A theoretical and experimental analysis of calcium speciation and precipitation in dairy ultrafiltration permeate

Rejection of dairy salts by a nanofiltration membrane

“…The increase in rejection with increasing pH may be explained by considering the phosphate speciation presented in our earlier work [19]. Using a calcium equilibrium model for an identical salt solution , we showed that while H 2 PO 4 − is the major species present within the pH range of 3-6, at higher pH values there become increasing concentrations of HPO 4 2− , and to a lesser extent CaHPO 4 .…”

“…The equilibrium between these species is dictated by the ionic strength, individual ion concentrations and pH of the solution. Interactions between some typical important dairy salts (Ca 2+ , PO 4 3− , Cit 3− ) have been described in detail in our earlier work [19]. The species formed must be considered separately when describing the rejection behaviour of a salt, as they may have a different charge or size to the free ion.…”

“…At 4 mM phosphate and a pH of 7 a sudden severe decline in flux indicated commencement of membrane fouling. This was due to super-saturation and crystallisation of calcium phosphate at the membrane surface [19]. Data points corresponding to this fouling instance have been omitted.…”

The influence of dairy salts on nanofiltration membrane charge

“…This was once again most likely due to the calcium ions interacting with the anionic species. In this case, the ionic calcium levels would be expected to drop significantly, as shown during calcium citrate speciation modeling in our earlier work (Rice et al, 2010), resulting in a lower calcium concentration for membrane adsorption. The negatively charged calcium species formed (i.e.…”

“…This might be expected since citrate was previously shown to have an effect on surface charge at low pH, when present as the sole multivalent ion in solution, whereas calcium did not. Furthermore, at this pH few ionic interactions between calcium and citrate ions occur in the free solution (Rice et al, 2010), and hence speciation effects would not be expected. When calcium was added to a phosphate system, there was no distinguishable change in zeta potential at pH of 3.5, probably because neither of these ions was able to individually influence the charge at this pH (Fig.…”