Mechanism of soft soil improvement by bio-enzymes: insights from enzyme-clay interactions

Bio-enzymes have been proposed as a soil improvement approach with potential environmental advantages. This study investigated the effects of bio-enzymes on the stabilization of soft soil. Given that bio-enzymes primarily interact with the clay of the soft soil, clay was extracted from the soft soil to comprehensively examine the interaction mechanism between bio-enzymes and clay. Results indicate that bio-enzymes improved the unconfined compressive strength (UCS) of soft soil, with a 38.7% increase at a bio-enzymes content of 1.5 mg protein g −1 dry soil, while reducing its specific surface area but causing a more negative zeta potential. Aggregation tests show that bio-enzymes adsorption increased clay aggregation. Increasing the protein concentration from 0 to 0.5 mg/mL increased the average hydrodynamic diameter by approximately 14.2% at an ionic strength of 20 mM and 29.6% at 150 mM. Meanwhile, the clay contact angle significantly increased by more than fourfold. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory predicts that the hydrophobic interaction lowered the interparticle energy barrier at lower ionic strengths and deepened primary minimum at higher ionic strengths, facilitating clay aggregation. X-ray photoelectron spectroscopy (XPS) indicated that bio-enzymes adsorption enriched hydrophobic groups and masked the underlying hydrophilic mineral surface signals, causing enhanced hydrophobic forces. Transmission electron microscopy (TEM) further revealed that the bio-enzymes adsorption may fill the interparticle voids between clay particles. The enhanced clay aggregates finally improved the mechanical strength and microstructural density of the soft soil. These findings could advance the understanding of bio-enzyme-based soil improvement.