Bentonite modification and its application in environmental governance

Due to structural factors, the inner surface area of natural bentonite is relatively large, and the inner surface brings huge inner surface energy, so that it has good adsorption capacity. Although the adsorption performance of bentonite is good, under the condition of unmodified, when the natural bentonite is in water, the interlayer ions will have a hydrolysis reaction, which reduces its adsorption capacity, which also limits the application of bentonite in pollution treatment, therefore, it is necessary to modify the bentonite.

1. Bentonite modification technology

(1) High temperature modification

After high temperature roasting, the water in various forms inside the bentonite will evaporate, and take away some impurities, so that the spatial structure of bentonite is expanded, and the internal pores are opened, creating space for the adsorption of pollutants. However, when the roasting temperature is too high, the high temperature will cause damage to the bentonite structure, reduce its porosity, and reduce the adsorption performance.

(2) Ultrasonic modification

Short-term ultrasound will make the bentonite structure loose, the layer spacing will be larger, and harmful heavy metal ions will be more likely to enter. Long-term ultrasonic will change the Si-O-Si bond on the surface of the bentonite crystal sheet, increase the contact opportunity between the metal ions and the aluminum oxygen position on the surface of the bentonite, and enhance the obligate adsorption of metal ions by bentonite.

(3) Metal modification and magnetic modification

Common metal modifiers are Fe and La, among which lanthanum-modified bentonite (LMB) is an adsorbent widely used in phosphorus treatment. With the development of technology, metal ions no longer play a role simply as a metal modifier, but are embedded in bentonite with the function of a magnetic agent.

(4) Acid modification

The bentonite interlayer ions were originally Na+, Ca2+, Mg2+, Al3+, etc., and acid modification is to soak bentonite with acid, precipitate interlayer cations, dredge the pores between bentonite, and make the adsorbate easier to diffuse internally. At the same time, H+ enters the bentonite interlayer to replace the original ions, weaken the bentonite interlayer force, and increase 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 the functional groups or organic matter in the organic matter to replace the cation of the bentonite layer, which not only makes the obtained modified bentonite transform into lipophilic and hydrophobic, but also increases the layer spacing, strengthens the dirt-holding capacity and ion exchange capacity. According to the principle of similarity compatibility, its surface adsorption capacity for organic pollutants is improved.

(6) Inorganic modification

Inorganic modification refers to the use of the interlayer positive ion exchangeable characteristics of bentonite, according to the hydrolysis reaction of inorganic materials, so that the metal ions in it enter the bentonite interlayer to exchange interchangeable positive ions, so as to prepare inorganic modified bentonite. After inorganic modification of bentonite, the layer spacing is significantly expanded, the specific surface 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 layer voids and positive ion exchangeable characteristics, first using inorganic polymers to expand its interlayer domain, and then using activators to change the surface properties of bentonite.

2. Application of modified bentonite in environmental governance

(1) Heavy metal pollutants

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 complex was studied. The results showed that with the increase of acid concentration, the adsorption capacity of Pb ions and Cu ions decreased to varying degrees. The adsorption of heavy metal pollutants by polyanionic cellulose and modified bentonite showed that the adsorption effect of heavy metal pollutants was good.

(2) Organic pollutants

The adsorption results were obtained by different adsorbate concentrations and different solid-liquid ratios, and the maximum unit adsorption capacity was 110mg/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 114.4mg/g at 25°C.

The alkali modification, salt modification and surfactant modification of bentonite were carried out, and it was found that the removal rates of methyl blue by three different modified bentonite were 73.25%, 81.62% and 85.06%, respectively. The cationic starch-bentonite system was used for adsorption treatment, and it was found that the cationic starch-bentonite system had obvious treatment effect on simple pollutants.

(3) Inorganic pollutants

It was found that the adsorption and flocculation performance of the mixed water samples was the best when the mixed water samples were treated with fiber cotton and then treated with chitosan modified bentonite. In agricultural production, nitrogen and phosphorus are important fertilizer raw materials and play an important role in food production, but in recent years, the loss of nitrogen and phosphorus in the soil has caused pollution to water resources. The rejection rate of natural soil is not high, while the interception rate of lanthanum-modified bentonite (LMB) is 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.