Investigation of formation mechanism of tufa deposits by chemical breakdown of legacy paper mill sludge

Abstract

This study investigates the formation mechanisms of anthropogenic tufa deposits resulting from the chemical breakdown of legacy paper mill sludge (PMS) at the former Dalmore Paper Mill site in Auchendinny, Scotland. Tufa, a form of calcium carbonate (CaCO₃), typically forms in natural freshwater environments; however, this research explores its precipitation through interactions between alkaline industrial waste (PMS) and atmospheric CO₂. Field and laboratory analyses were carried out to assess the geochemical characteristics of the stream water, as well as the mineralogical and isotopic composition of the associated tufa deposits. X-ray diffraction (XRD) analysis revealed that PMS is composed predominantly of calcite, while tufa samples also contain minor amounts of quartz and kaolinite. Stream water measurements indicated alkaline conditions (pH 8.27–8.98) and elevated calcium concentrations, with calcite saturation indices suggesting conditions favourable for carbonate precipitation. Stable isotope analysis of tufa deposits (δ¹³C −24.62‰ to −13.74‰; δ¹⁸O −17.50‰ to −7.66‰) revealed a dominant contribution from atmospheric CO₂, confirming a precipitation mechanism driven by CO₂ ingassing and hydroxylation reactions. The results support a model where rainwater infiltrates PMS heaps, leaching calcium into stream waters. As these calcium-rich waters mix with atmospheric CO₂, supersaturation occurs, leading to rapid calcite precipitation. The resulting tufa deposits exhibit laminated structures and high porosity, indicating episodic deposition under variable geochemical conditions. This study provides the first detailed evidence of tufa formation from PMS and suggests that such industrial waste materials can act as carbon sinks, capturing and mineralising atmospheric CO₂. These findings expand the understanding of anthropogenic carbonate systems and highlight the potential of PMS as a low-cost material for environmental remediation and carbon sequestration, supporting sustainable waste management strategies and contributing to climate mitigation goals.