Aggregate Formation in a Kaolinite Suspension during Microbiological Modification of Clay Surface

Abstract

An aggregate-forming effect of the microorganisms that modify a hydrophilic surface of minerals to an amphiphilic one was demonstrated in a model experiment: a sterile liquid medium was supplemented with the microorganism Bacillus velezensis washed off from river sand and with kaolinite, which contains potassium ions (up to 1%) in a sorbed state and in admixtures. An increase in the protein, carbon, and nitrogen contents was observable over two months of incubation; the C : N ratio reached a value close to 5. It was assumed that the microorganisms consuming potassium ion necessary for their vital activity produce protein compounds in the form of specific secretions of glycoprotein and polysaccharide natures. These compounds hydrophobized the surface of minerals (the contact angle increased from 20° to 40°), changed the specific surface area, and increased the share of microaggregate fractions (50–250 and 250–500 µm) by 5.4 and 1.5%, respectively. The incubation with B. velezensis hydrophobized the kaolinite surface as compared with the control specimens, most likely because of an amphiphilic nature of the microbial products. In turn, the hydrophobization of mineral surface induced a hydrophobic interaction between mineral particles thereby forming organomineral microaggregates.