DOLOMITE FILTRATION: A NEW METHOD OF REMOVING TOXIC METALS AND METALLOIDS FROM PETROLEUM PRODUCED WATER

The aim of this study is to elucidate the feasibility of using dolomite filtration for removing a mixture of toxic metals from PW characterized by high salinity. To this aim, we conducted experiments and reactive transport simulations to assess the removal of high concentrations of toxic metals (100 mg/L for each metal) commonly found in hypersaline produced waters (45,000–115,000 mg/L) using dolomite from Arbuckle Group. The focus was on Ba, Sr, Cd, Pb, and As, as well as on guar gum as a metal complexing additive. We found that salinity has a significant negative effect on the removal of Sr and Ba but a mild effect on the removal of Cd. This is due to chloro-complexion reactions that reduce the concentration of free metal ions in the aqueous phase, and thus their sorption on dolomite. In addition, an increase in pH due to the dolomite dissolution significantly enhances the removal of Cd compared to the removal of Ba and Sr. The formation of guar-gum complexes of metals in the aqueous phase inhibits the removal of Sr, Ba, and Cd by dolomite filtration, this inhibition is more significant for Cd than for Sr and Ba. The removal of the toxic metal is the function of sorption and precipitation reactions of metals on the dolomite surface. These reactions are kinetically controlled and are not limited by the availability of pore surface area, but by the combination of the alkalinity role and retention time of PW in the dolomite filter. The affinity sequence for the sorption of toxic metals at salinity levels of PW is Pb>Cd>As>Ba>Sr. An increase in these cations’ removal by increasing the number (length) of filters suggests that complete removal of these metals can be obtained by optimizing the relationship between the injection rate and dolomite filter size. Higher removal levels predicted for Sr than for Ba and over predictions of Cd removal using previously proposed surface complexation models for dolomite highlights the necessity of further experimental research to incorporate molecular scale surface crystal structure parameters into surface complexation models. Our findings have large implications for establishing a new economic PW treatment method and the elucidation of the fate of toxic metals present in PW disposed into deep dolomite saline aquifers.