Iron active filter membrane contact oxidation and iron removal principle

I. Introduction

In China's groundwater iron removal technology, the method of removing iron by aeration contact oxidation is widely used. The aeration contact oxygen removal method is to make the ferrous ions in the aerated groundwater enter the contact filter layer together with the dissolved oxygen without oxidation, and complete the oxidation and retention of the ferrous ions under the contact catalysis of the filter layer. Natural manganese sand removal is a kind of contact oxidation and iron removal method widely used in China; artificial rust sand and naturally formed rust sand iron removal method is another kind of contact oxidation and iron removal successfully tested in China in the 1970s. law.

In the past, the author has conducted systematic experiments and research on the natural manganese sand removal method. In recent years, the process of removing iron from artificial rust sand and naturally formed rust sand with quartz sand as carrier has been studied at home and abroad. These research results have developed a contact oxidation and iron removal process, improved the efficiency of the contact oxidation and iron removal process, and promoted the promotion and application of the contact oxidation and iron removal process.

There is a development process in the understanding of the mechanism of contact with oxidation and iron removal. Since the use of pyrolusite as a contact oxidation and iron removal filter for groundwater in the 1930s, manganese dioxide has been used as a catalyst, which is called classical theory. As early as the early 1960s, the author discovered the contact catalysis of "active filter membrane" in the process of studying the removal of iron from natural manganese sand. Later, after repeated tests of models and production tests, it was finally officially proposed in 1974. The filter membrane is in contact with the principle of oxidation and iron removal, which further deepens the understanding.

In recent years, the author has studied the basic characteristics of iron active filter membrane in contact with oxidation and iron removal. Experiments have shown that the new filter material has a certain ability to remove iron at the beginning, but it does not last for a period of time and the iron removal ability begins to fail. The concentration of iron in the filtered water increases accordingly; as the running time increases, the iron removal capacity of the filter material gradually increases, and the water quality after filtration becomes better, and finally the filter material has a stable iron removal capacity. Ultimately, it has a stable ability to remove iron. The filter material with stable iron removal ability is called “mature” filter material; the conversion process from new filter material to “mature” filter material is called “mature” process of filter material. In fact, the maturation process of the filter material is the process of formation and accumulation of the iron active filter membrane on the surface of the filter material. In this paper, the problems of iron removal of new filter materials, formation and accumulation of active membranes, and iron removal characteristics of active membranes in mature filter layers will be discussed.

Second, the new filter material for iron removal

The un-aerated oxygen-free iron-containing groundwater was filtered through a new filter layer, and it was found that the filter layer initially had a certain ability to remove ferrous ions. Figure 1 shows the removal of ferrous ions from water by new natural manganese sand. Where new quartz sand or anthracite remove ferrous ions, similar to natural manganese sand. The new filter material can remove iron under anaerobic conditions, indicating that the new filter material has an adsorption effect on ferrous ions in water.

The adsorption capacity of the new filter material for ferrous ions in water is related to the variety of filter materials. Table 1 shows the dynamic adsorption capacity of several new filter materials for ferrous ions in water under anaerobic conditions. It can be seen from Table 1 that the adsorption capacity of Mashan manganese sand is the largest and the quartz ore is the smallest.

Table 1 Dynamic adsorption capacity of new filter media for ferrous ions

Experiments have shown that ferrous ions adsorbed on the surface of the new filter material can be oxidized to high iron in the presence of dissolved oxygen. However, the high-iron hydroxide formed on the surface of the new filter material is very different in nature from the iron filter membrane having strong catalytic activity generated on the surface of the mature filter material. First, the high-iron hydroxide formed on the surface of the new filter material has a very dense structure. The comparison test between the new filter layer and the mature filter layer shows that the hydraulic impedance of the mature filter layer is 40 times higher than that of the new filter layer when the same amount of iron is trapped in the filter layer (2 g in one test). Therefore, the high-iron hydroxide formed on the surface of the new filter material is much denser than the active filter on the surface of the mature filter material.

Secondly, the high-iron hydroxide formed on the surface of the new filter material does not have strong contact catalytic activity. Figure 2 shows a comparison of the ripening processes of three new filter media. It can be seen from the figure that since the new filter material has a certain adsorption capacity, there is a certain iron removal effect at the initial stage of filtration, but when their adsorption capacity is gradually exhausted, the iron concentration of the filtered water is continuously increased. As the filtration and iron removal process progresses, an iron filter membrane having contact catalytic activity is formed on the surface of the filter material, and the filter material gradually matures due to the accumulation of the active membrane material on the surface of the filter material. The concentration of iron in the effluent of the filter layer begins to decrease again, thereby having a peak-like characteristic. The experiment found that although the adsorption capacities of the three new filter materials are very different, their maturity is basically the same. If the high iron hydroxide formed on the surface of the new filter material has contact catalytic activity. Then the new filter material with large adsorption capacity will retain more iron and should mature faster, that is, it has a shorter maturity period, but this is not the case. Therefore, the high-iron hydroxide formed on the surface of the new filter material does not have strong contact catalytic activity, and it has different properties from the iron filter material having strong catalytic activity on the surface of the mature filter material.

Third, the mature process of the filter material

After the iron-containing groundwater is aerated and oxygenated, it is filtered through the new filter layer. Because the new filter material has the adsorption capacity, it has a certain iron removal capacity. At the same time, the surface of the filter material begins to form a catalytically active iron filter. Therefore, the new filter material has the functions of adsorption iron removal and contact oxidation and iron removal in the mature process. At the initial stage of filtration of new filter materials, the effect of contact oxidation and iron removal is very small, so the main method is adsorption and iron removal. As the adsorption capacity of the filter material is consumed, the iron removal capacity is lowered, and the iron concentration of the filtrate effluent gradually increases. On the other hand, the iron removal ability of the active filter formed on the surface of the filter material is continuously increased. When the rate of increase of the iron removal capacity of the active filter exceeds the decrease rate of the capacity of the adsorption and removal of iron, the effluent of the filter layer contains iron. The concentration began to decline. Since the contact oxidation and iron removal process of the active filter membrane is an autocatalytic process, the increase of the iron removal capacity of the filter membrane has an accelerating feature, so that the process line of the iron concentration of the filter layer effluent has a slightly convex shape after the peak. Until the effluent concentration drops to the required value. After that, the iron concentration of the filtrate effluent is stabilized at a very low value, indicating that the filter material has matured. In this way, the maturation process of the filter material can be divided into three stages. The first stage is that the adsorption and iron removal of the new filter material predominates, which is called the adsorption section; the second stage is the catalytic iron removal of the iron active filter. The advantage, and has the characteristics of accelerated progress, called the accelerated catalytic section; the third stage is the stable catalytic iron removal of the iron active filter, called the stable catalytic section, as shown in Figure 3. The stable catalytic iron removal process continues for a considerable period of time, and the filter material is finally fully mature. The surface of a fully mature filter material is covered with a ferrous active filter and yellowed, so it is often called rust sand.

The adsorption capacity of the filter materials is different, and their maturation process is also different; the adsorption capacity is small, the adsorption phase is relatively short, and the peak of the process water concentration change of the filter layer is also large; the filter material with large adsorption capacity is compared with the adsorption phase. Long, the peak of the water is also small. When the adsorption capacity of the filter material is large, and the iron concentration of the groundwater is small, the peak of the effluent concentration may fall below the water quality standard. When the filter is put into production, the qualified water quality can be supplied.

Under the conditions shown in Figure 2, we also carried out the maturity test of Beidaihe quartz ore, Songhuajianghe sand, Heilongjiang bituminous coal and other filter materials, and the test results are basically consistent with Figure 2. The total length of the adsorption section and the accelerated catalytic section of the above six kinds of filter materials is about 4 to 5 days. At this time, the iron concentration of the filtrate effluent can be reduced to 0.3 mg/l or less, but the effluent water quality is not stable enough, after 7 days, All can stably remove iron.

In summary, the different types of filter materials only affect the effluent quality of the initial stage of iron removal, and basically do not affect the maturity of the filter material and the iron removal performance of the mature filter material, that is, for the mature filter material, different varieties As the carrier of the iron active filter membrane, the filter material has no difference. This provides a theoretical basis for the use of cheap filter materials such as quartz sand, river sand and anthracite in the process of contact oxidation and iron removal. The economic significance is very large. of. However, filter materials with large adsorption capacity, such as natural manganese sand, have good effluent quality in the early stage of iron removal, which is of practical significance. Quartz sand, anthracite and other filter materials with small adsorption capacity, the initial effluent water quality is poor at the initial stage of production, and measures to improve water quality and accelerate the maturity of filter materials are required.

The adhesion index of the surface of the filter material (the number of mg of iron attached to the surface of the 100 mg filter material) was used as an indicator of the maturity of the filter material. As mentioned above, since different filter materials have different adsorption capacities, the adsorption of oxidized iron on the surface of the filter material does not have catalytic activity. The filter material with large adsorption capacity makes the adhesion index reach a considerable value at the initial stage of iron removal, but the filter material does not have a corresponding "mature" degree. Therefore, the adhesion index is used as an indicator of the maturity of the filter material, and it is not universally applicable to filter materials having different adsorption capacities.

People are accustomed to use the iron concentration in the iron removal filter to fall below the drinking water quality standard (0.3mg/l) as a sign of filter maturity. Since the filter layers are all operated under certain conditions, this makes "mature" related to specific working conditions, without uniform standards, and is difficult to compare with each other, so it is also imperfect.

We believe that it is reasonable to use the contact oxidation reaction rate constant of the unit filter surface area or the contact catalytic activity coefficient of the filter layer as an indicator of the maturity of the filter material.

Fourth, the chemical composition of iron active filter membrane and its basic characteristics of catalysis

In the process of removing ferrous ions, an iron active filter membrane is gradually formed on the surface of the filter material. In a filtration cycle, if the amount of adhesion of the filter on the surface of the filter is greater than that in the backwash, the amount of iron on the surface of the filter increases, which causes the filter particles to gradually become larger. For the groundwater de-ironing water plant with high iron concentration, obvious thickening and granulation of the filter layer can be observed. In some water plants, the filter material is used for one year, and the particle size of some filter materials can be increased from 0.6 to 2.0 mm. It is 5 to 6 mm, and the volume is increased several times or even several times to become a rust ball. This rust ball is brownish yellow when wet and has a loose layer of iron hydroxide (filter) attached to the surface. Wash the filter, the surface of the rust ball is smooth and has a certain strength. The rust ball is cut open, and the interior is brown and black, which is round and dense. There is a small core made up of fine filter material in the rust ball, but there is also no core composed entirely of iron.

After the rust balls taken from the Jiamusi Water Plant were calcined, it was found to contain 88% of Fe ₂ O ₃ and 8% of SiO ₂ , and further contained various elements such as Ca, Mg, and Mn. The chemical composition of the loose iron filter outside the rust ball is the same as that of the rust ball. According to the process of rust ball formation, it can be concluded that the dense substance inside is gradually formed by the long-term accumulation of such a loose iron filter on the surface of the filter material.

We also analyzed the differential thermal and thermal weight loss of the fresh filter and the internal material of the rust ball, and measured their chemical composition as shown in Table 2. The sample of the fresh filter is iron sludge deposited by the filter backwash water (one day before the measurement). It can be seen from Table 2 that although the internal components of the iron filter and the rust ball have the same chemical composition, there are many differences in the chemical composition. It can be seen from the comparison that the process of accumulating the internal material of the rust ball from the iron filter membrane on the surface of the filter material is a process in which the crystal water gradually separates, and the appearance is from loose to dense.

In order to understand the difference between the catalytic activity of the filter and the internal material of the rust ball. The following comparative test was conducted. One filter tube is filled with a rust ball with a fresh filter membrane as a filter material, and the other filter tube is filled with a rust ball for washing the filter membrane as a filter material, so that they are subjected to a iron removal test under the same conditions.

Table 2 Chemical composition of iron active filter

Figure 4 shows the test results. It can be seen from the figure that the rust ball with fresh filter has good iron-reducing effect. The rust ball washed out of the filter has poor iron removal effect and has the same characteristics as the new filter material. It shows that only the filter material with loose surface of the rust ball has catalytic activity, while the total dense substance in the rust ball is No catalytic activity. This catalytically active loose iron membrane on the surface of the filter material is called an iron-active membrane.

The groundwater has a ferric concentration of 14 mg/l; the dissolved oxygen concentration is 7-8 mg/l; and the filtration rate is 10 m/h.

Experiments show that the activity of fresh iron active membrane is the strongest. With the prolongation of time, the iron membrane gradually ages and its catalytic activity gradually decreases. The experiment was carried out using a mature filter material, and the experimental results are shown in Fig. 5. It can be seen from the figure that after a few days of shutdown, the iron removal efficiency of the mature filter material has been greatly reduced, indicating that the iron filter will age and lose its catalytic activity over time. The dense substance inside the rust ball is formed by the long-term accumulation of the aging iron filter. Therefore, the catalysis of the iron active membrane on the surface of the filter material can only be achieved in the continuous iron removal process. The iron active filter membrane on the surface of the filter material is newly replenished in the process of filtering and removing iron, so that the new filter membrane is continuously covered on the original filter membrane, which keeps the membrane membrane fresh and has high catalytic activity. The old filter membrane gradually loses its catalytic activity and becomes a dense deposit on the surface of the filter material. The continuous renewal of the iron active filter membrane on the surface of the filter material is a necessary condition for the normal operation of the rust sand in contact with the oxidation and iron removal process.

It has been clarified that the process of contacting the iron active filter membrane with iron oxide is firstly the ferrous ion of the membrane ion exchange adsorption water, which can be expressed as follows:

Fe(OH) ₃ ·2H ₂ O+Fe ²⁺ = Fe(OH) ₂ (OFe) ·2H ₂ O ⁺ +H ⁺

When there is dissolved oxygen in the water, the adsorbed ferrous ions are rapidly hydrolyzed and oxidized under the catalysis of the active filter membrane, so that the catalyst is regenerated, and the reaction product acts as a catalyst to participate in the reaction, so the iron active filter membrane is contacted and oxidized. Iron is an autocatalytic process.

Fe(OH) ₂ (Ofe) ·2H ₂ O+1/4·O ₂ +9/2 ·H ₂ O= 2Fe(OH) ₃ ·2H ₂ O+ H ⁺

The iron mud in the backwash water was collected for analysis and found to be substantially free of ferrous compounds. It shows that the ferrous ions adsorbed by the active membrane can be rapidly oxidized to high iron.

According to the iron active filter membrane, the oxidation and iron removal is an automatic catalytic process. The iron which is trapped in the filter layer during the process of filtering and removing iron should have the catalytic effect, so that the contact layer of the filter layer can be obtained. improve. This is indeed the case. Figure 6 shows the variation of iron concentration in water along the depth direction of the filter layer during iron removal. Curve 1 is the concentration distribution 1 hour after the backwashing of the filter layer, and curve 2 is the case after 36 hours after the backwashing. It can be seen from the figure that the position of curve 2 is shifted upwards compared with the curve 1, indicating that the contact oxidation and iron removal ability of the filter layer is obviously improved with the accumulation of iron in the filter layer, which confirms the contact oxidation of the iron active filter membrane. Iron is the conclusion of an automated catalytic process.

V. Contact oxidation and iron removal rate of mature filter layer

The ferrous ions in the water are removed in the mature filter layer and undergo the following steps: the ferrous ions diffuse from the water to the surface of the filter; the ferrous ions are adsorbed by the active membrane on the surface of the filter; the adsorbed ferrous ions are hydrolyzed It is oxidized to form a high-iron hydroxide-iron active filter. Among the above steps, the slowest reaction rate will be the control step of the iron removal rate. Experiments have shown that the diffusion of ferrous ions to the surface of the filter material may be a controlling factor for the rate of iron removal. The experiment also showed that the active filter on the filter material only adsorbed ferrous ions in the water on the outer surface.

According to Fick's law, when the ferrous ion diffuses toward the surface of the filter, the diffusion rate is proportional to the difference in ferrous ion concentration (C-C') between the water and the surface of the filter, and inversely proportional to the thickness σ of the boundary layer on the surface of the filter. If the diffusion rate is taken as the iron removal rate and C' is considered to be negligible, then

-dc/dt=DS/D(C-C')≈DS/σ·C (1)

Where t is the time, t = ml / u;

l - the thickness of the filter layer;

m - filter layer porosity;

U——filter speed;

D——the diffusion coefficient;

S——the outer surface area of ​​the filter membrane per unit volume of the filter layer, S=6a(1-m)/d;

D——the particle size of the filter;

A——the shape factor of the filter material;

Σ—the thickness of the boundary layer;

C' - the concentration of ferrous ions on the surface of the filter.

Substituting the above parameters into equation (1)

-dc/dι=βC (2)

β=6Dam(1-m)/ σdu (3)

In the formula, β is called the contact catalytic activity coefficient of the filter layer.

When the water flows in a laminar flow state in the filter layer, the thickness of the boundary layer can be considered to be a certain value (σ=const). It can be known from the formula (3) that the catalytic activity coefficient of the filter layer is inversely proportional to the primary of the filtration rate. relationship.

When water flows in a turbulent state in the filter layer, it can be approximated that the thickness of the boundary layer is inversely proportional to the filtration rate.

σ=a/u (4)

Where a is the proportionality factor. Substituting equation (4) into equation (3),

β=6Dam(1-m)/ad (5)

That is, when turbulent flow, the iron removal effect is independent of the filtration speed, which can be regarded as inversely proportional to the zeroth power of the filtration speed.

When water is lower than the transition zone between laminar flow and turbulent flow in the filter layer, it can be considered that the catalytic activity coefficient of the filter layer is inversely proportional to the p-th power of the filtration rate.

β=6Dam(1-m)/bdup (6)

Where b is the proportionality factor; and 0

The flow state of water in the filter layer can be discriminated by the Reynolds number. Reynolds number is calculated as follows

Re=pdu/6μa(1-m) (7)

Then, when Re < 2, it is a laminar flow.

The relationship between the above-mentioned filter layer iron removal rate and the filter material particle size and the filtration rate has been obtained experimentally in the research of natural manganese sand removal. Now, we have made a theoretical argument.

The reaction rate (adsorption, oxidation, hydrolysis) of ferrous ions on the filter is proportional to the concentration of ferrous ions on the surface, so the rate of iron removal on the surface of the filter is

-Dc/dt=KSC' (8)

Where K is the reaction rate constant on the membrane per unit area.

When the iron removal process is stable, the surface reaction rate is equal to the diffusion rate, ie

KSC'=DS/σ(C-C') (9)

Thus C'=C/(1+Kσ/D) (10)

Substituting equation (10) into equation (8),

-Dc/dl=[K/(1+Kσ/D)]·[6am(1-m)/du·C] (11)

Comparing equations (11) and (2), we know that

β=[K/(1+Kσ/D)]·[6am(1-m)/du] (12)

It can be seen from the above formula that β increases with the increase of K, so both can be used as indicators for judging the maturity of the filter material.

Six, several conclusions

1. Through experiments on various kinds of filter materials such as natural manganese sand, quartz sand, river sand and anthracite, it is found that the new filter material has adsorption effect on iron ions in water. The adsorption capacity varies with the type of filter material, but it is adsorbed to the new filter material. The surface of the iron is oxidized and does not have catalytic properties. The adsorption capacity of the new filter material is large, and the iron removal quality is good at the initial stage of filtration.

2. Experiments have shown that the catalytic activity of ferrous ion oxidation is the iron active filter membrane naturally formed on the surface of the filter material during the iron removal process, and the formation speed is generally independent of the type of filter material. The chemical composition of the iron active filter membrane is Fe(OH)3·2H2O. The experiment confirmed that the iron active filter membrane is exposed to oxidation and iron removal process: the ferrous ions in the water are first adsorbed by the filter membrane, then oxidized and hydrolyzed to form a new active filter membrane, and participate in the reaction as a new catalyst, so the active filter membrane Iron removal is an automated catalytic reaction process. Experiments have shown that the iron retained in the filter layer during the iron removal process can increase the contact catalytic ability of the filter layer.

3. Experiments show that the “mature” process of the new filter material is the process of gradual accumulation of the iron active filter membrane on the surface of the filter material. The iron removal process of the mature filter material is essentially the iron removal process of the iron active filter membrane on the surface of the filter material. For mature filter materials, different kinds of filter materials serve as carriers for iron active filter membranes, and their effects are basically indistinguishable. The maturation process of the filter material can be divided into three sections: the adsorption section, the accelerated catalytic section and the stable catalytic section. It is recommended to use the reaction rate constant K on the surface area of ​​the unit filter or the contact catalytic activity coefficient β of the filter layer as an indicator for judging the maturity of the filter material.

4. Experimental studies have shown that the activity of fresh iron active membrane is the strongest, but the membrane is gradually dehydrated and aging, and its catalytic activity is gradually weakened. Therefore, the catalytic activity of the filter surface surfactant membrane is only continuous. This can only be achieved by filtering and removing iron.

5. Experiments have confirmed that the rate of contact oxidation and iron removal of the filter layer is controlled by the diffusion rate of ferrous ions to the surface of the filter membrane. Starting from the law of diffusion, the theory deduces the iron removal rate formula of the filter layer.

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