Nickel Recovery from Hyperaccumulator Plants Using a Chelating Resin

Abstract

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 DOWEX^TM 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 .

Introduction

Worldwide, huge areas are covered by ultramafic soils containing high amounts of nickel (Ni). Large amounts of mine tailings and industrial wastes are also available. At a time when metal demand is increasing, these secondary resources begin to gain interest. Some plants, called hyperaccumulator plants (HA), are able to grow on the ultramafic soils, despite their low fertility, and to accumulate nickel in their tissues. More than 450 species of Ni HA have been identified, in temperate and tropical areas. These plants have been initially considered for soil remediation [1] but now it is widely agreed that Ni can be recovered from their biomass. The concept of agromining is currently developing. Agromining is a process, consisting of growing HAs on ultramafic soils to extract Ni and then recovering it in different chemical forms [2, 3].

The pioneering approach of Li [4] has proven that Ni metal could be produced by pyrometallurgy from the plant Alyssum murale (A. murale). Since then, hydrometallurgical processes have been studied at the laboratory scale and larger [5]. The general methodology starts by burning the dry plant, harvested at its flowering stage, in order to obtain ash. In this way, Ni is concentrated up to 15–20% [6], which is far higher than any Ni ore. At the same time, organic matter is destroyed while energy is produced [7]. Then, Ni is extracted from the ash by acid leaching and the leach liquor is processed to obtain different possible products, the prime example being the production of ANSH (ammonium and nickel sulphate hexahydrate) [5, 8].

Currently, a new strategy is being investigated consisting of extracting Ni directly from the biomass by leaching without burning the plant. It has been proven that water leaching was sufficient, which is of great importance, since no chemicals are needed [9]. But the leaching solution contains not only Ni and the major ions but also a variety of organic molecules. Therefore, separation processes have to be developed to isolate Ni in a desired form, following an environmentally friendly approach. It has been proven that selective precipitation of nickel hydroxide from the plant leachate is not possible [9, 10]. The present contribution is focused on the investigation of the use of a chelating resin .

Experiments are presented using the hyperaccumulator plant A. murale. In the first step, the dry plant is leached with water, and in the second step, column experiments are performed with the chelating resin DOWEX^TM M4195 , to evaluate the possibility of recovering Ni by adsorption /elution.

Materials and Methods

(a)
Hyperaccumulator plant

The hyperaccumulator Alyssum murale (Brassicaceae family) was grown next to Progradec (Albania), on ultramafic soils and harvested at the flowering stage. The aerial parts were sun-dried and 500 kg of dry plant material was ground to a particle size of 2 mm. A few kilograms of this ground material was used for the experimental work described here.

(b)
Chemicals and Ni-chelating resin

Malic acid (>99%), malonic acid (>98%) were obtained from Fluka, citric acid (>99.5%) and oxalic acid (>99,5%) from Labkem, Ni acetate (>99%) and NiCl₂ (>98%) from Acros organics, HNO₃ (>65%) and HCl (37%) from Sigma Aldrich and H₂O₂ (>50%) from VWR. Ultra-pure water (Elga–Purelab Chorus) was used for all the experiments.

DOWEX^TM M4195 chelating resin was purchased from Sigma Aldrich. The matrix is made of macroporous styrene-divinylbenzene and the functional group is bis-picolylamine; the particle size is 20/50 US Mesh. This resin is able to adsorb many transition metal ions from solutions with pH less than 2.

(i)
Experimental set-up
The experimental set up was composed of a syringe pump, a glass column (GE Healthcare Life Science), on line pH (electrode Mettler Toledo, 1–11 pH, 0–80 °C T, Standard pH Meter Materlab PH210) and conductivity detectors and a fraction collector (Gilson 206 Fraction collector). Three columns were used depending on the experiments: column #1 (biomass leaching ), XK 50/20, 5 cm × 20 cm, id × length; column #2 (resin capacity), XK 16/20, 1.6 cm × 20 cm, id × length; column #3 (Ni recovery ), C 10/10, 1 cm × 10 cm, id × length. All column experiments were carried out at ambient temperature (20 °C).
(ii)
Biomass leaching
Column #1 was filled with 60 g of ground plant material (bed height: 13 cm, bed volume (BV1): 255 mL) and fed in upflow mode with ultra-pure water. The flow rate was 450 mL h⁻¹ (1.76 BV h⁻¹).
(iii)
Measurement of resin capacity for Ni
Column #2 was filled with 3 g of dry resin (bed height: 6.4 cm, bed volume (BV2): 5 mL). The column was pre-equilibrated with HCl, by injecting a solution of HCl (1.6 M) until pH at the column exit was equal to pH at the column inlet. Afterwards, the column was saturated by passing 25 BV of a NiCl₂ solution (40 mM, pH 2), and eluted by injecting HCl (1.6 M). The column was fed upwards in each case. The flow rate was 30 mL h⁻¹ (0.1 BV2 min⁻¹), except for elution, for which it was lowered to 0.07 BV2 min⁻¹. The resin loading of Ni was obtained by mass balance . During saturation, 44 fractions of 2.95 mL (0.6 BV2) were collected and further analyzed by ICP to determine the metal concentrations and by ion chromatography to determine the Cl^- ion concentrations. During elution, 30 fractions of 3.5 mL (0.7 BV2) were collected and analyzed by ICP.
(iv)
Nickel recovery from the leachate
Column #3 was filled with 18.5 g of dry resin (bed height 16.4 cm, bed volume (BV3): 30 mL) and pre-equilibrated with 1.6 M HCl. At the end of this stage, the column was fed by the plant leachate. The total leachate collected from the biomass leaching was injected (400 mL); 41 fractions of 9.9 mL (0.33 BV3) were collected and analyzed by ICP to determine metals concentration, by TOC meter to determine dissolved organic carbon concentration and by HPLC to determine low molecular weight carboxylic acids concentration.
Then the column was rinsed by injecting ultrapure water (678 mL) and elution was performed by injecting the 1.6 M HCl solution. The eluate was recovered in 44 fractions of 10 mL (15 BV3 total). The flow rate was 180 mL h⁻¹ (0.1 BV3 min⁻¹), except for elution, for which it was lowered to 0.07 BV3 min⁻¹ (126 mL h⁻¹), and in each case the column was fed upwards.

(d)
Chemical analyses

Solid samples (0.1 g) were digested with 8.5 mL of HNO₃ and 1.5 mL of H₂O₂ in a microwave oven (Milestone Start D Microwave Digestion System). Liquid samples (2 mL) were digested with 1 mL of HNO₃. Digestates were diluted to 50 mL with ultra-pure water and filtered (Phenomenex, regenerated cellulose, 25 mm disks, pore size 0.45 µm). Samples were analyzed by plasma emission spectroscopy (ICP-AES) (Thermo ICAP 6000 Series ICP Spectrometer). Quality controls were performed with standard solutions prepared from a multi-element certified solution (1000 mg L⁻¹ SCP sciences).

To monitor dissolved organic carbon (DOC), liquid samples (40 mL) were analyzed by an organic carbon analyzer TOC-V_CSH (Shimadzu).

Low molecular weight carboxylic acids (LMWCAs) were analyzed by HPLC (Shimadzu, prominence modular , 20A series): Kinetex F5 column (100 × 4.6 mm id, packing diameter: 2.6 µm) in series with an Aminex HPX 87H column (300 × 7.8 mm id, packing diameter: 9 µm); diode array detector at 214 nm; mobile phase: 2 mM H₂SO₄ at 0.3 mL min⁻¹, 60 min isocratic mode, temperature 30 °C. Standardization was performed with a solution containing malic, malonic, citric and oxalic acids and nickel acetate salts, at concentrations ranging from 0.5 to 10 mM for each species.

Results and Discussion

(a)
Nickel leaching from the dry biomass

Guilpain et al. [9] have extensively presented the results concerning the leaching of A. murale with water, but the main features are reviewed here for completeness. In this previous work, leaching had been performed in static and dynamic conditions, using batch and column reactors. Using a column enabled us to reach a high solid:liquid (S:L) ratio (16%).

The concentrations of the main elements in the entirety of the parts of the plant that are above ground (stems, foliage and lowers) and of the leachate are given in Table 1 which also lists the extraction yields calculated using Eq. (1).

Table 1

Concentrations of major elements in A. murale (stems, foliage and flowers) and in leachate obtained by column leaching (solid:liquid = 0.16 (wt:wt)) with ultrapure water, and extraction yields

	K	Ni	Ca	Mg	Fe
Concentration in plant (g kg⁻¹)	6.7 ± 0.5	5.1 ± 0.7	4.1 ± 0.6	2.4 ± 0.3	0.9 ± 0.2
Concentration in leachate (g L⁻¹)	0.966	0.561	0.280	0.190	9 × 10⁻⁴
Extraction yield (%)	90	73	43	49	1

$Extr.yield_{i} \left( \% \right) = \frac{{w_{i,l} }}{{C_{i,p} w_{p} }}\,x\,100$

(1)

where w_i,l is the mass of element i in solution after leaching (g), C_i,p the concentration of i in the plant tissues (g of i (g_{dry plant})⁻¹) and w_p the mass of dry plant in the batch reactor (g).

The leachate also contained 12 g L⁻¹ organic carbon. The breakthrough curves of organic carbon and nickel had the same shape, and it was demonstrated that the concentrations of Ni and organic carbon were in proportion.

Moreover, knowing that Ni is bound to low molecular weight carboxylic acids (LMWCA’s) in the plant, mainly malic and citric acids [11], these acids were monitored in the leachate. Malic, malonic, citric, acetic and oxalic acids were found at respective concentrations of 2.54, 1.20, 0.694, 0.149 and 0.099 g L⁻¹, accounting for 14% of the total organic carbon. Nickel speciation was modelled using the JChess software [12], which showed that about 97% of the nickel was accounted for by Ni bound in LMWCA complexes and 3% in the Ni²⁺ form [9].

(b)
Nickel recovery from the leachate

Ni could not be recovered from the leachate by selective precipitation of nickel hydroxide [9, 10]. Among the possible techniques, the use of a chelating resin has been selected as a conventional technique for Ni separation [13, 14]

(i)
Capacity of the chelating resin DOWEX ^TM M4195
Preliminary column experiments were run with different solutions of Ni chloride or sulphate dissolved in hydrochloric or sulphuric acids to adsorb Ni, at pH lower than 2 as prescribed in the literature. The adsorption and elution curves of Ni corresponding to the conditions described in section “Materials and Methods” are displayed in Fig. 1. Mass balances performed on these curves showed the effective adsorption capacity of Ni was 0.4 eq L⁻¹ (0.8 M) under these conditions. Adsorption capacity depends on pH and the solution composition; it has to be measured in different experimental conditions in order to be able to predict its variations.
Fig. 1
a Ni breakthrough curve obtained with column #2 preequilibrated with 1.6 M HCl and fed with a solution of 40 mM NiCl₂ acidified to pH 2 with 1.6 M HCl b Ni elution curve obtained by feeding column #2 with 1.6 M HCl
(ii)
Ni adsorption from the plant leachate
The breakthrough curves obtained by feeding column #3 (pre-equilibrated with 1.6 M HCl) with the plant leachate are plotted in Fig. 2. The major cations other than Ni (Mg, K, and Ca) were not adsorbed and broke through almost from the start (at 1 BV). The concentration of organic carbon also increased at 1 BV but did not reach the inlet concentration, showing that organic compounds were partly retained by the resin. Ni concentration remained close to zero until 5 BV had passed and sulphur concentration was rising very gradually, from the start of the test, but remained below 20% of its feed concentration at the end of the test, showing the retention of both Ni and sulphur (in the sulphate form).
Fig. 2
Breakthrough curves of the major cations (K, Ca, Ni, Mg), S and organic C from column fed with plant leachate

(iii)

Elution curves

The elution curves measured by feeding column #3 with 1.6 M HCl are plotted in Fig. 3. They show the recovery of the Ni, organic carbon and sulphur fixed by the resin during the adsorption step.

../images/468727_1_En_162_Chapter/468727_1_En_162_Fig3_HTML.gif — Fig. 3
Elution curves of Ni, organic C and S eluted with 1.6 M HCl

Mass balances are given in Table 2, showing that the resin selectively retained Ni, while the other cations were very little adsorbed. Organic carbon and sulphur were retained to a lesser extent. Eluted fractions were calculated compared to the amount of injected elements, meaning all Ni fixed is recovered whereas fractions of S and C were not totally eluted.

Table 2

Mass balances on major cations, organic C and S after adsorption /elution cycle

	K	Ni	Ca	Mg	C_org	S
Fed into column #3 (mmol)	9.89	3.84	2.80	3.14	372	1.34
Adsorbed quantity (mmol)	0.96	3.66	0.32	0.20	130	1.12
Adsorbed fraction (%)	10	95	11	6	35	84
Eluted quantity (mmol)	0.11	3.63	0	0.02	12.14	0.97
Eluted fraction (%)	1	95	0	1	3	72

These results show that Ni recovery from the plant leachate using the DOWEX^TM M4195 selective resin is very promising. A number of adsorption /elution cycles have to be performed in order to assess how many times the resin can be recycled before there is a decrease in performance or fouling, for example by some loaded components that are not effectively eluted.

Conclusions

These results have shown that Ni can be recovered from hyperaccumulator plants using the following steps:

Ni is transferred into aqueous solution by leaching the dry plant with water at room temperature ;
The leachate is injected into a column filled with the chelating resin DOWEX^TM M4195 pre-equilibrated with 1.6 M HCl;
Ni is recovered by acid elution.

Further work is needed. From the a scientific point of view, the functioning of the resin has to be better understood with the aid of further experiments and modeling , in order to optimize the operating conditions. The number of adsorption /elution cycles is an important issue to evaluate in order to assess the feasibility of the global approach. Operating conditions also need to be optimized and parameters such as loading and elution flowrates determined so that the process can be engineered to recover nickel efficiently and as completely as possible from the bio-adsorbent plant mass.

Acknowledgements

This work was supported by the French National Research Agency through the ANR-14-CE-04-0005 Project “Agromine” and by the French ministry of higher education and research. The work is also a contribution to the French National Research Agency through the national “Investissements d’avenir” program (ANR-10-LABX-21-LABEX RESSOURCES21). The authors are grateful to Prof. A. Bani (UAT, Albania) for providing A. murale, biomass. They also thank the GISFI and the STEVAL platforms and technical staff for the pre-processing of the plant biomass.

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