The purpose of modification of magnetic iron oxide nanoparticles is an eco-friendly, emerging and economical method for removing deltamethrin in the aqueous solution and wastewater effluents when compared with other adsorbent methods. Modified magnetic iron oxide nanoparticles were synthesized by co-precipitation and then coupled with 3-hydroxytyraminium chloride. The nano-sorbent was characterized by thermogravimetric analysis, elemental analysis, transmission electron microscopy, scanning electron microscope, Fourier transform infrared spectroscopy, zero point charge and surface area determination. Batch studies were conducted and adsorption equilibrium, kinetic and thermodynamic non-linear models were carried out. The resulting equilibrium data were tested with Langmuir and Freundlich non-linear isotherm models, and the results showed that the Langmuir isotherm fitted the data well. Kinetic studies were done with different initial deltamethrin concentrations, adsorbent dosage and temperature, and the data were assimilated with pseudo-first order, pseudo-second order and intra-particle diffusion kinetic equations, and it was found that the studied nano-sorbent processes followed the pseudo-second order kinetic equation. Thermodynamic analysis was also carried out to estimate the changes in free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0). The thermodynamic parameters revealed that the adsorption of deltamethrin into the nano-sorbent was spontaneous, feasible and showed an endothermic process.

INTRODUCTION

The presence of pesticides in water is a significant problem caused by agriculture (Bhanu et al. 2011). The outgoing pesticides from fields can get mixed with ground and surface water, which obviously can flow into drinking water (Bhanu et al. 2011). Pesticides ordinarily have complex aromatic molecular organisms that make them more difficult and stable to photodegrade and biodegrade (Esteve-Turrillas et al. 2007). Pyrethroid pesticides have been broadly applied in animal health, agriculture, garden and home pest control since their commercial development (Wolansky & Harrill 2008). They have powerful neurotoxic activity against insects and low toxicity for mammals (Wolansky & Harrill 2008). Accordingly, they are used as replacements for more toxic organophosphates and organochlorines (Perry et al. 2007). At present, pyrethroid pesticides account for 25% of the global insecticide market (Perry et al. 2007). Deltamethrin is one of the most broadly and frequently applied pyrethroids against a wide spectrum of insect pests of important crops, and in the control of household insect pests such as cockroaches, mosquitoes, termites, flies, and fleas (Bhanu et al. 2011). Deltamethrin is widely used in agriculture because of its persistence and residual activity (Bhanu et al. 2011). Deltamethrin has extreme adsorption capacity on particles. It is immobile in the environment and soluble in water. However, its use is still dangerous to the ecosystem (Cui et al. 2009). All these agents together make deltamethrin capable of being harmful to the ecosystem and human health (Cui et al. 2009). Therefore, it is essential to develop methods to adsorb and eliminate deltamethrin residues from water and wastewater (Bhanu et al. 2011).

Different physicochemical treatments that included ion exchange (Humbert et al. 2008), precipitation (Jia et al. 2006), coagulation (Sarkar et al. 2007), chlorination (Acero et al. 2008), ozonisation (Maldonado et al. 2006) and advanced oxidation (Sanches et al. 2010) processes, with heterogeneous and homogeneous catalysts, were applied with the intention of pesticide adsorption. But these processes were deemed to be expensive and ineffective. Additionally, most of them create significantly large amounts of sludge, which results in disposal problems (Jia et al. 2006). Adsorption is widely applied for the removal of pollutants from water and wastewater (Travlou et al. 2013). It is an alternative technology in water and wastewater treatment that is based on the suitability of various kinds of nanoparticles to prevent and concentrate hazardous pesticides from aqueous media (Zarandi et al. 2016). Recently, nano-sorption has emerged as an effective, eco-friendly, and economical process for the removal of pesticides (Paşka et al. 2014). Among nanoparticles, iron oxide magnetic nanoparticles are of particular interest, hence modified magnetic iron oxide nanoparticles (MMIONPs) are desirable compared to others. What is more, MMIONPs need less care and maintenance, and they are fairly inexpensive. Moreover, MMIONPs can be easily regenerated and reused. The suggested novel method has the advantages of precision accuracy, and high recovery.

The effective operation of the adsorption processes needs equilibrium adsorption data (Sarkar et al. 2007). The equilibrium isotherm plays a significant role in prediction of the model for analysis of adsorption systems (Zhang et al. 2015). Meanwhile, the mechanisms and behaviors of deltamethrin adsorption are completely studied by non-linear equilibrium isotherm models, adsorption kinetics and thermodynamic parameters. The purpose of the present study is to examine the nano-sorption of deltamethrin, and the effectiveness of deltamethrin adsorption was studied with modified and naked nano-sorbents and shown with non-linear equilibrium isotherm, kinetics and thermodynamic models with comparisons between the significance of modified and naked nano-sorbents.

MATERIALS AND METHODS

Reagents

Methanol, 3-mercaptopropyl trimethoxysilane (MTPMS), acetone, ethanol, allylglycidylether (AGE), ammonia, 2,2′-azoisobutyronitrile (AIBN), 3-hydroxytyraminium chloride, toluene, FeCl2.4H2O, and FeCl3.6H2O were the products of Merck (Darmstadt, Germany). 2-dimethyl acrylamide and the technical grade of deltamethrin (98%) were supplied by Aldrich (Steinheim, Germany). The properties of the deltamethrin included the C22H19Br2NO3 formula and a molecular weight of 505.21 g/mol. Deltamethrin was stored at +4 °C, due to its low solubility, and the stock solution (500 mg/L) was prepared by dissolving a suitable amount of deltamethrin with methanol. All the working solutions with the desired concentrations were made by diluting the stock solution with distilled water, and the pH was suited to the optimum value (7) for further use. 5 mg of MMIONPs was added and the combination was shaken for 60 min. The nano-sorbents were removed by magnetic field and the supernatant was filtered. The concentration of deltamethrin in the solution was measured at 250 nm using the UV/Vis spectrophotometer. The UV–visible spectrum was recorded using the Cary 50 UV/Vis1601 spectrophotometer (Shimadzu, Japan).

Synthesis of MMIONPs

The synthesis of MMIONPs at previous work is undertaken and the methods for synthesis of the nano-sorbent are briefly explained. The synthesis of iron oxide magnetic nanoparticles in a flask of distilled water included dissolved and , and the preparation was done by using chemical co-precipitation. An ammonia solution was added to the solution and maintained for 120 min at a temperature of 353 K coupled with stirring under a nitrogen atmosphere. Then the precipitate was washed with ethanol and water, and the resultant precipitate was decanted by an external magnet and dried in an oven at 313 K for 8 h. For surface modification by 3-mercaptopropyl trimethoxysilane, the obtained nanoparticles, anhydrous toluene and MTPMS were refluxed at 383 K for 2 days. The resultant precipitate was decanted and washed with anhydrous toluene and dried in an oven. The polymer grafting by AGE and 2-dimethyl acrylamide, using the mixture of AGE, 2-dimethyl acrylamide, AIBN, ethanol and the modified magnetic nanoparticles were poured into a two-necked flask and refluxed at 343 K for 420 min in a water bath under nitrogen atmosphere (Ghafari et al. in press). The precipitate was washed with distilled water and ethanol. Coupling of 3-hydroxytyraminium chloride was dissolved in a buffer solution (pH 5), and was placed in an incubator shaker at 313 K for 2 days. The precipitate was washed with buffer solution and distilled water, followed by drying at room temperature. Figure 1 shows the mechanisms of two different kinds of interactions between MMIONPs and deltamethrin. Hydrogen banding and Л–Л stacking interactions make further adsorption between the modified nano-sorbents and deltamethrin, while the interactions justify the modification of the nano-sorbents.
Figure 1

A schematic presentation of MMIONPs and deltamethrin interaction.

Figure 1

A schematic presentation of MMIONPs and deltamethrin interaction.

Equilibrium experiments

Nano-sorbent equilibrium experiments were performed by applying the required nano-sorbent by varying the initial deltamethrin concentrations from 5 mg/L to 100 mg/L, and agitating the solutions for 60 min at 200 rpm at 293, 303 and 313 K. The amount of deltamethrin in MMIONPs at equilibrium qe (mg/g) was calculated by (Sid Kalal et al. 2013): 
formula
1
where Co and Ce (mg/L) are the liquid-phase initial and equilibrium concentrations of the deltamethrin respectively. V (L) is the volume of the solution, and W (g) is the mass of applied MMIONPs.

Kinetic experiments

The kinetics of deltamethrin nano-sorption in the nano-sorbent were accomplished by applying different dosages of 3, 5 and 7 mg of nano-sorbent, and changing the initial deltamethrin concentrations from 20 mg/L to 40 mg/L. The amount of sorbed deltamethrin was calculated at different temperatures of 293, 303, 313 and 323 K. The amount of deltamethrin in MMIONPs at different times qt (mg/g) was calculated by (Tahir et al. 2016): 
formula
2
where Ct (mg/L) is the kinetic concentrations of the deltamethrin.

RESULTS AND DISCUSSION

Characterization of the MMIONPs

The surface morphology of the nano-sorbent was studied by a Transmission Electron Microscope (TEM) model-EM208 (Philips Company), and a Scanning Electron Microscope (SEM) model EM-3200 (KYKY Company). The SEM and TEM images of MMIONPs were obtained to ensure the suitability of MMIONPs morphology and its correct synthesis and modification, besides verifying that the nano-sorbent size is less than 100 nm within the nano-range. The SEM and TEM images of MMIONPs display a steady distribution of dark magnetic and spherical agglomerated nanoparticles with diameters less than 80 nm. It is worth noting that the rough and large surface supplied good and active sites for adsorbing deltamethrin (Figure 2).
Figure 2

TEM (a) and SEM (b) images of MMIONPs.

Figure 2

TEM (a) and SEM (b) images of MMIONPs.

Fourier transform infrared spectroscopy (FT-IR) spectra were reported on a FT-IR spectrometer Jasco/FT-IR-410 by using the KBr pellet method (Easton, Maryland). The peaks at 3,157 cm−1 (O-H) and 567 cm−1 (Fe-O) are for the synthesis of iron oxide magnetic nanoparticles. The peaks at 2,922 cm−1 (C-H) and 1,099 cm−1 (SiO) showed the modification done by MTPMS. The additional peaks at 2,928 cm−1 (aliphatic C-H), 1,669 cm−1 (C = O), 1,454 cm−1 (CH2) and 1,097 cm−1 (SiO and CO) are related to polymer grafting by AGE and 2-dimethyl acrylamide. The additional peaks for coupling by 3-hydroxytyraminium chloride at 2,930 cm−1 (aliphatic C-H) and 1,095 cm−1 (SiO and CO) confirmed the success of the synthesis. The FT-IR spectra of the MNPs before and after modification are shown in Figure 3.
Figure 3

FT-IR spectra of the MNPs (a) before and (b) after surface modification.

Figure 3

FT-IR spectra of the MNPs (a) before and (b) after surface modification.

The MMIONPs were also confirmed through an elemental analysis. The elemental analysis (CHNS) model VARRIO El (Linseis Company, Germany) applied in this study is described in the Thermo-Finnigan elemental analyzer manual. The elements of C, H, N, and S in the sample in a column holding oxidant at 1,050 °C were converted to CO2, H2O, N2, and S2, respectively. They were separated in a GC column holding a molecular sieve and determined by a thermal conductivity detector. The percentages of C, H, N, and S in the sample were discovered after drawing the calibration curve for data processing of the sample. The elemental analysis of MMIONPs resulted in: C, 16%; H, 3%; N, 5%; and S, 2%. The observed results of nitrogen and carbon percentages display the successful immobilization of the organic components in each step. The results obtained from the elemental analysis are in good agreement with the FT-IR results.

Thermal gravimetric analysis (TGA) was performed by using a TGA-50H (Shimadzu Corporation). TGA was carried out for the magnetic nanoparticles up to a temperature of 600 °C (Zhang et al. 2006; Banerjee & Chen 2007; Badruddoza et al. 2011) since most weight losses occurred within the range of 300–400 °C and no weight loss trend was evident beyond this temperature (Zhang et al. 2006; Banerjee & Chen 2007; Badruddoza et al. 2011). Figure 4 displays the two-step weight loss of up to 600 °C. One weight loss of up to 120 °C and another major weight loss from 200 °C to 550 °C are shown in the figure with their results. As shown in Figure 4, a high sensitivity was seen to occur at 600 °C, beyond which a weight loss of constant percentage was observed for the decomposition and degradation of the grafted polymer.
Figure 4

TGA of MMIONPs.

Figure 4

TGA of MMIONPs.

The Brunauer–Emmett–Teller (BET) method was appropriate for the surface area analysis of MMIONPs. N2 adsorption-desorption isotherms on MMIONPs at 77 K was measured on a Micrometrics ASAP2020, from which the BET surface area and pore structure of the MMIONPs were estimated and the corresponding result is shown in Figure 5. The BET surface area of MMIONPs gained was 240 m2g−1 and the total pore volume (VP) gained was 0.601 cm3g−1. Table 1 shows the comparison of the surface areas among different sorbents, and understanding a good specific surface area (240 m2g−1) of MMIONPs can provide more surface active sites, leading to an increase in adsorption performance.
Table 1

Comparison of surface areas with some literature

Sorbent Surface area (m2 g−1Reference 
Activated carbon derived from date stones (DSAC) 763.40 Hameed et al. (2009)  
Bituminous shale 11.0 Ayar et al. (2008)  
Chitosan-modified magnetic graphitized multi-walled carbon nanotubes (CS-m-GMCNTs) 39.20 Zhu et al. (2013)  
Corundum 0.17 Oudou & Hansen (2002)  
Ionically modified magnetic nanoparticles (PPhSi-MNPs) 105.7 Badruddoza et al. (2013)  
Kaolinite 23.5 Oudou & Hansen (2002)  
MMIONPs 240 Our sorbent 
Montmorillonite 31.8 Oudou & Hansen (2002)  
Polyfurfuryl alcohol composite 80 Esteve-Turrillas et al. (2007)  
Quartz 0.10 Oudou & Hansen (2002)  
Sorbent Surface area (m2 g−1Reference 
Activated carbon derived from date stones (DSAC) 763.40 Hameed et al. (2009)  
Bituminous shale 11.0 Ayar et al. (2008)  
Chitosan-modified magnetic graphitized multi-walled carbon nanotubes (CS-m-GMCNTs) 39.20 Zhu et al. (2013)  
Corundum 0.17 Oudou & Hansen (2002)  
Ionically modified magnetic nanoparticles (PPhSi-MNPs) 105.7 Badruddoza et al. (2013)  
Kaolinite 23.5 Oudou & Hansen (2002)  
MMIONPs 240 Our sorbent 
Montmorillonite 31.8 Oudou & Hansen (2002)  
Polyfurfuryl alcohol composite 80 Esteve-Turrillas et al. (2007)  
Quartz 0.10 Oudou & Hansen (2002)  
Figure 5

BET adsorption–desorption isotherms of MMIONPs.

Figure 5

BET adsorption–desorption isotherms of MMIONPs.

The zero point charge was done by a solid addition technique. The initial pH of the aqueous solution usually plays a significant role in the process of nano-sorption. This effect of the initial pH, mostly depends on the zero point charge (pHzpc) of the nano-sorbents. The zero point charge examined for MMIONPs is shown in Figure 6. The pHzpc was around pH 6.5 and the optimum pH was around 7 (Ghafari et al. in press). This finding indicates that Л-Л stacking and hydrogen banding exists between the solution and the nano-sorbent around the neutral pH.
Figure 6

The pHzpc of the MMIONPs.

Figure 6

The pHzpc of the MMIONPs.

The effect of contact time

The nano-sorbent effect of contact time on the nano-sorption of deltamethrin was examined by changing the contact time from 0 to 120 min (Ghafari et al. in press). The intra-particle diffusion plots separately given in a certain contact time ranged between 0 and 120 min with different parameters, which are represented in Figures 79.
Figure 7

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: nano-sorbent dosage = 5 mg; pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different initial concentrations, as well as its nano-sorption by the naked Fe3O4 in 20 mg/L of initial deltamethrin concentration.

Figure 7

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: nano-sorbent dosage = 5 mg; pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different initial concentrations, as well as its nano-sorption by the naked Fe3O4 in 20 mg/L of initial deltamethrin concentration.

Figure 8

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: V = 20 mL, pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different adsorbent dosages.

Figure 8

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: V = 20 mL, pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different adsorbent dosages.

Figure 9

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: V = 20 mL, nano-sorbent dosage = 5 mg, pH = 7; t = 1 h; and agitation = 200 rpm at different temperatures.

Figure 9

Non-linear kinetic modeling of pseudo-first order, pseudo-second order and intra-particle diffusion for deltamethrin nano-sorption by MMIONPs under the following conditions: V = 20 mL, nano-sorbent dosage = 5 mg, pH = 7; t = 1 h; and agitation = 200 rpm at different temperatures.

Effect of initial deltamethrin concentration

The effect of initial deltamethrin concentration on nano-sorbent was examined by changing the deltamethrin concentrations of 20, 30 and 40 mg/L at 293 K (Figure 7 and Table 2).

Table 2

Non-linear kinetic parameters for deltamethrin adsorption using naked Fe3O4 and MMIONPsa

Initial concentration (mg/L) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (g/mg·min) qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
20 naked Fe3O4 5.88 0.01 5.54 0.97 0.029 5.9 0.98 0.37 2.09 0.99 
MMIONPs 39.95 0.47 39.41 0.99 0.018 41.31 0.99 1.67 26.45 0.98 
30 54.91 0.32 54.50 0.99 0.008 57.84 0.99 2.92 31.31 0.97 
40 65.65 0.28 64.64 0.99 0.006 68.83 0.99 3.77 34.74 0.97 
Initial concentration (mg/L) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (g/mg·min) qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
20 naked Fe3O4 5.88 0.01 5.54 0.97 0.029 5.9 0.98 0.37 2.09 0.99 
MMIONPs 39.95 0.47 39.41 0.99 0.018 41.31 0.99 1.67 26.45 0.98 
30 54.91 0.32 54.50 0.99 0.008 57.84 0.99 2.92 31.31 0.97 
40 65.65 0.28 64.64 0.99 0.006 68.83 0.99 3.77 34.74 0.97 

aUnder the conditions: nano-sorbent dosage = 5 mg; pH 7; t = 1 h; agitation = 200 rpm; T = 293 K, at different initial concentrations.

Effect of nano-sorbent dosage

The effect of nano-sorbent dosage on nano-sorption of deltamethrin by MMIONPs was examined by changing the nano-sorbent dosages of 3, 5 and 7 mg, which is illustrated in Figure 8 and Table 3.

Table 3

Non-linear kinetic parameters for deltamethrin adsorption using MMIONPsa

Adsorbent dosage (mg) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (min–1qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
59.49 0.45 55.73 0.98 0.01 59.23 0.99 2.96 34.31 0.99 
40 0.47 39.41 0.99 0.02 41.31 0.99 1.67 26.45 0.98 
28.55 0.78 27.96 0.99 0.04 28.97 0.99 0.85 21.68 0.99 
Adsorbent dosage (mg) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (min–1qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
59.49 0.45 55.73 0.98 0.01 59.23 0.99 2.96 34.31 0.99 
40 0.47 39.41 0.99 0.02 41.31 0.99 1.67 26.45 0.98 
28.55 0.78 27.96 0.99 0.04 28.97 0.99 0.85 21.68 0.99 

aUnder the conditions: V = 20 mL, pH 7; t = 1 h; agitation = 200 rpm; T = 293 K, at different adsorbent dosages.

Kinetic modelling

The kinetic of adsorption plays a key role in planning the sorption systems and it is needed for selection of the optimum operating conditions of a full-scale batch process (Zhang et al. 2015). The kinetic study was presented in order to estimate the mechanism of pesticide adsorption to the adsorbent. The pseudo-first order, pseudo-second order and intra-particle diffusion kinetics models were also conformed in order to fit the experimental data. Recently, these models have been broadly applied to describe the adsorption of pollutants from water and wastewater in different fields (Tahir et al. 2016). What is more, the sequential examination of the controlling mechanism of the adsorption processes such as the pseudo-first order, pseudo-second order and intra-particle diffusion equations are used to model the kinetics of deltamethrin adsorption in the nano-adsorbent (Berizi et al. 2016).

A kinetic analysis of adsorption in the form of the pseudo-first order equation is (Berizi et al. 2016): 
formula
3
where k1 is the rate constant (min−1). The qe and qt are the amounts of deltamethrin adsorbed (mg·g−1) at equilibrium time and at time t, respectively.
The pseudo-second order model can be defined in the following form (Berizi et al. 2016): 
formula
4
where k2 (g·mg−1 min−1) is the equilibrium value constant of pseudo-second order adsorption.
The capability of intra-particle diffusion resistance affecting adsorption is examined by applying the intra-particle diffusion model (Berizi et al. 2016): 
formula
5
where Kid (mg/g min1/2) is the diffusion rate coefficient. C is the intercept and relates to the thickness of the boundary layer. The intra-particle diffusion model is broadly applied to predict the rate controlling step.

The pseudo-first order, pseudo-second order and intra-particle diffusion models for the adsorption of deltamethrin with MMIONPs and naked Fe3O4 in different initial concentrations, adsorbent dosages and temperatures are illustrated in Figures 79. The corresponding parameters are presented in Tables 24. Figure 7 and Table 2 present the kinetic modelling for deltamethrin adsorption by MMIONPs and naked Fe3O4 at different initial concentrations.

Table 4

Non-linear kinetic parameters for deltamethrin adsorption using MMIONPsa

Temperature (K) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (min–1qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
293 37.9 0.18 36.75 0.98 0.007 39.16 0.99 2.54 15.17 0.98 
303 39.25 0.29 38.92 0.99 0.011 41.28 0.99 2.15 21.76 0.97 
313 39.61 0.39 38.96 0.99 0.015 41.30 0.99 1.86 24.47 0.97 
323 39.95 0.47 39.41 0.99 0.018 41.31 0.99 1.67 26.45 0.98 
Temperature (K) qe,exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intra-particle diffusion
 
K1 (min–1qe,cal (mg/g) R2 K2 (min–1qe,cal (mg/g) R2 Kid (mg/g min1/2R2 
293 37.9 0.18 36.75 0.98 0.007 39.16 0.99 2.54 15.17 0.98 
303 39.25 0.29 38.92 0.99 0.011 41.28 0.99 2.15 21.76 0.97 
313 39.61 0.39 38.96 0.99 0.015 41.30 0.99 1.86 24.47 0.97 
323 39.95 0.47 39.41 0.99 0.018 41.31 0.99 1.67 26.45 0.98 

aUnder the conditions: V = 20 mL, nano-sorbent dosage = 5 mg, pH 7; t = 1 h; agitation = 200 rpm; at different temperatures.

By increasing the initial deltamethrin concentration, the deltamethrin adsorption amount was increased. At higher initial deltamethrin concentrations, the sorption sites of MMIONPs were available for deltamethrin adsorption. Although the available sites might be filled at a higher concentration, deltamethrin removal is related to the initial concentration. Different adsorptions between naked and modified nano-sorbents of 5.9 and 41.31 mg/g, in 20 mg/L of initial deltamethrin concentration, justified the significance of modification. According to Figure 7, a good agreement between the experimental and calculated amounts of qe is seen in the pseudo-second order model. The near-unity R2 values are higher for the pseudo-second order, which indicates the ability of this model to describe the adsorption processes of the different initial concentrations of deltamethrin by MMIONPs. Kinetic modelling of deltamethrin adsorption by MMIONPs for different adsorbent dosages is presented in Figure 8 and Table 3.

The results showed that adsorption capacities decrease as the adsorbent dosage is increased in the solution. The optimum dosage that resulted in removal of deltamethrin was 3 mg of MMIONPs nano-sorbent. The decrease in the sorption capacity by increasing the nano-sorbent dose was due to the higher surface area at the lower nano-sorbent dose, resulting in more collisions between the pesticide molecules and the particles of nano-sorbent sites. According to Figure 8, a good agreement between the experimental and calculated amounts of qe is observed in the pseudo-second order model. R2 values of the pseudo-second order are higher than those of other rival models. This finding proves the ability of the pseudo-second order model to describe the adsorption processes of different dosages of MMIONPs adsorbent. The kinetic model for deltamethrin adsorption by MMIONPs at different temperatures is presented in Figure 9 and Table 4.

By increasing the temperature, the mobility of deltamethrin in solution increased, so the amount of deltamethrin adsorption was enhanced. According to Figure 9, a good agreement between the experimental and calculated amounts of qe is evidenced in the pseudo-second order model.

The equation constant k2 and the equilibrium capacity were both increased by increasing the temperature from 293 K to 323 K. It could be described by the Arrhenius equation as follows (Hameed et al. 2009): 
formula
6
where k2 is the pseudo-second order rate constant, A is the pre-exponential factor, R is the gas constant and Ea is the activation energy of adsorption. The activation energy (Ea) for the deltamethrin gained from plotting k2 against T was 22.87 kJ mol−1 (Figure 10). The Ea calculated from the Arrhenius equation could be applied to evaluate the type of adsorption. The Ea between 0–50 kJ mol−1 indicated physisorption. Consequently, the amounts we gained showed that the adsorption is likely due to physisorption.
Figure 10

Arrhenius plot for the activation of adsorption.

Figure 10

Arrhenius plot for the activation of adsorption.

From the studied kinetic equations, it was obvious that the nano-sorption of deltamethrin by MMIONPs nano-sorbent was faster than naked Fe3O4 nano-sorbent. What is more, the deltamethrin adsorption by use of nano-sorbent did not follow the intra-particle diffusion. The R2 amounts of the pseudo-second order equation were closer to unity when assimilated with the pseudo-first order equation to the nano-sorbent for all the initial deltamethrin concentrations, adsorbent dosages and temperatures.

Equilibrium modelling

The deltamethrin adsorption equilibrium data were subjected to Langmuir and Freundlich isotherm models. The optimization of an adsorption process needs an understanding of the driving forces that control the interaction between the adsorbate and the adsorbent. In order to optimize the pesticide removal plan, it is necessary to settle the most suitable correlation between the equilibrium data.

The Langmuir model supposes that adsorption happens at homogeneous adsorption sites, and intermolecular forces decrease very fast at a distance from the adsorption surface (Khaloo et al. 2012). This model is further based on the assumption that all the adsorption sites are energetically the same. The Langmuir equation is presented in the following form (Khaloo et al. 2012): 
formula
7
where qmax (mg·g−1) is the maximum adsorption capacity comparable to the complete monolayer coverage on the surface and KL (L mg−1) is the Langmuir constant.
Langmuir parameters calculated from Equation (7) are listed in Table 5. The basic characteristics of the Langmuir equation can be distinguished in terms of a dimensionless separation factor and RL, which is defined as fallow (Khaloo et al. 2012): 
formula
8
Table 5

Non-linear isotherm parameters for the adsorption of deltamethrin into naked Fe3O4 and MMIONPsa

    Naked Fe3O4 MMIONPs
 
Temperature (K) 313 293 303 313 
Langmuir qm (mg/g) 22.3 26.7 27.5 41.7 
kL (L/mg) 0.12 0.28 0.35 0.40 
R2 0.99 0.99 0.99 0.99 
RL 0.07 0.03 0.02 0.02 
Freundlich 3.31 1.77 4.64 4.12 
KF(mg/g)(L/mg)1/n 5.64 6.02 11.28 15.34 
R2 0.99 0.98 0.99 0.99 
    Naked Fe3O4 MMIONPs
 
Temperature (K) 313 293 303 313 
Langmuir qm (mg/g) 22.3 26.7 27.5 41.7 
kL (L/mg) 0.12 0.28 0.35 0.40 
R2 0.99 0.99 0.99 0.99 
RL 0.07 0.03 0.02 0.02 
Freundlich 3.31 1.77 4.64 4.12 
KF(mg/g)(L/mg)1/n 5.64 6.02 11.28 15.34 
R2 0.99 0.98 0.99 0.99 

aUnder the conditions: nano-sorbent dosage = 5 mg; V = 20 mL; pH 7; t = 1 h; agitation = 200 rpm; T = 293 K, at different temperatures.

The values of RL show the type of isotherm to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1).

The Freundlich isotherm is used to understand the relationship by observing the non-ideal and reversible adsorption, and it is not limited to the formation of the monolayer. This isotherm model is an experimental equation used to observe heterogeneous and multilayer systems based on non-uniform distribution of adsorption affinities. Therefore, the experimental equation can be written as (Khaloo et al. 2012): 
formula
9
where KF (mg·g−1) (L·mg−1) 1/n is the Freundlich constant and 1/n is the heterogeneity agent.
The Langmuir and Freundlich non-linear isotherms for the adsorption of deltamethrin at different temperatures and the corresponding factors, along with the correlation coefficients with MMIONPs and naked Fe3O4, are presented in Table 5 and Figure 11.
Figure 11

Non-linear equilibrium modeling of Langmuir and Freundlich for deltamethrin nano-sorption by MMIONPs under the following conditions: nano-sorbent dosage = 5 mg; V = 20 mL; pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different temperatures and its nano-sorption by the naked Fe3O4 at a temperature of 313 K.

Figure 11

Non-linear equilibrium modeling of Langmuir and Freundlich for deltamethrin nano-sorption by MMIONPs under the following conditions: nano-sorbent dosage = 5 mg; V = 20 mL; pH = 7; t = 1 h; agitation = 200 rpm; and T = 293 K at different temperatures and its nano-sorption by the naked Fe3O4 at a temperature of 313 K.

Figure 11 shows a good agreement between the experimental and Langmuir isotherms. The qm values increased, especially by increasing the temperature. This trend shows that the increase in temperature enhances the studied nano-sorption processes. The values of the Langmuir separation factor RL were between 0 and 1, implying that the studied nano-sorption processes were favorable at all of the three temperatures. Thus, MMIONPs appear to be a good adsorbent for deltamethrin removal. Also, the amounts of n were in the range of 1 to 10, which again showed that the studied processes were favorable at all of the three temperatures. Since the R2 amounts of the Langmuir nano-sorbent isotherm were closer to unity, the Langmuir isotherm fitted the data well, and therefore the nano-sorbent had heterogeneous surfaces that were functional for increasing the binding properties of the nano-sorbent.

The adsorption equilibrium models of naked Fe3O4 and MMIONPs at 313 K were examined in order to verify the significance of the modified nano-sorbent. The qm of the naked Fe3O4 and MMIONPs of deltamethrin were 22.3 and 41.7 mg/g, respectively, with the results showing higher qm of MMIONPs. Thus, the modification by Fe3O4 was effective. Furthermore, the adsorption basically depended on the polymers on the surface of the MMIONPs.

Adsorption thermodynamic studies

With the purpose of studying the nature of pesticide adsorption, the thermodynamic factors of adsorption processes, such as the standard Gibbs free energy change (ΔG0), the standard enthalpy change (ΔH0), and the standard entropy change (ΔS0), were estimated. The adsorption data gained at different temperatures were applied to calculate potentially important thermodynamic properties, i.e. (ΔG0), (ΔH0), and (ΔS0) with the following equations (Monajjemi et al. 2001): 
formula
10
 
formula
11
 
formula
12
where K is the equilibrium constant; R (8.314 J·mol−1·K−1) is the universal gas constant and T (K) is the absolute temperature. Thus, the plot of ln K against 1/T should be a straight line. ΔH and ΔS values could be obtained from the slope and intercept of this plot. Table 6 shows the negative values of ΔG0 and positive ΔH0 gained, which shows that the deltamethrin adsorption process is feasible, spontaneous and endothermic. When the temperature increases from 293 K to 313 K, the magnitude of free energy change shifts to high negative amounts (from −14.04 to −15.47 kJ mol−1), demonstrating that adsorption is more spontaneous at high temperatures, and it is simultaneously a physisorption and chemisorption processes. The positive value of ΔS0 indicates that increased randomness at the solid/solution interaction appears in the internal structure of the adsorption of deltamethrin into the MMIONPs. It has been related that ΔG0 up to −20 kJ mol−1 are compatible with interaction between nano-sorption sites and the deltamethrin (physical adsorption), while ΔG0 with negative values under −40 kJ mol−1 involve the transfer or charge sharing from the solid-phase surface to the deltamethrin in order to form a coordinate bond (chemical adsorption) (Sid Kalal et al. 2013). The deltamethrin ΔG0 values obtained for this purpose are < −20 kJ mol−1, which shows that physical adsorption is the predominant mechanism in the sorption progress (Sid Kalal et al. 2013).
Table 6

Thermodynamic parameters for deltamethrin adsorbed by MMIONPs

Initial deltamethrin concentration (mg L−1ΔG0 (kJ/mol)
 
ΔS0 (J/(mol K)) ΔH0 (kJ/mol) 
293 K 303 K 313 K 
20 −14.04 −14.76 −15.47 71.62 6.95 
Initial deltamethrin concentration (mg L−1ΔG0 (kJ/mol)
 
ΔS0 (J/(mol K)) ΔH0 (kJ/mol) 
293 K 303 K 313 K 
20 −14.04 −14.76 −15.47 71.62 6.95 

CONCLUSIONS

In the present study, deltamethrin was nano-sorbed into MMIONPs. The new modified nano-sorbent displays high adsorption of deltamethrin. The synthesis of the MMIONPs is economical and feasible. The MMIONPs have good capability for enrichment of trace amounts of deltamethrin from large sample volumes. The MMIONPs also have other advantages such as high adsorption capacity, high chemical stability and considerable reusability. The operation of nano-sorbent for the sorption of deltamethrin from aqueous solutions has been established in batch techniques. The zero point charge of the nano-sorbent was found to be around pH 6.5. The experimental results were analyzed by applying the Langmuir and Freundlich non-linear adsorption isotherm models. Based on the Langmuir analysis, the monolayer adsorption capacity was found to be 26.7, 27.5 and 41.7 mg g−1 at 293, 303 and 313 K respectively. The Freundlich constant (n) was between 1 and 10, which again showed that the nano-sorption progress was favorable. The RL amounts indicated that the MMIONPs were favorable for the adsorption of deltamethrin. The kinetics of the studied nano-sorption processes followed the pseudo-second order rate equation with high correlation coefficients. ΔG0 described the spontaneity of the process. The positive value of ΔH0 showed that the processes were endothermic and also the positive value of ΔS0 showed the increased randomness of the adsorbate molecules on the solid surfaces compared to in the solution. Therefore, the modified nano-sorbent showed a high adsorption capacity and accordingly indicated their potential application for effective removal of other hazardous and carcinogenic contaminants from aqueous solutions.

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