Phosphate removal using sludge from fuller's earth production

Abstract

This study assesses the phosphate removal capacity and mechanism of precipitation or adsorption from aqueous solutions in batch experiments by an industrial sludge containing gypsum (CaSO 4·2H 2O) obtained as a by-product from a fuller's earth process. The potential capacity for phosphate removal was tested using various solution concentrations, pH values, reaction times, and amount of sludge. The maximum phosphate adsorption capacity calculated using the Langmuir equation was 2.0 g kg −1. The pH for the maximum adsorption by the sludge was neutral to alkaline (pH 7–12). Over 99% of phosphate was removed from a phosphate solution of 30 mg L −1 using 0.15 g of sludge in a 9-h reaction. Sulfate (SO 4 2−) concentration increased with increasing initial phosphate concentration, possibly because of dissolution of gypsum and adsorption of both sulfate and phosphate. At high phosphate concentration (>1000 mg L −1), relative constant concentration of Ca 2+ was not consistent with adsorption of the most important phosphate removal mechanism. Results suggest that precipitation of calcium phosphate is principally responsible for phosphate removal under its high concentration. Agglomerated precipitate in the reaction sludge was observed by SEM and identified as brushite (CaHPO 4·2H 2O) by XRD, FT-IR, and DTA. Based on thermodynamic considerations, it is suggested that the brushite will readily transform to more stable phases, such as hydroxyapatite (Ca 5(PO 4) 3·OH).