Soil improvement using plant-derived urease-induced calcium carbonate precipitation

TLDR

This study presents plant‑derived urease‑induced calcium carbonate precipitation as an alternative to MICP for soil improvement, suggesting its use for strengthening loose sand, mitigating liquefaction, and restoring limestone monuments and soft rock formations. The method uses a crude extract of crushed watermelon seeds as urease, combined with calcium chloride and urea, to precipitate calcium carbonate in Mikawa sand specimens, with UCS measured while varying CaCl2‑urea concentration, urease activity, curing time, and temperature. The study found that higher CaCl2‑urea concentrations and longer curing times raise UCS, achieving up to ~3.0 MPa after 14 days with 0.7 M CaCl2‑urea and 3.912 U/mL urease, and that watermelon‑seed urease can replace commercial urease for low‑impact soil improvement.

Abstract

This study addresses a soil improvement technique using plant-derived urease-induced calcium carbonate (CC) precipitation (PDUICCP) as an alternative to microbially induced carbonate precipitation (MICP). A crude extract of crushed watermelon (Citrullus lanatus) seeds was used as the urease source along with calcium chloride (CaCl2) and urea (CO (NH2)2) for CC precipitation. Test specimens (φ = 2.3 cm, h = 7.1 cm) made from commercially available Mikawa sand (mean diameter, D50 = 870 µm) were cemented, and estimated unconfined compressive strength (UCS) of several kPa to MPa was obtained by changing the concentration of CaCl2- urea, urease activity, curing time, and temperature. The increase of curing time and that of the CaCl2-urea concentration from 0.3 M to 0.7 M caused an increase in estimated UCS value. The average estimated UCS obtained after 14 days' curing time for 0.7 M CaCl2-urea and 3.912 U/mL urease was around 3.0 MPa and for 0.3 and 0.5 M CaCl2-urea and 0.877 U/mL urease, it was around 1.5–2.0 MPa at 25 °C. By changing each of the abovementioned parameters, it may be possible to apply this method for strength improvement of loose sand, to mitigate the liquefaction, protection and restoration of limestone monuments and statuaries, and artificial soft rock formations. Crude urease from crushed watermelon seeds has the potential to replace commercially available urease for carbonate precipitation and for use as a low environmental impact type soil improvement method.