Synthesis of nitrogen doped carbon quantum dots/magnetite nanocomposites for efficient removal of methyl blue dye pollutant from contaminated water

As a remedy for environmental pollution, a simple synthesis approach has been developed to prepare nitrogen doped carbon quantum dot/magnetite nanocomposites (Fe3O4@NCQDs NCs) using non-toxic and cost effective lemon juice as precursor for removal of organic dye pollutant. Fe3O4@NCQDs NCs were characterized by using UV-Vis spectroscopy, FTIR, XRD, FESEM, EDS, TEM, VSM and TGA/DTA. TEM results show spherical shaped Fe3O4@NCQDs NCs with an average particle size of 5 nm. Batch adsorption studies were done to investigate the tendency of the nanocomposites to remove representative methyl blue (MB) dye from aqueous solution. The effects of MB dye concentration, dosage of Fe3O4@NCQDs NC adsorbent, pH, contact time and temperature were optimized by varying one variable while all the other parameters were kept constant. The experiment showed rapid removal of MB dye within 20 minutes with an adsorption efficiency of over 90.84% under optimum conditions. The adsorption process fits the Freundlich isotherm model well with R2 and n values of 0.993 and 1.842, respectively, at 298 K indicating the feasibility of the adsorption process. The adsorption process is spontaneous and involves exothermic behaviour as confirmed by thermodynamic studies. From a kinetic study, it was found that the pseudo-second order model is more suitable to describe the adsorption process than the pseudo-first order model for adsorption of MB dye onto Fe3O4@NCQDs NCs.


Introduction
During the past decades, the release of large quantities of toxic, carcinogenic and non-biodegradable organic dye pollutants into aquatic systems has continuously increased due to rapid industrialization, civilization and agricultural activity. 1 The presence of organic dye pollutants even at trace levels in effluents is very dangerous to human and aquatic life.The organic dye pollutants remain highly visible, resistant to aerobic digestion and stable to oxidizing agents due to their complex chemical structure and synthetic origin. 2 Therefore, it is highly desirable to develop an environmental friendly, highly efficient and cost effective method for removal of organic dye pollutants from wastewater effluents even at trace levels.
Different treatment methods such as biological treatment, 3 occulation-coagulation, 4 adsorption, 5 membrane ltration 6 and oxidation 7 have been used to remove dyes from wastewater effluents.Among these methods, the adsorption method is the simplest, efficient and low cost method for removal of dyes from wastewater effluents with no generation of byproducts and various natural and synthetic materials have been used as adsorbents. 8,9Recently, nanoscience and nanotechnology research introduced nanoadsorbent with more efficient adsorption, low cost and recyclable properties.The most repeatedly investigated nanoadsorbent is magnetite nanoparticles (Fe 3 O 4 NPs) based nanomaterials due to their excellent magnetic, biocompatible properties, facile synthesis and ease with which they may be tuned and functionalized for specic applications. 10,11Moreover, the most important advantage of Fe 3 O 4 NPs is its easy separation and purication aer application by an external magnet due to its magnetic properties. 12,13owever, bare Fe 3 O 4 NPs could easily aggregate in aqueous system to reduce the energy associated with surface area to volume ratio and the strong dipole-dipole attraction between particles and easily undergo oxidation and thus limits their technological applications. 14,150][21][22][23] In preparation of Fe 3 O 4 -chitosan nanocomposites, the surface of magnetite modied with amine and hydroxyl groups on chitosan. 24As a class of newly emerging uorescent nanomaterials, carbon quantum dots (CQDs) have offered tremendous opportunities for a wide scope of applications due to its excellent properties like good stability and solubility in water, low cost and biocompatibility. 25,26Tremendous promise has been shown in different applications by compositing carbon quantum dots with nanoparticles. 27Similar to chitosan, nitrogen doped carbon quantum dots have reactive amine and hydroxyl groups which are amenable to chemical modications and therefore, in nanocompositing Fe 3 O 4 with nitrogen doped carbon quantum dots (NCQDs), amine and carboxyl groups on NCQDs modify the surface of Fe 3 O 4 and protect the nanocomposites from aggregation.In addition, nitrogen doped carbon quantum dots (NCQDs) can preserve structural stabilization of Fe 3 O 4 NPs as capping agent and improve the surface of the materials by providing important functional groups which are important for interaction of the nanocomposites with chemical pollutant in environment. 28n this paper, we report the design and synthesis of magnetic and eco-friendly Fe 3 O 4 @NCQDs NCs via coprecipitation using lemon juice as precursor.The as prepared Fe 3 O 4 @NCQDs NCs have been applied for efficient removal of methyl blue (MB) dye from contaminated water.The effects of various experimental conditions such as contact time, initial concentration, pH, temperature and adsorbent dosages on the removal efficiency of MB were evaluated through a batch adsorption experiments.

Chemicals and reagents
Iron(III) chloride hexahydrate (FeCl 3 $6H 2 O) and iron(II) sulphate heptahydrate (FeSO 4 $7H 2 O) were purchased from Merck, India.Fresh lemon fruits were purchased from the local store nearby Andhra University.100 mL stainless steel Teon lined autoclave was used for hydrothermal synthesis of NCQDs.Milli-Q water was used throughout the experiments.Ethylenediamine was purchased from LOBA Chemie, Mumbai, India.All the reagents used are analytical grade and used as received without any further purication.

Synthesis of NCQDs from lemon juice
Hydrothermal method was used to synthesis nitrogen doped carbon quantum dots (NCQDs) by taking 20 mL of lemon juice and 2 mL of ethylenediamine in a 100 mL Teon-lined stainless steel autoclave and heated at 200 C in furnace for 3 hours.The obtained black paste was dissolved in 15 mL of water and centrifuged at 3000 rpm for 15 minutes to remove insoluble matter.Dichloromethane was added to the brown solution formed and centrifuged at 3000 rpm for 20 minutes to remove unreacted organic moieties.The aqueous layer was separated from lower organic layer and centrifuged at 12 000 rpm for 20 minutes thrice to remove larger size particles and the brown yellowish solution was nally obtained.To further get the smaller particle size of NCQDs, cleaning was done using column chromatographic separation in help of silica gel and dichloromethane as solvent.The resulting NCQDs was characterized and used for preparation of novel Fe 3 O 4 @NCQDs NCs.

Synthesis of Fe 3 O 4 @NCQDs NCs
Syntheses of Fe 3 O 4 @NCQDs NCs were done via coprecipitation reaction.In the procedure, 100 mL aqueous solution of 2 : 1 molar ratio of metal salts Fe 3+ (1.1127 g FeCl 3 $6H 2 O) and Fe 2+ (0.5708 g FeSO 4 $7H 2 O) was added in 250 mL three neck round bottom ask and the reaction was carried out for one hour under constant stirring in atmospheric nitrogen at 80 C. To the reaction ask, 25 mL diluted NCQDs aqueous solution (5 mg mL À1 and 10 mg mL À1 ) was added and reaction continued for 30 minutes.Then 20 mL of 2 M NaOH was added drop wise.The reaction was allowed to continue under stirring for 2 hours at 80 C. Finally, the black precipitate was obtained and separated by decantation with help of external magnet, washed several times with Milli-Q water, and dried under vacuum at room temperature.Bare Fe 3 O 4 NPs was synthesized in the same procedure without using NCQDs.

Characterization
UV-Vis absorption spectra of the synthesised NCQDs, Fe 3 O 4 NPs and Fe 3 O 4 @NCQDs NCs were obtained using a UNICAM UV 500(Thermo Electron Corporation).Fourier transform infrared spectra (FTIR) were obtained over the range of 400-4000 cm À1 using a SHIMADZU-IR PRESTIGE-2 Spectrometer.X-ray powder diffraction (XRD) pattern were recorded using PANalyticalX'pert pro diffractometer using Cu-Ka1 radiation (45 kV, 1.54056 A; scan rate of 0.02 degree per s).The morphology and microstructures of the synthesized Fe 3 O 4 @NCQDs NCs were investigated by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM, Jeol/JEM 2100, LaB6) operated at 200 kV.Further morphology and composition of Fe 3 O 4 @NCQDs NCs were characterized using eld emission scanning electron microscopy (FESEM, Zeiss Ultra-60) equipped with X-ray energy dispersive spectroscopy (EDS).Magnetic property of the material was determined at room temperature using vibrating sample magnetometer (Lakeshore VSM 7410).Composition of the Fe 3 O 4 @NCQDs NCs was further conrmed by thermal analysis using thermogravimetric and differential thermal analysis (TGA/DTA) of Perkin Elmer STA 6000 with TG sensitivity of 0.2 mg and DTA sensitivity of 0.06 mV.

Adsorption studies
To study adsorption efficiency of Fe 3 O 4 @NCQDs NCs for representative methyl blue (MB) dye solution from polluted water, the usual batch adsorption experiments were carried out using a series of conical ask of 100 mL capacity under covered conditions to prevent contamination and removal of dye solution from the ask during stirring.The effects of MB dye concentrations, dosage of Fe 3 O 4 @NCQDs NCs adsorbent, contact time, pH and temperature were optimized by varying one variable while all other parameters kept constant.For isothermal studies, experiments were performed at 293, 298 and 303 K with various initial MB dye concentrations and optimum dosage of adsorbent, contact time and pH.The kinetic experiments were performed at optimum dosage, temperature, pH and dye concentrations at constant time intervals.In the procedure, 100 mL of 10 ppm dye solution was taken in ask and 50 mg of adsorbent added and stirred at 293 K temperature.Aer certain time (t) of adsorption, adsorbent was separated from solution using external magnet and the unadsorbed MB concentration in the solution was determined using a UV-Vis spectrophotometer at l max of 664 nm.The MB uptake and percentage adsorption were calculated using eqn ( 1) and (2).
where, C 0 and C e are the initial and equilibrium concentrations of dye in mg L À1 .Q e is the amount of dye in mg g À1 adsorbed onto unit mass of adsorbent at equilibrium.V is the volume of dye solution in millilitre (mL); and m is the mass of the adsorbent in gram (g).
3 Results and discussion

Synthesis and characterization
Fluorescent and highly water soluble nitrogen doped carbon quantum dots (NCQDs) were synthesized by hydrothermal method using lemon juice as precursor and ethylenediamine as coreagent (ESI: Fig. ESI1 †).In the process, the chemicals present in lemon juice such as citric acid and ascorbic acid undergoes carbonization forming amorphous graphitic carbon dots and then doped and functionalized by ethylenediamine to form NCQDs.The prepared NCQDs exhibits two typical absorption peaks at 245 nm and 353 nm as shown in Fig. 1a  Information regarding the surface functional group of NCQDs was investigated by Fourier transform infrared spectroscopy (FTIR).As can be seen in the spectrum (Fig. 1b), there are characteristic bands which can indicate the presence of C-O bond, OH, aliphatic C-H, N-H and C-N functional groups.][32] The morphological properties of NCQDs were conrmed by TEM (Fig. 2a)and as the result indicated, the NCQDs particles are well uniformly distributed quasi-spherical nanoparticles with narrow size distribution in diameter range of 2-9 nm with an average of 5.5 nm based on statistical analysis of more than 90 dots (Fig. 2b).The holes in the selected area electron diffraction (SAED) of the NCQDs (inset in Fig. 2 101) diffraction pattern of graphitic carbon, as shown in Fig. 2d which indicates the amorphous nature of the NCQDs. 33,34he functional group on NCQDs can play key role in preparing Fe 3 O 4 @NCQDs NCs via coprecipitation method, the presence of NCQDs avoid nucleation stage of coprecipitation and hence aggregation free nanocomposite formed.
A possible plausible formation mechanism of Fe 3 O 4 @-NCQDs NCs via this method is that the carboxylic and amino group on NCQDs chelated with Fe 3+ and Fe 2+ to form ferric and ferrous complex.In the presence of NaOH, there is also formation of bonds between OH À and (Fe 2+ , Fe 3+ ).With heating, HO À /Fe 3+ and OH À /Fe 2+ bonds dominate over COO À / Fe 3+ and COO À /Fe 2+ bonds, and as a result ferric hydroxide, Fe(OH) 3 and ferrous hydroxide, Fe(OH) 2 formed.Ferric hydroxide and ferrous hydroxide dehydrated forming magnetite (Fe 3 O 4 ) nanoparticle crystals upon heating.The carboxyl and amino group of NCQDs are attached on Fe 3 O 4 NPs surface through chelation to iron ions.To effectively form Fe 3 O 4 @-NCQD NCs, amino and carbonyl functional groups on surface of NCQDs interact with COO À /Fe 3+ and COO À /Fe 2+ through electrostatic interaction by forming bidentate coordinate covalent bond.As a result stable, relatively aggregate free and uniform sizes Fe 3 O 4 @NCQDs NCs were formed.The FTIR spectra of Fe 3 O 4 @NCQDs NCs synthesised using different proportion of NCQDs depicted in Fig. 3b.FTIR spectra show broad overlapping band around 3360 cm À1 , which can be attributed to the n (O-H) and n (N-H) stretching vibration of the hydroxyl and amine group of NCQDs.The band at 1050 cm À1 ascribed to the presence of an alcoholic C -O stretching. 35The bands at 1624 cm À1 and 1400 cm À1 are ascribed to asymmetric (n as ) and symmetric (n s ) stretching of the COO À respectively. 36he band at 1624 cm À1 is also due to the N -H bending mode of the amine group coupling with the n as C -O.The energy difference (Dn) between the n as (COO À ) and v s (COO À ) IR bands can reveal the interaction between the carboxylate head and the metal atom. 37The Dv (1624À1400 ¼ 224 cm À1 ) is ascribed to bridging and bidentate coordination, where the interaction between the COO À group and the Fe atom was covalent. 38The characteristic absorption peaks for Fe 3 O 4 NPs were observed at 580 cm À1 and 418 cm À1 which ascribed to the stretching vibrations of Fe 2+ -O and Fe 3+ -O bonds for Fe 3 O 4 NPs respectively. 39he crystallite structure of the as prepared NCQDs, Fe 3 O 4 NPs and Fe 3 O 4 @NCQDs NCs were determined by using X-ray diffraction (XRD) technique.Fig. 3d shows the XRD pattern of  220), (311), ( 222), ( 400), ( 422), ( 511), ( 440), ( 530), ( 622) and ( 444) respectively in which all peaks indexed to the inverse spinal phase of magnetite (JCPDS le, no.19-0629).In addition to Fe 3 O 4 NPs patterns, in the XRD pattern of Fe 3 -O 4 @NCQDs NCs (Fig. 3c), there is an additional weak peak at 22.64 which is characteristic of graphitic NCQDs and can be indexed to the (002) reection indicating good binding of Fe 3 O 4 NPs and NCQDs in formation of Fe 3 O 4 @NCQDs NCs.The sharp and strong peaks indicate high crystallinity of the as synthesized Fe 3 O 4 @NCQDs NCs. 40orphological study of the synthesised Fe 3 O 4 @NCQDs NCs was investigated using FESEM and TEM.As depicted in Fig. 4, FESEM images clearly showed that the Fe 3 O 4 @NCQDs NCs have nearly spherical shape with uniform distribution.The presence of iron (Fe), oxygen (O), carbon(C) and nitrogen (N) in EDS spectrum conrms the successful formation of Fe 3 O 4 @NCQDs NCs.
The representative TEM images of Fe 3 O 4 @NCQDs NCs were presented in Fig. 5a and b.It is clear from TEM images that Fe 3 O 4 @NCQDs NCs have spherical shape without any  aggregation.The particle size distribution of the as synthesized nanocomposites is shown in histogram (Fig. 5d) and the calculated average particle size based on over 100 particles is 5 nm.In addition, crystalline diffraction rings from the selected area diffraction (SEAD) patterns (Fig. 5c) demonstrated that the crystalline nature of the as prepared Fe 3 O 4 @NCQDs NCs.The inset in Fig. 5a obtained from HRTEM indicates the lattice space (0.44 nm) which is comparable to XRD results.

Effect of initial concentration.
To optimize this parameter, different concentrations (1.25-15 mg L À1 ) of 100 mL MB dye solution were used at (contact time ¼ 20 min), (pH ¼ 11), (adsorbent dose ¼ 50 mg) and (temperature ¼ 298 K).The effect of varying concentrations on adsorption is shown in Fig. 6a.The dye removal efficiency of Fe 3 O 4 @NCQDs NCs was dependent on the initial concentrations of the dye solution in that the maximum adsorption took place at 1.25 mg L À1 , which decreased up to 15 mg L À1 from 97.01 to 86.72% with increasing adsorbate concentration.The decrease in adsorption with an increase in dye concentration could be explained on the basis that MB removal depends on the availability of the binding sites on the Fe 3 O 4 @NCQDs NCs adsorbent surface.Total available adsorption sites for a xed amount of Fe 3 O 4 @NCQDs NCs were used at 7.5 mg L À1 concentration and therefore, 7.5 mg L À1 was taken as optimum initial concentration.
3.2.2Effect of adsorbent dosage.Optimization of dosage was carried out using 0.1-2.0g L À1 of Fe 3 O 4 @NCQDs NCs.MB dye concentration, pH, contact time and temperature were 7.5 mg L À1 , 11.0, 20 min and 298 K, respectively.The effect of dose on percept uptake of MB onto Fe 3 O 4 @NCQDs NCs is shown in Fig. 6b, indicating rapid increase in adsorption with increasing doses.MB adsorption increased from 60.28 to 94.49% at dosages of 0.1 and 2.0 g L À1 of Fe 3 O 4 @NCQDs NCs adsorbent.Adsorption was 90.84% at 0.5 g L À1 and on further increasing the dose, adsorption percentage was slightly increased with no signicance.Therefore, 0.5 g L À1 of Fe 3 -O 4 @NCQDs NCs was considered as optimum dosage.7.5 mg L À1 , 0.5 g L À1 , 11.00 and 298 K, respectively.The result showed that adsorption of the MB dye onto Fe 3 O 4 @NCQDs NCs adsorbent consisted of two phases; initial stage consisting of rapid adsorption (0-15 min) and nal stage with the relatively slow adsorption rate (Fig. 6c and d).The adsorption increased from 64.6 to 90.84% as the contact time was increased from 1 to 20 min.On further increasing the contact time up to 40 min adsorption increased to 91.78%, but this increase was insig-nicant and slow.Therefore, 20 min was considered to be the optimum time for MB dye adsorption onto Fe 3 O 4 @NCQDs NCs.
3.2.4Effect of pH.One of the most governing factors for removal of dye from water using adsorption process is pH. 41The effect of varying pH (2.0 to 12.0) on the adsorption of MB onto Fe 3 O 4 @NCQDs NCs was investigated with initial dye concentration of 7.5 mg L À1 , catalyst dosage of 0.5 g L À1 , and contact time of 20 min at 298 K (Fig. 7a and b).The pH of solution was adjusted using 0.1 M HCl/NaOH.The adsorption capacity increased continuously as the pH increased from 2-12.At acidic pH, lower adsorption of MB was observed due to the presence of excess H 3 O + ions competing with MB cations for the available adsorption sites which reduce the adsorbed amount.Therefore, pH of 11 was selected as optimum pH for adsorption of MB dye solution onto Fe 3 O 4 @NCQDs NCs.
3.2.5Effect of temperature.The adsorption studies were carried out at 293, 298 and 303 K, and the results of these experiments are presented in Fig. 7c.The solution temperature was controlled using water bath by using ice as cooling agent.The adsorption decreased almost for all concentrations of methyl blue when temperature was raised from 293 to 303 K.The decrease in adsorption with rise of temperature indicated exothermic nature of the adsorption process.

Isotherm modelling of adsorption
The adsorption data were analyzed by tting to isotherm models that are Langmuir, Freundlich and Temkin.The isotherm experiments were carried out at 293, 298 and 303 K with 100 mL MB solution of 2.5-10 mg L À1 concentrations, at solution pH 11.0 and adsorbent dosage of 0.5 g L À1 .
3.3.1 Langmuir isotherm model.The Langmuir model assumes that uptake of adsorbate occurs on a homogeneous surface by monolayer adsorption and that there is no interaction between adsorbent and adsorbate species. 42,43Langmuir mathematical equation is: where, C e (mg L À1 ) and Q e (mg g À1 ) have the usual meanings.Q 0 (adsorption capacity in mg g À1 ) is the amount of adsorbate that can be absorbed by a unit mass of the adsorbent for the formation of monolayer on the surface and 'b' is Langmuir constant, which is related to the affinity between the adsorbent and adsorbate.A separation factor which also known as dimensionless equilibrium parameter, R L and its value indicates the adsorption nature: unfavorable if R L > 1, linear if R L ¼ 1, favourable if 0 < R L < 1. 44 R L was calculated using the following equation: The calculated value of R L at 293, 298 and 303 K are 0.436, 0.428 and 0.414, respectively, indicating the suitability of Fe 3 -O 4 @NCQDs NCs for adsorption of MB dye solution from waste water.
3.3.2Freundlich isotherm model.The Freundlich model assumes that the uptake of adsorbate occurs on a heterogeneous adsorbent surface. 45The mathematical equation of Freundlich isotherm is expressed as: Freundlich constants, K F (adsorption capacity), and n (adsorption intensity) calculated from the slopes and intercepts of the Freundlich plots, log Q e vs. log C e (Fig. 8b).Magnitude of 'K F ' can be taken as a relative measure of adsorption capacity of Fe 3 O 4 @NCQDs NCs for the adsorption of MB.The Freundlich constant n (intensity of adsorption) varies with the heterogeneity of the adsorbent and for favourable adsorption 'n' values should be in the range 1-10. 46,47he values of Freundlich constant 'n' at 293, 298 and 303 K were 1.838, 1.842, and 1.811 are higher than unity suggesting feasibility of adsorption of MB onto the surface of Fe 3 O 4 @NCQDs NCs.The values of 'K F ' were 19.491, 17.069 and 16.317 mg g À1 , which clearly showed that 'K F ' decreased slightly from 293 to 303 K, indicating the decrease in the adsorption capacity at higher temperature.This is in agreement with Langmuir isotherm observations.The regression coefficients were more close to unity as compared to that of Langmuir isotherm showing better tting of the Freundlich model, which suggest adsorption of MB onto heterogeneous Fe 3 O 4 @NCQDs NCs surface.
3.3.3Temkin isotherm model.Temkin isotherm is based on the assumption that the heat of adsorption of all molecules in layer decreases linearly with coverage of adsorbent surface due to adsorbate-adsorbent interactions. 48The mathematical equation of the isotherm is expressed as: where, A T (L g À1 ) and b T (kJ mol À1 ) are Temkin isotherm constants, and 'R' and 'T' are the universal gas constant and absolute temperature (K), respectively.'A T ' is the equilibrium binding constant, related to the maximum binding energy and 'b T ' is a constant related to the heat of adsorption. 49he plot of Q e vs. ln C e is shown in Fig. 9c.The constants 'b T ' and 'A T ' were calculated from the slope and intercept of the plot, respectively, and are listed in ESI: Table ESI1 † along with regression coefficients.High magnitudes of 'A T ' and 'b T " indicated high interactions between MB and adsorbent.Therefore, the process might be chemisorption.The values of R 2 were well close to the unity showing good tting of the adsorption data to Temkin isotherm model.

Thermodynamic studies of adsorption
Thermodynamic studies are used to describe any reaction in a better way.In this work, thermodynamic studies were performed and the thermodynamic parameters (DG , DH , and DS ) were determined at 293, 298 and 303 K temperature using the equations: DG ¼ ÀRT ln K eq (9) ln K eq ¼ ÀDH /RT + DS /R (10)   where Q e is solid phase concentration at equilibrium (mg g À1 ), C e is equilibrium concentration of dye in solution (mg L À1 ) and K eq is equilibrium constant.The calculated free energy change (DG ), enthalpy change (DH ) and entropy change (DS ) parameters at different temperatures are presented in ESI: Table ESI2.† DH and DS values are calculated from the slope and intercept of the linear plots of ln K eq vs. 1/T, respectively (Fig. 8d).Negative values of DG indicated spontaneity and feasibility while negative values of entropy change DS and enthalpy change DH indicated the exothermic nature of adsorption of MB dye solution onto Fe 3 O 4 @NCQDs NCs.

Kinetics and mechanism of MB dye adsorption studies onto Fe 3 O 4 @NCQDs NCs
The kinetics of adsorption which shows the rate of transport of the dye from solution to the surface of adsorbent was investigated by pseudo-rst-order and pseudo-second order kinetic models.Linearized mathematical form of pseudo-rst order model 50 where k 1 (min À1 ) is pseudo-rst order rate constant, and Q t is the amount (mg g À1 ) of adsorbate on the adsorbent surface at time t.The slope and intercept of straight line plots of log(Q e À Q t ) vs. t (Fig. 9a) gave the values of (k 1 ¼ 0.228 min À1 ) and Q e (3.125 mg g À1 ), respectively, and are tabulated in ESI: Table ESI3.† R 2 ¼ 0.939 which is close to unity indicating tness of pseudo-rst order model.Linearized mathematical form of pseudo-second order model 51 where h is initial rate constant (h ¼ k 2 Q e 2 ) and k 2 is overall pseudo-second order constant.
According to the values of correlation coefficient and Q e (calc.)(ESI: Table ESI3 †) and Fig. 9b, the pseudo-second order model is found to be more suitable to describe the adsorption kinetic data than the pseudo-rst order model for adsorption of MB dye onto Fe 3 O 4 @NCQDs NCs.Hence, it has been conrmed that the adsorption process follows pseudo-second order kinetic behaviour.

Recyclability of adsorbent
Fe 3 O 4 @NCQDs NCs was used as adsorbent for removal of MB from aqueous solution.The used Fe 3 O 4 @NCQDs NCs can be easily separated from solution at end of application using external magnet.Recyclability of the Fe 3 O 4 @NCQDs NCs in MB removal was tested for 5 times and the results are depicted in Fig. 9c.The desorption process of MB was conducted by adjusting pH to 2 using 0.1 M HCl to remove MB form surface of Fe 3 O 4 @NCQDs NCs, then washing with acetone.The regenerated Fe 3 O 4 @NCQDs NCs adsorbent was used to evaluate the reusability of the Fe 3 O 4 @NCQDs NCs for adsorption times.The removal efficiency for the rst time 90.8% decreases to 79.2% at the h cycle.Our results suggested that the Fe 3 O 4 @NCQDs NCs can be reused over 5 times.

Conclusion
In conclusion, we have developed a cost effective method to prepare spherical shaped Fe 3 O 4 @NCQDs NCs using lemon extract as precursor.Superparamagnetic Fe 3 O 4 @NCQDs NCs have spherical morphology with an average particle size of 5 nm.Fe 3 O 4 @NCQDs NCs were used as adsorbent removal of MB dye pollutant from aqueous solution.Batch adsorption experiments showed enhanced rapid removal of MB dye within 20 minutes with adsorption efficiency of about 90.84% at optimum conditions.The adsorption data showed good tting to Freundlich with R 2 of 0.993 at 298 K temperature.The Freundlich constant 'n' value at 298 K was 1.842 which suggested the feasibility of adsorption of MB onto the surface of Fe 3 O 4 @NCQDs NCs.Thermodynamic studies indicated the spontaneity and exothermic nature of the adsorption process.From kinetics study, it was found that, the pseudo-second order model is more suitable to describe the adsorption kinetic data than the pseudo-rst order model for adsorption of MB dye onto Fe 3 O 4 @NCQDs NCs.
(black solid line) which extended with tail to visible region.The rst absorption peak at 245 nm could be assigned to p -p* transition of aromatic -C]Cbonds in the sp 2 hybridized domain of graphitic core and the other peak at 353 nm could be assigned to n -p* transition of -C]O, C-N, or -C-OH bonds which may be from hydroxyl (-COOH) or amine (-NH 2 ) groups on surface of NCQDs. 29The brown yellowish aqueous solution of NCQDs appears brilliant blue under ultraviolet irradiation (inset in Fig. 1(ii)) which indicate the bright luminescence of the prepared NCQDs.In Fig. 1a (blue broken line) indicate the emission spectra of the blue luminescent NCQDs; excitation at 360 nm and emission at 452 nm.
(a)) indicated the particle formation and only two bright spots observed showing the amorphous nature.Paper sheet layer like FESEM image in Fig. 2c conrmed the amorphous nature of NCQDs.Result from elemental composition analysis of EDS spectrum (Fig. ESI3a †) reveal the presence of C, O and N in the as synthesized material indicating well formation of nitrogen doped carbon quantum dots.X-ray diffraction (XRD) patterns show broad intense diffraction peak centered at 2q ¼ 23 and weak peak at 2q ¼ 42 which assigned to (002) and (

Fig. 1
Fig. 1 (a) UV-Vis absorption (solid black line) and emission spectra (blue broken line) inset at right (i) NCQDs solution at day light and (ii) NCQDs solution under ultraviolet radiation, (b) FTIR spectrum of NCQDs.
Fig. 3a shows the UV-visible absorption spectra of Fe 3 O 4 @-NCQDs NCs and Fe 3 O 4 NPs.The UV-visible spectrum show a broad absorption peak at 350 nm which extended to near IR region, which is primarily due to absorption and scattering of light by Fe 3 O 4 NPs. 14The strong absorption peak around 200 nm for Fe 3 O 4 @NCQDs NCs ascribed to p -p* transition of NCQDs, which indicates effective combining of Fe 3 O 4 NPs and NCQDs.

3 . 2 . 3
Effect of contact time.Time optimization for the maximum removal of MB dye onto Fe 3 O 4 @NCQDs NCs adsorbent was done by varying contact time (1-40 min).The initial concentration, adsorbent dosage, pH and temperature were
Both 'b' and 'Q 0 ' are characteristics of adsorbent and adsorbate pair.The plot of C e /Q e vs. C e at 293, 298 and 303 K is shown in Fig. 8a from which 'Q 0 'and 'b' values were evaluated from the slope and intercept of the plots.The values of 'Q 0 ' were observed to be 24.888,24.480 and 24.414 mg g À1 at 293, 298 and 303 K, respectively.These values slightly decreased with temperature, which indicated exothermic adsorption of MB onto Fe 3 O 4 @NCQDs NCs.The decrease in Langmuir constant, 'b' values from 2.679 to 1.834 with increase in temperature from 293 to 303 K indicated lower affinity of MB for Fe 3 O 4 @NCQDs NCs at higher temperature.The close to unity values of the regression coefficient, R 2 (0.946-0.989) indicated good ttings of Langmuir isothermal model (ESI: Table ESI1 †).

Fig. 7
Fig. 7 (a) UV-Vis absorption spectra at different pH (b) effect of pH on adsorption and (c) effect of temperature at different concentration on adsorption of MB on Fe 3 O 4 @NCQDs NCs.

Fig. 9
Fig. 9 (a) Pseudo-first order kinetic plot (b) pseudo-second order kinetic plot and (c) The recyclability of the Fe 3 O 4 @NCQDs NCs in the MB dye removal from aqueous solution for 5 successive cycles (Fe 3 -O 4 @NCQDs NCs ¼ 50 mg and MB ¼ 7.5 ppm).