SORPTION OF CADMIUM TO BACTERIAL EXTRACELLULAR POLYMERIC SEDIMENT COATINGS UNDER ESTUARINE CONDITIONS

Microbial extracellular polymeric substances (EPS) are ubiquitous feat ures in aquatic environments. Produced by surface-adherent bacteria an d microalgae, EPS are often present as coatings on surfaces of sedimen t particles and exhibit high affinities for divalent cationic metals. Thus, EPS sediment coatings may participate in the fate of potentially toxic metals. We coated particulate silica (8-10 mu m) with EPS produ ced by NISC1, a bacterium isolated from estuarine sediments, in order to measure the metal binding characteristics of these coatings. We use d the radioisotope Cd-109 to measure effects of salinity (salinity = 2 .5, 7.5, and 25 parts per thousand), Cd concentration ([Cd] = 0.038, 5 , and 50 mg/L), and pH (pH = 5, Ci, 7, 8, and 9) on Cd sorption to EPS -coated (EPS silica) silica and to noncoated silica (NC-silica). Also, Cd sorption by NISC1 EPS coatings was compared to coatings of polymer s formed by the bacterium Alteromonas atlantica and the alga Macrocyst is porifera. Under all circumstances, EPS coatings increased the affin ity of silica for Cd. Extracellular polymeric substance-particulate ag gregates rapidly sorbed up to 90% of Cd from aqueous solution. Extrace llular polymeric substance sediment coatings exhibited a maximum log d istribution coefficient (K-c) of 6.5 at 2.5 parts per thousand. Sorpti on of Cd to NC-silica was affected by salinity and metal concentration , whereas sorption of Cd to EPS-silica was only affected by salinity u nder high metal concentrations. Changes in pH had a dramatic effect on Cd sorption, with the proportion of free Cd to sorbed Cd changing fro m approximately 90% at pH 5 to 5% at pH 9. Desorption of Cd from EPS-s ilica was enhanced with increasing salinity. These experiments suggest that EPS coatings actively participate in binding dissolved overlying and pore-water metals in estuarine sediments.