Applied Clay Science 140 (2017) 157–164
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Adsorption of phosphate ions from aqueous solution by modiﬁed bentonite with magnesium hydroxide Mg(OH)2 Mohamed El Bouraie a,⁎, Alaa A. Masoud b a b
Central Laboratory for Environmental Quality Monitoring (CLEQM), National Water Research Center (NWRC), El-Qanater El-Khairiya 13621, Egypt Geology Department, Faculty of Science, Tanta University, Egypt
a r t i c l e
i n f o
Article history: Received 3 August 2016 Received in revised form 13 January 2017 Accepted 19 January 2017 Available online 16 February 2017 Keywords: Adsorption Phosphate Bentonite Modiﬁcation Adsorption model
a b s t r a c t This study investigated phosphate ions removal from aqueous solutions by using modiﬁed bentonite with magnesium hydroxide in batch system. Raw bentonite (RB) and Mg-modiﬁed bentonite (MB) were characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). Adsorption experiments were conducted on the adsorption of phosphate onto RB and MB in batch experiments. Phosphate ion removal by MB was pH dependent, and the optimum adsorption was observed at pH 7. The adsorption process was relatively fast and equilibrium conditions were established within 120 min at 45 °C. The results were analyzed according to the Langmuir and Freundlich isotherm equations. The adsorption data is well interpreted by the Langmuir isotherm. Phosphate solution at a concentration of 25 mg/L was adsorbed by MB, and the ﬁnal adsorption efﬁciency was N54%. The results showed that phosphate adsorption density of MB was high with the maximum adsorption density of 14.33 mg/m2, which suggested that MB was an excellent adsorbent for effective phosphate removal from water. Thermodynamically negative ΔG°, positive ΔH°, and positive ΔS° demonstrated the high afﬁnity, and endothermic adsorption process between MB and phosphate from aqueous solutions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Contamination of surface water with phosphate and its removal in water treatment had become increasing focus worldwide. Many dephosphorization studies had been made for aqueous solution, including biological, chemical precipitation, and adsorption (Yu et al., 2010). Of all phosphates removal techniques, adsorption was increasing attention and becoming an attractive technology because of its simplicity, low cost, ease of operation and handling, sludge free operation, and the capacity of regenerate and reuse solids. In this regard, many adsorbents had been explored such as: zeolite; bauxite reﬁnery residues (red mud; BauxsolTM); calcined dolomite; ﬂy ash; and ferric iron oxides (Fytianos et al., 1998; Wu et al., 2006; Huang et al., 2008; Despland et al., 2011). Phosphorus (P) is an essential, often limiting, nutrient for growth of organisms in most ecosystems. However, excessive supply of phosphorus from wastewater into water bodies, such as lakes, rivers and creeks cause eutrophication, resulting in the bloom of aquatic plants, growth of algae and depletion of dissolved oxygen. Phosphate removal from aqueous solution had been widely studied during the past decades. Typical ⁎ Corresponding author. E-mail addresses: [email protected]
(M. El Bouraie), [email protected]
http://dx.doi.org/10.1016/j.clay.2017.01.021 0169-1317/© 2017 Elsevier B.V. All rights reserved.
removal methods such as chemical and biological treatments had been successfully applied (Yeoman et al., 1988). Nevertheless, increasing attention had been paid to adsorptive removal of phosphate from aqueous solution (Hano et al., 1997; Donnert and Salecker, 1999). Eutrophication had become the primary water quality issue for most of the freshwater ecosystems in the world (Smith et al., 2006). N38% of water bodies in many regions of the world were considered to have eutrophication problems. One gram of phosphorous was required for every 7 g of nitrogen for the formation of the organic matter created in the process (Ross et al., 2008). Thus, the excess of bioavailable phosphorous was the key nutrient which was understood to lead to eutrophication of water bodies, resulting in increased aquatic plant and algal growth. Chemical methods for removing phosphate compounds caused product quality reducing and side effects on aquatic environments. Among the available methods for removing these pollutants, adsorption is still one of the most preferred methods, especially for efﬂuents with moderate and low pollutant concentration (Nameni et al., 2007). In the past years, there had been increasing interest in developing recyclable inorganic adsorbents, particularly from bentonite for efﬁcient removal of organic pollutants from aqueous solutions (Nouri, 2002). Raw bentonite is one of the abundant clay minerals at the earth's surface, which can be used as an effective adsorbent for many toxic substances in soil, water and air. Bentonite is effective adsorbent for cations
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
but it shows lower afﬁnity toward negative groups, like phosphate, due to the absence of effective adsorption sites for anions in water. Adsorption onto bentonite that contains montmorillonite appears to involve two distinct mechanisms: (i) an ion exchange reaction at permanentcharge sites, and (ii) formation of complexes with the surface hydroxyl groups. Modiﬁed bentonite is prepared by exchanging the naturally occurring interlayer monovalent and divalent cations like sodium and/or calcium with highly charged polymeric metal species such as magnesium. Recently, raw bentonite was successfully employed for the adsorption of metal ions and dyes. It has been considered as a potential adsorbent for the removal of pollutants from water. The effective application of bentonite for the water treatment is limited due to its surface area and presence of net negative charge leading to its low adsorption capacity. As a result in the present research work modiﬁcation of physical structure and chemical properties of bentonite results the maximization of its adsorption capacity (Tahir et al., 2010). A composite adsorbent, modiﬁed bentonite with Magnesium Hydroxide (MB), was proposed and studied in this research. The reason for choosing magnesium is that relative to Fe+3 or Al+3 ions, magnesium hydroxide has a higher afﬁnity for phosphate (Fan and Anderson, 2005). Bentonite, which had a high surface area, should provide an efﬁcient surface for magnesium hydroxide. At the same time, the efﬁciency of adsorption was greatly affected by the MB amount and its preparation conditions. MB was known to have high surface area and permanent porosity, which makes them attractive adsorbents (Jiang et al., 2004). The use of MB in environmental applications was gained increasing attention due to the versatility of the modiﬁcation process through which the porosity and properties of both pillar and bearing material can be modiﬁed to achieve desired product properties (Gurses et al., 2006). The objective of this study was to examine the feasibility of using Magnesium modiﬁed bentonite as adsorbents for phosphate removal from aqueous solutions. A modiﬁed bentonite based on natural bentonite, was prepared and phosphate uptake was evaluated vs. pH using adsorption efﬁciency and adsorption isotherms. The effects of temperature and thermodynamic study on phosphate adsorption capability were also investigated. RB was compared with MB used for phosphate ions removal from aqueous solutions. 2. Materials and methods 2.1. Chemicals All chemicals used in the present study were of analytical reagent grade. Sodium hydroxide and magnesium chloride were obtained from Merck, Germany. A stock solution of phosphate (40 mg/L) was prepared by dissolving 175.75 mg of KH2PO4, dried at 120 °C for 2 h, in 1 L of deionized water (Conductivity = 0.5 μS/m). Working standards were prepared by dilution of the stock solution. 0.1 M HCl and 0.1 M NaOH solutions were used for pH adjustment.
Table 1 Physical and chemical characteristics of raw and modiﬁed bentonite. Parameters
Physical characteristics Loss on ignition (%) Density Granulometry (μm) Capacity swelling Cg pH for 10 g/L Conductance (μS) for 0.5 g/L Speciﬁc surface area, BET (m2/g)
12.66 ± 0.06 c 1 ± 0.02 c 1–2 ± 0.03 c 8.31 ± 0.08 b 9.6 ± 0.02 c 63.5 ± 0.03 c 33.52 ± 0.02 b
12.31 ± 0.04 b 1.05 ± 0.02 a 1–1.5 ± 0.04 b 8.52 ± 0.06 c 7.8 ± 0.04 b 81.4 ± 0.02 c 51.72 ± 0.04 a
Chemical composition (wt%) SiO2 Al2O3 Fe2O3 MgO TiO2 CaO PO4 K2O Na2O MnO FeO C S
55.70 ± 0.03 c 21.5 ± 0.08 b 3.5 ± 0.09 a 5.6 ± 0.06 c 0.84 ± 0.08 b 3.15 ± 0.04 c 0.75 ± 0.05 c 0.51 ± 0.04 c 3.75 ± 0.08 b 1.56 ± 0.02 c 0.82 ± 0.04 b 1.47 ± 0.02 c 0.85 ± 0.03 a
59.5 ± 0.02 b 0.03 ± 0.05 c 3.65 ± 0.03 c 12.2 ± 0.05 a 0.92 ± 0.06 c 3.26 ± 0.03 a 13.01 ± 0.06 b 0.36 ± 0.03 a 2.89 ± 0.04 c 1.08 ± 0.01 b 0.54 ± 0.05 b 1.62 ± 0.06 c 0.94 ± 0.04 b
The same letters in the same column indicate no signiﬁcant difference (p N 0.05).
as on the outer surface and edges. The cation exchange capacity (CEC) of the sample was determined by the methylthioninium chloride method before puriﬁcation was about 0.512 meq/g, this value was low compared to pure bentonite sample. Therefore, the sample was not pure enough and the mass yield was 63.88% (Olu-Owolabiand and Unuabonah, 2011). The speciﬁc surface areas (BET) of the bentonite samples were determined by ASAP 2000 micropore analysis. The BET of RB was 33.52 m2/g. 2.3. Puriﬁed bentonite Bentonite particles were dispersed in water and heated at 75 °C in the presence of a solution composed of 2 M AlCl3 and 1 M NaOH. The purpose of this operation was to eliminate inorganic and organic compounds, various free cations found in the interlayer spaces, and then saturated with aluminium (Al3+) to ensure complete transformation into the aluminium form. After these exchanges, the slurry was stirred for 12 h at room temperature, ﬁltered, and washed repeatedly with deionized water (Arias and Sen, 2009). The suspension obtained was put in dialysis membranes to remove the chloride ions adsorbed onto the surface from the layers. The dialysis water was renewed until what the test with silver nitrate indicates the absence of chloride ions. Then, the puriﬁed bentonite suspension was dried in an oven at a temperature not exceeding 60 °C in order to be activated later. The CEC was determined by the methylene blue method after puriﬁcation activation was about 1.13 meq/g and the mass yield was 55.62%.
2.2. Raw bentonite 2.4. Modiﬁcation of bentonite by magnesium hydroxide RB was used as an adsorbent for the experiments. All bentonite samples were in the clod sized, forms when ﬁrst received. Latter, they were dried at 105 °C for 4 h in a drying air oven, and then were grounded using a porcelain mill to 74 μm for the experiments. The results of the mineralogical and chemical compositions of the materials are presented in Table 1. The basic structural unit of RB was composed of two tetrahedral coordinated sheets of silicon ions surrounding a sandwiched octahedral coordinated sheet of aluminium ions. The isomorphous substitution of Al3+ for Si4+ in the tetrahedral sheet and Mg2+ for Al3+ in the octahedral sheet results in a net negative surface charge on bentonite. Compared with other clay types, it has excellent adsorption properties and possesses adsorption sites available within its interlayer space as well
10 g of treated bentonite was immersed in 150 mL of 2.0 M magnesium hydroxide and the mixture was agitated at 90 °C for 6 h. The obtained powder was rinsed with 0.01 M HCl aqueous solution to remove the excess Mg(OH)2 precipitated on the outer surface of bentonite and further washed with deionized water. In some times, The colour on surface bentonite was turned from original light colour to dark brown, indicating the oxidation of the hydroxide into oxide phase at room temperature and the yield was mashed manually (Liu et al., 2011). Some inorganic materials can be used to improve the gelation property of bentonite. According to this study, ﬂowability and stability of RB was improved by the addition of Mg(OH)2. However, it is worth to mention that the addition of Mg(OH)2 into the suspension
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
signiﬁcantly increases the viscosity of bentonite suspensions. In addition, the solubility of Mg(OH)2 was too low, which led to regulate the viscosity behaviours of bentonite suspensions. This was an interesting point that the positive effect of Mg ions in bentonite suspensions, however, without emphasizing the effect of Mg(OH)2 particles in suspensions. Since Mg(OH)2 does not dissolve well in bentonite suspensions, the effect of dissolved Mg ions on suspension viscosity can be eliminated. In this study, the modiﬁcation process was performed to understand the interaction mechanism between bentonite particles and Mg(OH)2. Therefore, the BET surface area value increased from 33.52 to 51.72 m2/g. The higher surface area was obtained because of the removal of inorganic impurities by the Mg(OH)2 interaction. The higher values of the speciﬁc surface area were related to the agglomerated structure of magnesium hydroxide particles, which had been created during the modiﬁcation process. 2.5. Characterization of the adsorbents The morphological and chemical compositions of raw and modiﬁed bentonite were obtained by the scanning electron microscope (SEM) a JSM6360LV SEM with High-Low vacuum (JEOL) with the accelerating voltage of 20 kV with X-ray dispersive spectrometer (JXA8621 Superprobe; JEOL, Japan). The Fourier Transformed Infrared (FTIR) spectra of the bentonites were obtained using KBr wafers and SHIMADZU 8400S FTIR. The FTIR was used to identify the surface functional groups of two bentonites. 2.6. Phosphate adsorption experiments ions onto adsorbents were tested in a batch The adsorption of PO3− 4 equilibration by mixing 2 g of the sorbent in a 1 L phosphate solution with the initial concentrations (ranging from 0.05 to 25 mg/L). The experiments were carried out at pH of 6.0 and 7.0 for both bentonites (RB and MB) under the operating temperature (45 ± 0.1 °C), and agitation speed of 200 rpm within the equilibrium time of 120 min, unless otherwise speciﬁed by the study design (i.e., to examine the effects of pH, temperature, initial metal concentration, and contact time). After equilibration, supernatant were withdrawn and ﬁltered with 0.45 μm syringe driven ﬁlter (Millex-LH, PTFE, Millipore Corp., Ireland). The ﬁltrate concentration was measured by the molybdate blue spectrophotometric method (APHA, 2005) using a Lambda 25 UV/VIS spectrophotometer (Perkin-Elmer, Germany). The determination limit of the analytical method was 0.01 mg PO−3 4 /L. Blank samples with no adsorbent were perpetrated and monitored as a control. All experiments were run in triplicate. Phosphate removal efﬁciency (E) was calculated according to Eq. (1): E¼
ðC o −C e Þ 100 Co
al., 2012). All experiments were repeatedly performed in duplicate. The experimental error limit of duplicates was maintained at ±0.5%. 2.6.1. Adsorption isotherm Langmuir and Freundlich isotherm models were used to establish the relationship between the amount of adsorbed phosphate ion onto MB and its equilibrium concentration in aqueous system (Krishna and Bhattacharyya, 2002). Langmuir adsorption isotherm was based on the assumption of monolayer adsorption onto a surface with a ﬁnite number of identical sites (Sen and Sarzali, 2008). Langmuir isotherm could be arranged in its linear form as Eq. (3): Ce Ce 1 ¼ þ qe qm K L qm
V ðC o −C e Þ BET
Where V is the volume of solution (L), Co and Ce are the concentration of phosphate (mg PO−3 4 /L) before and after adsorption, respectively, and BET (m2/g) is the speciﬁc surface areas of adsorbent (Zehhaf et
Where Ce is the equilibrium concentration of phosphate (mg/L) and qe is the amount of adsorbed in milligram per unit speciﬁc surface area of adsorbent (mg/m2). qm and KL are Langmuir constant relating adsorption density (mg/m2) and the energy of adsorption (L/g), respectively. These constants can be calculated from the slope and intercept of the linear plots of Ce/qe versus Ce, respectively. The dimensionless parameter of adsorption (RL) (deﬁned as RL = 1/(1+ KLCo), where Co is the initial concentration was used as an indicator to assess the extent of adsorption). Depending on the RL value, there are four possibilities for adsorption: (1) favorable adsorption if 0 b RL b 1, (2) unfavorable adsorption when RL N 1, (3) linear adsorption for RL = 1, and (4) irreversible adsorption for RL = 0 (Sen and Dustin, 2011). The adsorption data were also ﬁtted to Freundlich isotherm, which is described by the linear form following Eq. (4): logqe ¼ logK F þ
1 logC e n
Where KF and n are Freundlich constants incorporating all factors affecting the adsorption density and intensity of adsorption, respectively. Values of KF and n were determined from the intercept and slope of the linear plot of log qe versus log Ce. 2.6.2. Thermodynamic parameters The removal of phosphate ions by MB was studied from thermodynamic viewpoint to ascertain the nature of adsorption process under the condition of the current study. To achieve this goal, three thermodynamic parameters, including Gibb's free energy (ΔG°), enthalpy change (Δ H°) and change in entropy (Δ S°), were determined by using Eqs. (5)–(8): KL ¼
Where E is the phosphate removal efﬁciency (%), Co is the initial con3 centration of phosphate (mg PO− 4 /L) and Ce is the equilibrium concentration. The phosphate adsorption kinetics were studied after speciﬁed period of time the ﬂasks were taken out for the analysis of phosphate concentration. The amount of adsorbed in milligram per unit speciﬁc surface area of adsorbent is called by adsorption density, qe 2 (mg PO−3 4 /m ) was commonly represented by using Eq. (2):
ΔG ° ¼ −RT ln K L ln K L ¼
ΔS ° ΔH ° − R RT
ΔG ° ¼ ΔH °−TΔS °
ð6Þ ð7Þ ð8Þ
Where Cs and Ce are the removed and remaining concentrations, respectively. KL is the distribution coefﬁcient for the adsorption, ΔS°, ΔH°, and ΔG° are the changes of entropy, enthalpy, and the Gibbs energy, T (K) is the temperature, and R (8.314 J/mol K) is the gas constant. The values of ΔH° and ΔS° were determined from the slopes and intercepts of the plots of ln KL versus 1/T (Eloussaief et al., 2011; Özdes et al., 2011).
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
Fig. 1. SEM micrographs of bentonites: raw bentonite (a), Mg-modiﬁed bentonite (b) and bentonite after adsorption (c).
3. Results and discussion 3.1. Characterization of adsorbents SEM images help understand the microscale surface morphology of the bentonite samples. As can be seen in Fig. 1, raw bentonite surface is relatively more smooth and reveal spongy appearance with irregular structure than modiﬁed bentonite after the treatment with magnesium ions, which lead to the formation of Mg(OH)2 clusters between the interlayer spaces of bentonite. These Mg(OH)2 clusters formed between the interlayers are rigid enough not only to prevent the interlayer spacing from collapsing, but also to generate pores larger than those of conventional zeolites. After adsorption, modiﬁed bentonite surfaces became swollen. This swelling could be the result of phosphate adsorption. The X-ray diffraction analysis (XRD) was mainly used to give data pertinent to basal spacing of minerals and established the fact of adsorption and provided information regarding the absorbed molecules into the basal spacing of bentonite particles. XRD analysis was carried out by mixing the modiﬁed and raw bentonite separately with magnesium hydroxide to evaluate the changes occurred in the interlaminar distance of bentonite resulting from their inter action with Mg(OH)2. The X-ray diffractograms of raw, Mg-modiﬁed bentonite and bentonite after adsorption of phosphate ions were shown in Fig. 2. The small angle (3°– 10°) of XRD diffraction peaks could be used to study the structure of mesoporous materials. XRD results showed that the exception of SiO2 after modiﬁcation of the adsorbent, other compounds of the adsorbent decreased (Flessner et al., 2001). Until a certain loading, Al2O3 were converted to AlCl3 during the modiﬁcation process. Fig. 2a shows that peaks observed around 2θ = 22°, 24°, 27°, 31°, 36° and 42° are belong to Al2O3 phase. Bentonite samples were mainly composed of dioctahedral (SiO2) and speciﬁcally montmorillonite with the d001 basal reﬂection exhibited at about 15.3 Å (Fig. 2a). The estimated montmorillonite contents of the samples were found 91%, and 86% for the samples RB and MB, respectively. Thus, both samples were characterized by high concentration of montmorillonite and low levels of impurities. The main (d001) reﬂection
of oxy-silica-montmorillonite was reduced and (d200) reﬂection of dolomite disappeared. However, the shift of (d001) reﬂection of MB to the right on x axis points to the phosphate ion exchange from interlayer space for anions. Increasing ratio of Mg(OH)2/bentonite also increased the number of active sites MgO which has high catalytic activity and basicity. The active site MgO is responsible for the presence of the magnesium hydroxide molecule between the silicate layers. However, with further increase of Mg(OH)2 loading, the interactions between Mg(OH)2 with internal layer of bentonite (Si\\O\\Al stretching groups) were excessive and during the modiﬁcation, a new phase of Si\\O\\Mg compound was formed. This new phase compound (Si\\O\\Mg) had lower catalytic activity and basicity than the MgO phase (Rahni et al., 2014). The XRD pattern of MB showed poor crystallinity, broad and less intense peaks as compared to the parent bentonite mineral due to the presence excess amount of Si-O-Mg compound, or an irregular stacking of the activated and non-activated layers and thus the structure of the resultant bentonite became amorphous (Eren and Afsin, 2008). From XRD diagrams, the peaks present as in Fig. 2c, attributed to the physical adsorbed PO3− 4 ions onto MB. According to the intensity and quantity of the diffraction peaks, it can be speculated that Mg(OH)2 on the surface of MB was gradually converted in situ into Mg3(PO4)2. In order to clarify the mechaions onto MB as shown in (Scheme 1). nism of adsorption of PO3− 4 The FTIR spectra of raw and modiﬁed bentonite samples in the wavenumber range of 4000–500 cm−1 were shown in Fig. 3. The spectrum of raw bentonite exhibited absorption bands at 3450 and 1650 cm−1 this was assigned to the stretching and bending vibrations of the OH groups for the water molecules adsorbed on bentonite surface, and a band at 3621 cm−1 which represented the stretching vibration of the hydroxyl groups coordinated to octahedral Al3 + cations (Sdiri et al., 2011). The raw bentonite spectrum also contained a band between 693 and 796 cm−1 which were attributed to orthodase and quartz respectively (Sdiri et al., 2010). The intensive band at 1088 cm−1 was assigned to the Si\\O stretching vibration, whereas the bands around 466 and 521 cm− 1 were ascribed to Si\\O\\Al (where Al was the octahedral cation) and Si\\O\\Si bending vibrations, respectively. The intensive bands near 521, 850 and 916 cm−1 attributed to Al\\Mg\\OH, Al\\Al\\OH and Al\\OH\\O\\Si stretching vibration respectively. FTIR spectra of Fig. 3b and c showed the decreasing intensity as a result of modiﬁcation bentonite with magnesium hydroxide which reﬂects the leaching of octahedral cation, such as Al3+ from the bentonite structure. The vibrational band near 3626 cm−1, assigned to stretching Mg-OH vibrations is typical for Mg-rich dioctahedral layered minerals. The reduction of the content of the octahedral cation was accompanied by a decrease of both OH bending vibrations at 850 and 916 cm−1. The stretch vibrations found within band range 1100–
Fig. 2. X-ray diffraction patterns of bentonites: raw bentonite (a), Mg-modiﬁed bentonite (b) and bentonite after adsorption (c).
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
Scheme 1. Adsorption mechanism of phosphate onto modiﬁed bentonite.
1200 cm− 1 appeared to represent functional group of phosphorus (P_O) belonging to phosphate ions as in Fig. 3c. The FTIR result was in clear agreement with the SEM and XRD studies which indicate sequential degradation of the bentonite sheet upon acid treatment.
3.2. Effect of pH Adsorption of phosphate by raw and modiﬁed bentonite was investigated in the pH range 4–9 while maintaining the other parameters constant. Fig. 4 showed the change in phosphate uptake by both bentonites at different initial pH levels. In general, Raw bentonite carry two types of electrical charge; one permanent negative charge generated from its structural components and a second variable surface charge which depends on the pH of the liquid phase. The adsorption efﬁciency of MB increased gradually in the pH range of 4–7 and reached a maximum value (54.6%) when the pH value was 7. At pH values over 8, the surface of MB became negatively charged thus phosphate adsorption drops dramatically (13.4%). Although the increased adsorption efﬁciency in this region was very pronounced for the MB, such increment was not observed for RB. The adsorption efﬁciency of RB changed very little within the pH range of 4–6 and decreased in solutions with higher pH values. Hence, with increased of pH values, the negative charges of the RB surface that made electrostatic repulsion between phosphate ions with SiO2 which had been formed at high pH. But in acidic pH, the surface is positively charged due to the high hydrated radius, that made electrostatic attraction with PO−3 4 ions. In the present study, the Mg(OH)2 modiﬁed of bentonite resulted to an increase of pH value at point of zero charge (PZC) from 8.4 to 9.7; that means for a wide range of pH values, the MB remains positively charged, favoring the electrostatic attraction to H2PO− 4 (pKa = 2.1) and to (pKa = 7.2) species. As we have already shown, the initial PZC HPO−2 4 of the RB was accompanied with a low afﬁnity for phosphate in
Fig. 3. FTIR spectra of bentonites: raw bentonite (a), Mg-modiﬁed bentonite (b) and bentonite after adsorption (c).
Fig. 4. Effect of pH on phosphate-uptake by raw and modiﬁed bentonites.
contradiction to the expected; the latter was attributed to the electrostatic attraction mechanism (solution pH b pH during the initial adsorption phase). PZC could change when the surface functional groups are altered due to their modiﬁcation. The surface charge was effected by the solution pH; at pH b pHPZC the surface was positively charged, while at pH N pHPZC was negatively charged. Hence, the chemical modiﬁcation of bentonite with Mg(OH)2 and the enhanced chemical or/and physical adsorption mechanisms should play a greater role on removal of phosphate (Mekhlouﬁ et al., 2013). The pH-dependent increase was due to increasing adsorption of formed phosphate anions, with pKa 7.2 (Papadas et al., 2009), and with the positively charged surface sites of bentonite. Meanwhile, the major decrease in adsorption was due to the repulsion between phosphate molecules and the abundance of OH− ions at higher pH values (Stathi et al., 2007). Another reason for this decrease was due to the particle interactions resulting from the high adsorbents dose, such as aggregations. Aggregations could lead to a decrease in the total surface area of the adsorbents and an increase in the diffusional path length. However, phosphate adsorption onto MB appeared to be more pH dependent than with RB. 3.3. Effect of contact time Fig. 5 showed the change in phosphate adsorption efﬁciency with time on raw and modiﬁed bentonite. Adsorption studies were continued for 250 min, and it was found that adsorption efﬁciency increased with the increasing contact time, and a large portion of the phosphate ions were adsorbed onto RB within a few minutes (20 min); whereas MB reached equilibrium almost at the end of 120 min. This was due to MB had more expanded sheets between the layers than RB, conﬁrmed the signiﬁcant role of chemical adsorption as a removal mechanism for phosphate. In MB, phosphate binding was lower at the beginning of the experiment, probably because of the surface interaction (physical
Fig. 5. Effect of the contact time on phosphate-uptake by raw and modiﬁed bentonites.
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
Fig. 6. Effect of phosphate concentration on adsorption density of raw and modiﬁed bentonites.
Fig. 8. Effect of temperature on adsorption densities of raw and modiﬁed bentonites toward phosphate.
adsorption) combined with slow dissolution of Mg(OH)2 that usually constitutes the ﬁrst step of phosphate adsorption. The adsorption may take place on the outer surface and in the interlayers of the bentonites. Therefore, phosphate uptake at the beginning was about 2.3 times higher for MB than RB. However, adsorption ability for MB reached ﬁve times higher value than that of RB at the end of the experiments.
ion increased with the increase in the adsorbent amount (adof PO−3 4 sorption density) due to instauration of the active sites on the adsorbent surface during the adsorption process (Zamparas et al., 2012).
3.4. Effect of phosphate concentration The removal of phosphate ions onto the adsorbents as a function of their concentrations was investigated at constant temperature (45 ± 0.1 °C) by varying the initial phosphate concentration from 0.05 to 25 mg/L while keeping all other parameters constant. Fig. 6 represented the change in adsorption behavior of both bentonites under different phosphate ion concentrations. The adsorption density increased with the increasing amounts of phosphate added onto RB and MB. At a low adsorbed rate of phosphate added (0.05 mg/L), the amounts of PO−3 4 onto RB was 0.002 mg/m2 (3.0%). However, at the high phosphate adsorbed onto RB was added (25 mg/L) rate, the amounts of PO−3 4 1.62 mg/m2 (6.49%). But in case of MB, amounts of phosphate adsorbed at the low (0.05 mg/L) was 0.015 mg/m2 (30%) and high (25 mg/L) rate added was 14.33 mg/m2 (57.32%). Increasing initial phosphate concen3 uptake onto adsorbents to a certain tration led to increase the PO− 4 point; then, a plateau occurred for both bentonites that indicated unavailability of adsorption sites (Kaya and Ören, 2005). 3.5. Effect of adsorbent amount The effect of adsorbent amount on the sorption of phosphate was analyzed kinetically over a range of 0.2 to 10 g. The percentages of adsorbed phosphate increased from 0.57% to 5.44% onto RB and from 7.46% to 54.42% onto MB, respectively (Fig. 7). The removal efﬁciency
Fig. 7. Effect of adsorbent amount on phosphate-uptake by raw and modiﬁed bentonites.
3.6. Effect of temperature The effect of temperature on phosphate adsorption was determined at temperatures ranging from 25 to 50 °C. As shown in Fig. 8, high temperature was advantageous for phosphate adsorption on both bentonites. Fig. 8 showed an increase in the adsorption density with an increase in temperature. This indicated that the adsorption reaction was of endothermic nature and the ion-exchange mechanism was favored at higher temperatures. The increases in adsorption density of MB at higher temperatures could be caused by the enlargement of pore size and/or modiﬁcation of the adsorbent surface (Yan et al., 2010). Likewise, decreased adsorption density of both bentonites at lower temperatures, could be caused by decreased pore size. Consequently, the maximum amounts of phosphate adsorbed on RB and MB were increased from 0.72 to 1.16 mg/m2 and 10.33 to 14.02 mg/m2, respectively. It was common that increasing temperature could create a swelling inﬂuence inside the adsorbent structure. 3.7. Adsorption isotherm Adsorption isotherms described the distribution of phosphate ion on RB and MB at equilibrium. Adsorption balance of phosphate ions was studied by the Freundlich and Langmuir adsorption isotherm. Langmuir isotherm was the indicator of active surface adsorption on the homogenous surface, while the Freundlich isotherm was used for heterogeneous surfaces (Dada et al., 2012). The linear form of Langmuir and Freundlich equations were as below. Fig. 9 illustrated the plots of phosphate ion where a straight line could be well observed between Ce/qe and Ce. This indicated that the experimental data followed Langmuir's isotherm. qm and KL were
Fig. 9. Langmuir isotherm plots for the adsorption of phosphate onto raw and modiﬁed bentonites.
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
Table 2 Langmuir and Freundlich isotherm parameters for phosphate ions adsorption onto Raw Clay and Modiﬁed Bentonite. Adsorbent Temperature (°C)
25 35 45 25 35 45
0.391 0.408 0.445 2.977 3.137 3.435
0.076 0.131 0.145 0.170 0.172 0.223
0.391 0.669 0.719 0.922 0.937 0.967
0.034 0.042 0.044 0.336 0.37 0.38
1.019 1.083 1.117 1.18 1.193 1.264
0.836 0.842 0.843 0.913 0.927 0.961
determined from slope and intercepts of the plots (Fig. 9) and were presented in Table 2. From the results, it was clear that the value of adsorption efﬁciency qm and adsorption energy b of the adsorbent increased with temperature. High temperatures increased the kinetic energy of phosphate and, hence, enhanced the mobility of phosphate ion. This led to a higher chance of phosphate being adsorbed onto the adsorbent and an increase in its adsorption density. Further, it conﬁrmed the endothermic nature of the processes involved in the system. To conﬁrm the adorability of the adsorption process, the separation factor (RL) had calculated. The values were found between 0 and 1 and conﬁrm that the ongoing adsorption process was favorable (Sen and Dustin, 2011). A further analysis of the Langmuir equation could be made on the basis of a dimensionless equilibrium parameter, RL, also known as the separation factor, given by (Sen and Khoo, 2013). The separation factor, RL had been calculated from Langmuir plot. It had been found that the calculated range of RL values from 0.133 to 0.478 for RB and 0.158 to 0.529 for MB system with the initial phosphate ion range of 0.05 to 25 mg/L. These RL values indicated favorable adsorption as it lie in the range 0 b RL b 1. The maximum Langmuir adsorption density of MB was more than RB (Sen and Khoo, 2013). The Freundlich isotherm model was considered to be appropriate for describing both multilayer sorption and sorption on heterogeneous surfaces (Dada et al., 2012). The Freundlich isotherm was linear if 1/n = 1 and, as 1/n decreased, the isotherm became more nonlinear. The units for qe and Ce should be consistent if parameters were to have any practical application (Dada et al., 2012). Linear plot of log qe versus log Ce showed that the adsorption of phosphate ion followed the Freundlich isotherm (Fig. 10). Values of KF and 1/n were found in the Table 2, showed the increase of positive charge on the surface that enhanced the electrostatic force between MB surface and phosphate ion, which increases the adsorption of phosphate. The values clearly showed that dominance in adsorption density. The intensity of adsorption was an indicative of the bond energies between phosphate and adsorbent and the possibility of slight chemical sorption rather than physical sorption. The possibility of multilayer
Fig. 11. Van't Hoff plot of adsorption equilibrium constant KL.
adsorption of phosphate through the percolation process could not be ruled out. However, the values of n were greater than one indicating the adsorption was much more favorable (Ho et al., 2002). As both Langmuir and Freundlich isotherm models could explain the adsorption, this suggested that the adsorption of phosphate was potentially monolayer (Feng et al., 2009). The values of 1/n b 1 conﬁrmed a favorable adsorption onto microporous adsorbent (Gunay et al., 2007). The results indicated that MB had higher maximum adsorption density for phosphate. Moreover, adsorption density for MB was found to have a higher KF value than RB, indicated that this phosphate had higher afﬁnity for MB than RB. Overall Langmuir isotherm model had higher regression coefﬁcient (R2) compared to the Freundlich isotherm model for both the systems.
3.8. Adsorption thermodynamics The thermodynamic equilibrium parameters (ΔG°, ΔH° and ΔS°) for the adsorption phosphate onto both bentonites at 45 °C were calculated at various temperatures from the ﬁt of the adsorption isotherms. A plot of ln KL versus 1/T was found to be linear as shown in Fig. 11. ΔH° and ΔS° were determined from the slops and intercepts of the plot (Fig. 11). For raw bentonite, the negative value of ΔG° reﬂects a more energetically favorable adsorption at low temperature (25 °C) as presented in Table 3. The decrease in Δ G° with the increase in temperature showed an increase in feasibility of sorption at higher temperature and so, indicated the nature of phosphate adsorption was not spontaneous. On the other hand, the negative value of ΔG° indicated the spontaneous nature of phosphate adsorption ions on MB. The positive value of ΔH° indicated that the process of adsorption was endothermic and irreversible, probably due to nonpolar interactions. The positive value of ΔS° suggested a high degree of disorderliness at the solid-solution interface during the adsorption process of phosphate onto MB. It also reﬂected the afﬁnity of the adsorbent for phosphate and suggested some structural changes in adsorbate and adsorbent (Donat et al., 2005).
Table 3 Thermodynamics parameters of phosphate ions adsorption onto Raw Clay and Modiﬁed Bentonite. Adsorbent Raw Ben
Fig. 10. Freundlich isotherm plots for the adsorption of phosphate onto raw and modiﬁed bentonites.
ΔS° (J/mol K)
25 35 45 25 35 45
−4.236 −4.101 −3.966 −3.737 −4.707 −5.678
−8.246 −8.246 −8.246 25.189 25.189 25.189
−13.457 −13.457 −13.457 97.066 97.066 97.066
M. El Bouraie, A.A. Masoud Applied Clay Science 140 (2017) 157–164
4. Conclusions Modiﬁed bentonite with Mg(OH)2 was an adsorbent for phosphate ions removal from aqueous solutions. RB and MB were used to remove phosphate from aqueous solution. The phosphate adsorption capabilities of RB were low, but high for MB. The pH effect, adsorbents concentration, equilibrium time, adsorption isotherms and thermodynamic adsorption, were examined. The results indicated that the modiﬁcation process has signiﬁcant effect on the removal efﬁciency of phosphate. Moreover, the highest adsorption density was found with 25 mg/L initial phosphate ion concentration, at 45 °C and pH 7, as 14.33 mg/m2. Adsorption equilibrium ﬁndings ﬁtted with the Langmuir isotherm better when compared to the Freundlich isotherm. Thermodynamically negative ΔG°, positive ΔH°, and positive ΔS° demonstrated the high afﬁnity and endothermic adsorption process between the adsorbent and the adsorbed. The adsorbent used in this study demonstrated a relatively good phosphate adsorption density and removal yield when compared to studies conducted with bentonite modiﬁed by different methods or with other adsorbents. However, RB was less efﬁcient compared to MB, so that MB was a proper alternative for the removal of contaminants in processes that need large quantities of adsorbents, because of its suitable characteristics, such as availability, inexpensive, reusability and proper ability in removing contaminants, its efﬁciency would be increased more, due to increase in the surface area. Acknowledgement Both authors would like to thank the staff of Central Laboratory for Environmental Quality Monitoring for their cooperation during measurements for providing necessary facilities to accomplish the work. Special thanks devoted to Central Laboratory for Tanta University to support present research and the required devices. References American Public Health Association (APHA), 2005. Standard Method for Examination of Water and Wastewater. 21st ed. (Washington DC, USA). Arias, F., Sen, T.K., 2009. Removal of zinc metal ion (Zn+2) from its aqueous solution by kaolin clay mineral: a kinetic and equilibrium study. Colloids Surf. A Physicochem. Eng. Asp. 348, 100–108. Dada, A.O., Olalekan, A.P., Olatunya, A.M., 2012. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms studies of equilibrium sorption of Zn unto phosphoric acid modiﬁed rice husk. J. Appl. Chem. 3, 38–45. Despland, L.M., Clark, M.W., Vancoc, T., Erler, D., Aragno, M., 2011. Nutrient and tracemetal removal by Bauxsol pellets in wastewater treatment. Environ. Sci. Technol. 45, 5746–5753. Donat, R., Akdogan, A., Erdem, E., Cetisli, H., 2005. Thermodynamics of Pb2+ and Ni2+ adsorption onto natural bentonite from aqueous solutions. J. Colloid Interface Sci. 286, 43–52. Donnert, D., Salecker, M., 1999. Elimination of phosphorus from municipal and industrial waste water. Water Sci. Technol. 40, 195–202. Eloussaief, M., Kallel, N., Yaacoubi, A., Benzina, M., 2011. Mineralogical identiﬁcation, spectroscopic characterization, and potential environmental use of natural clay materials on chromate removal from aqueous solutions. Chem. Eng. J. 168, 1024–1031. Eren, E., Afsin, B., 2008. An investigation of Cu (II) adsorption by raw and acid-activated bentonite: a combined potentiometric, thermodynamic, XRD, IR, DTA study. J. Hazard. Mater. 151, 682–691. Fan, H.J., Anderson, P.R., 2005. Copper and cadmium removal by Mn oxide-coated granular activated carbon. Sep. Purif. Technol. 45, 61–67. Feng, N.C., Guo, X.Y., Liang, S., 2009. Adsorption study of copper(II) by chemically modiﬁed orange peel. J. Hazard. Mater. 164, 1286–1292. Flessner, U., Jones, D.J., Rozière, J., Zajac, J., 2001. A study of the surface acidity of acidtreated montmorillonite clay catalysts. J. Mol. Catal. A Chem. 168, 247–256. Fytianos, K., Voudrias, E., Raikos, N., 1998. Modelling of phosphorus removal from aqueous and wastewater samples using ferric iron. Environ. Pollut. 101, 123–130. Gunay, A., Arslankaya, E., Tosun, I., 2007. Lead removal from aqueous solution by natural and pretreated clinoptilolite: adsorption equilibrium and kinetics. J. Hazard. Mater. 146, 362–371.
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