Adsorption Removal of Environmental Hormones of Dimethyl Phthalate Using Novel Magnetic Adsorbent

Magnetic polyvinyl alcohol adsorbent M-PVAL was employed to remove and concentrate dimethyl phthalate DMP. The M-PVAL was prepared after sequential syntheses of magnetic Fe3O4 (M) and polyvinyl acetate (M-PVAC). The saturated magnetizations of M, M-PVAC, and M-PVAL are 57.2, 26.0, and 43.2 emu g−1 with superparamagnetism, respectively. The average size of M-PVAL by number is 0.75 μm in micro size. Adsorption experiments include three cases: (1) adjustment of initial pH (pH0) of solution to 5, (2) no adjustment of pH0 with value in 6.04–6.64, and (3) adjusted pH0 = 7. The corresponding saturated amounts of adsorption of unimolecular layer of Langmuir isotherm are 4.01, 5.21, and 4.22 mg g−1, respectively. Values of heterogeneity factor of Freundlich isotherm are 2.59, 2.19, and 2.59 which are greater than 1, revealing the favorable adsorption of DMP/M-PVAL system. Values of adsorption activation energy per mole of Dubinin-Radushkevich isotherm are, respectively, of low values of 7.04, 6.48, and 7.19 kJ mol−1, indicating the natural occurring of the adsorption process studied. The tiny size of adsorbent makes the adsorption take place easily while its superparamagnetism is beneficial for the separation and recovery of micro adsorbent from liquid by applying magnetic field after completion of adsorption.

The natural attenuation mechanisms which include chemical breakdown, biodegradation, and photolysis can decompose the PAEs [25]. The halftimes vary from days to years, depending on the environment especially the temperature and the properties of PAEs such as structure and length of functional groups. However, it takes a long time. Thus, many techniques have been used to treat the PAEs-containing water and waste, including physical treatments of adsorption [32,33] and membrane processes such as ultrafiltration, nanofiltration, and reverse osmosis [34][35][36][37][38], biodegradation by microorganisms [25], and chemical processes of basecatalyzed hydrolysis, ultraviolet (UV) radiation, ozonation, combined UV radiation/ozonation, catalytic ozonation, and combined UV/catalytic ozonation [25,26,39,40]. Among the above said methods, adsorption can effectively remove the PAEs from the solution with PAEs concentrated on the solid adsorbents. The adsorption has been also applied to other emerging contaminants [41,42]. The exhausted adsorbents are regenerated for reuse. The treatment of waste regeneration solution containing high-concentration PAEs is then followed. It is usually carried out by the destruction processes such as biological and chemical treatments noted above. Concerning the improvement of adsorption rate, proper surface area, and easy recovery of adsorbent, adsorption using novel magnetic micro-nano size magnetic adsorbents has been developed and employed for the removal of inorganic pollutants [43][44][45][46]. This study applied the micro size magnetic polyvinyl alcohol (M-PVAL) for the adsorption removal of organic DMP. (M) was prepared by chemical coprecipitation method. Ferrous chloride and ferric chloride were firstly dissolved using distilled water at 85 ∘ C in nitrogen environment. It followed the addition of aqueous ammonia to form the magnetic suspension of precipitated Fe 3 O 4 . Oleic acid acting as a dispersion agent was immediately and slowly fed into the suspension liquid until the appearance of clear supernatant liquid. This thus yielded the magnetite M. It was then synthesized to form magnetic polyvinyl acetate (M-PVAC) by suspension polymerization. For performing this, PVAL was dissolved via distilled water at 60 ∘ C in nitrogen environment to provide background solution for polymerization. After addition of magnetite M, VAC, and divinyl benzene, the suspension polymerization proceeded at 70 ∘ C for about 6 h and then cooled to 25 ∘ C, forming M-PVAC. Washing with deionized water, its surface was modified by alcoholysis to produce a polymer adsorbent of magnetic polyvinyl alcohol M-PVAL. In alcoholysis, M-PVAC was suspended in methanol solution for 6 h to obtain M-PVAL. Detailed description of the above procedures can be found in Tseng et al. [45].

Isothermal
Adsorption. DMP solutions with various concentrations were prepared. 0.1 g M-PVAL adsorbent was added into each 50 mL DMP solution filled in 125 mL flask. The initial pH value (pH 0 ) of solution was adjusted to desired value using HCl or NaOH. The adsorptions with various initial DMP concentrations were conducted in a constant-bath shaker. A blank solution without M-PVAL adsorbent was tested along each batch of adsorptions.
After the adsorption of 8 h, a magnet was attached beneath the flask to adhere the M-PVAL adsorbent. The pH value of solution was measured. The magnetic separated solution was withdrawn using syringe and filtered with 0.22 m filter. 2 mL filtrate was collected for the measurement of concentration.  The success of synthesis of M-PVAL with functional group of -OH can be further justified by the peak of adsorption (inverse of transmittance) at 3400 cm −1 in FTIR diagram presented in Figure 2. The peak at 1266 cm −1 also reveals the binding of C-O. The same characteristics of nonmagnetic polyvinyl alcohol were also reported by Kaczmarek et al. [47] and Majumdar and Adhikari [48].  respectively. Most of the M-PVAL particles have size less than 1 m. The major physical characteristics of M-PVAL are summarized in Table 1. The particle porosity is 0.03, indicating the micro size M-PVAL is essentially nonporous. This ensures that the pore diffusion is negligible in adsorption process.

Isothermal Adsorption of DMP.
Langmuir, Freundlich, and D-R isotherms were tested to examine the adsorptions of DMP on M-PVAL for three cases, namely, Cases 1 and 3 with initial pH 0 adjusted at 5 and 7 and Case 2 without adjustment of pH 0 with pH value in 6.04-6.64, respectively. At equilibrium, the pH values for the three cases with pH 0 = 5, 6.04-6.64, and 7 increase to about 7.36-7.87, 6.9-8.05, and 7.42-8.39, respectively. This is consistent with the adsorption of slightly acidic DMP by base M-PVAL, which exhibits pH of 9.1 and zeta potential of −35.6 mv when dispersed in deionized (DI) water as illustrated in Figure 4. The values of 4 The Scientific World Journal  isotherm parameters of corresponding isotherms are listed in Table 2.
The Langmuir isotherm deduces the values of unimolecular layer L of 4.01, 5.21, and 4.22 g kg −1 for Cases 1, 2, and 3, respectively, indicating minor effect of adjustment of pH 0 on the saturation capacity L with Case 2 without adjustment of pH 0 yielding higher value. The adsorption equilibrium constants L exhibit difference with Cases 1 and 2 with adjustment of pH 0 giving higher values. The cause might be due to the effects of adjustment addition of HCl or NaOH on the adsorption. The balance of decrease of L while increase of L by the adjustment of pH 0 results in the close adsorption behaviors for the three cases as depicted in Figures 5, 6, and 7. The -squares of model fittings as shown in Figure 8 are greater than 0.94, indicating good agreement. The results thus suggest performing the adsorption without adjustment of pH 0 .
The heterogeneity factors F of Freundlich isotherm obtained for the three cases are 2.59, 2.19, and 2.59, respectively. All these values are greater than 1, revealing that the adsorption of the noted DMP/M-PVAL systems is favorable. The F values are about 0.6-0.95 (mg g −1 )(g m −3 ) −1/ F . The fittings of Freundlich isotherm as illustrated in Figure 9 are fairly satisfactory with -square higher than 0.81, which, however, is not as good as those of Langmuir isotherm. The good fitting of Langmuir isotherm for the adsorbent M-PVAL may be further justified by noting that the M-PVAL is tiny with number average particle size of 0.75 m which exhibits fast adsorption rate with low diffusion resistance thus in favor of the formation of thin monolayer.
Applying D-R isotherm for the three cases gives the saturation adsorption capacity D of 3. byÖzcan et al. for the adsorption of ion-exchange form [49].
The low values of D thus support that the adsorption process of DMP/M-PVAL in this study proceeds naturally. The above results indicate that the equilibrium of DMP/ M-PVAL exhibits saturation value. Further, among the three isotherms examined, the Langmuir isotherm shows the best agreement and thus is more appropriate to describe the adsorption equilibrium of DMP/M-PVAL system.

Conclusions
Some major conclusions may be drawn from the adsorption removal of DMP using superparamagnetic micro size adsorbent of M-PVAL examined in this study as follows:  separated from the treated liquid for the regeneration by applying externally magnetic field. (3) Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherms were tested to describe the equilibrium of DMP/M-PVAL for three cases: (1) adjusting initial pH (pH 0 ) at 5, (2) no adjustment of pH with pH 0 = 6.04-6.64, and (3)   Di-n-butyl phthalate DCHP: Di-cyclohexyl phthalate DEHP: Di-(2-ethyl hexyl) phthalate DEP: Di-ethyl phthalate DHP: Di-hexyl phthalate DIDP: Di-iso-decyl phthalate DINP: Di-iso-nonyl phthalate DMP: Dimethyl phthalate DNOP: Di-n-octyl phthalate : P Polyvinyl chloride pH 0 : Initial pH value : Adsorbate concentration in solid phase (mg g −1 or g kg −1 or mmol g −1 ) : Adsorbate concentration in solid phase at equilibrium (mg g −1 or g kg −1 or mmol g −1 ) L : U n i m o l e c u l a r l a y e r o f L a n g m u i r isotherm (mg g −1 or g kg −1 ) D : D-R isotherm constant denoting saturation adsorption capacity (mg g −1 or mmol g −1 ) : Universal gas constant (8.314 J mol −1 K −1 ) 2 L , 2 F , and 2 D : Correlation coefficients by means of fitting the experimental data to Langmuir, Freundlich, and D-R isotherms, respectively The Scientific World Journal 7 SEM: Scanning electron microscope SQUID: Superconducting quantum interference device : Absolute temperature (K) UV: Ultraviolet.