Kinetics and thermodynamics of NPX adsorption by γ-FeOOH in aqueous media

2.1 Materials and primary instrumentation

NPX, Α-methyl-6-methoxy-2-naphthaleneacetic acid, purity >98% (West Asia Reagent Company); acetonitrile, methanol, ethanol, chromatography pure (USA ACS Enke Chemical); ferrous sulfate, analytical pure (Chengdu Kelon Chemical Reagent); ammonia, analytical pure (Shanghai Titan Scientific. Co. Ltd); EDTA, analytical pure (Shanghai Gaochuang Chemical Technology Co Ltd.); NaOH, ferric chloride, silver nitrate, analysis pure (Guangzhou Chemical Reagent Factory); barium chloride, analytical grade (Guangdong West Long Chemical Reagent Factory); sulfuric acid, analytical grade (Hengyang City Kay Letter Chemical Reagent Co., Ltd.); All water used in the experiments was ultra-pure.

High-performance liquid chromatography: LC-20AT type, SHIMADZU; magnetic constant temperature stirrer: CJJ-931 type (Jiangsu Jintan Universal Science Instrument Factory); vacuum pump: SH-III type (Zhengzhou Great Wall Technology Trade Company); vacuum oven: DZF-6050, (Shanghai Heng Technology Instrument Limited Company); constant temperature oscillator: BHZ-B type (Shanghai Bo Xun Medical Biological Instrument limited Company); muffle furnace: GSL-1100-XS type (Fertilizer Crystal Material Technology Limited Company). Ultra-pure water system: Smart2 Pure ultra-pure water/pure water integrated system (Germany TKA); X-ray diffraction analyzer: D / MAX-Ultima IV (Japan Science Company).

2.2 Preparation and characterization of γ-FeOOH

2.3 γ-FeOOH adsorbed NPX experiment

Effect of initial NPX concentration: 500 ml of NPX solution, initially prepared at 5, 10, 20, 30, and 40 mg·L⁻¹ was prepared, and 0.1 g of γ-FeOOH was added and shaken in a 25 °C incubator.

Effect of temperature: 500 ml of NPX solution at a concentration of 10 mg·L⁻¹ was prepared, and 0.1 g of γ-FeOOH was added and tested at 25 °C, 35 °C, and 45 °C, respectively.

Effect of PH: 500 ml of NPX solution at a concentration of 10 mg·L⁻¹ was prepared, and the pH was adjusted to pH 3, 5, 7, and 9 with a 0.1 mol·L⁻¹ H₂SO₄ or 0.1 mol·L⁻¹ NaOH solution. Exactly 0.1 g of γ-FeOOH was added to the solution and agitated in a 25 °C incubator.

The above experiments were carried out at 100 r/min, for 15, 30, 45, 60, 75, 90, 120, 150, 180, 210, and 240 min, and a 10 ml was sampled by syringe and immediately passed through the 0.45 µm filter. Three groups were set up for the test. The filtrate was determined by high performance liquid chromatography.

2.4 Detection method

The filtrate concentration was determined by high performance liquid chromatography. The chromatographic conditions were as follows: The column was ZORBAX Eclipse XDS-C18 column (205 ∗ 4.6 mm, 5 µm), mobile phase was acetonitrile-pure water (70:30, volume ratio), detector was photodiode array detector (SPD-M20A), detection wavelength of 254 nm; flow rate of 1 ml·min-1, injection volume of 10μL, column temperature of 30 C.

XRD: the working voltage was 40 kV, the current was 30 mA, the radiation source was Cu Kα target, the wavelength was 0.15406 nm, the scanning range 2θ angle was 20°–80°, the step was 8°·min⁻¹, which was used to determine the XRD of γ-FeOOH.

2.5 Adsorption kinetics

In this experiment, Lagergren quasi-first-order kinetic equation, quasi-second-order kinetics equation and intergranular diffusion equation were used to fit the dynamics.

Lagergren quasi-first order kinetic equation:

Lagergren quasi-second-order kinetics equation:

Intergranular diffusion equation:

In these equations, q_e is the adsorption capacity at balance (mg·g⁻¹); q_t is the adsorption capacity at any time (mg·g⁻¹); and K₁ is the Lagergren first-order kinetics constant (min⁻¹); K₂ is the Lagergren second-order kinetics constant (g·min⁻¹·mg⁻¹); Kid is the rate constant of the intergranular diffusion equation (mg·g⁻¹·min^−1/2); and c is a constant related to the Boundary layer thickness (g·mg⁻¹·min⁻¹) (Chiou and Li, 2002).

2.6 Adsorption isotherms

The adsorption isotherm was fitted by the Freundlich adsorption isothermal model and Langmuir adsorption isothermal model.

Langmuir adsorption isotherm:

Freundlich adsorption isotherm:

In these equations, C_e is the solution concentration at equilibrium (mg·L⁻¹); q_e is the adsorption capacity of the adsorbent for equilibrium (mg·g⁻¹); q_m is the theoretical saturated adsorption capacity of the adsorbent (mg·g⁻¹); b is the empirical constant associated with the adsorption heat (L·mg⁻¹); K_F is the empirical constant associated with the adsorption capacity (mg^1−(1/n)·L^1/n·g⁻¹), 1/n is the empirical constant associated with the surface heterogeneity.

2.7 Adsorption thermodynamics

In these equations, ΔG⁰ is the Gibbs free energy change (kJ·mol⁻¹); ΔH⁰ is the enthalpy change (kJ·mol⁻¹); △S⁰ is the enthalpy change (J·mol⁻¹·K⁻¹); q_e is the adsorption capacity of the adsorbent at equilibrium (mg·g⁻¹); T is the adsorption temperature (K); R is the standard molar Gas parameters (8.314 J·mol⁻¹·K⁻¹) (Garcia-Delgado et al., 2012; Otero et al., 2004).

3.1 Characterization of γ-FeOOH

The XRD patterns of γ-FeOOH (Fig. 1(a)) showed that there were 12 peaks in the XRD patterns, which were (1 2 0), (0 1 1), (0 3 1), (1 1 1), (0 6 0), (2 0 0), (2 2 0) (1 5 1), (0 8 0), (0 0 2), (1 8 0), and (1 2 2). These peaks were associated with γ-FeOOH, which indicated that the prepared powder was a pure γ-FeOOH crystal phase.

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Fig. 1 Characterization of γ-FeOOH.

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Fig. 1(b) shows the SEM images of γ-FeOOH. As can be seen, γ-FeOOH was primarily in the form of a mixed crystal phase of particles (particle size of ∼50 nm) and short rods (∼200 nm in length). This was related to the pH control in the preparation of the γ-FeOOH (Farcasiu et al., 1991; Razali and Said, 2017). The morphology of the mixed crystal phase was relatively regular, indicating smooth and dispersible properties.

Fig. 1(c) depicts the N₂ adsorption-desorption isotherm and pore size distribution of γ-FeOOH with a specific surface area of 125.7 m²·g⁻¹. The adsorption-desorption curve had a significant hysteresis loop, indicating that the sample contained a mesoporous structure. The desorption curve BJH method was used to calculate the pore size distribution, and the pore size range was ∼45 nm (Kruk et al., 1997; Halim and Phang, 2017).

3.2 Effect of initial concentration of NPX on the adsorption properties of γ-FeOOH

In this study, the adsorption behavior of 0.2 g·L⁻¹ γ-FeOOH at an initial concentration of 5, 10, 20, 30, and 40 mg·L⁻¹ of NPX solution at 25 °C was studied. The results revealed that the adsorption capacity q_e of γ-FeOOH on NPX increased with higher initial concentrations when the solution pH was 4.71 during the same adsorption time. From Fig. 2(a), the slope of the adsorption curve was larger at 120 min. Following the onset of adsorption, which indicated that q_e increased rapidly. In 120–240 min, the adsorption curve became gentle, indicating that q_e growth was slow and the adsorption was gradually balanced. The diffusion driving force of the NPX molecules in water increased with the initial concentration. In the first stage of adsorption the diffusion driving force was larger, as NPX was more easily adsorbed on the adsorbent, and q_e increased rapidly. With the decreased concentration of NPX in water, the diffusion driving force was weakened, leading to the slow increase of q_e in the second stage (Sellaoui et al., 2017; Wahab and Adzmi, 2017).

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Fig. 2 Factors effect on the adsorption of NPX by γ-FeOOH.

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In addition, when the initial concentration of NPX was 5 mg·L⁻¹–30 mg·L⁻¹, q_e was obviously increased. The increase of q_e was decreased at 30 mg·L⁻¹–40 mg·L⁻¹. As the amount of adsorbent in the water was the same, the population of active sites that may be provided is constant. Hence, increasing the initial concentration of NPX resulted in more intense competition for NPX molecules for a finite number of active sites, and a concurrent decrease in qe (Gamba et al., 2017; Azizan et al., 2017).

3.3 Effect of temperature on adsorption of NPX by γ-FeOOH

In order to investigate the influence of temperature on the adsorption effect, the initial concentrations of 5, 10, 20, 30, and 40 mg·L⁻¹ NPX solution contained 0.2 g·L⁻¹ γ-FeOOH, at reaction temperatures of 25, 35, and 45 °C, respectively, with an adsorption time of 240 min, toward the determination of the concentration of NPX solution (Rahmat et al., 2017).

This study found that the temperature of γ-FeOOH adsorption concentration NPX solution produce a certain effect, as shown in Fig. 2(b). Within the scope of 25–45 °C, in the NPX solution for the different initial concentrations, it was obvious that γ-FeOOH qe increased with the elevation in temperature (Sa’at and Zaman, 2017). Thus, the rise in temperature was related to the γ-FeOOH adsorption of NPX. It may be speculated that the adsorption reaction was endothermic. Elevated temperatures likely accelerated the NPX thermal motion of molecules, causing an increased physical NPX adsorption rate on the γ-FeOOH. Simultaneously, raised temperatures may increase the amount of energy that is required during the adsorption process; the γ-FeOOH and NPX form stable chemical bonds between the molecules, which act to improve the q_e (De’nan et al., 2017; Hassan and Ismail, 2017).

3.4 Effect of pH on adsorption of NPX by γ-FeOOH

The pH of a solution may affect the degree of NPX dissociation in water, as well as the charge profile of the γ-FeOOH colloidal surface, is a critically influencing factor of the adsorption reaction. According to NPX pKa = 4.15, NPX existed primarily in molecular form when the pH was <4.15. On the contrary, NPX exists mainly in the form of NPX⁻, and the isoelectric point pH_ZPC value of γ-FeOOH was 8.47. When the pH was <8.47, the surface of the γ-FeOOH colloid was positively charged, where the smaller the pH was, the higher the positive charge. When the pH> was 8.47, the γ-FeOOH colloidal surface had a negative charge, where the greater the pH was, the stronger the negative charge (Qian et al., 2017; Hanedar et al., 2017).

In this experiment, the concentration of NPX in the solution was 10 mg·L⁻¹ and the concentration of on γ-FeOOH was 0.2 g·L⁻¹. The initial pH value was 3, 5, 7, and 9 respectively. Concurrently, the temperature was 25° C and the reaction time was 240 min. The concentration of NPX in the solution is shown in Fig. 2(c), where it can be seen that the effect of pH on the adsorption of NPX by γ-FeOOH can have a great influence: when pH < 7, the q_e of γ-FeOOH increased with pH from 17.83 mg·g⁻¹ to 28.05 mg·g⁻¹. When pH > 7, the q_e of γ-FeOOH decreased with the increase of pH, from 28.05 mg·g⁻¹ to 14.70 mg·g⁻¹. In particular, when pH = 5–7, the growth of qe was slow.

From the Henderson-Hasselbalch equation (see Eq. (9)), the proportion of the distribution of NPX and γ-FeOOH in the pH range of the experiment is shown in Table S2. When pH = 3, the positive charge on the surface of the γ-FeOOH colloid was the strongest; however, the ratio of NPX- was not high, and the electrostatic charge was prominent. When pH = 4.71–5, the positive charge of γ-FeOOH colloid was less potent than it was at pH = 3, and the ratio of NPX was ∼85%. Electrostatic adsorption and chemical bond adsorption were both affected. When pH = 7, the positive charge of γ-FeOOH colloid surface was weak, but still attained 96.72%. Under this condition, the proportion of NPX⁻ was 99.9%, and both of them participated in electrostatic adsorption and chemical bond adsorption had the largest proportion. When pH = 9, the surface of the γ-FeOOH colloid was negatively charged, and the ratio of NPX⁻ was close to 100%. The rejection phenomenon of the same charges existed in γ-FeOOH and NPX, and the adsorption capacity was small (Ragot and Schubert, 2008; Teng and Zhao, 2017).

3.5 Kinetic analysis of the adsorption of γ-FeOOH to NPX

According to the experimental data of “effect of initial concentration of NPX on the adsorption performance of γ-FeOOH”, the Lagergren quasi-first-order kinetic model (Fig. S1(a)) and the quasi-second-order kinetic model (Fig. S1(b)) were fitted, respectively. According to formulas (1) and (2), the fitting results are shown in Table 1. It may be seen from Table 1 that the quasi-first-order kinetic model was fitted with experimental data R² = 0.8193–0.9573, and the equilibrium adsorption capacity was different from the experimental results (△q_e = 12.53–34.75 mg·g⁻¹), which cannot reasonably reflect the adsorption of γ-FeOOH to NPX. The quasi-second-order kinetic model was R² > 0.99, and the equilibrium adsorption was close to the experimental result (△q_e = 0.11–1.18 mg·g⁻¹), which can well describe the adsorption process of γ-FeOOH to NPX.

The quasi-first order kinetics model requires the amount of saturated adsorption prior to fitting, which for practical applications requires a long time, and is impossible to accurately predict. Concomitantly, the quasi-first-order dynamic model for considering the diffusion process of material in the particles is the only control step of the adsorption rate. The quasi-second-order kinetics model contains all the adsorption processes, such as the diffusion of the external liquid film, the surface adsorption, and the internal diffusion of the particles, which reflects the adsorption and binding of the substance at the adsorption point. The adsorption process of γ-FeOOH to NPX was in accordance with the quasi-second-order kinetics model, indicating that γ-FeOOH and NPX, through electronic sharing or electronic exchange, formed chemical bonds and resulted in chemical adsorption, which constituted the adsorption process control steps (Ding et al., 2005; Pei et al., 2013). The analysis in Section 3.4 is consistent that the quasi-first order kinetics model, but is only suitable for describing the initial stage of the adsorption process (Khan et al., 2017).

The experimental data were fitted with the intergranular diffusion model (Fig. S2). According to Eq. (3), the results are shown in Table 2. It may be seen that there was no linear relationship between qt-t^1/2, which could be divided into three stages.

In other, words, the adsorption reaction was carried out in three steps. The initial stage was membrane diffusion, where K_i1 was the largest, and c₁ was the smallest. The second stage was the internal diffusion stage, K_i2 < K_i1, c₂ < c₁. The third stage was the equilibrium stage, with K_i3 being the smallest, c₃ (R² > 0.95). The second stage exhibited a good linear relationship (R² > 0.95); however, c ≠ 0, which means not through the origin, indicated that the internal diffusion process was the control step of the adsorption reaction, but not the only control step (Aharoni and Sparks, 1991; Hameed and El-Khaiary, 2008; Muthukumar and Mohan, 2004; Wang et al., 2009; Aslam et al., 2017).

During the adsorption process, K_i decreased continuously, indicating that the diffusion rate decreased step by step; however, C_i increased continuously, signifying that during the adsorption process, the boundary layer surrounding γ-FeOOH was increasing. This was due to the early adsorption; γ-FeOOH showed a large number of active sites, such that NPX was easily adsorbed by γ-FeOOH. In the adsorption media, with the increase of the number of NPX on γ-FeOOH, the diffusion resistance increased, adsorption became sluggish and the boundary layer gradually increased. At the conclusion of the final adsorption, the number of NPX on γ-FeOOH was gradually stabilized and showed a steady state (Gros et al., 2017).

3.6 Isothermal analysis of γ-FeOOH adsorption

The experimental data on the effect of temperature on the adsorption of NPX by γ-FeOOH were analyzed by isothermal adsorption. According to Eqs. (4) and (5), the Langmuir adsorption isothermal model (see Fig. S3(a)) and Freundlich adsorption isothermal model (see Fig. S3(b)) were proposed. The fitting results are shown in Table 3. Langmuir and Freundlich isotherms can describe the adsorption process well. It may be seen from the model that the adsorption effect of γ-FeOOH on NPX was increased with elevated temperature, which was consistent with the experimental results. Among them, the Langmuir isotherm model exhibited a high correlation, which indicated that the adsorption of NPX was associated with single molecular layer adsorption, and interactions between NPX molecules did not occur. According to the Freundlich isotherm model, n = 1.59–2.07, which were all higher than 1, which indicated that NPX was easily adsorbed on γ-FeOOH; (Mi et al., 2012) K increased with rising temperatures, indicating that the elevated temperature was conducive to the increase of adsorption intensity and adsorption capacity.

3.7 Thermodynamic analysis of γ-FeOOH adsorption

Interactions between the adsorbent and the adsorbate may generally be analyzed via the following six mechanisms: van der Waals force, hydrophobic bond force, chemical bond force, hydrogen bond force, dipole moment, and ligand exchange, etc. specific adsorption heat data, such as is shown in Table S3. In practical applications, the type of force may be determined by the heat of adsorption in the adsorption process.

In this study, we investigated the adsorption mechanism of 0.2 g·L⁻¹ γ-FeOOH at the initial NPX solution concentration of 10 mg·L⁻¹ at T = 297, 307, and 317 K. According to Eq. (6), it can be calculated ΔG, (7) and (8) use lnK_C ∼ 1/T to map, which can plot the available thermodynamic curve as in Fig. 3. The thermodynamic parameters as shown in Table S4, ΔG < 0 indicated that the adsorption process is spontaneous. The larger the T was, the larger the |ΔG|, which translated to increasing heat making the impetus of adsorption greater and more conducive to the adsorption process. ΔH > 0 indicated that the adsorption was an endothermic process, and |ΔH| > 60 kJ·mol⁻¹, which meant that adsorption was based chemical bond forces. ΔS > 0 indicated that the degree of freedom increased during the adsorption process. During the adsorption process, NPX molecules in the solution were adsorbed onto the surface of the γ-FeOOH from the disorder state and became regular. The degree of freedom was subsequently decreased, which is the process of entropy reduction. The process of H₂O desorption onto the surface of the γ-FeOOH was from an orderly to disordered state, which is the process of entropy increase. Since the NPX molecular volume was much larger than H2O, when a NPX molecule adsorbed onto γ-FeOOH, multiple water molecules adsorbed on the γ-FeOOH were desorbed. The entropy of H₂O desorption increased with the decrease in entropy caused by NPX adsorption, so ΔS > 0 (Abbaspour et al., 2017; Agrawal and Sahu, 2006; Arias et al., 2017; Jing and Li, 2012; Konggidinata et al., 2017).

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Fig. 3 Thermodynamic curve for adsorption of NPX on γ-FeOOH.

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