1. Introduction {#sec1}
===============

In the modern era of industrial development, environmental problems have drawn more and more attention all over the world. Discharge of chemical industries such as textiles, paper, color photography, printing, dye, and food industries that comprise large amounts of contaminants such as dyes and paints pose a risk to agriculture and the food chain and consequently to human health. Independent of the source, a large number of nondegradable pollutants are being discharged in the ecosystem, especially water.^[@ref1]−[@ref5]^ Synthetic dyes generally have complex aromatic molecular structures and are more resistant toward biodegradation and oxidizing agents. Addition of such less ecofriendly dyes into aquatic systems can lead to several disadvantages and hazards such as hindrance of light penetration as well as mutagenic changes. This in turn leads to increase in chemical oxygen demand and biological oxygen demand of water system.^[@ref6]−[@ref8]^ Therefore, treatment of dye wastewater prior to discharge into water resources of the environment is a critical task.^[@ref9]^

Methylene blue (MB), malachite green (MG), and rhodamine are dyes with commercial significance and widely used for dyeing textile fibers like cotton, wool, and so forth. These dyes are also used as analytical reagents. MB is also used in temporary hair coloring. Although not strongly hazardous, it still has various adverse effects on being consumed.^[@ref10]^ MG is a common cationic dye belonging to the triphenylmethane and it is an important indestructible dye. Along with being carcinogenic, the detrimental impact of MG includes excess sweating, eye burn, and so forth.^[@ref11]^ Rhodamine is a relatively hydrophobic dye widely used as a colorant for textiles and food stuffs. Various physical chemical and biological methods have been manifested to eliminate dyes from wastewater. These include adsorption, ozonation, ion exchange, membrane and oxidative processes, and several others. Adsorption technique stands out as a convenient and cost-effective technique to remove dyes and for effluent treatment, among all the demonstrated methods.^[@ref12]^ Till date, numerous adsorbents have been developed in order to remove dyes from aqueous solutions. Some examples are activated carbon,^[@ref13]^ MCM-41 and MCA,^[@ref14]^ graphene oxide,^[@ref15]^ mesoporous SiO~2~,^[@ref16]^ polymer-clay,^[@ref17]^ alumina,^[@ref18]^ agricultural wastes,^[@ref19]^ bioadsorbents, nanoadsorbents,^[@ref20]−[@ref22]^ hydrogels,^[@ref23],[@ref24]^ and aerogels.^[@ref25],[@ref26]^ Because of a higher surface area, and ability of surface modification, nanoadsorbents exhibit greater efficiency and higher removal rate of pollutants from wastewater.^[@ref21],[@ref27]^ Magnetic nanoparticles (MNPs) such as Fe~3~O~4~ are beneficial for environmental applications because of the rapid dissociation of target molecules from the samples easily by application of an external magnetic field.^[@ref28],[@ref29]^

Despite this, there are a very few reports related to a logical approach for simultaneous adsorption of multiple dyes. In this study, we have developed a simple synthesis of magnetic adsorbents that can rapidly remove noxious pollutants like dyes within few minutes to hours of application.

In the past, we have chemically conjugated MNPs with β-cyclodextrin (CD) using isocyanate linkages.^[@ref30],[@ref31]^ The resulting nanoconjugates were capable of successfully encapsulating not only hydrophobic drugs but also hydrophilic drugs.^[@ref32]^ Inspired by these results we attempted to develop a magnetic nanoadsorbent conjugated with the CD polymer, capable of simultaneous removal of hydrophobic as well as hydrophilic dyes.

β-CD is a natural cyclic oligosaccharide with hydrophilic outer and hydrophobic inner cavity, giving rise to a phenomenal capacity to form inclusion complexes in solution with several organic molecules through host--guest interaction.^[@ref33],[@ref34]^ Hence, β-CD-based polymers have been utilized for removing disperse and reactive dyes from aqueous solutions.^[@ref35]^

For this study, a superparamagnetic nanoadsorbent (SPNA) was synthesized by covalent conjugation of MNPs with a cross-linked CD--maleic anhydride copolymer. The efficiency of SPNA as an adsorbent for hydrophobic as well as hydrophilic dyes was assessed under varying concentrations, times, and pHs. The sorption kinetics and probable mechanism of the adsorption were determined and the data were fitted into various isotherm models.

2. Results and Discussion {#sec2}
=========================

2.1. Synthesis of SPNA {#sec2.1}
----------------------

The SPNA was synthesized according to the schematic shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}. Cross-linking of CD with maleic anhydride leads to decrease in its solubility, making it more suitable as an adsorbent. The CD polymer (CD--MA) is synthesized by cross-linking CD with maleic anhydride via an oxoanion intermediate by nucleophilic substitution reaction. The oxoanion is formed because of the generation of strong alkaline conditions by addition of NaH, which causes deprotonation of the hydroxyl group at the C2 position of the anhydrous glucose unit. The next step is functionalization of iron oxide nanoparticles with CD--MA. MNPs have hydroxyl groups on their surface which react with the free carboxyl groups of CD--MA via ester linkages to give the final product.

![Schematic for synthesis of SPNA.](ao-2019-00881t_0001){#fig1}

2.2. NMR Spectra Analysis {#sec2.2}
-------------------------

The structure of the CD--MA polymer was verified by proton nuclear magnetic resonance (NMR) ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}). The spectra show characteristic peaks of protons for free acids at 6.36 ppm. The vinylic protons of maleic monoesters were observed at 6.46 and 6.92 ppm. The protons of β-CD from a repetitive unit of glucose appear at 4.91 ppm for H1, 3.50 ppm for H2, 3.68--3.82 ppm for H3, H5, H6. and 3.43--3.47 ppm for H4.^[@ref36]^ The NMR spectra of maleic anhydride and CD are provided in the [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf) (Figures S1 and S2).

![NMR spectra for CD--MA.](ao-2019-00881t_0007){#fig2}

2.3. FT-IR Spectra Analysis {#sec2.3}
---------------------------

The IR spectra of β-CD shows a characteristic broad band within the range of 3300--3500 cm^--1^, which corresponds to the presence of hydroxyl groups ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}). The IR spectra for SPNA also show a broad band at 3461--3173 cm^--1^ because of −OH stretching vibrations, which corresponds to the incorporation of CD moieties on SPNA. The absorption band at 1854 cm^--1^ corresponding to carbonyl stretching frequencies is observed in the spectra for maleic anhydride. However, in the spectra of SPNA the C=O stretching frequency is observed at 1725 cm^--1^; the shift is corresponding to the formation of ester linkages. Moreover, the absorption band at 2829.6 cm^--1^ corresponds to asymmetric C--H stretching vibration, which further confirms that an ester bond is formed. The peak at 1589 cm^--1^ signifies the presence of C=C in maleic anhydride; the same peak is observed at 1574 cm^--1^, thus proving the attachment of a maleic anhydride unit.

![IR spectra of MNPs, β-CD, MA, and SPNA.](ao-2019-00881t_0008){#fig3}

The peaks at 1023 and 1152 cm^--1^ correspond to the antisymmetric (C--O--C) vibrations and coupled (C--C/C--O) stretching vibration. The peak observed at 772 cm^--1^ in the spectra of both MNPs and SPNA indicates successful conjugation of the polymer on the surface of iron oxide nanoparticles.

2.4. X-ray Diffractogram Analysis {#sec2.4}
---------------------------------

The XRD diffractogram of SPNA ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}) showed peaks at 30.30, 35.56, 56.68, and 62.75 corresponding to (220), (311), (511), and (440) planes of Fe~3~O~4~, respectively. This leads to a conclusion that the incorporation of polymer does not change the crystal phase of MNPs.^[@ref37],[@ref38]^

![XRD pattern for fresh SPNA and recycled SPNA.](ao-2019-00881t_0009){#fig4}

2.5. HR-TEM Analysis {#sec2.5}
--------------------

The high-resolution transmission electron microscopy (HR-TEM) images ([Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}) show the size distribution of MNPs and SPNA. Pure MNPs were observed as nanosized aggregates having a size range of 20--30 nm. However, SPNA appeared as spherical and discrete monodispersed particles having a narrow size distribution of 50--60 nm. This suggests that the thickness of the CD--MA polymer on MNPs is approximately 20--30 nm. Moreover, image C shows a distinct layer of light field contrast, which arises because of the polymer coating on the surface of the iron oxide nanoparticles. This also provides an evidence for the successful coating of CD--MA on MNPs.

![HR-TEM images for (A) pure MNPs and (B,C) SPNA.](ao-2019-00881t_0010){#fig5}

2.6. Dynamic Light Scattering Analysis {#sec2.6}
--------------------------------------

The hydrodynamic diameter of SPNA was determined using dynamic light scattering experiments. At 25 °C, the diameter was observed to be 57 nm. Moreover, the particles were monodispersed, which is concordant with the data of HR-TEM ([Figure S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf)).

2.7. VSM Analysis {#sec2.7}
-----------------

The performance of magnetic materials can be assessed by hysteresis loop determinations at ambient temperature using a vibrating sample magnetometer (VSM). The VSM measurements of bare MNPs and SPNA are shown in [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}. The magnetic properties are measured as a function of magnetic field, time, and temperature. It can be observed from the M--H loops that SPNA shows ferromagnetic behavior.

![Room-temperature magnetization curves of SPNA (Inset: photograph showing separation of SPNA using an external magnet).](ao-2019-00881t_0011){#fig6}

A decrease in the magnetization value was observed in the case of SPNA, which is due to coating of polymer on MNPs. However, the material could be easily dispersed and withdrawn with the help of an external magnet. This observation suggests that the magnetism of the iron oxide nanoparticles at the core is retained even after conjugation with CD--MA.

2.8. TGA Analysis {#sec2.8}
-----------------

The nanoadsorbent was analyzed with thermogravimetric analysis (TGA) ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}) for the assessment of its thermal properties. Three stages of degradation were observed for the adsorbent. A mass loss of 4.5 % corresponding to removal of adsorbed water molecules was observed below 200 °C. In the second stage of degradation in a temperature range of 200--350 °C about 36.8% mass loss is observed, attributed to breaking of ester linkages.^[@ref39]^ The final degradation of 11.4% in the 360--500 °C range occurs as a result of decomposition of CD molecules.^[@ref40]^

![TGA curve of SPNA.](ao-2019-00881t_0012){#fig7}

2.9. BET Analysis {#sec2.9}
-----------------

The Brunauer--Emmet--Teller (BET) adsorption isotherm of SPNA shown in [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"} exhibits the shape of the type II isotherm as per the IUPAC classification, thus suggesting that these particles are mostly nonporous or macroporous.^[@ref41]^ Thus, SPNA has the possibility of showing physisorption or monolayer adsorption.^[@ref42]^ The BET specific surface area, total pore volume, and average pore diameter of sample is presented in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}. The BET adsorption isotherm of pristine iron oxide nanoparticles is provided in the [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf) (Figure S4).

![Nitrogen adsorption--desorption isotherm of SPNA.](ao-2019-00881t_0013){#fig8}

###### BET Parameters of SPNA

         BET plot             
  ------ ---------- --------- ----------
  SPNA   14.96      40.7369   0.143870

2.10. Selectivity of Adsorption (Effect of Cavity of CD) {#sec2.10}
--------------------------------------------------------

The CD--MA polymer facilitates the uptake of hydrophilic as well as hydrophobic dye molecules via various chemical interactions, making SPNA a versatile adsorbent. The hydrophobic cavity of CD is likely to encapsulate the hydrophobic dye rhodamine 6G (R6G) more than the cationic dyes because of which it is expected to get adsorbed to a greater extent. However, the results from the adsorption experiments show an interesting trend. The removal of hydrophilic dyes is more than hydrophobic dyes. A comparison further shows that removal of MG is the maximum amongst the three dyes and removal efficiency for R6G is the least. The higher adsorption efficiency for MG can be attributed to the small spatial prohibition for the structure of MG molecules. This leads to rapid interaction of MG molecules with SPNA. However, although there is availability of the hydrophobic cavity of CD for R6G the steric hindrance caused because of the large spatial size causes the encapsulation within the cavity difficult and hence less adsorption as compared to MB and MG. Various other factors which lead to this phenomenon have been explained further.

2.11. Effect of SPNA Dosage {#sec2.11}
---------------------------

The quantity of the adsorbent corresponds to the available active sites and hence plays an important role in the process of adsorption. The effect of SPNA quantity on the percentage removal of the dyes was evaluated at pH 7.0 for 24 h. The results of this study are shown in [Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}. It is observed that the maximum percentage removal was obtained using 20 mg of SPNAs for all the three dyes. The increase in the removal efficiency from 10 to 20 mg corresponds to the higher availability of active interaction sites because of the large surface area with the increase in the quantity of adsorbent. Further increase in dosage causes a decrease in the removal efficiency. This can be explained by two reasons. First, the magnetic property of the adsorbent results in formation of aggregates during the process of adsorption. This causes a decrease in the exposed surface area available for adsorption, thus affecting the removal efficiency. Second, an increase in the amount of the absorbent causes a decrease in the rate at which the active adsorption sites are utilized by the adsorbate molecules, which lowers the percentage removal. Thus, 20 mg is the optimum quantity of the adsorbent.

![Effect of SPNA dosage on the percentage removal of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB, and R6G at 25 °C and pH-7.0.](ao-2019-00881t_0014){#fig9}

2.12. Effect of Contact Time {#sec2.12}
----------------------------

[Figure S5](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf) shows the effect of contact time on the adsorption of dyes by SPNAs at 1, 5, 10, 15, 20, 25, and 30 min (pH 7.0 and 25 °C). It was observed that the equilibrium is attained in 30 min. The removal efficiency of the three dyes are different at the end of 30 min, which is because of the difference in the interaction of the dyes with the adsorbent. The removal of MG is considerably fast as compared to that of MB and R6G. The process of adsorption can be elaborated in three stages; first, the dye molecules migrate thorough the solution to reach the adsorbent surface. After this, certain dye molecules are adsorbed via the van der Waals force. Electrostatic interactions between the cationic dye and the adsorbent consist of the last stage. As the process of adsorption begins the removal increases with time, which corresponds to the attachment via van der Waals forces and then via electrostatic interactions when the dye is in close proximity to the adsorbent. After lapse of time, the vacant sites cannot be further occupied by the dye molecules. This corresponds to the attainment of equilibrium.

2.13. Effect of pH {#sec2.13}
------------------

The pH of the solution plays an important role during the process of dye adsorption. This occurs because pH affects the adsorbent as well as the dye molecules, for instance the surface charge of the adsorbent, the structure of dye molecules, and the extent to which ionization of dyes occurs.^[@ref12]^ The effect of pH is illustrated in [Figure S6](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf). It was observed that maximum adsorption of the dye is obtained under basic conditions (pH 9). This phenomenon occurs because of the cationic nature of MG and MB. On the other hand, the surface of the adsorbent is negatively charged because of the presence of dissociated functional groups. The presence of hydroxyl groups in protonated form (OH) at lower pH values causes a weak interaction between the dyes and adsorbent. In this case, these exists a competition between H^+^ ions and cationic dye molecules, which is unfavorable for the adsorption of dye molecules. However, on increasing the pH of the solution the deprotonation aids formation of (O−) groups on the adsorbent surface, thus increasing its negative charge density. Thus, the binding of cationic dye molecules is facilitated by electrostatic interaction between the tdye andhe adsorbent.^[@ref43]^

2.14. Effect of Initial Concentration {#sec2.14}
-------------------------------------

The effect of initial concentration illustrated in [Figure S7](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf) shows that the adsorption rate increases rapidly before the concentration reaches the maximum of 30 mg/L for MG and 20 mg/L for MB and R6G. The rate of adsorption then decreases and becomes constant. This phenomenon occurs because initially because of the higher concentration of the dye there is an increase in the mass gradient between the solution and the adsorbent. This is the driving force for the migration of dye ions from the bulk of the solution to the surface of the adsorbent. After attainment of equilibrium the decrease in number of active sites does not favor further adsorption, which explains the decrease and further constant nature of the graph.^[@ref44]^

2.15. Effect of Temperature {#sec2.15}
---------------------------

The dye removal increases with the increase of temperature. In the case of MG, % Re increases from 87.9 to 97.2% by increasing the temperature by 10 °C.

Similarly, for MB the removal increases from 80.2 to 85.1% and for R6G it is from 30.5 to 37.3% as shown in [Figure [10](#fig10){ref-type="fig"}](#fig10){ref-type="fig"}. An increase in temperature not only enhances the mobility of dye molecules but also provides sufficient energy, which facilitates the interaction between the active sites and dye molecules. This causes an increase in the adsorption of dye at higher temperatures.

![Effect of temperature on dye removal efficiency at 35 and 45 °C.](ao-2019-00881t_0002){#fig10}

2.16. Adsorption Isotherms {#sec2.16}
--------------------------

The relationship between the adsorption capacity (*q*~e~) and concentration of solution after adsorption at a particular temperature is illustrated using an adsorption isotherm. They are imperative for designing an adsorption system and its data analysis. In this context, adsorption capacities of the dyes onto SPNA were evaluated by Langmuir and Freundlich adsorption isotherm models ([Figures S8 and S9](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf)).where, *C*~e~ (mg L^--1^) is the equilibrium concentration of dyes, *q*~e~ (mg g^--1^) is the adsorption capacity of SPNAs at equilibrium; *q*~m~ is the maximum adsorption capacity (mg g^--1^), and *K*~L~ (L/mg) and *n* are the Freundlich equilibrium adsorption constants.

The calculated results of various parameters of the adsorption isotherm models are shown in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. It can be concluded from the values of the correlation coefficient that the dyes follow the Langmuir isotherm model. A monolayer adsorption proceeds at certain homogeneous sites on the adsorbent, and all these sites are energetically identical.^[@ref45]^ The results obtained from the adsorption isotherm modeling studies are in agreement with the observations obtained from BET surface area analysis.

###### Isotherm Constants for the Adsorption of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB, and R6G at 25 °C pH 7.0

        Langmuir   Freundlich                         
  ----- ---------- ------------ ------- ------ ------ -------
  MG    18.52      0.0635       0.997   2.69   1.94   0.897
  MB    11.43      0.0851       0.992   3.99   1.73   0.712
  R6G   5.94       1.4237       0.942   2.19   0.02   0.832

2.17. Adsorption Kinetics {#sec2.17}
-------------------------

Adsorption kinetics validates the linear relationship between the adsorption capacity and time. Several models were employed to ascertain the kinetics of adsorption of the dyes' SPNAs. Zero-order, first-order, second-order, third-order, pseudo-first-order, pseudo-second-order models were applied to fit the experimental data. Of all these models, the pseudo-first-order and pseudo-second-order kinetics are studied in detail for establishing the sorption kinetics. The pseudo-first- and -second-order models are expressed as follows^[@ref34]^Here, *q*~e~ and *q*~*t*~ (mg g^--1^) are the amount of adsorbed dye at equilibrium and at time *t* (min), respectively. The pseudo-first-order rate constant is *k*~1~ and pseudo-second-order rate constant is expressed as *k*~2~ (mg min g^--1^).

The values of correlation coefficients (*R*^2^) were introduced to ascertain the suitability in a particular model. From the values of *R*^2^ it was concluded that the adsorption kinetics in the case of SPNA follows the pseudo-second-order model for all the three dyes as represented in [Figure [11](#fig11){ref-type="fig"}](#fig11){ref-type="fig"}. Thus, it can be said that the chemisorption process is the rate-determining step in the adsorption of dyes on SPNA. The process of adsorption/desorption process is controlled by the occurrence or absence of chemical interactions of the dye molecules with the adsorbent^[@ref46]^

![(A) Zero-order (B) first-order (C) second-order (D) third-order (E) pseudo-first-order (F) pseudo-second-order kinetic curves for MB, MG, and R6G.](ao-2019-00881t_0003){#fig11}

2.18. Simultaneous Adsorption Studies of Hydrophilic and Hydrophobic Dyes {#sec2.18}
-------------------------------------------------------------------------

The results of simultaneous adsorption of MG and R6G show that adsorption capacity of MG is higher than that of R6G. The removal efficiency of MG and R6G was found to be 94.58 and 26.48%, respectively. The simultaneous adsorption effect of both the dyes on SPNA was determined by using the ratio of adsorption capacities (*R*~q~)where *q*~b~, *i* and *q*~m~, *i* are the uptake of dye *i* in the binary system and monocomponent system having the same initial concentration, respectively.

There can be three probable cases in simultaneous adsorption: (i) if *R*~q~ \> 1, synergism, which suggests that the adsorption of dye is enhanced by the presence of other dyes; (ii) if *R*~q~ \< 1, antagonism, that is, the adsorption of dye is suppressed by the presence of other dyes, and (iii) if *R*~q~ = 1, noninteraction, the adsorption of each dye is independent of each other.

In this case, we observe the value of *R*~q~ of less than 1, which suggests that it is a case of antagonism. Thus, the presence of MG affects the adsorption of R6G and a phenomenon of competitive adsorption is observed between both the dyes. This observation can be explained by two plausible mechanisms (i) out of the various chemical interactions formed between the dyes and SPNA, MG forms hydrogen bonding with the adsorbent, which is a much stronger interaction. However, R6G is unable to form hydrogen bonding with SPNA. (ii) The large size of R6G hinders its rapid encapsulation within the cavity as compared to MG. Hence, MG predominates in competitive adsorption.^[@ref47]^ Thus, maximum uptake of MG occurs from the mixture of the solution because of its stronger affinity with the SPNA surface.

2.19. Desorption and Recyclability {#sec2.19}
----------------------------------

The reuse of the adsorbent is an important index for defining the efficiency of the adsorbent. In order to assess the reusability of the adsorbent successive adsorption/desorption cycles were carried out.^[@ref48],[@ref49]^ Results show that the adsorption capacity decreases after five cycles for all the three dyes ([Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}). The desorption experiments were carried out in an ethanol water mixture, which is generally the system used for desorption of dyes under study.^[@ref50]^ It was observed that more than 90% of the dye was effectively removed during the desorption cycles.

![Percentage removal of dyes after successive desorption/adsorption cycles.](ao-2019-00881t_0004){#fig12}

The IR spectra ([Figure [13](#fig13){ref-type="fig"}](#fig13){ref-type="fig"}) and XRD diffractogram ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}) of the regenerated material shows no significant shifts in the bands or 2θ values. The same characteristics peaks are observed in the IR of the regenerated SPNA as those observed in fresh SPNA. However, there is a decrease in the intensities of the IR peaks, which may be due to minor degradation caused to SPNA during the process of desorption. However, the degradation does not affect the removal efficiency of the material and hence the recycling does not compromise the performance of the adsorbent for dye removal.^[@ref35]^ Similar observations were seen in XRD diffractogram.

![IR spectra of the recycled adsorbent.](ao-2019-00881t_0005){#fig13}

2.20. Adsorption Mechanism {#sec2.20}
--------------------------

The probable interaction of SPNA with different dyes is shown in [Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}. The mechanism of adsorption is governed by various factors like the structure and functional behavior of adsorbate molecules and surface characteristics of the adsorbent. The adsorbate in this case is the dye molecule, which is planar and can be readily adsorbed on the adsorbent via the van der Waals force and hydrogen bonding interactions in addition to CD cavity. Further, in the case of the cationic dyes the electrostatic forces of interaction play a role in the sorption process.

![Interaction of the dyes with the adsorbent.](ao-2019-00881t_0006){#fig14}

The cavity of CD can encapsulate all the three types of organic dyes.^[@ref9]^ However, lower adsorption of R6G suggests that its larger size and hydrophobic nature prevent any other interaction with the SPNA. On the other hand, MB and MG are able to bind to the CD--MA polymer via electrostatic forces of interaction, van der Waals forces, and hydrogen bonding in addition to the host--guest interaction with the cavity of CD.

The elemental composition of the adsorbent prior to and after adsorption/desorption cycles can be used to compare the characteristic changes of dye-loaded adsorbents ([Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}).The higher content of carbon after adsorption suggests the successful adsorption of the dyes on the surface of SPNA. The presence of elements like nitrogen and sulphur on adsorbent + MB, adsorbent + MG and adsorbent + R6G further proves their presence on the adsorbent by virtue of adsorption. Thus, it can be assumed that the changes in the composition of elements correspond to adsorption of the dyes, which may be due to these possible interactions.

###### Elemental Composition of the Adsorbent Determined before and after Adsorption Following Four Adsorption/Desorption Cycles

  materials         C (wt %)   Fe (wt %)   O (Wt %)   N (Wt %)   S (Wt %)
  ----------------- ---------- ----------- ---------- ---------- ----------
  bare adsorbent    30.39      7.05        62.57      0          0
  adsorbent + MG    46.75      6.76        31.92      14.75      0
  adsorbent + R6G   40.28      22.58       31.27      5.86       0
  adsorbent + MB    36.71      34.46       23.29      5.41       0.12

2.21. Comparison with Other Adsorbents {#sec2.21}
--------------------------------------

The values of maximum adsorption capacities of the dyes on SPNA obtained from experiments have been compared with other results reported in literature ([Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"}). It is observed that SPNA has a greater adsorption capability as compared to other adsorbents. The results prove that the SPNA can be considered a promising adsorbent for dye removal, especially MG. The literature values also reveal that the lower adsorption efficiency in case of R6G is probably due to its larger size.

###### Comparison of Maximum Adsorption Capacities of SPNA with Other Adsorbents

  adsorbent                            adsorbate   *Q*~m~ (mg/g)   references
  ------------------------------------ ----------- --------------- ------------
  ZnO particles                        MG          13.6            ([@ref51])
  RGO                                  MG          13.52           ([@ref52])
  SPNA                                 MG          18.52           this work
  EDTA-β-CD                            MB          0.262           ([@ref35])
  *C. racemosa* var. cylindracea       MB          5.23            ([@ref53])
  CAS                                  MB          8.52            ([@ref54])
  SPNA                                 MB          11.43           this work
  trichodermaharzianummycelial waste   R6G         3.40            ([@ref55])
  Na^+^-montmorillonite                R6G         0.40            ([@ref56])
  SPNA                                 R6G         5.94            this work

3. Conclusions {#sec3}
==============

In summary, this work has established a proficient and simple approach to prepare a novel magnetic nanoadsorbent (SPNA), which can efficiently remove organic dyes particularly MB, MG, and R6G. The results indicate that SPNA is an excellent nanoadsorbent for removal of dyes with maximum removal efficiency of 97.2, 85.1, and 37.3% for MB, MG, and R6G, respectively. The main advantage of the maleic anhydride cross-linked CD polymer is that it facilitates simultaneous adsorption of hydrophobic as well as hydrophilic dyes. The detailed investigation of adsorption behavior of the SPNA exhibited that the adsorption process was dominated by the van der Waals interactions between the SPNA and dye molecules. The removal efficiency is regulated by concentration, adsorbent dose, pH, and temperature. The adsorption kinetic studies reveal that removal of organic dye is a spontaneous process and obeys the pseudo-second-order model. The equilibrium data were best fitted with the Langmuir isotherm model. Furthermore, SPNA can be regenerated and reused for five cycles with a slight decrease in removal efficiency and can be easily separated by magnetic decantation. Thus, convenient, highly efficient, and technically feasible properties make this novel nanoadsorbent a promising low-cost adsorbent for dye removal. It can be concluded from the results that this SPNA can be used for environmental remediation effectively.

4. Materials and Methods {#sec4}
========================

4.1. Materials {#sec4.1}
--------------

β--CD (≥97% purity), maleic anhydride, iron oxide (Fe~3~O~4~) nanoparticles (97% purity), and acetone were purchased from Sigma-Aldrich, India. *N*,*N*-Dimethylformamide (DMF) was purchased from Qualigens, Bombay, India. MB, MG, and R6G were obtained from Fisher Scientific, Navi Mumbai, Laboratory Sulab Reagent, Baroda, and Loba Chemie Laboratory Reagents and Fine Chemicals, Mumbai, India. Analytical grade reagents were used as received. The solutions of dye were prepared using de-ionized water.

4.2. Synthesis of SPNA {#sec4.2}
----------------------

For the synthesis, CD (500 mg) was dissolved in DMF (25 mL), and then solid NaH (15 mg) was slowly added into the solution with vigorous stirring. Further, the stirring was continued at room temperature for 6 h. Then, solid MA (50 mg) was slowly added to the solution of formed β-CD oxoanions. The reaction mixture was continuously stirred in a sealed round-bottom flask in an oil bath at a controlled temperature (100 °C) for 1 h. To this mixture DCC (50 mg) and DMAP (5 mg) were added. Iron oxide nanoparticles (50 mg) were dispersed in 10 mL of DMF and this dispersion was added to the above reaction mixture. The reaction was stirred for 24 h. The product was precipitated in acetone.

4.3. Characterization of SPNA {#sec4.3}
-----------------------------

The NMR spectrum for CD--MA polymer was recorded on a Bruker Avance III (400 MHz) NMR spectrophotometer using D~2~O as the solvent. A PerkinElmer IR spectrophotometer was used to record FTIR spectra of SPNA as KBr discs at room temperature. The X-ray diffractograms were recorded on a D2 Phase 2 Bruker X-ray powder diffractometer at room temperature. VSM analysis of SPNA was performed by using Lakeshore VSM 7410 at room temperature. In order to accomplish HR-TEM analysis a JEOL (Jem-2100) electron microscope was used at an acceleration voltage of 200 kV. Dynamic light scattering measurements were carried out on a Beckman Coulter Delso Nano. Energy-dispersive X-ray (EDX) analysis of the vacuum-dried SPNA was recorded by the model-JSM-5610 LV. The Surface area and porosity of SPNA were measured using a volumetric adsorption system (Micromeritics Instrument Corporation, USA, model ASAP 2020) using N2 adsorption/desorption isotherms at 77 K up to 1 bar. Before measurements, the samples were activated (degassed) by heating at the rate of 1 K min^--1^ up to 295 K under vacuum. The surface area was calculated using the BET method and the porosity by the Barrett--Joyner--Halenda method. TGA was performed using TG--DTA 6300 INCARP EXSTAR 6000 at a heating rate of 10 °C/min in the temperature range of 30--500 °C with nitrogen atmosphere maintained throughout the measurement.

4.4. Adsorption of Dyes {#sec4.4}
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To evaluate the adsorption capacity of SPNA, two hydrophilic dyes, MB, and MG, and a hydrophobic dye, R6G, were used as model dyes for adsorption studies. The adsorption process was carried out in 100 mL conical flasks containing 10 mL of dye solutions. To determine the optimum quantity of SPNA to be used in subsequent experiments, different amounts (10, 15, and 20 mg) of SPNA was added in 50 mg/L of prepared dye solutions individually. All the solutions were placed on magnetic stirring for different time intervals, aliquots were collected, and further measured for content of unadsorbed dye using a UV--vis spectrophotometer. For determining optimum concentration of dyes to be used in successive experiments, different concentrations (10--50 mg/L) of dyes were prepared and optimum amount of SPNA was added to the solutions. All the solutions were placed on a shaker for 24 h at 27 °C and shaking speed of 160 rpm to attain equilibrium condition. The supernatant was separated by applying an external magnetic field and analyzed using a UV--visible spectrophotometer at the wavelength range of 400--800 nm. To determine optimum pH for adsorption, 10 mL of 30 mg/L MB, MG, and R6G were taken with 50 mg of SPNA at different pH and kept on a shaker for equilibrium study, supernatants were collected after adsorption, and analyzed using a Jasco V-730 UV--visible spectrophotometer (λ~max~ = 664 for MB, λ~max~ = 617 for MG and λ~max~ = 526 for R6G).The adsorption capacity, removal percentage, and adsorption at particular time "*t*" were calculated using the following equations^[@ref9]^where *q*~e~ denotes the adsorption capacity (mg/g), q~*t*~ is the adsorption capacity at a particular time "*t*" (mg/g), and % R~e~ is the removal percentage of dyes; *C*~0~ represents the initial concentration (mg/L), *C*~e~ is the equilibrium concentration (mg/L), and *C*~*t*~ denotes the concentration at time "*t*" (mg/L) of dyes in aqueous solution; "*V*" denotes the volume of the solution (L), and "*m*" denotes the weight of the adsorbent (g). All the adsorption experiments were performed in triplicate and mean values were reported.

4.5. Simultaneous Adsorption Studies of Hydrophilic and Hydrophobic Dyes {#sec4.5}
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As the adsorbent was able to adsorb hydrophilic dyes with greater efficiency it was interesting to investigate the simultaneous adsorption of hydrophilic and hydrophobic dyes in a binary mixture. For this purpose, a binary system of MG-R6G having an initial concentration of 30 mg L^--1^, pH 7.0 and was used. To ensure that both the dyes reach an equilibrium, time of 24 h was selected for these binary experiments.

4.6. Desorption Experiment and Reusability {#sec4.6}
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SPNA (20 mg) was placed in a conical flask containing 50 mg/L dye solutions individually and solutions were magnetically stirred for 5 h. Dye solutions were separated using an external magnetic field and final concentrations of dyes were determined. The SPNA was desorbed by washing it under constant stirring with 1:1 ethanol and water three times, respectively, dried, and stored to start a new adsorption batch.^[@ref57]^

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.9b00881](http://pubs.acs.org/doi/abs/10.1021/acsomega.9b00881).^1^H NMR spectra of maleic anhydride and CD, DLS profile for SPNA, nitrogen adsorption--desorption isotherm of pristine iron oxide nanoparticles, effect of contact time on the percentage removal of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB, and R6G at 25 °C and pH-7.0, effect of pH on the percentage removal of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB and R6G at 25 °C, effect of initial concentration on adsorption of dyes on SPNA, Langmuir adsorption isotherm for the removal of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB, and R6G at 25 °C pH 7.0, Freundlich adsorption isotherm for the removal of 30 mg·L^--1^ of MG, 20 mg·L^--1^ of MB, and R6G at 25 °C pH 7.0 ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00881/suppl_file/ao9b00881_si_001.pdf))

Supplementary Material
======================

###### 

ao9b00881_si_001.pdf

The authors declare no competing financial interest.

The authors are thankful to Solvay Research and Innovation Centre, GIDC-Savli Vadodara, Gujarat, for providing financial assistance. The authors acknowledge SAIF, IIT-Chennai, for providing VSM facilities, SAIF, IIT-Mumbai, for HR-TEM, CHARUSAT for XRD measurements, and Department of Metallurgical and Materials engineering for EDX analysis, respectively. The authors would like to acknowledge SERB-DST, New Delhi, for providing UV--visible and fluorescence instrumental facilities under the research grant sanctioned to Dr. Vinay Kumar Singh (reference no. EMR/2016/003172) as well as DST-FIST program for NMR facilities.
