Study of the Effect of Leucine on Calcium Carbonate Precipitation in a Circular Economy Perspective

Abstract

This paper studies the crystallization of calcium carbonate in the presence of leucine—a green additive. The effect of leucine on calcium carbonate precipitation kinetic is particularly interesting since CaCO3 is a valuable product worthy to be recovered from industrial liquid wastes (e.g., desalination brines) in the circular economy approach. Experiments have been performed in a laboratory scale plant with a supersaturation range which spams from 2 to 120 and two different leucine concentration (0.520 × 10−3 and 1.041 × 10−3 mol/L). Results obtained have been compared with previous published ones, carried out without any additives. From the measurements of induction times for calcium carbonate nucleation, it was established that in solution, the leucine favors the precipitation of calcium carbonate, so it can be considered a promoter in calcium carbonate crystallization and this behavior enhances when raising its concentration in solution. Interfacial tension was determined for both leucine concentration levels, and the values obtained are in the range 51–84 mJ/m2.

Investigating the Effect of Leucine on Calcium Carbonate Nucleation Mechanisms in the View of a Circular Economy

“…Experimental data have been reported in Figures 3 and 4, which show induction time measurements when 0.00104 and 0.00052 M leucine concentrations, respectively, are added in solution. Results at T = 25 °C, published in a previous paper, 20 have been reported as a reference. As can be seen from Figures 3 and 4, at a fixed leucine concentration, whatever it is, 0.00052 or 0.00104 M, induction time curves translate upward as the temperature is lowered from 35 to 15 °C; that is, a decrease in temperature slightly results in an increase in induction times for calcium carbonate nucleation, thus retarding the crystallization.…”

“…Considerations about the species in solution, together with the calculation of supersaturation ratio have been reported in another paper, particularly taking into account the Debye–Hückel equation and the dependence of pH on the temperature …”

“…The experimental apparatus is sketched in Figure 1: in another paper 20 you may find a thorough discussion about the optical method used to measure the induction time for nucleation, together with the procedure for preparing the solution to be filled in the crystallizer.…”

“…Different temperatures (T = 15, 25, and 35 °C) were considered for all experimental runs. Further details about the concentration of CaCl 2 •2H 2 O and NaHCO 3 are reported in previous work: 20 their values together with two different concentrations of leucine added in solution (0.520 × 10 −3 and 1.041 × 10 −3 mol/L corresponding to 68.2 and 136.5 ppm, respectively) have allowed the performing of a supersaturation ratio range from 2 to 180.…”

“…Considerations about the species in solution, together with the calculation of supersaturation ratio have been reported in another paper, 20 particularly taking into account the Debye−Huckel equation 21 and the dependence of pH on the temperature. 22 The supersaturation ratios can be calculated using the detailed numerical model reported in the Appendix, which accounts for all the equilibria that settle in aqueous solution, namely, Ca +2 and CO 3 − .…”