Batch and column experiment were performed to investigate

the removal of cadmium, copper, and zinc from synthetic

stormwater by compost-amended sand. The batch sorption capacities for Cd and

Zn are 2.13 mg/g and 3.82 mg/g, respectively, for Minnesota Compost 1 and 0.02 and

0.07 mg/g, respectively for sand. Copper precipitates as tenorite (CuO) at the pH of the

stormwater (7.2), so a sorption capacity was not computed. Column studies using four

different ratios of compost (0, 10, 30, and 50%, by volume) in sand were conducted to

develop metal breakthrough curves. The breakthrough curves for Cd and Zn were fit to

the Thomas Model. The resulting sorption capacities are 0.07, 0.23, 0.37, 0.78 mg Cd/g

and 0.10, 0.23, 0.33, 0.61 mg Zn/g for 0, 10, 30, and 50% compost fractions, respectively.

These sorption capacities, when adjusted for mass of sand and compost, are consistent

with the sorption capacities determined from the batch experiments. Copper was

removed in the columns due to filtration and no breakthrough occurred in the duration

of the study. The phosphorus concentrations exiting the columns were initially high (0.5

mg P/L), but then decreased to a steady state value of 0.20 - 29 mg P/L (that exceeded

the influent value of 0.13 ± 0.03mg P/L) for the remainder of the experiment. Overall,

the results suggest that bioretention cells are not likely to fail because of loss of dissolved

toxic metal removal capacity as calculated breakthrough times on the order of hundreds

of years far exceed the typical design life of engineering systems of 30 years. Lastly,

the compost that is key to metals removal may actually release nutrients (i.e., phosphorus).

Thus, it is important to consider the installation of alternative media beneath the

compost-amended sand to remove phosphorus, such as iron-amended sand. Such hybrid

approaches require more investigation.

 

ADVISORS: John Gulliver and Ray Hozalski