Removal of Cationic Tolonium Chloride Dye Using Fe 3 O 4 Nanoparticles Modified with Sodium Dodecyl Sulfate

M. H. Abedi,a M. Ahmadmoazzam,b,c and N. Jaafarzadehb,d,* aDepartment of Chemical Engineering, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran bDepartment of Environmental Health Engineering, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran cStudent Research Committee, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, IR Iran dEnvironmental Technologies Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran


Introduction
Dye removal is one of the most important parameters in wastewater treatment. 1Dyes must be removed not only because of their appearance, but also dyes can combine with chlorine and form halogenated compounds that are harmful and carcinogenic. 2,3Many industries, including textile, dyeing, paper making, cosmetics, pharmaceuticals, etc., use these compounds extensively. 4Discharge of waste containing these compounds into the environment is harmful to human health and damages the ecosystem, because they are usually non-biodegradable and toxic. 5Therefore, appropriate techniques for treating these types of wastewater are required.Due to their low biodegradability features, biological treatment methods are not usually able to effectively remove the dye compounds from the wastewater. 6Different methods, including various physicochemical methods, such as coagulation-flocculation, 7 ultra and nanofiltration, 8,9 reverse osmosis, 10 electrocoagulation, [11][12][13] advanced oxidation processes, [14][15][16] chemical oxidation, 17 and adsorption on dif-ferent materials 18 have been studied by many researchers.Most of the proposed methods for the removal of dyes have low efficiency, according to the time required for removal.Adsorption methods by nanomaterials are excellent techniques for dye removal from wastewater due to their efficiency, high surface area, high reactivity properties, and catalytic potential, in addition to the simplicity of design. 19,20Adsorption by metal nanoparticles is an environmentally friendly technology, which has been considered in recent years as a suitable method to remove organic contaminants and heavy metal ions from water and wastewater. 21In recent years, the use of Fe 3 O 4 magnetic nanoparticles (Fe 3 O 4 NPs) has been considered in water and wastewater treatment due to its many advantages. 22,23In addition to high adsorption ability, magnetic nanoparticles have the advantage of easy separation by a strong magnetic field, which is important in terms of their recycling and reuse. 24By applying a magnetic field to a magnetic fluid, the magnetic particles stick together and form a solid.The main problem with this method is that the fine particles dispersed in the liquid tend to agglomerate and form larger particles, and therefore reduce their magnetic features. 25In order to overcome this problem and increase their application, the Fe 3 O 4 surface is usually modified with different agents. 26,27Modification of the surface of nanoparticles with agents, such as polymers or surface-active compounds (surfactants), can prevent their agglomeration.Surfactants attach to the nanoparticles and create a repulsive force between them thus preventing agglomeration.9][30] The Fe 3 O 4 NPs modified with SDS have been reported in 2011 for removal of safranin O dye from aqueous solutions. 30Faraji et al. reported the SDS-coated Fe 3 O 4 NPs as a solid-phase extraction adsorbent for the extraction of Hg(II) from water samples. 29Furthermore, high-efficiency extraction of fluoxetine using the Fe 3 O 4 NPs modified with SDS from aquatic and urine samples have been reported in 2011. 28n this study, we aimed to modify the Fe 3 O 4 NPs surface with sodium dodecyl sulfate as a facile modification way to improve the adsorption efficiency of the magnetic nanoparticles, which exhibit a high efficiency for tolonium chloride removal from aqueous solutions.The effect of different parameters, such as pH, dye concentration, temperature, and ions intervention on process performance was investigated.The experimental data were also analyzed to study the adsorption kinetics and isotherms.

Analytical methods
The concentration of the tolonium chloride dye was measured using a UV-Vis spectrophotometer (Varian Cary 100, USA) with quartz cells, measuring the absorbance at λ max = 632 nm.The pH was measured with a portable pH-meter (Mettler Toledo-Seven Easy, Switzerland).A magnet with strength of 1.5 Tesla was used to separate the nanoparticles from solution.
Preparation of magnetic Fe 3 O 4 -SDS Sodium dodecyl sulfate 0.5 % solution (%w/v) was prepared by adding 0.5 g of sodium dodecyl sulfate salt to a 100-mL volumetric balloon, and dissolved in distilled water.In order to cover the iron oxide nanoparticles with the SDS, one mL of 0.5 % SDS solution was added to a beaker containing 0.6 grams of iron oxide nanoparticles, and stirred with a glass rod for two minutes. 28,30The resulting magnetic Fe 3 O 4 -SDS was then separated from the solution with a magnet, and washed several times with deionized water to remove the residual surfactant.

Batch adsorption experiments
The adsorption experiments of tolonium chloride on Fe 3 O 4 -SDS were performed through a batch process.The dye solution, containing 20 mg L -1 , was prepared by dissolving 0.02 g tolonium chloride in 1000 mL deionized water.The solution of 0.1 M phosphoric acid and 0.1 M NaOH were used for the preparation of phosphate buffer solution with pH = 6, and used to adjust the initial pH of the solution.Firstly, 10 mL of 20 mg L -1 tolonium chloride solution and 3 mL of buffer solution were added to a 100-mL volumetric balloon and filled with distilled water.The concentration of the dye in prepared and control solutions was measured with a UV-Vis spectrophotometer at λ max = 632 nm.Thereafter, 0.6 g of Fe 3 O 4 -SDS was added to an Erlenmeyer flask containing 100 mL of the prepared dye solution and stirred for two minutes.After stirring, the flask was placed in the magnetic field for adsorbent separation.The adsorption capacities (mg g -1 ) and removal efficiency (%) were finally calculated by equations ( 1) and ( 2), respectively: o ( ) where q e (mg g -1 ) is the amount of dye adsorbed on the adsorbent (adsorption capacities), C o and C t are concentrations of the dye (mg L -1 ) at the initial time and at time t, respectively, V (mL) is the volume of dye solution, C e is the equilibrium concentration of dyes (mg L -1 ), and m (g) is the weight of Fe 3 O 4 -SDS.

Effect of pH variation
The effect of pH variation on the adsorption of TC onto Fe 3 O 4 -SDS was evaluated in a pH range of 3-12.This pH range was selected because when Fe 3 O 4 NPs are dissolved at a pH less than 3, it causes darkening of the solution. 28Also, at a pH higher than 12, the magnetic nanoparticles shift to colloidal particles and do not respond to magnetic fields.As shown in Fig. 3, the adsorption of the dye increased with pH increasing from 3 to 6, and then decreased as the pH increased from 6 to 12.The maximum TC removal of 98.1 % was obtained at pH 6.According to the zeta potential, the Fe 3 O 4 particles have isoelectric point at pH 6.5. 31Therefore, the positively charged surface of Fe 3 O 4 NPs in the solution created suitable conditions for higher adsorption of anionic surfactants; hence, the cationic dye removal increased. 28Also, removal reduction at lower pH (< 6) can be due to the competition of H + ions with the cationic dye, which prevented the complete adsorption of the dye cations onto the negative clouds surrounding the nanoparticles due to repulsive force. 32In addition, at alkali pH, more available hydroxyl ions in solution will encompass the cationic dye molecules and decrease the positively charged at the dye surface.Therefore, the electrostatic attraction decreased between the ionic adsorbent head groups (-SO 3 -) and cationic dye molecules surrounded by hydroxyl ions. 30,33

Effect of magnetic nanoparticles doses and SDS concentration
The effect of adsorbent doses on the adsorption efficiency of the dye was studied with the addition of 0.25 and 0.65 g of the Fe 3 O 4 NPs to 100 mL of the dye solution.As shown in Fig. 4a, with the use of more nanoparticles, because of the presence of more adsorption sites, the dye removal efficiency also increased from 66.3 to 98.2 % for nanoparticles mass of 0.25 and 0.6 g, respectively, and then it was almost fixed.Therefore, 0.6 g of the Fe 3 O 4 NPs was used as an optimum amount for the next experiments.
In addition, the effect of the SDS concentration for modification of the surface of Fe 3 O 4 NPs was investigated in a concentration range of 0.1 to 0.62 % (w/v).The surfactant can help the adsorption of the cationic dye on the surface of the Fe 3 O 4 by increasing the negative charge on the adsorbent surface. 25,29According to the obtained results, dye molecules were efficiently removed with a higher amount of SDS added to the solution, and the dye removal increased to 98.7 % with the maximum amount of 0.62 % (w/v) (Fig. 4b).This phenomenon was due to the fact that the repulsion force had decreased between the cationic dye and positive surface charge of the Fe 3 O 4 NPs when more adsorbent was coated with the SDS. 30As the removal ef- Removal (%) pH ficiency was constant with the SDS addition, more than 0.52 % (w/v), this value was selected for the next experiments.

Temperature effect
According to the literature, temperature is a main factor in the adsorption process. 5,32Therefore, the temperature effect on TC adsorption efficiency onto Fe 3 O 4 -SDS was studied in the temperature range of 15-45 °C at 100 mL TC solution (2 mg L -1 ), and 0.6 g Fe 3 O 4 -SDS (Fig. 5).Removal of TC dye revealed that the process was relatively dependent on temperature.The data trend showed that the dye removal increased with temperature from 15 to 30 °C, and then a reduction in removal efficiency was observed at the temperature above 30 °C, which verified that TC dye adsorption was an exothermic process.The highest removal efficiency was 93.3 % at a temperature of 30 °C.At lower temperature, a lower kinetic energy of the dye molecules could cause an insufficient collision between TC and adsorbent surface, and decrease the removal efficiency. 34Also, lower adsorption at temperatures above 30 °C might be due to the higher kinetic energy of the TC molecules than the adsorption potential between TC molecules and adsorbent surface, which weakens the absorbing force. 35,36In addition, the desorption process of the adsorbed molecules may start with temperature rising. 32

Effect of stirring times and TC dye initial concentration
Experiments were carried out at different stirring times (1 to 7 min) to study the contact time effect on the removal efficiency of TC dye onto Fe 3 O 4 -SDS.As shown in Fig. 6, at the beginning of the experiment, adsorption efficiency rapidly increased with time and showed the maximum removal of 98.1 % after 3 min reaction, and then slightly decreased.This was because, at the beginning of the experiment, all the adsorbent sites were empty, and therefore, the dye molecule adsorption on the adsorbent surface increased with increasing time.Over time, with the saturation of adsorption sites, adsorption reduced and eventually reached adsorption and desorption equilibrium. 6,37  The effect of initial dye concentration on the removal efficiency of TC dye at different concentrations (0.6 to 4 mg L -1 with fixed 0.6 g of Fe 3 O 4 -SDS) is indicated in Fig. 7.The results showed that the removal percentage increased as the concentration increased from 0.6 to 2 mg L -1 , and then decreased with higher concentration.This phenomenon can easily be explained in terms of the fact that each adsorbent has a specific adsorption site that is saturated with increasing concentrations above a certain level.Studies from other researchers showed similar results. 34,38,39sorbent reusability study Fig. 8 shows the ability of Fe 3 O 4 -SDS of adsorbing a certain amount of TC during 5 consecutive cycles.The removal rate eventually decreased to 92.5 % after repeating the process three times, and by repeating the process for the fourth cycle the removal efficiency decreased finally to 72 %.At the last cycle, the adsorption was measured after washing the adsorbent with methanol.The result showed that the removal rate increased to 86.5 % after desorption step with methanol.

Effect of NaCl concentration and ions intervention
The effect of NaCl concentration (as an electrolyte) on the removal efficiency of the TC onto Fe 3 O 4 -SDS was studied (Fig. 9).In addition, to investigate the effect of ionic strength, varying amounts of Cl -, CO 3 2-, Mg 2+ , Ca 2+ , and K + were added to the solution.Interfering effects of three dyes (Rose Bengal, Amaranth, and Thionine) on the removal efficiency of the TC was also studied.The dye removal rate of 150 mL dye solution (2 mg L -1 ) was 98.6 %, and this amount was considered as a reference for comparison with other results.If other result variations are less than ±5 % of the reference amount, the results are acceptable and the expected ion concentration is not disturbing.Table 1 shows the maximum concentration for every case in which the TC removal varied less than ±5 % of the reference amount.The results showed that the interfering agents have little effect on the TC removal process even at high concentrations.Finally, to evaluate the effectiveness of Fe 3 O 4 -SDS nanoparticles in real terms, three petrochemical effluents were used as the reaction matrix and the removal efficiency was measured under the previously mentioned optimal conditions.As previous tests have shown, the disturbing ions in wastewater have little impact on process performance (Table 2).

Kinetics and adsorption isotherms
In adsorption experiments, a kinetic analysis is very important to understand the details and mechanisms of the adsorption functions. 40The adsorption  kinetic analysis predicts the adsorption rate in addition to the capacity of adsorbents for the removal of contaminants, which are important for designing and modeling these processes.The contact time required for complete adsorption is achieved using adsorption rate.Analyzing the kinetic data for adsorption of the TC onto Fe 3 O 4 -SDS was determined using the pseudo-first-order and pseudo-second-order models for expression of the adsorption rate. 41he kinetic studies were done in a 250-mL glass beaker filled with 100 mL TC solution (2 mg L -1 ) added to 0.6 g Fe 3 O 4 -SDS at 30 °C, and 100 rpm stirring speed.The kinetic model rate constants were calculated using the line equations obtained from the kinetic models plotting and pseudo-first-order and pseudo-second-order model equations (equation 3 and 4, respectively) as follows: 1 t e log( ) log( ) 2.303 where q e and q t are adsorption capacity at equilibrium and time t in mg g -1 , and k 1 and k 2 are pseudo-firstorder and pseudo-second-order rate constants in min -1 and g mg -1 min -1 , respectively.Fig. 10 shows the fitting of kinetic data to the pseudo-secondorder kinetic model.The obtained value for the rate constant (k 2 ) and adsorption capacity at equilibrium (q e ) for pseudo-second-order model were 2.81 g mg -1 min -1 and 6.66 mg g -1 , respectively.Also, k 1 and q e were calculated as 0.152 min -1 and 0.21 mg g -1 , respectively, for pseudo-first-order model.The higher value of the rate constant (k 2 ) and equilibrium adsorption capacity (q e ) for pseudo-second-order model showed the best fitting of the pseudo-second-order model for adsorption of the TC onto Fe 3 O 4 -SDS, and confirmed the higher adsorption potential in pseudo-second-order model due to the chemisorption mechanism via electrostatic attraction. 30n this study, the equilibrium isotherm equations of Langmuir, Freundlich, and Temkin were used to describe the adsorption behavior and predict the adsorption pattern in a batch study.150 mL of the TC solution in the concentration range of 2-20 mg L -1 was added to a glass jar containing 0.6 g of adsorbent.The amount of the TC adsorbed onto the Fe 3 O 4 -SDS was obtained after reaching the equilibrium time.In order to select the most appropriate isotherm equations, they were drawn in linear form and isotherm constants were then obtained.Linear forms of the adsorption isotherm models are as follows: -Langmuir equation 42 e e e max max where q e (mg g -1 ) is the equilibrium concentration of the adsorbed dye onto the adsorbent, C e (mg L -1 ) the equilibrium concentration of the dye in solution, q max (mg g -1 ) the maximum monolayer capacity of the adsorbent, b (L mg -1 ) the Langmuir constant and related to the strength of the adsorption energy.In the Langmuir isotherm model, the q max and b values can be obtained by linear regression of e e C q versus C e in which max 1 q and max 1 q b ⋅ are slope and intercept of plotted linear graph, respectively (Fig. 11a).
-Freundlich equation 43 e f e 1 log log( ) log where k f (L g -1 ) and n (dimensionless) are the Freundlich constant and show the adsorption capacity of the adsorbent and the heterogeneity factor, respectively.Then n and k f values can be calculated respectively from the slope and intercept of the line by drawing logq e versus logC e (Fig. 11b).
-Temkin equation 44 e e log q A B C = + ⋅ (7)   where A (adsorption potential in the initial state (mg g -1 )), and B (stability of adsorption (L mg -1 )) are the Temkin isotherm constants, and are respectively the intercept and slope of the plotted linear regression of q e versus logC e (Fig. 11c).dlich (R 2 = 0.9645) and Temkin (R 2 = 0.9922) models.The Langmuir constant values were calculated as 111.11mg g -1 and 0.53 L mg -1 , respectively.The higher values for the Langmuir constant (b) showed that more surface of adsorbent was covered by adsorbed molecules due to the stronger tendency of the molecules to the surface. 45,46The q max value is the maximum capacity of the adsorbent and its high value confirmed an appropriate adsorption capacity for TC adsorption by the Fe 3 O 4 -SDS. 4

Conclusions
Iron oxide nanoparticles modified with sodium dodecyl sulfate surfactant (Fe 3 O 4 -SDS) were successfully utilized as an efficient adsorbent to re-move the tolonium chloride dye from wastewater samples.Maximum removal of the dye was achieved at 98 % in optimum conditions (pH = 6; 0.6 g Fe 3 O 4 -SDS; 30 °C temperature, and 3 min stirring time).In this study, the cations interference was very low and no significant impact of real wastewater cations was observed on removal efficiency.Additionally, NaCl (as an electrolyte), showed no significant impact on the dye removal up to 0.8 molar concentration, which increases the industrial application of the process.The kinetic study showed the best fitting of the pseudo-second-order model for adsorption of the TC onto Fe 3 O 4 -SDS and confirmed chemisorption mechanism.The Langmuir model showed better fitting of the data for TC adsorption by the Fe 3 O 4 -SDS compared with the Freundlich and Temkin models.In addition, Fe 3 O 4 -SDS nanoparticles are easily synthesizable, magnetically recoverable and regenerable, which increases their practical applications.

AcKNoWlEDGMENT
The present paper was prepared from MSc thesis in Chemical Engineering: Environmental Engineering.This research received no specific grant from funding agencies in the public, commercial, or non-profit sectors.

F
i g . 1 -TEM image of the purchased Fe 3 O 4 F i g . 2 -Molecular structure of tolonium chloride (a) and sodium dodecyl sulfate (b)

F i g . 3 -
Effect of pH variation on removal efficiency of TC onto Fe 3 O 4 -SDS.Test conditions: 100 ml Tc solution (2 mg l -1 ); 0.6 g of Fe 3 O 4 covered with one ml of 0.5 % SDS solution and 2 min stirring time (100 rpm).

F i g . 4 -
Effect of magnetic nanoparticles amount and SDS concentration on removal efficiency of TC onto Fe 3 O 4 -SDS.Test conditions: pH = 6; 100 ml Tc solution (2 mg l -1 ), and 2 min stirring time (100 rpm).

F i g . 9 -Ta b l e 2 -
Effect of NaCl concentration on removal efficiency of TC onto Fe 3 O 4 -SDS.Test conditions: pH = 6; 100 ml Tc solution (2 mg L -1 ), and 3 min stirring time (100 rpm).Ta b l e 1 -Effect of ions and dyes intervention on TC removalType of studied ions and dyesMaximum concentration without interference (mg L -1 ) Fe 3 O 4 -SDS performance for TC removal from pet-

Fig. 11 shows
the calculated coefficients for the adsorption isotherms of the TC onto the Fe 3 O 4 -SDS.Comparison of the obtained values for constants and regression coefficients in accordance with these three models offered the Langmuir model (R 2 = 0.9975) as better fitting of the data for TC adsorption by the Fe 3 O 4 -SDS compared with the Freun-

F i g . 1 0
-Plotting the pseudo-second-order kinetic model in optimum R e f e r e n c e s F i g .11 -Graph plotted for calculating q max and b in the langmuir model (a), k f and n in Freundlich model (b), and A and B constants in Temkin model (c)