CN113023949A

CN113023949A - Method for removing hexavalent chromium through filtration and reinforcement by catalytic reduction coupling membrane

Info

Publication number: CN113023949A
Application number: CN202110286327.XA
Authority: CN
Inventors: 赵晟锌; 孔星宇; 陈忠林; 高珊; 沈吉敏; 康晶; 王斌远; 恩乔亚·噢萨
Original assignee: Harbin Institute of Technology
Current assignee: Harbin Institute of Technology
Priority date: 2021-03-17
Filing date: 2021-03-17
Publication date: 2021-06-25
Anticipated expiration: 2041-03-17
Also published as: CN113023949B

Abstract

A method for removing hexavalent chromium by filtration and reinforcement of a catalytic reduction coupling membrane belongs to the technical field of water treatment. The method comprises the following steps: adding NaBH into the mixed solution with the initial pH of less than 3.07 under the condition of continuous stirring at 650rpm₄A powder wherein the temperature is kept constant within 23-27 ℃ resulting in a reduction of Cr (VI); continuously stirring, stopping carrying out the reduction reaction for 5min, and standing for 15 min-24 h to obtain a heterogeneous solution; and filtering through an MCE membrane. The invention adopts a two-step method for removing Cr (VI) in a Cr (VI) -Ox coexisting system: reduction self-precipitation treatment and filtration treatment. Common Fe (III) and Al (III) are selected as catalysts, the cost is low, and NaBH can be obviously enhanced under the non-strong acid condition₄The reduction effect on high-concentration Cr (VI) and the natural precipitation effect of Cr (III) in a Cr (III) -Ox coexisting system (suitable concentrations of Fe (III) and Al (III) ensure that the TCr removal rate is between 98.49 and 99.90 percent).

Description

Method for removing hexavalent chromium through filtration and reinforcement by catalytic reduction coupling membrane

Technical Field

The invention belongs to the technical field of water treatment, and particularly relates to a method for removing hexavalent chromium by filtration and reinforcement of a catalytic reduction coupling membrane.

Background

Chromium is one of important strategic materials, and is widely applied to industries such as metallurgy, chemistry, electroplating, wood preservation, leather tanning and the like due to the excellent characteristics of hardness, wear resistance, high temperature resistance, corrosion resistance and the like. The toxicity of chromium is related to the valence state, and the chromium in the wastewater mainly exists in Cr (VI) and Cr (III), wherein Cr (VI) not only has high toxicity, but also has extremely strong mobility, so that the Cr (VI) is reduced into Cr (III) and then is removed by a precipitation method, which is generally considered as the best recovery of Cr (VI), and is also a practical treatment process commonly used for the wastewater of Cr (VI). However, with the continuous development and updating of surface treatment technology, a large number of organic compounds are widely used in various industries, including natural organic ligands such as oxalic acid, citric acid, tartaric acid, formic acid, etc., and synthetic organic ligands such as EDTA, various surfactants, pharmaceutical intermediates, etc. These organics and Cr (III) can form polynuclear coordination compounds in water, thus becoming stable Cr (III) complex wastewater, hindering the conversion of Cr (III) to solid phase, and the standard discharge of chromium is difficult to achieve by adopting an alkaline precipitation method, which provides a challenge to the traditional chemical precipitation treatment of Cr (VI).

The influence of the organic matters on the treatment of the chromium-containing wastewater is mainly shown as follows: firstly, organic ligands with reducing ability (such as oxalic acid, citric acid, tartaric acid and other organic matters containing alpha-hydroxyl) and added reducing agents can generate oxidation-reduction reaction with Cr (VI), but the rate of reducing Cr (VI) by the organic ligands with reducing ability is very slow; and secondly, the organic ligand usually contains N or O atoms capable of providing lone pair electrons, and can form a metal organic complex with Cr (III) generated by reduction or coexisting cations, so that the precipitation process of Cr (III) is influenced. At present, many researches are carried out on the influence of organic matters on the Cr (VI) reduction effect, Mario et al consider the influence of organic reducing agents commonly existing in nature, such as alpha-hydroxycarboxylic acid, alpha-carbonyl carboxylic acid, phenols and ascorbic acid, and complexing agents, such as EDTA and acetylacetone, on the reduction of Cr (VI) by zero-valent iron, and the results show that the reduction rate of Cr (VI) is improved by nearly 50 times at the highest by the organic reducing agents under the condition of low initial concentration and pH of Cr (VI). Liu et al showed that the activity and durability of zero-valent iron could not be effectively improved in the presence of citric acid, and that the Cr (VI) reduction rate could be significantly increased only by introducing photo-excitation. However, most of their studies only consider the strengthening effect of organic molecules on the reduction of cr (vi) by the reducing agent, and do not consider the subsequent removal of cr (iii). The residual cr (iii) will be converted to cr (vi) in certain circumstances, which is not negligible. Therefore, it is only the actual elimination of Cr (VI) contamination that the residual Cr (III) is removed from the wastewater. Therefore, it is necessary to combine the Cr (VI) reduction and the total Cr (TCr) removal for a new redox system consisting of Cr (VI) -organic compound-reducing agent.

The treatment of industrial waste water containing cr (vi) -organic compounds is a long-standing concern in the environmental field. Oxalic acid and its salts are commonly used in many industrial processes such as chemical synthesis, pharmaceuticals, wood preservation, cleaning and treating of metal surfaces, textile printing, leather processing, and metallurgy, to name a few. Due to its wide application in industrial processes, it may coexist with cr (vi) in various wastewaters of wood preservation, dyeing and plating, etc. Oxalic acid and its salts, although relatively simple in structure, contain-COO^-The functional group, O atom, can provide lone pair electrons to produce complexation with Cr (III). Therefore, the treatment of industrial wastewater containing Cr (VI) -oxalic acid is a long-standing concern.

In the past, it was found that the coexisting system of Cr (VI) -oxalate (Ox) is treated with NaBH₄After treatment, the reduction rate and TCr removal rate of Cr (VI) are closely related to the concentration of Ox, and high concentration of Ox (more than 1mM) has strengthened NaBH₄The effect of reducing Cr (VI), but the low concentration (less than 1mM) of Ox not only has no obvious effect on the reduction synergy of Cr (VI), but also strongly influences the turbidity of the system, so that the generated Cr (III) is difficult to precipitate, thereby influencing the removal of TCr.

Disclosure of Invention

The invention aims to solve the problem that hexavalent chromium in industrial wastewater is difficult to remove, and provides a method for removing hexavalent chromium by filtration and reinforcement through a catalytic reduction coupling membrane.

NaBH₄As a common strong reducing agent, acids and metal salts or complexes thereof can be used as homogeneous catalysts to accelerate the hydrolysis thereof. Under the catalysis of Fe or Al, NaBH is adopted₄A single system with high concentration of Cr (VI) in the treated water is feasible, and the generated Cr (III) can be removed by a natural sedimentation method in a short time. Therefore, aiming at the problem of removing Cr (VI) in a low-concentration Ox and Cr (VI) coexisting system, the invention develops Fe and Al catalytic NaBH₄The method for synchronously removing turbidity and dirt by micro-membrane filtration efficiently reduces Cr (VI) and solves the problem of Cr (III) residue.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a method for removing hexavalent chromium by filtration and reinforcement through a catalytic reduction coupling membrane comprises the following steps:

the method comprises the following steps: adding NaBH into the mixed solution with the initial pH of less than 3.07 under the condition of continuous stirring at 650rpm₄Powder, wherein the temperature is kept constant within 23-27 ℃, resulting in the reduction of hexavalent chromium; controlling the molar concentration ratio of hexavalent chromium to oxalate, ferric iron, trivalent aluminum and silicate in the mixed solution to be 1.92 mM: 0 to 0.40 mM: 0 to 3.0 mM: 0 to 3.0 mM: 0 to 3.0 mM;

step two: continuously stirring, stopping carrying out the reduction reaction for 5min, and standing for 15 min-24 h to obtain a heterogeneous solution containing suspended matters (SS);

step three: and filtering through an MCE membrane.

Compared with the prior art, the invention has the beneficial effects that: the invention adopts a two-step method for removing Cr (VI) in a Cr (VI) -Ox coexisting system: reduction self-precipitation treatment and filtration treatment. Common Fe (III) and Al (III) are selected as catalysts, the cost is low, and NaBH can be obviously enhanced under the non-strong acid condition₄The reduction effect on high-concentration Cr (VI) and the natural precipitation effect of Cr (III) in a Cr (III) -Ox coexisting system (suitable concentrations of Fe (III) and Al (III) ensure that the TCr removal rate is between 98.49 and 99.90 percent). The introduction of Fe (III) and Al (III) increases the size of Cr (III) suspended substances, and Cr (III) can be effectively and almost completely retained by the micro-filtration membrane. The filtration treatment can effectively further remove the suspended Cr (III) concentration, and ensure that the TCr concentration is below 1 mg/L. The introduction of Fe (III) and Al (III) can make more Ox stay in the liquid phase, which is beneficial to the reuse of the Ox.

Drawings

FIG. 1 is a graph of the effect of Ox concentration on Cr (VI) conversion;

FIG. 2 is a graph showing the effect of Ox concentration on TCr removal in a multi-component mixed solution by natural sedimentation for 15 min;

FIG. 3 is a graph showing the effect of Ox concentration on turbidity;

FIG. 4 is a graph showing the effect of self-precipitation of Fe, Al and Si;

FIG. 5 is a graph showing the effect of concentration of Ox on the natural settling of TCr in a multi-component mixed solution for 15min, followed by filtration through a microfiltration membrane having a pore size of 0.45 μm;

FIG. 6 is a graph showing the comparison of the concentrations of Fe, Al and Si, all at 2.0 mM;

FIG. 7 is NaBH₄Graph of the effect of dosing on turbidity of the system;

FIG. 8 is NaBH₄Graph of the effect of dosing on system endpoint pH;

FIG. 9 is NaBH₄Influence diagram of dosage on TCr removal in each component mixed solution;

FIG. 10 is a graph of the effect of Fe ion concentration on Cr (VI) conversion and TCr removal;

FIG. 11 is a graph of the effect of Al ion concentration on Cr (VI) conversion and TCr removal;

FIG. 12 is a graph of the effect of Si ion concentration on Cr (VI) conversion and TCr removal;

FIG. 13 is a graph of the effect of ion concentration on turbidity;

FIG. 14 is a graph of the effect of settling time on TCr removal;

FIG. 15 is a graph of the effect of settling time on turbidity;

FIG. 16 is a graph of the effect of precipitation time on Fe, Al and Si precipitation rate;

FIG. 17 is Y₁An internal residual map of the response model;

FIG. 18 is Y₃An internal residual map of the response model;

FIG. 19 is Y₁An outer residual map of the response model;

FIG. 20 is Y₃An outer residual map of the response model;

FIG. 21 is Y₁Response model Fe and Al interaction diagram;

FIG. 22 is Y₁Interaction diagram of response models Fe and Ox;

FIG. 23 is Y₁Interaction diagram of response models Al and Ox;

FIG. 24 is Y₃Response model Fe and Al interaction diagram;

FIG. 25 is Y₃Response model Fe and Al interaction diagram;

FIG. 26 is Y₃Response model Fe and Al interaction diagram;

FIG. 27 is a graph showing the change in suspended state of Fe and Cr in the suspension in 20 experiments;

FIG. 28 is a graph showing the change in suspended Al and Cr in the suspension in 20 experiments;

FIG. 29 shows the yield of H under the influence of Fe₂A change curve along with time and a first-order dynamics model diagram thereof;

FIG. 30 shows the yield H under the influence of Al₂A change curve along with time and a first-order dynamics model diagram thereof;

FIG. 31 shows H production under the influence of Si₂A change curve along with time and a first-order dynamics model diagram thereof;

FIG. 32 is NaBH₄Coupling ofA change chart of turbidity of the mixed solution of each component after membrane filtration treatment;

FIG. 33 is a graph showing the particle size distribution of the suspension in each heterogeneous system;

FIG. 34 is NaBH₄Carrying out coupling membrane filtration treatment on a TCr diagram in the multi-component mixed solution;

FIG. 35 is an infra-red spectrum of a suspension; wherein (a) suspended matter from a Cr system; (b) suspensions from the Ox-Cr system; (c) suspensions from the Si-Ox-Cr system; (d) suspensions from the Fe-Ox-Cr system; (e) suspensions from the Al-Ox-Cr system;

fig. 36 is a graph showing the change in the concentration of dissolved Ox.

Detailed Description

The technical solution of the present invention is further described below with reference to the drawings and the embodiments, but the present invention is not limited thereto, and modifications or equivalent substitutions may be made to the technical solution of the present invention without departing from the spirit of the technical solution of the present invention, and the technical solution of the present invention is covered by the protection scope of the present invention.

Aiming at the problem of difficult Cr (VI) treatment in a Cr (VI) -Ox coexistence system, the method takes the cations Fe and Al as catalysts and optimizes the NaBH catalysis by utilizing Center Combination Design (CCD)₄Reducing and removing Cr (VI), and coupling with microfiltration membrane filtration technology to realize complete interception of suspended Cr (III), thereby achieving the purpose of synchronously removing turbidity and dirt and further strengthening the removal of TCr.

The method is used for removing TCr in a Cr (VI) and Ox coexisting solution system. The factors of Ox concentration, Fe (III) concentration and Al (III) concentration on NaBH₄Removing the influence of Cr (VI) in Cr (VI) -Ox, taking the concentration of Ox, Fe (III) and Al (III) as main factors, and establishing NaBH by adopting a central combination design method (CCD)₄And a prediction model for removing the TCr is removed, the turbid and sewage simultaneous removal effect of a subsequent microfiltration membrane is investigated, and the reason for strengthening the removal of the TCr by the catalytic chemical reduction coupling membrane filtration is clarified. The results show that low concentrations (0.3mM) of Fe (III), Al (III) vs. NaBH₄The reduced Cr (VI) has weak strengthening effect, and because of the existence of Ox, the reduced Cr (III) forms high-turbidity suspended substances and cannot be naturally precipitated, so that the removal of TCr is reduced to almost zero. High concentration (2-3 mM) of Fe (III), Al (III) on NaBH₄The reduced Cr (VI) has stronger strengthening effect, most of the generated Cr (III) can be removed by a natural precipitation method, but a small amount of residual Cr (III) still exists in the solution, and the residual Cr (III) can be almost completely removed by combining with a microfiltration membrane, because suspended matters formed by Fe (III) -Cr (III) or Al (III) -Cr (III) have larger grain diameter and are easy to be intercepted by the microfiltration membrane. The development of a novel catalytic chemical reduction/membrane filtration combined method is beneficial to reducing the retention of TCr in the environment.

The first embodiment is as follows: the embodiment describes a method for removing hexavalent chromium by filtration and reinforcement through a catalytic reduction coupling membrane, which comprises the following steps:

the method comprises the following steps: adding NaBH into the mixed solution with the initial pH of less than 3.07 under the condition of continuous stirring at 650rpm₄A powder wherein the temperature is kept constant within 23-27 ℃ resulting in a reduction of Cr (VI); controlling the molar concentration ratio of hexavalent chromium to oxalate, ferric iron, trivalent aluminum and silicate in the mixed solution to be 1.92mM (100 mg/L): 0 to 0.40 mM: 0 to 3.0 mM: 0 to 3.0 mM: 0 to 3.0 mM; the pH is preferably less than 3, and the treatment effect is relatively better;

step two: continuously stirring, stopping carrying out reduction reaction for 5min, and standing for 15 min-24 h to obtain a heterogeneous solution containing SS;

step three: and filtering through an MCE membrane.

The second embodiment is as follows: in a specific embodiment, a method for removing hexavalent chromium by filtration and reinforcement through a catalytic reduction coupled membrane includes the step one in which NaBH is added₄The mixing ratio of the powder to the mixed solution is 0.01-0.10 g: 550 mL.

The third concrete implementation mode: in the third step, the diameter of the MCE membrane is one or more of 1 μm,0.45 μm and 0.1 μm, and when a plurality of MCE membranes are used for filtration, the MCE membranes can be used in sequence.

Example 1:

aiming at the removal of Cr (VI) in 550mL of 100mg/L Cr (VI) -Ox mixed solution, NaBH is adopted₄The coupled membrane filtration method is carried out by the following four steps.

Firstly, the alloy containing Cr (VI) (100mg/L), SiO₃ ^2-Pouring the mixed solution of (Si) (0.3mM) and Ox (0-0.30 mM) into a 1L double-neck flat-bottom flask, adding Fe (III) and Al (III) to form a multi-component mixed solution, and adjusting the pH to 3.05 +/-0.02. Wherein the molar concentration ratio of Cr (VI) to Ox, Fe (III), Al (III) and Si is 1.92 mM: 0-0.30 mM: 1.65 mM: 1.65 mM: 0.3 mM.

Secondly, the magnetic stirrer is turned on, the rotating speed is set to be 650rpm, and a certain amount of NaBH is added into the multi-component mixed solution₄The powder (0.10g) was stirred for 5min and Cr (VI) was reduced, wherein the temperature of the reaction system was kept constant at 25. + -. 2 ℃.

And thirdly, standing for 15min after stopping stirring, and finishing the flocculation and precipitation processes.

And finally, obtaining a suspension containing a small amount of SS on the upper layer after the floc is precipitated, and filtering the suspension by using a 0.1-micron MCE membrane. The removal rate of TCr is between 98.49% and 99.90%, and the reduction product Cr (III) is almost removed by 100%.

The following is a specific implementation process of the invention:

a mixture of insoluble small solid particles larger than 100nm suspended in a liquid is called a suspension. The concentrations of TCr, total Fe (TFe), total Al (TAl) and Total Si (TSi) in the suspension were determined by ion-coupled plasma emission spectrometry (ICP-OES). Prior to measurement, the acidity was adjusted to ensure that the suspended Cr, Fe, Al and Si were completely dissolved prior to the assay. The concentration of residual cr (vi) was determined spectrophotometrically at 372nm using a method based on the stable and fixed molar absorption coefficient of cr (vi) by itself in an alkaline environment. The conversion concentration of Cr (VI) is calculated based on the difference in the concentrations of residual Cr (VI) and initial Cr (VI) in the same phase. The concentration of Cr (III) in the aqueous solution was calculated from the difference in the concentrations of TCr and Cr (VI) in the liquid phase, and filtration treatment was carried out using a Mixed Cellulose Ester (MCE) microporous membrane filter having a pore size of 0.1 μm before the measurement. The measurement of TCr, TFe and TAl in the dissolved state was carried out by the ICP-OES method, and the measurement was carried out by filtration using a Mixed Cellulose Ester (MCE) microporous membrane filter having a pore size of 0.1. mu.m. The suspension and the filtrate obtained after filtration were subjected to turbidity measurement using a turbidimeter. The functional group measurements of the solid samples were characterized by FTIR using the conventional KBr tabletting method.

1. Effect of Ox concentration on Cr removal and System turbidity

Mixing Fe (III), Al (III) and silicate (represented by Si) with Cr (VI) -Ox to obtain a mixed solution, and adding 0.1g of NaBH₄The treatment was carried out, and FIG. 1 shows the effect of the concentration of Ox on the conversion of Cr (VI) in each mixed solution. As can be seen, the conversion rates of Cr (VI) in the different mixed solutions are not very different. However, when the concentration of Ox was 0.20mM or more, the removal rate of TCr in each mixed solution was low (FIG. 2). It is known that Cr (VI) is difficult to remove by flocculation and precipitation, and therefore, the low removal rate of TCr indicates that Cr (III) obtained by reduction has poor natural precipitation effect. When the concentration of Ox in the mixed solution is 0.10mM, the mixed solution with Fe (III) or Al (III) has the TCr removal rates of 38.05 percent and 41.52 percent respectively, which are respectively less than 51.67 percent and 54.33 percent of the control group (the mixed solution without Ox), and the Cr (III) also has poor self-precipitation effect.

The reason why cr (iii) has a poor self-settling effect is that it participates in the formation of a suspension that is difficult to settle in each system in which Ox is present. The turbidity of each system is shown in FIG. 3. When the concentration of Ox in the mixed solution is 0.2mM or more, the turbidity of the system with 0.2mM Fe or 0.2mM Al is higher than that of the other two systems, and the high turbidity is probably attributed to the fact that Fe (III) or Al (III) is also in suspension, as can be seen from the precipitation rate of Fe (III) or Al (III) (FIG. 4), in combination with the phenomenon that Cr (III) is not precipitated, thereby forming Cr (III) -Fe (III) -Ox or Cr (III) -Al (III) -Ox multi-complex which is difficult to precipitate.

After each suspension was filtered through a 0.45 μm microfiltration membrane (FIG. 5), the removal of TCr was improved to various degrees, that is, Cr (III) which did not settle naturally could be retained by the 0.45 μm microfiltration membrane to various degrees. In particular in systems in which Fe (III) or Al (III) is present, the TCr removal is almost identical to the Cr (VI) conversion. However, in systems where no fe (iii) or al (iii) is present, the retained cr (iii) gradually decreases as the Ox concentration in the system increases.

In conclusion, 0.30mM Fe (III), Al (III) or Si has no significant promoting effect on the conversion of Cr (VI) in Cr (VI) -Ox, and can not effectively assist the precipitation of Cr (III), but rather, Fe (III) or Al (III) is involved in the suspension to increase the turbidity of the system, thereby bringing the TCr removal to a lower level. The micro-filtration membrane can solve the problem of Cr (III) removal.

2、NaBH₄Effect of dosage on Cr removal and System turbidity

Adding NaBH with different mass into four mixed solutions containing Cr (VI) -Ox₄Powder, heterogeneous system obtained by continuous stirring for 5min, natural sedimentation treatment, FIG. 6 shows NaBH₄Effect of dosing on TCr removal. For a single solution in which only Cr (VI) is present, NaBH₄The increase in (b) did not significantly improve the TCr removal, which means that NaBH was used for the reduction of Cr (VI)₄The utilization rate gradually decreases. The reason is that there is an excess of NaBH₄Participate in the side reaction of hydrolysis. In the mixed solution with Fe or Al, NaBH is carried out₄When the adding amount is increased to more than 0.06g, the increasing trend of the TCr removal rate is slowed down, and the highest removal rates are 83.13 percent and 89.55 percent respectively, relative to the same NaBH₄Dosed mixed solution system without Fe or Al present, NaBH₄The utilization rate is improved. And the mixed solution with Si is subjected to NaBH₄After the treatment, TCr was hardly removed, and it was found that 2.0mM of Si was not effective in promoting NaBH, like Fe or Al, when Ox was present together with Cr (VI)₄And removing the TCr.

The turbidity of each heterogeneous system is shown in figure 7. Comparison of identical NaBH₄The turbidity of each mixed solution system was highest in the presence of 2.0mM Si, and their turbidity varied with NaBH₄The dosage is increased and gradually reduced (67.5 to 45NTU), the pH of the end point of each solution of the system is 7.1 to 9.2 (figure 8), Cr (III) is easy to form hydroxide precipitate, but Si, Cr (III) obtained by reduction and Ox form a suspension with higher turbidity due to the existence of Si and Ox. In the presence of Fe or Al, when NaBH₄When the amount of the addition is 0.06g or more, the turbidity can be kept at a low level.

The removal of TCr in each system is shown in FIG. 9, which is a result of subjecting the suspension obtained by the natural sedimentation to membrane filtration. TCr in the Ox-Cr (VI) mixed solution is gradually increased to the maximum of 25.13-44.10 percent from the almost unremovable TCr before being filtered, but is still lower than the same NaBH₄An amount of a single solution of Cr (VI) free of Ox is added. The yield of Cr (III) in Cr (VI) solution is known to be influenced by NaBH₄The effect of the amount of addition and the presence or absence of 0.2mM Ox was small. For both Ox-Cr (VI) and Cr (VI) solutions, the Cr (III) production was nearly identical and there was a difference in TCr removal, indicating that the presence of Ox was detrimental to the filtration removal of Cr (III). However, TCr removal of the Ox-Cr (VI) mixed solution is accompanied by NaBH₄The increase in dosage tends to be gradual, which may be related to the end-point pH, as the end-point pH increases, cr (iii) in the presence of Ox is more easily filtered. The TCr in the multi-component mixed solution of Si-Ox-Cr (VI) is increased to 41.00-47.19 percent from the almost unremovable state before filtration. With NaBH₄The adding amount is increased from 0.02g to 0.10g, and the removal fluctuation of TCr is small, which can indicate that a suspension system with Si exists, and the interception effect of the 0.45 mu m microfiltration membrane on Cr is not greatly influenced by the end point pH (7.1-9.2). When NaBH₄When the amount of the compound is 0.06g or more, the multi-component mixed system containing Fe or Al is excellent in TCr removal performance after filtration (more than 89.61% and 92.75%, respectively). And NaBH₄The reason why the addition amount was 0.04g or less and the TCr removal rate was low may be related to the fact that the end point pH was in the acidic range (FIG. 8).

3. Effect of Fe, Al and Si concentrations on Cr removal and System haze

Fe (III), Al (III) or Si with different dosages are respectively added into the Cr (VI) -Ox mixed solution, the Cr (VI) solution without Ox is taken as a control group, and the conversion rate and the TCr removal rate of each system Cr (VI) are shown in figures 10-12. With increasing Fe (III) addition (FIG. 10), the conversion of Cr (VI) in both Fe-Ox-Cr (VI) and Fe-Cr (VI) mixed solutions increased gradually, and Al also showed similar behavior (FIG. 11). The Cr (VI) and Cr (VI) -Ox solutions with the same Fe content have similar Cr (VI) conversion rate. However, from the TCr removal result, when the amount of Fe added is 0.3-1 mM, the difference between the TCr removal rate and the Cr (VI) conversion rate in the Ox-Cr (VI) mixed solution is large, which indicates that the produced Cr (III)It was possible to remove them completely from the solution, i.e. Ox affected the sedimentation of cr (iii), which is also evidenced by the high turbidity they exhibit (fig. 13). The two solutions of Cr (VI) and Cr (VI) -Ox with the same Al content, wherein the Al content is between 0.3 and 1mM, and the conversion rate of Cr (VI) in the solution containing Ox is slightly larger than that of Cr (VI) solution without Ox, which shows that Ox can cooperate with Al to promote NaBH₄The conversion to Cr (VI), but this synergistic acceleration of Ox becomes weaker with increasing Al addition. For Al (FIG. 11) added in 2-3 mM, both in Cr (VI) solution and in Ox-Cr (VI) solution, the TCr removal rate and Cr (VI) conversion rate are very close, and the solution system can keep low turbidity (FIG. 13). Thus, high Al loadings with enhanced NaBH₄The Cr (VI) is reduced, and the Cr (III) precipitation is promoted and the turbidity of the system is reduced, which seems to be irrelevant to the existence of 0.2mM Ox.

For each solution system with Si present (fig. 12), the Si concentration had a greater effect on the removal of TCr. In the Cr (VI) solution system without Ox, the Cr (III) precipitate generated in the Cr (VI) solution with low content of Si (0.3mM) is hardly influenced by Si, but the turbidity gradually increases along with the increase of the Si concentration, and the Si is shown to be more and more unfavorable for the precipitation of the Cr (III), although part of the Cr (III) can be removed by self-precipitation. The Cr (VI) solution system in the presence of 0.2mM Ox did not precipitate the Cr (III) formed, regardless of the Si concentration. And because of the presence of Si, the Ox-cr (vi) mixed system forms a suspension with lower turbidity than the cr (vi) only system, i.e. cr (iii) is more prone to conversion to soluble cr (iii).

4. Effect of settling time on TCr removal and turbidity

The above experimental results show that the Ox-Cr (VI) is subjected to NaBH₄The heterogeneous system obtained after the treatment was allowed to settle for 15min, the system was in a high turbidity state due to the presence of suspended matter, while the presence of low amounts (0.3mM) of Fe, Al or Si resulted in a higher turbidity of the system. FIGS. 14 to 15 show four kinds of multicomponent mixed solutions of Ox-Cr (VI), Fe-Ox-Cr (VI), Al-Ox-Cr (VI), and Si-Ox-Cr (VI), respectively, which are processed by NaBH₄After the treatment, the turbidity of each system generally decreased toward the lower level in the course of the natural sedimentation process, as can be seen from the graphThe potential, TCr, after 16h of removal, tends to stabilize. The TCr in the Si-Ox-Cr (VI) solution system is hardly removed within 24h, and the TCr removal rate is gradually increased for the other three multi-component mixed solution systems, which indicates that the Cr (III) is gradually settled. In the initial 2h, the sedimentation rate of Cr (III) is the fastest in the Ox-Cr system with Al, the sedimentation rate of Cr (III) is the fastest in the Ox-Cr system without Fe and Al for 4-6 h, the sedimentation rate of Cr (III) is the fastest in the solution with Fe for 6-12 h. FIG. 16 shows the precipitation of Fe, Al and Si, similar to the TCr removal trend in the same solution system, thus illustrating that slow co-precipitation of Fe (III) and Cr (III), Al (III) and Cr (III) occurs. In the past, it was found that Fe and Al contribute to the rapid precipitation of the formed cr (iii), and it was found that the reason for limiting the rapid precipitation of Fe or Al and cr (iii) is the co-existing Ox in the solution. Based on the above results, it was found that it is difficult to rapidly precipitate the formed Cr (III) in a short time by the self-precipitation alone.

5. Center combination design and evaluation of Cr removal effect under multi-factor coexistence

In order to establish the optimal experimental conditions and prediction model for removing TCr by three main factors, the interaction among Fe, Al and Ox on NaBH is researched₄The influence of removing TCr by a one-step method reveals the relationship among Cr, Fe, Al and Ox, a response surface method based on Central Combination Design (CCD) is selected, the ratio of removing TCr and suspended Cr by natural precipitation is taken as a research index, and three important factors of Fe concentration (A), Al concentration (B) and Ox concentration (C) are taken as variables after all other test conditions are determined. The experiments were performed at five equidistant levels, encoded as-1.682, -1, 0, 1 and 1.682. Table 1 shows the relationship between the actual value and the encoded value of each factor. Table 2 shows the natural sedimentation (one-step method, Y)₁) And natural sedimentation in combination with membrane filtration (two-step process, Y)₂) The following experiments with the removal of TCr gave a total of 20 experiments. The difference between two syndromes (Y)₁，Y₂) Representing the ratio of Cr in suspension (Y)₃)。

5.1 regression model building and testing

The results of the tests were analyzed using two tests, one being the sum of squares of the sequence models (Table 3) and the other being statistics of the summary of the models(table 4) in order to select the correct mathematical model of the two responses from the different linear, interactive, quadratic and cubic models. Examination of the two reactions showed that the linear and interactive (2FI) models had higher p-values and lower determinants (R) than the quadratic model²). Despite the R of the cubic model²The values are higher, but the model built with it was found to be out of shape and not suggested for use. Therefore, a quadratic model was chosen to adequately describe these three variable pairs Y₁And Y₃The influence of (c). In addition, R of two responses²And adjusted R²All values are close to 1 (Y)₁0.98 and 0.97, respectively, Y₃0.93 and 0.86, respectively), predicted R²Value 0.8932 (Y)₁) And 0.79 (Y)₃) Respectively with adjusted R²Value 0.97 (Y)₁) And 0.86 (Y)₃) The two quadratic models are matched, so that the two quadratic models have good fitting performance, and the observed value and the predicted value have high correlation. That is, the two models fit well with experimental data, and experimental results in the research range can replace actual experiments with regression equations.

The assumptions of normality and independence of the error terms in the two mathematical models were verified. The data points on the graph are very close to straight lines (FIGS. 17-18), and the conclusion of normal distribution of the data can be drawn. It can be observed in fig. 19-20 that the errors in the two response variables are independent of each other, independent of the order in which the experimental tests were performed. Analysis of variance (ANOVA) was used to estimate the statistical significance of the developed secondary models by F-test. As can be seen from tables 5 and 6, the two response surface regression equations have significant statistical significance, and thus two regression models are suitable. Furthermore, a "loss of fit p value" greater than 0.05 indicates that the loss of fit is not significant relative to the "pure error" of the design point for the replicate experiment (6 experiments), Y is not significant₁And Y₃The "missing F-fit value" of (a) is such a large possibility that it occurs due to noise as 10.38% and 93.13%, respectively.

Tables 5-6 also describe the linear, interactive and quadratic effects of these factors on the TCr removal and suspended Cr ratios. Statistical analysis of the TCr removal response shows that fe (iii) and al (iii) concentrations are very significant linear terms, and their interactions and quadratic terms are also very important. There are five significant terms for the suspended Cr fraction. Where Ox is the only meaningful linear term. Both the two quadratic and interaction terms of fe (iii) concentration and al (iii) contribute to the presentation of a statistically very significant effect. In addition, the interaction term of the concentration of Fe (III) with the concentration of Ox is an important factor. Based on the quadratic model, a quadratic polynomial is used as a functional relationship between the actual factors and the predicted response, as shown in equations (1) to (2).

TCr removal Rate (%)

＝39.11+32.38A+40.16B-58.06C-10.90AC+8.78AC+2.68BC-4.27A²-6.23B²+64.60C² (1)

Suspended state Cr (%)

＝0.86+1.16A-3.71B+45.82C+1.56AB-7.06AC-3.72BC-0.72A²+0.61B²-50.42C² (2)

TABLE 1 actual and coded values of the three factors

TABLE 1 Experimental results for three independent variables from a Central Combination Design (CCD)

a TCr removed from natural sedimentation; b, removing TCr in MCE membrane filtration with the aperture of 0.1 mu m after natural sedimentation; c from Y₂And Y₁The ratio of the suspended Cr is obtained from the difference.

TABLE 3 sum of squares of sequence models

TABLE 4 statistics of model rollups

TABLE 5 response surface Y₁Results of the analysis of variance of

Source	Sum of squares	Degree of freedom	Mean square	F value	P value
							Model (model)	796.86	9	88.54	67.95	<0.0001	**
Concentration of A-Fe	211.38	1	211.38	162.22	<0.0001	**
							Concentration of B-Al	325.2	1	325.2	249.57	<0.0001	**
Concentration of C-Ox	2.88	1	2.88	2.21	0.1682
							AB	389.07	1	389.07	298.58	<0.0001	**
AC	1.42	1	1.42	1.09	0.3212
							BC	0.13	1	0.13	0.1	0.7563
A²	107.42	1	107.42	82.44	<0.0001	**
							B²	228.89	1	228.89	175.66	<0.0001	**
C²	0.78	1	0.78	0.6	0.4572
							Residual	13.03	10	1.3
LackofFit	10.05	5	2.01	3.38	0.1038
							PureError	2.98	5	0.6
CorrectionTotal	809.89	19

Very significant:, p <0.01, significant:, 0.01< p <0.05, not significant: p >0.05

TABLE 6 response surface Y₃Results of the analysis of variance of

Source	Sum of squares	Degree of freedom	Mean square	F value	P value
							Model (model)	18.58	9	2.06	14.04	0.0001	**
Concentration of A-Fe	0.033	1	0.033	0.22	0.646
							Concentration of B-Al	0.12	1	0.12	0.83	0.3839
Concentration of C-Ox	3.04	1	3.04	20.68	0.0011	**
							AB	7.94	1	7.94	54.01	<0.0001	**
AC	0.92	1	0.92	6.24	0.0315	*
							BC	0.26	1	0.26	1.74	0.2167
A²	3.06	1	3.06	20.82	0.001	**
							B²	2.19	1	2.19	14.87	0.0032	**
C²	0.47	1	0.47	3.23	0.1025
							Residual	1.47	10	0.15
LackofFit	0.28	5	0.056	0.23	0.9313
							PureError	1.19	5	0.24
CorrectionTotal	20.05	19

5.2 contour plot analysis of the effects of Fe, Al and Ox concentrations on TCr removal and suspended Cr fraction

The interaction between the three factors affecting the TCr removal rate and the suspended Cr ratio is shown in the two-dimensional contour plots of FIGS. 21-23. The contour plot of FIG. 21 shows the interaction of Fe concentration and Al concentration on TCr removal, while the third variable (Ox concentration) is fixed at zero level (0.20 mM). The results show that when the Fe concentration is in the central region of Al concentration, the removal rate of TCr reaches the highest point in the central region of Al concentration. In addition, when the concentration of Fe is high, the concentration of Fe should not be too high, so that a good effect of removing TCr can be obtained. Similarly, when the Al concentration is set to a higher level, the Fe concentration should not be set too high. When the Al concentration was at a zero level (1.65mM), the interaction between the Fe concentration and the Ox concentration was as shown in the contour diagram of FIG. 22. When the amount of Fe added was set in the center region, the removal rate of TCr decreased with the increase in Ox concentration, indicating that the increase in Ox concentration was detrimental to the removal of TCr. When Ox is fixed in the central region, the removal rate of TCr increases and then decreases as the Fe concentration increases. The results showed that in the low concentration region, the removal of TCr was promoted with the gradual increase in Fe concentration, while in the high concentration region, the removal of TCr was suppressed. The interaction of Al concentration and Ox concentration on TCr removal is shown in fig. 23. Their interaction has a similar effect on TCr removal as Fe and Ox concentrations.

The influence of the interaction among the three factors on the proportion of the suspended Cr is analyzed. As can be seen from the contour diagram (fig. 24) describing the interaction of the Fe concentration and the Al concentration, when the Fe concentration is set in the central region, the proportion of suspended Cr decreases first and then increases as the Al concentration increases. Therefore, the Al concentration can decrease the proportion of Cr in suspension at low concentration, and can increase the proportion of Cr in suspension at high concentration. The Al concentration was fixed at zero level (fig. 25), and when the Fe concentration was in the central region, the proportion of Cr in the suspended state increased with the increase in the oxygen concentration; when the concentration of Ox is higher, the proportion of suspended Cr is increased and then decreased along with the increase of the concentration of Fe. Further, the contour plot of fig. 26 shows the effect of the interaction between Al concentration and Ox concentration on the suspended Cr ratio. The lower proportion of suspended Cr occurs in the low Ox concentration region, while the Fe concentration is in the central region.

The molar masses of the Fe, Al and Cr in the suspension state in 20 groups of experiments are detected, as shown in fig. 27-28, and it can be known that the Cr (III) in the suspension state has a certain concomitant relationship with the Fe or Al, and the change trends are basically consistent, so that the removal of the Fe or Al in the suspension state can be inferred to be beneficial to the removal of the Cr (III) in the suspension state.

6. Potential mechanism of influence of Ox on TCr removal

6.1 NaBH under the influence of Fe, Al and Si₄Hydrolysis kinetics of

NaBH₄The autohydrolysis rate is very slow in the absence of catalyst. NaBH₄Possible mechanisms of autohydrolysis are described in equations 3-7. As shown in equation 4, the reduction of the number of protons in the environment adversely affects the hydrolysis reaction, and if the number of protons participating in the reaction increases, NaBH can be prevented₄Produce H₂A slowing of the rate. However, the pH of the system was artificially controlled without the use of buffer solutions during hydrolysis, and in fact, what we observed was that with NaBH₄The increase in hydrolysis time, the increase in alkalinity in the solution, and this experimental phenomenon can be explained by equation 7: tetrahydroxyborate (B (OH)) produced as a by-product of formula 6₄ ^-Can be used as reactant to generate tetraborate ion (B)₄O₇ ^2-) (equation 7), this process consumes 2 protons, which gradually shifts the reaction solution to alkalinity. Generation of base to make H in solution⁺The reduction of ions makes step 2 the limiting step.

As shown in FIGS. 29 to 30, NaBH is not added to the solution containing Fe (III) or Al (III)₄The hydrolysis was maintained only by the initially supplied acidity (pH 3), and a constant hydrolysis rate was maintained for the initial 1min, but as the reaction time progressed, the reaction rate decreased, which was related to the above-mentioned phenomenon of pH rise in the solution. Addition of Fe (III) and Al (III) to NaBH₄Hydrolysis to produce H₂The rate is greatly improved. In the kinetic model, NaBH is described by a first order kinetic model₄The hydrolysis kinetics behavior of (a). For H within 1min₂The yield data is fitted, and the first order kinetic fitting result shows that H is produced along with the increase of the concentration of Fe (III) or Al (III)₂The rate constant gradually increases. The added metal can be used as a Lewis acid center, and the Lewis base can be easily absorbed. Thus, OH in solution^-The molecules can be absorbed by the Fe or Al region by electron donor, which slows OH^-The limitation of step 2 is further increased by increasing NaBH₄The rate of the hydrolysis reaction, which is also responsible for the rapid reduction of cr (vi). Presence of Si to NaBH₄The hydrolysis rate had no significant effect (FIG. 31), and thus had no significant effect on the conversion of Cr (VI). As shown in the inset, the linear regression was good with correlation coefficients greater than 0.99 at different Fe or Al concentrations (mM), i.e. within the initial 1min, the first order kinetics followed a good linear regression. Although the reactions within 1-5 min can still be well described by first order kinetic equations, their rate constants are lower than those of the first 1 min. This may be associated with the catalytic action of Fe (III) or Al (III) being lost in the subsequent 4min, while NaBH₄Hydrogen is generated only by autohydrolysis. Although the reactions within 1-5 min can still be well described by a first order kinetic equation, the rate constants of the reactions are reduced compared with those within the first 1 min. This may be associated with the disappearance of the catalytic action of Fe or Al after 4min, NaBH₄The hydrogen is produced by self-hydrolysis.

The method comprises the following steps:

step two:

step three:

step four:

step five:

6.2 Effect of interception by Fe, Al and Si

All four suspensions were passed sequentially through 1 μm,0.45 μm and 0.1 μm microfiltration membranes, showing a decrease in turbidity before and after membrane filtration in the same solution system (FIG. 32), indicating that different sized suspensions can be retained by different sized microfiltration membranes. In the case of the Ox-Cr solution system in which Fe (III) or Al (III) exists, most of the suspended matter is retained by the 1 μm membrane filter, and the turbidity is reduced to 0.68NTU and 0.58NTU respectively by the last stage of membrane filtration (0.1 μm) (FIG. 32). However, for the system without Fe (III) or Al (III), i.e., Ox-Cr and Si-Ox-Cr, most of the suspended matter was retained on the 0.1 μm microfiltration membrane, and filtered through the last stage microfiltration membrane (0.1 μm), and the turbidity was reduced to 1.79NTU and 7.08NTU, respectively.

The above-mentioned size distribution of the suspension was further confirmed by the results of the particle size distribution (FIG. 33, which shows that NaBH was used₄In the suspension obtained after treatment of Si-Ox-Cr (VI), suspensions of sizes smaller than 0.1 μm are present, and in the Ox-Cr (VI) system suspensions of sizes smaller than 0.1 μm are also present, the median particle size of the latter being slightly larger than that of the former, which is why the turbidity is still relatively large after filtration through a 0.1 μm filter. And via NaBH₄The suspension obtained after the treatment of Fe-Ox-Cr (VI) and Al-Ox-Cr (VI) is mainly composed of suspended substances with the size of more than 0.45 μm.

By detecting the Cr content in each filtrate (fig. 34), the Cr (III) in the suspension with Fe or Al can be basically removed after being filtered by a 1 μm microfiltration membrane, and the obtained filtrate continues to sequentially pass through 0.45 μm and 0.1 μm microfiltration membranes, so that the TCr removal is not obviously improved, thereby indicating that when Fe or Al exists, Cr (III) mainly exists in large-size (larger than 1 μm) suspended matters, which is beneficial to the Cr (III) being intercepted by a large-pore-size membrane to be removed. In the absence of Fe or Al, a part of Cr (III) forms small-sized suspended matters even completely dissolved due to the existence of Ox, which is attributed to the complexation of Cr (III) ions with Ox to form a surface mixed valence dimer complex bridged by Ox, and the internal electron transfer of the complex leads to the better solubility of oxides. The presence of Si causes some of the cr (iii) to form smaller particles. The small size of the suspension will not contribute to the total retention of Cr (III) by the microfiltration membrane, although 0.1 μm microfiltration membranes can remove most of the Cr (III).

6.3 identification of solid product

From the results of the elemental analysis (table 7), it is understood that the content of C element is small in the system in which Fe or Al exists, and the content is the lowest in the system in which Al exists. Since the element C is derived from Ox, the content of C can be used as an indication of the content of Ox, and the element H is derived mainly from OH^-Can be used as an index OH^-And (4) content. From the C/H ratio, it is found that the Ox ratio is decreased and OH is decreased in the suspended matter due to the presence of Fe and Al^-The ratio is increased, which may be in the presence of Fe or Al, OH^-The substitution of the moiety Ox by the ligand which is more cationic. The stability constant of the Fe (III) -OH complex is greater at all levels than that of the Fe (III) -Ox complex, lg beta at 25 deg.C₁、lgβ₂And lg beta₃Values of 11.87, 21.17 and 29.67, respectively, and lg beta of the Fe (III) -Ox complex₁、lgβ₂And lg beta₃The values of (a) are 9.4, 16.2 and 20.2, respectively. As does Al.

Infrared spectroscopic analysis was performed after drying the suspended matter trapped on the membrane. The infrared spectra (b) to (e) of the four suspensions are shown in FIG. 35. The suspensions from the Ox-Cr (VI) and Si-Ox-Cr (VI) systems have similar IR spectra, while Fe-Ox-Cr (VI) and Al-Ox-Cr (VI) have similar IR spectra. Compared with the system without Ox, absorption peaks ascribed to Ox were found in all four suspensions, and the positions of the absorption peaks were 1708cm each^-1，1685cm^-1，1407cm^-1，1275cm^-1And 805cm^-1. Since the content of Ox in the solid cannot be accurately and quantitatively analyzed by infrared, the content of Ox is semi-quantitatively analyzed, i.e. 1483cm in the same infrared spectrum^-1As reference peak (only belonging to Cr (OH))₃·xH₂O)，1275cm^-1And 805cm^-1The intensity of the absorption peak at (Ox only) is divided by the intensity of the reference peak to obtain a specific value, and the specific value is obtained by countingAs a result of comparison of the values, 1275cm of Ox was found to be present in the suspended matter containing Fe or Al^-1And 805cm^-1The absorption peak becomes weak. Al is similar to that of Fe. This means that in the suspension with Fe and Al present, the Ox content is reduced relative to the Cr content. This result is mutually corroborated with the results of the elemental analysis experiments (Table 7).

TABLE 7 analysis results of elements in suspension of each system

Name (R)	Cwt(％)	Hwt(％)	C/Hratio
				Suspensions of the Ox-Cr (VI) system	2.80±0.01	3.322±0.088	0.843±0.027
Suspension of Fe-Ox-Cr system	1.62±0.01	3.387±0.005	0.478±0.005
				Suspensions of the Al-Ox-Cr system	1.46±0.02	3.661±0.040	0.397±0.001
Suspensions of the Si-Ox-Cr system	2.79±0.01	3.315±0.083	0.842±0.025

6.4 identification of Ox in liquid phase

To clarify the presence of Ox in NaBH₄The reduction of Cr (VI) and the subsequent Cr (III) self-precipitation, and the change of the dissolved state Ox in the self-precipitation time of 24h was monitored by ion chromatography (FIG. 36). The concentration of Ox varies in three systems within 0.25-24 h of precipitation.

At the initial time of precipitation, there is a minimum concentration of dissolved Ox in the three heterogeneous systems, and at subsequent times, the dissolved Ox gradually increases, i.e., a portion of Ox undergoes precipitation and re-dissolution. Ox is known to be very hydrophilic, and its precipitation is due to its ability to combine with a portion of the suspended cations that tend to convert to a precipitate, forming an insoluble complex. Insolubilization is achieved on the one hand by the tendency of the cations in the complex to convert themselves to precipitate and on the other hand by the fact that the amount of hydrophilic Ox is not sufficient to complex all the cations in solution. The hydrophobic end of the cation in the complex and the hydrophilic end of Ox, thereby forming a suspension that is difficult to precipitate, resulting in a solution in a highly turbid state. However, as Ox dissolves, the complex sites bound by the suspended metal cations are released, so that Cr (III), C (III) -Fe (III) and Cr (III) -Al (III) in the three solution systems gradually sink, and the turbidity gradually decreases.

This curve can also be used to describe the distribution ratio change of Ox in the solid phase (or suspension state) and liquid phase. For Ox in the Cr-Ox system, its proportion in the liquid phase is always smaller than that in the solid phase, while for Ox in the Fe-Cr-Ox and Al-Cr-Ox systems, its proportion in the liquid phase is always larger than that in the solid phase, that is, in the presence of Fe or Al, Ox tends to remain in the liquid phase more. This may be related to the competitive complexing of Ox with Fe or Al to cr (iii) and thus the formation of suspended matter. Under the condition of no Fe or Al, the content of dissolved Ox is lower than that of a system with Fe or Al within the time range of 0-24 h, 53.75% of Ox participates in the formation of suspended matters or precipitates when naturally settling for 24h, and under the condition of Fe or Al, the proportions of the Ox participating in the formation of the suspended matters or precipitates are respectively 11.28% and 6.30%.

NaBH₄The coupled membrane filtration removes Cr (VI) in Cr (VI) -Ox, and is influenced by the concentrations of Fe (III) and Al (III). The Fe (III) and Al (III) concentrations affect both the reduction of Cr (VI) and the removal of TCr. Low concentration (0.3mM) of Fe (III), Al (III) vs. NaBH₄The reduced Cr (VI) has weak strengthening effect, and because of the existence of Ox, the reduced Cr (III) forms suspension with high turbidity and cannot be naturally precipitated, so that the removal of TCr is reduced to almost zero. High concentration of Fe (III), Al (III) to NaBH₄The reduced Cr (VI) has stronger strengthening effect, most of Cr (III) can be removed by a natural precipitation method, but a small part of residual Cr (III) still exists in the solution, and the residual Cr (III) can be effectively removed by combining with a microfiltration membrane, because Fe (III) -Cr (III) or Al (III) -Cr (III) can form suspended matters with larger grain diameter and can be easily intercepted by the microfiltration membrane.

Claims

1. A method for removing hexavalent chromium by filtration and reinforcement of a catalytic reduction coupling membrane is characterized by comprising the following steps: the method comprises the following steps:

the method comprises the following steps: adding NaBH into the mixed solution with the initial pH of less than 3.07 under the condition of continuous stirring at 650rpm₄Powder, wherein the temperature is kept constant within 23-27 ℃, resulting in the reduction of hexavalent chromium; controlling the molar concentration ratio of hexavalent chromium to oxalate, ferric iron, trivalent aluminum and silicate in the solution to be 1.92 mM: 0 to 0.40 mM: 0 to 3.0 mM: 0 to 3.0 mM: 0 to 3.0 mM;

step two: continuously stirring, stopping carrying out reduction reaction for 5min, and standing for 15 min-24 h to obtain a heterogeneous solution containing suspended matters;

step three: and filtering through an MCE membrane.

2. A catalytic reduction coupling membrane according to claim 1The method for removing hexavalent chromium by filtration and reinforcement is characterized by comprising the following steps: in step one, the NaBH₄The mixing ratio of the powder to the mixed solution is 0.01-0.10 g: 550 mL.

3. The method for removing hexavalent chromium through filtration and reinforcement by using the catalytic reduction coupling membrane according to claim 1, wherein the method comprises the following steps: in the third step, the aperture of the MCE membrane is one or more of 1 μm,0.45 μm and 0.1 μm.