Agronomy of metal crops used in agromining

Authors: NKRUMAH, PHILIP - University Of Queensland; Chaney, Rufus; MOREL, JEAN-LOUIS - Université De Lorraine

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 7/10/2016
Publication Date: N/A
Citation: N/A

Interpretive Summary: Agromining or Phytomining is the growth of metal accumulator plant biomass as a metal crop. When plants can accumulate 1% of a metal in their harvest able shoots, the amount of metals removed per year could achieve soil decontamination. Further, for some elements, the ash of the plant shoots could be a high grade commercial metal ore. This manuscript summarizes the development of the concept of phyto/agromining and the extensive basic and field trials conducted to develop improved genetic materials and agricultural practices to make phyto/agromining a commercial technology. Research on Alyssum species showed that conventional plant breeding can significantly improve crop Ni concentration and shoot yield. The fertilizer requirements and soil analyses to demonstrate needed fertilizers were tested with ultramafic/serpentine soils. These soils are very infertile if not previously farmed, deficient and P, N, K, and Ca, and sometimes deficient in Mo and B, so good agronomy is needed for profitable phyto/agromining. In addition, recovery of metals from the plant shoots has been tested. Hyperaccumulator plant ash has been very successfully used as an ore in pyrometallurgy at industrial scale, and hydrometallurgy purification of the metals from plant biomass has been achieved at laboratory scale. Production of high quality nickel salts could provide higher economic return that production of Ni metals. Hypernickelophore plants accumulate Ni into their shoots with only other plant nutrients so metal recovery is easy, while attempting to recover Ni from the soils where these plants grow well (ultramafic soils) using traditional metal recovery is very difficult because the soil are high in Fe and Mn oxides and Mg-silicate which interfere with Ni recovery even using high pressure sulfuric acid leaching of the soils. The recent recession after 2007 has hindered commercialization because the value of Ni dropped from >$20/kg to about 9$/kg while fertilizer costs have risen. Great promise seems available for phyto/agromining of Ni from extensive tropical ultramafic soils where locally adapted hypernickelophores have been increasingly identified at ultramafic soil locations when they were examined. Species which can be coppiced (harvested annually with leaves) offer high Ni yields and profit.

Technical Abstract: This review of the agronomy of metal crops used in agromining/phytomining summarizes the history of the development of phytomining and the experimental work to identify the agronomic practices most important to high annual nickel yield when hypernickelophore (accumulate over 1% Ni in dry shoots). The idea was published in 1983, patents issued in 1998 and later, but the technology has not been commercialized more recently due to the recession in metals prices. Although hyperaccumulators show large natural variation for trace metal accumulation, collection of diverse germplasm followed by normal plant breeding techniques to improve the ‘metal crop’ is clearly a key step in developing agromining. Higher soil metal phytoavailability will always be a desired property of soils intended for commercial agromining. Subsoil metal increases shoot yield, shoot Ni concentration and Ni quantity in shoots of Ni ‘metal crops’. Tilling soil to maximize root penetration, adequate inorganic fertilization and appropriate plant densities are more important for developing efficient agromining approaches with Alyssum Ni hyperaccumulator species than organic soil amendments. However, the extreme conditions of some substrates may require the application of amendments to improve local soil conditions necessary for normal plants growth. Co-cropping (growing a legume or grass with the metal crop) may not be useful in Ni agromining; but may have pronounced positive effects on Cd ‘metal crops’. The management of propagation and harvest will necessarily be dependent upon the species being used for agromining. The feasibility of agromining for Ni is being proved from laboratory and field experiments, while the demonstration of its applicability to other strategic elements (e.g. Co, Mn, rare earths) is underway and should borrow the same general approach. Because most of the existing agromining field trials have occurred in temperate regions, mainly on A. murale and A. corsicum, ongoing pioneering study in Sabah, Malaysia using tropical species will be critical to determine whether the trends in temperate regions could be confirmed in a wet tropical environment.