Investigation on the removal of entacapone from contaminated water using magnetic activated carbon

Fe₃O₄ nanoparticles were prepared by a surfactant-free sonochemical reaction [43]. 1 g of FeCl₂ and 5 g of NAC were added to 100 ml distilled water and then 20 ml of 2 M NH₃ solution was added dropwise to the solution under ultrasonic waves (60 W) for 3 min. The synthesized precipitate of MNAC was separated using a magnet and then rinsed with de-ionized water and finally dried at 60 °C in an oven. Then NAC and MNAC were analyzed by scanning electron microscope (SEM).

In each adsorption experiment, 50 ml of ENT solutions at different concentrations was transferred into the reactor. The Erlenmeyer flask containing Synthetic pharmaceutical solution was conducted at the shaking speed of 150 rpm. The pH of ENT solution was regulated with 0.1 N HCl or 0.1N NaOH and then different amounts of MNAC were weighed and added in solution flask. When the adsorption process was Performed, MNAC was filtered using a magnet and the find concentration was analyzed using Ultraviolet-Visible spectrophotometer (DR 5000 - Hach) at a wavelength of 308 nm. The effects of different parameters such as pH [2–10], ENT initial concentration (10–100 mg l⁻¹), adsorption dose (0.04–0.24 g l⁻¹), reaction time (2–60 min), and temperature (10 °C–40 °C) were examined. Thus adsorption isotherms and kinetic experiments at different ENT concentrations were studied. The adsorption capacity amounts of the MNAC (q_t, mg g⁻¹) and removal percentage (R%) were estimated according to the following equations.

$$\begin{eqnarray}&&{\rm{qt}}=\displaystyle \frac{{\rm{V}}({{\rm{C}}}_{0}-{{\rm{C}}}_{{\rm{t}}})}{{\rm{m}}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&R \% =\displaystyle \frac{({{\rm{C}}}_{0}-{{\rm{C}}}_{{\rm{t}}})}{{{\rm{C}}}_{{\rm{t}}}}\times 100\end{eqnarray} \tag{ 2 }$$

In the above equations qt (mg g⁻¹) is the adsorption capacity of MNAC for ENT at the time 't'. C₀ and C_t (mg l⁻¹) denote the contaminant concentration in the aqueous phase at initial and time 't' of the contact time. V (L) is the solution volume and m (g) is the mass of the adsorbent used.

The equilibrium adsorption isotherm results of magnetic activated carbon were evaluated by Langmuir, Freundlich, and Temkin models.

2.3.1. Langmuir isotherm model

The Langmuir sorption isotherm model is utilized to monolayer adsorb on homogenously distributed adsorption sites on an adsorbent surface. The Langmuir equation is given in the following equation:

$$\begin{eqnarray}&&\displaystyle \frac{{{\bf{c}}}_{{\bf{e}}}}{{{\bf{q}}}_{{\bf{e}}}}=\displaystyle \frac{1}{{{\bf{k}}}_{{\bf{L}}}\,{{\bf{q}}}_{{\bf{m}}}}+\displaystyle \frac{1}{{{\bf{q}}}_{{\bf{m}}}}{{\boldsymbol{c}}}_{{\bf{e}}}\end{eqnarray} \tag{ 3 }$$

Where q_e (mg g⁻¹) is the amount of pharmaceuticals that can be adsorbed by magnetic carbon at equilibrium, K_L (L mg⁻¹) is Langmuir constant, C_e (mg l⁻¹) is the ENT concentration at equilibrium, q_m (mg g⁻¹) is the theoretical maximum adsorption capacity.

2.3.2. Freundlich isotherm model

The Freundlich sorption isotherm is applied to non-ideal and multilayer adsorption.

$$\begin{eqnarray}&&\mathrm{Ln}\left({{\rm{q}}}_{{\rm{e}}}\right)={{\rm{LnK}}}_{{\rm{f}}}+\,\displaystyle \frac{1}{{\rm{n}}}\,\mathrm{ln}\,{{\rm{C}}}_{{\rm{e}}}\end{eqnarray} \tag{ 4 }$$

In this equation, K_F and n are Freundlich constants, K_F (mg^1−(1/n) L^1/n g⁻¹) is the sorption capacity of the MNAC and n is related the sorption intensity.

2.3.3. Temkin isotherm model

The Tempkin isotherm model assumes that the heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent–adsorbate interactions. The linear form of Temkin isotherm is:

$$\begin{eqnarray}&&{{\rm{q}}}_{e}={{\rm{K}}}_{{\rm{t}}}\,\mathrm{ln}\left({\rm{A}}\right)+{{\rm{K}}}_{{\rm{t}}}\,\mathrm{ln}\,{{\rm{C}}}_{{\rm{e}}}\end{eqnarray} \tag{ 5 }$$

where K_t is adsorption constants of Temkin model and related to the heat of the adsorption process. The plot of q_e versus Ln (C_e) permits the determination of the constants K_t and A and R².

To investigate the adsorption mechanism of various concentrations ENT onto MNAC, the Pseudo-first order, Pseudo-second-order and Weber-Morris model was used.

The pseudo-first order model kinetic model equation is expressed as follows [44]:

$$\begin{eqnarray}&&\mathrm{ln}\left({q}_{e}-{q}_{t}\right)=\,\mathrm{ln}\,{q}_{e}-{K}_{1}t\end{eqnarray} \tag{ 6 }$$

The linearized form of pseudo second order rate is expressed by the following equation [45, 46]:

$$\begin{eqnarray}&&\displaystyle \frac{{\rm{t}}}{{{\rm{q}}}_{{\rm{t}}}}=\displaystyle \frac{1}{{{\rm{k}}}_{2}\,{{\rm{q}}}_{{\rm{e}}}^{2}}+\displaystyle \frac{1}{{{\rm{q}}}_{{\rm{e}}}}{\rm{t}}\end{eqnarray} \tag{ 7 }$$

Where, q_e and q_t are the amount of adsorption capacity at equilibrium and time t (mg g⁻¹) and k₁ and k₂ are constants of adsorption rate.

The value of k₁ and q_e were calculated from the slope and intercept of the plot of ln(q_e–q_t) versus t. Similarly, the value of k₂ and q_e can be determined from the slope and intercept of the linear plot of t/q_t versus.

Moreover, intra-particle diffusion model which usually used based on the theory was applied in our study. This model proposed by Weber and Morris with the following equation [47]:

$$\begin{eqnarray}&&{{\rm{q}}}_{{\rm{t}}}={{\rm{K}}}_{{\rm{idt}}}^{0.5}+{\rm{I}}\end{eqnarray} \tag{ 8 }$$

where q_t (mg g⁻¹) is the amount of ENT adsorbed at time 't', K_id (mg g⁻¹ min^−1/2) is the intra-particle diffusion rate constant and I is constant.

The earlier works exhibited that pomegranate wood carbon is excellent adsorbent for removing organic compounds [28]. Textural characteristics of NAC are reported in table 1. NAC has high BET specific surface area (1029 m² g⁻¹) and mean pore diameter of 2.46 nm. This AC can be categorized in the mesoporous adsorbent. The SEM images micrographs of the synthesized NAC, Fe₃O₄, and MNAC nanoparticles are shown in figure 1. As in previous studies, SEM morphology of NAC demonstrates fibrous-like structure with parallel lines and long channels (figure 1(a)). Figure 1(b) shows the SEM images of the sonochemically prepared Fe₂O₃ under explained conditions. The SEM image of magnetic activated carbon in figure 1(c), confirms the modification of NAC by iron nanoparticles. Separation of carbon from solutions were done using an external magnetic field (figure 2). As shown in figure 2, the carbon is completely magnetized and simply separated from the aqueous solution. We examined the adsorption capacity of NAC and MNAC to compare the ability of these adsorbents in removing BET and results indicated that adsorption capacity was equal (data not shown).

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The pH of the pharmaceutical solution has a significant impact on the adsorption of ENT onto NH4Cl-modified activated carbon. The optimum pH which maximum percent of removal occurs was examined, figure 3. shows the effect of initial pH on ENT adsorption efficiency. The solutions with different pH in the range of 2 to 10 were investigated. When the pH solution was changed from 2 to 6, the removal percent was increased from 56% to over 86.3 % and higher than 6 the amount of adsorption was reduced to 37%. The optimum pH is related to affect the chemistry of ENT molecule and the charged surface of the magnetic active carbon [48]. In pH values lower than 6 the ENT molecules are protonated and give positively charged, also considering that the pH_pzc of MNAC was 6.6, a positive charge created on the surface at pH under pH_zpc. Thereby the competition between H⁺ and positively charged ENT for the limited MNAC and Electrostatic repulsion between positively charged drug and positively charged adsorbent reduce the percentage of adsorption [49]. The increase in adsorption amount at pH 6 (around pH_ZPC of MNAC) is due to decreased positive charge of MNAC surface and number of H⁺ ions in solution observed. At pH values higher than 6, OH⁻ increase in the Entacapone solution and excretion forces exist between ENT ion and adsorbent negatively charged surface result reduces adsorption. These findings are in accordance with those reported by Alahabadi et al In this study, adsorption was attributed to the physical π-π electron-donor-acceptor interactions [29].

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To determine the optimal point, agitation speeds were investigated in a range from 0 rpm to 250 rpm. Observations show that the percentages adsorbed of ENT increased as the agitation speed was increased up to 150 rpm thence the increase was minor. This effect can be attributed to the decrease in boundary layer thickness around the adsorbent particles.

Figure 4 shows the removal of ENT by MNAC as a function of adsorbent dosage. This effect was studied at various amounts of MNAC from 0.04 to 0.24 g l⁻¹ and 50 ml of 50 mg l⁻¹ Entacapone solution at pH 6.0 and 293 K. The initial increase in adsorption percent from 15.5% to 86.3% can be justified to increased surface area and the accessibility of more adsorption sites [50]. However, with further increase in adsorbents mass, there is no significant difference in removal percentage. this trend can be explained by unsaturation of adsorption sites through the adsorption reaction and decrease in the total surface area of the adsorbent due to the particle interactions such as aggregation. Results show that the dose of 0.16 g l⁻¹ of magnetic active carbon was the best choice for removal [51, 52].

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Usually, the contact time between adsorbent and adsorbate is considerable significance in the Wastewater filtration by adsorption. In this study the effect of contact time on adsorption was tested using MNAC. For this purpose, 50 ml of solutions containing ENT (50 mg l⁻¹) at pH 6.0 and 293 K were contacted with 0.16 g l⁻¹ of MNAC (figure 5). The result showed the adsorption capacities respectively increased up to10 min and then it was stable. This behavior suggests that at the initial stage of sorption, many surface sites for adsorption is unsaturation. after 10 min the driving force for mass transfer between the ENT molecules and the MNAC decreases. Similar results were obtained by Srivastava et al [53].

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To study the thermodynamics of ENT adsorption, the adsorption of Entacapone on MNAC was investigated at 283, 388, 293,298, 303, 308, 313,318 and 323 K. For this purpose, 25 ml portions of 50 mg l⁻¹ ENT solution at pH 6.0 contacted a with 0.004 g of MNAC, and the results showed in figure 6. By addition of temperature from 283 to 308°k increased the sorption of ENT from 60% to 94%. This increasing trend shows that the process is an endothermic process. The rising temperature increases the mobility of the ENT molecules and Facilitate Entacapone transfer by the boundary liquid layer [54]. ENT adsorption by MNAC at the higher temperature is an exothermic process because by further increase in temperature, adsorption decreases. This phenomenon occurs due to the release of the adsorbed ENT into the drug solution. These findings are in accordance with those reported by Zheng et al [55].

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The equilibrium of adsorption of various concentrations of ENT on MNAC was investigated, initial Entacapone concentration was investigated in the range 50–500 mg l⁻¹. In this study adsorption capacity and removal percentage of MNAC (0.16 g l⁻¹) was evaluated at pH value of 6.0 and 293 K for 360 min. As shown in figure 7 by increasing concentration the removal percentages of ENT decrease (figure 7(a)), while the adsorption capacities of ENT increase (figure 7(b)). In Previous studies, the maximum adsorption capacity of amoxicillin and Atrazine onto NAC was 437 and 714.3 mg g⁻¹ which are much less than this these present study (1666.67 mg g⁻¹) [28, 56]. Comparison the various adsorbents with MNAC showed that magnetic active carbon has a high absorption capacity [10, 57–60]. According to these results, it was determined by increased pharmaceutical concentration, the driving force for mass transfer between the solution and adsorbent increases [61, 62]. The high removal percentages in lower initial concentrations show MNAC efficiency for entacapone removal.

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Adsorption equilibrium was studied. In this study, three isotherm models (Langmuir, Freundlich, and Temkin) were used to evaluate. Isothermal models describe the interactive type between Pharmaceutical molecules and adsorbents. Information of the Langmuir, Freundlich, and Temkin isotherm models are shown in table 2. The adsorption isotherms data of ENT showed the Langmuir model had better correspondence than others models, which assumed that the ENT adsorption onto MNAC occurs monolayer adsorption process on a surface containing a finite number of identical sites [63].

Kinetics of adsorption of various concentrations (10, 50 and 100 mg l⁻¹) ENT onto MNAC was analyzed. For this purpose, 50 ml of 50 mg l⁻¹ Entacapone solution at pH 6.0 were contacted with 0.4 g l⁻¹ of MNAC at 293 K, and the results were conducted in figure 8. Based on this figure, the removal efficiency is a function of contact time. As the initial ENT concentration increases, the adsorption time is increased. The complete adsorption of 10 mg l⁻¹ ENT on MNAC was obtained at a contact time of 2 min, concentrations of 50 and 100 mg l⁻¹ respectively are completed at contact times of 15 and 35 min, which represents the adsorption is a rapid process.

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Parameters of different kinetics models for adsorption of Entacapone by MNAC at different initial concentrations and 293 K are shown in table 3. The determination coefficient (R²) obtained for pseudo-second order models is higher than pseudo first order and Weber-Morris models which indicates that the pseudo second order model provides a good correlation for the adsorption of ENT onto MNAC. In this model rate of adsorption was dependent more on the availability of the adsorption sites than ENT concentration in solution. Similar results were obtained on the adsorption of amoxicillin onto NAC by Moussavi et al [28].