Calcium–phosphorus interactions at a nano-structured silicate surface

Abstract

Nano-structured calcium silicate (NCS), a highly porous material synthesized by controlled precipitation from geothermal fluids or sodium silicate solution, was developed as filler for use in paper manufacture. NCS has been shown to chemisorb orthophosphate from an aqueous solution probably obeying a Freundlich isotherm with high selectivity compared to other common environmental anions. Microanalysis of the products of chemisorption indicated there was significant change from the porous and nano-structured morphology of pristine NCS to fibrous and crystalline morphologies and non-porous detritus. X-ray diffraction analysis of the crystalline products showed it to be brushite, CaHPO42H2O, while the largely X-ray amorphous component was a mixture of calcium phosphates. A two-step mechanism was proposed for the chemisorption of phosphate from an aqueous solution by NCS. The first step, which was highly dependent on pH, was thought to be desorption of hydroxide ions from the NCS surface. This was kinetically favoured at lower initial pH, where the predominant form of phosphate present was H2PO(-)4, and led to decreased phosphorus uptake with increasing pH. The second step was thought to be a continuing chemisorption process after stabilization of the pH-value. The formation of brushite as the primary chemisorption product was found to be consistent with the proposed mechanism.

Batch and continuous phosphate uptake studies employing a ferrimagnetic calcium silicate hydrate composite

“…The activation energy for this ageing step process been reported as between 95-105 kJ mol À1 . 10,11 It has also been shown by Lai that reinforced NCSH material exhibits similar adsorption characteristics to those observed by Southam. It was found that the material possessed a maximum sorption capacity for phosphate of 1.94 mmol P per g NCSH.…”

“…Of the various calcium phosphates, the apatites have the lowest solubility; pK-values for hydroxyapatite, Ca 5 (PO 4 ) 3 (OH), range from 54.4 to 58.5 at 25 C. 18 Despite the low solubility of the apatites, calcium and phosphate are readily found in solution at concentrations far exceeding that for the precipitation of apatite phases, due to the large activation energy of the precipitation step. 19,20 Phosphate sorption by nano-structured calcium silicate hydrate Southam, in 2004Southam, in , 9 2005 10 and 2008 11 conducted extensive studies of the characteristics of nano-structured calcium silicate hydrate (NCSH) with regard to its sorption of phosphate. 19 The phosphate ion, PO 4 3À is not encountered in this initial precipitate, which subsequently ages to form the apatite phase over a period of several hours.…”

Phosphate Removal from Aqueous Solutions Using Calcium Silicate Hydrate Prepared from Blast Furnace Slag

“…19,21) Synthetic CSH compounds has recently been examined in phosphorus recovery from wastewater as crystal seeds, since Ca 2 + and OH − ions released from CSH react with phosphate species to produce HAP crystals on the surface. 22,23) Since the slag-made CSH has a higher specific surface area and unique composition, it is expected to show a superior performances for phosphorus adsorption compared with pure synthetic analogue previously reported. Furthermore, application of the slag-made CSH as an alternative low-cost adsorbent for phosphorus would not only lead to a cost-effective phosphorus removal and recovery process, but also contribute to the sustainable management of BF slags produced in iron-making industry.…”

“…These combined analyses conclusively suggest that the phosphate ions are fixed onto the adsorbent surface in the form of amorphous calcium phosphate species, where the surface exposed Ca 2 + sites and Ca 2 + ions eluted into aqueous solution are the main active species. 22,23) …”

Synthesis and Enhanced Phosphate Recovery Property of Porous Calcium Silicate Hydrate Using Polyethyleneglycol as Pore-Generation Agent

“…This is because the original dense pore structure can be blocked by HAP crystallization quickly at the beginning of reaction stage. The dense pore structure and poor solubility of CSH are therefore the two key factors of phosphate recovery [24]. …”