Impact of prechlorination pH on Pb(II)-NOM complexation and lead release in drinking water distribution systems

The presence of Pb in drinking water resulting from corrosion of lead pipes, solder and fixtures poses a significant threat to public health and safety. Pb is a potent neurotoxin that can impair the cognitive development of children at low concentrations in the bloodstream. During the corrosion of distribution system components containing Pb, precipitate scales form on the interior surfaces of pipes in the distribution system providing a protective barrier between the bulk solution and the pipe surface; the amount of dissolved Pb²⁺ is controlled by the thermodynamics and kinetics of precipitation/dissolution of the particular Pb scales in the system. The solubility and composition of Pb scales phases residing on the pipe surfaces are a function of the background water chemistry (including pH, redox potential and ligand types and concentration). Therefore, considering the conditional stability of lead scales is crucial when making changes to water treatment processes, as alterations in water quality can significantly disrupt the stability of scales in the DWDS, leading to elevated lead concentrations. In distribution systems that employ monochloramine to provide residual disinfection, the formation of Pb(II) scales such as cerussite (PbCO₃) and hydroxyapatite are favored depending on the background water composition (e.g., phosphates added for corrosion protection or carbonate present). Utilities that chloraminate typically implement a prechlorination step in which free chlorine reacts for a specified time prior to ammonia addition to meet disinfection requirements. During the prechlorination step, free chlorine can react with natural organic matter (NOM) to promote Pb release. The formation of NOM-Pb complexes increases the total soluble Pb in distribution systems containing Pb scales. However, before NOM enters the drinking water distribution system (DWDS) oxidation or nucleophilic substitution of NOM functional groups during prechlorination can decrease the extent of Pb(II)-NOM complexation. The relative impact of these processes on Pb(II) concentrations in drinking water distribution systems must be understood for effective Pb control in DWDS’s containing NOM. In this study, the impact of Pb-NOM complexation on Pb(II) release from cerussite (PbCO₃), a Pb scale phase that can form in the DWDS, is studied in batch dissolution experiments with and without prechlorination. Prechorination of the water containing NOM for a 30-minute contact time decreased the amount of Pb released in the presence of NOM with greater reduction at lower pH. The trends in pH were consistent with the greater reactivity between free chlorine and NOM at lower pH. Additionally, excitation emission matrices (EEMs) and near edge x-ray fine structures (NEXAFS) spectroscopy confirmed that chlorination of the NOM led to destruction of electron rich aromatic moieties that function as prominent ligands in Pb(II)-NOM complexation Further, it was found that fewer in number of disinfection byproducts (DBPs) precursors were generated upon reaction of chlorine and NOM at lower pH conditions. These results demonstrate the potential for chlorination to reduce Pb(II)-NOM complexation. The concentrations of total soluble Pb observed in the batch dissolution experiments ranging from 40 ppb to 600 ppb are not representative of dissolved Pb concentrations in the water distribution system as a Pb concentration of 15 ppb necessitates the implementation of corrosion control measures. Further studies are needed to assess the relative impact of Pb(II)-NOM complexation under continuous flow conditions that more closely mimic pipe flow through conditions representative of water distribution system conditions.