(90c) Implementation of Ion Exchange Processes on Industrial Waste Streams for CO2 Mineralization

Economic development and population growth will lead to an increase in global energy consumption. Large coal-fired power plants (>500 MW) account for almost half of the total CO₂ emissions from fossil fuel combustion, and nearly 26% of the global fossil fuel and industry CO₂ emissions. Long-term management of the more than 36 Gt of CO₂ that is emitted globally per year is urgently needed. CO₂ mineralization is an attractive method of sequestering CO₂ because it captures CO₂ as an insoluble carbonate mineral, mimicking the natural process of rock weathering. Here, we present a pathway for accelerated CO₂ mineralization enabled by divalent cations sourced from liquid streams.

In this work, we investigate a mineralization route that converts flue gas CO₂ to precipitated calcium carbonate (PCC) using waste aqueous streams as the calcium source. The proposed mineralization route integrates CO₂ utilization and waste liquid management at coal power plants. Maintaining alkalinity during carbonation is the primary challenge in this process. Instead of relying on the consumption of costly and unsustainable sources of alkalinity (e.g., sodium hydroxide), we used regenerable solids to increase the pH of water by ion exchange. The increase in alkalinity creates a more thermodynamically favorable environment for calcite precipitation.

The capacities for ion exchange of various commercially available zeolites and ion exchange resins in CO₂-saturated water were studied through batch equilibrium and column ion-exchange experiments. During ion exchange, H⁺ in the water exchanges with Na⁺, Ca²⁺ or K⁺ in the zeolite or resin. Batch equilibrium isotherms for H⁺–Na⁺ exchange were collected by first adding varying amounts of the ion exchange solid (0 – 1.0 kg per kg of water) to CO₂-saturated water (pCO₂ = 1 atm, pH 4) to determine the maximum H⁺–Na⁺ exchange achievable. Ion-exchange column studies were performed to determine breakthrough capacity of the system. In both systems, an increase in pH coupled with an increase in Na concentration was observed, demonstrating the feasibility of forward ion exchange. Zeolitic materials increased the alkalinity of the solution from pH 4 to around 10, whereas ion exchange resins containing weakly acidic functional groups (e.g., carboxylic acid) exhibited higher affinity for H⁺, increasing pH to 11.3 and surpassing the pH shift induced by the zeolitic materials.

The extent of calcite precipitation was evaluated by mixing the alkaline CO₃^2--rich water obtained after passing through the ion-exchange column and a CaCl₂ solution. Calcite yields are as expected from chemical thermodynamics. Calcium carbonate yields found experimentally were about 26 and 8 mmol per kg water for the weakly acidic ion exchange resin and zeolitic materials, respectively.

The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric inorganic bases (e.g., sodium hydroxide) to induce alkalinity for the consequent precipitation of CaCO₃. Instead, weakly acidic ion exchange resins commonly used for water hardness removal were alternatively used to induce pH swings in the aqueous solutions. The high calcium carbonate yields obtained for the materials examined demonstrate the potential application of this process. In addition, the successful operation of the calcium carbonate production process at standard temperature and pressure conditions support its potential for industrial implementation.