which is consistent with the presence of high
affinity sites that result in higher Kd values at low concentrations.
In a few cases, the N values were clearly higher than 1, which is in
agreement with the observed sigmoidal-shaped isotherms (e.g.,
MOW-Cu or DOM-Pb). In this case, the DOC content might compete
with the solid phase for the sorption of metals, leading to lower Kd
values than expected at low concentrations.