Study on adsorption of hexavalent chromium by composite material prepared from iron-based solid wastes

Material characterization results

To investigate the structural composition of NMC-2, XRD analysis plots were performed. Figure 1a shows the XRD pattern of the NMC-2 composite before adsorption. The XRD pattern shows the corresponding strong and narrow peaks, from which it can be seen that the peaks of broad diffraction NMC-2 can correspond to the standard cards of Fe, C, Fe₇C₃, Fe₂C, and FeC, indicating that the synthesized adsorbent is an iron-carbon composite. It can be indicated that mesoporous nitrogen-doped composites were formed during the carbonization process. During the experiments, it was found that the materials are magnetic, probably because of the presence of Fe, FeC, Fe₇C₃, Fe₂C. Due to the magnetic properties of this type of material, rapid separation and recovery can be obtained under the conditions of an applied magnetic field, which allows easy separation of the adsorbent and metal ions from the wastewater¹⁵.

From the adsorption–desorption curves of adsorbent N₂ in Fig. 1b, it can be seen that the NMC-2 isotherm belongs to the class IV curve, and the appearance of H3-type hysteresis loops is observed at the medium pressure end, and H3 is commonly found in aggregates with laminar structure, producing slit mesoporous or macroporous materials, which indicates that N₂ condenses and accumulates in the pore channels, and these phenomena prove that NMC-2 is a porous material¹⁶. Figure 1c shows the pore size distribution of the adsorbent NMC-2 obtained according to the BJH calculation method, from which it can be seen that the pore size distribution is not uniform in the range, and most of them are concentrated below 20 nm, while according to Table 1, the specific surface area of the original sample of Fenton sludge and fly ash is 124.08 m²/g and 3.79 m²/g, respectively, and the specific surface area of NMC-2 is 228.65 m²/g. The Fenton The pore volume of the original samples of Fenton sludge and fly ash were 0.18 cm³/g and 0.006 cm³/g respectively, while the pore volume of NMC-2 was 0.24 cm³/g. The pore diameters of the original sample of Fenton sludge and fly ash were 5.72 nm and 6.70 nm respectively, while the pore diameter of NMC-2 was 4.22 nm. The above data indicated that the synthetic materials have increased the specific surface area and pore volume compared with the original samples, indicating that the doping of nitrogen can increase the specific surface area of the material. Since the pore size of mesoporous materials is 2–50 nm, NMC-2 is a porous material with main mesopores. Thanks to the large specific surface area provided by the mesopores, the material has a large number of active sites, and in addition, the mesopores can store more Cr(VI)¹⁶, which contributes to efficient removal.

The morphological analysis of the material surface using SEM can see the surface structure and the pore structure of NMC-2. And Fig. 2a–d shows the swept electron microscope image of NMC-2. Figure 2a shows that the surface of the material is not smooth, and there are more lint-like fiber structures. The fibers in Fig. 2b are loosely and irregularly arranged, which may be due to the irregular morphology caused by the small particles of the NMC-2 sample. As shown in Fig. 2c and Fig. 2a there are more pores generated on the surface of NMC-2, which may be due to the addition of K₂CO₃ to urea and, Fenton sludge solution to generate CO₂¹⁷.

These pores can provide many active sites, which is consistent with the results derived in Fig. 1, where NMC-2 is a mesoporous-dominated porous material, and also demonstrates that the addition of urea can provide a nitrogen source for the material, providing abundant active sites. Figure 2j depicts the TEM of NMC-2. the TEM images show that the synthesized NMC-2 has a folded structure with a surface covered by a carbon film, and the HRTEM (Fig. 2e) also confirms this result with a lattice spacing of 0.13, 0.15, 0.20, 0.23, 0.24, and 0.25 nm, corresponding to the (4 5 2) and (1 0 2) of C, the (2 0 1) of FeC) surface, the (2 1 0) surface of Fe₇C₃, the (5 3 1) surface of Fe₂C, and the (2 0 1) surface of FeC, which also confirms the synthesis of the above substances. The corresponding EDS spectra of the dark field diagram NMC-2 were obtained from Fig. 2j, and the EDS spectra proved the presence of various elements: carbon (C) (Fig. 2f) from fly ash, iron (Fe) (Fig. 2g) from Fenton sludge, nitrogen (N) (Fig. 2h) from urea, and the presence of (O) (Fig. 2i), further confirming the successful preparation of NMC-2.

The type of functional groups and chemical bonding on the surface of the material can be analyzed by IR spectrogram analysis. Figure 3b shows the FTIR image of NMC-2 adsorbent 3440 cm⁻¹ wide and strong absorption peak is due to the stretching vibration of –OH, there is a large amount of –OH present on the surface of the material; the peak appearing at 1640 cm⁻¹ is –COOH. Characterization reveals that the –OH absorption peak is wider^18,19. In addition, the absorptions at 1390 cm⁻¹ and 1000 cm⁻¹ were attributed to the bending of –OH vibrations of alcohols and phenol and the stretching vibration of C–O²⁰. The above results indicate that the surface of NMC-2 contains a large number of oxygen-containing functional groups, and these functional groups can provide many active sites for the removal of Cr(VI). It was also found that the weak peaks corresponding to 573 cm⁻¹ and 550 cm⁻¹ were attributed to Fe–O groups²¹. The stretching of Fe–O may be due to the oxidation of loaded Fe⁰ and Fe²⁺ to Fe³⁺²². Figure 3a shows the Fenton sludge and fly ash FTIR images. It can be seen from the figure that the surfaces of Fenton sludge and fly ash contain a large number of oxygen-containing functional groups, the surface functional groups of the two raw materials are more abundant, and the functional groups of NMC-2 around 3441 cm⁻¹, 1632 cm⁻¹, and 1400 cm⁻¹ are not significantly different from those of the raw materials, and the C–H stretching vibration peaks of NMC-2 around 873 cm⁻¹ and 698 cm⁻¹ is not obvious, which may be because the material the C–H bond on the surface of the raw material was oxidized to C–O in the process of synthesis.

Cr(VI) adsorption experiment

Selection of adsorbent

To select the best adsorbent, Cr(VI) adsorption tests were performed on four adsorbents. Figure 4a shows the effect of Fenton sludge and the urea addition on the adsorption efficiency. The Cr(VI) removal rates of the four adsorbents were ranked from low to high: MC-1 < NMC-0.5 < NMC-1 < NMC-2. By comparing the Cr(VI) removal rates of MC-1 and NMC-1, it can be seen that the Cr(VI) removal rate of NMC-1 with 1 g of urea added was 48.8%. Cr(VI) removal rate of MC-1 with no urea was added, and its Cr(VI) removal rate was 17.8%, indicating that the addition of urea has a facilitating effect on the removal of Cr(VI). The addition of urea during the preparation of the adsorbent plays a role in providing a nitrogen source, indicating that doping with nitrogen can effectively improve the adsorption capacity of the material because the nitrogen-containing functional groups can provide more active sites and thus adsorb more Cr(VI). The comparison of the Cr(VI) removal rates of NMC-0.5, NMC-1, and NMC-2 showed that the Cr(VI) removal rate of Fenton sludge in NMC-0.5 was 33.3% with 0.5 g. The Cr(VI) removal rate of Fenton sludge in NMC-1 was 48.8% with 1 g. The Cr(VI) removal rate of Fenton sludge in NMC-2 was 64.3% with 2 g. It shows that the adsorption of material on Cr(VI) is related to the Fenton sludge dosage, and the higher the Fenton sludge dosage, the higher the adsorption efficiency. Meanwhile, in the adsorption of material with magnetite, it is found that NMC-2 has strong magnetism, and the adsorbent with stronger magnetism is easier to be magnetically separated and regenerated after adsorption, whether the adsorbent is easily separated and regenerated is very important in practical application, considering the magnetic separation performance and adsorption performance of the material, and choosing NMC-2 were selected for subsequent experiments considering the magnetic separation performance and adsorption performance of the material.

Effect of pH value

The pH is one of the factors that affect the performance of the adsorbent. The solution pH affects the protonation/deprotonation of the adsorbate and the surface charge of the adsorbent⁴. The effect of pH from 6.0 to 10.0 on the removal rate of Cr(VI) was investigated under the condition that other factors were kept constant. Figure 4b shows the test of the effect of pH on Cr(VI) adsorption, from the figure, it can be seen that NMC-2 has a higher Cr(VI) removal rate of 80.36% at pH = 10, and the Cr(VI) removal rate from pH = 6 to pH = 8 decreases continuously from the performance and the adsorption performance from pH = 8 to pH = 10 increases. Because pH determines the form of Cr(VI) in solution, Cr(VI) exists mainly in the form of CrO₄²⁻ and HCrO₄⁻ in aqueous solution, 1 < pH < 6.5, HCrO₄⁻ is the main form; pH > 6.5, CrO₄²⁻ is the main form. The decrease of removal rate from pH = 6 to pH = 8 is because the surface of NMC-2 is rich in oxygen-containing functional groups, which can form hydrogen bonds with HCrO₄⁻, and it has lower adsorption free energy than CrO₄²⁻, and is easily adsorbed; secondly, due to the acidic environment, the surface –COOH and -OH of NMC-2 can be protonated with H⁺, forming positively charged functional groups –OH²⁺, –COOH²⁺, which can bind the anions HCrO₄⁻ and CrO₄²⁻ through electrostatic interaction, resulting in higher adsorption of Cr(VI)^14,18,23,24. When the pH was increased from 6 to 8, the alkalinity was strengthened and HCrO₄⁻ was gradually converted to CrO₄²⁻, which bound more oxygen-containing functional groups than HCrO₄⁻ and reduced the adsorption, while OH– in the solution was easily bound to the acidic functional groups on the surface of NMC-2, which reduced the uptake of CrO₄²⁻²³. When pH = 8 to pH = 10 the removal rate of Cr(VI) gradually increased from 68.81 to 80.36%. Due to the large presence of CrO₄²⁻ at increasing pH, at the same time a part of Cr(VI) was adsorbed and another part of Cr(VI) reacted with Fe⁰ and Fe²⁺ would form Cr(III) with a large amount of OH⁻, and Cr(III) and OH⁻ further formed precipitation. According to the above results, the optimal pH value for the experiment is pH = 10.

Effect of time

The effect of time from 0 to 1200 min on the removal rate of Cr(VI) was studied under the condition that other factors were kept constant. Figure 4c shows the test of the effect of time on the adsorption of Cr(VI). As can be seen from the figure, the reaction process of Cr(VI) on NMC-2 is divided into fast, diffusion, and equilibrium stages²⁵. When 0–60 min, the adsorption is in the fast stage and the removal rate of Cr(VI) increases rapidly because the material has a porous structure, abundant surface functional groups, and active sites, which lead to the rapid adsorption of Cr(VI) by electrostatic force attraction. When 60–720 min, it belongs to the diffusion stage, the removal rate reaches 75.84% at 360 min, and then the removal rate slows down and decreases because the material surface functional groups and Cr(VI) are desorbed or the kinetics change leading to the reaction from fast to slow²⁶. After that, the removal rate increased slowly and the active sites on the material surface were saturated until the equilibrium state was reached at 1200 min when the Cr(VI) removal rate could reach 80.36%.

Effect of initial concentration

The effect of concentration from 0 to 300 mg/L on the removal rate of Cr(VI) was studied under the condition that other factors were kept constant. The initial concentration also has an important effect on the adsorption, which not only affects the amount of Cr(VI) loaded on the NMC material, thus indirectly affecting the electron transport process, but also severely affects the distribution of heavy metal ions in the solution, and the removal rate of Cr(VI) can be maximized only under the optimum conditions²⁷.

Figure 4d shows the effect of concentration on Cr(VI) adsorption, when the initial concentration increased from 30 to 100 mg/L, the removal rate of Cr(VI) gradually increased, probably because the initial concentration in the solution was low, the surface functional group of NMC-2 was more than Cr(VI) in solution, and Cr(VI) in solution was able to rapidly adsorb on NMC-2, resulting in a higher removal rate²⁸. When the initial concentration increased from 100 to 200 mg/L, the Cr(VI) removal rate gradually decreased from 62.96 to 18.37%. It may be due to the increase in the initial concentration, the adsorption saturation of the functional groups on the surface of NMC-2 was reached in a short time, the adsorption rate slowed down, and the Cr(VI) removal rate decreased¹⁷. The results showed that the adsorption effect was better when the initial concentration was 100 mg/L.

Effect of temperature

Investigate the effect of temperature 273–315 K on the removal rate of Cr(VI) while keeping other factors constant. Figure 4e shows the effect of temperature on Cr(VI) adsorption, temperature is an important influencing factor, it will directly affect the removal rate of Cr(VI), as can be seen from the figure, when T = 318 K, the removal rate is only 37.26%, when T = 303 K, the removal rate is 59.30%, and the highest removal rate is 80.26% under the condition of T = 298 K. Therefore, the removal rate of Cr(VI) decreases with the increase in temperature, and the adsorption effect is better at T = 298 K.

Adsorption kinetics and isotherms

The pseudo-primary kinetic model, pseudo second kinetic model, and modified Elovich model have fitted for the process of Cr(VI) removal by NMC-2, and the results are shown in Fig. 5a–c, and the correlation coefficients of the three kinetic models are shown in Table S2. The maximum adsorption amount of 70.87 mg/L was obtained, which was different from the experimental amount of 80.36 mg/L. This shows that the pseudo primary kinetic model is not suitable for describing the adsorption process of NMC-2 on Cr(VI).

The modified Elovich model was used to reflect the desorption process of non-uniform surface chemisorption. As shown in Table S2, the correlation coefficient R² obtained by fitting the modified Elovich model was relatively small at 0.75593, and the maximum adsorption amount obtained by calculation was 58.58 mg/L, which differed significantly from the experimentally obtained adsorption amount of 80.36 mg/L. This shows that the modified Elovich model is not suitable to describe the adsorption process of NMC-2 on Cr(VI).

In addition Table S2 also calculates the relevant parameters of the pseudo second kinetic model, and the overall fitting order is: pseudo second kinetic model > pseudo-first-order kinetic model > Elovich model according to the decision coefficient R². The pseudo second kinetic model gives the best fit (R² > 0.9). It can be seen that the correlation coefficient R² obtained by fitting the pseudo second kinetic model is 0.90658, which is closer to 1. Meanwhile, the maximum saturation adsorption amount calculated is 73.74 mg/L, respectively, which is less different from the experimentally obtained adsorption saturation adsorption value, and the pseudo second kinetic model has the best correlation with the NMC-2 removal of Cr(VI) system. It indicates that the kinetics of Cr(VI) adsorption by NMC-2 is more consistent with the pseudo second kinetic model. Therefore, it indicates that the kinetics of Cr(VI) adsorption by NMC-2 is more consistent with the pseudo second kinetic model, which further indicates that the process is a chemisorption process involving electron sharing or electron exchange, and the adsorption rate is controlled by chemisorption^29,30,31. Also, this conclusion is in agreement with those obtained by other scholars using other adsorbents for Cr(VI) removal^19,21,32. The adsorption reaction is a chemisorption accompanied by physical adsorption.

Table 2 compares the ability of the prepared NMC-2 composites with other materials for the removal of Cr(VI). The experimental adsorption amount of NMC-2, Q_max = 393.79 mg/g, was obtained by fitting the integral method, and the integral equation is shown below.

where Q_max, maximum adsorption capacity; t_e, time to reach adsorption equilibrium, min; v, adsorbed solution volume, mL.

It was found through Table 2 that NMC-2 exhibited greater Cr(VI) adsorption capacity, indicating that the prepared composite can be used as a potential adsorbent for the effective cleaning of industrial wastewater with Cr(VI).

Adsorption thermodynamic

Adsorption isotherm refers to the relationship between adsorption capacity and gas phase pressure or concentration in the adsorption process under constant temperature. The most commonly used are Freundlich and Langmuir adsorption isotherms. Freundlich adsorption isotherm shows that the multilayer adsorption has a non-uniform distribution of functional groups, and the adsorbed molecules interact with each other. The formula is as shown in Equation S11. The Langmuir adsorption isotherm shows that the adsorbent forms a monomolecular layer on the surface of the adsorbent, each active site is the same and there is no interaction between adsorbed molecules, and the formula is shown in Equation S12³⁶.

The equilibrium data are fitted by the Langmuir and Freundlich isotherm models, and the isotherm fitting curve is shown in Fig. S2, and it can be found that the Langmuir fitting curve R² = 0.7978, the Freundlich fitting curve R² = 0.9213, and the Freundlich R² is closer to 1. It can be explained that the distribution of active sites on the surface of the adsorbent prepared in this study is not uniform, and there is a non-uniform surface condition on the surface in this experiment^30,37. The adsorption process may be multilayer adsorption, and there are interactions between adsorbed Cr(VI) molecules. From Table S3, it can be seen that n =  − 1.45, 1/n =  − 0.689 < 1, and physical adsorption exists in the surface adsorption process³⁸.

The adsorption thermodynamics can reflect the change of energy before and after adsorption, the adsorption thermodynamic parameters are as follows, in which the Gibbs free energy (ΔG), enthalpy (ΔH) entropy is calculated (ΔS), and the calculation equation is S13–S15. The fitting curve of lnK_d and 1/T is shown in Fig. S3. It can be seen that the slope after fitting is 8145.26, the intercept is − 24.55, and the fitting curve R² = 0.90925, indicating a good linear correlation. The thermodynamic parameters obtained by calculation are shown in Table S4. It can be seen from the table that ΔG is negative, indicating that the adsorption process is spontaneous, at the same time, with the increase in temperature, the absolute value of ΔG decreases, indicating that low temperature is conducive to adsorption³⁹. At the same time, the negative value of ΔH indicates that the adsorption process is an exothermic reaction, which is mainly monolayer adsorption accompanied by multilayer adsorption, which is consistent with the results of the thermodynamic model of adsorption. Generally, when ΔH is between 2.1 and 20.9 kJ/mol, which indicates that the adsorption is mainly physical; when ΔH is between 20.9 and 418.4 kJ/mol, it indicates that the adsorption is mainly chemical⁴⁰. As can be seen from Table S4, the adsorption of NMC-2 is mainly chemical adsorption, which conforms to the fitting result of the kinetic model.

Reaction mechanism

Figure 6c shows the XPS spectrum of Fe 2p after NMC-2 adsorption, XPS was used to further verify the valence state of Fe in the sample and the results show peak centers at 710.2 eV and 723.7 eV, indicating the presence of Fe³⁺ 2p3/2 and Fe³⁺ 2p1/2³³. Also, the XPS analysis results revealed two additional peaks with higher energy at 714.8 eV and 728.4 eV as satellite peaks. It indicates the presence of trivalent iron oxides on the surface of NMC-2 after adsorption, which is consistent with the results obtained in XRD for the presence of FeC.

The reaction mechanism was determined by XPS measurements of the surface composition and material valence of NMC-2 before and after Cr(VI) adsorption. Figure 6f shows the full spectra of carbon, nitrogen, oxygen, and iron. The full spectrum fully demonstrates the entry of Fe and N elements into porous carbon during the carbon thermal process⁴¹. After adsorption, a peak of 578.08 eV appears, which is due to the presence of Cr 2p, which means that Cr(VI) is adsorbed by NMC-2, which also proves the XRD results in Fig. 1a. The Cr 2p peaks in Fig. 6a are mainly assigned to Cr(VI) 587.84 eV and 590.14 eV and Cr(III) 574.25 eV and 577.83 eV. The XPS results indicate that part of the highly toxic Cr(VI) will be adsorbed by NMC-2 and part of Cr(VI) will be reduced to Cr(III), which is further evidence that the reaction. This is further evidence that not only the adsorption reaction but also the redox reaction exists. Figure 6d shows the XPS spectra of N 1s before adsorption, which shows five fission peaks corresponding to N1, 397.77 eV (nitrogen bonding of metals), N2, 398.99 eV (pyridine-N), N3, 399.93 eV (pyrrole-N) and N4, 402.44 eV (graphite-N) and N5, 405.72 eV (nitrogen oxides), respectively^42,43. The presence of the above five fission peaks after the adsorption of Cr(VI) can also be observed from the suction of Fig. 6e. A comparison of the results in Fig. 6d–e revealed that the peak area of N2 increased from 8.0 to 16.0%, N3 increased from 11.9 to 18.0%, N4 increased from 12.4 to 31.2%, the peak area content of N5 decreased from 64.8 to 31.9% after adsorption, and the above results indicated that pyridine-N, pyrrole-N, graphite-N, and nitrogen oxides provided the adsorption provided the driving force. Figure 6b shows the XPS spectrum of Fe 2p before adsorption, from which it can be found that before the adsorption of Cr(VI), Fe 2p was divided into three peaks: 711.38, 714.08, and 725.27 eV, which confirms the presence of Fe, Fe²⁺ and Fe³⁺⁴³. After adsorption, Fe 2p peaks appeared at 710.2, 714.8, 723.7, and 728.4 eV, and no Fe, Fe²⁺, but Fe³⁺ were observed in the adsorbed peaks (Fig. 6f). This indicates that Fe, Fe²⁺, and Cr(VI) have undergone redox reactions to form Fe³⁺. To further illustrate the reaction mechanism, the Cr(III) and Cr(VI) ratios were calculated using XPS data as shown in Table S5. The results showed that Cr(III) and Cr(VI) accounted for 60.65% and 39.35% of Cr, respectively, indicating the presence of oxidation behavior during the reaction and the predominance of redox reaction^43,44.

Based on the above analysis, the reaction mechanism of Cr(VI) is summarized as follows: firstly, a part of Cr(VI) is adsorbed by the negative charge on the surface of NMC-2 due to electrostatic gravitational force. Meanwhile, the surface of NMC-2 is rich in functional groups and has a large specific surface area, which is beneficial to enhance the removal rate of Cr(VI)^43,45,46. Secondly, Fe and Fe²⁺ transfer electrons and react with the redox reaction of Cr(VI) through the porous channels on the surface of NMC-2 (Equations 2-3). Finally, as the Cr(VI) reaction generates a large amount of OH⁻, resulting in Cr(III) can be present in the form of a precipitate. The reaction mechanism is shown in Fig. 7.

Source: Ecology - nature.com