Transport and Fate of Ureolytic Sporosarcina pasteurii in Saturated Sand Columns: Experiments and Modelling

Abstract

Abstarct Despite a broad application of ureolytic bacteria in many bioremediation and biocementation processes, very limited studies have reported their transport and retention behaviors under various physical–chemical–biological conditions. In this study, we report transport and retention of Sporosarcina pasteurii in saturated sand, based on a series of column breakthrough experiments under different conditions including ionic strengths (ISs: 0.5 mM–1 M), flow velocity (50, 100, 200 cm/h), bacteria optical density (OD600 = 1.0, 0.48), column length (280 mm, 150 mm), and changes in IS conditions (0.5 M CaCl2 or deionised water). We use a two-site kinetic model, representing (1) attachment on grain surfaces, and (2) straining at crevices and constrictions, to quantify and predict the bacterial attachment and straining. Model parameters were calibrated by tracer (NaCl) breakthrough curves (BTCs) and bacteria BTCs at different IS/velocity conditions. The model was then applied to successfully predict the bacteria BTCs at lower initial bacteria density (OD600 = 0.48) and for shorter column lengths (150 mm). We demonstrated that higher ionic strength (from 0.5 to 1000 mM) dramatically enhanced the retention efficiency of S. pasteurii through an enhancement of attachment (from 9.4 to 69.6%) and straining (from 8.1 to 34.2%), whilst the bacterial survival and the urease activity were unaffected at high IS conditions (500 and 1000 mM NaCl) within 5 h. Increasing flow velocity (from 50 to 200 cm/h) caused a decrease in attachment (from 39.5 to 22.4%) and decrease in straining (from 40.5 to 19.3%) as a result of the increased hydrodynamic shear forces, which tends to reduce the attachment at the secondary minimum and decrease the extent of flow stagnation regions for straining. Lower initial bacteria OD600 (from 1.0 to 0.48) enhanced the attachment (from 31.8 to 40.9%) and the straining (from 22.9 to 42.2%) as a result of reducing the site-blockage effect. In addition, 0.5 M CaCl2 with a stronger IS increased the retention of in the column, whilst deionised water with a lower IS caused bacterial release. These findings provide useful information for a better understanding of the transport and fate of Sporosarcina pasteurii in saturated soil, and can be used to optimise bioaugmentation strategy and cementation efficiency for soil improvement. Article Highlights Transport of S. pasteurii in sands is highly affected by ionic strength, flow velocity, bacteria density, and even column size Straining was enhanced (from 8.1% to 34.2%) if increasing IS (from 0.5 to 500 mM) without affecting bacterial survival Bacteria coagulation among 2–3 bacterial cells occurs under ISs of 500 and 1000 mM without forming large flocculation