Method for removing Cr(VI) and / or As(III) from wastewater by monodisperse iron-organic acid-light synergistic removal

Abstract

The present invention belongs to the field of wastewater treatment and specifically discloses a method for removing Cr(VI) and / or As(III) from wastewater by using monodisperse iron, organic acid and light. The method comprises mixing wastewater containing Cr(VI) and / or As(III) with monodisperse iron and an organic acid, reacting the mixture under light-assisted conditions, and then separating the solid and liquid to obtain treated water from which Cr(VI) and / or As(III) has been removed. The monodisperse iron is obtained by calcining iron phthalocyanine at a temperature of 450 to 750° C. The process of the present invention can achieve synergy and improve the removal effect of Cr(VI) and / or As(III).

Description

Technical Field

[0001] The invention belongs to the technical field of water treatment, and in particular relates to a method for removing Cr(VI) and / or As(III) from wastewater. Background Art

[0002] The heavy metals chromium (Cr) and arsenic (As) are two major carcinogenic pollutants commonly found in wastewater. Acidic mine drainage (AMD), generated by biochemical oxidation during mining operations and tailings dumping, contains significant amounts of these heavy metals. The acidic atmosphere of the AMD system imparts strong corrosion resistance and mobility to these heavy metals, posing a significant threat to the ecological environment and human health. The toxicity and mobility of these heavy metals depend primarily on their valence state. Chromium primarily exists as trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Cr(III) is less toxic and has poor mobility. Chromium removal strategies rely on reducing Cr(VI) to Cr(III), followed by removal via adsorption or precipitation. Arsenic typically exists as trivalent arsenic (As(III)) and pentavalent arsenic (As(V)). As(III) is highly toxic and mobile, so oxidation is the primary method used to convert As(III) to As(V), followed by removal via adsorption. However, the coexistence of Cr(VI) / As(III) requires strong redox properties for treatment technologies and processes, which greatly increases the difficulty of removing complex heavy metal water bodies in a one-step process.

[0003] To achieve the simultaneous redox conversion of Cr(VI) / As(III), current effective removal methods require complex equipment, treatment processes, and / or high doses of chemicals, making them unsuitable for treating large-scale AMD systems like those found in mines. For example, Chinese patent application CN201510305104.8 discloses an “electrochemical water treatment method for the simultaneous reduction of chromium and arsenic oxide,” CN201310613475.3 discloses a “treatment device for wastewater containing arsenic and chromium,” and CN201310390648.X discloses a “treatment method for wastewater containing chromium and arsenic.” These methods utilize replicated process technology or large amounts of chemical reagents (such as hydrogen peroxide, ferrous sulfate, and sodium sulfite) for the simultaneous removal of Cr(VI) / As(III), resulting in low efficiency and excessive energy consumption. Conventional heavy metal adsorption methods, while capable of removing chromium and arsenic, often suffer from issues such as varying adsorption selectivity and difficulty recycling the adsorbent. For example, Chinese patent application CN202211412367.5, "Preparation and Application of Lanthanum Zirconium MOFs as Dual-Functional Adsorbents for Arsenic and Chromium," CN201910958408.2, "A Schmidt Mineral for Removing Trivalent Arsenic and Hexavalent Chromium Pollution," and CN201610047579.6, "A Method for Adsorbing Hexavalent Chromium and Arsenic from Water Using Modified Grapefruit Peel," all involve the preparation of complex adsorbents. Therefore, the simultaneous removal of Cr(VI) and As(III) in an economical and simple manner is of great environmental significance. Summary of the Invention

[0004] In response to the problem of unsatisfactory Cr(VI) / As(III) removal in wastewater, the present invention aims to provide a method for synergistically removing Cr(VI) and / or As(III) from wastewater using a monodisperse iron-organic acid-light process, aiming to improve the Cr(VI) / As(III) removal effect in wastewater, as well as to improve treatment selectivity and cyclic removal stability.

[0005] A method for synergistically removing Cr(VI) and / or As(III) from wastewater using monodisperse iron, organic acid, and light, comprising mixing wastewater to be treated containing Cr(VI) and / or As(III) with monodisperse iron and an organic acid, reacting the mixture under the assistance of light, and subsequently performing solid-liquid separation to obtain treated water from which Cr(VI) and / or As(III) has been removed.

[0006] The monodisperse iron is obtained by calcining iron phthalocyanine at a temperature of 450-750°C.

[0007] The present invention innovatively obtains monodisperse iron by calcining iron phthalocyanine at a temperature of 450 to 750°C, and innovatively combines the monodisperse iron with an organic acid and light irradiation. This unexpectedly achieves synergy, improves the removal of Cr(VI) and / or As(III) in wastewater, and achieves the simultaneous removal of Cr(VI) and As(III). Furthermore, the method of the present invention has excellent removal selectivity and cyclic removal stability.

[0008] In the present invention, the wastewater to be treated is any wastewater containing Cr(VI) and / or As(III).

[0009] In the present invention, thanks to the excellent treatment advantages of the treatment method, compared with the existing removal methods, it has a better simultaneous removal advantage for the wastewater to be treated that contains Cr(VI) and As(III).

[0010] In the present invention, the wastewater to be treated may be mine wastewater containing Cr(VI) and / or As(III).

[0011] In the present invention, there is no particular requirement for the contents of Cr(VI) and As(III) in the wastewater to be treated. For example, considering the process value of the treatment, the Cr(VI) content is 5 g / L or more, preferably 10 g / L or more, and more preferably 15 to 50 g / L. The As(III) content is 10 g / L or more, preferably 20 g / L or more, and more preferably 30 to 100 g / L.

[0012] In the present invention, monodisperse iron obtained by calcining iron phthalocyanine is innovatively used for the treatment of Cr(VI) and As(III), and it is further found that combining it with organic acid and light irradiation can unexpectedly further enhance the removal effect, improve the removal efficiency, removal selectivity and cycle stability.

[0013] In the present invention, iron phthalocyanine is innovatively used for calcination, and the calcination temperature is combined to obtain monodisperse, highly stable iron elemental particles. During the treatment with organic acid and light irradiation, the iron elemental particles will not be etched by the organic acid, but on the contrary can unexpectedly achieve synergy and enhance the removal effect, selectivity and cycle stability.

[0014] In the present invention, the iron phthalocyanine is obtained by pyrolysis reaction of a mixed raw material of an iron source, a nitrogen source, pyromellitic anhydride, a swelling agent, and a catalyst;

[0015] Preferably, the iron source is a water-soluble ferric salt, preferably ferric chloride;

[0016] Preferably, the nitrogen source is at least one of urea, melamine, and dicyandiamide;

[0017] Preferably, the swelling agent is at least one of ammonium chloride, ammonium carbonate, ammonium nitrate, and sodium carbonate;

[0018] Preferably, the catalyst is at least one of ammonium molybdate and ammonium tungstate.

[0019] In the present invention, the molar ratio of the iron source, the nitrogen source, the pyromellitic anhydride, the swelling agent and the catalyst is 1:10-15:1-2:2.5-4:0.01-0.02.

[0020] In the present invention, the temperature of the pyrolysis reaction may be 200-300°C, preferably 210-240°C.

[0021] In the present invention, the pyrolysis reaction time may be 0.5 hours or longer, and further may be 1 to 5 hours.

[0022] In the present invention, the product of the pyrolysis reaction can be washed and then calcined.

[0023] In the present invention, iron phthalocyanine is used as a precursor for calcination treatment, and the calcination temperature is further controlled. In this way, a monodisperse iron material with a special physical and chemical structure can be unexpectedly obtained. The monodisperse iron material can resist the acid corrosion of organic acids. In addition, the monodisperse iron material can achieve synergy with organic acids and light irradiation, and can unexpectedly further improve the removal efficiency, removal rate, removal selectivity and cycle stability of Cr(VI) and As(III).

[0024] Preferably, the calcination stage is carried out in a protective atmosphere, preferably at least one of nitrogen and an inert gas.

[0025] Preferably, the temperature of the calcination stage is 500-700°C, more preferably 550-650°C; studies have shown that under preferred conditions, the surface and physicochemical structure of monodisperse iron can be further modified, which helps to further improve its synergistic effect with organic acids and light assistance, and can further improve its Cr and As degradation ability.

[0026] Preferably, the calcination time is 1-3 hours.

[0027] In the present invention, the amount of monodisperse iron can be adjusted according to the pollution situation. For effect and cost considerations, it can be above 0.05 g / L, preferably 0.1-0.5 g / L, and more preferably 0.2-0.5 g / L.

[0028] In the present invention, the organic acid is a monocarboxylic acid or a polycarboxylic acid having 2 to 10 carbon atoms, preferably one of citric acid, oxalic acid, lactic acid, malic acid and tartaric acid or a mixture thereof.

[0029] In the present invention, the dosage of the organic acid can be adjusted according to the pollution situation. For effect and cost considerations, the dosage can be above 1 mM / L, preferably 1.5-5 mM / L, and more preferably 2-4 mM / L.

[0030] In the present invention, the pH of the system in the treatment stage is controlled between 2 and 10, preferably between 6 and 7.

[0031] In the present invention, the light is natural light, for example, sunlight;

[0032] Preferably, the light irradiation intensity is 500-3000 watts per square meter, further 800-1500 watts per square meter.

[0033] Preferably, the monodisperse iron solid obtained by solid-liquid separation is recycled. In the present invention, the technical process can achieve excellent cyclic processing stability.

[0034] Beneficial effects

[0035] Given the complex and difficult-to-treat nature of current heavy metal wastewater systems, conventional water treatment technologies are unable to thoroughly purify heavy metals from wastewater in a single step. Research conducted by the present invention demonstrates that the product of calcined iron phthalocyanine can unexpectedly effectively improve the removal of Cr(VI) and / or As(III). Furthermore, combining it with organic acids and light irradiation unexpectedly achieves synergistic effects, further improving the removal efficiency, effectiveness, selectivity, and recycling stability of Cr(VI) and / or As(III).

[0036] The method described in the present invention has the advantages of high efficiency, green environmental protection, simple process operation, and easy large-scale production. It has the advantages of simple preparation process, high efficiency, green environmental protection, and easy scale-up. It provides an effective solution for one-step and deep water purification of composite heavy metal waste and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is an electron microscope characterization image of the monodisperse iron catalyst in Example 1, wherein Figure 1 -a is a scanning electron microscope image of a monodispersed iron catalyst sample, with a scale of 5 μm. Figure 1 -b is a high-angle annular dark-field scanning transmission electron microscopy image of a monodisperse iron catalyst sample, with a scale of 2 nm.

[0038] Figure 2 is the X-ray crystal diffraction pattern of the monodisperse iron catalyst in Example 1.

[0039] Figure 3The monodispersed iron catalyst in Example 1 cooperates with oxalic acid to remove Cr(VI) and As(III) under conditions simulating natural light, wherein Figure 3 -a is the reduction efficiency of Cr(VI), Figure 3 -b is the oxidation efficiency of As(III), Figure 3 -c is the removal efficiency of total chromium in the solution, Figure 4 -d is the removal efficiency of total arsenic in the solution.

[0040] Figure 4 The monodisperse iron catalyst in Example 2 removes Cr(VI) and As(III) under various conditions. Figure 4 -a is the reduction efficiency of Cr(VI) at various dosages of monodisperse iron catalysts, Figure 4 -b is the oxidation efficiency of As(III) at various dosages of monodisperse iron catalysts, Figure 4 -c is the reduction efficiency of Cr(VI) at various oxalic acid dosages, Figure 4 -d is the oxidation efficiency of As(III) at various oxalic acid dosages, Figure 4 -e is the reduction efficiency of Cr(VI) at various pH values, Figure 4 -f is the oxidation efficiency of As(III) at various pH values, Figure 4 -g is the reduction efficiency of different small molecule organic acids on Cr(VI), Figure 4 -h is the oxidation efficiency of As(III) by different small molecule organic acids.

[0041] Figure 5 The monodisperse iron catalyst in Example 3 cooperates with oxalic acid to remove Cr(VI) and As(III) under conditions of simulated natural light, under the influence of multiple anions and under multiple cycles. Figure 5 -a is the reduction efficiency of Cr(VI) under the influence of different anions, Figure 5 -b is the oxidation efficiency of As(III) under the influence of different anions, Figure 5 -c is the reduction efficiency of Cr(VI) under multiple cycles of monodisperse iron catalyst and oxalic acid system, Figure 5 -d is the oxidation efficiency of As(III) under multiple cycles of the monodisperse iron catalyst and oxalic acid system.

[0042] Figure 6 The test results of each group in Example 4 are shown in FIG.

[0043] Figure 7 The test results of each group in Comparative Example 5 are shown; DETAILED DESCRIPTION

[0044] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, but the protection scope of the present invention is not limited to the following embodiment.

[0045] The present invention provides a method for removing Cr(VI) and As(III) by using monodisperse iron in combination with a small molecule organic acid, which comprises the following steps:

[0046] (1) Ferric chloride, urea, pyromellitic anhydride, ammonium chloride and ammonium molybdate are uniformly mixed in a mortar according to a certain mass ratio, and then pyrolyzed at low temperature in a muffle furnace;

[0047] (2) washing the product after low-temperature pyrolysis in step (1), removing inorganic salts and uncrosslinked organic matter in the product, and drying it in a vacuum drying oven to obtain a precursor (ferrophthalocyanine);

[0048] (3) placing the dried precursor from step (2) in a corundum magnetic boat, introducing an inert gas into a tube furnace for high-temperature pyrolysis to obtain a monodisperse iron catalyst (monodisperse iron);

[0049] (4) adding the monodisperse iron catalyst and small molecule organic acid obtained in step (3) into the wastewater containing chromium and arsenic composite heavy metals in a certain dosage, and simultaneously reducing the chromium and oxidizing the arsenic in the wastewater with the assistance of simulated sunlight.

[0050] Furthermore, the ferric chloride, urea, pyromellitic anhydride, ammonium chloride and ammonium molybdate reagents described in step (1) are all anhydrous and analytically pure, and the added mass ratios are respectively 1:10-15:1-2:2.5-4:0.01-0.02 in molar ratio.

[0051] Furthermore, the conditions for the low-temperature pyrolysis in step (1) are a pyrolysis temperature of 210-240° C., a heating rate of 1-5° C. per minute, and a pyrolysis time of 1 to 5 hours.

[0052] Furthermore, the washing in step (2) is to filter and wash the pyrolysis product in batches and multiple times using three solvents: methanol, deionized water and acetone.

[0053] Furthermore, the inert gas in step (3) is one of high-purity argon, high-purity nitrogen or high-purity carbon dioxide gas or a mixture thereof, and the gas flow rate into the tube furnace is 0.25-1.0 liters per minute.

[0054] Furthermore, the high-temperature pyrolysis (calcination) conditions in step (3) are as follows: a pyrolysis temperature of 450-750° C., a heating rate of 2-10° C. per minute, and a calcination time of 1-3 hours.

[0055] Furthermore, the small molecule organic acid in step (4) is one of oxalic acid, citric acid, lactic acid, malic acid and tartaric acid, or a mixture thereof.

[0056] Furthermore, the chromium and arsenic in step (4) are one or a mixture of hexavalent chromium (Cr(VI)) and trivalent arsenic (As(III)).

[0057] Furthermore, the sunlight in step (4) is simulated by a photocatalytic instrument through a xenon lamp, and the light radiation intensity is 500-3000 watts per square meter.

[0058] The following is a specific implementation plan:

[0059] Example 1

[0060] Step (1): Preparation of monodisperse iron catalyst:

[0061] A mixture of ferric chloride (0.5 g), urea, pyromellitic anhydride, ammonium chloride and ammonium molybdate (molar ratio: 1:12:1.5:3:0.015) was heated to 220°C (marked as temperature T1) at a heating rate of 2.0°C / min, and thermally decomposed for 3 hours. Subsequently, it was filtered and washed three times with three solvents of methanol, deionized water and acetone, and vacuum dried. Then, it was heated to 600°C (marked as temperature T2) under a high-purity argon atmosphere with a gas flow rate of 0.5 liters per minute and a heating rate of 5.0°C / min, and thermally decomposed for 2 hours to finally obtain a monodisperse iron catalyst, wherein the electron microscopy characterization is as follows Figure 1 As shown. Figure 1 -a It can be seen that the monodisperse iron catalyst exists in a layered structure similar to multiple graphene. Figure 1 The uniformly dispersed bright spots in -b show that the iron atoms in the monodisperse iron catalyst are anchored on the surface of the carbon-based support in a single dispersed form.

[0062] The monodisperse iron catalyst was characterized by X-ray crystallography to determine the presence of iron species in the catalyst. X-ray crystallography revealed that the monodisperse iron catalyst existed primarily as a graphitic carbon phase, with no peaks associated with iron species observed. This suggests that the iron was anchored in a monodisperse manner on the surface of the carbon-based support.

[0063] Step (2)

[0064] The monodispersed iron catalyst obtained in step 1 was used in conjunction with oxalic acid to conduct a photocatalytic experiment on Cr(VI) / As(III) mixed heavy metals.

[0065] Reaction conditions: monodisperse iron catalyst dosage of 0.2 g / L, oxalic acid dosage of 2 mM / L, Cr(VI) concentration of 20 mg / L, As(III) concentration of 40 mg / L, reaction volume of 100 mL, solution pH of 6.5, light irradiation intensity of 1000 W / m 2 , the reaction time is 30min. The test results are shown in Figure 3 of a monodisperse iron-oxalate-natural light group.

[0066] Comparative Example 1

[0067] Compared with Example 1, the only difference is that in step 2, the oxalic acid is missing, and the other operations and parameters are the same as in Example 1. Figure 3 Monodisperse iron + natural light group in.

[0068] Comparative Example 2

[0069] Compared with Example 1, the only difference is that in step 2, the monodisperse iron catalyst is missing, and the other operations and parameters are the same as those in Example 1. Figure 3 The oxalic acid + natural light group.

[0070] Comparative Example 3

[0071] Compared with Example 1, the only difference is that in step 2, the treatment process is carried out under light-proof conditions, and the other operations and parameters are the same as those in Example 1. Figure 3 Monodisperse iron + oxalate groups in.

[0072] Comparative Example 4

[0073] Compared with Example 1, the only difference is that in step 2, the monodisperse iron and oxalic acid are not added during the treatment process, and the other operations and parameters are the same as those in Example 1. Figure 3 Natural light group.

[0074] Figure 3 -a and 3-b show that the monodispersed iron catalyst can remove 20 mg / L of Cr(VI) and 40 mg / L of As(III) in 30 min of reaction time in synergistic effect with oxalic acid under the assistance of simulated sunlight. Figure 3 -c and 3-d show that the removal of chromium from the solution is through the oxidation of hexavalent chromium to form trivalent chromium, while the removal of arsenic is first achieved by reducing trivalent arsenic to pentavalent arsenic, and then some of the pentavalent arsenic ions are adsorbed by the monodisperse iron catalyst.

[0075] In summary, oxalic acid and natural light alone have basically no degradation ability, and the effect of dispersing iron is also limited. However, combining the three can unexpectedly achieve synergy and significantly improve the simultaneous degradation of Cr(VI) and As(III).

[0076] Example 2

[0077] This example explores the conditions for removing Cr(VI) / As(III) mixed heavy metals by using the monodispersed iron catalyst obtained in Implementation Case 1 in collaboration with natural small molecule organic acid.

[0078] Group A: Compared with Example 1, the only difference is that the amount of monodisperse iron in step 2 is changed, and the other operations and parameters are the same as Example 1; the test results of Cr(VI) and As(III) are shown in Figure 4 -a and 4-b.

[0079] Figure 4 -a and 4-b show that Cr(VI) and As(III) can be removed within 30 min at a dosage of 0.2 g / L of monodisperse iron catalyst.

[0080] Group B: Compared with Example 1, the only difference is that the amount of oxalic acid in step 2 is changed, and the other operations and parameters are the same as Example 1; the test results of Cr(VI) and As(III) are shown in Figure 4 -c and 4-d.

[0081] Figure 4 -c and 4-d show that Cr(VI) and As(III) are greatly affected by the dosage of oxalic acid, and the removal of Cr(VI) / As(III) can be achieved when the dosage of oxalic acid is higher than 2 mM / L.

[0082] Group C: Compared with Example 1, the only difference is that the pH in step 2 is changed. Other operations and parameters are the same as Example 1; the test results of Cr(VI) and As(III) are shown in Figure 4 -e and 4-f.

[0083] Figure 4 -e and 4-f show that the initial pH value of the solution has little effect on the removal of Cr(VI) / As(III) heavy metals, and the acidic environment is conducive to the removal of Cr(VI) / As(III).

[0084] Group D: Compared with Example 1, the only difference is that the type of organic acid in step 2 is changed, and the other operations and parameters are the same as Example 1; the test results of Cr(VI) and As(III) are shown in Figure 4 -g and 4-h.

[0085] Figure 4 -g and 4-h showed that different natural small molecule organic acids synergized monodispersed iron catalysts for the removal of Cr(VI) / As(III) heavy metals, among which oxalic acid, citric acid and tartaric acid were able to effectively remove Cr(VI) / As(III) within 30 min.

[0086] Example 3

[0087] This example explores the anionic effects and cyclic performance of the monodisperse iron catalyst obtained in Example 1 in combination with oxalic acid on the removal of mixed heavy metals such as Cr(VI) and As(III). Specifically, in Step 2, solid-liquid separation is performed after treatment to recover the solid catalyst, which is then used as the monodisperse iron catalyst in Step 2 and recycled for the treatment of the next batch of wastewater. During this cyclic treatment, the wastewater parameters, catalyst, oxalic acid, and light irradiation conditions in Step 2 are the same as those in Step 2 of Example 1.

[0088] Figure 5 -a and 5-b show different concentrations of HCO3 - ,NO3 - ,H2PO4 - ,SO4 2- and Cl - It has little effect on the removal of Cr(VI) / As(III) heavy metals. Figure 5 -c and 5-d show that the system can still remove more than 90% of Cr(VI) / As(III) heavy metals after 5 cycles.

[0089] Example 4 - Calcination Temperature Screening Case

[0090] Compared with Example 1, the only difference is that the temperature T2 is changed. The experimental groups are:

[0091] A: T2 is 500℃;

[0092] B: T2 is 600℃;

[0093] C: T2 is 700℃;

[0094] Other operations and parameters are the same as in Example 1;

[0095] Test results see Figure 6 .

[0096] pass Figure 6 It can be seen that the monodisperse iron catalyst prepared at temperature T2 = 600°C has the best removal effect on Cr(VI) / As(III), and the removal efficiency reaches 100% within 30 minutes.

[0097] Comparative Example 5

[0098] Compared with Example 1, the only difference is that the monodisperse iron is replaced by the following components (the amount of introduced iron element is the same as that of Example 1); the comparative groups are:

[0099] Group A: ferrous chloride was used to replace the monodisperse iron, wherein the amount of iron introduced was the same as that in Example 1;

[0100] Group B: ferric chloride was used to replace the monodisperse iron, wherein the amount of iron introduced was the same as in Example 1;

[0101] Group C: an equal amount of the monodisperse iron was replaced by a carbon skeleton (the product obtained in step 1 without adding ferric chloride), wherein the operation and parameters were the same as in Example 1;

[0102] Group D: Commercial ferrosoferric oxide was used to replace the monodisperse iron, and the amount of iron introduced was the same as in Example 1;

[0103] Group E: Commercial ferric oxide was used to replace the monodisperse iron, and the amount of iron introduced was the same as in Example 1;

[0104] Group F: Commercial titanium dioxide was used to replace the monodisperse iron, and the amount of iron introduced was the same as in Example 1;

[0105] Group G: Carbon-encapsulated iron replaces the monodisperse iron;

[0106] The preparation process of the carbon-coated iron is as follows:

[0107] 0.2 g of ferric chloride was mixed with 2 g of chitosan and then calcined in a tubular furnace. The mixture was pyrolyzed at 600 °C with a temperature increase rate of 5.0 °C per minute under a high-purity argon atmosphere for 2 hours at a gas flow rate of 0.5 liters per minute.

[0108] It can be seen that the material obtained by calcining iron phthalocyanine according to the present invention, combined with organic acid and light irradiation, can unexpectedly achieve synergy and obtain excellent Cr(VI) and / or As(III) removal effect.

Claims

1. A method for removing Cr(VI) and / or As(III) from wastewater by synergistically using monodisperse iron, organic acid and light, characterized in that: The wastewater to be treated containing Cr(VI) and / or As(III) is mixed with monodisperse iron and an organic acid, and reacted under light assistance, followed by solid-liquid separation to obtain treated water from which Cr(VI) and / or As(III) has been removed; The monodisperse iron is obtained by calcining iron phthalocyanine at a temperature of 450-750°C.

2. The method according to claim 1, wherein The wastewater to be treated contains Cr(VI) and As(III).

3. The method according to claim 2, wherein In the wastewater to be treated, the content of Cr(VI) is above 5 g / L.

4. The method according to claim 3, wherein In the wastewater to be treated, the content of Cr(VI) is above 10 g / L.

5. The method according to claim 4, wherein In the wastewater to be treated, the content of Cr(VI) is 15-50 g / L.

6. The method according to claim 1, wherein In the wastewater to be treated, the content of As(III) is above 10 g / L.

7. The method according to claim 6, wherein In the wastewater to be treated, the content of As(III) is above 20 g / L.

8. The method according to claim 7, wherein In the wastewater to be treated, the content of As(III) is 30-100 g / L.

9. The method according to claim 1, wherein The iron phthalocyanine is obtained by subjecting a mixed raw material of an iron source, a nitrogen source, pyromellitic anhydride, a swelling agent and a catalyst to a pyrolysis reaction.

10. The method according to claim 9, wherein The iron source is a water-soluble trivalent iron salt.

11. The method according to claim 10, wherein The iron source is ferric chloride.

12. The method according to claim 9, wherein The nitrogen source is at least one of urea, melamine and dicyandiamide.

13. The method according to claim 9, wherein The swelling agent is at least one of ammonium chloride, ammonium carbonate, ammonium nitrate and sodium carbonate.

14. The method according to claim 9, wherein The catalyst is at least one of ammonium molybdate and ammonium tungstate.

15. The method according to claim 9, wherein The molar ratio of the iron source, the nitrogen source, the pyromellitic anhydride, the swelling agent and the catalyst is 1:10-15:1-2:2.5-4:0.01-0.

02.

16. The method according to claim 9, wherein The temperature of the pyrolysis reaction is 200-300°C.

17. The method according to claim 9, wherein The temperature of the calcination stage is 500-700°C.

18. The method according to claim 17, wherein The temperature of the calcination stage is 550~650℃.

19. The method according to claim 17, wherein The calcination time is 1-3h.

20. The method of claim 17, wherein: The calcination stage is carried out in a protective atmosphere.

21. The method according to claim 20, wherein The protective atmosphere in the calcination stage is at least one of nitrogen and an inert gas.

22. The method of claim 1, wherein The amount of the monodisperse iron is above 0.05 g / L.

23. The method according to claim 22, wherein The dosage of the monodisperse iron is 0.1-0.5 g / L.

24. The method according to claim 23, wherein The amount of monodisperse iron used is 0.2-0.5 g / L.

25. The method of claim 1, wherein The organic acid is a unit or polycarboxylic acid having 2 to 10 carbon atoms.

26. The method of claim 25, wherein: The organic acid is one of citric acid, oxalic acid, lactic acid, malic acid and tartaric acid or a mixture thereof.

27. The method of claim 1, wherein: The amount of organic acid used is 1 mM / L or more.

28. The method of claim 27, wherein: The dosage of organic acid is 1.5~5 mM / L.

29. The method of claim 28, wherein The dosage of organic acid is 2-4 mM / L.

30. The method of claim 1, wherein The light is natural light.

31. The method of claim 1, wherein The irradiation intensity of the light is 500-3000 watts per square meter.

32. The method of claim 1, wherein The monodispersed iron solid obtained by solid-liquid separation is recycled.

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