Removal of Methylene Blue and Neutral Red from Aqueous Solutions by Surfactant-Modified Magnetic Nanoparticles as Highly Efficient Adsorbent Mohammad Bagher Gholivand, a Yadollah Yamini, b Manijeh Dayeni, a and Shahram Seidi c a Faculty of Chemistry, Razi University, Kermanshah, Iran; mbgholivand@gmail.com (for correspondence) b Department of Chemistry, Tarbiat Modares University, Tehran, Iran c Department of Analytical Chemistry, Faculty of Chemistry, K.N. Toosi University of Technology, Tehran, Iran Published online 22 June 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12174 Sodium dodecyl sulfate-coated magnetic nanoparticles were utilized as the inexpensive and effective adsorbents to remove neutral red and methylene blue (two cationic dyes) from aqueous solutions. Factors affecting adsorption, includ- ing the amounts of adsorbent and surfactant, pH, and con- tact time, were investigated by an orthogonal central composite design. The kinetic processes of the dyes adsorption were described by applying the pseudo-first-order and the pseudo-second-order models. It was found that the pseudo- second-order kinetic equation described the data of dye adsorption with a good correlation (R 2 > 0.999) which indi- cated a chemisorptions mechanism. The extent of dye removal enhanced with increasing initial dye concentration. The adsorption of dyes at concentrations of 30–70 mg/L reached to equilibrium within 35 min. The adsorption data were represented by Langmuir isotherm model. In addition, the maximum monolayer capacities, (q max ), of methylene blue and neutral red onto the modified nanoparticles were found to be as high as 294.12 and 416.67 (mg/g), respec- tively. These values are higher than the adsorption capacities of some other adsorbents that have recently been published in the literature. In addition, the required contact time to achieve equilibrium was reduced by the modified nanopar- ticles. VC 2015 American Institute of Chemical Engineers Environ Prog, 34: 1683–1693, 2015 Keywords: magnetic nanoparticles, methylene blue,neutral Red, sodium dodecyl sulfate, removal INTRODUCTION Dyes are widely used in various industries, including dye- stuffs, paper, textile, rubber, plastic, cosmetic, pharmaceuti- cal, and food for coloring their products. As a result, a considerable amount of water contamination is generated by them. Most of the dyes are toxic and have mutagenic and carcinogenic effects influencing the health of human, plants, and aquatic beings [1,2]. In addition, synthetic dyes form very toxic compounds by reaction with metal ions and lead to many water-borne diseases [3,4]. The Ecological and Toxi- cological Association of dyes (ETAD) and organic pigment manufacturer reported that 98% of dyes have an LC50 (lethal concentration for 50% of the test animals) value of >1 mg/L and among them 59% have an LC50 value of >100 mg/L [5]. Most of the dyes have good stability and low degradability owing to their chemical structures. Therefore, they resist against many chemicals and considered nonoxidizable under many conventional physical and biological treatments [6]. Thus, their removal is one of the indispensable processes in wastewater treatment. Conventional methods for dye remov- ing from wastewaters include flocculation, electrokinetic, coagulation, ion exchange, membrane filtration, oxidation, irradiation, and ozonation [7–10]. Most of these techniques are expensive, time consuming, and cannot effectively be used for the treatment of a wide range of dye-containing wastewaters. Among these methods, adsorption is an avail- able method which has superior to the others in terms of ini- tial cost, simplicity of design, and ease of operation, without producing harmful substances [11–13]. Consequently, various potential adsorbents have been implemented for the removal of specific organics from water. Recently, the researchers are interested in finding more suitable, efficient, inexpensive, and easily accessible types of adsorbents. Magnetic iron oxide nanoparticles (MNPs) have extensively been studied as a suitable absorbents with large surface area and small diffu- sion resistance for the separation and removal of chemical species such as pollutants, metals, dyes, and gases [14–17]. In addition, MNPs make separation easier and faster by being recovered with an external magnet without additional centrif- ugation or filtration process [18]. Surface modification of magnetic nanoparticles is an important key for different applications and can be accomplished by physical/or chemi- cal adsorption of different organic compounds such as sur- factants. Ionic surfactants can be adsorbed on the surface of a support with opposite surface charge as monolayer (hemi- micelles) or bilayer (admicelles) to form versatile sorbents with new interaction abilities. The hydrocarbon chains of surfactants provide hydrophobic or chain–chain interactions for hydrophobic analytes, whereas the polar groups absorb ionic analytes via electrostatic interaction or hydrogen bond- ing. Because of these two types of interactions, this method is highly suitable for amphiphilic anlytes by the formation of mixed analyte–sorbent aggregates [19,20]. Methylene blue (MB) and neutral red (NR) are the two types of cationic dyes that are largely applied for dying of cotton, wood, and silk [21]. Their effluents are aesthetic VC 2015 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.34, No.6) DOI 10.1002/ep November 2015 1683