RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS
Title: COST-EFFECTIVE PHYTOEXTRACTION OF CD FROM RICE SOILS USING THLASPI CAERULESCENS
| Angle, J - DEPT NAT RESOURCES, UMD |
| Wang, A - DEPT NAT RESOURCES, UMD |
| Mcintosh, M - DEPT NAT RESOURCES, UMD |
| Reeves, R - MASSEY UNIVERSITY, NZ |
Submitted to: Ecosystem Health
Publication Type: Abstract Only
Publication Acceptance Date: June 28, 2005
Publication Date: September 10, 2005
Citation: Chaney, R.L., Angle, J.S., Wang, A., Mcintosh, M.S., Reeves, R.D. 2005. Cost-effective phytoextraction of cd from rice soils using thlaspi caerulescens. [abstract]. Abstract book of the International Symposium of Phytoremediation and Ecosystem Health, Hangzhou, China. September 10-13, 2005. pp. 33-34.
Many areas where rice is produced in Asia have become Cd+Zn contaminated by mine wastes suspended in irrigation water. Research has shown that subsistence rice farmers are more susceptible to soil Cd contamination than other exposed groups, partly because of the low bioavailability of Fe and Zn in rice diets. And partly because Zn is 100 times higher than Cd in the soil but is not increased in rice grain while Cd can be increased 100-fold depending on pH and redox during grain filling. Growing garden foods on such soils has not caused human Cd disease apparently because of the high fraction of home-grown rice in diets of rice farmers, and the high bioavailability of Cd in such diets. Thus, practical and cost-effective methods to alleviate Cd risk from contaminated rice soils are needed. Soil removal or soil covering has been effective in remediation of mine waste contaminated areas in Japan, but the cost was about US$2.5 million/ha. Soil cover (60-100 cm) can reduce Cd accumulation in rice grain because roots are near the surface, but other crops may accumulate Cd from covered contaminated soils. Looking at all alternatives to remediate the human Cd risk of mine waste contaminated rice soils, phytoextraction appears to be the most cost-effective.
We have previously described progress in our development of a Cd phytoextraction technology. Although ‘Prayon’ Thlaspi caerulescens removed Cd slowly, southern France strains of this species can remove 10-20-fold higher amounts of Cd per year. And although the yield is low compared to many crop plants, the bioconcentration of Cd from soils is remarkable. Adjusting soil pH from near neutral to 5.5-6.0 gives maximum annual Cd removal. With two test soils containing 5 and 25 mg Cd kg-1 (and 450 and 1500 mg Zn kg-1), growing southern France types of T. caerulescens for six months in pots removed 45 and 37% of the total soil Cd. Shoot Cd at optimum pH was about 350 mg kg-1 for the lower Cd soil and 1500 mg Cd kg-1 for the higher Cd soil. We believe that 3-5 annual crops of Thlaspi harvested at mid- flowering stage could remove Cd effectively and allow production of rice with acceptable Cd levels even with less than optimum rice soil management. Results from recent research to further develop and demonstrate the use of southern France type Thlaspi caerulescens in commercial Cd phytoextraction will be described.
Besides mine waste contaminated rice soils, soils with contamination or mineralization by sources with high Cd:Zn ratio promote plant uptake of Cd by several mechanisms. Healthy plants with high Cd levels (10 mg Cd kg-1 dry weight of lettuce, spinach, etc.) have been grown on natural soils enriched by marine shale parent materials, or on mine soils from phosphate mining (phosphate ores have about 0.10-0.16 g Cd per 1 g Zn, far higher than Zn-mine waste contaminated soils (0.01 g Cd per 1 g Zn). Other crops do not appear to be capable of carrying enough Cd into diets, or to strongly affect dietary Cd bioavailability such that they can cause risk to humans even from home-grown garden foods.
Because Cd metal has little value, the combination of biomass energy and recovered Zn and Cd in Thlaspi biomass will not pay for the remediation service. But compared to the cost of soil removal and replacement, or 60 cm soil cover with uncontaminated soil, Thlaspi Cd phytoextraction is very low cost. Breeding improved cultivars of Thlaspi caerulescens is expected to increase annual Cd removal appreciably. The biomass can be disposed in solid waste landfills if the value of energy and recovered metals are not sufficient to justify transport and processing. Managing fields to produce maximum annual Cd removal would be improved by paying farmers for the plant Cd they deliver in biomass.
Rice soils which produce rice unsafe for lifetime consumption by farm families is produced on contaminated soils in Japan, China, Korea, Thailand and some other regions not yet evaluated. Such soils have also been used to produce high Cd tobacco that can cause adverse Cd effects to those who smoke home-grown tobacco products. Government agencies that regulate crop and soil safety need to procure Cd phytoextraction services for this technology to be more fully developed and additional management practices needed for even more cost-effective Cd phytoextraction can be identified.