Inorganic Flocculant-Based Soybean Urease Extraction and its Effect on Biomineralization

Enzyme induced carbonate precipitation (EICP) based on self-extracted crude soybean urease solution (CSUS) is a promising method for soil improvement. However, the deionized water-extracted CSUS usually contains a large amount of impurities that easily lead to bioclogging during the biogrouting process, resulting in a nonuniform biomineralization effect. In this study, a purification method using inorganic flocculants is proposed to extract CSUS with relatively high purity and urease activity (UA) for EICP method. Seven commonly used inorganic flocculants were adopted in this study, including KAl(SO4)2•12H2O, AlCl3•6H2O, Al2(SO4)3•18H2O, Fe2(SO4)3, poly aluminum chloride, poly ferric sulfate, and poly aluminum ferric chloride. Three sets of tests, including CSUS extraction tests, solution tests, and sand column treatment tests, were conducted to investigate the feasibility and validity of this purification method. The test results show that inorganic flocculants could effectively reduce the turbidity of the extracted CSUS, avoid the bioclogging during biogrouting, and thus improve the biomineralization effect of the CSUS-based EICP method. Compared with deionized water-based CSUS, at least 60% of the impurities in CSUS can be removed at optimal flocculant contents when the soybean powder content is 100 g/L. On the other hand, the flocculants would also cause a reduction in the UA of the extracted CSUS. Considering the UA, turbidity, and biomineralization effect of the extracted CSUS, the optimal inorganic flocculants and their contents are recommended to be 3.0 g/L for KAl(SO4)2•12H2O, 2.0 g/L for AlCl3•6H2O, 2.5 g/L for Al2(SO4)3•18H2O under the tested conditions, respectively.