X-ray absorption spectroscopy and monitoring of soil enzyme activity have been used to monitor the effects on lead speciation of plant growth and added chicken guano. The approach could allow gun shooting clubs to clean up land contaminated with ammunition lead.

Yohey Hashimoto of the Department of Bioresource Science at Mie University in Tsu, Hiroki Matsufuru of Gifu Prefectural Institute for Bioengineering in Gifu, Masaki Takaoka of the Department of Urban and Environmental Engineering at Kyoto University, Kyoto, and Takeshi Sato of the Japan Synchrotron Radiation Research Institute in Hyogo, published details of their work in a recent issue of the Journal of Environmental Quality.

Chemical immobilization is one of the most immediate methods for the remediation of soils contaminated with metals. It is inexpensive and quickly reduces environmental problems. Hashimoto and colleagues have now assessed the immobilization effects of poultry waste and growth of the guinea grass species Panicum maximum Jacq on soils and specifically lead speciation in this soil.

The researchers point out that much research has been carried out elsewhere to find specific additives that might amend the characteristics of soil and so immobilise toxic metal species. "The primary role of immobilizing amendments is to alter the original soil metals to more geochemically stable phases via sorption, precipitation, and complexation processes," the team explains. Researchers have suggested that cost-effective amendments would include industrial by-products rather than expensive manufactured chemicals.

To test the viability of the remediation process, the team obtained soil contaminated with lead from the top 200 mm of their local shooting range and compared values with uncontaminated soil from 200 to 750 mm subsurface. They found that standard analysis showed that remediation reduced the toxicity of the surface soil samples by 90% versus the subsurface control soil. Chicken waste alone was effective in reducing lead mobility but was improved still further when guinea grass was grown in the samples too.

The team used extended X-ray absorption fine structure (EXAFS) analysis to look in detail at the lead species present before and after remediation. "The EXAFS spectroscopy experimentation was conducted using Beamline BL01B1 at the SPring-8, in Hyogo, Japan, with support from the Japan Synchrotron Radiation Research Institute," the team writes.

They found that remediation reduced the proportion of lead carbonate and organolead species in the samples through conversion to what they describe as "geochemically stable" species of Pb₅(PO₄)₃Cl at 30 to 35% of the total lead species.

Tests of soil enzyme activity were also carried out and the team found that plant growth and addition of chicken waste increased the activities of dehydrogenase and phosphatase in the surface soil by 2.7- and 1.1 times, respectively, for each enzyme when compared to activity in untreated control samples.

They point out that, "In shooting range soils, carbonate-bearing lead minerals, including cerussite and hydrocerussite are dominant through the weathering processes of lead bullets. Therefore, the reduction of carbonate-associated lead species and their transformation into more geochemically stable phases can be primarily targeted for lead immobilization of shooting range soils."

"The use of [animal waste] amendments in combination with plant growth may have potential as an integrated remediation strategy that enables lead immobilization and soil biological restoration in shooting range soils," the team concludes.

The team points out that the exact mechanism by which plant growth and soil amendment with chicken guano actually immobilises lead species. They also provide details of the optimum rate at which the alkaline waste product should be applied to avoid increasing the lead ion transport potential, which would be like shooting oneself in the foot as it would release more lead from ammunition as a mobile form into the soil.

Related links:

• J Environ Quality, 2009, 38, 1420-1428
• Bioresources, Mie Uni

Article by David Bradley