Influence of humic acid on the transport behavior of bacteria in quartz sand

Abstract

The significance of natural organic matter (NOM) on the transport of bacteria in packed porous media (quartz sand) was examined in both NaCl and CaCl 2–NaCl mixing solutions at pH 6.0. Three representative cell types (with EPS), Rhodococcus sp. QL2 (Gram-positive, non-motile), Escherichia coli BL21 (Gram-negative, non-motile), and E. coli C3000 (Gram-negative, motile), were utilized to systematically determine the influence of NOM (Suwannee River humic acid (SRHA)) on cell transport behavior. To investigate the significance of SRHA on transport of bacteria without EPS on cell surfaces, experiments for treated cells with the removal of EPS from cell surfaces were also performed. The breakthrough plateaus for all examined bacteria with the presence of SRHA (1 mg L −1) in solutions were higher than those with the absence of SRHA under all examined conditions, indicating that the presence of SRHA in solutions enhanced cell transport regardless of cell types (Gram-negative or Gram-positive), motility (non-motile or motile), presence or absence of EPS on cell surfaces, and solution chemistry (ionic strength and ion valence). Zeta potentials for bacteria and quartz sand with the presence of SRHA were similar as those without SRHA present in solutions, suggesting that SRHA did not alter the surface charge of bacteria or sand, thus the enhanced cell transport by SRHA was not likely driven by alteration in the surface charge of either cell or quartz sand. SRHA pre-equilibration experiments demonstrated that the site competition by a portion of SRHA and the repelling deposition by suspended SRHA contributed to the decreased cell deposition observed with the presence of SRHA in bacteria suspension.