Microbial-induced carbonate precipitation using urea hydrolysis is a relatively new improvement technique for granular soils. An important factor in achieving uniform calcite deposition (and hence consistent improvements in geomechanical properties) throughout the treated soil mass is the protocol adopted for injecting the reagents of ureolytic bacteria, urea and calcium. This paper reports a laboratory study investigating a technique to treat two loose medium quartz sands using different injection strategies. Staged injection including retention periods, and with a pressure head applied during injection of the bacterial cell solution, proved most effective. Sand specimens were treated using different concentrations of bacterial cell and urea–calcium chloride solutions and for a single injection cycle. Measured strength and stiffness values from unconfined compression tests ranged from 50 to 240 kPa and from 6 to 56 MPa, respectively. Permeability coefficient values were reduced by up to approximately one order of magnitude. Hence, a single injection cycle adhering to the proposed treatment method did not significantly affect the drainage capacity of the sand media. Greater improvements in stiffness and strength were achieved for lower bacterial cell and higher cementation solution concentrations, with a higher molarity of urea (non-equimolar solutions) proving even more effective. These findings were confirmed by scanning electron microscope observations.

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Efficient and sustainable destruction of per- and polyfluoroalkyl substances (PFAS) in solid matrices remains a critical challenge due to the limitations of conventional treatment methods. This study investigated the performance of natural piezoelectric tourmaline (TM) as a co-reagent for the mechanochemical degradation (MCD) of perfluorooctanoic acid (PFOA), and its applicability to PFAS-contaminated soils. TM-assisted MCD achieved 97% PFOA degradation and 96% reduction in total organic carbon (TOC) after 6 h of milling, indicating substantial PFOA destruction. Fluorine mass-balance analysis using combustion ion chromatography and quantitative fluorine nuclear magnetic resonance (19F qNMR) confirmed the conservation of total fluorine (TF) during TM-assisted MCD, with fluorine initially present only as total organic fluorine (TOF). TOF was primarily converted to inorganic fluoride (IF; 95.8%), indicating extensive PFOA mineralization. The approach was further evaluated for artificially contaminated soil with four representative PFAS: (PFOA, perfluorooctane sulfonic acid (PFOS), perfluorobutanoic acid (PFBA), and perfluorobutane sulfonate (PFBS)) at 10 and 1.5 µg g-1 each. At 10 µg g-1, PFAS degradation reached 90% with MCD alone and 92% when soil was amended with 20% (w/w) TM after 4 h of milling, whereas at 1.5 µg g-1, degradation reached 88% with MCD alone but decreased to 66% with TM owing to particle agglomeration, which reduced milling efficiency. Overall, TM was highly effective for solid-phase PFOA degradation, whereas its role in soil was constrained by TM-soil particle interactions, highlighting its potential as an environmentally benign co-reagent for PFAS destruction while emphasizing the importance of soil effects in practical applications.