Insight into the effect of pH-adjusted acid on thermodynamic properties and crystallization sequence during evaporative-crystallization process of hydrolyzed urine.

The evaporative-crystallization process (ECP) is a frequently used approach for complete nutrient recovery from human urine, and crystallization sequence is related to the selection of seed and the optimization of crystallization process. In this study, three hydrolyzed urine (HU) samples, which were acidified to an initial pH of 4 with HCl, H2SO4, and H3PO4, were used to recover crystallized products by ECP, their crystallization process and thermodynamic properties during ECP were compared, and the detailed crystallization sequence was analyzed using the PHREEQC-2 simulation. The results showed that the pH-adjusted acid has a significant effect on crystal precipitation, and the new crystal in HCl-4-HU, H2SO4-4-HU, and H3PO4-4-HU first appeared at volume concentration factors (CFV) of 19.61, 9.90, and 9.96, respectively. Furthermore, the simulated crystallization process characteristics of HU by PHREEQC-2 have a good fit with the actual experimental data, and crystallization sequence of HCl-4-HU, H2SO4-4-HU, H3PO4-4-HU during ECP were NH4Cl (CFV from 10.25 to 100) / NaCl (CFV from 71.43 to 100), NH4NaSO4 (CFV from 10.25 to 55.56) / NH4Cl (CFV from 20 to 100) / (NH4)2SO4 (CFV from 40.45 to 100), NH4H2PO4 (CFV from 10.25 to 100) / NaH2PO4 (CFV from38.46 to 55.5) / NaCl (CFV from 45.46 to 100), respectively. The present study clearly reveals the crystallization sequence and thermodynamic properties of nutrient elements in acidified HU, which provides an important theoretical basis for the optimization of crystallized products obtained from HU for future study.