Making a meal of metal

Simple application of bonemeal may prove to be an effective, low-cost and sustainable way to remediate soils contaminated by metal pollutants, the BA hears.

Geoscientist Online 10 September 2008

Thousands of sites across the UK suffer from contaminated soil created by previous industrial use - such as mining, engineering works or lead smelting operations writes Ted Nield. Development of this land has become a necessity as the need for new housing outstrips the supply of greenfield sites; but if the polluted land is not dealt with properly through remediation of the soil, dream homes can become an owner’s nightmare. In recent years contaminated land has become a very real issue for homeowners in Ashford (Kent), Cambridge, Stratford (London), Shrewsbury, Hartlepool and Ross-shire.

Metals present in soils can represent a potential hazard to humans and the environment if they are readily released into solution, or are otherwise rendered available to biological processes. At the moment, methods of treating metal-contaminated soils isolate the soil from the ecosystem by capping or removing the soil and dumping it elsewhere. These methods are expensive and unsustainable, so we need alternative solutions - new ways of reducing contamination in situ, without removing the soil.

It now looks likely that one solution will come with the help of a phosphate source, such as bonemeal, which has the ability to immobilise metals. When released into the soil, phosphate combines with the metals to precipitate as a stable metal phosphate. Due to the low solubility of metal phosphates, this process is effectively irreversible and locks up the contaminated metals.

However it is a subject for debate whether bonemeal does in fact provide the optimum phosphate source. The choice of remediation medium is a critical step to the development of this new technology, so scientists are working towards experimental results on bonemeal treated metal-contaminated soils to either confirm or refute the existence of metal-phosphate interactions.

Early results from work reported to the BA by Dr Paul Schofield (Researcher in Mineral Sciences at the Natural History Museum, London) suggest that bonemeal could indeed be the key. Using the microfocus spectroscopy beamline at the Diamond synchrotron in Oxfordshire (picture), the scientists have been able to pinpoint precisely the mixed-metal phosphate grains within bonemeal-remediated contaminated soil.

Schofield says: “This removes the need for tedious and expensive separation processes that often involve heavy organic liquids. This far we have identified a number of grains that are potentially mixed metal phosphates and spectroscopic data has allowed a tentative confirmation that these phases are indeed mixed-metal phosphates.”

The ultimate goal is to achieve unambiguous identification of mixed-metal phosphates formed in situ within the soil. This will thus allow us to model the stability of these phases and thus to assess confidently and accurately the potential of this remediation technology at different metal contaminated sites.

“To date, our work in this area has been confined to bulk experiments that have produced results that were not definitive for a number of reasons. The level of detail that we are achieving with our research at Diamond is bringing us much closer to revealing the exact nature of the bonemeal/metal contaminated soil interactions, thus answering definitely a question that has been puzzling us for some years – namely, can a low cost recycled material like bonemeal help us towards sustainable treatment of metal-contaminated soils?”

“Continuing our microfocus spectroscopy studies at Diamond will allow us to obtain high resolution spectroscopic and diffraction data that will reveal the atomic structure of these mixed-metal phosphates. When we are at a stage whereby we can unambiguously identify the mixed-metal phosphates we can explore the potential for this cheap remediation technology to be applied to a range of different soil environments, in which the soils have differing metal sources, acidity, organic content and environmental conditions.

“Beyond this we will explore the potential for bonemeal to be used as a barrier material preventing toxic metal bearing waters from interacting with uncontaminated soils and ecologies, and also as direct additions to mine wastes, heaps and tips locking up metals before they become mobilized into the environment.

Making a meal of metal

Simple application of bonemeal may prove to be an effective, low-cost and sustainable way to remediate soils contaminated by metal pollutants, the BA hears.

Geoscientist Online 10 September 2008

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