Adsorption method to remove ammonia nitrogen pollutants from wastewater

The principle of ammonia nitrogen removal is mainly the adsorption of non-ionic ammonia and the ion exchange with ionic ammonia. The main factors affecting the removal of ammonia nitrogen include the action time with the adsorbent solution, the amount of adsorbent, the ammonia nitrogen concentration in the solution, the particle size of the adsorbent and the temperature of the solution.

 

Adsorption method to remove ammonia nitrogen pollutants from wastewater

The principle of ammonia nitrogen removal is mainly the adsorption of non-ionic ammonia and the ion exchange with ionic ammonia. The main factors affecting the removal of ammonia nitrogen include the action time with the adsorbent solution, the amount of adsorbent, the ammonia nitrogen concentration in the solution, the particle size of the adsorbent and the temperature of the solution.

In wastewater treatment practice, many wastewaters still fail to meet the discharge standards after secondary treatment, and further deep treatment of the secondary effluent is required. For this reason, the adsorption method is also used for deep denitrification.

Zeolite adsorption method has been successfully industrialized in the United States and Japan. It mainly uses fixed bed adsorption columns, with clinoptilolite as the adsorbent, particle size of 0.8-1.7mm, air velocity of 5-10h-1, inlet ammonia nitrogen concentration of 20mg/L, and outlet ammonia nitrogen concentration of <1mg/L.

Several commonly used ammonia nitrogen adsorbents

1. Activated carbon

Activated carbon (AC) is a common adsorption material, mainly in granular and powdered form. Activated carbon can be made from animals, plants, coal, petroleum, pulp waste liquid, waste synthetic resin and other organic residues, etc., after crushing and adding adhesive to form, and then heated dehydration, carbonization and activation. Activated carbon has a huge specific surface area and particularly developed micropores. Usually, the specific surface area of activated carbon is as high as 500-1700m2/g. The micropore volume of activated carbon is about 0.15-0.9mL/g, and the surface area accounts for more than 95% of the total surface area.

The adsorption of activated carbon is mainly physical adsorption, but due to the presence of surface oxides, some chemical selective adsorption is also carried out. Activated carbon is currently a commonly used adsorbent in wastewater treatment.

Among them, granular carbon has the largest amount due to its simple process and convenient operation. Granular carbon used abroad is mostly coal or fruit shell amorphous carbon, and columnar coal carbon is mostly used in China. Activated carbon mainly adsorbs nitrogen in the form of ammonia molecules in water, has no selectivity, and has limited adsorption capacity, so the denitrification efficiency is very low.

2. Activated carbon fiber

Activated carbon fiber (ACF) is a new type of adsorption functional material. It is made of lignin, cellulose, phenolic fiber, polypropylene fiber, asphalt fiber, etc., and is made by carbonization and activation.

Compared with activated carbon, ACF has a unique microporous structure, higher surface area and specific surface area, multiple functional groups, and a smaller average pore diameter. It has been widely used in wastewater, waste gas treatment, water purification and other fields through physical adsorption, chemical adsorption and physical and chemical adsorption.

The micropore volume of fibrous activated carbon accounts for about 90% of the total pore volume, and most of its micropore diameters are about 1nm, without excessive pores and macropores. The specific surface area is generally 600-1200m2/g, and can even reach 3000m2/g. Activated carbon fiber has a fast desorption and regeneration rate, a short time, and its performance remains unchanged, which is better than activated carbon.

ACF finished products can be made into shapes such as felt, cloth, and paper. They are more flexible in practical applications, easier to operate, and have a certain strength and resistance to breakage. They overcome the problems of grooves and sedimentation formed during the operation of granular and powdered activated carbon.

Like activated carbon, activated carbon fiber has no selectivity in adsorption and is mainly used to adsorb organic pollutants. However, the adsorption of ammonia nitrogen is limited and it is generally used for comprehensive wastewater treatment in refineries.

3. Zeolite

Zeolite (Zeolite) is a natural and inexpensive ion exchange material with a wide distribution and high mining volume. There are many types of naturally produced zeolites, among which clinoptilolite (Clinoptilolite) and mordenite (Mordenite) are the main ones.

Zeolite is a porous aluminosilicate mineral with a water-containing frame structure. The most basic structure of the zeolite skeleton is silicon oxygen (SiO4) tetrahedron and aluminum oxygen (AlO4) tetrahedron. Zeolite has an open skeleton structure, the volume of the crystal cavity is about 40% to 50% of the total volume, and the pore size is mostly below 1nm.

Zeolite has a large specific surface area (400 to 800m2/g zeolite), second only to activated carbon, and can be used as an adsorbent, ion exchanger, etc. Zeolite has a preferential selective adsorption effect on polar, unsaturated and easily polarizable molecules, and zeolite has acid resistance, alkali resistance, thermal stability and other properties, and is often used in industrial wastewater treatment.

Zeolite has a large adsorption capacity and purification effect. The cation exchange capacity of clinoptilolite and mordenite is 223meq100g (milligram equivalent) and 213meq100g respectively. Because natural zeolite contains impurities, the exchange capacity of higher purity zeolite is no more than 200meq100g, generally 100 to 150meq100g.

Clinoptilolite has good selectivity for NH4+-N in ion exchange and quantitative treatment, so it can be used in sewage ammonia nitrogen removal treatment process in engineering, and the denitrification rate can reach 90% to 97%. The industrial zeolite ammonia removal device is relatively simple, generally a cylindrical filter.

The main methods for regenerating zeolite are chemical solution regeneration using NaOH or NaCl solution and combustion regeneration under high temperature conditions of 500℃ to 600℃ to convert NH4+ in zeolite into NH3 gas.

After repeated regeneration, zeolite has little effect on the adsorption and exchange capacity of NH4+, but due to the coexistence of cations such as Ca2+ in sewage, the exchange capacity of zeolite will be irreversibly reduced.

4. Humic acid adsorbent

Humic acid is a negatively charged polyelectrolyte with a large molecular weight of organic matter. Humic acid substances can be used to treat industrial wastewater, especially heavy metal wastewater and radioactive wastewater, to remove ions.

The properties and structure of humic acid itself determine its adsorption performance for cations. The adsorption of cations by humic acid includes ion exchange, integration, surface adsorption, condensation and other effects, including both chemical adsorption and physical adsorption.

There are two major categories of humic acid substances used as adsorbents: one is natural weathered coal, peat, lignite, etc. rich in humic acid, which are used directly as adsorbents or as adsorbents after simple treatment; the other is to use humic acid-rich substances with appropriate binders to make humic acid resins, granulate and form them, and use them in tubular or tower adsorption devices.

5. Coal-based adsorbents

Activated carbon is a high-quality adsorbent, but it is expensive and difficult to regenerate, which limits its use. Therefore, it is of great significance to develop and manufacture cheap substitutes. Lignite, long flame coal, anthracite, etc. have high porosity and specific surface area, and there is a great prospect for using them to prepare coal-based adsorbents.

Coal-based adsorbents have a removal effect on metal ions, cyanide, volatile phenols, etc. in wastewater, have strong adsorption capacity, and are low in price and do not require regeneration. The sulfonated coal is generated by reacting coal with concentrated sulfuric acid or fuming sulfuric acid, which can be used to treat TNT wastewater. After adsorption, it can be easily desorbed with acetone or ethyl acetate plus ethanol and can be reused repeatedly.