Recovery of phosphate from calcium-containing aqueous solution resulting from biochar-induced calcium phosphate precipitation

Abstract

Nutrient pollution due to excessive phosphorus entering waterways has a significant detrimental public health and environmental impact. Conventional phosphate treatment methods can, themselves, have unintended environmental consequences. Here we present a novel treatment method which operates at near neutral pH (pH = 6.5) using unmodified biochar for the recovery of phosphorus from an aqueous solution containing calcium ions. Biochar produced by the pyrolysis of grapevine cane at different temperatures were prepared, characterized, and investigated as seed particles for the precipitation of calcium phosphate. The removal of phosphate was only observed in the presence of both biochar and calcium ions in the aqueous solution. X-Ray Diffraction analysis reveals that phosphate was adsorbed in the form of brushite (CaHPO3) onto the biochar. The phosphate adsorption capacities were 37.5, 32.0, 25.9, and 16.9 mg.g-1 from biochar prepared at 700, 600, 500, and 400 °C respectively. Kinetic modelling suggests phosphate is adsorbed by a slow multi-step process. Desorption studies indicated that the phosphate was recoverable by acidity modulation at moderate pH levels. 30% of the adsorbed phosphate from the biochar was released at pH = 5 and up to 97% was released at pH = 4. Precipitation of recoverable calcium phosphate at near neutral conditions is a significant improvement in utility over conventional phosphate precipitation methods, and therefore has potential application in the recycling of phosphate from aqueous solutions.