Changes in ionic strength, whether from mixing with saline waters, road salt, or from the large osmotic adjustment needed for the Microtox ™ toxicity assay, affect the aqueous chemistry of stormwater runoff and receiving waters. Such changes can alter the partitioning of metals between dissolved and colloidal phases. The presence of humic substances, commonly found in environmental waters, can also affect metal/colloid interactions. Batch experiments were performed to evaluate the effects of humic acid, NaCl (2% w/v), and an artificial stormwater solution (ASW) (I=0.001) on the sorption of Cu and Zn to kaolinite (100 mg/L) and iron oxide (40 mg/L). Total metal concentrations ranged from 0.25 to 12 mg/L.

Sorption to iron oxide and kaolinite in a DIW solution was low for both Cu and Zn. Equilibration of kaolinite and iron oxide with humic acid prior to the addition of metals greatly increased sorption of both metals to both minerals (as high as 90% retention at low concentrations). Equilibration with ASW in addition to humic acid did not affect the copper isotherms at low (< 1 mg/L) concentrations but lowered zinc retention by 20-40 %. Addition of 2% NaCl decreased the sorption of both metals to both minerals by 30-70%, lowering the isotherms notably. The effect of NaCl was greater for Zn.

The results indicate that the partitioning of Cu and Zn between the dissolved and colloidal phases is strongly influenced by the presence of humic substances through the formation of coatings on the minerals. Small changes in ionic strength in the environment (similar to the addition of the ASW) will affect the partitioning of Zn, but will have less effect on Cu partitioning. A large increase in NaCl will greatly decrease the sorption of Cu and Zn to colloidal phases. The combined effects of these factors will influence the bioavailability of metals as well as their potential for transport and sedimentation as stormwater moves through the environment and into receiving waters.