Adsorption of Cd to natural biofilms in the presence of EDTA: effect of pH, concentration, and component addition sequence.

Both dissolved organic matters (DOM) and natural biofilms are important substances in controlling the behavior of trace metals in natural aquatic environments. In this study, ethylenediaminetetraacetic acid (EDTA) was selected as a typical DOM to investigate the effect of DOM on the adsorption of trace metals to the biofilms in natural waters. The adsorption of Cd to biofilms, including adsorption isotherm at a fixed pH (pH = 6.0) and pH-edge adsorption (pH ranging from 4.3 to 9.0) with different adsorption sequences, was determined without EDTA and in the presence of EDTA ([EDTA] = 0.5 μmol/L for isotherms measurement and [EDTA] = 0.5 and 2.0 μmol/L for pH-dependent adsorption). The presence of EDTA generally decreased the adsorption of Cd, and the effect was determined by solution pH, concentration of EDTA, and adsorption sequence. Higher concentration or higher pH usually resulted in a more significant decrease. The influence of adsorption sequence on the effect of EDTA was insignificant in lower pH range, while the adsorption usually decreased in the order of Cd only adsorption > Cd first adsorption > EDTA first adsorption ≈ simultaneous adsorption in higher pH range. The effect of EDTA could be attributed to the conversion of Cd speciation, the competition with the biofilms for Cd, and the dissolution of Mn oxides from the biofilms. EDTA affected the adsorption of Cd to natural biofilms, and the effect could be fairly significant. The role of Mn oxides in determining the behavior of trace metals might be underestimated.