Abstract

Punica granatum carpellary membrane (PGCM) has been chemically modified with 2,4-dinitrophenylhydrazine by one step reaction process. Pristine PGCM and modified Punica granatum carpellary membrane (MPGCM) have been employed as adsorbents for removal of cadmium (II) ions from aqueous solution. Modification on the surface of PGCM was characterized by Fourier transform infrared, scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction techniques. Removal of Cd (II) ions was carried out at various optimized conditions, i.e. adsorbent dose (20 mg/l), pH (7), contact time (2 h), and temperature (303 K). The Langmuir isotherm model was the best fit with a high value of regression coefficient (R2 = 0.991–0.999) compared with the Freundlich and Temkin models. Langmuir maximum adsorption capacity for cadmium was found to be 111.11 mg/g for PGCM and 142.85 mg/g for MPGCM. Kinetics of adsorption of cadmium onto MPGCM was best described by pseudo-second-order kinetics model compared with pseudo-first-order. The negative value of free energy change showed that adsorption was highly spontaneous and favorable. Also the positive value of entropy and enthalpy showed that the process of adsorption was associated with increased randomness and endothermic in nature. Mechanism of removal of cadmium ions from water by MPGCM has also been explicated.

INTRODUCTION

Elements such as lead, mercury, cadmium and arsenic have many environmental implications and cause major human health problems in different parts of the world. The World Health Organization (WHO) has identified ten chemicals of major public health concern that include cadmium, arsenic, lead and mercury (WHO 2017) which when present at higher concentrations than permissible limits pose adverse effects on various biological systems. It was strongly recommended that there should be a multi-sectoral action plan to protect the environment and human health from the harmful effects of chemicals. Some of the greatest threats to human health come from contaminated water.

Discharges of water from industries and households are the main sources of water pollution. Heavy metals such as lead, arsenic, cadmium and chromium are the main constituents of various industrial processes (Chowdhury et al. 2016). Cadmium metal is most widely used in pigments, smelting, metal plating, cadmium nickel batteries, alloy industries and phosphate fertilizers (Kumar & Chawla 2014; Kumar et al. 2015). The presence of cadmium in water is a very serious issue because cadmium is non-biodegradable in nature and has a tendency to bio-accumulate in the biological systems which may lead to serious acute or chronic diseases (Kumar & Chawla 2014). Therefore it is necessary to develop some methods and techniques for the removal of cadmium from industrial effluents before their discharge into water bodies.

Many techniques have been developed for removal of heavy metal ions from water. Conventional methods used for the removal of cadmium and other heavy metal ions from water are chemical oxidation and reduction method, ion-exchange, filtration, chemical precipitation, electrochemical treatment, membrane technologies, reverse osmosis, coagulation-sedimentation, reverse osmosis, and adsorption (Chawla et al. 2015; Ihsanullah et al. 2016; Kaur et al. 2016; Kyzas et al. 2016; Wang et al. 2016; Zhao et al. 2016). However, some methods are quite expensive, complicated to handle and less efficient for water treatment at large scale. Among these techniques, adsorption is one of the preferred methods for removal of heavy metal ions from water/wastewater because of its low cost, and because it is easy to operate and highly efficient. Various synthetic materials such as nano-materials are used as adsorbent for removal of cadmium from water but these are expensive and also pose certain environmental implications (Mahmoud et al. 2016; Zhao et al. 2016). A low cost material with high efficiency to remove cadmium from water is still a challenge and more efforts are required in this direction. Many plant waste materials and peels have also been used as natural adsorbents worldwide for the treatment of wastewater contaminated with heavy metals. Availability of natural adsorbents is another important aspect as some of the adsorbents showing high adsorption capacity are not available everywhere. Pomegranate is one of the easily available commercial fruits in India. Moreover, in some parts of India, especially in central and southern India, it is available throughout the year. Pomegranate belong to the genus Punica, order Myrtales and family Lythraceae. Few researchers have used only the pomegranate peel for the removal of heavy metals such as chromium (Najim & Yassin 2009), iron (Moghadam et al. 2013), lead (Alam et al. 2012), cadmium (Kumar & Kumar 2015), lead and copper (El-Ashtoukhy et al. 2008) from water or wastewater using the adsorption method.

The pomegranate fruit has several anatomical parts such as peel, carpellary membrane, seeds and arils. Half of the total fruit weight corresponds to the peel. The carpellary membrane, which separates the seeds within the fruit, acts as a protective membrane and contains poly-phenols including gallic acid (trihydroxy benzoic acid), ellagic acid (EA) (hexahydroxydiphenic acid), gallagic acid condensed tannins, catechins, gallocatechins and prodelphinidins, flavonoids (luteolin, quercetin, kaempferol, gallagic, EA glycosides, EA, punicalagin, punicalin, pedunculagin), proanthocyanidin, ellagitannin compounds, minerals and complex polysaccharides (Kulkarni et al. 2004; Mohammad & Kashani 2012). The aim of this study is to use Punica granatum carpellary membrane (PGCM) in its pristine form and chemically modified PGCM with 2,4-dinitrophenylhydrazine (MPGCM) for the removal of cadmium metal ions from water and wastewater. In this study, PGCM and MPGCM were used as new adsorbents to investigate the effect of the various physiochemical parameters such as pH, contact time, adsorbent dose, temperature, etc., on adsorption of cadmium ions. Different techniques were employed for characterization of both adsorbents, i.e. pristine PGCM and modified MPGCM, before and after the adsorption process.

MATERIALS AND METHODS

Experimental

Reagent grade 2,4-dinitrophenylhydrazine (2,4-DNP) from E. Merck Ltd, India, was used for the modification of PGCM. AR/GR grade cadmium nitrate [Cd(NO3)2] was received from E. Merck Ltd, India and was used without further purification. The stock solution of cadmium nitrate of 1,000 mg/l was prepared in double distilled water. The solutions of various concentrations (20 to 100 mg/l) were prepared by dilution of the stock solution. 0.1 N HNO3 and 0.1 N NaOH were obtained from LOBA Chemie, India and used for adjusting pH while carrying out pH experiments. The surface area, pore diameter and pore volume of the adsorbents were determined by the Brunauer–Emmett–Teller (BET) method. Regeneration of adsorbents was carried out by 20% H2SO4. Thermodynamic studies were carried out at 297 K to 333 K at optimized conditions. All used apparatus was firstly washed with aqua regia then with double distilled water.

Modification of adsorbent

PGCM was carefully separated from the pomegranate fruit collected from Gurgaon, India and washed with double distilled water several times. Initially, it was sun dried followed by drying in an oven at 373 K. The dried sample was finally crushed, ground properly and sieved to get the fine particle size of the adsorbent. The fine powdered sample was stored in closed glass bottles for further use. PGCM was modified by 2,4-DNP. For surface modification: 2.5 g of 2,4-DNP was mixed in 50 ml of ethanol with 1–2 drops of sulfuric acid and filtered to obtain a clear solution of 2,4-DNP (Figure 1(a)). This solution was mixed with PGCM powder and stirred on a magnetic stirrer for 8 to 10 hours at 323–333 K. After stirring, the solution was filtered and washed several times with ethanol and dried in the oven for further use.

Figure 1

Schematic diagram (a) modification of PGCM by 2,4-dinitrophenylhydrazine, (b) removal of cadmium (II) ions by ion exchange mechanism with MPGCM and (c) removal of cadmium (II) ions by coordination mechanism with MPGCM.

Figure 1

Schematic diagram (a) modification of PGCM by 2,4-dinitrophenylhydrazine, (b) removal of cadmium (II) ions by ion exchange mechanism with MPGCM and (c) removal of cadmium (II) ions by coordination mechanism with MPGCM.

Characterization and adsorption studies

The Fourier transform infrared (FTIR) spectrum was recorded by FTIR spectrophotometer in the range of 500–4,000 cm−1 (Shimadzu). X-ray diffraction (XRD) analysis (RIGAKU) of the adsorbent and scanning electron microscope (SEM) analysis (JEOL JSM-6100) was carried out to study the morphology of the adsorbent.

Adsorption of cadmium (II) ions on PGCM and MPGCM were studied using the batch mode adsorption method by shaking a known amount of PGCM and MPGCM adsorbent with cadmium (II) ion solution in 50 ml reagent bottles or glass vials. The adsorption study was carried out systematically in terms of various parameters such as initial cadmium ion concentration (20–100 mg/l), different pH (1–8), different time intervals (20 min to 180 min), various adsorbent doses (0.1 g/l to 0.5 g/l) and various temperatures (297–333 K) at 150 rpm. After proper shaking of the contents in the reagent bottles, adsorbent powder was filtered from the solution using Whatman filter paper no 41. The residual cadmium ions concentration in aqueous solution before and after equilibrium was determined by atomic absorption spectrophotometer (AA-6200, Shimadzu). The equilibrium data obtained in the batch adsorption experiments was used to calculate the amount of cadmium (II) ions adsorbed per unit mass of PGCM and MPGCM (qe in mg/g) using the following equation:  
formula
(1)
where Co is initial concentration of cadmium (II) ions in aqueous solution (mg/l), Ce is final concentration of cadmium (II) ions in aqueous solution (mg/l), qe is amount of cadmium (II) ions adsorbed per unit mass of PGCM or MPGCM (mg/g), V is volume of aqueous solution of cadmium (II) ions in liters and m is mass of PGCM or MPGCM powder in grams.

The adsorption rate was evaluated by fitting the experimental data into kinetic models such as pseudo-first-order and pseudo-second-order kinetic model (Lagergren 1898; Demirbas et al. 2002). From the experiments, pseudo-first-order rate constant (k1), pseudo-second-order rate constant (k2) and equilibrium uptake capacity (qe) were calculated.

RESULTS AND DISCUSSION

Characterization studies of PGCM and MPGCM

The characterization of modified PGCM and adsorption of cadmium (II) ions on PGCM and MPGCM were investigated by different techniques and the results obtained are discussed in the following sections.

SEM analysis

SEM delivers images with information about the topography of samples by scanning the surface of the sample with a focused electron beam. Figure 2 shows SEM images of PGCM (Figure 2(a) and 2b)) and MPGCM (Figure 2(c) and 2(d)). before and after adsorption of cadmium (II) ions, respectively. Figure 2(a) and 2(c) show sample PGCM and MPGCM at 1000X magnification, respectively. It can be seen clearly that there are number of vacant sites available. However, in Figure 2(b) and 2(d), these sites are occupied or covered by the cadmium (II) ions due to adsorption.

Figure 2

SEM images of (a) PGCM, (b) PGCM-Cd(II), (c) MPGCM, (d) MPGCM-Cd(II).

Figure 2

SEM images of (a) PGCM, (b) PGCM-Cd(II), (c) MPGCM, (d) MPGCM-Cd(II).

XRD analysis

XRD analysis is used to explore the degree of crystallinity in the materials. In this technique, a beam of X-rays falls on the sample and the scattered beam is analyzed. The X-rays scattered by the atoms of the material constructively interfere with each other to determine various characteristics of the crystalline materials. XRD spectra of PGCM and MPGCM adsorbents were recorded and are shown in Figure 3(a) and 3(b). It can be seen in both spectra that there are two peaks at 2θ of 0.18, 0.68 for PGCM and 0.19, 0.73 for MPGCM. The broadening of diffraction peaks and the absence of other diffraction peaks in higher regions indicates the amorphous nature of the material.

Figure 3

XRD analysis of (a) PGCM and (b) MPGCM.

Figure 3

XRD analysis of (a) PGCM and (b) MPGCM.

FTIR spectroscopy analysis

Surface modification of PGCM was confirmed by FTIR analysis. The IR spectra of the PGCM and MPGCM were recorded on Fourier transform spectrometer in the mid-infrared region (4,000–650 cm−1). Many functional groups give characteristic absorption peaks in the specific range. However, signals corresponding to functional groups may vary over a wide range due to the complex interaction between the coordinating atoms of the modifying reagent, i.e. 2,4-DNP, with other functional groups on the surface of adsorbent and cadmium ions. The FTIR spectra of the PGCM and MPGCM are shown in Figure 4(a) and 4(b) before adsorption of cadmium (II) ions and Figure 4(c) and 4(d) after adsorption of cadmium (II) ions.

Figure 4

FTIR spectra of (a) PGCM, (b) MPGCM, (c) PGCM-Cd(II), and (d) MPGCM-Cd(II).

Figure 4

FTIR spectra of (a) PGCM, (b) MPGCM, (c) PGCM-Cd(II), and (d) MPGCM-Cd(II).

FTIR spectrum of PGCM exhibited characteristic bands of carbonyl stretching (1,729 cm−1) and –OH stretching (3,169 cm−1) of poly phenols on the surface of the adsorbent. However, the FTIR spectrum of MPGCM exhibited all the characteristic bands of PGCM with additional bands at 3,174 cm−1, 3,348 cm−1 (stretching of Ar–NH, –NH, =NH), 1,521 cm−1 (asymmetric stretching of Ar–NO2), 1,223 cm–1 (stretching vibration of C–N) and 923 cm−1 (twisting, bending of –NH, =NH) (Mohammad & Kashani 2016). Intensity of carbonyl stretching (1,729 cm−1) peaks of PGCM was observed to be decreased which confirmed the modification of PGCM due to reaction of the carbonyl group with 2,4-dinitrophenylhydrazine. The IR spectra of PGCM and MPGCM adsorbents were slightly different after adsorption of cadmium (II) ions. Intensity of carbonyl stretching (1,729 cm−1) peaks of PGCM was further slightly decreased with higher wave numbers (1,733 cm−1) which can be attributed to the interaction of cadmium ions with carbonyl group. Also a new band was observed at 726.23 cm−1 due to Cd–O stretching which confirms the bonding of cadmium ions on the surface of PGCM adsorbent with additional characteristics of a weak band of CdO at 564 and 490 cm−1 (Abd El-Salaam & Hassan 1982; Aldwayyan et al. 2013). On other hand, a new broad band at 840 cm−1 due to Cd–N stretching was observed in the IR spectrum of cadmium loaded MPGCM which could be bonding of cadmium ions with the nitrogen of dinitrophenylhydrazine.

EDX analysis

The energy dispersive X-ray spectroscopy (EDX) detects X-rays emitted from the sample during bombardment by an electron beam to characterize its elemental composition. The EDX detector measures the relative abundance of emitted X-rays versus their energy and the spectrum of X-ray energy versus counts so obtained allow the determination of elemental composition of the sample. EDX spectra of PGCM and MPGCM before and after adsorption are shown in Figure 5(a)5(d). EDX spectrum of PGCM and MPGCM did not exhibit any characteristic peak for cadmium (II). However, both PGCM and MPGCM after adsorption of cadmium (II) exhibited an additional peak of cadmium (II) ions at 3.1 keV in the spectrum, which confirmed the adsorption of Cd(II) on the surface of PGCM and MPGCM (Table 1).

Table 1

Elemental composition of PGCM and MPGCM before and after cadmium (II) ions adsorption

S. no. PGCM
 
MPGCM
 
Element Wt.% before adsorption Wt.% after adsorption Element Wt.% before adsorption Wt.% after adsorption 
Carbon 22.93 36.81 Carbon 13.53 32.7 
Potassium 1.18 0.01 Oxygen 79.81 64.77 
Oxygen 75.76 59.71 Potassium 3.64 0.12 
Silicon 0.01 0.42 Sulfur 1.38 0.02 
Chlorine 0.12 0.02 Copper 1.27 0.1 
Cadmium – 2.28 Calcium 0.37 0.01 
– – – Cadmium – 3.03 
S. no. PGCM
 
MPGCM
 
Element Wt.% before adsorption Wt.% after adsorption Element Wt.% before adsorption Wt.% after adsorption 
Carbon 22.93 36.81 Carbon 13.53 32.7 
Potassium 1.18 0.01 Oxygen 79.81 64.77 
Oxygen 75.76 59.71 Potassium 3.64 0.12 
Silicon 0.01 0.42 Sulfur 1.38 0.02 
Chlorine 0.12 0.02 Copper 1.27 0.1 
Cadmium – 2.28 Calcium 0.37 0.01 
– – – Cadmium – 3.03 
Figure 5

EDX spectra of (a) PGCM, (b) PGCM-Cd(II), (c) MPGCM and (d) MPGCM-Cd(II).

Figure 5

EDX spectra of (a) PGCM, (b) PGCM-Cd(II), (c) MPGCM and (d) MPGCM-Cd(II).

BET surface area analysis

The BET method is used to determine the total surface area of adsorbents (Brunauer et al. 1938). Barrett–Joyner–Halenda (BJH) adsorption-desorption analysis helps in understanding the proportion of pore area and pore volume (Villarroel-Rocha et al. 2014). Surface area of PGCM and MPGCM was analyzed by the BET method. It was observed that the surface area decreased from 10.85 m²/g for PGCM to 5.058 m²/g for MPGCM. The corresponding pore sizes were 71 Å and 52.81 Å and pore volumes were 3.262 × 10−3 and 2.786 × 10−3 cc/g, respectively, for PGCM and MPGCM. The decrease in all the surface parameters supports the surface modification of adsorbent which may be attributed to the occupation of pores of adsorbent PGCM by molecules of 2,4-DNP via a condensation reaction between functional groups on the surface and 2,4-DNP. A similar observation was reported by Sayari & Hamoudi (2001).

Adsorption studies of cadmium (II) ions on adsorbent

Adsorption experiments were carried out systematically in order to optimize the set of parameters to achieve the best performance of the adsorbents, PGCM and MPGCM, and the results obtained are discussed in the following sections.

Effect of adsorbent dose

It is important to study the effect of adsorbent dose on adsorption in order to optimize the adsorption conditions. From Figure 6(a) it can be seen that the amount of cadmium (II) ions adsorbed per unit mass (qe) initially increases with adsorbent dosage as availability of adsorption sites increases with increasing dose. The effect of adsorbent dose for removal of cadmium (II) ions was examined by varying the dose between 10 mg/l and 60 mg/l for 60 ppm cadmium (II) ions solution and contact time up to 2 h. It was observed that the adsorption did not change significantly beyond the adsorbent dose of 20 mg/l for both PGCM and MPGCM due to reduction in concentration gradient (Kumar et al. 2011). However, the value of qe remained constant when the adsorbent dose increased to 60 mg/l for both adsorbents. The increase in adsorption with increasing MPGCM adsorbent dose is due to the increase in availability of adsorption sites with functional groups of the modifying reagent. Moreover, further increase in the MPGCM adsorbent dose hardly increases the adsorption because of overlapping of the modified adsorption groups as a result of overloading on the surface of adsorbent particles (Kumar & Gayothri 2009). Therefore, adsorbent dose of 20 mg/l was considered as the optimum dose for all adsorption studies.

Figure 6

Effect of (a) adsorbent dosage, (b) initial cadmium (II) ions concentration and (c) pH for adsorption of cadmium on PGCM and MPGCM.

Figure 6

Effect of (a) adsorbent dosage, (b) initial cadmium (II) ions concentration and (c) pH for adsorption of cadmium on PGCM and MPGCM.

Effect of initial cadmium (II) ions concentration

The adsorption capacity of adsorbent was also affected by the initial concentration of cadmium (II) ions. The effect of initial cadmium (II) ions concentration on adsorption was studied by taking the various concentrations of cadmium (II) ions solution ranging from 20 mg/l to 100 mg/l with optimum adsorbent dose of 20 mg/l and contact time of 2 h (Figure 6(b)). When the concentration of metal is lower, the adsorption was observed to be quite significant because of availability of more active sites on the surface of the adsorbent.

Effect of contact time

Contact time plays a crucial role in optimizing the adsorption condition of cadmium (II) ions on the surface of the adsorbent. In this study, the effect of contact time on both of the adsorbents was investigated at constant adsorbent dose (20 mg/l) for 60 mg/l cadmium (II) solution from 20 minutes to 180 minutes. The adsorption increased rapidly with increase in time and became constant at 120 minutes for both of the adsorbents. The adsorption increases in the beginning because of availability of more adsorption sites on the surface of the adsorbent. After 120 minutes, the adsorption becomes constant because most of the sites were occupied by cadmium (II) ions from solution.

Effect of pH

pH of the solution is the main parameter which affects the removal of any heavy metal ions from solution. The adsorption study was carried out with initial pH varying from 1 to 8 at other constant parameters (adsorbent dose = 20 mg/l, contact time = 120 min, Co = 60 mg/l and T = 303 K). The solution pH was adjusted by the addition of 0.1N HNO3 for acidic solution and 0.1N NaOH for alkaline solution. The maximum adsorption of cadmium (II) ions was found at pH 7 using both adsorbents as shown in Figure 6(c). The adsorption capacity of both adsorbents was found to increase with increasing pH of solution up to pH 7. However, at lower pH, hydrogen ions compete with the cadmium ions for adsorption sites which makes the adsorbent surface positive and at pH higher than 8, precipitation of cadmium ions as hydroxide decreases the adsorption of cadmium (II) ions. It is important to mention that the pH of solution after equilibrium decreased very little (>0.32) and it was as a result of replacement of H+ ions on functional groups on the adsorbent by cadmium ions.

Effect of temperature

Temperature has significant effect on adsorption of cadmium (II) ions on the adsorbent surface. The temperature study was carried out within the temperature range of 297 K to 333 K at other constant parameters (adsorbent mass = 20 mg/l, contact time = 120 min, initial concentration = 60 mg/l and pH = 7). It was observed that adsorption of cadmium (II) ions increases with the increase in temperature. This is due to the combined effect of mobility of the ions and activity of the sites which increases with increase in temperature.

Adsorption isotherms

Adsorption capacity of the adsorbent is determined by applying various isotherms such as Langmuir isotherm, Freundlich isotherm and Temkin isotherm. These isotherms are quite helpful to evaluate the equilibrium relationship between the metal ions in solution and outer surface of the adsorbent.

The Langmuir isotherm

The standard linear form of Langmuir isotherm may be described as follows (Kaur et al. 2016):  
formula
(2)
where Ce is equilibrium concentration of cadmium (II) ions, qe is amount of cadmium (II) ions adsorbed at equilibrium (mg/g), qmax is maximum adsorption capacity on the monolayer surface (mg/g), and KL is equilibrium constant related to energy of adsorption. Langmuir constant (KL) is the essential characteristic of Langmuir isotherms and used to determine the dimensionless separation factor (RL). Dimensionless separation factor RL can be calculated as follows:  
formula
(3)
where Co is the initial concentration of cadmium ions (mg/l) and KL is Langmuir constant or equilibrium constant. The value of dimensionless separation factor (RL) indicates the favorability of the isotherm whether irreversible, favorable, linear or unfavorable. If RL = 0 isotherm will be irreversible; if RL = 1, isotherm will be linear; if 0 < RL <1, isotherm will be favorable and if RL >1 isotherm will be unfavorable. The RL values for cadmium (II) ion adsorption by PGCM and MPGCM were found to be less than one but greater than zero for all experiments, indicating favorability of the isotherm (Table 2). Maximum adsorption capacity (qmax in mg/g) and Langmuir constant (KL in l/g) can be evaluated from the slope and intercept of the linear plot of Ce versus Ce/qe (Figure 7(a)).
Table 2

Parameters of isotherm models for adsorption of cadmium (II) ions on PGCM and MPGCM

Adsorbent Isotherm model
 
Langmuir
 
Freundlich
 
Temkin
 
qmax (mg/g) KL (l/g) R2 R2 R2 
PGCM 111.11 0.029 0.991 1.44 4.89 0.990 22.21 0.34 0.984 
MPGCM 142.85 0.022 0.999 1.33 4.47 0.992 24.85 0.31 0.983 
Adsorbent Isotherm model
 
Langmuir
 
Freundlich
 
Temkin
 
qmax (mg/g) KL (l/g) R2 R2 R2 
PGCM 111.11 0.029 0.991 1.44 4.89 0.990 22.21 0.34 0.984 
MPGCM 142.85 0.022 0.999 1.33 4.47 0.992 24.85 0.31 0.983 

Freundlich isotherm

The standard linear form of Freundlich isotherm can be described as follows (Kaur et al. 2016):  
formula
(4)
where K (mg/g) is Freundlich constant related to the adsorption capacity, qe is amount of cadmium ions adsorbed at equilibrium (mg/g), Ce is concentration of cadmium (II) ions at equilibrium concentration, 1/n is heterogeneity factor of adsorption, n is related to adsorption intensity and measure of the deviation from linearity of adsorption. Freundlich constant K (mg/g) and qe (mg/g) can be evaluated from slope and intercept of the linear plot of ln Ce and ln qe (Figure 7(b)). Heterogeneity factor n may be varied from 1 to 10 for classification of favorable adsorption (Chantawong et al. 2003). The value of 1/n if lower than one indicates a normal adsorption whereas the value 1/n if greater than one indicates adsorption is much more favorable and known as cooperative adsorption. The Freundlich isotherm is found to be linear with slope 1/n and intercept ln KF (Figure 7(b)) for adsorption of cadmium (II) ions on the surface of PGCM and MPGCM. The results are summarized in Table 2. From the data it can be seen that the value of 1/n is lower than one which indicates a normal adsorption. Apart from the homogeneous surface on PGCM, the Freundlich equation is also suitable for a highly heterogeneous surface on MPGCM which can be seen by the smaller values of 1/n indicating greater heterogeneity.
Figure 7

Isotherm models, (a) Langmuir isotherm, (b) Freundlich isotherm, (c) Temkin isotherm, for adsorption of cadmium on PGCM and MPGCM.

Figure 7

Isotherm models, (a) Langmuir isotherm, (b) Freundlich isotherm, (c) Temkin isotherm, for adsorption of cadmium on PGCM and MPGCM.

Temkin isotherm

The adsorption potential of adsorbent/adsorbate interactions can be evaluated by the Temkin isotherm which explicitly takes into account the interactions of adsorbate ions of the solution and the surface of the adsorbent (Kaur et al. 2016). The standard linear form of the Temkin isotherm can be described as follows:  
formula
(5)
where A is the equilibrium binding constant, B is also a constant that can be calculated by B=RT/b, b is the Temkin constant which is related to heat of sorption (J/mol), T is the temperature (K) and R is the ideal gas constant (8.314 J/mol/K), Ce is concentration of the adsorbate at equilibrium (mg/l), qe is the amount of adsorbate adsorbed at equilibrium (mg/g). Constants A (l/min) related to the maximum binding energy and B related to heat of adsorption can be evaluated from slope and intercept of the linear plot of qe versus ln Ce. The Temkin isotherm was found to be linear with slope B and intercept B ln Ce for adsorption of cadmium (II) ions on the surface of PGCM and MPGCM (Figure 7(c)). The results are summarized in Table 2. From Table 2, it can be seen that the value of B is 22.21 J/mol for PGCM and 24.85 J/mol for MPGCM indicating the process of physical adsorption. Low values of B in this study indicate a weak interaction between adsorbent and adsorbate supporting a mechanism of ion exchange.

From the comparative analysis of results, specifically regression correlation coefficients, it can be concluded that the Langmuir model was best fitted for cadmium (II) ions on the surface of PGCM and MPGCM compared with Freundlich and Temkin models. The regression correlation coefficient of the Langmuir equation is more close to unity compared with the other two isotherms studied. The maximum adsorption capacity (qmax) and Langmuir constant (KL) determined from the slope and intercept of the Langmuir plot are found to be 111.11 mg/g and 0.0293 l/mg for PGCM and 142.85 mg/g and 0.02243 l/mg for MPGCM, respectively.

Kinetics of adsorption

The adsorption rate can be evaluated by fitting the experimental data into different kinetic models and predicting information about the physiosorption or chemisorption of the adsorbate. The distinctive nature of equilibrium kinetic profiles was studied to determine the rate-limiting steps involved in the process of adsorption of cadmium (II) ions. Different models, pseudo-first-order and pseudo-second-order kinetic models (Lagergren 1898; Demirbas et al. 2002), were applied to determine the rate constant for adsorption.

Pseudo-first-order kinetic model

The pseudo-first-order kinetic model to determine the rate constant for adsorption can be expressed as (Lagergren 1898):  
formula
(6)
where qe and qt refer to amount of cadmium (II) ions adsorbed (mg/g) at equilibrium and at any time t, respectively and k1 (L/mg/min) is equilibrium rate constant of pseudo-first-order. The standard linear form of the first-order kinetic equation is  
formula
(7)

Pseudo-second-order kinetic model

The pseudo-second-order kinetic model to determine the rate constant for adsorption can be expressed as (Demirbas et al. 2002):  
formula
(8)
where qe and qt refer to amount of cadmium (II) ions adsorbed (mg/g) at equilibrium and at any time, t, respectively and k2 (g/mg/min) is equilibrium rate constant of pseudo-second-order. The standard linear form of pseudo-second-order kinetic equation is:  
formula
(9)
The value of qe and k1 were calculated from the slope and intercept of the linear plot of ln (qeqt) versus time (Figure 8(a)). However, the values of qe and k2 for pseudo-second-order kinetics were calculated from the slope and intercept of the linear plot of t/qt versus time (Figure 8(b)). The results so obtained are summarized in Table 3.
Table 3

Parameters of adsorption kinetics of cadmium (II) ions on PGCM and MPGCM

Adsorbent Pseudo-first-order kinetics
 
Pseudo-second-order kinetics
 
  
qe k1 R2 qe k2 R2 qe, exp. (mg/g) 
PGCM 15.08 0.019 0.965 43.47 0.0027 0.997 41.0 
MPGCM 11.38 0.017 0.950 47.60 0.0023 0.992 42.0 
Adsorbent Pseudo-first-order kinetics
 
Pseudo-second-order kinetics
 
  
qe k1 R2 qe k2 R2 qe, exp. (mg/g) 
PGCM 15.08 0.019 0.965 43.47 0.0027 0.997 41.0 
MPGCM 11.38 0.017 0.950 47.60 0.0023 0.992 42.0 
Figure 8

Kinetic models (a) pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, (c) Van't hoff equation, for adsorption of cadmium on PGCM and MPGCM adsorbents.

Figure 8

Kinetic models (a) pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, (c) Van't hoff equation, for adsorption of cadmium on PGCM and MPGCM adsorbents.

Results showed that the value of the regression correlation coefficient (R2) for pseudo-second-order reaction kinetics was found to be very close to that of the pseudo-first-order rate kinetic equation (Table 3). The qe values calculated using pseudo-second-order kinetics were found to be in good agreement with the experimental qe values which also proved that the kinetics of adsorption of cadmium onto PGCM and MPGCM followed the pseudo-second-order model.

Thermodynamics study

Thermodynamic studies have been carried out to evaluate spontaneity and feasibility of adsorption. Effect of temperature on the adsorption of cadmium (II) ions using PGCM and MPGCM has been investigated in terms of thermodynamic parameters, such as free energy change (ΔG0), enthalpy change (ΔH0) and entropy change (ΔS0) which were calculated according to the following equations:  
formula
(10)
 
formula
(11)
where qe is amount of cadmium (II) ions absorbed per unit mass of adsorbent (mg/g), Ce is concentration of cadmium (II) ions in the solution at equilibrium (mg/l), KD is the thermodynamic constant for the adsorption reaction at equilibrium, R is universal gas constant (8.314 J/mol/K), and T is absolute temperature (Kelvin).
Gibbs free energy change (ΔG0) of adsorption was calculated using the following equation:  
formula
(12)
Change in the enthalpy (ΔH0) and entropy (ΔS0) were calculated from the slope and intercept of a linear plot of ln KD versus 1/T (Figure 8(c)).

The thermodynamic parameters are summarized in Table 4. From the data analysis, it was found that the values of enthalpy (ΔH0) and the entropy changes (ΔS0) are positive which indicated that adsorption of cadmium (II) ions on PGCM and MPGCM is endothermic in nature and associated with an increase in randomness associated with the adsorption phenomenon. The positive value of ΔS0 reflected the high affinity of the adsorbents PGCM and MPGCM towards cadmium (II) ions and is also attributed to the addition of water molecules previously adsorbed on the surface of adsorbent and displaced by cadmium (II) during adsorption. Also, the negative values of ΔG0 at various temperatures indicated the spontaneous nature of the adsorption process.

Table 4

Thermodynamic parameters for adsorption of cadmium (II) ions on PGCM and MPGCM

Adsorbent Thermodynamic parameters
 
ΔGo (kJ/mol) ΔHo (kJ/mol) ΔSo (kJ/mol/K) 
Temp (K) 297 303 313 323 333 – – 
PGCM −1.55 −1.66 −1.85 −2.04 −2.23 4.09 0.019 
MPGCM −1.57 −1.70 −1.93 −2.16 −2.39 5.26 0.023 
Adsorbent Thermodynamic parameters
 
ΔGo (kJ/mol) ΔHo (kJ/mol) ΔSo (kJ/mol/K) 
Temp (K) 297 303 313 323 333 – – 
PGCM −1.55 −1.66 −1.85 −2.04 −2.23 4.09 0.019 
MPGCM −1.57 −1.70 −1.93 −2.16 −2.39 5.26 0.023 

Mechanism

Carpellary membrane, which separates the seeds within the fruit acting as a protective membrane, contains flavonoids such as Luteolin, quercetin, kaempferol, gallagic acid, EA glycosides, EA, punicalagin, punicalin, and pedunculagin (Kulkarni et al. 2004; Mohammad & Kashani 2012). In the present study, the functional groups of flavonoids on the surface of the carpellary membrane were used for surface modification. 2,4-dinitrophenylhydrazine was used as surface modifying reagent which reacted with the carbonyl functional groups of flavonoids to form 2,4-dinitrophenylhydrazone, resulting in red color compounds (Figure 1 (a)) on the surface of the adsorbent. The surface modified adsorbent, MPGCM, was then applied for the removal of cadmium (II) ions from aqueous solution.

The rate of removal of cadmium ions from water depends on availability of binding sites and ion exchange capacity of the adsorbent. Ion exchange capacity of poly-phenols depends on the electron withdrawing atom attached to the carbon atom to facilitate the ease of breaking of the O–H bond and this explains the very low ion exchange capacity of poly-phenols (Ibezim-Ezeani et al. 2010). Removal of cadmium by PGCM was facilitated either by adsorption or by ion exchange mechanism by the existing carboxylic group (Figure 1(b)). However, modification on the surface of PGCM by 2,4-dinitrophenylhydrazine facilitates the adsorption of cadmium ions from water on to its surface through ion exchange as well as coordination mechanism (Figure 1(b) and 1(c)).

Regeneration of the adsorbents

Sometimes it is more economical to discard the adsorbent rather than regenerate it especially when the cost of adsorbent is very low. However, such disposal of used adsorbents would be hazardous for the environment as well as human beings. In the present work, although the cost of raw adsorbent and its surface modification is very low, the regeneration was carried out in recognition of the need for environmental safety. Regeneration of PGCM and MPGCM was carried out by using a desorbing agent, H2SO4 (Cant & Walker 1977). It was found that as the concentration of H2SO4 was increased initially, there was also an increase in efficiency of the desorption process up to a certain concentration (20% H2SO4). It was also observed that high concentration of H2SO4 may deteriorate the adsorbent. In two cycles of treatment with desorbing agent 20% H2SO4, desorption of cadmium (II) from used adsorbent was achieved after 10 to 15 minutes with ∼90% efficiency. After chemical interaction with H2SO4, adsorbents were washed with de-ionized water and dried at 100 °C for 24 hours, and stored in a dry place for reapplication.

Comparison of PGCM and MPGCM adsorbent with other natural adsorbents

Finally, a comparison of PGCM and MPGCM with natural and surface modified adsorbents, previously reported in the literature was made and is shown in Figure 9. On analyzing the figure, maximum adsorption capacities (qmax) for removal of cadmium ions from water by PGCM and other natural adsorbents under a given set of conditions follow the following order:

Figure 9

Comparison of maximum adsorption capacity of (a) PGCM with other natural adsorbents and (b) MPGCM with other surface modified adsorbents.

Figure 9

Comparison of maximum adsorption capacity of (a) PGCM with other natural adsorbents and (b) MPGCM with other surface modified adsorbents.

PGCM (111.11) > medlar peel (98.14) > orange rind (peel) (83.33) > black gram husk (49.74) > Tamarix articulata wastes (41.6) > cactus cladodes (30.42) > grape stalk (27.88) > redwood bark (27.6) > rice husk (21.28) > bagasse (17.73) > tea industry waste (11.29) > barley hull (6.06) for unmodified natural adsorbents.

Similarly, the performance of MPGCM and previously reported surface modified natural adsorbents can also be arranged in the following order:

MPGCM (142.85) > wheat bran-sulfuric acid (101) > sawdust (Cedrus deodara wood)-NaOH (73.62) > corncob-citric acid (55.2) > rice husk-NaOH (20.24) > corncob-HNO3 (19.3) > pomegranate peel-HCHO (18.5) > rice husk-NaHCO3 (16.18) > wheat bran-water washed (15.71) > rice husk-epichlorohydrin (11.12) > rice husk-washed (8.58) > sugarcane bagasse-water washed (6.79) > corncob-washed (5.09) for modified natural adsorbents.

From Figure 9 it can be seen that the maximum adsorption capacity of peel of the natural adsorbent is very high compared with other outer parts of the natural adsorbents. This may be due to the presence of more active functional groups in the peel of the products to interact with heavy metal ions in water (Babel & Kurniawan 2003). It is practically quite difficult to compare or correlate the result of one adsorbent with another adsorbent because of varying experimental conditions. However, the data gives the general idea about the nature of adsorbents at normal or ambient conditions. Adsorption capacity of modified forms of natural adsorbents was also found to be higher than the unmodified form because of the presence of additional functional or other groups on the surface of the adsorbent (Ngah & Hanafiah 2008). The adsorption capacities of PGCM and MPGCM adsorbents are found to be better than other commercially available or modified natural adsorbents.

CONCLUSION

The surface modification of PGCM with 2,4-dinitrophenylhydrazine by one-step reaction process was successfully carried out to prepare MPGCM. Both adsorbents were applied for the removal of cadmium (II) ions from aqueous solution. Different isotherm models such as Langmuir isotherm, Freundlich and Temkin isotherm models have been applied for adsorption studies. Langmuir isotherm model was the best fitted model with high value of regression coefficient. The Langmuir maximum adsorption capacity for cadmium ions on PGCM and MPGCM was 111.11 mg/g and 142.85 mg/g, respectively. Pseudo-second-order kinetic model was the best fitted for the adsorption of cadmium ions on both the adsorbents. The adsorption process was endothermic and spontaneous in nature. This study demonstrates that natural PGCM and MPGCM can be used cost-effectively for removal of cadmium ions from water. Further studies can be elaborated for the removal of other contaminants present in water. The performance of both adsorbents was found to be comparable with and superior in many respects to other adsorbents already reported in the literature.

ACKNOWLEDGEMENTS

The authors would like to thank Manav Rachna International University Faridabad, India, for providing support for this study.

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