Chemical and physical controls of toxic metals removal from petroleum produced water by dolomite filtration
Abstract
Produced water (PW) from oil and gas reservoirs contains orders of magnitude higher concentrations of toxic metals (heavy metals, metalloids, and alkaline earth metals) than seawater, shallow groundwater, and conventional wastewater. The high concentration of total dissolved salts (TDS) and complex composition of PW hinder the utilization of conventional water treatment technologies to remove toxic metals from PW at an economic cost. To reduce and/or replace the use of expensive conventional technologies to remove high concentrations of toxic metals from huge volumes of PW, the goal of this research was: 1) to assess the potentiality of using dolomite filters made of compressed dolomite grains to remove toxic metals from PW, 2) to determine the chemical (alkalinity, pH, and salinity) and physical (injection rate, dolomite surface area) controls of toxic metals removal by dolomite filtration, 3) to elucidate the changes in the morphology, and mineral and elemental composition of the dolomite surface due to precipitation/co-precipitation and/or sorption reactions of toxic metals, and 4) to mathematically model and simulate the removal of toxic metals from PW by dolomite filtration. The focus was on the most common and abundant toxic metals found in PW (e.g., Sr, Ba, As, Cd, and Pb). The attained removal levels of toxic metals are comparable to those obtained with synthetic materials. It was found that 1) the in-situ generation of alkalinity from the dissolution of dolomite controls the precipitation/coprecipitation of toxic metals as carbonate minerals, 2) the increases of pH that results from the dissolution of dolomite promotes sorption reactions of toxic metals on dolomite, 3) Ba and Sr are removed from PW preferentially via sorption reactions, Pb and As are removed mostly via precipitation/co-precipitation reactions of carbonate minerals, and the removal mechanism of Cd is highly dependent on the PW composition (alkalinity and pH). Carbonate precipitates from PW have complex compositions that reflect the composition of PW, and the lack of thermodynamic data for those complex carbonate minerals hinders the application of reactive transport models to reproduce experimental results. The findings of this research have large implications toward the establishment of a new dolomite filtration technology to treat PW for toxic metals at an economic cost, as well as to predict the fate of toxic metals present in PW commonly disposed into deep dolomite saline aquifers, and/or to recover critical metals from PW by dolomite filtration.
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- OSU Dissertations [11222]