Assessing atmospheric CO₂ capture with legacy paper mill waste in Scotland

Global warming over the past 70 years has been driven by rising atmospheric CO₂ levels, largely resulting from industrialization. During this period, large quantities of alkaline waste materials were generated, many of which have the potential to capture atmospheric CO₂ through mineral carbonation, hence offsetting some of these industrial emissions. One such material is paper mill sludge (PMS), a by-product of paper production. Significant volumes of legacy PMS exist worldwide, offering an untapped resource for carbon sequestration. To assess its carbon capture potential, this study maps and quantifies legacy PMS deposits in Scotland, a region with a long history of paper-making. Using historical records and GIS-based spatial analysis, 23 PMS deposits were identified across Scotland, primarily concentrated in the central and northeastern regions. The total volume of these deposits was estimated at 1,450,745 m³. X-ray diffraction (XRD) analysis revealed that PMS samples are composed predominantly of calcite (∼95%), indicating near-complete carbonation. This equates to the sequestration of approximately 1.72 million tonnes of atmospheric CO₂ since deposition. Spatial analysis examined the co-location of PMS deposits with designated ecological and cultural protection zones, revealing minimal overlap. This underscores the need for targeted management strategies to safeguard these carbon sinks from urban development or land-use changes that could release stored CO₂ back into the atmosphere.