Optimization of Enzyme-Mediated Calcite Precipitation as a Soil-Improvement Technique: The Effect of Aragonite and Gypsum on the Mechanical Properties of Treated Sand

Abstract

Abstract:The effectiveness of magnesium as a substitute material in enzyme-mediated calcite precipitation was evaluated. Magnesium sulfate was added to the injecting solution composed of urea, urease, and calcium chloride. The effect of the substitution on the amount of precipitated materials was evaluated through precipitation tests. X-ray powder diffraction and scanning electron microscopy analyses were conducted to examine the mineralogical morphology of the precipitated minerals and to determine the effect of magnesium on the composition of the precipitated materials. In addition to calcite, aragonite and gypsum were formed as the precipitated materials. The effect of the presence of aragonite and gypsum, in addition to calcite, as a soil-improvement technique was evaluated through unconfined compressive strength tests. Soil specimens were prepared in polyvinyl chloride cylinders and treated with concentration-controlled solutions, which produced calcite, aragonite, and gypsum. The mineralogical analysis revealed that the low and high concentrations of magnesium sulfate effectively promoted the formation of aragonite and gypsum, respectively. The injecting solutions which produced aragonite and calcite brought about a significant improvement in soil strength. The presence of the precipitated materials, comprising 10% of the soil mass within a treated sand, generated a strength of 0.6 MPa.

Applicability of Natural Zeolite for NH-Forms Removal in Enzyme-Mediated Calcite Precipitation Technique

“…The precipitated amounts, ranging from 2.2-8.8% of the soil mass and corresponding to the strength of 24-305 kPa, were obtained by one to three PV injections. In comparison to the previous studies, in which the EMCP technique was conducted without zeolite [10,34], the precipitated amount and the strength obtained in this work were relatively similar. With a low concentration of reagents (R = 0.5 mol/L), the strength derived from Putra et al [34] was slightly higher than that obtained in this study.…”

“…Unconfined compressive strength (UCS) tests were performed to evaluate the applicability of the grouting materials for improving the soil strength. The experimental procedures developed by Putra et al [10] were followed in this work. Sand specimens of silica sand with e max , e min , coefficient of uniformity (CU), and specific gravities (Gs) of 0.899, 0.549, 1.550, and 2.653, respectively, were used in this study.…”

“…Urease with concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0 g/L were used to dissociate the 0.5 and 1.0 mol/L of urea in the mixed solution of the reagent composed of urea-CaCl2. The test tube results are shown as a precipitation ratio: the ratio of the actual mass of precipitated minerals to the theoretical mass of the maximum precipitation of CaCO3 in a solution of 30 mL, in Figure 3 [10,33]. The reagent concentration of 0.5 mol/L produced a higher precipitation ratio than that of 1.0 mol/L.…”

“…In this study, the urea and CaCl 2 were obtained from Kanto Chemical (Tokyo, Japan), while the enzyme urease (020-83242, Kishida Chemical, Osaka, Japan) with the urease activity of 2950 U/g, was purified from jack bean meal. The precipitated amount, corresponding to the different combinations of reagent and urease concentrations, were evaluated in our previous work [10,33]. Urease with concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0 g/L were used to dissociate the 0.5 and 1.0 mol/L of urea in the mixed solution of the reagent composed of urea-CaCl 2 .…”

“…In this technique, an enzyme of urease is employed to dissociate the urea into ammonium and carbonate ions. The produced carbonate ions are precipitated as calcium carbonate crystals in the presence of calcium ions [10]. The reactions of the urea hydrolysis and the calcite formation are shown in Equations (1)- (5 The EMCP technique produces ammonia and ammonium as the byproduct of urea hydrolysis (see Equations (1) and (2)).…”