Discrepancies between pH and Corrosive Indices of Hypersaline Effluents

Determining the pH of hypersaline wastewater is essential for regulatory compliance and the feasibility of geochemical and treatment processes. Hypersaline solutions have high ionic strength, which introduces deviations among measured proton activity, its concentration, and potential acidity. Here, we evaluate these deviations with theoretical simulations and by modeling hypersaline lithium brines from South America and produced waters from U.S. oil and gas operations (n > 60 000). We show that when the ionic strength increases above 2 m, proton activity coefficients steadily increase above unity. This causes the pH value to overestimate proton concentration. As this discrepancy increases, the pH alone cannot be used to accurately evaluate the solution’s corrosivity by features such as the saturation index (SI) or calcium carbonate precipitation potential (CCPP). Our analysis reveals that 5% of the investigated produced waters showed pH values of <6 but positive CCPP or SI values, undermining pH as a single regulatory indicator of corrosivity. Accurate evaluation of hypersaline effluent corrosivity requires the utilization of advanced tools such as the PHREEQC software with the Pitzer model. This approach ensures a more reliable characterization of the potential hypersaline effluent corrosivity and thus more efficient management and policy.