- Email: [email protected]
Arsenic removal from contaminated water by ultrafine δ-FeOOH adsorbents
Chemical Engineering Journal 237 (2014) 47–54
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Arsenic removal from contaminated water by ultrafine d-FeOOH adsorbents Márcia C.S. Faria a, Renedy S. Rosemberg b, Cleide A. Bomfeti b, Douglas S. Monteiro b, Fernando Barbosa a, Luiz C.A. Oliveira c, Mariandry Rodriguez b, Márcio C. Pereira b, Jairo L. Rodrigues b,⇑ a b c
Departamento de Análises Clínicas, Toxicológicas e Bromatológicas, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, USP, 14040-903 Ribeirão Preto, São Paulo, Brazil Instituto de Ciência, Engenharia e Tecnologia, Universidade Federal dos Vales Jequitinhonha e Mucuri, Teófilo Otoni 39803-371, Minas Gerais, Brazil Departamento de Química, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil
h i g h l i g h t s Efficient d-FeOOH adsorbents were prepared by a simple and cheap method. d-FeOOH nanoparticles exhibited high adsorption capacity for arsenic. Arsenic was better adsorbed by d-FeOOH nanoparticles at acidic pH. d-FeOOH is a promising adsorbent for heavy metal in contaminated water.
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 3 October 2013 Accepted 5 October 2013 Available online 12 October 2013 Keywords: Adsorption Iron oxides Environmental remediation Nanoparticles
a b s t r a c t d-FeOOH nanoparticles were prepared by a fast, simple and cheap synthesis method for use as an adsorbent for As(V) in water. Rietveld refinement on XRD pattern confirmed that d-FeOOH was successful synthesized. TEM images evidenced that the average particle sizes for d-FeOOH is 20 nm, which provided a high surface area of 135 m2 g1 and average pore sizes of 18 nm, as verified with BET measurements. Zeta potential revealed that the point of zero charge of d-FeOOH is 8.4, which favored the As(V) adsorption on the d-FeOOH surface even at neutral pH. The As(V) adsorption capacity of d-FeOOH was estimated to be 37.3 mg g1 at pH 7. The kinetics data were best fitted with a pseudo-second order, thus suggesting chemical adsorption on the surface and pores of d-FeOOH nanoparticles. The interaction between As(V) and d-FeOOH nanoparticles was suggested to be mainly inner sphere complexes. The adsorption isotherm obtained at pH 7 was best fitted to the Langmuir and Redlich–Peterson models and, therefore, a non-ideal monolayer adsorption model for As(V) on d-FeOOH nanoparticles was proposed. The small particle size, high surface area and adsorption capacity make d-FeOOH a promising adsorbent for toxic metals in contaminated water. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Arsenic contamination in natural water is a worldwide problem because of its known toxicity and health hazards. The US Environmental Protection Agency (USEPA) has classified arsenic as a Class ‘A’ human carcinogenic and because of the serious health problems caused by arsenic, in 2001, it reduced the maximum arsenic content in drinking water from 50 lg L1 to 10 lg L1 [1]. Thus, several technologies such as oxidation and filtration [2], biological oxidation followed by removal for iron and manganese oxides [3,4], co-precipitation followed by coagulation [5], sedimentation and filtration [6], ion exchange through suitable cation and
⇑ Corresponding author. Tel./fax: +55 33 3522 6037. E-mail address: [email protected] (J.L. Rodrigues).
anion exchange resins [7], adsorption [8–10] and membrane technology including reverse osmosis [11], nanofiltration [12] and electrodialysis [13] have been used to remove arsenic from water. Among the technologies to remove the arsenic from contaminated water, adsorption process has been the most widely used because of its high removal efficiency, easy operation, low cost and sludge-free. Recently, several studies have focused in the development of novel materials based on iron oxides [14,15], activated carbon [16,17], alumina [18], zeolites [19], clays [20], etc. to adsorb arsenic from water. Among these reported materials, iron oxides have attracted special attention due to its low cost and good adsorption capacity for arsenic species. In attempt to improve the adsorption capacity of iron oxides for arsenic in water several studies have been reported. Sun et al. [15] showed that the adsorption capacity of akaganèite (b-FeOOH) can
1385-8947/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.10.006
48
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
be enhanced by isomorphic substitution of Fe3+ by Zr4+ on its structure. At pH 7, the maximum adsorption capacity for As(V) by akaganèite was 60 mg g1. Mg2+-doped a-Fe2O3 exhibited adsorption capacity for As(V) of 10 mg g1 [21]. Composites of iron oxides and carbonaceous materials also have been used as a strategy to increase the adsorption capacity of iron oxides. For example, composites of Fe3O4-reduced graphite oxide-MnO2 removed 12.22 mg As(V) per gram of catalyst [22]. A high adsorption capacity for As(V) was achieved in the presence of Fe3O4/graphene composite (180.3 mg g1) [23]. Composites of iron oxides and TiO2 or Al2O3 presented adsorption capacity for As(V) of 7.8 and 54.55 mg g1, respectively [24,25]. On the other hand, natural iron oxides have provided very lower adsorption capacity (0.02– 0.4 mg g1) because of its low specific surface area [14,26,27]. Polymorphs of FeOOH such as goethite (a-FeOOH), lepidocrocite (c-FeOOH) and akaganèite (b-FeOOH) have been described as good adsorbents for As(V). Mohapatra et al. [28] synthesized pure goethite and with different dopant cations (Cu, Ni and Co) for As(V) adsorption. The higher adsorption capacity was obtained for 2.7% Cu(II)-doped goethite (19.55 mg g1) at pH 2. Pure goethite exhibited adsorption capacity of 3.32 mg g1 at pH 7. Akaganèite with 330 m2 g1 surface area exhibited adsorption capacity as high as 120 mg g1 [29]. Adsorption capacities of 25.17 mg g1 have been reported for lepidocrocite with specific area of 103.9 m2 g1 [30]. Another polymorph, perhaps less known, constitutes d-FeOOH, which has the same crystallographic structure as CdI2. Crystalline d-FeOOH with small particle size, high specific area and narrow pore size distribution can be prepared in the laboratory by a simple and easily reproducible method. It is the unique polymorph of FeOOH that is ferrimagnetic, thus it can be easily recovered after use in catalysis or adsorption process using a simple magnet [31]. These characteristics make d-FeOOH a suitable candidate for use as an adsorbent for heavy metals in aqueous medium. To the best of our knowledge, only one study [32] has reported the use of d-FeOOH as an adsorbent for As(V). However, the aim of that work was provide a vibrational spectroscopy database for arsenic speciation in aqueous solution and at the several iron oxides-solution interfaces. No detailed study on the use of d-FeOOH for adsorption of heavy metals, more specifically arsenic, in water has been reported. In this work, crystalline d-FeOOH with small particle size and high surface area was prepared by chemical precipitation method for use as an effective adsorbent for the removal of As(V) from water under neutral pH conditions. The adsorption behavior of As(V) onto d-FeOOH nanoparticles was investigated in detail.
Geigerflex diffractometer equipped with a graphite diffractedbeam monochromator. Silicon was used as an external standard. The Rietveld structural refinement was performed with FULLPROF 2012 program. The morphology of the produced materials was monitored with transmission electron microscopy (TEM), using a JEOL transmission electron microscope model JEM 2000EXII. The surface area of d-FeOOH nanoparticles was determined by the BET method, using N2 adsorption/desorption in an Autosorb 1 Quantachrome instrument. The total pore volume was estimated from the amount of nitrogen adsorbed at P/P0 = 0.95; and the pore size distribution was calculated based on the BJH theory. Surface charge of d-FeOOH nanoparticles was judged by zeta potential measurement on a Malvern Zetasizer 2000 HS (Malvern, UK). 2.3. Batch adsorption tests 2.3.1. Adsorption Isotherm Adsorption experiments were conducted at temperature 25 ± 1 °C and pH 7 ± 0.3 (adjusted by 0.1 M NaOH or HCl). 40 mL of As(V) solutions with a concentration range of 0–20 mg L1 were used. Then, 10 mg of d-FeOOH nanoparticles were added into the above arsenic solutions. The equilibrium experiments were carried out in constant temperature (25 ± 1 °C) for 24 h. Then, the sample solution was separated by an external magnet. The initial and residual concentrations of As(V) were determined with an Inductively Coupled Plasma Mass Spectrometry (ICP–MS) ELAN DRC II (PerkinElmer Life and Analytical Sciences, USA). The specific amount of arsenic adsorbed was calculated from the following equation:
qe ¼
Vðc0 ce Þ m
ð1Þ
where qe (mg g1) is the equilibrium adsorption capacity, c0 and ce (mg L1) are the initial and equilibrium concentration of the adsorbate in solution, respectively, V is the volume (L) of arsenic solution and m is the mass (g) of adsorbent used in the experiments. 2.3.2. Adsorption kinetics The rate of the arsenic adsorption was studied by batch experiments with varying reaction times. In short, 10 mg of d-FeOOH nanoparticles were thoroughly mixed with 40 mL of 2.5, 5, 10 or 20 mg L1 As(V) solution. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment.
2. Experimental 2.1. Synthesis of d-FeOOH nanoparticles The synthesis of d-FeOOH was carried out as described by Pereira et al. [33] and Chagas et al. [34] with some modifications. In short, crystalline d-FeOOH was prepared by adding 50 mL of 2 M NaOH alcoholic solution into 50 mL of solution containing 5.5604 g of FeSO4(NH4)2SO46H2O. After the formation of green rust precipitated, 10 mL of 30% H2O2 was immediately added with stirring. The precipitate turned reddish brown within a few seconds, indicating the formation of d-FeOOH nanoparticles. The precipitate was washed with distilled water several times and dried in a vacuum desiccator at room temperature. 2.2. Materials characterization Powder X-ray diffraction (XRD) data were collected from 20° to 70° 2h by using Cu Ka (k = 1.540560 Å) radiation in a Rigaku
2.3.3. Effects of pH Experiments to determine the effect of solution pH values on arsenic adsorption were carried out by adding 10 mg of d-FeOOH nanoparticles into the vessel containing 40 mL of 10 mg L1 As(V) solution. The pH values of the aqueous solution were adjusted to 5, 7 or 9 using 0.01 M HCl or NaOH solutions. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment. 2.3.4. Effects of common anions The effects of common anions on As(V) removal were investigated in the presence of 0, 50 and 500 mg L1 NaNO3. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment.
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
3. Results and discussions 3.1. Characterization Fig. 1 shows the Rietveld refinement on the XRD data for the synthesized d-FeOOH nanoparticles. The Rietveld refinement yielded a profile residual factor, Rp, of approximately 3.1%, indicative of good quality refinement model. This XRD profile was fitted with a pseudo-Voigt function to obtain the characteristic structural parameters of d-FeOOH, which was identified by its 100, 101, 102 and 110 reflections that are consistent with the JCPDS File 13–87. The hexagonal lattice parameters for the d-FeOOH nanoparticles were found to be a = 2.9412 Å and c = 4.5278 Å. From the Rietveld refinement, the average of apparent crystallite size for this d-FeOOH nanoparticles was determined as being 16(3) nm. The grain morphology of the as-synthesized d-FeOOH nanoparticles is shown in Fig. 2. The formation of agglomerated irregular and pseudo-spherical morphologies can be clearly observed from Fig. 2. The average particle sizes estimated from TEM images are 20(2) nm. The N2 adsorption–desorption isotherm for the d-FeOOH nanoparticles is shown in Fig. 3. The isotherm exhibited high N2 adsorption and reached 340 cm3 g1 at p/p0 = 0.95, which is an indicative of a material with high pore volume. d-FeOOH nanoparticles exhibited a type IV isotherm, characteristic of mesoporous materials and a type H1 hysteresis loop, which is related to
Fig. 1. Rietveld refinement on the XRD pattern of d-FeOOH.
49
the filling of the mesopores due to capillary condensation. In addition, d-FeOOH nanoparticles presented a high specific surface area (135 m2 g1), which is an important characteristic of this material for use as an adsorbent for arsenic. The pore size distribution (Fig. 3, inset) calculated from desorption branch of the N2-isotherms by the BJH method, showed an average of pore size of 18 nm, thus confirming the presence of mesopores in d-FeOOH nanoparticles. 3.2. Batch adsorption tests 3.2.1. Kinetics of As(V) adsorption on d-FeOOH nanoparticles The kinetics of As(V) adsorption by d-FeOOH nanoparticles is shown in Fig. 4a. Preliminary studies on the rate of arsenic removal, at pH 7, by d-FeOOH nanoparticles indicated the about of 97%, 84%, 50% and 28% of the total arsenic was removed from the 2.5, 5, 10 and 20 mg L1 As(V) aqueous solution after 60 min adsorption, respectively, suggesting that the adsorption process of As(V) on the d-FeOOH surface is rapid in the initial stage and gradually decreases with the progress of adsorption until the equilibrium is reached. This could be assigned to the high external surface area of d-FeOOH due to its small particle size. The effect of initial As(V) concentration on equilibrium adsorption time showed that the time of equilibrium adsorption as well as the time required to achieve a definitive fraction of equilibrium adsorption is dependent of the initial As(V) concentration (Fig. 4a).
Fig. 3. Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) for the d-FeOOH nanoparticles.
Fig. 2. (a and b) TEM images of d-FeOOH nanoparticles.
50
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
Fig. 4. (a) Kinetics of As(V) adsorption on d-FeOOH nanoparticles and (b) pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles. Initial As(V) concentrations = 2.5–20 mg L1; adsorbent dose = 0.25 g L1; pH = 7.0 ± 0.3; shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C.
At low concentrations of As(V), i.e. 2.5 and 5 mg L1, the equilibrium is reached at the first 1 and 2 h, respectively, whereas for the higher concentrations, i.e. 10 and 20 mg L1, the equilibrium is reached after 4 h adsorption. The initial As(V) concentration provides an important driving force to overcome all mass transfer resistances of the As(V) between the aqueous and solid phases. Higher initial concentration of As(V) provided higher adsorption capacity. In order to elucidate the adsorption mechanism the kinetics data were fitted using two different models based on (i) Lagergren’s pseudo-first order equation (Eq. (2)) [35] and (ii) Ho’s pseudo-second-order equation (Eq. (3)) [36].
ln ðqe qt Þ ¼ ln qe k1 t
ð2Þ
t 1 1 ¼ þ qt qe k2 q2e
ð3Þ
where qe and qt are the amounts of adsorbed arsenic (mg g1) at equilibrium and at any time t (min), respectively. k1 (min1) and k2 (g mg1min1) are the equilibrium rate constants for pseudo first- and second-order adsorption, respectively. It was found that pseudo-first order kinetic model did not fit the adsorption of arsenic on d-FeOOH nanoparticles. Plots of ln (qe qt) vs. t displayed non-linearity indicative of higher order kinetics (plots not shown). On the other hand, the pseudo-second order model effectively described the observed kinetics, with plots of t/qt vs. t displaying strong linearity (Fig. 4b and Table 1), suggesting that the adsorption rate was governed by chemical adsorption of As(V) onto the active d-FeOOH sites [37]. qe and k2 can be then determined from the slope and intercept of the plot shown in Fig. 4b, respectively, and the obtained results are shown in Table 1. These results suggest that the initial adsorption rate is strongly dependent on the initial concentration of As(V). The initial adsorption rate is faster as the initial concentration of As(V) decreases, because of the most of active adsorption sites in d-FeOOH nanoparticles is unoccupied at low As(V) concentrations.
The pseudo-second order rate model is a good approximation of reaction kinetics, but it does not provide information about the rate-controlling step. In order to check whether the film or pore diffusion was the controlling step in the adsorption process, the experimental data were fitted to the Weber–Morris model (Eq. (4)) [38].
qt ¼ kint t1=2
where kint is the intraparticle diffusion rate constant. According to this model, if a straight line through the origin is obtained when t1/2 is plotted against qt, intraparticle diffusion and one surface binding process is the rate-limiting step. Non-linearity means that multiple processes are limiting the overall rate of adsorption, and these rate-limiting processes can be distinguished as distinct linear portions of the data over the time period in which they are exerting the control on the overall process [39]. Fig. 5 shows that the straight lines do not pass through the origin, indicating that two steps governed As(V) adsorption on d-FeOOH nanoparticles rather than one. The first linear part of the plot had a steeper qt vs. t1/2 slope, which suggests the diffusion of As(V) from the bulk solution to the exterior surface of d-FeOOH nanoparticles. The second linear part had a lower qt vs. t1/2 slope, which was due to interparticle diffusion of As(V) from the solution to the mesopores of d-FeOOH. Increasing the initial As(V) concentration (Fig. 5) increased the intraparticle diffusion parameters in both adsorption regions, which indicates that the driving force increases as the As(V) concentrations increase.
Table 1 Adsorption kinetic parameters for As(V) adsorption on d-FeOOH nanoparticles at pH 7.0(±0.3) and at 25.0(±0.5) °C. h0 = k2 q2e = initial adsorption rate, as t approaches 0. [As(V)] (mg L1)
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
2.5 5 10 20
0.024 0.003 0.0009 0.0004
10.07 21.84 37.7 43.1
2.43 1.43 1.28 0.74
0.9997 0.9986 0.9880 0.9540
ð4Þ
Fig. 5. Intraparticle As(V) diffusion modeling onto d-FeOOH nanoparticles.
51
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
3.2.2. Effect of pH on the kinetics of As(V) adsorption The effect of pH on As(V) adsorption on d-FeOOH nanoparticles is summarized in Fig. 6. The As(V) adsorption capacity of d-FeOOH nanoparticles decreased with increasing solution pH (Fig. 6 and Table 2), which may due, at least in part, to the changes of pH-dependent electrostatic force existing between the d-FeOOH surface and arsenic species. In order to determine the changes in the charges of d-FeOOH surface with the pH, zeta potential measurements were performed. Fig. 7 showed that the point of zero charge (pHpzc) of d-FeOOH is 8.4. Thus, at pH 5 and 7 the d-FeOOH surface is protonated („FeAOHþ 2 , „ means surface), which reflected the positive potential of 40 and 18 mV observed in Fig. 7, respectively. This favored the electrostatic attraction force with negatively charged H2 AsO 4 ; thus improving the adsorption capacity of d-FeOOH nanoparticles at these pH values. At pH 9, repulsions between negatively charged (2.4 mV) d-FeOOH surface („FeAO) and HAsO2 readily decreased the adsorption 4 capacity of d-FeOOH nanoparticles. 3.2.3. Effect of ionic strength on the As(V) adsorption The effect of ionic strength on As(V) removal by d-FeOOH nanoparticles is illustrated in Fig. 8. Adsorption capacity decreased from 37.7 mg g1 in the absence of nitrate to 29.9 and 29.5 mg g1 in the presence of 50 and 500 mg L1 nitrate (Table 3). The decrease in the arsenate removal can be explained by the consumption of the available binding sites of the d-FeOOH and reduction in electrostatic attraction. However, when the NaNO3 increased from 50 to 500 mg L1, the removal of As(V) practically did not change, suggesting that the surface interaction between As(V) and d-FeOOH is mainly inner sphere in nature. When changing the ionic strength of the As(V) solution, non-specifically adsorbed ions were more sensitive to the change of ionic strength than specifically adsorbed ions because electrolytes can also form outer-spheres complexes with the d-FeOOH through electrostatic forces [40]. Müller et al. [32] have also suggested from Raman spectroscopy that the mechanism of As(V) adsorption onto different iron oxides follows an inner-sphere, bidentate-binuclear complexation model.
Fig. 7. Zeta potential measurement for the d-FeOOH nanoparticles.
d-FeOOH nanoparticles was 37.3 mg g1. This value was comparable to many of the reported adsorption capacities for iron oxides in the state of art (Table 4). The high As(V) adsorption capacity was due to the small particle size and high surface area of d-FeOOH nanoparticles. Moreover, the mesoporous d-FeOOH nanoparticles are magnetic and their dispersion in water can be easily separated by applying an external magnetic field. These nanoparticles showed saturation magnetization of 13.2 emu g1. Upon magnetic separation and stirring with aqueous NaOH solution. It is interesting to highlight that this high adsorption capacity was found for the pure and unmodified d-FeOOH and, therefore, several possibilities to improve the d-FeOOH adsorption capacity based on materials chemistry still are open for further studies. To obtain more information about the arsenic adsorption by d-FeOOH nanoparticles, Langmuir, Freundlich and Redlich–Peterson isotherm models were tested. The Freundlich model (plots not shown) did not fit well to the obtained data. The best results were obtained for the Redlich–Peterson (R2 = 0.9778) and Langmuir
3.3. Adsorption isotherms To determine the adsorption capacity of d-FeOOH nanoparticles, the equilibrium adsorption of arsenic was studied as a function of arsenic concentration. Fig. 9 showed that initially the isotherm rises sharply indicating that the accessible sites in d-FeOOH are available for adsorption. However, as As(V) concentration increased, site saturation of d-FeOOH surface occurred and a plateau was reached, indicative of that all active adsorption sites in d-FeOOH were occupied. The adsorption capacity of As(V) by
Table 2 Adsorption kinetic parameters for 10 mg L1 As(V) adsorption on d-FeOOH nanoparticles at different pH and at 25.0(±0.5) °C. pH
5 7 9
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
0.0012 0.0009 0.0006
41.3 37.7 23.3
2.05 1.28 0.33
0.9958 0.9880 0.9030
Fig. 6. (a) Effect of pH on As(V) adsorption onto d-FeOOH nanoparticles. (b) Pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles at different pH. Initial As(V) concentration = 10 mg L1; Adsorbent dose = 0.25 g L1, shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C.
52
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
Fig. 8. (a) Effect of ionic strength on As(V) adsorption onto d-FeOOH nanoparticles. (b) Pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles at different ionic strength. Initial As(V) concentration = 10 mg L1; adsorbent dose = 0.25 g L1, shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C, pH = 7.
Table 3 Adsorption kinetic parameters for 10 mg L1 As(V) adsorption on d-FeOOH nanoparticles in the presence of different NaNO3 concentrations, pH 7 and at 25.0(±0.5) °C. [NaNO3] (mg L1)
0 50 500
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
0.0009 0.0016 0.0015
37.7 29.9 29.5
1.28 1.43 1.31
0.9880 0.9926 0.9941
Fig. 9. As(V) adsorption isotherm onto d-FeOOH nanoparticles. The data is fitted with the (a) Langmuir isotherm and (b) Redlich–Peterson isotherm.
(R2 = 0.9634) isotherm models (Fig. 9 and Table 5). Langmuir isotherm equation was given as follows (Eq. (5)):
qeq ¼
kL Q m C e ; 1 þ kL C e
ð5Þ
where Qm (mg g1) and Ce (mg L1) are maximum adsorption capacity and the concentration at equilibrium respectively, kL is Langmuir constant, which represents the energy of adsorption (L mg1). Redlich–Peterson isotherm was expressed below (Eq. (6)):
qeq ¼
k1 C e 1 þ k2 C ae
ð6Þ
where qeq (mg g1) and Ce (mg L1) are the amount adsorbed per unit mass of the adsorbent and concentration at equilibrium, respectively. k1 (L g1) and k2 (L mg1) are the Redlich–Peterson isotherm constants, and a is an exponent that describes the system heterogeneity; it lies between 0 and 1.
The Redlich–Peterson isotherm is an empirical equation that may be used to represent adsorption equilibrium over a wide concentration range. The mechanism of adsorption is a hybrid model featuring both Langmuir and Freundlich isotherms and it does not follow ideal monolayer adsorption [41]. Eq. (5)can be simplified to the Langmuir isotherm when a = 1. Since the a value obtained from the Redlich–Peterson was 0.97 in this study, we suggest that the As(V) adsorption on d-FeOOH surface followed a non-ideal monolayer adsorption model. 4. Conclusions An efficient adsorbent for As(V) based on d-FeOOH nanoparticles was prepared by a simple and environmentally friendly method. d-FeOOH had a high adsorption capacity for As(V), 37.3 mg g1, at neutral pH. This high adsorption capacity was assigned to the small particle size and the high specific surface area of d-FeOOH. Moreover, d-FeOOH can be characterized as a low cost and easily
53
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54 Table 4 Comparison between adsorption capacities for d-FeOOH and others iron oxides. Adsorbent
BET area (m2 g1)
pH
Adsorption (mg g1)
Adsorption (mg m2)
Reference
d-FeOOH Zr-doped b-FeOOH a-FeOOH/siderite c-FeOOH treated at 250 °C Fe3O4 Fe3O4-reduced graphite oxide-MnO2 TiO2/Fe2O3 Fe2O3/activated carbon Ultrafine hematite (5 nm) c-Fe2O3/TiO2 Mg-doped a-Fe2O3 Natural hematite Ascorbic acid-coated Fe3O4 Fe3O4/graphene Schwertmannite
135 220.77 95.8–110 109.2 245 113.7 133.5 821 162 153.9 438 0.38 178.48 98.2 206.1
7 7 7 7 7 7 7 7 7 7 7 7 7 7 7
37.3 60 115–121 38 41 12.22 7.8 17 47 33 10 0.02 16.56 180.3 34
0.28 0.27 1.2 0.35 0.17 0.11 0.06 0.02 0.29 0.21 0.02 0.05 0.09 1.84 0.16
This work (15) (42) (30) (43) (22) (24) (44) (45) (46) (21) (14) (47) (23) (48)
Table 5 Isotherm parameters obtained by non-linear regression fit for the d-FeOOH nanoparticles. Langmuir isotherm
Redlich–Peterson isotherm
qm (mg g1)
kL (L mg1)
R2
K1 (L g1)
K2 (L mg1)
a
R2
37.30
20.12
0.9634
819.02
23.04
0.97
0.9778
available adsorbent, as the starting reagents are cheap. Because of the aforesaid, d-FeOOH holds considerable promise to be used as an adsorbent for heavy metals in contaminated water. Acknowledgments The authors are grateful to CNPQ (including grants #470738/ 2011-1; 302755/2011-0; 550341/2012-9; 309455/2012-0) and FAPEMIG and FAPESP for financial support. References [1] EPA, 2001. Cost Analyses for Selected Groundwater Cleanup Projects: Pump and Treat Systems and Permeable Reactive Barriers. U.S. EPA, Office of Solid Waste and Emergency Response. EPA-542-R-00-013, February 2001. [2] O.X. Leupin, S.J. Hug, Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron, Water Res. 39 (2005) 1729–1740. [3] I.A. Katsoyiannis, A.I. Zouboulis, Application of biological processes for the removal of arsenic from groundwaters, Water Res. 38 (2004) 17–26. [4] I.A. Katsoyiannis, A.I. Zouboulis, M. Jekel, Kinetics of bacterial As(III) oxidation and subsequent As(V) removal by sorption onto biogenic manganese oxides during groundwater treatment, Ind. Eng. Chem. Res. 43 (2004) 486–493. [5] X. Meng, G.P. Korfiatis, C. Christodoulatos, S. Bang, Treatment of arsenic in Bangladesh well water using a household co-precipitation and filtration system, Water Res. 35 (2001) 2805–2810. [6] A.S. Sheoran, V. Sheoran, Heavy metal removal mechanism of acid mine drainage in wetlands: a critical review, Min. Eng. 19 (2006) 105–116. [7] J.E. Greenleaf, J. Lin, A.K. Sengupta, Two novel applications of ion exchange fibers: arsenic removal and chemical-free softening of hard water, Environ. Prog. 25 (2006) 300–311. [8] D. Mohan, C.U. Pittman Jr., Arsenic removal from water/wastewater using adsorbents—a critical review, J. Hazard. Mater. 142 (2007) 1–53. [9] X. Luo, C. Wang, L. Wang, F. Deng, S. Luo, X. Tu, C. Au, Nanocomposites of graphene oxide-hydrated zirconium oxide for simultaneous removal of As(III) and As(V) from water, Chem. Eng. J. 220 (2013) 98–106. [10] X. Luo, C. Wang, S. Luo, R. Dong, X. Tu, G. Zeng, Adsorption of As(III) and As(V) from water using magnetite Fe3O4-reduced graphite oxide-MnO2 nanocomposites, Chem. Eng. J. 187 (2012) 45–52. [11] R.Y. Ning, Arsenic removal by reverse osmosis, Desalination 143 (2002) 237– 241. [12] Y. Sato, M. Kang, T. Kamei, Y. Magara, Performance of nanofiltration for arsenic removal, Water Res. 36 (2002) 3371–3377. [13] A.B. Ribeiro, E.P. Mateus, L.M. Ottosen, G. Bech-Nielsen, Electrodialytic removal of Cu, Cr, and As from chromated copper arsenate-treated timber waste, Environ. Sci. Technol. 34 (2000) 784–788.
[14] J. Giménez, J. Pablo, M. Martínez, M. Rovira, C. Valderrama, Reactive transport of arsenic (III) and arsenic(V) on natural hematite: experimental and modeling, J. Colloid. Interface Sci. 348 (2010) 293–297. [15] X. Sun, C. Hu, X. Hu, J. Qu, M. Yang, Characterization and adsorption performance of Zr-doped akaganèite for efficient arsenic removal, J. Chem. Technol. Biotechnol. 88 (2013) 629–635. [16] Q.L. Zhang, T.C. Lin, X. Chen, N.Y. Gao, A method for preparing ferric activated carbon composites adsorbents to remove arsenic from drinking water, J. Hazard. Mater. 148 (2007) 671–678. [17] D.Q.L. Oliveira, M. Gonçalves, L.C.A. Oliveira, L.R.G. Guilherme, Removal of As (V) and Cr (VI) from aqueous solutions using solid waste from leather industry, J. Hazard. Mater. 151 (2008) 280–284. [18] C. Han, H. Li, H. Pu, H. Yu, L. Deng, S. Huang, Y. Luo, Synthesis and characterization of mesoporous alumina and their performances for removing arsenic(V), Chem. Eng. J 217 (2013) 1–9. [19] V. Swarnkar, R. Tomar, Use of surfactant-modified zeolites for arsenate removal from pollutant water, J. Disp. Sci. Technol. 33 (2012) 913–918. [20] A. Anjum, P. Lokeswari, M. Kaur, M. Datta, Removal of As (III) from aqueous solutions using montmorillonite, J. Anal. Sci. Methods Instrum. 1 (2011) 25–30. [21] W. Tang, Y. Su, Q. Li, S. Gao, J.K. Shang, Mg-doping: a facile approach to impart enhanced arsenic adsorption performance and easy magnetic separation capability to –Fe2O3 nanoadsorbents, J. Mater. Chem. A 1 (2013) 830–836. [22] X. Luo, C. Wang, S. Luo, R. Dong, X. Tu, G. Zeng, Adsorption of As (III) and As (V) from water using magnetite Fe3O4-reduced graphite oxide–MnO2 nanocomposites, Chem. Eng. J. 87 (2012) 45–52. [23] A.K. Mishra, S. Ramaprabhu, Ultrahigh arsenic sorption using iron oxidegraphene nanocomposite supercapacitor assembly, J. Appl. Phys. 112 (2012) 104315–104316. [24] M. D’Arcy, D. Weiss, M. Bluck, R. Vilar, Adsorption kinetics, capacity and mechanism of arsenate and phosphate on a bifunctional TiO2–Fe2O3 bi-composite, J. Colloid Interface Sci. 364 (2011) 205–212. [25] T. Basu, K. Gupta, U.C. Ghosh, Performances of As(V) adsorption of calcined (250 °C) synthetic iron(III)–aluminum(III) mixed oxide in the presence of some groundwater occurring ions, Chem. Eng. J. 183 (2012) 303–314. [26] W. Zhang, P. Singh, E. Paling, S. Delides, Arsenic removal from contaminated water by natural iron ores, Miner. Eng. 17 (2004) 517–524. [27] H. Guo, D. Stuben, Z. Berner, Removal of arsenic from aqueous solution by natural siderite and hematite, Appl. Geochem. 22 (2007) 1039–1051. [28] M. Mohapatra, S.K. Sahoo, S. Anand, R.P. Das, Removal of As(V) by Cu(II)-, Ni(II)-, or Co(II)-doped goethite samples, J. Colloid Interface Sci. 298 (2006) 6–12. [29] E.A. Deliyanni, D.N. Bakoyannakis, A.I. Zouboulis, K.A. Matis, Sorption of As (V) ions by akaganèite-type nanocrystals, Chemosphere 50 (2003) 155–163. [30] E. Repo, M. Makinen, S. Rengaraj, G. Natarajan, A. Bhatnagard, M. Sillanpaa, Lepidocrocite and its heat-treated forms as effective arsenic adsorbents in aqueous medium, Chem. Eng. J. 180 (2012) 159–169. [31] I.S.X. Pinto, P.H.V.V. Pacheco, J.V. Coelho, E. Lorençon, J.D. Ardisson, J.D. Fabris, P.P. Souza, K.W.H. Krambrock, L.C.A. Oliveira, M.C. Pereira, Nanostructured dFeOOH: an efficient Fenton-like catalyst for the oxidation of organics in water, Appl. Catal. B 119 (2012) 175–182. [32] K. Müller, V.S.T. Ciminelli, M.S.S. Dantas, S. Willscher, A comparative study of As (III) and As (V) in aqueous solution and adsorbed on iron oxy-hydroxides by Raman spectroscopy, Water Res. 44 (2010) 5660–5672. [33] M.C. Pereira, E.M. Garcia, A.C. Silva, E. Lorençon, J.D. Ardisson, E. Murad, J.D. Fabris, T. Matencio, T.C. Ramalho, M.V.J. Rocha, Nanostructured d-FeOOH: a novel photocatalyst for water splitting, J. Mater. Chem. 21 (2011) 10280– 10282. [34] P. Chagas, A.C. Silva, E.C. Passamani, J.D. Ardisson, L.C.A. Oliveira, J.D. Fabris, R.M. Paniago, D.S. Monteiro, M.C. Pereira, d-FeOOH: a superparamagnetic material for controlled heat release under AC magnetic field, J. Nanopart. Res. 15 (2013) 15–44.
54
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
[35] S. Lagergren, About the theory of so-called adsorption of soluble substances, Kungliga Svenska Vetenskapsakademien Handlingar 24 (1998) 1–39. [36] Y.S. Ho, Second-order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and nonlinear methods, Water Res. 40 (2006) 119–125. [37] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. [38] W.J. Weber, J.C. Morris, Kinetics of adsorption of carbon from solutions, J. Sanit. Eng. Div. 89 (1963) 31–60.
[39] V. Belessi, G. Romanos, N. Boukos, D. Lambropoulou, C. Trapalis, Removal of reactive red 195 from aqueous solutions by adsorption on the surface of TiO2 nanoparticles, J. Hazard. Mater. 170 (2009) 836–844. [40] L. Zheng, A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal, Water Res. 37 (2003) 4351–4358. [41] R.K. Prasad, S.N. Srivastava, Sorption on distillery spent was onto fly ash: kinetics and mass transfer studies, Chem. Eng. J. 146 (2009) 90–97.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Arsenic removal from contaminated water by ultrafine d-FeOOH adsorbents Márcia C.S. Faria a, Renedy S. Rosemberg b, Cleide A. Bomfeti b, Douglas S. Monteiro b, Fernando Barbosa a, Luiz C.A. Oliveira c, Mariandry Rodriguez b, Márcio C. Pereira b, Jairo L. Rodrigues b,⇑ a b c
Departamento de Análises Clínicas, Toxicológicas e Bromatológicas, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, USP, 14040-903 Ribeirão Preto, São Paulo, Brazil Instituto de Ciência, Engenharia e Tecnologia, Universidade Federal dos Vales Jequitinhonha e Mucuri, Teófilo Otoni 39803-371, Minas Gerais, Brazil Departamento de Química, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil
h i g h l i g h t s Efficient d-FeOOH adsorbents were prepared by a simple and cheap method. d-FeOOH nanoparticles exhibited high adsorption capacity for arsenic. Arsenic was better adsorbed by d-FeOOH nanoparticles at acidic pH. d-FeOOH is a promising adsorbent for heavy metal in contaminated water.
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 3 October 2013 Accepted 5 October 2013 Available online 12 October 2013 Keywords: Adsorption Iron oxides Environmental remediation Nanoparticles
a b s t r a c t d-FeOOH nanoparticles were prepared by a fast, simple and cheap synthesis method for use as an adsorbent for As(V) in water. Rietveld refinement on XRD pattern confirmed that d-FeOOH was successful synthesized. TEM images evidenced that the average particle sizes for d-FeOOH is 20 nm, which provided a high surface area of 135 m2 g1 and average pore sizes of 18 nm, as verified with BET measurements. Zeta potential revealed that the point of zero charge of d-FeOOH is 8.4, which favored the As(V) adsorption on the d-FeOOH surface even at neutral pH. The As(V) adsorption capacity of d-FeOOH was estimated to be 37.3 mg g1 at pH 7. The kinetics data were best fitted with a pseudo-second order, thus suggesting chemical adsorption on the surface and pores of d-FeOOH nanoparticles. The interaction between As(V) and d-FeOOH nanoparticles was suggested to be mainly inner sphere complexes. The adsorption isotherm obtained at pH 7 was best fitted to the Langmuir and Redlich–Peterson models and, therefore, a non-ideal monolayer adsorption model for As(V) on d-FeOOH nanoparticles was proposed. The small particle size, high surface area and adsorption capacity make d-FeOOH a promising adsorbent for toxic metals in contaminated water. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Arsenic contamination in natural water is a worldwide problem because of its known toxicity and health hazards. The US Environmental Protection Agency (USEPA) has classified arsenic as a Class ‘A’ human carcinogenic and because of the serious health problems caused by arsenic, in 2001, it reduced the maximum arsenic content in drinking water from 50 lg L1 to 10 lg L1 [1]. Thus, several technologies such as oxidation and filtration [2], biological oxidation followed by removal for iron and manganese oxides [3,4], co-precipitation followed by coagulation [5], sedimentation and filtration [6], ion exchange through suitable cation and
⇑ Corresponding author. Tel./fax: +55 33 3522 6037. E-mail address: [email protected] (J.L. Rodrigues).
anion exchange resins [7], adsorption [8–10] and membrane technology including reverse osmosis [11], nanofiltration [12] and electrodialysis [13] have been used to remove arsenic from water. Among the technologies to remove the arsenic from contaminated water, adsorption process has been the most widely used because of its high removal efficiency, easy operation, low cost and sludge-free. Recently, several studies have focused in the development of novel materials based on iron oxides [14,15], activated carbon [16,17], alumina [18], zeolites [19], clays [20], etc. to adsorb arsenic from water. Among these reported materials, iron oxides have attracted special attention due to its low cost and good adsorption capacity for arsenic species. In attempt to improve the adsorption capacity of iron oxides for arsenic in water several studies have been reported. Sun et al. [15] showed that the adsorption capacity of akaganèite (b-FeOOH) can
1385-8947/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.10.006
48
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
be enhanced by isomorphic substitution of Fe3+ by Zr4+ on its structure. At pH 7, the maximum adsorption capacity for As(V) by akaganèite was 60 mg g1. Mg2+-doped a-Fe2O3 exhibited adsorption capacity for As(V) of 10 mg g1 [21]. Composites of iron oxides and carbonaceous materials also have been used as a strategy to increase the adsorption capacity of iron oxides. For example, composites of Fe3O4-reduced graphite oxide-MnO2 removed 12.22 mg As(V) per gram of catalyst [22]. A high adsorption capacity for As(V) was achieved in the presence of Fe3O4/graphene composite (180.3 mg g1) [23]. Composites of iron oxides and TiO2 or Al2O3 presented adsorption capacity for As(V) of 7.8 and 54.55 mg g1, respectively [24,25]. On the other hand, natural iron oxides have provided very lower adsorption capacity (0.02– 0.4 mg g1) because of its low specific surface area [14,26,27]. Polymorphs of FeOOH such as goethite (a-FeOOH), lepidocrocite (c-FeOOH) and akaganèite (b-FeOOH) have been described as good adsorbents for As(V). Mohapatra et al. [28] synthesized pure goethite and with different dopant cations (Cu, Ni and Co) for As(V) adsorption. The higher adsorption capacity was obtained for 2.7% Cu(II)-doped goethite (19.55 mg g1) at pH 2. Pure goethite exhibited adsorption capacity of 3.32 mg g1 at pH 7. Akaganèite with 330 m2 g1 surface area exhibited adsorption capacity as high as 120 mg g1 [29]. Adsorption capacities of 25.17 mg g1 have been reported for lepidocrocite with specific area of 103.9 m2 g1 [30]. Another polymorph, perhaps less known, constitutes d-FeOOH, which has the same crystallographic structure as CdI2. Crystalline d-FeOOH with small particle size, high specific area and narrow pore size distribution can be prepared in the laboratory by a simple and easily reproducible method. It is the unique polymorph of FeOOH that is ferrimagnetic, thus it can be easily recovered after use in catalysis or adsorption process using a simple magnet [31]. These characteristics make d-FeOOH a suitable candidate for use as an adsorbent for heavy metals in aqueous medium. To the best of our knowledge, only one study [32] has reported the use of d-FeOOH as an adsorbent for As(V). However, the aim of that work was provide a vibrational spectroscopy database for arsenic speciation in aqueous solution and at the several iron oxides-solution interfaces. No detailed study on the use of d-FeOOH for adsorption of heavy metals, more specifically arsenic, in water has been reported. In this work, crystalline d-FeOOH with small particle size and high surface area was prepared by chemical precipitation method for use as an effective adsorbent for the removal of As(V) from water under neutral pH conditions. The adsorption behavior of As(V) onto d-FeOOH nanoparticles was investigated in detail.
Geigerflex diffractometer equipped with a graphite diffractedbeam monochromator. Silicon was used as an external standard. The Rietveld structural refinement was performed with FULLPROF 2012 program. The morphology of the produced materials was monitored with transmission electron microscopy (TEM), using a JEOL transmission electron microscope model JEM 2000EXII. The surface area of d-FeOOH nanoparticles was determined by the BET method, using N2 adsorption/desorption in an Autosorb 1 Quantachrome instrument. The total pore volume was estimated from the amount of nitrogen adsorbed at P/P0 = 0.95; and the pore size distribution was calculated based on the BJH theory. Surface charge of d-FeOOH nanoparticles was judged by zeta potential measurement on a Malvern Zetasizer 2000 HS (Malvern, UK). 2.3. Batch adsorption tests 2.3.1. Adsorption Isotherm Adsorption experiments were conducted at temperature 25 ± 1 °C and pH 7 ± 0.3 (adjusted by 0.1 M NaOH or HCl). 40 mL of As(V) solutions with a concentration range of 0–20 mg L1 were used. Then, 10 mg of d-FeOOH nanoparticles were added into the above arsenic solutions. The equilibrium experiments were carried out in constant temperature (25 ± 1 °C) for 24 h. Then, the sample solution was separated by an external magnet. The initial and residual concentrations of As(V) were determined with an Inductively Coupled Plasma Mass Spectrometry (ICP–MS) ELAN DRC II (PerkinElmer Life and Analytical Sciences, USA). The specific amount of arsenic adsorbed was calculated from the following equation:
qe ¼
Vðc0 ce Þ m
ð1Þ
where qe (mg g1) is the equilibrium adsorption capacity, c0 and ce (mg L1) are the initial and equilibrium concentration of the adsorbate in solution, respectively, V is the volume (L) of arsenic solution and m is the mass (g) of adsorbent used in the experiments. 2.3.2. Adsorption kinetics The rate of the arsenic adsorption was studied by batch experiments with varying reaction times. In short, 10 mg of d-FeOOH nanoparticles were thoroughly mixed with 40 mL of 2.5, 5, 10 or 20 mg L1 As(V) solution. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment.
2. Experimental 2.1. Synthesis of d-FeOOH nanoparticles The synthesis of d-FeOOH was carried out as described by Pereira et al. [33] and Chagas et al. [34] with some modifications. In short, crystalline d-FeOOH was prepared by adding 50 mL of 2 M NaOH alcoholic solution into 50 mL of solution containing 5.5604 g of FeSO4(NH4)2SO46H2O. After the formation of green rust precipitated, 10 mL of 30% H2O2 was immediately added with stirring. The precipitate turned reddish brown within a few seconds, indicating the formation of d-FeOOH nanoparticles. The precipitate was washed with distilled water several times and dried in a vacuum desiccator at room temperature. 2.2. Materials characterization Powder X-ray diffraction (XRD) data were collected from 20° to 70° 2h by using Cu Ka (k = 1.540560 Å) radiation in a Rigaku
2.3.3. Effects of pH Experiments to determine the effect of solution pH values on arsenic adsorption were carried out by adding 10 mg of d-FeOOH nanoparticles into the vessel containing 40 mL of 10 mg L1 As(V) solution. The pH values of the aqueous solution were adjusted to 5, 7 or 9 using 0.01 M HCl or NaOH solutions. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment. 2.3.4. Effects of common anions The effects of common anions on As(V) removal were investigated in the presence of 0, 50 and 500 mg L1 NaNO3. Solutions were in contact with d-FeOOH nanoparticles for a period of time between 0 and 360 min. The sample solution of residual arsenic concentration was analyzed at the different adsorption time with an ICP–MS equipment.
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
3. Results and discussions 3.1. Characterization Fig. 1 shows the Rietveld refinement on the XRD data for the synthesized d-FeOOH nanoparticles. The Rietveld refinement yielded a profile residual factor, Rp, of approximately 3.1%, indicative of good quality refinement model. This XRD profile was fitted with a pseudo-Voigt function to obtain the characteristic structural parameters of d-FeOOH, which was identified by its 100, 101, 102 and 110 reflections that are consistent with the JCPDS File 13–87. The hexagonal lattice parameters for the d-FeOOH nanoparticles were found to be a = 2.9412 Å and c = 4.5278 Å. From the Rietveld refinement, the average of apparent crystallite size for this d-FeOOH nanoparticles was determined as being 16(3) nm. The grain morphology of the as-synthesized d-FeOOH nanoparticles is shown in Fig. 2. The formation of agglomerated irregular and pseudo-spherical morphologies can be clearly observed from Fig. 2. The average particle sizes estimated from TEM images are 20(2) nm. The N2 adsorption–desorption isotherm for the d-FeOOH nanoparticles is shown in Fig. 3. The isotherm exhibited high N2 adsorption and reached 340 cm3 g1 at p/p0 = 0.95, which is an indicative of a material with high pore volume. d-FeOOH nanoparticles exhibited a type IV isotherm, characteristic of mesoporous materials and a type H1 hysteresis loop, which is related to
Fig. 1. Rietveld refinement on the XRD pattern of d-FeOOH.
49
the filling of the mesopores due to capillary condensation. In addition, d-FeOOH nanoparticles presented a high specific surface area (135 m2 g1), which is an important characteristic of this material for use as an adsorbent for arsenic. The pore size distribution (Fig. 3, inset) calculated from desorption branch of the N2-isotherms by the BJH method, showed an average of pore size of 18 nm, thus confirming the presence of mesopores in d-FeOOH nanoparticles. 3.2. Batch adsorption tests 3.2.1. Kinetics of As(V) adsorption on d-FeOOH nanoparticles The kinetics of As(V) adsorption by d-FeOOH nanoparticles is shown in Fig. 4a. Preliminary studies on the rate of arsenic removal, at pH 7, by d-FeOOH nanoparticles indicated the about of 97%, 84%, 50% and 28% of the total arsenic was removed from the 2.5, 5, 10 and 20 mg L1 As(V) aqueous solution after 60 min adsorption, respectively, suggesting that the adsorption process of As(V) on the d-FeOOH surface is rapid in the initial stage and gradually decreases with the progress of adsorption until the equilibrium is reached. This could be assigned to the high external surface area of d-FeOOH due to its small particle size. The effect of initial As(V) concentration on equilibrium adsorption time showed that the time of equilibrium adsorption as well as the time required to achieve a definitive fraction of equilibrium adsorption is dependent of the initial As(V) concentration (Fig. 4a).
Fig. 3. Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) for the d-FeOOH nanoparticles.
Fig. 2. (a and b) TEM images of d-FeOOH nanoparticles.
50
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
Fig. 4. (a) Kinetics of As(V) adsorption on d-FeOOH nanoparticles and (b) pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles. Initial As(V) concentrations = 2.5–20 mg L1; adsorbent dose = 0.25 g L1; pH = 7.0 ± 0.3; shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C.
At low concentrations of As(V), i.e. 2.5 and 5 mg L1, the equilibrium is reached at the first 1 and 2 h, respectively, whereas for the higher concentrations, i.e. 10 and 20 mg L1, the equilibrium is reached after 4 h adsorption. The initial As(V) concentration provides an important driving force to overcome all mass transfer resistances of the As(V) between the aqueous and solid phases. Higher initial concentration of As(V) provided higher adsorption capacity. In order to elucidate the adsorption mechanism the kinetics data were fitted using two different models based on (i) Lagergren’s pseudo-first order equation (Eq. (2)) [35] and (ii) Ho’s pseudo-second-order equation (Eq. (3)) [36].
ln ðqe qt Þ ¼ ln qe k1 t
ð2Þ
t 1 1 ¼ þ qt qe k2 q2e
ð3Þ
where qe and qt are the amounts of adsorbed arsenic (mg g1) at equilibrium and at any time t (min), respectively. k1 (min1) and k2 (g mg1min1) are the equilibrium rate constants for pseudo first- and second-order adsorption, respectively. It was found that pseudo-first order kinetic model did not fit the adsorption of arsenic on d-FeOOH nanoparticles. Plots of ln (qe qt) vs. t displayed non-linearity indicative of higher order kinetics (plots not shown). On the other hand, the pseudo-second order model effectively described the observed kinetics, with plots of t/qt vs. t displaying strong linearity (Fig. 4b and Table 1), suggesting that the adsorption rate was governed by chemical adsorption of As(V) onto the active d-FeOOH sites [37]. qe and k2 can be then determined from the slope and intercept of the plot shown in Fig. 4b, respectively, and the obtained results are shown in Table 1. These results suggest that the initial adsorption rate is strongly dependent on the initial concentration of As(V). The initial adsorption rate is faster as the initial concentration of As(V) decreases, because of the most of active adsorption sites in d-FeOOH nanoparticles is unoccupied at low As(V) concentrations.
The pseudo-second order rate model is a good approximation of reaction kinetics, but it does not provide information about the rate-controlling step. In order to check whether the film or pore diffusion was the controlling step in the adsorption process, the experimental data were fitted to the Weber–Morris model (Eq. (4)) [38].
qt ¼ kint t1=2
where kint is the intraparticle diffusion rate constant. According to this model, if a straight line through the origin is obtained when t1/2 is plotted against qt, intraparticle diffusion and one surface binding process is the rate-limiting step. Non-linearity means that multiple processes are limiting the overall rate of adsorption, and these rate-limiting processes can be distinguished as distinct linear portions of the data over the time period in which they are exerting the control on the overall process [39]. Fig. 5 shows that the straight lines do not pass through the origin, indicating that two steps governed As(V) adsorption on d-FeOOH nanoparticles rather than one. The first linear part of the plot had a steeper qt vs. t1/2 slope, which suggests the diffusion of As(V) from the bulk solution to the exterior surface of d-FeOOH nanoparticles. The second linear part had a lower qt vs. t1/2 slope, which was due to interparticle diffusion of As(V) from the solution to the mesopores of d-FeOOH. Increasing the initial As(V) concentration (Fig. 5) increased the intraparticle diffusion parameters in both adsorption regions, which indicates that the driving force increases as the As(V) concentrations increase.
Table 1 Adsorption kinetic parameters for As(V) adsorption on d-FeOOH nanoparticles at pH 7.0(±0.3) and at 25.0(±0.5) °C. h0 = k2 q2e = initial adsorption rate, as t approaches 0. [As(V)] (mg L1)
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
2.5 5 10 20
0.024 0.003 0.0009 0.0004
10.07 21.84 37.7 43.1
2.43 1.43 1.28 0.74
0.9997 0.9986 0.9880 0.9540
ð4Þ
Fig. 5. Intraparticle As(V) diffusion modeling onto d-FeOOH nanoparticles.
51
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
3.2.2. Effect of pH on the kinetics of As(V) adsorption The effect of pH on As(V) adsorption on d-FeOOH nanoparticles is summarized in Fig. 6. The As(V) adsorption capacity of d-FeOOH nanoparticles decreased with increasing solution pH (Fig. 6 and Table 2), which may due, at least in part, to the changes of pH-dependent electrostatic force existing between the d-FeOOH surface and arsenic species. In order to determine the changes in the charges of d-FeOOH surface with the pH, zeta potential measurements were performed. Fig. 7 showed that the point of zero charge (pHpzc) of d-FeOOH is 8.4. Thus, at pH 5 and 7 the d-FeOOH surface is protonated („FeAOHþ 2 , „ means surface), which reflected the positive potential of 40 and 18 mV observed in Fig. 7, respectively. This favored the electrostatic attraction force with negatively charged H2 AsO 4 ; thus improving the adsorption capacity of d-FeOOH nanoparticles at these pH values. At pH 9, repulsions between negatively charged (2.4 mV) d-FeOOH surface („FeAO) and HAsO2 readily decreased the adsorption 4 capacity of d-FeOOH nanoparticles. 3.2.3. Effect of ionic strength on the As(V) adsorption The effect of ionic strength on As(V) removal by d-FeOOH nanoparticles is illustrated in Fig. 8. Adsorption capacity decreased from 37.7 mg g1 in the absence of nitrate to 29.9 and 29.5 mg g1 in the presence of 50 and 500 mg L1 nitrate (Table 3). The decrease in the arsenate removal can be explained by the consumption of the available binding sites of the d-FeOOH and reduction in electrostatic attraction. However, when the NaNO3 increased from 50 to 500 mg L1, the removal of As(V) practically did not change, suggesting that the surface interaction between As(V) and d-FeOOH is mainly inner sphere in nature. When changing the ionic strength of the As(V) solution, non-specifically adsorbed ions were more sensitive to the change of ionic strength than specifically adsorbed ions because electrolytes can also form outer-spheres complexes with the d-FeOOH through electrostatic forces [40]. Müller et al. [32] have also suggested from Raman spectroscopy that the mechanism of As(V) adsorption onto different iron oxides follows an inner-sphere, bidentate-binuclear complexation model.
Fig. 7. Zeta potential measurement for the d-FeOOH nanoparticles.
d-FeOOH nanoparticles was 37.3 mg g1. This value was comparable to many of the reported adsorption capacities for iron oxides in the state of art (Table 4). The high As(V) adsorption capacity was due to the small particle size and high surface area of d-FeOOH nanoparticles. Moreover, the mesoporous d-FeOOH nanoparticles are magnetic and their dispersion in water can be easily separated by applying an external magnetic field. These nanoparticles showed saturation magnetization of 13.2 emu g1. Upon magnetic separation and stirring with aqueous NaOH solution. It is interesting to highlight that this high adsorption capacity was found for the pure and unmodified d-FeOOH and, therefore, several possibilities to improve the d-FeOOH adsorption capacity based on materials chemistry still are open for further studies. To obtain more information about the arsenic adsorption by d-FeOOH nanoparticles, Langmuir, Freundlich and Redlich–Peterson isotherm models were tested. The Freundlich model (plots not shown) did not fit well to the obtained data. The best results were obtained for the Redlich–Peterson (R2 = 0.9778) and Langmuir
3.3. Adsorption isotherms To determine the adsorption capacity of d-FeOOH nanoparticles, the equilibrium adsorption of arsenic was studied as a function of arsenic concentration. Fig. 9 showed that initially the isotherm rises sharply indicating that the accessible sites in d-FeOOH are available for adsorption. However, as As(V) concentration increased, site saturation of d-FeOOH surface occurred and a plateau was reached, indicative of that all active adsorption sites in d-FeOOH were occupied. The adsorption capacity of As(V) by
Table 2 Adsorption kinetic parameters for 10 mg L1 As(V) adsorption on d-FeOOH nanoparticles at different pH and at 25.0(±0.5) °C. pH
5 7 9
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
0.0012 0.0009 0.0006
41.3 37.7 23.3
2.05 1.28 0.33
0.9958 0.9880 0.9030
Fig. 6. (a) Effect of pH on As(V) adsorption onto d-FeOOH nanoparticles. (b) Pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles at different pH. Initial As(V) concentration = 10 mg L1; Adsorbent dose = 0.25 g L1, shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C.
52
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
Fig. 8. (a) Effect of ionic strength on As(V) adsorption onto d-FeOOH nanoparticles. (b) Pseudo-second order model for adsorption of As(V) on d-FeOOH nanoparticles at different ionic strength. Initial As(V) concentration = 10 mg L1; adsorbent dose = 0.25 g L1, shaking speed = 100 rpm; temperature = 25.0 ± 0.5 °C, pH = 7.
Table 3 Adsorption kinetic parameters for 10 mg L1 As(V) adsorption on d-FeOOH nanoparticles in the presence of different NaNO3 concentrations, pH 7 and at 25.0(±0.5) °C. [NaNO3] (mg L1)
0 50 500
Pseudo-second-order k2 (g mg1min1)
qe (mg g1)
h0 (mg g1 min1)
R2
0.0009 0.0016 0.0015
37.7 29.9 29.5
1.28 1.43 1.31
0.9880 0.9926 0.9941
Fig. 9. As(V) adsorption isotherm onto d-FeOOH nanoparticles. The data is fitted with the (a) Langmuir isotherm and (b) Redlich–Peterson isotherm.
(R2 = 0.9634) isotherm models (Fig. 9 and Table 5). Langmuir isotherm equation was given as follows (Eq. (5)):
qeq ¼
kL Q m C e ; 1 þ kL C e
ð5Þ
where Qm (mg g1) and Ce (mg L1) are maximum adsorption capacity and the concentration at equilibrium respectively, kL is Langmuir constant, which represents the energy of adsorption (L mg1). Redlich–Peterson isotherm was expressed below (Eq. (6)):
qeq ¼
k1 C e 1 þ k2 C ae
ð6Þ
where qeq (mg g1) and Ce (mg L1) are the amount adsorbed per unit mass of the adsorbent and concentration at equilibrium, respectively. k1 (L g1) and k2 (L mg1) are the Redlich–Peterson isotherm constants, and a is an exponent that describes the system heterogeneity; it lies between 0 and 1.
The Redlich–Peterson isotherm is an empirical equation that may be used to represent adsorption equilibrium over a wide concentration range. The mechanism of adsorption is a hybrid model featuring both Langmuir and Freundlich isotherms and it does not follow ideal monolayer adsorption [41]. Eq. (5)can be simplified to the Langmuir isotherm when a = 1. Since the a value obtained from the Redlich–Peterson was 0.97 in this study, we suggest that the As(V) adsorption on d-FeOOH surface followed a non-ideal monolayer adsorption model. 4. Conclusions An efficient adsorbent for As(V) based on d-FeOOH nanoparticles was prepared by a simple and environmentally friendly method. d-FeOOH had a high adsorption capacity for As(V), 37.3 mg g1, at neutral pH. This high adsorption capacity was assigned to the small particle size and the high specific surface area of d-FeOOH. Moreover, d-FeOOH can be characterized as a low cost and easily
53
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54 Table 4 Comparison between adsorption capacities for d-FeOOH and others iron oxides. Adsorbent
BET area (m2 g1)
pH
Adsorption (mg g1)
Adsorption (mg m2)
Reference
d-FeOOH Zr-doped b-FeOOH a-FeOOH/siderite c-FeOOH treated at 250 °C Fe3O4 Fe3O4-reduced graphite oxide-MnO2 TiO2/Fe2O3 Fe2O3/activated carbon Ultrafine hematite (5 nm) c-Fe2O3/TiO2 Mg-doped a-Fe2O3 Natural hematite Ascorbic acid-coated Fe3O4 Fe3O4/graphene Schwertmannite
135 220.77 95.8–110 109.2 245 113.7 133.5 821 162 153.9 438 0.38 178.48 98.2 206.1
7 7 7 7 7 7 7 7 7 7 7 7 7 7 7
37.3 60 115–121 38 41 12.22 7.8 17 47 33 10 0.02 16.56 180.3 34
0.28 0.27 1.2 0.35 0.17 0.11 0.06 0.02 0.29 0.21 0.02 0.05 0.09 1.84 0.16
This work (15) (42) (30) (43) (22) (24) (44) (45) (46) (21) (14) (47) (23) (48)
Table 5 Isotherm parameters obtained by non-linear regression fit for the d-FeOOH nanoparticles. Langmuir isotherm
Redlich–Peterson isotherm
qm (mg g1)
kL (L mg1)
R2
K1 (L g1)
K2 (L mg1)
a
R2
37.30
20.12
0.9634
819.02
23.04
0.97
0.9778
available adsorbent, as the starting reagents are cheap. Because of the aforesaid, d-FeOOH holds considerable promise to be used as an adsorbent for heavy metals in contaminated water. Acknowledgments The authors are grateful to CNPQ (including grants #470738/ 2011-1; 302755/2011-0; 550341/2012-9; 309455/2012-0) and FAPEMIG and FAPESP for financial support. References [1] EPA, 2001. Cost Analyses for Selected Groundwater Cleanup Projects: Pump and Treat Systems and Permeable Reactive Barriers. U.S. EPA, Office of Solid Waste and Emergency Response. EPA-542-R-00-013, February 2001. [2] O.X. Leupin, S.J. Hug, Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron, Water Res. 39 (2005) 1729–1740. [3] I.A. Katsoyiannis, A.I. Zouboulis, Application of biological processes for the removal of arsenic from groundwaters, Water Res. 38 (2004) 17–26. [4] I.A. Katsoyiannis, A.I. Zouboulis, M. Jekel, Kinetics of bacterial As(III) oxidation and subsequent As(V) removal by sorption onto biogenic manganese oxides during groundwater treatment, Ind. Eng. Chem. Res. 43 (2004) 486–493. [5] X. Meng, G.P. Korfiatis, C. Christodoulatos, S. Bang, Treatment of arsenic in Bangladesh well water using a household co-precipitation and filtration system, Water Res. 35 (2001) 2805–2810. [6] A.S. Sheoran, V. Sheoran, Heavy metal removal mechanism of acid mine drainage in wetlands: a critical review, Min. Eng. 19 (2006) 105–116. [7] J.E. Greenleaf, J. Lin, A.K. Sengupta, Two novel applications of ion exchange fibers: arsenic removal and chemical-free softening of hard water, Environ. Prog. 25 (2006) 300–311. [8] D. Mohan, C.U. Pittman Jr., Arsenic removal from water/wastewater using adsorbents—a critical review, J. Hazard. Mater. 142 (2007) 1–53. [9] X. Luo, C. Wang, L. Wang, F. Deng, S. Luo, X. Tu, C. Au, Nanocomposites of graphene oxide-hydrated zirconium oxide for simultaneous removal of As(III) and As(V) from water, Chem. Eng. J. 220 (2013) 98–106. [10] X. Luo, C. Wang, S. Luo, R. Dong, X. Tu, G. Zeng, Adsorption of As(III) and As(V) from water using magnetite Fe3O4-reduced graphite oxide-MnO2 nanocomposites, Chem. Eng. J. 187 (2012) 45–52. [11] R.Y. Ning, Arsenic removal by reverse osmosis, Desalination 143 (2002) 237– 241. [12] Y. Sato, M. Kang, T. Kamei, Y. Magara, Performance of nanofiltration for arsenic removal, Water Res. 36 (2002) 3371–3377. [13] A.B. Ribeiro, E.P. Mateus, L.M. Ottosen, G. Bech-Nielsen, Electrodialytic removal of Cu, Cr, and As from chromated copper arsenate-treated timber waste, Environ. Sci. Technol. 34 (2000) 784–788.
[14] J. Giménez, J. Pablo, M. Martínez, M. Rovira, C. Valderrama, Reactive transport of arsenic (III) and arsenic(V) on natural hematite: experimental and modeling, J. Colloid. Interface Sci. 348 (2010) 293–297. [15] X. Sun, C. Hu, X. Hu, J. Qu, M. Yang, Characterization and adsorption performance of Zr-doped akaganèite for efficient arsenic removal, J. Chem. Technol. Biotechnol. 88 (2013) 629–635. [16] Q.L. Zhang, T.C. Lin, X. Chen, N.Y. Gao, A method for preparing ferric activated carbon composites adsorbents to remove arsenic from drinking water, J. Hazard. Mater. 148 (2007) 671–678. [17] D.Q.L. Oliveira, M. Gonçalves, L.C.A. Oliveira, L.R.G. Guilherme, Removal of As (V) and Cr (VI) from aqueous solutions using solid waste from leather industry, J. Hazard. Mater. 151 (2008) 280–284. [18] C. Han, H. Li, H. Pu, H. Yu, L. Deng, S. Huang, Y. Luo, Synthesis and characterization of mesoporous alumina and their performances for removing arsenic(V), Chem. Eng. J 217 (2013) 1–9. [19] V. Swarnkar, R. Tomar, Use of surfactant-modified zeolites for arsenate removal from pollutant water, J. Disp. Sci. Technol. 33 (2012) 913–918. [20] A. Anjum, P. Lokeswari, M. Kaur, M. Datta, Removal of As (III) from aqueous solutions using montmorillonite, J. Anal. Sci. Methods Instrum. 1 (2011) 25–30. [21] W. Tang, Y. Su, Q. Li, S. Gao, J.K. Shang, Mg-doping: a facile approach to impart enhanced arsenic adsorption performance and easy magnetic separation capability to –Fe2O3 nanoadsorbents, J. Mater. Chem. A 1 (2013) 830–836. [22] X. Luo, C. Wang, S. Luo, R. Dong, X. Tu, G. Zeng, Adsorption of As (III) and As (V) from water using magnetite Fe3O4-reduced graphite oxide–MnO2 nanocomposites, Chem. Eng. J. 87 (2012) 45–52. [23] A.K. Mishra, S. Ramaprabhu, Ultrahigh arsenic sorption using iron oxidegraphene nanocomposite supercapacitor assembly, J. Appl. Phys. 112 (2012) 104315–104316. [24] M. D’Arcy, D. Weiss, M. Bluck, R. Vilar, Adsorption kinetics, capacity and mechanism of arsenate and phosphate on a bifunctional TiO2–Fe2O3 bi-composite, J. Colloid Interface Sci. 364 (2011) 205–212. [25] T. Basu, K. Gupta, U.C. Ghosh, Performances of As(V) adsorption of calcined (250 °C) synthetic iron(III)–aluminum(III) mixed oxide in the presence of some groundwater occurring ions, Chem. Eng. J. 183 (2012) 303–314. [26] W. Zhang, P. Singh, E. Paling, S. Delides, Arsenic removal from contaminated water by natural iron ores, Miner. Eng. 17 (2004) 517–524. [27] H. Guo, D. Stuben, Z. Berner, Removal of arsenic from aqueous solution by natural siderite and hematite, Appl. Geochem. 22 (2007) 1039–1051. [28] M. Mohapatra, S.K. Sahoo, S. Anand, R.P. Das, Removal of As(V) by Cu(II)-, Ni(II)-, or Co(II)-doped goethite samples, J. Colloid Interface Sci. 298 (2006) 6–12. [29] E.A. Deliyanni, D.N. Bakoyannakis, A.I. Zouboulis, K.A. Matis, Sorption of As (V) ions by akaganèite-type nanocrystals, Chemosphere 50 (2003) 155–163. [30] E. Repo, M. Makinen, S. Rengaraj, G. Natarajan, A. Bhatnagard, M. Sillanpaa, Lepidocrocite and its heat-treated forms as effective arsenic adsorbents in aqueous medium, Chem. Eng. J. 180 (2012) 159–169. [31] I.S.X. Pinto, P.H.V.V. Pacheco, J.V. Coelho, E. Lorençon, J.D. Ardisson, J.D. Fabris, P.P. Souza, K.W.H. Krambrock, L.C.A. Oliveira, M.C. Pereira, Nanostructured dFeOOH: an efficient Fenton-like catalyst for the oxidation of organics in water, Appl. Catal. B 119 (2012) 175–182. [32] K. Müller, V.S.T. Ciminelli, M.S.S. Dantas, S. Willscher, A comparative study of As (III) and As (V) in aqueous solution and adsorbed on iron oxy-hydroxides by Raman spectroscopy, Water Res. 44 (2010) 5660–5672. [33] M.C. Pereira, E.M. Garcia, A.C. Silva, E. Lorençon, J.D. Ardisson, E. Murad, J.D. Fabris, T. Matencio, T.C. Ramalho, M.V.J. Rocha, Nanostructured d-FeOOH: a novel photocatalyst for water splitting, J. Mater. Chem. 21 (2011) 10280– 10282. [34] P. Chagas, A.C. Silva, E.C. Passamani, J.D. Ardisson, L.C.A. Oliveira, J.D. Fabris, R.M. Paniago, D.S. Monteiro, M.C. Pereira, d-FeOOH: a superparamagnetic material for controlled heat release under AC magnetic field, J. Nanopart. Res. 15 (2013) 15–44.
54
M.C.S. Faria et al. / Chemical Engineering Journal 237 (2014) 47–54
[35] S. Lagergren, About the theory of so-called adsorption of soluble substances, Kungliga Svenska Vetenskapsakademien Handlingar 24 (1998) 1–39. [36] Y.S. Ho, Second-order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and nonlinear methods, Water Res. 40 (2006) 119–125. [37] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. [38] W.J. Weber, J.C. Morris, Kinetics of adsorption of carbon from solutions, J. Sanit. Eng. Div. 89 (1963) 31–60.
[39] V. Belessi, G. Romanos, N. Boukos, D. Lambropoulou, C. Trapalis, Removal of reactive red 195 from aqueous solutions by adsorption on the surface of TiO2 nanoparticles, J. Hazard. Mater. 170 (2009) 836–844. [40] L. Zheng, A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal, Water Res. 37 (2003) 4351–4358. [41] R.K. Prasad, S.N. Srivastava, Sorption on distillery spent was onto fly ash: kinetics and mass transfer studies, Chem. Eng. J. 146 (2009) 90–97.