Simultaneous Removal of Divalent Heavy Metals from Aqueous

Article pubs.acs.org/IECR

Simultaneous Removal of Divalent Heavy Metals from Aqueous Solutions Using Raw and Mechanochemically Treated Interstratified Montmorillonite/Kaolinite Clay Ksenija R. Kumrić,† Anđelka B. Đukić,‡ Tatjana M. Trtić-Petrović,† Nikola S. Vukelić,§ Zoran Stojanović,∥ Jasmina D. Grbović Novaković,‡ and Ljiljana Lj. Matović*,‡ †

Laboratory of Physics and ‡Laboratory of Materials Sciences, Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia § Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11158 Belgrade, Serbia ∥ Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, P.O. Box 377, 11000 Belgrade, Serbia S Supporting Information *

ABSTRACT: The removal of Pb(II), Cd(II), Cu(II), and Zn(II) from aqueous solutions using (un)modified Serbian interstratified montmorillonite/kaolinite clay as an adsorbent was investigated. The clay was modified by mechanochemical activation for different time periods. X-ray diffraction patterns and particle size distributions were used to characterize the samples. Batch adsorption studies were conducted to optimize various conditions. The adsorption equilibrium was established within 60 min, and the maximum adsorption occurred in the pH range of 4.5−6.5. The milled clays exhibited greater equilibrium adsorption capacities (qe) for all of the metals than the raw clay. A difference in qe values for clays milled for 2 and 19 h could be observed only for initial concentrations (Ci) of ≥100 mg dm−3. This was related to the amorphization (i.e., exfoliation) of 19-hmilled clay particles. The adsorption equilibrium data of heavy metals on both raw and modified clays fit the Langmuir equation, although there were changes in the microstructure of the clay. The mechanochemical treatment of the clay reduced the amount of adsorbent necessary to achieve a highly efficient removal of heavy metals by a factor of 5. Thus, the mechanochemically treated interstratified clay can be considered as an efficient adsorbent for the simultaneous removal of divalent heavy metals.

1. INTRODUCTION Wastewaters containing heavy metals as contaminants originate from a large number of metal-related industries and mines. Heavy metals are toxic and nonbiodegradable, and their presence in streams and lakes leads to bioaccumulation in living organisms, causing health problems in animals, plants, and human beings.1,2 To avoid water pollution, treatment, that is, the removal of heavy-metal ions from industrial wastewaters, is needed before disposal. Several conventional techniques are used for removing heavy metals from aqueous solutions, such as chemical precipitation, ion exchange, solvent extraction, reverse osmosis, and adsorption.3 Compared with other conventional techniques, adsorption appears to be an attractive process in view of its efficiency and simplicity of operation in the treatment of wastewaters containing heavy metals, as well as the availability of a wide range of adsorbents.4 The adsorbent should have a strong affinity for the target metal ions (binding them irreversibly under ambient conditions) and, simultaneously, the ability to release the same metals under different conditions so that it can be regenerated for further use.5 Large-scale sorption processes for the treatment of wastewaters containing heavy metals demand inexpensive, nontoxic, and available sorbents with known kinetic parameters and sorption characteristics.6 Clay minerals are good adsorbents for metal ions from aqueous solutions owing to their high cationexchange capacities, high abundance and local availability, nontoxicity, chemical and mechanical stability, Brønsted and © XXXX American Chemical Society

Lewis acidity, low costs, and ability to be recycled. There is growing interest in research on both natural untreated and modified clay minerals, including attempts to improve the characteristics of the materials regarding the removal of heavymetal ions from industrial wastewaters.7 To enhance the heavymetal-ion sorption properties of clay minerals, different techniques of modification have been applied: acid activation,8−10 intercalation and pillaring,9,10 mechanochemical activation,11−13 and chemical modification using inorganic- or organic-based complex-forming ions.14,15 In comparison with other methods, mechanochemical activation represents an environmentally friendly and inexpensive method of modification. Structural changes induced by milling, such as fragmentation, distortion, particle size reduction followed by an increase in specific surface area, peeling off of layers, exfoliation of particles, abrasion, and amorphization, can lead to the higher cationexchange capacity of the clays.12,16−18 Although the influence of mechanical milling on changes in microstructure and morphology of the clay structure has been examined in detail,12,13,17,18 only a limited number of studies have investigated the use of this modification technique for the Received: January 23, 2013 Revised: May 22, 2013 Accepted: May 22, 2013

A

dx.doi.org/10.1021/ie400257k | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

2.2. Chemicals. ZnCl2, Pb(NO3)2, Cd(NO3)2·4H2O, and Cu(NO3)2·2.5H2O were purchased from Sigma-Aldrich (St. Louis, MO). Potassium chloride, sodium acetate, sodium hydroxide, and nitric acid were purchased from Lach Ner (Brno, Czech Republic). All chemicals were of analyticalreagent grade. Deionized water was supplied from a Milli-Q water purification system (Millipore, Bedford, MA). Heavy-metal stock solutions containing 500 mg dm−3 concentrations of Zn(II), Pb(II), Cd(II), and Cu(II) were prepared separately for each metal by dissolving an adequate amount of salt in 0.1 dm3 of deionized water. The stock solutions were stable for months at room temperature. Working heavy-metal-ion solutions, with total metal-ion concentrations in the range of 25−600 mg dm−3, were prepared shortly before each experiment by appropriate dilution of the stock solutions with deionized water. The pH of the working solution was adjusted by using either 0.1 mol dm−3 HNO3 or 0.1 mol dm−3 NaOH. 2.3. Adsorption Study. Batch experiments were carried out at room temperature by mixing 0.05 g of natural clay adsorbent and 25 cm3 of working multimetal ion solution in closed polyethylene bottles. The total Zn(II), Pb(II), Cd(II), and Cu(II) ion concentration in the working solution was 50 mg dm−3 (12.5 mg dm−3 each of the investigated metals) at pH 5.5. The initial pH was adjusted with 0.1 mol dm−3 HNO3. The samples were shaken on a laboratory shaker (Promax 2020, Heidolph, Schwabach, Germany) for 60 min at a stirring speed of 200 rpm for better mass transport with high interfacial area of contact. After that, the liquid phases were separated from the solid phases by filtration through a 0.45-μm microporous membrane filter (Membrane Solutions LLC, Plano, TX). The residual concentration of heavy-metal ions in each aliquot was determined with a 797 VA Computrace polarography system (Metrohm, Herisau, Switzerland) using the Metrohm’s procedure for the voltammetric determination of zinc, cadmium, lead, and copper in water samples (no. 231/2 e). The concentrations of desorbed metal cations [Na(I), K(I), Ca(II), Mg(II)] were analyzed on an 861 Advanced Compact ion chromatography (IC) system (Metrohm, Herisau, Switzerland) with a conductivity detector and a Metrosep C2 analytical column (Metrohm, 150 mm × 4 mm). In the initial heavymetal-ion solution, the concentrations of Na(I), K(I), Ca(II), and Mg(II) ions were 0.1, 0.1, 0.2, and 0.5 mg dm−3, respectively. The effects of different experimental parameters, such as contact time (30 s−24 h), solution pH [(1.8 ± 0.1)−(6.5 ± 0.1)], initial total metal-ion concentration (25−600 mg dm−3), amount of adsorbent (0.5−10 g dm−3), and milling time of clay (0−19 h), were investigated with respect to the removal efficiency of metal ions. The concentrations of Zn(II), Pb(II), Cd(II), and Cu(II) ions in multimetal solution were set to be equal. All experiments were carried out in duplicate, and the data obtained were used for analysis. 2.4. Calculations. 2.4.1. Metal Uptake. The amount of metal ion adsorbed at time t per unit mass of the investigated natural clay (qt) and the removal efficiency of particular metal ion (E) were evaluated using the equations

improvement of heavy-metal-ion sorption properties of clays. Vdović et al. examined the changes in the surface properties of montmorillonite, kaolinite, and mica during grinding using the technique of high-energy milling. The initial capacity of 142 cmol kg−1 for montmorillonite, for example, was increased to 175 cmol kg−1 after 16 min of grinding. Further milling produced a great reduction of the cation-exchange capacity to 10 cmol kg−1; this was related to the decrease in specific surface area, which was the result of agglomeration and amorphization.12 Suraj et al. and Hongo et al. reported increased adsorption of Pb(II) ions after mechanochemical treatment of raw kaolinite and vermiculite clay minerals, respectively.19,20 Hongo et al. attributed the results to an increased number of surface hydroxyl groups available for adsorption, the generation of an amorphous silica phase, and appreciable distortion of the layers. The fact that almost the same amount of Pb(II) adsorbed for the samples treated for 8 and 4 min was attributed to two conflicting factors: the increase in the surface hydroxyl groups and the destruction of the periodic clay structure.20 Nenadović et al. reported that, after mechanochemical modification of diatomaceous earth, the Pb(II) sorption capacity was considerably improved. The immobilization efficiency of Pb(II) increased from 22% for the unmilled sample to 81% for the 5-h-milled sample at a ball-to-powder ratio (BPR) of 4:1.21 The increase in the immobilization efficiency of Pb(II) was attributed to the amorphization of the material. To the best of our knowledge, simultaneous heavymetal-ion removal on mechanochemically activated clay has not been reported before. The aim of the present study was to evaluate the usefulness of both locally available raw interstratified montmorillonite/ kaolinite clay and mechanochemically treated clay as adsorbents for the simultaneous removal of Pb(II), Cd(II), Cu(II), and Zn(II) ions from aqueous solutions. The effects of various experimental parameters such as contact time, solution pH, initial metal-ion concentration, amount of adsorbent, and milling time were investigated. The sorption behavior of the selected heavy-metal ions onto milled clay samples was correlated with changes in the microstructure induced by milling. The findings were compared with similar ones in the literature. Finally, the potential of milled clay as an adsorbent in heavy-metal removal was compared to that of raw natural clay.

2. EXPERIMENTAL SECTION 2.1. Adsorbent. The raw natural clay used as an adsorbent in this study was obtained from the mine Bogovina, located near the town of Bor in Eastern Serbia. Mechanochemical activation was used for clay modification. The ball-milling process was performed in air in a Turbula type 2TC mixer using a hardened steel vial and balls with the BPR fixed at 4:1. The milling times tested were 1, 2, 10, and 19 h. The crystal phases present in the samples were identified by powder X-ray diffraction (XRD) using a Siemens Kristallflex D500 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The scan range was 2θ = 2−75°, and the scanning rate was 0.02°/s. Angular correction was performed with a high-quality Si standard. For qualitative and quantitative analyses of the X-ray powder diagrams, Powder Cell 2.4 software was used.22 A Malvern 2000SM Mastersizer laser scattering particle size analysis system was used to obtain quantitative clay particle size distributions. The specified resolution range of the system was submillimeter to 2 mm.

qt = B

⎛ C i − Ct ⎞ ⎜ ⎟V ⎝ W ⎠

(1)



Article

Figure 1. XRD patterns of (a) raw clay and (b−e) clays milled for (b) 1, (c) 2, (d) 10, and (e) 19 h. MMT, montmorillonite; MMT/K, montmorillonite/kaolinite; Q, quartz; C, calcite.

⎛ C − Ce ⎞ E (%) = 100⎜ i ⎟ ⎝ Ci ⎠

adsorption related to the adsorption energy, adsorption capacity, and degree of heterogeneity of the adsorbent’s surface, respectively. The parameters of the Langmuir isotherm, qm and KL, and the Freundlich isotherm, n and KF, were evaluated from the slope and intercept of linear plots of Ce/qe versus Ce and ln qe versus ln Ce, respectively.

(2)

where Ci is the initial metal-ion concentration (mg dm−3), Ct is the metal-ion concentration at time t (mg dm−3), Ce is the equilibrium concentration of metal ion (mg dm−3), V is the volume of the solution (dm3), and W is the mass of clay (g). 2.4.2. Adsorption Isotherms. The distribution of the adsorbate on the adsorbent surface at equilibrium can be fundamentally explained by the adsorption isotherm.23 In this study, the experimental equilibrium data of the investigated metal ions on raw and milled clays were analyzed by the two most commonly used isotherm models, namely, the Langmuir and Freundlich isotherms. The Langmuir isotherm assumes that the uptake of metal ions occurs on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions and is given by the equations qe =

3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. The characteristics of the raw and milled natural clay were determined by XRD and particle size distribution (PSD) studies. Changes in the structure of the clay induced upon milling were correlated with the changes in sorption behavior of the selected heavymetal ions. 3.1.1. Microstructural Characterization: XRD Study. Figure 1a shows the XRD pattern of the raw, unmilled clay. In the lowangle range (2θ = 5−20°), broad and weak peaks were noticed. The main component of the sample had broad reflections close to the basal reflections of kaolinite or a kaolinite-containing mixed-layer phase. The peaks located at the positions 2θ = 6.94° (d001 = 12.58 Å) and 2θ = 12.22° (d001 = 7.23 Å) correspond to the reflections from (001) planes of interstratified clay minerals montmorillonite/kaolinite (MMT/K). Quartz (Q) and calcite (C) at the positions 2θ = 20.94°, 26.43°, and 50.19° and 2θ = 29.36°, respectively, were present as concomitant minerals. It was found that interstratification of kaolinite/smectite in clay is common in nature. It is widely recognized that peaks of interstratified kaolinite/smectite are broad and very often low in intensity as well.26−30 Broad peaks and a high background signal at small angles indicate the existence of complex interstratifications in clays.29 Kaolinitic phases with d001 > 7.15 Å might refer to fine kaolinite with a small thickness of coherent scattering domains or to kaolinite that is in mixed layers.30 Mixed layered kaolinite/ smectite is formed by the dissolution of smectite and crystallization of kaolinite layers.31,32 Because of the partial overlapping of the reflections from the different phases that are interstratified, a quantitative analysis of

qmKLCe 1 + KLCe

(3)

Ce C 1 = + e qe qmKL qm

(4)

whereas the Freundlich isotherm expresses multilayer adsorption on a heterogeneous surface, accompanied by interaction between adsorbed molecules,24,25 and is given by the equations qe = KFCe1/ n ln qe = ln KF +

(5)

1 ln Ce n

(6)

In eqs 3−6, qe is the amount of heavy-metal ions adsorbed per unit mass of adsorbent at equilibrium (mg g−1); qm is the maximum adsorption capacity or amount of metal ions adsorbed per unit mass of adsorbent that is required to cover the sorbent surface completely as a monolayer (mg g−1); and KL, KF, and n are the Langmuir and Freundlich constants of C



Article

the sample is difficult, so different treatments were applied to achieve shifts of the basal reflection positions.31−33 Randomly interstratified kaolinite/smectite is easy to identify by comparing the XRD pattern of an air-dried sample with that of a sample that has been treated with ethylene glycol. After decomposition of the peak at d001 = 7.2 Å, it was found that the reflection corresponds to mineral kaolinite and interstratified kaolinite/smectite.31 Similar results for the interstratification of minerals (2:1) were obtained in other studies.32,33 After 1 h of milling, in the 2θ = 5−20° range, two separate peaks were located at the positions 2θ = 5.88° (d001 = 15.005 Å) and 2θ = 12.41° (d001 = 7.124 Å). They are attributed to montmorillonite (MMT) and clay composite (MMT/K) with kaolinite as the predominant mineral phase (Figure 1b). The obtained results indicate the partial separation of montmorillonite and kaolinite. To the best of our knowledge, separation of interstratified clay mineral particles by mechanical milling has not been previously found. With an increase in the ball-milling time, the separation and delamination of minerals in the sample were more pronounced: Diffraction maxima corresponding to the (001) reflections of montmorillonite and MMT/K clay composite, after 2 h of milling, were shifted to smaller angles [2θ = 5.83° (d001 = 15.15 Å), 2θ = 12.24° (d001 = 7.22 Å)] (Figure 1c). After 10 h of milling, a pronounced amorphization process was noticed (Figure 1d). This process was followed by peak broadening, that is, a decrease in the intensity of the (001) reflection of montmorillonite and montmorillonite/kaolinite composite, which implied a further separation of the layers of the clay minerals. The milling time of 10 h led to a shift of the peak position of montmorillonite to the even smaller angle of 2θ = 5.43° (d001 = 16.27 Å), whereas the peak of MMT/K composite was barely indicated (Figure 1d). With prolonged milling time, the level of crystallinity decreased and made the typical clay structure disappear. The complete amorphization of the clay minerals is clearly visible in the XRD pattern of the 19-h-milled sample (Figure 1e). Structural changes induced by milling, such as fragmentation, distortion, particle size reduction followed by an increase in specific surface area, peeling off of layers, exfoliation of particles, abrasion, and amorphization, have been widely observed .12,16−18,34−36 3.1.2. Microstructural Characterization: PSD Study. Figure 2 shows the particle size distribution (PSD) curves of the unmilled clay and the clays milled for different times (1, 2, 10, and 19 h). The unmilled sample has a broad monomodal distribution of particles in the range from 0.4 to 200 μm, with mean particle size of 27 μm. The milling leads to changes in the shape of the distribution curve and also to a reduction of the mean particle sizes: In the case of the samples milled for 1 and 2 h, the distribution curves narrow but mostly retain a monomodal shape, with mean particle sizes of 9.5 and 8.8 μm, respectively. On the other hand, the particle size distribution curve of the sample milled for 10 h shows bimodal character with mean particle sizes of 8.4 μm (∼95%) and 115 μm (∼5%). The particle size distribution curve of the sample milled for 19 h shows monomodal character with a mean particle size higher than in the samples milled for shorter periods of time (17.1 μm). According to the literature data,37 interparticle interactions depend on the forces of attraction and repulsion between powder particles and become dominant when the particle size is smaller than 20 μm. Milling increases the specific surface area,

Figure 2. Particle size distributions of raw unmilled clay (RC) and of clay milled for 1, 2, 10, and 19 h (MC 1h, MC 2h, MC 10h, and MC 19h, respectively), obtained by laser scattering.

and thus, the cohesive and adhesive interactions present between powder particles can lead to their agglomeration. The tendency for agglomeration of particles was confirmed by milling: The 19-h-milled sample had a higher mean particle size (17.1 μm) than the clays treated for shorter periods of time (∼9 μm). Also, the appearance of the second maximum of the bimodal distribution curve of the 10-h-milled sample with a mean particle size of 120 μm can be attributed to the agglomeration of particles during the milling process. 3.2. Optimization of the Adsorption Parameters. To determine the optimal conditions for the efficient simultaneous removal of heavy-metal ions from aqueous solutions, raw interstratified montmorillonite/kaolinite clay was used as an adsorbent. The effects of contact time, solution pH, and initial total metal-ion concentration were investigated. The determined optimal conditions (adsorption equilibrium time and pH of the aqueous solution) were used for the simultaneous adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) ions onto mechanochemically treated interstratified montmorillonite/ kaolinite clay, and the obtained results were compared to the adsorption onto untreated clay. 3.2.1. Effect of Contact Time. The effect of contact time on the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) onto the raw natural clay was investigated at different contact times ranging from 30 s to 24 h (Figure 3). As can be seen in Figure 3, the equilibrium adsorption was established after 60 min of contact, and the maximum uptake on the raw clay for all investigated metals was reached: 94.7% (6.0 mg g−1) for Pb(II), 89.2% (5.4 mg g−1) for Cu(II), 61.6% (3.8 mg g−1) for Zn(II), and 60.8% (3.7 mg g−1) for Cd(II). After equilibrium was reached, the contact time no longer had an influence on the metal-ion adsorption, and the removal efficiencies and adsorption capacities remained constant over the time period observed (24 h). In addition, it is evident in Figure 3 that the adsorption rate increased rapidly in the first 5 min of adsorption, and 88.4% of the Pb(II), 77.7% of the Cu(II), 58.0% of the Zn(II), and 55.8% of the Cd(II) were adsorbed during this period of time. This means that, after only 5 min of adsorption, the removal efficiencies of Zn(II), Pb(II), Cd(II), and Cu(II) exceeded 85% of their values at equilibrium. This can be explained by the fact D



Article

with lower pH. Thus, with decreasing pH of the solution, especially for pH < 3, the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) ions decreased dramatically, and the removal efficiency of each investigated heavy-metal ion decreased to less than 10%. This can be explained by the fact that the active sites of the clay are more protonated at a lower pH and less available for binding with the metal ions. However, as the pH increases, the active sites become more accessible for the adsorption of the positively charged metal ions through electrostatic forces of attraction.39−41 Similar results were observed for the adsorption of heavy-metal ions using different clay samples.24,41 The changes in the final pH values of the solution, with regard to the initial pH values, and the concentrations of metal ions released to water [Na(I), K(I), Ca(II), Mg(II)] after adsorption equilibrium was reached, are presented in Table 1. It Table 1. pH Change of the Solution at the Adsorption Equilibrium of Pb(II), Cu(II), Zn(II), and Cd(II) Ions on Raw Clay at Different Initial pH Values and Concentrations of Ions Released into the Solution

Figure 3. Effect of contact time on the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) by the natural clay adsorbent at an initial total metal-ion concentration of 50 mg dm−3. Conditions: pH, 5.5; stirring speed, 200 rpm; concentration of adsorbent, 2 g dm−3.

metals released into water [mg dm−3 (mequiv dm−3)]

pH

that the number of available adsorption sites on the surface is the highest at the beginning of adsorption process, whereas a further increase in contact time decreases the number of available adsorption sites on the clay, which gradually interact with the metal ions. The obtained results imply that the sorption mainly took place at the surface of the sorbent during the initial stage. The adsorption time of 60 min was used for the rest of the study. 3.2.2. Effect of pH. The pH of an aqueous solution strongly affects the adsorption of heavy metals onto clay minerals.38 The effect of the solution pH on the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) ions using the raw natural clay was investigated at varying pH values from 1.8 to 6.5. The obtained results are presented in Figure 4. It is evident that the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) ions onto the investigated natural clay was highly pH-dependent, with maximum removal efficiencies of 94.4%, 87.9%, 61.2%, and 60.6%, respectively, at the pH values between 4.5 and 6.5. The amount of adsorbed heavy-metal ions showed a declining trend

initial

final

2.0 3.1 4.0 5.5 6.3

2.5 6.1 6.3 6.3 6.4

Na+ 12.7 10.7 10.5 10.7 10.4

(0.55) (0.47) (0.46) (0.47) (0.45)

K+ 5.3 3.8 3.5 3.6 3.6

(0.14) (0.10) (0.09) (0.09) (0.09)

Ca2+ 212.9 (10.62) 20.4 (1.02) 8.6 (0.43) 6.7 (0.33) 6.1 (0.30)

Mg2+ 4.9 2.8 2.1 1.5 1.3

(0.40) (0.23) (0.17) (0.12) (0.11)

can be seen that the pH of the final stage was higher than that of the initial stage. The final pH values tended to be ∼6.3 for the initial pH values ranging from 3.0 to 6.3. The increase in pH can be attributed to the dissolution of calcite present in the raw clay (XRD pattern, Figure 1) and the existence of corresponding carbonate equilibria. Because calcite dissolution strongly depends on pH, the extent of its dissolution was measured by the increment in Ca(II) concentration in the solution. Obviously, the dissolution of calcite was maximal at pH 2.0 when the highest increase in Ca(II) concentration was observed. With increasing initial pH, the concentration of Ca(II) ions in the solution decreased. According to the literaure data,42,43 the sorption of Ca(II) ions is significantly favored over Cd(II) and Zn(II) adsorption onto montmorillonite. Therefore, it can be assumed that the lower removal of Cd(II) and Zn(II) compared to Pb(II) and Cu(II) came from the presence of competing Ca(II) ions. A more detailed investigation of the simultaneous adsorption of Ca(II), Zn(II), and Cd(II) ions onto the raw clay used in this study is necessary to better interpret the possible competition between Ca(II) and Zn(II) ions, as well as Ca(II) and Cd(II) ions. The concentration of sorbed heavy metals (∼0.83 mequiv dm−3 at pH 6.5) versus the sum of concentrations of released cations (0.95 mequiv dm−3 at pH 6.5) indicates that cation exchange is the primary mechanism involved in the simultaneous removal of Pb(II), Cu(II), Zn(II), and Cd(II) ions by the investigated raw clay. 3.2.3. Effect of Initial Metal-Ion Concentration. The effect of the initial metal-ion concentration on the raw natural clay adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) was studied by varying the total metal-ion concentration from 25 to 600 mg dm−3, that is, by varying the respective metal-ion concentration from 6.2 to 150 mg dm−3 in the multimetal solution at pH 5.5

Figure 4. Effect of pH on the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) onto raw natural clay at an initial total metal-ion concentration of 50 mg dm−3. Conditions: contact time, 60 min; stirring speed, 200 rpm; concentration of adsorbent, 2 g dm−3. E



Article

Table 2. Langmuir and Freundlich Parameters for the Adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) Ions onto Raw Natural Clay (RC)a Langmuir fitting

a

heavy metal

qm

Pb(II) Cu(II) Zn(II) Cd(II)

21.1 ± 0.3 16.4 ± 0.3 8.1 ± 0.3 6.0 ± 0.5

Freundlich fitting 2

KL 0.44 0.42 0.21 0.26

± ± ± ±

R 0.15 0.22 0.10 0.15

0.99 0.99 0.99 0.97

KF 6.7 4.7 2.4 2.2

± ± ± ±

R2

n 0.3 0.4 0.3 0.3

3.65 3.30 3.70 3.85

± ± ± ±

0.35 0.45 0.51 0.63

0.95 0.91 0.91 0.92

Conditions: pH, 5.5; adsorbent concentration, 2 g dm−3; and contact time, 60 min.

determined optimal conditions for the raw clay in section 3.2. The effect of the milling time of clay on the removal efficiency of heavy metals is presented in Figure 5.

while keeping all other parameters constant. As expected, the removal efficiencies of the investigated metal ions decreased with increasing initial metal-ion concentration in the aqueous solution, because of the saturation of the available adsorption sites on the clay. However, the equilibrium amount of metal ion adsorbed per unit mass of clay, qe, increased with increasing initial metal-ion concentration, because of the increase in the driving force of the metal ions toward the active sites on the adsorbent. In the case of Cd(II) ions, a drop in sorption at Ci > 200 mg dm−3 can be observed (see Table S1 in the Supporting Information). The occurrence of a drop in the adsorption of Cd(II) ions, compared to other metal ions, is possibly a consequence of the fact that the other ions adsorb more readily on the adsorption sites, thus leaving no available adsorption sites for Cd(II) ions.6,44 To determine the surface properties and affinity of the adsorbent, the experimental data were analyzed using the Langmuir (eq 4) and Freundlich (eq 6) isotherm models. The corresponding Langmuir and Freundlich parameters and correlation coefficients are reported in Table 2. According to the results summarized in Table 2, it can be seen that the Langmuir plots have higher correlation coefficients than the Freundlich plots, suggesting that the Langmuir isotherm is a good model for the simultaneous sorption of all investigated metals. The obtained experimental data fit the Langmuir model in the whole range of concentrations, with correlation coefficients greater than 0.97 for all of the investigated heavy metals, with the exception of Cd(II). The obtained experimental data for Cd(II) ions fit the Langmuir model satisfactorily (R2 = 0.97) only for initial total metal-ion concentrations of ≤200 mg dm−3. The results suggest that the adsorption occurs through the formation of a monolayer coverage of metal ions on the surface of the investigated natural unmilled clay. The Langmuir monolayer adsorption capacities of Pb(II), Cu(II), Zn(II), and Cd(II) ions on the raw natural clay were estimated to be 21.1, 16.4, 8.1, and 6.0 mg g−1, respectively. The obtained qm values of the selected heavy-metal ions led to the conclusion that the sequence of selectivity is Pb > Cu > Zn > Cd. This is the same order as predicted by the Irving−Williams series for the ease of adsorption/desorption of heavy metals.45 3.3. Comparative Study of Adsorption on Untreated and Mechanochemically Treated Clay. 3.3.1. Effect of Different Milling Times of Natural Clay. The adsorption behavior of Pb(II), Cd(II), Cu(II), and Zn(II) ions on natural clay was determined on both raw and milled clay samples and expressed as the removal efficiency for each metal ion in solution. The samples of natural clay were milled for 1, 2, 10, and 19 h, and 2 g/L of each adsorbent was mixed with 25 cm3 of 50 mg dm−3 multimetal ion solution at pH 5.5. The mixture was shaken at 200 rpm for 60 min, then the adsorbent was removed by filtration, and the final metal-ion concentration in the supernatant was determined. These were previously

Figure 5. Effect of milling time of natural clay on the removal efficiency of heavy-metal ions from aqueous solutions by raw clay (RC) and by clay milled for 1, 2, 10, and 19 h (MC 1h, MC 2h, MC 10h, and MC 19h, respectively).

The mechanochemical treatment of the raw clay led to a reduction in the particle size by a factor of 2.8−3.2, depending on the duration of milling. It can be seen in Figure 5 that the treated samples showed a marked improvement in their ability to adsorb heavy-metal ions when compared to the raw clay and, as expected, the removal efficiencies increased with increasing milling time of the clay. The increase in the removal efficiencies of Pb(II), Cd(II), Cu(II), and Zn(II) ions was particularly pronounced for the clay samples milled for 1 h (MC 1h) and 2 h (MC 2h) and less pronounced for the samples milled for 10 h (MC 10 h) and 19 h (MC 19h). The removal efficiencies of Pb(II), Cu(II), Zn(II), and Cd(II) ions using the raw clay (RC) sample were 94.7%, 89.2%, 61.6%, and 60.8%, and using the 2h-milled clay, the E values increased to 99.5%, 99.2%, 95.1%, and 93.8%, respectively. A further increase in the milling time (10 and 19 h) of the clay slightly improved the removal efficiencies of Zn(II) and Cd(II) ions, whereas there was no influence on the adsorption of Pb(II) and Cu(II) ions. The improvement of the adsorption ability of mechanochemically treated clay samples compared to the raw clay can be attributed to the increased surface available for adsorption. This is due to the decrease in the clay particle size, which causes a larger number of edge sites, as well as exfoliation of the clay mineral particles (see sections 3.1.1 and 3.1.2). The results show that modified natural clay enables a more efficient (>97%) simultaneous removal of heavy-metal ions from aqueous solutions than raw unmodified clay and can be F



Article

increasing amount of natural clay and, hence, the removal efficiency of metal ions increased. Comparing the influence of the adsorbent concentration on the removal efficiency of metal ions on the raw and milled clay samples (2 and 19 h), it is demonstrated that the adsorbent concentration has a stronger influence on the adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) ions on the raw clay than the milled clays. The maximum removal efficiency for the unmilled sample was attained at the adsorbent concentration of 10 g dm−3, whereas the same removal effect, using the 2- and 19-h-milled clays, was achieved at an adsorbent concentration of 2 g dm−3. Any further addition of milled clay over the concentration of 2 g dm−3 did not cause any significant change in adsorption. Thus, the modification of the used natural clay by mechanical milling not only improved the adsorption characteristics of the adsorbent, in terms of the removal efficiency of the investigated heavy-metal ions, but also reduced, by a factor of 5, the amount of adsorbent necessary to achieve the highly efficient removal of Pb(II), Cu(II), Zn(II), and Cd(II) ions from aqueous solutions. 3.3.3. Adsorption Isotherms. To determine the adsorption isotherms of Pb(II), Cu(II), Zn(II), and Cd(II) ions using 2and 19-h-milled natural clay, the effect of the initial metal-ion concentration was investigated. The total initial metal-ion concentration was varied from 25 to 600 mg dm−3, with a constant adsorbent amount of 2 g dm−3. The obtained results are presented in Table S2 (Supporting Information), and the data converted to the Langmuir and Freundlich isotherms are reported in Table 3. The results presented in Tables S1 and S2 (Supporting Information) show that the milled clays enable greater qe values for all of the metals in comparison with qe values obtained for adsorption on raw clay in the whole range of initial metal-ion concentrations investigated. Also, difference in qe values of 2and 19-h-milled clays can be observed (see Table S2 in the Supporting Information) only for higher initial metal-ion concentrations, Ci ≥ 100 mg dm−3. For example, at the initial total metal-ion concentration of 400 mg dm−3, the qe values for the adsorption on the 19-h-milled clay are 11−37% higher, depending on the particular heavy-metal ion, in comparison with qe values obtained from 2-h-milled adsorbent clay. This can be related to the amorphization (i.e., exfoliation) of the 19h-milled clay particles, which makes them more accessible at higher ion concentrations, although the sample milled for 19 h showed a greater mean particle size caused by agglomeration.

considered as an economical and efficient sorbent for heavymetal-contaminated wastewaters. 3.3.2. Effect of Adsorbent Concentration. The effect of the adsorbent concentration was studied using raw and 2- and 19h-milled clays as adsorbents. The initial total metal-ion concentration was fixed at 50 mg dm−3 (pH 5.5), and the amount of clay was varied from 0.5 to 10 g dm−3. The results obtained for Pb(II) and Zn(II) ions are shown in panels a and b, respectively, of Figure 6, and for the other two ions, Cu(II)

Figure 6. Comparison of the effect of adsorbent concentration on the adsorption of (a) Pb(II) and (b) Zn(II) ions onto raw clay (RC) and 2- and 19-h-milled clay (MC 2h and MC 19h, respectively) at an initial total metal-ion concentration of 50 mg dm−3. Conditions: pH, 5.5; contact time, 60 min; stirring speed, 200 rpm.

and Cd(II), the obtained dependencies were similar. The results show that, upon increasing the adsorbent concentration, the amounts of adsorbed metal ions (i.e., their removal efficiencies) increased. The results can be explained by the fact that the number of available adsorption sites increased with

Table 3. Langmuir and Freundlich Parameters for the Adsorption of Pb(II), Cu(II), Zn(II), and Cd(II) Ions onto 2- and 19-hMilled Clay (MC 2h and MC 19h, respectively)a Langmuir fitting heavy metal

a

qm

Freundlich fitting R2

KL


27.2 21.3 14.7 11.3

± ± ± ±

0.7 0.6 0.8 0.6

0.43 0.42 0.20 0.20

± ± ± ±

0.23 0.25 0.13 0.15


42.6 34.6 16.0 14.1

± ± ± ±

1.1 0.8 0.8 0.6

0.72 0.69 0.22 0.22

± ± ± ±

0.39 0.42 0.16 0.14

MC 2h 0.99 0.99 0.98 0.98 MC 19h 0.99 0.99 0.98 0.99

KF

R2

n

10.0 ± 0.5 8.1 ± 0.3 5.6 ± 0.4 5.1 ± 0.3

4.47 4.57 5.15 6.35

± ± ± ±

0.70 0.46 0.90 1.30

0.88 0.95 0.86 0.82

13.6 ± 0.6 12.1 ± 0.7 6.5 ± 0.3 6.1 ± 0.2

3.20 3.81 5.62 6.07

± ± ± ±

0.37 0.68 0.72 0.75

0.93 0.85 0.92 0.92

Conditions: pH, 5.5; adsorbent concentration, 2 g dm−3; and contact time, 60 min. G



Article

highly efficient removal of metal ions by a factor of 5. As determined by XRD and PSD studies, these improvements can be attributed to the increased surface available for adsorption, because of the decrease in the clay particle size, as well as the exfoliation of the clay mineral particles. The adsorption data for all metal ions on the raw and mechanochemically treated clays fit the Langmuir isotherm model. The Langmuir monolayer adsorption capacities of Pb(II), Cu(II), Zn(II), and Cd(II) ions increased with milling, and for the 19-h-milled clay, the values were 50.5%, 52.6%, 49.4%, and 57.4% higher, respectively, than the values for the raw clay sample. The obtained results represent a fundamental study of the simultaneous sorption of heavy-metal ions from aqueous solutions by raw and mechanochemically activated interstratified montmorillonite/kaolinite clay. The study shows that the investigated clay, as an economical and efficient sorbent, has potential for application in the treatment of heavy-metalcontaminated wastewaters. Further investigations will be directed toward applications to real samples and the enhancement of the adsorption properties of the studied clay.

These two conflicting parameters limit a further increase in sorption properties by milling. Similar results were reported for Pb(II) adsorption onto vermiculite subjected to mechanochemical treatment using a vibration mill.20 The isotherm data presented in Table 3 show that the obtained experimental results fit the Langmuir isotherm model better than the Freundlich isotherm model, because the Langmuir plots have higher correlation coefficients (R2 > 0.95). This suggests that the uptake of Pb(II), Cu(II), Zn(II), and Cd(II) ions occurs on a homogeneous surface by monolayer adsorption, as was found for the adsorption on raw clay, independently of the microstructural changes induced by milling (partial separation of interstratified layers, delamination, agglomeration, etc.). There is no significant difference in the correlation coefficients for the Langmuir model applied to milled clays compared to raw clay, except for Cd(II) ions. Using the raw clay as an adsorbent, the experimental data for Cd(II) ions fit the Langmuir model only for Ci ≤ 200 mg dm−3, whereas using milled clays as adsorbents, the experimental data fit the Langmuir model for the whole range of investigated Ci values. The Langmuir monolayer adsorption capacities of Pb(II), Cu(II), Zn(II), and Cd(II) ions were estimated to be 27.2, 21.3, 14.7, and 11.3 mg g−1 and 42.6, 34.6, 16.0, and 14.1 mg g−1, respectively, for the 2- and 19-h-milled clay. The sequence of selectivity stayed in the same order as for the raw clay (Pb > Cu > Zn > Cd). Comparing the adsorption capacities (reported in the form of monolayer adsorption capacities, qm) of the investigated clay and various natural adsorbents reported in the literature for the simultaneous removal of heavy metals,6,24,46−49 the 19-h-milled interstratified MMT/K clay exhibits a high removal capacity for the investigated cations. (Table S3 in the Supporting Information lists the qm values of various natural adsorbents.) This suggests that mechanochemically activated interstratified MMT/K clay can be used as a good adsorbent. When compared to other methods of material modification, mechanochemical treatment represents an economical, environmentally friendly, and efficient modification technique. Milling with different kinds of additives or lubricants deserves serious consideration as a possibility for further improvement of the adsorption capacity of the investigated interstratified MMT/K clay.

■

ASSOCIATED CONTENT

S Supporting Information *

Equilibrium concentrations of Pb(II), Cu(II), Zn(II), and Cd(II); removal efficiencies and equilibrium amounts adsorbed per unit mass of the raw (RC) and 2- and 19-h-milled clay (MC 2h and MC 19h, respectively); plots of qe versus Ce and Langmuir plots; and comparison of the adsorption capacities of the investigated clay and various natural adsorbents. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +381 60 324 1789. Fax: +381 11 3408 224. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the support to this work provided by the Ministry of Education and Science of Serbia through the projects No. III 45006 and No. III 45012.

4. CONCLUSIONS Interstratified montmorillonite/kaolinite clay from the Serbian mine Bogovina is an effective adsorbent for the simultaneous removal of Pb(II), Cu(II), Zn(II), and Cd(II) from aqueous solutions. Batch experiments were performed to study the effects of contact time, solution pH, initial metal-ion concentration, milling time of the clay, and amount of adsorbent. The results showed that equilibrium was established after 60 min of contact. The adsorption of metal ions is highly pH-dependent and increases with increasing pH of aqueous solution. The maximum removal efficiencies and adsorption capacities of the selected metal ions were obtained at pH values between 4.5 and 6.5. The modification of the clay by mechanochemical activation significantly improved the adsorption behavior of the adsorbent. The removal efficiencies (E) and adsorption capacities (qe) of the investigated heavy metals increased with increasing milling time of the clay and were greater than the E and qe values obtained using raw clay. In addition, the milled clays reduced the amount of adsorbent necessary to achieve the

■

H

LIST OF SYMBOLS Ce = equilibrium concentration of metal ion in the aqueous solution (mg dm−3) Ci = initial metal-ion concentration in the aqueous solution (mg dm−3) Ct = metal-ion concentration in the aqueous solution at any time t (mg dm−3) E = removal efficiency (%) KF = Freundlich constant [(mg g−1) (dm3 mg−1)1/n] KL = Langmuir constant (dm3 mg−1) n = Freundlich constant (g dm−3) qe = equilibrium amount of metal ion adsorbed per unit mass of adsorbent (mg g−1) qm = maximum monolayer adsorption capacity (mg g−1) qt = amount of metal ion adsorbed per unit mass of adsorbent at any time t (mg g−1) R2 = correlation coefficient dx.doi.org/10.1021/ie400257k | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX


Article

V = volume of the aqueous solution (dm3) W = mass of the adsorbent (g)

■

(22) Krausand, W.; Nolze, G. POWDER CELLA Program for the Representation and Manipulation of Crystal Structures and Calculation of the Resulting X-ray Powder Patterns. J. Appl. Crystallogr. 1996, 29, 301. (23) Cho, D.-W.; Jeon, B.-H.; Chon, C.-M.; Kim, Y.; Schwartz, F. W.; Lee, E.-S.; Song, H. A Novel Chitosan/Clay/Magnetite Composite for Adsorption of Cu(II) and As(V). Chem. Eng. J. 2012, 200−202, 654. (24) Jiang, M.-q.; Jin, X.-y.; Lu, X.-Q.; Chen, Z.-l. Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) onto Natural Kaolinite Clay. Desalination 2010, 252, 33. (25) wa Mulange, D. M.; Garbers-Craig, A. M. Stabilization of Cr(VI) from Fine Ferrochrome Dust Using Exfoliated Vermiculite. J. Hazard. Mater. 2012, 223−224, 46. (26) Schmehl, W. R.; Jackson, M. L. Interstratification of Layer Silicates in Two Soil Clays. Clays Clay Miner. 1955, 4, 423. (27) Sakharov, B. A.; Lindgreen, H.; Salynand, A. L.; Drits, A. MixedLayer Kaolinite−Illite−Vermiculite in North Sea Shales. Clay Miner. 1999, 34, 333. (28) Churchman, G. J.; Slade, P. G.; Self, P. G.; Janik, L. J. Nature of Interstratified Kaolin−Smectites in Some Australian Soils. Aust. J. Soil Res. 1994, 32, 805. (29) Schultz, L. G.; Shepard, A. O.; Blackmon, P. D.; Starkey, H. C. Mixed-Layer Kaolinite−Montmorillonite from the Yucatan Peninsula, Mexico. Clays Clay Miner. 1971, 19, 137. (30) Vingiani, S.; Righi, D.; Petit, S.; Terribile, F. Mixed-layer Kaolinite−Smectite Minerals in a Red−Black Soil Sequence from Basalt in Sardinia, Italy. Clays Clay Miner. 2004, 52, 473. (31) Hong, H.; Churchman, G. J.; Gu, Y.; Yin, K.; Wang, C. Kaolinite−Smectite Mixed-Layer Clays in the Jiujiang Red Soils and Their Climate Significance. Geoderma 2012, 173−174, 75. (32) Omotoso, O. E.; Mikula, R. J. High Surface Areas Caused by Smectitic Interstratification of Kaolinite and Illite in Athabasca Oil Sands. Appl. Clay Sci. 2004, 25, 37. (33) Srivastava, P.; Parkash, B.; Pal, D. K. Clay Minerals in Soils as Evidence of Holocene Climatic Change, Central Indo-Gangetic Plains, North-Central India. Quat. Res. 1998, 50, 230. (34) Dudek, T.; Cuadros, J.; Huertas, J. Structure of Mixed-Layer Kaolinite−Smectite and Smectite-to-Kaolinite Transformation Mechanism from Synthesis Experiments. Am. Mineral. 2007, 92, 179. (35) Brindley, G. W. Experimental Methods. In X-ray Identification and Crystal Structures of Clay Minerals; Mineralogical Society: London, 1951. (36) Christidis, G.; Dunham, A. C. Compositional Variations in Smectites: Part I. Alteration of Intermediate Volcanic Rocks. A Case Study from Milos Island, Greece. Clay Miner. 1993, 28, 255. (37) Visser, J. van der Waals and Other Cohesive Forces Affecting Powder Fluidization, Chapter 1. Powder Technol. 1989, 58, 1. (38) Chromium(VI) Handbook; CRC Press: Boca Raton, FL, 2005. (39) Wang, C.; Liu, J.; Zhang, Z.; Wang, B.; Sun, H. Adsorption of Cd(II), Ni(II), and Zn(II) by Tourmaline at Acidic Conditions: Kinetics, Thermodynamics, and Mechanisms. Ind. Eng. Chem. Res. 2012, 51, 4397. (40) Unuabonah, E. I.; Adebowale, K. O.; Olu-Owolabi, B. I.; Yang, L. Z.; Kong, L. X. Adsorption of Pb(II) and Cd(II) from Aqueous Solutions onto Sodium Tetraborate-Modified Kaolinite Clay: Equilibrium and Thermodynamic Studies. Hydrometallurgy 2008, 93, 1. (41) Abollino, O.; Aceto, M.; Malandrino, M.; Sarzanini, C.; Mentasti, E. Adsorption of Heavy Metals on Na-montmorillonite. Effect of pH and Organic Substances. Water Res. 2003, 37, 1619. (42) Van Bladel, R.; Halen, H.; Cloos, P. Calcium−Zinc and Calcium−Cadmium Exchange in Suspensions of Various Types of Clays. Clay Miner. 1993, 28, 33. (43) Bradl, H. B. Adsorption of Heavy Metal Ions on Soils and Soils Constituents. J. Colloid Interface Sci. 2004, 277, 1. (44) Adebowale, K. O.; Unuabonah, I. E.; Olu-Owolabi, B. I. The Effect of Some Operating Variables on the Adsorption of Lead and Cadmium Ions on Kaolinite Clay. J. Hazard. Mater. 2006, 134, 130. (45) Horowitz, A. J. A Primer on Sediment-Trace Element Chemistry, 2nd ed.; Lewis Publishers: Chelsea, U.K., 1991.

REFERENCES

(1) An, Y.-J.; Kim, Y.-M.; Kwon, T.-I.; Jeong, S.-W. Combined Effect of Copper, Cadmium, and Lead upon Cucumis sativus Growth and Bioaccumulation. Sci. Total Environ. 2004, 326, 85. (2) Oliveira Ribeiro, C. A.; Vollaire, Y.; Sanchez-Chardi, A.; Roche, H. Bioaccumulation and the Effects of Organochlorine Pesticides, PAH and Heavy Metals in the Eel (Anguilla anguilla) at the Camargue Nature Reserve, France. Aquat. Toxicol. 2005, 74, 53. (3) Fu, F.; Wang, Q. Removal of Heavy Metal Ions from Wastewaters: A Review. J. Environ. Manage. 2011, 92, 407. (4) Babel, S.; Kurniawan, T. A. Low-cost Adsorbents for Heavy Metals Uptake from Contaminated Water: A Review. J. Hazard. Mater. 2003, 97, 219. (5) Kubilay, S.; Gurkan, R.; Savran, A.; Sahan, T. Removal of Cu(II), Zn(II) and Co(II) Ions from Aqueous Solutions by Adsorption onto Natural Bentonite. Adsorption 2007, 13, 41. (6) Kwon, J.-S.; Yun, S.-T.; Lee, J.-H.; Kim, S.-O.; Jo, H. Y. Removal of Divalent Heavy Metals (Cd, Cu, Pb, and Zn) and Arsenic(III) from Aqueous Solutions Using Scoria: Kinetics and Equilibria of Sorption. J. Hazard. Mater. 2010, 174, 307. (7) Zhao, G.; Wu, X.; Tan, X.; Wang, X. Sorption of Heavy Metal Ions from Aqueous Solutions: A Review. Open Colloid Sci. J. 2011, 4, 19. (8) Bhattacharyya, K. G.; Gupta, S. S. Removal of Cu(II) by Natural and Acid-Activated Clays: An Insight of Adsorption Isotherm, Kinetic and Thermodynamics. Desalination 2011, 272, 66. (9) Bhattacharyya, K. G.; Gupta, S. S. Adsorption of a Few Heavy Metals on Natural and Modified Kaolinite and Montmorillonite: A Review. Adv. Colloid Interface Sci. 2008, 140, 114. (10) Gupta, S. S.; Bhattacharyya, K. G. Removal of Cd(II) from Aqueous Solution by Kaolinite, Montmorillonite and Their Poly(oxo zirconium) and Tetrabutylammonium Derivatives. J. Hazard. Mater. 2006, 128, 247. (11) Suryanarayana, C. Mechanical Alloying and Milling. Prog. Mater. Sci. 2001, 46, 1. (12) Vdović, N.; Jurina, I.; Škapin, S. D.; Sondi, I. The Surface Properties of Clay Minerals Modified by Intensive Dry Milling Revisited. Appl. Clay Sci. 2010, 48, 575. (13) Perrin-Sarazin, F.; Sepehr, M.; Bouaricha, S.; Denault, J. Potential of Ball Milling to Improve Clay Dispersion in Nanocomposites. Polym. Eng. Sci. 2009, 49, 651. (14) Sidheswara, O.; Bhat, A. N.; Ganguli, P. Interaction of Salts of Fatty Acids into Kaolinite. Clays Clay Miner. 1990, 38, 29. (15) Sang-Mo, K.; Dixon, J. B. Preparation and Application of Organo-Minerals as Sorbents of Phenol, Benzene and Toluene. Appl. Clay Sci. 2002, 18, 111. (16) Hrachová, J.; Madejová, J.; Billik, P.; Komadel, P.; Fajnor, V. Š. Dry Grinding of Ca and Octadecyltrimethylammonium Montmorillonite. J. Colloid Interface Sci. 2007, 316, 589. (17) Lee, Y. C.; Kuo, C. L.; Wen, S. B.; Lin, C. P. Changes of Organo-Montmorillonite by Ball-Milling in Water and Kerosene. Appl. Clay Sci. 2007, 36, 265. (18) Ramadan, A. R.; Esawi, A. M. K.; Gawad, A. A. Effect of Ball Milling on the Structure of Na+-Montmorillonite and OrganoMontmorillonite (Cloisite 30B). Appl. Clay Sci. 2010, 47, 196. (19) Suraj, G.; Iyer, C. S. P.; Rugmini, S.; Lalithambika, M. The Effect of Micronization on Kaolinites and Their Sorption Behaviour. Appl. Clay Sci. 1997, 12, 111. (20) Hongo, T.; Yoshino, S.; Yamazaki, A.; Yamasaki, A.; Satokawa, S. Mechanochemical Treatment of Vermiculite in Vibration Milling and Its Effect on Lead(II) Adsorption Ability. Appl. Clay Sci. 2012, 70, 74. (21) Nenadović, S.; Nenadović, M.; Kovačević, R.; Matović, Lj.; Matović, B.; Jovanović, Z.; Grbović Novaković, J. Influence of Diatomite Microctructure on Its Adsorption Capacity for Pb(II). Sci. Sintering 2009, 41, 309. I



Article

(46) Elouear, Z.; Bouzid, J.; Boujelben, N.; Feki, M.; Jamoussi, F.; Montiel, A. Heavy Metal Removal from Aqueous Solutions by Activated Phosphate Rock. J. Hazard. Mater. 2008, 156, 412. (47) Sahu, R. C.; Patel, R.; Ray, B. C. Adsorption of Zn(II) on Activated Red Mud: Neutralized by CO2. Desalination 2011, 266, 93. (48) Baker, H. M.; Massadeh, A. M.; Younes, H. A. Natural Jordanian Zeolite: Removal of Heavy Metal Ions from Water Samples Using Column and Batch Methods. Environ. Monit. Assess. 2008, 157, 319. (49) Pentari, D.; Perdikatsis, V.; Katsimicha, D.; Kanaki, A. Sorption Properties of Low Calorific Value Greek Lignites: Removal of Lead, Cadmium, Zinc and Copper Ions from Aqueous Solutions. J. Hazard. Mater. 2009, 168, 1017.

J


Simultaneous Removal of Divalent Heavy Metals from Aqueous

Recommend Documents