The observed trends in dissolved copper concentrations
matched the students’ predictions that only the bioreactors
containing both the soil (bacteria) and the leaves (carbon)
displayed a decreasing trend in dissolved copper concentrations
(Figure 2). These results led the students to conclude that the
likely removal mechanism was the precipitation of Cu as CuS(s).
These data also demonstrated a 20% decrease in dissolved copper
concentrations in all bioreactors containing soil, compared
to those without soil, following their assembly on day 0. The
students postulated that since only the dissolved copper was
measured, copper may bind to soil surfaces, thereby leading
to copper removal from the dissolved phase. Previous research
has shown that in fact copper does have a high affinity for the
organic matter in soil (12, 13). However, the reduction in the
copper concentration was smaller than the subsequent decline of
the copper in the full treatment bioreactors during the duration
of the experiment. These data provided students an opportunity
to discuss whether the addition of carbon alone was sufficient
as a remediation technique and to consider the relative time
scales of the effects.
In contrast to copper, dissolved iron concentrations increased
by 70–160% in three of the five bioreactors (Figure
3). This trend was not anticipated by the students, but