Microwave-assisted preparation of manganese dioxide modified activated carbon for adsorption of lead ions

Industrial wastewater contains many kinds of heavy metals, such as Cr, Pb, Cu, Cd, and so on, which are considered to be persistent, bioaccumulative and harmful substances (Manes & Hofer 1982; Chansuvarn & Jainae 2017; Mallakpour & Motirasoul 2018). As far as the current development is concerned, although various wastewater treatment technologies can be applied, some of the wastewater will be directly or indirectly discharged into the environment. Besides, the presence of heavy metals poses serious threat to ecological environment security due to their higher concentration and strong mobility in the environment (Mishra et al. 2004; Peng et al. 2015). Langmuir (1917) mentioned that fractions of heavy metals in the environment include exchangeable, weak acid-soluble fractions, reducible fractions, oxidizable fractions and residual fractions. In addition, exchangeable fractions and weak acid soluble fractions were regarded as bioavailable metal fractions; that is, unstable fractions, which can be absorbed by microbes and plants, and finally concentrated in the human body through the food chain. They combine with many substances in the environment to form complexes, which can migrate and transform in the water body and greatly increase solubility, thereby causing secondary pollution (Pera-Titus et al. 2004; Pourjavid et al. 2014). What is noteworthy is that reducing the heavy metals concentration and converting from unstable fractions to stable fractions may do well in weakening their risk (Mishra et al. 2004).

Now, various remediation technologies have been developed for metals removal from wastewaters (Tangahu et al. 2011; Khushk et al. 2020). Common methods for handling heavy metal ions include a biological method, chemical deposition, redox method, osmotic filtration method and adsorption method (Manes & Hofer 1982; Chansuvarn & Jainae 2017). Adsorption is the most economical, widely available and effective method for which activated carbon is the adsorbent of choice. Therefore, it is of great significance to develop an efficient adsorbent for wastewater treatment.

Multiple researches have shown that manganese dioxide has better adsorption performance, and its unique octahedral structure can absorb heavy metal ions. Hydrated manganese dioxide is a kind of manganese oxide commonly used in water treatment; nevertheless, the particles have the disadvantage of easy condensation, and the average particle size is 80 nm (Tangahu et al. 2011; Wan et al. 2016; Mallakpour & Motirasoul 2018). The easy agglomeration of manganese dioxide will cause its adsorption effect to be greatly reduced. Although the adsorption effect of activated carbon on lead ions is improved, the activated carbon is modified with nitric acid and doped with manganese dioxide. The loading of manganese dioxide on activated carbon can also avoid the defects of condensation (Mishra et al. 2004; Peng et al. 2015; Sellaoui et al. 2019).

Manganese dioxide has a relatively high specific surface area and a strong affinity for heavy metal ions plays an important role in the adsorption process (Langmuir 1917; Pera-Titus et al. 2004). Compared with other adsorbents, manganese dioxide has a certain selectivity to heavy metals (Pourjavid et al. 2014). The new ecological hydrated manganese dioxide studied by S. P. Mishra and colleagues has a minimum particle size of about 10 nm and the largest particle size was 200 nm (Langmuir 1917). This is solved by supporting the MnO₂ on activated carbon, as well as enhancing the adsorption capacity of activated carbon to heavy metal ions (Wang et al. 2015, 2017).

In this study, a method for preparing new adsorbent (MnO₂-AC) was proposed. The purpose of this study is to manufacture an adsorbent based on activated carbon with high efficiency to adsorb heavy metal ions. Consequently, the objectives of this work are to: (1) propose a method that through nitric acid modification and microwave heating, can load manganese dioxide on activated carbon. (2) The effects of MnO₂-AC on lead ion adsorption were studied. (3) To explore the adsorption mechanism of MnO₂-AC for lead ions.

All chemicals in this study are an analytical grade (AR) or better, purchased from Aladdin Industrial Corporation, including activated carbon KMnO₄, MnO₄·4H₂O, HNO₃, etc.

In order to study the properties of activated carbon before and after modification and the best modification conditions, the following experiments were carried out. Firstly, 1.2 g activated carbon was soaked with different concentrations of nitric acid 200 mL (2 mol/L, 4 mol/L, 6 mol/L, 8 mol/L,10 mol/L). After that, the soaked materials were washed and filtered. Secondly, the material was put into a boxed microwave oven and heated to 400, 500, 600, 700, and 800 °C with a power of 960 W and kept warm for 30, 45, and 60 min. Finally, after washing and drying, lead ions were adsorbed and compared with unmodified activated carbon to select the optimal modification and heating conditions.

MnO₂-AC was synthesized by the coprecipitation method through mixing the KMnO₄ and MnSO₄·4H₂O. The procedures of preparation processing are mainly as follows. Firstly, activated carbon was modified by adding 100 mL nitric acid solution with a concentration of 2 mol/L. After washing and filtering, it was heated to 700 °C with the power of 960 W in the microwave oven and kept warm for 45 min. Secondly, 0.02 mol KMnO₄ was made up in a 200 mL solution and mixed with 12 g activated carbon. Activated carbon was allowed to fully absorb potassium permanganate for about 30 min until the solution faded. The MnSO₄·4H₂O solution prepared at 0.02 mol was add to a 50 mL burette to control the flow rate. The MnSO₄·4H₂O solution was slowly dropped from the burette into a beaker containing activated carbon, which fully adsorbed the KMnO₄ and was fully stirred using a magnetic stirring apparatus. Finally, the material was repeatedly washed with ultrapure water 3 times and put into an electrothermal blow dry oven at 80 degree Celsius for 10 h. The dried material will be used for batch sorption experiments and characterization detection. The preparation flow chart is shown in Figure 1.

Batch sorption experiments were carried out using intermittent testing with powdered MnO₂-AC in a 200 mL conical flask. Typically, the given quality of sorbent (0.02 g) was dispersed into 100 ml Pb(II) solution with a concentration of 100, 150, 200, 250, and 300 mg/L respectively. Adjust the solution pH with HNO₃ and KOH. A thermostatic orbit incubator shaker equipped with gas bath was used to vibrate the flasks at 180 rpm and 298 K taking samples every 20 min for 3 h. After that, samples were taken every 1 h for 6 h. The data was used to investigate the adsorption kinetics. Remaining material was vibrated for 15 h. Preliminary kinetic tests showed that 24 h was sufficient to reach adsorption equilibrium.

Lead ions concentration in the aqueous phase was analyzed using a TAS-990 flame atom adsorption spectrophotometer (Persee Inc., China), while an atom fluorescence spectrophotometer (AFS-230E) (Haiguang Co., Ltd) was used in other cases. Lead ion solution was prepared by ultrapure water and Pb(NO₃)₂ with HNO₃ to be the basic solution.

The surface morphology of the carrier AC and MnO₂ was observed by using a scanning electron microscope (SEM). The crystallinity of the loaded MnO₂ particles was characterized by X-ray diffraction (XRD). The chemical structures of MnO₂-AC and AC were analyzed by Fourier transform infrared spectroscopy (FTIR) over the range of 400–4,000 cm⁻¹. The pore structure properties were determined by an N₂ adsorption-desorption apparatus at 77 K. The specific surface area (S_BET) was estimated from N₂ adsorption-desorption data using the Brunauer-Emmett-Teller (BET) equation. The pore distributions were analyzed by Density-Functional-Theory (DFT) method. Energy spectrum analysis was conducted by X-ray photoelectron spectroscopy (XPS) and the original binding energy was adjusted according to the C1s peak. A graphics curve fitting program was used to fit the XPS results.

Different mass ratios of manganese dioxide and activated carbon have different adsorption properties. The molar ratio of carbon to manganese dioxide was explored, and the ratio is as follows: C: MnO₂ = 0.2:0.1; C: MnO₂ = 0.5:0.1; C: MnO₂ = 1:0.1; C: MnO₂ = 1.5:0.1; C: MnO₂ = 2:0.1. It can be observed in Figure 2 that the adsorption capacity is largest when AC: MnO₂ is 1:0.1. The mass ratio is 12:8.694. The optimal ratio of adsorbed lead ions is 664 mg/g.

When the proportion of MnO₂ increases, there is too much MnO₂ per unit area on the activated carbon, which results in blockage of the activated carbon pores, and manganese dioxide coagulates into agglomerates, thereby reducing the utilization efficiency of the manganese dioxide and activated carbon. When the ratio of MnO₂ decreases, the binding capacity of manganese dioxide to Pb decreases, resulting in a decrease in the maximum adsorption capacity. Therefore, the ratio 1:0.1 was selected for subsequent research.

There are two purposes for using nitric acid to modify activated carbon: (1) to remove impurities from the surface and pore diameter of activated carbon. (2) Nitrates in nitric acid can provide more adsorption sites for subsequent loads. The best modification conditions are: nitric acid concentration of 8 mol/L, microwave heating temperature of 700 °C. The specific surface area was 940 m²/g, which was 263% higher than the original activated carbon. Using activated carbon after nitric acid activation as the carrier to load manganese dioxide and synthesize MnO₂-AC material can improve the load of manganese dioxide. Activated carbon before activation is shown in Figure 3(a). There are many impurities on the surface of activated carbon, the pores are blocked by impurities, and the specific surface area of activated carbon is small. The activated carbon before activation is shown in Figure 3(b). The impurities on the surface of activated carbon are removed and the pore diameter is increased. Meanwhile, the pore diameter of activated carbon is slightly larger than before due to heating, which is conducive to the increase of specific surface area of activated carbon and the load of manganese dioxide.

Figure 4(a) and 4(b) are the modified activated carbon. It can be observed that the surface of activated carbon is clean, free of impurities and rich in pore diameter. In the preparation process, potassium permanganate is fully absorbed by the activated carbon at first. After the solution fades, drop by drop manganese sulfate solution is added and manganese dioxide is precipitated in the pore diameter of the activated carbon to prevent the flocculation of manganese dioxide (Xu et al. 2013). Meanwhile, manganese dioxide is loaded on the activated carbon. The SEM image is shown in Figure 4(c); it is obvious that MnO₂ is generated in the aqueous solution and adsorbed in the pore structure of activated carbon (You et al. 2017). At the same time, it can be observed that the pore distribution of activated carbon is uniform and the structure is dense. As shown in Figure 4(d), manganese dioxide particles are evenly distributed on the activated carbon and their size is much smaller than the pore size of activated carbon. Therefore, loading manganese dioxide on activated carbon can effectively avoid the flocculation of manganese dioxide (Zhang et al. 2014).

In this study, the S_BET of MnO₂-AC was calculated by N₂ adsorption-desorption isotherms (Figure 5), and its surface structure parameters are shown in Table 1. As can be seen from the table, the S_BET of activated carbon modified by nitric acid has been greatly improved compared with that of commercially activated carbon. As the pore structure of MnO₂-AC is blocked by nano-manganese dioxide particles, the S_BET of MnO₂-AC is reduced compared with activated carbon modified by nitric acid (Zhang et al. 2017).

The average pore size of activated carbon and MnO₂-AC treated with nitric acid is 1.239 nm and 0.942 nm, respectively, and the micropore surface area is 0.246 mL/g and 0.197 mL/g, respectively. Compared with the commercially available activated carbon, the pore size is reduced and the proportion of micropores is increased, which indicates that the pore structure of activated carbon is more abundant after nitric acid modification. MnO₂-AC also has the same characteristics (Zhang et al. 2016). Because manganese dioxide is loaded on activated carbon, its average pore size is smaller (Zhou et al. 2018).

Figure 6 shows the pore size distribution of the three activated carbons. It is obvious that nitric acid-modified activated carbon and MnO₂-AC have a more microporous structure, while commercial activated carbon has almost no microporous structure.

Figure 7 depicts the maximum adsorption capacity of three kinds of activated carbon to Pb(II). The maximum adsorption capacity of the original activated carbon is 56 mg/g, the maximum adsorption capacity of nitric acid-modified activated carbon is 363 mg/g, and the maximum adsorption capacity of the MnO₂-AC is 740 mg/g. The modification by nitric acid and load of MnO₂ can significantly improve the adsorption capacity of activated carbon to Pb(II). Compared with the original activated carbon, the maximum adsorption capacity of nitric acid-modified activated carbon can be increased by 5 times, while MnO₂-AC can be increased by about 14 times. This shows that MnO₂-AC can greatly improve the adsorption capacity of activated carbon to Pb(II). The following research focuses on MnO₂-AC, studying the conditions affecting its adsorption and exploring the adsorption mechanism.

The pH value is one of the important factors affecting the adsorption process, which will affect the surface properties of the adsorbent, the chemical properties of the solution and the morphology of heavy metal ions, so it is of great importance to study the influence of pH value on the adsorption capacity. The effect of solution pH value on lead ion adsorption is shown in Figure 8.

The adsorption capacity of MnO₂-AC on lead ions increases with the increase of solution pH. It reaches its maximum value when pH = 6, and then decreases slowly, and the maximum adsorption value is 664 mg/L. The transition metal oxide surface is rich in hydroxyl groups. At a low pH (pH = 2–6), the surface of the adsorbent is surrounded by a high concentration of hydronium ions, competing with metal ions for the adsorption point. At a low pH, the surface of MnO₂ is highly protonated, making the surface positively charged, thus reducing the material's adsorption capacity and increasing electrostatic repulsion. With the increase of pH value, the adsorbent surface was negative on the whole, and the adsorption capacity of metal ions was enhanced.

Figure 9 shows the adsorption equilibrium isothermal curve of the adsorption of Pb(II) by MnO₂-AC. It can be seen from the figure that the adsorption of Pb(II) by MnO₂-AC is rapid at the initial stage, followed by a slow adsorption process. When the lead ion concentration was lower than 300 mg/L, the adsorption amount increased with the increase in concentration. When the lead ion concentration was higher than 300 mg/L, the adsorption capacity of MnO₂-AC increased slowly and gradually became stable, indicating that the adsorption of Pb(II) on its surface tended to be saturated. After calculation, the maximum adsorption capacity of MnO₂-AC for the lead ions is 667 mg/g.

Table 2 is the fitting parameters of two isothermal models Langmuir and Freundlich for the adsorption Pb(II) of MnO₂-AC. The correlation coefficient R² of Langmuir and Freundlich isothermal model and adsorption curve fitting were 0.9917 and 0.9825, respectively. The closer the correlation coefficient is to 1, the higher the fit. Therefore, the adsorption of Pb(II) by MnO₂-AC was more consistent with the Langmuir model, and its adsorption mechanism was single-layer adsorption. The maximum adsorption capacity of MnO₂-AC for Pb(II) was calculated by the Langmuir model fitting as 667 mg/g. Compared with the adsorbents adsorbed by Pb(II) in previous studies, the MnO₂-AC in this study has certain advantages and can be used as an efficient adsorbent for the removal of heavy metal ions in water.

This is consistent with the fitting results and analysis of the pseudo-second-order kinetic model: in the initial stage of rapid adsorption (k_i.1), more than 50% of Pb(II) in the solution was adsorbed at the surfactant sites of the adsorbent. Secondly, the adsorption rate (k_i.2) of the second stage decreased significantly, and the removal rate of metal ions was close to the maximum. This may be due to the adsorption process of metal ions entering the internal pore structure of the adsorbent, and the ions entering the micropores of the adsorbent. The diffusion resistance also increases with the increase of metal ions, and the adsorption gradually increases.

MnO₂-AC has a more complex pore structure than the modified activated carbon and commercial activated carbon. Due to the loading performance of manganese dioxide, MnO₂-AC has higher activity and finer distribution volume, which can help to make good contact with metal ions and reduce metal ions. Resistant to solution diffusion, these results indicate that manganese dioxide is effective for the modification of activated carbon (Chen & Yeh 2005).

X-ray diffraction spectrum analysis is used to study the crystal phase and structure of the adsorbent. The X-ray diffraction pattern before and after the modification of activated carbon by manganese dioxide and after the adsorption of Pb(II) by MnO₂-AC is shown in Figure 12. The X-ray diffraction pattern of MnO₂-AC shows multiple peaks, namely C and MnO₂, which are located in 18.10, 26.50, 37.50, 39.000, 40.20, 49.80, 60.3 and 65.1. It can be observed that after manganese dioxide is loaded on the activated carbon, all the diffraction peaks except the carbon peak all point to manganese dioxide (JCPDS No.44-0141).

After the adsorption of lead ions, there was a significant peak of lead ions (JCPDS No.23-0345). This indicates that lead ions are adsorbed on the surface of MnO₂-AC. In order to further explore the adsorption mechanism, infrared analysis is carried out on lead ions (Kim et al. 2013; Yuan et al. 2014).

The infrared spectra before and after the adsorption of Pb(II) by MnO₂-AC are shown in Figure 13. In the case of MnO₂-AC, the spectrum showed absorption bands of 1,386.36, 1,648.69 and 3,431.71 cm⁻¹. The peak at 1,386.36 cm⁻¹ can be attributed to the vibration of the combination of hydroxyl group and manganese atoms; the peak value at 1,648.69 cm⁻¹ was attributed to the bending vibration of the C-C bond; the widened band at 3,431.71 cm⁻¹ can be attributed to the bending vibration of the adsorbed molecular water and the stretching vibration of the hydroxyl group. After adsorption of Pb(II) by MnO₂-AC, the hydroxyl bands containing Pb molecules were significantly shifted from 3,431.71 and 1,386.36 cm⁻¹ to 3,449.76 and 1,403.78 cm⁻¹, as shown in the figure. The observation results show that the interaction between metal ions and hydroxyl groups on the surface of the adsorbent can achieve the effect of adsorbing metal ions. At the same time, the significant movement of the hydroxyl band may be attributed to the formation of the spherical complex between heavy metal ions and manganese dioxide, forming the Mn-O-Pb bond.

XPS was used to analyze the chemical composition and structural changes of MnO₂-AC before and after adsorption of lead ions. The XPS spectra of lead ion adsorption by MnO₂-AC are shown in Figure 14(a), with 284.8 eV, 532.4 eV, 642.3 eV and 654.1 eV, 143.3 eV and 137.8 eV. The corresponding elements at the crest are C1s, O1s, Mn2p_3/2 and Mn2p_1/2, Pb_5/2 and Pb_7/2, respectively. It is noted that after the adsorption of Pb(II) by MnO₂-AC, the binding energy of Mn increases with adsorption (Kushwaha et al. 2012). As shown in Figure 14(b), after adsorption of Pb(II), the binding energy of MnO₂-AC at Mn increased from 641.98 and 653.48 eV to 642.3 and 654.1 eV, respectively. This may be the result of the Mn-O-Pb bond formed on the surface of the adsorbent, resulting in a decrease in the density of the electron cloud outside the nucleus around Mn atoms, thus increasing the binding energy of Mn2p. As shown in Figure 14(c), it was also observed that the binding energy of Pb4f in Pb(NO₃)₂ was 138.82 and 143.77 eV, respectively. After adsorption, the binding energy decreased to 137.88 and 143.28 eV, respectively, and the transition from the high to low region was obvious, indicating that there was a specific interaction between Pb(II) and MnO₂-AC. In addition, the peak observed at 137.88 eV was consistent with the Pb-O bond at 138.0 eV, further confirming that Pb(II) and manganese dioxide in MnO₂-AC formed an intraplate complex. No clear evidence of Pb(II) being oxidized into Pb(IV) or forming other solid phases was observed from the XPS spectrum, indicating that the main mechanism of MnO₂-AC removing Pb(II) was attributed to adsorption (Fiedor et al.1998; Sofer et al. 2014).

This paper compared the adsorption capacity of Pb(II) on original activated carbon, modified activated carbon and manganese dioxide-doped activated carbon, and the results showed that the modified nitric acid activated carbon can significantly improve the adsorption capacity of Pb(II) on the activated carbon, while the modified activated carbon doped by manganese dioxide can further improve the adsorption capacity of lead ions. The optimal doping ratio of manganese dioxide on activated carbon was explored, the molar ratio was AC:MnO₂ = 1:0.1. The effect of pH on adsorption was also explored, the results showed that the maximum adsorption capacity of MnO₂-AC on Pb(II) increased with the increase of pH value, and reached the maximum adsorption capacity when pH was 6, and then decreased with the continued increase of pH value.

In order to study the adsorption principle of MnO₂-AC on Pb(II), this paper explored the adsorption kinetics of MnO₂-AC on Pb(II), and conducted XRD, XPS and FTIR tests on the samples before and after Pb(II) adsorption. The principle of adsorption can be divided into two parts. When the concentration of lead ions in solution is high, the adsorption of Pb(II) by MnO₂-AC is in the sphere, and a large number of manganese dioxide particles in the apertures of activated carbon have a negative charge. After electrostatic adsorption of Pb(II), it is fixed in the structure of activated carbon. With saturation of adsorption, the adsorption rate of Pb(II) gradually decreases. At this time, the main adsorption principle is internal diffusion of ions, and Pb(II) is mainly fixed by electrostatic adsorption.

The authors would like to express their gratitude to the Specialized Research Fund for the National Natural Science Foundation of China (21966019), Yunnan Ten Thousand Talents Plan Industrial Technology Talents Project (2019-1096), Yunnan Ten Thousand Talents Plan Young & Elite Talents Project (2018–73), Yunnan Science and Technology Planning Project (2018IB020), Yunnan Science and Technology Planning Project (2018IC085).

All relevant data are included in the paper or its Supplementary Information.