Efficient removal of some anionic dyes from aqueous solution using a polymer-coated magnetic nano-adsorbent

The presence of organic contaminants in water causes some serious problems to aquatic life and human health disorders even in trace amounts (Chen & Wu 2014). Among various organic contaminants, discharge of synthetic dyes into the hydrosphere possess a significant source of pollution due to their recalcitrant nature. This gives an undesirable color to water bodies which will reduce sunlight penetration and disturbs photochemical and biological cycles of aquatic life (Wong et al. 2004). Synthetic dyes are widely used in many fields of advanced technology, e.g., in various kinds of the textile, paper, leather tanning, food processing, plastics, cosmetics, rubber, printing, and dye manufacturing industries. The release of synthetic dyes to the environment poses challenges to environmental scientists. These concerns have led to new and strict regulations concerning colored wastewater discharge as well as developing more efficient treatment technologies.

Various methods, such as adsorption, advanced oxidation processes, biodegradation, coagulation, and the membrane process, have been suggested to handle dye removal from water. All these processes have some advantages or disadvantages over the other methods (Khataee & Kasiri 2010; Chen & Wu 2014). A balanced approach is, therefore, needed to look into the worthiness on choosing an appropriate method which can be used to remove the dye in solution. The adsorption method is the most applied in the removal of organic dyes and pigments from wastewaters since it can produce high-quality water, and it can be employed as a process that is economically feasible (Madrakian et al. 2011). Many textile industries use commercial activated carbon for the treatment of dye waste. Although activated carbon is commonly used as an adsorbent for color removal, its main disadvantage is its high production and treatment costs (Afkhami et al. 2007; Madrakian et al. 2011). Thus, many researchers throughout the world have focused their efforts on optimizing adsorption and developing novel alternative adsorbents with higher adsorption capacities and lower costs. In this regard, much attention has recently been paid to nanotechnology.

Nanometer-sized materials are widely used for the effective adsorption of different chemical species from water samples (Madrakian et al. 2013a; Kyzas & Matis 2015). The magnetic nanoparticle as an efficient adsorbent with a large specific surface area and small diffusion resistance has been recognized (Ngomsik et al. 2005). The magnetic separation provides a suitable route for online separation, where particles with affinity to target species are mixed with the heterogeneous solution. Upon mixing with the solution, the particles tag the target species. External magnetic fields are then applied to separate the tagged particles from the solution. Iron oxide nanoparticles (i.e., magnetite, maghemite, etc.) are attractive examples of magnetic nanoparticles. The synthesis of magnetite nanoparticles has been intensively developed not only for its high fundamental scientific interest but also for many technological applications in biology (Xie et al. 2004), medical applications (Ahmadi et al. 2015), bioseparation (Bucak et al. 2003); and separation and preconcentration of various anions and cations (Afkhami & Norooz-Asl 2009; Madrakian et al. 2012), due to their novel structural, electronic, magnetic, and catalytic properties. Recently, employing magnetite nanoparticles with modified surfaces has attracted the high attention of researchers for removal of cationic and anionic dyes from water (Ambashta & Sillanpää 2010; Madrakian et al. 2013c).

Herein, a novel magnetic adsorbent has been developed for dye removal and wastewater treatment purposes. In this regard, magnetite nanospheres were synthesized using the solvothermal method and further surface modification was performed using a synthetic polymer. The adsorbent was successfully utilized for removal of some anionic dyes (i.e., reactive yellow 145, reactive blue 19, and reactive red 195) from aqueous samples.

Reactive yellow 145 (RY145), reactive blue 19 (RB19), and reactive red 195 (RR195) anionic dyes were purchased from Sigma-Aldrich Company (St Louis, MO, USA). Table 1 shows some chemical information of the investigated dyes. All of the other chemicals used were of analytical reagent grade and were purchased from Merck Company (Darmstadt, Germany). Double distilled water (DDW) was used throughout the work. The investigated dyes' stock solutions were prepared in DDW from their sodium salts and the working standard solutions of different dyes' concentrations were prepared daily by diluting the stock solution with DDW. Britton–Robinson (B-R) universal buffer was used for pH adjustment of the working solutions.

Table 1

Chemical information of the investigated anionic dyes

Name .	Molecular formula .	Molecular weight (g mol−1) .	λmax (nm) .	Structure .
RY145	C28H20ClN9Na4O16S5	1,026.25	421
RB19	C22H16N2Na2O11S3	626.55	602
RR195	C31H19ClN7Na5O19S6	1,136.32	542

Name .	Molecular formula .	Molecular weight (g mol−1) .	λmax (nm) .	Structure .
RY145	C28H20ClN9Na4O16S5	1,026.25	421
RB19	C22H16N2Na2O11S3	626.55	602
RR195	C31H19ClN7Na5O19S6	1,136.32	542

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The size, morphology, and structure of the synthesized nanospheres were characterized by transmission electronic microscopy (TEM, Philips-CMC-300 KV) and scanning electron microscope (SEM, MIRA FEG-SEM, and TESCAN). The crystal structure of the synthesized nanospheres was determined by an X-ray diffractometer (XRD, 38066 Riva, d/G. via M. Misone, 11/D (TN) Italy) at ambient temperature. The magnetic properties of the synthesized nanospheres were measured using a vibrating sample magnetometer (VSM, 4 in. Daghigh Meghnatis Kashan Co., Kashan, Iran). The mid-infrared spectra of the synthesized nanospheres in the region of 4,000–400 cm⁻¹ were recorded by a Fourier transform infrared spectrometer (FT-IR, Perkin-Elmer model Spectrum GX) using KBr pellets. A single beam ultraviolet (UV)-mini-WPA spectrophotometer was used for the determination of dye concentration in the solutions. A Metrohm model 713 pH meter was used for pH measurements. A 40 kHz universal ultrasonic cleaner water bath (RoHS, Korea) was used.

MNSs were synthesized by the solvothermal reduction method with minor modifications (Deng et al. 2005). Typically, FeCl₃.6H₂O (1.35 g) was dissolved in ethylene glycol (40.0 mL) to form a clear solution, followed by the addition of sodium acetate (3.6 g) and polyethylene glycol (1.0 g). The mixture was ultrasonicated vigorously for 30 min and then refluxed at 180°C for 8 h, and then allowed to cool down to room temperature. The black products were washed several times with ethanol and DDW water and then dried at 60°C for 6 h (Figure 1(b)).

In order to prepare the AMNSs, the amidoamine monomer was polymerized in the presence of MNSs (0.5 g, as the magnetic core), ammonium persulfate (0.1 g, as the initiator), and ethylene glycol dimethacrylate (50 μL, as the cross-linking monomer) in 30 mL DDW at 85°C for 12 h. Then, the product was separated using a magnet and washed with methanol and DDW to remove unreacted reagents.

Adsorption is a physicochemical process that involves the mass transfer of a solute from the liquid phase to the adsorbent surface. The adsorption capacities of adsorbents were calculated from the difference between the initial and the final concentration at any intermediate time. The sorption dynamics of the adsorption by AMNSs were tested with the pseudo-first order and the pseudo-second order kinetic models (Madrakian et al. 2013a).

To study the adsorption kinetics of the investigated dyes on AMNSs, 50.0 mg L⁻¹ initial concentration of corresponding dye solutions which had been stirred in the presence of 0.02 g adsorbents at pH = 5.0 and for different time ranges (0–50 min) were used at room temperature. The solution was separated by magnetic decantation to remove adsorbent and analyzed spectrophotometrically.

The capacity of the adsorbent is an important factor that determines how much sorbent is required to remove quantitatively a specific amount of the dye from solution. For measuring the adsorption capacity of AMNSs, the absorbent was added into dye solutions at various concentrations (under optimum condition), and the suspensions were stirred at room temperature until the equilibrium was reached, followed by magnetic removal of the absorbent. An adsorption isotherm describes the fraction of the sorbate molecules that are partitioned between the liquid and the solid phase at equilibrium. Adsorption of the dyes by AMNS adsorbent was modeled using Freundlich (Freundlich & Heller 1939) and Langmuir (Langmuir 1916) adsorption isotherm models. The remaining dye in the supernatants was measured spectrophotometrically at the wavelength of the maximum absorbance of each dye, and the results were used to plot the isothermal adsorption curves.

To evaluate the possibility of regeneration and the reuse of AMNS adsorbent, desorption experiments were performed. Dye desorption from the AMNSs was conducted by washing the dyes loaded on AMNSs using 2.0 mL of pure methanol, sodium hydroxide aqueous solution (0.1 M) and acetonitrile. For this purpose, 2.0 mL of eluent was added to 0.02 g of dye loaded AMNSs in a beaker. Then, the AMNSs were collected magnetically from the solution. The concentration of dyes in the desorbed solution was measured spectrophotometrically. Results showed that 0.1 M sodium hydroxide aqueous solution was an effective eluent for desorption of the dyes. It is notable that the equilibrium of desorption was achieved within about 10 min, which was fast, similar to the adsorption equilibrium. This was due to the absence of internal diffusion resistance. After elution of the adsorbed dyes, the adsorbent was washed with DDW and vacuum dried at 50°C overnight and reused for the dye removal.

The synthesized AMNSs were fully characterized using XRD, SEM, TEM, VSM, and FT-IR measurements. Then, batch experiments were used for evaluation and optimization affecting various parameters such as pH, contact time, and nanosphere dosage.

One of the important factors affecting the removal of the dyes from aqueous solutions is the pH of the solution. The dependence of dyes' molecules sorption on pH is related to both the dyes' chemistry in the solution and the ionization state of the functional groups of the sorbent which affects the availability of binding sites (Madrakian et al. 2011). All of the investigated dyes are anionic dyes. In the case of the adsorbent, the responsible parameter is the point of zero charge (pH_PZC). The point of zero charge is a characteristic of metal oxides (hydroxides) and is of fundamental importance in surface science. It is a concept relating to the phenomenon of adsorption and describes the condition when the electrical charge density on a surface is zero. The surface charge of AMNSs with primary amine groups (belong to the functional monomer) and hydroxyl groups (belong to MNSs) is largely dependent on the pH of the solution. The pH_PZC is caused by the amphoteric behavior of hydroxyl and surface amino groups, and the interaction between surface sites and the electrolyte species. When brought into contact with aqueous solutions, hydroxyl groups of surface sites can undergo protonation or deprotonation, depending on the solution pH, to form charged surface species.

The dependence of the adsorption of the dyes on the modified nanospheres' amount was studied at room temperature and pH 5.0 by varying the adsorbent amount from 0.01 to 0.05 g in contact with 50.0 mL solution of 50.0 mg L⁻¹ of the dyes with agitation time of 45 min. The results showed that increasing the amount of AMNSs increases the removal efficiencies of the dyes due to the availability of higher adsorption sites. The adsorption reached a maximum with 0.02 g of the adsorbent, and maximum percentage removal was about 98%.

The removal of the dyes by adsorption on AMNSs was found to be rapid at the initial period (in the first ≈5th min) and then to become slow and stagnate with the increase in the contact time (≈5th to ≈15th min), and nearly reached a plateau after approximately the 20th min of the experiment. Different kinetic parameters of the dyes' adsorption onto AMNSs are shown in Table 2. All the experimental data showed better compliance with the pseudo-second order kinetic model regarding higher correlation coefficient value (R² > 0.98) and lower root mean square (RMS) value. Moreover, the q values (q_{e, cal}) calculated from the pseudo-second order model were more consistent with the experimental q values (q_{e, exp}) than with those calculated from the pseudo-first order model. Hence, it could be found that the pseudo-second order kinetic model was more valid to describe the adsorption behavior of the dyes onto AMNSs.

The higher correlation coefficient obtained for the Langmuir model, for all of the investigated dyes, and lower RMS values indicates that the experimental data are better fitted to this model, and adsorption of the investigated anionic dyes on AMNS adsorbent is more compatible with Langmuir assumptions, i.e., adsorption takes place at specific homogeneous sites within the adsorbent. The Langmuir model is based on the physical hypothesis that the maximum adsorption capacity consists of a monolayer adsorption, that there are no interactions between adsorbed molecules, and that the adsorption energy is distributed homogeneously over the entire coverage surface. This sorption model serves to estimate the maximum uptake values where they cannot be reached in the experiments.

According to the results (Table 3), the maximum amounts of the dyes that can be adsorbed by AMNSs were found to be 221.4, 201.6, and 135.3 mg g⁻¹ at pH 5.0 in the case of RR195, RY145, and RB19, respectively. As the results show, the capacity factor for RR195 and RY145 is higher than that for RB19. The difference in capacity may be due to the difference in the structure of dyes and the number of the anionic functional groups.

In Table 4, we compared the ability of our inexpensive adsorbent with other adsorbents in the removal of the dyes from aqueous solutions. The results show that AMNSs are a better adsorbent compared to some of the adsorbents.

In this work, a magnetic adsorbent was developed for dye removal purposes. The prepared magnetic adsorbent is well dispersed in the water medium and be easily separated magnetically from the medium after the dyes' adsorption process. The rapid adsorption rate is mainly attributed to the polymer structure and functional groups on the adsorbent providing a large surface area and good affinity for the facile and fast adsorption of dye molecules. The Langmuir isotherm model well fitted the adsorption data. As the calculated capacity factors of AMNSs show, they are a very good adsorbent for removing the investigated anionic dyes. The results of this study suggest that the developed adsorbent can be considered as an alternative adsorbent for wastewater treatments and controlling environmental pollution.

The authors gratefully acknowledge the financial support provided by Centre for Analytical Chemistry-KAVA Research Institute (CAC-KRI, Project No. Y34CS024/2013). The authors also acknowledge the Research Laboratory of Spectrometry & Micro/Nano Extraction of Iran University of Science and Technology (RLSMNE-IUST) for their support.