The management of alkaline (pH 11–12.5) leachate is an important issue associated with the conditioning, afteruse or disposal of steel slags. Passive in-gassing of atmospheric CO2 is a low cost option for reducing Ca(OH)2 alkalinity, as Ca(OH)2 is neutralised by carbonic acid to produce CaCO3. The relative effectiveness of such treatment can be affected by both the system geometry (i.e. stepped cascades versus settlement ponds) and biological colonization. Sterilized mesocosm experiments run over periods of 20 days showed that, due to more water mixing and enhanced CO2 dissolution at the weirs, the cascade systems (pH 11.2 → 9.6) are more effective than settlement ponds (pH 11.2 → 11.0) for lowering leachate alkalinity in all the tested conditions. The presence of an active microbial biofilm resulted in significantly more pH reduction in ponds (pH 11.2 → 9.5), but had a small impact on the cascade systems (pH 11.2 → 9.4). The pH variation in biofilm colonized systems shows a diurnal cycle of 1 to 1.5 pH units due to CO2 uptake and release associated with respiration and photosynthesis. The results demonstrate that, where gradient permits, aeration via stepped cascades are the best option for neutralisation of steel slag leachates, and where feasible, the development of biofilm communities can also help reduce alkalinity.