Implementation of Ion Exchange Processes for CO2 Mineralization Using Produced Water Streams

Large coal-fired power plants (>500 MW) account for 46% of global CO₂ emissions, and long-term management of this CO₂ to is urgently needed mitigate global temperature increases. Sequestration of CO₂ within stable mineral carbonates (e.g., CaCO₃) represents an attractive emission reduction strategy because it offers a leakage-free alternative to geological storage of CO₂ in an environmentally friendly form. This work describes a mineralization process in which divalent cations are sourced from various waste streams (e.g., produced water and brackish water) and alkalinity is induced via regenerable ion-exchange materials. In our process, aqueous streams with pH > 8 are produced by contacting fresh water with various ion-exchange materials (e.g., Na form zeolites or ion exchange resins). These streams are mixed with produced water containing high concentrations (~0.1 M) of Ca²⁺ leading to the precipitation of solid calcium carbonate (PCC). This process has the advantages of using regenerable solids in a simple and continuous process to increase the pH of water by ion exchange instead of relying on the consumption of costly and unsustainable sources of alkalinity (e.g., sodium hydroxide).

The pH shift in our process is effected by the exchange of protons for Na⁺ ions. Thus, the Na⁺-H⁺ exchange isotherms and adsorption rates of 13X and 4A zeolites and TP-207 and TP-260 resins in CO₂-saturated water (pCO₂ = 1 atm, pH 4) were studied via batch equilibrium experiments. Adsorption isotherms obey the Langmuir model, with the resins achieving higher capacities than zeolites: 2.4 mmol H⁺/g for TP-207, 2.2 mmol H⁺/g for TP-260, 1.8 mmol H⁺/g for 4A and 1.6 mmol H⁺/g for 13X. Consequently, ion exchange resins were capable of producing aqueous solutions with of pH = 11.3 and 10.9 for TP-207 and TP-260 respectively, compared to a pH = 10.2 and 9.8 for solutions contacted with 4A and 13X respectively. Multi-component mixtures of Ca²⁺ and Mg²⁺ cations in CO₂-saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution significantly inhibited H⁺ uptake because of their larger field strength, reducing capacities to as low as 1.2 mmol H⁺/g for resins and 0.8 mmol H⁺/g for zeolites. These results indicate the alkalinity inducing reaction for this process must occur in the absence of divalent cations to achieve the desired pH for CO₂ mineralization.

The extent of calcite precipitation was evaluated experimentally by mixing the alkaline CO₃^2--rich water solution that is obtained from the ion-exchange column, with a simulated liquid waste stream solution (1.4 M NaCl, 0.1 M CaCl₂, 0.056 M MgCl₂, 0.01 M CaSO₄, 0.0001 M KCl, 0.00044 FeCl₂). Experimental calcite yields were 26 mmol/L for cation exchange resins and 8 mmol/L for zeolitic materials, with the formation of goethite (an iron-hydroxide phase, FeOOH) as the primary contaminant phase (99% calcite, 1% goethite). Yields calculated via Gibbs Energy minimization were 26.5 mmol/L and 8.8 mmol/L for the resin and zeolite systems, respectively, indicating that the experimental process was able to achieve thermodynamic maximum production of calcite.

The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric bases to induce alkalinity for the precipitation of CaCO₃. The high calcium carbonate yields (1 ton CO₂/day utilizing a produced water flow rate = 6.2 L/min and a total bed volume = 0.02 m³) obtained for the materials examined and the successful operation at standard temperature and pressure conditions support their potential for industrial implementation.