Essay: Developing efficient adsorbents for cadmium removal

The present study explores four adsorbents prepared from watermelon rind as a lignocellulosic material for developing efficient adsorbents for cadmium removal from aqueous solution and environmental real samples. Batch adsorption experiments were performed to study the effect of physical and chemical treatments on cadmium adsorption properties of the powder. The isotherm analysis revealed that the Langmuir model could describe the adsorption data. The pseudo-second order model better fitted the obtained kinetics data and the adsorption process was spontaneous and exothermic. The results suggested that modified watermelon rind had the potential to become a promising technique for in situ cadmium metal-contaminated water treatment.
1. Introduction
Water pollution is one of major environmental concerns specially pollution by heavy metals. For many years, heavy metals in water are the main preoccupation because of its toxicity towards human beings, aquatic-life and also the environment. The increase in the industrial activities has intensified this kind of pollution and deterioration of some ecosystems [1]. Cadmium as a heavy metal is registered as List 1 in the Dangerous Substances Directive (2006/11/EC) of the European Union. The cadmium quality standards are 0.1 mg/L for marine and estuaries water and 0.05 mg/L for freshwater. Cadmium is extensively used in many industries may mainly the manufacture of nickel–cadmium batteries, pesticides, fertilizers, the electroplating industry, pigments and dyes and textile operations [2]. Cadmium after ingestion can causes renal dysfunction, hypertension and anaemia [3]. Thus, a competent treatment of the polluted wastewater is highly required. Traditional treatment methods include membrane filtration, electrolysis, chemical precipitation, coagulation, chemical oxidation, ion exchange, ozonation are sometimes restricted because of technical or even economic restrictions [4]. In addition, the operational procedures of these methods are complicated and time consuming. Iorder to reduce the operational period and treatment costs, many studies have been conducted by using abundantly available natural wastes to seek alternative cheap methods. Recently, a few common adsorbents have been developed to remove cadmium from aqueous solution, such as dead calcareous skeletons [5], rubber tires [6], corn stalk [7], maize straw [8], olive fruit stones [9], switch grass [10], oyster shells [11] and apple pomace [12].
Watermelon rind is one of the agriculture wastes, and it is not used as industrial raw material but incinerated or abandoned. Interestingly, watermelon rind consists of pectin, cellulose, proteins, and carotenoids [13]. These polymers are rich in functional groups such as hydroxyl (cellulose), carboxylic (pectin) and amine (proteins), and can easily bind cadmium ions. Based on these characteristics, watermelon rind used as potential low cost adsorbent. In the present study, watermelon was used as an adsorbent for better cadmium removal. Its sorption properties for cadmium ions were explored. Batch mode studies were used to study the effect of pH, adsorbent dose, contact time, and concentration of metal. Application of prepared adsorbents into real environmental samples was performed to investigate applicability to cadmium removal from real samples.
2. Materials and methods
2.1. Preparation of adsorbents
2.1.1. Raw watermelon:
Watermelon was purchased from local grocery store in Jeddah, Saudi Arabia. Firstly, watermelon rind has been cut into pieces of size of 4X4 cm. The watermelon pieces were washed with running tap water for several times to remove any external dust particles or impurities and air dried under supervision in the sun for 5 days. Once it was completely dried, the watermelon rind powdered into smaller grains by using domestic Mixer (ELEKTA-EFBG-1586 model). The resulting grains were passed through a 430 μm sieve. Before any treatment, the dried water melon powder was divided into 4 portions. One portion is left as it is and named WM (raw watermelon).
2.1.2. Watermelon microwave radiation assisted in deionized water:
The second portion of watermelon powder, 25 g, was added to 400 mL deionized water and subjected to microwave radiation for 5 min. The reactions were carried out in a domestic microwave oven (Samsung, 2450MHz, 800W). They were then cooled to room temperature, filtered and washed with deionized water. The residue were dried in an oven for 24 hr at 60°C, grinded and sieved to particle size less than 433 μm. The resulting powder treated with microwave radiation in absence of any chemicals, just deionized water, was called WMW .
2.1.3. Watermelon microwave radiation assisted in presence of NaOH:
The third portion powder was treated as follow: 30 g of WM was added to 300 mL of 1M NaOH and placed in an oven domestic microwave at 800 Watt for 5 min. After being returned to room temperature, samples were thoroughly rinsed with deionized water to remove excess alkali and finally oven-dried at 60°C for 24 h. The dried alkali treated watermelon were crushed and sieved to particle size less than 430 μm and desighned as WMA.
2.1.4. Watermelon microwave radiation assisted in presence of H2O2
The last WM portion and the fourth, is treated with hydrogen peroxide as follow: about 60g of WM was suspended in 500 mL deionized water. The pH of the mixture were adjusted by 0.1M NaOH to be 10, then 150 mL H2O2 (30%) was added to the mixture. It was then radiated for 15 min in microwave. The adsorbent obtained was filtered, washed with deionized water and dried at 60°C for 24 h in an oven. Finally, it was crushed and sieved to keep only particles size 430 μm. This adsorbent was named as WMH.
2.2. Chemical reagents and instruments
All reagents used in this study were of analytical grade were used without further purification. All the chemicals were obtained from BDH. A stock solution of cadmium (II) was prepared by dissolving a predetermined amount of cadmium chloride in 1 L of deionised water. All solutions used in the adsorption experiments were prepared by diluting the cadmium (II) stock solution to the required concentrations. The pH of each feed solution was adjusted to a required pH value using 0.1 M HCl or 0.1 M NaOH. The concentration of residual cadmim (II) in the liquid solution was determined with the inductively coupled plasma-optical emission spectrometry (ICP-OES Optima 4100 DV), Perkin Elmer, USA. FT-IR spectrophotometric analysis was performed with the KBr disc technique using a Jasco model FT-IR 310 (Japan). Scanning electron microscope (SEM) images were taken using a JEOL JSM 6360 LV electron microscope.
2.3. Adsorption process:
The uptake of cadmium by the watermelon adsorbents was carried out using a batch method. Adsorption experiments for cadmium (II) ion on the prepared adsorbents were studied by adding a known quantity of the treated watermelon powder to cadmium ion solutions having initial metal concentration of 50-500 mg/L in 50 mL sample solutions. The mixture was left for 24 hours until equilibrium followed by filtration and metal analysis. Adsorption experiments were carried out in batch mode at ambient temperature; 20°C. The amount of cadmium (II) at equilibrium, qe was calculated from the mass balance equation given as below:
qt= (C0-Ct)V/m (1)
where qt (mg/g) is the amount of cadmium (II) per mass unit of watermelon adsorbents at certain time t, C0 and Ct (mg/L) are the initial and at time t concentration of cadmium (II), respectively, V is the volume of the solution (mL), and m is the mass of watermelon adsorbent (mg). The percent cadmium removal by WM, WMW, WMA and WMH (Adsorption %) was calculated for each equilibration by the expression presented as:
Adsorption% = (C0-Ct) / C0 X 100 (2)
2.3.1. Determination of adsorption parameters
2.3.1.1. Effect of pH
Adsorption experiments were conducted in the pH range 2.02-10.09 keeping all other parameters constant (Cd(II) concentration = 100 mg/L, adsorbent dose 0.1g/50mL; contact time 24h; temperature 20°C). The solution pH was adjusted to the required value with 0.1M HCl or NaOH and pH was measured using a pH meter (Mitller Toledo, model FE20-USA).
2.3.1.2. Effect of adsorbent dosage
The adsorption of Cd(II) onto WM, WMW, WMA and WMH was studied by changing the quantities of the adsorbents from 1 to 5g/L in the test solution while keeping the initial Cd(II) concentration at100mg/L, temperature=20°C, pH 5.58 and equilibrium time 24h.
2.3.1.3. Effect of cadmium concentration and isotherms of adsorption
To establish adsorption isotherms and the effect of cadmium concentration, aqueous solutions of Cd(II) were prepared, in a concentration range from 50 to 500 mg/L. Once the equilibrium has been reached, the quantity of adsorbed Cd(II), as well as the residual concentration of cadmium ions in solution, was evaluated.
2.3.1.4. Kinetic studies
Raw or treated watermelon (0.1 g) was put into 50 mL of a 100 mg/L Cd(II) solution. This system was left at various predetermined intervals at temperature 20°C. The residual concentration of cadmium (II) was determined for different contact times (from 3 min to 150 min) so that the equilibrium time can be evaluated.
2.3.1.5 Thermodynamic parameters
The influence of the temperature in the adsorption of Cd(II) on watermelon materials have been studied. For this series of experiments, the maximum quantity adsorbed by 0.1 g of substrate in contact with an aqueous solution of Cd(II) at 100 mg/L was determined in a temperature range from 20 to 35°C.
3. Results and discussion
3.1. IR spectroscopy and scanning electron micrograph
The FT-IR analysis provides information on the chemical structure of the adsorbent materials. This analysis is performed in the range of 400-4000cm-1. FT-IR spectra of raw watermelon rind (WM), hot water treated (WMW), NaOH-treated (WMA) and H2O2-treated powder (WMH) under microwave radiation are depicted in Fig. 1. All spectra show a weak band observed at 2918cm-1 is attributed to the symmetric and asymmetric C-H stretching vibration of aliphatic acids. The observed wide absorption band at 3329-3340 cm-1 corresponds to the O-H stretching mode of hydroxyl groups and adsorbed water. The asymmetry and position of this O-H band at low wave number values indicates the presence of strong hydrogen bonds [14]. The strong beak with high intensity that observed in the region 1016-1022 cm-1 corresponds to C-O stretching vibration of carboxylic acids and alcohols.
FT-IR spectrum of the raw material (WM) shows band at 1238 cm-1, predicting the presence of phenolic and lactonic groups, while free and esterified carboxylic groups may be indicated from the bands at 1732 and 1607cm-1 which may be useful in identifying pectins. For WMW, WMA and WMH, slight shifts of weak absorption bands characterizing the previous reported groups were found. Moreover some of the detected absorption bands either disappeared or became less predominant due to the used method of treatment. The carboxylic peak observed at 1732 cm-1 for the raw material (WM) is disappeared in the IR-spectrum of WMA. This can be taken as an evidence for the decrease of surface acidity upon treating with alkali. However, the acidic groups became more predominant in the IR- spectra of WMW and WMH. This indicates the increase of surface acidity due to treatment the raw watermelon material with hot water or H2O2. It should be mentioned that, either the slight shift in wavenumber associated with particular groups or increase of some functional groups may be attributed to the successful modification upon different treating methods. These treatment methods aimed to remove lignin and part of hemicellulose which lead to increase of pores population and then decrease the crystalinity of cellulose.
SEM pictures of four kinds of watermelon rind (WM, WMW, WMA and WMH) before and after adsorption of cadmium are shown in Fig. 2. According to SEM observation, the surface of raw WM seems to be relatively flat, whereas WMA, WMW and WMH have more extensive surface area. Characterization using SEM results showed that the surface structure of watermelon before and after adsorption of Cd (II) obtained is different from each other. The surface is smoother after adsorption, probably due to the deposition of cadmium by physical sorption or a progressive change in watermelon surface mineralogy.
3.2. Effect of pH
Fig. 3 shows that Cd(II) solutions were precipitated above pH 10. As can be seen the percentage adsorption of cadmium on watermelon adsorbents, WM, WMW, WMA and WMH increased as pH of the solution was increased and reach to maximum value at pH 9.12. Adsorption of metal ion onto adsorbent material depends on both the nature of adsorbent surface and species distribution of the metal cation. Under the condition of higher solution pH, the solubility of any metal complexes decrease sufficiently, which, subsequently, leads to precipitation and complicating the adsorption process to a great extent. At pH 2.02 the sorption was 15.66 %, 18.62%, 18.87% and 17.86% on WM, WMW, WMA and WMH, respectively. Increasing, the pH by 2 units, the uptake increased to 55.80%, 71.77%, 66.22% and 69.39% on WM, WMW, WMA and WMH, respectively. This sharp increase in the biosorption efficiency could be explained in two ways. Firstly, at low pH values, protons compete with Cd(II) ions for sorption sites on the watermelon surface; thereby a sharp increase in the final solution pH was observed for those having low initial pH values. Secondly, for each hydrolyzable metal ion, there is a critical pH range, which is often 2 units wide; where the metal uptake efficiency increased from a very low value to maximum value. This pH value is commonly known as the sorption edge [15]. As the pH increased the competition became less fierce and uptake increased. The removal efficiency of Cd(II) increased with increasing solution pH from 4.15 to 9.12 and then showed a decreasing trend when pH was higher than the optimal pH. This decreasing trend in adsorption capacity due to formation of soluble hydroxyl complexes which result in precipitation of Cd(II) as Cd(OH)2. Similar results have been reported [16].
3.3. Effect of adsorbent dosage
Fig. 4 presents the adsorbent dose profile versus Cd(II) adsorbed per unit mass. It was observed that the percentage adsorption increased as the adsorbent dosage was increased over the range 1 g to 5 g/L. This is due to fact that an increase in adsorbent dosage increases the number of active sites available for adsorption process. As seen from results, the point of saturation was attained at dose of 4g/L. This can be interpreted based on Van derWaals interactions occurring between the negative charged sites on the watermelon surface and the positively charged cadmium ions. It is evident that for the quantitative removal of different value of cadmium in 50 mL a high dosage of watermelon is required. The data clearly shows that the WMW is more effective than WMH, WMA and WM for removal of cadmium. Thus treatment with hot water under radiation of domestic microwave is the more efficient method. This may be due to heating under microwave radiation accelerates the removal of lignin and hemicellulose in deionized water.
3.4. Effect of initial cadmium concentration
As shown in Fig. 5 the adsorption capacities of the four watermelon adsorbents increased with increasing cadmium concentration while the adsorption yields of cadmium showed the opposite trend. When the initial cadmium concentration was increases from 50-500 mg/L, the loading capacity increased from 17.62 to 94.98 mg/g, 22.26 to 66.74mg/L, 19.57 to 54.39mg/L and 22.24 to 55.75mg/L for WM, WMW, WMA and WMH, respectively. Increasing the mass transfer driving force and therefore the rate at which cadmium molecules pass from the bulk solution into the watermelon particle surface. On the other hand, the removal capacity of cadmium decreases as the initial cadmium concentration increases (Fig. 5). At low concentration, there will be unoccupied active sites on the watermelon surface, and when the initial metal concentration increases, the active sites required for adsorption of the pollutant molecules will disappear [16]. The equilibrium uptake and adsorption yield were highest for both WMW and WMH, the two lines are almost coincide together, but one can barely see that the removal capacity of WMW is the highest. This may be due to the greater microporous structure and therefore less crystallinity of cellulose in hot water treated watermelon under microwave radiation. This confirmed what is conducted by adsorbent dosage effect.
3.5. Adsorption isotherms:
Experimental isotherm are important for describing sorption capacity to facilitate evaluation of the feasibility of this process for a given adsorbent, for selection of the most appropriate adsorbent and for preliminary determination of adsorbent dosage requirements. Furthermore, the isotherm plays a critical role in the predictive modeling procedures for design of sorption systems and analysis. Various adsorption isotherm models were proposed. Among these the most used models to describe the process in water and wastewater applications were developed by (i) Langmuir, (ii) Freundlich, (iii) Temkin and (iv) Dubinin- Radushkvich.
(i) Langmuir isotherm
It is an empirical isotherm model derived from a proposed kinetic mechanism and it is based on four hypotheses: (a) The surface of the adsorbent is uniform, that is, all the adsorption sites are equal. (b) Adsorbed molecules do not interact. (c) All adsorption occurs through the same mechanism. (d) At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of pollutant, only on the free surface of the adsorbent [16].
The Langmuir isotherm is described mathematically by the Eq. 3 [17]:
Ce/qe = (1/qLKL) + (1/qL) Ce (3)
where qL is the monolayer adsorption capacity of adsorbent (mg/g) whereas KL is the Langmuir adsorption constant (L/mg). Thus, a plot of Ce/qe versus Ce gives a straight line of intercept 1/(qLKL) and slope 1/qL. The plotting of cadmium ions adsorbed (qe) by watermelon rind against the equilibrium concentration of cadmium ions (Ce) in solution gives the equilibrium isotherm, Fig. 6 and the values of qL and KL were calculated and tabulated in Table 1. As can be seen from the table and Fig. 7, high values of correlation coefficient for WM, WMW, WMA and WMH indicate that the sorption of Cd (II) ions fit well into the Langmuir model. The values of qL which are higher for WMW and WMH adsorbents confirms that the sorption capacity of the microwave irradiated hot water treated and H2O2-treated watermelon were higher than that of the raw and other treated biomass. Although both lines are almost identical in the Fig. 6, but numerical values of Langmuir adsorption constants showed that the sorption capacity of WMW is the highest.
Based on the further analysis of Langmuir equation, the fundamental features of the Langmuir isotherm can be described in terms of a dimensionless constant, separation factor or equilibrium parameter RL, which is defined by the following expression [18]:
RL = 1/(1+KL Cmax) (4)
where Cmax is the highest initial cadmium concentration in the solution (mg/L). The RL parameter is considered as a robust mark of the adsorption and there are four probabilities for the RL value: (i) for favourable adsorption 0 < RL < 1, (ii) for unfavourable adsorption RL > 1, (iii) for linear adsorption RL = 1 and (iv) for irreversible adsorption RL = 0. For this study, the values of RL obtained that presented in Table 1 for raw and treated watermelon adsorbents are less than unity i.e. 01. Therefore the biosorption process of cadmium (II) onto raw and treated watermelon rind is favourable[19].
(ii) Freundlich isotherm
The Freundlich isotherm was broadly used to describe adsorption process in liquid and for adsorption on heterogeneous surface with multilayer adsorption. This model assumes that as the adsorbate concentration increases, the concentration of adsorbate on the adsorbent surface also increases. The Freundlich isotherm is described by the following empirical Eq. 5 [20]:
Log qe = log KF + (1/n) log Ce (5)
where KF is a constant indicative of the relative adsorption capacity of the adsorbent, and n is adsorption intensity related to the surface heterogeneity. Both KF and n can be determined by the linear plot of log qe versus log Ce. If n = 1 then the partition between the two phases are independent of the adsorbate concentration. If value of 1/n is below one it indicates a normal adsorption process. On the other hand, 1/n being above one indicates cooperative adsorption process [21]. 1/n is a heterogeneity parameter. The smaller 1/n, the greater the expected heterogeneity. This expression reduces to a linear adsorption isotherm when 1/n = 1. If n lies between one and ten, this indicates a favorable adsorption process. As tabulated in Table 1, the values of R2 is ranged from 0.906 t0 0.955 indicating that the Freundlich isotherm was not the good fit for biosorption process of cadmium onto watermelon surface. However, the comparatively higher values of KF obtained for WMW and WMH show that the metal ions bond strongly onto the surface of the biosorbents. Also, it can also be seen in Table 1 that the values of n are situated in the range of 1–10, demonstrating that adsorption of cadmium onto watermelon is favorable process.
(ii) Temkin model
Another adsorption isotherm model, Temkin isotherm, was also used to fit the
experimental data. Unlike the Langmuir and Freundlich equations, the Temkin isotherm takes into account the interactions between adsorbents and metal ions to be adsorbed and is based on the assumption that the free energy of adsorption is a function of the surface coverage.The Temkin model equation assumes that the heat of adsorption of all the molecules in the layer decreases linearly due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the binding energies, up to some maximum binding energy [22]. It is expressed by the following relation (6):
qe = B ln A + B ln Ce (6)
where constant B=RT/b is related to the heat of adsorption, R the universal gas constant(J/mol.K), T the temperature(K), b the variation of adsorption energy (J/mol) and A is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy. A plot of qe versus ln Ce enables the determination of the isotherm constants B and A from the slope and the intercept, respectively. Corresponding to these linear plots, the Temkin isotherm parameters are calculated and summarized in Table 1. The regression equation and R2 values for Temkin model was observed that this isotherm also gave very good description of the sorption process cadmium onto Watermelon rind, over the range of concentration studied.
(iv) Dubinin-Radushkevich isotherm
This isotherm model was chosen to estimate the characteristic porosity of the applied adsorbents and the apparent energy of sorption process. The model is described by the Eq. 7 below:
ln qe = ln qm – β ɛ2 (7)
Where, qm is the Dubinin-Radushkevich isotherm constant related to the maximum adsorption capacity, β is a cosfficient related to the mean free energy of adsorption E (kJ/mol) and ɛ is the Polanyi potential, where:
E = (-2β)-1/2 (8)
ɛ = RT (1+1/Ce) (9)
To predict the adsorption effect, that is, physical or chemical adsorption, the mean free energy, E, of adsorption was calculated by Eq. 8. It is reported that, when the value of E is below8 kJ/mol, the adsorption process can be considered as the physical adsorption. In contrast, if the value of E is located in the range of 8–16 kJ/mol, it follows ion-exchange and higher values of 24.7±3.2 kJmol indicates strong chemisorptions formation between the adsorbent and the adsorbate [23].
As shown in Table 1, it can be found that the values of linear regression coefficient, R2, are located in the range of 0.854 – 0.9215, suggesting that these experimental data fitted worse with the Dubinin-Radushkevich equation. This finding indicates that cadmium(II) adsorption on WM, WMW, WMA and WMH does not follow the Dubinin-Radushkevich adsorption model. The apparent energy of adsorption and Dubinin-Radushkevich isotherm constants are shown on Table 1. The high values of qm for WMW and WMH show high sorption capacity. The values of the apparent energy, E, of cadmium adsorption on watermelon are in range of 0.11 to 0.32 KJ/mol which depict physisorption process. In order to further support the assertion that physical adsorption is the predominant mechanism, the values of enthalpy (ΔHº) were estimated.
On comparing the values of the correlation coefficient, R2, for the four tested isotherms, it can be observed that the biosorption data of Cd(II) on watermelon rind fitted well with the Langmuir isotherm followed by Temkin, Freundlich isotherm and least in Dubinin-Radushkevich isotherm. It is obvious from Table 1 that by comparing the maximum adsorption capacities of cadmium (II) on watermelon, WMW have great potential for the removal of Cd(II) ions from aqueous solutions. Thence, upon treatment of the biosorbent with different treatment methods, microwave irradiated water treated watermelon is the efficient method for removal of cadmium(II) ions from aqueous solutions.
Table 1. Values of parameters of each isotherm model used.
Isotherm Model Adsorbents Parameter R2
Langmuir WM
WMW
WMA
WMH qL=53.48
KL=0.035
RL=0.054
qL=68.97
KL=0.054
RL=0.036
qL=57.14
KL=0.042
RL=0.045
qL=68.03
KL=0.053
RL=0.036 0.999
0.997
0.999
0.997
Freundlich WM
WMW
WMA
WMH KF=7.332
n=2.760
KF=16.323
n=3.989
KF=11.505
n=3.619
KF=16.007
n=3.976
0.906
0.945
0.922
0.955
Temkin WM
WMW
WMA
WMH A=0.526
B=9.816
A=1.899
B=10.287
A=0.898
B=9.590
A=1.818
B=10.211 0.965
0.991
0.974
0.995
Dubinin-Radushkevich WM
WMW
WMA
WMH qm=44.23
E=0.11
qm=55.46
E=0.29
qm=47.14
E=0.16
qm= 54.53
E=0.32
0.922
0.868
0.901
0.854
3.6. Kinetic studies
The kinetics studies of any adsorption system describe the rate of adsorbate uptake on adsorbent and it controls the equilibrium time. The kinetic parameters are helpful for the prediction of uptake rate, which gives important information for designing and modeling the sorption processes.
The adsorption kinetic curves of the cadmium ions on WM, WMW, WMA and WMH are shown in Fig. 8. As can be seen, a large amount of cadmium ions could be adsorbed within a short time (10 min), which is an advantage of using watermelon rind as biosorbent. The rate limiting step of cadmium adsorption by watermelon was examined by fitting the experimental (removal percentage) to various conventional kinetic models including pseudo first and second-order, Elovich and intraparticle diffusion kinetic models.
(i) Pseudo-first-order kinetic model
The pseudo-first-order model can be expressed by the following relation [24]:
ln(qe − qt ) = ln qe − (k1/2.303)t (10)
where k1 is the first-order rate constant and qe and qt are the amounts of cadmium adsorbed at equilibrium and time t (mg/g), respectively. The values of ln(qe −qt) are calculated from the experimental data and plotted against t, k1 is calculated from the slope of the plot. The correlation coefficient values in addition to qe values (experimental and calculated) for cadmium removal by watermelon are summarized in Table 2. The low value of correlation coefficients (in the range of 0.106–0.430 for the four adsorbents WM, WMW, WMA and WMH) and the non-reasonable difference between the experimental and calculated adsorption capacity (qe) shows that this model fails to interpret the experimental data.
(ii) Pseudo-second-order kinetic model
The pseudo-second-order kinetic model has been widely used to predict adsorption kinetics and can be expressed as [25]:
t/qt=1/(k2qe2)+(1/qe)t (11)
where the constant k2 (g/mg min) is the rate constant of pseudo-second-order adsorption model. The values of t/qt are plotted against t as shown in Fig. 9. Fitting kinetics parameters of adsorption of cadmium onto watermelon according to pseudo-second-order model (Eq. 11) are tabulated in Table 2. As seen from the results, the pseudo-second-order kinetic model fits the experimental data quite well, the correlation coefficients values, R2, reach the unity and the experimental and theoretical uptakes are in good agreement. This indicates the applicability of the second-order kinetic model to describe the adsorption process of cadmium onto watermelon. The calculated qe values were 31.35, 40.16, 34.13 and 39.37 for WM, WMW, WMA and WMH, while the experimental values were 31.20, 40.01, 34.11 and 39.24 for the same order.
Table 2. Values of parameters of each kinetic model used.
kinetic Model Adsorbents Parameter R2
Pseudo-first-order WM
WMW
WMA
AMH
qe=2.49
k1=0.011
qe = 1.39
k1=0.023
qe = 1.58
k1= 0.020
qe = 4.9568
k1=-0.00576 0.106
0.345
0.430
0.292
Pseudo-second-order WM
WMW
WMA
WMH qe = 31.35
k2=0.058
qe =40.16
k2=0.053
qe = 34.13
k2= 0.067
qe = 39.37
k2= 0.0371 1
1
1
1
Elovich WM
WMW
WMA
WMH α=2.34×105
ß= 0.51
α=9.26×105
ß=0.43
α=14.6×105
ß=0.52
α=4.66×105
ß=0.417 0.541
0.504
0.540
0.590
Intraparticle-diffusion WM
WMW
WMA
WMH kint=0.562
C= 26.15
kint = 0.667
C= 33.99
kint = 0.550
C= 29.08
kint = 0.703
C= 32.808
0.330
0.304
0.330
0.380
(iii) Elovich equation
The Elovich kinetic model is described by the following relation [26]:
qt=1/ß ln (αß) + (1/ß) ln t (12)
This model gives an useful information on the extent of surface activity and activation energy for chemisorption (g/mg). The parameters (α) and (β) can be calculated from the slope and intercept of the linear plot of qt versus ln(t). The obtained R2 values of this model, Table 2, were 0.541, 0.504, 0.540 and 0.590 for WM, WMW, WMA and WMH, respectively. This great deviation from linearity reflects that this model suggested by Weber and Moris does not fit kinetic data.
(iv) Intraparticle diffusion equation
Possibility of involvement of intraparticle diffusion model as the sole mechanism was investigated according to Weber-Moris Eq. 13[27]:
qe=C + kint t1/2 (13)
where kint (mg/g min0.5) is the intraparticle diffusion rate constant and C is the boundary layer thickness. If intraparticle diffusion occurs, then qt versus t1/2 will be linear and if the plot passes through the origin, then subsequently, the rate limiting step is only due to the intraparticle diffusion. Otherwise, some other mechanisms along with the intraparticle diffusion mechanism is also involved. From Table 2, the constant C was found to increase from 26.15 in raw watermelon, WM to be 29.08, 32.80 and 33.99 in WMA, WMH and WMW, respectively. This change in C value is belong to increase in thickness of the boundary cadmium layer and decrease the chance of the external mass transfer and subsequently distinct increase in the amount of internal mass transfer. The values of kint obtained from the plots indicate that the intraparticle diffusion model is not applicable. Since the plots of qt versus t1/2 do not pass through zero and depending on the poor determination coefficients, R2 , it can be conclude that the intraparticle diffusion is not the rate determining step of the adsorption mechanism.
3.7. Thermodynamic study
Thermodynamic parameters give advantageous information about the adsorption nature of the present work. The thermodynamic constants such as change in free energy (ΔGº), enthalpy (ΔHº) and entropy (ΔSº) give useful view about the feasibility and the spontaneous nature of the sorption process and generally can be obtained from the following mathematical expressions (Eq. 14):
ΔGº = – RT ln KC (14)
ln KC = – ΔGº /RT= – (ΔHº /RT) + (ΔSº /R) (15)
where R is the gas constant (8.314 J/mol.K), T is the absolute temperature (K) and KC is the thermodynamic equilibrium constant and can be obtained from the relation [28]:
KC = C∂/Ce
where C∂ is mg of cadmium (II) adsorbed per liter and Ce is the equilibrium cadmium (II) concentration of solution (mg/L). Both ΔHº and ΔG◦ can be obtained from the slope and intercept of Van’t Hoff plot of lnKC versus 1/T. The data are tabulated in Table 3 and demonstrated in Fig. 10. Generally, the negative values of ΔHº show exothermic nature of the adsorption, the negative ΔSº values suggest the decrease in cadmium ions concentration, while the negative ΔGº values show spontaneous feasible adsorption process and the decrease in its value by rising temperature show the physical nature adsorption. The negative values of ΔHº was estimated as 19.72, 19.99, 19.92 and 19.48 kJ/mol for WM, WMW, WMA and WMH, respectively. The magnitude of ΔHº gives an idea about the type of adsorption process, where physical adsorption falls into the rangeof 2.1–20.9 kJ/mol while the heats of chemisorption generally falls into a higher range of 80–200 kJ/mol [29]. The results obtained in this study is the normal consequence of the combination of physical adsorption, which takes place through electrostatic interactions and is confirmed by the mean free energy, E, that conducted by using relation 8. The negative values of ΔS°, -63.73J/mole K, -57.45 J/mole K, -62.58 J/mole K and -56.52 J/mole K, suggested a decrease in randomness at the watermelon/solution interface during the adsorption of cadmium ions on the watermelons’ surface. The negative entropy of the adsorption and immobilization of Cd(II) on the watermelons’ surface may be attributed to the decrease in the degree of freedom of the cadmium ions.
Table 3. Values of thermodynamic parameters for the adsorption of cadmium ions by WM, WMW, WMA and WMH.
Adsorbent T(K) Kc ΔG°(kJ/mol) ΔH°(kJ/mol) ΔS°(J/mol K)
WM 293
298
303
308 1.66
1.22
1.14
1.10 -1.23
-0.50
-0.33
-0.24 -19.72
-63.73
WMW 293
298
303
308 4.00
2.84
2.68
2.63 -3.38
-2.59
-2.49
-2.47 -19.99
-57.45
WMA 293
298
303
308 0.76
0.36
0.34
0.33 -1.86
-0.89
-0.85
-0.85 -19.92
-62.58
WMH 293
298
303
308 3.65
2.56
2.44
2.41 -3.15
-2.33
-2.25
-2.25 -19.48
-56.52
3.8. Application of watermelon adsorbents to real environmental samples
To explore the applicability of watermelon adsorbents to remove cadmium ions from real environmental water samples, WM, WMW, WMA and WMH were subjected to three real water samples: Red sea water, Al-Arbaien lake water and tap water. Location map demonstrated in Fig. 11, is taken via Google earth program. In the three real samples, the concentration of Cd(II) was under the detection limit of the ICP-OES, so the three real water samples were spiked with appropriate concentration of cadmium ions to obtain final concentration of 100 mg/L. By comparing the amount of cadmium adsorbed by watermelon in environmental samples with those adsorbed in model water, it is found that that removal efficiency of Cd(II) in real environmental samples were slightly lower than those recorded for synthetic model. Similar trend were reported [30]. As shown in Fig. 12, the removal of cadmium onto WM adsorbent in synthetic model water was of 62.40% while the removal in real water was ranged from 58.01% in sea water to 61.27% in tap water. For WMW adsorbent, synthetic model water was recorded 80.01% cadmium removal while real water recorded 76.57% for sea water, 77.88% for lake water and 79.01% for tap water. WMA adsorbent showed a cadmium removal efficiency of 68.22% in synthetic water, while in real water samples the removal was ranged from 65.27% in sea water to 67.06% in tap water. WMH adsorbent recorded an efficiency of cadmium removal in model water of 78.48% while the removal was ranged from 74.45% in sea water to 76.26% in tap water. The adsorption of Cd(II) onto watermelon adsorbents from Red sea water and Al-Arbaien lake water was slightly low compared with tap water. This is may be attriuted to the high concentration of Na+, K+, Ca2+, and Mg2+ in the sea water sample (Fig.12), which compete with the Cd(II) adsorption on the watermelon binding sites. Therefore, it could be concluded that the adsorbents studied were efficient removers for the cadmium metal ions.
4. Conclusion
The results of this study indicated that the modified watermelon is a good adsorbent for the removal of cadmium from aqueous solutions and environmental real samples. The obtained results showed that the maximum percentage removal of cadmium metal ions was recorded by microwave radiated watermelon in presence of H2O2 and water. The adsorption kinetics fitted well with pseudo-second-order model and the adsorption isotherms could be well described by the Langmuir isotherm model for cadmium ions. The adsorption process was spontaneous, exothermic and physical in nature.
The applicability of native and modified watermelon adsorbents to remove cadmium ions was investigated using three real environmental samples: Red sea water, Al-Arbaien lake water and tap water. The results suggested that modified watermelon rind had the potential to become a promising technique for in situ cadmium metal-contaminated water treatment.