What are hyperaccumulating plants?
Definition of hyperaccumulator
The term “hyperaccumulator” was firstly defined by Jaffré and co-workers who described the spectacular accumulation of Ni in the plant Sebertia accuminata (Jaffre et al., 1976). The threshold concentrations for hyperaccumulators are in Table 1.1, such as 1,000 mg metals kg-1 (0.1 %) dry mass for As, Co, Ni, Cu, Pb and U, except for Mg and Zn, which it is 10,000 mg kg-1, Ag and Au is 1 mg kg-1, and Cd, Se and TI of 100 mg kg-1 (Baker et al., 2000; Brooks, 1998; Sheoran et al., 2009).
Types of hyperaccumulators
Up to now, there are over 500 plant species (approximately 0.2 % of all known species) that were reported with the abilities to hyperaccumulate metals (As, Cd, Co, Cu, Mn, Ni, Pb, Sb, Se, Ti, Zn) (Kramer, 2010; Sarma, 2011). Most of these plants (about 70 %) are Ni-hyperaccumulating plants, particularly from the genus of Alyssum. Among Ni-hyperaccumulators, hypernickelophores have the ability to store more than 10,000 µg Ni g-1 of dry matter in their tissues without heavy metal toxicity symptoms (Baker and Brooks, 1989; Brooks, 1977, Van der Ent et al., 2013).
Mechanism of metal hyperaccumulation
Metal hyperaccumulation by plants is achieved through the coordination of several processes: bio-activation in the rhizosphere, root absorption and compartmentation, xylem transport (metal uptake by shoot) and distribution and sequestration (Clemens et al., 2002; Krämer, 2010; Verbruggen et al., 2009; Zhao and McGrath, 2009).
The major processes involved in mechanism of heavy metal hyperaccumulation by plants are displayed in Fig.1.2. Most heavy metals are largely immobile and their bioavailability to plant root is restricted. Internal and external factors affected the bioavailability of heavy metals for plant uptaking and biomass increasing were investigated, such as soil pH, fertilizers, chelator-buffered nutrient solution (Bani et al., 2007; Centofanti et al., 2011; Chaney et al., 2007; Li et al., 2003a; Qui et al., 2008; Smolinska and Cedzynska, 2007). Some scientists tried to develop transgenic hyperaccumulators to obtain phytoextraction (Chaney et al., 2007; Cherian and Oliveira, 2005; Clemens et al., 2002).
Methods of improving the ability of metal accumulation
Although hyperaccumulation ability seems to act whatever the edaphic conditions are, many factors can improve metal phytoextraction. Therefore, there has been a growing interest in investigating the associated factors improving the accumulation abilities of the hyperaccumulating plants (Bhargava et al., 2012; Chaney et al., 2008; Chaney et al., 2000; Hsiao et al., 2007; Li et al., 2003a; Reeves and Adiguzel, 2008; Sheoran et al., 2011). The major factors affecting heavy metal accumulation ability are as follows:
Soil pH affects not only the types of soils (acid or alkaline), and also the bioavailable metal cations quantity in the soils. The group of Chaney R.L. (USDA) investigated the pH effect on metal accumulation in two Alyssum species. pH treated by 0.01 M Sr(NO3)2 in the Quarry and Welland (Typic Epiaquoll; Canadian classification, Terric Mesisol) lead to a decline of extractable Ni with the increase in pH (Sr(NO3)2-extractable of Ni is 4.85 % at pH of 5.6 but 2.07 % at pH of 7.3 (Kukier et al., 2004; Li et al., 2003b). And pH of about 6 was the best pH to have the maximum Ni phytoextraction from A. murale and A.corsicum grown on serpentine soil (Qui et al., 2008).
Fertilizers are often used to increase the crop production, such as phosphorus (P) was used to enhance the vegetable-rice crop and potato production (Alam and Ladha, 2004; Rosen et al., 2014). Based on the same idea, inorganic and organic nitrogen, phosphorus (such as N, NP and NPK etc.) were studied to increase the metal up taking capacity of the hyperaccumulators (Bani, 2009; Bani et al., 2007; Chaney et al., 2007; Chaney et al., 2005; Li et al., 2012). For instance, NH4+-N was the best fertilizer for P. vittata as it can phytoextract maximum of As from the soil (Liao et al., 2007).
Using chelating agents to enhance metal uptake by hyperaccumulator plants has been studied (Chaney et al., 1998; Chaney et al., 2007; Karami and Shamsuddin, 2010; Leštan et al., 2008). For example, EDTA (ethylenediamine tetraacetic acid) was known as the best chelate to improve certain metals (Cr, Cd, Ni, Hg etc.) uptake by hyperaccumulators, especially for Pb (Leštan et al., 2008; Smolinska and Cedzynska, 2007). EDTA and DTPA (diethylenetriamine pentaacetate) have shown their effectiveness to increase Cr and Ni levels in the soil solution (Bani et al., 2014; Bani et al., 2009; Hsiao et al., 2007). However, chelating agents may cause unacceptable contaminant leaching. As a consequence, we should have to balance the positive and negative sides of using chelating agents (Centofanti et al., 2011; Chaney et al., 2007; Smolinska and Cedzynska, 2007).
Tailoring rhizosphere microorganisms communities to improve metal phytoextraction Rhizobacteria may affect hyperaccumulator plants growth and metal phytoextraction, then work on tailoring rhizosphere microorganisms communities to improve metal phytoextraction was carried out (Cabello-Conejo et al., 2014; Lucisine et al., 2014). The conclusion was that the underground rhizosphere microbial communities could improve the values of biomass and Ni phytoextraction, such as a strain of Arthrobacter nicotinovorans SA40.
Phytoextraction and the other phytoremediation techniques
Phytoextraction is one of the techniques of phytoremediation. Phytoremediation consists in using plants to absorb, accumulate, degrade, volatilize or immobilize soil pollutants (Macek et al., 2007; Salt et al., 1998; Sarma, 2011; Schroeder et al., 2010; Wendy Ann Peer et al., 2005). As illustrated in Fig.1.3, the different types of phytoremediation are phytostabilization, phytodegradation, phytovolatilization (evapotranspiration), and phytoextraction (Macek et al., 2007; Marques et al., 2009; Nwoko, 2010; Sarma, 2011; Schroeder et al., 2010).
rhizosphere (Cheraghi et al., 2011; Salt et al., 1995). For instance, plants can prevent water erosion, reduce wind erosion, and stabilize and restrain mine tailings (Mendez and Maier, 2008).
Phytodegradation consists in using metabolic activities within plant tissues or enzymes released from roots to break down the contaminants (Jiang et al., 2010; Macek et al., 2007; Marmiroli et al., 2006). For example, plants of Brassicacae, Brassica napus (rape) and Brassica oleraccea (cabbage), and a grass, Festuca rubra (red fescue) were cultivated on a sediment to study (Caille et al., 2005). And also many plants are able to break down organic compounds such as polycyclic aromatic hydrocarbons (PAH) using a biodegradation process (Biache et al., 2013; Cébron et al., 2013; Joner et al., 2004).
Phytovolatilization aims at using plants to take up contaminants from soil or water to the leaves, and evaporate, or volatilize to release them directly or modify the form of pollutants into the atmosphere (Macek et al., 2007; Salt et al., 1998). For example, poplar trees can volatilize 90 % of trichloroethylene (TCE) by converting it to chlorinated acetates and CO2. Phytovolatilization can be used to remediate selenium polluted soils and waters by plants (Orchard et al., 2000; Zayed et al., 2000).
Phytoextraction consists in using plants to remove dangerous contaminants, metals or metalloids, from soils or water; pollutants are transferred to the aerial plant tissue, and the pollution level can be lowered by harvesting the biomass (Chaney et al., 2000; Garbisu and Alkorta, 2001; Robinson et al., 2003; Tang et al., 2012).
Phytoextraction is a continuous process based on the capacity of hyperaccumulator plants to gradually accumulate metals into their biomass. Metals present in soils, or more precisely, the available fraction of these metals are taken up by plant roots to be translocated to the aerial parts. It depends on several key factors: the extent of soil contamination, metal availability for uptake by roots (bioavailability), ability of selected plants to grow and accumulate metals under the specific climatic and soil conditions of the site being remediated, etc. (Angle et al., 2001; Bani, 2009; Chardot et al., 2005; Do Nascimento and Xing, 2006; Li et al., 2003b; Mahmood, 2010).
Serpentine soils containing nickel
Ultramafic soils are extensively present in many regions worldwide (Cuba, New Caledonia, Australia, Turkey, Brazil, China etc.). In Europe, they are mainly found in the Balkans (Fig.1.5: Albania, Greece, Bosnia and Serbia etc.) (Bani et al., 2010). Serpentine soil is a specific kind of ultramafic soils – deriving from serpentinite rocks – which are usually shallow, coarse, and have special physical and chemical properties (Rajakaruna and Bohm, 2002), e.g. low nutrient status, cation imbalances, moisture stress, soil instability, high surface temperature effects, and high metal concentrations. Iron (Fe), magnesium (Mg), and silicon (Si) as well as nickel (Ni), chromium (Cr) and cobalt (Co) often occur in large amounts but such for nitrogen (N), phosphorus (P), potassium (K) and boron (B) are usually deficient (Shallari et al., 1998). In serpentine soils, Ni and Cr concentrations are usually a few g kg-1 and can reach more than 10 g kg-1 (Alves et al., 2011; Brooks, 1987; Cheng et al., 2011; Proctor, 1999). In Albania, values of more than 3.0 g kg-1 of Ni are often recorded on the major ultramafic soil types (Bani, 2009).
Table of contents :
Chapter 1: Bibliography study
2 What are hyperaccumulating plants?
2.1 Definition of hyperaccumulator
2.2 Types of hyperaccumulators
2.3 Mechanism of metal hyperaccumulation
2.4 Methods of improving the ability of metal accumulation
3.1 Definition of phytoextraction
4.1 Nickel in soils: what are the potential resources for phytomining?
4.1.1 Serpentine soils containing nickel
4.1.2 Main sources of nickel pollution in soils
4.1.3 Ni speciation in soils
4.1.4 Health risks due to nickel contaminated soils
4.1.5 Nickel contaminated soil remediation
4.2 Agronomy of phytomining
4.3 Recovery of Ni from hyperaccumulator plants
4.3.1 Producing pure Ni from hyperaccumulators
4.3.2 Producing Lewis acid catalysts from hyperaccumulators
4.3.3 Producing nickel double salt from hyperaccumulators
4.3.4 Hydrothermal process of extracting nickel from the Ni-hyperaccumulators
4.4 Feasibility of nickel phytomining
5 Hypothesis and objectives
Chapter 2: Characterizations of plants and ashes from Ni-hyperaccumulators
2 Materials and methods
2.1 Sampling and preparation of the plants
2.2 Sampling of the ashes
2.3 Mineralization and ICP-AES analysis of biomass and ashes
2.4 X-ray diffraction analysis
2.5 Particle size distribution, SEM and EDX analysis
3 Results and discussion
3.1 Mass distribution of biomass among stems, leaves, flowers and seeds of the different taxa
3.2 Elemental concentration of the hyperaccumulators
3.3 Optimization of the furnace treatment
3.3.1 Influence of temperature on ashes composition
3.3.2 Influence of combustion duration on ash composition
3.4 Concentrations in the ashes of the 14 hyperaccumulators
3.5 XRD of the ashes of the hyperaccumulators
3.6 Particle size distribution, SEM and EDX analysis
Chapter 3: Optimization of the process to produce ANSH (ammonium nickel sulfate hexahydrate) from ashes of A. murale
2 Part I-Ash washing stage
2.2 Materials and methods
2.2.1 Composition of the raw ashes
2.2.2 Influence of the washing duration
2.2.3 Influence of ash mass fraction
2.2.4 Influence of the stirring speed
2.2.5 Influence the washing methods
2.3 Results and discussion
2.3.1 Influence of the stirring duration
2.3.2 Influence of the ash mass fraction
2.3.3 Influence of the stirring speed
2.3.4 Influence of the washing methods
2.3.5 Dissolution equilibrium calculations in the washing step
3 Part II-Acid leaching
3.2 Materials and methods
3.2.1 Experiments to test acid leaching at room temperature
3.2.2 Acid leaching at high temperature
3.3 Results and discussion
3.3.1 Controlling the pH
3.3.2 Recovery rates of Mg and Ni from A12
3.3.3 Selective leaching
3.3.4 Optimal parameters for acid leaching
4 Part III – Leachate purification before crystallization
4.2 Materials and methods
4.2.2 Adding NaF to eliminate Mg
4.3 Results and discussion
4.3.2 NaF addition to remove Mg followed by evaporation
5 Part IV- Crystallization
5.2 Materials and methods
5.2.1 Different methods of crystallization
5.3 Results and discussion
Chapter 4: New ideas of producing nickel products from A. murale biomass
2 Part I-Producing nickel products from A. murale ash
2.2 Materials and methods
2.2.1 Preparing NiSO4 at pH between 4 and 5
2.2.2 Producing NiS, ANSH salt and other nickel products
2.3 Results and discussion
2.3.1 Production of ANSH with NiS as an intermediate product
3 Part II–Producing nickel products directly from A. murale plants
3.2. Materials and methods
3.2.1 Extracting nickel from A. murale plants in deionized water at 100 °C
3.2.2 Ni extraction by heating with ultrasonic wave or autoclave
3.2.3 Producing nickel sulfide, nickel sulfate
3.3. Results and discussion
3.3.1 Extraction of the major elements
3.3.2 Organic acids in the boiled water
3.3.3 Comparison of different heating methods
3.3.4 Trial to prepare NiS from the aqueous solution
3.3.5 Ni extraction from the boiled solution at pH 1
Conclusion & perspectives