title: Understanding the controls on ecotoxic metal and clumped isotope compositions of slag-derived tufas
Creators: name: Holdsworth, Christopher Michael
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Slag, hyperalkaline, slag leachate, pollution, toxic metals, carbonates, anthropogenic tufas, CO2 sequestration, clumped isotopes, CO2 hydroxylation, kinetic fractionation, LA-ICP-MS, Howden Burn, Consett.

description: Steel slag is a by-product of steel production comprised of Ca, Mg and Al-silicates and oxides. Over 500 million tons of slag is produced every year and multi-million-ton accumulations of legacy slag heaps can be found at former and existing steel works across the UK. The weathering of legacy slag releases potentially ecotoxic metals such as V and Cr into the environment and precipitates calcium carbonate (CaCO3). These precipitates become enriched in leached metals, sequestering them from the leachate solution. Analysis of leachate CaCO3 accumulations in these environments can provide insights into the evolution of slag leaching through time and the controls on the ecosystem services (i.e. ecotoxic metal and atmospheric CO2 sequestration) CaCO3 precipitation can provide. This study combines hydrochemical, bulk-tufa and intra-tufa chemical analysis with tufa clumped isotope palaeothermometry to understand the controls on ecotoxic metal and isotope compositions of slag-derived tufas. Samples were taken from the Howden Burn watercourse, which drains a &gt;20million ton slag heap of the now-closed Consett steelworks in north east England. Hydrochemical data were characterised by a decrease in watercourse pH downstream, a loss of some slag-derived elements downstream (e.g. Al, As, Mo, Se and V) and an inverse relationship between the concentration of slag-derived elements and rainfall. The downstream decrease in pH and loss of certain elements was accompanied by lower concentrations of slag-derived elements in mid- and down-stream tufa samples analysed during bulk- and intra-tufa analysis. These trends are indicative of the buffering effect that CaCO3 precipitation has upon leachate pH. Relative chronologies of V and Cr tufa concentrations and clumped isotope compositions provided by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis and clumped isotope palaeothermometry, respectively demonstrate the variation of rainfall dilution through time. Laser data is characterised by concentration spikes of V and Cr &gt;1000mg/kg. These spikes are of higher magnitude and frequency in upstream tufa samples relative to mid- and down-stream samples. Variation between tufa sample concentrations reflects the buffering effect of tufa precipitation upon pH downstream, thus reducing the ability of downstream tufas to sequester V, Cr and other slag-derived elements. The heterogeneity of V and Cr within individual tufa samples likely represents the variation in leachate pH through time that results from varying extents of rainfall and dilution. Carbon (δ13C), oxygen (δ18O) and clumped isotope (Δ47) data is characterised by δ13C and δ18O depletion and a subsequent enrichment of Δ47 clumped isotope values of the order of ~0.100 – 0.150‰, compared to expected equilibrium values calculated from the historic meteoric water δ18O record. The variation in the extent of this δ13C and δ18O depletion and Δ47 enrichment, or isotope disequilibrium, is also likely representative of the variation in leachate dilution from rainfall through time. Dilution of hydroxide and Ca concentrations in leachate waters reduces the extent of CO2 hydroxylation and subsequent isotope disequilibrium from kinetic fractionation, producing tufa isotope data that distributes along an isotope end-member mixing line. As such, the data presented in this study suggests that variations in ecotoxic metal and clumped isotope compositions of slag-derived tufas through space (inter-sample) are indicative of the buffering effect of CaCO3 precipitation. In contrast, variations in ecotoxic metal and clumped isotope compositions of slag-derived tufas through time (intra-sample) are indicative of the variation in leachate dilution from rainfall.