Effects of metal ions on THMs and HAAs formation during tannic acid chlorination

The present study was implemented to examine THM and HAA formation from the chlorination of tannic acid (TA) in the presence of common metal ions (e.g., Mg2+, Fe2+, Mn2+, Cu2+, Al3+) typically found in water sources. The inhibition efficiency of THM formation followed the order Mg2+ < Cu2+ < Fe2+ < Mn2+ < Al3+ and the effect of HAA enhancement followed the order Mg2+ < Mn2+ < Cu2+ < Al3+ < Fe2+. Similar to the case of single metal ion, the presence of multiple metal ions exerted analogous influence on the THM and HAA formation but without synergistic enhancement or inhibition. The investigation of nuclear magnetic resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopy indicated that the inhibitory effect of metal ions on THM formation was proportional to the changes in electron densities of active sites of tannic acid while the promoting effect of metal ions on HAA formation was more related to the generation of hydroxyl radical caused by the presence of metal ions. This study suggests that two different mechanisms could be involved in the effects of metal ions on THM and HAA formation from TA chlorination.

► Metal ions inhibit THMs formation but enhance HAAs formation during TA chlorination.
► Metal ions change electron densities of active sites and OH formation.
► Mechanisms were proposed regarding metal ions effects on THMs and HAAs formation.