinetic model and pseudo-second-order kinetic model (Fig. 4). The results are shown in table 4. It can be seen that the obtained R2 values of the pseu- do-second-order model were better than those of the pseudo-first-order model for both adsorbents, suggesting that the adsorption process is second-order. Moreover, the calculated qe values were much closer to the experimental

values in the pseudo-second-order kinetic model than the pseudo-first-order kinetic model indicating that the ad- sorption process is second-order. As seen in table 4, when the initial ion concentration increases from 10 to 200 µg/ ml, the pseudo-second-order constants (k2) decreased for MP-SBA-15 and AP-SBA-15, respectively. This indicates that the available active sites on the adsorbents are satu- rated rapidly by As (III) and As (V).

Adsorption capacities for both sorbents towards As (III) and As (V) were too close to those values obtained by Langmuir adsorption isotherms confirms the results ob- tained were reliable especially when compared to those experimentally produced.

Figure 4. Pseudo-first order and Pseudo-second order kinetics

models of As(III) an As(V) onto MP- and AP-SBA-15 sorbents.

Table 4. Kinetic parameters for the adsorption of As(III) and As(V) on the adsorbents.

Adsorbents/Adsorbate C0 µg/ mL qe (exp) (mg/g) The pseudo first-order The pseudo second-order
k (min-1)

1

qe (cal) (mg/g) R2 k (min-1)

2

qe (cal) (mg/g) R2
MP-SBA-15 / As(III) 10 4.99 0.150 3.86 0.9663 0.094 5.25 0.9935
MP-SBA-15 / As(III) 50 19.40 0.099 9.51 0.9944 0.045 19.68 0.9981
MP-SBA-15 / As(III) 100 33.77 0.063 21.34 0.9545 0.010 34.10 0.9765
MP-SBA-15 / As(III) 200 42.17 0.079 31.09 0.9850 0.007 44.02 0.9877
MP-SBA-15 / As(V) 10 5.00 0.173 1.73 0.9967 0.354 5.06 0.9995
MP-SBA-15 / As(V) 50 24.99 0.231 4.45 0.9960 0.221 25.11 0.9999
MP-SBA-15 / As(V) 100 49.01 0.152 15.96 0.9817 0.039 49.58 0.9996
MP-SBA-15 / As(V) 200 96.95 0.076 34.16 0.9961 0.009 97.56 0.9970
AP-SBA-15 / As(III) 10 5.00 0.175 2.04 0.9985 0.348 5.07 0.9995
AP-SBA-15 / As(III) 50 24.96 0.206 13.72 0.9765 0.064 25.36 0.9996
AP-SBA-15 / As(III) 100 49.69 0.130 24.69 0.9756 0.021 50.52 0.9989
AP-SBA-15 / As(III) 200 94.80 0.083 52.32 0.9635 0.005 96.95 0.9924
AP-SBA-15 / As(V) 10 4.97 0.131 4.05 0.9772 0.088 5.18 0.9935
AP-SBA-15 / As(V) 50 16.95 0.097 8.37 0.9952 0.045 17.24 0.9972
AP-SBA-15 / As(V) 100 22.50 0.081 11.15 0.9917 0.030 22.75 0.9962
AP-SBA-15 / As(V) 200 24.00 0.076 12.36 0.9481 0.029 24.19 0.9963

Table 5. Thermodynamic parameters for the adsorption of As(III) and As(V) on the adsorbents.

Adsorbents/Adsorbate Temp. (K) ΔGo (kJ/mol) ΔHo (kJ/mol) ΔSo (J/mol.K) ΔEo (kJ/mol)
MP-SBA-15 / As(III) 298 -15.95 3.68 65.7 3.51
MP-SBA-15 / As(III) 308 -16.51
MP-SBA-15 / As(III) 318 -17.22
MP-SBA-15 / As(III) 328 -17.90
MP-SBA-15 / As(V) 298 -20.27 15.23 119.3 4.65
MP-SBA-15 / As(V) 308 -21.57
MP-SBA-15 / As(V) 318 -22.80
MP-SBA-15 / As(V) 328 -23.84
AP-SBA-15 / As(III) 298 -3.08 0.86 13.2 5.00
AP-SBA-15 / As(III) 308 -3.21
AP-SBA-15 / As(III) 318 -3.32
AP-SBA-15 / As(III) 328 -3.48
AP-SBA-15 / As(V) 298 -1.84 3.29 17.2 3.79
AP-SBA-15 / As(V) 308 -1.98
AP-SBA-15 / As(V) 318 -2.20
AP-SBA-15 / As(V) 328 -2.34

Cite this article: Idris S A M. Enhanced Removal of Arsenic Species from Water by Functionalised Mesoporous Hybrid Material. J J Hydrology. 2015, 1(2): 009.

Adsorption Thermodynamics

The thermodynamic parameters of the adsorption pro- cess, such as free energy (ΔG0) of the adsorption, enthalpy change and entropy change and activation energy were determined and are listed in table 5. Negative values of ΔG0 for both types of adsorption mean that the process is spontaneous. The value of was found to be positive confirming the endothermic nature of the adsorption process. The positive values of show the increased ran- domness at the solid/solution interface with some struc- tural changes in the adsorbate and adsorbent, this was more when MP used as functional groups as the values of much higher. This give an idea about the interaction was more at lower temperatures. The activation energy values for the adsorption of MP-SBA-15 towards As(V) and AP- SBA-15 towards As(III) were found to be 4.65 kJ/mol and

5.00 kJ/mol, respectively were higher than MP-SBA-15 towards As(III) and AP-SBA-15 towards As(V) 3.51 kJ/ mol and 3.79 kJ/mol, respectively indicate that enhancing the adsorption need less energy for MP-SBA-15 towards As(III) and AP-SBA-15 towards As(V) which confirm that the performance of adsorption of MP-SBA-15 towards As(V) and AP-SBA-15 towards As(III) at room tempera- ture is better.

Removal of As(III) and As(V) from Contaminated, Multi-Element, Solutions

Table 6. The efficiency of MP-SBA-15 and AP-SBA-15 to extract

As(III) and As(V) in multi elements sample

Heavy Metals Initial concentra-

tion (μg mL-1)

Concentration of metal found

(μg mL-1)

MP-SBA-15 AP-SBA-15
Al 1.0 0.008 0.187
As(III) 1.0 0.634 0.003
As(V) 1.0 0.001 0.359
Ba 1.0 0.027 0.024
Cd 1.0 0.002 0.587
Co 1.0 0.128 0.007
Mn 1.0 0.224 0.027
Ni 1.0 0.828 0.018
Pb 1.0 0.251 0.009
Zn 1.0 0.059 0.012

Excellent performance for extraction of As(III) and As(V)

was also demonstrated for MP-SBA-15 and AP-SBA-15 in the presence of other potentially toxic metal ions (PTMs) as seen in table 6. Standard solutions containing 1.0 μg cm-3 of a range of selected metal ions (Al, Ba, Cd, Co, Mn, Ni, Pb, and Zn) were prepared and stirred for 2 h with of MP- SBA-15 and AP-SBA-15 (50 mg) at pH value around 7. In spite of other metal ions being present at same concentra- tions than As (III) and As (V), extraction efficiency values of more than 99.7 % was achieved for removal of As(III) and As(V) by AP-SBA-15 and MP-SBA-15; which implied that the As(III)-N and As(V)-S co-ordinating bond were not appreciably hindered by the presence of other co-or- dinating ions at concentration used for this experiment. These simple experiments provide further evidence of the high performance, and selectivity, of AP-SBA-15 and MP- SBA-15 for As (III) and As(V).

Conclusion

In this study, MP-SBA-15 and AP-SBA-15 were prepared and the adsorption of As(III) and As(V) ions by these ad- sorbents was investigated. The equilibrium data well fitted the Langmuir sorption isotherms (R2 values are closer to one), and the maximum adsorption capacity of As(III) and As(V) reached 98.2 mg/g and 97.2 mg/g for MP-SBA-15 for As(V) extracted and AP-SBA-15 when As(III) extract- ed, respectively. The adsorption process is fitted by pseu- do-second-order kinetic model. The pseudo-second-order constants (k2) decreased for MP-SBA-15 and AP-SBA-15, respectively. This indicates that the available active sites on the adsorbents are saturated rapidly by As(III) and As(V) thus the performance will be better and rapidly at low arsenic concentration. The adsorption thermody- namic parameters revealed that the uptake reactions of both As(III) and As(V) to adsorbents are spontaneous, en- dothermic and randomness adsorption behaviour at the solid/solution interface with some structural changes in the adsorbate and adsorbent has observed by the positive values of .

Even the adsorption capacity were high for both sorbents and pre-treatment of solution will permit either almost a complete removal of As(III) or As(V), a sorbent that can be used to extract a wide range of other PTMs from water. No reduction in performance was observed even when re- moving As(III) or As(V) from water which contained oth- er potentially toxic metal ions at higher concentrations of other metal ions which suggest that both sorbents could be used for environmental remediation.

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