I. Introduction

With the development of China's industry and the increase of population, environmental pollution has gradually intensified, especially the pollution of water sources, posing a great threat to the health of urban residents. Studies have shown that most of the organic matter contained in contaminated water sources is harmful to human health or has the potential to form chlorine disinfection by-products (DBPs) that have a “triad effect” on the human body. The conventional water treatment process is limited by its purification function. Organic compounds in water, heavy metal ions is low and algae removal capacity, especially for low molecular weight water soluble organics removal less effective. With the intensification of water pollution and the continuous improvement of people's living standards, people's requirements for drinking water quality are more stringent, and the corresponding water supply quality standards are also constantly improving. The purification of contaminated water has become a very important and urgent new topic. Quartz sand is a water purification material widely used in various water treatment, sewage treatment and environmental treatment. However, due to the small 孑L gap on the surface of the quartz sand filter material, the specific surface area and isoelectric point are lower. Negatively charged under normal conditions makes it very unsatisfactory for the removal of toxic substances (heavy metal ions, etc.), bacteria, viruses and organic matter in water.

In recent years, the researchers have proposed the idea of ​​modifying ordinary quartz sand filter materials. By attaching different functional substances to the surface of quartz sand to improve the properties of the surface of the filter material, a modified filter with excellent adsorption performance and certain mechanical strength is prepared. material. In view of China's current economic level, this technology will have broad application prospects in water treatment.

Second, modified quartz sand filter material

(1) Modified filter material

The modified filter material is coated with a layer of modifier (usually metal oxide and hydroxide) by chemical reaction on the surface of the ordinary quartz sand filter material, thereby changing the physical and chemical properties of the surface of the original filter material particles to improve the filter material. The ability to adsorb certain special substances and enhance the interception capacity of the filter material to achieve the purpose of improving the quality of the effluent. Practice has shown that the modified filter material can fully exert the ability to increase the specific surface area of ​​the filter material and enhance the adsorption. In order to achieve the enhanced adsorption and oxidation purification functions produced by the surface coating during the contact with various organic substances, bacteria and algae in the water. The modified filter material can not only adsorb macromolecules and colloidal organic matter, but also adsorb and oxidize heavy metal ions and small molecule soluble organic substances in water. Thereby achieving the goal of comprehensively improving water quality.

(two) adsorbent

There are many kinds of materials suitable for the filter adsorbent, and commonly used include Fe-based, Al-based oxides or hydroxides, manganese oxides, magnesium oxides, and rare earth metal complexes. Among them, Fe-based and Al-based hydrated oxides and hydroxides are the most widely used adsorbents. Active magnesium oxide has a large specific surface area and voids, and thus has a strong adsorption capacity. Rare earth metal complexes are representative of new materials. It can improve the surface structure of the conventional filter material, increase the adsorption position of the filter material surface, and have a good removal effect on impurities in the water (such as heavy metal ions and some inorganic ions). However, rare earth metal compounds are not well combined with carriers as adsorbents, which limits their widespread use to some extent.

In addition, high molecular organic compounds containing various special groups are applied to the surface of the filter. Or in combination with a metal oxide, the use of functional groups thereon to improve the adsorption performance of the filter material is also the current research direction.

Third, the application of modified filter material in water treatment

(1) Modified filter material to remove heavy metal ions from water

In the adsorption test of Zn metal oxide modified filter it found that in addition to zinc in the experiment, the pH value is closely related to the removal of aluminum oxide coated sand modified with filter: when the effluent pH is less than 4. The effect of removing zinc from aluminum coated sand is not as good as that of quartz sand; when it is greater than 4, the removal rate of zinc increases with the increase of pH value; when it is greater than 9, the removal rate of zinc can reach 100%.

The adsorption experiments of Pb2+, Cd2+, Cu2+ and C were carried out on the sand column covered with iron oxide. The results showed that the heavy metal ions Pb, Cd ²⁺ , Cu and Cr ³⁺ in the influent water were at pH 8. At .5, it can be almost completely removed, and the removal rate of the general sand is extremely low; for the heavy metal ions in the complex state, the sand has almost no removal effect. The iron oxide-coated modified sand column can remove almost all of the complexed heavy metal ions when processing 40 times of the filter bed volume of water. For particulate heavy metals, under high hydraulic loading conditions. Generally, the sand column and the modified sand column have a 70% removal rate for the particles. However, the removal rate is permanently achieved in the modified sand column, and is quickly reduced to 25% in general sand materials; compared with the general sand column, the modified sand particles can also increase the running time of the filter bed and reduce the washing frequency; When the water is backflushed, the acid in the column (pH 3) is used to regenerate the medium in the column.

(2) Modified filter material to remove anionic harmful substances in water

1. Modified filter material removes arsenic from water

Arsenic is a toxic substance. Some surface waters contain arsenic due to pollution from pesticides, solid waste, and industrial wastewater; some groundwater is contaminated with arsenic by passing through arsenic-rich formations. Inorganic arsenic toxicity is greater than organic arsenic Trivalent arsenic is more toxic than pentavalent arsenic. The World Health Organization stipulates that the trivalent arsenic content in drinking water is less than 0.01 ms.

In the experiment of removing arsenic by using iron oxide coated sand modified filter material, it was found that the arsenic removal effect was remarkable, the removal rate could reach above 95%, and the law of low pH value and high removal rate was followed.

The iron hydroxide modified sand was used to adsorb As ̈ and Ass* in water. The results showed that the ratio of adsorbent and arsenic concentration was appropriate and the pH value was appropriate. The arsenic removal rate can be made 92%.

2. Modified filter material removes selenium from water

SeO ₃ ^2- and SeO ₄ ^2- are the main forms of Se in water, and when Se is excessive, it is harmful to the human body. Wen-hui Kuant et al. found that the use of alumina-covered sand (AOCS) as an adsorbent can effectively remove Se(IV) and Se(VI) from water. The pH of the preparation affected the adsorption of Se by AOCS. The pH value was 5.98. The removal rate of Se(IV) and Se(vI) by AOCS was the highest, and the ratio was 1.05mg Se(IV)/gAOCS and 0.63mg respectively. Se(VI)/gAOCS. The adsorption isotherm parameters of Se(IV) and Se(VI) are related to the pH value of the solution during adsorption. The maximum adsorption amount decreases with increasing pH value within a certain pH range. The Se removal rate is close to 100% under appropriate pH, adsorption dose and adsorption time.

3. Modified filter material to remove fluorine from water

Gao Naiyun et al. used the iron oxide coated sand modified filter to remove fluorine. It was found that under low pH conditions (pH less than 5.O), the fluorine removal rate can reach more than 90%, but with the pH value of water. The increase in fluorine removal rate has been reduced; the fluorine adsorption isotherm is a typical Langmuir type.

(3) Modified filter material to remove organic matter from water

Chang et al. used iron oxide coated sand to remove organic matter and found that under acidic conditions, iron oxide coated sand can effectively adsorb natural organic matter (NOM); Ca2+, Mg2+, CI', SeO ^{₂ -} , and HCO in water ^- Ions, even at high concentrations, have no effect on the capacity of the NOM adsorbed.

The NOM performance of three different oxide coated sands and uncoated sands was compared. Experimental studies have shown that the effect of coating aluminum sand and coated iron sand prepared by high temperature on NOM is better than that prepared by alkaline precipitation method. Iron sand is applied, and all three kinds of coated sand are superior to uncoated sand; the effect of three different coated sands after backwashing is no different from that before backwashing.

Static and dynamic adsorption experiments were carried out on the effects of four different modified filter materials on the removal of organic matter in water. The results show that the removal effect of modified filter materials on organic matter is always better than that of original filter materials. The removal rate of organic matter by coated iron sand is 66%.

(4) Modified filter material to remove microorganisms from water

Cover the sand with iron hydroxide and aluminum hydroxide. The filter material is modified. The lkg modified filter material was placed in the column (volume 0.7L), and the removal rate of Escherichia coli, Vibrio cholerae, poliovirus 1, and large intestinal phage MS-2 in water was above 99%, in 30d In the test, 120 L of water was treated, and this removal effect was maintained. After passing through 192 L of water, the removal rate of Escherichia coli, coliform phage MS-2, and poliovirus 1 was still 80%, 99.9%, and 90%. Also. The modified filter material can also remove intestinal bacteria and large intestine phage in raw water. At room temperature, the removal rate of the tested microorganisms (such as intestinal bacteria, large intestine phage MS-2, E. coli) was above 99%; while ordinary sand particles had almost no removal effect on microorganisms. In addition, the cover-related ingredients were not detected in the effluent. The cover is very stable.

The quartz sand was covered with AI(OH) ₃ , Fe(OH) ₃ , lime and 8-MnO. The results showed that the pH value was in the range of 6.3 to 9 and the sand covered by A1(OH) ₃ adsorbed poliovirus 1 The result is the best. The experiment of removing poliovirus by using AI(OH) covering sand as filter medium (volume 0.0063L), the virus removal rate was 98.9% when passing 0.2L water; virus removal when treating 0.5L water The rate is 93.97%.

A1 (0H), Fe (OH) ₃ modified quartz sand filter to remove viruses in water. The experiment showed that the removal rate of poliovirus 1 was 86%±1% when the pH of the filter material covering both A1(OH) ₃ and Fe(OH) ₃ was 7 . The removal rate of the large intestine phage MS-2 was 100%.

(5) Modified filter materials to remove other pollutants

The removal of trace phenol from the modified filter material was studied. It was found that the modified filter material had a certain adsorption effect on phenol, and the adsorption efficiency of phenol increased gradually with the decrease of pH value, but the removal rate was not high. This indicates that the adsorption of phenol on the surface of the modified filter material follows the mechanism of coordination exchange adsorption, and the pre-plated potassium permanganate has obvious strengthening effect on the filtration of phenol by the modified filter material.

The removal of algae by modified filter material was studied. It was found that although the modified sand had a slightly better direct filtration effect on algae-containing water than quartz sand. However, the filtration effect of modified sand on algae-containing water after coagulation and sedimentation is obviously better than that of quartz sand. In particular, the filtration interception capacity is much larger than that of ordinary quartz sand. The study also found that residual aluminum in algae-containing water after coagulation precipitation promoted the removal of algae particles.

Fourth, the existing problems

(1) The filter material is immersed in water for a long time in the filtration process, how to ensure the stability of the modified filter material. That is, the strong adhesion of the metal oxide/hydroxide to the surface of the filter material is a key technical problem that needs to be solved in research and practical application. Currently. In foreign countries, iron or manganese oxides are generally sintered on the surface of quartz sand by high temperature calcination. The rich surface hydroxyl groups formed by the iron oxide in water increase the adsorption capacity. In the future research. We should focus on the research of various preparation methods, especially the modification of common filter materials by various substance complexes, and the reference to nano-preparation technology is also worth exploring.

(2) Seeking a suitable carrier. It has been found abroad that the modified filter material made of olivine instead of ordinary quartz sand has much better performance than the modified filter material made of quartz sand. In China, other natural filter materials can be used instead of quartz sand according to local conditions, or the modified sand and other adsorption carriers can be co-sintered (such as diatomaceous earth , pozzolan, etc.) to improve the removal effect of the modified filter material.

(3) So far, there are not many substances removed by modified filter materials. The removal of harmful substances such as ammonia nitrogen and phenol in water has yet to be studied. And because of the different methods of production by various researchers, the modified filter materials produced tend to have large differences in performance. Therefore, how to find an effective preparation method, industrialize the production of modified filter materials, and form an independent product to promote its wide application in water treatment is a subject that needs to be further studied and has great significance.

V. Outlook

At present, domestic and foreign experts and scholars are committed to researching the preparation method and carrier of modified filter materials and how to broaden the adsorption range of modified filter materials in the preparation process and improve the adsorption efficiency, so as to prepare modified filter materials with better performance. The modified filter material can effectively remove heavy metal ions, organic matter, bacteria and viruses, algae, phenol and other toxic and harmful substances in water, and is very suitable for the current status of water treatment in China. Therefore, it will have a wide application prospect in water treatment.

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