Baptiste Laubie

The effect of plant density in nickel-phytomining field experiments with Alyssum murale in Albania

Ultramafic vertisols cover large areas in Albania and offer opportunities for phytomining. We undertook a field experiment with native Alyssum murale on two representative Vertisols at a distance of 20 km from each other (Pojske and Domosdove, Albania), to test the effect of planting density (transplanted seedlings) on a phytomining cropping system. Both areas were cleared in late summer 2012 and then ploughed and the soils were characterised. At Domosdove, an area of 0.5 ha was planted with local native seedlings at a density of six plants per square metre in September 2012. Spontaneous plants that had germinated in Spring 2012 were left to grow without any competition from other plants on a second 0.1-ha plot at Domosdove. All plots were weeded manually in the autumn of 2012 and spring of 2013. Individual plants occupied ~1 m2 at maturity. At Pojske, an area of 0.3 ha was also planted in September 2012 with local native seedlings of A. murale at a density of four plants per square metre. Plants grown at initial densities of four and six plants per square metre did not fully cover the ground; gaps were filled in naturally by a second spontaneous generation of A. murale seedlings (recruits) that had germinated in Autumn 2012. Other weeds were eliminated with herbicides. At Domosdove, at densities of one and six plants and at Pojske of four plants per square metre, the biomass yield was 10, 5 and 10 t ha–1, respectively. Concentration of phytoextracted nickel was 77, 41 and 112 kg ha–1. We suggest that a density of four plants per square metre is suitable for phytoextraction with native populations of A. murale. A. murale can be a weed itself and lower the nickel phytoextraction yield. Plants responded differently in their native environment than in previous field trials in North America.

Processing of bio-ore to products

Hyperaccumulator plants may contain valuable metals at concentrations comparable to those of conventional metal ore and can be significantly upgraded by incineration. There is an incentive to recover these metals as products to partially counter-balance the cost of disposing the contaminated biomass from contaminated soils, mine tailings, and processing wastes. Metal recovery is included in the agromining chain, which has been developed over the past two decades for Ni and Au. More than 450 Ni-hyperaccumulator species are currently known and some grow quickly providing a high farming yield. Nickel recovery involves an extraction step, ashing and/or leaching of the dry biomass, followed by a refining step using pyro- or hydrometallurgy. The final products are ferronickel, Ni metal, Ni salts or Ni catalysts, all being widely used in various industrial sectors and in everyday life. Gold can be recovered from mine tailings using a number of plant species, typically aided by a timed addition of an Au-chelating extractant to the soil. Dry biomass is ashed and smelted. This approach enables the treatment of resources that could not be effectively processed using conventional methods. In addition to nickel and gold, the recovery of other metals or elements (e.g. Cd, Zn, Mn, REEs) has been investigated. Further effort is required to improve process efficiency and to discover new options tailored to the unique characteristics of hyperaccumulator plant biomass.

Element Case Studies: Rare Earth Elements

The growing demand of strategic resources, e.g. rare earth elements (REEs), for development of modern technologies has spurred an increase in mining activities and consequently a release of REEs into the environment, posing a potential threat to human health. Phytoremediation, regarded as an in situ and low-cost means to remediate polluted soils, uses the growth and harvest of hyperaccumulator plants that take up high concentrations of metals in their shoots, allowing metal removal from contaminated soil (phytoextraction) or commercial production of high-value metals (phytomining). In this chapter, we review the discovery of REE hyperaccumulators worldwide, particularly focusing on the fern species Dicranopteris dichotoma that preferentially takes up light REEs. Though less understood, mechanisms of REE uptake, translocation, and distribution in hyperaccumulator plants are also discussed. Finally, taking D. dichotoma as an example, we estimate the phytomining potential for REEs using this species, based on its biomass production, REE concentrations in the ash, and current market prices of REEs.

REE Recovery from the Fern D. Dichotoma by Acid Oxalic Precipitation After Direct Leaching with EDTA

Agromining is a phytotechnology aiming at producing commercial metal compounds from low-grade ores thanks to hyperaccumulating plants. The fern Dichranopteris dichotoma is a rare earth element (REE) hyperaccumulator, which naturally grows on former mine tailings in China. It accumulates up to 0.35% of REEs in its aerial parts. Different hydrometallurgical processes are currently developed to recover these elements directly from the biomass or from ashes after combustion. The process presented here consists of a direct extraction by EDTA solution, followed by precipitation with acid oxalic. Optimal precipitation pH and influence of organic matter are determined by modelling and by experimental studies. The final solid contains 4.3% of REEs, with calcium as the main cationic impurity (0.45% of the precipitate). The recovery yield is similar for major REEs and is around 70%. After optimization, upscaling of the process will allow the agromining development to recover REEs from secondary resources with more environmentally friendly techniques.

Nickel Recovery from Hyperaccumulator Plants Using a Chelating Resin

A wide variety of plants are able to extract nickel (Ni) from soils and accumulate this metal in their foliage. Hydrometallurgical processes have been developed to recover Ni in the form of metal or salts, starting from the plant A. murale. They include: (1) a combustion step, to obtain ash containing 15–20% Ni; (2) a leaching step; and (3) purification. That whole flowsheet has been thoroughly investigated, and the processes scaled up to the pilot scale. Another strategy is presented here, consisting of recovering Ni without burning the plant. Ni can be extracted from the dry plant by water leaching, but the leachate is a multi-element solution from which Ni has to be separated. Selective precipitation of Ni hydroxide is not possible since Ni is bound to organic ligands. In this work, the plant leachate is processed by adsorption on the chelating resin DOWEXTM M4195. The results showed that Ni was selectively retained whereas the other main cations were not. Nickel could be recovered at the elution step. This methodology needs to be further investigated but these initial results are very encouraging and open the possibility of Ni recovery by agromining.