Mobility and speciation of chromium in the urbanindustrial environment
Chromium (Cr) is an important metal which was has been used since the industrial revolution for various applications. It can be toxic dependant on the species present and the production of Cr chemicals has left localised Cr pollution where waste from Cr chemical works was dumped. The fate of Cr, which has leached out of these waste sites, with soils, sediments and waters is dependent on its speciation and the process that leads to the changes in Cr species in the environment is currently poorly understood. The city of Glasgow in the West of Scotland was recognised as an industrial centre from the industrial revolution in the 1800s through to the late 1960s. It hosted large amounts of heavy industry gaining it a reputation as industrial powerhouse. Some of the first industries to appear along the banks of the River Clyde were weaving and dying industries. For the dying process Cr chemicals were required to fix colour pigments to the cloth so in the 1820s J&J White’s Shawfield Chromium Works started to meet this demand. White developed and patented a novel way to extract Cr from chromite ore which lead to it becoming one of the largest producers of Cr chemicals in the world during its heyday. This new process involved roasting the ore with a diluent, an alkali carbonate, improving the penetration of air into the melt and leading to the more efficient conversion of insoluble trivalent Cr (Cr(III)) to soluble hexavalent Cr (Cr(VI)). This soluble Cr(VI) was leached out and converted to the desired product. This high lime process led to the production of a waste product, called chromite ore processing residue (COPR), which is highly alkaline and contained approximately 4% (w/w) of Cr of which ~30% is in the form of the significantly more toxic Cr(VI). The waste material was used as backfill at certain locations in the south of the city, including Polmadie, where it was used in football terracing and to fill in old pits. It is estimated that there was 2.5 million tonnes of this toxic waste material produced before the factory shut in 1968. Research in the late 1990s and early 2000s characterised COPR in the waste disposal sites and ways to remediate it but there was limited focus on the fate of Cr once it gets into the wider environment. This project seeks to determine the biogeochemical and environmental factors that influence the preservation of Cr(VI) and its transport once it leaves the waste disposal sites, along with its bioaccessibility in Cr polluted soils. The study site was the Polmadie Burn, a small stream which runs alongside Richmond Park and which drains areas of COPR waste before meeting the River Clyde. Previous work has demonstrated that it has transported a large quantity of Cr from these former industrial sites into the River Clyde and concentrations therein are close to or sometimes slightly in excess of UK Environmental Quality Standards for surface waters. Initial characterisation in this study determined Cr(VI) concentrations of <4180 μg L-1 in the Polmadie Burn waters, <1200 mg kg- 1 in the underlying sediments, and <2190 mg kg-1 in the banking soils. Total Cr concentrations in the soil and sediment were <6310 and <14900 mg kg-1, respectively. Sequential extraction results showed that 63 ± 10 and 52 ± 5 % of Cr is associated with organic matter in the soils and sediments, respectively, with most of the rest associated with iron hydroxides. The drying of soil and sediments can alter their physical and chemical properties leading to release of metals bound to them. To investigate the drying of Polmadie soils and sediments due to prolonged dry spells, drying experiments were constructed to investigate the release of Cr when soil and sediments were rewetted after different drying periods. These results showed that, in general, low concentrations of Cr were released with a maximum of 0.11 and 0.008% of the Cr released from soils and sediments, respectively. They also reveal that drying of sediments resulted in the release of Cr(VI) due to the oxidation of Cr(III) by freshly precipitated manganese oxides which form in the sediment. This process was not observed in the soils. Sorption of Cr(VI) to the soil, however, was a much more favourable process as over 96% of Cr(VI) was sorbed to the soil after a 48 h shaking period. This was not a reversible process as Cr(VI) sorbed to the banking soil was not released when the Cr loaded soil was shaken in 0.01 M calcium chloride solution showing that the Polmadie soils are a sink for Cr(VI) providing a natural resource for immobilising Cr(VI) in ground water. The inhalation bioaccessibility of Cr in Polmadie banking soils was quantified to estimate the risk to human health. It was found that 59.3 ± 40 and 16.3 ± 3 mg kg-1 of bioaccessible Cr and Cr(VI), respectively, was present in the <10 μm fraction of the banking soil. This was equivalent to 15.1 ± 10 and 19.0 ± 8.3% of the total Cr and the total Cr(VI) in the <10 μm fraction being bioaccessible via inhalation. The risk to people living around the area was concluded to be minimal as the potential for creating inhalable dust from the soil was low but further work is required looking into the airborne concentrations of Cr in the area to fully quantify the airborne Cr concentrations. This research has examined the fate of Cr in the environment once it leaches out of COPR waste. It has shown that low concentrations of Cr is leached from the Polmadie sediments and soils with the organic rich soil is able to irreversibly sorb Cr(VI) from solution. Upon drying both the soils and sediments release a small proportion of the Cr in them when rewetting. This shows that the Polmadie Burn sediment and banking soil is a sink for Cr by immobilising it and also lowers its toxicity by reducing soluble Cr(VI) to Cr(III). Although this research has used the Polmadie Burn as a case study it gives a better understanding of the fate and species of Cr in organic rich soils. It will also help predict the mobility and speciation of Cr present in such soils giving a better understanding of the risk of Cr in these soils to human health.