Due to structural factors, natural bentonite has a relatively large internal surface area, and the internal surface brings huge internal surface energy, which makes it have better adsorption capacity. Although the adsorption performance of bentonite is good, but under the condition of unmodified, when the natural bentonite is in water, the interlayer ion will appear hydrolysis reaction, reduce its adsorption capacity, which limits the application of bentonite in pollution treatment, therefore, it is necessary to modify the bentonite.
1, Bentonite modification technology
(1) high temperature modification
The water film on the surface of natural bentonite has a hindrance to 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 spatial structure of bentonite is expanded and the internal pores are opened, creating space for the adsorption of pollutants. However, when the calcination temperature is too high, the high temperature will damage the structure of bentonite, reduce the porosity and reduce the adsorption performance.
(2) Ultrasonic modification
Short-term ultrasound will make bentonite loose structure, layer spacing becomes larger, harmful heavy metal ions are more likely to enter; long-term ultrasound will make the bentonite crystal layer surface Si-O-Si bonds change, increase the metal ions and bentonite surface aluminum oxygen contact opportunities, so that bentonite on the specific adsorption of metal ions has been enhanced.
(3) metal modification and magnetic modification
The common metal modifications are Fe and La, and lanthanum modified bentonite (LMB) is a widely used adsorbent for phosphorus treatment. With the development of technology, metal ions are no longer simply used as metal modifiers, but are embedded in bentonite as magnetic agents.
(4) Acid modification
The interlayer ions of bentonite were originally Na, Ca2, Mg2, Al3, etc. Acid modification is to soak bentonite with acid, precipitate interlayer cations, dredge the pores between bentonite, and make the adsorbate more easily diffuse inside. At the same time, H enters the bentonite layer, replaces the original ion, weakens the interaction between the bentonite layer, and increases the cation exchange capacity (CEC) and adsorption capacity.
(5) Organic modification
Natural bentonite is hydrophilic because of the existence of interlayer ions, which is not conducive to the adsorption of organic pollutants. Organic modification is the use of functional groups in organic matter or organic matter to replace the cation of bentonite layer, which not only makes the resulting modified bentonite into lipophilic hydrophobicity, but also increases the layer spacing and strengthens the capacity of pollution and ion exchange. According to the principle of similar compatibility, the surface adsorption capacity of organic pollutants is improved.
(6) inorganic modification
Inorganic modification refers to the use of bentonite interlayer positive ion exchangeable characteristics, according to the hydrolysis reaction of inorganic materials, so that the metal ions into the bentonite layer to replace the interchangeable positive ions, so as to produce inorganic modified bentonite. After bentonite is modified by inorganic, 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 layer gap and positive ion exchange characteristics, the first use of inorganic polymers to expand its interlayer domain, and then use the activator 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 complex was studied. The results showed that with the increase of acid concentration, the adsorption capacity of Pb ion and Cu ion decreased in different 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 ion was better.
(2) Organic pollutants
Using sodium bentonite to adsorb parsley aldehyde, the adsorption results were obtained by different adsorbate concentrations and different solid-liquid ratios, and the maximum unit adsorption capacity was 110 mg/g. ANTONELLI et al. used thermally modified bentonite (CVL) to study the adsorption capacity of ciprofloxacin and found that the maximum unit adsorption capacity could reach 114.4 mg/g at 25 ℃.
The bentonite was modified by alkali, salt and surfactant respectively, and the removal rates of methyl blue were 73.25%, 81.62% and 85.06%, respectively. Cationic starch-bentonite system was used for adsorption treatment, and it was found that the cationic starch-bentonite system had obvious effect on the treatment of simple pollutants.
(3) Inorganic pollutants
It was found that the mixed water sample was treated with fiber cotton first, and then the mixed water sample was treated with chitosan modified bentonite, which had the best adsorption and flocculation performance. In agricultural production, nitrogen and phosphorus, as important chemical 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 natural soil was not high, while that of lanthanum modified bentonite (LMB) reached 93.9%. Therefore, the lanthanum modified bentonite can not only ensure the retention of nitrogen and phosphorus in the soil, but also control the pollution of agricultural production, which has a good application prospect.