Comparative Studies of Diclofenac Sodium (NSAID) Adsorption on Wheat (Triticum aestivum) Bran and Groundnut (Arachis hypogaea) Shell Powder using Vertical and Sequential Bed Column

Wheat bran and groundnut shell powder have been used to study the mechanism of diclofenac sodium adsorption from aqueous solution using batch as well as column modes and maximum uptake is 84.3% for wheat bran and 82.4% for groundnut shell powder at pH 6, drug concentration 1 mg/L at 298 K for 30min. Isotherm and error analysis reveals that Freundlich and Langmuir isotherms fitted well. Kinetic studies show that the adsorption process follows second-order kinetics and thermodynamic study shows endothermic adsorption process. Column adsorption study is important for industrial scale adsorption and column studies have been carried out using vertical bed and sequential bed adsorption columns at pH 6 which is the optimum pH for maximum adsorption for batch experiments. Vertical and sequential bed columns setup is simple and economical which provides flow under gravity. The effect of varying inlet feed concentration and flow rate on the breakthrough and exhaustion time of columns has been studied to determine the bed capacities of both columns. Thomas model and Yoon-Nelson models fitted well with experimental data for continuous flow column studies.

Presence of analgesics and antipyretics in aquatic bodies has been determined as an emerging issue due to the adverse effects on the aqueous environment as well as human health [1-5]. Among the most consumed non-steroidal, anti-inflammatory, non-prescription drugs, diclofenac sodium [5] is the most frequently detected component in pharmaceutical wastewaters and surface water in the range 0.05-1 μg/L [1,6-8]. Diclofenac sodium, 2-[(2, 6-chlorophenyl) amino], a phenylacetic acid derivative is used for the treatment of degenerative joint diseases [9-11], however, this drug is highly toxic for vultures and cattle [10,12]. Various methods proposed for the removal of diclofenac from aqueous media include biodegradation [13], advanced oxidation and electrocoagulation, and electro-oxidation [1,14,15]. However due to the complexity of the structure of diclofenac, biodegradation is not effective, and advanced oxidation forms harmful byproducts [5] (Chemical structure 1). As a result, the adsorption process due to its high selectivity at low concentrations, easy regeneration of adsorbent, and cost-effectiveness [2,9] has gained importance. A variety of adsorbents have been studied [16], of which activated carbon is the most commonly used, but a major drawback is the cost of adsorbent as well as the regeneration cost [9]. As such there is a need for cost-effective adsorbents for the removal of drugs [17]. Literature reports the effective use of freely available agricultural by-products, as adsorbents for the removal of drugs [2,18-21] because they are inexpensive, freely available in bulk, have good adsorption potential, are selective in nature and there is no need of regeneration. The present study is aimed at studying the adsorption potential of wheat (Triticum aestivum) bran and groundnut (Arachis hypogaea) shell powder for the removal of diclofenac sodium from aqueous solution. Most of the reports on the removal of pharmaceutical pollutants pertain to batch studies but the equilibrium data obtained from batch adsorption experiments are not sufficient for designing a water treatment system on an industrial scale. For continuous operation [22] few reports are available on the use of sequential bed columns [22]. Keeping in mind the above factors, continuous column studies have also been conducted using vertical as well as sequential bed columns, as the set-up is simple and economical, no feed pumps or other mechanical devices are required, as the solution is made to flow under gravity, and can be applicable for small scale industries.

Preparation of adsorbent

Preparation of wheat (Triticum aestivum) bran: Wheat bran (Bagrry’s) was washed, dried, crushed, and sieved to achieve a particle size range of 250-420 microns, and used as an adsorbent [23].

Preparation of groundnut (Arachis hypogaea) shell powder: Groundnuts were deseeded and shells were washed, dried in sunlight for 2-3 days, and finely ground. After sieve analysis 250-420 micron size particles were used as adsorbent [24,25].

Preparation of adsorbate solution

Diclofenac sodium injection (INTAS Pharmaceuticals Ltd.) of a 3 mL vial, containing 75 mg of the drug was used to prepare the stock solution of diclofenac sodium having a drug concentration of 100 mg/L in distilled water. Working solutions of 0.1 - 1.0 mg/L were obtained by dilution.

Estimation of diclofenac sodium

The drug solution prepared by dilution is analyzed spectrophotometrically, at a maximum wavelength of 276 nm [6,26] using a U.V- visible spectrophotometer (Shimadzu 2450, Japan) with 1 cm optical path length quartz cells. Calibration curves were plotted and distilled water was used as blank.

Batch adsorption studies

Batch experiments have been conducted by adding a known quantity of adsorbent (i.e 10 mg) in 10 mL of diclofenac solution in the concentration range 0.1-1.0 mg/L (keeping detection limit of the drug in wastewater), pH (5-11) under constant stirring for regular time intervals (i.e 5 to 30 min., until equilibrium is attained). The pH of the solution was adjusted using 0.1M HCl/NaOH. After adsorption experiments, the concentration of the supernatant was determined using a UV-visible spectrophotometer, (Shimadzu 2450, Japan) [27]. Each set of experiments was done in triplicate to check the reproducibility of results.

Column adsorption studies

Continuous flow column studies have been conducted using vertical as well as sequential bed adsorption columns by feeding the drug solution of varying influent concentration (0.6, 0.8, 1.0 mg/L) at different feed flow rates (5 mL/min., 7.5 mL/min. and 10 mL/min.) at pH 6 for both the adsorbents. The adsorption column was operated until the column had been exhausted or saturated (i.e. effluent concentration of the drug was approximately reaching 90% of C_o (feed concentration) i.e. (C_t/C_o = 0.9) [22].

Experimental setup

Vertical column: A vertical column of 1.5 cm diameter and 20 cm height containing adsorbent up to a depth of 1 cm was set up (Figure 1) (1.2 gm wheat bran and 1 gm groundnut shell powder). The downflow mode was used to collect the effluent from the bottom at regular time intervals and analyzed using U.V- a visible spectrophotometer [22].

Sequential bed column: The sequential bed column setup of 4 glass tubes of 1.5 cm diameter in a sequential manner was used in which each step contains 1/4^th part of the total amount of adsorbent that is taken in the vertical bed column (Figure 1) [22]. Samples were collected from the bottom at regular time intervals for the determination of adsorbate concentration.

Breakthrough curves

The breakthrough curves analyses of continuous flow adsorption columns were studied using determined effluent concentration at different time intervals and plots were obtained C_t/C_o against contact time (t), where (C_t) is effluent concentration and (C_o) is adsorbate inlet concentration. The breakthrough point was obtained at C_t = 0.5C_o and saturation at C_t = 0.9C_o [22].

Characterization of adsorbents

Proximate analysis: The moisture content is found to be 7.8% for wheat bran and 9.7% for groundnut shell powder at 100 °C. Samples were further heated to get constant weight and ash content was found to be 3.90% for wheat bran and 7% for groundnut shell powder at 600 °C [24].

Sieve analysis: Sieve analysis was carried out and particles of 250-420 microns in size were used as adsorbent in both cases [24].

X-ray diffraction (XRD): XRD (Figure 2a) of wheat bran shows the amorphous nature of the sample with no prints of crystallinity [28]. Groundnut shell powder (Figure 2c) indicates broad diffraction peaks with traces of amorphous cellulose in the sample [24].

BET: The Brunauer-Emmet-Teller (BET) analysis of both adsorbents was performed by initial degassing at 120°C [22]. The surface area is 112 m²/g the pore volume is 0.10 cm³/g and the mean pore size is 5.14 nm of wheat bran. The surface area is 123 m²/g, the pore volume is 0.19 cm³/g, and the mean pore size of 6.17 nm groundnut shell powder.

Scanning Electron Microscopy (SEM): The SEM images show that wheat bran (Figure 2b) exhibits a platelet-like structure and groundnut shell powder (Figure 2d) has a porous rough surface [29] and exhibits a flake-like structure [30].

FTIR: FTIR of wheat bran (Figure 3a) shows bands band at 3433 cm^-1, 2932 cm^-1, and 1649 cm^-1 corresponding to the (O-H) and chemisorbed water, stretching vibrations of C-H group and carbonyl group (C=O). The peak at 1151 cm^-1 is assigned to (-CN) and at 1029 cm^-1 for the C-O bond of cellulose [23]. After adsorption (Figure 3b) presence of diclofenac sodium shows due to 3290 cm^-1, 1666 cm^-1, and 1579 cm^-1. The peak at 1373-1337 cm^-1 corresponds to C-N stretching [31] of diclofenac after adsorption and at 749 cm^-1 for diclofenac charaC_teristic [31].

In groundnut, shell powder (Figure 3c), the peak at 3405, 1731, and 1638 cm^-1 is due to (O-H) and chemisorbed water, hemicelluloses, and lignin respectively [22]. After adsorption (Figure 3d) the peak at 747 cm^-1 shows the presence of diclofenac.

Effect of pH and point zero charge (pHpzc)

To study the effect of pH on the removal of diclofenac pH was varied in the range 5-11 (precipitation occurs at pH 4.8 and beyond pH 11) for a contact time of 30 min., drug concentration (0.1-1.0 mg/L) at 25 ºC. The adsorption of diclofenac increases with an increase in pH and maxima obtained at pH 6 for both the adsorbents (Figure 4). For diclofenac sodium (pKa 4.1). Below pH 4, cationic diclofenac sodium, and above pH 4 anionic diclofenac was predominate [32,33]. Point zero charge, pHpzc was found to be 6.2 for wheat bran and 6.4 for groundnut shell powder [22,23]. Above pH 4, electrostatic attraction between anionic diclofenac and positively charged adsorbent is the predominant mechanism at pH 6 for both adsorbents. Above pH 6, a negatively charged adsorbent surface causes electrostatic repulsion for diclofenac sodium; which leads to a decrease in adsorption [34].

Effect of initial concentration and contact time

The effect of initial drug concentration and contact time has been studied for the concentration range 0.1 - 1.0 mg/L at varying pH (5-11), with contact time 0 to 30 min. at 25 ºC. Figure 5 shows that with the increase in concentration and contact time, the amount adsorbed increases, and equilibrium was obtained within 30 min. in both cases, which may be attributed to larger surface site availability at early stages and higher dilution. With time, available sites get occupied, leading to a decrease in the adsorption rate [27].

Adsorption isotherm modeling

The experimental data at equilibrium conditions were analyzed using Freundlich (Eq.1), Langmuir (Eq.2), and Temkin (Eq.4) adsorption isotherm models.

$log q_e = log K_f + \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right. log C_e$ (1)

$\raisebox{1ex}{$C_e$}\!\left/ \!\raisebox{-1ex}{$q_e$}\right. = \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$K_L$}\right. + \raisebox{1ex}{$a_L$}\!\left/ \!\raisebox{-1ex}{$K_L$}\right. C_e$ (2)

where Ce is the equilibrium concentration (mgL^-1) and q_e is the equilibrium amount adsorbed (mgg^-1), n and K_f are Freundlich isotherm (Figure 6a) constants, K_L related to the affinity of the binding sites (Lmg^-1), aL the Langmuir isotherm (Figure 6b) constant. Langmuir model can be expressed in terms of separation faC_tor or equilibrium parameter RL,

$R_L= \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$(1+a_L C_o)$}\right.$ (3)

The values of RL are found to be 0.034 and 0.042 for wheat bran and groundnut shell powder respectively indicating that favorable adsorption occurs in each case [27].

$q_e = B_T ln A_T + B_T ln C_e$ (4)

Where A_T is the equilibrium binding constant (Lg^-1) corresponding to maximum binding energy, B_T = (RT)/bT, is constantly related to the heat of adsorption (Jmol^-1), bT is Temkin isotherm (Figure 6c) constant, T is the absolute temperature (K) and R is the universal gas constant, 8.314 J mol^-1 K^-1. The values of isotherm constants are presented in Table 1. Freundlich and Langmuir isotherm models show favorable and monolayer adsorption and effectiveness of the adsorbent [20,34]. A straight line plot of q_e vs. ln C_e was obtained, which indicates the applicability of the Temkin isotherm model [32].

Error analysis for isotherm studies

To determine the best fit of isotherm models, five different error functions of non-linear regression can be used [24], as follows:

Sum of squared errors (SSE)

$SSE= {\displaystyle {\sum }_{i=1}^n{(q_{e, cal}- q_{e, exp})}^2}$ (5)

Sum of absolute errors (SAE)

$SAE= {\displaystyle {\sum }_{i=1}^n│q_{e, cal}-q_{e, exp}{│}_i}$ (6)

Average relative error (ARE)

$ARE= \frac{100}{n}{\displaystyle {\sum }_{i=1}^n│\frac{{\text{q}}_{\text{e},\text{ cal}}-{\text{q}}_{\text{e},\text{ exp}}}{{\text{q}}_{\text{e},\text{ exp}}}│}$ (7)

Hybrid fraC_tional error function (HYBRID)

$HYBRID= {\displaystyle {\sum }_{i=1}^n{\left[\frac{{(q_{e, exp}- q_{e, cal})}^2}{q_{e, exp}}\right]}_i}$ (8)

Marquardt’s percent standard deviation (MPDS)

$MPSD= {\displaystyle {\sum }_{i=1}^n{\left[\frac{q_{e, exp}- q_{e, cal}}{q_{e, exp}}\right]}^2}$ (9)

The values of the five error functions have been calculated and a comparison among error function values (Table 2) suggests that Freundlich and Langmuir isotherm models, with high correlation coefficients and low error function values, are a better fit as compared to the Temkin model for the adsorption of diclofenac on both the adsorbents under study [24].

Kinetic studies

The equilibrium data obtained optimum conditions for both the adsorbents have been analyzed for kinetic studies using pseudo first order (Figure 7a) (Eq. 10), second-order (Eq. 11) rate equations, and the possibility of intraparticle diffusion, using Morris Weber model (Eq. 12).

$log (q_e–q) = log q_e - k_{ad} X t/ 2.303$ (10)

$\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$q$}\right. = \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$K_2$}\right. X \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$q_e{}^2$}\right. + \raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$q_e$}\right.$ (11)

where q_e and q (mg g−1) are the amounts of drug adsorbed at equilibrium and at any time taken for study respectively, t (min) is the time of contact and kad is the adsorption rate constant (min^-1), K₂ is the equilibrium rate constant of pseudo-second order adsorption (gmg^-1 min^-1). The values of kinetic constants are given in Table 1. A straight line plot (Figure 7b) obtained from the plot of t/qt vs. t shows a better fit of pseudo-second-order kinetics [24].

$q=K_{p } x t^{1/2}$ (12)

where q is the amount of drug adsorbed at different time intervals (mgg^-1), K_p is the intraparticle diffusion constant (mgg^-1 min^-1) and t is contact time (min.). K_p is calculated from the slope of the linear plot of q vs t^1/2 and is given in Table 1.

A straight line plot (Figure 7c) has been obtained but does not pass through the origin, which indicates that intraparticle diffusion occurs but is not the rate-determining step [27].

Effect of temperature

The effect of temperature was investigated for the temperatures; 25º, 30º, 35º, 40º, and 45 ºC. The adsorption capacity increases from 0.0079 to 0.0082 mg/g for wheat bran and 0.0072 to 0.0081 mg/g for groundnut shell powder with an increase in temperature from 298K to 318K respectively. The enhancement in adsorption capacity with an increase in temperature occurs due to an increase in the number of sorption sites generated because of the breaking of some internal bonds near the active surface sites of the adsorbent in each case [9].

Thermodynamic studies

The following equations have been used to determine thermodynamic parameters for the adsorption process.

$△G^o= -RT\ln K_c$ (13)

$\ln K_c= \raisebox{1ex}{$△S^o$}\!\left/ \!\raisebox{-1ex}{$R$}\right.- \raisebox{1ex}{$△H^o$}\!\left/ \!\raisebox{-1ex}{$RT$}\right.$ (14)

K_c is the equilibrium constant given by C_soild/C_liquid; where C_soild is the solid phase equilibrium concentration and C_liquid is the liquid phase equilibrium concentration of adsorbate (mg/L), T is the temperature in degree Kelvin, and R is gas constant. The values of change in enthalpy (∆H⁰) (J/K/mol) and entropy (∆S⁰) (J/K^-1mol^-1) are 0.109 and 0.122 for wheat bran and 0.203 and 0.115 for groundnut shell powder. The value of ∆G⁰ is found to be (-2.24 x 103) and (-2.22 x 103) for wheat bran and groundnut shell powder respectively. The negative values of ∆G⁰ indicate feasibility and spontaneous nature, and positive values of ∆H⁰ and ∆S<⁰ for both adsorbents suggest the endothermic nature of adsorption and increase in randomness at solid solution interface during adsorption [9,27].

Column adsorption study

Effect of feed Concentration and flow rate: An attempt has been made to study the effect of influent concentration and the flow rate on the efficiency of the columns. The concentrations have been varied between 0.8 to 1.0 mg/L at a flow rate of 5 mL/min. and it was observed that the removal efficiency decreases with an increase in concentration for vertical as well as sequential adsorption columns, therefore at higher dilution larger volumes can be treated due to a decrease in concentration gradient and mass transfer coefficient [24,34,35]. The adsorption capacities of wheat bran and groundnut shell powder are 0.0067 mg/g and 0.0050 mg/g for vertical bed column and 0.0061 mg/g and 0.0060 mg/g for sequential bed column for 0.8 mg/L feed concentration. For feed concentration 0.9 mg/L, the adsorption capacities of wheat bran decrease to 0.0055 mg/g and for groundnut shell powder to 0.0040 mg/g for the vertical bed column. The values are 0.0050 mg/g and 0.0039 mg/g for wheat bran and groundnut shell powder sequential bed column. This further decreases to 0.0041 mg/g and 0.0037 mg/g for vertical bed and 0.0048 mg/g and 0.0035 mg/g for sequential bed column for wheat bran and groundnut shell powder respectively for 1.0 mg/L feed concentration at a flow rate of 5 mL/min. In all the cases.

To study the effect of flow rate the flow rates have been varied from 5 mL/min. to 10 mL/min., at a constant influent concentration of 0.8 mg/L in both the columns, it has been observed that at a higher flow rate, the column was saturated earlier probably due to lesser residence time of solution in bed which resulted in lesser mass transfer and lesser adsorption [24]. For fixed feed concentration of 0.8 mg/L, when the flow rate was increased from 5 mL/min. to 7.5 mL/min. to 10 mL/min., the adsorption capacities of wheat bran have been found to decrease from 0.0067 mg/g to 0.0050 mg/g to 0.0042 mg/g for vertical bed column and from 0.0061 mg/g to 0.0055 mg/g and 0.0044 mg/g for sequential bed column. Similarly for groundnut shell powder adsorption capacities decrease from 0.0050 mg/g to 0.0040 mg/g to 0.0038 mg/g for vertical bed and from 0.0060 mg/g to 0.0040 mg/g to 0.0037 mg/g for sequential bed column.

The breakthrough time corresponding to C_t/C_o = 0.5 and the exhaustion time corresponding to C_t/C_o = 0.90 were obtained for drug concentration 0.8 mg/L for both the adsorbents using vertical as well as sequential columns. breakthrough point occurred after 30 min and exhaustion time was 60 min for wheat bran using vertical column and 40 min. and 80 min. for the sequential bed column while for groundnut shell powder breakthrough occurred at 40 min and 70 min. For vertical and sequential adsorption columns and the exhaustion time was 50 min and 60 min for vertical and sequential adsorption columns respectively (Figure 8). As the drug concentration increased, the breakthrough curve became steeper, and the breakthrough time decreased [24,36].

Modeling of breakthrough curves

The data obtained have been subjected to analyses using the Thomas model and Yoon–Nelson model.

Thomas model: The Thomas model is a widely used method, based on breakthrough results, to study column performance. The linear form of the model is given as:

$ln\left[\raisebox{1ex}{$C_o$}\!\left/ \!\raisebox{-1ex}{$C_t$}\right.- 1\right]= K_{TH}{q}_o\raisebox{1ex}{$M$}\!\left/ \!\raisebox{-1ex}{$Q$}\right.- K_{TH} C_o t$ (16)

Where K_TH is Thomas rate constant (L min^-1mg^-1); qo the maximum solid-phase concentration of the solute (mg g^-1) (adsorption capacity of the bed), M is the mass of adsorbent (g); Q is the flow rate (L min^-1). The values of K_TH and the qo (Table 3) can be determined from the plot of [ln(Co/C_t)-1] against time at a given flow rate [24]. As the initial drug concentration increases the values of K_TH (L min^-1mg^-1) and adsorption capacity, qo (mg/g) shows a significant decrease, whereas, K_TH and qo increase with an increase in flow rate, for both the columns, which may be attributed to the higher driving force due to difference between the solid and liquid phase concentration of adsorbate [24,19].

The Yoon–Nelson model: This model is a simple theoretical model used to prediC_t the rate constant and time required for a 50 % adsorbate breakthrough. The linearized model is expressed as:

$\ln \raisebox{1ex}{$C_t$}\!\left/ \!\raisebox{-1ex}{$C_o$}\right.- C_t= K_{YN}t- \tau K_{YN}$ (17)

where K_YN is the rate constant (l mg^-1), τ the time required for 50% adsorbate breakthrough (min). The constants K_YN and τ can be determined from a plot of [ln(C_t/C_o)-C_t] against time t at any given adsorption conditions (Table 3). It has been observed that the value of K_YN increases and τ, time taken for 50% breakthrough decreases with an increase in initial drug concentration and flow rate [24,39,40].

At present wheat bran and groundnut shell powder have been used as agricultural waste adsorbents for the removal of diclofenac sodium and maximum uptake is 84.3% for wheat bran and 82.4% for groundnut shell powder at pH 6, drug concentration 1 mg/L at 298 K for 30 min. As shown by SEM studies both the adsorbents have an amorphous nature and rough surface so the possibility of drug adsorption is more which is further confirmed by FTIR results. As mentioned in the literature the drug concentration detected in wastewater is in the range of micrograms per liter that’s why a study has been conducted at low concentrations of adsorbate solution. The equilibrium adsorption capacities for continuous flow column studies using both sequential bed and vertical columns show that both the adsorbents can be used as efficient adsorbents for pharmaceutical pollutant removal. Adsorption capacity was found to be slightly greater for sequential bed columns as compared to vertical columns, which shows that the sequential bed column can be efficiently used in small-scale industries for the removal of water pollutants, also the setup is simple and economical. On the other hand, a comparative study of both the adsorbents shows that both the adsorbents are effective for the removal of drugs, whereas wheat bran shows slightly greater adsorption capacity for the removal of diclofenac sodium.

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