Biosynthesis and Characterization of Iron Nanoparticles for Effective Adsorption of Cr ( VI )

In this study, iron nanoparticles (FeNPs) were synthesized via a green method using loquat (Eriobotrya japonica) leaves aqueous extract as a renewable reducing agent. .e synthesized FeNPs were characterized by DLS, XRD, FT-IR, SEM/EDX, and TEM analysis, and then, they were used as an adsorbent for Cr(VI) removal from aqueous solutions. Batch adsorption experiments were carried out to investigate the optimum adsorption parameters such as the initial pH of the solution, temperature, initial Cr(VI) concentration, and adsorbent concentration. .e optimum adsorption conditions were determined as initial pH 3.0, temperature 45°C, and adsorbent concentration 1 g/L. Also, a linear increase was observed in adsorbed Cr(VI) amounts with the increasing initial Cr(VI) concentrations..e biosynthesized FeNPs showed the high removal levels higher than 90% for Cr(VI) adsorption at a wide range of initial Cr(VI) concentrations (50–500mg/L). .e experimental equilibrium data were modelled with Langmuir and Freundlich isotherm models, and it was found that experimental equilibrium data could be well described by the Langmuir isotherm model. .e maximum monolayer coverage capacity of FeNPs for Cr(VI) adsorption was found to be 312.5mg/g. .e pseudo-first-order and the pseudo-second-order kinetic models were applied to the experimental adsorption data, and it was concluded that the data were defined as the best agreement with the pseudo-second-order kinetic model. Weber–Morris model was used to investigate the effect of mass transfer on the adsorption of Cr(VI) onto FeNPs; it was observed that both the film (boundary layer) and intraparticle diffusion affected the studied adsorption process. .e thermodynamic studies suggested that Cr(VI) adsorption onto FeNPs was endothermic and nonspontaneous, and the positive ΔS value indicated increased disorder at the solid-solution interface during the adsorption.


Introduction
Water pollution is one of the most important burning environmental issues among the various kinds of environmental pollution since water resources are contaminated by a variety of organic, inorganic, and biological pollutants due to day-to-day utilization and potential applications of water at domestic, industrial, and agricultural areas all over the world [1,2].
e heavy metals (arsenic, zinc, copper, nickel, mercury, cadmium, lead, and chromium) are the best known nonbiodegradable water pollutants which are extremely toxic and dangerous for the ecosystem, agriculture, and human health [3].
e chromium ions exist in industrial wastewaters both trivalent and hexavalent forms.Cr(VI) ion exists in different depending on the pH and the Cr(VI) concentration.Otherwise, Cr(III) ion is relatively less toxic than Cr(VI) ion.e Cr(III) species in aqueous solutions is present in the forms of Cr 3+ , Cr(OH) 2+ , Cr(OH) 2+ , Cr(OH) 3 , or Cr(OH) 4−  depending on the solution pH [4,5].
Cr(VI) is widely used in paints and pigments, chrome chemicals, leather tanning, electroplating industries, cement, and photography industries that lead to production of great quantities of effluents containing the toxic Cr(VI) ions [6].Moreover, Cr(VI) is classified as the top 16th hazardous substance by the Agency for Toxic Diseases Registry (ATSDR) due to its carcinogenic properties and toxicity degree [4].It is quite harmful to the living organisms because it has the potential to pass through cell membranes and reduces to reactive intermediates.
ese reactive intermediates can attack DNA, proteins, and membrane lipids, and they cause damage to the cellular functions [7].e acceptable discharge level for the hexavalent chromium ion in industrial wastewaters is reported as 0.1 mg/L by the United States Environmental Protection Agency (US EPA), and the permitted Cr(VI) ion limit in drinking water is 50 µg/L according to the World Health Organization (WHO) data [4,5].erefore, to reduce Cr(VI) ion to the standard level, effective, eco-friendly and economic treatment methods should be improved.
ere are commonly used treatment methods for discharging chromium ions from wastewaters such as adsorption, biosorption, coagulation, chemical precipitation, reverse osmosis, ion exchange, extraction, and membrane filtration techniques.Among them, adsorption offers a good opportunity for the removal of the chromium ion from wastewaters because of its low cost, simple operation, the capability of the trace level pollutant removal, and high efficiency [4,6].
Recently, the usage of metal/metal oxide nanoparticles such as zero-valent iron, Fe 3 O 4 , TiO 2 , ZnO, and their composites as a sorbent has attracted great attention in the treatment of various heavy metals like chromium.e advantage of these functional nanomaterials is high adsorption capacity that is associated with their large specific surface area and high reactivity in the adsorption processes [3].
Biosynthetic approaches are quite popular for the synthesis of metal nanoparticles using microorganisms, enzyme, and plant extracts, due to their predominant advantages compared to traditional chemical synthesis methods including toxic and expensive compounds such as NaBH 4 as the reducing agent.Among biosynthetic procedures, the exploitation of plant extracts has a privileged position for nanoparticle preparation since plant leaves are inexpensive, natural, environmentally friendly, harmless, readily available, and suitable for scale-up steps.Loquat (Eriobotrya japonica) is an Asian yellow fruit belonging to the Rosaceae family, and its leaves are used widely for medicinal purpose against cough and asthma diseases in Turkey [8,9].Loquat leaves are very rich in terms of phenols, flavonoids, triterpene acids, and polysaccharides.ese special compounds act as a bioreduction agent for metallic ions.
To the best of our knowledge, loquat leaves extract was used for the synthesis of silver and gold nanoparticles [10,11], but this is the first study reported in literature about usage of loquat leaves aqueous extract for the green synthesis of FeNPs.However, biosynthesized zero-valent iron nanoparticles [12]; crystalline and amorphous Fe and Al-oxides such as hematite [α-Fe 2 O 3 ], goethite [α-FeOOH], and alumina [α-Al 2 O 3 ] [4,13]; and composite materials including iron minerals such as Fe 3 O 4 magnetite graphene oxide encapsulated in calcium alginate beads [7] and Femodified activated carbon [14] were reported as effective adsorbents for removal of Cr(VI) ions.
e aim of the present study was the synthesis of iron nanoparticles (FeNPs) using Eriobotrya japonica leaves aqueous extract as a reducing/capping agent and investigating the Cr(VI) adsorption efficiency.For this purpose, the systematic characterization studies of synthesized FeNPs were carried out by DLS, XRD, FT-IR, SEM/ EDX, and TEM analysis methods, and then, the adsorbent properties of FeNPs for Cr(VI) removal were investigated in a batch system.

Materials and Methods
2.1.Materials.All the chemicals used in experiments were of analytical grade, and they were used without further purification.FeSO 4 •7H 2 O to be used in the synthesis of FeNPs and 1,5-diphenylcarbazide were supplied from Carlo Erba and acetone (>99.5%), and HCl (37%) were provided from Sigma-Aldrich.A stock solution of 1000 mg/L of Cr(VI) was prepared by dissolving the appropriate amount of K 2 Cr 2 O 7 (Carlo Erba) in 1000 mL of distilled water.e solution with desired Cr(VI) concentration was prepared by appropriate dilutions from stock Cr(VI) solution.e pH values of the solutions were adjusted with 0.1 N hydrochloric acid and 0.1 N sodium hydroxide.

Biosynthesis of FeNPs. Loquat (Eriobotrya japonica)
leaves were collected from loquat trees grown in Mersin, Turkey.Aqueous extract of E. japonica leaves was prepared by heating a certain amount of air-dried leaves as its concentration will be 60 g/L, at 100 °C, with 500 ml of distilled water for 1 h.Subsequently, the filtered aqueous extract was added to 0.1 mol/L FeSO 4 •7H 2 O solution in 1 : 1 volume ratio, and the final mixture was kept under vigorous stirring for 3 h [15].e immediate changing of the colour of the solution from clear to intense black indicated the formation of FeNPs.e formed nanoparticles were separated by evaporating water from solution on a hot plate and then collected nanoparticles, washed several times with deionized water, and dried in an oven at 120 °C overnight.

Characterization of FeNPs.
e mean hydrodynamic particle-size and size distribution of the green-synthesized FeNPs were measured by dynamic light scattering (DLS) analysis with the Malvern Zetasizer.e functional groups of FeNPs before and after adsorption were determined by Fourier transform infrared (FT-IR) spectrometer in the range of 4000-400 cm −1 .e crystal structure was observed by X-ray powder diffraction (XRD) analysis, using nickelfiltered Cu Kα radiation in a Philips XPert MPD apparatus operated at 40 kV and 30 mA, in the 2θ range of 10 °-90 °. e morphology of the FeNPs was analyzed by scanning electron microscope (SEM) analysis with Zeiss/Supra 55 SEM and transmission electron microscope (TEM).For the TEM study, nanoparticle samples were sonicated in analytical grade methanol and mounted on 200-mesh formwarecoated copper grid and allowed to dry.TEM images were obtained using the instrument (JEOL JEM 1011 TEM) which was operated at an acceleration voltage of 80 kV, and mean diameters of particles were determined using the Image J software program.

2
International Journal of Chemical Engineering e elemental identification and quantitative compositional information of the adsorbent were determined by Zeiss/Supra 55 energy dispersive X-ray analyzer (EDX) before and after adsorption.e pH at the point of zero charge (pH PZC ) of prepared FeNPs was obtained by measuring the ζ (zeta) potential at different ranges of pH (2.5-12) with Zetasizer (Malvern Zetasizer Nano ZS model).
e pH values of the FeNPs suspensions were adjusted from 2.5 to 12 by adding 0.1 M HCl or 0.1 M NaOH solutions as required.

Batch Adsorption Studies.
e adsorption experiments were carried out in 250 mL Erlenmeyer flasks containing 100 mL of Cr(VI) adsorption solution.0.1 g of FeNPs, except for adsorbent concentration experiments, was contacted with 100 mL of Cr(VI) solution at the known initial Cr(VI) concentration and initial pH.en, the flasks were agitated at a constant temperature and shaking rate for 480 min which is more than ample time for adsorption equilibrium.Samples were taken before mixing the FeNPs, metal bearing solution, and at predetermined time intervals for determining the unadsorbed metal ion concentrations in the solution.Samples were centrifuged at 3500 rpm for 5 min, and the supernatant liquid was analyzed.
Experiments were repeated for different initial pH values (3-8), initial Cr(VI) concentration (50-500 mg/L), temperature (25-55 °C), and adsorbent concentration (0.5-3 g/L) values.e adsorbed amount (q e (mg/g)) and the percentage of adsorption (adsorption (%)) at equilibrium were calculated as follows: where C 0 (mg/L) and C e (mg/L) are the initial and equilibrium concentrations of Cr(VI) solution, respectively; m (g) is the mass of the adsorbent; and V (L) is the volume of the liquid phase.

Analysis Method of Cr(VI).
e unadsorbed Cr(VI) concentration was analyzed in a spectrophotometer (Cheibos UV-Vis model) according to the standard colorimetric method, using 1,5-diphenylcarbazide as the complexing agent at the wavelength of 540 nm [16].

Error Functions.
e goodness of fit of the kinetic and isotherm equations to the experimental data was evaluated using the chi-square test and (χ 2 ) and root-mean-square error (RMSE).An error function is defined to enable the optimization process and to evaluate the fit of the isotherm equation to the experimental data.e chi-square test (χ 2 ) and root-mean-square error (RMSE) values were calculated using the Microsoft Excel software by the following equations: where q i,cal and q i,exp refer to the theoretical and experimental adsorption capacities (mg/g), respectively, and n is the number of experimental observations.

Characterization Studies.
e mean hydrodynamic diameter of the FeNPs was found to be approximately 171.2 nm with a polydispersity index (PDI) value of 0.217 by DLS analysis.Particle-size distribution of the greensynthesized FeNPs by DLS analysis is presented in Figure 1.
e SEM images of the synthesized FeNPs at different magnifications are presented in Figures 2(a)-2(e) before and after adsorption, respectively.SEM images revealed that FeNPs were synthesized successfully.e mean particle size of the synthesized FeNPs was determined to be 89 nm by the Image J software program.As seen from Figures 2(a)-2(c), the formed nanoparticles have irregular spherical and porous morphology which is indicating a chain structure, and nanoparticles are quite agglomerated.According to Figures 2(a)-2(c), it was seen that the agglomeration tendency of nanoparticles gave rise to the formation of nanoparticles which had higher mean hydrodynamic diameters.erefore, the mean hydrodynamic diameters of nanoparticles were found as higher than 100 nm by DLS analysis may be due to the absorption effects and particle scattering because of agglomeration [17].After Cr(VI) adsorption, as shown from Figures 2(d) and 2(e), it was observed that the nanoparticles were more agglomerated, and the pores were closed due to the adsorption of Cr(VI) anions onto the adsorbent surface.
In order to check the morphology and support the formation of synthesized FeNPs, TEM analysis was carried out.e obtained TEM micrograph is presented in Figure 3, and the image which was taken at higher magnification (60KX) is presented in the inset of Figure 3.According to the TEM image, it was shown that the spherical particles were obtained whose diameters were 55.4 nm, 70.31 nm, and 114.29 nm, from left to right, respectively.Moreover, it was shown from the image which was presented in the inset of Figure 3, the agglomeration behavior was observed leading to the formation of particles with bigger sizes. is result supported the SEM images.
Elemental analysis results of loquat leave extracts and FeNPs before and after adsorption are given in Table 1.According to Table 1, the elemental and quantitative weight composition (wt.%) of synthesized FeNPs were composed of 28.70% Fe, 37.53% O, 18.31% C, 12.55% S, 2.642% K, and 0.243% Mg elements.EDX analysis results revealed high Fe and O elements content and the presence of C, O, K, and Mg elements which were derived from aqueous leaves extract.
International Journal of Chemical Engineering e C signals are attributed to the polyphenol groups and other C-containing molecules [15].Also, S element must be originating from extract and FeSO 4 •7H 2 O precursor used in the synthesis of FeNPs.After adsorption, the presence of the Cr element in the elemental composition confirmed the binding of the Cr(VI) ions onto the FeNPs surface.
e XRD pattern of synthesized FeNPs is given in Figure 4, where the peaks at 2θ � 25.9 °, 28.3 °, and 35.6 °correspond to iron oxohydroxide, maghemite (c-Fe 2 O 3 ), and magnetite (Fe 3 O 4 ), respectively.Also, the intensity peak at 2θ � 17.56 °was identified to belong to an organic matter in polyphenols reported in the literature [15,18].As seen from Figure 4, the relatively weak peaks in the XRD diagram indicate that synthesized nanoparticles have amorphous grains [15].Moreover, some studies in the literature have reported that FeNPs synthesized by the plant extract are amorphous [15,18].Also, after Cr(VI) adsorption, the XRD diagram of green-synthesized FeNPs indicated that the material was completely amorphous (figure not shown).e differences in the XRD patterns of FeNPs between before and after adsorption may be due to change in the crystallinity and phase of the FeNPs during the chemisorption of Cr(VI).
FT-IR study of crude extract and FeNPs before and adsorption was carried out to investigate the functional groups responsible for adsorption.e absorption bands were observed in all the samples, as presented in Figures 5,  6(a), and 6(b), respectively.
In the FT-IR spectrum of loquat leaves extract (Figure 5), the broad band at 3270.9 cm −1 may correspond to the surface -OH group which is related to the phenolic compounds.e bands that observed at the wavenumbers of 2928 cm −1 and  [12,19].e band at 1720 cm −1 is identified as the carbonyl group from dimerized saturated aliphatic acids [20], and the bands at 1595.36 cm −1 and 1378.12cm −1 are attributed to C�O, C-O stretching and the presence of C-O stretching in carboxyl groups, respectively.e broad band at 1243 cm −1 corresponds to the C-O-C or C�O stretching vibration, and the band at 1036 cm −1 may be assigned to C-O-C and O-H absorption [20].Moreover, the bands observed at 889.03, 763.93, and 599.63 cm −1 correspond to phenol groups and bending vibrations of aromatics, carboxylic acids, and amides [12].
ese peaks demonstrate the presence of phenolics could be probably responsible for the bioreduction of Fe 2+ ions and formation of FeNPs.
According to the FT-IR spectrum of synthesized FeNPs (Figure 6(a)), the bands at 3236.66 cm −1 and 2155 cm −1 are attributed to the O-H group related to the phenolic compounds and C-H stretching vibrations of -CH 2 and CH 3 functional groups.e band at 1605 cm −1 corresponds to C�O stretching, and the band at 1077.78 cm −1 is due to C-O stretching [21].Moreover, the peaks at 400 cm −1 -550 cm −1 indicated the tetrahedral and octahedral Fe-O bonds and 596.17

Effects of Environmental Conditions on the Cr(VI)
Adsorption onto FeNPs

Effect of Initial pH.
e initial pH of the solution is an important factor in the adsorption capacity due to its impact on the ionic forms of chromium in solutions and the surface properties of the adsorbent [7].e effect of the initial pH on the adsorption of Cr(VI) ions onto FeNPs is presented in Figure 7.According to Figure 7, Cr(VI) removal by green-synthesized FeNPs is a pH-dependent adsorption system.e maximum equilibrium uptake value was found to be 99.28 mg/g at pH 3.0 and then started to gradually decrease when the pH value was increased up to 8.0.e formation of di erent species of Cr(VI) in aqueous solutions was given by the following equations: In this study, the active form of Cr(VI) was chosen as HCrO 4 − because of obtaining high removal in the acidic medium.International Journal of Chemical Engineering e attitude of the maximum adsorption capacity at pH 3.0 can be explained by the isoelectric point of adsorbent.
e isoelectric point of FeNPs (pH pzc ) was determined as 4.0 by the zeta (ζ) potential measurement.e change of the zeta potential with pH is presented in Figure 9.
e surface of adsorbent was charged positively at the lower pH values than the isoelectric point of adsorbent and adsorption capacity increased due to strong electrostatic attraction between positively charged adsorbent surface and negatively charged HCrO 4 − ions.Conversely, at the higher pH values than the isoelectric point of adsorbent, the number of negatively charged sites increased, and the number of positively charged sites decreased, so Cr(VI) adsorption capacity decreased due to the electrostatic repulsion between adsorbent surface and HCrO 4 − ions.Metal ion adsorption onto hydrous solids such as metal oxides is a complex reaction between surface sites and adsorbent.e developed surface sites for a hydrated adsorbent were formulated as follows: where S represents the FeNPs surface sites and S−OH 2 + , S−OH, and S−O − refer to protonated, neutral, and deprotonated surface hydroxyl functional groups, respectively.In this work, at the pH value below pH pzc , the positively charged adsorbent surface (S−OH 2+ ) accelerated the uptake of the anionic HCrO 4 − adsorption after which the Coulombic interaction forces can easily occur [23].
erefore, two possible reactions would be suggested as given below in equations ( 5) and ( 6) about the adsorption mechanism depending on the interaction between HCrO 4 − ions and the surface sites of adsorbent at the pH value below pH pzc [4]: for pH < pH pzc (6) where � SOH 2 + (HCrO 4 − ) and � SHCrO 4 are the bonding complexes.

Effect of Initial Cr(VI) Concentration.
e initial metal ion concentration is an important factor on the adsorption capacity in terms of providing the higher driving force to overcome mass transfer resistances of the metal ions between the aqueous and solid phases.
e effect of initial Cr(VI) concentration on adsorption and removal efficiency of FeNPs in the studied initial Cr(VI) concentrations was depicted in Figure 10(a) and 10(b), respectively.As seen from Figure 10(a), the adsorption capacities increased linearly when the initial concentrations of Cr(VI) were increased.By increasing the initial concentration of Cr(VI), the number of collisions with FeNPs increases which consequently enhances the rate aggregation and provides more surface binding sites available for Cr(VI) adsorption.e equation of q e versus C 0 was found as q e (mg/g) � 0.9453 * C 0 (mg/L), (R 2 � 0.997) at optimum temperature 45 °C, initial pH 3.0, 1.0 g/L adsorbent concentration, and 480 min equilibrium time for the range of the studied initial Cr(VI) concentration.According to the equation, the higher removal amounts can be obtained at higher initial Cr(VI) concentrations since the adsorption process has not shown a saturation trend in the range of 50-500 mg/L of initial Cr(VI) concentrations, yet.Also, as seen in Figure 10(b), high adsorption percentages (>90%) were obtained for Cr(VI) adsorption in the studied initial Cr(VI) concentration range.

Effect of Adsorbent Concentration.
e adsorbent concentration effect on the percentage of adsorption and equilibrium uptake is presented in Figure 11.According to Figure 11, the equilibrium uptakes of FeNPs for Cr(VI) concentrations decreased with the increasing adsorbent concentration from 0.5 g/L to 3 g/L.However, the adsorption percentage increased up to 1.0 g/L of adsorbent concentration and then slightly remained constant with further increase in adsorbent concentration.e decrease in uptake values with increasing adsorbent concentration may arise from the interaction between adsorbent particles such as aggregation, resulting from high adsorbent concentration.
e agglomeration of the adsorbent particles would lead to a decrease in the active surface area of the adsorbent and an Zeta potential (mV) pH International Journal of Chemical Engineering increase in diffusional path length.Hence, the lower adsorption capacities were observed at high adsorbent concentrations.As a result, the optimum adsorbent concentration was selected as 1.0 g/L for Cr(VI) ion adsorption onto FeNPs.e effect of temperature on the adsorption is presented in Figure 12.According to Figure 12, it was seen that high adsorption capacity was obtained with an increase in temperature from 25 °C to 45 °C, and then, adsorption capacity moderately remained constant at 55 °C.
erefore, the optimum temperature value for Cr(VI) adsorption was determined as 45 °C.Based on the results, high operation temperature indicated that the studied adsorption process was endothermic nature.
e temperature effect on the studied adsorption process was also evaluated with thermodynamic studies by calculating the entropy change (ΔS), enthalpy change (ΔH), and Gibbs energy change (ΔG).ΔG, ΔH, and ΔS were determined using the following equations [24]: where ΔH and entropy change ΔS were from the slope and intercept of the linear plot of ln K c versus 1/T according to the Van't Hoff equation (equation (7) while ΔG was calculated according to equation ( 8). e equilibrium constant (K c ) values were calculated from the following equation [1]: where C ad,e and C e are the adsorbed metal ion concentration (mg/L) and the unadsorbed metal ion concentration at equilibrium (mg/L), respectively.When the adsorbent q e (mg/g) Figure 12: Effect of temperature (initial pH 3, initial Cr(VI) concentration 200 mg/L, and adsorbent concentration 1 g/L).International Journal of Chemical Engineering concentration is 1.0 g/L, C ad,e is equal to q e at the studied temperature.
In this study, the linear form of Van't Hoff equation for the adsorption of Cr(VI) onto FeNPs was found as ln K c � −8041x(1/T) + 23.03 with the regression coefficient 0.999 (data not shown), and the thermodynamic parameters were calculated and are presented in Table 2.
e calculated ΔG values had positive indicating that adsorption of Cr(VI) on FeNPs was nonspontaneous.e positive value of ΔS suggested randomness of the adsorbed Cr(VI) species at the solid-solution interface during adsorption [5].Moreover, the calculated ΔH values higher than 42 kJ/mol indicated the process results from chemisorption of the Cr(VI) ions onto FeNPs [25].Consequently, the thermodynamic parameters such as Gibb's free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) supported the results of the temperature effect.

Equilibrium, Kinetic, and Mass Transfer Modelling.
e Langmuir and Freundlich isotherm models are commonly used to describe the adsorption data and get the idea about the interactions of adsorbed materials with the adsorbent [26].
e Langmuir isotherm model suggests monolayer adsorption occurring on a homogeneous surface comes from the finite number of similar active sites, and the Freundlich isotherm model is valid for adsorption to take place on a heterogeneous surface [25].
e well-known linearized forms of Langmuir and Freundlich isotherm models are given as follows: Langmuir: 1 Freundlich: ln q e � ln K F + 1 n  ln C e .(10) e linear forms of Langmuir and Freundlich isotherm models were applied to experimental equilibrium data at different temperatures.
e isotherm constants calculated from the isotherm equations with regression coefficients and error function values are summarized in Table 3.
Also, the experimental and predicted isotherms for Cr(VI) adsorption at 45 °C and initial pH 3 are given in Figure 13.As seen from Table 3 and Figure 13, Langmuir isotherm model was in better agreement with experimental equilibrium data due to the high regression coefficients and lower χ 2 and RMSE error function values.
is result demonstrated that the adsorption occurred at specific homogeneous sites within the adsorbent forming monolayer coverage of Cr(VI) at the surface of FeNPs.e maximum monolayer coverage capacity of adsorbent was determined as 312.5 mg/g at 45 °C.As seen from Table 3, the maximum monolayer coverage capacity values of FeNPs for Cr(VI) were increased by increasing temperature may be due to the endothermic nature of the studied adsorption process.
e adsorption capacity of green-synthesized FeNPs in this study for the removal of Cr(VI) was compared with those of other adsorbents reported in the literature, and their adsorption capacities are presented in Table 4.According to Table 4, synthesized FeNPs in this study have a good adsorption capacity in comparison with most of the other adsorbents.However, the exact comparison of adsorption capacity of adsorbents is difficult due to the differences in optimum experimental conditions and physicochemical properties of adsorbents such as functional groups and specific surface areas.
e adsorption kinetics were elucidated by correlating the adsorption kinetic data of the Cr(VI) onto FeNPs using the linear forms of the pseudo-first-order and the pseudosecond-order kinetic models [30].e linearized forms of pseudo-first-order and the pseudo-second-order kinetic models were given in the following equations: pseudo-first-order: log q e − q t  � log q e  − k 1 t 2.303 , (11) pseudo-pseudo-second-order: For the adsorption of Cr(VI) onto FeNPs, the parameters of the pseudo-first-order and pseudo-secondorder kinetic models with regression coefficients are presented in Tables 5 and 6, respectively.From Tables 5  and 6, the adsorption kinetics of Cr(VI) onto FeNPs were better described by the pseudo-second-order kinetic model due to the consistency of the experimental and calculated q e values and high regression coefficients and lower error functions than the pseudo-first-order model at all initial Cr(VI) concentrations.Based on the assumptions of the pseudo-second-order model, the chemisorption was the rate-determining step in the adsorption of Cr(VI) on FeNPs.
To evaluate the boundary layer and intraparticle diffusion mechanism between Cr(VI) ions and FeNPs, Weber-Morris model was used, and model parameters and regression coefficients are summarized in Table 7. Weber-Morris intraparticle diffusion model is expressed by the following equation: where I is the intercept related to the boundary layer effect and K i is the intraparticle diffusion rate constant which can be evaluated from the slope of the linear plots of q t versus t 0.5 .If Weber-Morris plot of q t versus t 0.5 is linear and passes through the origin, the intraparticle diffusion is the sole ratedetermining step, but when the plots in the figure are multilinear with three distinct regions; in that case, there are three different kinetic mechanisms.e initial region corresponds to the external surface uptake; the second stage International Journal of Chemical Engineering relates the gradual uptake indicating intraparticle diffusion as the rate-limiting step and final plateau region represents equilibrium uptake [31].
Based on the results, it was observed a multilinear plot (figure not shown) indicated that both intraparticle and film diffusion were effective on the adsorption of Cr(VI) on FeNPs.Moreover, according to Table 7, it was seen that the internal (K i ) and external diffusion constants (I) gradually increased with the increasing initial Cr(VI) concentration.

Conclusions
In summary, iron nanoparticles were biosynthesized by using a green, reducing agent, loquat leaves aqueous extract,    (ii) e optimum adsorption conditions were determined as initial pH 3.0, temperature 45 °C, and adsorbent concentration 1.0 g/L.e uptake values at equilibrium increased linearly with the increasing the initial Cr(VI) concentration.(iii) e adsorption equilibrium data were best fitted to the Langmuir isotherm model.e monolayer coverage capacity calculated from the Langmuir isotherm model was found to be 312.5 mg/g.(iv) e thermodynamic parameters ΔG, ΔH, and ΔS were calculated as positive, and results indicated that the studied adsorption process was nonspontaneous, endothermic, and increasing in the randomness of adsorbed species.(v) e adsorption kinetic data followed the pseudosecond-order kinetic model, and both intraparticle and film diffusion were effective on the adsorption process.(vi) Based on the results, the biosynthesis method could be suggested for preparing iron nanoparticles in terms of eco-friendly, cost-effective, and easy-tohandle synthesis process with large scale, through the abundance of loquat or other plant leaves.
(vii) Consequently, green-synthesized FeNPs can be a good candidate for Cr(VI) removal owing to their high adsorption capacity.

Nomenclature b:
A constant related to the affinity of the binding sites (L/mg) C e : Unadsorbed Cr(VI) metal ion concentration at equilibrium (mg/L) C 0 : Initial Cr(VI) metal ion concentration (mg/L) K F : Freundlich constant indicating adsorption capacity ((mg/g)/(L/mg) 1/n ) K i : Intraparticle diffusion rate constant (mg/g•min 0.5 ) k 1 : Pseudo-first-order kinetic rate constant (1/min) k 2 : Pseudo-second-order kinetic rate constant (g/ mg•min) q e : Adsorbed amount per unit mass of adsorbent (mg/g) q e,cal 1 : Calculated adsorbed amount per unit mass of adsorbent from pseudo-first-order kinetic model (mg/g) q e,cal 2 : Calculated adsorbed amount per unit mass of adsorbent from pseudo-second-order kinetic model (mg/g) q e,exp : Experimental adsorbed amount per unit mass of adsorbent (mg/g) q t : Adsorbed amount per unit mass of adsorbent at any time (mg/g) Q o : Maximum monolayer coverage capacity of adsorbent (mg/g) 1/n: Freundlich constant indicating adsorption intensity.

Figure 2 :Figure 1 :
Figure 2: SEM images of FeNPs at different magnifications (a-c) before and (d, e) after adsorption.
cm −1 and 458.88 cm −1 refer to Fe-O stretches of Fe 3 O 4 and Fe 2 O 3 [2, 21].After Cr(VI) adsorption, as seen from Figure 6(b), a slight change was observed in the peaks due to chemisorption of Cr(VI) ions.Based on the FT-IR results, the adsorption mechanism of the Cr(VI) ions onto FeNPs may result due to the chemical interaction between -OH and -C�O functional groups on the FeNPs surface and Cr(VI) ions in the solution [2].

Figure 8 :
Figure 8: e concentration diagram for Cr(VI) species depends on pH.

Figure 9 :
Figure 9: e isoelectric point of the prepared FeNPs.

3. 2 . 4 .
Effect of Temperature.e effect of temperature on adsorption was investigated in the range of 25-55 °C temperature values.

Figure 13 :
Figure 13: Comparison of the experimental and predicted isotherms for Cr(VI) adsorption (temperature 45 °C and initial pH 3).
(i) e characterization studies confirmed the synthesis of FeNPs successfully.e synthesized FeNPs had irregular spherical morphology, and they were relatively agglomerated.e XRD spectrum showed the diffraction peaks belong to mainly iron hydroxide and iron oxyhydroxide.According to FT-IR spectrums, the peaks refer to Fe-O stretches of Fe 3 O 4 and Fe 2 O 3 .

Table 1 :
Elemental analysis results for FeNPs before and after adsorption.

Table 2 :
ermodynamic parameters of the adsorption system.

Table 3 :
e isotherm model constants and regression coefficient.

Table 4 :
Comparison of results obtained in this study for the adsorption of Cr(VI) with those of other adsorbents.

Table 5 :
Kinetic model parameters with regression coefficients and error functions for the pseudo-first-order kinetic model.