2489
Carbonate Cementation via Plant Derived Urease
Cimentation carbonatée par l’utilisation d’uréase issue de plantes
Hamdan N., Kavazanjian Jr. E., O’Donnell S.
School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 85287-5306; PH:
(480) 965-3997
ABSTRACT: The use of plant-derived urease enzyme to induce calcium carbonate (CaCO
3
) cementation has been demonstrated
through laboratory column tests. Benefits of the use of plant-derived urease over the use of microbially-generated urease to induce
carbonate cementation include the small size of the enzyme, which permits penetration into finer grained soils and makes the process
less sensitive to bioplugging, and the availability of 100% of the carbon in the substrate for conversion to CaCO
3
. The laboratory
column tests employed both Ottawa 20-30 silica sand and finer-grained F-60 silica sand. The laboratory column specimens were
prepared in a variety of manners and showed varying degrees of cementation and carbonate yield. Triaxial tests performed on
cemented specimens showed significant strength increases over non-cemented specimens. These tests confirm the feasibility of using
plant-derived urease to induce carbonate cementation in sand and provide valuable insight into the factors that must be considered in
developing practical applications for ureolytic carbonate precipitation using plant-derived urease enzyme.
RÉSUMÉ : La cimentation de sable par du carbonate de calcium (CaCO
3
) produit par l’enzyme uréase obtenue à partir de plantes a
été réalisée en laboratoire. Les avantages d’utiliser de l’uréase obtenue de plantes plutôt que de l’uréase produite microbilogiquement
pour produire la cimentation carbonatée sont la petite taille de l’enzyme qui permet la pénétration dans les sols fins et rend le
processus moins sujet au colmatage biologique et la disponibilité à 100% du carbone présent dans le substratum pour conversion en
CaCO
3
. Des essais en colonnes ont été réalisés sur deux sables de silice dits Ottawa 20-30 et F-60 (plus fin). Les échantillons ont été
préparés de différentes manières et ont atteint des degrés de cimentation variés et des productions de carbonate différentes. Les
résultats des essais de compression triaxiale sur des échantillons cimentés et des échantillons non-cimentés indiquent que les premiers
sont beaucoup plus résistants. Ces essais confirment que l’uréase obtenue à partir de plantes peut être utilisée pour induire une
cimentation carbonatée dans les sables. De plus ces essais ont permis de d’identifier les facteurs à considérer pour développer des
applications pratiques pour l’utilisation de la précipitation carbonatée « uréolytique » en utilisant l’uréase issue de plantes.
KEYWORDS: carbonate, cementation, urease, calcite, soil improvement
1. INTRODUCTION
1.1
Background
The potential for using plant-derived urease enzyme to cement
sands by inducing calcium carbonate (CaCO
3
) precipitation has
been demonstrated through a series of laboratory column tests
on two different gradations of silica sand. The use of
microbially induced carbonate precipitation (MICP) to cement
cohesionless soils has recently received substantial attention
from geotechnical researchers (Burbank et al. 2012, Chou et al.
2011, Dejong et al. 2010, Harkes et al. 2010, van Paassen et al.
2010). The MICP mechanism most often discussed in the
literature and most advanced in terms of field application is
hydrolysis of urea (ureolytic hydrolysis). MICP via ureolytic
hydrolysis relies on microbes to generate urease enzyme, which
then serves as a catalyst for the precipitation reaction. The use
of plant-derived urease (enzymatic ureolytic hydrolysis) to
induce CaCO
3
precipitation eliminates the need for microbes in
the CaCO
3
precipitation process.
Besides eliminating the need to nurture urease-producing
microbes, enzymatic ureolytic hydrolysis offers several other
advantages over ureolytic MICP. Applications of ureolytic
MICP on clean sands in laboratory column tests and limited
field tests have encountered significant practical difficulties,
including bioplugging (permeability reduction accompanying
induced mineral precipitation) and generation of a toxic waste
product (ammonium salt) (Harkes et al. 2010, van Paassen et al.
2008). Bioplugging not only limits the distribution of
precipitation agents within the soil but also makes flushing of
the waste product from the soil a difficult, energy intensive task.
Due to these limitations, mass stabilization of soil using
ureolytic MICP remains problematic. Furthermore, the microbes
that produce the urease enzyme cannot readily penetrate the
pores of soils smaller than medium to fine sand, limiting the
minimum grains size of soils amenable to ureolytic MICP to
clean fine sands or coarser graded soils. The small size (on the
order of 12 nm) of the urease enzyme suggests that CaCO
3
precipitation by enzymatic ureolytic hydrolysis will be less
susceptible to bio-plugging and will be able to penetrate finer
grained soils, perhaps into the silt-sized particle range,
compared to MICP processes.
1.2
Sustainability of Ground Improvement Practices
Finding effective solutions to ground improvement challenges is
becoming increasingly complex due to sustainability
considerations. Established materials and methods often need
to be either replaced or supplemented by innovative materials
and environmentally-friendly practices to address sustainability
considerations. One example of a common building material
that poses significant sustainability concerns is Portland cement.
Portland cement is widely used in ground improvement
applications. Unfortunately, Portland cement production is
extremely energy intensive and a major source of emissions of
carbon dioxide (CO
2
), as well as of sulfur and nitrogen oxides.
MICP has been explored recently as an alternative to Portland
cement for ground improvement. Reductions in the use of
