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Removal of fluoride ions using a polypyrrole magnetic nanocomposite influenced by a rotating magnetic field

The impact of a varying rotating magnetic field in stimulating adsorption of fluoride ions onto a polypyrrole magnetic nanocomposite synthesized via in situ a polymerization process was evaluated. Under the effect of a rotating magnetic field, improved removal of adsorbate (10 mg L−1) from aqueous solution using the polypyrrole magnetic nanocomposite was observed, with a maximum removal of 78.2% observed at a magnetic field intensity of 0.019 T. Particle aggregation resulting from the force owing to the gradient on the particles as the magnetic field was increased resulted in improved fluoride removal. This aggregation of particles leads to an improved chain collision and expanse of particle interaction with the fluoride solution. The process of adsorption of fluoride by the PPy/Fe3O4 nanocomposite followed both the Freudlich isotherm and the Temkin isotherm. Interestingly, under the effect of the rotating magnetic field, the adsorption process was best described by the Freundlich isotherm.


Introduction
The presence of uoride in groundwater is a natural process, which is inuenced by geological and hydrogeological circumstances. 1 In water, organic chemicals and heavy metals pose severe threats to humans and the environment, owing to their non-biodegradability and high toxicity. 2 Owing to the increased pollution of water bodies, good drinking water quality has been a big challenge in recent years. For humans and animals, uoride is a vital element which relates to the total amount consumed. 3 It also threatens the existence of living organisms, in specic humans. 4 In small amounts, uoride is important for dental and bones growth in humans at concentrations of about 0.5-1.0 mg L À1 but it leads to dental and skeletal uorosis when intake through food and drinks exceed 1.5 mg L À1 . [3][4][5] Once the maximum allowable concentration for uoride in wastewater by the World Health Organization (WHO) is exceeded, there is a need for innovative technologies that are environmentally friendly and cost-effective to be employed for uoride removal from aqueous solution. 6,7 The adsorption process is a unique method employed for heavy metals ions removal from solutions by placing them directly on the precise chosen adsorbent surface. 8 The adsorption process can be improved signicantly by taking advantage of using peripheral factors like electric elds, ultrasonic waves, irradiation and magnetic eld. In the treatment of water, the magnetic eld has been used as a valuable tool owing to their easy access, capability, efficiency, effective energy consumption and little impact on the environment. 2 The effects of applied magnetic eld method on nonmagnetic colloidal particles in aqueous systems, relating to the physiochemical properties prior and aer magnetic eld exposure have been described by some researchers. [9][10][11] A number of research relating to the effect of magnetic eld on the adsorption processes have been reported. Khiadani, et al., 2013 observed an improved removal in the turbidity of Pb, Zn, Cd and PO 4 ions from urban runoff treated with a magnetic eld. 12 Zhang, et al., 2005 also reported in their various studies an increase in the amount zinc, and copper(II) adsorbed onto Na-rectorite, Ca-rectorite and Kaolinite when a magnetic eld was applied. [13][14][15] The rate of strontium and radium adsorbed onto monosodium titanate was also observed to be enhanced with the application of a magnetic eld, with an increased removal observed in the early contact time. 16 Increase removal of an organic compound onto clay modied with iron under the inuence of the magnetic eld was reported by Tireli et al., 2014. This enhanced organic compounds removal was ascribed to increased mobility of the adsorbent due to the improved orientation of the adsorbent molecules in the aqueous medium. 17 Improvement in the removal of mercury ions captured by y ash under a magnetic eld, which was ascribed to possible changes in either the adsorbate or adsorbent was also reported by He et al., 2015. These changes were ascribed to the orbital angular and intrinsic spin angular momentums interacting with the magnetic eld, resulting in the modication in the energy of the atom. 18 The research purpose of this work is to study the noninvasive use of weak anti-clockwise rotating magnetic eld generated from a modied magnetic eld reactor and its inuence in stimulating adsorption of uoride ions onto magnetic nanocomposite from aqueous solution. Consequently, this study aims to dene a suitable kinetic model applicable to the treatment of uoride due to the limitations in previous studies using a magnetic eld.

Materials
All reagents used were of analytic reagent rating and freshly prepared before further use. Pyrrole monomer (Py), sodium uoride, ferric chloride and magnetite (Fe 3 O 4 ), sodium uoride (NaF) were procured from Sigma-Aldrich, South Africa. A stock solution of uoride (1000 mg L À1 ) was prepared by dissolving 2.21 g of sodium uoride in 1 litre of deionized water. Standard uoride solutions were prepared by diluting of the stock solution to precise concentration.

Synthesis of PPy/Fe 3 O 4 nanocomposite
Polypyrrole magnetic nanocomposite was synthesized using the procedure described by Aigbe et al., 2018. 19 To have a good distribution of magnetite in deionized water, 0.4 g of magnetite was added to 80 millilitres of deionized water and the solution was ultrasonicated for 30 min. Then, 0.8 mL of pyrrole and 6 g FeCl 3 were added into the nanoparticles suspended solution and shaken manually continuously for 5 minutes. For polymerization reaction to take place, the solution was kept for three hours at room temperature. The synthesized nanocomposite (black powder) was washed with deionized water continuously during ltration until the ltrate became colourless. To stop reaction within the ltrate, acetone was injected. The nanocomposite was dehydrated under vacuum at 60 C for 24 hours.

Characterization of nanoadsorbent
The nanoadsorbent crystalline structure was determined with a SmarLab X-ray diffractometer using Cu-Ka radiation (wavelength (l) ¼ 1.540593 A) from 10-90 . The FTIR spectra of the nanoadsorbent prior and aer adsorption were measured on PerkinElmer Vertex 70 Spectrometer with wavenumber range of 500-3000 per centimetres (cm À1 ). The surface morphology, size distribution and the elementary conguration of the nanoadsorbent were determined using a high-resolution transmission electron microscopy (JEOL JEM-2100), which was equipped with energy dispersive X-ray (EDX) and a scanning electron microscopy (Leo-Zeiss). The surface area of the nanocomposite was determined using Brunauer-Emmett-Teller (BET) measured using nitrogen adsorption-desorption method at a lowtemperature on a Micromeritics ASAP 2020 (Micromeritics USA). The magnetic property of the nanocomposites was measured at room temperature using the Bruker-Electron Spin Resonance (ESR) spectrometer. The elementary composition of PPy@Fe 3 O 4 nanocomposite was also established using the X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra device.

Experimental setup
Experiments were conducted using a modied magnetic eld reactor shown in Fig. 1. The modied reactor was made up of Fig. 1 Magnetic field reactor. a 1.1 kW 8-pole three-phase induction motor, with the windings of the stator having height and width of 160 Â 14.39 mm. The reaction column with internal diameter, external diameter and height of 66 Â 100 Â 25 mm and with a maximum capacity of 100 mL, was held into the air-gap of the induction motor using the rotor of the induction motor, which was modied for this purpose. To supply power (current) to the windings of the stator, a 10 A variable AC power source was used. To maintain a constant temperature within the setup, a 220 volt, 34 watt fan mounted on a retort stand was used. The rotating magnetic eld generated by the stator windings in the air-gap was varied using the power source, with a maximum of 0.027 T magnetic eld measured using a digital gaussmeter.

Adsorption experiments inuenced by magnetic eld
To study the inuence of controlling parameters like pH, contact time, adsorbent dosage, and initial uoride concentrations on uoride ions adsorption under the effect of rotating magnetic eld (RMF) were conducted using the batch method.
To study the pH effect on the adsorption of uoride ions by the nanoadsorbent spun by a rotating magnetic eld of 0.019 T, the pH of 10 mg L À1 uoride solutions were adjusted from pH 2-10 using 0.1 M NaOH or HCI. The inuence of adsorbent dosage on uoride ions adsorption was performed under the inuence of a magnetic eld of 0.019 T by stimulation varying amount of adsorbent of 0.025-0.150 g with 10 mg L À1 at pH 6. To evaluate the magnetic eld effect on uoride ions (10 mg L À1 ) adsorption onto polypyrrole magnetic nanocomposite, the magnetic exposure time was varied between 20 and 60 min at varying MF of 0.012-0.026 T at pH 6. Aer adsorption experiments, each sample was withdrawn by means of a syringe and ltered through a 0.45 micrometre (mm) syringe lter. The uoride concentration in each sample was measured using the ionselective electrode (Thermo Scientic Orion ISE meter) with a low-level TISAB buffer. The percentage adsorption efficiency of uoride ions removed was calculated using eqn (1): where C initial and C equilibrium are the initial concentration and equilibrium concentration of uoride respectively. Each experiment was conducted in triplicate (three samples), with the average value reported to check for reproductivity of the data points, which are depicted in the respective graphs. The standard deviation was calculated and provided in the form of error bars in the plotted graphs.
To study the initial concentration effect on the adsorption of uoride ions under the inuence of the magnetic eld, uoride initial concentrations of 20-100 mg L À1 were spun with 0.1 g nanoadsorbent using a xed magnetic eld of 0.019 T for 24 hours. For kinetic experiments, 0.05 g of polypyrrole magnetic nanocomposite was mixed with uoride concentrations of 10-60 mg L À1 using a xed magnetic eld of 0.019 T at a magnetic exposure time of 5-90 min. The amount of adsorbate adsorbed by the adsorbent (q e ) and the amount of adsorbate adsorbed per unit mass of adsorbent at a given time (q t ) were evaluated using eqn (2) and (3): where C initial , and C equilibrium represents the initial and equilibrium concentrations of adsorbate solution (mg L À1 ), V is the volume of adsorbate (L) and m is the mass of adsorbent used (g).
where C initial and C time are the initial and the bulk phase uoride concentrations at time t (mg L À1 ), V is the volume of adsorbate (L) and m is the mass of the adsorbent used (g).
To understand the inuence of the nanoadsorbent surface charge on the removal of uoride, the nanoadsorbent point of zero charge (pH pzc ) was determined by altering the pH of 50 mL of 0.01 M NaCl solution between pH 2-12 using 0.1 M NaOH or HCl. PPy@Fe 3 O 4 nanocomposite dosage of 0.05 g was added to the NaCI solution and spun using a magnetic eld of 0.019 T for 3 hours. The nal pH was measured at the end of each experiment. The pHpzc of PPy@Fe 3 O 4 nanocomposite was determined from the plot of nal pH against the initial pH. Where the values of the nal pH against the initial pH were constant, the pHpzc of the adsorbent was determined.

Effect of Co-existing anions
The effect of co-existing anions (CI À1 , NO 3 À , SO 4 2À and PO 4 3À ) on the adsorption of uoride onto PPy@ Fe 3 O 4 nanocomposite under the inuence of 0.019 T MF was explored using 0.1 g to treat 10 mg L À1 solutions at pH 6 for 2 hours. The percentage of uoride removed under the effect of each anion was determined using eqn (1).

Desorption study
Desorption study was carried out using 0.1 g of PPy@Fe 3 O 4 nanocomposite for the adsorption of 10 mg L À1 (50 mL) of uoride at pH 6 under MF of 0.019 T. The PPy@Fe 3 O 4 nanocomposite aer adsorption was separated from the uoride solution using a 0.45 micrometre (mm) syringe lter. The adsorbent was then spun with MF in 50 mL distilled water to remove unadsorbed uoride. To desorb uoride from the adsorbent, the adsorbent was spun in a 0.   22 The average crystalline size (D n ) of the nanoadsorbent according to the FWHM of the (311) diffraction peak was determined using the Debye-Scherrer formula given in eqn (4):

Results and discussion
where k is the shape factor, l is the X-ray wavelength (1.540593 nm), b is the full-width half-maximum (FWHM) in radians, and cos q is the cosine of the Bragg angle. The average crystalline size of PPy@Fe 3 O 4 nanocomposite was determined to be approximately 19 nm, which is consistent with the TEM result. 23,24 Comparing the XRD patterns of Fe 3 O 4 and the magnetic nanocomposite before adsorption, the main component of the nanocomposite was observed to be the crystalline magnetite (Fe 3 O 4 ). 25 Shi in the characteristic peaks to higher intensity aer adsorption was observed with the application of MF (Fig. 2c). This was due to the interaction between the magnetic nanocomposite with the magnetised aqueous solution. 26 There was no visible modication in the XRD crystalline pattern of the nanoadsorbent prior and aer uoride adsorption under the effect of the magnetic eld.

FTIR spectra of adsorbent.
To determine the functional groups of individual material, infrared spectroscopy of PPy/Fe 3 O 4 nanocomposite, PPy/Fe 3 O 4 nanocomposite aer uoride ions adsorption, Fe 3 O 4 , and PPy were performed as shown in Fig. 3. The spectra of the adsorbent have characteristic peaks of the oxidised polypyrrole and Fe 3 O 4 . The observed adsorption peak at 546 cm À1 was ascribed to the vibration of the Fe-O band, which is a characteristic peak of Fe 3 O 4 (Fig. 3a). 27 The characteristic peaks of polypyrrole was observed at 1693, 1566, 1481, 1286, 1197, 1047, 924 and 795 cm À1 (Fig. 3b). 4,[28][29][30] The adsorption peaks observed at 1697, 1535, 1452, 1290, 1168, 1031, 957, 779 and 552 cm À1 were characteristic peaks of PPy/ Fe 3 O 4 nanocomposite before uoride adsorption. Aer uoride adsorption, most of the functional groups undergoes a red shi with the peaks of the PPy component of the nanocomposite shiing to 1699, 1542, 1463, 1297, 1168, 1089, 960, 785 and 552 cm À1 . These adsorption peaks were ascribed to the C]N bond, C]C stretch, C-N stretch, C-H or C-N in-plane deformation, C-C vibration, C-H in-plane deformation, C-C out of plane deformation vibration and out of plane C-H vibration of pyrrole. 21,[31][32][33] The change in the IR peak values aer adsorption was ascribed to the development of a chemical bond between functional groups existing on the nanocomposite and uoride.
3.1.3 TEM and SEM analysis of adsorbent. The TEM image of the adsorbent is shown in Fig. 4. The morphology and size of the adsorbent shows a core/shell structure, with the Fe 3 O 4 (core) being encircled by the PPy shell, with the PPy/Fe 3 O 4 nanocomposite being polydispersed (Fig. 4a). The nanocomposite was observed to be spherical in shape, with smooth uniform morphology, caused by the deagglomerating effect of the polymer coating the nanoparticles (Fig. 4b). The average particle size of the TEM image (Fig. 4a) was determined to be approximately 25 AE 11.7 nm using ImageJ soware. The average particle size distribution was tted using the Gaussian function shown in Fig. 4c. The EDX spectrum of the nanoadsorbent shows the presence of C, N, O, Fe and CI, which are the main elements the nanoadsorbent. The low intensity of the Fe and O lines indicated that the magnetic core of the nanocomposite (Fe 3 O 4 ) was appropriately covered by polypyrrole (Fig. 4d). The appearance of the uoride peak in the PPy/Fe 3 O 4 nanocomposite treated with uoride (Fig. 4e) conrms uoride adsorption onto the adsorbent. The Cu signal observed in the spectrum comes from the copper grid used for TEM.
3.1.4 BET analysis. The BET surface area (m 2 g À1 ), pore diameter (nm) and pore volume (cm 3 g À1 ) of the nanoadsorbent obtained from the nitrogen adsorption and desorption isotherms (Fig. 5). The result shows a characteristic type IV hysteresis loop, which shows the mesoporous characteristics of PPy@Fe 3 O 4 nanocomposite as categorized by IUPAC. The adsorbent has a hysteresis loop that closes at the relative pressure of 0.88P/P o . 34,35 The pore size distribution of the nanoadsorbent is depicted in Fig. 5b. The polypyrrole magnetic nanocomposite displays a predominant peak at 3.3 and 5.8 nm, with an average pore diameter of 6.2 nm. The BET surface area, pore volume and average pore diameter of PPy@Fe 3 O 4 nanocomposite are 28.77 m 2 g À1 , 0.06 cm 3 g À1 and 15.82 nm, respectively.
3.1.5 Magnetic properties of nanoadsorbent. The ESR spectra of the nanoadsorbent carried out at room temperature is shown in Fig. 6. The shape of the lines is symmetrical, with a resonance signal that is wide and broad. The resonance eld (H r ) and peak to peak linewidth before and aer adsorption were determined to be about 3039, 1350 and 1200 Gauss respectively. In signal identication, an unidentied signal can be of valuable assistance when the g-factor is determined. The gfactor for iron(III) (Fe 3+ ) was calculated to be 1.4-3.1 and 2.0-9.7 for low and high spin complexes. The g-factor of the nanoadsorbent before and aer adsorption was determined to be approximately 2.22 Gauss, which was due to Fe 3+ spin interactions. Such interactions show superparamagnetic behaviour categorized by the presence of clusters. The shapes and the eld location were equal to a typical magnetic nanoparticle suspension and are consistent with superparamagnetic iron oxide nanoparticles ESR spectra. [36][37][38][39] Fitting the line shape of the nanoadsorbent to the Gaussian and Lorentzian functions, a combination of 59% Gaussian function and 41% Lorentzian This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 595-609 | 599 function was observed (Fig. 6b). The Gaussian t was characteristic of ferromagnetic resonance, which signies that the main percentage of the PPy@Fe 3 O 4 nanocomposite was magnetic.  scan XPS spectra, the main elements of the adsorbent were C (271 eV), O (520 eV), Cl (200 eV), N (400 eV), and Fe (710 eV), which were attributed to C 1s, O 1s, Cl 2p, N 1s, and Fe 2p respectively. Aer adsorption of uoride, a new peak observed at 685 eV was attributed to F 1s. The existence of the uoride peak aer adsorption, conrms the adsorption of uoride onto the PPy@Fe 3 O 4 nanocomposite.

pH effect on adsorption of uoride ions
It is important to note that pH is a vital parameter that controls adsorption at the water-adsorbent interface, 42 as it affects the speciation of metal ions, the surface charge of nanoadsorbent and the degree of sorbent ionisation. 43 The pH effect on the uoride adsorption was conducted under the effect of an RMF of 0.019 T at a pH range of 2-10 using 0.1 M HCI or NaOH. The effect of pH on the adsorption of uoride was interpreted using the pH PZC of the nanoadsorbent and the pK a of HF. For HF, the dissociation factor is 3.17 as uoride ions occur as anions in solutions at pH > 3.17. The pH PZC of the nanoadsorbent was determined to be 3.2. At pH < 3.2, the nanoadsorbent have   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 595-609 | 601 a positive surface charge, while at pH > 3.2, the nanoadsorbent have a negative surface charge (Fig. 8a). From the result in Fig. 8b, the optimum removal of uoride ions was observed at pH 6. Outside this pH, the percentage of uoride ions removed was signicantly reduced at lower or higher pH values. The range of pH used has a trivial effect on uoride ions protonation and the nanoadsorbent surface chemistry. Hence, the improved removal observed at pH 6 was ascribed to the chemical speciation of uoride in water 3 and the nullication of the negative charge on the nanoadsorbent surface by greater hydrogen concentration at lower pH values. 44 The H + ions predominate at lower pH and form a bond with uoride to form HF. The pK a of HF is 3.16. When the solution pH was less than 3.16, the weakly charged HF ions dominate, the adsorbate was not readily adsorbed to the adsorbent, resulting in the decrease in uoride adsorption. But, when the pH value was increased above 3.16, uoride ions dominate, and uoride was readily adsorbed. Consequently, a steady rise in uoride removal was observed aer this pH value. 45,46 At high pH values, the stability of met-allouoro complexes declines and the free uoride anions dominates. The deprotonation of uoride adsorption sites at higher pH (>pH 6) leads to a decrease in the quantity of uoride ions removed, owing to the strong competition for binding sites on the adsorbent between OH À and uoride ions. 47-50

Effect of adsorbent dosage
The effect of adsorbent dosage on uoride adsorption was evaluated using the dosage range of 0.025-0.150 g under the effect of RMF of 0.019 T is shown in Fig. 9. A substantial intensication in the amount of uoride ion adsorbed onto the nanoadsorbent, with a percentage removal of 34.5-81.5% observed with a corresponding mass of nanoadsorbent used. This increase removal of uoride ions was due to increased adsorption sites on the nanoadsorbent surface and sorptive surface area as the adsorbent dosage was increased. The adsorption of uoride was relatively constant at higher dose because of saturation of pore volume and surface. 44

Effect of initial concentration
Result of the effect of initial adsorbate concentration (20-100 mg L À1 ) under the effect of RMF of 0.019 T, while keeping the pH and sorbent mass constant is shown in Fig. 10. With the rise in the initial adsorbate concentration, the exchange between adsorbate and adsorption sites on the PPy@Fe 3 O 4 nanocomposite was facilitated due to the diffusion of anions to the adsorption sites progressing more rapidly. The resistance of the mass transfer of uoride ions between the aqueous and solid phase can be overcome, by the initial adsorbate concentration providing the driving force needed. This result in improved equilibrium sorption pending the adsorbent saturation being attained. 40 3.5. Effect of rotating magnetic eld on uoride ions adsorption at magnetic exposure times Fig. 11 shows the results of RMF effect of 0.012-0.028 T on uoride ions adsorption onto the nanoadsorbent at magnetic exposure times of 20 and 60 min. Enhanced adsorption of uoride ions by the nanoadsorbent was observed as the intensity of RMF was enhanced. Improved adsorption was also observed at exposure time of 60 min (78.2%) when related to exposure time of 20 min (74.3%) at MF intensity of 0.019 T. A trivial drop in the amount of uoride ions removed was observed as the magnetic eld was increased from 0.019-0.027 T. The impact of the MF on polypyrrole magnetic nanocomposite behaviour in aqueous solution and during adsorption of uoride ions was determined in a cylindrical microchannel using a complementary metal-oxidesemiconductor (CMOS) camera placed in-line with a LED light at a frame rate of 100 fps. Magnetic force as a result of the eld gradient caused particles movement in the direction of higher eld strength. Fig. 11b and c show the transport of particles in the direction of low and high magnetic eld intensities (0.012 and 0.019 T). 51 When no MF was applied to the particles, the particles were observed to be polydispersed in the uoride solution as shown in Fig. 11d. The size of particles cluster formed depends on the velocity, with a large uniform cluster of particles travelling at a fast pace observed (Fig. 11c). At low MF This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 595-609 | 603 intensity, a non-uniform cluster of particles with slow dri velocities was observed. The particles spinning efficiency at low MF intensity was also observed to decrease signicantly owing to chain breakage (cluster of particles), as sections of these fragmented particles cluster were unt to return to the initial chain formation leading to the reduced external area for uoride ions adsorption. The lateral binding of chains driving into contact by the effect of MF was observed to signicantly increase the chain polydispersity with intensication in the magnetic eld leading to increase collision of chains and improved particle contact area with the uoride solution. 19,31,33

Adsorption isotherms
Information on the surface properties of the adsorbent, its behaviour and isotherm studies were evaluated in the presence of RMF as shown in Fig. 12, the Langmuir, Freundlich and Temkin isotherms models were tted to the experimental data. The Langmuir isotherm describes the adsorption of a solute from a liquid solution. The Freundlich isotherm is an experimental calculation used to dene the sorption on the varying surface. 52 The Temkin isotherm model provides valuable understanding into the adsorption mechanism, as the model describes the adsorbent adsorbate interactions and the adsorption energy. This model also assumes that the heat of adsorption of all molecules declines linearly with the rise in adsorbent surface exposure and the even distribution of binding energies up to the determined binding energy is characterized by adsorption. 53,54 The linearized form of all isotherm models are shown in eqn (5)- (7): ln q e ¼ ln K F þ 1 n ln C e (6) where q m (mg g À1 ), C e (mg L À1 ) and b (L mg À1 ) represents the maximum monolayer adsorption capacity, the equilibrium uoride concentration and the Langmuir isotherm constant relating to the affinity of the binding sites at a given temperature. K F and n are the Freundlich isotherm constants relating to the adsorption capacity and the adsorption intensity of the adsorbent. T is the absolute temperature (K), R is molar gas constant (8.314 J mol À1 K À1 ), K T (L mg À1 ) is the equilibrium binding constant corresponding to the maximum binding energy and b T (kJ mol À1 ) is the adsorption heat. The constants K T and b T were determined from the intercept and slope of the straight line of the plot of q e against ln C e . The adsorption energy (b T ) for polypyrrole magnetic nanocomposite under magnetic eld inuence was negative (exothermic reaction) as the b T value was higher than 8 kJ mol À1 which indicates a chemical reaction process, and uoride adsorption takes place on the varied surface of the adsorbent. 55,56 The correlation coefficients (R 2 ) of the Freundlich isotherm model (0.98381) was determined to be over and above that of the Langmuir isotherm (0.89992) and Temkin isotherm models (0.97579). The adsorbent adsorption capacity and the adsorption intensity (n) was determined from the linearized Freundlich isotherm model to be 1.11740 and 1.34171 at 304 K, which shows favourable adsorption of uoride ions by the magnetic nanocomposite under the inuence of MF (Table 1).

Adsorption kinetics in magnetic eld
An important parameter that represents the adsorption efficiency is the adsorption kinetics. 57 The most signicant feature in the design of adsorption systems is the estimation of the rate at which the adsorption process takes place for certain systems. 58 The rate of uoride adsorption onto polypyrrole  magnetic nanocomposite plotted as a function of initial uoride concentration is shown in Fig. 13a. Adsorption was observed to be relatively fast, reaching equilibrium at 10 min, as the adsorption capacity of the adsorbent increased with increase in time.
The time-dependent adsorption data were analysed using the linearized form of the pseudo-rst and second-order kinetic models were used. The linearize forms of the pseudo-rstkinetics and second-order-kinetic models are shown in eqn (8) and (9): where k 1 and k 2 are the rate constants of adsorption for the pseudo-rst and second-order kinetic models in g mg À1 min À1 , q t is the amount of uoride ions adsorbed by the adsorbent at time t in mg g À1 and q e is the adsorption capacity at equilibrium in mg g À1 . The correlation coefficient values (R 2 ) obtained for uoride adsorption using the pseudo-second-kinetic model were in the range of 0.99933 and 0.99999, with this model best describing the adsorption process taking place under the effect of the MF. The results show a decrease in the k 2 values with increase in the q e values as the initial adsorbate concentration is improved under the effect of MF (6.02-24.70 mg g À1 ) and 3.70 mg g À1 when no MF was applied (control). The estimated theoretical value of q e (second-order-kinetic model) was in excellent agreement with the q e values obtained experimentally.  Therefore, uoride adsorption onto PPy@Fe 3 O 4 nanocomposite in the presence of MF was controlled kinetically using the pseudo-second-order model instead of the pseudo-rst-order model, as the chemical adsorption was described by the second-order-kinetic model as the rate-limiting process. Due to a good degree of mixing and extent dispersion of nanoparticles using MF, adsorption process was affected by two mechanisms; which was the rapid adsorption due to electrostatic attraction followed by slow gradual adsorption of pollutants onto nanocomposite surface by complexation as seen in Table 2. The presence of anions in water can hinder the process of adsorption. The impact of co-existing anions such as chloride, nitrate, phosphate and sulphate on uoride adsorption onto PPy@Fe 3 O 4 nanocomposite was studied under the inuence of MF as shown in Fig. 14. Chloride and nitrate were observed not to have a signicant effect on uoride adsorption, while phosphate and sulphate had a signicant effect on uoride adsorption onto the adsorbent. The sequence of affinity for anion adsorption onto the nano adsorbent was in the order of CI À1 > NO 3 À > SO 4 2À > PO 4 3À . 60 Chloride and nitrate ions form external sphere surface complexes. Hence, they have less interference with the adsorption of uoride onto the adsorbent. But, uoride, sulphate and phosphate ions form both internal and external sphere surface complexes. Consequently, the presence of sulphate and phosphate ions slightly reduces uoride adsorption as a result of competitive interaction for the adsorption site. 40,61

Desorption study
The reuse of an adsorbent depends primarily on the simplicity with which the adsorbate is released from the active sites on the adsorbent. Result of the percentage of uoride removed from PPy@Fe 3 O 4 nanocomposite aer three cycles of desorption is shown in Fig. 15

Conclusion
The magnetic eld is considered as a potential technology to deal with the harmful effects of uoride ions in wastewater. From the results obtained, signicant improvement was observed from the amount of uoride ions removed using magnetic nanocomposite under the inuence of magnetic eld, with a maximum uoride ion adsorption observed at magnetic eld intensity of 0.019 T. At different magnetic exposure time, removals of 74.2-78.2% and 72.6-74.3% were observed at pH 6 at varying magnetic eld intensity. The adsorption process was best dened by the Freundlich isotherm model and the kinetics study results showed that the sorption process followed the pseudo-second-order kinetic model under the effect of the magnetic eld.

Conflicts of interest
There is no conict to declare.