Application of Laser-Induced Breakdown Spectroscopy to quantify the changes in plasma characteristics in calcite-precipitated soils

Abstract

The changes in calcium carbonate precipitation of modified soil by Enzyme Induced Calcite Precipitation (EICP) were investigated using Laser-Induced Breakdown Spectroscopy (LIBS). This experiment enabled elemental composition analysis of the soil samples. The plasma electron density and temperature were also extracted from the collected spectra of distinct soil samples i.e., W1 and W2 treated with EICP. The type of the crystal of CaCO3 deposited during the treatment process was investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. This research was conducted by utilizing an Nd: YAG laser with a wavelength of 532 nm, a pulse duration of 6 ns, and an energy of 25 mJ, from where the plasma temperature and electron density of modified soil throughout several treatment cycles were analyzed. The spectra were collected from different locations within the sample and the spectral range considered for the study was 370 nm-800 nm. The plasma electron density and plasma temperature determined using LIBS spectral data increase with the increase in the treatment cycle. With the increase in the treatment cycle, the deposition of CaCO3 was found to increase and correspondingly plasma temperature increased by 220 % for soil W1 and 117.3 % for soil W2. A similar rise in plasma electron density was noticed at the highest cycle of the treatment process; plasma electron density is augmented by 106.4 % for W1 and 21.50 % for W2. This investigation of the LIBS spectral data information indicates a positive correlation between the number of treatment cycles and the plasma temperature, meaning that as the treatment cycles rise, the plasma temperature also increases indicating a larger deposition of CaCO3. The findings are valuable in the domain of geotechnical engineering, where lasers are extensively employed for the examination of the soil profile and can prove to be a useful method for the characterization of the modified soils.