Fluoride, natural organic matter, and particles : the effect of ligand competition on the size distribution of aluminum precipitates in flocculation

Abstract

Fluoride occurs at elevated concentrations naturally in surface and ground waters around the world. If consumed at low concentrations in drinking water (< 1.5 mg/L), fluoride is shown to reduce the occurrence of dental caries and the Centers for Disease Control and Prevention named fluoridation of public water systems one of the 10 Great Public Health Achievements of the 20th Century (CDC, 1999). However, prolonged exposure to high concentrations of fluoride (> 2.0 mg/L) causes adverse health effects to teeth and bones. For this reason the United State Environmental Protection Agency (USEPA) enacted a maximum contaminant level (MCL) for fluoride at 4.0 mg/L. This rule is currently under review following a recent risk assessment and may be lowered. If the MCL were lowered, water systems previously meeting treatment standards would suddenly find themselves out of compliance and will need to implement additional treatment to meet the new standard. Defluoridation by alum coagulation is a proposed defluoridation method. However, the interaction between fluoride and natural organic matter (NOM) and their effects on the particle size distribution of aluminum precipitates is not well understood. Because the particle size distribution of aluminum precipitates is an important parameter in the efficiency of sedimentation and filtration systems, a thorough understanding of these interactions and their potential effect on sedimentation and filtration is needed to inform the implementation of defluoridation by alum coagulation. This work utilized a series of jar tests on synthetic surface water to determine the effect of fluoride and NOM on the particle size distribution of aluminum precipitates. It was found that fluoride caused the volume distribution of aluminum precipitates to shift toward smaller particle sizes. However, NOM caused the formation of a larger number of aluminum precipitates, which resulted in a dramatic increase in the total volume of precipitates. When both fluoride and NOM were in the system, a combination of the two effects was observed: the volume distribution shifted toward smaller particle sizes but the peak of the distribution shifted toward a greater volume, indicating both smaller particles were being formed and a greater overall volume of particles precipitated.