Removal of Two Waterborne Pathogenic Bacterial Strains by Activated Carbon Particles Prior to and after Charge Modification

Abstract

Waterborne diseases constitute a threat to public health despite costly treatment measures aimed at removing pathogenic microorganisms from potable water supplies. This paper compared the removal of Raoultella terrigena ATCC 33257 and Escherichia coli ATCC 25922 by negatively and positively charged types of activated carbon particles. Both strains display bimodal negative zeta-potential distributions in stabilized water. Carbon particles were suspended to an equivalent external geometric surface area of 700 cm2 in 250 mL of a bacterial suspension, with shaking. Samples were taken after different durations for plate counting. Initial removal rates were less elevated for the positively charged carbon particle than expected, yielding the conclusion that bacterial adhesion under shaking is mass-transport limited. After 360 min, however, the log-reduction of the more negatively charged R. terrigena in suspension was largest for the positively charged carbon particles as compared with the negatively charged ones, although conditioning in ultrapure or tap water of positively charged carbon particles for 21 days eliminated the favorable effect of the positive charge due to counterion adsorption from the water. Removal of the less negatively charged E. coli was less affected by aging of the (positively charged) carbon particles, confirming the role of electrostatic interactions in bacterial removal by activated carbon particles. The microporous, negatively charged coconut carbon performed less than the mesoporous, positively charged carbon particle prior to conditioning but did not suffer from loss of effect after conditioning in ultrapure or tap water.

Interaction forces between waterborne bacteria and activated carbon particles

“…terrigena and E. coli are both negatively charged under the ionic strength conditions of the experiments, with population-averaged zeta potentials of −41.8 ± 7.5 mV and −25.5±9.5 mV, respectively [22]. NAC-treatment did not have a significant effect on the zeta potential of R. terrigena.…”

“…In Fig. 5, we relate the percentage retraction curves revealed by traversing an activated carbon particle through the outer bacterial surface layer with the previously determined, macroscopic bacterial removal in a batch assay [22]. In Fig.…”

“…Bacterial contact angles were determined with water, formamide, methylene iodide and α-bromonaphthalene using the sessile drop technique for R. terrigena prior [22] to and after NAC treatment as well as for E. coli [22]. Briefly, bacteria were resuspended in ultrapure water and deposited onto a 0.45 µm pore size filter (Millipore) using negative pressure.…”

“…Average zeta potentials of the bacterial strains in stabilized water (pH 6.8) were determined by particulate microelectrophoresis, using a Lazer Zee Meter 501 (PenKem), discarding heterogeneous subpopulations, as found for R. terrigena and E. coli [22].…”

“…The log-reduction of R. terrigena and E. coli established by activated, mesoporous carbon particles (CA-10, acidic wood-based activated carbon; RGC, basic wood-based activated carbon; PVAM, RGC carbon particle coated with polyvinyl amine) in a batch test[22] as a function of the percentage of retraction curves, showing adhesion between bacteria and activated carbon particles by the atomic force microscope. Linear and rank correlation coefficients amount both 0.72.…”