Kothuri, MahindraDevatha, C. P.2026-01-242023https://idr.nitk.ac.in/handle/123456789/18840Waste management is an intricate and pressing global challenge that demands our immediate attention and concerted efforts to address the environmental impacts resulting in widespread pollution and adverse effects on human health. To combat these challenges, it is imperative to develop innovative and eco-friendly solutions that mitigate the negative consequences of waste and promote sustainable practices for a cleaner and healthier future. Waste materials, including biomedical ash and ferrochrome ash, contain toxic heavy metals such as Pb, Ni, and Cr, which pose risks to the environment and human health. Hence, the present study aims to assess the effect of biomineralization to immobilize the heavy metals and allowing the safe application of biomedical ash (BMA) and ferrochrome ash (FCA) in enhancing the properties of black cotton soil (BCS). The research objectives encompass evaluating microbial growth in waste ashes, identifying the optimal protein source for bacterial urease production, optimizing nutrient medium composition for enhanced biomineralization, and analyzing the impact of biomineralization on the engineering properties of waste ashes as environmentally friendly soil stabilizers. To address these objectives, a three-stage methodology has been adopted. In the first stage, the investigation focused on the presence of bacteria in waste ashes. Indigenous urease-positive bacteria were isolated using the serial dilution technique with Christensen's agar medium and identified through 16SrRNA analysis. The study identified Bacillus cereus and Lysinibacillus sphaericus as suitable urease-positive bacteria for biomineralization in BMA and ferrochrome ash FCA, respectively. The performance of different legumes as protein sources were compared by monitoring pH, optical density, and urease activity over time. Blackgram and soybean were identified as the most suitable protein sources for bacterial growth and urease activity. The second stage of the research involved integrating biomineralization into biomedical ash for stabilizing black cotton soil. Response surface methodology (RSM) using a central composite design (CCD) was employed to model the role of protein, vitamin, and carbon sources in urease activity. The study determined the optimal combination ii of 23.47 g/L black gram, 3.45 g/L yeast extract, and 0.03 g/L dextrose while also observing that the dosage of protein and vitamin sources significantly impacted bacterial growth and urease activity. The production of mineralized biomedical ash consisted of combining equimolar urea & calcium chloride (1 M) and the acquired bacteria with biomedical ash. Leachate extracted from mineralized biomedical ash demonstrated reduced concentrations of Hg (97 %), Cr (96 %), Zn (97 %), Pb (93 %), Fe (94 %), Cu (93 %), Cd (98 %), Ba (87 %), As (96 %), Ti (88 %), and Se (86 %), indicating the effectiveness of biomineralization in immobilizing heavy metals. To assess the influence of mBMA on soil characteristics, black cotton soil (BCS) was subjected to different proportions (10 %, 20 %, 30 %, and 40 %) of mBMA. At 40 % soil replacement (m40) by mBMA, the soil’s liquid limit (LL), plastic limit (PL), plasticity index (PI), and free swell index (FSI) were 47 %, 34 %, 13 %, and 14 %, respectively. The corresponding values in the same order are 53 %, 25 %, 27 %, and 114 % for BCS. The optimal moisture content (OMC) shifted from 22 % for BCS to 26 % for m40. The corresponding maximum dry density (MDD) reduced from 1.596 g/cm3 for BCS to 1.458 g/cm³ for m40. These values indicate improved soil consistency and reduced compressibility due to adding mBMA to BCS. Notably, the highest unconfined compressive strength (UCS) of 147 kPa was observed for m30 (30 % BCS replaced with mBMA). UCS of BCS was determined as 35 kPa. Characterization studies (XRD, FEGSEM, FTIR, TGA) were conducted on mBMA. X-ray diffraction analysis detected significant amounts of calcite while scanning electron microscope images revealed the presence of dense matter connecting the ash particles, which was identified as calcite formed during biomineralization. Fourier transform infrared absorption bands corresponding to carbonates further supported the occurrence of biomineralization. A 10 % weight reduction in the characteristic thermal decomposition range for calcium carbonate also confirmed its presence due to biomineralization. Calcite was identified in mBMA through XRD, with peaks observed at 23.03 °, 29.38 °, 25.47 °, 31.34 °, 35.98 °, 39.41 °, 43.15 °, 45.44 °, 48.48 °, and 57.40 °. Dense matter connecting the ash particles was observed in FESEM images of mBMA. It is believed to be the calcite formed during biomineralization. The carbonate presence was backed iii by the FTIR absorption bands at 711.6 cm-1, 873.6 cm-1, and 1420.3 cm-1. A 10% weight reduction in the characteristic thermal decomposition range (570 °C to 660 °C) for calcium carbonate confirmed its advent during biomineralization. In the third stage of the present study, FCA was employed in conjunction with biomineralization to stabilize BCS. Through optimization of the quadratic model, an ideal combination of 20 g/L soybean, 3 g/L yeast extract, and 0.125 g/L dextrose was determined for maximum optical density (1.946) and urease activity (27 m.mol urease/min). To stabilize the black cotton soil, soil bacteria were used with ferrochrome ash. The study involved assessing the extent to which ferrochrome ash could replace the black cotton soil and analyzing the impact of bacterial optical density, urea, and calcium chloride on enhancing the soil's unconfined compressive strength. The soil composite with the highest UCS of 350 kPa (TC5) comprised 40 % FCA, a bacterial medium with an optical density of 1.12, 0.5 g urea, and 0.5 g calcium chloride. A quadratic model was employed to investigate the impact of ferrochrome ash, bacterial density, calcium chloride, and urea concentrations on the unconfined compressive strength. The model indicated FCA as the primary contributor for the UCS improvement. The leachate of TC5 demonstrated reductions in heavy metal concentrations with efficiencies of 95% for Ni, 97% for Cu, 98% for Fe, 99% for Cr and Zn, and 100% for Pb, Cd, Ti, Hg, and As. The XRD analysis of TC5 revealed peaks at 20.61 °, 26.39 °, 28.08 °, 29.20 °, 31.08 °, 36.29 °, 39.21 °, 42.20 °, 43.16 °, 49.92 °, and 55.17 °. Additionally, peaks of FCA were observed at 28.32°, 40.54 °, and 50.23 °, while peaks of BCS were observed at 20.96 °, 26.72 °, 36.63 °, 39.58 °, 40.41 °, 42.53 °, 45.92 °, and 50.24 °. SEM of TC5 contained rhombohedral crystals of calcium carbonate and spherical particles of FCA on the flaky surface of the clayey soil. The FTIR profile of TC5 contained a characteristic absorption band for carbonate at 1457 cm-1, which was absent in the spectra of BCS and FCA. Other bands at 993 cm-1 and 1633 cm-1 indicate C-S-H formation. TGA analysis of TC5 exhibited an 18% weight loss in the temperature range iv of 590 °C to 810 °C, indicating the decomposition of calcium carbonate formed during biomineralization. These findings have important implications for waste management strategies, providing valuable insights into the potential of BMA and FCA with biomineralization in mitigating environmental risks and deriving value from waste.enBacterial urease productionBiomineralizationBiomedical ashBlack cotton soilFerrochrome ashHeavy metalsSoil stabilizationThesis