Impact of solution chemistry on morphology of enzyme-induced calcium carbonate precipitate

Abstract

Enzyme-induced calcium carbonate precipitation (EICP) through the urea hydrolysis pathway has been widely studied for various applications. The EICP solution comprises urea, a calcium source (usually calcium chloride) and the enzyme urease. This study addressed the effect of the chemical concentration of the EICP solution on the morphology of the calcium carbonate product. This was achieved by varying the concentration of urea–calcium chloride and urease activity. The duration of the reaction was the third variable. The precipitation efficiency and the interface shearing resistance were reported. Precipitation efficiency decreased as the concentration of urea–calcium chloride reached beyond 0.75 mol/l. The calcium carbonate polymorph was predominantly calcite. Its crystal size and shape did, however, vary, depending on the precipitation conditions. The findings showed that the urease activity promoted the formation of rhombohedral calcite in the presence of adequate calcium ions and urea. Spherical calcite was formed when the urease activity was further increased. The morphology of calcite evolved from a single, uniform, smooth spherical crystal to a polycrystalline formation with orthorhombic protrusions. The crystals tended to grow as the reaction time increased, resulting in aggregation, when the urease levels crossed 30 kU/l. It was noted that spherical crystals exhibited stronger interface shearing resistance than rhombohedral crystals.