Ureolysis-Driven Carbonate Precipitation

Ureolysis is the enzymatic hydrolysis of urea, and it is the most widely studied biochemical route to microbially induced calcite precipitation. The reaction is catalysed by the enzyme urease, which is produced by a range of soil bacteria.

The reaction chain

Urea hydrolysis proceeds in two steps:

1. Urease cleaves urea into ammonia and carbamate.
2. Carbamate hydrolyses spontaneously to a second molecule of ammonia and carbonic acid.

The net effect on the pore solution is twofold. Ammonia production raises the local pH, shifting the carbonate equilibrium toward the carbonate ion. Carbonic acid supplies dissolved inorganic carbon. Together these changes increase the saturation index with respect to calcium carbonate.

In the presence of dissolved calcium — commonly supplied as calcium chloride — the solution becomes supersaturated and calcite precipitates:

• The calcium cation combines with carbonate to form CaCO₃.
• Precipitation occurs preferentially on nucleation sites, including bacterial cell surfaces and existing grain surfaces.

Why ureolysis dominates MICP practice

Ureolysis is favoured in engineering applications because it is fast, produces a large pH shift per unit of substrate, and requires only urea and a calcium salt as feed — both inexpensive and readily available. The bacteria that carry it can be grown in bulk and delivered as a suspension.

Practical consequences and drawbacks

• Ammonia by-product. Ureolysis releases ammonia, which can volatilize as a gas or accumulate as ammonium in the pore water. This is the principal environmental objection to ureolytic MICP and the main reason alternative metabolisms are investigated.
• Feed composition matters. The relative concentrations of urea and calcium, and the rate at which they are supplied, determine whether precipitation occurs in the desired location or prematurely in the injection line.
• Uniformity is difficult. Precipitation tends to concentrate near the injection point, so achieving even treatment across a large volume requires careful cycling.

Evidence status in this wiki

This page describes the ureolysis route as it is generally understood in the published literature. The source dossier that prompted it — Microbially Induced Calcite Precipitation: The source archive Materials Dossier (Provisional Record) — was not readable in full at the time of writing, and no specific strains, concentrations, molarities or cycle counts from that dossier are reproduced here.

Related

• Microbially Induced Calcite Precipitation (MICP) — the process this route serves.
• Bio-Cementation — the engineering applications.
• Calcium Carbonate (Calcite, CaCO₃) — the precipitated mineral.
• Enzyme-Induced Carbonate Precipitation (EICP) — the cell-free variant that uses extracted urease.