ARSENIC REMOVAL FROM HIGH SALINITY WASTEWATER THROUGH BARITE CO-PRECIPITATION

Produced waters from oil and gas operations contain a wide range of contaminants, such as Ba, Ra, Sr, and As, released from interaction with subsurface brines. The high salinity of produced waters complicates the treatment process. Chemical co-precipitation of contaminant ions with Ba²⁺ or SO₄^2- in barite (BaSO₄) has been shown to effectively remove contaminants, despite solution conditions being below the saturation index of the contaminant ions. We focus on anion uptake of As(V) into BaSO₄ through substitution of HAsO₄^2- with SO₄^2- at varying NaCl concentrations. In batch experiments at 0.001, 0.5, 1.0, and 1.5 M NaCl, BaSO₄ particles were precipitated in the presence of 0.009 M Na₂HAsO₄ solutions by mixing 1.4 mM BaCl₂ with 1.4 mM Na₂SO₄. Individual BaSO₄ particles were mapped for Ba, S, and As using a Synchrotron X-ray Fluorescence Nanoprobe. Elemental maps showed relatively homogeneous distributions of As at ~2 wt. % throughout the BaSO₄ particles at salinities below 1.0 M NaCl. However, at 1.5 M NaCl, BaSO₄ particles had ~2 wt. % As in the particle center, but obtained a ~20 wt. % As-rich zone near the edge.

We attribute As zonation to differences in activities of ions at varying salinities, and consequently differences in saturation indices (SI) of our initial solutions. At low salinities and high SI, particles precipitate rapidly with a large number of nuclei, producing small, relatively uniform particle sizes, and as a result the reaction path of the system does not pass through the solution conditions that support high As uptake. In contrast, at high salinities and low SI, particles nucleate and precipitate slowly, allowing older particles to grow larger than newer particles over time, and producing solution conditions that result in high As uptake. A geochemical model implementing layer-by-layer growth described in the Doerner-Hoskins model was developed to depict the growth of BaSO₄ particles and uptake of As at varying salinities, and to examine controls on the amount of As removal from solution.