This study synthesizes polyethyleneimine-glutaraldehyde (PEI-GA) resins using different amounts of GA to crosslink with a certain amount of PEI and compares these adsorbents for the adsorption of Cu(II) (cations) and Se(VI) (anions). Moreover, the stability of adsorption affinity of PEI-GA resins stored in open or sealed conditions is also studied. Results show that the amount of GA for PEI crosslinking does not affect the adsorption performance for Se(VI), especially when PEI/GA mass ratio is less than 2, while for Cu(II), the increase on GA amount decreases Cu(II) adsorption capacity. This difference is directly correlated to the change in the adsorption mechanism from electrostatic attraction to chelation. The primary and secondary amine groups on PEI can easily react with CO2 in the air to form carbamate, potentially affecting the adsorption performance of PEI. Results also indicate that the adsorption efficiency for Se(VI) is hardly affected by the storage condition, while that for Cu(II) decreases significantly after 20-day storage compared to the freshly prepared ones. In addition, all of the adsorbents can selectively remove Se(VI) from Se(VI)-As(V) system and Cu(II) from Pb(II)-Cu(II) system, indicating that the crosslinking has no significant influence on the selectivity.