Chemosphere 99 (2014) 207–215
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Equilibrium and kinetics studies of arsenate adsorption by FePO4 M. Hamayun 1, T. Mahmood 1, A. Naeem ⇑, M. Muska 1, S.U. Din 1, M. Waseem 1 National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar, Pakistan
h i g h l i g h t s FePO4 is a potential adsorbent for anion-exchange removal of arsenate. Adsorption mechanism involves the exchange of arsenate with OH ions of surface. Both film diffusion and pore diffusion are involved in kinetics adsorption mechanism. FTIR and EDX studies validate the adsorption of arsenate by FePO4.
a r t i c l e
i n f o
Article history: Received 15 July 2013 Received in revised form 12 October 2013 Accepted 14 October 2013 Available online 23 November 2013 Keywords: Adsorption Arsenic Sorption maxima Kinetics Ferric phosphate
a b s t r a c t The present work is focusing on removal of arsenate from aqueous solution using FePO4. The equilibrium study regarding the removal of arsenic by FePO4 was carried out at 298, 308, 318 and 328 K. Langmuir parameters were found to increase with the increase in temperature indicating that the adsorption is favorable at high temperature. Kinetic study of arsenate adsorption on FePO4 was also carried out at different temperatures and at pH 6 and 8. Different kinetic models were used to the kinetic data amongst which pseudo second order model was best fitted. The mechanism of the adsorption kinetics was investigated by employing intraparticle diffusion and Richenberg models. The energy of activation (Ea) was found to be 30 and 35.52 kJ mol1 at pH 6 and pH 8, respectively, suggesting chemisorption nature of the adsorption process. The negative entropic values of activation signified the existence of entropy barrier while the positive DG# values indicated the existence of energy barrier to be crossed over for the occurrence of a chemical reaction. Both the spectroscopic studies and increase in equilibrium pH reveal the anion exchange removal of arsenate from aqueous solution to the solid surface. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Soils with high concentration of heavy metals affect ground water, surrounding ecosystem, human health and agriculture productivity (Houben et al., 2013). Arsenic is a metalloid which is a dangerous water pollutant throughout the world. Arsenic is mobilized by natural weathering reactions, biological activity, geochemical reactions, volcanic emissions and other anthropogenic activities. Soil erosion and leaching contribute to 612 108 and 2380 108 g year1 arsenic, respectively, in dissolved and suspended forms in the oceans (Mohan and Pittman, 2007). Most environmental arsenic problems are the result of mobilization under natural conditions. However, mining activities, combustion of fossil fuels, use of arsenic pesticides, herbicides, crop desiccants and use of arsenic additives to livestock feed are the other sources of arsenic. It occurs in natural waters in both inorganic and organic forms, such as monomethyl arsenic acid (MMAA), dimethyl arsenic ⇑ Corresponding author. Tel./fax: +92 91 9216766. 1
E-mail address: [email protected] (A. Naeem). Fax: +92 91 9216766.
0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.075
acid (DMAA), and arseno-sugars. The inorganic form of arsenic is more toxic than the organic form. Arsenic usually occurs in two valence states (Zhang et al., 2013), arsenite [As (III)] and arsenate [As (V)]. In natural waters, arsenite species primarily consist of arsenious acid (H3AsO3), while arsenate species consist of H2AsO1 4 and HAsO2 4 (Jeong et al., 2007; Mohan and Pittman, 2007). The use of arsenic contaminated water for a long period gives rise to serious diseases in the human beings. The chronic health effects associated with arsenic include cancer of lung, bladder, liver, kidney, skin pigmentation, nerve tissue injuries and cardiovascular diseases (Ansone et al., 2013). Several countries like Bangladesh, Taiwan, Argentina, Mexico, Chile, Mongolia, China, Hungary, Thailand, USA, Germany, New Zealand, South Africa, Pakistan and India are at the risk due to arsenic contamination of water (Manjare et al., 2005). Considering its health and toxicological effects, many regulatory agencies have revised the maximum contaminant level (MCL) for arsenic in drinking water from 0.05 to 0.01 mg L1 (Gregor, 2001). The US Environmental Protection Agency recommends that public water systems must comply with the standard of 10 lg L1 for arsenic instead of 50 lg L1 used for years (Yang et al., 2007; Gupta et al., 2012).
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Several techniques such as coagulation/precipitation, reverse osmosis, ion exchange, and adsorption effectively lower arsenic concentration in aqueous solutions. Some of these techniques may produce large amounts of chemical sludge which need further treatment before being disposed of. Amongst the above mentioned technologies adsorption has played a significant role in removing the pollutants from the aqueous system. It has been observed that arsenate is well adsorbed onto the surface of minerals (Payne and Abdel-Fattah, 2005). Several researchers have used different sorbents for the uptake of arsenate from the aqueous phase which include, arbuscular mycorrhizal fungal (Leung et al., 2013), oyster shell (Rahman et al., 2008), activated carbon (Chuang et al., 2005), iron-treated activated carbon and zeolites (Payne and Abdel-Fattah, 2005), iron-imended bio sand filters (Chiew et al., 2009), iron-sulfide coated sand (Han et al., 2013), ferrihydrite-coated sand (Mähler and Persson, 2013), binary mixed oxide of iron and silicon (Mahmood et al., 2012), mesoporous alumina (Kim et al., 2004), granular titanium dioxide (Bang et al., 2005), chelating resin (Balaji et al., 2005), and mixed valent iron oxides (Mishra and Farrell, 2005). Iron containing compounds have been thoroughly studied for the arsenic removal from water because of their greater affinity for the arsenic adsorption (Gupta et al., 2012). Like other iron bearing compounds, iron (III) phosphate is insoluble solid material which exists in various amorphous, crystalline, and intermediate state of crystallinity (Mustafa et al., 1999). Iron (III) phosphate possesses pH-dependent surface charge properties. It is considered an important sink for arsenate adsorption from the aqueous system. It is inexpensive and convenient to use. However, literature regarding arsenate adsorption by iron phosphate has been ignored. Hence in the present investigations, an attempt was made to test FePO4 for the arsenate adsorption from the aqueous solution. In the current study adsorption equilibrium experiments were conducted at 298, 308, 318 and 328 K which also correspond to the seasonal temperature changes in the Indian subcontinent where arsenic contamination is a serious problem of concern. This study also gives a comprehensive account of the adsorption kinetics at different temperatures, which is an essential parameter in the adsorption process because it gives an insight of the time span which is required for the complete saturation of the adsorbent surface. 2. Experimental 2.1. Reagents and equipments All the reagents used in this study were of analytical grade. Iron (III) nitrate nanohydrate was provided by Scharlau, while arsenic standard solution and ortho phosphoric acid were purchased from Merck. Stock solutions of test reagents were prepared in a milli Q water. All the glassware and sample bottles were soaked in 10% HNO3 and then rinsed with milli Q water. 2.2. Preparation of FePO4 FePO4 was precipitated by mixing trisodium phosphate with solution of ferric nitrate according to the following reaction (Naeem et al., 2007),
FeðNO3 Þ3 þ Na3 PO4 ! FePO4 þ 3NaNO3
ð1Þ
The mixture was equilibrated for a period of 1 h at 40 °C with continuous stirring. The pH of the system was maintained at pH 6 by adding required concentration of HNO3 and NaOH solution. Thick yellow precipitate was formed which was then subjected to washing by dialyzing with the Mili Q water. The precipitate was filtered and washed until filtrate was free of nitrate anions. The sample thus prepared was air dried and then oven dried at 110 °C
for 24 h (Mustafa et al., 1999). The dried sample was ground and passed through a mesh size of 200 and stored in a sealed reagent bottle for further investigation. 2.3. Characterization The point of zero charge (PZC) was determined using 0.1 M NaCl solution as a background electrolyte. Forty milliliters of 0.1 M NaCl was taken in different flasks and the pH of solutions were adjusted in the pH range of 2–12 by addition of 0.1 M HNO3/NaOH solutions. Ferric phosphate (0.1 g) was added to each flask and these flasks were placed in shaker bath for 24 h at 298 K and the final pH of the suspension was noted. PZC of the sample was determined by plotting the difference between initial and final pH vs. initial pH (Mullet et al., 1999; Mahmood et al., 2011a). Air dried samples of FePO4 was mixed with KBr crystals and the mixture ground to fine powder, using agate mortar. The resulting powder was passed into a disc under vacuum. Fourier transform infrared (FTIR) spectra of the FePO4 samples before and after arsenate adsorption were recorded using SHIMADZU 8201PC, FTIR spectrophotometer (Lakshmipathiraj et al., 2006). The Bruner–Emmett–Teller (BET) surface area of FePO4 was determined using Quantachrome NOVA 2200e surface area analyzer in the presence of nitrogen as purging gas. The degassing of the sample was carried out by subjecting the sample to a preliminary heating at 473 K for 2 h and then the surface area of FePO4 was calculated (Waseem et al., 2013). The surface morphology of the sample was analyzed through scanning electron micrograph (SEM) model JSM 5910 (JEOL Japan) equipped with an energy dispersive X-ray (EDX) micro analyzer model INCA 200 (UK) at 20 keV. The SEM pictures and EDX spectra of FePO4 before and after arsenate adsorption were analyzed. Thermogrvimetric analysis (TGA) and differential thermogrvimetric (DTG) analysis of the FePO4 were carried out using TG/DTA analyzer, Perkin Elmer model 6300. The sample of FePO4 was heated up to 1000 °C at a heating rate of 5 °C min1 (Yang et al., 2013). 2.4. Adsorption equilibrium and kinetic study The equilibrium studies of arsenate sorption onto FePO4 were conducted at four different temperatures (298, 308, 318, and 328 K) according to the method given in the literature (Naeem et al., 2002; Das et al., 2013). The adsorption study was launched by varying the arsenic concentration in the range of 10–100 mg L1 at pH 6. FePO4 (0.1 g) was added to a reaction vessel containing 40 mL of arsenate solution. The pH of solution was adjusted to pH 6 by using HNO3/NaOH solutions. The reaction vessels were then equilibrated in a shaking bath at 120 rpm for a period of 24 h. The equilibrium pH of the suspension was noted which were filtered and analyzed for the determination of arsenate in the filtrate. The amount of arsenate in the filtrate was determined using atomic absorption spectrometer AAnalyst 800 (Perkin Elmer). Some controlled adsorption experiments in the absence of FePO4 were also conducted. The kinetic study was carried out in the same way as the adsorption equilibrium studies at pH 6 and at pH 8. In kinetic studies, the solution of specified flask was filtered at a pre-determined interval of time which was analyzed for arsenate by atomic absorption spectrophotometer. 3. Results and discussions 3.1. Characterization The PZC of FePO4 determined by the salt addition method was found to be 6.3 (Fig. S1) which is comparable in magnitude to
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3.2. Equilibrium study The adsorption equilibrium study for the removal of arsenic by FePO4 was conducted at 298, 308, 318 and 328 K by varying the arsenic concentration in the range of 10–100 mg L1. Most of the surface, ground and industrial waste water bears different temperature and different concentration level of arsenate, therefore the above wide range of temperature and concentration has been chosen for the present findings to match with the natural water system. The adsorption capacity of FePO4 increases with increase in initial arsenic concentration (Fig. 1a). The increase in adsorption capacity with increase in arsenic concentration is an outcome of the increase in driving force because of the concentration gradient build up between the surface of FePO4 and arsenate solution (Kumar et al., 2012). It is also important to mention here that the % removal of arsenate decreases with the increase in the initial arsenate concentration (Fig. 1a). This is attributed to the fixed quantity of the FePO4 being used during the course of this investigation. The uptake of arsenate by FePO4 increases with the increase in temperature which indicates the endothermic behavior of the adsorption reaction (Fig. 1b). Different reasons for the efficient adsorption at high temperature have been mentioned in the literature. Wu et al. (2008) suggested that the increase in adsorption with temperature was due to the increase in the rate of intraparticle of metal ions into the pores of adsorbent at higher temperature. Al-Degs et al. (2008) were of the opinion that the increase in adsorption with temperature is due to the increased penetration of adsorbate inside the micropores of the surface or the creation of new active sites. The increase in the level of arsenic sorption with temperature in the present case may be due to increase in the activity of the sorption sites on the adsorbent surface as well as enlargement of the pores as is also suggested by Borah et al.
2
80
1.5
60
1
40
q x 104 (mol g-1) %removal
0.5
% removal
100
q
104 (mol g-1)
(a) 2.5
20
0
0 0
1.5
3
4.5
6
7.5
9
Ce 104 (mol L-1)
(b) 2.5 104 (mol g-1)
2.1 1.7 298K 308K 318K 328K
1.3
q
the reported value of PZC (5.1) for AlPO4 (Mustafa et al., 2006). The functional groups on the surface of adsorbent were investigated by FTIR spectra. FTIR spectra of FePO4 are given in Fig. S2. The spectra seem to be simple and all the peaks can be interpreted easily. Band in the range of 900–1200 cm1 is assigned to P–O stretching vibration. Similarly, the band at 1600–1700 cm1 can be assigned to O– H bending whereas a broad band in the range of 3000–3500 cm1 is attributed to O–H stretching vibrations (Mustafa et al., 1999; Biswas et al., 2009; Muniz et al., 2009). The BET surface area of FePO4 was observed to be 80 m2 g1 which is exactly the same as was observed for potassium titanium oxophosphate (Biswas et al., 2009) and is greater than AlPO4 (48 m2 g1), FePO4 (60 m2 g1) and CrPO4 (57 m2 g1) reported by Naeem et al. (2002) and Mustafa et al. (1999). The SEM images of FePO4 are displayed in Fig. S3. It can be seen that the surface of pure FePO4 is porous in nature (Fig. S3a). The adsorbent particles are non homogeneous in respect of size and shape. The EDX results of FePO4 before and after arsenate adsorption are presented in Fig. S4. It is obvious from Fig. S4a that the pure sample of FePO4 has significant amount of iron in addition to phosphorus and oxygen. TGA of FePO4 was carried out up to 1000 °C. The weight loss was observed to take place in three steps (Fig. S5). The first weight loss is computed to be 1.85% at 41.44 °C and the second weight loss (15.18%) occurs at 153.55 °C. These two weight losses in the temperature range 41–154 °C may be attributed to the loss of physiosorbed water. The above mentioned two weight losses are abrupt while a third slow weight loss in temperature range 150–300 °C is most probably due to the loss of interstitial water. Moreover, the DTG curve further augments the conclusions drawn from the TGA curve.
0.9 0.5 0
2
4
6
8
Ce 104 (mol L-1) Fig. 1. Effect of (a) initial arsenate concentration and (b) temperature on the sorption of arsenate at pH 6 by FePO4.
(2009) for the sorption of arsenic (V) from aqueous solution by acid modified carbon black. Moreover, the uptake of arsenate onto FePO4 was accompanied by the increase in final pH of the system. This indicates that the OH ion is released from the solid surface into the aqueous phase according to the following mechanism.
ð2Þ
The Langmuir, Dubinin–Radushkevich (D–R) and Freundlich models were tested to the experimental data. The Freundlich model was excluded because of its poor correlation coefficient (R2) and lack of agreement between the experimental and theoretical values of adsorption capacity. Therefore, equilibrium adsorption data collected at different temperatures was explained with the help of Langmuir and Dubinin–Radushkevich models. Both the models were tested to probe into the mechanism of arsenic sorption onto FePO4. The linear form of the conventional Langmuir model may be written as;
Ce 1 Ce ¼ þ qm K L qm q
ð3Þ
where q is the adsorption of aresnate (mol g1), Ce is the equilibrium concentration (M), qm is the maximum adsorption capacity (mol g1) and KL is the binding energy constant (L mol1). The values of qm and KL were calculated from the slopes and intercepts of the plots of Ce /q vs. Ce (Fig. 2a) respectively. Langmuir constants and correlation coefficients (R2) are listed in Table S1. Langmuir
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model was best fitted to experimental data on the basis of good R2 values and the close agreement between theoretical and experimental values of adsorption capacity. The adsorption maxima (0.198 mmol g1) of FePO4 is comparable in magnitude with the data reported in the literature for the arsenic adsorption onto modified calcined bauxite (Bhakat et al., 2006b), activated carbon (Chuang et al., 2005), manganese wad (Mohapatra et al., 2006) and iron-oxide impregnated carbon (Maiti et al., 2010). The values of qm and KL (Table S1) increase with the increase in temperature indicating that adsorption is favorable at high temperature and the process is endothermic in nature. Similar trend in the variation of qm and KL values with temperature was reported elsewhere (Reichenberg, 1953; Naeem et al., 2009; Zhu et al., 2009). D-R model is used to distinguish between physiosorption and chemisorption and also to assess the mechanism of arsenate adsorption on ferric phosphate. This model is written as follows (Ranjan et al., 2009),
ln q ¼ ln qm bF 2
ð4Þ
1þ1 Ce
ð5Þ
F ¼ RT ln
where q, qm and Ce have their usual meanings as mentioned earlier, F is the Polanyi Potential, R is universal gas constant (8.314 J mol1 K1). The data was interpreted by the D–R model which showed a good fit to the experimental data with R2 6 0.96. The values of the constants b and qm were calculated from slope and intercept of the plot of ln q vs. F2 (Fig. 2b). The mean free energy (E) of adsorption can be calculated from the b value using the following mathematically expression;
of E lie between 8 and 16 kJ mol1 while the sorption process is physical in nature if the values of E < 8 kJ mol1 (Mohan and Pittman, 2006). In this study the values of E are found to be within the energy range of 8–16 kJ mol1 (Table S2). The values of E verify the adsorption of arsenate on ferric phosphate is chemisorption in nature. Similar findings have been reported for the biosorption of arsenic by rice polish (Ranjan et al., 2009). 3.3. Kinetic study In the present study, the effect of contact time on the detoxification of water from arsenic was studied at pH 6 and pH 8 by ferric phosphate at 298, 308, 318 and 328 K. The data plotted between the amount of arsenate adsorbed against time (t) at pH 6 and at pH 8 has been shown in Fig. 3. The curves in this figure show the rapid arsenate adsorption in the very beginning which can be attributed to the relative large number of available vacant sites on the ferric phosphate surface as compare to the later hours (Kadirvelu et al., 2005; Uysal and Ar, 2007). The adsorption curves are smooth and continuous leading to saturation which suggest the monolayer coverage of arsenate on the surface of ferric phosphate. Similar findings have been reported by Salim et al. (2007). The uptake of arsenate by ferric phosphate is time-dependent. In the initial 300 min (5 h), the adsorption of arsenate was rapid and then declined significantly as the reaction approached equilibrium. No further uptake of arsenate by ferric phosphate was observed after the equilibrium time of 960 min (16 h). The rate of arsenate adsorption onto FePO4 at all temperatures follows the trend pH 6 > pH 8. The relatively higher adsorption at pH 6 (Fig. 3a) than
(a) 16
The value of E gives important information about the ion exchange process. The adsorption type is ion exchange if the values
12
(a)
5 298 K
Ce/q (g L-1)
4
qt 105 (mol g -1)
ð6Þ
E ¼ ð2bÞ0:5
298 K
318 K 328 K
308 K
3
308 K
8
4
318 K 328 K
2
0 0
4
8
Time
1 0 0
2
4
6
8
10
(b)
12
16
10-2 (min)
1
Ce × 104 (mol L-1) 0.8
qt 104 (mol g -1)
(b) -8
Ln q
-9 298 K
-10
308 K
0.6
298 K
0.4
308 K 318 K 328 K
0.2
318 K 328 K
0 -11 250
0 350
450
550
650
750
850
5
10
Time
15
20
10-2 (min)
F2 (kJ mol-1) Fig. 2. (a) Langmuir and (b) D–R plots for arsenate adsorption by FePO4.
Fig. 3. Kinetics for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
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(a) 12 10
t/qt 10-6 (min g mol-1)
pH 8 (Fig. 3b) may be attributed to the attractive forces between the arsenate anions and the positively charged surface of ferric phosphate. Another reason may be the competition of OH ions with the arsenate anions for the same adsorption sites of FePO4 as the pH increases, which results in a decrease in sorption capacities at pH 8. The equilibrium time is observed to be same for both the pH values which reveal that the time required for the surface saturation is independent of pH. Similar observations were reported by Chen et al. (2007) for the anion-exchange sorption of arsenate by iron-impregnated carbon. The adsorption kinetic data was subjected to both the pseudo first and pseudo-second order kinetic models (Borah et al., 2008; Rao et al., 2008). However, the pseudo-first-order kinetic model was failed to explain the data because of the poor R2 values and lack of coincidence between the experimental and theoretical adsorption capacities of FePO4.
8 6 298K
4
308K 318K
2
328K
0 0
4
8
Time
12
16
10-2 (min)
(b) 24
3.4. Pseudo-second order model
t 1 1 ¼ þ ðtÞ qt k2 q2e qe 1
ð7Þ 1
1
where k2 (g mol min ) is the rate constant and qe (mol g ) is the equilibrium adsorption capacity. The values k2 and qe were calculated from intercept and slope of Fig. 4. The magnitude of k2 and the experimental and calculated values of qe are listed in Tables S3 and S4. On the basis of R2 P 0.99 and close agreement between experimental and theoretical values of the qe, the pseudo-second order equation shows a good fit to the adsorption kinetic data. Similar kind of behavior has been observed by Borah et al. (2009). The increasing trend in the rate constant (k2) values with increase in temperature is similar to that reported by Maiti et al. (2010). The comparison of adsorption capacities at pH 6 and pH 8 (Tables S3 and S4) indicate that the qe values at pH 6 are almost three times greater than at pH 8. The greater values of qe at pH 6 are consistent with the observation of Mondal et al. (2007) which suggests that arsenate adsorption is maximum in the pH range of 5–7. The increase in theoretical values of qe with the increase in temperature is typical to the chemisorption nature of the process (Chiew et al., 2009). According to Bhakat et al. (2006a) the current experimental data follows pseudo second order model, hence it is concluded that chemisorption is the possible route of arsenate adsorption onto FePO4. 3.5. Adsorption kinetics mechanism The uptake of adsorbate species from the liquid to the solid phase is carried out in three consecutive steps (Al-Degs et al., 2008). Firstly, film diffusion occurs. Secondly, particle diffusion takes place. The third step is the adsorption which is being very rapid in nature and cannot be taken into account for the rate determining step (Singh and Pant, 2006). In order to understand the rate controlling step, the experimental data was subjected to the following different models; 3.5.1. Intraparticle diffusion study (Weber and Morris model) The model of intraparticle diffusion is of great concern because it plays a significant role in the rate determining step in the equilibrium adsorption process. During the batch mode of adsorption, the transport of sorbate species into the pore of sorbent is often the rate controlling step. The rate constants of intraparticle diffusion (kid) at different temperatures were determined by using the
t/qt 10-6 (min g mol-1)
20
Pseudo-second order kinetic model (Anirudhan and Radhakrishnan, 2008) was applied to the data in its linearized form as follow,
16 12 8
298K 308K 318K
4
328K
0 0
4
8
Time
12
16
10-2 (min)
Fig. 4. Pseudo second order plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
following equation (Oladoja et al., 2008) and are depicted in Tables S5 and S6.
qt ¼ kid t0:5 þ C
ð8Þ
where kid is the intraparticle diffusion rate constant (mol g1 min0.5). The intercept (C) of the plot reflects the boundary layer effect. The larger the C values the greater is the contribution of the surface sorption in the rate controlling step i.e. the greater the boundary layer effect. If the plot of qt vs. t0.5 is linear and passes through the origin then the intraparticle diffusion is the rate-limiting step. Secondly, if the plot of qt vs. t0.5 does not pass through the origin then the intraparticle diffusion is not the sole rate-limiting step (Ahn et al., 2009). In the present study the plots of qt vs. t0.5 (Fig. 5), according to Eq. (8), do not pass through the origin, which demonstrate that intraparticle diffusion is not the only rate-limiting step. Also, the plots show an initially curved portion, which suggest film diffusion (boundary layer) and the subsequent linear portion attribute to the intraparticle diffusion (Singh and Pant, 2006). The mechanism of arsenate adsorption by ferric phosphate is still a point of conjecture and both the processes of surface adsorption and intraparticle diffusion seem to be involved in determining the rate-controlling step. The present findings are in agreement with the literature data (Al-Degs et al., 2008; Tripathy and Raichur, 2008; Amin, 2009; Xu et al., 2009). 3.5.2. Richenberg model The Richenberg model (Reichenberg, 1953) was tested to differentiate between film and intraparticle diffusion which may be written as follow,
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M. Hamayun et al. / Chemosphere 99 (2014) 207–215
(a) 18
(a)
3
qt
2
12 298 K
9
Bt
105 (mol g-1)
15
308 K 298K
1
6
308K
318 K
318K
3
328 K
328K
0
0 2
7
12
17
22
27
32
37
0
42
2
4
Time
t1/2 (min)0.5
6
8
10
10-2 (min)
(b) 0.5
(b) 10
0.3
4
318K 328K
308 K
0.1
318 K 328 K
-0.1
0 0
5
10
15
20
t1/2
25
30
35
0
40
1 6 X 1 expðm2 Bt Þ p2 m m2
ð9Þ
qt qe
ð10Þ
Rearranging the above equation gives
F values > 0:85 Bt ¼ 0:4977 lnð1 FÞ pffiffiffiffi
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!2 p2 F 3
p p
4
Time
where F is the fractional attainment of equilibrium at time t and is obtained by the following expression;
and for F values < 0:85 Bt ¼
2
(min)0.5
Fig. 5. Intraparticle diffusion plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
F¼
308K
Bt
298 K
2
F ¼1
298K
6
qt
105 (mol g-1)
8
ð11Þ
ð12Þ
where qt is the amount adsorbed at any time t (min) and Bt is a mathematical function of F and m is an integer that defines infinite series solution. The linear plots of Bt against t at different temperatures differentiate film diffusion and particle diffusion. If a plot of Bt vs. t is linear passing through the origin, then the rate limiting step is a particle diffusion, otherwise, it is governed by film diffusion. It is obvious from Fig. 6 that the plots are straight lines and do not pass through the origin. Hence, it is summarized that film diffusion is the main rate limiting step for the sorption of arsenate by ferric phosphate. Similar findings were reported for the sorption of Cu+2 from aqueous solution by cedar saw dust and crushed brick (Djeribi and Hamdaoui, 2008). However, to further probe into the mechanism of adsorption whether, it is kinetically controlled by film or pore diffusion, the
6
8
10
10-2 (min)
Fig. 6. Richenberg plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
following equations were tested to assess the values of film diffusion (D1) and pore diffusion (D2),
1=2 qt D1 ¼6 t 1=2 qe pa2
ð13Þ
D1 is calculated from the slope of the plot of qt/qe vs. t1/2 for arsenate adsorption on FePO4 as shown in the representative Fig. S6a.
! q 6 D2 p2 ln 1 t ¼ ln 2 t qe p a2
ð14Þ
D2 is calculated from the slopes of the plots of ln (1 qt/qe) vs. t for arsenate adsorption onto FePO4 as presented in the representative Fig. S6b. The values of D1 and D2 are shown in Tables S5 and S6 at different temperatures and pHs. Increase in temperature shows an increase in both D1 and D2 values at pH 6 and pH 8. The large negative exponential values of the same order of magnitude indicate that both the film and pore diffusions are involved in the adsorption mechanism. Further, since both D1 and D2 values increase with temperature which validate the increase in adsorption capacity with increase in temperature (Karthikeyan et al., 2005). 3.6. Activation energy In the present findings, Arrhenius equation has been employed to evaluate activation energy (Ea) for the arsenate sorption onto FePO4. This equation may be written as follows (Xu et al., 2009).
M. Hamayun et al. / Chemosphere 99 (2014) 207–215
(a) 7 pH 6
6
lnk2
pH 8
5
4
3 3
3.1
3.2
3.3
3.4
T-1 10 3 (K-1)
ln (k 2 /T)
(b) 2
(8.314 J mol1 K1). The values of DS# and DH# were calculated from the slope and intercept of the plots of ln (k2/T) vs. T1 (Fig. 7b). The values of DG#, DH# and DS# are given in Table S7. The magnitude of DS# describes the boundary of energy over which reactant molecules must pass as activated complexes (Rao et al., 2008). The negative DS# indicates the existence of energy barrier in the system. Similar observations have been reported in the literature (Mohapatra et al., 2009). The values of DH# are positive (Table S7) which shows that sufficient energy is needed to cross the energy barrier for the adsorption of arsenate onto FePO4. The values of DG# are found to be positive at both pH 6 and pH 8. The positive DG# values indicate the existence of energy barrier to be crossed over for the occurrence of a chemical reaction. Moreover, the magnitude of DG# increases with the increase in temperature (Table S7). It reveals that the higher temperature provides more energy to enhance the adsorption rate and the diffusion of the arsenate anions into the interior of FePO4 (Mahmood et al., 2011b).
1.5
3.8. Spectroscopic evidences
1 3
3.1
3.2
3.3
3.4
T-1 10 3 (K-1) Fig. 7. (a) Arrhenius and (b) Eyring plots for arsenate adsorption on ferric phosphate at pH 6 and pH 8 at different temperatures.
ln k2 ¼ ln A
Ea RT
ð15Þ
where A is the pre-exponential factor, R is gas constant, and T is the absolute temperature (K). Straight lines were obtained by plotting ln k2 against reciprocal of absolute temperature (T1) (Fig. 7a). The magnitude of Ea was ascertained from the slopes of Fig. 7a at pH 6 and pH 8 respectively. The values of Ea were found to be 30 and 35.52 kJ mol1 at pH 6 and pH 8, respectively (Tables S3 and S4). If the magnitude of Ea is lesser than 42 kJ mol1, diffusion control process is involved, but if the magnitude of Ea is greater than 42 kJ mol1 then chemically controlled process governs the reaction. This is because the temperature dependence of the pore diffusivity is relatively weak. Here in case of arsenate adsorption onto FePO4, the diffusion process refers to the movement of the solute to an external surface of adsorbent and not diffusivity of material along micropore wall surfaces in a particle (Al-Ghouti et al., 2005; Mahmood et al., 2011b). 3.7. Thermodynamic parameters The enthalpy of activation (DH#) and entropy of activation (DS#) were calculated using the Eyring’s Eq. (16) while, the free energy of activation (DG#) can be calculated by using Eq. (17).
k2 ln ¼ T
213
" !# ln kB DS# DH # þ hp R RT
DG# ¼ DH# T DS#
The spectroscopic evidences in respect of the present adsorption studies include; (i) The appearance of a new band in the FTIR spectrum of the FePO4 after arsenic adsorption is shown in Fig. S2b. This new band in the region of 820–860 cm1 was assigned to the arsenate anions as was also observed by Mondal et al. (2008) while studying the arsenic removal from simulated ground water by Fe+3 impregnated activated carbon. (ii) SEM of the FePO4 after arsenate adsorption (Fig. S3b) shows low porosity which also confirms the sorption of arsenate onto FePO4. (iii) Moreover, the appearance of a new arsenic peak in the EDX spectrum of FePO4 after adsorption (Fig. S4b) also verifies the adsorption of arsenate onto the solid phase. Similar findings were reported by Haron et al. (2008) for sorption of arsenic by cerium-exchanged zeolite. 4. Conclusion This work demonstrated that FePO4 is a viable material for the treatment of arsenic containing aqueous solution. Adsorption of arsenic on FePO4 was favorable in lower pH range at higher temperature. Arsenic adsorption on ferric phosphate was found to increase with increase in temperature. The adsorption kinetics data followed pseudo-second-order kinetic model suggesting chemisorption nature of the process. Intraparticle diffusion model confirmed that intraparticle is not a fully operative mechanism, whereas the Richenberg model suggested film diffusion to be the rate limiting step. The value of Ea was found to be in the range of chemisorption process. The thermodynamic parameter showed that DH# is the main controlling factor for governing the adsorption reaction. Spectroscopic studies confirmed that anion exchange process is responsible for the uptake of arsenate onto FePO4. Appendix A. Supplementary material
ð16Þ
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2 013.10.075.
ð17Þ
References
As defined earlier k2 is the pseudo second order rate constant (g mol1 min1), kB is the Boltzmann constant (1.3807 1023 J K1), hp is the Plank’s constant (6.6261 1034 J s), T is the absolute temperature (K) and R is the general gas constant
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Equilibrium and kinetics studies of arsenate adsorption by FePO4 M. Hamayun 1, T. Mahmood 1, A. Naeem ⇑, M. Muska 1, S.U. Din 1, M. Waseem 1 National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar, Pakistan
h i g h l i g h t s FePO4 is a potential adsorbent for anion-exchange removal of arsenate. Adsorption mechanism involves the exchange of arsenate with OH ions of surface. Both film diffusion and pore diffusion are involved in kinetics adsorption mechanism. FTIR and EDX studies validate the adsorption of arsenate by FePO4.
a r t i c l e
i n f o
Article history: Received 15 July 2013 Received in revised form 12 October 2013 Accepted 14 October 2013 Available online 23 November 2013 Keywords: Adsorption Arsenic Sorption maxima Kinetics Ferric phosphate
a b s t r a c t The present work is focusing on removal of arsenate from aqueous solution using FePO4. The equilibrium study regarding the removal of arsenic by FePO4 was carried out at 298, 308, 318 and 328 K. Langmuir parameters were found to increase with the increase in temperature indicating that the adsorption is favorable at high temperature. Kinetic study of arsenate adsorption on FePO4 was also carried out at different temperatures and at pH 6 and 8. Different kinetic models were used to the kinetic data amongst which pseudo second order model was best fitted. The mechanism of the adsorption kinetics was investigated by employing intraparticle diffusion and Richenberg models. The energy of activation (Ea) was found to be 30 and 35.52 kJ mol1 at pH 6 and pH 8, respectively, suggesting chemisorption nature of the adsorption process. The negative entropic values of activation signified the existence of entropy barrier while the positive DG# values indicated the existence of energy barrier to be crossed over for the occurrence of a chemical reaction. Both the spectroscopic studies and increase in equilibrium pH reveal the anion exchange removal of arsenate from aqueous solution to the solid surface. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Soils with high concentration of heavy metals affect ground water, surrounding ecosystem, human health and agriculture productivity (Houben et al., 2013). Arsenic is a metalloid which is a dangerous water pollutant throughout the world. Arsenic is mobilized by natural weathering reactions, biological activity, geochemical reactions, volcanic emissions and other anthropogenic activities. Soil erosion and leaching contribute to 612 108 and 2380 108 g year1 arsenic, respectively, in dissolved and suspended forms in the oceans (Mohan and Pittman, 2007). Most environmental arsenic problems are the result of mobilization under natural conditions. However, mining activities, combustion of fossil fuels, use of arsenic pesticides, herbicides, crop desiccants and use of arsenic additives to livestock feed are the other sources of arsenic. It occurs in natural waters in both inorganic and organic forms, such as monomethyl arsenic acid (MMAA), dimethyl arsenic ⇑ Corresponding author. Tel./fax: +92 91 9216766. 1
E-mail address: [email protected] (A. Naeem). Fax: +92 91 9216766.
0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.075
acid (DMAA), and arseno-sugars. The inorganic form of arsenic is more toxic than the organic form. Arsenic usually occurs in two valence states (Zhang et al., 2013), arsenite [As (III)] and arsenate [As (V)]. In natural waters, arsenite species primarily consist of arsenious acid (H3AsO3), while arsenate species consist of H2AsO1 4 and HAsO2 4 (Jeong et al., 2007; Mohan and Pittman, 2007). The use of arsenic contaminated water for a long period gives rise to serious diseases in the human beings. The chronic health effects associated with arsenic include cancer of lung, bladder, liver, kidney, skin pigmentation, nerve tissue injuries and cardiovascular diseases (Ansone et al., 2013). Several countries like Bangladesh, Taiwan, Argentina, Mexico, Chile, Mongolia, China, Hungary, Thailand, USA, Germany, New Zealand, South Africa, Pakistan and India are at the risk due to arsenic contamination of water (Manjare et al., 2005). Considering its health and toxicological effects, many regulatory agencies have revised the maximum contaminant level (MCL) for arsenic in drinking water from 0.05 to 0.01 mg L1 (Gregor, 2001). The US Environmental Protection Agency recommends that public water systems must comply with the standard of 10 lg L1 for arsenic instead of 50 lg L1 used for years (Yang et al., 2007; Gupta et al., 2012).
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Several techniques such as coagulation/precipitation, reverse osmosis, ion exchange, and adsorption effectively lower arsenic concentration in aqueous solutions. Some of these techniques may produce large amounts of chemical sludge which need further treatment before being disposed of. Amongst the above mentioned technologies adsorption has played a significant role in removing the pollutants from the aqueous system. It has been observed that arsenate is well adsorbed onto the surface of minerals (Payne and Abdel-Fattah, 2005). Several researchers have used different sorbents for the uptake of arsenate from the aqueous phase which include, arbuscular mycorrhizal fungal (Leung et al., 2013), oyster shell (Rahman et al., 2008), activated carbon (Chuang et al., 2005), iron-treated activated carbon and zeolites (Payne and Abdel-Fattah, 2005), iron-imended bio sand filters (Chiew et al., 2009), iron-sulfide coated sand (Han et al., 2013), ferrihydrite-coated sand (Mähler and Persson, 2013), binary mixed oxide of iron and silicon (Mahmood et al., 2012), mesoporous alumina (Kim et al., 2004), granular titanium dioxide (Bang et al., 2005), chelating resin (Balaji et al., 2005), and mixed valent iron oxides (Mishra and Farrell, 2005). Iron containing compounds have been thoroughly studied for the arsenic removal from water because of their greater affinity for the arsenic adsorption (Gupta et al., 2012). Like other iron bearing compounds, iron (III) phosphate is insoluble solid material which exists in various amorphous, crystalline, and intermediate state of crystallinity (Mustafa et al., 1999). Iron (III) phosphate possesses pH-dependent surface charge properties. It is considered an important sink for arsenate adsorption from the aqueous system. It is inexpensive and convenient to use. However, literature regarding arsenate adsorption by iron phosphate has been ignored. Hence in the present investigations, an attempt was made to test FePO4 for the arsenate adsorption from the aqueous solution. In the current study adsorption equilibrium experiments were conducted at 298, 308, 318 and 328 K which also correspond to the seasonal temperature changes in the Indian subcontinent where arsenic contamination is a serious problem of concern. This study also gives a comprehensive account of the adsorption kinetics at different temperatures, which is an essential parameter in the adsorption process because it gives an insight of the time span which is required for the complete saturation of the adsorbent surface. 2. Experimental 2.1. Reagents and equipments All the reagents used in this study were of analytical grade. Iron (III) nitrate nanohydrate was provided by Scharlau, while arsenic standard solution and ortho phosphoric acid were purchased from Merck. Stock solutions of test reagents were prepared in a milli Q water. All the glassware and sample bottles were soaked in 10% HNO3 and then rinsed with milli Q water. 2.2. Preparation of FePO4 FePO4 was precipitated by mixing trisodium phosphate with solution of ferric nitrate according to the following reaction (Naeem et al., 2007),
FeðNO3 Þ3 þ Na3 PO4 ! FePO4 þ 3NaNO3
ð1Þ
The mixture was equilibrated for a period of 1 h at 40 °C with continuous stirring. The pH of the system was maintained at pH 6 by adding required concentration of HNO3 and NaOH solution. Thick yellow precipitate was formed which was then subjected to washing by dialyzing with the Mili Q water. The precipitate was filtered and washed until filtrate was free of nitrate anions. The sample thus prepared was air dried and then oven dried at 110 °C
for 24 h (Mustafa et al., 1999). The dried sample was ground and passed through a mesh size of 200 and stored in a sealed reagent bottle for further investigation. 2.3. Characterization The point of zero charge (PZC) was determined using 0.1 M NaCl solution as a background electrolyte. Forty milliliters of 0.1 M NaCl was taken in different flasks and the pH of solutions were adjusted in the pH range of 2–12 by addition of 0.1 M HNO3/NaOH solutions. Ferric phosphate (0.1 g) was added to each flask and these flasks were placed in shaker bath for 24 h at 298 K and the final pH of the suspension was noted. PZC of the sample was determined by plotting the difference between initial and final pH vs. initial pH (Mullet et al., 1999; Mahmood et al., 2011a). Air dried samples of FePO4 was mixed with KBr crystals and the mixture ground to fine powder, using agate mortar. The resulting powder was passed into a disc under vacuum. Fourier transform infrared (FTIR) spectra of the FePO4 samples before and after arsenate adsorption were recorded using SHIMADZU 8201PC, FTIR spectrophotometer (Lakshmipathiraj et al., 2006). The Bruner–Emmett–Teller (BET) surface area of FePO4 was determined using Quantachrome NOVA 2200e surface area analyzer in the presence of nitrogen as purging gas. The degassing of the sample was carried out by subjecting the sample to a preliminary heating at 473 K for 2 h and then the surface area of FePO4 was calculated (Waseem et al., 2013). The surface morphology of the sample was analyzed through scanning electron micrograph (SEM) model JSM 5910 (JEOL Japan) equipped with an energy dispersive X-ray (EDX) micro analyzer model INCA 200 (UK) at 20 keV. The SEM pictures and EDX spectra of FePO4 before and after arsenate adsorption were analyzed. Thermogrvimetric analysis (TGA) and differential thermogrvimetric (DTG) analysis of the FePO4 were carried out using TG/DTA analyzer, Perkin Elmer model 6300. The sample of FePO4 was heated up to 1000 °C at a heating rate of 5 °C min1 (Yang et al., 2013). 2.4. Adsorption equilibrium and kinetic study The equilibrium studies of arsenate sorption onto FePO4 were conducted at four different temperatures (298, 308, 318, and 328 K) according to the method given in the literature (Naeem et al., 2002; Das et al., 2013). The adsorption study was launched by varying the arsenic concentration in the range of 10–100 mg L1 at pH 6. FePO4 (0.1 g) was added to a reaction vessel containing 40 mL of arsenate solution. The pH of solution was adjusted to pH 6 by using HNO3/NaOH solutions. The reaction vessels were then equilibrated in a shaking bath at 120 rpm for a period of 24 h. The equilibrium pH of the suspension was noted which were filtered and analyzed for the determination of arsenate in the filtrate. The amount of arsenate in the filtrate was determined using atomic absorption spectrometer AAnalyst 800 (Perkin Elmer). Some controlled adsorption experiments in the absence of FePO4 were also conducted. The kinetic study was carried out in the same way as the adsorption equilibrium studies at pH 6 and at pH 8. In kinetic studies, the solution of specified flask was filtered at a pre-determined interval of time which was analyzed for arsenate by atomic absorption spectrophotometer. 3. Results and discussions 3.1. Characterization The PZC of FePO4 determined by the salt addition method was found to be 6.3 (Fig. S1) which is comparable in magnitude to
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3.2. Equilibrium study The adsorption equilibrium study for the removal of arsenic by FePO4 was conducted at 298, 308, 318 and 328 K by varying the arsenic concentration in the range of 10–100 mg L1. Most of the surface, ground and industrial waste water bears different temperature and different concentration level of arsenate, therefore the above wide range of temperature and concentration has been chosen for the present findings to match with the natural water system. The adsorption capacity of FePO4 increases with increase in initial arsenic concentration (Fig. 1a). The increase in adsorption capacity with increase in arsenic concentration is an outcome of the increase in driving force because of the concentration gradient build up between the surface of FePO4 and arsenate solution (Kumar et al., 2012). It is also important to mention here that the % removal of arsenate decreases with the increase in the initial arsenate concentration (Fig. 1a). This is attributed to the fixed quantity of the FePO4 being used during the course of this investigation. The uptake of arsenate by FePO4 increases with the increase in temperature which indicates the endothermic behavior of the adsorption reaction (Fig. 1b). Different reasons for the efficient adsorption at high temperature have been mentioned in the literature. Wu et al. (2008) suggested that the increase in adsorption with temperature was due to the increase in the rate of intraparticle of metal ions into the pores of adsorbent at higher temperature. Al-Degs et al. (2008) were of the opinion that the increase in adsorption with temperature is due to the increased penetration of adsorbate inside the micropores of the surface or the creation of new active sites. The increase in the level of arsenic sorption with temperature in the present case may be due to increase in the activity of the sorption sites on the adsorbent surface as well as enlargement of the pores as is also suggested by Borah et al.
2
80
1.5
60
1
40
q x 104 (mol g-1) %removal
0.5
% removal
100
q
104 (mol g-1)
(a) 2.5
20
0
0 0
1.5
3
4.5
6
7.5
9
Ce 104 (mol L-1)
(b) 2.5 104 (mol g-1)
2.1 1.7 298K 308K 318K 328K
1.3
q
the reported value of PZC (5.1) for AlPO4 (Mustafa et al., 2006). The functional groups on the surface of adsorbent were investigated by FTIR spectra. FTIR spectra of FePO4 are given in Fig. S2. The spectra seem to be simple and all the peaks can be interpreted easily. Band in the range of 900–1200 cm1 is assigned to P–O stretching vibration. Similarly, the band at 1600–1700 cm1 can be assigned to O– H bending whereas a broad band in the range of 3000–3500 cm1 is attributed to O–H stretching vibrations (Mustafa et al., 1999; Biswas et al., 2009; Muniz et al., 2009). The BET surface area of FePO4 was observed to be 80 m2 g1 which is exactly the same as was observed for potassium titanium oxophosphate (Biswas et al., 2009) and is greater than AlPO4 (48 m2 g1), FePO4 (60 m2 g1) and CrPO4 (57 m2 g1) reported by Naeem et al. (2002) and Mustafa et al. (1999). The SEM images of FePO4 are displayed in Fig. S3. It can be seen that the surface of pure FePO4 is porous in nature (Fig. S3a). The adsorbent particles are non homogeneous in respect of size and shape. The EDX results of FePO4 before and after arsenate adsorption are presented in Fig. S4. It is obvious from Fig. S4a that the pure sample of FePO4 has significant amount of iron in addition to phosphorus and oxygen. TGA of FePO4 was carried out up to 1000 °C. The weight loss was observed to take place in three steps (Fig. S5). The first weight loss is computed to be 1.85% at 41.44 °C and the second weight loss (15.18%) occurs at 153.55 °C. These two weight losses in the temperature range 41–154 °C may be attributed to the loss of physiosorbed water. The above mentioned two weight losses are abrupt while a third slow weight loss in temperature range 150–300 °C is most probably due to the loss of interstitial water. Moreover, the DTG curve further augments the conclusions drawn from the TGA curve.
0.9 0.5 0
2
4
6
8
Ce 104 (mol L-1) Fig. 1. Effect of (a) initial arsenate concentration and (b) temperature on the sorption of arsenate at pH 6 by FePO4.
(2009) for the sorption of arsenic (V) from aqueous solution by acid modified carbon black. Moreover, the uptake of arsenate onto FePO4 was accompanied by the increase in final pH of the system. This indicates that the OH ion is released from the solid surface into the aqueous phase according to the following mechanism.
ð2Þ
The Langmuir, Dubinin–Radushkevich (D–R) and Freundlich models were tested to the experimental data. The Freundlich model was excluded because of its poor correlation coefficient (R2) and lack of agreement between the experimental and theoretical values of adsorption capacity. Therefore, equilibrium adsorption data collected at different temperatures was explained with the help of Langmuir and Dubinin–Radushkevich models. Both the models were tested to probe into the mechanism of arsenic sorption onto FePO4. The linear form of the conventional Langmuir model may be written as;
Ce 1 Ce ¼ þ qm K L qm q
ð3Þ
where q is the adsorption of aresnate (mol g1), Ce is the equilibrium concentration (M), qm is the maximum adsorption capacity (mol g1) and KL is the binding energy constant (L mol1). The values of qm and KL were calculated from the slopes and intercepts of the plots of Ce /q vs. Ce (Fig. 2a) respectively. Langmuir constants and correlation coefficients (R2) are listed in Table S1. Langmuir
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M. Hamayun et al. / Chemosphere 99 (2014) 207–215
model was best fitted to experimental data on the basis of good R2 values and the close agreement between theoretical and experimental values of adsorption capacity. The adsorption maxima (0.198 mmol g1) of FePO4 is comparable in magnitude with the data reported in the literature for the arsenic adsorption onto modified calcined bauxite (Bhakat et al., 2006b), activated carbon (Chuang et al., 2005), manganese wad (Mohapatra et al., 2006) and iron-oxide impregnated carbon (Maiti et al., 2010). The values of qm and KL (Table S1) increase with the increase in temperature indicating that adsorption is favorable at high temperature and the process is endothermic in nature. Similar trend in the variation of qm and KL values with temperature was reported elsewhere (Reichenberg, 1953; Naeem et al., 2009; Zhu et al., 2009). D-R model is used to distinguish between physiosorption and chemisorption and also to assess the mechanism of arsenate adsorption on ferric phosphate. This model is written as follows (Ranjan et al., 2009),
ln q ¼ ln qm bF 2
ð4Þ
1þ1 Ce
ð5Þ
F ¼ RT ln
where q, qm and Ce have their usual meanings as mentioned earlier, F is the Polanyi Potential, R is universal gas constant (8.314 J mol1 K1). The data was interpreted by the D–R model which showed a good fit to the experimental data with R2 6 0.96. The values of the constants b and qm were calculated from slope and intercept of the plot of ln q vs. F2 (Fig. 2b). The mean free energy (E) of adsorption can be calculated from the b value using the following mathematically expression;
of E lie between 8 and 16 kJ mol1 while the sorption process is physical in nature if the values of E < 8 kJ mol1 (Mohan and Pittman, 2006). In this study the values of E are found to be within the energy range of 8–16 kJ mol1 (Table S2). The values of E verify the adsorption of arsenate on ferric phosphate is chemisorption in nature. Similar findings have been reported for the biosorption of arsenic by rice polish (Ranjan et al., 2009). 3.3. Kinetic study In the present study, the effect of contact time on the detoxification of water from arsenic was studied at pH 6 and pH 8 by ferric phosphate at 298, 308, 318 and 328 K. The data plotted between the amount of arsenate adsorbed against time (t) at pH 6 and at pH 8 has been shown in Fig. 3. The curves in this figure show the rapid arsenate adsorption in the very beginning which can be attributed to the relative large number of available vacant sites on the ferric phosphate surface as compare to the later hours (Kadirvelu et al., 2005; Uysal and Ar, 2007). The adsorption curves are smooth and continuous leading to saturation which suggest the monolayer coverage of arsenate on the surface of ferric phosphate. Similar findings have been reported by Salim et al. (2007). The uptake of arsenate by ferric phosphate is time-dependent. In the initial 300 min (5 h), the adsorption of arsenate was rapid and then declined significantly as the reaction approached equilibrium. No further uptake of arsenate by ferric phosphate was observed after the equilibrium time of 960 min (16 h). The rate of arsenate adsorption onto FePO4 at all temperatures follows the trend pH 6 > pH 8. The relatively higher adsorption at pH 6 (Fig. 3a) than
(a) 16
The value of E gives important information about the ion exchange process. The adsorption type is ion exchange if the values
12
(a)
5 298 K
Ce/q (g L-1)
4
qt 105 (mol g -1)
ð6Þ
E ¼ ð2bÞ0:5
298 K
318 K 328 K
308 K
3
308 K
8
4
318 K 328 K
2
0 0
4
8
Time
1 0 0
2
4
6
8
10
(b)
12
16
10-2 (min)
1
Ce × 104 (mol L-1) 0.8
qt 104 (mol g -1)
(b) -8
Ln q
-9 298 K
-10
308 K
0.6
298 K
0.4
308 K 318 K 328 K
0.2
318 K 328 K
0 -11 250
0 350
450
550
650
750
850
5
10
Time
15
20
10-2 (min)
F2 (kJ mol-1) Fig. 2. (a) Langmuir and (b) D–R plots for arsenate adsorption by FePO4.
Fig. 3. Kinetics for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
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M. Hamayun et al. / Chemosphere 99 (2014) 207–215
(a) 12 10
t/qt 10-6 (min g mol-1)
pH 8 (Fig. 3b) may be attributed to the attractive forces between the arsenate anions and the positively charged surface of ferric phosphate. Another reason may be the competition of OH ions with the arsenate anions for the same adsorption sites of FePO4 as the pH increases, which results in a decrease in sorption capacities at pH 8. The equilibrium time is observed to be same for both the pH values which reveal that the time required for the surface saturation is independent of pH. Similar observations were reported by Chen et al. (2007) for the anion-exchange sorption of arsenate by iron-impregnated carbon. The adsorption kinetic data was subjected to both the pseudo first and pseudo-second order kinetic models (Borah et al., 2008; Rao et al., 2008). However, the pseudo-first-order kinetic model was failed to explain the data because of the poor R2 values and lack of coincidence between the experimental and theoretical adsorption capacities of FePO4.
8 6 298K
4
308K 318K
2
328K
0 0
4
8
Time
12
16
10-2 (min)
(b) 24
3.4. Pseudo-second order model
t 1 1 ¼ þ ðtÞ qt k2 q2e qe 1
ð7Þ 1
1
where k2 (g mol min ) is the rate constant and qe (mol g ) is the equilibrium adsorption capacity. The values k2 and qe were calculated from intercept and slope of Fig. 4. The magnitude of k2 and the experimental and calculated values of qe are listed in Tables S3 and S4. On the basis of R2 P 0.99 and close agreement between experimental and theoretical values of the qe, the pseudo-second order equation shows a good fit to the adsorption kinetic data. Similar kind of behavior has been observed by Borah et al. (2009). The increasing trend in the rate constant (k2) values with increase in temperature is similar to that reported by Maiti et al. (2010). The comparison of adsorption capacities at pH 6 and pH 8 (Tables S3 and S4) indicate that the qe values at pH 6 are almost three times greater than at pH 8. The greater values of qe at pH 6 are consistent with the observation of Mondal et al. (2007) which suggests that arsenate adsorption is maximum in the pH range of 5–7. The increase in theoretical values of qe with the increase in temperature is typical to the chemisorption nature of the process (Chiew et al., 2009). According to Bhakat et al. (2006a) the current experimental data follows pseudo second order model, hence it is concluded that chemisorption is the possible route of arsenate adsorption onto FePO4. 3.5. Adsorption kinetics mechanism The uptake of adsorbate species from the liquid to the solid phase is carried out in three consecutive steps (Al-Degs et al., 2008). Firstly, film diffusion occurs. Secondly, particle diffusion takes place. The third step is the adsorption which is being very rapid in nature and cannot be taken into account for the rate determining step (Singh and Pant, 2006). In order to understand the rate controlling step, the experimental data was subjected to the following different models; 3.5.1. Intraparticle diffusion study (Weber and Morris model) The model of intraparticle diffusion is of great concern because it plays a significant role in the rate determining step in the equilibrium adsorption process. During the batch mode of adsorption, the transport of sorbate species into the pore of sorbent is often the rate controlling step. The rate constants of intraparticle diffusion (kid) at different temperatures were determined by using the
t/qt 10-6 (min g mol-1)
20
Pseudo-second order kinetic model (Anirudhan and Radhakrishnan, 2008) was applied to the data in its linearized form as follow,
16 12 8
298K 308K 318K
4
328K
0 0
4
8
Time
12
16
10-2 (min)
Fig. 4. Pseudo second order plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
following equation (Oladoja et al., 2008) and are depicted in Tables S5 and S6.
qt ¼ kid t0:5 þ C
ð8Þ
where kid is the intraparticle diffusion rate constant (mol g1 min0.5). The intercept (C) of the plot reflects the boundary layer effect. The larger the C values the greater is the contribution of the surface sorption in the rate controlling step i.e. the greater the boundary layer effect. If the plot of qt vs. t0.5 is linear and passes through the origin then the intraparticle diffusion is the rate-limiting step. Secondly, if the plot of qt vs. t0.5 does not pass through the origin then the intraparticle diffusion is not the sole rate-limiting step (Ahn et al., 2009). In the present study the plots of qt vs. t0.5 (Fig. 5), according to Eq. (8), do not pass through the origin, which demonstrate that intraparticle diffusion is not the only rate-limiting step. Also, the plots show an initially curved portion, which suggest film diffusion (boundary layer) and the subsequent linear portion attribute to the intraparticle diffusion (Singh and Pant, 2006). The mechanism of arsenate adsorption by ferric phosphate is still a point of conjecture and both the processes of surface adsorption and intraparticle diffusion seem to be involved in determining the rate-controlling step. The present findings are in agreement with the literature data (Al-Degs et al., 2008; Tripathy and Raichur, 2008; Amin, 2009; Xu et al., 2009). 3.5.2. Richenberg model The Richenberg model (Reichenberg, 1953) was tested to differentiate between film and intraparticle diffusion which may be written as follow,
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M. Hamayun et al. / Chemosphere 99 (2014) 207–215
(a) 18
(a)
3
qt
2
12 298 K
9
Bt
105 (mol g-1)
15
308 K 298K
1
6
308K
318 K
318K
3
328 K
328K
0
0 2
7
12
17
22
27
32
37
0
42
2
4
Time
t1/2 (min)0.5
6
8
10
10-2 (min)
(b) 0.5
(b) 10
0.3
4
318K 328K
308 K
0.1
318 K 328 K
-0.1
0 0
5
10
15
20
t1/2
25
30
35
0
40
1 6 X 1 expðm2 Bt Þ p2 m m2
ð9Þ
qt qe
ð10Þ
Rearranging the above equation gives
F values > 0:85 Bt ¼ 0:4977 lnð1 FÞ pffiffiffiffi
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!2 p2 F 3
p p
4
Time
where F is the fractional attainment of equilibrium at time t and is obtained by the following expression;
and for F values < 0:85 Bt ¼
2
(min)0.5
Fig. 5. Intraparticle diffusion plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
F¼
308K
Bt
298 K
2
F ¼1
298K
6
qt
105 (mol g-1)
8
ð11Þ
ð12Þ
where qt is the amount adsorbed at any time t (min) and Bt is a mathematical function of F and m is an integer that defines infinite series solution. The linear plots of Bt against t at different temperatures differentiate film diffusion and particle diffusion. If a plot of Bt vs. t is linear passing through the origin, then the rate limiting step is a particle diffusion, otherwise, it is governed by film diffusion. It is obvious from Fig. 6 that the plots are straight lines and do not pass through the origin. Hence, it is summarized that film diffusion is the main rate limiting step for the sorption of arsenate by ferric phosphate. Similar findings were reported for the sorption of Cu+2 from aqueous solution by cedar saw dust and crushed brick (Djeribi and Hamdaoui, 2008). However, to further probe into the mechanism of adsorption whether, it is kinetically controlled by film or pore diffusion, the
6
8
10
10-2 (min)
Fig. 6. Richenberg plot for arsenate adsorption on ferric phosphate at (a) pH 6 and (b) pH 8 at different temperatures.
following equations were tested to assess the values of film diffusion (D1) and pore diffusion (D2),
1=2 qt D1 ¼6 t 1=2 qe pa2
ð13Þ
D1 is calculated from the slope of the plot of qt/qe vs. t1/2 for arsenate adsorption on FePO4 as shown in the representative Fig. S6a.
! q 6 D2 p2 ln 1 t ¼ ln 2 t qe p a2
ð14Þ
D2 is calculated from the slopes of the plots of ln (1 qt/qe) vs. t for arsenate adsorption onto FePO4 as presented in the representative Fig. S6b. The values of D1 and D2 are shown in Tables S5 and S6 at different temperatures and pHs. Increase in temperature shows an increase in both D1 and D2 values at pH 6 and pH 8. The large negative exponential values of the same order of magnitude indicate that both the film and pore diffusions are involved in the adsorption mechanism. Further, since both D1 and D2 values increase with temperature which validate the increase in adsorption capacity with increase in temperature (Karthikeyan et al., 2005). 3.6. Activation energy In the present findings, Arrhenius equation has been employed to evaluate activation energy (Ea) for the arsenate sorption onto FePO4. This equation may be written as follows (Xu et al., 2009).
M. Hamayun et al. / Chemosphere 99 (2014) 207–215
(a) 7 pH 6
6
lnk2
pH 8
5
4
3 3
3.1
3.2
3.3
3.4
T-1 10 3 (K-1)
ln (k 2 /T)
(b) 2
(8.314 J mol1 K1). The values of DS# and DH# were calculated from the slope and intercept of the plots of ln (k2/T) vs. T1 (Fig. 7b). The values of DG#, DH# and DS# are given in Table S7. The magnitude of DS# describes the boundary of energy over which reactant molecules must pass as activated complexes (Rao et al., 2008). The negative DS# indicates the existence of energy barrier in the system. Similar observations have been reported in the literature (Mohapatra et al., 2009). The values of DH# are positive (Table S7) which shows that sufficient energy is needed to cross the energy barrier for the adsorption of arsenate onto FePO4. The values of DG# are found to be positive at both pH 6 and pH 8. The positive DG# values indicate the existence of energy barrier to be crossed over for the occurrence of a chemical reaction. Moreover, the magnitude of DG# increases with the increase in temperature (Table S7). It reveals that the higher temperature provides more energy to enhance the adsorption rate and the diffusion of the arsenate anions into the interior of FePO4 (Mahmood et al., 2011b).
1.5
3.8. Spectroscopic evidences
1 3
3.1
3.2
3.3
3.4
T-1 10 3 (K-1) Fig. 7. (a) Arrhenius and (b) Eyring plots for arsenate adsorption on ferric phosphate at pH 6 and pH 8 at different temperatures.
ln k2 ¼ ln A
Ea RT
ð15Þ
where A is the pre-exponential factor, R is gas constant, and T is the absolute temperature (K). Straight lines were obtained by plotting ln k2 against reciprocal of absolute temperature (T1) (Fig. 7a). The magnitude of Ea was ascertained from the slopes of Fig. 7a at pH 6 and pH 8 respectively. The values of Ea were found to be 30 and 35.52 kJ mol1 at pH 6 and pH 8, respectively (Tables S3 and S4). If the magnitude of Ea is lesser than 42 kJ mol1, diffusion control process is involved, but if the magnitude of Ea is greater than 42 kJ mol1 then chemically controlled process governs the reaction. This is because the temperature dependence of the pore diffusivity is relatively weak. Here in case of arsenate adsorption onto FePO4, the diffusion process refers to the movement of the solute to an external surface of adsorbent and not diffusivity of material along micropore wall surfaces in a particle (Al-Ghouti et al., 2005; Mahmood et al., 2011b). 3.7. Thermodynamic parameters The enthalpy of activation (DH#) and entropy of activation (DS#) were calculated using the Eyring’s Eq. (16) while, the free energy of activation (DG#) can be calculated by using Eq. (17).
k2 ln ¼ T
213
" !# ln kB DS# DH # þ hp R RT
DG# ¼ DH# T DS#
The spectroscopic evidences in respect of the present adsorption studies include; (i) The appearance of a new band in the FTIR spectrum of the FePO4 after arsenic adsorption is shown in Fig. S2b. This new band in the region of 820–860 cm1 was assigned to the arsenate anions as was also observed by Mondal et al. (2008) while studying the arsenic removal from simulated ground water by Fe+3 impregnated activated carbon. (ii) SEM of the FePO4 after arsenate adsorption (Fig. S3b) shows low porosity which also confirms the sorption of arsenate onto FePO4. (iii) Moreover, the appearance of a new arsenic peak in the EDX spectrum of FePO4 after adsorption (Fig. S4b) also verifies the adsorption of arsenate onto the solid phase. Similar findings were reported by Haron et al. (2008) for sorption of arsenic by cerium-exchanged zeolite. 4. Conclusion This work demonstrated that FePO4 is a viable material for the treatment of arsenic containing aqueous solution. Adsorption of arsenic on FePO4 was favorable in lower pH range at higher temperature. Arsenic adsorption on ferric phosphate was found to increase with increase in temperature. The adsorption kinetics data followed pseudo-second-order kinetic model suggesting chemisorption nature of the process. Intraparticle diffusion model confirmed that intraparticle is not a fully operative mechanism, whereas the Richenberg model suggested film diffusion to be the rate limiting step. The value of Ea was found to be in the range of chemisorption process. The thermodynamic parameter showed that DH# is the main controlling factor for governing the adsorption reaction. Spectroscopic studies confirmed that anion exchange process is responsible for the uptake of arsenate onto FePO4. Appendix A. Supplementary material
ð16Þ
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2 013.10.075.
ð17Þ
References
As defined earlier k2 is the pseudo second order rate constant (g mol1 min1), kB is the Boltzmann constant (1.3807 1023 J K1), hp is the Plank’s constant (6.6261 1034 J s), T is the absolute temperature (K) and R is the general gas constant
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