CO₂-Based Treatment of Alkaline Industrial Wastes: Pollution Mitigation and Resource Recovery
Background and Rationale:
Alkaline industrial wastes such as bauxite residue (red mud), steel slags, concrete crusher fines, combustion ashes, and chromium ore processing residue (COPR) pose significant environmental challenges due to their high pH and potential to leach toxic metals. These materials often exceed pH values of 11–13, leading to mobility of contaminants like arsenic, vanadium, and chromium, and hindering natural attenuation processes. Recent research suggests that CO₂ dosing can neutralize alkalinity through carbonation reactions, forming stable carbonates and reducing pH. This approach not only mitigates environmental risks but may also enable selective recovery of valuable metals such as gallium and rare earth elements (REEs), which are often present in trace amounts in these wastes.
Research Objectives:
- Determine optimal CO₂ dosing rates for effective pH neutralization across different waste types and leachates.
- Assess the potential for pH rebound post-treatment due to residual alkalinity or dissolution of secondary phases.
- Evaluate the removal efficiency of toxic elements (As, V, Cr) following CO₂ treatment.
- Investigate the recovery potential of valuable metals (V, Ga, REEs) from treated residues and leachates.
- Develop mechanistic models to describe carbonation reactions and metal speciation changes.
Methodology and Approach:
The proposed research will begin with a comprehensive characterization of alkaline industrial wastes, including bauxite residue, steel slags, concrete crusher fines, combustion ashes, and chromium ore processing residue (COPR). Representative samples will be collected and analysed to determine their mineralogical composition using X-ray diffraction (XRD), elemental profiles via inductively coupled plasma mass spectrometry (ICP-MS), and leaching behaviour through standard protocols such as the Toxicity Characteristic Leaching Procedure (TCLP). This initial phase will establish a baseline understanding of the chemical and physical properties of each waste type, informing subsequent treatment strategies.
Following characterization, CO₂ treatment experiments will be conducted to assess the potential for carbonation to neutralize alkalinity and immobilize contaminants. Both batch and column setups will be employed to simulate different environmental and operational conditions. CO₂ will be introduced in gaseous and dissolved forms, with careful control of dosing rates, flow regimes, and exposure durations. Key parameters such as pH, redox potential (Eh), electrical conductivity, and carbonate formation will be monitored throughout the experiments to evaluate treatment efficacy and reaction kinetics.
A critical component of the study will be post-treatment analysis, focusing on the stability of the treated materials and the potential for pH rebound over time. Simulated environmental conditions will be used to assess long-term behaviour, and leachates will be analysed for both toxic and valuable metals. Advanced analytical techniques, including ICP-MS for concentration measurements and speciation tools such as X-ray Absorption Near Edge Structure (XANES) and electron microscopy (SEM/TEM), will be used to determine the chemical forms and mobility of elements like arsenic, vanadium, chromium, gallium, and rare earth elements (REEs).
To explore the resource recovery potential, selective leaching and precipitation methods will be applied to extract gallium and REEs from treated residues and leachates. The purity and yield of recovered metals will be evaluated, alongside a preliminary economic assessment to determine the feasibility of integrating recovery into waste treatment workflows.
Finally, the project will incorporate geochemical modelling and simulation using tools such as PHREEQC to replicate carbonation reactions and predict long-term stability of treated materials. These models will also be used to simulate metal speciation and mobility under varying environmental conditions, contributing to the development of predictive frameworks for both contaminant immobilization and resource recovery.
Expected Outcomes:
The research is expected to deliver several key outcomes:
- Development of waste-specific CO₂ dosing protocols that optimize treatment efficiency.
- Improved understanding of pH rebound mechanisms and strategies to mitigate long-term alkalinity.
- Quantitative data on the immobilization of toxic metals and the recovery of valuable elements, supporting both environmental protection and resource circularity.
- Creation of scalable treatment frameworks that can be adapted for industrial application, contributing to sustainable waste management and carbon utilization strategies.
Significance and Impact:
This research will contribute to sustainable waste management, carbon utilization, and critical raw material recovery. It aligns with circular economy principles and supports decarbonization strategies in heavy industries. In addition, this project aligns strongly with NERC’s remit through its focus on pollution mitigation, environmental geochemistry, and sustainable resource recovery. It addresses the transformation and long-term fate of pollutants in alkaline industrial wastes, including the immobilization of toxic metals and the recovery of valuable elements. The use of CO₂ for treatment and stabilization intersects with NERC’s interests in pollution, ecotoxicology, hydrogeology, and Earth surface processes, while the investigation of leachate chemistry and pH dynamics supports NERC’s focus on water quality and soil science. The project also contributes to environmental biotechnology and technology for environmental applications.
Candidate:
A strong candidate for this PhD project would ideally have a multidisciplinary background combining elements of environmental science, geochemistry, and chemical or environmental engineering. Given the technical nature of the work, the candidate should be comfortable with laboratory-based experimentation, including sample preparation, analytical techniques (e.g., XRD, ICP-MS, SEM), and handling of industrial waste materials. Experience or interest in geochemical modelling (e.g., using PHREEQC or similar software) would be highly beneficial, as would familiarity with environmental remediation technologies and metal recovery processes. A good understanding of aqueous chemistry, mineralogy, and pollution science is desirable, particularly in the context of alkaline waste behaviour and contaminant mobility.
The project also requires a candidate who is analytically minded, capable of interpreting complex datasets and integrating experimental results with theoretical models. Since the research aligns with several areas of the NERC remit—including pollution mitigation, water quality, hydrogeology, and environmental technology—a candidate with a strong interest in environmental sustainability, resource recovery, and climate-related innovation would be particularly well-suited. Finally, good communication skills and the ability to work across disciplines (e.g., collaborating with geoscientists, chemists, and engineers) will be important for the successful delivery and impact of the project.
Suggested References:
Alkaline residues and the environment: a review of impacts, management practices and opportunities. https://www.sciencedirect.com/science/article/pii/S0959652615013396
Behavior of aluminum, arsenic, and vanadium during the neutralization of red mud leachate by HCl, gypsum, or seawater. https://pubs.acs.org/doi/full/10.1021/es4010834
Hydraulic and biotic impacts on neutralisation of high-pH waters. https://www.sciencedirect.com/science/article/pii/S0048969717313566