A method for photooxidation of trivalent arsenic in sewage using biochar

Abstract

The invention discloses a method for photo-oxidizing trivalent arsenic in sewage by using biochar, and belongs to the field of sewage remediation. The method is to add biochar to sewage containing high concentration of As(III) and pH of 8.0-11.0, and expose it to visible light with a certain light intensity, stir it in the air, and oxidize As(III) in the sewage to As(V). In the invention, biochar can accelerate the activation of oxygen in the air to produce active oxygen species ROS (mainly H2O2) under the action of visible light, and at the same time stimulate the photo-oxidation of the oxidation active groups (mainly quinone groups) on the biochar, thereby accelerating the oxidation of trivalent arsenic As(III) in alkaline sewage to pentavalent arsenic As(V). Compared with the lightless condition, the oxidation effect of biochar on trivalent arsenic As(III) in alkaline sewage under the action of visible light is better. At the same time, the raw material source of the invention is wide, the economic cost is low, and it is environmentally friendly and clean; the light source can be selected from solar energy or LED; it can be applied to alkaline sewage containing high concentration of trivalent arsenic, and can also achieve in-situ oxidation, simple operation, and easier operation.

Description

Technical Field

[0001] The invention belongs to the technical field of arsenic-containing wastewater treatment, and in particular relates to a method for photo-oxidizing trivalent arsenic in wastewater by utilizing biochar. Background Art

[0002] Arsenic is one of the most common environmental pollutants in the world. It is highly toxic, carcinogenic and teratogenic, and is listed as a Class I carcinogen by the World Health Organization's International Agency for Research on Cancer (IARC). Mining development and natural weathering can lead to the dissolution of arsenic-containing minerals, produce arsenic-containing wastewater, and cause large-scale arsenic pollution. Arsenic exists in water mainly in the form of arsenite (As(III)) and arsenate (As(V)), among which trivalent arsenic As(III) has stronger solubility, toxicity and mobility than pentavalent arsenic As(V). At present, the common methods for treating arsenic-containing wastewater include adsorption, precipitation, ion exchange, membrane separation, etc. Among them, adsorption is widely used because of its simple operation, high efficiency and low cost. However, most adsorbent materials have good removal effects on As(V) but low removal effects on As(III). Therefore, oxidation pretreatment to convert As(III) into As(V) and then adsorb and remove it has become an effective way to remove As(III). Commonly used oxidants include potassium permanganate, ozone, hydrogen peroxide, ultraviolet rays and chlorine. Due to their strong oxidizing properties, ozone and hydrogen peroxide can not only oxidize As(III) but also oxidize organic matter in water to produce by-products; ultraviolet oxidation consumes a lot of energy and is not suitable for small water treatment systems; chlorine can react with organic matter during oxidation and produce chlorine-containing by-products that are harmful to the human body. For example: Patent document CN101508479A discloses a method for oxidizing trivalent arsenic in a solution, which is to add potassium permanganate to the solution to oxidize the trivalent arsenic to pentavalent arsenic. When the pH value of the solution is between 12 and 13.5, potassium permanganate solid reagent is directly added and air is introduced at the same time to oxidize the trivalent arsenic. The concentration of trivalent arsenic in the solution is 25 g / L or less. The pH range of the solution of this method is narrow, so the range of solution values ​​that can be treated is narrowed.

[0003] In recent years, biochar has attracted much attention as a new type of carbon-rich material. It is mainly obtained by pyrolyzing different types of agricultural or food waste under oxygen-limited conditions. Due to the excellent pore structure of biochar and the presence of a large number of surface functional groups on the surface, it has the ability to efficiently adsorb organic and inorganic pollutants. Carbon is also a strong light-absorbing material. Therefore, biochar has photocatalytic properties and has the ability to produce reactive oxygen species (ROS) under light. ROS plays an important role in the transformation of pollutants in the photochemical process of natural aquatic systems. In recent years, people have paid more and more attention to the photocatalytic properties of biochar. For example, patent document CN112010285A discloses a method for degrading enrofloxacin using ball-milled biochar as a photocatalyst, and patent document CN 212283424 U discloses a technical device for photocatalytic waste gas treatment using biochar. However, these studies mainly focus on the feasibility of biochar for degradation and transformation of organic pollutants under light, and there are relatively few studies on the overall role and relative importance of redox transformation of inorganic pollutants. Biochar has multifunctional components, such as redox substances, inorganic minerals, functional groups, etc., which can potentially oxidize and fix As(III). Although patent document CN 110204030B discloses a method for oxidizing trivalent arsenic in groundwater using biochar, the oxidation capacity of biochar itself for As(III) is still relatively low.

[0004] Therefore, this study provides a method for oxidizing As(III) in water by visible light photocatalytic biochar, which can quickly improve the oxidation capacity of biochar for As(III) with low energy consumption, no by-products, and no secondary pollution. Summary of the invention

[0005] In order to solve the above shortcomings and deficiencies, the purpose of the present invention is to provide a method for using visible light to photocatalyze biochar to oxidize trivalent arsenic As(III) in alkaline sewage. This method can quickly improve the oxidation of trivalent arsenic As(III) by biochar, and has low energy consumption compared to other oxidation technologies, no by-products, and no secondary pollution. Biochar can accelerate the activation of oxygen in the air to produce reactive oxygen species ROS (mainly H 2 O 2 ), and at the same time stimulate the photooxidation of the oxidative active groups (mainly quinone groups) on the biochar, thereby accelerating the oxidation of trivalent arsenic As(III) to pentavalent arsenic As(V) in alkaline wastewater.

[0006] The present invention provides a method for oxidizing trivalent arsenic As(III) in alkaline sewage by using visible light photocatalytic biochar. The method comprises the following steps: adding biochar to sewage containing As(III) with a pH value of 8.0-11.0, exposing the sewage to visible light with a certain light intensity, placing the biochar in the air, stirring the biochar, and oxidizing the As(III) in the sewage to As(V); the biochar is obtained by pyrolyzing the raw materials at 350-400°C and isolating the raw materials from oxygen for 2-4 hours, and then washing the obtained solid with deionized water.

[0007] Furthermore, in the above technical solution, the intensity of the visible light is 80-150 mW.

[0008] Furthermore, in the above technical solution, the wavelength of the visible light illumination is 350-780nm.

[0009] Furthermore, in the above technical solution, the biochar can accelerate the activation of oxygen in the air under the action of visible light to produce reactive oxygen species ROS (mainly H 2 O 2 ), and at the same time stimulate the photooxidation of the oxidative active groups (mainly quinone groups) on the biochar, thereby accelerating the oxidation of trivalent arsenic As(III) to pentavalent arsenic As(V) in alkaline wastewater.

[0010] Furthermore, in the above technical solution, the biochar is made of waste crop straw as raw material. The waste crop straw includes one or two or more of rice straw, corn straw, wheat straw and sorghum straw.

[0011] Furthermore, in the above technical solution, in the wastewater containing trivalent arsenic As(III), the concentration of trivalent arsenic As(III) is 0.1-50 mg / L.

[0012] Furthermore, in the above technical solution, the biochar is added to the sewage containing trivalent arsenic As(III), and the mass of the biochar added to each liter of the sewage containing trivalent arsenic As(III) is 0.2-2.5 g.

[0013] Furthermore, in the above technical solution, the stirring time is 3 to 24 hours.

[0014] The present invention further provides an application of biochar as a photocatalyst for oxidizing trivalent arsenic As(III) in sewage. The biochar is applied as a photocatalyst for oxidizing trivalent arsenic As(III) in sewage.

[0015] Furthermore, in the above technical solution, when the concentration of trivalent arsenic As(III) in the sewage is 10 mg / L and the pH is 11.0, the oxidation rate of biochar photooxidation reaches 100% within 4 hours.

[0016] Beneficial effects of the present invention

[0017] The biochar in the present invention can accelerate the activation of oxygen in the air under the action of visible light to produce reactive oxygen species ROS (mainly H 2 O 2 ), and at the same time stimulate the photo-oxidizability of the oxidative active groups (mainly quinone groups) on the biochar, thereby accelerating the oxidation of trivalent arsenic As(III) in alkaline sewage to pentavalent arsenic As(V). Since many industrial sectors such as chemical industry, pesticide, papermaking, and smelting will produce a large amount of high-concentration alkaline sewage, the present invention can quickly oxidize trivalent arsenic As(III) in sewage without producing other by-products, so as to avoid reaction with other pollutants and affect further treatment of sewage. In addition, the present invention uses cheap and harmless farmland straw as biochar raw material, which is convenient and easy to obtain, has low economic cost, and is environmentally friendly and clean. The light source used is a visible light source. In actual operation, a solar light source can be used or an LED power supply can be used (no ultraviolet light source is required), which is safer and more convenient. It can be applied to alkaline sewage containing a high concentration of trivalent arsenic As(III), and can also achieve in-situ oxidation, which is simple to operate and easier to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a process flow chart of biochar photo-oxidation of trivalent arsenic As(III) in wastewater.

[0019] Figure 2 It is the scanning electron microscope spectra of the biochar obtained in Examples 1 to 5 and Comparative Examples 1, 2-1, 3, and 4 under the conditions of oxygen isolation and pyrolysis at 350° C. in a muffle furnace for 4 hours.

[0020] Figure 3 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater oxidized by biochar when the pH is 8.0 in Example 1 and Comparative Example 1, respectively, under light and no light conditions.

[0021] Figure 4 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater oxidized by biochar when the pH is 9.5 under light and no light conditions, respectively, in Example 2 and Comparative Example 2-1.

[0022] Figure 5 This is the FTIR spectrum of biochar before and after reaction when the pH is 9.5 in Example 2.

[0023] Figure 6 The H generated in the biochar system under light and no light conditions when the pH value is 9.5 in Example 2 and Comparative Example 2-1, respectively. 2 O 2 content.

[0024] Figure 7 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater oxidized by biochar under light conditions when the pH value is 9.5 in Example 2 and Comparative Example 2-2, respectively, when the biochar is pyrolyzed at 350°C for 4 hours and the biochar is pyrolyzed at 400°C for 1 hour.

[0025] Figure 8 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater oxidized by biochar at pH 11.0 in the presence and absence of light in Example 3 and Comparative Example 3, respectively.

[0026] Fig. 9 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater oxidized by biochar under light conditions of pH 11.0 when the concentration of trivalent arsenic As(III) in wastewater in Examples 3, 4 and 5 is 10 ppm, 20 ppm and 30 ppm respectively.

[0027] Fig.10 Schematic diagram of the oxidation efficiency of trivalent arsenic As(III) in wastewater in comparative example 4 when the pH value is 8.0, 9.5, and 11.0, respectively, under light conditions only and without the addition of biochar. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] The present invention adds biochar to sewage with a pH value of 8.0 to 11.0 and contains high concentrations of As(III), and exposes it to visible light with a certain light intensity in the air, and stirs the reaction for several hours to oxidize As(III) in the sewage to As(V). Figure 1 Process flow chart.

[0030] Biochar can accelerate the activation of oxygen in the air under the action of visible light to produce reactive oxygen species (ROS) (mainly H 2 O 2 ), and at the same time stimulate the photooxidation of the oxidative active groups (mainly quinone groups) on the biochar, thereby accelerating the oxidation of trivalent arsenic As(III) to pentavalent arsenic As(V) in alkaline wastewater.

[0031] The sources of wastewater containing trivalent arsenic used in the following Examples 1, 2, 3, 4, 5 and Comparative Examples 1, 2-1, 2-2, 3, 4 are the same.

[0032] The biochar used in the following Examples 1, 2, 3, 4, 5 and Comparative Examples 1, 2-1, 3, 4 was prepared by the following method:

[0033] The raw material of biochar is rice straw, which is taken from a farm in Jiangsu, China. Before pyrolysis, the rice straw is repeatedly washed with deionized water, air-dried, and pyrolyzed in a muffle furnace at 350℃ for 4h under oxygen-free conditions. The generated biochar is cooled to room temperature and washed twice with deionized water. After washing, it is dried at 60℃ for 24h using a vacuum dryer. After drying, the biochar is ground in a mortar and can pass through a 2mm sieve and stored in a desiccator away from light.

[0034] Figure 2 This is the SEM image of biochar. It can be seen that the rice straw biochar has an uneven block morphology, and the block particles are small in size (about 30-120μm). Some particles are thin flakes with smooth surfaces, and there are many particles with obvious mesh structures on the surface. They should be formed by roasting the core of rice straw.

[0035] Example 1

[0036] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 8.0. The mixture was stirred at room temperature 25℃ for 12h under the condition of 500W xenon lamp (350-780nm). 2mL of samples were taken at 0h, 1h, 2h, 4h, 6h, 8h, 10h, and 12h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22μm filter. The trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0037] like Figure 3 As shown, when the pH value in sewage is 8.0, under visible light illumination, the oxidation rate of As(III) can reach 70.5% within 12 hours.

[0038] Comparative Example 1

[0039] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 8.0. The mixture was stirred at room temperature 25℃ for 12h in the dark. 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 8h, 10h, and 12h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals. The trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0040] like Figure 3As shown, when the pH of the sewage is 8.0, under dark conditions, the oxidation rate of As(III) within 12 hours is only 15.6%. Compared with Example 1, it can be seen that under the condition of pH=8.0, visible light illumination can accelerate the efficiency of biochar oxidation of As(III) in sewage.

[0041] Example 2

[0042] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 9.5. The mixture was stirred at room temperature 25℃ for 6h under the condition of 500W xenon lamp (350-780nm). 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22μm filter. The trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0043] like Figure 4 As shown, when the pH value in sewage is 9.5, under visible light illumination, the oxidation rate of As(III) can reach 95.5% within 6 hours.

[0044] Figure 5 FTIR spectra of biochar before and after reaction at pH 9.5. It can be seen from the figure that there are a large number of oxidative active groups on the surface of biochar. After the reaction, we can clearly see that the quinone group C=O peak on the surface of biochar decreases, which indicates that the quinone group C=O participates in the oxidation of As(III) during the reaction.

[0045] Figure 6 When the pH value was 9.5, the H produced in the biochar system under light and light-free conditions was 2 O 2 It can be seen from the figure that compared with dark conditions, biochar will produce more H in the reaction process under light conditions. 2 O 2 (Active oxygen), H 2 O 2 As(III) can be directly oxidized to As(V).

[0046] Comparative Example 2-1

[0047] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 9.5. The mixture was stirred at room temperature 25℃ for 6h in the dark. 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals. Atomic fluorescence (AFS) was used to analyze trivalent arsenic As(III).

[0048] like Figure 4 As shown, under dark conditions, the oxidation rate of As(III) within 6 hours is only 22.8%. Compared with Example 2, it can be seen that under pH=9.5 conditions, visible light illumination can significantly accelerate the efficiency of biochar oxidation of As(III) in wastewater.

[0049] Comparative Example 2-2

[0050] The biochar material is changed in this comparative example. The pyrolysis temperature and time of the biochar material in this comparative example are different from those of the biochar material in Example 2 in the muffle furnace. The raw material for preparing biochar is rice straw, which is taken from a farm in Jiangsu, China and is the same type of biochar material as Examples 1 to 2. Before pyrolysis, the rice straw is repeatedly washed with deionized water, air-dried, and pyrolyzed at 400°C in a muffle furnace for 1 hour under oxygen-free conditions. The biochar to be produced is cooled to room temperature and washed twice with deionized water. After washing, it is dried at 60°C for 24 hours using a vacuum dryer. After drying, the biochar is ground using a mortar and can pass through a 2 mm sieve and stored in a desiccator away from light.

[0051] The reaction conditions in the other comparative examples were the same as those in Example 2, specifically, 0.5 g of biochar was placed in 250 mL of wastewater containing 10 mg / L trivalent arsenic As(III), the pH was adjusted to 9.5, and the reaction was stirred at room temperature 25°C for 6 h under a 500 W xenon lamp (350-780 nm). 2 mL of samples were taken at 0 h, 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, and 6 h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22 μm filter head. Trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0052] like Figure 7 As shown, when the biochar material was pyrolyzed at 400°C for 1 h and the pH of the sewage was 9.5, the oxidation rate of As(III) could only reach 84.6% within 6 h under visible light illumination.

[0053] Example 3

[0054] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 11.0. The mixture was stirred at room temperature 25℃ for 6h under the condition of 500W xenon lamp (350-780nm). 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22μm filter. The trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0055] like Figure 8 As shown, when the pH value of sewage is 11.0, under visible light illumination, the oxidation rate of As(III) can reach 98.7% within 3 hours.

[0056] Comparative Example 3

[0057] 0.5g of biochar was placed in 250mL of wastewater containing 10mg / L trivalent arsenic As(III), and the pH was adjusted to 11.0. The mixture was stirred at room temperature 25℃ for 6h in the dark. 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals. Atomic fluorescence (AFS) was used to analyze trivalent arsenic As(III).

[0058] like Figure 8 As shown in the figure, under dark conditions, the oxidation rate of As(III) within 6 hours was only 71.7%. It can be seen that under pH=11.0 conditions, visible light illumination can significantly improve the efficiency of biochar oxidation of As(III) in wastewater.

[0059] Example 4

[0060] 0.5g of biochar was placed in 250mL of wastewater containing 20mg / L trivalent arsenic As(III), and the pH was adjusted to 11.0. Under the condition of a 500W xenon lamp (350-780nm), the reaction was stirred at room temperature 25℃ for 6h. 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22μm filter head. Trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0061] like Fig. 9 As shown, under light conditions, when the As(III) in the sewage is 20 mg / L, the oxidation rate of As(III) can reach 100% within 6 hours.

[0062] Example 5

[0063] 0.5g of biochar was placed in 250mL of wastewater containing 30mg / L trivalent arsenic As(III), and the pH was adjusted to 11.0. Under the condition of 500W xenon lamp (350-780nm), the reaction was stirred at room temperature 25℃ for 6h. 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, 5h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering the biochar with a 0.22μm filter head. Trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0064] like Fig. 9 As shown in the figure, under light conditions, when the As(III) in the sewage was 30 mg / L, the As(III) oxidation rate within 6 hours also reached 76.1%. It can be seen that under alkaline conditions, visible light catalytic biochar oxidation of high concentration As(III) in sewage still has a good oxidation rate.

[0065] Comparative Example 4

[0066] 250mL of wastewater containing 10mg / L trivalent arsenic As(III) was measured, and the pH was adjusted to 8.0, 9.5 and 11.0 respectively. No biochar was added, and the reaction was stirred at room temperature 25℃ for 6h under the condition of 500W xenon lamp (350-780nm). 2mL of samples were taken at 0h, 0.5h, 1h, 2h, 3h, 4h, and 6h, and 5 drops of methanol were added to eliminate the influence of the generated hydroxyl radicals after filtering with a 0.22μm filter head to separate the biochar. Trivalent arsenic As(III) was analyzed by atomic fluorescence (AFS).

[0067] like Fig.10 As shown in the figure, without adding biochar, when the pH of the sewage was 8.0, 9.5 and 11.0, the oxidation rate of As(III) within 6 hours under visible light illumination was basically 0. This shows that visible light alone can basically not oxidize trivalent arsenic in sewage, and the above biochar is used as a photocatalyst to oxidize trivalent arsenic As(III) in sewage.

[0068] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for photo-oxidation of trivalent arsenic in alkaline wastewater using biochar, characterized in that: The method comprises adding biochar to sewage containing As(III) with a pH value of 8.0 to 11.0, exposing the sewage to visible light with a certain light intensity, placing the biochar in the air, stirring the biochar, and oxidizing the As(III) in the sewage to As(V); the biochar is obtained by pyrolyzing the raw material at 350 to 400° C. and isolating the raw material from oxygen for 2 to 4 hours, and then washing the obtained solid with deionized water; The intensity of the visible light is 80mW to 150mW; The wavelength of the visible light illumination is 350-780nm; In the wastewater containing trivalent arsenic As(III), the concentration of trivalent arsenic As(III) is 0.1-50 mg / L; The biochar is added into the sewage containing trivalent arsenic As(III), and the mass of the biochar added into the sewage containing trivalent arsenic As(III) is 0.2-2.5 g per liter of trivalent arsenic As(III); The stirring time is 3 to 24 hours.

2. The method for photo-oxidation of trivalent arsenic in wastewater using biochar according to claim 1, characterized in that: The biochar is made from waste crop straws, which include one or two or more of rice straws, corn straws, wheat straws and sorghum straws.

3. The use of biochar for photo-oxidation of trivalent arsenic in wastewater according to claim 1, characterized in that: The biochar is used as a photocatalyst for the oxidation of trivalent arsenic As(III) in sewage.

4. The use of biochar for photo-oxidation of trivalent arsenic in wastewater according to claim 3, characterized in that: When the concentration of trivalent arsenic As(III) in the sewage is 10 mg / L, the oxidation rate of biochar photo-oxidation reaches 100% within 4 hours.

Images