Experimental study on the effects of different factors on the crystallization rate and products of Mn-Ca co-crystallization.

Abstract

Calcium and manganese are common ions that pollute drinking water and, therefore, ingestion may seriously harm human health. However, despite its importance, few studies into the synergistic removal of calcium and manganese have been reported. Calcium and manganese have similar chemical properties and, consequently, can be removed by co-crystallization, as the exact crystalline phase formed by this process and the process rate depend on the pH, the dissolved oxygen (DO) content, and the concentrations of the component ions. In this work, we experimentally studied the co-crystallization of Mn and Ca using an automatic potentiometric titrator. We found that the concentration of Mn2+ can be reduced from 3.0 to <0.1 mg/L by the co-crystallization of Mn and Ca at pH 10.5 and a DO content of 8.5 mg/L. In addition, the crystallizations of Ca and Mn are mutually inhibitory; the crystallization process of Mn is obviously divided into two stages: crystal nucleation and crystal growth. Increasing the pH, decreasing the DO content, and decreasing the Mn ion concentration increase the rate of CaCO3 crystallization, whereas the opposite changes increase the rate of Mn crystallization. Furthermore, Mn-Ca co-crystallization leads to the formation of various substances, including single crystals (CaCO3/MnCO3), mixed crystals (CaMnCO3), and Mn oxides (MnxOy/Mn(OH)O). Our findings regarding the effects, precipitation rates, and precipitation mechanisms of Mn-Ca co-crystallization serve as an important guide for the optimization and control of Mn-Ca co-crystallization processes.