Magnetite nanoparticle decorated reduced graphene oxide for adsorptive removal of crystal violet and antifungal activities

This work reports the synthesis and application of magnetic rGO/Fe3O4 NCs using a pod extract of Dolichos lablab L. as areducing agent. GO was synthesized by a modified Hummers method, however GO was reduced using the plant extract to produce rGO. The as-synthesized rGO/Fe3O4 NCs were characterized by UV-vis spectrophotometer, Fourier transform infrared (FT-IR) spectroscopy, FT-Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy supported with energy dispersed X-ray spectroscopy (FESEM-EDX), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The synthesis of magnetic rGO/Fe3O4 NCs was confirmed from characterization results of FT-Raman, TEM and VSM. The FT-Raman results showed the D and G bands at 1306.92 cm−1 and 1591 cm−1 due to rGO and a peak at around 589 cm−1 due to Fe3O4 NPs that were anchored on rGO sheets; TEM results showed the synthesis of Fe3O4 with an average particle size of 8.86 nm anchored on the surface of rGO sheets. The VSM result confirmed the superparamagnetic properties of the rGO/Fe3O4 NCs with a saturation magnetization of 42 emu g−1. The adsorption capacity of rGO/Fe3O4 NCs towards crystal violet (CV) dye was calculated to be 62 mg g−1. The dye removal behavior fitted well with the Freundlich isotherm and the pseudo-second-order kinetic model implies possible chemisorption. Besides, rGO/Fe3O4 NCs showed antifungal activities against Trichophyton mentagrophytes and Candida albicans by agar-well diffusion method with a zone inhibition of 24 mm and 21 mm, respectively. Therefore, rGO/Fe3O4 NCs can be used as an excellent adsorbent to remove organic dye pollutants and kill pathogens.


Introduction
Graphite oxide, also known as graphitic oxide or graphitic acid, is a compound of carbon, oxygen and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers. 1 Most procedures have been reliant on strong oxidizing mixtures containing one or more concentrated acids and oxidizing materials during the synthesis of graphitic oxide. 2,3 The bulk material disperses in basic medium to yield monomolecular sheets or one atom thick in a closely packed honeycomb two dimensional (2D) lattice known as GO. 4 Hence, graphene is the single-layer form of graphite. The combination of GO or rGO with magnetite produces Fe 3 O 4 -graphene hybrid which has a planar geometry, high conductivity, fascinating carrier transport properties, large surface area, strong magnetism, low cost and environmentally benign nature which has opened an opportunity for various applications. [5][6][7][8] Therefore, rGO/Fe 3 O 4 hybrids have reported for catalysis, water purication, sensors, toxic heavy metal removal, capacitors, bio-medical diagnosis therapy, microwave absorption, water desalination, photocatalysis and antimicrobial. 7,[9][10][11][12][13][14][15] The hybrid combination of magnetic Fe 3 O 4 NPs and graphene sheets best produces rGO/ Fe 3 O 4 NCs that have recognized properties such as great dispersibility, large surface area, superparamagnetism and fabulous extraction capacity. 16 The p-p stacking, hydrogen bonding and electrostatic interaction between rGO/Fe 3 O 4 NCs and dye molecules are responsible for the adsorptive removal of dye molecules from aqueous solution. 17 Moreover, preceding reports indicated that rGO/Fe 3 O 4 NCs can efficiently remove toxic heavy metals, 7,18 uoroquinolones, 19 organic dyes 16,20 and MB dye via degradation. 17 In general, centrifugation and ltration methods are used to separate the adsorbent material from aqueous solution 21 even though these applications are time-consuming and require extra cost. 22 Compared with traditional centrifugation and ltration methods, magnetic separation method is an efficient, fast and economic method for the separation of magnetic adsorbents from the medium aer the adsorption treatment of pollutants is completed. 23 Hence, it is important to synthesize magnetically separable and high surface area rGO/Fe 3 O 4 NCs for removal of water pollutants and treat pathogens. Diverse methods have been used to synthesize rGO/Fe 3 O 4 NCs, such as chemical co-precipitation, 13,14 solvothermal reduction, 11,24 chemical reduction, 7 and green synthesis. 12,15,25 Different types of adsorbents have been used as way of removal of organic compounds and metals from aqueous solution. For example, Harijan and Chandra 26 have stated facile fabrication of magnetite graphene composite through thermal reduction of graphene oxide to graphene in alkali medium with extreme sorption capacity for Cr 6+ (5.5 mg g À1 ) at pH ¼ 6.6. The graphene sheets prohibited agglomeration of the Fe 3 O 4 NPs while enabling a good dispersion of the Fe 3 O 4 NPs and at the same time the specic surface area of the composite is considerably enhanced. Similarly, Sun et al. 11 have reported one step solvothermal synthetic route of rGO/Fe 3 O 4 NCs for excellent removal of toxic dyes, i.e., rhodamine B and malachite green with removal efficiency of 91% and 94%, respectively. He et al. have reported waste biolms biosorbents to treat Cd 2+ from aqueous solutions. The maximum adsorption capacity of dry waste biolms for Cd 2+ is 42 mg g À1 when the initial concentration of Cd 2+ is 50 mg L À1 . 27 Bionanomaterials such as Saccharomyces cerevisiae and nano Fe 3 O 4 encapsulated in a sodium alginate-polyvinyl alcohol matrix and Penicillium doped with nano Fe 3 O 4 entrapped in polyvinyl alcohol-sodium alginate gel beads have been reported to remove atrazine from aqueous solution by biodegradation. 28,29 Moreover, synthesis of rGO/Fe 3 O 4 NCs through a facile and environmental benign approach is a prime concern for practical application. Plant extracts currently attract a tremendous research interest for synthesis of rGO/Fe 3 O 4 NCs owing to environmentally friendly and ability to reduce GO into rGO. Gurunathan et al. 30 have recorded reduction of GO into rGO with spinach leaf extract as a simultaneous reducing and stabilizing agent. The plant extracts of Solanum trilobatum 17 and Averrhoa carambola 12 were previously reported as reducing agent for synthesis of rGO/ Fe 3 O 4 NCs. To the best of our information, Dolichos lablab L. has not been reported to synthesize rGO or rGO/Fe 3 O 4 NCs. Dolichos lablab L. is a plant that is extensively scattered in India that contains lectins sugar in the form of mannose/glucose specic, 31 galactose specic, 32 crude lipid, crude protein, insoluble dietary bre, soluble dietary bre, carbohydrate, and amino acids. 33 For this reason, the existence of many alternative phytoconstituents of the plant motivated the researchers to prepare rGO/Fe 3 O 4 NCs for different applications.
Herein, in this study, the researchers have synthesized rGO/ Fe 3 O 4 NCs using a facile co-precipitation method of Fe 3 O 4 onto rGO sheet in the presence of extract of Dolichos lablab L. for the adsorptive removal of crystal violet (CV) and antifungal activity against pathogens. The pod extract of Dolichos lablab L. was used as reducing and capping agent during preparation of rGO and rGO/Fe 3 O 4 NCs. The crystal structure, electronic property and surface morphology of the as-synthesized materials were investigated by UV-vis, FT-IR, FT-Raman, powder XRD, FESEM-EDX, TEM and VSM techniques. The CV dye was selected as a model water pollutant to investigate the adsorption capacity of rGO/Fe 3 O 4 NCs. Besides, Trichophyton mentagrophytes and Candida albicans pathogens were chosen to examine the antifungal activities.

Materials
All the chemicals in this paper were analytical grade acquired from Merck, HiMedia and Sigma-Aldrich and used without further purication.

Preparation of aqueous pod extract of Dolichos lablab L
Pod layers were rst detached from the seeds and cut into pieces by hand and shade dried in the laboratory for 21 days. Dried pods were grinded into powder using the Bajaj (Gx8) mixer grinder. The optimization of percent aqueous plant extract was taken based on our previous work. 34 To prepare 1% pod extract of Dolichos lablab L., 1 g pod powder was poured into 250 mL Erlenmeyer ask containing 100 mL Milli-Q water and heated at 70 C for 20 min. The solution broth was permitted to cool and later ltered with Whatman no. 42 lter paper to create yellowish solution. Lastly, the plant extract suspension was preserved in a refrigerator at 4 C for further utilize.

Synthesis of graphene oxide and reduced graphene oxide
GO was processed by modied Hummers strategy. 1 To prepare rGO from GO, pod extract of Dolichos lablab L. was used as a reducing agent. 100 mg of GO was added into round bottomed ask containing 50 mL Milli-Q water and sonicated for 30 min. Meanwhile 10 mL of 1% pod extract of Dolichos lablab L. was added and reuxed at 80 C while stirring for 12 h to produce a black precipitate. Finally, rGO was collected by centrifugation at 12 000 rpm and dried at 100 C in vacuum oven.

Synthesis of rGO-magnetite NCs using pod extract of Dolichos lablab L
The rGO/Fe 3 O 4 NCs were fabricated through co-precipitation using FeSO 4 $7H 2 O and FeCl 3 $6H 2 O in the presence of GO in alkaline medium according to the procedure in ref. 35. One (1) mg mL À1 dispersed GO was exfoliated into 100 mL of iron source solution containing both FeSO 4 $7H 2 O and FeCl 3 $6H 2 O salts (1 : 2 molar ratio) and it was sonicated for 1 h. Ten (10) mL of 1% pod extract of Dolichos lablab L. solution was added slowly to the above suspension of GO at 30 C with vigorous stirring under nitrogen atmosphere for 1 h. In short period, 10 mL of ammonia solution was added drop wise to modify the mixture pH to a value of 10. Moreover, reducing agents in the plant extract may reduce Fe 3+ as well. Thereupon, the mixture was heated to 80 C for 12 h to form dark colored solution indicating formation of rGO/Fe 3 O 4 NCs. Finally, black colored rGO/Fe 3 O 4 NCs was produced, separated, washed with Milli-Q water and ethanol, and then dried in hot air oven at 60 C for 12 h.

Characterization
The UV-visible absorption spectra were recorded using a (UNI-CAM UV 500, Thermo Electron Corporation) spectrophotometer in the range of 200-800 nm. FT-IR spectrum was recorded over the range of 4000-400 cm À1 using a SHIMADZU-IR PRESTIGE-2 spectrometer. Raman spectrum was obtained using FT-Raman spectroscopy (Bruker RFS 27, USA) with laser source Nd (YAG 1064 nm) at 2 cm À1 resolution. 16+ mW laser power was irradiated on rGO/Fe 3 O 4 NCs to collect Raman spectrum over the wide range 4000-50 cm À1 . XRD patterns were recorded by PANalytical X'pert pro diffractometer at 0.02 degree per sec scan rate using Cu Ka 1 radiation (l ¼ 1.5406Å, 45 kV, 40 mA). TEM images were acquired through (TEM model FEI TECNAI G2 S-Twin) at an accelerating voltage of 200 kV. The morphology of the sample was characterized using FE-SEM (FE-SEM, Zeiss Ultra-60) equipped with EDX. VSM was used to examine the magnetic property of rGO/Fe 3 O 4 NCs (Lakeshore Cryotronics, Inc., Idea-VSM, model 7410, USA).

Batch mode adsorption studies
The adsorption experiments of CV dye on rGO/Fe 3 O 4 NCs took place with different initial concentrations of CV dye at different pH values at room temperature. 100 mL (5, 10, 15, 20 mg L À1 ) of CV dye solution was placed into 250 mL beaker and the pH (4-12) of the solution was maintained by using 0.1 M HCl/0.1 M NaOH solution. Aerward, 20 mg rGO/Fe 3 O 4 NCs was added to the beaker and kept agitated in an incubator for 220 min until equilibrium was established. The initial concentration of the dye was reported as Co and in the consecutive 20 min time interval, concentration of the dye was recorded as C e . The amount of CV dye adsorbed per gram of adsorbent at time, t, q t (mg g À1 ) and equilibrium, q e (mg g À1 ) were determined using (eqn (1) and (2)).
is the mass of rGO/Fe 3 O 4 NCs adsorbent (g) and 'h' is the dye removal percent (%). The adsorption isotherm was studied using Langmuir and Freundlich isotherm models, while, the kinetics was studied using pseudo-rst and pseudo-secondorder kinetic models.

Adsorption isotherms
To determine the maximum adsorption capacity of rGO/Fe 3 O 4 NCs, the adsorption isotherm was analyzed by Langmuir 36 and Freundlich 37 isotherm models. The Langmuir adsorption isotherm argues consistent adsorbent surface activity and unilayer adsorption, with partial adsorbent active sites, is revealed as in (eqn (4)).
where, 'C e ' is the equilibrium concentration of CV in the aqueous solution (mg L À1 ), 'K L ' is the Langmuir adsorption constant (L mg À1 ) correlated to heat of adsorption, 'q max ' is the maximal adsorption capacity of the adsorbent (mg g À1 ), and 'q e ' is the amount of CV adsorbed per mass of adsorbent at equilibrium (mg g À1 ).
The Freundlich isotherm signies surface heterogeneity of the adsorbent as well as multilayer coverage on the surface. The linear equation of Freundlich isotherm is expressed as in (eqn (5)).
where, 'q e ' is the quantity of dye adsorbed per unit weight of adsorbent (mg g À1 ), 'C e ' is the equilibrium concentration of dye in the bulk solution (mg L À1 ), 'K F ' is the Freundlich constant indicative of the relative adsorption capacity of the adsorbent (mg g À1 ) (L mg À1 ) 1/n and 'n' is adsorption intensity in the Freundlich equation.

Adsorption kinetics
The physical and chemical properties of the adsorbent as well as mass transfer mechanisms are among the several prominent parameters to determine the adsorption mechanism. To revise the effect of adsorption time on CV dye removal by rGO/Fe 3 O 4 NCs, the mechanism of the adsorption process was studied by tting pseudo-rst-order and pseudo-second-order reactions to the experimental data. The adsorption equilibrium time of CV by rGO/Fe 3 O 4 NCs was 200 min. The Lagergren pseudo rstorder kinetic model 38 is known as in (eqn (6)).
where, 'q e ' and 'q t ' are amount of CV dye adsorbed at equilibrium and time, t (mg g À1 ), 'k 1 ' is rate constant of pseudo-rst-order kinetic model (min) and 't' is time (min). Likewise, the pseudosecond-order kinetic model 39 is expressed as in (eqn (7)).
where, 'k 2 ' is the rate constant of pseudo-second-order (g mg À1 min À1 ), 'q t ' is the amount of CV dye adsorbed on surface of rGO/Fe 3 O 4 NCs at time, t (mg g À1 ), 'q e ' is the equilibrium sorption capacity (mg g À1 ).

Antifungal activity against Trichophyton mentagrophytes and Candida albicans
The antifungal activity was carried out using agar-well diffusion method by employing 24 h cultures with given rGO/Fe 3 O 4 NCs. 40 The medium was sterilized by autoclaving at 120 C (15 lb per in 2 ). About 20 mL of the nutrient agar medium/potato dextrose agar seeded with the respective fungal strains were transferred aseptically into each sterilized Petri plate. The plates were le at room temperature for solidication. Each plate, a single well of 6 mm diameter was made using a sterile borer. The rGO/Fe 3 O 4 NCs were freshly reconstituted with suitable solvent (DMSO) and tested at various concentrations (2.5 mg mL À1 , 5 mg mL À1 , 10 mg mL À1 ). The samples (50 mL) and the control along with standard (clotrimazole (5 mg mL À1 )) were placed in 6 mm diameter well. Then the fungal plates were incubated at 28 AE 2 C. Activity diameter of the zone of inhibition was measured using Himedia antibiotic zone scale.   In this study, one pot coprecipitation green method was used to synthesize rGO/Fe 3 O 4 NCs using iron salts (Fe 3+ /Fe 2+ ) in 2 : 1 molar ratio, GO and pod extract of Dolichos lablab L. A systematic schematic representation of synthesis of rGO/Fe 3 O 4 NCs for the possible application of removal of CV dye is represented in Fig. 1. Fig. 2 depicts the absorption peak of the synthesized GO, rGO and rGO/Fe 3 O 4 NCs by using UV-vis spectrophotometer. The absorption peak of GO at 228 nm is due to p to p* transition of the aromatic C]C bond, 41 Fig. 2(a). The reduction of GO using plant extract red shied the peak to 245 nm is possible indication of synthesis of rGO, Fig. 2(b). Furthermore, a peak is observed for rGO/Fe 3 O 4 NCs at around 267 nm, Fig. 2(c). Fig. 3 Fig. 3(a)). GO showed characteristic peaks at 3280 cm À1 , 1702 cm À1 and 1194 cm À1 due to O-H, C]O and epoxy functional groups, respectively, 42 Fig. 3(b). The peak intensity 1194 cm À1 is very much decreased in the rGO spectrum implies pod extract of Dolichos lablab L. is excellent reducing agent. The O-H stretching peaks in Fig. 3(a-c)     functional group was used as reducing agent. In addition, FT-IR spectrum of rGO/Fe 3 O 4 NCs showed characteristic peaks at 1184 and 820 cm À1 due to C-O stretching and CH 2 rocking, respectively. The two peaks at 509 and 403 cm À1 were due to Fe-O stretching in rGO/Fe 3 O 4 NCs, 43 Fig. 3(d and e). The plant extract was used as reducing agent to facilitate reduction and formation of Fe 3 O 4 NPs on the surface of rGO sheets without the need to use hazardous reducing agent such as hydrazine.

FT-IR analysis.
3.1.3. FT-Raman analysis. FT-Raman spectroscopy is used to characterize carbon containing materials and to identify amorphous and crystalline carbon structures. 44 GO was allowed to react with Fe 2+ /Fe 3+ solutions in the presence of pod extract of Dolichos lablab L. as a reducing agent to produce rGO/Fe 3 O 4 NCs. The alkaloid, phenol, avonoid, amino acid, protein, terpenoid and saponin are the already identied constituents of Dolichos lablab L. that are responsible for reducing GO to rGO. 45 The FT-Raman spectrum of rGO/Fe 3 O 4 NCs is depicted in Fig. 4 and shows a clear formation of the nanocomposite. The FT-Raman shis at 1306.92 cm À1 (D band) and 1591 cm À1 (G band) had ratio of I D /I G ¼ 1. 19. The D band is assigned to the breathing mode of the k-point phonons with A 1g symmetry of disordered graphite structure, whereas the G band introduces the E 2g vibration mode between two sp 2 carbon atoms. 46 The three peaks at 269, 589, and 767 cm À1 were related to Fe-O vibration of E g and A 1g modes in Fe 3 O 4 . Hence, this result also proved magnetite nanoparticles were decorated on surfaces of rGO sheets. 47 Similarly, two peaks were absorbed at 1591 and 1307 cm À1 for magnetite-rGO composites synthesized by using hydrazine hydrate as a reducing agent. 7 The G band of rGO (1601 cm À1 , not shown here) is red shied by 10 cm À1 to 1591 cm À1 in rGO/Fe 3 O 4 NCs.
3.1.4. Powder XRD analysis. The phase structures of GO, rGO, Fe 3 O 4 NPs and rGO/Fe 3 O 4 NCs were characterized through powder XRD as indicated in Fig. 5. The XRD pattern of GO showed strong and intense peak at 2q ¼ 10 with a lattice reection of (001), Fig. 5(a). The XRD analysis proved formation of rGO and displayed a broad and intense peak at 2q ¼ 21 with a lattice reection of (002) in Fig. 5(b). Similar broad peak was observed in the angle between 15 -30 during synthesis of rGO using heating coconut shell. 48 The XRD pattern of  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 34916-34927 | 34921 and 62.9 with diffraction lines matching to (220), (311), (400), (422), (511) and (440), respectively, are consistent with the standard XRD data for the face centered cubic (fcc) Fe 3 O 4 structure (JCPDS card no. 19-0629). The peak at 2q ¼ 21 (002) which is seen in the XRD pattern of rGO/Fe 3 O 4 NCs (Fig. 5(d)), conrmed Fe 3 O 4 NPs were anchored on the surfaces of rGO sheets. 49 The average crystallite size of the manufactured rGO/ Fe 3 O 4 NCs was calculated to be 38 nm using the well-known Scherrer equation (eqn (8)).
where, 'D (hkl) ' is the average crystallite diameter (nm), 'k' is Scherrer constant (0.94), 'l' is the X-ray wavelength, 'b' is the half width of XRD diffraction line, and 'q' is the Bragg's angle in degrees.
3.1.5. Morphology and structural analysis. The morphological looks and surface characteristics of rGO/Fe 3 O 4 NCs were studied by using FESEM-EDX. Fig. 6(a-c) depicts FE-SEM images of Fe 3 O 4 NPs that were consistently distributed on the surface of rGO sheets. The size of the Fe 3 O 4 NPs from the image sources range from 9.75 to 14.85 nm. The EDX spectrum in Fig. 6(d) clearly identied the elemental existence of Fe, C and O with 27.94%, 34.51% and 37.55% by atomic mass, respectively. The TEM analysis was used to study the morphology and size of the synthesized rGO/Fe 3 O 4 NCs. Fig. 7(a and b) clearly shows spherical shaped Fe 3 O 4 NPs are homogeneously anchored at the surface of disorderly distributed rGO sheets. The SAED patterns of rGO/Fe 3 O 4 NCs in Fig. 7(c) showed both polycrystalline nature of Fe 3 O 4 and disorderly oriented nature of rGO sheets. The average particle size of Fe 3 O 4 in rGO/Fe 3 O 4 NCs was calculated to be 8.86 nm using (ImageJ) soware, Fig. 7(d).
There was small sign of aggregation of Fe 3 O 4 NPs observed on the surface of rGO sheets. Hence, the large surface area to volume ratio of the NCs causes the rGO/Fe 3 O 4 NCs to have an efficient adsorption property towards CV dye pollutant.
3.1.6. VSM analysis. The magnetic property of rGO/Fe 3 O 4 NCs was measured by VSM at room temperature and conrmed its superparamagnetic property. Fig. 8 shows the hysteresis loop of rGO/Fe 3 O 4 NCs with a magnetic saturation (M s ) 42 emu g À1 which was lower than that of L-Met capped Fe 3 O 4 NPs 50 indicated Fe 3 O 4 NPs were anchored on the surface of rGO sheets.

Adsorption study of CV dye
3.2.1. Adsorption isotherms. Fig. 9(a) shows UV-vis absorption spectra of CV dye adsorbed on rGO/Fe 3 O 4 NCs as a function of time, while Fig. 9(b) shows effect of pH on dye removal. The increase in the pH of dye solution (4,7,10,12) resulted increase in the dye removal efficiency of the adsorbent by (34,64,90, 95%), respectively. The dye removal efficiency is lower at pH ¼ 4 because the rGO/Fe 3 O 4 NCs surface is positively charged. Similarly, Duman et al. 21 have reported adsorption of CV dye onto magnetic OMWCNT-Fe 3 O 4 NPs is lowest at pH ¼ 2  (smaller than pH PZC ). This is due to the electrostatic repulsion between positively charged adsorbent surfaces and positively charged CV dye. However, at the higher pH solution (pH ¼ 12), the surface of the rGO/Fe 3 O 4 NCs became negatively charged which resulted to electrostatic forces of attraction between CV dye molecules and the adsorbent that increased adsorption efficiency to 95%. Fig. 10, depicts adsorption isotherm of CV dye on rGO/Fe 3 O 4 NCs using different initial CV dye concentration at pH ¼ 10. Dye adsorption increased with increasing initial concentration of organic dye pollutant. Fig. 11, describes Langmuir and Freundlich adsorption isotherm models of CV on the surface of rGO/Fe 3 O 4 NCs. The correlation coefficients (r 2 ) and other parameters from the models are presented in Table 1. The adsorption isotherm better tted to Langmuir (r 2 ¼ 0.998) than Freundlich (r 2 ¼ 0.992). Hence, the adsorption mechanism was monolayer, i.e., physical intermolecular attraction between the adsorbent and adsorbate predominated. Similar results were reported for the adsorption isotherms of various pollutantadsorbent systems. 51,52 The monolayer adsorption maximum capacity (q max ) of CV on rGO/Fe 3 O 4 NCs was found to be 62 mg g À1 . Hence, current study shows rGO/Fe 3 O 4 NCs has better adsorption capacity than Fe 3 O 4 NPs alone towards CV dye 53 due to increased number of sites of functional groups in rGO/Fe 3 O 4 NCs.
3.2.2. Adsorption kinetics. The linear plots of pseudo-rstorder and pseudo-second-order kinetic models for adsorption of CV dye on rGO/Fe 3 O 4 NCs are shown in Fig. 12. The kinetic model parameters and constants for the adsorption of CV dye on the surface of the adsorbent at room temperature are represented in Table 2. Adsorption of CV dye on surface of rGO/ Fe 3 O 4 NCs follows pseudo-second-order kinetics implies that the rate determining step is chemisorption. 54 Similar results were reported for the adsorption kinetics of various pollutants onto activated carbon cloth. 55-57 Therefore, adsorption was facilitated due to the interaction of cationic dye (CV) and rGO     Pseudo-rst-order Pseudo-second-order k 1 (min À1 ) q t , cal (mg g À1 ) r 2 k 2 (g mg À1 min À1 ) q t , cal (mg g À1 ) h (mg g À1 min À1 ) r 2  mentagrophytes with inhibition zone of 16 AE 0.8 mm, but no zone of inhibition was observed against Candida albicans. In addition, green synthesized ZnS NPs using Phyllanthus niruri plant extract and thorn-like ZnO NPs synthesized by sol-gel method both showed antifungal activity against Candida albicans with maximum inhibition zone of 32 mm and 20 AE 1.5 mm, respectively. 63,64 Hence, the as-synthesized rGO/Fe 3 O 4 NCs have showed better antifungal activity against the above studied fungus and therefore can be used as antimicrobial applications.

Conclusions
Eco-friendly, cost-effective and single-step green coprecipitation synthesis of magnetically separable rGO/Fe 3 O 4 NCs using pod extract of Dolichos lablab L. as reducing agent was reported. The result of TEM analysis showed the average particle size of Fe 3 O 4 NPs in the as-synthesized rGO/Fe 3 O 4 NCs was 8.86 nm. The maximum CV dye removal efficiency of the adsorbent was calculated to be 95% at pH ¼ 12. The adsorption mechanism well tted to Langmuir than Freundlich isotherm and followed pseudo-second-order kinetics. Dolichos lablab L. mediated rGO/Fe 3 O 4 NCs can be used as efficient removal of toxic dyes and metals from contaminated water and antifungal against microorganism pathogens.

Conflicts of interest
There are no conicts to declare.