Removal of rhodamine B and Cr(VI) from aqueous solutions by a polyoxometalate adsorbent

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Contents lists available at ScienceDirect

Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Removal of rhodamine B and Cr(VI) from aqueous solutions by a polyoxometalate adsorbent Feng Li a , Yong Chen a , Haimei Huang a , Wei Cao b , Taohai Li a,∗ a

College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China b Department of Physics and Chemistry, University of Oulu, PO Box 3000, Oulu FIN-90014, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history:

A polyoxometalate framework [Ni(bipy)2 ]2 (HPW12 O40 ) was synthesized via an efficient chem-

Received 25 March 2015

ical coprecipitation method. The adsorption ability was investigated by removing rhodamine

Received in revised form 17 May

B from aqueous solutions. The adsorption ability was extensively explored by varying the

2015

experimental conditions such as contact time, pH, temperature, adsorbent dose, initial dye

Accepted 20 May 2015

concentration and the reusability. Intra-particle diffusion model was employed to determine

Available online 29 May 2015

the rate limiting step of the adsorption process. Langmuir isotherm model was found suitable for the adsorption system, and the adsorption process obeys the pseudo-second-order

Keyword:

kinetics. Moreover, this adsorbent can be reused almost without any loss of adsorption

Polyoxometalate

capacity. It also has a multiplex adsorption capability as well as a satisfactory tolerance

Coprecipitation

against changes in pH/dye concentration. The [Ni(bipy)2 ]2 (HPW12 O40 ) was found able to

Adsorption

remove Cr(VI) ions from aqueous solution.

Rhodamine B

© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Dyes are widely used in many industries, such as textile, paper and plastics. As a consequence, substantial volumes of waste water are contaminated by the dyes (Ali and Sreekrishnan, 2001; Banat et al., 1996; Mahmoodi and Arami, 2009; Pokhrel and Viraraghavan, 2004; Robinson et al., 2001; Thompson et al., 2001). Nowadays, there are more than 10,000 kinds of dyes available commercially (Gong et al., 2005). However, many of them are toxic and even carcinogenic (Wang et al., 2005). The molecules of dyes usually have synthetic origins and complex aromatic structures, making them more stable but difficult to be biodegraded. As a result, the dyes in wastewater can’t be degraded naturally. The aquatic life and food chain are subsequently affected. As one of the key dyes, rhodamine B (RhB) has been widely used in dying wax paper, typing paper, silk, wool, leather, jute, cotton, acrylic and straw. The waste water from these industries is highly colored, and needs to be decontaminated for recycles.

∗

Wastewater treatments are normally handled through physical, chemical, biological purification methods, or combinations of these methods in order to recover partial or all lost properties of wastes (Alkan et al., 2004). Generally speaking, physical and chemical methods include adsorption, ion exchange and membrane filtrating. These methods are effective for removing dyes without producing redundant by-products. The RhB can be removed from aqueous solution through various methods such as photodegradations and different adsorptions (Al-Rashed and Al-Gaid, 2012; Wei et al., 2008; Xie et al., 2008; Zhuang et al., 2010). Among them, the adsorption method is found primarily effective because dyes are not only moderately stable to light and heat, but also resistant to oxidation and biodegradation. Besides, the adsorption technique works effectively without additional pretreatments before its usage, and will not produce unwanted by-products. Therefore, adsorption has been employed extensively in industrial processes for separation and purification. A lot of adsorbents have been synthesized.

Corresponding author. Tel.: +86 731 58292202; fax: +86 731 8292251. E-mail address: [email protected] (T. Li). http://dx.doi.org/10.1016/j.cherd.2015.05.030 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Among them, many commercially available species can be found, such as activated carbon (Romero-Anaya et al., 2010; Tseng et al., 2010), silica gel (Tzvetkova et al., 2010), activated alumina and polymeric porous materials frameworks (Camacho et al., 2010; Costanzo et al., 2010; Hamada et al., 2007; Xu et al., 2008). However, these adsorbents suffer from either low or limited adsorption capacity or inefficient extraction. Moreover almost all of them encounter with problems of recycling and reuse. To the best of our knowledge, highperformance and low-cost adsorbents with easy extractable and regenerate properties are scares, despite of strong desires of the market and industry. From another hand, the design and synthesis of metal–organic frameworks (MOFs) have attracted considerable interests due to their outstanding porosities with large surface areas up to 5500 m2 /g, and remarkable capabilities of adsorptions (Wu et al., 2007; Krawiec et al., 2006; Huo and Yan, 2012). The MOFs are generated from the coordination of organic ligands to metal centers. Similar to organic ligands, polyoxometalates (POMs) have intriguing structures and wide potential applications in a variety of areas, including gas storage, separations, catalysis, magnetism, and nonlinear optical materials (Banerjee et al., 2008; Ockwig et al., 2005). The organic ligands can coordinate the metal centers forming the metal–organic skeletons of MOFs. Such structural and material properties make them suitable as functional secondary building blocks (SBUs) for the design and construction of MOFs (James, 2003; Eddaoudi et al., 2001). As a result, many hybrid materials based on POMs have been developed (Magueres et al., 1994; Noguchi et al., 2006; Nomiya et al., 2010; Song et al., 2007; Uchida, 2006; Zhai et al., 2007). In this article, we reported on the removal of the RhB dye and Cr(VI) by adsorption technique using a POM as the adsorbent. The POM was synthesized from the reaction of bipyridine, NiCl2 ·6H2 O and phosphotungstic acid (H3 PW12 O40 ·xH2 O) in water batch. It was found that the present adsorbent is easily produced, low-cost, massively productive and effective to adsorb the pollutants.

2.

Materials and methods

2.1.

Materials, equipment and procedure

Rhodamine B (C.I. = 45,170; chemical formula: C28 H31 ClN2 O3 ; molecular weight = 479.01 g/mol; maximum wavelength = 553 nm) was used as the adsorptive, and was not further purified. The concentrations of rhodamine B dye were determined spectrophotometrically by using a double beam UV spectrophotometer (Lambda 25, PerkinElmer). The dye concentrations were estimated by monitoring the absorbance at max = 553 nm, while its molecule existed as positive ion in aqueous medium (Fig. S1). The pH value of the solution was adjusted by using 0.1 M HCl and 0.1 M NaOH solutions. All compounds were of analytic reagent grade. Batch experiments were carried out in a glass beaker of 200 mL volume. Stock solution of rhodamine B (1000 mg/L) was prepared by dissolving RhB (1.000 g) in 1.0 L of distilled water. Experimental solutions of desired concentrations were obtained by successive dilutions of the stock solution. Initial dye concentrations were varied from 10 to 35 mg/L. To study effects of pH on dye adsorption, experiments of solutions with different pH values were conducted. Different amounts of adsorbents were used

193

for the initial dye concentration of 25 mg/L for the purpose of testing the dose influences on dye adsorption. Temperature dependent absorbability of the POM was carried out at different temperatures. A common adsorbent dose of 0.1 g/100 mL and initial pH 4.7 was used in all the experiments. For the measurements of the repeated absorbability experiment: 0.5 g NPW sample was dispersed in 100 mL RhB solution (25 mg/L) and performed as described above. After each run of reactions, the NPW sample was separated from the solution, washed thoroughly with ethanol three times and dried overnight at 80 ◦ C. Then, the separated NPW sample was returned to the adsorbing reaction system, and each recycling test was conducted under the same conditions. The chromium sample was prepared by dissolving potassium dichromate (K2 Cr2 O7 ) in distilled water. The adsorption of Cr(VI) was performed, and the concentration of free chromium(VI) ions was determined spectrophotometrically by monitoring a purple–violet emission induced by the 1,5-diphenylcarbazide in acidic solution as the complex agent. Specific surface area of the adsorbent determined from a BET surface area analyzer gave a pore size distribution curve of the proposed adsorbent. The operational mechanism is based on the nitrogen equilibrium adsorption isotherm at 77 K. Fourier transform infrared spectroscopy (FTIR, Make: Perkin Elmer, USA, Model: FT-IR1710) analysis was carried out to confirm the crystallizations (crystalline or amorphous) as well as the bond stretching of the adsorbent. The scanning electron microscope (SEM) images were recorded on a JSM-6610LV microscope. The ultraviolet absorption spectral analysis was performed on a Perkin Elmer Lambda 25 Ultraviolet spectrometer, while the element analysis on a Vario EL Element analyzer. Thermogravimetric analysis (TGA) curves were collected from a TGA 50 thermogravimetric analyzer at a heating rate of 20 ◦ C/min in dynamic nitrogen atmosphere (60 mL/min). A digital pH meter (pH/Ion meter metrohm model E-20-C) was used to measure the pH value of the solution. A standard technique was used to determine the RhB concentration which was evaluated with a calibration curve. The amount of dye adsorbed per unit weight of adsorbent (qt (mg/g)) at a time interval t, and percentage dye removal efficiency R were calculated as follows: qt =

(C0 − Ct )V M

(1)

R=

(C0 − Ct ) × 100 C0

(2)

In order to study the adsorption isotherm, 0.1 g of asprepared NPW was kept in contact to 100 mL dye solution of 25 mg/L at different temperatures for 50 min. Stirring at a constant speed was applied. After solutions reaching the equilibrium, the amount of dye adsorbed (mg/g) on the absorbent surface was measured through a microbalance. Blank experiments were carried out with dye solution but without adsorbent to ensure that no dye was adsorbed onto the walls of the beakers. All experiments were repeated once to minimize uncertainties in the measurements.

2.2.

Synthesis of [Ni(bipy)2 ]2 (HPW12 O40 )

In a typical synthesis of the [Ni(bipy)2 ]2 (HPW12 O40 ) powders, Bipyridine (2.73 g, 25Yc excess), NiCl2 ·6H2 O (1.66 g) were refluxed for 72 h in 95Yc ethanol. The obtained solution was

194

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Fig. 1 – (a) FTIR of prepared adsorbent, (b) TGA of prepared adsorbent, (c) XRD patterns of NPW.

filtered, sealed with a plastic wrap, and volatilized to form crystalline. The crystalline was then filtered, washed with distilled water, ethanol and ether, and dried at 60 ◦ C for 12 h. The obtained powder was named A. A heteropolyacid (Phosphotungstic acid) aqueous solution was added to the aqueous solution of powder A slowly until a molar ratio of 1:1 was reached. Later the mixed solution was heated at 80 ◦ C for 1 h. The solution was filtered, and the precipitate was washed with distilled water, ethanol and ether three times. The washed precipitation was dried in vacuum at 60 ◦ C for 12 h. Yield (based on HPW) is 93.42%. Anal. Calcd. results for [Ni(bipy)2 ]2 (HPW12 O40 ) (in %) are: C 13.252; H, 0.914; N, 3.101;

C/N, 4.273. And found values (%) are: C, 13.238; H, 0.922; N, 3.104; C/N,4.265.

3.

Results and discussion

3.1.

Characterizations of the adsorbent

The FTIR spectrum of absorbent was depicted in Fig. 1a. Comparing with the FTIR spectrum of bipy, the peak at 1600 cm−1 of bipy shifts to 1580 cm−1 in the FTIR spectrum of absorbent, and the face bending vibration at 750 cm−1 to 759 cm−1 . The

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

195

isostructural with POMs. Characterizations through BET, elemental analysis, TGA, XRD, SEM and FTIR have confirmed the success of the synthesis route and the purity of the NPW.

3.2.

Fig. 2 – Effect of contact time on the removal of RhB Dye: 25 mg/L, adsorbent dose: 0.1 g/100 mL, pH: 4.7 ± 0.1, stirring speed: 400 rpm and temperature of 25 ± 2 ◦ C.

shifts are attributed to the coordination of two nitrogen atoms of bipyridine to Ni, which limits the vibration of the ring of bipy, and increases the energy. In addition, the shoulder peak evolves to a distinctive peak, denoting the formation of the complexes. The bands observed at about 3420 cm−1 could be assigned to the stretching vibration of N H. The trough around 1080 cm−1 represented the P O stretching. The characteristic peaks around 978, 898 and 817 cm−1 were assigned to (W O), (W Ob W) and (W Oc W) of the PW12 O40 3− polyanion, respectively (the Ob and Oc are the bridging oxygen atoms of Keggin structure) (Yan et al., 2014). It is concluded that the material keeps the original Keggin structure. The IR spectra show that this material possesses several surface functional groups. The SEM micrograph of the adsorbent at 20.0 K × magnification was depicted in Fig. S1. A fleeciness surface can be observed. Fig. S2 shows the nitrogen equilibrium adsorption isotherm at 77 K. The specific surface area of the product adsorbent prepared based on BET surface area analyzer was 61.279 m2 /g with a pore volume of 0.24 cm3 /g. The pore size distribution of NPW was estimated by using the Barrett–Joyner–Halenda method. A maximum pore diameter of 1.006 nm was determined. Fig. 1b shows the TGA curve of the compound. From the figure, the NPW was found stable when the temperature was lower than 73 ◦ C. The quality loss of 3.68% at the range of 73–420 ◦ C is attributed to loss of free water molecules. A weight loss of 7.46% from 420 ◦ C to 556 ◦ C was found in accordance with the percentage of pyridine in NPW (7.45%). Thus it can be concluded that the loss of pyridine leads to decreases of the NPW quantities. The characteristic temperature of the adsorbent is around 670 ◦ C and the decomposition rate of NPW is the fastest at this temperature. The adsorbent synthesized in the present experimental condition has a good thermal stability. The XRD pattern of the as-synthesized NPW is almost identical to the simulated one of [Cu(bipy)2 ]2 (HPW12 O40 ) pattern (Fig. 2c) (Li et al., 2007). These results show that the as-synthesized materials and [Cu(bipy)2 ]2 (HPW12 O40 ) are

Effect of contact time

In the design of economical wastewater treatment systems, equilibrium time is one of the most important parameters. In Fig. 2, the variation in percentage RhB removal was plotted as a function of contact time. The RhB solution volume was 100 mL with an initial concentration of 25 mg/L in neutral pH, and the NPW concentration was 0.1 g/100 mL of NPW. From the figure the extent of dye removal by the NPW increases with the contact time. The removal rate of dye by adsorption on NPW is fast at the initial period of contact time, and then becomes slow with the contact time (Fig. S3). This may be due to the strong attractive forces between the dye molecules and the adsorbent. During the adsorption process, fast diffusion onto the external surface is followed by pore diffusions into the intraparticle matrix to reach a rapid equilibrium. We selected the characteristic absorption of the RhB to monitor the adsorptive process. The equilibrium time was found ∼40 min when the maximum dye adsorption capacity was reached. It seems that at higher contact time due to saturation of adsorbent surface (due to the accumulation of the adsorption sites by the dye ions), total adsorbent surface area and further the diffusion pathway decreased significantly. Thus, for all equilibrium adsorption studies, the equilibration period was kept 40 min. Furthermore, we also found that the NPW are effective in adsorbing the methyl orange (MO) (Fig. S4). As shown in Fig. S5, it is clear that the composite has different adsorption ability toward different organic dyes. While 92% of RhB is degraded in 40 min (given by the solution color dispersion), however, only less than 13.47% of MO is within the same period. The above results can be attributed to that POMs where a large number of negative charges encapsulated in the neutral framework may have a stronger force with the positive charges of dyes (Raji and Anirudhan, 1997). Thus, different dye molecules have different adsorption characteristics on the NPW.

3.3.

Effect of adsorbent dosage

Adsorbent dosage is an important parameter because this factor determines the capacity of an adsorbent for a given initial concentration of organic dye solutions. The effect of adsorbent dosage was studied on RhB removal by keeping all other experimental conditions unchanged. With different adsorbent amount in the range of 20–120 mg, the results obtained were shown in Fig. 3(a). The removal ratio of dye increases with the quantities of the adsorbent. This is attributed to the increased adsorbent surface area of mesopore and the availability of more adsorption sites. The maximum removal percentage is 92% when the adsorbent dose is 120 mg. However, as shown in Fig. 3(b), the adsorption capacity decreases with the adsorbent dose. The amount adsorbed per unit mass of the adsorbent doesn’t increase considerably after the dosage quantity of 100 mg. Such a trend is tentatively attributed to the adsorption sites which remains unsaturated during the adsorption process (Raji and Anirudhan, 1997). Subsequent experiments were carried out using 0.1 g/100 mL of NPW.

196

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Fig. 3 – (a) Effect of adsorbent dose on dye removal. Dye concentration: 25 mg/L, pH: 4.7 ± 0.1, time: 40 min, stirrer speed: 400 rpm and temperature of 25 ± 2 ◦ C. (b) Effect of adsorbent dose on dye uptake. Dye concentration: 25 mg/L, pH: 4.7 ± 0.1, time: 40 min, stirrer speed: 400 rpm and temperature of 25 ± 2 ◦ C.

3.4. RhB

Effect of initial dye concentration on adsorption of

The initial concentration provides an important driving force to overcome all mass transfer resistances of the dye between the aqueous and solid phases. The adsorption is strongly influenced by the concentration of the solution, while the adsorptive reactions are directly proportional to the concentration of the solution. To determine proper RhB adsorption, initial concentrations of RhB solutions were changed and time intervals were assessed until no adsorption of RhB on adsorbent took place. Effects of initial dye concentration on its removal efficiency and the actual amount of adsorbed dye were investigated in the concentration range of 10–35 mg/L. The results were shown in Fig. 4. For the 100 mL of RhB solution with initial concentration of 10 mg/L, more than 96% of adsorption was reached, whereas for solution with initial concentration of 25 mg/L, the RhB removal percent was 91%. The results indicate that the actual amount of dye adsorbed per unit mass of the adsorbent increases with its concentration (Fig. 4(a)). This is due to the driving force concentration

gradient increases with the initial dye concentration. However, because of the competition of RhB molecules for the active adsorption sites, the adsorption process will be significantly slowed down following the RhB concentration increment. These phenomena show that the mass gradient between the solution and adsorbent (driving force for the transfer of dye molecules from bulk solution to the particle surface) decrease significantly with the initial RhB concentration.

3.5.

Effect of initial pH

The pH of the dye solution plays an important role in the whole adsorption process, and especially impacts on the adsorption capacity (Huang et al., 2011). The pH value of the aqueous medium serves in the form of adsorptive uptake of the dye color. This may be due to its effect on the surface of the adsorbent and ionization/dissociation of the adsorptive molecules. Correspondingly, the interaction between dye molecules and active centers on the adsorbent is mainly affected by ionization states of the functional groups of both dye molecules and adsorbent surface. The pH of the solution may change the

Fig. 4 – (a) Variation of dye removal for different initial concentrations of RhB. pH: 4.7, 40 min agitation time at speed of 400 rpm in temperature of 25 ± 2 ◦ C, 0.1 g/100 mL of adsorbent. (b) Variation of dye uptake for different initial concentrations of RhB. pH: 4.7, 40 min agitation time at speed of 400 rpm in temperature of 25 ± 2 ◦ C, 0.1 g/100 mL of adsorbent.

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

197

leads to the decreased percentage of adsorption when pH was lowered from 6.0 to 4.0. When solution pH is above 6.0, the carboxyl group gets ionized and the zwitterions form of RhB is formed (Deshpande and Kumar, 2002). The zwitterion form of RhB in water may increase dimerization of the RhB. Rhe dimerized molecules are too big that they cannot enter most of the pore structures of NPW. Thus, the RhB removal rate is reduced. When the pH is above 7.0, excessive OH− ions compete with COO− ones in binding with N+ slowing down the aggregations of RhB (Li et al., 2010). Therefore, an increase in the adsorption of RhB on the NPW can be observed at pH > 7.0. At higher pH, the RhB contains chloride anion. The halogen ion is exchanged to have a displacement reaction with the NaOH in producing NaCl (aq) and RhB-S+ OH (aq). The salt of NaCl has been reported as an origin to decrease the adsorptions of RhB-S+ OH (aq) on the NPW (Hameed and Ahmad, 2009). Fig. 5 – Effect of solution pH on adsorption of RhB. Dye: 25 mg/L; adsorbent dose: 0.1 g/100 mL; 40 min agitation time at speed of 400 rpm in temperature of 25 ± 2 ◦ C. surface charge of the adsorbent, the degree of ionization of the adsorbate molecule, and the extent of dissociation of functional groups on the active sites of the adsorbent (Nandi et al., 2009). The removal percentage of RhB by the adsorbent at different pH values in the pH range of 2–10 was depicted in Fig. 5. The changes of RhB removal percentages were not obvious within 40 min though the pH values were tuned from 2.0 to 10.0. The highest removal percentage for RhB is 97.0% at pH 2.0, and the lowest removal percentage is 58.5% at pH 7.0. The uptake of RhB all pH from 2.0 to 10.0 is up to 60%. Therefore, both acidic and basic solutions are suitable for RhB removal. RhB is an aromatic amino acid with amphoteric characteristics due to the presence of both the amino group ( NHR2 ) and the carboxyl group ( COOH). Thus, the charge state of RhB is dependent on solution pH. At pH values lower than 4, the RhB ions are of cationic and monomeric molecular form (Deshpande and Kumar, 2002). Consequently, the RhB can enter the pores leading to the decreased percentage of adsorption when pH was set from 4.0 to 2.0. The electrostatic repulsion between cationic RhB and positively charged NPW

3.6.

Reusability

The reusability of the NPW adsorbent was investigated by putting 100 mL of dye solution with a concentration of 25 mg/L in 0.5 g of adsorbent at room temperature (298 K) for 25 min. As shown in Fig. 6, the adsorbent is capable of removing RhB dye up to 80.4% over 7 cycles, and 41.9% over 10 times. As the reaction being proceeded, the amount of collected adsorbent recycled is continuously reduced. Meanwhile the incomplete ethanol extraction of the adsorbed dye reduces the dye removal. This property is commercially significant in improving the material values and reducing the adsorption cost.

3.7.

Effect of temperature

The temperature dependence of adsorption reactions offers possibilities to study the enthalpy and entropy changes during the adsorption. The removal of dyes by adsorbent was studied at the temperature range of 293–313 K to determine the adsorption isotherms and thermodynamic parameters (Fig. 7). As shown in Fig. 7, for the dye concentration of 25 mg/L using 0.1 g adsorbent, the effect of temperature doesn’t seem to be very pronounced at the temperature scale of 293–313 K. At

Fig. 6 – Effect of reusability on RhB adsorption at C0 : 25 mg/L at pH 4.7, adsorbent dose: 0.1 g/100 mL, 40 min agitation time at speed of 400 rpm at the temperature of 25 ± 2 ◦ C. (a) The adsorption behaviors versus time of the NPW (from the undermost curve to the topmost one: 1st to 10th are respectively loaded). (b) The removal percent of RhB versus recycle times.

198

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Table 1 – Langmuir and Freundlich adsorption isotherm constants for the adsorption of RhB on adsorbent at different temperatures. Isotherm model

Temperature (K) 293

Langmuir Qm (mg/g) Ka (mL/g) R2 Freundlich n Kf ((mg/g)/(mg/L)n ) R2

227.4343 121.951 0.9722 −2.882 0.001312 0.9088

298

303

308

313

227.0288 144.928 0.9917

223.6917 125.000 0.9873

229.9717 125.000 0.9756

235.3063 138.889 0.9777

−0.258 0.5523 0.9530

−0.318 0.4807 0.9237

−0.291 0.5514 0.9070

−0.228 0.5921 0.8903

the temperature stage of 308–313 K, the decreasing trend of adsorption with temperature is mainly due to the weakening of adsorptive forces between the active sites of adsorbent and the dye ions, as well as between the adjacent molecules of adsorbed phase. This type of adsorption is likely classified reversible. In this stage, adsorption dependence on temperature would be comparatively weaker than desorption (Juang et al., 1997). The results show that adsorption is exothermic initially (293–303 K), and it turns into endothermic process afterwards (303–308 K). In the case of exothermic adsorption, the extent of dye adsorption decreases despite of the system temperature increase. In the case of endothermic adsorption, the extent of dye adsorption increases with the temperature, ruling out the possibility of chemisorptions (Hema and Arivoli, 2007). Rehman et al. (2013) also reported similar endothermicexothermic driven trend during the adsorption of brilliant green dye on red clay.

3.8. Adsorption equilibrium study of RhB adsorption on NPW The adsorption isotherm is a fundamental concept in describing the interactive behavior between solutes and adsorbent. It also gives comprehensive information about the nature of interaction. Two different isotherm models of Langmuir and Freundlich models are applied in this study to analyze the experimental parameters of adsorption equilibrium, and to

deduce important parameters of the surface properties and affinity to the RhB molecules. The linear form of Langmuir isotherm model reads (Langmuir, 1918): Ce 1 Ce = + , qe KL Qm Qm

where KL is the Langmuir adsorption constant (L/mg), and Qm the theoretical maximum adsorption capacity (mg/g). KL is a measure of the affinity between adsorbate and adsorbent, and it is related to the free energy of adsorption (Wang et al., 2006). The linear form of Freundlich isotherm model (Freundlich, 1906) is written as follows (Freundlich, 1906): log qe = log KF + n log Ce ,

(4)

where KF ((mg/g)/(mg/L)n ) and n are isotherm constants indicating the capacity and intensity of the adsorption, respectively. 1/n indicates the heterogeneity factor. All the parameters obtained from the experiments for the adsorption of RhB on the adsorbent at 293 K, 298 K, 303 K, 313 K and 318 K in the binary component system are determined and tabulated in Table 1. The Langmuir and Freundlich isotherms are given along with their correlation coefficients. From the table, values of Qm increase with the temperature. This confirms the process is endothermic. As shown in Table 1, the Langmuir isotherm model has higher correlation coefficients (R2 > 0.99) than the Freundlich model. Therefore, we tentatively conclude that the RhB adsorption scheme on the present adsorbent follows Langmuir adsorption model.

3.9.

Fig. 7 – Effect of temperature on RhB adsorption at C0 : 25 mg/L, pH: 4.7, adsorbent dose: 0.1 g/100 mL, 40 min agitation time at speed of 400 rpm in various temperatures.

(3)

Adsorption kinetic study of RhB on NPW

It is a physical chemistry process for adsorption that involves the mass transfer of a solute (adsorbate) from the fluid phase to the adsorbent surface (Silva et al., 1906). Studies of the adsorption kinetics are desirable because they will provide information about the adsorption mechanism. The pseudofirst-order and pseudo-second-order model were tested for the adsorption of RhB onto NPW. The best-fit model was selected based on the linear regression correlation coefficient (r2 ) values. The adsorption of dye on solid surface may be explained by an initial rapid binding of dye molecules on the adsorbent surface followed by relatively slower intra-particle diffusion. The pseudo-first-order kinetic model was employed to predict adsorption kinetics of dyes (Ghaedi et al., 2011). It is assumed that the rate of changes of solute uptake with time is directly proportional to the difference in saturation

199

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Table 2 – Adsorption kinetic parameters of RhB onto adsorbent at different temperatures. T (K)

qe (exp) mg/g Pseudo-first-order K1 (min−1 ) qe (cal.), mg/g R1 2 Pseudo-second-order K2 (g/mg min) H qe (calc.) (mg/g) R2 2 Intraparticle diffusion kid (mg/g min1/2 ) C R32

293

298

303

308

313

227.4695

227.0288

223.6917

229.9717

227.4243

0.0372 26.882 0.9413

0.0337 29.674 0.9491

0.0400 25.000 0.9373

0.0452 22.124 0.9817

0.0445 22.472 0.9675

0.0039 256.409 256.410 0.9969

0.0041 243.902 243.902 0.9963

0.0041 243.902 243.902 0.9992

0.0038 263.158 263.158 0.9978

0.0038 263.158 263.158 0.9990

13.577 153.22 0.9049

15.871 119.09 0.9775

18.278 103.50 0.9530

21.814 86.92 0.9638

21.387 84.52 0.9804

concentration and the amount of solid uptake with time. A linear form of pseudo-first-order model is described by Lagergren in the form of (Arami et al., 2005)

log(qe − qt ) = log qe −

 k  1 2.303

h = k2 qe2 t.

(5)

Here qt is the adsorbed amount of dye at time t (mg/g), and k1 is the equilibrium rate constant of pseudo-first-order adsorption (min−1 ). The values log (qe − qt ) were calculated from the kinetic data (Fig. 2(b)). The plot log (qe − qt ) versus t is expected to be a straight line. The line slope and intercept allow calculating adsorption rate constant and equilibrium adsorption capacity. Most of the experimental data points are along the straight line. Some of data points deviate from line. This may be due to film boundary layers reduced during the stirring. The calculated values of respective parameters of first order kinetic model are listed in Table 2. The correlation coefficients are lower, indicating the pseudo-first-order model is not sufficient in predicting the kinetics of RhB adsorption onto NPW. The kinetic data were further fitted by using Ho’s pseudosecond-order kinetics model for adsorption process with chemisorptions (Senturk et al., 2010). The linear form of sorption kinetics can be presented by pseudo-second-order model as: t t 1 + = qt qe (k2 qe2 )

The second-order rate constants were used to calculate the initial sorption rate (h) given by

In this equation, h is the initial sorption rate (mg/g min) (Özacar and S¸engil, 2003). The rate constant for intra-particle diffusion (kid ) is given by qt = kid t1/2 + C

(8)

where qt is the amount adsorbed (mg/g) at time t. The qt vs. t1/2 curve for different dyes was plotted in Fig. 8. As can be seen from the figure, there is a linear part, and then an increasing ˘ trend, followed by a plateau (Dogan et al., 2004). The linear one can be owned to the intraparticle diffusion, while the bulk diffusion may result in initial curved part and the plateau at the equilibrium. This implies that the adsorption of the dye is a combined process between dye molecules from solution to the pores of the particles through the particle solution interface, and the adsorption on the available surface of adsorbent. The particle rate constants calculated from Fig. 8 are 13.577, 15.871, 18.278, 21.814 and 21.387 mg/g min1/2 at the temperature of

(6)

where k2 is the rate constant of second-order adsorption (g/mg min). The plot of t/qt versus t gives a straight line with the slope of 1/qe and intercept 1/(k2 qe2 ). The values of qe and k2 were determined from the slope and intercept values, respectively. They were presented in Table 2. The correla  tion coefficients for the second-order kinetics model R22 were 0.9969, 0.9963, 0.9992, 0.9978 and 0.9990 at the temperature of 293 K, 298 K, 303 K, 313 K and 318 K, respectively. As shown in Table 2, the calculated values are very close to those values obtained from experiments. Therefore, it may be concluded that the adsorption scheme of RhB on our adsorbent is categorized within the pseudo-second-order kinetic model. The process is consequentially attributed chemisorptions controlled.

(7)

Fig. 8 – Intraparticle diffusion plots of dyes onto NPW (T = 298 K, adsorbent = 0.1 g/100 mL, C0 = 25 mg/L).

200

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Table 3 – Comparison of proposed method with some previously reported RhB removal. . Material

Adsorption capacity (mg/g)

[Ni(bipy)2]2(HPW12O40) ␣-Bi2 Mo3 O12 Neem sawdust Orange peel Banana peel Sugarcane dust

22.75 2.73 2.355 14.3 20.6 3.240

293 K, 298 K, 303 K, 313 K and 318 K, respectively. It is found that kid increases with the temperature.

3.10.

Various adsorbent for RhB adsorption

The absorption capabilities of the present adsorbent are further compared to previously reported results (Annadurai et al., 2002; Khattri and Singh, 1999, 2000; Martínez-de La Cruz and Obregón Alfaro, 2009). As shown in Table 3, the quantity of RhB absorbed the present adsorbent (per unit) is much superior or comparable to other adsorbents: Our adsorbent achieves the adsorption of 22.75 mg dyes by per gram of NPW at the temperature of 298 K, while adsorptions of other adsorbents are limited to 20 mg/g. This fundamental study will be helpful for further application in designing a batch adsorption system for the treatment of water coming from dying industries. In addition, it is worth mention that the satisfactory tolerance against the variations in pH condition of the adsorption performance of NPW extract RhB from water should not be neglected.

3.11.

The adsorption of Cr(VI)

The adsorption of toxic metal is important for the application of an adsorbent. Removal of Cr(VI) ions from aqueous solution using the NPW was investigated preliminarily. The effect of time on the adsorption of Cr(VI) was depicted in Fig. 9. Experiments were carried out with the same time intervals. As shown in Fig. 9, the absorbance decreases with the contact time. A fast adsorption process happens within the first 30 min, and the equilibrium reaches the maximum value after 5 h. The removal percentage of Cr(VI) tops at 69%.

Contact time 40 min 20 min 30 min 24 h 24 h 30 min

4.

Source Present work Martínez-de La Cruz and Obregón Alfaro (2009) Khattri and Singh (1999) Annadurai et al. (2002) Annadurai et al. (2002) Khattri and Singh (2000)

Conclusions

In summary, highly efficient NPW adsorbent for the adsorption of rhodamine B dye from aqueous solution has been demonstrated in terms of the adsorption isotherm, kinetics, and reuse of the sorbent. The adsorption capacity has been investigated in different conditions with various operating parameters such as adsorbent dose, contact time, pH, initial dye concentration and temperature. The simple and high-yield preparation, high adsorption capacity, excellent reusability, good tolerance against variations in realistic adsorption environments and handy operations make NPW a promising adsorbent for the adsorption and removal of RhB and Cr(VI) from aqueous solution.

Acknowledgment Authors acknowledge with thanks the financial support of Provincial Natural Science Foundation of Hunan, China (13JJ6041; 2015JJ2138), the National Natural Science Foundation of China (21343008), the Open Project Program of State Key Laboratory of Structural Chemistry, China (no. 20150018), Oulu University Strategic Grant, and Magnus Ehrnrooth Foundation. Authors thank M. Kokkonen for his help of language improvements.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cherd.2015.05.030.

References

Fig. 9 – UV–vis plots of Cr(VI) with adsorbent (0.5 g) at agitation time.

Ali, M., Sreekrishnan, T.R., 2001. Aquatic toxicity from pulp and paper mill effluents: a review. Adv. Environ. Res. 5, 175–196. ˘ Alkan, M., Demirbas, Ö., C¸elikcapa, S., Dogan, M., 2004. Sorption of acid red 57 from aqueous solution onto sepiolite. J. Hazard. Mater. 116, 135–145. Al-Rashed, S.M., Al-Gaid, A.A., 2012. Kinetic and thermodynamic studies on the adsorption behavior of Rhodamine B dye on Duolite C-20 resin. J. Saudi Chem. Soc. 16, 209–215. Annadurai, G., Juang, R.S., Lee, D.J., 2002. Use of cellulose-based wastes for adsorption of dyes from aqueous solutions. J. Hazard. Mater. 92, 263–274. Arami, M., Limaee, N.Y., Mahmoodi, N.M., Tabrizi, N.S., 2005. Removal of dyes from colored textile wastewater by orange peel adsorbent: equilibrium and kinetic studies. J. Colloid Interface Sci. 288, 371–376. Banat, I.M., Nigam, P., Singh, D., Marchant, R., 1996. Microbial, decolorization of textile-dye containing effluents: a review. Bioresour. Technol. 58, 217–222. Banerjee, R., Phan, A., Wang, B., Knobler, C., Furukawa, H., O’Keeffe, M., Yaghi, O.M., 2008. High-throughput synthesis of zeolitic imidazolate frameworks and application to CO2 capture. Science 319, 939–943.

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Camacho, L.M., Torres, A., Saha, D., Deng, S.G., 2010. Adsorption equilibrium and kinetics of fluoride on sol–gel-derived activated alumina adsorbents. J. Colloid Interface Sci. 349, 307–313. Costanzo, J.A., Ober, C.A., Black, R., 2010. Evaluation of polymer matrices for an adsorptive approach to plasma detoxification. Biomaterials 31, 2857–2865. Deshpande, A.V., Kumar, U., 2002. Effect of method of preparation on photophysical properties of Rh-B impregnated sol–gel hosts. J. Non-Cryst. Solids 306, 149–159. ˘ Dogan, M., Alkan, M., Türkyılmaz, A., Özdemir, Y., 2004. Kinetics and mechanism of removal of methylene blue by adsorption onto perlite. J. Hazard. Mater. 109, 141–148. Eddaoudi, M., Moler, D.B., Li, H., 2001. Modular chemistry: secondary building units as a basis for the design of highly porous and robust metal–organic carboxylate frameworks. Acc. Chem. Res. 34, 319–330. Freundlich, H., 1906. Über die adsorption in lösungen. Engelmann Leipzig 57, 385–470. Ghaedi, M., Hassanzadeh, A., Nasiri, K.S., 2011. Multiwalled carbon nanotubes as adsorbents for the kinetic and equilibrium study of the removal of Alizarin red S and Morin. J. Chem. Eng. Data 56, 2511–2520. Gong, R., Li, M., Yang, C., Sun, Y., Chen, J., 2005. Removal of cationic dyes from aqueous solution by adsorption on peanut hull. J. Hazard. Mater. 121, 247–250. Hamada, K., Kaneko, T., Chen, M.Q., 2007. Size-selective material adsorption property of polymeric nanoparticles with projection coronas. Chem. Mater. 19, 1044–1052. Hema, M., Arivoli, S., 2007. Comparative study on the adsorption kinetics and thermodynamics of dyes onto acid activated low cost carbon. Int. J. Phys. Sci. 2, 10–17. Huang, C.Y., Song, M., Gu, Z.Y., Wang, H.F., Yan, X.P., 2011. Probing the adsorption characteristic of metal–organic framework MIL-101 for volatile organic compounds by quartz crystal microbalance. Environ. Sci. Technol. 45, 4490–4496. Hameed, B.H., Ahmad, A.A., 2009. Batch adsorption of methylene blue from aqueous solution by garlic peel, an agricultural waste biomass. J. Hazard. Mater. 164, 870–875. Huo, S.H., Yan, X.P., 2012. Metal–organic framework MIL-100 (Fe) for the adsorption of malachite green from aqueous solution. J. Mater. Chem. 22, 7449–7455. James, S.L., 2003. Metal–organic frameworks. Chem. Soc. Rev. 32, 276–288. Juang, R.S., Wu, F.C., Tseng, R.L., 1997. The ability of activated clay for the adsorption of dyes from aqueous solutions. Environ. Technol. 18, 525–531. Khattri, S.D., Singh, M.K., 1999. Color removal from dye wastewater using sugar cane dust as an adsorbent. Adsorpt. Sci. Technol. 17, 26982. Khattri, S.D., Singh, M.K., 2000. Colour removal from synthetic dye wastewater using a bioadsorbent. Water Air Soil Pollut. 120, 283–294. Krawiec, P., Kramer, M., Sabo, M., Kunschke, R., Fröde, H., Kaskel, S., 2006. Improved hydrogen storage in the metal–organic framework Cu3 (BTC)2 . Adv. Eng. Mater. 8, 293–296. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403. Li, L., Liu, S., Zhu, T., 2010. Application of activated carbon derived from scrap tires for adsorption of Rhodamine B. J. Environ. Sci. 22, 1273–1280. Li, T.H., Lü, J., Gao, S.Y., Cao, R., 2007. Two novel grid networks based on Keggin-type polyoxometalate clusters assembled through weak Cu· · ·O interactions. Inorg. Chem. Commun. 10, 551–554. Magueres, P.L., Ouahab, L., Golhen, S., 1994. Diamagnetic and paramagnetic keggin polyoxometalate salts containing 1-D and 3-D decamethylferrocenium networks: preparation, crystal structures and magnetic properties of [Fe(C5 Me5 )2 ]4 (POM)(solv)n (POM = [SiMo12 O40 ]4− , [SiW12 O40 ]4− , [PMo12 O40 ]4− , [HFeW12 O40 ]4− ; solv = H2 O, C3 H7 ON, CH3 CN). Inorg. Chem. 33, 5180–5187.

201

Mahmoodi, N.M., Arami, M., 2009. Degradation and toxicity reduction of textile wastewater using immobilized titania nanophotocatalysis. J. Photochem. Photobiol., B: Biol. 94, 20–24. Martínez-de La Cruz, A., Obregón Alfaro, S., 2009. Synthesis and characterization of nanoparticles of ␣-Bi2 Mo3 O12 prepared by co-precipitation method: Langmuir adsorption parameters and photocatalytic properties with rhodamine B. Solid State Sci. 11, 829–835. Nandi, B.K., Goswami, A., Purkait, M.K., 2009. Adsorption characteristics of brilliant green dye on kaolin. J. Hazard. Mater. 161, 387–395. Noguchi, R., Hara, A., Sugie, A., Nomiya, K., 2006. Synthesis of novel gold(I) complexes derived by AgCl-elimination between [AuCl(PPh3 )] and silver(I) heterocyclic carboxylates, and their antimicrobial activities, molecular structure of [Au(R,S-Hpyrrld)(PPh3 )] (H2 pyrrld = 2-pyrrolidone-5-carboxylic acid). Inorg. Chem. Commun. 9, 355–359. Nomiya, K., Yoshida, T., Sakai, Y., Nanba, A., Tsuruta, S., 2010. Intercluster compound between a tetrakis{triphenylphosphinegold(I)} oxonium cation and a keggin polyoxometalate (POM): formation during the course of carboxylate elimination of a monomeric triphenylphosphinegold(I) carboxylate in the presence of POMs. Inorg. Chem. 49, 8247–8254. Ockwig, N.W., Delgado-Friedrichs, O., O’Keeffe, M., Yaghi, O.M., 2005. Reticular chemistry: occurrence and taxonomy of nets and grammar for the design of frameworks. Acc. Chem. Res. 38, 176–182. Özacar, M., S¸engil, I.A., 2003. Enhancing phosphate removal from wastewater by using polyelectrolytes and clay injection. J. Hazard. Mater. 100, 131–146. Pokhrel, D., Viraraghavan, T., 2004. Treatment of pulp and paper mill wastewater—a review. Sci. Total Environ. 33, 37–58. Raji, C., Anirudhan, T.S., 1997. Kinetics of Pb(II) adsorption by polyacrylamide grafted sawdust. Ind. J. Chem. Technol. 4, 157–162. Rehman, M.S.U., Munir, M., Ashfaq, M., Rashid, N., Nazar, M.F., Danish, M., Han, J.I., 2013. Adsorption of Brilliant Green dye from aqueous solution onto red clay. Chem. Eng. J. 228, 54–62. Robinson, T., McMullan, G., Marchant, R., Nigam, P., 2001. Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresour. Technol. 77, 247–255. Romero-Anaya, A.J., Lillo-Ródenas, M.A., Linares-Solano, A., 2010. Spherical activated carbons for low concentration toluene adsorption. Carbon 48, 2625–2633. Senturk, H.B., Ozdes, D., Duran, C., 2010. Biosorption of rhodamine 6G from aqueous solutions onto almond shell (Prunus dulcis) as a low cost biosorbent. Desalination 252, 81–87. Silva, J.P., Sousa, S., Rodrigues, J., Antunes, H., Porter, J.J., Gonc¸alves, I., Dias, S.F., 1906. Adsorption of acid orange 7 dye in aqueous solutions by spent brewery grains. Sep. Purif. Technol. 40, 309–315. Song, Y.F., Abbas, H., Ritchie, C., McMillian, N., 2007. From polyoxometalate building blocks to polymers and materials: the silver connection. J. Mater. Chem. 17, 903–908. Thompson, G., Swain, J., Kay, M., Forster, C.F., 2001. The treatment of pulp and paper mill effluent: a review. Bioresour. Technol. 77, 275–286. Tseng, R.L., Wu, K.T., Wu, F.C., Juang, R.S., 2010. Kinetic studies on the adsorption of phenol, 4-chlorophenol and 2,4-dichlorophenol from water using activated carbons. J. Environ. Manage. 91, 2208–22014. Tzvetkova, P., Vassileva, P., Nickolov, R., 2010. Modified silica gel with 5-amino-1,3,4-thiadiazole-2-thiol for heavy metal ions removal. J. Porous Mater. 17, 459–463. Uchida, N.S., 2006. Structures and sorption properties of ionic crystals of polyoxometalates with macrocation. Chem. Lett. 35, 688–693.

202

chemical engineering research and design 1 0 0 ( 2 0 1 5 ) 192–202

Wang, S., Boyjoo, Y., Choueib, A., 2005. A comparative study of dye removal using fly ash treated by different methods. Chemosphere 60, 1401–1407. Wang, S., Li, H., Xu, L., 2006. Application of zeolite MCM-22 for basic dye removal from wastewater. J. Colloid Interface Sci. 295, 71–78. Wei, X., Xu, G., Ren, Z.H., Xu, C.X., Shen, G., Han, G.R., 2008. PVA-assisted hydrothermal synthesis of SrTiO3 nanoparticles with enhanced photocatalytic activity for degradation of RhB. J. Am. Ceram. Soc. 91, 3795–3799. Wu, H., Zhou, W., Yildirim, T., 2007. Hydrogen storage in a prototypical zeolitic imidazolate framework-8. J. Am. Chem. Soc. 129, 5314–5315. Xie, T.H., Sun, X.Y., Lin, J., 2008. Enhanced photocatalytic degradation of RhB driven by visible light-induced MMCT of Ti(IV) O Fe(II) formed in Fe-doped SrTiO3 . J. Phys. Chem. C 112, 9753–9759.

Xu, M.C., Zhou, Y., Huang, J.H., 2008. Adsorption behaviors of three polymeric adsorbents with amide groups for phenol in aqueous solution. J. Colloid Interface Sci. 327, 9–14. Yan, A.X., Yao, S., Li, Y.G., Zhang, Z.M., Lu, Y., Chen, W.L., Wang, E.B., 2014. Incorporating polyoxometalates into a porous MOF greatly improves its selective adsorption of cationic dyes. Chem. Eur. J. 20, 6927–69233. Zhai, Q.G., Wu, X.Y., Chen, S.M., Zhao, Z.G., Lu, C.Z., 2007. Construction of Ag/1,2,4-triazole/polyoxometalates hybrid family varying from diverse supramolecular assemblies to 3-D rod-packing framework. Inorg. Chem. 46, 5046–5058. Zhuang, J.D., Dai, W.X., Tian, Q.F., Li, Z.H., Xie, L.Y., Wang, J.X., Liu, P., 2010. Photocatalytic degradation of RhB over TiO2 bilayer films: effect of defects and their location. Langmuir 26, 9686–9694.