Clay minerals heavy metal adsorption materials


release time:

2019-12-10

Compared with the disadvantages of poor heat resistance, poor radiation resistance and high cost of resin adsorbents, Guangxi clay mineral materials have obvious advantages for environmental adsorbents, such as cheap, abundant resources, easy handling, high strength and stability. It is a research hotspot of environmental adsorption materials in recent years, with huge application potential. It has become a trend to use Nanning clay minerals and their processing materials to deal with heavy metal pollution.

 

1. Environmental functional properties of clay minerals

Clay minerals refer to some hydrous phyllosilicate minerals mainly containing aluminum and magnesium, including bentonite, kaolin, vermiculite, sepiolite, illite, palygorskite, zeolite, diatomite, attapulgite, etc. .

The structure of clay minerals contains freely exchangeable inorganic cations, and some electrons of oxygen atoms are exposed on the crystal surface, so that clay minerals have two diametrically opposite abilities: good adsorption performance and self-purification ability.

During the formation of clay minerals, isomorphous substitutions often occur in their tetrahedral or octahedral structures, resulting in charge imbalance, and due to crystal breakage, oxygen atoms are exposed on their fracture surfaces. These properties make clay minerals The crystal face has a permanent negative charge, which attracts metal ions, and can produce coordination and combine to achieve the purpose of removal.

At the same time, the characteristics of small particles, unsaturated charges, huge specific surface area and the existence of adsorbed water and cations in the interlayer domain determine the ideal adsorption properties of clay minerals.

2. Application of clay minerals in heavy metal adsorption

A large number of studies have shown that the direct use of clay minerals (such as vermiculite, bentonite, sepiolite, etc.) or after simple activation treatment has shown a good adsorption effect on heavy metal ions.

(1) Bentonite

The surface of bentonite sheets has permanent negative charges, weak interlayer bonding force, and exchangeable cations. This unique structure endows bentonite with excellent cation exchange and adsorption properties. For example, bentonite is more efficient at removing copper, zinc, chromium, and nickel ions than illite and kaolin. Peng Ronghua et al. conducted hot acid modification of bentonite original soil and found that when the ion concentration was less than 45 mg/L, the removal rate of lead and cadmium ions was as high as 98%.

(2) Kaolin

The layers of kaolin are connected by hydrogen bonds, which are tightly bound and have no exchangeable cations. The original adsorption performance is not ideal. It needs to be modified to give full play to the adsorption performance of kaolin. Wu Pingxiao et al. found that the modification of kaolin with humic acid can improve the adsorption performance of three heavy metals: Cu2+, Cd2+, and Cr3+.

(3) Sepiolite

The upper and lower layers of the sepiolite structural unit are arranged in one-dimensional growth, which makes it have a fibrous body shape, a large specific surface area and abundant pores, showing good adsorption performance. After it is modified by heating (baking, roasting), hydrothermalization, acid treatment and ion exchange, its specific surface area increases, ion exchange capacity increases, and it has stronger adsorption capacity.

(4) Vermiculite

Vermiculite is very similar to montmorillonite in structure and layout, and has a larger specific surface area and higher ion exchange capacity. The mechanism of its adsorption of heavy metal ions is mainly cation exchange. Guo Jixiang and others found that, as an adsorbent for heavy metals in water, vermiculite has the advantages of large specific surface area, large adsorption capacity, light weight, and better adsorption effect than diatomite and sepiolite.

(5) Diatomaceous earth

Diatomite shell has a large number of nano-scale micropores arranged in an orderly manner. It is a natural nano-material and can absorb impurities equivalent to 3-4 times its own mass. Natural diatomite has adsorption effect on Pb2+, Cu2+, Cd3+. Through modification treatment, the adsorption capacity of diatomite can be further improved and its adsorption effect can be enhanced.