Does bonemeal have a role in removing metals from contaminated soil?

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

Bonemeal in a slide used for microEXAFS work at Diamond, copyright Schofield, Mosselmans & Hodson

It now looks likely that one solution will come with the help of a phosphate source, such as bonemeal, which has the ability to immobilize 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.

Debate is currently ongoing as to whether bonemeal does in fact provide the optimum phosphate source. Obviously the choice of remediation medium is a critical step to the development of this new technology, so we are working towards experimental results on bonemeal treated metal-contaminated soils that either confirm or refute the existence of metal phosphate interactions.

Early results from our work on the microfocus spectroscopy beamline have allowed us to precisely pinpoint the mixed-metal phosphate grains within the bonemeal-remediated metal contaminated soil, negating the requirement for tedious and expensive separation processes that often involve heavy organic liquids. Thus 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 allow us to model the stability of these phases and to confidently and accurately assess 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, answering a question that has been puzzling us for 8 years; can a low cost recycled material like bonemeal help us towards sustainable treatment of metal-contaminated soils?

There are thousands of sites across the UK with contaminated soil due to previous industrial use such as mining, engineering works or lead smelting operations. 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 give their owners nightmares when the extent of the contamination is brought to light. In the last 5 years, contaminated land has become a very real issue for homeowners in Ashford (Kent), Cambridge, Stratford (London), Shrewsbury, Hartlepool and Ross-shire, to name just a few sites.

At the moment, methods of treating metal-contaminated soils essentially isolate the soil from the ecosystem by capping or removal of the soil and dumping it elsewhere. These methods are expensive and unsustainable so we need alternative solutions.

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.

Dr Paul Schofield, Natural History Museum, London.