Biogas and calcite handling in calcium fluoride precipitation from aqueous solution guided by thermodynamic...

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 kg_CO2/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.