Influence of different chemical treatments on transport of Alcaligenes paradoxus in porous media

Abstract

Seven chemicals, three buffers, and a salt solution known to affect bacterial attachment were tested quantify their abilities to enhance the penetration of Alcaligenes paradoxus in porous media. Chemical treatments included Tween 20 (a nonionic surfactant that affects hydrophobic interactions), sodium dodecyl sulfate (an anionic surfactant), EDTA (a cell membrane permeabilizer that removes outer membrane lipopolysaccharides), sodium PP1 (a surface charge modifier), sodium periodate (an oxidizer that cleaves surface polysaccharides), lysozyme (an enzyme that cleaves cell wall components), and proteinase K (a nonspecific protease that cleaves peptide bonds). Buffers included MOPS [3-(N-morpholino)propanesulfonic acid], Tris, phosphate, and an unbuffered solution containing only NaCl. Transport characteristics in the porous media were compared by using a sticking coefficient, α, defined as the rate at which particles stick to a grain of medium divided by the rate at which they strike the grain. Tween 20 reduced α by 2.5 orders of magnitude, to α = 0.0016, and was the most effective chemical treatment for decreasing bacterial attachment to glass beads in buffered solutions. Similar reductions in α were achieved in unbuffered solutions by reducing the solution ionic strength to 0.01 mM. EDTA, protease, and other treatments designed to alter cell structures did not reduce α by more than an order of magnitude. The number of bacteria retained by the porous media was decreased by treatments that made A. paradoxus more hydrophobic and less electrostatically charged, although α was poorly correlated with electrophoretic mobility and hydrophobicity index measurements at lower α values. Filtration model calculations indicate that α values of <0.01 are necessary to achieve bacterial transport over distances large enough to facilitate bioremediation of contaminated groundwater aquifers. Chemical treatments that reduce α from 0.61 (1 mM MOPS buffer; ionic strength, 70 mM) to 0.0016 would increase bacterial penetration from 0.16 to 60 m, with only a 2-log reduction in cell concentration under typical groundwater and soil conditions (assuming a water velocity of 1 m day-1 and an average soil grain diameter of 500 μm).