Phosphate in the environment : its analysis and removal using advanced materials

Amorphous calcium silicate (ACS), a novel silicate based advanced material, has been developed as filler for use in paper manufacture. It has a large surface area and unique nano-structure that has been identified as having great promise as a sorbent. ACS has shown an affinity for chemisorption of orthophosphate from an aqueous solution, obeying a Freundlich isotherm with high selectivity for orthophosphate over other common environmental anions. Removal efficiencies of up to 100% phosphate were achieved at environmental and waste stream concentrations of between 0.01 and 100 mg P L. This provided significant efficiency improvements over other sorbent-based methods for the removal of phosphate from wastewater. Chemisorption studies indicated that loadings up to 1.9 mmol P g ACS (ca. 18 w/w% as PO 4 3-) were achievable. Determination of activation energies from studies of phosphate and hydrogen ion concentration showed that dihydrogen orthophosphate was the kinetically favoured form of phosphate chemisorbed. It was also chemisorbed with higher loadings, and hence greater efficiency, than other forms of orthophosphate. It was initially assumed that tricalcium phosphate or hydroxyapatite would be the preferred product due to its lower solubility and higher stability. As such, PO 4 3- would be preferentially form chemisorbed, however this was contradicted by the kinetics studies above. Surface analysis indicated there was significant change from the porous and amorphous morphology of untreated ACS to fibrous and crystalline morphologies and a non-porous detritus in the product of chemisorption. Chemical analysis of the crystalline material showed it to be brushite, calcium monohydrogren phosphate dihydrate. A mechanism was proposed for the chemisorption of phosphate from an aqueous solution by ACS, which sees a two-step process. The first step, which was highly dependent on pH, was thought to be desorption of hydroxide from or ion-exchange of calcium on the surface. This was kinetically favoured at lower initial pH where the predominant form of phosphate present was H2PO4- and led to decreased uptake with increasing pH. The second step was thought to be a continuing chemisorption process after stabilisation of pH. The formation of brushite as the primary chemisorption product was found to be due to the inhibition of crystallisation of other calcium phosphates caused by the presence of silicon. Desorption of phosphate from the spent ACS was achieved to moderate levels by repetitive agitation in aqueous solutions and with various chemical treatments. This desorption indicated that there was an exchangeable phosphate fraction present in the spent ACS, which had potential for reuse as a nutrient-rich soil additive. The selectivity of ACS for the chemisorption of orthophosphate over other ions was utilised in the construction of an ACS ion selective electrode (ISE) for the quantitative potentiometric determination of orthophosphate in aqueous solutions. ACS was also utilised as the active material in a quartz crystal microbalance (QCM) for the gravimetric determination of orthophosphate and showed consistent linearity according to its tendency to chemisorb phosphate according to the Freundlich Isotherm, though with moderate selectivity. These techniques provided some success for the quantitative determination of orthophosphate in environmental waters.