Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 9/27/2005
Publication Date: 9/15/2006
Citation: Wang, A., Chaney, R.L., Angle, J.S., Mcintosh, M.S. 2006. Using hyperaccumulator plants to phytoextract soil Cd. In: Mackova, M., Dowling, D.N., Macek, T., editors. Phytoremediation and Rhizoremediation. Dordrecht, Netherlands: Springer Verlag. p. 103-114. Interpretive Summary: Soils are so contaminated by Cd and Zn that humans are at risk from Cd in plants grown on the soil. This is especially true for subsistence rice farm families, or tobacco growers because these crops accumulate Cd to high levels which are bioavailable to humans. Phytoextraction is a promising cost effective method to remediate the Cd risk of such soils. We have demonstrated management methods for growing Thlaspi caerulescens on contaminated soils, and selected improved genotypes which perform well. This paper reviews information about which soils need to be remediated, and how Thlaspi can be managed for high annual Cd removal. In a recent test, thlaspi grown for 6 months removed 37% of total Cd from a smelter-contaminated soil which contained 25 mg Cd/kg dry soil. The remarkable ability to accumulate Cd in the presence of normal levels of geogenic Zn is the key for this success. Even with Thlaspi tolerating 2.5% shoot Zn, with geogenic Zn-Cd contamination, the maximum Cd in most plants shoots would be 250 mg/kg, the ratio of the soil. The 10-fold greater Cd accumulation ability in strains from southern France give higher annual removals and shorter remediation periods. For the large areas of Cd contaminated rice paddy soils, this is the most cost-effective technology reported to date. In addition to showing effective Cd phytoextraction, we have investigated the potential for adverse effects on the environment if one lowers soil pH to increase annual Cd removal. If pH is lowered too greatly, soil microbes can be killed. But we found that maximum annual removal could be managed at pH levels between 5.5 and 6 without harming soil microbes. At lower pH yield of T. caerulescens is also reduced strongly especially root exploration of the strongly acidic soil. Thus over-acidification is counter-productive. Appropriate management obtains maximum yield and Cd hyperaccumulation without harming soil microbial populations.
Technical Abstract: This paper reviews development of methods to phytoextract Cd from contaminated soils which now cause risk to humans and wildlife. Thlaspi caerulescens from southern France hyperaccumulate 10-times higher Cd levels than any other plants reported for soils contaminated by Zn mine wastes or smelter emissions. Rice and tobacco grown on such soils are a risk to humans. We tested the effect of adjusting soil pH on phytoextraction of Cd from two soils and concluded that reducing pH is an effective method to enhance Cd phytoavailability and T. caerulescens uptake for both Cd and Zn. It also can greatly shorten the time span for phytoremediation to be complete and has the potential to overcome the problems associated with the long time requirement of phytoremediation. Using sulfur to reduce soil pH is advantageous over chelating-induced methods because it is cost-effective and with few toxic effects on plant growth. The proper and effective pH range for maximum metal uptake may differ for individual soils and therefore must be identified to ensure successful phytoremediation. For Cd phytoextration, the higher Cd:Zn accumulation in plants allows Cd removed at high rates without Zn phytotoxicity reducing yields (which limits annual Cd removed by Prayon types). We also investigated potential adverse effects of acidification to increase annual Cd phytoextraction by testing for effects on soil microbial populations and activities. When pH was lowered to 5.5 and below, microbial populations declined and fungi populations increased. By liming to pH 6.5 and incubation, we found that soil microbial populations can be restored as long as pH was not adjusted too low. The pH level at which persistent adverse effects of acidification occurred varied in the soils tested, perhaps due to the level of exchangeable aluminum in the soils. Well managed Thlaspi caerulescens phytoextraction is cost-effective and causes no persistent adverse effects on the soil ecosystem.