Integrated Critical Minerals Recovery from Produced Water and CO2 Sequestration

Abstract

Produced water (PW), a high-salinity byproduct generated during oil and gas production, poses substantial environmental management challenges while also representing an underutilized secondary resource for carbon fixation and critical mineral recovery. The objective of this dissertation was to develop integrated and sustainable approaches for the high-value utilization of PW, with particular emphasis on maximizing its carbon mineralization capacity and enabling the recovery of critical minerals, including lithium (Li) and rare earth elements (REEs). First, a two-step strategy was developed for selective recovery of REEs from PW through precipitation-based preconcentration followed by dissolution–reprecipitation purification. Different precipitants were systematically evaluated for REE preconcentration from the highly saline PW matrix, and the resulting solids were comprehensively characterized by SEM-EDS, XRD, Raman spectroscopy, TGA, and FTIR. NH3·H2O and NaOH exhibited similar separation behavior with a REE recovery of 75% at pH 7.0 and provided the best selectivity by minimizing the co-precipitation of alkaline earth metals, whereas Na2CO3 and oxalic acid caused substantial Ca/Sr co-precipitation and therefore lower selectivity. Mechanistic investigations showed that Sc tended to precipitate as hydroxide phases in the NH3·H2O and NaOH systems, whereas the other REEs were recovered mainly through adsorption by iron hydroxides. These distinct pathways were further supported by species distribution calculations and Johnson-Mehl-Avrami (JMA) model analysis. Because direct desorption of the hydroxide-derived preconcentrates was ineffective, purification was achieved by acid redissolution followed by selective oxalate reprecipitation. Among the tested acids, HCl provided the best purification performance than HNO3 and H2SO4, yielding 67% TREE recovery with 27% product purity under 300 mg/L oxalic acid. Economic analysis indicated that the recovered REE product had an estimated value of 0.625 USD/t PW and a net benefit of 0.017 USD/t PW. This work establishes both a practical process and a mechanistic framework for trace REE recovery from complex produced water systems. Secondly, the feasibility of using PW for CO2 mineralization under ambient conditions was demonstrated through systematic parametric investigations. Under optimal conditions, including an initial pH of 12.0, a final pH of 8.0, and a CO2 flow rate of 0.4 L/min, each ton of PW sequestered 28.75 kg of CO2, corresponding to 90% of its total carbon mineralization potential. The conversion efficiencies of Mg2+, Ca2+, Sr2+, and Ba2+ reached 68.0%, 92.8%, 99.2%, and 98.7%, respectively. Phase transformation analyses revealed that the major metal species evolved from hydroxides to metastable carbonates and ultimately to stable carbonates or bicarbonates, with MgCO3, CaCO3, SrCO3, and BaCO3 identified as the primary products. This work demonstrates that PW can serve as an effective medium for simultaneous wastewater treatment and permanent CO2 sequestration, while also generating valuable carbonate byproducts. Furthermore, an efficient Li recovery process was developed for low-grade brines with ultrahigh sodium (Na) interference using a synergistic solvent extraction system consisting of LIX 54 and TBP in kerosene. This system was specifically designed for challenging feed solutions representative of PW-related brines, including a solution containing 0.1 g/L Li and 75.5 g/L Na, corresponding to an exceptionally high Na/Li molar ratio of 226.6. Under optimized conditions, the LIX 54/TBP system achieved high Li extraction with minimal Na co-extraction, resulting in excellent Li/Na selectivity in a single contact at room temperature. The strongest synergistic effect was obtained at a 1:1 molar ratio of LIX 54 to TBP. In addition, co-extracted Na was effectively removed by deionized-water scrubbing, and Li was fully stripped using 0.5 mol/L HCl. The system also exhibited excellent stability and reusability over multiple cycles. Mechanistic investigations indicated that the extracted Li species could be represented as LiAB, where A and B correspond to deprotonated LIX 54 and TBP, respectively. TBP was found to promote the enolization of LIX 54, strengthen Li coordination, and improve the lipophilicity of the extracted complex, thereby accounting for the enhanced extraction efficiency and Li/Na selectivity. The applicability of the process was further confirmed using real PW, demonstrating its feasibility for Li recovery from complex low-grade brines. A preliminary economic analysis demonstrated a net benefit of 2.83–3.65 $/t PW. Overall, this dissertation establishes an integrated framework for transforming PW from an environmental liability into a multifunctional resource for critical mineral recovery and CO2 sequestration. A staged valorization strategy is proposed for PW valorization, in which REEs are selectively recovered at near-neutral pH, the remaining alkaline earth-rich stream is further treated at higher pH for CO2 mineralization and carbonate formation, and the resulting metal-simplified brine is then used for selective lithium recovery. This work advances sustainable process development for wastewater valorization and contributes to circular resource utilization, decarbonization, and critical mineral supply resilience.