Sorption of cadmium to bacterial extracellular polymeric sediment coatings under estuarine conditions

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AbstractMcrobial extracellular polymeric substances (EPS) are ubiquitous features in aquatic environments. Produced by surface‐adherent bacteria and microalgae, EPS are often present as coatings on surfaces of sediment 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 μm) with EPS produced by NISC1, a bacterium isolated from estuarine sediments, in order to measure the metal binding characteristics of these coatings. We used the radioisotope 109Cd to measure effects of salinity (salinity = 2.5, 7.5, and 25‰), Cd concentration([Cd] = 0.038, 5, and 50 mg/L), and pH (pH = 5, 6, 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 polymers formed by the bacterium Alteromonas atlantica and the alga Macrocystis porifera. Under all circumstances, EPS coatings increased the affinity of silica for Cd. Extracellular polymeric substance‐particulate aggregates rapidly sorbed up to 90% of Cd from aqueous solution. Extracellular polymeric substance sediment coatings exhibited a maximum log distribution coefficient (Kd) of 6.5 at 2.5‰. Sorption of Cd to NC‐silica was affected by salinity and metal concentration, whereas sorption of Cd to EPS‐silica was only affected by salinity under high metal concentrations. Changes in pH had a dramatic effect on Cd sorption, with the proportion of free Cd to sorbed Cd changing from approximately 90% at pH 5 to 5% at pH 9. Desorption of Cd from EPS‐silica was enhanced with increasing salinity. These experiments suggest that EPS coatings actively participate in binding dissolved overlying and pore‐water metals in estuarine sediments.