**CuFe2O4**/**Polyaniline (PANI) Nanocomposite for the Hazard Mercuric Ion Removal: Synthesis, Characterization, and Adsorption Properties Study**

**Saad S. M. Hassan 1,\*, Ayman H. Kamel 1,\* , Amr A. Hassan 1,2 , Abd El-Galil E. Amr 3,4,\* , Heba Abd El-Naby <sup>1</sup> , Mohamed A. Al-Omar <sup>3</sup> and Ahmed Y. A. Sayed <sup>3</sup>**


Academic Editors: Chiara Bisio and Monica Pica Received: 11 May 2020; Accepted: 9 June 2020; Published: 12 June 2020

**Abstract:** Copper ferrite nano-particles (CuFe2O4) were synthesized, characterized, modified with polyaniline to form CuFe2O4/PANI nano-composite. They were used as new adsorbents for the removal of the hazardous mercuric ions from aqueous solutions. High resolution transmission electron microscope (HR-TEM), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) and Brunauer–Emmett–Teller (BET) were used for the characterization of the synthesized CuFe2O<sup>4</sup> nano-particles (NPs) in presence and absence of PANI nano-composite. The synthesized CuFe2O4NPs were of spherical shape with an average size of 10.8 nm. XRD analysis displayed crystal peaks for CuFe2O4NPs and amorphous peaks CuFe2O4/PANI nano-composite due to the existence of polyaniline layer. Contact time, adsorbent dose, solution pH, adsorption kinetics, adsorption isotherm and recyclability were studied. The method at the optimum conditions exhibited high performance with high mercury removal percentage of up to 99% with a maximum adsorption capacity 12.5 and 157.1 mg/g for CuFe2O<sup>4</sup> and CuFe2O4/PANI, respectively. The adsorption processes were fitted to Langmuir isotherms. The adsorption behavior of CuFe2O4@PANI composite towards Hg2<sup>+</sup> ions is attributed to the soft acid–soft base strong interaction between PANI and Hg(II) ions. High stability and enhanced re-usability are offered using CuFe2O4@PANI composite due to its enhanced removal efficiency. No significant removal decrease was noticed after five adsorption–desorption cycles. In addition, it possesses an easy removal from aqueous solutions by external magnetic field after adsorption experiments. These indicated the enhancement of polyaniline to the surface of CuFe2O<sup>4</sup> toward the adsorption of mercury from aqueous solutions.

**Keywords:** CuFe2O<sup>4</sup> nano-particles; CuFe2O4/PANI composite; mercury (II) removal; adsorption

#### **1. Introduction**

A clean water resource is a vital and necessary goal for the whole world. Toxic heavy metals like Hg, Pb, Cd, and Ni are considered the most dangerous environmental pollutants in the water, thus becoming of prior anxiety because of their toxicity and non-biodegradability to plants, animals and human [1,2]. Mercury is one of these heavy metals that can cause serious environmental and health problems as chronic and acute poisoning. It exists in different forms such as metallic Hg, Hg+, Hg2+,


and organic mercury containing phenyl, methyl, and ethyl groups, etc. It causes different diseases such as Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and damaging of the immune system and kidneys. Mercury is considered as prior hazardous pollutant by the Agency for Toxic Substances and Disease Registry [3]. One of its natural sources is the volcanoes that produce almost half of the mercury emissions released in atmosphere. It is also produced from different industrial sources such as pharmaceuticals, chloralkali, plastic, textile, paint, rubber, paper, cement, electronic industry, coal combustion, fertilizers, oil refining, and rubber processing [4,5]. The other half is generated by humans by various means including 65% in combustion, 11% in the production of gold, 6.8% in the production of non-ferrous metal, 6.4% in the production of cement, 3.0% in the waste disposal including municipal waste, and 3.0% in the production of caustic soda [6]. According to the World Health Organization (WHO), 1 µg/L is the maximum permissible concentration of Hg(II)in drinking water [7]. According to the European Union (EU), the maximum acceptable level of Hg(II) is 5 µg/L for wastewater discharge [8–10]. River and lake water in the nearby industries may contain mercuric discharges which are fatal for aquatic as well as for human life. These discharges could accumulate in the stomach and remain non digestible resulting in the formation of cancerous diseases. Long-term exposure to mercury could cause serious damage to nerves, brain, kidney, lung irritation, eye irritation, skin rashes, vomiting, and diarrhea [11].Researchers have been used a lot of techniques to get rid of heavy metals in particularly mercury ion from waste water such as sorption and filtration [12], ion exchange [13,14], chemical precipitation [6], adsorption [15–17], solid phase extraction [18], and adsorption process using nano-materials [19–27]. The adsorption technique is the most effective and commonly used due to its high removal efficiency and cheapness.

Recently, there is a focus on the application of nano-materials in the removal of different environmental pollutants. This is based on their distinctive properties such as high surface area, high adsorption, and special photoelectric property. However, they are suffering from difficulty of their separation from aqueous solutions due to their small particle size which restricts the application in water treatment. So, it is preferable using magnetic nano-materials that can be easily separated from solution with external magnetic field [28–30].

Magnetic nano-materials possess adsorbent properties that qualify them for use as promising adsorbent materials, which open up a wide field for engineering separation applications. These magnetic nano-particles can be separated based on their nanostructures due to the easy direction of magnetization, which will vary depending on the arrangement of the atoms in the magnetic structure [31–33]. Applying a low density magnetic field stimulates the magnetization of the material and therefore makes the use of magnetic force possible, but when the magnetic field is cut off, the magnetization immediately decreases to zero. This last point is important for the release of particles after adsorption of the waste [34,35]. The main drawback of using magnetic nano-particles is the low potential pollutant removal ability. To invade this defect, the surface of magnetic nano-particles has been modified. The surface properties of nano-particles can be greatly enhanced after this modification. This is preferred through the Van der Waals interaction between the modified material and the reduced solvent shielding of the ions in the interlamellar environment.

Polyaniline (PANI) has attracted much attention because of its several unique properties [36–38]. It is highly stable in air and soluble in various solvents and exhibits dramatic changes in its electronic structure and physical properties in the protonated state. It also shows magnetic behavior because of its high spin density [39,40].

In the present work, modification of CuFe2O<sup>4</sup> nano-particles (NPs) with polyaniline was used as a novel adsorbent for mercury removal in aqueous solutions. The nano-particles were synthesized, characterized, and used as an adsorbent for mercury removal under optimum conditions. The removal efficiency of the prepared adsorbents was investigated, and their adsorption and desorption behaviors towards mercury species were studied.

#### **2. Results and Discussion** *2.1. Adsorbent Characterization*

#### *2.1. Adsorbent Characterization* 2.1.1. X-ray Diffraction Pattern

#### 2.1.1. X-ray Diffraction Pattern The phase identification of CuFe2O4 and CuFe2O4/PANI nano-composites was illustrated by X-

The phase identification of CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composites was illustrated by X-ray diffraction (XRD) as shown in Figure 1. All of the high intensity peaks are indexed and refined as tetragonal structure with I41/amd space group, which is consistent with standard Joint Committee on Powder Diffraction Standards (JCPDS) card no. 34-0425.The obtained XRD pattern exhibits good crystallinity for CuFe2O4. The reflection plans (101), (112), (211), (220), (303), and (224) coincide with the tetragonal spinel phase for CuFe2O<sup>4</sup> with a characteristic peak appears at 2θ 35.5◦ . The reflection plans (010), (100), and (110) coincide with the amorphous phase of standard data for polyaniline. It is apparent that the broad diffraction peak centered at 2θ value 25.3◦ (110) in Figure 1 is the characteristic peak of the PANI layer. This can be ascribed to the periodicity parallel and perpendicular to the polymer chains, respectively [41]. The characteristic peak of CuFe2O<sup>4</sup> still appears at 35.5◦ and little shift for the other peaks when doped with PANI. The average crystalline size of the prepared nano-composite was calculated using Scherrer's equation [42]: ray diffraction (XRD) as shown in Figure 1. All of the high intensity peaks are indexed and refined as tetragonal structure with I41/amd space group, which is consistent with standard Joint Committee on Powder Diffraction Standards (JCPDS) card no. 34-0425.The obtained XRD pattern exhibits good crystallinity for CuFe2O4. The reflection plans (101), (112), (211), (220), (303), and (224) coincide with the tetragonal spinel phase for CuFe2O4 with a characteristic peak appears at 2*θ* 35.5°. The reflection plans (010), (100), and (110) coincide with the amorphous phase of standard data for polyaniline. It is apparent that the broad diffraction peak centered at 2*θ* value 25.3° (110) in Figure 1 is the characteristic peak of the PANI layer. This can be ascribed to the periodicity parallel and perpendicular to the polymer chains, respectively [41]. The characteristic peak of CuFe2O4 still appears at 35.5° and little shift for the other peaks when doped with PANI. The average crystalline size of the prepared nano-composite was calculated using Scherrer's equation [42]: *D* = 0.9 *λ*/*β cos θ* (1)

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$$\mathbf{p} = 0.9 \,\,\lambda \vert \mathcal{S} \cos \theta \tag{1}$$

where d is the average crystalline size, λ is the wavelength of CuKα*,*β is the full width at half maximum (FWHM) of most intense diffraction peak (211), and θ is the Bragg's angle. The average particle size is estimated to be 10.8 and 23.4 nm for CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composites, respectively. particle size is estimated to be 10.8 and 23.4 nm for CuFe2O4 and CuFe2O4/PANI nano-composites, respectively.

**Figure 1.** Pattern of both CuFe2O4 and CuFe2O4/PANI nano-composites. **Figure 1.** Pattern of both CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites.

#### 2.1.2. High Resolution Transmission Electron Microscopy (HRTEM) 2.1.2. High Resolution Transmission Electron Microscopy (HRTEM)

TEM images for CuFe2O4 nano-particles showed spherical shaped nano-particles with small agglomeration and nano sizes of 10.8 nm that coincides with the XRD result. The particles are dense and regularly distributed with clear boundary between neighboring particles as observed in Figure 2a. TEM images of CuFe2O4/PANI nano-composite revealed the light shell nature of PANI in which dark core copper ferrite particles are embedded as shown in Figure 2b. TEM images for CuFe2O<sup>4</sup> nano-particles showed spherical shaped nano-particles with small agglomeration and nano sizes of 10.8 nm that coincides with the XRD result. The particles are dense and regularly distributed with clear boundary between neighboring particles as observed in Figure 2a. TEM images of CuFe2O4/PANI nano-composite revealed the light shell nature of PANI in which dark core copper ferrite particles are embedded as shown in Figure 2b.

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**Figure 2.** Images of (**A**) CuFe2O<sup>4</sup> and (**B**) CuFe2O<sup>4</sup> /PANI nano-composites.

#### **Figure 2.** Images of (**A**) CuFe2O4 and (**B**) CuFe2O4/PANI nano-composites. 2.1.3. Brunauer–Emmett–Teller (BET)

**V o lu m e (c c /g )**

2.1.3. Brunauer–Emmett–Teller (BET) The N2 adsorption–desorption experiment at 77 K for CuFe2O4 and CuFe2O4/PANI nanocomposites are shown in Figure 3. The figure shows an adsorption isotherm of the type IV with a hysteresis loop that is associated with capillary condensation within the mesoporous regions [43], with a hysteresis loop type H3, which is usually indicative of aggregates of platelet particles or adsorbents containing slit pores. The initial part of the isotherm (until *p/p<sup>o</sup>* ≈ 0.4) can be attributed to monolayer/multilayer adsorption because it follows the same path of desorption, which demonstrates weak adsorbate–adsorbent interactions. The hysteresis loop begins at *p/p<sup>o</sup>* = 0.4 and it ends at *p/p<sup>o</sup>* = 0.95; the hysteresis loop exhibits limited adsorption. This phenomenon is related to the presence of particles that are not rigidly joined together. The BET surface area and pore volume of the nano-composite are recorded in Table 1. The pore size of CuFe2O4 nano-particles was about 9.9 nm. It was regarded as a mesoporous material of surface area 44.7 m<sup>2</sup> /g and pore volume of 0.11 cm<sup>3</sup> /g. For CuFe2O4/PANI nano-composites, the BET surface area is lower, around 30.8 m<sup>2</sup> /g, due to the lower cumulative volume of pores (0.06 cm<sup>3</sup> /g). The N<sup>2</sup> adsorption–desorption experiment at 77 K for CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composites are shown in Figure 3. The figure shows an adsorption isotherm of the type IV with a hysteresis loop that is associated with capillary condensation within the mesoporous regions [43], with a hysteresis loop type H3, which is usually indicative of aggregates of platelet particles or adsorbents containing slit pores. The initial part of the isotherm (until *p*/*p <sup>o</sup>* ≈ 0.4) can be attributed to monolayer/multilayer adsorption because it follows the same path of desorption, which demonstrates weak adsorbate–adsorbent interactions. The hysteresis loop begins at *p*/*p <sup>o</sup>* = 0.4 and it ends at *p*/*p <sup>o</sup>* = 0.95; the hysteresis loop exhibits limited adsorption. This phenomenon is related to the presence of particles that are not rigidly joined together. The BET surface area and pore volume of the nano-composite are recorded in Table 1. The pore size of CuFe2O<sup>4</sup> nano-particles was about 9.9 nm. It was regarded as a mesoporous material of surface area 44.7 m<sup>2</sup> /g and pore volume of 0.11 cm<sup>3</sup> /g. For CuFe2O4/PANI nano-composites, the BET surface area is lower, around 30.8 m<sup>2</sup> /g, due to the lower cumulative volume of pores (0.06 cm<sup>3</sup> /g). **Figure 2.** Images of (**A**) CuFe2O4 and (**B**) CuFe2O4/PANI nano-composites. 2.1.3. Brunauer–Emmett–Teller (BET) The N2 adsorption–desorption experiment at 77 K for CuFe2O4 and CuFe2O4/PANI nanocomposites are shown in Figure 3. The figure shows an adsorption isotherm of the type IV with a hysteresis loop that is associated with capillary condensation within the mesoporous regions [43], with a hysteresis loop type H3, which is usually indicative of aggregates of platelet particles or adsorbents containing slit pores. The initial part of the isotherm (until *p/p<sup>o</sup>* ≈ 0.4) can be attributed to monolayer/multilayer adsorption because it follows the same path of desorption, which demonstrates weak adsorbate–adsorbent interactions. The hysteresis loop begins at *p/p<sup>o</sup>* = 0.4 and it ends at *p/p<sup>o</sup>* = 0.95; the hysteresis loop exhibits limited adsorption. This phenomenon is related to the presence of particles that are not rigidly joined together. The BET surface area and pore volume of the nano-composite are recorded in Table 1. The pore size of CuFe2O4 nano-particles was about 9.9 nm. It was regarded as a mesoporous material of surface area 44.7 m<sup>2</sup> /g and pore volume of 0.11 cm<sup>3</sup> /g. For CuFe2O4/PANI nano-composites, the BET surface area is lower, around 30.8 m<sup>2</sup> /g, due to the lower cumulative volume of pores (0.06 cm<sup>3</sup> /g).

**Figure 3.** N2 adsorption–desorption isotherms of coupled CuFe2O4 and CuFe2O4/PANI (polyaniline) **Relative pressure(p/p<sup>o</sup> ) Relative pressure(p/p<sup>o</sup> ) Figure 3.** N2 adsorption–desorption isotherms of coupled CuFe2O4 and CuFe2O4/PANI (polyaniline) nano-composites. **Figure 3.** N<sup>2</sup> adsorption–desorption isotherms of coupled CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI (polyaniline) nano-composites.

nano-composites. **Table 1.** General surface characteristics of CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites obtained by N<sup>2</sup> adsorption at 77 K.


N2 adsorption at 77 K.

**Sample Surface Area(m<sup>2</sup>**

#### 2.1.4. Fourier Transforms–Infrared Spectroscopy (FTIR) The FTIR spectra of CuFe2O4 and CuFe2O4/PANI nano-composites are shown as supplementary material in Supplementary Materials Figure S1. CuFe2O4 spectrum has only a characteristic peak at

The FTIR spectra of CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composites are shown as supplementary material in Supplementary Materials Figure S1. CuFe2O<sup>4</sup> spectrum has only a characteristic peak at 574.9 cm−<sup>1</sup> of M–O bond while CuFe2O4/PANI spectrum has several characteristics peaks corresponding to polyaniline. These include peaks at 3419.1 cm−<sup>1</sup> assigned for N–H stretching, 1561.1 cm−<sup>1</sup> assigned to stretching vibration of C=C, 1469.7 cm−<sup>1</sup> assigned to stretching vibration of C–C, 1298.7 cm−<sup>1</sup> C–N stretching vibrations, 1135.8 cm−<sup>1</sup> for C–H bending mode and 777 cm−<sup>1</sup> assigned to the wagging of =C–H. Hence the obtained results confirm the presence of copper ferrite nano-particles doped PANI. 574.9 cm−1 of M–O bond while CuFe2O4/PANI spectrum has several characteristics peaks corresponding to polyaniline. These include peaks at 3419.1 cm−1 assigned for N–H stretching, 1561.1 cm−1 assigned to stretching vibration of C=C, 1469.7 cm−1 assigned to stretching vibration of C–C, 1298.7 cm−1 C–N stretching vibrations, 1135.8 cm−1 for C–H bending mode and 777 cm−1 assigned to the wagging of=C–H. Hence the obtained results confirm the presence of copper ferrite nano-particles doped PANI.

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**Table 1.** General surface characteristics of CuFe2O4 and CuFe2O4/PANI nano-composites obtained by

**/g) Average Pore Volume(cm<sup>3</sup>**

CuFe2O4/PANI 30.8 0.06 17.8 CuFe2O4 NP 44.7 0.11 9.9

**/g) Average Pore Diameter (nm)** 

#### 2.1.5. Thermal Analysis Thermal-gravimetric analysis (TGA) of CuFe2O4/PANI is presented in Figure S2. It showed an

2.1.5. Thermal Analysis

Thermal-gravimetric analysis (TGA) of CuFe2O4/PANI is presented in Figure S2. It showed an overall weight loss of 35% in the range of 25–800 ◦C. A weight loss before 100 ◦C is noticed in the TGA curve due to residual water evaporation. Another weight loss is noticed within the ranging from 310 to 480 ◦C and 480 to 630 ◦C due to the thermal degradation of the lower and the higher weight PANI chains, respectively. overall weight loss of 35% in the range of 25–800 °C. A weight loss before 100 °C is noticed in the TGA curve due to residual water evaporation. Another weight loss is noticed within the ranging from 310 to 480 °C and 480 to 630 °C due to the thermal degradation of the lower and the higher weight PANI chains, respectively.

#### *2.2. Adsorption Study 2.2. Adsorption Study*

Nano-composite particles consisting of CuFe2O<sup>4</sup> and that doped with PANI were prepared and tested as adsorbing substances to remove mercuric ions from aqueous solutions and some industrial waste water. Nano-composite particles consisting of CuFe2O4 and that doped with PANI were prepared and tested as adsorbing substances to remove mercuric ions from aqueous solutions and some industrial waste water.

#### 2.2.1. Effect of Mercury Concentration 2.2.1. Effect of Mercury Concentration

The removal efficiency of mercury ions using CuFe2O4NPs was 82% beginning from 10 up to 120 µg/mL. The adsorption performed at pH value 7 for 30 mL of the adsorbent solution stirred for 120 min. While CuFe2O4/PANI nano-composites exhibit higher removal efficiency of 99.5% when varying the concentration of Hg2<sup>+</sup> from 10 to 32 µg/mL. It begins to decrease to 92.3% upon increasing the concentration of mercury up to 120 µg/mL (Figure 4). The removal efficiency of mercury ions using CuFe2O4NPs was 82% beginning from 10 up to 120 µg/mL. The adsorption performed at pH value 7 for 30 mL of the adsorbent solution stirred for 120 min. While CuFe2O4/PANI nano-composites exhibit higher removal efficiency of 99.5% when varying the concentration of Hg2+ from 10 to 32 µg/mL. It begins to decrease to 92.3% upon increasing the concentration of mercury up to 120 µg/mL (Figure 4).

**Figure 4.** Effect of Hg2+ concentration on adsorption optimization using CuFe2O4 and CuFe2O4/PANI nano-composites sorbents; Conditions: (V= 30 mL, contact time = 2 h for CuFe2O4 and 1 h for CuFe2O4/PANI, adsorbent amount =0.1 g CuFe2O4 and 0.2 g CuFe2O4/PANI and pH 7). **Figure 4.** Effect of Hg2<sup>+</sup> concentration on adsorption optimization using CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites sorbents; Conditions: (V = 30 mL, contact time = 2 h for CuFe2O<sup>4</sup> and 1 h for CuFe2O<sup>4</sup> /PANI, adsorbent amount = 0.1 g CuFe2O<sup>4</sup> and 0.2 g CuFe2O<sup>4</sup> /PANI and pH 7).

#### 2.2.2. Effect of Contact Time

As shown in Figure 5, the concentration of Hg(II) ions was studied relative to the contact time of each adsorbent. It was found that the time required to obtain more than 80% of Hg(II) removal was 2 h for CuFe2O4. However, in case of CuFe2O4/PANI composite, the time required to achieve the equilibrium was one hour with a removal percentage of 99.5%. To examine the adsorption mechanism, 2.2.2. Effect of Contact Time

**Removal %**

2.2.2. Effect of Contact Time

kinetics is the vital feature. Pseudo first order and second order models were fitted as the practical kinetics data. The obtained results were presented in Table 2. The adsorption process for both CuFe2O<sup>4</sup> and CuFe2O4/PANI composite obeyed the second order model. mechanism, kinetics is the vital feature. Pseudo first order and second order models were fitted as the practical kinetics data. The obtained results were presented in Table 2. The adsorption process for both CuFe2O4 and CuFe2O4/PANI composite obeyed the second order model. **100 110**

equilibrium was one hour with a removal percentage of 99.5%. To examine the adsorption

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As shown in Figure 5, the concentration of Hg(II) ions was studied relative to the contact time

As shown in Figure 5, the concentration of Hg(II) ions was studied relative to the contact time of each adsorbent. It was found that the time required to obtain more than 80% of Hg(II) removal was 2 h for CuFe2O4. However, in case of CuFe2O4/PANI composite, the time required to achieve the equilibrium was one hour with a removal percentage of 99.5%. To examine the adsorption mechanism, kinetics is the vital feature. Pseudo first order and second order models were fitted as

**Figure 5.** Effect of contact time on adsorption optimization using CuFe2O4 and CuFe2O4/PANI nanocomposites sorbents (V = 30 mL, Hg2+ concentration = 25 µg/mL, adsorbent amount = 0.1 g CuFe2O<sup>4</sup> and 0.2 g CuFe2O4/PANI and pH 7). **Figure 5.** Effect of contact time on adsorption optimization using CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites sorbents (V = 30 mL, Hg2<sup>+</sup> concentration = 25 µg/mL, adsorbent amount = 0.1 g CuFe2O<sup>4</sup> and 0.2 g CuFe2O<sup>4</sup> /PANI and pH 7). composites sorbents (V = 30 mL, Hg2+ concentration = 25 µg/mL, adsorbent amount = 0.1 g CuFe2O<sup>4</sup> and 0.2 g CuFe2O4/PANI and pH 7). **Table 2.** Adsorption kinetics parameters.

**Table 2.** Adsorption kinetics parameters. **Table 2.** Adsorption kinetics parameters. **Adsorbent Pseudo-First Order Second Order** 


#### To optimize the amount of adsorbent NPs, different amounts from each adsorbent in the range 2.2.3. Effect of Adsorbent Amount To optimize the amount of adsorbent NPs, different amounts from each adsorbent in the range

2.2.3. Effect of Adsorbent Amount

of 0.05 to 0.3 g were put in contact with 30 mL of 25 µg/mL Hg2+ solutions of pH 7 and 60 min contact time. As shown in Figure 6, it was observed that the maximum adsorption (i.e., 99.5% removal efficiency) was attained after using 0.2 and 0.1 g for CuFe2O4NPs and CuFe2O4/PANI composite, respectively. To optimize the amount of adsorbent NPs, different amounts from each adsorbent in the range of 0.05 to 0.3 g were put in contact with 30 mL of 25 µg/mL Hg2<sup>+</sup> solutions of pH 7 and 60 min contact time. As shown in Figure 6, it was observed that the maximum adsorption (i.e., 99.5% removal efficiency) was attained after using 0.2 and 0.1 g for CuFe2O4NPs and CuFe2O4/PANI composite, respectively. of 0.05 to 0.3 g were put in contact with 30 mL of 25 µg/mL Hg2+ solutions of pH 7 and 60 min contact time. As shown in Figure 6, it was observed that the maximum adsorption (i.e., 99.5% removal efficiency) was attained after using 0.2 and 0.1 g for CuFe2O4NPs and CuFe2O4/PANI composite, respectively.

**Figure 6.** Effect of CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composite sorbents amount on adsorption optimization (V = 30 mL, Hg2<sup>+</sup> concentration = 25 µg/mL, contact time =2 h for CuFe2O<sup>4</sup> and 1 h for CuFe2O<sup>4</sup> /PANI and pH 7).

#### 2.2.4. Effect of pH

The pH is an essential parameter for Hg2<sup>+</sup> adsorption due to its relevance to Hg speciation, as well as the interactions between Hg species and adsorbent surfaces. When the feed water pH was 2.2.4. Effect of pH

CuFe2O4/PANI and pH 7).

forming negative charges on the CuFe2O4 surface.

varied from 6.0 to 9.0, Hg2<sup>+</sup> removal efficiency of CuFe2O4/polyaniline remained at ~99.5% (Figure 7A). For CuFe2O4NPs the removal percentage of Hg(II) ions became constant until pH reaches 7. This can be explained that at higher pH values, oxygen-containing groups (e.g., –OH) are ionized to –O–, forming negative charges on the CuFe2O<sup>4</sup> surface. PANI-HCl), nitrogen atoms of imine groups were preferentially bound by protons, causing the PANI surfaces carrying positive charges.

CuFe2O4/PANI composite, is around 4–6 and 6, respectively. CuFe2O4/PANI composite had net negative charges at pH > 6.0 and positive charges at pH < 6.0. At low pH values (e.g., pH < 5.0 for

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**Figure 6.** Effect of CuFe2O4 and CuFe2O4/PANI nano-composite sorbents amount on adsorption optimization (V = 30 mL, Hg2+ concentration = 25 µg/mL, contact time =2 h for CuFe2O4 and 1 h for

The pH is an essential parameter for Hg2+ adsorption due to its relevance to Hg speciation, as well as the interactions between Hg species and adsorbent surfaces. When the feed water pH was varied from 6.0 to 9.0, Hg2+ removal efficiency of CuFe2O4/polyaniline remained at ~99.5% (Figure 7A). For CuFe2O4NPs the removal percentage of Hg(II) ions became constant until pH reaches 7. This can be explained that at higher pH values, oxygen-containing groups (e.g.,–OH) are ionized to –O–,

**Figure 7.** (**A**) Effect of pH on adsorption optimization using CuFe2O4 and CuFe2O4/PANI nanocomposite sorbents (V= 30 mL, Hg2+ concentration= 25 µg/mL, adsorbent amount = 0.1 g CuFe2O4 and 0.2 g CuFe2O4/PANI and contact time = 2 h for CuFe2O4 and 1 h for CuFe2O4/PANI). (**B**) Plots of the zeta potential as a function of pH for CuFe2O4/PANI and PANI. **Figure 7.** (**A**) Effect of pH on adsorption optimization using CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composite sorbents (V= 30 mL, Hg2<sup>+</sup> concentration= 25 µg/mL, adsorbent amount = 0.1 g CuFe2O<sup>4</sup> and 0.2 g CuFe2O<sup>4</sup> /PANI and contact time = 2 h for CuFe2O<sup>4</sup> and 1 h for CuFe2O<sup>4</sup> /PANI). (**B**) Plots of the zeta potential as a function of pH for CuFe2O<sup>4</sup> /PANI and PANI.

*2.3. Adsorption Isotherms*  Langmuir (Equation (2), Freundlich (Equation (3), and Temkin (Equation (4) models were applied to calculate the sorption of Hg2+ ions for both CuFe2O4 and CuFe2O4/PANI nano-composite (Figures 8 and 9). Based on zeta potential results (Figure 7B),the Point of Zero Charge for both PANI and CuFe2O4/PANI composite, is around 4–6 and 6, respectively. CuFe2O4/PANI composite had net negative charges at pH > 6.0 and positive charges at pH < 6.0. At low pH values (e.g., pH < 5.0 for PANI-HCl), nitrogen atoms of imine groups were preferentially bound by protons, causing the PANI surfaces carrying positive charges.

#### *2.3. Adsorption Isotherms*

*Log Qt* = (1/*n*) *log Ct*+ *log kF* (3) Langmuir (Equation (2), Freundlich (Equation (3), and Temkin (Equation (4) models were applied to calculate the sorption of Hg2<sup>+</sup> ions for both CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composite (Figures 8 and 9).

$$\mathbf{1}/\mathbf{Q}\_t = \mathbf{1}/\mathbf{X}\_m b \mathbf{C}\_t + \mathbf{1}/\mathbf{X}\_m \tag{2}$$

1/*Qt* = 1/*Xmb Ct* + 1/*Xm* (2)

$$
\text{Log Q}\_l = (1/n)\log \text{C}\_l + \log k\_{\text{F}} \tag{3}
$$

$$Q\_l = (RT/B\_T)\ln{C\_l} + (RT/B\_T)\ln{K\_T} \tag{4}$$

intensities *n* and *Kf* (mg/g), and the constants of Freundlich model, were calculated. Temkin constants, where: *Q<sup>t</sup>* is adsorption capacity at equilibrium (mg/g), *C<sup>t</sup>* is equilibrium concentration of the Hg2<sup>+</sup> solution (µg/mL), *t* (min) is contact time, *X<sup>m</sup>* (mg/g) is maximum monolayer adsorption capacity and *b* (L/mg) is the adsorption equilibrium constant. Relative adsorption capacities and sorption intensities *n* and *K<sup>f</sup>* (mg/g), and the constants of Freundlich model, were calculated. Temkin constants, *B<sup>T</sup>* (kJ/mol) and *K<sup>T</sup>* (L/mg) whose are constants of heat of sorption and maximum binding energy were estimated. A 30 mL of different mercury concentrations ranging from 10 to 200 µg/mLwere tested under the optimum conditions and the adsorption was expressed by three equilibrium models: Langmuir, Freundlich, and Temkin to illustrate the adsorption capacity and adsorption behavior. The theory of Langmuir assumes that the adsorption occurs by monolayer on the surface of the adsorbent with the same adsorption sites (homogeneous surface), while Freundlich is an empirical theory at which the adsorption occurs by multilayer on the surface of the adsorbent with different adsorption sites (heterogeneous surface). Temkin assumed that there are indirect interactions between adsorbate molecules and the heat of adsorption of all molecules decrease linearly with increasing

composites.

Langmuir

Freundlich

surface coverage [44]. The results are summarized in Table 3 and confirm the reasonable adsorption capacity of the used nano-composite material and follows Langmuir isotherm model. Temkin bT 0.371 0.162 K J/mol R2 0.980 0.997

**Table 3.** Isotherm constants for the adsorption of mercury onto CuFe2O4 and CuFe2O4/PANI nano-

**Model Parameters CuFe2O4 CuFe2O4/PANI Unit** 

0.998 0.999

0.997 0.995

Xm 12.5 157.1 mg/g B 0.561 0.153 L/mg

N 1.06 1.34 mg/g Kf 2.75 5.24 mg/g

KT 0.34 0.744 L/mg

*Molecules* **2020**, *25*, x 8 of 15

*BT* (kJ/mol) and *KT* (L/mg) whose are constants of heat of sorption and maximum binding energy were estimated. A 30 mL of different mercury concentrations ranging from 10 to 200 µg/mLwere tested under the optimum conditions and the adsorption was expressed by three equilibrium models: Langmuir, Freundlich, and Temkin to illustrate the adsorption capacity and adsorption behavior. The theory of Langmuir assumes that the adsorption occurs by monolayer on the surface of the adsorbent with the same adsorption sites (homogeneous surface), while Freundlich is an empirical theory at which the adsorption occurs by multilayer on the surface of the adsorbent with different adsorption sites (heterogeneous surface). Temkin assumed that there are indirect interactions between adsorbate molecules and the heat of adsorption of all molecules decrease linearly with increasing surface coverage [44]. The results are summarized in Table 3 and confirm the reasonable adsorption capacity

of the used nano-composite material and follows Langmuir isotherm model.

R2

R2

**Figure 8.** (**A**) Langmuir, (**B**) Freundlich, and (**C**) Temkin isotherms for mercury removal using CuFe2O4 nano-particles (NPs). **Figure 8.** (**A**) Langmuir, (**B**) Freundlich, and (**C**) Temkin isotherms for mercury removal using CuFe2O<sup>4</sup> nano-particles (NPs). *Molecules* **2020**, *25*, x 9 of 15

**Figure 9.** (**A**) Langmuir, (**B**) Freundlich, and (**C**) Temkin isotherms for mercury removal using CuFe2O4/PANi nano-composite. **Figure 9.** (**A**) Langmuir, (**B**) Freundlich, and (**C**) Temkin isotherms for mercury removal using CuFe2O<sup>4</sup> /PANi nano-composite.

CuFe2O4/PANI nano-composite. The removal percentage of the studied ions using CuFe2O4NPs and CuFe2O4/PANI nano-composite is shown in Figure 10. From the mentioned results, CuFe2O4/PANI nano-composite revealed an enhanced removal power towards inorganic mercury than CuFe2O4NPs

**Figure 10.** Removal of different metal ions with CuFe2O4 and CuFe2O4/PANI nano-composite.

**Fe2+ Hg2+ Hg<sup>+</sup> Cr6+ Ag<sup>+</sup>**

**0**

**20**

**R em oval %**

**40**

**60**

**80**

**100**

> Fe2+~ Cr6+ > Cu2+ > Pb2+. No remarkable adsorption for Ag<sup>+</sup>

**Pb2+ Cu2+**

**CuFe2O<sup>4</sup> CuFe2O<sup>4</sup> /PAN**

, Hg2+, Fe2+, Cu2+, Cr6+, Pb2+, and Ag<sup>+</sup>

~ Cr6+ > Ag<sup>+</sup>

was

>> Pb2+. No

ions using

*2.4. Competitive Adsorption of Different Heavy Metals* 

affinity order was: Hg2+ > Hg<sup>+</sup>

only.

*2.5. Regeneration* 

The adsorption of some metal ions such as Hg<sup>+</sup>

**1 /Q t**

**0 .1 2 A**


**Table 3.** Isotherm constants for the adsorption of mercury onto CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites. **3 0**

**1 .0**

**1 .2**

**lo g Q t**

**1 .4**

**1 .6 B**

**0 .5 1 .0 1 .5**

**lo g C <sup>t</sup>**

*Molecules* **2020**, *25*, x 9 of 15

#### *2.4. Competitive Adsorption of Di*ff*erent Heavy Metals 2.4. Competitive Adsorption of Different Heavy Metals*

**1 /c <sup>t</sup>**

**4 0 C**

The adsorption of some metal ions such as Hg+, Hg2+, Fe2+, Cu2+, Cr6+, Pb2+, and Ag<sup>+</sup> was investigated. CuFe2O4NPs revealed an affinity order: Fe2<sup>+</sup> > Hg2<sup>+</sup> > Hg<sup>+</sup> ~ Cr6<sup>+</sup> > Ag<sup>+</sup> >> Pb2+. No remarkable adsorption for Cu2<sup>+</sup> ions using CuFe2O4NPs. For CuFe2O4/PANI nano-composite, the affinity order was: Hg2<sup>+</sup> > Hg<sup>+</sup> > Fe2+~ Cr6<sup>+</sup> > Cu2<sup>+</sup> > Pb2+. No remarkable adsorption for Ag<sup>+</sup> ions using CuFe2O4/PANI nano-composite. The removal percentage of the studied ions using CuFe2O4NPs and CuFe2O4/PANI nano-composite is shown in Figure 10. From the mentioned results, CuFe2O4/PANI nano-composite revealed an enhanced removal power towards inorganic mercury than CuFe2O4NPs only. The adsorption of some metal ions such as Hg<sup>+</sup> , Hg2+, Fe2+, Cu2+, Cr6+, Pb2+, and Ag<sup>+</sup> was investigated. CuFe2O4NPs revealed an affinity order: Fe2+ > Hg2+ > Hg<sup>+</sup> ~ Cr6+ > Ag<sup>+</sup> >> Pb2+. No remarkable adsorption for Cu2+ ions using CuFe2O4NPs. For CuFe2O4/PANI nano-composite, the affinity order was: Hg2+ > Hg<sup>+</sup> > Fe2+~ Cr6+ > Cu2+ > Pb2+. No remarkable adsorption for Ag<sup>+</sup> ions using CuFe2O4/PANI nano-composite. The removal percentage of the studied ions using CuFe2O4NPs and CuFe2O4/PANI nano-composite is shown in Figure 10. From the mentioned results, CuFe2O4/PANI nano-composite revealed an enhanced removal power towards inorganic mercury than CuFe2O4NPs only.

**Figure 10.** Removal of different metal ions with CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composite.

#### **Figure 10.** Removal of different metal ions with CuFe2O4 and CuFe2O4/PANI nano-composite. *2.5. Regeneration*

*2.5. Regeneration*  The adsorbent material was regenerated after each adsorption cycle of mercury by washing with 0.1 M acetic acid. After five cycles of regeneration, the efficiency of CuFe2O<sup>4</sup> NPs for the removal of Hg<sup>+</sup> ions remains 82.0% however, there was a decrease in the removal efficiency of the CuFe2O4/PANI sorbent reached to 85.3% as shown in Figure 11.

*Molecules* **2020**, *25*, x 10 of 15

The adsorbent material was regenerated after each adsorption cycle of mercury by washing with 0.1 M acetic acid. After five cycles of regeneration, the efficiency of CuFe2O4 NPs for the removal of Hg+ ions remains 82.0% however, there was a decrease in the removal efficiency of the CuFe2O4/PANI

**Figure 11.** Mercury removal by CuFe2O4 and CuFe2O4/PANI nano-composites after regeneration**. Figure 11.** Mercury removal by CuFe2O<sup>4</sup> and CuFe2O<sup>4</sup> /PANI nano-composites after regeneration.

#### *2.6. Comparison with Other Sorbents for Mercury Removal 2.6. Comparison with Other Sorbents for Mercury Removal*

Water pollution becomes a critical issue around the world, and heavy metals contribute to major pollution in water. The application of nano-materials for the removal of mercuric ions from water has attracted significant attention. Table 4 summarizes some of reported sorbents used for mercury removal included the present work. The prepared copper ferrite loaded by polyaniline provides higher maximum adsorption capacity [12,17,18,20,26], short contact time [12,20,23,27], andhigher removal percentage [12,18–21,24–27]. Water pollution becomes a critical issue around the world, and heavy metals contribute to major pollution in water. The application of nano-materials for the removal of mercuric ions from water has attracted significant attention. Table 4 summarizes some of reported sorbents used for mercury removal included the present work. The prepared copper ferrite loaded by polyaniline provides higher maximum adsorption capacity [12,17,18,20,26], short contact time [12,20,23,27], andhigher removal percentage [12,18–21,24–27].



#### *2.7. Mechanism of Adsorption*

Polyaniline Nanotubes 0.8239 60 min 90 [26] Iron oxide nanoparticles NR 24 h 87 [27] CuFe2O<sup>4</sup> CuFe2O4/PAN 12.5 157.1 120 min 60 min 82 99.5 This work *2.7. Mechanism of Adsorption*  The adsorption mechanism of Hg2<sup>+</sup> ions using CuFe2O<sup>4</sup> and CuFe2O4/PANI composite is shown in Figure 12. The adsorption mechanism can be explained in two ways. Physical adsorption can be occurred on the surface of PANI layer or in the porosity of the adsorbent or chemical adsorption through the interaction between the PANI base layers with mercuric ions. In addition, at the working pH value, oxygen-containing groups (e.g., –OH) in CuFe2O<sup>4</sup> can be ionized to –O−, forming negative charges on the CuFe2O<sup>4</sup> surface and enhance the favorable adsorption of Hg2<sup>+</sup> ions.

charges on the CuFe2O4 surface and enhance the favorable adsorption of Hg2+ ions.

*Molecules* **2020**, *25*, x 11 of 15

The adsorption mechanism of Hg2+ ions using CuFe2O4 and CuFe2O4/PANI composite is shown in Figure 12. The adsorption mechanism can be explained in two ways. Physical adsorption can be occurred on the surface of PANI layer or in the porosity of the adsorbent or chemical adsorption through the interaction between the PANI base layers with mercuric ions. In addition, at the working

, forming negative

**Figure 12.** Schematic of Hg2+ adsorption mechanism. **Figure 12.** Schematic of Hg2<sup>+</sup> adsorption mechanism.

#### **3. Materials and Methods 3. Materials and Methods**

#### *3.1. Materials 3.1. Materials*

For the experimental purpose, all chemicals used were of analytical reagent grade, 98–99%. Metal nitrates, chloride, and sulfate were of the highest purity and supplied by Sigma-Aldrich (St. Louis, MO, USA). Polyethylene glycol 6000 (PEG), potassium hydroxide, and ammonium peroxydisulfate were purchased from Fluka (Ronkonoma, NY, USA). Aniline was purchased from Central Drug House Ltd. (New Delhi, India) and distilled prior to use. All the chemicals were used as received without any further purification. For the experimental purpose, all chemicals used were of analytical reagent grade, 98–99%. Metal nitrates, chloride, and sulfate were of the highest purity and supplied by Sigma-Aldrich (St. Louis, MO, USA). Polyethylene glycol 6000 (PEG), potassium hydroxide, and ammonium peroxydisulfate were purchased from Fluka (Ronkonoma, NY, USA). Aniline was purchased from Central Drug House Ltd. (New Delhi, India) and distilled prior to use. All the chemicals were used as received without any further purification.

#### *3.2. Apparatus 3.2. Apparatus*

High-resolution transmission electron microscopy (HRTEM) images were taken by JEOL-JEM-2100 electron microscope instrument (Osaka, Japan). The prepared adsorbents were characterized by X-ray diffraction (XRD) which were carried out by BRUKER D2 PHASER 2nd generation X-ray diffractometer (Berline, Germany) using CuK*α*,*β* radiation (λ = 0.154 nm) in the angular region of 2*θ* = 4–80°. Operation conditions were 40KV, 40 mA and scanning speed of 8°/min. The Brunauer– Emmett–Teller (BET) surface area measurements were carried out by N2 adsorption–desorption at 77 K using Nova 3200 s (Florida, FL, USA) unite instrument, in the relative pressure (*p/p<sup>o</sup> )* at 0.25104. Fourier Transform Infrared (ATR-FTIR) was used to obtain the spectra in a spectral range of 4000– 500 cm−1. Inductively coupled argon plasma (ICAP 6500 Duo, Thermo Scientific, Abingdon, UK) as High-resolution transmission electron microscopy (HRTEM) images were taken by JEOL-JEM-2100 electron microscope instrument (Osaka, Japan). The prepared adsorbents were characterized by X-ray diffraction (XRD) which were carried out by BRUKER D2 PHASER 2nd generation X-ray diffractometer (Berline, Germany) using CuKα,β radiation (λ = 0.154 nm) in the angular region of 2θ = 4–80◦ . Operation conditions were 40KV, 40 mA and scanning speed of 8◦ /min. The Brunauer–Emmett–Teller (BET) surface area measurements were carried out by N<sup>2</sup> adsorption–desorption at 77 K using Nova 3200 s (Florida, FL, USA) unite instrument, in the relative pressure (*p*/*p o )* at 0.25104. Fourier Transform Infrared (ATR-FTIR) was used to obtain the spectra in a spectral range of 4000–500 cm−<sup>1</sup> . Inductively coupled argon plasma (ICAP 6500 Duo, Thermo Scientific, Abingdon, UK) as used for mercury ion evaluation.

#### used for mercury ion evaluation. *3.3. Preparation of CuFe2O4Nano-Particles*

*3.3. Preparation of CuFe2O4Nano-Particles*  The synthesis of the nano-particles was done by using the co-precipitation technique [45]. Briefly 11.7 mmol CuSO4 and 14.98 mmol FeCl3 were dissolved in 200 mL 1 wt.% PEG solution. The solution was kept under stirring for about one hour to insure the equilibrium between all the components. To the above mixture, 4M KOH was added drop-wise with vigorous stirring until reaching a pH 9. The mixture was kept under magnetic stirring for another two hours then aged overnight. The precipitate was filtered, washed with distilled water until it was free from Cl<sup>−</sup> and SO<sup>4</sup> 2− ions and dried at 70 °C The synthesis of the nano-particles was done by using the co-precipitation technique [45]. Briefly 11.7 mmol CuSO<sup>4</sup> and 14.98 mmol FeCl<sup>3</sup> were dissolved in 200 mL 1 wt.% PEG solution. The solution was kept under stirring for about one hour to insure the equilibrium between all the components. To the above mixture, 4M KOH was added drop-wise with vigorous stirring until reaching a pH 9. The mixture was kept under magnetic stirring for another two hours then aged overnight. The precipitate was filtered, washed with distilled water until it was free from Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> ions and dried at 70 ◦C for two h. The precipitated was then calcined at 600 ◦C in air for 3 h and then ground using agate motor to obtain a fine powder.

#### motor to obtain a fine powder. *3.4. Preparation of CuFe2O4*/*PANI Nano-Composite*

The polyaniline copper ferrite nano-composite was prepared using chemical polymerization method by dispersing 2 g of the previously prepared CuFe2O<sup>4</sup> nano-particles in 200 mL of 2M HCl and stirred vigorously at room temperature for 10 min. A 4.5 mL aliquot of distilled aniline monomer was added under continuous stirring for 30 min. To the above suspension, 20 mL of 19.7 mmol (NH4)2S2O<sup>8</sup> solution was added drop-wisely, as a polymerization initiator. An immediate color change of the

for two h. The precipitated was then calcined at 600 °C in air for 3 h and then ground using agate

solution to blue green was observed. The suspension was stirred to complete the polymerization process for about 1 h. The copper ferrite doped PANI was separated on a filter paper, rinsed with distilled water, and finally dried at 100◦C in an electrical oven. The produced powder has a green color which represents emeraldine salt of polyaniline.

#### *3.5. Removal of Mercury from Waste Water*

A range of mercury (II) ion (10–120 µg/mL) was prepared. For the adsorption studies different amounts of either CuFe2O<sup>4</sup> or CuFe2O4/PANI nano-composite (ranged from 0.05 to 0.3 g) were added to 30 mL of the prepared solution at room temperature and pH 7. These solutions were stirred for a contact time varied from 15 min to 2 h. After adsorption, the solutions were filtered and the adsorbent material was separated. The concentration of Hg2<sup>+</sup> ion was evaluated before and after the removal of mercury by inductively coupled argon plasma.

The removal percentage of mercury was calculated using the following equation:

$$\langle Bernoulli \rangle\_0 = ((\mathbf{C}\_0 - \mathbf{C}\_l)) \langle \mathbf{C}\_0 \times 100 \rangle \tag{5}$$

where, *C*<sup>0</sup> and *C<sup>t</sup>* are the mercury concentration in µg/mL at initial and after time *t*, respectively.

#### **4. Conclusions**

CuFe2O4/PANI nano-composite was successfully prepared and its adsorption properties towards Hg2<sup>+</sup> ions removal were checked. An X-ray diffractometer, TEM, and BET were used to characterize the prepared nano-composites. The crystallite size of the synthesized CuFe2O<sup>4</sup> and CuFe2O4/PANI nano-composite was 10.2 and 23.4 nm, respectively. Under the optimum conditions, CuFe2O4/PANI offer higher removal efficiency than CuFe2O<sup>4</sup> for Hg+/ Hg+<sup>2</sup> ions which were 95.3 and 99.5%, respectively. Both adsorbents followed the second order model and Langmuir model with adsorption capacity of 12.5 and 157.1 mg/g for CuFe2O<sup>4</sup> and CuFe2O4/PANI composite, respectively. After five cycles of regeneration, the efficiency of CuFe2O4NPs for the removal of Hg<sup>+</sup> ions remains 82.0% however, there was a decrease in the removal efficiency of the CuFe2O4/PANI sorbent reached to 85.3%with lower efficiency and good performance when used again after five cycles. These materials were successfully applied for the removal of Hg2<sup>+</sup> ions with a high efficiency over other studied heavy metals.

**Supplementary Materials:** The following are available online, Figure S1: FTIR spectra of (A) CuFe2O<sup>4</sup> and (B) CuFe2O<sup>4</sup> /PANI nanocomposites; Figure S2: Thermal-gravimetric analysis (TGA) of CuFe2O<sup>4</sup> /PANI.

**Author Contributions:** The listed authors contributed to this work as described in the following: H.A.E.-N., A.A.H., S.S.M.H., and A.H.K. gave the concepts of the work, interpretation of the results, the experimental part and prepared the manuscript; A.H.K., S.S.M.H., and A.E.-G.E.A. cooperated in the preparation of the manuscript and A.H.K. and S.S.M.H. performed the revision before submission. A.E.-G.E.A., M.A.A.-O., A.Y.A.S. revealed the financial support for the work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by King Saud University, Project No. RSP-2020/66.

**Acknowledgments:** Authors are grateful to King Saud University for funding the work through Researchers Supporting Project (Project No. RSP-2020/66).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Sample Availability:** Samples of the compounds are available from the authors.

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*Article*
