CN111348775B

CN111348775B - Method for removing As (III) in wastewater by reinforced coagulation

Info

Publication number: CN111348775B
Application number: CN202010176798.0A
Authority: CN
Inventors: 王晓萌; 周立祥; 徐佳蕊
Original assignee: Nanjing Agricultural University
Current assignee: Nanjing Agricultural University
Priority date: 2020-03-13
Filing date: 2020-03-13
Publication date: 2022-08-26
Anticipated expiration: 2040-03-13
Also published as: CN111348775A

Abstract

The invention belongs to the field of wastewater treatment, and discloses a method for removing As (III) in wastewater by reinforced coagulation, which mainly comprises the following steps: (1) adding a certain amount of calcium peroxide into the As (III) -containing wastewater, and stirring; (2) and (4) adding an inorganic coagulant for coagulation treatment. In the treatment process, the calcium peroxide plays a role of an oxidant to convert As (III) into As (V), and the pH environment in the water body solution is improved by slow hydrolysis of the calcium peroxide in the water body solution, so that the pH value is maintained at a slightly alkaline level beneficial to coagulating sedimentation in the coagulating sedimentation process, the coagulating efficiency is improved, and the high-efficiency coagulating efficiency and the high-efficiency oxidizing efficiency are realized. The method is simple to operate, low in cost and free of additional equipment investment, and can be widely applied to advanced treatment of arsenic-containing industrial wastewater and treatment of arsenic-polluted underground water or drinking water.

Description

Method for removing As (III) in wastewater by reinforced coagulation

Technical Field

The invention belongs to the field of water treatment, and relates to a method for removing As (III) in wastewater by reinforced coagulation.

Background

Arsenic (As) and its compounds are highly toxic and are recognized As highly toxic carcinogens. Arsenic-containing groundwater has become a significant global problem, and prolonged exposure to arsenic contamination can cause a variety of cancers and other diseases. Arsenic is present in water primarily in trivalent and pentavalent forms, with as (iii) being more toxic, mobile in the environment, and difficult to remove than as (v).

At present, methods for removing As (III) mainly comprise coagulation precipitation, adsorption, membrane separation and the like, wherein the coagulation technology is very widely applied to the water and wastewater treatment process due to simple operation and low price. However, the coagulation technology has the problem of low As (III) removing efficiency in the As (III) treatment process.

In view of the above problems, researchers have developed technologies for oxidizing as (iii) to as (v) with an oxidizing agent and removing the same by adsorption or coagulation. For example, the application of Chinese patent No. 200310107197.0, published as 6/8/2005, proposes a technology of oxidizing As (III) into As (V) by hydrogen peroxide, and removing the As (III) by conventional coagulation and adsorption; the application of Chinese patent No. 200310106665.2, published as 2005, 4-month, and 27-day, proposes a method for improving the subsequent coagulation efficiency by oxidizing As (III) with potassium permanganate as an oxidizing agent; chinese patent No. 201210561013.7, an application with the publication date of 2013, 4, month and 10 proposes a method for removing As (V) by catalytically oxidizing As (III) by titanium dioxide under ultraviolet light and combining with rust adsorption; the Chinese patent application No. 2019103211100, published as 2019, 6 and 28, proposes a method for removing As (III) by catalytic oxidation of sulfite and ferric salt, and the scheme is as follows: adding sulfite and ferric salt into water containing arsenic to be treated, adjusting the pH value to 4-6, stirring for reaction, and standing at room temperature to obtain effluent. The principle of this application lies in: in a slightly acidic aerobic solution, iron ions (Fe) are utilized ³⁺ ) Can catalyze the reaction of oxygen and sulfite to generate sulfur oxygen free radical which can efficiently oxidize arsenic (As) in positive trivalent state in water ³⁺ ) Or arsenic-containing organic matter (ASA), which can efficiently convert the arsenic in the positive trivalent state into the positive pentavalent arsenic and remove the arsenic in a short time through the adsorption of iron ions.

And As the application of Chinese patent application No. 201110308990.1, published As 2013, 4, month and 17, a method for treating arsenic-containing wastewater by pre-oxidation-composite coagulation precipitation-filtration is provided, wherein under the condition that the wastewater contains trivalent arsenic, a certain amount of oxidant is added into a reaction tank for pre-oxidation to convert all the trivalent arsenic into pentavalent arsenic, a pre-oxidation step is not needed for the wastewater only containing the pentavalent arsenic, then a certain amount of ferric salt aqueous solution is added, the molar ratio of Fe/As is controlled to be 5-50, acid or alkali is added to adjust the pH to be 6-8, the mixture is rapidly stirred for 0.1-1 min to generate ferric arsenate precipitation, meanwhile, the ferric salt is hydrolyzed to generate hydroxyl iron to promote ferric arsenate coagulation and precipitation, 0.1-10 mg/L of organic polymer flocculant aqueous solution is added to rapidly stirred for 0.1-1 min, the mixture is slowly stirred for 5-10 min to precipitate for 10-30 min, and supernatant is discharged. The method of the above application converts all trivalent arsenic into pentavalent arsenic by pre-oxidizing with an oxidant, and this strategy converts trivalent arsenic into pentavalent arsenic which is more easily coagulatively precipitated to improve coagulation efficiency, although it is adjusted to a suitable pH value before coagulation to facilitate the exertion of oxidation efficiency, during coagulation, due to the hydrolysis of the coagulant, the pH of the solution in the treatment process is inevitably lowered, thereby resulting in the reduction of coagulation efficiency, for example, the coagulation efficiency is further improved by continuously adjusting the pH in the treatment process, thereby undoubtedly increasing the complexity of the treatment process and increasing the treatment cost.

In order to solve the problems in the coagulation treatment of As (III), a simple and effective method for removing As (III) by reinforced coagulation is urgently needed to be developed, the efficiency of the coagulation treatment of As (III) is improved, the cost is effectively saved, and the method has very important significance for ensuring the water environment safety.

Disclosure of Invention

1. Problems to be solved

Aiming at the problem of low efficiency of coagulation treatment of As (III) in the prior art, the invention provides a method for removing As (III) by utilizing calcium peroxide to strengthen coagulation, on one hand, the calcium peroxide plays a role of an oxidant to convert As (III) into As (V), on the other hand, the calcium peroxide is slowly hydrolyzed in a water body solution to improve the pH environment in the water body solution, so that the pH value in the coagulation precipitation process is maintained at a slightly alkaline level beneficial to the coagulation precipitation, and the subsequent coagulation efficiency is improved.

Further, aiming at the problems that the input amount of the oxidant is increased easily and the cost is overhigh in the mode of completely converting As (III) into As (V) by using a strong oxidant in the prior art under the condition of high content of As (III), the method of the invention ensures that the oxidation rate of calcium peroxide to As (III) is lower than 100 percent in the pre-oxidation process, namely, the As (III) is not completely converted into As (V), and the proper oxidation degree is matched with the adjustment of calcium peroxide to pH value, so that the high-efficiency coagulation efficiency can be achieved, and the cost is effectively saved.

2. Technical scheme

In order to solve the problems, the technical scheme adopted by the invention is as follows:

the invention provides a method for removing As (III) by reinforced coagulation, which comprises the following steps:

1) adding a certain amount of calcium peroxide into the As (III) -containing wastewater, and stirring for reaction for a period of time to oxidize As (III) by the calcium peroxide;

2) adding a certain amount of inorganic coagulant, stirring for reaction, and standing for precipitation.

According to the method, on one hand, calcium peroxide plays a role of an oxidant and converts As (III) into As (V), on the other hand, calcium peroxide is slowly hydrolyzed in a water body solution, the pH environment in the water body solution is improved (the pH of the solution is increased to 7-10 after the calcium peroxide is hydrolyzed), the pH value in the coagulating sedimentation process is maintained at a slightly alkaline level beneficial to coagulating sedimentation, and the subsequent coagulating efficiency is improved.

The method has the advantage of adjusting the pH of the solution, and is more suitable for treating the water containing the acidic and neutral arsenic.

In addition, the process of oxidizing As (III) by calcium peroxide and the process of removing As by coagulation simultaneously (oxidation and coagulation are synchronously generated), the invention utilizes the slow hydrolysis effect of calcium peroxide in water body, not only effectively adjusts the pH value of the solution in the coagulation process, but also can continuously oxidize As (III) which is not coagulated and removed into As (V) in the coagulation process, thereby achieving the purpose of continuously improving the coagulation efficiency. Under the condition, the coagulation has high As removing efficiency, the generated floccule is large and the sedimentation is fast, and the As content in the supernatant reaches the standard specified by China and the international health organization.

As a further improvement of the invention, the oxidation rate of the As (III) by the calcium peroxide in the step 1) is lower than 100%.

The method of the invention ensures that the oxidation rate of the As (III) by the calcium peroxide is lower than 100 percent in the pre-oxidation process, namely the calcium peroxide is not completely converted into the As (V), so the calcium peroxide can be continuously oxidized in the subsequent coagulating sedimentation process.

As a further improvement of the invention, the oxidation rate of the calcium peroxide to As (III) in the step 1) is kept between 32 and 85 percent. Preferably from 42% to 82%.

As a further improvement of the invention, the adding concentration of the calcium peroxide is 15-100 mg/L, and/or the adding concentration of the inorganic coagulant is 50-400 mg/L.

As a further improvement of the present invention, the inorganic coagulant comprises an iron-based and/or titanium-based coagulant.

As a further improvement of the invention, the inorganic coagulant comprises any one or a combination of ferric chloride, ferric sulfate, polymeric ferric chloride, polymeric ferric sulfate, titanium tetrachloride, titanium sulfate, polymeric titanium chloride and polymeric titanium sulfate. Preferably polymeric ferric sulphate.

As a further improvement of the invention, when the inorganic coagulant is titanium polychloride, the adding amount of the calcium peroxide is preferably 40-50 mg/L.

As a further improvement of the invention, the pH value of the wastewater containing As (III) is in the range of 4-9, and/or the concentration of As (III) in the wastewater is 0.1-1.0 mg/L. The preferable pH range is 4-7.

As a further improvement of the invention, the method comprises the following specific steps:

1) adding a certain amount of calcium peroxide into the wastewater containing As (III), and stirring at the rotating speed of 200-500 rpm for 2-5 min to oxidize part of As (III) in the solution into As (V);

2) adding a certain amount of inorganic coagulant, quickly stirring for several minutes, slowing down the stirring speed, continuously stirring for a certain time, and standing to obtain supernatant effluent.

As a further improvement of the invention, in the step 2), the rapid stirring speed is 200-500 rpm, the rapid stirring time is 3-5 min, the slow stirring speed is 40-80 rpm, the slow stirring time is 10-30 min, and the standing time is 20-60 min.

3. Advantageous effects

Compared with the prior art, the invention has the beneficial effects that:

(1) the method for removing As (III) in wastewater by reinforced coagulation disclosed by the invention has the advantages that calcium peroxide plays a role of an oxidant in the treatment process, and As (III) is converted into As (V), and calcium peroxide is slowly hydrolyzed in a water body solution, so that the pH environment in the water body solution is improved, the pH value in the coagulation precipitation process is maintained at a slightly alkaline level beneficial to coagulation precipitation, and the subsequent coagulation efficiency is improved.

(2) The method for removing As (III) in wastewater by reinforced coagulation simultaneously generates the process of oxidizing As (III) by calcium peroxide and the process of removing As by coagulation (synchronously generates oxidation and coagulation), so the method not only effectively adjusts the pH value of the solution in the coagulation process by utilizing the slow hydrolysis effect of the calcium peroxide in the water body, but also continuously oxidizes As (III) which is not removed by coagulation into As (V) in the coagulation process, thereby achieving the purpose of continuously improving the coagulation efficiency.

(3) According to the method for removing As (III) in wastewater by enhanced coagulation, calcium peroxide is added and stirred for reaction, so that the oxidation rate of As (III) by calcium peroxide is lower than 100%, and all As (III) is not converted into As (V), therefore, the As (III) can be continuously oxidized by calcium peroxide in the subsequent coagulating sedimentation process.

(4) Compared with the prior art, the method for removing As (III) in wastewater by reinforced coagulation has the characteristics of remarkable economy, universality, high efficiency, simple and convenient operation and the like. The adopted calcium peroxide as an oxidant has no toxicity, and the selected coagulant is generally applicable to a water treatment process. The method does not need to additionally increase treatment equipment, does not change the original treatment process of a water plant, and can be applied in a large scale.

(5) The method for removing As (III) in wastewater by reinforced coagulation can effectively promote the improvement of coagulation efficiency under different initial pH conditions, different As (III) content conditions and different types of inorganic coagulants, has universal applicability and stability, and is beneficial to popularization.

Drawings

FIG. 1 shows the direct coagulation of titanium sulfate with CaO under different initial pH conditions ₂ Comparing the As (III) removal efficiency by pre-oxidation reinforced coagulation;

FIG. 2 shows the direct coagulation of titanium sulfate with CaO at different initial As (III) concentrations ₂ Comparing the As (III) removal efficiency by pre-oxidation reinforced coagulation;

FIG. 3 shows direct coagulation and CaO in different titanium tetrachloride dosages ₂ Comparing the As (III) removal efficiency of the pre-oxidation reinforced coagulation;

FIG. 4 shows different CaO ₂ Directly coagulating and CaO mixing the titanium polychloride under the condition of dosage ₂ Comparing the As (III) removal efficiency by pre-oxidation reinforced coagulation;

FIG. 5 shows CaO ₂ The removal efficiency of As is compared and the pH value of the solution is changed in the pre-oxidation reinforced coagulation and calcium hydroxide alkali-regulating coagulation processes;

Detailed Description

The invention is further described with reference to specific examples.

Example 1

Step 1), adding 15mg/L of CaO into a water sample with the content of As (III) of 1.0mg/L and the initial pH values of 4, 5, 6, 7, 8 and 9 respectively ₂ Continuously stirring at the rotating speed of 200rpm for 5 min; after completion, CaO was measured ₂ The oxidation rate of as (iii) was measured, and CaO was added at an initial pH of 4, 5, 6, 7, 8, and 9, respectively ₂ The oxidation rates for As (III) were 32%, 34%, 51%, 63%, 72% and 85%, respectively. Namely CaO in step 1) ₂ Pre-oxidation (incomplete oxidation) of As (III) is generated.

Step 2), respectively adding 50mg/L titanium sulfate into the water sample treated in the step 1), stirring at the rotating speed of 200rpm for 5min, adjusting the rotating speed to 40rpm, continuously stirring for 10min, standing for 20min, and then measuring the concentration of residual arsenic in the supernatant.

Meanwhile, setting a direct coagulation comparison experiment aiming at each pH value water sample, namely performing the coagulation experiment of the step 2) under the same condition, and comparing CaO under the same condition ₂ The As removal efficiency in the pre-oxidation reinforced coagulation and direct coagulation processes is improved. Titanium sulfate direct coagulation and CaO under different initial pH conditions ₂ As shown in figure 1, the pre-oxidation reinforced coagulation pair As (III) removal efficiency pair is shown in figure 1, and the method disclosed by the invention has the advantages that the titanium sulfate coagulation removal efficiency is obviously improved, particularly under the condition of initial pH 4-7, the titanium sulfate coagulation removal efficiency is higher, although CaO under the condition ₂ The oxidation rate of As (III) is obviously lower than that of the alkaline condition. The results of this example show that by adding calcium peroxide as a coagulation enhancing reagent in advance, pre-oxidation of as (iii) and removal of as (v) by coagulation can be performed simultaneously, and slow hydrolysis of the added calcium peroxide can provide a suitable pH environment for the coagulation precipitation, and can also continuously oxidize as (iii) in the subsequent coagulation precipitation process, and at the same time, high coagulation efficiency and oxidation efficiency can be achieved. In addition, the method can achieve the aim of improving the coagulation efficiency without completely realizing 100 percent of oxidation rate of As (III), effectively saves reagents and reduces cost.

Example 2

Step 1), adding 20mg/L CaO into a water sample (pH 5.0) containing As (III) ₂ Continuously stirring at the rotating speed of 250rpm for 2.5 min;

and 2) adding 80mg/L of titanium sulfate coagulant into the water sample, stirring at the rotating speed of 400rpm for 3min, adjusting the rotating speed to 60rpm, continuously stirring for 20min, standing for 30min, and determining the residual arsenic concentration in the supernatant.

Five groups of parallel experiments with initial As (III) concentrations of 0.1, 0.2, 0.5, 0.8 and 1.0mg/L were set according to the above experimental procedures, and CaO in each group was measured after the completion of step 1) ₂ Oxidation rate of As (III); the results were: CaO at initial As (III) concentrations of 0.1, 0.2, 0.5, 0.8 and 1.0mg/L, respectively ₂ As (III) was oxidized at 81%, 82%, 62%, 42% and 35%, respectively.

Meanwhile, a direct coagulation comparison experiment is set for five groups of parallel experiments, namely, the coagulation experiment of the step 2) is only carried out under the same condition, and CaO is compared under the same condition ₂ The removal efficiency of As in the pre-oxidation reinforced coagulation and direct coagulation processes is improved. Comparison ofThe effect is shown in fig. 2, and can be obtained as follows: under the conditions of different initial As (III) concentrations, the method can obviously improve the As removal efficiency of titanium sulfate without completely realizing the total oxidation of As (III), and is stable and effective in treating water samples with different As (III) concentrations.

Example 3

Step 1), adding 100mg/L CaO into a water sample (pH 7.0) with As (III) content of 1.0mg/L ₂ Continuously stirring at 500rpm for 2min, and measuring CaO after the step 1) is finished ₂ The rate of oxidation of As (III) was 82%.

And step 2), adding a titanium tetrachloride coagulant into the water sample, stirring at the rotating speed of 500rpm for 3min, adjusting the rotating speed to 80rpm, continuously stirring for 30min, standing for 40min, and measuring the concentration of the residual arsenic in the supernatant.

Aiming at the steps, five groups of parallel experiments with titanium tetrachloride adding concentrations of 50, 100, 200, 300, 400 and 500mg/L are set, and meanwhile, comparison experiments are set in the five groups of parallel experiments respectively, namely, only the coagulation experiment in the step 2) is carried out under the same conditions. Comparative CaO under identical conditions ₂ The removal efficiency of As in the pre-oxidation reinforced coagulation and the direct coagulation process is compared, the comparison result is shown in figure 3, and the result shows that: even if the adding amount of the coagulant is increased, the As (III) removing efficiency of more than 80 percent cannot be achieved through a single coagulation process, and the method can obviously reduce the using amount of the coagulant and obviously improve the As (III) removing capacity.

Example 4

Step 1), adding a certain amount of CaO into a water sample (pH 7.0) with the As (III) content of 1.0mg/L ₂ Continuously stirring at the rotating speed of 350rpm for 3.5 min;

and 2) adding 200mg/L of polymeric titanium chloride coagulant into the water sample, stirring at the rotating speed of 300rpm for 4min, adjusting the rotating speed to 60rpm, continuously stirring for 20min, standing for 60min, and measuring the residual arsenic concentration in the supernatant.

Setting CaO aiming at the steps ₂ Five groups of parallel experiments with the adding concentrations of 15mg/L, 30mg/L, 50mg/L, 70mg/L and 100mg/L are respectively arranged aiming at the five groups of parallel experimentsComparative experiment, i.e. only the coagulation experiment of step 2) was performed under the same conditions.

After the step 1) is finished, CaO in each group is measured ₂ As (III) was oxidized, and as a result, it was found that: in the presence of CaO ₂ CaO is added in the amount of 15mg/L, 30mg/L, 50mg/L, 70mg/L and 100mg/L respectively ₂ The oxidation rates for As (III) were 32%, 44%, 56%, 71% and 82%, respectively. Different CaO ₂ Directly coagulating and CaO by adding the poly-titanium chloride ₂ As (III) removal efficiency of pre-oxidation enhanced coagulation pair As is shown in FIG. 4, as can be seen from FIG. 4, CaO ₂ After pretreatment, the removal efficiency of the titanium polychlorid to As is obviously improved, and the CaO is lower ₂ The addition amount can obviously improve the coagulation efficiency, and the As (III) is not required to be completely oxidized into As (V) when the same treatment effect is achieved.

Example 5

Step 1), adding 40mg/L CaO into a water sample (pH 4.0) with the As (III) content of 1.0mg/L ₂ Continuously stirring at the rotating speed of 200rpm for 5 min; after the step 1) is finished, CaO is measured ₂ The As (III) oxidation rate was 42%.

And 2) adding 100mg/L of ferric salt coagulant into the water sample, stirring at the rotating speed of 400rpm for 3min, adjusting the rotating speed to 50rpm, continuously stirring for 30min, standing for 40min, and determining the concentration of residual arsenic in the supernatant.

Aiming at the steps, five groups of parallel experiments are set, wherein the types of the added ferric salt coagulants are ferric chloride, ferric sulfate, polymeric ferric chloride and polymeric ferric sulfate respectively; under the same conditions, CaO ₂ As the removal efficiency of As in the pre-oxidation enhanced coagulation and direct coagulation processes is shown in the following table 1, CaO can be obtained for different inorganic coagulant types ₂ The treatment shows obvious effect of strengthening arsenic removal, and the type of the coagulant has certain influence on the As removal treatment efficiency of the synchronous oxidation coagulation process.

TABLE 1 direct coagulation of four iron salt coagulants with CaO ₂ Comparison of As (III) removal rate by pre-oxidation enhanced coagulation

Comparative example

Step 1), 30mg/L of CaO is added into water samples with the initial pH values of 4, 5, 6, 7, 8 and 9 and As (III) content of 1.0mg/L ₂ Continuously stirring at the rotating speed of 250rpm for 3 min; after completion, CaO was measured ₂ As (III) oxidation rate, the results are: CaO at an initial pH of 4, 5, 6, 7, 8, 9 ₂ As (III) was oxidized at 35%, 37%, 53%, 65%, 75% and 88%, respectively.

For step 1), a comparison group added with calcium hydroxide to adjust coagulation is set for each pH aqueous solution, and Ca (OH) is used in the comparison group ₂ Replacement of CaO ₂ The other conditions were the same.

And 2) adding 70mg/L titanium sulfate into the water sample, stirring at the rotating speed of 300rpm for 3min, adjusting the rotating speed to 60rpm, continuously stirring for 20min, standing for 30min, and determining the residual arsenic concentration in the supernatant.

CaO of the present invention ₂ Comparison of As removal efficiency and pH change of the solution in the pre-oxidation enhanced coagulation and the coagulation with the addition of calcium hydroxide As shown in FIG. 5 shows that Ca (OH) ₂ Replacement of CaO ₂ Then, the pH of the solution is adjusted to be alkaline, the pH conversion degree of the solution is the same, but the removal efficiency of the synchronous oxidation reinforced coagulation process to As (III) is still higher than that of the coagulation condition after the alkali adjustment, which shows that the CaO of the invention ₂ The strengthening coagulation treatment process is far larger than the improvement effect of adjusting the pH value to the As removal rate through the alkali liquor, the treatment process of the invention relates to synchronous oxidation and coagulation processes, and the high-efficiency removal efficiency can be achieved without completely realizing the complete oxidation of As (III).

Claims

1. A method for removing As (III) in wastewater by reinforced coagulation is characterized in that: the method comprises the following steps:

1) adding a certain amount of calcium peroxide into the As (III) -containing wastewater, and stirring for reaction to oxidize As (III) by the calcium peroxide; the pH value of the As (III) -containing wastewater is 4-9, and/or the concentration of As (III) in the wastewater is 0.1-1.0 mg/L; the oxidation rate of calcium peroxide to As (III) in the step 1) is kept at 32-85%; the adding concentration of the calcium peroxide is 15-100 mg/L;

2) adding a certain amount of inorganic coagulant, stirring for reaction, standing for precipitation, and/or adding the inorganic coagulant with the concentration of 50-400 mg/L.

2. The method for removing As (III) in wastewater by reinforced coagulation as claimed in claim 1, wherein: the inorganic coagulant comprises an iron-based coagulant and/or a titanium-based coagulant.

3. The method for removing As (III) in wastewater by enhanced coagulation according to claim 2, which is characterized in that: the inorganic coagulant comprises any one or combination of ferric chloride, ferric sulfate, polymeric ferric chloride, polymeric ferric sulfate, titanium tetrachloride, titanium sulfate, polymeric titanium chloride and polymeric titanium sulfate.

4. The method for removing As (III) in wastewater by reinforced coagulation as claimed in claim 3, wherein: when the inorganic coagulant is titanium polychlorid, the adding amount of calcium peroxide is 40-50 mg/L.

5. The method for removing As (III) in wastewater by reinforced coagulation as claimed in claim 4, wherein: the method comprises the following specific steps:

1) adding a certain amount of calcium peroxide into the As (III) -containing wastewater, and stirring at the rotating speed of 200-500 rpm for 2-5 min to oxidize part of As (III) in the solution into As (V);

2) adding a certain amount of inorganic coagulant, quickly stirring for a certain time, slowing down the stirring speed, continuously stirring for a certain time, and standing for a certain time to obtain supernatant effluent.

6. The method for removing As (III) in wastewater by reinforced coagulation as claimed in claim 5, wherein: in the step 2), the rapid stirring speed is 200-500 rpm, the rapid stirring time is 3-5 min, the slow stirring speed is 40-80 rpm, the slow stirring time is 10-30 min, and the standing time is 20-60 min.