Study on Adsorption of Cu and Ba from Aqueous Solutions Using Nanoparticles of Origanum (OR) and Lavandula (LV)

Wild herbs (Origanum (OR) and Lavandula (LV)) were used as environment-friendly adsorbents in this study. The adsorbents were used for adsorption of Cu and Ba from water. The adsorption of heavy metals onto OR and LV was dependent on particle size, dose, and solution pH. The diameter of adsorbent particles was less than 282.8 nm. The adsorption follows second-order kinetics. Langmuir and Freundlich models have been applied to describe the equilibrium data, and the thermodynamic parameters, the Gibbs free energy, ∆G°, enthalpy, ∆H°, and entropy, ∆S°, have been determined. The positive value of ∆H° suggests that the adsorption of heavy metals by the wild herbs is endothermic. The negative values of ∆G° at all the studied temperatures indicate that the adsorption is a spontaneous process. It can be concluded that OR and LV are promising adsorbents for the removal of heavy metals from aqueous solutions over a range of concentrations.


Introduction
e adsorption of toxic substances resulting from the accumulation of industrial wastes is important and one of the most dangerous challenges facing the environment and society nowadays. One of the most significant contaminants affecting water resources is heavy metals. ese metal ions present a significant risk to animals and humans because of their high toxicity at both low and high concentrations in soil and water. e search for new techniques to remove these contaminants has involved both chemical and biological methods.
Contaminating materials have become increasingly dangerous with increasing technological development, and need for diverse heavy metals, for example, in ore processing and other modern industries, prompted the organizations concerned with the preservation of the environment to develop restrictions and laws for laboratories and concerning the treatment industrial waste before its release into the environment so that toxic materials do not exceed the allowable limits. Researchers have, thus, devoted efforts to find effective methods for the removal of contaminants from waste, but these processes are often economically expensive; therefore, we must find ways to bypass conventional and high-cost advanced adsorption technologies. e adsorption of contaminants on the surfaces of solids is an effective method for heavy metal remediation. Activated carbon is considered to be an efficient, competitive material for this task. However, the cost of production is still high; thus, many researchers have begun to search for alternative adsorbents made from local natural materials. Plants are one type of alternative material that can be used to remove heavy metal ions from water systems and soil. e current proliferation of technology and development of science has been an enormous boon for humans. ey have become dependent on technology and scientific development in various aspects such as daily activities, trade, industry, and work. Heavy metals are structural elements such as lead, zinc, arsenic, cadmium, copper, titanium, cobalt, lithium, aluminum, and mercury and can be in the form of metals or dissolved salts. ese metals are present in the environment in air, water, and soil. For example, factory chimneys release metal oxides into the air, thus transmitting heavy metal pollution to humans, animals, and plants. In addition, car exhausts release lead oxides, resulting from the combustion of tetraethyl lead, into the atmosphere, and this is one of the most widespread routes to leading contamination of marine organisms with metals and the transit of these contaminants via sea fishing to humans and animals. Furthermore, agricultural soil is one of the most important sources of food polluted by heavy metals, which arises through the irrigation of crops with polluted water or the use of pesticides. In this situation, the metals are transmitted through the vascular system of plants and fruits. erefore, field crops irrigated with drainage water polluted by heavy metals are one of the most important and most dangerous sources for the entry of toxic heavy metals into the human body [1].
ere are many methods for adsorption of heavy metals from the environment, both chemical and physical. However, some of these are not economically feasible. erefore, it is necessary to investigate low-cost, effective alternatives. e adsorption of heavy metals by the adsorption technology is a good alternative, and it is used in the treatment of wastewater and soil. To compare the adsorbent substances, the cost, as well as effectiveness, must be considered. Activated carbon is a highly effective alternative used for the adsorption of heavy metals from wastewater, but it is soluble under extremely acidic conditions [2].
e aim of this work is to study the adsorption of Cu and Ba from water using microparticles of Origanum (OR) and Lavandula (LV), which are considered environmentally safe and low-cost. In this study, we use thermodynamic, kinetic, equilibrium, and adsorption isotherms in the analysis of the adsorption behavior.

Materials.
All the chemicals used in this study were of analytical grade. Cu(Cl) 2 ·4H 2 O (98%) and Ba(Cl 2 )·4H 2 O (90%) were purchased from Sigma-Aldrich (Germany). (Figure 1) is a leafy plant used as a kind of aromatic spices and flavorings in foods, as well as the benefits of medical oregano. It also contains a variety of antioxidant compounds, such as rosmarinic acid. e chemicals in oregano are those that give it distinctive aroma and flavor. It includes thymol, albinin, limonene, carvacrol, and carophyllene [9].

Preparation of the Adsorbents. Origanum
Lavandula (LV Figure 1) is a plant that grows and prefers sunny open areas. Lavender is used for insomnia, nervousness, and depression [10]. e wild herbs used in this study were purchased from a perfumer. e dried herbs were then cut into small pieces and milled to a powder using a laboratory planetary ball mill (DECO-PBM-V-0.4 L). e powders were sieved into particles of 50, 100, 200, 300, and 400 μm using an Octagon D200 digital sieve shaker. e adsorbents were stored in glass bottles for further use without any pretreatment.

Methods.
For the adsorption study, distilled water was used to prepare various solutions at the desired concentrations from the stock solution. e adsorption experiments were performed in a series of flasks containing 100 mL solutions of metal ions at desired concentration and mass of adsorbent herbs. e mixtures were shaken for 16 h at 120 rpm using a shaker ("Rotaterm" orbital and linear shaker). e mixtures were filtered, and the heavy metals concentrations were determined by inductively coupled plasma (ICP) mass spectrometry. e effect of adsorbent doses from 0.1 to 2 g on the metal adsorption was studied. e initial concentration was 10 mg/L. e effect of pH on the heavy metal adsorption process was investigated at pH values from 3 to 12, adjusted either with 1 M·HCl or 1 M·NaOH using a pH meter, to monitor the change. In addition, the effect of the adsorbent particle size was investigated from 400 to less than 50 μm, and the contact time was varied between 1 and 3 h. e effect of temperature was studied from 25°to 60°C for 3 h using 0.5 g of the adsorbent at an initial metal concentration of 10 mg/L. e adsorption equilibrium was investigated for different metal concentrations between 1 and 100 mg/L. In addition, kinetic experiments were conducted with 10 mg/L metal and 0.5 g of the adsorbent at room temperature with stirring at 120 rpm for 6 h. e adsorption at equilibrium, q e (mg/g), was calculated by using the following equation: where C 0 and C e (mg/L) are the liquid-phase heavy metal concentrations initially and at equilibrium, respectively; V is the volume of the solution (L); and m is the mass of the dry adsorbent (g). e percentage of metal adsorption (Ads HM %) from the solution was calculated as follows: e pseudo-first-order and pseudo-second-order adsorption kinetic models were applied to describe the behavior of the adsorbent. e equilibrium data were then fitted using the Langmuir and Freundlich isotherm models. e thermodynamic parameters calculated to describe the adsorption process onto OR and LV include the changes in standard enthalpy (∆H°), standard entropy (∆S°), and standard free energy (∆G°).

Characterization of the Adsorbents.
Fourier-transform (FT-IR) analysis was used to determine the functional groups in the adsorbents. e particle size of adsorbents was measured using an XRD. e initial metal concentrations of the adsorbents were determined for Cu and Ba from ICP measurements. Figure 2 shows the FT-IR absorption bands of the adsorbent. e peaks indicate the presence of functional groups containing O-H and N-H at 3300 cm −1 wavelength and carboxylic acid (−COOH) groups at 1600 cm −1 and 1150 cm −1 wavelengths, which play a major role in the adsorption process and efficiently adsorb metal ions via ion exchange [11]. e mineral composition of biomass ash commonly shows a lowest amount of Cu and Ba (not exceeding 0.072 mg/L) in OR, as does LV, and hence the Cu and Ba were selected for further study (Figure 3). e smallest size obtained after milling the wild herbs was 50 μm. Figure 4 shows the size of adsorbent particles that had been ground to less than 50 μm. e diameter of the particle is less than 282.8 nm, which is smaller than the size reached by milling, and smaller particles can increase the effectiveness of adsorption. e small particle size was characterized by fine porosity and a large internal surface area, while the powder form has larger diameter pores and smaller internal surface area. A smaller pore size results in stronger and greater adsorption capacity because the small adsorbent particle size reduces the pathway for both mass transport and internal diffusion of the adsorbent inside the adsorbents [12].

Effect of Particle Sizes of Adsorbent.
e experimental effect of particles on metal adsorption was investigated for five sizes (400 to less than 50 μm) at room temperature and initial pH (5 and 7 for Cu and Ba, respectively). Figure 5 indicates that metal adsorption increased with decreasing particle size for OR and LV. is behavior can be attributed to the increased internal surface area with decreasing particle size [12].
is behavior arises because most of the internal surface of these particles can be used for adsorption. e smaller particle size gives higher adsorption rates because metal ions have a short path for transfer inside the pores of the small particle adsorbents [13].
However, it is expected that the use of the smaller particle size should give greater removal rate because of the increased surface area, and as the particle size increases, the number of small pores increases. e particle dimensions determine the propagation distance, because the dimensions of the adsorbent particles obtained by standard sieves depend on the particle length and width; thus, this explains the decreased adsorption of metals with increasing adsorbent particle size [14].

Effect of Adsorbent Dosage.
e effect of the adsorbent doses on heavy metal adsorption by OR and LV was studied. As shown in Figure 6, the adsorption percentage of Cu and Ba increased with increase of adsorbent doses. e increase in adsorption percentage with increasing adsorbent dose arises because of the increased availability of exchange sites or surface areas at higher concentrations of adsorbents [15]. Similar behavior has been observed in previous studies and arises from the effect of interactions between mineral ions and adsorbent. e increased dose is consistent with the greater area and larger number of adsorption sites [16]. Figure 7, the percent adsorption of Cu and Ba increased significantly with increasing pH from 3 to 5, and then stabilized until reaching pH 7; after that, the adsorption was a slight decrease above pH 7 for both adsorbents OR and LV [15].

Adsorption Kinetic Model
To understand the rate and type of adsorption on adsorbents, we studied several kinetic models.

Pseudo-First-Order Kinetic Model.
e pseudo-firstorder model is expressed by the following equation: log q e − q t � log q e − where q e and q t represent the amount of metal ions adsorbed at equilibrium and at time, t, respectively and K 1 is pseudofirst-order rate constant. As shown in Figure 8 and Table 1, the regression coefficient (R 2 ≤ 0.96) indicates that the experimental data accurately support the first-order model to describe the adsorption kinetics of Cu ions, but do not support it with Ba ions. However, we found that the typical values of q m are lower than the experimental values for Cu ions.
is indicates that both metal ions and the adsorbent are involved   in the adsorption process [17].
erefore, the first-order model may not be suitable for explaining the adsorption kinetics of ions on the adsorbents. Similar results were obtained for the kinetics of adsorption of different metal ions on other adsorbents [17][18][19].

Pseudo-Second-Order Kinetic Model.
e pseudosecond-order model is expressed by the following equation: where K 2 is the pseudo-second-order rate constant. e experimental data showed the best fit with the highest correlation coefficients (R 2 ≥ 0.996) for the pseudosecond-order model for each of the metal ions. In addition, the experimental values were well matched with the calculated data (Table 1). erefore, the experimental data accurately support the pseudo-second-order kinetic model for the adsorption of metal ions.
is finding indicates that the rate-limiting step was not the resistance of the boundary layer [8].
us, the ratelimiting step is chemisorption involving valence forces through the exchange of electrons between the metal ions and the different functional groups in the adsorbent [20].
is is consistent with the observed rapid adsorption. As shown in Figure 8, the kinetic analysis for the adsorption of metal ions on the catalysts revealed one step: the rate of adsorption in the equilibrium state. is phenomenon indicates that the chemical adsorption is the dominant step on the adsorption process [11,21]. e adsorption half-time (t1/2) for the adsorption of half the metal ions in minutes was between 13.45 and 21.80 min, indicating significant affinity of the adsorbent molecules for the adsorbate [8].

Adsorption Isotherm
To understand the mechanism of metal ion adsorption from an aqueous solution by the adsorbents, we tested several adsorption isotherm models.
e Langmuir adsorption isotherm was applied in accordance with the following equation: where C e is the adsorbate equilibrium concentration, q e is the observed adsorption capacity at equilibrium, q m is the maximum adsorption capacity, and K L is the adsorption equilibrium constant. e Langmuir adsorption isotherm showed a linear relationship between C e /q e and C e for the adsorption of metals on OR and LV (Figure 9), and the slope and R 2 data are presented in Table 2. e plot exhibited good correlation coefficient and linearity. e R 2 values are very close to unity, indicating strong correlation with the Langmuir adsorption isotherm. e maximum adsorption capacity of the adsorbent, q m (mg/g), for metal ions is high, which can be explained by their noncovalent interaction with ions. Another reason may be the presence of carboxylic acid-containing sugars in the herbs, which bind the metal ions [22]. So, the values of R L were calculated from the following equation: R L ≤ 1, indicating that the Langmuir isotherm is appropriate for the adsorption process. For a single adsorption system, C 0 is usually the highest concentration of the liquid phase encountered [23].
where n is the Freundlich equilibrium coefficient; values of log q e are plotted against log C e (Figure 9). e slope and R 2 data are presented in Table 2. In the slope of the Freundlich plot, 1/n is greater than 1, which indicates that adsorption is cooperative. Higher values of K F and n indicate a higher affinity for the adsorption of metals [4]. Values of 1/n < 1 indicate the favorable adsorption of all metals, and values of n within the range 1-10 represent good adsorption and a favorable physical process. Higher values of K F indicate a high adsorption intensity [24]. From the Langmuir and Freundlich isotherms, it is clear the metal ions were strongly adsorbed in the pores of the herbs.

Thermodynamic Parameters
In order to understand the nature of adsorption, the thermodynamic parameters such as free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) could be calculated using the following equations: e values of the thermodynamic parameters are shown in Table 3. e linearized van 't Hoff plots of ln K eq versus 1/T are shown in Figure 10. e values of ∆H°and ∆S°were determined from the slope and intercept of the linear plot.
e parameters indicate that the adsorption process is spontaneous and endothermic. e positive value of ∆S°i ndicates an increase in the degrees of freedom (or disorder) of the adsorbed species for Cu and Ba ions.
e negative values of ∆G°indicate that the thermodynamic process was spontaneous and feasible for Cu and Ba ions [8]. Moreover, the increase in negative ∆G°values with increasing temperature shows an increased probability of adsorption at higher temperatures, which is consistent with an earlier report [25].

Conclusion
In this study, the adsorption of Cu and Ba onto OR and LV was investigated with respect to the particle size, dosage of the adsorbent, and pH. Characterization of the adsorbents revealed the presence of carboxylic groups that were involved in the adsorption of the metal ions. Adsorption was maximum with 50 μm particle size and 0.5 g of the adsorbent. Metal ion adsorption onto wild herbs follows the pseudo-first-order and pseudo-second-order kinetic models. e adsorption isotherm data for the Cu and Ba ions fitted well with the Langmuir and Freundlich models. e thermodynamic parameters show that the adsorption process was feasible, spontaneous, and endothermic in nature for Cu and Ba. Hence, the wild herbs can be used as a low-cost adsorbent for the removal of heavy metal ions from aqueous solutions. Further study is warranted to evaluate the potential of the adsorbents for heavy metal removal from aqueous solutions.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Bioinorganic Chemistry and Applications 7