Linear and Nonlinear Approach on Kinetic Studies in the Adsorption of Ciprofloxacin onto Banana Peels-base Carbon from Aqueous Solution

Fresh water is an important element for living beings to survive and contributes to the quality of our lives. Since surface and groundwater are the primary water sources for home and industrial applications, keeping an eye on their quality is a vital issue. The toxins are currently polluting the water environment. One of the most concerning concerns regarding contamination resulting from the pharmaceutical industry is pharmaceutical effluent; these industries use biologically active substances to treat diseases that negatively influence the ecosystem, human health, and ecology [1]-[2]. Personal care products, which are the leading causes of pharmaceutical contamination in the water environment, include lotions, detergents, hair dyes, lipsticks, cosmetics, creams, bath soaps, dental care products, shampoos, toothpaste, sunscreens, fragrances, and other household items. The goal is to improve the quality of daily life [3]-[4]. Highly clean water is necessary for numerous processes, such as material processing and purification, cooling systems, and production. Water is a crucial raw element in the chemical and pharmaceutical industries. The many different kinds of water that require treatment as part of the water management process include feedwater for utilities, process water, wastewater, water recycling, water from byproduct treatment, water from desalination, and water for agriculture [5]. "Process wastewater" refers to wastewater generated by ongoing industrial activities. Process wastewater is a broad category of water created during the manufacturing process, the purification of raw materials, and the purification of products, byproducts, intermediates, or waste products. Many research articles have been published on treating pharmaceutical wastewater treatment methods. Treatment techniques, however, only allow for the recovery of components from waste; they cannot lower costs or environmental pollution. Several evaluations have been published on the lack of treatment options, the makeup of pharmaceutical wastewater, and research on lowering wastewater treatment costs and minimizing environmental contamination [6]-[11]. But till now no optimized condition or low-cost effective absorbent has not been proposed in the literature.

To observe the scenario of pharmaceutical wastewater treatment, we decide to use low-cost adsorbent materials to treat the wastewater. From the inspiration of our previous works [12]-[14], we used common agricultural waste materials banana peels, usually throughout from bananas. These banana peels are collected locally and made as carbon adsorbent by the pyrolysis process. The carbon materials produced from banana peels were used as adsorbents in the batch adsorption process. The four different conditions were applied: concentration, time duration of the process, temperature, and the pH of the solution. The kinetics studies on the batch adsorption process are the most important steps [15]-[17]. The kinetic theory explains the nature of the adsorption process, the chemical reaction rate, and the adsorbents' physical or chemical properties under system operating circumstances. For this reason, we examine the various linear formulations of the pseudo-first- and pseudo-second-order kinetic equations utilized in the adsorption kinetic models. By calculating the kinetic parameters and comparing our experimental results of the adsorption of ciprofloxacin on banana peel carbon materials, we can address the nonlinear and linear kinetics for pseudo-first and pseudo-second-order kinetic models in this current inquiry.

The prepared banana peel-based Activated carbon (BPAC) was characterized by XRD as shown in Fig. 1. The XRD patterns of banana peel-based activated carbon exhibited sharp peaks indicating changes in the activated carbon structure. The XRD figure ensured that the pyrolysis process at 800 ºC prepared banana peel-based activated carbon thermally decomposed to crystalline graphitic form.

The activated carbon derived from banana peels was used as an adsorbent; the adsorption of ciprofloxacin from its aqueous solution was examined by utilizing the batch technique to optimize the concentration of adsorbate, concentration of adsorbent, contact time, pH, and shaking rate. The parameters were chosen based on the location of the maximal adsorption. The concentration of BPAC vs the percentage of ciprofloxacin removed from its aqueous solution is shown in Figure 2. The figure shows that the percentage of ciprofloxacin removed from its aqueous solution rose with increased BPAC concentration, with 200 mg being the ideal BPAC concentration. A noteworthy 96.7% of the ciprofloxacin was removed with 200 mg of the BPAC.

The ciprofloxacin concentration versus the ciprofloxacin aqueous solution clearance percentage is displayed in Figure 3. As the figure illustrates, the percentage of ciprofloxacin removed from its aqueous solution reduces as the concentration of the antibiotic increases. The maximum percentage of ciprofloxacin elimination from its aqueous solution—86%—was seen at the 50 mg/L starting concentration.

The impact of duration (time) on this elimination process is depicted in Figure 4. As a result, the removal % grew as the period increased. After 70 minutes, 95.6% of the ciprofloxacin in its aqueous solution had been removed. In this adsorption process, the impact of rate shaking was investigated. Figure 5 illustrates how 98.7% more ciprofloxacin was removed when the shaking rate was increased—the rate of shaking at 150 rpm. Ciprofloxacin is known to be partly soluble in water, and the elimination percentage rises with longer shaking times. Additionally, pH impact was examined and depicted in Figure 6. The maximum percentage of ciprofloxacin removed from its aqueous solution was found at pH 7.35.

Since ciprofloxacin is acidic and only partially soluble in water, the maximum clearance % is achieved at pH 7.35. The surface shape of the BPAC is shown in Figure 7, both before and after ciprofloxacin was adsorbed. The BPAC's surface was highly rough and full of cavities prior to adsorption, but during adsorption, the surface turned a milky white colour, resembling a blowing mushroom. The BPAC's milky surface validated the adsorption procedure for extracting ciprofloxacin from its aqueous solution.

Several kinetic models were used to analyze the time-dependent batch process adsorption experimental data of ciprofloxacin on BPAC. The elimination of ciprofloxacin through adsorption on BPAC was examined experimentally using various nonlinear and linear forms of pseudo-first- and pseudo-second-order models. The software Origin Lab was utilized to compute the kinetic parameters in nonlinearThe Origin Lab program software was used to calculate kinetic parameters in nonlinear equations. The pseudo-first-order Lagergren’s kinetic equation [21] is as follows:

And the pseudo-second-order equation is as below [22]:

Here, dt = at time t, the amount of ciprofloxacin adsorbed (mg/g), de = at equilibrium, the amount of ciprofloxacin adsorbed (mg/g), t = duration time in minutes, k1 = first-order kinetics model rate constant (min⁻¹), k2 = second-order kinetics model rate constant (g/mg, min).

Integrating the equations (3) and (4), with conditions at t= 0, dt = 0, and dt = dt at t = t, and rearrangements, the equations may be represented to some kinetic models, in nonlinear and linear forms which are shown in Table 1. From the listed equations in Table, pseudo-first-order (PFO) is one nonlinear and one linear equation, on the other hand, there is one nonlinear and four types of linear kinetic equations for pseudo-second-order (PSO).

Usually, in the adsorption kinetics study, the linear forms of pseudo-first-order and type I pseudo-second-order equations shown in Table -1 are used. Table 2 presents the non-linear forms of Pseudo-First -order and Pseudo-Second-Order kinetic models for the adsorption information of ciprofloxacin onto banana peel-based activated carbon which was plotted qt vs t shown in figures 8 and the determined constants and theoretically calculated qe. The pseudo-first-order linearized expression kinetic model has been shown in Figure 9 and plotted log(qe-qt) vs t and the determined parameters are reported in Table 2. Similarly, the Pseudo Second-Order kinetic rate constant k2 and theoretical qe were calculated from the linear plots of t/qt vs t, 1/qt vs 1/t, qt vs qt/t, and qt/t vs qt for PSO type I, II, III, and IV expressions, respectively, and are shown in Fig. 10-14, and calculated parameters are summarized in Table 2.

To compare different models, the coefficient of determination R² and Chi-square test χ² were used as determining tools for the best-fit kinetic equations which were calculated by the following expressions for linear equations:

and Chi-square for nonlinear equations as below

Here, qt,exp (mg/g) is the experimental value of adsorption at any time t, qt, cal (mg/g) is the calculated value of adsorption at any time t, q̄t,exp (mg/g) is the average of the experimental value of the adsorption at any time t, qe (mg/g) is the calculated equilibrium capacity from the models used. The values of R² and Chi-square χ² values for linear and nonlinear equations are presented in Table 2.

The data obtained for a nonlinear model is a smaller number by comparison with the experimental data, i.e.; χ² would be smaller & vice versa, on the other hand, in the linear models, the value of R² which is less than 1, is considered a favorable condition.

The values of Chi-square (χ²) analysis for non-linear models of Pseudo-First-Order and Pseudo-Second-Order were found to be 0.03107 and 0.01964, respectively, indicating the PSO model seems to be more plausible for the calculation of kinetic parameters for the adsorption of ciprofloxacin onto BPAC.

Similarly, the lower R² value of 0.9103 (Table 2) was determined indicating that the pseudo-first-order kinetic model in linear form is not an appropriate model to explain the sorption kinetic data of the ciprofloxacin on BPAC. The adsorption kinetics data was also explained by one nonlinear and four linear models of the pseudo-second-order equations, parameters, and plotted parameters summarized in Table 1, and results are reported in Table 2. Considering R² values in Table 2, Pseudo-Second-Order linear Type III & IV is the best-fitted rate expression in terms of maximum R² value of 0.9986 & 0.9986 respectively. Besides the PSO linear type I, and II have lower R² values of 0.9982, and 0.9968, respectively, were observed. The PSO type I equation is the most commonly used equation by various researchers for different sorption systems [12]-[13]. The calculated values of the rate constant for Pseudo-Second-Order linear Type III & IV (shown in Table 2) were close to each other. Besides calculating rate constant values for the PSO linear type I, and II were also closer to each other. In the pseudo-first-order kinetic model, the values of qe PFO nonlinear were 4.75 times higher than the PFO linear one. The qe values obtained by PFO non-linear equation were 13.345 mg/g and the linear equation was 2.8106 mg/g, the results indicating the PFO linear model does not apply to the adsorption data of the ciprofloxacin on BPAC. However, the PFO nonlinear model was closer to the experimental value than the linear one and may be an option for its applicability. The experimentally determined value of the qe was in close agreement with the value obtained using the nonlinear PSO model (16.245 mg/g). Methylene blue adsorption on activated carbon was reported to have a similar outcome [6]. Compared to the experimental value, the PSO model type IV linear equations yield noticeably different values (4.4799 mg/g). On the other hand, the values obtained for PSO Type III with the slightest departure from the experiment were nearly identical to those obtained for the other three linear models, namely type I, type II, and type III. While having a lower coefficient of determination (0.9968), the qe value determined by PSO Type II is closer to the experimental value. Thus, the PSO, Type III equation should be a preferable choice for the evaluation of kinetic parameters for the adsorption of ciprofloxacin on BPAC when using linear equations, based on the minimum difference between the theoretically and experimentally determined qe values and the maximum observed value of R². The pseudo-second-order kinetic model predicts the behavior over the complete adsorption range, which implies that chemical sorption or chemisorption is the rate-limiting phase. Under these circumstances, adsorption capacity rather than adsorbate concentration determines the adsorption rate.

Utilizing experimental data, several linear kinetic models were investigated to resolve intricate issues about the linearization of a non-linear form. The data obtained indicates that using a non-linear expression to ascertain the kinetic parameters was a more suitable approach, given that the error distribution remains constant throughout the execution process. One must optimize specific equation parameters while applying a nonlinear model to minimize the Chi-square value as much as feasible. The pseudo-second-order kinetic model provided a more satisfactory explanation for ciprofloxacin sorption. According to the study, the pseudo-first-order and pseudo-second-order kinetics models non-linear forms performed better than their linear counterparts. Of the four linear forms of pseudo-second order investigated; Type III appears to be a more suitable choice for determining the kinetic parameters involved in the adsorption of ciprofloxacin on BPAC.