In vitro screening of crude urease extract from native Australian plants and weeds for bio-cementation of sands

Abstract

Urease enzymes from plants are directly utilised in enzyme-induced calcite precipitation (EICP) to catalyse calcium carbonate (CaCO3) precipitation between soil particles to improve their strength and stiffness. EICP does not require microbial culture and may be applicable for finer soils with their smaller size. However, most studies on EICP utilise purified urease enzymes, which are often rare, expensive and limited to food-grade beans or seeds, making the technique less cost-effective. To find alternative and cheaper sources, crude urease extracts from diverse plant species, particularly weeds and Australian native plants, were obtained, identified and characterised using a series of in vitro experiments. All selected plant species contain a considerable amount of urease enzyme, exhibiting different urease activity and calcium carbonate precipitation. EICP treatment by different crude extracts showed different levels of hydraulic conductivity, unconfined compressive strength (UCS), wind and raindrop erosion resistance. However, the crude extract from an Australian weed, matured paddy melon seeds (M-PMS), had the highest specific activity of 8997 U/mg and was further used for EICP treatment of six different soil types. The UCS of treated soils was influenced by the soil types. For similar calcium carbonate content, the strength and stiffness of the EICP-treated specimens were significantly influenced by the confining stress and degree of saturation in undrained triaxial conditions. Also, the mass loss, strength and average calcium carbonate decreased with increasing cycles of wetting–drying, freezing–thawing and elevated temperature. When compared with purified urease enzyme, M-PMS produced comparable strength and durability resistance in the treated samples.