Iodine adsorption characteristics of activated carbon obtained from Spinacia oleracea (spinach) leaves

Spinach leaves powder was modified by activation with conc. H2SO4 and characterized by using FTIR, optical microscopy, XRD analysis, and methylene blue adsorption method. The maximum specific surface area measured by the Methylene blue adsorption method was 499 m2/g. The adsorption of iodine was investigated by varying parameters like pH, adsorbent dose, contact time, and I2 concentration. The adsorption process was fitted to the Langmuir model controlled by pseudo second order kinetics with a constant rate value of 0.00305 g/(mg·min). The maximum adsorption capacity was 909.091 mg/g at pH 10. The ∆G value was -25 kJ/mol, which confirmed the physico chemcal adsorption process.

A study for Iodine recovery processes utilized various types of adsorbents for recovery purposes. Adsorption of iodine on activated carbon was generally expected in the form of I 2 . Activated coconut carbon particles were used to adsorb iodide through its pores. Alumina impregnated with copper and chromium oxides was used as adsorbent material for the recovery of iodine in both aqueous and gas phases. Strongly basic anion exchange resin and activated carbon particles were used as adsorbent materials to recover iodine from brines. The recent topic of excitement for most researchers was to recover iodine from aqueous solution with low-cost adsorbent materials instead of highly expensive ion exchange resins [10 -16]. A study for the adsorption of Cr, Sr, and I on microalgae and aquatic plants showed that among 188 strains examined from microalgae, aquatic plants, and unidentified algae species, five strains were selected as highly positive radioactive eliminators for iodine ions. The maximum elimination of radioactive iodine was 66 % by the Nostoc Commune Tir4 strain [7]. A study for iodine recovery by a granulated activated carbon from diluted aqueous solutions, such as natural brines and bitterns, examines its iodine extraction followed by desorption using sodium hydroxide solution. Maximum 97.5 % iodine was adsorbed and showing a good fit with Freundlich adsorption isotherm under optimal conditions of pH 2, agitation speed 200 rpm, particle size of carbon 0.5 -1.2 mm, carbon dosage 2 g/L, contact time 3 hours and initial concentration of iodine solution 30 ppm. Maximum desorption of 98.9 % was achieved under the optimal conditions of NaOH solution temperature (0.5 %) 55 °C, agitation speed 300 rpm, contact time 6 hour, NaOH solution volume, and washing water volume 10 times the iodine solution volume and the maximum number of washing stages of 2. The iodine number of activated carbon used was 850 mg/g [10]. A study for adsorption of iodine on nylon -6 showed that nylon -6 was imparted with antibacterial activity by adsorption of iodine. The amount of iodine adsorbed was found to be very high as compared to the theoretical amount of iodine required to form a single layer on the surface of fibers. Iodine adsorption from solution in acetone did not follow Langmuir adsorption, but in case of adsorption of iodine from vapors, it was appeared to follow pseudo -Langmuir adsorption isotherm. In the vapor phase, iodine was adsorbed on nylon mainly in molecular form while in the presence of a solvent, iodine change to ionic form Ithat may form complex with more iodine molecules to form polyiodide ions like I 3 -, I 5 etc. [17]. A study for adsorption of iodine on cationic cross -linked starches investigated the optimal value of adsorption capacity of 4.54 and 4.61 mmol/g at a higher value of the degree of substitution 0.54 and the highest temperature 35 °C, in accordance with Langmuir adsorption isotherm for adsorption of iodine on cationic cross -linked starch chloride (CCSCl) and cation cross -linked starch iodide (CCSI), respectively [18]. The ion exchange mechanism between anionic species of iodine and quaternary ammonium groups and favorability of iodine adsorption on both CCSCl and CCSI was confirmed by the calculated values of Dubinin -Radushkevich adsorption energy E DR and the Freundlich constant n F and was spontaneous process. The experimental data of iodine adsorption on cross -linked starch (CS) at equilibrium (Q B,BXF ) varied from 0.20 to 0.29 mmol/g and was physical and was not suitably described by Langmuir adsorption modes. The Freundlich and Dubinin -Radushkevich adsorption models described the iodine adsorption on both cation-cross -linked starch (CCS) and cross -linked starch (CS) with the same approximation (R 2 > 0.95) [18]. A study for the application of methylene blue and iodine adsorption in the measurement of specific surface area by four acid and salt treated activated carbons estimated, mesoporous structural parameters (SMB) in 10 -3 km 2 / kg to range between 14.545 -15.100, 13.548 -14.011, 12.313 -13.970 and 14.275 -14.551 for the groundnut shell (GS), shea nutshells (SS), poultry droppings (PD), and Poultry waste (PW) sorbents, respectively. Their corresponding micropore level and degree of activation were presented as iodine adsorption number (in 10 -3 molar of iodine per gram of activated carbon) following the range of 2.156 -2.171, 2.174 -2.191, 2.163 -2.193 and 2.157 -2.168. The data were widely different from those of their respective pyrolyzed materials and were only slightly lower than those of commercial reference carbon (SMB 15.627 × 10 -3 km 2 /kg and IAN 2.230 mmol/g). In each case, it was seen that activation after carbonization further increased the total surface area. Generally, iodine number, surface area, degree of activation, and expected adsorption effectiveness in removal of small-sized adsorbate was seen higher in the case of smaller volume of iodine adsorbed, and the reverse was also true [19]. Improvisation of adsorption using low-cost adsorbent materials such as readily available agricultural waste and their physico-chemical modification in order to introduce surface functional groups is one of the most significant ways to enhance the adsorption efficiency of the adsorbent. Only a few studies concentrate on summarizing the various techniques used to produce activated carbon from spinach leaves. Therefore, in this study, acid-activated carbon prepared from the chemical treatment of spinach leaves powder with concentrated H 2 SO 4 is used to remediate iodine from aqueous solution.

Materials:
Waste spinach leaves needed for the study were collected from agricultural lands and local vegetable markets of Janakpur, Nepal. Required chemicals were obtained from different manufacturers mentioned at particular places, and the reagents were AR/LR grades and were used without further purification.

Preparation of adsorbent from spinach leaves:
The spinach leaves sample was washed with distilled water. The washed sample was dried in shadow until a constant weight was achieved. The dried sample was ground to obtain a fine powder. The powdered sample was stored in a labeled container, and it was abbreviated as DSL (dry spinach leaves powder). 30 g of dried DSL was mixed with 60 mL of concentrated sulfuric acid (96 %) in an 800 mL round bottom flask (The specific surface area of the adsorbent obtained from refluxing with H 3 PO 4 was lower than that of H 2 SO 4 [20]). The mixture was refluxed at 100 °C in an oil bath by three pathways. In the first case, it was refluxed continuously for 6 hours, and then it was left for 18 hours in contact with H 2 SO 4 for further soaking. In the second and third cases, the same process was repeated for 2 and 3 days. After completion of the reaction, a black product obtained in each case was washed with distilled water repeatedly until neutrality. The neutral black products obtained were dried in a convection oven at 70 °C for 24 hours. The final products thus obtained were again ground to get a fine powder. All the three black products containing acid -activated carbon as a major constituent was abbreviated as ASL-1 (acid -activated carbon prepared from spinach leaves powder with 1 day treatment), ASL-2 (acid -activated carbon prepared from spinach leaves powder with 2 days' treatment) and ASL-3 (acid -activated carbon prepared from spinach leaves powder with 3 days' treatment). The obtained amount of activated carbon was in the range of 5 to 5.5 g per 30 g of the dry spinach powder (i.e., the percentage yield was between 16.6 to 18.3) while the ash content was neither separated nor analyzed quantitatively. Preparation of approximately 0.1 M stock solution of iodine: 20 g of KI (Merck Life Science Private Limited, India) was dissolved in 400 mL of distilled water taken in a beaker. 12.8 g of A.R. grade iodine was dissolved in a previously prepared solution of KI. The mixture was stirred with a glass rod continuously until the complete dissolution of iodine occurred. The resulting solution was then transferred in to 1000 mL volumetric flask. The volume was made up to mark with distilled water. The solution was made homogeneous by shaking, and then it was kept in the dark and cold place. Thus, the prepared solution was standardized using K 2 Cr 2 O 7 (Merck Life Science Private Limited, India). Preparation of working solution of iodine: Working solutions of lower concentrations ranging from 50 to 900 ppm were prepared by the serial dilution of standardized stock solution of iodine (Merck Life Science Private Limited, India) with distilled water. Preparation of reagents: Sodium thiosulphate solution (approx. 0.1 N): 0.1 N sodium thiosulphate (Qualigens Fine Chemicals, India) was taken in a 250 mL volumetric flask and diluted up to the mark with distilled water. The prepared solution was standardized with 0.1 N K 2 Cr 2 O 7 solution as a primary standard. It was then diluted to appropriate lower concentrations by dilution of stock solution with distilled water. KI solution (approx. 10 %): About 10 g of A.R. grade potassium iodide was dissolved in distilled water in 100 mL volumetric flask and volume were made up to the mark. Starch solution (approx. 1%): A paste of 1.0 g of soluble starch (Thomas Baker Chemicals Private Limited, India) was prepared with distilled water. This paste was then transferred in to a 100 mL of boiling distilled water in a beaker with constant stirring by a glass rod. The mixture was boiled until a clear solution was obtained, cooled, and stored in a stoppered bottle. The freshly prepared starch solution was used as far as it was possible. Buffer solutions of pH 4.0, 7.0 and 9.2 were prepared by dissolving buffer tablets (Hi Media Laboratories Private Limited, India) in 100 mL volumetric flasks with distilled water. Preparation of 1000 mg/L stock solution of methylene blue: 1.0 g of accurately weighed and dried methylene blue (Merck Life Science Private Limited, India) was dissolved in 1000 mL volumetric flask containing a little amount of distilled water, and then the volume was made up to mark with distilled water. The working solution of methylene blue was prepared by serial dilution of 1000 mg/L stock solution.

Determination of λ max and calibration curve for methylene blue solution:
First of all, 1000 mg/L stock solution was diluted to 100 mg/L with distilled water in a 250 mL volumetric flask by taking a 25 mL solution. After that, the resulting solution was diluted to 10 mg/L in a 500 mL volumetric flask by taking a 50 mL solution, and the volume was made up to the mark with distilled water. From 100 ppm methylene blue solution, 2.5, 5.0, 7.5, 10.0, 12.5, 17.5, 20.0, 22.5 mL solutions were prepared in 25 mL volumetric flask, respectively to prepare 1 to 9 ppm solutions. The solution having intermediate concentration (5 mg/L) was taken for the determination of λ max . The measurement was carried from 600 to 680 nm using a spectrophotometer (2306, Electronics, India) by setting the blank solution at zero absorbance. The maximum absorbance was obtained at 665 nm. After finding out the value of λ max , the wavelength is set at 665 nm, and the absorbance of the solution of different concentrations (1 to 10 mg/L) was measured. Thus, the obtained plot between absorbance and concentration of the solution is known as a calibration curve for the methylene blue solution. Characterization of adsorbent materials: The phase detection was done using X-ray Diffractometer with monochromatic Cu Kα radiation (D2 phaser Diffractometer, Bruker, Germany, at Nepal Academy of Science and Technology NAST). Functional groups present in samples before and after adsorption of iodine were analyzed using Fourier transform infrared spectroscopy (IR Tracer 100, Shimadzu, Japan at Central Department of Chemistry, Tribhuvan University). Samples, before and after adsorption of iodine, were analyzed using optical microscopy (proCAM Radical Scientific India RXLr-4) to determine structural parts, subcomponents & porosity of the adsorbent materials.

Determination of specific surface area of the ASLs:
The Langmuir adsorption isotherm model was used to determine the surface area of spinach leaf powder adsorbent material. For this 25 mg of ASLs was added to the reagent bottle containing varying concentrations of methylene blue solutions from 10 to 300 mg/L. The solutions were agitated in a mechanical shaker for 24 hours. After 30 minutes, the supernatant solutions were pipetted out. The absorbance of resultant supernatant solutions was measured at 665 nm using a spectrophotometer. The Q max value was calculated from Langmuir adsorption isotherm. Adsorption studies: Effect of pH: Initially was determined an optimal concentration of iodine by trial error method and it was found to be 692 mg/L. In order to study the effect of pH on iodine adsorption, 50 mL of 692 mg/L of iodine solution was poured in to a series of stoppered bottles and the pH of the solutions were varied from 2 to 10.5 using the pH meter with the help of 0.1 M NaOH and 0.1 M HCl. Flasks containing 50 mg of ASL-2 in each were shaken for 24 hours in a mechanical shaker at 220 rpm. After shaking, each equilibrated solution was filtered instantly using Whatman No. 41 filter paper. Direct titration method was used to determine the initial and equilibrium concentrations of iodine. In each case, the adsorption efficiency was determined with the help of the calculated value of initial and equilibrium concentration.

Effect of initial concentration:
The effect of iodine concentration on the adsorption was studied under optimum pH. 50 mg of ASL-2 was added to the 50 mL of iodine solution having initial concentrations ranging from 50 -908 mg/L. The equilibration time for iodine removal was less than 50 minutes for each. Each solution was agitated at 220 rpm until the fulfillment of equilibration time. After shaking, each solution was filtered instantly using Whatman No. 41 filter paper. Direct titration method was used in order to determine the initial and equilibrium concentrations of iodine. In each case, the two most widely used models of Langmuir and Freundlich were tested for studying the adsorption isotherm.

Effect of reaction time:
The effect of kinetics on the adsorption of iodine removal was studied. Here, 50 mg of ASL-2 was added to the 50 mL of iodine solution having an initial concentration of 169.069 mg/L. The solution was agitated in a mechanical shaker at 220 rpm for different time durations ranging from 5 min. to 24 hours. After shaking, each solution was filtered instantly. Direct titration method was applied for the determination of initial and equilibrium concentrations of iodine. In each case, the data obtained were tested with pseudo-firstorder and pseudo -second -order kinetic models.

Characterization of adsorbent materials:X-ray diffraction (XRD) analysis:
The XRD pattern of raw spinach leaves to powder and the samples obtained by two days' treatment with concentrated sulfuric acid at 100 °C was recorded and is shown in Fig. 1. These obtained results revealed that bonded O-H groups are present in an abundant amount in ASL, and these groups are responsible for adsorption. These groups in bio -char of adsorbent may serve as H + donor. Therefore, deprotonated bonded O-H groups may be involved in coordination with metallic and non-metallic ions. The spectra analysis of ASLSs showed that there were a clear band shift and a decrease in intensity of the band at 3298 cm -1 , 2914 cm -1 , and 1600 cm -1 in the sequence ASL-2 > ASL-3 > ASL-1. These are due to the high temperature in the activation process that broke some more intermolecular bonds in ASL-2 compared with ASL-1. Less satisfactory parameters for ASL-3 compared with ASL-2 may be due to an overdose of contact time of adsorbent with concentrated H 2 SO 4 . Hence, ASL-2 being the best adsorbent among 3 activated carbons, it was utilized for carrying out the adsorption process for the adsorption of iodine. Optical microscopic images: Adsorbents before and after adsorption with iodine as well as raw sample were analyzed using an optical microscope. The resulting images at ten times magnification are shown in Fig. 3.

Fig. 3. Optical images of DSL and ASL-2 before and after adsorption of iodine
The optical image of the activated carbon indicated the presence of a homogeneously distributed pore structure, and this porous nature of adsorbent materials was found to decrease with the adsorption of iodine.

Determination of λ max for methylene blue solution:
From the absorbance versus wavelength plot for the methylene blue solution, the maximum absorbance was observed at 665 nm, which is in close agreement with the values reported in the literature [22].

Calibration curve of the methylene blue solution:
The absorbance versus concentration plot for the methylene blue solution is shown in Fig. 4. It was found to be linear up to 10 ppm and obeyed Beer -Lambert's law. In order to measure the absorbance of a highly concentrated methylene blue solution, further dilution was required.

Specific surface area determination:
For the determination of specific surface area, "Methylene blue adsorption method" was adopted widely for various natural solids such as activated carbon, charcoal, graphite, and silica, etc. [23]. This method is simple, rapid, easy, reliable, and cheap. Therefore, this method can be performed in any laboratory easily. In this method, a fixed amount of adsorbent is added to the different concentrations of methylene blue solution, and the specific surface area is determined by using Langmuir adsorption isotherm [23]. By assuming monolayer adsorption of methylene blue on to the surface of sorbent particle, the specific surface area is calculated as: Where, S MB is the specific surface area in m 2 /g, Q max is the mass in gram of methylene blue adsorbed per gram of sorbent, a MB is the area occupied by one molecule of methylene blue in m 2 /molecule, N is the Avogadro's number (6.023×1023 molecules/mol), and M is the methylene blue molar mass, i.e., 373.9 g/mol [23,24]. Here, the Q max is equivalent to the equilibrium adsorption capacity of the Langmuir equation. From the slopes of linearized Langmuir curves (i.e., C e / Q e as a function of C e ) for ASL-1, ASL-2, and ASL-3 the specific surface area of the adsorbent materials was determined. The specific surface area of the adsorbent materials is shown in the bar diagram of Fig. 5.
The specific surface area values are in the range of 436, 499, and 476 m 2 /g for ASL-1, ASL-2, and ASL-3, respectively. The lowest surface area of ASL-1 in comparison with ASL-2 may be due to not completed of activation of spinach in one day while in case of 3 days activation, more ash content was formed, which may have blocked the pores of the adsorbent ASL-3.

Effect of pH:
The results obtained for the adsorptive removal of iodine as a function of pH are presented in Fig. 6. The adsorption yield of adsorbent was changed when the pH of a solution containing iodine was increased continuously. Adsorption % and adsorption capacities were observed the maximum at pH 10 and minimum at 11.5 for iodine. In general, at low pH, concentration, and mobility of H + ions are high. So they get preferentially adsorbed on the adsorbent rather than the other adsorbate molecules. Hence, adsorption of Iodine ions at very low pH was found the minimum.
Among various species such as I 2 , I -, and I 3 that are present in the aqueous iodine solution, at alkaline pH or in the presence of NaOH small amount of I 2 that present in the solution react with Ito form I 3 - [25]. The experimental optimum equilibrium pH value for iodine adsorption was 10. The fluctuation of experimental optimum pH value with respect to reference value may be due to the adsorption of iodine in the form of I 3 -. The decrease in iodide adsorption below this pH value may be due to the combined effect of blocking of adsorption sites with a huge number of H + ions and lowering of the Iodide content in the solution below initial pH 8. The decrease in iodide adsorption at an equilibrium pH value greater than 10 may be due to desorption of I 2 at higher concentration of NaOH resulted due to inter ionic repulsion in between the iodide ions and hydroxyl ions. Batch adsorption isotherm studies: In 1918, Langmuir gave a relation between the amount adsorbed and the concentration for a unimolecular layer and is known as Langmuir adsorption isotherm [26]. Langmuir assumed monolayer surface coverage of adsorbent molecules with energetically equivalent adsorption sites where there was no interaction between the adsorbed molecules. The general form of equation representing the Langmuir's adsorption isotherm is given as: Where, Q e is equilibrium adsorption capacity (mg/g), X is the mass of adsorbate (mg), M is the mass of adsorbent (g), Q max is maximum adsorption capacity (mg/g), b is a constant related to adsorption energy (L/ mg) and C e is the equilibrium concentration of the ions adsorbed (mg/L). The linearized form of equation (2) is: Thus, C b /Q b values are plotted as ordinate against C e as abscissa to 1/Q max get a straight line with a slope equal to and intercept on the ordinate equal to 1/Q max b . Here, slope and intercept are used for the determination of Q max and b, respectively. The essential characteristics of the Langmuir isotherm can be explained in terms of a dimensionless separation factor (K L ) which describes the type of isotherm, as defined by [27].
Where, C i is the initial concentration of the adsorbate in mg/L and K L is the Langmuir equilibrium parameter. Shape of isotherm and nature of adsorption process is indicated by K L , [K L > 1, unfavorable, K L =1, linear, 0 < K L < 1, favorable, K L = 0, irreversible]. Hence, for favorable isotherms, the K L value should lie between 0 and 1 [28,29]. Freundlich adsorption isotherm: Freundlich, in 1909, proposed an empirical equation to represent, in general, the adsorption relationship and is classically called Freundlich adsorption isotherm. According to Freundlich, the heterogeneous adsorbent surface containing a non-uniform distribution of heat of adsorption is suggested to adsorb more than one ion per binding site forming multilayer adsorption [30]. It is expressed as: Where: Q e is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), C e is the equilibrium concentration of the adsorbate (mg/L), K F , and n are Freundlich isotherm constants related to adsorption  capacity and intensity of adsorption respectively. The value of 1/n generally lies between 0 and 1. On taking logarithms on both sides of equation (5), Thus, K F and n can be determined, if the values of log Q e are plotted as ordinate against log C e as abscissa. The slopes and intercepts of the linearized Langmuir and Freundlich plots were used to calculate the Langmuir and Freundlich parameters and tabulated as shown in Table 1. Iodine adsorption versus equilibrium concentration plot for I 2 is shown in Fig. 7. Langmuir isotherm was better fitted by the isotherm data in comparison with Freundlich isotherm. Therefore, adsorption on the surface of ASL-2 was monolayer adsorption with a very high coefficient of determination (R 2 = 0.9994). The maximum uptake capacity (Q max ) for I 2 was 909.091 mg/g. The obtained Q max value was significantly higher than most of the other adsorbent reported in the literature. Error analysis for isotherm studies: In order to evaluate the fit between experimental data and the various isotherm models, linear regression is the widely used approach where the closeness of the fit is determined from the value of the coefficient of determination (R 2 ). Linear regression analysis, however, transforms nonlinear isotherm equations in to their linearized forms and this transformation violates the error variance and  Fig. 7. The adsorption isotherm of iodine onto activated carbon obtained from spinach normality assumptions of standard least squares. In this sense, the non-linear chi-square analysis, x 2 , shown in equation (7) can give more accurate results since this method compares all isotherms on the same abscissa and ordinate [28].
where, Q b,cal is the equilibrium capacity obtained from expressions describing the different models (mg/g) and Q b,bxp is the equilibrium capacity (mg/g) obtained from the experimental data. Smaller x 2 values confirm a better correspondence between obtained data and isotherm model. In addition, non-linear chi-square analysis can also be used in the case where linear regression analysis leads to inadequate conclusions [28]. The smaller value of x 2 for the Langmuir model than that of the Freundlich model also confirmed the Langmuir adsorption isotherm as the best fitting model in each case. From Table 1, ∆G value calculated was -25 kJ/ mol. The negative value for free energy (∆G) confirmed the spontaneous, physico -chemical adsorption of I 2 on ASL-2. The K L values obtained from Eqn. (7) for I 2 adsorption with using Langmuir, parameters were in the ranges between 0.01 to 0.09 which are between 0 and 1, indicating favorable adsorption. Batch kinetic studies: A number of adsorption kinetic models have been introduced in order to investigate the reaction kinetics. Among them, pseudo-first-order and pseudo-second-order kinetic models are the most frequently used models. The adsorption capacity can also be used to describe the adsorption mechanism of the metal ions on the adsorbent surface where it is difficult to analyze using correlation coefficient values [31]. Pseudo-first-order kinetic model: Pseudo-first-order kinetic model states that the rate of adsorption at any time (dQ/dt) is directly proportional to the amount of remaining unoccupied surface site (i.e. Q e -Q t ). The differential form of pseudo-first-order rate equation (Lagergren, 1898) is generally expressed as below [32]: Where, Q e and Q t are the amounts of adsorbate adsorbed per unit weight (mg/g) of adsorbent at equilibrium and at the time (t), respectively and K 1 (min -1 ) is the first order Lagergren adsorption rate constant. The linearized form of the equation obtained by integrating equation (8) for the boundary conditions t = 0 to t = t and Q t = 0 to Q t = Q t is given as: Hence, Q e and K 1 can be calculated from the slope and intercept of the graph plotted between log(Q e -Q t ) and time. Pseudo-second-order kinetic model: Pseudo-second -order kinetic model states that the rate of adsorption at any time (dQ/dt) is directly proportional to the square of the amount of remaining unoccupied surface sites [i.e. (Q e -Q t ) 2 ] [33]. Therefore, the generalized form of kinetic rate law can be written as follows: Where, Q e and Q t are the amount of adsorbate adsorbed per unit weight of adsorbent (mg/g) at equilibrium and at the time (t), respectively and K 2 (g/ mg · min) is the pseudo-second-order adsorption rate constant. The linearized form of the equation obtained after integrating for the boundary conditions t = 0 to t and Q t = 0 to Q t , and rearranging of the integrated rate law for a pseudo-second-order reaction is given as: Where, t is contact time and h (= K 2 Q e 2 ) is the initial adsorption rate (mg/g·min). Hence, K 2 and Q e values can be calculated from the slope and intercept of the graph plotted between (t/Q t ) and t. The kinetic parameters obtained from the slope and intercepts of pseudo-first and pseudo-second-order rate equations for the adsorption of iodine on to the activated carbon obtained from spinach leaves (ASL-2) are presented in the following Table 2. The kinetic curve representing the equilibrium adsorption capacity versus time is shown in Fig. 8. The amount of adsorption is sharply increased with increasing contact time in the initial stage for I 2 and then gradually increased to reach an equilibrium value in less than 50 minutes. A negligible effect on the amount of adsorption was observed on further increasing contact time. The initial sharp increase of adsorption capacity is due to the availability of a greater number of active sites at the initial stage. The number of active sites decreases with time due to adsorption and it reaches an equilibrium value after a certain interval of time. Therefore, the equilibrium adsorption capacity was gradually increased to reach equilibrium after a certain interval of time.

Mechanism of iodine adsorption:
The maximum adsorption at pH 10 and the free energy of adsorption (∆G) in the range of -25 kJ/mol indicate the complex pattern of adsorption mechanism involving more than one mechanism. In aqueous solution, iodine can be present in three species, such as elemental (I 2 ), Iand I 3 -. The I 3 is expected to be stable under very oxidized conditions, whereas elemental iodine is stable at moderately oxidized, acidic conditions [34]. Thus, it is expected that the adsorptive species of iodine on the present adsorbent at pH 10 is I 3 and is in agreement with a previous report [35]. The adsorption process is followed by both physisorption and chemisorption. Loosely bonded hydroxyl groups available for the ion exchange process are responsible for chemisorption while, the pores available in activated carbon are responsible for physisorption (The analysis of the FTIR spectra of the activated carbon before and after the adsorption of iodine is shown in Fig. 10). At pH 10 there is some amount of NaOH present in the solution which reacts with iodine as: On porous adsorbents, overall adsorption processes are controlled either by film diffusion (external mass transfer of adsorbate from solution to the external surface of the adsorbent) alone or, in combination with pore diffusion (intraparticle mass transfer of adsorbate in to the adsorbent pores). The intraparticle diffusion model proposed by Weber and Morris identifies the process explaining rate-determining step. The general form of the equation used is shown in equation (13).
Where, K id (mg/gh 0.5 ) is the intraparticle diffusion rate constant. The K id and C can be calculated from the slope and intercept of the graph plotted between Q t and t 0. 5  by passing through the origin. If the whole plot doesn't pass through the origin, it indicates that the mechanism of intraparticle diffusion is not only the rate-limiting step of the adsorption process. The multiple linear sections in the plot indicate the involvement of other interaction mechanisms [36,37]. A plot of iodine uptake versus square root of time is shown in Fig. 9. The Q t versus plot for each ion does not pass through the origin, showing that the intraparticle diffusion model was not the rate-limiting step. The K id and R 2 values obtained from the plot are 1.9464 and 0.9095 respectively. Fig. 9. The plot of Q t versus for the adsorption of iodine on ASL-2 The FTIR spectra of the activated carbon before and after the adsorption of iodine are shown in the following Fig. 10. Fig. 10. FTIR Spectra of activated carbon obtained from the spinach before and after Iodine adsorption No broadband of hydroxyl peak was detected in the spectra obtained after the adsorption of iodine ions, even in the range 3280-3460 cm -1 . This diminishing of hydroxyl peak after adsorption may be due to the involvement of the hydroxyl group in the adsorption process. A clear peak of C=C of arene and C-O-C linkage at 1564-1640 cm -1 and 1030 cm -1 in ASL-2 are diminished after iodine adsorption showing that C=C of arene interacts with the generated iodine species and cross-linking between hydroxyl groups of different polyphenols are no longer stable after iodine adsorption. Peaks of background CO 2 seen at 2200-2400 cm -1 were mostly diminished after iodine ions adsorption showing that iodine ions are preferentially adsorbed on the adsorbent expelling out CO 2 from their adsorption sites.

CONCLUSION
In this study, low-cost adsorbent materials were prepared from waste spinach leaves by chemical treatment with conc. H 2 SO 4 . Adsorbent materials were characterized with various tools and techniques available. The adsorption of I 2 was a pH-dependent process and was found maximum at pH 10. The equilibration time for adsorption of iodine on ASL-2 was less than 50 minutes. The adsorption data followed pseudo-second-order kinetics with a rate constant of 0.00305 g/(mg·min), indicating chemisorption as the rate-limiting mechanism. The equilibrium adsorption capacity determined from the Langmuir model was 909.091 mg/g. The negative ∆G value of -25 kJ/mol obtained from Langmuir equation indicated that the adsorption process was a spontaneous, feasible, and physio-chemical type.