Use of surfactants for efficient removal of calcium phosphate particles from water
Authors: Radovenchyk Vіacheslav , Hordiienko Kateryna , Radovenchyk Yaroslav , Krysenko Tamara 
Published: September 2022

Today, softening of water in offices and private homes is mainly carried out by the ion exchange method, despite its significant negative impact on the hydrosphere. The development of alternative safe technologies in this field is extremely relevant today. Such an alternative can be considered the use of effective reagents that transfer hardness ions into the solid phase.

The use of phosphates as such reagents allows, depending on the conditions, to reduce the residual hardness of water to the level of 0.1 mg-eq/dm3 or less. An important problem of this technology is the efficiency of separating the solid phase from the mother liquor. This step can be implemented by advocating or filtering. Using model calcium solutions treated with appropriate doses of sodium phosphate and flocculants, the most effective of them in settling processes were selected. As flocculants, polyacrylamide was used - as a nonionic flocculant, Magnofloc - 336 from Ciba - as an anionic flocculant, Zetag - 7692 from Ciba - as a cationic flocculant.

The flocculant Magnofloc - 336 turned out to be the most effective in settling highly dispersed particles of calcium phosphate. At a flocculant concentration of 30 mg/dm3 for 10 min of settling, the apparent volume of the solid phase decreases to 30 % of the initial volume of the suspension and stabilizes at this value. Other types of flocculants and other doses were less effective.

A somewhat different situation is observed when separating the solid phase by filtration. The greatest influence on the filtration rate is caused by the temperature, the hydrogen index and the ratio between the components. Only temperatures below 20 °C significantly affect the rate of filtration. This is especially noticeable at a temperature of 5 °C. At this temperature, the volume of filtrate is 8-20 minutes behind the volume of filtrate of distilled water. At 15 °C, this difference is smaller, and at temperatures above 20 °C, the curves generally overlap. In the pH range of 5-9, there is no significant effect on the filtration rate. Only in a strongly alkaline environment do the necessary filtering times increase significantly, which, in our opinion, is due to the formation of a significant number of amorphous particles of various composition, which can block the pores of the filters. With the stoichiometric ratio of the components, the curve of the change in the filtration rate practically coincides with the curve of the change in the filtration rate of distilled water under the same conditions. When the ratio changes both downward and upward, the conditions for filtering the suspension deteriorate. And the more the ratio differs from stoichiometry, the more developed the structure of the solid phase is and the worse its separation from the liquid phase is. As the ratio between the components changes, so does the pH of the zero charge of the solid particles. If the stoichiometric ratio of the components is at pH 8.15, then when the ratio decreases to 0.5, it decreases to pH 7.41, and when it increases to 2, it increases to pH 8.64. A change in the ratio of components affects not only the sign of the surface charge, but also its magnitude, which cannot help but affect the efficiency of solid phase separation. The type and dosage of flocculants determined to be most effective in settling do not always provide the same effect in filtration. Suspensions without the addition of flocculants have the highest filtration speed. The study of the effect of flocculants at their concentrations of 10 and 30 mg/dm3 showed that none of its different types and at their different concentrations contributes to an increase in the filtration rate. Obviously, significant doses of flocculants contribute to the rapid calming of the porous medium of the filters and cause a decrease in the rate of transport of the liquid phase.

The obtained results make it possible to determine effective reagents for removing calcium ions from natural waters and the optimal conditions for their use and serve as a basis for the development of effective and environmentally safe technologies for softening natural waters.