Modification of bentonite and its application in environmental management
Due to structural factors, the inner surface area of natural bentonite is relatively large, and the inner surface brings huge inner surface energy, which makes it have better adsorption capacity. Although the adsorption performance of bentonite is good, under unmodified conditions, when natural bentonite is in water, the interlayer ions will undergo hydrolysis reaction, reducing its adsorption capacity, which also limits the application of bentonite in pollution treatment. Therefore, there are It is necessary to modify the bentonite.
1. Bentonite modification technology
(1) High temperature modification
The water film on the surface of natural bentonite hinders the adsorption of pollutants. After high temperature roasting, the water in various forms inside the bentonite will evaporate and take away some impurities, so that the space structure of the bentonite is expanded and the internal pores are opened. Create space for the adsorption of pollutants. However, when the calcination temperature is too high, the high temperature will cause damage to the structure of bentonite, which will reduce the porosity and the adsorption performance.
(2) Ultrasonic modification
Short-term ultrasound will loosen the structure of bentonite, increase the interlayer spacing, and make it easier for harmful heavy metal ions to enter; long-term ultrasound will change the Si-O-Si bond on the surface of the bentonite crystal sheet, increasing the metal ions and the surface of the bentonite. Therefore, the obligate adsorption of metal ions by bentonite is enhanced.
(3) Metal modification and magnetic modification
Common metal modifiers are Fe and La, among which lanthanum-modified bentonite (LMB) is a widely used adsorbent for phosphorus treatment. With the development of technology, metal ions no longer simply function as metal modifiers, but are embedded in bentonite with the function of magnetic agents.
(4) Acid modification
The interlayer ions of bentonite are originally Na+, Ca2+, Mg2+, Al3+, etc. The acid modification is to soak the bentonite with acid to precipitate the interlayer cations, dredge the pores between the bentonites, and make the adsorbate more easily diffused inside. At the same time, H+ enters between the bentonite layers, replacing the original ions, weakening the interlayer force of the bentonite, and increasing the cation exchange capacity (CEC) and adsorption capacity.
(5) Organic modification
Due to the existence of interlayer ions, natural bentonite is hydrophilic, which is not conducive to the adsorption of organic pollutants. Organic modification is to use functional groups or organic substances in organic matter to replace the cations of the bentonite layer, which not only makes the obtained modified bentonite into lipophilic and hydrophobic, but also increases the interlayer spacing, strengthens the dirt holding capacity and ion exchange. ability. According to the similar compatibility principle, its surface adsorption capacity for organic pollutants is improved.
(6) Inorganic modification
Inorganic modification refers to the use of the exchangeable characteristics of positive ions between the layers of bentonite. According to the hydrolysis reaction of inorganic materials, the metal ions enter the layers of bentonite to replace exchangeable positive ions, so as to prepare inorganic modified ions. Sexual bentonite. After inorganic modification of bentonite, the interlayer spacing is significantly expanded, the specific surface area is increased, and the adsorption effect is significantly improved.
(7) Inorganic-organic composite modification
Inorganic-organic composite modification refers to the use of bentonite's large interlayer voids and exchangeable positive ions, first using inorganic polymers to expand its interlayer domains, and then using activators to change the surface characteristics of bentonite.
2. Application of modified bentonite in environmental management
(1) Heavy metal pollutants
The bentonite was modified with different concentrations of hydrochloric acid, nitric acid, phosphoric acid and sulfuric acid, and the adsorption effect of modified bentonite on CdZn-Pb-Cu quaternary composite was studied. It was found that with the increase of acid concentration, the adsorption capacity of Pb ion and Cu ion decreased to varying degrees. The adsorption of heavy metal pollutants by polyanionic cellulose and modified bentonite was studied, and it was found that the adsorption effect of Pb ions was better.
(2) Organic pollutants
The sodium bentonite was used to adsorb carvaldehyde, and the adsorption results were obtained through different adsorbate concentrations and different solid-liquid ratios. The maximum unit adsorption capacity was 110 mg/g. ANTONELLI et al. used thermally modified bentonite (CVL) to study the adsorption performance of ciprofloxacin and found that the maximum unit adsorption capacity could reach 114.4 mg/g at 25 °C.
Alkali modification, salt modification and surfactant modification were carried out on bentonite respectively, and it was found that the removal rates of methyl blue by three different modified bentonites were 73.25%, 81.62% and 85.06% respectively. The adsorption treatment of cationic starch-bentonite system was used, and it was found that the cationic starch-bentonite system had obvious treatment effect on simple pollutants.
(3) Inorganic pollutants
The study found that the adsorption and flocculation performance was the best when the mixed water samples were first treated with fiber cotton, and then treated with chitosan-modified bentonite. In agricultural production, nitrogen and phosphorus, as important fertilizer raw materials, play an important role in food production, but in recent years, the loss of nitrogen and phosphorus in soil has caused pollution to water resources. The retention rate of the two is not high in natural soil, while the retention rate of lanthanum-modified bentonite (LMB) reaches 93.9%. Therefore, lanthanum-modified bentonite can not only ensure the retention of nitrogen and phosphorus in the soil, but also control the pollution of agricultural production, and has a good application prospect.