Biogas and calcite handling in calcium fluoride precipitation from aqueous solution guided by thermodynamic simulation with PHREEQC

Abstract

In this innovative study, biogas has been associated with calcium carbonate [CaCO3] to promote the precipitation of fluorite, aiming at the treatment of wastewater with high content of fluoride. The work associates distinct sources of calcium and CO2 for the precipitation of fluorite according to previous simulation with the free software PHREEQC. Considering the reaction at equilibrium, the minimal predicted Ca dosage was 215 mg/L, lower than the 430 mg/L that was experimentally determined, independent of Ca source. The simultaneous use of CaCO3 and CO2 from distinct gas sources (pure CO2, 1:1 CO2:N2, and biogas) exhibited high performance permitting the reduction of fluoride content from 134 to 10 mg/L, with low gas consumption. The biogas consumption of 66.0 mmol/L, equivalent to 33.4 mmol/L of CO2 (1.47 kgCO2/m3 treated wastewater) was predicted, indicating that the biogas storage bag of 700 L would be able to treat 469 L of wastewater. Furthermore, the inert fraction of biogas (CH4) did not impact the reaction and it may be used after the reaction as an alternative source of power, equivalent to 8.25 kWh/m3 treated wastewater. Final solids were composed by fluorite and non-dissolved calcite, confirming the predictions obtained by PHREEQC.