Formation of carbonaceous and nitrogenous iodinated disinfection byproducts from biofilm extracellular polymeric substances by the oxidation of iodide-containing waters with lead dioxide

Abstract

Lead dioxide (PbO2) is an important form of lead mineral scales in drinking water pipes. Iodide (I−) widely presents in source waters and can be thermodynamically oxidized by PbO2 to the reactive iodine species (I2/HOI). Biofilm extracellular polymeric substances (EPS) are nonnegligible precursors of disinfection byproducts (DBPs). The aim was to study the oxidation of I− by PbO2 and formation of iodinated DBPs (I-DBPs) from EPS. At a high molar ratio of PbO2 to I− (> 100), the observed rate constants of I− oxidation decreased as pH increased from 6.0 to 9.0 with an H+ dependence of 0.79, and the rate constant (k) was 1.6 × 1011 M−2.79 s−1. Most of formed I2/HOI (> 92%) was transformed to organic iodine in the presence of EPS. EPS had a lower formation potential (FP) of carbonaceous I-DBPs (C-IDBPs), while a higher that of nitrogenous I-DBPs (N-IDBPs) than HA, resulting in a higher Chinese Hamster Ovary cell cytotoxicity. Generally, the formation of I-DBPs decreased with the increase of pH due to the reduction of surface positive charge and electrochemical driving force. PbO2 dose and I− concentration also had a significant effect on the I-DBPs formation. EPS proteins had a higher FP of both C- and N-IDBPs than polysaccharides on account of more electrophilic sites and higher nitrogen content. In proteins, aspartic acid was the main contributor to triiodomethane and iodoacetic acids formation, whereas aspartic acid, asparagine and tyrosine were the major precursors of diiodoacetonitrile and diiodoacetamide. The study helps to improve the control strategy of I-DBPs when biofilm outbreaks in lead-containing water pipes.