Investigation on Porous Carbon-Loaded MnO for Removing Hexavalent Chromium from Aqueous Solution

Abstract

Porous carbon-loaded MnO was prepared via a combination of the sol–gel method and the chemical blow molding method using polyvinylpyrrolidone (PVP) and manganese nitrate as starting materials. SEM, EDX, TEM, FTIR, XRD, XPS, nitrogen adsorption–desorption, and elemental analysis were used to assess its physical and chemical characteristics. Furthermore, the adsorption property of porous carbon-loaded MnO for hexavalent chromium (Cr(VI)) in polluted water was investigated in detail. The results demonstrated that large numbers of MnO nanoparticles were evenly mounted on the surfaces of carbon walls, with a uniform distribution of C, N, and O elements. The BET surface area was 46.728 m²/g, and the pore sizes of porous carbon ranged from 2 nm to 10 nm. Additionally, abundant surface functional groups were found in porous carbon-loaded MnO, a result consistent with XPS data and applicable to the adsorption of heavy metals from aqueous solutions containing Cr(VI). The Freundlich model fitted the adsorption isotherm well, and the pseudo−second−order model precisely matched the adsorption kinetics. According to the study results, the adsorption was multilayer, and the adsorption process involved an endothermic reaction. These results indicate that this is a feasible way to synthesize a high−efficiency adsorbent for the removal of harmful heavy−metal ions from wastewater.

1. Introduction

Due to their high toxicity, high mobility, and poor biodegradation, heavy metals have caused significant damage to the global ecological environment [1,2,3,4]. Among the heavy metals, chromium has caused the most serious pollution in water because of its extensive usage in the electroplating, printing, and dyeing industries. Chromium has many valence states, with trivalence and hexavalence (Cr(VI)) being most common. However, the toxicity of Cr(VI) is 100 times stronger than that of trivalent chromium, and it exhibits high solubility in water. Moreover, Cr(VI) is easily absorbed into the human body and may cause inheritable genetic defects and carcinogenicity, as well as posing a persistent risk to the environment. Therefore, how to effectively deal with Cr(VI) pollution of aqueous solutions has become a focus of research [5,6].

At present, the main methods for removing Cr(VI) from aqueous solutions include chemical reduction [7,8,9], ion exchange [10], membrane separation [11,12], and adsorption [13,14,15,16,17,18,19]. The main disadvantages of removal methods include high sludge production, in the case of chemical reduction; easy contamination of resin, in the case of ion exchange; and serious membrane contamination and high operating costs, in the case of membrane separation. Among all the methods, the adsorption method stands out for its simple operation, good selectivity, and high removal rate, making it suitable for treating wastewater with low concentrations of Cr(VI). Compared to other adsorbents, carbonbased porous materials have garnered significant attention due to their excellent anticorrosion properties, high thermal stability, large specific surface areas, and ease of surface modification. For example, Mahmoud et al. utilized graphene oxidemodified shrimp shell magnetic biochar as an effective biosorbent for Cr(VI) removal by utilizing waste materials [16]. Yi et al. also synthesized magnetic biochar enriched with lowvalent iron to eliminate Cr(VI) from wastewater, promoting easy recycling [20]. Additionally, several studies have shown that the presence of manganese in adsorption materials enhances adsorption of Cr(VI) with uniform adsorbent distribution [21,22,23]. However, most of these materials are seriously affected by agglomeration of nanoscale particles with low specific areas. Generally, the adsorption performance of adsorbents is determined by their pore structures and their surface physical and chemical properties.

Based on the above elaboration, porous carbon has a large specific surface area, and exhibits good thermal stability and excellent anti−corrosion performance, while manganese is beneficial for adsorbing Cr(VI). Therefore, in the present study, a novel adsorbent of porous carbon-loaded MnO was synthesized by a combination of the sol–gel method and the chemical blow molding method, using PVP and manganese nitrate as raw materials. The adsorbent pore structure was tailored according to the adsorption requirements. At the same time, the removal mechanism of Cr(VI) in aqueous solution was investigated in detail.

2. Materials and Methods

2.1. Chemicals

Polyvinyl Pyrrolidone (PVP, Mw = 1,300,000, Aladdin), Mn(NO₃)₂, 98% H₂SO₄, 85% H₃PO_4, and 36~38% HCl were used in the experiments. Other chemicals used in this study were of analytical−laboratory grade. The Cr(VI) stock solutions were produced by dissolution of potassium dichromate (K₂Cr₂O₇) in ultrapure water.

2.2. Preparation of Porous Carbon-Loaded MnO

The combination of the sol–gel method and the chemical blow molding method which was used to prepare porous carbon-loaded MnO can be described as follows: First, 3 g PVP and 3 g Mn(NO₃)₂ were added to 300 mL deionized water and stirred for 2 h at room temperature. Then, the solution was stirred in an oil bath at 85–90 °C until the gel state was formed. The gel was then heated at 200 °C for 1 h in Ar to form xerogel. Thereafter, it was heated to 700 °C at a rate of 3 °C/min in Argon and carbonized for 3 h. Finally, the sample was ground into powder, and stored for later use. The preparation process for porous carbon-loaded MnO is illustrated in Figure 1.

2.3. Characterization of Porous Carbon-Loaded MnO

The surface morphology of porous carbon-loaded MnO was observed using a scanning electron microscope (SEM, Zeiss Sigma 300, Oberkochen, Germany), and elemental distribution was determined by energy-dispersive X−ray spectroscopy (EDX, Zeiss Smartedx, Oberkochen, Germany). Transmission electron microscopy (TEM, JEOL JEM−2100, Tokyo, Japan) was employed to investigate the microstructure and to obtain detailed information about porous carbon-loaded MnO. The functional groups on the surface of porous carbon-loaded MnO were identified by Fourier-transform infrared spectrum (FTIR, Thermo Scientific, Nicolet Is5, Waltham, MA, USA). Specific surface area and pore size distribution were determined by the N₂ adsorption–desorption method at 77K (BET specific area analyzer, Micromeritics Instruments, ASAP 2020HD88, Norcross, GA, USA). The chemical state and the composition of the elements on the surface of porous carbon-loaded MnO were determined using an X−ray photoelectron spectrometer (XPS, Thermo Scientific, Nicolet Is5, Waltham, USA). An X−ray diffractometer (XRD, Bruker AXS D8 Advance, Karlsruhe, Germany) was employed to identify the crystalline phase of materials. Potential was measured by a zeta potentiometer (Malvern, Zetasizer Nano ZS90, Malvern, UK). Weight loss of material in the air was determined by a thermogravimetric analyzer (TA Instruments, Q500, New Castle, DE, USA).

2.4. Adsorption Experiment

First, each of the bottles was filled with 100 mL of potassium dichromate solution and 0.2 g of porous carbon-loaded MnO. Then, the pH of the solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH to a desired value. Next, the bottles were shaken for a desired time on a rotary shaker at 120 rpm. The absorbance of the sample was measured by the 1,5−diphenylcarbohydrazide spectrophotometric method using an A360 UV−visible spectrophotometer(Shanghai, China). The amount of Cr(VI) adsorption at equilibrium q_e (mg/g) was calculated using the following Equation (1):

$${\mathrm{q}}_{\mathrm{e}}=\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}/\mathrm{m}$$

(1)

where C₀ and C_e (mg/L) are the initial and equilibrium concentrations, respectively, of Cr(VI); V (L) expresses the volume of Cr(VI) solution; m (g) is the adsorbent mass; and q_e represents the adsorbed Cr(VI) amount (mg/g).

The efficiency with which Cr(VI) was removed by the porous carbon−loaded MnO material was calculated using the following Equation (2):

$$\mathrm{R}=({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})/{\mathrm{C}}_0\times 100\%$$

(2)

where R is the removal efficiency of Cr(VI); and C₀ and C_e (mg/L) are the initial and equilibrium concentrations, respectively, of Cr(VI).

3. Results and Discussion

3.1. Structure Characterization

3.1.1. Phase Structure of the Material

To determine the crystal phase structure of the material, XRD tests were conducted, and the results are shown in Figure 2a. It can be seen that the diffraction peak with 2θ at 40.5° is the strongest, corresponding to the (200) plane of MnO, and that the diffraction peaks with 2θ at 34.9°, 58.7°, 70.2°, 73.8° were caused by the (111), (220), (311), (222) planes of MnO. The diffraction peaks of the material are consistent with JCPDS No.07−0230, which suggests that porous carbon-loaded MnO was successfully prepared. In addition, porous carbon-loaded MnO was further analyzed by Raman spectroscopy. In Figure 2b, the peak at 643.7 cm⁻¹ is considered to be the symmetric Mn−O stretching of MnO, and the peaks at 1330.0 and 1590.5 cm⁻¹ correspond to the D−band and G−band of carbon materials, respectively. The I_D/I_G ratio is around 0.87. Along with the invisible weak XRD carbon peak at about 26°, the Raman wide peaks indicate a low degree of graphitization and the existence of many defects in the carbon materials. These results indicate that porous carbon-loaded MnO is amorphous carbon as a whole, and that nitrogen doping increases the height of the basic structureunits of the turbostratic structure of the carbon material, improving the adsorption performance of the material.

3.1.2. Morphology and Elemental Distribution

To reveal the surface morphology and elemental distribution of porous carbon−loaded MnO, SEM, and EDX tests were carried out. The results can be found in Figure 3. Figure 3a shows that porous carbon-loaded MnO has large numbers of outer and inner pores and a welldeveloped pore structure. Therefore, the material has more active sites, which makes its adsorption performance better. Figure 3b shows that porous carbon-loaded MnO is mainly composed of C, O, Mn, and N. The specific content is shown in

Table 1. Element content in porous carbon-loaded MnO.

Element	Weight%	Atomic%
C K	33.70	57.62
N K	2.29	3.36
O K	16.60	21.31
Mn K	47.40	17.71

. The weight percentage of Mn is 47.40%, and the weight percentage of carbon is 33.70%, while the maximum atomic percentage of carbon is 57.62%. Figure 3c is a realtime elemental surface distribution map obtained during energy spectrumanalysis. Figure 3d shows realtime surface distribution maps for each single element; these show distribution of C, N, O, and Mn within porous carbon-loaded MnO, further proving that MnO particles are uniformly embedded in the carbon matrix. The thin walls of porous carbon shorten the distance required for Cr(VI) to reach MnO, and then promote the adsorption of heavy metals.

Thermogravimetric characterization was carried out to determine the percentage of Mn and the nature of functional groups present on the surfaces of the adsorbents. The results are illustrated in Figure 4. The TG curve indicates that the material exhibits a constant weight loss, with a value of 54.03% recorded at 1000 °C. Three distinct peaks can be observed in the DTG curve. The peak at 85.7 °C is mainly due to the evaporation of free water in the material. The peak at 351.1 °C is mainly attributed to the oxidation of C and the loss of N−containing groups. Finally, the peak at 938.8 °C is because of the conversion of Mn₃O₄ to MnO. These findings are consistent with the results of the energy spectrum analysis, considering the error threshold of EDX.

3.1.3. Surface Structure and Properties

The structure of porous carbon-loaded MnO was observed using TEM, as illustrated in Figure 5. In Figure 5a, there are visible a large number of uniformly distributed nanoparticles in a thin carbon wall with a particle size of about 10 nm. HRTEM images of porous carbon-loaded MnO are displayed in Figure 5b, in which a spacing of the lattice measuring 0.223 nm corresponds to the (200) facet of MnO which is mounted in amorphous and porous carbon, in accordance with the results of XRD and Raman analyses.

Because the pore characteristic is one of the most important properties of adsorbents, the specific surface area, pore structure, and pore size distribution were tested, as shown in Figure 6a,b. Figure 6a shows the nitrogen adsorption–desorption curve. According to the work of Sing et al. [24], the isotherm was assigned to type IV. In addition, the BET surface area was calculated to be 46.728 m²/g based on the nitrogen adsorption–desorption test. Figure 6b shows the DFT pore diameter distribution. This indicates that the pore size of porous carbon-loaded MnO is 2–10 nm, and thus may be classified among the mesopores. This is beneficial to the adsorption of Cr(VI) because pores within a 2–10 nm size range allow Cr(VI) ions to easily transmit, and also provide a large specific surface area. Therefore, compared with other adsorbents such as granular KMnO₂NH₂ [25], porous carbon-loaded MnO with this special honeycomb structure has a higher removal efficiency. This is confirmed in Section 3.2 below, where an efficiency level of 97.6% at pH 2.0 is reported.

Surface physical and chemical properties are directly related to the adsorption performance of the adsorbent. Therefore, the surface functional groups of porous carbon−loaded MnO were analyzed by Fourier-transform infrared spectroscopy (FTIR), as illustrated in Figure 6c. It can be seen from Figure 6c that the surface functional groups are abundant. The absorption peak at 3410 cm⁻¹ is attributed to the stretching vibrations of NH₂ and -NH, which are produced by both manganese nitrate and PVP in the reaction process. The absorption peak at 2929 cm⁻¹ corresponds to the reverse stretching vibration of CH₂, the absorption peak at 1848 cm⁻¹ is induced by the stretching vibration of C=O, and the absorption peak at 1206 cm⁻¹ corresponds to the stretching vibration of CO. All these functional groups are generated during the carbonization and decomposition processes of PVP. The absorption peak at 1580 cm⁻¹ is attributed to the reverse stretching vibration of NO₂, which is mainly attributed to the decomposition of manganese nitrate and PVP at high temperature. Furthermore, the peaks at 592 cm⁻¹ and 470 cm⁻¹ belong to the fingerprint region.

The chemical composition of porous carbon-loaded MnO was investigated by XPS, The results, as illustrated in Figure 7a, showed the existence of Mn, N, O, and C, in correspondence with the EDX results. The highresolution spectrum of Mn 2p yielded two peaks located at 641.5 and 653.4 eV, consistent with the Mn (2P3/2) and Mn (2P1/2) in the MnO bond shown in Figure 7b. Figure 7c shows the high−resolution C1s spectrum, which can be fitted with three peaks located at 284.7, 285.4, and 287.2 eV, corresponding to C−C, CH₂−CO, and C=O. These findings are in accordance with the FTIR spectra results. Figure 7d shows a highresolution N1s spectrum which has been partitioned into four peaks, 398.3, 400.2, 400.3, and 403.1 eV, corresponding, respectively, to pyridinic, NH/NO, pyrrolic, and Noxide produced by decomposition of manganese nitrate and PVP.

3.2. Adsorption Studies

Solution pH can change the surface status and the substance type of an adsorbent [26,27]. Therefore, it was necessary to investigate the effect of pH on the adsorption property. Figure 8a shows the effect of solution pH on the adsorption property of porous carbon-loaded MnO. Based on preexperiments, considering both the removal rate and adsorption capacity of hexavalent chromium, an adsorption dosage of 2 g/L was selected. It can be seen from Figure 8a that C_e/C₀ increases with rising pH. When the pH value is 2.0, C_e/C₀ is only 2.4%, and removal efficiency reaches 97.6%; when the pH value increases to 5, C_e/C₀ increases to 55.4%, and when the pH value increases to 9, C_e/C₀ reaches 76.0%. The results show that the adsorption of Cr(VI) on porous carbon-loaded MnO is favorable under acidic conditions, while the adsorption effect on Cr (VI) is poor under neutral and alkaline conditions. In general, Cr(VI) mainly exists in aqueous solution as three anions: CrO₄²⁻, HCrO₄^2−, and Cr₂O₇²⁻. There is a balance between them when the external environment changes from an alkaline solution to an acidic solution. This balance may be expressed as follows:

$$2\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}+2{\mathrm{H}}^{+}\iff 2\mathrm{H}\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{-}\iff \mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(3)

In alkaline or neutral solution, Cr(VI) mainly exists as CrO₄²⁻. When the concentration of H⁺ in the solution is gradually increased, HCrO₄²⁻ is first generated and then transformed into Cr₂O₇²⁻. Under an acidic condition, the increase in H⁺ concentration in the solution leads to the protonation of NH₂ and NH, so that NH₃⁺ and NH₂⁺ are formed, making the surface of porous carbon-loaded MnO positively charged [28,29,30]. At the same time, Cr(VI) exists in the form of Cr₂O₇²⁻ in the solution, and the electrostatic attraction between cations and anions is enhanced; thus, the adsorption capacity of porous carbon-loaded MnO increases. In addition, with increased acidity, a redox reaction occurs between the Cr(VI) in the solution and the porous carbon-loaded MnO. Gradually, increasing numbers of groups are mounted at the defect in the material, improving its adsorption property. However, when pH increases, the hydroxyl groups on the surface of porous carbon-loaded MnO pile up significantly, so that the surface of porous carbon-loaded MnO exhibits negative charges, and like charges repel each other. This weakens the effect between Cr(VI) and porous carbon-loaded MnO, and is therefore not conducive to adsorption. We may state, then, that the efficiency with which Cr(VI) is removed by porous carbon-loaded MnO is reduced in an alkaline environment. Simultaneously with the above, zeta potential at different pH values was measured, as shown in Figure 9. It can be determined from Figure 8 that the point of zero charge of the material is at pH 5.27. Therefore, when pH is less than 5.27, the surface of the material carries a positive charge, but when pH is greater than 5.27, the surface of the material carries a negative charge. This is consistent with the results shown in Figure 8a. However, considering that the pH of local polluted water is around 6, pH 6 was selected instead of pH 2 for the subsequent study of the absorption isotherm and kinetics which we now describe, as follows:

The adsorption process is always accompanied by endothermic and exothermic reactions, and the solution temperature is also an important factor affecting the adsorption process. Figure 8b shows the effect of solution temperature on adsorption property. The results show that C_e/C₀ is 74.6% at 15 °C, but decreases to 62.3% at 45 °C. The higher the solution temperature, the smaller the value of C_e/C₀ and the better the adsorption property. This makes it clear that the endothermic reaction occurs in the adsorption process.

The nonlinear Langmuir and Freundlich models are often used for adsorption isotherm fitting [21,31]. In this study, they were used to fit the adsorption equilibrium data of Cr(VI) on the surface of porous carbon-loaded MnO. These can be represented by the following equations:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}}$$

(4)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}}$$

(5)

where q_max is the maximum adsorption capacity, mg/g; K_L is the Langmuir constant, related to the energy of adsorption; K_F is the Freundlich constant, related to the adsorption capacity; and 1/n is a heterogeneity factor.

The fitting results are shown in Figure 8c, and the fitting parameters are listed in

Table 2. Langmuir and Freundlich model parameters for adsorption.

Langmuir Model			Freundlich Model
qmax (mg/g)	KL (L/mg)	R2	KF	n	R2
0.9416	1.0277	0.8349	0.4643	3.214	0.9339

. It can be seen that the correlation coefficient of the Freundlich model is 0.9339, which is larger than that of the Langmuir model. Therefore, the Freundlich model can better fit the adsorption isotherm, suggesting that removal of Cr(VI) occurs on surfaces exhibiting a heterogeneous energy distribution with formation of multilayers [2].

To estimate the adsorption rates, the kinetic results were fitted using pseudo−first−order and pseudo−second−order models, which can be expressed using Equations (6) and (7), respectively.

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}})$$

(6)

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{e}}^2{\mathrm{k}}_2\mathrm{t}}{1+{\mathrm{q}}_{\mathrm{e}}{\mathrm{k}}_2\mathrm{t}}}$$

(7)

where q_t expresses the amount of Cr(VI) adsorbed at time t, mg/g; the parameter k₁ is the pseudo−first−order rate constant; k₂ corresponds to the pseudo−second−order rate constant; and q_e²k₂ is the initial sorption rate.

The kinetic fitting results are shown in Figure 8d, and the results of the kinetic parameters obtained from the fittings are summarized in

Table 3. Kinetic parameters obtained from fittings with pseudofirst-order and pseudo-secondorder models.

Pseudo−First−Order Model			Pseudo−Second−Order Model
qe (mg/g)	k1	R2	qe (mg/g)	k2	R2
0.4878	0.0288	0.8933	0.5838	0.0565	0.9301

. It can be seen that the correlation coefficient of the pseudo−second−order model is 0.9301, which is larger than that of the pseudo−first−order model. Therefore, the pseudo−second−order model can better describe the adsorption data. This indicates that the rate−controlling step of the process mainly involves valence forces between the adsorbent and the adsorbate [32].

The strong van der Waals force and redox reaction between the walls of porous carbon-loaded MnO and Cr(VI) ions was also attributed to the adsorption reaction. The low−valent manganese ions contained in MnO can act as electron donors to reduce Cr(VI) to Cr(III), the reduced Cr(III) can be immobilized by complexation with oxygen−containing functional groups, and precipitate such as Cr(OH)₃ was generated on the surface of porous carbon-loaded MnO. The possible adsorption mechanism is shown in Figure 10.

4. Conclusions

Porous carbon-loaded MnO containing numerous functional groups was prepared using a combination of the sol–gel method and the chemical blow molding method. It was found to have a thin amorphous carbon wall and a welldeveloped pore structure, and to be mainly composed of mesopores. The elements C, N, O, and Mn were evenly distributed, and large numbers of nanoMnO particles were embedded in the interlinked carbon walls. Porous carbon-loaded MnO demonstrated better adsorption properties for Cr(VI) in an acidic environment, with removal efficiency reaching 97.6% at pH 2.0. Through experimental and theoretical analysis, it was found that electrostatic attraction, van der Waals forces, and redox reactions were the main adsorption pathways. Based on these results, we may state that porous carbon-loaded MnO exhibits favorable physical and chemical properties as an adsorbent of Cr(VI). It was expected that this study would provide a theoretical basis for the preparation of carbon materials with better adsorption properties by chemical blow molding. Porous carbon-loaded MnO prepared by chemical blow molding should have extensive application value in the removal of heavy metals from mine tailings’ wastewater and metallurgical wastewater, as well as wastewater from chemical, leather, and other industries.