Comparison of the adsorption gap between zeolite and activated carbon

Zeolite itself has a multi-regular pore structure, and its pores are comparable to the molecular size of general substances, with a diameter of about 0.3~1nm, and its specific surface area is as high as 400~800m2/g. The special physical structure makes zeolite have two adsorption characteristics: selective adsorption and efficient adsorption. Relying on its strong adsorption properties, zeolite has been widely used in many fields such as water treatment, soil remediation, green building materials, petroleum processing, and daily chemical industry, and its adsorption properties are still being further explored by researchers.

The special structure of zeolite

The basic building blocks of zeolites are silica-oxygen tetrahedra and alumina-oxygen tetrahedra, whose structure in the plane is shown in Figure 1 below, where the silica-oxygen tetrahedra are linked to each other by sharing all the oxygen.

Microscopic observation shows that the zeolite structure can be divided into 3 parts: the aluminosilicate lattice; the interconnected pores (pores or cavities) in the lattice; and the cations and water molecules in the pores or cavities. This special structure of zeolite determines its strong adsorption property, so it is now widely used as an excellent adsorbent.

Adsorption properties of zeolites

Unlike traditional solid adsorbents (e.g., activated carbon), zeolite adsorption has two distinctive features, namely, selective adsorption and efficient adsorption.

Selective adsorption

The pore size of zeolite is uniformly distributed and mostly below 10 nm, which is comparable to the molecular size of general substances, and thus has the selective adsorption property of molecular sieve. In general, the large central cavities and pore channels of zeolites are filled with water molecules that form hydrated spheres around exchangeable cations.

Typically, zeolites will lose water when heated at 350-400°C for several hours or longer. At this point, molecules with an effective diameter small enough to pass through the pores will be readily adsorbed by the zeolite in the dehydration pores and central cavities, while molecules with a diameter too large to enter the pores will be rejected, which is known as the sieving ability of the "molecular sieve". 

In the structure of zeolite, the negative charges on the anion lattice and the positive charge centers of the balanced cations do not overlap in space, so the zeolite has a large electrostatic attraction inside. It is this electrostatic force that makes zeolites have preferential selective adsorption for polar, unsaturated and easily polarized molecules. In addition, molecular sieve has a selective effect on the polar size of molecules, the more polar can be the more easily polarized substances, the easier to be adsorbed.

According to a fishery company test it can be seen that the same weight of zeolite and activated carbon were put into the pond at the same time, the adsorption data of the two groups of materials can be compared. In the better absorption efficiency of ammonia was the experimental group using zeolite, which could reduce to 5.57 mg/L of toxic gas, a reduction of 58.1% compared to the original sample, but under the same conditions, activated carbon only reduced 1.49 mg/L of toxic gas, a reduction of 15.9% compared to the original sample. 

In addition, the pH value in the water samples treated with activated carbon gradually increased from 7.2 to 9.13 during the experiment. the reason was that the activated carbon adsorbed nitrogen ions, resulting in a super high pH value, while the water samples treated with zeolite had a relatively stable pH value.

The test once again proved that zeolite can be used as a high-quality adsorbent in water treatment and other fields, and has strong performance in adsorbing harmful substances and heavy metals, and its pH value is more stable and safe in the process.