Delimiting soil chemistry thresholds for nickel hyperaccumulator plants in Sabah (Malaysia)

Nickel hyperaccumulator plants have been the focus of considerable research because of their unique ecophysiological characteristics that can be exploited in phytomining technology. Comparatively little research has focussed on the soil chemistry of tropical nickel hyperaccumulator plants to date. This study aimed to elucidate whether the soil chemistry associated with nickel hyperaccumulator plants has distinctive characteristics that could be indicative of specific edaphic requirements. The soil chemistry associated with 18 different nickel hyperaccumulator plant species occurring in Sabah (Malaysia) was compared with local ultramafic soils where nickel hyperaccumulator plants were absent. The results showed that nickel hyperaccumulators in the study area were restricted to circum-neutral soils with relatively high phytoavailable calcium, magnesium and nickel concentrations. There appeared to be a ‘threshold response’ for the presence of nickel hyperaccumulator plants at >20 μg g−1 carboxylic-extractable nickel or >630 μg g−1 total nickel, and >pH 6.3 thereby delimiting their edaphic range. Two (not mutually exclusive) hypotheses were proposed to explain nickel hyperaccumulation on these soils: (1) hyperaccumulators excrete large amounts of root exudates thereby increasing nickel phytoavailability through intense rhizosphere mineral weathering; and (2) hyperaccumulators have extremely high nickel uptake efficiency thereby severely depleting nickel and stimulating re-supply of Ni from diffusion from labile Ni pools. It was concluded that since there was an association with soils with highly labile nickel pools, the available evidence primarily supports hypothesis (2

The long road to developing agromining/phytomining

The concept of phytomining is a natural extension of botanical prospecting and the study of metal biochemistry and biogeography of metal hyperaccumulator plants. Some elements may be phyto-extracted to remediate soils, but the recovered biomass would have little economic value (Cd, As, etc.) and disposal of the biomass would be a cost. A few elements may have sufficient economic value in phytomining biomass to support commercial practice (Ni, Co, Au). The development of phytomining requires (1) selection of high-biomass hyperaccumulator plant species; (2) evaluation of genetic diversity and breeding of improved strains with higher yields of the phytoextracted element; (3) development of agronomic practices to maximize economic return; and (4) development of methods to recover the phytomined element from the plant biomass. Plant species and methods for phytomining of soil Ni have been demonstrated for several species and locations (temperate and tropical climates). Production of Ni metal in an electric arc furnace smelter, and of Ni(NH4) 2SO4 using a hydrometallurgical method, have been demonstrated. Full commercial phytomining of Ni is beginning in Albania using Alyssum murale, and major trials in Malaysia are underway using Phyllanthus securinegioides. Variable prices of commodity metals add confusion to the development of commercial phytomining