Accumulation of metals,such as cadmium(Cd),lead and zinc,in soils arising from natural processes and human activities is a well-recognized threat to human health and environmental quality.In agricultural soils,metal transfer from soil to plant and thus into the food chain is of particular concern.When community relocation,soil removal and/or switching to non-food crop production are not viable options for addressing soil contamination problems in a region,the only satisfactory approach is to identify measures that can remediate the contaminated soil in-situ and/or render the contaminant metals unavailable for plant uptake.We are taking this approach in relation to Cd contaminated rice paddy soils in NW Thailand.The aim is to utilize biochar,the charcoal-like material generated by low temperature pyrolysis of organic substances,as a soil amendment.We are conducting ongoing experiments with biochar generated from rice husks and wheat straw.These involve characterizing the sorption-desorption capacity of the materials in relation to Cd and determining,using isotopic exchange techniques,the degree to which soil amendment with biochar can reduce the availability of the metal.Batch equilibrium studies with finely ground biochar and Cd solutions ranging up to 530 mg/L(10:1 liquid:solid ratio,24h equilibration time,centrifuged and supernatant filtered at 0.45 ?m followed by ICP-OES analysis)indicated that both rice and wheat biochar had Cd sorption capacities in excess of 5000 mg/kg.Freundlich isotherms were generated and were found to be good models for the sorption data.Desorption of residues with de-ionised water(10:1 liquid:solid ratio,24h equilibration time)revealed that greater than 99.5%of the sorbed Cd was retained by both biochar types.Isotope exchange studies(E value method)using the stable Cd isotope 111-Cd were conducted on contaminated paddy soils from Mae Sot,Thailand(total Cd~60 mg/kg),that had been amended with 0,1,5,10,15 and 20%rice biochar by mass.Solids(1 g)were equilibrated with 20 mL 0.01M MgCl2 then spiked with 0.7 ?g 111-Cd and allowed to equilibrate for 1 week before being centrifuged,solutions filtered and analysed by ICP-MS.The isotopic ratios of 111-Cd and 114-Cd in solutions of spiked and non-spiked samples were used to determine exchangeable Cd concentration(i.e.the E value)in treated and untreated soils.Available Cd in untreated soil was 41 mg/kg but was reduced to 27 mg/kg(corrected for soil mass)at the highest amendment rate.