Author
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Chaney, Rufus |
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MAHONEY, MICHELE - Us Environmental Protection Agency (EPA) |
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Submitted to: Meeting Proceedings
Publication Type: Proceedings Publication Acceptance Date: 6/13/2014 Publication Date: 7/15/2014 Citation: Chaney, R.L., Mahoney, M. 2014. Phytostabilization and phytomining: Principles and successes. Meeting Proceedings. Proc. Life of Mines Conference, July 15-17, 2014, Brisbane Australia Paper 104. Interpretive Summary: Mine and smelter industries often caused severe metal contamination of soils or left residues from mining which are barren and eroding in the environment. The engineering approach to solve these problems has been removal of the contaminated soils to a landfill, or application of deep layers of cover soil, which is very expensive and consumes topsoil from elsewhere. Research on risk assessment of soil metals, and for methods to remediate the soils at lower costs, have identified improved science to deal with such sites, especially the extensive sites where hundreds or thousands of acres are hazardous. Usually the hazardous condition results from strongly acidic pH which causes the metals to be highly phytoavailable and phytotoxic. Research showed that making the soils calcareous can greatly reduce metal phytoavailability and with use of organic soil amendments, can allow persistent revegetation of the sites with common revegetation species. High Pb in soils may be inactivated by adding phosphate as part of the soil amendments needed for remediation. Because most of such sites have eroded, it is not simple to restore the original ecosystem, and if highly contaminated in a naturally acidic region, it is not possible to remediate the site without making the site alkaline. Simply making the site calcareous to prevent harm from soil metals changes the ecosystem. Thus, “revitalization” of such sites is the new goal, developing a soil and plant cover which can persist without additional investment, and which is safe for wildlife. Added organic amendments such as manures and composts add not only organic matter, organic-N and phosphate, but microbes needed for a living soil. The overall method using byproducts and organic amendments to remediate sites in situ is called Phytostabilization. Research has identified pH management and organic amendment methods which allow effective revitalization of nearly all metal contaminated sites from mining and smelting. An alternative to phytostabilization is phytoextraction. Phytoextraction is the use of plants which can hyperaccumulate metals from soils into their shoots where they can be harvested and removed from the site. If the metal in the shoots has no economic value, the technology is called “Phytoextraction”. If the metal in the shoots has economic value, the technology is called “phytomining”. For Cd, Se and As contaminated sites, elements in the biomass are unlikely to pay for the costs of remediation. But phytoextraction can provide a much lower cost remediation than alternatives. Early claims about Pb phytoextraction were shown to be useless in the real world. Phytoextraction methods are needed for Cd-contaminated rice soils, etc., which cause human disease. One alternative to hyperaccumulators for Cd phytoextraction is use of willow and similar trees to produce biomass energy which also removes metals slowly from the site because the energy production pays for the phytoextraction. Phytomining is especially promising for nickel because Ni rich soils, mine wastes and smelter contaminated lands are extensive in many countries. A technology for Ni phytomining was developed in 2001, but not commercialized by the patent licensee. With the incipient expiration of the patents, Ni phytomining will be possible again in both temperate and tropical lands. This meeting is considering these alternatives to landfilling hazardous soils. The methods discussed can provide great savings and in the case of Ni phytomining, even make a profit while remediating contaminated soil. Technical Abstract: Mine and ore beneficiation wastes and smelter contaminated soils often cause phytotoxicity and threaten adverse environmental effects if not remediated. Science has clarified both the risks from soil metals and methods to alleviate those risks that can be applied at low cost. Phytoremediation is a family of plant and agronomy based technologies to deal with environmental aspects of sites requiring remediation. If valid risk assessment indicates that metals comprise risk, phytostabilization may be used to alleviate that risk for most contaminated sites. In particular, Zn-Pb, Cu and Ni mine and smelter sites are readily remediated by making the soil calcareous and fertile, with added organic and metal sorbent amendments which include soil microbes to “revitalize” the soil. In each case, the soil fertility and causes of phytotoxicity should be clarified by agricultural soil extractions, and the combination of amendments required to improve fertility and reverse phytotoxicity identified and found locally. In contrast with phytostabilization, Phytoextraction/Phytomining uses rare metal hyperaccumulator plants and agronomic management to maximize annual phytoextraction of soil elements into plant shoots for harvest, ashing, and use as a metal ore. Plants which accumulate over 2% Ni in shoots, and yield 10-20 t ha-1 dry biomass yield 200-400 kg Ni ha-1, which has value greater than all common agricultural crops. Considering the value of elements, existence of hyperaccumulator plants, and demonstrated agronomic management for Ni phytomining, it is clear that mine sites and Ni mineralized sub-ore-grade serpentine soils can be phytomined economically as an agricultural mining technology. A team developed agronomic methods and improved cultivars of Alyssum which can be profitably grown on temperate zone Ni mineralized or contaminated soils. Plant species for tropical ultramafic soil materials have been identified. Phytoextraction is also strongly needed to remove Cd from rice soils to prevent human disease from soil Cd. |
