The Impact of Aluminum on the Initiation and Development of MnOx(s) Coatings for Manganese Removal

Many treatment facilities remove soluble Mn by an autocatalytic adsorption-oxidation process involving manganese oxide (MnOx(s))-coated filter media and a free chlorine residual known as the natural greensand effect (NGE). In recent years, significant amounts of aluminum (Al) have been found integrated into MnOx(s) coatings on media from drinking water treatment facilities worldwide. The primary objective of this study was to characterize MnOx(s) coatings developed in the presence and absence of Al, and to further define the role played by Al in the coatings' initiation and development. A secondary objective of the study was to examine the potential for pre-filter oxidation of Mn and formation of nano-size MnOx(s) particles, which would be destabilized by Al(OH)3(s) and captured in the filter. This material could act as a seed for coating formation and help explain the integration of Al into MnOx(s) coatings. Bench-scale column tests were conducted to examine Mn removal and backwash composition, while centrifugation and ultrafiltration were utilized to examine the potential for rapid Mn oxidation. Results indicate that the presence of Al augments the initiation and development of MnOx(s) coatings. The backwash solids of columns loaded with Al were composed of a mixture of Mn and Al, suggesting that active adsorption-oxidation sites were present in the Al(OH)3(s) floc captured by the filter. These results suggest at least a small amount of pre-filter MnOx(s) formation by contact with free chlorine; further, that Al(OH)3(s) solids present may destabilize these negatively charged solids into a form that is important to MnOx(s) coating formation.