Application and Research Progress of Expanded Modified Python in Water Pollution Treatment

2023-01-28

The natural silicate mineral vermiculite is widely distributed in nature, has the characteristics of strong cation exchange capacity and large specific surface area, and has outstanding advantages in water pollution restoration after modification.

Expansion and exfoliation are not determined by the mineral properties of vermiculite itself. The multiphase mixed natural vermiculite composed of hydrous biotite and other minerals has the characteristics of expansion and exfoliation when exposed to high temperature or hydrogen peroxide.

In the process of thermal modification or chemical modification, other minerals embedded in the vermiculite block the escape of gas between the vermiculite layers, so that the interlayer pressure continues to increase, the pressure will open the vermiculite, the gas overflows, and the vermiculite Swelling and flaking. The biggest advantage of expanded vermiculite is that its specific surface area is larger, and the divalent metal ions in the octahedron between the vermiculite layers exchange with aluminum ions, the tetravalent silicon in the tetrahedron is replaced by aluminum, and the overall cation exchange capacity of the vermiculite increases. , cation exchange and exposed silanol groups and aluminum alcohol groups are the main mechanisms for the interaction between vermiculite and water pollutants.

1. Thermal Modification

The heating process of vermiculite leads to the removal of the water absorbed by the vermiculite itself and the bound water between the layers. After continuing to increase the temperature, the vermiculite structure is destroyed and collapsed until the temperature rises to remove the hydroxyl group and gradually transforms into enstatite. Below 400°C, the basic structure of vermiculite will not be destroyed, and if the temperature continues to rise, it will cause cracks in vermiculite until the phase transition. Thermal modification causes changes in the specific surface area and pore size, which affects the adsorption of pollutants. When dealing with heavy metals, it can play a role by forming hydroxide precipitates on the surface of vermiculite through ion exchange.

The modified vermiculite prepared by Researchers et al. by adding 4mol% H2SO4 glycerol at 580°C has a Langmuir adsorption capacity of 476.2mg/kg. The composition of vermiculite also affects the amount of vermiculite adsorption. Compared with the adsorption effect of nickel on the thermal modification of Brazilian vermiculite, the Chinese vermiculite under the same modification conditions has a higher content of mica, higher expansion rate, larger porosity, and better adsorption effect.

2. Microwave/H2O2 Modification

Microwave modified vermiculite is beneficial to the adsorption and removal of non-polar pollutants and heavy metals. It is more energy-saving and shorter than heat treatment. Vermiculite is not easy to embrittle after microwave modification, and microwaves cause vibration of polar molecules in vermiculite. Frictional heat is generated, and the water between the vermiculite layers and the adsorbed water are rapidly vaporized, increasing the internal pressure and causing the vermiculite to expand, which avoids the defect of uneven heating of thermal modification, because the heated and expanded part acts as a heat insulation barrier to block external heat Transfer to the part far away from the heat source.

Factors affecting the effect of microwave modification include microwave power, processing time, and particle size. H2O2 decomposes to produce oxygen and water vapor to promote the separation of vermiculite layers and break them into small pieces. Therefore, H2O2 soaking can make vermiculite expand and fall off. The higher the concentration of the soaking solution, the higher the degree of vermiculite exfoliation. The hydroxide ions released from the surface of the vermiculite neutralize the charges, so that the pH of the modified vermiculite mixture is lower than that of the original vermiculite, and part of the hydroxyl groups are lost during the modification process, resulting in the active sites of the modified vermiculite being lower than those of the original vermiculite. Raw vermiculite will decline.

After the vermiculite was modified by microwave for 40s at 480W, its volume expanded 7 times. Researchers found that larger vermiculite particles and higher microwave power can obtain higher expansion ratio. Researchers mixed hydrogen peroxide and microwave to obtain expanded modified vermiculite, which provided more action points for Na2S modification in the later stage.