RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS
Title: Improved Understanding of Hyperaccumulation Yields Commercial Phytoextraction and Phytomining Technologies
| Angle, J Scott - UNIV OF GEORGIA |
| Broadhurst, C Leigh - UNIV OF MARYLAND |
| Peters, Carinne - UNIV OF MARYLAND |
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 28, 2007
Publication Date: August 31, 2007
Citation: Chaney, R.L., Angle, J., Broadhurst, C., Peters, C.A. 2007. Improved Understanding of Hyperaccumulation Yields Commercial Phytoextraction and Phytomining Technologies. Journal of Environmental Quality. 36:1429-1443.
Interpretive Summary: Phytoextraction is a promising soil remediation technology which uses plants to remove contaminants from soils. Many different directions of research have been followed in the last 15 years during which research has been undertaken. The present review considers methods which have been found to achieve significant phytoextraction of soil metals and discusses reasons for failure of other attempts. In particular, natural element hyperaccumulator plants can absorb and accumulate in their shoots on the order of 100-times higher concentrations of elements (e.g., Zn, Cd, Ni, Co, Mn, Tl, Se, etc.) than normal crop species. Although some have low annual yields, the high accumulation allows high annual removal of elements from the contaminated soils. Another significant point is the recovery of metals from the biomass for sale, or use as a feed supplement, as a fertilizer, or for biomass energy production can offset the costs of soil remediation. In the case of Ni, growing hyperaccumulator plants on mineralized or contaminated soils allows production of biomass, and ash of the biomass, which is a high grade ore for Ni which can enter into normal Ni metal production technologies. Alternatively, the same hyperaccumulator plant biomass can be used as a Ni fertilizer for deficient crops. Mechanisms that plants use to achieve hyperaccumulation are summarized in the manuscript. Many of the failures in phytoextraction research result from study of plants with no useful metal accumulation ability, or the requirement of addition of chelating agents to dissolve soil metals and assist transport in plants. Application of chelating agents causes leaching of metals to groundwater and is unacceptable; further the cost is extreme ($30,000/ha-year). Phytoextraction has been demonstrated to be an effective and cost effective technology for soil remediation if hyperaccumulator plants are used.
This paper reviews progress in phytoextraction of soil elements and illustrates the key role of hyperaccumulator plant species in useful technologies. Much research has focused on elements which are not practically phytoextracted (Pb); on addition of chelating agents which cause unacceptable contaminant leaching and is cost prohibitive; and on plant species which offer no useful phytoextraction capability (e.g., Brassica juncea Czern). Ni phytoextraction by Alyssum hyperaccumulator species, which have been developed into commercial phytomining technologies, are discussed in more detail. Ni is ultimately accumulated in vacuoles of leaf epidermal cells which provides defense against some insect predators and plant diseases. Constitutive up-regulation of trans-membrane element transporters appears to be the key process which allows these plants to achieve hyperaccumulation. Cd phytoextraction is needed for rice soils contaminated by mine wastes and smelter emissions with 100-fold more Zn than Cd. Although many plant species can accumulate high levels of Cd in the absence of Zn, when Zn is 100-fold the Cd present, only Thlaspi caerulescens from southern France can phytoextract useful amounts of Cd. Production of biomass with value as ore or fertilizer or improved food (Se) or feed supplement may offset costs of phytoextraction crop production. Transgenic phytoextraction plants has been achieved for Hg, but not for other elements. Although several researchers have been attempting to clone all genes required for effective hyperaccumulation of some elements, success appears years away; such demonstrations will be needed to prove we have identified all necessary processes in hyperaccumulation.