Laboratory investigation of aluminum solubility and solid-phase properties following alum treatment of lake waters

Abstract

Water samples from two southern California lakes adversely affected by internal nutrient loading were treated with a 20 mg/L dose of Al 3+ in laboratory studies to examine Al solubility and solid-phase speciation over time. Alum [Al 2(SO 4) 3·18 H 2O] applications to water samples from Big Bear Lake and Lake Elsinore resulted in a rapid initial decrease in pH and alkalinity followed by a gradual recovery in pH over several weeks. Dissolved Al concentrations increased following treatment, reaching a maximum of 2.54 mg/L after 17 days in Lake Elsinore water and 0.91 mg/L after 48 days in Big Bear Lake water; concentrations in both waters then decreased to <0.25 mg/L after 150 days. The solid phase was periodically collected and analyzed using X-ray diffraction (XRD), differential scanning calorimetry-thermogravimetric analysis (DSC-TGA), scanning electron microscopy (SEM), and surface area analyses to investigate the nature of the reaction products and crystallinity development over time. Poorly ordered, X-ray amorphous solid phases transformed over time to relatively well-ordered gibbsite, with strong diffraction peaks at 4.8 and 4.3 Å. XRD also indicated the formation of a second (possibly aluminosilicate) crystalline phase after 150 days in Lake Elsinore water. Surface areas also decreased over time as crystals reordered to form gibbsite/microcrystalline gibbsite species. DSC-TGA results suggested that the initially formed amorphous Al(OH) 3 underwent transformation to >45% gibbsite. These results were supported by geochemical modeling using Visual MINTEQ, with Al solubility putatively controlled by amorphous Al(OH) 3 shortly after treatment and approaching that of microcrystalline gibbsite after about 150 days. These findings indicate that Al(OH) 3 formed after alum treatment undergoes significant chemical and mineralogical changes that may alter its effectiveness as a reactive barrier to phosphorus release from lake sediments.