Metal Oxide Modification
Electrostatic attraction due to the electronegativity of vermiculite surface is the main physical adsorption. Surface area, pH and oxide loading are the main factors affecting the adsorption. According to the properties of pollutants, the effects of load and experimental conditions on vermiculite can be considered from coordination exchange, electrostatic repulsion, Lewis acid-base neutralization, hydrogen bonding, and complexation. For example, under acidic conditions, modified vermiculite surface with positive charge, easy to adsorb anionic pollutants, and cation electrostatic repulsion; The surface of the modified vermiculite is negatively charged and has strong binding ability with heavy metal ions in the higher pH environment without forming metal precipitation.
Yao Jun et al. used Fe3O4 magnetic modification of vermiculite to remove heavy metal Pb2+, desorption by magnet, and recycling. Ferric oxide modified vermiculite can increase the zero charge point of vermiculite and increase the surface charge and hydroxyl content. Silanol groups combine with some metal oxides and participate in the process of heavy metal adsorption. In addition, metal oxides with amphoteric characteristics, such as MnO2, also depend on the initial pH value of the target pollutant. Sari et al. claimed that the modification of manganese dioxide adsorbed Ag+ and significantly increased the negative charge. ChenMeiqing et al. loaded the vermiculite with relatively uniform fish scale MnO2 and used fixation, ion exchange, and oxidation reactions to remove thallium contamination in water.
Metal Modification
The difference of modified metal ions will affect the electrostatic force with water, and then affect the expansion temperature and size of vermiculite. In addition to ion exchange and electrostatic interaction, it can also precipitate and coordinate bond with contamination according to its characteristics. The modified ions such as lanthanum, iron and zirconium cause the base spacing of vermiculite to become wider. The metal ions with high valence enter the interlayer of vermiculite, exchange with cations such as magnesium, aluminum and calcium, and change the surface charge.
According to Thanos et al., lead nitrate modification can transform Cr6+ into insoluble lead chromate immobilized on the surface of modified vermiculite. Lima et al. increased the Ca2+ removal rate of vermiculite from 36.5% to 97.1% by using 20g/L sodium modified vermiculite adsorbent. Aluminum and iron loaded on the surface of vermiculite by salt modification, hydration and hydroxylation can play the role of both metal and clay minerals, and increase the phosphate adsorption effect.
Karathanasis et al. showed that mineral clays rich in hydroxyl interlayer vermiculite and montmorillonite had a significant adsorption effect on organophosphorus and inorganic phosphorus, and had a better adsorption effect than free aluminum or iron hydroxide, and organophosphorus had a higher adsorption effect than inorganic phosphorus.
Rare earth metals play a huge role in many industrial production fields. By checking the associated minerals of rare earth in nature, we can find the substances that are most easily bound to rare earth metals and the pollutants that are most easily adsorbed in the environment. In addition, the effect of coexisting ions can be ignored only when the concentration difference between target pollutants and other ions is large. Wei Ya huang et al. used La(OH)3 to change benign expansive vermiculite to remove phosphate, and the adsorption capacity of modified benign vermiculite was one order of magnitude higher than that of original vermiculite. Some lanthanum ions hydrolyze to form lanthanum hydrate to reduce the charge and promote more lanthanum into vermiculite.
