Mechanisms of ligand removal and competition in aluminum salt coagulation systems : insights using ATR-FTIR spectroscopy

Fluoride, while beneficial in moderate to low doses, has detrimental health effects associated with chronic exposure to elevated concentrations from sources such as drinking water. Natural organic matter (NOM) is ubiquitous in many drinking water sources and can contribute to the formation of carcinogenic disinfection by-products (DBPs). In an era marked by increasing concerns for drinking water quality and increased costs to produce high quality drinking water, small water systems (SWSs) are the most vulnerable due to their lack of financial and operational resources. Aluminum salt coagulation processes have the potential to remove both fluoride and NOM, and would be advantageous to a SWS since the technology has been widely used for decades. However, the removal mechanisms and interactions among competing contaminants remain poorly understood. This study aims to elucidate the mechanisms of removal and better understand the competition that occurs in a coagulation system by using a combination of spectroscopic insights at the solid surfaces and macroscopic insights from aqueous removal levels. Attenuated total reflectance Fourier transform infrared spectroscopy provided insights at the solid-aqueous interface where it was concluded that sulfate and pyromellitate (an NOM surrogate) adsorb via outer-sphere complexation while fluoride, silicate, and carbonate create inner-sphere surface complexes. It was also evident that silicate accumulated and polymerized at the aluminum hydroxide surface with time. Both fluoride and pyromellitate exhibited greater removal efficiencies in waters containing sulfate versus chloride, suggesting that sulfate has a higher affinity for the surface than chloride. Fluoride and pyromellitate removals were also reduced at the higher pH of 7.5 due to increased aluminum solubility and competition with inner-sphere complexed carbonate. Pyromellitate removal is significantly impacted by the presence of fluoride whereas fluoride removal is marginally impacted by the presence of pyromellitate. This was explained by the inner-sphere complex that fluoride forms in comparison to the weaker electrostatic interaction of pyromellitate. Silicate was observed to reduce fluoride and pyromellitate removals both initially and with time due to competition for surface bonding sites. Over time, the accumulation and polymerization of silicate is believed to be responsible for desorption of fluoride and pyromellitate.