Effects of pCO2 on the Removal of Fluoride from Wastewater by Calcite

• Published in: Journal of environmental engineering (New York, N.Y.) Vol. 139; no. 8; pp. 1053 - 1061
• Main Authors: Sleap, Scott B, Turner, Brett D, Krabbenhøft, Kristian, Sloan, Scott W
• Format: Journal Article
• Language: English
• Published: Reston, VA American Society of Civil Engineers 01.08.2013
• Subjects: Applied sciences; Calcite; Calcium carbonate; Carbon dioxide; Exact sciences and technology; Fluorides; General purification processes; Groundwaters; Mathematical models; Natural water pollution; Other industrial wastes. Sewage sludge; Pollution; Stirring; Technical Papers; Transport; Wastes; Wastewater treatment; Wastewaters; Water treatment and pollution; Stirring; Carbon dioxide; Wastewater management; Limestone; Fluorides; Calcite; Modeling; Waste water; Transport process; Inorganic Compounds; Industrial waste; Pollution; Partial pressure; Decontamination; Batchwise; Groundwater; pH; Industrial Wastes; Remediation; Reactor; Ground water; Inorganic chemicals
• ISSN: 0733-9372; 1943-7870
• DOI: 10.1061/(ASCE)EE.1943-7870.0000710

AbstractFree-drift batch reactor experiments using calcite (limestone, CaCO3) were used to study fluoride removal through precipitation as fluorite (CaF2) from solutions with concentrations reflective of an industrially contaminated site. The influence of CO2 partial pressure (pCO2), stirring rate, and fluoride concentration were investigated in this paper. Equilibrium modeling shows that in wastewaters with high fluoride concentrations (∼2,000  mg/L), the flux of CO2(g) to CO2(aq) could not keep up with the consumption of CO2(aq), resulting in an initial disequilibrium with experimental pH reaching equilibrium quickly, while fluoride removal lagged. Increasing stirring rate significantly decreased the extent of disequilibrium and the time at which the CaCO3-fluoride-CO2 system attained equilibrium due to the increased rate of transport of dissolved CO2 to the CaCO3 surface, and simultaneously the rate of transport of the dissolved CaCO3 to the bulk solution. Optimal fluoride removal occurs at pCO2∼10−0.52 [30% (mol% CO2)] with 96% of the initial 2,000  mg/L fluoride load removed in less than 80 min with a stirring rate of 300 revolutions per minute. Increasing pCO2 to ∼100 (100% CO2) resulted in very little gain, less than 2%, in fluoride removal, or in the time required to reach equilibrium and therefore significant remediation cost savings can be obtained by using pCO2 30% when compared to 100%.