Adsorptive Removal Behavior of Pb (II) and Cr (VI) Pollutants from an Aqueous Environment onto Polyaniline-Modified MIL100(Fe)

Abstract

The present work introduces a new sorbent, so-called PANI/MIL100(Fe), for removing Pb (II) and Cr (VI) from wastewater. The successful preparation of PANI/MIL100(Fe) was verified via Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) characterizations. This adsorptive material showed a microporous structure with surface area magnitudes of up to 261.29 m²/g, a total pore volume of 0.2124 cm³/g, and a pore size distribution of around 1.2 nm. The maximum adsorption capacities of PANI/MIL100(Fe) for Cr (VI) (pH = 2) and Pb (II) (pH = 6) were obtained as 72.37 and 81.76 mg/g, respectively. The isotherm modeling assessments illustrated that the sorption of Pb (II) and Cr (VI) was consistent with the Sips model (R² > 0.99), while the adsorption kinetics were suited to a pseudo-second-order model (R² > 0.95). Thermodynamic studies of both metal ions demonstrated the spontaneous and endothermic nature of the process. The reusability of the PANI/MIL100(Fe) indicated promising adsorption properties for Pb (II) and Cr (VI) up to three cycles. Moreover, the XRD and FT-IR spectroscopy results after adsorption/desorption showed excellent stability of the adsorbent and physisorption mechanism, wherein electrons were exchanged between the PANI/MIL-100(Fe) and Pb (II) and Cr (VI). Finally, the findings suggested that PANI/MIL100(Fe) can be considered an efficient and environmentally friendly adsorbent to remove Pb (II) and Cr (VI) from wastewater.

1. Introduction

Nowadays, water-soluble metal ions in wastewater treatment have attracted considerable attention. A substantial amount of wastewater with toxic heavy metals is directly or indirectly discharged into the environment, causing heavy metallic ions to be accumulated in ecosystems and causing intensive environmental problems. Heavy metal pollution can harm the brain, kidneys, nervous system, and other organs, even at low concentrations [1].

Cr (VI) and Pb (II) are common heavy metal contaminants with a high degree of toxicity, stability, and mobility in industries, namely battery manufacturing, metallurgy, metal plating, mining, fertilizers, pesticides, and paper industries [1,2]. Chromium in the food chain can lead to changes in human physiology and accordingly cause serious health problems including simple skin irritation and lung cancer. Lead can create intensive dysfunction in the kidneys, liver, and reproductive system [1]. Generally, unlike organic pollutants, which are chemically oxidized or biodegraded, heavy metal contaminants with high toxicity are usually not easily degradable, and their treatment is difficult. Therefore, an appropriate strategy is needed to develop a new system for tracing the metal ions and their efficient removal from liquid phases.

At present, conventional technologies used for removing chromium and lead ions from aqueous solutions mainly consist of chemical precipitation, membrane filtration, electrochemistry, biological treatment, ion exchange, and adsorption [3,4,5,6,7]. The adsorption method is a technique for removing lead and chromium ions in water via ion exchange, complexation, and adsorption by an adsorbent. This method has the advantages of low cost, low energy consumption, high efficiency, lower environmental impacts, and better regeneration capability. Therefore, the adsorption technology has broad application prospects in dealing with metal pollution in water [8]. This process depends on a lot of factors, one of the most important of which is pH, because it can alter both the surface charge characteristics of the adsorbent and metal ion species distribution in an aqueous solution [9]. For example, Rodríguez et al. [10] examined the removal of heavy metals (Fe, Cu, Zn, and Al) from acid mine drainage water in the mining areas of the Huelva province, Spain, and reported that at a pH of 3.5, most of the iron was removed while Zn and Cu were partially removed. Moreover, at a pH of 5.5, the removal of all the above metals increased considerably.

Metal–organic framework (MOF) material is a porous material that has been developed rapidly recently. In 1995, Yaghi and Li [11] synthesized MOF materials for the first time. MOFs exhibit excellent performance for the adsorption of ions from aqueous media [12,13,14,15], gas storage and separation [16], sensors [17], photochemical properties [18], catalysis [19], and drug delivery [20], owing to their high porosity and surface areas, fascinating topology structures, abundant unsaturated sites, diverse post-modification potentials, adjustable hole size, and tailored chemistry. MOFs displayed excellent performance on the adsorption of intended pollutants owing to the presence of coordinatively saturated or unsaturated metal sites [1]. In the structures of MOFs, metal coordination centers are linked by multi-dentate organic ligands to produce three-dimensional extensile networks to create high porosity.

MIL100(Fe), as an iron-trimesic MOF, is a promising agent for water purification because of (I) the little toxicity of Fe ions, (II) light and low-cost accessibility of synthesis precursors, (III) powerful hydrolytic stability of coordinative structure contributed by strong Lewis acidity of trivalent iron, (IV) the simplicity of synthesis approaches and their environmental aspects, and (V) great unsaturated functional groups of organic linkers’ potential for adsorption of metal ions [21].

A high number of systematic research works have been carried out on the adsorptive removal of heavy metal ions using MOFs. For instance, Rahimi utilized the Cu-based MOF for the sorption of Pb (II) and Cd (II) and reported that the sorption capacity was considerably greater than that of mordenite zeolite (MOR) or graphene oxide (GO) [22]. Zhongmin et al. [8] prepared magnetic orderly mesoporous α-Fe₂O₃ nanoclusters using thermal decomposition of MIL-100(Fe) to examine the sorption attributes on As (III) and As (V). Their findings demonstrated that the highest sorption capacity of As (III) and As (V) was found to be 109.89 and 181.82 mg/g after 30 min, respectively. Moreover, some conductive polymers such as polypyrrole (PPy) and polyaniline (PANI) were adopted to modify the MOFs to promote conductivity and enhance the separation, due to their low cost, non-toxic, high electrical, and optical attributes.

Polyaniline (PANI) is one of the most important nanostructured polymers with π–π conjugated structures, which in recent years attracted great attention from researchers owing to its low cost of monomer, high electrical conductivity, superior environmental stability, simple synthesis, and polar functional groups [23]. PANI has also demonstrated an excellent sorption performance to remove heavy metal contaminants from wastewaters because of imine functionality, high value of amine (–NH) and marvelous groups in the polymer chains, and good redox properties, as well as a large surface area to volume ratio [24]. Karthik et al. [25] evaluated the polyaniline-grafted chitosan particles for adsorption of Pb (II) and Cd (II) in a batch system. Their results showed that the highest sorption capacity achieved was 16.07 and 14.33 mg/g for Pb (II) and Cd (II), respectively, based on the Langmuir isotherm. Meanwhile, suitable pH was 6 for high sorption of Pb (II) and Cd (II) ions. A large number of studies have been conducted to modify PANI with many materials to increase the adsorption capacity and make them more processable in the treatment techniques of water [26]. Composite materials demonstrate superior thermal and chemical stabilities, sorption capacities, granulometric properties, and also have higher selectivity in the removal of heavy metal ions compared to pure inorganic and organic materials [27].

This study aims to combine MOF materials with polyaniline in a certain way to ensure the improvement of adsorption capacity. The catalytic and photocatalytic behaviors of PANI/MIL100(Fe) composite have been examined in the various reactions, but its potential on the sorption performance of hazardous ions was barely reported. Thus, in this research, the effects of PANI/MIL100(Fe) on Pb (II) and Cr (VI) adsorption were systematically investigated. In this regard, firstly, PANI/MIL100(Fe) was synthesized under ambient conditions and characterized by elemental analysis, XRD, FESEM, FT-IR spectroscopy, and BET. Then, the effects of the solution’s pH, sorption isotherm, kinetic, and thermodynamic properties were addressed to detect the optimum conditions for the removal of Cr (VI) and Pb (II). Moreover, the XRD and FT-IR spectroscopies after adsorption/desorption were used to study the adsorbent stability and mechanism of the reaction.

2. Materials and Methods

2.1. Chemicals

Iron (III) nitrate nonahydrate ≥99% (Fe(NO₃)₃·9H₂O), 1,3,5-benzenetricarboxylic acid (TMA; H₃BTC, 98%), hydrogen fluoride (HF, 48.0%), nitric acid (HNO₃, 65%–68%), hydrochloric acid (HCl, Merck), ammonium peroxodisulfate (APS), and aniline were all purchased from Merck (Darmstadt, Germany). Aniline monomer was distilled under a reduced pressure. All chemical reagents used in this study were employed without purification. In this study, potassium bichromate (K₂Cr₂O₇) and lead nitrate (Pb(NO₃)₂) were utilized for providing the stock solutions.

2.2. Fabricate of PANI/MIL-100(Fe) Composite

The MIL-100(Fe) used in this research was fabricated by the improved procedure according to the literature [1]. Concisely, 8.0 Fe: 5.36 TMA: 24 HF: 4.8 HNO₃: 2240 H₂O were first blended and sealed in a 60 mL Teflon-lined autoclave and conducted hydrothermal reaction at 160 °C for five days. Then, the synthesized composition was purified by solvent extraction route employing hot ethanol/water. The obtained powders were filtered, washed via distilled/deionized (DD) water, and dried under electric oven at 60 °C for 12 h (Figure 1a) [28].

The PANI/MIL-100(Fe) composite was prepared through an in situ chemical oxidative polymerization monomer of aniline with the help of oxidizing agent of ammonium peroxydisulfate (Figure 1b). First, 0.3 g of MIL-100(Fe) was poured into 80 mL of DD water and ultrasonicated for 20 min. After this, aniline monomer (0.0644 M), HCl (1 M), and 50 mL of DD water were poured into dispersion. After 20 min, APS (0.0208 M) was poured slowly into the reaction mixture with continuous stirring at 0–5 °C. Within a short time, the color of the mixture changed to greenish black. The polymerization process was conducted in an ice bath for almost 24 h. After this process, the powder was purified by centrifugation (6000 rpm) using the reaction solvent to pull out any impurities. Ultimately, the resultant composite was dried at 50 °C [29].

2.3. Characterization Techniques

The X-ray diffraction (XRD) patterns were assessed using an X’Pert MPD X-ray diffractometer (Philips PANalytical, Almelo, The Netherlands) at Cu Kα irradiation. A Bruker Tensor 27 FT-IR spectrometer (Bruker, Billerica, MA, USA) was applied to record Fourier-transform infrared spectroscopy (FT-IR) of the materials. The surface morphology of composite was examined utilizing Zeiss-EM10C Transmission Electron Microscope (TEM, Oberkochen, Germany) with an accelerating voltage of 80 kV. The Brunauer–Emmett–Teller (BET) analyses were used for determining the pore size distributions and specific surface areas via a N₂ adsorption/desorption instrument (Belsorp-mini II, Osaka, Japan) technique. The zeta potential was measured using a microprocessor pH meter (pH211, Hanna Instruments Inc., Woonsocket, RI, USA).

2.4. Pb (II) and Cr (VI) Removal Experiments

2.4.1. Adsorption Study

To study the sorption performance of Pb (II) and Cr (VI) via the PANI/MIL-100(Fe) adsorbent, firstly a certain concentration of Cr (VI) and Pb (II) aqueous stock solution (200 ppm) was achieved from dissolving a certain amount of Pb(NO₃)₂ and K₂Cr₂O₇ in DD water. The obtained aqueous solutions were also produced at the required concentrations of Cr (VI) and Pb (II) ions. The 0.1 M/0.5 M HCl or NaOH was applied for adjusting the initial pH. The initial and residual Cr (VI) contents were determined by a 2,2′-Diphenylcarbonic dihydrazide method on Ultraviolet–visible (UV-Vis) spectrophotometer (model UNICO 2100, Ridge Road, Suite E Dayton, NJ, USA) at 540 nm wavelength. In this study, a known concentration of stock solution (ranging from 0.0–2.0 ppm) was used to plot the calibration curve [26]. At the end of Pb (II) adsorption, the adsorbent was removed, and residual ion concentrations were determined using an atomic absorption spectrometer (Thermo Elemental’s SOLAAR S Series, Waltham, MA, USA). The removal rate (R) of Cr (VI) and Pb (II) was calculated by Equation (1) [30]:

$$\mathrm{R}\left(\%\right)=\frac{\left(C_0-C_e\right)}{C_0}\times 100$$

(1)

where C₀ and C_e show the initial and residual concentration of Cr (VI) and/or Pb (II) ions (ppm), respectively.

2.4.2. Sorption Isotherms, Kinetics, and Thermodynamics Studies

In the isotherm experiments, test solutions (10 mL) of Pb (II) (20–150 ppm) and Cr (VI) (20–50 ppm) were mixed with 10 mg of PANI/MIL-100(Fe). The ion solution’s pH was regulated to 2.0 for Cr (VI) and 6.0 for Pb (II), and the experiments were conducted at temperatures of 298, 308, and 318 K consecutively for 120 min. To perform an adsorption kinetics study, 1 mg/mL of PANI/MIL-100(Fe) adsorbent was dispersed in Pb (II) and Cr (VI) solutions with a prearranged initial concentration (pH = 2 for Cr (VI) and pH = 6 for Pb (II)) for dissimilar time intervals (2, 4, 8, 12, 16, 20, 30, 40, 50, 60, 90, and 120 min). At the end of sorption, the concentration of Cr (VI) and Pb (II) ions was analyzed, and the percent uptake (q_e, mg/g) of metal ions was computed by following Equation (2) [31]:

$${\mathrm{q}}_{\mathrm{e}}=\frac{\left(C_0-C_e\right)V}{m}$$

(2)

where q_e mentions the magnitude of Cr (VI) or Pb (II) adsorbed per gram of adsorbent at equilibrium (mg/g), C₀ denotes the initial concentration of ions (ppm), C_e shows the concentration of residual ions (ppm), m stands for the mass of adsorbent (mg), and V (mL) refers to the solution volume.

2.4.3. Reusability

Commonly, adsorbent stability in the flow of its reaction is regarded as one of the paramount factors for its practical utility. The reusability test of PANI/MIL-100(Fe) was consummated with 1 mg/mL of the adsorbent immersed in a 20 ppm solution of metal ions at room temperature for 90 min. By the end of each test, the solid precipitate was washed with DD water and dried at 298 K, prepared to be used in the other tests. In this study, 0.1 M NaOH/HCl was applied to regenerate PANI/MIL-100(Fe) adsorbent for Cr (VI) and Pb (II), respectively, after 4 h at 170 rpm followed by separation and washing by DD water [32]. When the first cycle of regeneration was finished, the adsorbents were washed three times with DD water to attain a neutral pH. This adsorption/desorption cycle was reiterated three times.

3. Results and Discussions

3.1. Characterization

FT-IR spectrum was performed to describe the material’s functional groups (Figure 2). In the IR spectra of PANI/MIL-100(Fe), the broad bands at the zone of 1382–1627 cm⁻¹ are relevant to the existence of symmetrical and asymmetrical stretching vibration of the carboxylate coordinated to metallic ions in the MIL-100(Fe) [33]. The band of 1118 cm⁻¹ is pertinent to the benzene vibration in H₃BTC [28]. Further, as one could observe the peaks with regard to C–H, the C=C groups in benzenoid become obvious at 711 and 758 cm⁻¹ [33,34].

The stretching modes at 1545 and 1468 cm⁻¹ are due to the C=N and C=C for the quinoid and benzenoid rings of PANI, respectively. The mode at 1301 cm⁻¹ is relevant to the C–N in the secondary aromatic amines. As well, the absorption peak of 1240 cm⁻¹ is attributed to the N–H bending vibrations. Finally, a broad band of secondary N–H stretching is assigned at 3440 cm⁻¹ [35].

The structure of samples before and after three successive cycles was recorded using XRD (Figure 3), which indicates no structural transformation occurred. As demonstrated in Figure 3a, the characteristic diffraction peaks at 2θ = 11.07°, 12.63°, 14.3° 20.16°, 24°, and 27.7° were allocated to MIL-100(Fe) in the composite [28]. Moreover, the characteristic bands of PANI were located at 14.2°, 20°, and 25.2° (Figure 3a), concurring with the (011), (021), and (200) lattice planes [34]. The results support the successful formation of the PANI/MIL-100(Fe).

Furthermore, XRD patterns for Pb (II) and Cr (VI) (2theta of 0 to 80) in the regeneration stage to ensure stability are implied in Figure 3b,c. XRD patterns for PANI/MIL-100(Fe) had no sharp peak from 2θ of 40 to 80°. The relative intensity of the observed peaks for PANI/MIL-100(Fe) is slightly decreased after treatment with HCl/NaOH as a reagent. Before and after the adsorption, for the case Pb (II), very minor changes were recorded in the crystal structures of the composite. Moreover, the same results were attained for Cr (VI). These facts verify that PANI/MIL-100(Fe) has high stability. To put it another way, the crystal structure of the PANI/MIL-100(Fe) was not affected in the acidic/basic condition.

The SEM picture of the PANI/MIL-100(Fe) was exhibited in Figure 4, which depicted orderly polyhedron or octahedron shapes of MIL-100(Fe). The PANI appeared well-covered onto the surface of MIL-100(Fe) and penetrated the slot of the crystals, which means that most PANI is produced inside the hole of MIL-100(Fe) [36]. It is proven that after the encapsulation of PANI within the framework, even under the acidic prepared condition, the surface structure of the MIL-100(Fe) crystals remains almost intact, which affirms the stability of the MIL-100. Moreover, the stability of the PANI/MIL-100(Fe) was confirmed via XRD patterns after desorption of Cr (VI) and Pb (II) [37].

Based on the results shown in Figure 5, the N₂ adsorption/desorption of PANI/MIL-100(Fe) exhibited type IV isotherms with microporous distributions. The pore size distribution of the composite was around 1.2 nm. The S_BET and S_Long of the PANI/MIL-100(Fe) were determined to be 159.47 and 261.29 m²/g, respectively. PANI/MIL-100(Fe) exhibited a much smaller surface area than bulk MIL-100(Fe) (S_BET = 1843.9 m²/g, our previous work) [28]. This may be owing to the filling of PANI inside the MIL-100(Fe) gaps in the course of the synthesis process. In addition, PANI/MIL-100(Fe) has an amount of 0.2124 cm³/g for the total pore volume (p/p₀ = 0.990), with a magnitude of 5.32 nm for the mean pore diameter.

3.2. pH Influences and Adsorption Mechanism

The point of zero charge, or pH_PZC, is the pH magnitude in which electric charge on the adsorbent’s surface is nil. pH_PZC is a vital factor and can help to obtain a better understanding of the electrostatic interaction between adsorbate molecules with the surface of the adsorbent. The initial pH of solution vs. ∆pH plot for PANI/MIL-100(Fe) is displayed in Figure 6. For this test, 1 mg/mL of the adsorbent was immersed in a solution of NaCl (0.01 M) at room temperature for 48 h [38]. A HCl and NaOH solution (0.01 M) was used to adjust the pH between 2–8. The value of pH_PZC achieved was 2.38, which indicated that the pH of ~2.4 below the adsorbent surface was positively charged, and above this magnitude the surface charge was negative. Speciation of the adsorbed ions and adsorbent surface chemistry are affected by the solution pH, which in turn can influence the material’s sorption features.

The effects of pH on the Pb (II) and Cr (VI) adsorption onto PANI/MIL-100(Fe) were assessed over the pH ranging from 2 to 6 (Figure 7). In the acidic condition, Cr (VI) exists as HC_rO₄⁻ and Cr₂O₇²⁻ (pH < 5.0), while CrO₄²⁻ preponderantly exists at pH > 5.0; accordingly, it can result in significant anions adsorption onto the positively charged surface of the adsorbent via electrostatic attraction. On the contrary, the negatively charged surface groups on the adsorbent at pH > pH_PZC are led to repel the anions in the solution. It can be noted that the Cr (VI) sorption was reduced with rising pH from 2 to 6; consequently, a pH solution of 2 was chosen for the sorption experiments of Cr (VI). For the Pb (II) ion, the equilibrium sorption capacity was bound to be negligible at low pH because of the competitive sorption between Pb⁺² ions and H₃O⁺, which enhanced with elevating pH from 2 to 6. In addition, the precipitation of Pb (II) as hydroxides, i.e., Pb (OH)₃⁻, Pb (OH)₂, and Pb (OH)⁺, occurs at pH = 7. Hence, a pH solution of 5.5–6 is suggested for examinations of the Pb (II) removal [39].

3.3. Study of Kinetic Models

In order to study the regulation of Pb (II) and Cr (VI) sorption onto PANI/MIL-100(Fe), pseudo-first and pseudo-second-order models were applied to assess the kinetic information. The kinetic different models, such as pseudo-first-order, pseudo-second-order, and Weber–Morris models (Figure A1, Figure A2 and Figure A3), were fitted to the experimental data to investigate the sorption mechanism of Pb (II) and Cr (VI) onto PANI/MIL-100(Fe).

Table 1. Kinetics, isotherms, and other equations used in this study.

Model/Equation	Parameter	Ref
Pseudo-first-order kinetic $\mathrm{Ln}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\mathrm{Lnq}}_{\mathrm{e}}-{\mathrm{K}}_1\mathrm{t}$	qt (mg/mL): amount of adsorbate adsorbed at time t qe (mg/g): equilibrium sorption capacity K1 (1/min): pseudo-first-order equilibrium rate constant	[37]
Pseudo-second-order kinetic ${\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{q}}_{\mathrm{e}}^2{\mathrm{K}}_2\mathrm{t}}{1+{\mathrm{K}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}$	K2 (g/mg min): pseudo-second-order equilibrium rate constant	[38]
Intraparticle diffusion ${\mathrm{q}}_{\mathrm{t}}={\mathrm{K}}_{\mathrm{id}}{\mathrm{t}}^{0.5}+\complement$	Kid (mg/g min1/2): intraparticle diffusion rate constant C: boundary layer obtained by extrapolation of the linear portion of the plot of qt versus t0.5	[37]
Langmuir ${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{max}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_e}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$ ${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_0}$	qmax (mg/g): maximum sorption capacity KL (L/mg): Langmuir sorption équation constant Ce (mg/L): equilibrium adsorbate concentration in solution C0 (mg/L): initial adsorbate concentration in solution RL: separation factor	[42]
Freundlich ${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{\frac{1}{{\mathrm{n}}_{\mathrm{f}}}}$	KF (L/mg)1/n(mg/g): Freundlich parameter 1/nf: constant corresponded to adsorption intensity	[42]
Temkin ${\mathrm{q}}_{\mathrm{e}}={\mathrm{B}}_{\mathrm{T}}\mathrm{Ln}\left({\mathrm{K}}_{\mathrm{T}}{\mathrm{C}}_{\mathrm{e}}\right)$ ${\mathrm{B}}_{\mathrm{T}}=\frac{R\mathrm{T}}{{\mathrm{b}}_{\mathrm{T}}}$	KT (L/mg): Temkin adsorption potential BT (KJ/mol): heat of sorption	[42]
Sips ${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{K}}_{\mathrm{s}}{\mathrm{q}}_{\max }{\mathrm{C}}_{\mathrm{e}}^{\frac{1}{{\mathrm{n}}_{\mathrm{s}}}}}{1+{\mathrm{K}}_{\mathrm{s}}{\mathrm{C}}_{\mathrm{e}}^{\frac{1}{{\mathrm{n}}_{\mathrm{s}}}}}$	Ks (L/mg): Sips model constant 1/ns: Sips model exponent	[25]
${\mathrm{K}}_{\mathrm{c}}=\frac{C_s}{C_e}$ $\Delta G°=-R\mathrm{T}\mathrm{Ln}\left({\mathrm{K}}_{\mathrm{c}}\right)$ $\mathrm{Ln}\left({\mathrm{K}}_{\mathrm{c}}\right)=\frac{\Delta S°}{R}-\frac{\Delta H°}{R\mathrm{T}}$	Kc: distribution coefficient Cs (mg/g): amount of ion adsorbent ΔG° (kJ/mol): change in standard free energy ΔS° (kJ/mol): standard entropy ΔH° (kJ/mol): standard enthalpy R (8.314 J/mol K): universal gas constant	[42]
Modified Arrhenius $\mathsf{\theta }=\left[1-\frac{C_e}{C_0}\right]$ $S^{*}=\left(1-\mathsf{\theta }\right){\mathrm{e}}^{-\left(\frac{E_a}{R\mathrm{T}}\right)}$	Ea: activation energy S*: sticking probability θ: surface coverage	[35]

summarizes the mathematical expression of the kinetic parameters used in this study [40,41].

The measured kinetic parameters are summarized in

Table 2. Kinetic parameters for Pb (II) and Cr (VI) adsorption at the initial dye concentration of 50 (mg/L) onto MIL-100(Fe)/PANI.

Kinetic Model	Parameters	Pb (II)	Cr (VI)
		50	50
Pseudo-first-order	K1 (1/min)	0.8425	0.3420
	qe (mg/g)	34.072	17.570
	R2	0.9390	0.8812
Pseudo-second-order	K2 (1/min)	0.0453	0.0246
	qe (mg/g)	35.408	19.000
	R2	0.9648	0.9575
Intraparticle diffusion	Kid,1 (mg/g min1/2)	1.4887	1.7300
	C1 (mg/g)	26.809	8.8330
	R2	0.9796	0.9040
	Kid,2 (mg/g min1/2)	0.9563	0.6715
	C2 (mg/g)	28.921	13.690
	R2	0.9274	0.8264
	Kid,3 (mg/g min1/2)	1.5188	0.2072
	C3 (mg/g)	24.936	17.780
	R2	0.9110	0.7231

. In a comparative analysis to the pseudo-first-order model, the quantities of R² of the pseudo-second-order model are higher and much closer to 1 for both metals. Moreover, for the fitting result of the pseudo-second-order model, it was evident that the q_e calculated was close to experimental values (for Pb (II): q_{e, cal} = 35.40 mg/g q_{e, exp} = 40 mg/g; for Cr (VI): q_{e, cal} = 19.00 mg/g q_{e, exp} = 19.97 mg/g). According to these finding, the pseudo-second-order model indicates a good description for the sorption of Cr (VI) and Pb (II) onto PANI/MIL-100(Fe) [42].

To gain a better understanding of the reaction mechanism, an intraparticle diffusion model (IPD) was utilized. By fitting the data with the IPD model (Figure A3 and Table 2), the adsorption can be divided into three successive linear stages: (1) External mass transfer. In this form, the sorption rate is high, and instantaneous sorption occurs in the first step. Moreover, the mass transfer impetus promotes with the increment of the initial concentration of pollutants, causing a growth of the sorption rate and K value. (2) Interfacial diffusion is the gradual sorption step, where intraparticle diffusion is controlled. The sorption sites of PANI/MIL-100(Fe) are slowly saturated, the concentration of the solution decreases, and the internal diffusion resistance increases. (3) Intraparticle diffusion. This portion represents the final equilibrium step; the IPD starts to slow down owing to the extremely low solute concentration in solution [43,44].

3.4. Experimental Design and Adsorption Isotherms

Isotherm models including the Langmuir, Freundlich, Temkin, and Sips models were used to expose the equilibrium correlation among the concentration of Pb (II) and Cr (VI) ions in solution and the concentration of metal ions retained in the PANI/MIL-100(Fe) at a given temperature. Table 1 summarizes the mathematical expression of isotherms used in this study [28,45].

Nonlinear fitting data for the sorption of Cr (VI) and Pb (II) ions by PANI/MIL-100(Fe) are illustrated in Figure A4 and Figure A5, with detailed fitting parameters summarized in

Table 3. Isotherm parameters for Pb (II) and Cr (VI) adsorption onto MIL-100(Fe)/PANI.

Isotherm Parameters	Temperature (K)
	Pb (II)			Cr (VI)
	298 K	308 K	318 K	298 K	308 K	318 K
Langmuir
qmax (mg/g)	67.787	69.103	71.426	39.450	39.515	39.995
KL (L/mg)	0.3747	0.5088	0.7185	6.1265	8.8427	12.233
RL	0.0326	0.0242	0.0173	0.0046	0.0032	0.0023
R2	0.9552	0.9563	0.9541	0.8437	0.8555	0.9489
Freundlich
KF (L/mg)1/n(mg/g)	28.113	30.982	33.725	29.325	30.732	32.124
nf	4.6339	4.9407	5.0787	5.7175	6.2932	7.4515
R2	0.8878	0.8860	0.9070	0.9904	0.9794	0.9091
Temkin
BT (kJ/mol)	10.591	10.305	10.332	5.325	4.950	4.411
KT (L/mg)	9.9494	15.632	22.894	301.37	616.43	1803.79
R2	0.9699	0.9719	0.9835	0.9981	0.9979	0.9583
Sips
qmax (mg/g)	76.934	77.909	81.760	72.370	58.554	43.865
KS (L/mg)	0.4079	0.5116	0.6183	0.7101	1.203	4.079
ns	1.4799	1.5069	1.6017	3.2154	2.8224	1.6795
R2	0.9879	0.9947	0.9953	0.9965	0.9939	0.9903

. Two important factors used to evaluate the fitted model were adjusted: R-Square value (R²) and theoretical maximum sorption capacity (q_m). It is obvious that the Sips isotherm with an R² magnitude of 0.9953 at a temperature of 318 K for Pb (II) displays the superior results compared with other models. The R² of Cr (VI) in various models (Freundlich, Temkin, and Sips) at low temperature, 298 K, was almost the same, indicating that a mixed adsorption mechanism occurred in the sorption process of Cr (VI). In the same vein, the theoretical maximum adsorption capacity (q_max) for the uptake of Pb (II) and Cr (VI) that was obtained was 81.76 and 72.37 mg/g, respectively. The R_L magnitudes between 0 and 1; 0.0326, 0.0242, and 0.0173 for Pb (II); and 0.0046, 0.0032, and 0.0023 for Cr (VI) displayed favorable sorption of both two metal ions onto PANI/MIL-100(Fe) at all temperatures.

In relation to the Freundlich isotherm, the K_F value obtained 28.11, 30.98, and 33.72 for Pb (II) and 29.32, 30.73, and 32.12 for Cr (VI), respectively. Moreover, all the magnitudes of the Freundlich exponent n were greater than 1 (Table 3), which depicted that the heterogeneous process is favorable.

The magnitude of B_T derived from the Temkin isotherm can give information on the chemical or physical nature of the sorption mechanism. In this regard, physisorption mechanisms are reported to have B_T magnitudes smaller than 20 kJ/mol, whereas the range of 80 to 400 kJ/mol implies chemical sorption [9]. Nevertheless, according to the obtained B_T values, it can be obviously concluded that a physisorption occurred in the sorption of Cr (VI) and Pb (II) ions on the PANI/MIL-100(Fe) adsorbent. Moreover, based on the equilibrium binding constant, the increase of K_T values with the rise of temperature reveals that the binding strength between the adsorbate and adsorbent is being strengthened.

In the case of the Sips model, q_max values were 76.93, 77.90, and 81.76 mg/g for Pb (II) and 72.37, 58.55, and 43.865 mg/g for Cr (VI) at the temperatures of 298, 308, and 318 K, respectively.

3.5. Adsorption Thermodynamics

The mathematical expression of thermodynamic factors such as standard Gibb’s free energy (ΔG°), enthalpy (ΔH°), entropy change (ΔS°), and activation energy (E_a) were summarized in Table 1 [36,43]. The magnitudes of ΔS° and ΔH° can be estimated by the intercept and slope of a graph of Ln (K_c) vs. 1/T (Figure 8) [46]. The calculated magnitudes of thermodynamic factors were displayed in

Table 4. Thermodynamic parameters for the adsorption of Pb (II) and Cr (VI) onto MIL-100(Fe)/PANI.

Heavy Metal Ions	ΔG° (kJ/mol)					ΔH° (kJ/mol)	ΔS° (J/mol·K)	Ea (kJ/mol)
	298 °K	308 °K	313 °K	318 °K	323 °K
Pb (II)	−4.77167	−5.68388	−6.33805	−7.13721	−8.17302	+32.9148	+126.012	+30.3047
Cr (VI)	−5.85144	−6.40522	−6.81851	−7.27772	−7.79501	+17.1887	+76.9815	+16.0097

.

All negative magnitudes of the ΔG° indicate that the sorption of Cr (VI) and Pb (II) within the PANI/MIL-100(Fe) is spontaneous. When the temperature of the process rises, the absolute magnitudes of Gibb’s free energy increase, which shows the degree of spontaneity enhancement. The standard enthalpy magnitudes of Cr (VI) and Pb (II) adsorbed by PANI/MIL-100(Fe) are 17.18 and 32.91 kJ/mol, respectively. According to positive magnitudes of the ΔH°, high temperatures are desirable for both metal ions sorption; in other words, results indicated that the sorption mechanism is an endothermic process. Analogously, all positive magnitudes for the ΔS° indicate that the grade of freedom of the sorption system rises with the enhancement of reaction temperature [47].

Table 4 reported the E_a defined from the slopes of the straight lines of the modified Arrhenius diagrams from Figure 9. Physisorption and chemisorption are two principal types of adsorptions. Customarily, the range of energies in the physisorption processes is between 5 and 40 kJ/mol, which may be due to the involvement of only weak forces in this process. The rate of chemisorption is affected by temperature, with a limited E_a in the range of 40–800 kJ/mol [48]. Hence, we can express that the deliberated E_a is an authentic amount for physisorption. According to the E_a values, it can be reasoned that the rate-determining stage in the sorption of Cr (VI) and Pb (II) is physisorption, wherein electrons are exchanged between the PANI/MIL-100(Fe) and Pb (II) and Cr (VI).

3.6. Reusability of the Adsorbent

In order to assay the reuse and recycling properties of the PANI/MIL-100(Fe) (favorable features of a superb adsorbent), reusability analysis was conducted; the results are illustrated in Figure 10. The PANI/MIL-100(Fe) could maintain good sorption uptake after three cycles, still attaining 90% and 87.4% removal for Pb (II) and Cr (VI) after the third recycle. For both metals, the loss of the removal efficiency was nearly 9% after three runs.

3.7. Removal Mechanism of Cr (VI) and Pb (II) on PANI/MIL-100(Fe)

The heavy metal ions and PANI/MIL-100(Fe) sorption mechanisms can be imputed in two aspects: one is the sorption capability excited by a large specific surface area; the other is the ion-exchange reaction amid the heavy metal ions and the functional groups on the surface of the adsorbent (Figure 11). Pb (II) sorption using coordination and electrostatic interaction on the surfaces of PANI/MIL-100(Fe) is originated from the presence of C=N (PANI) and –COO⁻ (MIL100) functional groups [28,49]. A key factor of the adsorption mechanism in PANI is the hydrogen atom of the –NH– group. In this work, the protonation of ample amine (–NH–) and imine (=N–) groups in the polyaniline chain leads to the production of the –NH²⁺– and =NH⁺– acidic sites in the PANI chain, which ultimately can be the active sorption sites for HCrO₄⁻ and Cr₂O₇²⁻. Hereupon, there exists an acid–base interaction amid the Cr (VI) and the acidic –NH²⁺– and =NH⁺– moieties of the MIL-100(Fe)/PANI [46].

The removal mechanism of Pb (II) and Cr (VI) onto PANI/MIL-100(Fe) was examined by FT-IR [47]. According to the FT-IR spectrum, before the adsorption process (Figure 2), it is evident that the –NH– modes of PANI/MIL-100(Fe) were at 1240 and 3440 cm⁻¹, and the symmetric stretching of the carboxyl mode was at 1627 cm⁻¹, while after sorption of heavy metal ions on MIL-100(Fe)/PANI, the locations of all modes shifted impressively (Figure 12). Subsequently, they diminished to 1127, 3401, and 1623 cm⁻¹ after Cr (VI) was adsorbed and to 1121, 3395, and 1621 cm⁻¹ after Pb (II) was adsorbed, respectively. This case can be due to the interaction among functional groups on sorbent and metal ions, comprising surface compilations to form metal complexes or electrostatic. Moreover, the –NH– and carboxyl modes of PANI/MIL-100(Fe) diminished to 1151, 3228, and 1582 cm⁻¹ for Cr (VI) desorption and 1119, 3402, and 1624 cm⁻¹ for Pb (II) desorption.

Especially, statuses of all peaks after adsorption/desorption changed impressively to lower wavenumbers, indicating that the amine (–NH–) and –COO⁻ functional groups had a great affinity for Pb (II) and Cr (VI), respectively. Moreover, after sorption of heavy metal ions by MIL-100(Fe)/PANI, locations of multiple peaks in the scope of 450–650 cm⁻¹ did not change considerably.

4. Conclusions

A new adsorptive PANI/MIL-100(Fe) material was facilely prepared using the hydrothermal method. Firstly, the SEM, BET, XRD, and FT-IR characterizations verified the successful synthesis of PANI/MIL-100(Fe). The functional group and high surface area (S_Long = 261.29 m²/g) of the prepared composite were, respectively, validated using FT-IR and BET analysis. The XRD patterns of PANI/MIL-100(Fe) before and after ion adsorption implied the adsorbent stability in the regeneration step. Furthermore, the obtained PANI/MIL-100(Fe) composite displayed high uptake ability toward Cr (VI) adsorption (72.37 mg/g) and Pb (II) adsorption (81.76 mg/g) at pH = 2 and 6, respectively. The equilibrium and kinetic data of both Pb (II) and Cr (VI) ions adsorption were well-fitted using the Sips isotherm and pseudo-second-order kinetic models that showed the sorption rate dependency upon the availability of the sorption active sites. Furthermore, thermodynamic parameters demonstrated that the sorption mechanism of both Pb (II) and Cr (VI) ions are endothermic, spontaneous, and based on physisorption. The results of the regeneration process showed the high ability of PANI/MIL-100(Fe) adsorbent for both metal ions sorption after three cycles. According to the successful results on the artificial wastewater sample on the removal of target elements, the experimental results will be assessed on the actual wastewater from one of the chemical industries’ effluents as a next step.