Wastewater containing pharmaceutical residual components must be treated before being discharged to the environment. This study was conducted to investigate the efficiency of tungsten-carbon nanocomposite in diclofenac removal using design of experiment (DOE). The 27 batch adsorption experiments were done by choosing three effective parameters (pH, adsorbent dose, and initial concentration) at three levels. The nanocomposite was prepared by tungsten oxide and activated carbon powder in a ratio of 1 to 4 mass. The remaining concentration of diclofenac was measured by a spectrometer with adding reagents of 2, 2′-bipyridine, and ferric chloride. Analysis of variance (ANOVA) was applied to determine the main and interaction effects. The equilibrium time for removal process was determined as 30 min. It was observed that the pH had the lowest influence on the removal efficiency of diclofenac. Nanocomposite gave a high removal at low concentration of 5.0 mg/L. The maximum removal for an initial concentration of 5.0 mg/L was 88.0% at contact time of 30 min. The results of ANOVA showed that adsorbent mass was among the most effective variables. Using DOE as an efficient method revealed that tungsten-carbon nanocomposite has high efficiency in the removal of residual diclofenac from the aqueous solution.

INTRODUCTION

The control of environmental pollution becomes more important with the increase of population and development of cities and industries. Wastewater is one of the environmental pollution cases that should be managed through sanitary procedures. Wastewater contains organic, inorganic, and biological materials which can threaten public health and result in environmental pollution; therefore, it should be correctly disposed of and collected (Ng & Jern 2006).

In recent years, the presence of active pharmaceutical components in the environment, especially in water resources, has been known as one of the important topics in environmental chemistry (Gulkowska et al. 2008). When drugs are used by humans, they are usually absorbed in the body, and many of these materials with their metabolites are excreted through urine and thus enter the municipal wastewater networks without any metabolism or minor changes (Seifrtová et al. 2009). Due to the chemical structure of the drugs, they can remain resistant until entering the aquatic environment. Consequently, by consumption of polluted water, pharmaceutical residues can cause harmful effects in aquatic organisms and the environment (Cunningham et al. 2009).

Non-steroidal anti-inflammatory pharmaceuticals are among the most widely consumed drugs. These drugs inhibit cyclooxygenase and have three important features of reducing inflammation, analgesic, and antipyretic. Salicylates such as diclofenac are the most widely used antipyretic drugs. Diclofenac is a non-steroidal anti-inflammatory drug taken for controlling symptoms of arthritis or joint inflammation such as rheumatoid arthritis, arthritis and spondylitis, ligament sprain, muscle contusion, gout, migraine, dental pain, backache, and pain after surgery.

Diclofenac is available in different dosage forms such as 25, 50, and 100 mg tablets, 50 and 100 mg suppositories, 75 mg ampoules and 1% topical gel. This drug can be taken two or three times a day. The maximum dosage of diclofenac is 150 mg/day. After its consumption by humans, approximately 15% of this drug is excreted without any metabolism (Landsdorp et al. 1990). The results of the study which was conducted by Manu (2012) showed that the amount of diclofenac was 0.14–1.48 μg/L, in wastewater 0.59 μg/L in surface water, and 28.4 μg/L in aquatic environments (Manu 2012). Conventional wastewater treatment plants are not especially designed to remove drugs, so these chemicals often remain in the wastewater after treatment. Drugs either enter the sludge digesters, where they are then adsorbed by activated sludge and can play an inhibitory role in the activity of anaerobic bacteria biodegradation, or pass into the conventional wastewater treatment processes and permeate through the earth, groundwater (Gartiser et al. 2007), surface water resources, and drinking water. The occurrence of anti-inflammatory and analgesic drugs in sewage and fresh water was reviewed by Ziylan and Ince. Also, they conducted a survey of their elimination by chemical, biochemical and physical treatment processes (Ziylan & Ince 2011).

Hitherto, different physical, chemical, and biological methods have been evaluated to remove drug residues from aquatic environments (Dehghani et al. 2012). Recently, diclofenac decomposition in aqueous solutions has been reported by Solar UV-Fenton, UV/H2O2, UV-A/TiO2, Solar/TiO2/SiO2, Solar/TiO2 and electro-oxidation (Manu 2012). These methods are mostly expensive and lead to production of unwanted products. Therefore, it is better to select a suitable adsorbent to remove it from aqueous environments. The elimination rate of diclofenac in wastewater treatment plant and the role of sorption during the treatment were reported in a review study (Vieno & Sillanpää 2014). The adsorption method is better than a destructive process because adsorption methods, compared with destructive methods which produce toxic and unwanted by-products, are more safe, economic, and environment-friendly (Ehrampoush et al. 2015). One of the most common adsorbents for removing drugs and organic compounds from aqueous environments is activated carbon, which has high adsorption capacity in the form of powder. But it is difficult to separate after absorbing operations, so activated carbon nanocomposites have been made using nanotechnology to overcome this problem. Recently, nanotechnology has introduced types of nanomaterials to the water treatment industry in order to reduce contamination. These nanomaterials have had high efficiency in removing many organic and inorganic pollutants. Nanomaterials like carbon nanotubes and metal nanocomposites as an adsorbent have extremely high adsorption capacity and are applied to remove heavy metals, organic materials and biological impurities (Savage & Diallo 2005). Nanocomposites contain multi-phase materials in which at least one of their components has one dimension of less than 100 nm. In these materials, when one phase reaches nano-scale, the features of nanocomposites than conventional composites of the same phase change. Recently, compositions of metals and carbon have been studied, targeting developing improved properties such as surface catalytic activity for new applications. It is entrenched that the dominant contribution in metal-carbon composites comes from covalent bonds rather than metallic and ionic bonds; carbon between 2p orbital and the metal's d orbital. The experimental conditions were firstly obtained by a design of experiment (DOE) method. Then, the tungsten-carbon composite forms a stable lattice, which in the present study was prepared by the simultaneous spray method, and then its efficiency was investigated to remove redial diclofenac from aqueous solutions.

MATERIALS AND METHODS

Materials and equipment

Tablets containing 50 mg diclofenac sodium were purchased from Sobhan Pharmaceutical Company, Iran. Its physico-chemical properties are represented in Table 1.

Table 1

Physico-chemical properties of diclofenac

ParametersDiclofenac
Formula C14H11CL2NO2 
Molecular weight 296.16 g/mol 
Structure  
pka 4.15 
Water solubility (25°C) 23.73 mg/L 
ParametersDiclofenac
Formula C14H11CL2NO2 
Molecular weight 296.16 g/mol 
Structure  
pka 4.15 
Water solubility (25°C) 23.73 mg/L 

Chemical reagents in this study including HCl, NaOH, tungsten oxide (WO2), 2, 2′-bipyridine (C10H8N2), activated carbon, and ferric chloride (FeCl2), with high purity and laboratory grade, were purchased from Merck Company.

A spectrometer SP-3000 PLUS-UV/VIS was used to measure diclofenac, a pH meter (MicroBench Ti 2100) was applied to adjust the pH of the solutions, an ultrasonic device (D-78224 Singen/Htw) was also used to separate the particles and to homogenize the solution, and a shaker (INNOVA 40R) at the speed of 150 RPM was applied to mix the suspensions. A scanning electron microscope (SEM) (Phenom Holland) was employed to observe the surface morphology of synthesized nanocomposite. For this technique, the samples were held in the sample holder of SEM and five tests were performed on the synthesized nanocomposite. All solids were weighed by a digital scale with precision of ±0.0001 g. Devices, according to the relevant catalog, were calibrated before using them for analysis. In order to study the interactions and the main effects of factors in adsorption process, the obtained results from the experiments were analyzed by statistical test of analysis of variance (ANOVA) using Minitab 16 software. The results of the ANOVA test were considered by statistical parameters of the F-values and P-values. In general, a larger F-value and a smaller P-value indicate a more significant coefficient.

Preparation of adsorbent

Tungsten-carbon nanocomposite was prepared by sonochemistry method with partial modification (Bang & Suslick 2007). Briefly, tungsten oxide and activated carbon powder were weighed with a ratio 1 to 4 atomic mass by a digital scale and each one was separately made up to a specific volume of 100 mL in a volumetric flask. Then, the two solutions were mixed in a ratio of 1 to 2. The obtained solution was heated to a temperature of 80°C by a heater. Afterward, the suspension was placed under ultrasound waves for 30 min to separate the particles. Next, the mixture was sprayed in the oven for 3 h at a temperature of 110 °C for the remaining solid particles in the container. The solid sample was rinsed twice with distilled water, then twice with 95% ethanol, and then it was dried at 65 °C for one night.

Point of zero charge

To determine pHZPC of nanocomposite adsorbent, the changes of pH solutions were checked out with and without adsorbent at pH of 2, 4, 6, 8, 10, and 12. For each pH experiment, 0.5 g of the adsorbent was added to 100 ml solution of 0.005 M sodium chloride that their pH solutions were adjusted by 0.1 M HCl or NaOH in the mentioned pHs. The secondary pH of each solution was measured after 24 h at 25°C. Then, the changes of pH were plotted against the initial pH and the adsorbent pHZPC was determined based on the intersection of the two curves. The pH of zero point charge is the pH at which the curve crosses the straight line that fits the points pHi = pHf.

Adsorption studies

The removal process is influenced by different factors such as contact time, pH, different concentrations of diclofenac, and the amount of tungsten-carbon nanocomposite. At first, a stock solution was prepared by dissolving one tablet containing 50 mg diclofenac in 1 L of distilled water. Then, the standard solutions of 5.0, 10.0, and 15.0 mg/L were prepared using stock solutions of 50 mg/L. For investigation of parameters, the effect of contact time (5, 15, 30, 60, 90, and 120 min) was firstly optimized in initial pH of the solution, a concentration of 5.0 ppm and the adsorbent dose of 0.1 g in 100 mL. Afterward, the experimental design method was used to reduce the number of experiments according to selection of factors. The parameters and their levels are presented in Table 2.

Table 2

The coded values and range of variables for adsorption of diclofenac

FactorNameLevel
123
pH pH 3.0 5.0 7.0 
C0 Initial concentration (mg/L) 5.0 10.0 15.0 
Adsorbent dose (g/100 mL) 0.1 0.3 0.5 
FactorNameLevel
123
pH pH 3.0 5.0 7.0 
C0 Initial concentration (mg/L) 5.0 10.0 15.0 
Adsorbent dose (g/100 mL) 0.1 0.3 0.5 

Twenty-seven experiments were performed in laboratory scale at constant volume of 100 mL in a batch process. Nanocomposite adsorption efficiency was measured by mixing different doses of tungsten-carbon nanocomposite of 0.1, 0.3, and 0.5 in 100 mL of diclofenac solutions with diclofenac concentrations of 5.0, 10.0, and 15.0 mg/L and at pHs of 3.0, 5.0, and 7.0. Then, the samples were stirred using a shaker at the speed of 150 RPM at the temperature of 25 °C, after which the suspensions were filtrated using filter paper after the contact time of 30 min. Finally, the residual of diclofenac was measured in clear filtrated solutions.

Diclofenac measurement

In order to determine the residual concentration of diclofenac, 1 mL of ferric chloride and 1 mL of 2, 2′-bipyridine were added to the filtered samples. The absorbance of the solution was measured using a spectrometer in the wavelength of 520 nm. The removal efficiency was calculated applying data obtained from the spectrophotometer according to Equation (1). 
formula
1
where RE is the removal efficiency, A0 is the initial absorption of the solution, and Ae is the absorption after the removal process.

RESULTS

Characterization

The SEM images of tungsten-carbon nanocomposite with two magnifications are depicted in Figure 1.
Figure 1

SEM images of tungsten-carbon nanocomposite; (a) magnification of 1,200, (b) magnification of 20,000.

Figure 1

SEM images of tungsten-carbon nanocomposite; (a) magnification of 1,200, (b) magnification of 20,000.

Figure 1(a) indicates that the synthesized nanocomposite is porous and has a homogeneous structure with deep pores. The shiny points on the surface of Figure 1(b) with nano size about 100 nm are related to the mineral tungsten oxide and are adjoined in a spherical shape to the surface of activated carbon.

The changes of pH solution against the initial pH of solution in the presence and absence of the adsorbent are depicted in Figure 2. This figure shows an intersection in the pH of 6 that was known as the end point of the pHzpc.
Figure 2

Changes of pH against initial pH to obtain pHzpc of nanocomposite.

Figure 2

Changes of pH against initial pH to obtain pHzpc of nanocomposite.

pHzpc is an important property of solid materials and indicates the electrical neutrality of the adsorbent surface at the particular value of pH. It was obvious that the zero point charge of nanocomposite is 6.0. As a result, the adsorbent surface charge is positive at pH lower than 6.0 and creates a strong electrostatic force with negative ions; therefore, it leads to removal of negative ions. At pH higher than 6.0, due to the OH adsorption, the adsorbent charge is negative, which leads to an increase in removing of cations.

Effect of contact time

Figure 3 shows the effect of contact time on diclofenac sodium removal efficiency by nanocomposite adsorbent. As it can be seen, increasing of contact time to 30 min results in an increase in the removal rate, and after that an increase of time leads to nearly constant removal rate. So, the optimal contact time was considered as 30 min, which can be used in future experiments.
Figure 3

The effect of contact time on diclofenac removal efficiency (0.1 g of adsorbent and pH = 5.0).

Figure 3

The effect of contact time on diclofenac removal efficiency (0.1 g of adsorbent and pH = 5.0).

Statistical studies

Table 3 shows the condition of experiments and results of the efficiency of nanocomposite in the removal of diclofenac that was obtained from experimental data.

Table 3

Sample number, level of variables and diclofenac removal efficiency

TestpHC0 (mg/L)m(g)RE%
15 0.5 44.8 
15 0.3 23.9 
10 0.5 71.0 
10 0.3 23.9 
10 0.1 44.6 
0.5 88.0 
15 0.1 46.0 
15 0.3 45.5 
15 0.5 47.2 
10 15 0.1 38.0 
11 0.1 42. 6 
12 0.3 49.1 
13 15 0.5 29.5 
14 0.3 53.3 
15 0.5 80.7 
16 10 0.1 51.0 
17 10 0.3 53.4 
18 0.5 79.6 
19 0.1 40.0 
20 0.3 46.3 
21 10 0.3 47.9 
22 10 0.5 75.9 
23 15 0.3 41.3 
24 10 0.5 83.8 
25 10 0.3 71.3 
26 0.1 48.8 
27 10 0.5 80.1 
TestpHC0 (mg/L)m(g)RE%
15 0.5 44.8 
15 0.3 23.9 
10 0.5 71.0 
10 0.3 23.9 
10 0.1 44.6 
0.5 88.0 
15 0.1 46.0 
15 0.3 45.5 
15 0.5 47.2 
10 15 0.1 38.0 
11 0.1 42. 6 
12 0.3 49.1 
13 15 0.5 29.5 
14 0.3 53.3 
15 0.5 80.7 
16 10 0.1 51.0 
17 10 0.3 53.4 
18 0.5 79.6 
19 0.1 40.0 
20 0.3 46.3 
21 10 0.3 47.9 
22 10 0.5 75.9 
23 15 0.3 41.3 
24 10 0.5 83.8 
25 10 0.3 71.3 
26 0.1 48.8 
27 10 0.5 80.1 

The results of experimental design were interpreted along with ANOVAs, which are tabulated in Table 4.

Table 4

The results of ANOVA for diclofenac removal

FactorsStatistic
DFaCoeffbSE coeffcFP
pH −0.419 2.001 0.34 0.724 
16.490 2.001 42.46 0.000 
−8.346 2.001 10.81 0.005 
pH*M 0.501 0.133 1.83 0.217 
pH*C 0.124 0.133 0.74 0.593 
M*C 0.218 0.094 1.29 0.352 
Error     
Total 26     
FactorsStatistic
DFaCoeffbSE coeffcFP
pH −0.419 2.001 0.34 0.724 
16.490 2.001 42.46 0.000 
−8.346 2.001 10.81 0.005 
pH*M 0.501 0.133 1.83 0.217 
pH*C 0.124 0.133 0.74 0.593 
M*C 0.218 0.094 1.29 0.352 
Error     
Total 26     

aDegrees of freedom.

bCoefficient.

cStandard error of coefficient.

For more obvious analysis, main effects' curves of the parameters were plotted by Minitab software. The main effects of each factor on the diclofenac adsorption process by tungsten-carbon nanocomposite are shown in Figure 4.
Figure 4

The main effects of each factor on diclofenac adsorption process by tungsten-carbon nanocomposite adsorbent.

Figure 4

The main effects of each factor on diclofenac adsorption process by tungsten-carbon nanocomposite adsorbent.

The curves of the main effects show the deviation from the mean between high and low levels of each main parameter. When the factor effect is positive, adsorption efficiency increases by changing the factor from low level to high level. In return, if the factor effect is negative, adsorbent efficiency will decrease when the variable is at high level.

Also, the effects of interactions of parameters on diclofenac removal were studied as shown in Figure 5. In general, parallel lines show lack of interaction while deviation from parallelism indicates the interactions (Salmani et al. 2016).
Figure 5

The effects of each parameter interaction on diclofenac adsorption process by tungsten-carbon nanocomposite.

Figure 5

The effects of each parameter interaction on diclofenac adsorption process by tungsten-carbon nanocomposite.

DISCUSSION

The experiments can be carefully planned by an experimental design method so that the most accurate results are achieved using the minimum number of tests. In this approach, not only can the results be analyzed quickly, but also the interactions of variables with each other can be measured by DOE method. This method is considered as one of the strongest methods in a special design for optimizing the effective factors on the processes. Hence, we performed the experiments based on the DOE for considering main effect and interactions of variables as the statistical response. The optimum conditions were found as contact time 30 min, pH = 5, initial concentration 5 mg/L, and adsorbent dose, 0.5 mg/L solution, for which the removal efficiency was obtained as 88.0% in this condition.

Scanning electron microscopy provides some information about samples such as surface features, shape, size, and position of particles on the surface of the material. The image of adsorbent (Figure 1) indicates that tungsten-carbon granules have a heterogeneous morphology with different pore sizes. The pores of an adsorbent are an effective parameter in the adsorption process. In a study conducted by Shamohammadi, the relationship between adsorption rates and diameter of porosity showed that there was a good correlation between the adsorption rate and the average diameter pores of the adsorbent. Their results indicated that increase of the pores diameter led to decrease of adsorption rate (Shamohammadi et al. 2008).

pH is one of the important chemical factors and affects the adsorption process by the impact of the adsorbent surface charge. When a suitable pH is selected in the adsorption process, the adsorption efficiency will increase to maximum removal. At acidic pH, adsorbents absorb hydrogen ions and form a layer of positive charge. Therefore, cation adsorption involves an ion exchange on the surface of the adsorbent, and anion adsorption takes place on another layer. In basic solutions, adsorbent has a negative charge on the surface and cation adsorption takes place in the second layer, so anion adsorption is slower (Sharma 2012). This phenomenon relates to the chemical structure of the adsorbent at pH of zero point charge (pHzpc). At pHzpc, positive and negative charges on adsorbent functional groups are equal and adsorption takes place on the adsorbent surface. The anion adsorption on the surface of adsorbent is faster at pHs lower than pHzpc. Furthermore, at higher than pHzpc levels, the anions appear in the form of hydroxide on the surface and the cation adsorption on the surface of adsorbents is faster. The role of pH was investigated by the curve of the main effects on the adsorption process (Figure 4). The main effect curve of pH showed a little deviation from the mean between high and low levels of pH main parameter. Increasing of pH from 3.0 to 5.0 (level 1 to level 2) leads to a small increase in adsorption efficiency from 52.6% to 54.6%, and by increasing the pH to 7.0 (level 3), adsorption efficiency shows a slight decline to 51.7%. Also, from the result of Table 4, it can be seen that the P-value of pH is much higher than 0.05; therefore, the effect of pH was not statically significant. It is observed that pH has a small negative coefficient, and so this is not effective in the diclofenac adsorption process. Generally, all ranges of pHs are suitable for adsorption of diclofenac using tungsten-carbon nanocomposite adsorbent. But, the best pH for optimum adsorption of pH was the pH between pKa (4.2) and pHzpc (6.0).

Contact time is one of the most important factors in an adsorption process that is applied for designing of large systems. As Figure 3 represents, from the early stages of experiment to 30 min, the adsorption rate was high; this may result from an increase in the number of available vacant sites in adsorbent, but gradually over time, which is after 30 min, the diclofenac adsorption rate had a constant trend. This results in the accumulation of diclofenac particles on the active sites, which leads to a decrease in the number of active sites as well as the number of adsorbate particles in solution. As a conclusion, it can be noted that the concentration gradient between adsorbate in solution and on the surface of adsorbent increases in the early stage of the experiment. The concentration decreases gradually with longer times, and so an equilibrium adsorption rate occurs at times greater than 30 min.

Adsorbent mass is another important parameter that is applied to determine pollutant adsorption capacity by adsorbent. The statistical results of Table 3 showed that P-value of adsorbent mass is less than 0.05, so the adsorbent mass has a significant effect on diclofenac removal. Also, based on the results of the factor analysis (Table 4), it is found that adsorbent mass has the greatest positive coefficient. It indicates that adsorbent dose has a strong positive effect on the adsorption process. The experimental results (Table 3) showed that increasing the adsorbent dose from 0.1 (low level) to 0.5 (high level) leads to the increase in diclofenac adsorption from 38.0% to 88.0%. It is clear that increasing of adsorbent mass leads to an increase in removal rate because with the increase in adsorbent mass, the number of adsorption active sites increases for adsorption in solution. Consequently, the contact surface between adsorbent and pollutant increases and an increase in diclofenac removal efficiency occurs, reciprocally.

Initial concentration of pollutant is another main effect on the adsorption process that should be considered. Table 3 represented that initial diclofenac concentration has a negative coefficient, thus causing a negative effect on the diclofenac adsorption process using tungsten-carbon nanocomposite adsorbent. In other words, most adsorption efficiency occurs at a low level (level of 1) and the adsorption process is more suitable at lower concentration. Also, the P-value of initial concentration is less than 0.05; in other words, the factor of initial concentration has a significant effect on diclofenac removal. The results in Table 3 revealed that increasing the initial concentration from 5.0 (low level) to 15.0 (high level) leads to the decrease in the diclofenac removal from 88.0% to 29.8%. In fact, adsorbent has limited active sites, so that at high concentration of the pollutant, these active sites are saturated with the diclofenac particles at first, and then the removal efficiency decreases at high concentration of diclofenac. Moreover, diclofenac molecules seem to be large; when they are absorbed on the adsorbent, their large volume causes space prohibition, and this prevents adsorption of more molecules by active sites of adsorbent. This space prohibition becomes severe and causes reduction of removal efficiency.

In Figure 5, some lines are not parallel, and it provides evidence for effective interactions between the parameters. It is clear that there is a significant interaction between adsorbent mass and pH as well as diclofenac concentration and pH on removal efficiency by tungsten-carbon nanocomposite.

CONCLUSION

One of the main aims for a batch adsorption process is to find the optimum values of the parameters which affect the adsorption capacity and removal efficiency. This condition must be made with the minimum of time and with a minimum of other effective parameters. In this study, the optimization of the adsorption process for diclofenac removal from aqueous solutions by nanocomposite tungsten-carbon is presented by DOE. It was observed that the removal efficiency of diclofenac from aqueous solution was significantly influenced by time and adsorbent mass. From the interaction plots of removal efficiency, it can be seen that the interaction of parameters is important. DOE was used to find the optimum values of the main effective factors such as adsorbent mass and initial concentration in order to obtain maximum values of removal efficiency. For maximum values of removal efficiency, the ratio of adsorbent and concentration must be set to the maximum values in the process, and pH had a little effect on it. Using DOE as an efficient method revealed that tungsten-carbon nanocomposite has high efficiency in the removal of residual diclofenac from the aqueous solution.

CONFLICT OF INTERESTS

The authors declare that they have no conflict of interest.

AUTHORS’ CONTRIBUTIONS

Z.R. was the main investigator and synthesized the nanocomposite. Z.R. and M.H.S. drafted the manuscript. M.H.S. and M.M. were guides of study. M.H.E. was the advisor of study. H.A.S. contributed to data analysis. All authors read and approved the final manuscript.

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