Method for removing trace thallium in wastewater by using iron-modified graphene nanobiochar to strengthen high ferrate system

Abstract

The application discloses a method for removing trace thallium in wastewater by using a high ferrate system reinforced by iron-modified graphene nanobiocarbon, and belongs to the technical field of wastewater treatment. The method comprises the following steps: modifying biocarbon with iron to obtain iron-modified nanobiocarbon, loading graphene on the iron-modified nanobiocarbon, confining and doping iron particles in graphene nanometer, and obtaining iron-modified graphene nanobiocarbon. The application firstly confines and dopes iron particles in graphene nanometer, constructs a multifunctional material with high adsorption capacity and multiple active sites, and couples a potassium ferrate process to construct an adsorption-oxidation synergistic thallium removal system.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment, specifically relating to a method for removing trace thallium from wastewater based on an iron-modified graphene nano-biochar enhanced ferrate system. Background Technology

[0002] Thallium (Tl) is a rare but relatively widespread heavy metal element in the Earth's crust, known for its extremely high acute and chronic toxicity. The lethal dose for adults is only 8–10 mg / kg, and it exhibits significant bioaccumulation, potentially leading to fatal consequences under extreme exposure conditions. Thallium possesses both lithophile and chalcophile geochemical properties, thus often occurring in association with various metallic elements in sulfide deposits. With the acceleration of global industrialization, human activities have become a major source of thallium pollution in water bodies, particularly from the mining and beneficiation of sulfide polymetallic ores, non-ferrous metal smelting, coal-fired power generation, cement manufacturing, and the discharge of related industrial waste. However, currently, the concentration of thallium in thallium-containing industrial wastewater is generally low, and existing treatment technologies have significant limitations, including difficulty in consistently meeting increasingly stringent emission standards (0.002 mg / L), inconsistent treatment effectiveness, and the potential for secondary pollution. As the world accelerates its efforts to achieve carbon neutrality by the middle of this century, the production scale of key metal elements such as lithium and rare earth elements is expected to expand significantly. This industrialization process may inadvertently exacerbate the risk of thallium pollution to aquatic ecosystems through related mining, beneficiation, and smelting processes. Summary of the Invention

[0003] This invention proposes a method for treating thallium-containing wastewater using a combined system of graphene nano-doped iron particles and potassium ferrate, overcoming the limitations of a single system and improving the efficiency of thallium removal and system stability.

[0004] The first objective of this invention is to provide a method for preparing iron-modified graphene nano-biochar, which involves modifying biochar with iron to obtain iron-modified nano-biochar, and then loading graphene onto the iron-modified nano-biochar, thereby confining the graphene nanoparticles with iron particles to obtain iron-modified graphene nano-biochar.

[0005] Preferably, the iron-modified nano-biochar is obtained by soaking biochar in an iron ion solution for reaction, then drying the soaked biochar, and then pyrolyzing it.

[0006] Further optimization involves soaking Fe / biochar in a 0.1M FeCl3·6H2O solution at a mass ratio of 1:10, stirring for 1 hour, followed by ultrasonic treatment for 1 hour, then allowing it to stand overnight. Finally, the iron-soaked biochar is dried at 60°C to constant weight, and then dried at 5°C / min. −1Pyrolysis was performed at a heating rate of ~500℃, and the temperature was maintained for 1 h. The sample was collected and pulverized to obtain iron-modified biochar.

[0007] Preferably, the iron-modified graphene nano-biochar is obtained by adding graphene oxide, sodium citrate and iron-modified biochar to water for reaction, then drying, washing with water, and then drying again.

[0008] Further preferred, the iron-modified graphene nano-biochar is prepared by adding 0.12g of graphene oxide, 0.24g of sodium citrate, and 1.08g of iron-modified biochar to deionized water and homogenizing. The mixture is then dried at 90°C for 2 hours, washed with deionized water, dried in an oven at 70°C for 8 hours, and then freeze-dried for 48 hours to obtain iron-modified graphene nano-biochar.

[0009] The second objective of this invention is to provide a method for removing trace amounts of thallium from wastewater based on an iron-modified graphene nano-biochar enhanced ferrate system, which involves using iron-modified graphene nano-biochar and potassium ferrate in combination to treat thallium-containing wastewater.

[0010] Preferably, in the system for treating thallium-containing wastewater, the content of the iron-modified graphene nano-biochar is 5-100 mg / L.

[0011] Preferably, in the system for treating thallium-containing wastewater, the content of potassium ferrate is 10-500 mg / L.

[0012] Preferably, in the system for treating thallium-containing wastewater, the content of the iron-modified graphene nano-biochar is 20-40 mg / L, and the content of the potassium ferrate is 75-125 mg / L.

[0013] The beneficial effects of this invention are as follows:

[0014] (1) For the first time, graphene nanoparticles were confined and doped with iron particles to construct a multifunctional material with high adsorption capacity and multiple active sites. The potassium ferrate process was coupled to construct an adsorption-oxidation synergistic thallium removal system.

[0015] (2) By using the nano-confinence effect of graphene-doped iron nanoparticles, the catalytic oxidation activity of Fe(VI) on thallium ions and the lattice fixation ability of biochar on thallium can be improved.

[0016] (3) Compared with single adsorption or oxidation systems, this system improves the efficiency of thallium removal from wastewater and enhances system stability. By leveraging the complementary functions between materials and reagents, it provides a new approach to address the bottleneck issues of "limited removal efficiency and risk of secondary pollution" in industrial thallium pollution control.

[0017] (4) Provide new materials and new ideas for the efficient purification of thallium in complex water bodies at both the micro-interface reaction and macro-removal behavior levels. Attached Figure Description

[0018] Figure 1 (a) and (d) are SEM and EDS of the original biochar, respectively; (b) and (e) are SEM and EDS of the original biochar after iron modification, respectively; (c) and (f) are SEM and EDS of the iron-modified graphene nano-biochar, respectively.

[0019] Figure 2 (a) Adsorption isotherms of original biochar, iron-modified biochar, and iron-modified graphene nano-biochar; (b) Adsorption isotherm of original biochar and BJH adsorption pore size distribution; (c) Adsorption isotherm of iron-modified biochar and BJH adsorption pore size distribution; (d) Adsorption isotherm of iron-modified graphene nano-biochar and BJH adsorption pore size distribution.

[0020] Figure 3 It includes the zeta potential of original biochar, iron-modified biochar (FeBC), and iron-modified graphene nano-biochar.

[0021] Figure 4 The treatment of 100 μg / L Tl wastewater was achieved by using 30 mg / L iron-modified graphene nano-biochar (FeGB-gel) and 100 mg / L potassium ferrate alone or in combination.

[0022] Figure 5 (a) Fe2p spectrum before reaction; (b) Fe2p spectrum after reaction;

[0023] Figure 6 These are the Tl4f spectra before and after the reaction;

[0024] Figure 7 These are the FTIR spectra before and after the reaction. Detailed Implementation

[0025] The following embodiments are further illustrations of the present invention, but not limitations thereof.

[0026] Example 1:

[0027] 1. Material preparation:

[0028] 1.1 Preparation of raw biochar (BC)

[0029] Rice husk residue was washed with clean water, crushed to a particle size of 2 mm, dried at 80℃ for 24 h, and then pyrolyzed at 500℃ under oxygen conditions for 1 h to obtain primitive biochar (BC).

[0030] 1.2 Preparation of Iron-Modified Biochar (FeBC)

[0031] Dry raw biochar was soaked in 0.1M FeCl3·6H2O solution at a Fe / biochar mass ratio of 1:10, stirred with a magnetic stirrer for 1 hour, then ultrasonically treated at room temperature for 1 hour, and then allowed to stand overnight. Finally, the iron-soaked biochar was dried at 60°C to constant weight, and then dried at 5°C for 1 minute. −1 Pyrolysis was performed at a heating rate of ~500℃, and the temperature was maintained for 1 h. The sample was collected and pulverized (100 mesh) for further modification to obtain iron-modified biochar (FeBC).

[0032] 1.3 Preparation of Iron-Modified Graphene Nano-Biochar (FeGB-gel)

[0033] 8.0 g of graphene oxide suspension (containing 0.12 g of graphene oxide), 0.24 g of sodium citrate, and 1.08 g of magnetized iron-modified biochar (FeBC) were added to 50 mL of deionized water and homogenized for 30 min. The mixture was then dried at 90°C for 2 h to form iron-modified graphene nano-biochar (FeGB-gel). The sample was washed three times with deionized water, then dried in an oven at 70°C for 8 h, followed by freeze-drying for 48 h. Finally, the FeGB-gel was pulverized (100 mesh) and stored for further experiments.

[0034] 2. Treatment of thallium-containing wastewater using a combined system of iron-modified graphene nano-biochar and potassium ferrate: At room temperature (25°C), 1. Iron-modified graphene nano-biochar as the variable group: iron-modified graphene nano-biochar (FeGB-gel) at concentrations of 5 mg / L to 100 mg / L (in-system concentration) and potassium ferrate at 100 mg / L (final system concentration) were added to wastewater containing thallium (100 μg / L), with no iron-modified graphene nano-biochar added as the control; 2. Potassium ferrate as the variable group: potassium ferrate at concentrations of 10 mg / L to 500 mg / L (in-system concentration) and iron-modified graphene nano-biochar at 30 mg / L (final system concentration) were added to wastewater containing thallium (100 μg / L), with no potassium ferrate added as the control; 3. Iron-modified graphene nano-biochar (FeGB-gel) at a final system concentration of 30 mg / L and potassium ferrate at a final system concentration of 100 mg / L, or iron-modified graphene nano-biochar (FeGB-gel) at a final system concentration of 30 mg / L and potassium ferrate at a final system concentration of 100 mg / L, were respectively applied to wastewater containing thallium (100 μg / L). The mixture was stirred at 400 r / min, and nitric acid and sodium hydroxide were used to maintain a stable pH range of 6-8 during the treatment process. The reaction time was 30 minutes.

[0035] 3. Determination of thallium removal rate in thallium-containing wastewater: After the reaction, an appropriate amount of solution was extracted from 1.5–2 cm below the liquid surface using a syringe. The solution was filtered through a 0.45 μm filter membrane and acidified with 2% nitric acid. The Tl concentration was determined using inductively coupled plasma mass spectrometry (ICP-MS). By comparing the Tl content in the wastewater before and after the reaction, the thallium removal rate of the iron-modified graphene nano-biochar-potassium ferrate combined system was obtained.

[0036] 4. Structural Characterization: The morphology of the original iron-modified graphene nano-biochar (FeGB-gel) was observed using scanning electron microscopy (SEM); the morphology and lattice fringes of the nanoparticles before and after the reaction were observed using transmission electron microscopy (TEM), and the changes in sample structure during the processing were analyzed; the changes in the chemical valence state of the material surface before and after the reaction were analyzed using X-ray photoelectron spectroscopy (XPS); and the types and changes of functional groups in the material before and after the reaction were analyzed using Fourier transform infrared spectroscopy (FTIR). Figure 1As can be seen from Figure (a), the original biochar (BC) exhibits a relatively typical morphological characteristic of plant-derived char materials, retaining the original tissue skeleton structure of rice husks. A distinct layered or sheet-like structure can be observed, with regularly arranged groove-like textures in local areas. Its surface is relatively smooth with limited roughness, the pore structure is mainly composed of fissures, there are few surface attachments, and the degree of granulation is not obvious. (d) The high O content in Figure (b) indicates that the rice husk itself is rich in oxygen-containing structures such as Si-O, CO, and C=O, suggesting that BC is a pure carbon-oxygen system without the introduction of exogenous metals. This is consistent with the surface morphology with fewer particles observed in SEM. Figure (b) shows that after modification, the morphology of the iron-modified original biochar (FeBC) changes significantly. The original regular layered structure is covered by a large number of fine particles, exhibiting obvious granulation and agglomeration characteristics. The surface roughness increases significantly, with a large number of nano-sized particles uniformly or semi-uniformly loaded on the carbon surface. The original pore and groove structures are partially filled or obscured. These particles are usually iron species deposited on the biochar surface or grown in situ. (e) The figure shows that Fe is the dominant element in iron-modified biochar, indicating that iron species are highly enriched on the surface of biochar. The decrease in O content may be due to the decomposition of functional groups during pyrolysis. (c) Iron-modified graphene nanobiochar (FeGB-gel) exhibits obvious multi-scale composite structure characteristics and is the most complex of the three. The wrinkled and sheet-like structures can be clearly observed in Figure (c), which typically correspond to the morphological characteristics of graphene or reduced graphene oxide (GO). The graphene sheets have a coating, bridging, or supporting effect, constructing a three-dimensional open structure. The iron-based nanoparticles are uniformly anchored on the graphene sheets or biochar surface, forming a hierarchical pore structure with mesoporous-macroporous synergy. The internal space is more open, which is conducive to reactant diffusion and mass transfer. (f) The EDS figure shows an increase in O content, which may be due to the introduction of a large number of oxygen-containing functional groups (-COOH / -OH / C=O) after loading graphene oxide. Therefore, this composite system combines the porous framework and structural stability of biochar, the high reactivity of iron species, and the high specific surface area and excellent electron transport capacity of graphene. This structure is particularly advantageous for the adsorption-catalysis-redox synergistic mechanism and is especially suitable for efficient water treatment or heavy metal conversion systems.

[0037] Depend on Figure 2 It can be seen that all three types of biochar exhibit type IV isotherms, with a sharp increase in adsorption capacity in the high-pressure region (P / P0>0.8), indicating the presence of a large number of mesopores and macropores. The hysteresis loop shape indicates a complex pore structure, which is conducive to material transport. Figure 2(a) It can be seen that iron-modified graphene nano-biochar (FeGB-gel) > iron-modified biochar (FeBC) > original biochar (BC). Under the same relative pressure, FeGB-gel has the highest nitrogen adsorption capacity and the highest position on the curve. This indicates that iron-modified graphene nano-biochar has a better pore volume / specific surface area. (b)-(d) BJH pore size distribution shows that all three materials are mainly mesoporous (pore size concentrated in 10-100 nm). Among them, (b) original biochar shows an underdeveloped pore structure, uneven pore size distribution, and the fewest adsorption sites; (c) iron-modified biochar shows a better pore structure than original biochar, but its performance is weaker; (d) iron-modified graphene nano-biochar has a more uniform pore size distribution and a higher proportion of 3-4 nm mesopores. It is beneficial for adsorbing small molecule pollutants such as heavy metals, with high pore wall utilization, while retaining ~20 nm large mesopores to ensure that the adsorbed substances can quickly diffuse into the interior without affecting the mass transfer efficiency.

[0038] Table 1. Specific surface area and pore volume of three types of biochar

[0039] As shown in Table 1, the overall trend of specific surface area is: original biochar (BC) < iron-modified biochar (FeBC) < iron-modified graphene nano-biochar (FeGB-gel), while the pore volume tends to be stable between the latter two.

[0040] Depend on Figure 3 It is evident that all three types of biochar carry a negative charge, a typical characteristic of the ionization of oxygen-containing functional groups (such as carboxyl and hydroxyl groups). The charge intensity ranking is: iron-modified graphene nano-biochar (-44.2 mV) > iron-modified biochar (-25.1 mV) > original biochar (-17.4 mV). Among them, iron-modified graphene nano-biochar has a surface charge of -44.2 mV, exhibiting strong electrostatic repulsion between particles. It can maintain a dispersed state in solution for a long time, is not prone to sedimentation, and is less likely to caking in water treatment, thus maintaining the exposure of highly reactive sites. The more negative surface charge of iron-modified graphene nano-biochar also provides a stronger electrostatic attraction to cationic heavy metal pollutants.

[0041] Depend on Figure 4 It can be seen that when FeGB-gel is used alone, the removal rate is about 30-40% after 30 minutes, showing a relatively gentle upward curve; when K2FeO4 is used alone, the removal rate is moderate, rising rapidly in the first 15 minutes and then slowing down, reaching about 60-70% after 30 minutes; when the two are used together to form a thallium removal system, the removal rate is significantly the highest, showing a rapid increase followed by stabilization. The removal rate of thallium exceeds 99% after 30 minutes, meaning that the FeGB-gel and K2FeO4 combined system produces a synergistic effect of 1+1>2.

[0042] Depend on Figure 5 It can be seen that before the reaction, Fe(II) in FeGB-gel was 12.22% and Fe(III) was 87.78%. After being used in combination with 100 mg / L potassium ferrate for thallium removal, Fe(II) was 18.13% and Fe(III) was 81.87%. That is, the proportion of detectable ferrous iron on the surface increased from about 12% to about 18%, an increase of ~5.9 percentage points (about 48% relative increase). This change indicates that the valence state of iron on the surface or near the surface of the reaction system has undergone a detectable shift to ferrous iron; that is, although Fe(III) still dominates, the Fe(II) content has indeed increased.

[0043] Depend on Figure 6 It was found that no obvious Tl 4f characteristic peak was detected in the sample before the reaction; only a signal close to background noise was observed, indicating that no detectable thallium species were present on the material surface before the reaction. This result shows that the iron-modified graphene nano-biochar itself did not undergo background adsorption of thallium, providing a reliable control basis for the analysis of thallium valence state changes after the reaction. After the reaction, the sample showed a clear bimodal structure in the Tl 4f region, indicating that thallium was successfully enriched on the material surface. Among them, the oxidation state distribution of Tl(III) was 49.6%, and Tl(I) accounted for 50.4%, indicating that mixed valence state thallium substances coexisted after treatment. The presence of Tl(III) confirms that ferrate effectively oxidized Tl during the reaction. + At the same time, combined with a large amount of residual Tl + as well as Figure 6 The fact that the removal rate of thallium after the reaction reached over 99% indicates that some thallium was removed before oxidation. + Tl has been directly fixed to the surface of iron-modified graphene nano-biochar, or due to interfacial electron transfer with the carbon-based matrix. + Partial reduction occurred. Therefore, the simultaneous presence of Tl(I) and Tl(III) highlights the synergistic effect of oxidation and adsorption co-precipitation mechanisms in the removal of thallium.

[0044] To elucidate the synergistic mechanism of iron-modified graphene nano-biochar and potassium ferrate in removing thallium from water, Fourier transform infrared spectroscopy (FTIR) was performed on the materials before and after the reaction. The results are as follows: Figure 7 As shown.

[0045] In the sample before the reaction, 3438 cm -1A broad and distinct absorption peak appears nearby, which can be attributed to the stretching vibration of hydroxyl groups (-OH), mainly originating from phenolic hydroxyl and carboxyl hydroxyl groups on the surface of biochar and graphene, as well as Fe-OH groups on the surface of iron species. The presence of this peak indicates that the material surface is rich in oxygen-containing functional groups, providing active sites for subsequent metal ion complexation and redox reactions. After the reaction, the intensity of this absorption peak decreases and broadens to some extent, indicating that hydroxyl groups are consumed during Tl removal, possibly participating in Tl degradation. + Surface complexation and potassium ferrate-induced electron transfer reactions.

[0046] In the 1630 cm⁻¹ region, the unreacted sample exhibited a relatively clear absorption peak, which can be attributed to the C=C framework vibrations in the aromatic structure (characteristic of the aromatic domains in graphene and biochar) and the stretching vibrations of carboxyl or conjugated carbonyl groups (C=O), and may also include the HOH bending vibrations of a small amount of adsorbed water. This characteristic peak is very prominent in the original material, reflecting the presence of a conjugated structure between the biochar aromatic framework and graphene. After the reaction, the peak intensity decreased and the peak shape became flatter, indicating that the electronic environment of the conjugated system and oxygen-containing functional groups on the material surface changed, suggesting that graphene and the biochar framework participated in the electron transfer process during Tl removal.

[0047] A weak absorption peak was observed near 1388 cm⁻¹, which can be attributed to the carboxylate (-COO₂). - Symmetric stretching vibration (νs-COO) - The characteristic peak is usually associated with the carboxyl group structure coordinated with metal ions. The appearance of this characteristic peak indicates that iron species may be anchored to the surface of carbon-based materials through carboxyl groups, forming a Fe-OOC- structure. The changes in the intensity and morphology of this peak after the reaction indicate that the carboxyl group participates in the complexation or bridging fixation process of Tl, possibly forming -COO-Fe-Tl or related surface complex structures, thereby promoting the stable removal of Tl.

[0048] In the 1100-1000 cm⁻¹ region, the unreacted sample showed a relatively obvious absorption peak, which can be attributed to the stretching vibration of CO (alcohols or phenols) and the vibrational characteristics of the Fe-OC bond. This is a typical sign of successful iron loading and chemical bonding with the carbon matrix. After the reaction, the absorption peak in this region was significantly weakened and underwent some degree of splitting, indicating that the Fe-OC structure participated in the redox reaction during the reaction process. The iron active sites underwent valence state transformation under the action of potassium ferrate and interacted with Tl.

[0049] In 600-500 cm -1In the low wavenumber region, characteristic vibrational peaks of Fe-O or Fe-O-Fe can be observed in the unreacted sample, reflecting the presence of iron oxides or iron hydroxyl species. After the reaction, the absorption peak intensity in this region is enhanced and broadened, indicating that new metal-oxygen bonds may be formed on the material surface during the reaction, such as Fe-O-Tl or Tl-O related depositional phases, further confirming that Tl is fixed on the material surface in the form of precipitation or co-deposition after oxidation.

[0050] In summary, the FTIR results indicate that in the synergistic system of potassium ferrate and iron-modified graphene nano-biochar, the hydroxyl (-OH) and carboxyl groups (-COO) on the material surface... - Both the Fe-OC structure and Fe-O bonds are involved in the removal of the heavy metal thallium. The removal of Tl is not a simple physical adsorption, but is achieved through the synergistic effect of multiple mechanisms such as surface complexation, electron transfer, and oxidative deposition, which is highly consistent with the excellent thallium removal performance of this system.

Claims

1. A method for preparing iron-modified graphene nano-biochar, characterized in that, The process involves modifying biochar with iron to obtain iron-modified nano-biochar, then loading graphene onto the iron-modified nano-biochar, thereby confining the graphene nanoparticles with iron particles to obtain iron-modified graphene nano-biochar.

2. The preparation method according to claim 1, characterized in that, The iron-modified nano-biochar is obtained by soaking biochar in an iron ion solution, then drying the soaked biochar, and then pyrolyzing it.

3. The preparation method according to claim 2, characterized in that, Fe / biochar was soaked in a 0.1M FeCl3·6H2O solution at a mass ratio of 1:10, stirred for 1 hour, then ultrasonically treated for 1 hour, and allowed to stand overnight. Finally, the iron-soaked biochar was dried at 60°C to constant weight, and then dried at 5°C / min. −1 Pyrolysis was performed at a heating rate of ~500℃, and the temperature was maintained for 1 h. The sample was collected and pulverized to obtain iron-modified biochar.

4. The preparation method according to claim 1, 2 or 3, characterized in that, The iron-modified graphene nano-biochar is prepared by adding graphene oxide, sodium citrate and iron-modified biochar to water for reaction, then drying, washing with water, and drying again.

5. The preparation method according to claim 4, characterized in that, The iron-modified graphene nano-biochar was prepared by adding 0.12g of graphene oxide, 0.24g of sodium citrate, and 1.08g of iron-modified biochar to deionized water and homogenizing the mixture. The mixture was then dried at 90°C for 2 hours, washed with deionized water, dried in an oven at 70°C for 8 hours, and then freeze-dried for 48 hours to obtain the iron-modified graphene nano-biochar.

6. An iron-modified graphene nano-biochar prepared according to any one of the preparation methods of claims 1-5.

7. A method for removing trace thallium from wastewater based on an iron-modified graphene nano-biochar enhanced ferrate system, characterized in that, The method involves using the iron-modified graphene nano-biochar as described in claim 6 in combination with potassium ferrate to treat thallium-containing wastewater.

8. The method according to claim 7, characterized in that, In the system for treating thallium-containing wastewater, the content of the iron-modified graphene nano-biochar is 5-100 mg / L.

9. The method according to claim 7, characterized in that, In the system for treating thallium-containing wastewater, the potassium ferrate content is 10-500 mg / L.

10. The method according to claim 7, characterized in that, In the system for treating thallium-containing wastewater, the content of the iron-modified graphene nano-biochar is 20-40 mg / L, and the content of the potassium ferrate is 75-125 mg / L.

Images