Wastewater Treatment Basic Terminology: What Is Diatomite?
Formula: SiO2. Diatomite (diatomaceous earth) consists of amorphous SiO2 together with small amounts of Fe2O3, CaO, MgO, Al2O3 and organic impurities. It is usually pale yellow or light grey, soft, highly porous and very light. Industrially it serves as thermal insulation, filter media, filler, abrasive, raw material for water glass, decolorizing agent, filter aid and catalyst support.
Diatomite as a Catalyst Support
The support performance of diatomite depends mainly on its SiO2 content. In an industrial vanadium catalyst, for instance, the active component is V2O5, the promoter is an alkali-metal sulfate and the support is refined diatomite. Experiments show that SiO2 stabilizes the active component and that this effect is strengthened as the K2O or Na2O content rises. Catalyst activity is also related to the dispersion of the active phase and to the pore structure of the support. After acid treatment the oxide impurities in diatomite decrease, the SiO2 content rises and both specific surface area and pore volume increase, which is why refined diatomite outperforms natural diatomite as a support.
Origin: the Siliceous Remains of Diatoms
Diatomite is formed from the siliceous remains of unicellular algae collectively called diatoms; in essence it is hydrated amorphous SiO2. Diatoms live in both fresh water and salt water and are very diverse. They are generally divided into the centric diatoms (Centrales) and the pennate diatoms (Pennales), and each order contains many genera.
Natural diatomite consists mainly of SiO2. High-grade material is white and often contains more than 70% SiO2. A single diatom is colourless and transparent; the colour of the deposit as a whole depends on clay minerals and organic matter, so composition differs from one deposit to another.
Diatomite is a fossil deposit formed after diatoms — unicellular plants — died and accumulated over roughly 10,000 to 20,000 years. Diatoms are among the earliest protists on Earth and live in seawater or lake water. Their remains settle and, under specific geological conditions, a diatomite deposit is formed.
Physical and Chemical Properties
Its distinctive properties are high porosity, low density, a large specific surface area, relative incompressibility and chemical stability. Crushing, sorting, calcination, air classification and impurity removal change the particle-size distribution and surface properties, after which the material suits paints, coatings additives and many other industrial uses.
Mineralogically, diatomite is composed mainly of opal and its varieties, followed by clay minerals — hydromica (illite) and kaolinite — and mineral detritus such as quartz, feldspar and biotite, plus organic matter. Organic content ranges from a trace to more than 30%. Colours range from white, greyish white, grey and light greyish brown to yellow and green. The material is fine, loose, light, porous, strongly water-absorbent and permeable.
Typical physical data: density 1.9–2.3 g/cm3; bulk density 0.34–0.65 g/cm3; specific surface area 40–65 m²/g; pore radius 50–3000 nm; pore volume 0.45–0.98 cm³/g; water absorption 2–4 times its own volume; melting point 1650–1750 °C. Impurities such as Al2O3 and Fe2O3 may form low-melting eutectics and lower the melting point to 1400–1600 °C.
Chemically, diatomite is mainly SiO2 with minor Al2O3, Fe2O3, CaO and MgO, trace elements such as Ba, Be, Cr and Ni, and a certain amount of organic matter.
Opal and Associated Minerals
Opal (from the Latin opalus) is a hydrous silica gel mineral formed when part of the water is removed from a silica hydrogel, and is commonly described as a colloidal mineral. Opal itself is colourless; translucent varieties show a milky opalescence, and adsorbed coloured impurities and ions give it various tints — iron staining produces yellowish brown to brown, carbonaceous staining greyish black. Opal occurs as dense masses, stalactites, nodules, concretions and spherulitic aggregates, and as secondary fillings in sedimentary rocks. On dehydration, ageing or stress it develops cracks and a brecciated appearance and converts into quartz or chalcedony.
Clay minerals and carbonaceous matter are the main associated minerals. Clay minerals occur as microscopic flakes around the diatom particles and act as a cement where they dominate. Carbonaceous matter occurs as spots, lumps or layers and is peat or lignite of very low metamorphic grade that still preserves plant structure.
Adsorption Performance and Surface Hydroxyl Groups
The adsorption performance of diatomite is closely related to the hydroxyl groups on its surface and in its micropores and improves as their number increases. When ambient conditions change — at high temperature in particular — these hydroxyl groups are transformed, and the transformation strongly affects adsorption. The hydroxyl groups are also chemically active: they allow functional groups to be bound or grafted onto the surface, which alters adsorption behaviour and makes chemical modification a practical route to better performance.
Physical characteristics largely determine adsorption. Capacity generally rises with specific surface area, and the diffusion rate of adsorbates inside the pores increases with pore diameter, helping the system reach equilibrium. At a fixed pore volume, however, specific surface area is inversely proportional to pore diameter: larger pores mean lower surface area and lower equilibrium capacity. At a fixed pore diameter, a larger pore volume raises capacity.
Modification Methods
Calcination. For diatomite with a high loss on ignition, calcination is a very effective way to improve adsorption. High-temperature calcination markedly raises the SiO2 content, enlarges the pore diameter and increases surface acid strength. Calcination and acid washing both help to increase specific surface area and pore volume.
Acid modification. Strong-acid etching removes impurities from the surface of the diatom particles, increases purity, lowers density, enlarges pore volume and specific surface area, and markedly improves the pore structure — and therefore the adsorption performance.
Inorganic composite modification. Xiang Hongxia and co-workers modified diatomite with sodium carbonate and saturated calcium chloride, then combined the modified material with manganese-resistant bacteria to adsorb Mn(II). The composite showed a clearly improved uptake, with a saturated adsorption capacity of 56.18 mg/g.
Organic modification. Organic modification means grafting functional macromolecules onto the diatomite surface to raise adsorption capacity. Li Menlou and co-workers treated diatomite with 10% cetyltrimethylammonium bromide; capacity rose from 39.3 mg/g to 61.1 mg/g, an increase of 35.68%.
Main Industrial Applications
Filter aid. This is one of the largest uses of diatomite, with the widest range of grades, applications and volumes. Filter aids are powdered products made from high-grade diatomite; they raise filtration rate and clarify the filtrate. Because diatomite removes bacteria, impurities and odours, it gives stable performance and good adaptability, and is widely used in brewing and pharmaceutical production — the classic beer filter aid.
Functional filler. Diatomite is non-toxic and harmless and settles slowly in liquids, so it is added to other materials to improve strength, acid resistance, abrasion resistance and stability. Worldwide consumption as a functional filler is second only to filter aids, at about 20% of total output.
Wastewater treatment. Adsorption is effective for both domestic and industrial wastewater. Diatomite has high adsorption and decolorizing capacity and costs less than activated carbon, making it a promising substitute. Dosing raw diatomite directly into wastewater gives low removal efficiency, however; calcination, compounding or modification before use greatly improves purification efficiency.
Catalyst support. A large specific surface area, high porosity and resistance to acid, heat and abrasion make diatomite an ideal catalyst support. It is used in hydrogenation, dehydrogenation, oxidation and reduction reactions — for example as the support of nickel hydrogenation catalysts, of vanadium catalysts in sulfuric acid production and of phosphoric acid catalysts in the petroleum industry. It can also be impregnated to make heterogeneous catalysts for biodiesel synthesis from waste vegetable oil, used as a modified carrier to improve biofilm activity and stability in moving-bed biofilm reactors, or loaded with platinum-group or rare-earth metals for automotive exhaust treatment.
Coatings. Diatomite is specified by many large international coatings producers and is widely used in diatom mud, emulsion paints, interior and exterior wall coatings, alkyd and polyester systems, and especially architectural coatings. It levels film gloss, improves abrasion and scratch resistance, controls moisture and odours, and contributes to air purification, sound insulation, waterproofing and thermal insulation. Its large pore volume shortens drying time and reduces the amount of resin required, lowering cost — a cost-effective matting agent.
Agriculture. Diatomite is pH-neutral, non-toxic, well suspended, strongly adsorbent and light, with an oil absorption of 115% and a fineness of 325–500 mesh. It mixes evenly, does not clog spray lines, retains moisture in soil, loosens it and extends the effect of pesticides and fertilizers. It is also used in compound fertilizers for fruit trees, vegetables, flowers and field crops.
Rubber. Diatomite is used as a filler in tyres, rubber hoses, V-belts, rollers, conveyor belts and car mats. It increases rigidity and strength, gives a sedimentation volume of 95%, and improves heat resistance, abrasion resistance, thermal insulation and ageing resistance.
Paper. At a fineness of 120–1200 mesh, diatomite makes paper smoother, lighter and stronger and reduces swelling caused by humidity changes. In cigarette paper it regulates the burning rate without toxic side effects; in filter paper it improves filtrate clarity and speeds up filtration.
Feed. Diatomite is used as an additive in feed for pigs, poultry, fish and birds. It is pH-neutral and non-toxic, disperses evenly, binds with feed pellets instead of segregating, aids digestion, adsorbs and removes bacteria from the digestive tract, and improves water quality and survival rates in aquaculture ponds.
Building materials and indoor air quality. Diatomite is used for thermal and acoustic insulation, tiles and ceramics, and as a cement additive that raises compressive strength. Diatomite-based interior and exterior coatings and diatom mud release no harmful chemicals, are non-combustible, sound-insulating, waterproof, light and thermally insulating, and also regulate humidity, remove odours and purify indoor air. After Japan's indoor-air problems of the 1980s, regulators tightened limits on emissive building materials and encouraged low-emission alternatives; diatomite wall materials were launched on the Beijing market at China's second Green Building Materials Expo in October 2003, presented as removing free formaldehyde, benzene, ammonia and VOCs as well as pet and household odours.
