Purifying Water Containing Both Anionic and Cationic Species Using a

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Purifying Water Containing Both Anionic and Cationic Species Using a (Zn, Cu)O, ZnO, and Cobalt Ferrite Based Multiphase Adsorbent System Niya Mary Jacob,† Praveena Kuruva,† Giridhar Madras,‡ and Tiju Thomas*,† †

Materials Research Centre, Indian Institute of Science, Bangalore 560012, Karnataka, India Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India

‡

S Supporting Information *

ABSTRACT: For the purpose of water purification, novel and low-cost adsorbents which are promising replacements for activated carbon are being actively pursued. However, a single-phase material that adsorbs both cationic and anionic species remains elusive. Hence, a low-cost, multiphase adsorbent bed that purifies water containing both anionic and cationic pollutants is a desirable alternative. We choose anionic (Congo red, Orange G) and cationic (methylene blue, malachite green) dyes as model pollutants. These dyes are chosen since they are widely found in effluents from textile, leather, fishery, and pharmaceutical industries, and their carcinogenic, mutagenic, genotoxic, and cytotoxic impact on mammalian cells is well-established. We show that ZnO, (Zn0.24Cu0.76)O and cobalt ferrite based multiphase fixed adsorbent bed efficiently adsorbs model anionic (Congo red, Orange G) and cationic (methylene blue and malachite green) pollutants, and their complex mixtures. All adsorbent phases are synthesized using room-temperature, high-yield (∼96−100%), green chemical processes. The nanoadsorbents are characterized by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET) surface area analysis, and ζ potential measurements. The constituent nanophases are deliberately chosen to be beyond 50 nm, in order to avoid the nanotoxic size regime of oxides. Adsorption characteristics of each of the phases are examined. Isotherm based analysis shows that adsorption is both spontaneous and highly favorable. ζ potential measurements indicate that electrostatic interactions are the primary driving force for the observed adsorption behavior. The isotherms obtained are best described using a composite Langmuir−Freundlich model. Pseudo-first-order, rapid kinetics is observed (with adsorption rate constants as high as 0.1−0.2 min−1 in some cases). Film diffusion is shown to be the primary mechanism of adsorption.

1. INTRODUCTION For the purpose of adsorptive purification of water, novel adsorbents that can effectively replace activated carbon are actively being pursued.1−3 At present, a low-cost-material system that adsorbs both cationic and anionic pollutants and their complex mixtures remains an elusive target. In cases where amphoteric adsorbents4 are reported, the number of process steps involved in the synthesis makes both viability and cost a serious concern.4 Use of activated carbon, which is hitherto the most attractive and versatile adsorbent, is impeded primarily by its prohibitive costs.1,2 Some progress has been made to reduce the cost of activated carbon (e.g., using bamboo derived activated carbon5). Natural products based adsorbents such as bottom ash, deoiled soya, fly ash, and red mud6−8 have been investigated for removal of toxic organic pollutants (e.g., aromatic dyes). On the other hand, membrane based approaches have emerged as alternatives to traditional adsorbents.9−11 However, chemical modification of membranes that are needed to ensure long-term and efficient ultrafiltration, while also minimizing the tendency for membrane fouling, is a significant challenge.10 The multistep processes9 involved in fabrication and engineering of such membranes raise questions about its viability for use in rapid and low-cost water treatment. This is especially true when one considers the current tonnage of wastewater generated across the globe. However, for specialized and small-scale applications (e.g., protein separa© 2013 American Chemical Society

tions), these membrane technologies are both viable and promising.9 A new direction in membrane technology for water treatment involves development of hybrid membranes with inorganic inclusions (such as ZnO TiO2), to improve membrane performance.11 Considering the current state of affairs, it is reasonable to say that membrane technology presents its own set of challenges. Hence, efficient and low-cost adsorbents (single phase or multiphase) that effectively adsorb both anionic and cationic species continue to be of interest, despite recent developments in alternate technologies. In recent years, several groups have explored rare earth oxides due to their high adsorptive capacities.12−14 However, rare earth based technologies are inherently expensive, and rare earth compounds tend to be toxic. While their toxicological impacts are still a subject of intense research, it is quite evident that unregulated autophagy (a catabolic mechanism) is the most common cytotoxic impact of rare earth oxides.15,16 In comparison, clay17,18 based adsorbents seem more viable and ecofriendly.19 Polymer based adsorbent systems20,21 also hold promise for viable water remediation, especially in cases where the polymer is easy to produce, inexpensive, and biodegradable. Received: Revised: Accepted: Published: 16384

August 20, 2013 October 7, 2013 October 28, 2013 October 28, 2013 dx.doi.org/10.1021/ie402727z | Ind. Eng. Chem. Res. 2013, 52, 16384−16395

Industrial & Engineering Chemistry Research

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diseases.39 Orange G (OG), which is used as a food colorant in several parts of the globe, is found to be genotoxic.40 Hence, the dyes we have chosen for this study represent significant threat to human ecology and the overall environment. Remediation of water bodies contaminated with these organic pollutants (and related species) is of utmost importance. We show that composites of ZnO, CuO, and cobalt ferrite have appreciable adsorptive capacities, despite their large sizes (>40 nm) and low specific surface area. We choose to synthesize each of these components using room-temperature, green chemical approaches. Using this integrated system, rapid adsorption kinetics is observed. Unlike CoFe2O4, ZnO and related composites are nonmagnetic. Hence, we choose to immobilize the four materials presented here on an adsorbent bed. Some spatial separation between the phases is necessary, since surfaces of the various adsorbents interact strongly, reducing the overall adsorptive capacity of the intimate multiphase mixture (physical mixture). We notice that the multiphase adsorbent bed system reported here separates both cationic and anionic species efficiently. The adsorbent loading necessary is somewhat high when compared to granular activated carbon (GAC), due to the low specific area of the constituents.

Likewise earth abundant transition metal (TM) based oxides can also be used to obtain industrially viable adsorbents, especially if such systems are synthesized using green chemical, one-pot routes. TM oxides synthesized using such routes are good candidates for large-scale production. Hence, in this work, we choose to focus on a few earth abundant, transition metal binary and ternary oxides. Adsorptive abilities sensitively depend on the specific surface area and surface chemical details of the adsorbents.22 The surface chemistry of oxide composites is very rich.23 Hence, oxide composites lend themselves to easy tuning of ζ potentials,24,25 which in turn can be used to engineer their adsorptive capacities. The general trend in recent years has been to minimize particle sizes. However, nanotoxicity is a concern when it comes to practical use of nanomaterials. Recent toxicological reports suggest that even benign transition metal oxides (e.g., TiO2) can be cytotoxic when the particle size is below ∼30 nm.26,27 Hence, for development of practical and industrially viable adsorbents, it is essential to not just maximize functionality (i.e., adsorptive capacity) but also operate in the benign size regime. In our work, we focus on binary and ternary oxide nanoparticles in the size regime ≥ 50 nm to avoid the nanotoxic regime. Magnetic nanoadsorbents are a class of adsorbents that have the inherent advantage of being magnetically separable. This makes these systems more practical than other nanoadsorbents (which often require centrifugal separation, especially when gravity separation is not feasible). In fact recognition of this fact has resulted in pursuit of magnetic photocatalysts as well.28 In large water treatment plants, where the volume of water remedied is huge, centrifugal separation is not feasible. Fe3O4 based systems are the most widely studied magnetic nanoadsorbents at the moment.29−32 Ferrites have been particularly useful in elimination of heavy metal ions such as As.33 Recent reports suggest that cobalt ferrite (CoFe2O4) is a promising alternative to Fe3O4. However, in all previous reports of CoFe2O4, long reflux processes are employed in order to synthesize the material.34−36 From the point of view of green technologies, it is more appealing to pursue room-temperature (soft chemical) approaches to make viable magnetic nanoadsorbents. Hence, we use a room-temperature process to synthesize cobalt ferrite with promising adsorptive capacities. We show that the adsorptive capacities of cobalt ferrite (made using co-precipitation) depend sensitively on the pH of the synthesis solution. In this study, we use anionic (Congo red, Orange G) and cationic (methylene blue, malachite green) dyes as model pollutants. These dyes are deliberately chosen since they are widely used in both textile and leather industries. Also effluents from pharmaceutical industries, fish-breeding farms, and food coloring and biotechnology industries often contain these dyes. The presence of these dye systems in water bodies raises serious concerns since they are carcinogenic, mutagenic, genotoxic, and cytotoxic to mammalian cells.37−40 Hence, contamination from these (and related) dye systems is an ecological and human health concern. For example, both malachite green (MG) and methylene blue (MB), which are used in pharmaceutical industries, when ingested in high concentrations can cause hemolytic anemia, skin desquamation in infants, nausea, fever, abdominal−chest pain, hypotension, and bluish discoloration of skin, etc.37,38 On the other hand, MB is a severe eye irritant, which in high concentrations can induce corneal and conjunctival injury.41 Congo red (CR) is known to cause neurodegenerative disorders, such as Alzheimer’s, Parkinson’s, and Huntington’s

2. EXPERIMENTAL SECTION Materials Synthesis. Both ZnO and (Zn0.24Cu0.76)O nanocomposites are synthesized using a room-temperature, solid-state metathesis route using ZnCl2 (98%), CuCl2·2H2O (99%), and NaOH (98%) without further purification. For ZnO synthesis (eq 1a), stoichiometric quantities of ZnCl2 and NaOH are finely ground using pestle and mortar. In our experiment, 4.91 g of NaOH and 8.36 g of ZnCl2 (both finely ground) are mixed to yield 4.72 g of ZnO. Grinding this mixture gives rise to an exothermic reaction, which results in ZnO formation through a two-step metathesis and disproportionation reaction (synthesis yield ∼ 93%).42,43 ZnCl 2 + 2NaOH → ZnO + 2NaCl + H 2O

(1a)

x ZnCl 2 + (1 − x)CuCl 2 + 2NaOH → (ZnxCu1 − x)O + 2NaCl + H 2O

(1b)

In the case of (Zn0.24Cu00.76)O nanocomposites (eq 1b), ZnCl2 is added first to NaOH, which is followed by addition of CuCl2· 2H2O. Stoichiometric amounts of finely ground precursors are used. In our experiments, 12 g of ZnCl2, 8.8 g of NaOH, and 2.95 g of CuCl2·2H2O yielded 8.8 g of (Zn0.24Cu00.76)O nanocomposites (yield ∼ 96%). The reaction products are thoroughly washed using deionized (DI) water, filtered, and dried at room temperature. The composites are annealed at 200 °C for 2−3 h. Cobalt ferrite nanoparticles are synthesized by a roomtemperature co-precipitation method using Co(NO3)2·6H2O (99%), Fe(NO3)3·9H2O (98%), and NaOH (97%) precursors. A 0.2 M amount of Co(NO3)2·6H2O and 0.4 M Fe(NO3)3·9H2O solutions (prepared in deionized water) is mixed using a magnetic stirrer (stirring rate, 800−900 rpm). To this mixture, freshly prepared 2 M NaOH solution is added in a dropwise manner until the required pH is obtained. The pH of the solutions is measured using an Oakton pH tester (Eutech Instruments). Cobalt ferrite samples that precipitated out at pH 10 and pH 12.5 are used in this work. These materials will be called cobalt ferrite (pH 10) and cobalt ferrite (pH 12.5), respectively. The precipitate obtained is washed eight times using deionized water and dried in an oven at 100 °C. 16385

dx.doi.org/10.1021/ie402727z | Ind. Eng. Chem. Res. 2013, 52, 16384−16395


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light scattering technique. The materials are dispersed in DI water and sonicated for 20 min prior to the analysis. All of the ζ potential measurements of as-synthesized materials are performed at pH 7. Adsorption Experiments. We have choosen the four dyes MB, MG, OG, and CR for the adsorption experiments. The structural formulas of these dyes are given in Figure 1b(i−iv). Both OG and CR are anionic, while MB and MG are cationic. The cationic dyes (MB and MG) are acidic, and the anionic dyes (OG and CR) are basic in nature because of the way they associate with the H+ and OH− ions present in water.44 MB is a heterocyclic aromatic chemical compound with maximum absorption of light (λmax ) at 670 nm, while MG is a triarylmethane dye with λmax = 617 nm. OG is a synthetic azo dye (λmax ∼ 480 nm), and CR is a secondary diazo dye (λmax ∼ 498 nm). All of the experiments are carried out in the dark to avoid photolysis. The variation of adsorptive capacity with respect to adsorbent doses is used to determine the optimum loading. In all four cases (i−iv), the optimum adsorbent loading is found to be 1.5 g·L−1 (for 25 ppm dye solutions). Loading of 6 g/L adsorbent is found to be sufficient for concentrated solutions with ≥100 ppm dye concentration. Adsorption of (i) CR on ZnO, (ii) MG on (Zn0.24Cu0.76)O, (iii) MB on cobalt ferrite (pH = 12.5), and (iv) OG on cobalt ferrite (pH = 10) are found to be favorable. Since CoFe2O4 is magnetic, use of a magnetic stirrer prevents the proper assaying of its adsorptive capacity. Hence, for adsorption experiments involving cobalt ferrite or related composites, a thermostat orbital shaker (Shalom Instruments, Model SLM-INC-OS-16) is used. For all of the adsorption experiments, the orbital shaker is operated at 200 rpm and the temperature is maintained at ∼300 K. In cases where the components of the adsorbent are nonmagnetic, magnetic stirring is used (stirring rate, 300−350 rpm). For determining the adsorption kinetics, aliquots of the adsorbent−dye mixture are taken at regular intervals of time. The adsorbent is centrifuged out, and the concentration of the remaining dye in the solution is determined using a UV−vis spectrophotometer (Shimadzu UV-1700 Pharmaspec spectrometer). The adsorption activity of the synthesized material is compared with commercially activated carbon manufactured by E. Merc India Pvt. Ltd.). All of the adsorption experiments are carried out in the natural pH of the dye (refer to Supporting Information Table I).

In this study we aim to synthesize a material which can efficiently adsorb both anionic and cationic dyes. Hence, we use a combination of the above-mentioned adsorbents, i.e., ZnO, (Zn0.24Cu0.76)O, cobalt ferrite (pH 10), and cobalt ferrite (pH 12.5). The mixture of adsorbents, used without any postprocessing, is referred to as the physical mixture. On the other hand, the adsorbent bed consists of four glass plates (dimensions of each plate: 8 cm in length and 2.5 cm in width) kept beside each other (Figure 1a). Each of the plates has a different adsorbent phase

Figure 1. (a) Schematic representation of the “adsorbent bed” used. (b) Structures of (i) methylene blue, (ii) malachite green, (iii) Orange G, and (iv) Congo red.

immobilized on it. The bed made of four glass plates is placed at the bottom of the adsorbent chamber to ensure that the adsorbent phases do not mix with one another, since this reduces the observed absorptive capacity. Materials Characterization. The morphology of the assynthesized materials is studied using a field emission scanning electron microscope (FESEM, Karl Zeiss Ultra 55), equipped with an energy dispersive X-ray (EDX) analysis facility. The samples prepared are desiccated overnight to ensure complete removal of moisture. Gold sputtering is done prior to imaging, in order to ensure that sample charging does not interfere with electron microscopy. Crystal structures are examined by X-ray diffraction (XRD) obtained using PAN analytical X’pert PRO. The instrument is operated at 40 kV and 30 mA, and the 2θ range is chosen to be 20−80°. All of the diffractograms are obtained using a Cu Kα (wavelength = 1.54 Å) source. The specific surface area is measured using a Smart Sorb surface area analyzer (Smart Instruments Co. Pvt. Ltd.) by the nitrogen adsorption−desorption method at liquid nitrogen temperature. The samples are preheated at 120 °C for 4 h. The surface charge of the as-synthesized materials is analyzed using a ζ potential analyzer (Brookhaven Instruments Corp., ZetaPALS). It determines the ζ potential using phase analysis

3. RESULTS AND DISCUSSION Structural Studies. Figure 2 shows XRD patterns of each of the phases explored in this work. The lattice constants of all four materials are calculated using the positions of the diffraction peaks. The full width at half-maximum (fwhm) of the characteristic peaks ((100) for ZnO peak and (002) peak for CuO) is used to determine the crystallite size using the Debye− Scherrer equation. Metathesis reaction clearly results in phase pure ZnO (lattice constants a = b = 3.3 Å and c = 5.2 Å; crystallite size D = 91.5 nm; JCPDS card no.80-0075). This is consistent with reports by Cao et al. and Cheng et al.42,43 The XRD pattern of (Zn0.24Cu0.76)O confirms that it is a biphasic mixture of ZnO and CuO; there are no peaks corresponding to Cu2O. The lattice constants and crystallite size (D) of the ZnO phase are as follows: a = b = 3.2 Å, c = 5.2 Å, and D = 53 nm. The lattice constants and crystallite size of CuO are as follows: a = 4.6 Å, b = 3.4 Å, c = 5.1 Å, and D = 60 16386



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(SSAs) in the case of ZnO and (Zn0.24Cu0.76)O are significantly lesser than those associated with the cobalt ferrite samples (pH 10 and 12.5; Table 1). Given the average particle sizes, this low Table 1. Specific Surface Areas (SSAs) of the Four Adsorbents Reported material

SSA (m2/g)

ZnO (Zn0.24Cu0.76)O cobalt ferrite (pH 10) cobalt ferrite (pH 12.5)

13.3 31.3 115.6 138.9

specific surface area of ZnO and (Zn0.24Cu0.76)O is an indicator of significant agglomeration tendency in these material systems. The relatively high SSA associated with cobalt ferrite indicates that it has a lower tendency to agglomerate. Adsorption Kinetic and Isotherm Analysis. We find that ZnO is an efficient adsorbent of CR, while (Zn0.24Cu0.76)O adsorbs MG efficiently. Likewise cobalt ferrite (pH 10) is an efficient adsorbent of OG, while cobalt ferrite (pH 12.5) adsorbs MB. The adsorption behavior of cobalt ferrite (pH 10) and cobalt ferrite (pH 12.5) is very different; the reason for it very likely lies in the slight difference in composition. EDX suggests that both co-precipitates are nominally nonstoichiometric. Cobalt ferrite (pH 10) is slightly oxygen rich with a Co:Fe:O stoichiometric ratio of 1:1.95:3.93. On the other hand, cobalt ferrite (pH 12.5) is minimally oxygen deficient (Co:Fe:O = 1:2:3.98). The primary aim of this work is to arrive at a mixture that is capable of adsorbing both anionic and cationic species. To achieve this aim, each of these constituent adsorbents is separately analyzed. The results obtained are used to arrive at a composite system to achieve the desired sorption behavior. Adsorption Kinetics. Adsorption kinetics describes the rate of uptake of a dye by the adsorbent material. Parts a−d of Figure 4 show the adsorption of the four different dyes CR, MG, OG, and MB by the adsorbents ZnO, (Zn0.24Cu0.76)O, and cobalt ferrites (pH 10 and 12.5), respectively. In this study we find that all of the adsorption processes are pseudo-first-order (eq 2). The pseudofirst-order adsorption kinetics is modeled using the following:

Figure 2. XRD of as synthesized samples of (a) ZnO, (b) (Zn0.24Cu0.76)O, (c) cobalt ferrite (pH 10), and (d) cobalt ferrite (pH 12.5)..

nm (JCPDS card nos.80-0075 and 80-1917). The ZnO obtained is hexagonal and CuO is monoclinic with β = 99.6°. The diffraction pattern of cobalt ferrite obtained by coprecipitation method is consistent with JCPDS card no.221086.45 CoFe2O4 belongs to a cubic crystal system (a = 8.3 Å) and has an inverse spinel structure type. The crystallite sizes of cobalt ferrite (pH 10) and cobalt ferrite (pH 12.5) are 26 and 20 nm, respectively. Morphological Studies. Scanning electron microscopic (SEM) images of both ZnO and (Zn0.24Cu0.76)O show that particles are in the 50−70 nm size regime. From a practical point of view, having large particles is advantageous since nanotoxicity usually sets in below 30 nm in the case of ZnO.46 From Figure 3a,b it is clear that the particles form agglomerates.

ln(qe − qt ) = ln(qe) − kt

(2)

In the above equation, qe and qt are the amounts of dye adsorbed by the adsorbent (mg/g) at equilibrium and at time t (min); k is the first-order rate constant. qe and qt are defined as follows: qe =

(c0 − ce)V M

(3)

(c 0 − c t )V (4) M c0 and ce are the initial and equilibrium concentrations of dye (mg/L). V is the volume (in liters) of the dye solution used for adsorption. M is the amount of adsorbent used in grams. In the case of CR’s adsorption on ZnO, the rate constants and its variation with respect to concentration are shown in Table 2a. This system is a good representative of all of the dye−adsorbent systems studied in this work, since trends observed here are seen in other systems as well. For a similar tabulation for other dye− adsorbent systems, the reader may refer to Table 2b−d. Ideally first-order kinetics results in a rate constant which is independent of concentration. However, in our studies, we see a minor qt =

Figure 3. SEM images of (a) ZnO, (b) (Zn0 0.24Cu0.76)O, (c) cobalt ferrite (pH 12.5), and (d) cobalt ferrite (pH 10).

Cobalt ferrite (both pH 10 and 12.5) samples have significant polydispersity, as can be seen in the SEM images. The average particle sizes are 27 and 43 nm for cobalt ferrite (pH 10) and cobalt ferrite (pH 12.5), respectively. However, the pH 12.5 sample is less polydisperse (variance/mean size = 6) when compared to the pH 10 sample (variance/mean size = 7). Surface Area Analysis. Brunauer−Emmett−Teller (BET) measurements on these samples show that specific surface areas 16387



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Figure 4. Variations of dye concentration with time during adsorption experiments performed at room temperature: (a) adsorption of CR on ZnO, (b) MG on (Zn0.24Cu0.76)O, (c) MB on CoFe204 (pH 12.5). and (d) OG on CoFe2 04 (pH 10). Cdye represents the dye concentration.

Table 2. (a−d) Variation of Rate Constants with Respect to Initial Dye Concentration (c0)a adsorbent + dye

C0 (mg/L)

k (min −1)

qe (mg/g)

α1

α2

β

15.6 32.1 44.3 55

8.2 10.4 10.4 13.3

1.2 0.57 0.35 0.24

6.8 18.2 29.7 55.4

(a) ZnO + CR

100 200 300 500

0.37 0.34 0.18 0.05

(Zn0.24Cu0.76)O + MG

25 50 70 100

0.05 0.03 0.03 0.03

cobalt ferrite (pH = 12.5) + MB

25 75 150 200

0.1 0.11 0.11 0.15

cobalt ferrite (pH = 10) + OG

100 200 300 400

0.1 0.11 0.17 0.11

(b) 9.6 20.6 32.0 14.1

0.72 0.96 1.1 1.7

0.03 0.06 0.04 0.03

24 16 27.5 56.6

12.3 10.2 20.2 26.4

0.83 3.2 5.2 9.3

1 0.86 0.52 0.31

0.83 3.7 10 30

0.22 0.14 0.34 0.21

22.7 41.4 45.8 69

(c)

(d) 15.9 31.6 42.7 50

5 5.8 15.6 14.5

a Equilibrium concentration (qe) and relative intraparticle diffusion parameter (β) are shown as well. Adsorption of (a) CR on ZnO, (b) MG on (Zn0.24Cu0.76)O, (c) MB on cobalt ferrite (pH = 12.5), and (d) OG on cobalt ferrite (pH = 10). Pseudo-first-order kinetics with film diffusion being the primary adsorption process is observed in all cases.

concentration ranges over which the adsorbent remains effective (Figure 4a−d).48 In all four cases a large decrease in the concentration of the dye is observed in the initial stages (∼10 min) of adsorption; the system usually reaches equilibrium after 30−100 min. The rapid adsorption phase observed at the outset is likely due to the large number of active sites present at the beginning. As time progresses, the number of active adsorption sites decreases rapidly. This is in fact responsible for the pseudofirst-order kinetics observed.

variation of rate constant with dye concentration. Even in the case where the variation is the highest, i.e., in case of CR adsorption on ZnO, the variation of k with respect to concentration is only 8 × 10−4 min−1·mg−1·L; this results in an order less than 2. This is why it is reasonable to call the observed adsorption kinetics (in all systems studied here) as pseudo-firstorder.47,48 Adsorption experiments are carried out with different initial dye concentrations to check the adsorption capacity and 16388



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reports on CoFe2O4 based adsorbent systems use low concentrations of dye (10−30 ppm),54 which is due to the rapid adsorption site saturation that occurs in ferrites. However, in the system reported here, adsorption site saturation is not observed until 400 ppm (Figure 4d), and the qe/SSA value (0.43 mg/m2) is encouraging (qe ∼ 50 mg/g and SSA ∼115 m2/g at dye concentration of 400 ppm). Also in this work, we obtain reasonably good qe without using a low-dimensional phase (e.g., graphene). The performance of this ferrite can be further improved by enhancing SSA, while retaining the advantage that comes with being in the benign size regime. We are currently pursuing this direction by enhancing ζ potentials, using green and soft chemical approaches. Adsorption Isotherm. Isotherm analysis gives information about the maximum adsorption capacity of adsorbent. The Langmuir isotherm (eq 5) assumes monolayer adsorption of the adsorbate molecules on the adsorbent. Another major assumption in the Langmuir model is that all of the adsorption sites are energetically equivalent. On the other hand Freundlich proposed a multilayer, nonequivalent adsorption site model for adsorption over surfaces. In the Freundlich model (eq 6), the heterogeneity of the adsorbent system is quantified by the value of the parameter nc. To incorporate the advantages of both isotherms, a composite isotherm known as the Langmuir− Freundlich isotherm (eq 7) can be used.48

In the case of CR adsorption on ZnO we observe rapid adsorption kinetics. The adsorption efficiency is ∼100% which is achieved in 20−30 min, when the initial dye concentration (c0) ≤ 300 mg/L (Figure 4a). However, for c0 ≥ 400 mg/L, the dye concentration reduces by half in the initial stages (∼7 min.) and no considerable change in the concentration of the dye in the solution is observed after 1 h of stirring. Hence, in cases where the dye concentration exceeds 400 mg/L, equilibrium adsorption is rapidly attained. We also observe that the amount of dye removed at equilibrium (qe) increases from 15.6 to 55 mg/g with the increase in dye concentration from 100 to 500 mg/L. So it is clear that the removal of dyes depends on the concentration of the dye, which is consistent with results reported for other adsorbents as well.47 Recent reports on ZnO based adsorbents involve binary oxide systems (e.g., ZnO−Al2O3),49 or polymer based composites (e.g., chitosan−polyaniline/ZnO).50 ZnO loaded on activated carbon has also been studied.51 These investigations involve activation of surface charge, and enhancement of porosity of the material. This is done in order to achieve adsorbents with minimal adsorbent loading, maximum adsorption efficiency in short time, and high qe value. In order to compare our results on ZnO and (Zn, Cu)O with these reports,49−51 we calculate the qe per unit area of the adsorbent (i.e., qe/SSA). The qe of ZnO adsorbents reported here is 55 mg/g, while the SSA is 13.3 m2/g (CR concentration, 500 ppm). In this material, qe varies linearly with respect to concentration when the dye concentration is 1, which shows that the fastest diffusion process observed corresponds to film diffusion (refer to Table 2a−d). In fact large values for β indicate large intraparticle diffusion resistances, and hence high pore resistance in all cases.48 A large value for β is reasonable in this case, since the materials reported here are not porous. To highlight this, it may be noted that for porous metal oxide adsorbents β is often less than 4.48 However, we observe β to be several times higher (e.g., 55.4 in the case of CR adsorption on ZnO). Hence, the observed adsorption is clearly a film diffusion phenomenon, due to the lack of porosity in the reported samples. Table 3 and Figure 6 clearly indicate that the adsorption of dye by the adsorbent is a multistep process and is indicated by different rate constants (kid) at different intervals of time (refer to Supporting Information Table IIb−d). The intraparticle diffusion rate constant is kid. The thickness of the boundary layer is proportional to C. Variation of both kid and C with respect Table 3. Variation in Intraparticle Diffusion Rate Constant (kid) with Respect to Time, Suggesting a Multistep Adsorption Process, with Each Step Being Dominant at Different Times for ZnO (Adsorbent) + CR (Dye)a region 1 region 2 region 1 region 2 region 3 region 1 region 2 region 3

C0 (mg/L)

Kid (mg g−1min−1/2)

C (mg/g)

200

19.7 2.9 21.6 9 2.3 18.6 9.3 2.7

0 23 0 16 26 0 17 38

300

500

a

In fact the number of steps increases from 2 to 3, when the concentration of the dye increases. An increase in C (refer to eq 9) with respect to time indicates an increase in boundary film thickness.. 16391



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another; hence, the overall surface chemistry of the physical mixture is very different from that of the individual components. Next, we study the adsorption of dye mixtures on individual adsorbents. These adsorption experiments are carried out in 50 mL of the dye mixture (equal volume of 25 mg/L of MB, MG, OG, and CR). The adsorbent loading chosen is 1.5 g/L, since it is the optimum adsorbent loading. We find that the cationic and anionic dye mixtures show complexation, and flocculation, which itself results in some separation from water. This occurs very rapidly and spontaneously. This is consistent with the kinetics and thermodynamics of dye association reported so far.59 For example, complexation and separation due to dye association alone reduces the concentration of OG from 25 to 15 mg/L. Figure 8a represents the adsorption of the dye mixture on a single-phase adsorbent bed made up of ZnO alone. We notice that only the λmax ∼ 616 nm and λmax ∼ 481 nm peaks reduce in intensity with the progression of the adsorption experiment. This is the case with (Zn0.24Cu0.76)O also (Figure 8b). This implies that CR and MG are being adsorbed when ZnO or (Zn0.24Cu0.76) O are used, but the other two dye systems are not separated. In the case of cobalt ferrite (pH 10), only the λmax ∼ 481 nm peaks reduce significantly in intensity, indicating significant OG adsorption. Cobalt ferrite (pH 10) based adsorption also results in very minimal MG adsorption (λmax ∼ 616 nm; Figure 8c). On the other hand, cobalt ferrite (pH 12.5) significantly adsorbs MB but does not immobilize any other dye. Further, in the case of cobalt ferrite (pH 12.5), adsorption is followed by desorption (Figure 8d). From the above discussion, we infer that specific and limited adsorption and desorption of adsorbed dyes are problems associated with single-phase adsorbent beds. In stark contrast, a multiphase adsorbent bed (Figure 1a) results in adsorption of all four dyes, and desorption is completely avoided (Figure 9). Use of this immobilized adsorbent bed eliminates the possibility of physical mixing of the four adsorbents, and hence helps in retaining the adsorptive capacities of each of the adsorbents. When the multiphase adsorbent bed is used, the overall result is encouraging since both anionic and cationic dyes and their mixtures are efficiently adsorbed. The whole study is carried out in a one-pot reaction chamber. Associated dye complexes get adsorbed on the adsorbent thin films at a rate that is ∼2 times the rate at which the individual dyes get adsorbed. For example, when the dye concentration is 14 mg/L, the instantaneous adsorption rate constant of OG on cobalt ferrite (pH 10) is 0.17 min−1. On the other hand, the instantaneous adsorption rate for the 14 mg/L dye complex (formed by mixing the two cationic and two anionic dyes used in this study) is ∼0.35 min−1. Using the four-phase adsorbent bed, over 80% of cationic and anionic species, and their complex mixtures, is efficiently removed. This makes our result of practical relevance.

Figure 7. (a) Absorbance spectra as evidence for association of dyes in solution. (b) Concentrations of MB and MG are kept constant (25 mg/ L), while concentrations of CR and OG are varied. (c) Concentrations of CR and OG are fixed, while concentrations of MB and MG are varied.

dyes results in a substantial broadening of the spectrum, coupled with a reduction in the overall absorbance. This indicates strong associations between the dyes.58 In Figure 7b, the concentration of cationic dyes is kept constant (25 mg/L), and the anionic dye concentrations are varied. To obtain Figure 7c, anionic dye concentration is fixed and the cationic dye concentration is varied. From Figure 7b,c, it is clear that the similarly charged species also interact strongly with one another. Hence, cationic dyes interact among themselves, while also interacting with anionic dyes. Similarly anionic dyes interact with other anionic dyes, while interacting strongly with cationic dyes. However it is the cation−anion complexes (formed due to electrostatic interactions), which are responsible for complexation, flocculation, and separation of the dyes.58 We notice that physical mixtures of the four adsorbent phases behave very differently when compared to the individual components and are ineffective as adsorbents. This indicates that the phases in the composite are strongly interacting with one

4. CONCLUSION Adsorptive capacities of room-temperature synthesized ZnO, (Zn0.24Cu0.76)O and cobalt ferrite (pH 10 and 12.5) are reported. In all cases pseudo-first-order kinetics is observed, with primary contribution to adsorption coming from film diffusion. Isotherm based thermodynamic analysis shows that adsorption is highly favorable and spontaneous. The electrostatic origin of the observed adsorption is indicated by analysis of ζ potentials. The isotherms suggest that the overall adsorptive capacity per unit area (qe/SSA) is comparable to values reported recently. However, since the systems studied here have large particle 16392



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Figure 8. Adsorption of the mixture of four dyes on individual adsorbents: (a) ZnO, (b) (Zn0.24Cu0.76)O, (c) cobalt ferrite (pH 10), and (d) cobalt ferrite (pH 12.5).

a larger specific surface area. To achieve this, we are currently pursuing adaptations of the “green” and soft chemical synthetic processes presented here.

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ASSOCIATED CONTENT

S Supporting Information *

Figure showing the distribution of ζ potentials in (Zn0.24O0.76)O at pH 7 and tables listing the natural pH of the dye solution with respect to the concentration of the dye and variation in the intraparticle diffusion rate constants. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Association of dyes followed by adsorption using multiphase adsorbent films (made of ZnO, (Zn0.24Cu0.76)O, cobalt ferrite (pH 10), and cobalt ferrite(pH 12.5)), resulting in removal of dyes.

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AUTHOR INFORMATION

Corresponding Author

sizes (done to avoid nanotoxic regime), the overall adsorptive capacity is lower. The dyes in this study (anionic, Congo red and Orange G; cationic, methylene blue and malachite green) are deliberately chosen since they are widely found in industrial effluents, and their carcinogenic, mutagenic, genotoxic, and cytotoxic impacts on mammalian cells are well-established. These pollutants and their complex mixtures are separated from water using a fourphase adsorbent bed, made using the above-mentioned nanoadsorbents. In fact association between cationic and anionic moieties results in some elimination of pollutants. The adsorbent bed further improves the elimination of mixtures of model pollutants (removal efficiency ∼ 80−85%). Results presented here are relevant for practical water treatment, since wastewater always contains a mixture of anionic and cationic species. Efforts are underway to synthesize adsorbents which are in the benign size regime, while ensuring

*E-mail: [email protected].: +91-994-584-7407. Fax: +91802-360-7316. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

N.M.J. acknowledges Mr. S. Varadharajaprumal and Mr. Manikant Singh from the Centre of Nano Science and Engineering (CeNSE, IISc) for their technical support. T.T. thanks Prof. Marcus Winterer (University of DusibergEssen, Germany) and Dr. Peter Knut Lundquist (Heinrich Heine University, Düsseldorf, Germany) for fruitful discussions. He also thanks the Department of Science and Technology (DST) for lending financial support in the form of Grant No. DST 01117. 16393



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Purifying Water Containing Both Anionic and Cationic Species Using a

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