COPRECIPITATION REACTIONS FOR TREATMENT OF INDUSTRIAL WASTEWATERS

Abstract

Coprecipitation is the substitution of one element for another during chemical precipitation and can be an effective treatment method for industrial wastewaters contaminated with high concentrations of metals. Predicting contaminant incorporation during coprecipitation reactions remains a challenge due to kinetic and mechanistic effects that shift incorporation away from the thermodynamic prediction. In this dissertation, I advance the understanding of contaminant incorporation during coprecipitation with barite and calcium carbonate through experimental data and thermodynamic and kinetic modeling.

In Chapter 2, I present novel nano-scale resolution chemical composition maps obtained from synchrotron-based X-ray fluorescence (XRF) spectroscopy of barite particles coprecipitated in the presence of strontium. Thermodynamic modeling of (Ba,Sr)SO4 solid solutions was done using solid solution-aqueous solution (SS-AS) theory. The results show that the Sr content in (Ba,Sr)SO4 solid solutions varies widely among particles and even within a single particle. We observed substantial Sr incorporation that is far larger than thermodynamic models predict, likely indicating the formation of metastable solid solutions. These results suggest that coprecipitation offers significant potential for designing treatment systems for aqueous metals’ removal in desired metastable compositions.

In Chapter 3, I examine a wider range of solution conditions including saturation index, salinity, and cation/anion ration, and their effect on Sr incorporation in (Ba,Sr)SO4 solid solutions using bulk XRF. In every experiment we found more strontium incorporation than predicted by SS-AS theory. Sr removal can be enhanced by increasing the saturation index of barite, decreasing salinity, and decreasing the Ba2+:SO42-. I also present a kinetic model of coprecipitation based on the growth rate of individual particles that provides better predictions of Sr incorporation than SS-AS thermodynamic theory.

In Chapter 4, I present measurements of incorporation of metals into calcium carbonate particles in the context of remediating coal fly ash leachate through CO2 mineralization. The results show consistently higher trace element incorporation than predicted by a thermodynamic model. The degree to which trace element incorporation was enhanced over the thermodynamic prediction was correlated to the solubility of the pure endmember. These results suggest that carbonate precipitation may be a more effective strategy to immobilize toxic elements than is commonly assumed.