Land is an undervalued resource in many countries. It has and continues to be widely contaminated by industry. And yet we rely on healthy soil to underpin our industrial economies. In addition, wastelands are a blight on the landscape and are very likely to have a negative impact on a community’s sense of wellbeing. People prefer greenspace to wasteland. However current technologies for remediation of contaminated brownfield sites are expensive and unsustainable. Wastelands, for which there is little or no economic driver for regeneration, are frequently overlooked and remain barren.
Our research involves both science and social science and we are working with local communities to develop a sustainable methodology for regenerating wastelands (see www.robustdurham.org.uk). The sustainable technologies involve using clean by-products from industry which include compost and Water Treatment Residual (WTR). WTR is a by-product from the drinking water treatment process generated all over the world by the Water Industry. WTR contains the redox active minerals, iron and manganese oxides. These minerals are already present in the soil and form a large part of the soil’s natural defence mechanism against contaminants. By adding more of these minerals to the soil we can boost the soil’s defence mechanism against pollution.
Our work has shown for the first time that manganese oxides are capable of oxidatively breaking down ecotoxic organic contaminants eg tarry substances like anthracene. Anthracene can be converted to anthraquinone on the surface of manganese oxide minerals under the wetting and drying sequences which are common in soil. Anthraquinone is less ecotoxic than anthracene and importantly it can be broken down into harmless substances by soil microorganisms. Since wetting and drying sequences are predicted to become more prevalent by climate change models, the importance of the drying process in the break down of organic substances on manganese oxide minerals has important implications for the role of manganese oxides in land remediation.
We have also shown that manganese oxides are good at immobilising potentially toxic elements such as lead and arsenic (see Figure 1).
Figure 1: Electron Microprobe X-Ray Maps showing As sorption (also Pb and Zn) on the edge of the spherical manganese oxide-coated sand grain (image by Karen Hudson-Edwards from Birkbeck, University of London).
We have studied the surface of these manganese oxide coated sands using many spectroscopic techniques. Figure 2 shows an image of two trenches which were ‘dug’ into a Mn oxide coated grain using focused ion beam spectroscopy. Gallium ions are used to ‘dig’ the tiny trenches by blasting away the manganese oxide surface. The trenches are only 20µm deep which is one fifth the width of an average human hair. The cross section generated reveals the delicate features present within the layered manganese oxide coating. We are interested in how organic contaminants and natural organic matter is intercalated within these layers of manganese oxide.
Figure 2: Focused Ion Beam image of two trenches ‘dug’ into the surface of manganese oxide mineral (image by Leon Bowen, Durham University).
By adding these minerals to the soil we are able to improve the health of the soil by immobilising potentially ecotoxic elements and oxidatively breaking down organic contaminants. Since iron and manganese oxide minerals are available as a waste product from industry, our technology is cheap and therefore makes remediation of many of these brownfield sites a more realistic proposition.
The Philip Leverhulme Prize will allow me to concentrate on using more spectroscopic techniques in order to understand the role of Mn oxides in stabilising Soil Organic Matter. This future work may have important implications for land-based geoengineering techniques for climate change mitigation.
Dr Karen Johnson
Karen was awarded a Philip Leverhulme Prize in 2011; providing £70,000 over 24 months.