Column Adsorption Studies for the Removal of Cr ( VI ) Ions by �thylamineModi�ed Chitosan Carbonized Rice �us� Composite Beads withModelling and Optimization

The objective of this present study is the optimization of process parameters in adsorption of Cr(VI) ions by ethylamine modified chitosan carbonized rice husk composite beads (EAM-CCRCBs) using response surface methodology (RSM) and continuous adsorption studies of Cr(VI) ions by ethylamine modified chitosan carbonized rice husk composite beads (EAM-CCRCBs). The effect of process variables such as initial metal ion concentration, adsorbent dosage and pH were optimized using RSM in order to ensure high adsorption capacity at low adsorbent dosage and high initial metal ion concentration of Cr(VI) in batch process. The optimum condition suggested by the model for the process variable such as adsorbent dosage, pH and initial metal ion concentration was 0.14 g, 300 mg/L and pH2 with maximum removal of 99.8% and adsorption capacity of 52.7 mg/g respectively. Continuous adsorption studies were conducted under optimized initial metal ion concentration and pH for the removal of Cr(VI) ions using EAM-CCRCBs. The breakthrough curve analysis was determined using the experimental data obtained from the continuous adsorption. Continuous adsorption modelling such as bed depth service model and Thomson model were established by fitting it with experimental data.


Introduction
Chromium compounds are widely used in various industries such as electroplating, leather tanning, mining, aluminium conversion coating, operation dyes and pigments [1,2].e indiscriminate discharge of chromium metals into water resources causes serious health effect to human and environment because of its toxic nature.Chromium exists in trivalent form and hexavalent form in aqueous systems.Cr(III) ions are non toxic and play an essential role in the metabolism of plant and animals.Cr(VI) ions are highly toxic.Inhalation of Cr(VI) ions leads to the carcinogenetic problem.Other health effects of Cr(VI) ions are the skin allergy liver and stomach problems [3].e tolerance limit for the discharge of Cr(VI) ions into surface water is 0.1 mg/L and in potable water is 0.05 mg/L [4].us the removal of Cr(VI) ions becomes mandatory.
Various methodologies such as electrochemical, ion exchange, membrane �ltration, evaporation, solvent extraction, emulsion per traction technology, reverse osmosis and chemical coagulation, and adsorption are available for the removal of Cr(VI) ions [4].
Adsorption process is one of the efficient methods for Cr(VI) removal due to its simplicity, sludge-free operation, easiness in handling, availability of various adsorbents, and more efficiently in removal of heavy metals at lowerconcentration levels [5].

Materials and Methods
2.1.Reagents.Raw rice husk was obtained from a local rice mill.Chitosan was purchased from Pelican Biotech Industry, India.e chemicals used in this study such as nitric acid, sulphuric acid, acetic acid, sodium hydroxide, acetone, ethanol, isopropanol, ethylamine, and epichlorohydrin were supplied by Merck, India.Potassium dichromate was used for the preparation of Cr(VI) stock solution.e AR grade of 1,5diphenylcarbazide was used for analyzing chromium.Double distilled water was used to prepare all the solutions.

�reparation o� Et���amine Modi�ed C�itosan Car�oni�ed
Rice Husk Composite Beads (EAM-CCRCBs).e procedure for preparation and characterization of EAM-CCRCBs was described in our previous study [28].According to that procedure, the rice husk was thoroughly washed with distilled water and dried in hot air oven at 100 ∘ C for 5 hrs.e dried sample was treated with 70% concentrated nitric acid (1 : 1 by weight) at 70 ∘ C for 90 mins.Aer acid treatment, the sample was kept for overnight and then subjected to heat under a controlled atmosphere of nitrogen from ambient temperature to 600 ∘ C at a constant heating rate of 5 ∘ C/min in a tubular furnace for 4 hrs [29].Chitosan gel was prepared by dissolving 3 g of chitosan in 100 mL of 2% acetic acid.3 g of carbonized rice husk was added to the chitosan gel and kept in a rotary shaker for 12 hrs at 200 rpm.e chitosan carbonized rice husk gel solution was dropped into 0.5 mol/L of NaOH solution which was remained for 12 hrs and washed with distilled water to remove excess NaOH, which is dried for further use.
e prepared CCRCB was introduced into the 60 mL of isopropanol in a conical �ask to obtain a suspension with the CCRCB �oating on the surface.A mixture of 5 mL epichlorohydrin and 100 mL of water solution of acetone (volume ratio of acetone to water is 1 : 1) was added to the suspension and kept in temperature bath at 60 ∘ C for 2 hrs.en, it was �ltered, and the solid beads were transferred into the mixture of 150 mL of water solution of ethanol (volume ratio of ethanol to water is 1 : 1) and 5 mL of ethylamine, which is kept in temperature bath at 50 ∘ C for 10 hrs.en, it was washed and dried.

Adsorption Studies.
Batch adsorption studies were conducted to determine the adsorption capacity and percentage removal of Cr(VI) ions using EAM-CCRCBs.A desired quantity of EAM-CCRCBs was added to 25 mL of known concentration of Cr(VI) ions and pH in a 100 mL volumetric �ask and kept in a rotary shaker at agitation speed of 200 rpm.e supernatant liquid samples were separated by centrifuging the sample and then analyzed by using Jasco UV spectrophotometer at 540 nm to calculate the adsorption capacity and percentage removal.Experiments were repeated in triplicates, and the average percentage deviation was found to be 3-5%.e amount of adsorption at equilibrium   (mg/g) and percentage removal (%) were calculated as follows: where  0 and   are the initial and equilibrium concentrations (mg/L),  is the volume of solution (1),   is the adsorbed quantity (mg/g),  is the weight of adsorbent (g), and   is the solution concentration at the end of the adsorption process (mg/L).

Response Surface
Modeling.RSM is a statistical method based on the multivariate nonlinear model that has been widely used for the optimization of process variables of adsorption and also used to determine the regression model equations and operating conditions from the appropriate experiments.It is also useful in studying the interactions of the various parameters affecting the process [30][31][32][33].e standard RSM design called central composite method (CCD) was applied in this present study to determine the optimum process variables for adsorption of Cr(VI) ions using EAM-CCRCBs by using the design expert soware (Version 8.0.Stat-Ease) statistical package.e CCD was used for �tting a quadratic equation by multiple regression procedure which requires only a minimum number of experiments for modelling [30,31].e CCD consists of a 2  factorial runs (coded to the usual ± notation) with 2 axial runs (±, 0, 0,  , 0, (0, ±, 0, 0,  0,  , (0, 0,  , ± and   center runs (six replicates, 0, 0, 0,  , 0). e number of factors  increases the number of runs for a complete replicate of the design which is given as folloes: Basically the optimization process involves three major steps: (1 performing the statistically designed experiments, (2 estimating the coefficients in a mathematical model, and (3 predicting the response and checking the adequacy of the model [32,33]. An empirical model was developed to correlate the response to the adsorption of Cr(VI) ions from aqueous solution using EAM-CCRCBs based on second order as folloes: where  is the predicted response,  ′ 0 is the constant coef-�cient,   is the linear coefficients,   is the interaction coefficients,   is the quadratic coefficients, and   ,   are the coded values.

Continuous Adsorption Studies
. e continuous adsorption studies were conducted in a glass column with internal diameter of 1.5 cm and length of 40 cm.e EAM-CCRCBs was packed between the glass wool and glass beads in order to prevent the wash out of the adsorbent.A known quantity of adsorbent was then placed in the column to yield the desired bed height (25 cm and 15 cm) of the adsorbent.Potassium dichromate solution of known concentration (300 mg/L) was channelled into the column using a peristaltic pump in up�ow manner at the desired �ow rate (25 mL/min and 50 mL/min).Samples were collected from the exit of the column at different time intervals and analyzed for Cr(VI) ions using a UV-Vis Spectrophotometer (JASCO) by monitoring the changes in absorbance at a wavelength of maximum absorbance of 540 nm.Operation of the column was stopped when the effluent Cr(VI) ion concentration exceeded 99.5% of its initial concentration.

Development of Regression Model Analysis.
In this present investigation, the CCD of 3 variables such as adsorbent dosage (g), pH, and initial concentration (mg/L), each with �ve levels (±1 for the factorial points, 0 for the centre points, and ± for the axial points) were chosen as independent variables with designated coded factors as , , and , respectively, and the variables are presented in Table 1.A total of 20 experiments were necessary to estimate the coefficients of each model using linear regression analysis.e two dependent output responses, namely, percentage removal ( 1  and adsorption capacity (mg/g) ( 2  were obtained from the independent input variables for CCD and are presented in Table 2. e quadratic models were suggested by the soware for percentage removal ( 1  and adsorption capacity ( 2  of Cr(VI) ions removal using EAM-CCRCBs adsorbent due to the higher-order polynomial and are reported in Tables 3 and  4, respectively.
e quadratic model obtained for the percentage removal ( 1  and adsorption capacity ( 2  of Cr(VI) ions using EAM-CCRCBs adsorbent in terms of coded factors was reported as follows: ( e analysis of variance corresponding to (5) is reported in Tables 5 and 6.In general, the statistics "" value with low-probability "P" value represents high signi�cance of the regression model.e model F value of 43.48 and 109.79 implies that the model is signi�cant for percentage removal and adsorption capacity, respectively.
For percentage removal, , , ,   Based on (5), the actual and predicted plots for percentage removal and adsorption capacity of Cr(VI) ions using EAM-CCRCBs are shown in Figures 1 and 2. e values of  2 and  2 adj were found to be 0.9831 and 0.9679 for percentage removal as well as 0.9776 and 0.9574 for adsorption capacity of Cr(VI) ions using EAM-CCRCBs.adsorption capacity of Cr(VI) ions using EAM-CCRCBs were visualized through three-dimensional views of response surface plots and are shown in Figures 3-5.

e Combined Effect of pH and Adsorbent Dosage. e combined effect of pH and adsorbent dosage on percentage removal and adsorption capacity of Cr(VI) ion using EAM-
CCRCBs is shown in Figures 3(a) and 3(b), respectively.e maximum percentage removal and adsorption capacity of Cr(VI) ion using EAM-CCRCBs was obtained at pH 2. is may be due to the surface positive functional groups of the EAM-CCRCBs adsorbent carrying the oxyanions (negatively charged) of Cr(VI) ions by electrostatic force of attraction.At higher pH, the increased negative charges on the adsorbent surface decreased the attraction of oxyanions of CrO 4 −2 on the adsorbent [34].e maximum percentage removal of 99.8% and adsorption capacity of 52.7 mg/g at constant pH of 2 initial concentration of 300 mg/L at EAM-CCRCBs dosage of 0.14 g was obtained.concentration was kept constant for all varying dosages.e increase in percentage removal may be due to the complete utilization of all active sites in the adsorbent dosage by metal ions [18].e maximum percentage removal of 92.7% and adsorption capacity of 58.34 mg/g at constant pH of 3.5 and initial concentration of 374.02 mg/L at 0.14 g of EAM-CCRCBs dosage was obtained.5(a) and 5(b) represent the combined effect of initial metal ion concentration and pH on percentage removal and adsorption capacity of Cr(VI) ion using EAM-CCRCBs, respectively.e adsorption capacity of Cr(VI) ions was increased with increased metal ion concentrations, and the percentage removal of Cr(VI) ions was decreased with increased metal ion concentrations.is may be due to the availability of active sites is sufficient to occupy the metal ions at lower concentrations.e maximum percentage removal of 92.3% and adsorption capacity of 63.73 mg/g at constant pH of 2 and initial concentration of 374.02 mg/L at EAM-CCRCBs dosage of 0.13 g was obtained.

Optimization by RSM.
In order to achieve the maximum adsorption of Cr(VI) ions using EAM-CCRCBs, the optimum process variables were found from the developed mathematical model.e optimum condition suggested by the model for the process variable such as adsorbent dosage, and initial metal ion concentration was 0.14 g, and 300 mg/L at pH 2 with maximum removal of 99.8% and adsorption capacity of 52.7 mg/g, respectively, and is shown in Table 7.

Continuous Adsorption
Studies.e performance of a �xed bed column can be described through the concept of breakthrough curve analysis.e time to reach the breakthrough point and shape of the breakthrough curve are very important characteristics for determining the operation and the dynamic responses of an adsorption column.In our previous batch adsorption study and present optimization using RSM studies, both show that the percentage removal of Cr(VI) ion is maximum at pH 2. us, the effect of �ow rate and bed height was conducted at the solution pH of 2.

Effect of Flow
Rate. e effect of �ow rate for the adsorption Cr(VI) ions using EAM-CCRCBs was studied at various �ow rates of 25 mL/min and 50 mL/min at bed height of 25 cm, at an inlet concentration 300 mg/L and is shown in Figure 6.From Figure 6 it is observed that the rapid uptake of metal ion initial stages, later on the rate decreases slowly and �nally it reached saturation.For higher �ow rate, the rate of reaching the breakthrough time is faster whereas in lower �ow rate the rate of reaching the breakthrough time is slower.is may be due to the residence time �istribution of in�uent concentration to the adsorbent is greater in lower �ow rate [35,36].

Effect of Bed
Height.e adsorption of Cr(VI) ions in the packed bed column is largely dependent on the bed height, which is directly proportional to the quantity of EAM-CCRCBs in the column.e effect of bed height on breakthrough curve analysis was studied by varying the bed height to 15 cm, 20 cm, and 25 cm.e adsorption breakthrough curves were obtained by varying the bed heights at a �ow rate of 25 mL/min and an inlet Cr(VI) ion concentration of 300 mg/L.e breakthrough curve is presented in Figure 7. Faster breakthrough curves were observed for a bed height of 15 cm compared to the bed height of 20 cm and 25 cm.Higher bed contain more adsorbent; therefore, more binding sites will be available for the Cr(VI) ions to attach, which makes the rate of reaching of breakthrough time lesser [35,36].

Effect of Initial Metal Ion
Concentration. e effects of the two initial Cr(VI) ions concentrations (100 mg/L and 300 mg/L) on the adsorption process at a constant �ow rate of 25 mL/min and �xed bed height of 25 cm are shown in Figure   8.It can be deduced that, at a lower inlet concentrations, a slower breakthrough curve and the highest treated volume are obtained.e breakthrough point for 100 mg/L and 300 mg/L of Cr(VI) ions inlet concentrations occurred aer 360 min and 150 min respectively.and20 min, respectively.e slow transport of Cr(VI) ions onto EAM-CCRCBs was due to the lower concentration gradient and resulted in a slower breakthrough curve [35,36].Conversely, a higher concentration of Cr(VI) ions has been shown to lead to a higher driving force for Cr(VI) ions to overcome the mass transfer resistance in the liquid phase.Consequently, quick saturation of the available binding sites for Cr(VI) ions has caused the breakthrough time to decrease with the increasing inlet Cr(VI) ions concentration.

Bed Depth Service Time (BDST).
BDST model is used to predict the bed capacity by utilizing the different breakthrough values [37].e modi�ed version of the equation used in this evaluation is given as follows: where  is the time (mins),   is the adsorption capacity (mg/L),  0 is the inlet concentration of Cr(VI) ions (mg/L),  is the linear velocity of Cr(VI) ions across the column (cm/min),  is the bed depth (cm),   is the rate constant in BDST model (L/mg⋅min), and   is the effluent concentration of the Cr(VI) ions (mg/L).A plot of  versus  is expected to yield a linear curve in which  0 and   could be evaluated, from the slope and -axis intersection point, respectively.
BDST analysis was done, and the linear plot of this model is given in Figure 9. From Figure 9, the values of   and   were determined to be 150 mg/L and 0.0167 L/mg⋅min, respectively.Besides that, the correlation coefficient value ( 2 = 1.000) shows that this model is applicable.e constants obtained from this model could be utilized to scaling up the process of this �xed bed column [38].

omson Model
. omas developed a model for adsorption processes in which external and internal diffusion limitations are not present [34].e linearized form of the omas model [36] can be expressed as where   is the omas rate constant (mL/min⋅mg),   is the adsorption capacity of Cr(VI) ions uptake (mg/g),  0 is the inlet Cr(VI) ions concentration (mg/L),   is the effluent Cr(VI) ions concentration at time  (mg/L),  is the mass of adsorbent (g),  is the inlet �ow rate (mL/min), and  is the �ow time (min).e value of  0 /  is the ratio of inlet to outlet Cr(VI) ions concentrations.A linear plot of ln [(  /  ) − 1] against time () was drawn to determine the values of   and   from the interception point and slope of the plot, respectively.e data obtained from the experiment were �tted to the omas model using (7) and are shown in Figure 10 [39].From Figure 10, it is observed when the inlet Cr(VI) ions concentration increased from 100 mg/L to 300 mg/L, the   decreased from 0.004982 to 0.00367 mL/min⋅mg while the   increased from to 1730.89 to 2535.07 mg/g.is may be due to the higher driving force of the higher inlet Cr(VI) ions concentration [40].e  2 values for 100 mg/L and 300 mg/L were 0.9632 and 0.9492, respectively, which shows that omas model �ts well with the experimental data.omas model predicts the monolayer adsorption which also conformed with our earlier batch adsorption studies where the experimental data �ts well with Langmuir isotherm.

Conclusion
RSM is an effective tool for optimizing the process variable.e optimized condition was obtained for the removal of Cr(VI) ions using EAM-CCRCBs was 300 mg/L of initial concentration, solution pH of 2, and adsorbent dosage of 0.14 g/25 mL.e breakthrough curve analysis from continuous adsorption studies reveals that the slower breakthrough time reached for lesser initial concentration, slower �ow rate, and higher bed height.Bed Height service model and omson model were well �tted with experimental data.omson model reveals the monolayer adsorption which also conformed with our earlier batch adsorption studies where the experimental data �tted well with Langmuir adsorption isotherm.

4 F 1 :
e actual and predicted plot for percentage removal of Cr(VI) ions using EAM-CCRCBs.

5 F 2 :
e actual and predicted plot for adsorption capacity of Cr(VI) ions using EAM-CCRCBs.
d s o r b e n t d o s a g e ( g ) (b) F 3: e combined effect of pH and adsorbent dosage on (a) percentage removal and (b) adsorption capacity of Cr(VI) ions using EAM-CCRCBs.

2 AF 4 :
it ia l c o n c e n tr a ti o n ( p p m ) : a d s o r b e n t d o s a g e ( g ) it ia l c o n c e n tr a ti o n ( p p m ) Adsorption capacity (mg/g) e combined effect of initial concentration and adsorbent dosage on (a) percentage removal and (b) adsorption capacity of Cr(VI) ions using EAM-CCRCBs.

Figures 4 (F 5 :
a) and 4(b) indicate the combined effect of initial metal ion concentration and adsorbent dosage on percentage removal and adsorption capacity of Cr(VI) ion using EAM-CCRCBs, respectively.e adsorption capacity was decreased with increased adsorbent dosage, and percentage removal of Cr(VI) ions was increased with increased adsorbent dosage.e decrease in adsorption capacity might be attributed to the shortage of metal ion concentration in the solution since the initial metal ion it ia l c o n c e n tr a ti o n ( p p m ) it ia l c o n c e n tr a ti o n ( p p m ) e combined effect of initial concentration and pH on (a) percentage removal and (b) adsorption capacity of Cr(VI) ions using EAM-CCRCBs.

F 10 :
omas plot for the adsorption of Cr(VI) ions on EAM-CCRCBs.
2, 2, and  2 are signi�cant model terms.For adsorption capacity, , , , ,  2 ,  2 are signi�cant model terms.Values greater than 0.1000 indicate that the model terms are not signi�cant. T 1: Variables and levels considered for the adsorption of Cr(VI) using EAM-CCRCBs by CCD.
T 3: Model summary statistics for percentage removal of Cr(VI) using EAM-CCRCBs.Analysis of variance (ANOVA) for response surface quadratic model for percentage removal of Cr(VI) using EAM-CCRCBs.
Where  is the �sher value which is the ratio of mean square of the term to the mean square of the residual� and  is low-probability value.T 6: Analysis of variance (ANOVA) for response surface quadratic model for adsorption capacity of Cr(VI) using EAM-CCRCBs.
T 7: Optimized process variables value for adsorption of Cr(VI) ions by EAM-CCRCBs.