Sorption of rare earth coordination compounds

Rare earth elements (REEs) are valuable and strategically important in many high-technology areas, such as laser technology, pharmacy and metallurgy. The main methods of REE recovery are precipitation, extraction and sorption, in particular ion exchange using various sorbents, which allow to perform selective recovery and removal of associated components, as well as to separate rare earth metals with similar chemical properties. 
The paper examines recovery of ytterbium in the form of coordination compounds with Trilon B on weakly basic anion exchange resin D-403 from nitrate solutions. In order to estimate thermodynamic sorption parameters of ytterbium anionic complexes, ion exchange process was carried out from model solutions under constant ionic strength specified by NaNO3, optimal liquid to solid ratio, pH level, temperatures 298 and 343 K by variable concentrations method. Description of thermodynamic equilibrium was made using mass action law formulated for ion exchange equation and mathematically converted to linear form. Values of equilibrium constants, Gibbs free energy, enthalpy and entropy of the sorption process have been calculated. Basing on calculated values of Gibbs energy, a sorption series of complex REE ions with Trilon B was obtained over anion exchange resin D-403 from nitrate solutions at temperature 298 K. Sorption characteristics of anion exchange resin have been estimated: total capacity, limiting sorption of complex ions, total dynamic capacity and breakthrough dynamic capacity.

Authors of paper [7] studied recovery of Nd, Gd and Ho on synthesized resins, the functional group of which is represented by primary amine surface groups with REM-binding ligands: phosphonoacetic acid (PAA), N, N-bis (phosphonomethyl) glycine (BPG) and diethylenetriaminepentaacetic dianhydride (DTPADA). In order to estimate thermodynamic characteristics, a series of tests was performed with solutions, containing 0.3-300 mg/l of REEs at constant HCl concentration of 0.5 mol/l and a fixed pH value. Solutions were mixed at 300 rpm for 24 hours with ion exchange resin (10 g/l) at three temperatures: 20, 60 and 100°С. Sorption isotherms were obtained in constant presence of NaCl (0.5 mol/l) and fixed pH values, dependent on synthesized anion resin: 6.5 for amine, PAA, BPG or 2.5 for DTPADA, at temperatures 20, 60 and 100 °С. Thermodynamic description was made using Langmiur and Freundlich models. As a result, the authors calculated values of equilibrium constants, Gibbs free energy, enthalpy and entropy of Nd, Gd and Ho extraction.
In paper [14] the authors examined separation of metals based on ion exchange displacement of metal ions (Se, Re, Mo, La, Sc, Ce (IV)) from the solid phase on ion exchange resins Purolite A170, Lewatit M500 and Lewatit SP112. Effective separation of cerium (IV) and lanthanum was observed on Lewatit MonoPlus SP 112 (Na + ). Ion exchanger saturation was conducted under dynamic conditions with a lanthanum nitrate solution. Then elution with cerium (IV) nitrate solution was carried out. In the course of experiment, it was observed that cerium (IV) displaced 96-97 % of lanthanum from the solid phase of the resin. The process of frontal-gradient displacement of lanthanum by cerium (IV) was a part of the developed technology. Total coefficient of cerium and lanthanum separation using designed extraction method was 4300. Distribution coefficients for cerium and lanthanum were 1000 and 0.23, respectively. This method allows to extract cerium (IV) with the purity of 99.9 %.
Likewise, for extraction of such metals as thorium (IV) and uranium (IV) in the study [17] authors used solid sorbents with a resin matrix in the form of monomer 2, 4, 6-triformylphloroglucine and 2,5-diaminobenzenesulfonic acid (COF) and functional groups COF-SO 3  SO ] than it was for COF-SO 3 H. The authors separated thorium (IV), uranium (IV), cerium (III) and europium (III) and obtained the following coefficients of element separation: S Th/U = 9.4, S Th/Eu = 9.4 and S Th/Ce = 10.3.
Problem statement. Analysis of literature references allows to come to a conclusion that REM recovery from multisalt systems [11] is a relevant topic due to the complex character of REM sources. Values of obtained thermodynamic characteristics of the sorption process, namely of ion exchange, define not only conditions of running the process but also technological parameters of ion exchangers for REE separation and removal of associated components. Basing on Gibbs free energy of ion exchange process, it is possible to predict the efficiency of lanthanide recovery from aqueous salt solutions. Taking into account that REE recovery is usually performed from solutions of mineral inorganic acids, used as leaching solutions for phosphogypsum or other REM-containing materials, the paper focuses on ytterbium recovery in the form of anionic complexes with Trilon B on weakly basic anion exchange resin D-403 from nitric acid solutions. Experiment methodology. Static sorption examination. Examination of REE sorption was carried out from model solutions, containing anion complexes of ytterbium with Trilon B (EDTA ytterbiate ions), obtained by adding Trilon B to ytterbium nitrate solution with a precise mole ratio of 1:1 in accordance with the reaction stoichiometry [9]. Н 2 Tr 2-+ Yb 3+ ↔[YbTr] -+ 2H + . (1) Variable concentrations method was used at L/S ratio = 5 (solution -20, anion resin -4 cm 3 ), рН = 3 and constant ionic strength I, equal to 1 mol/kg (NaNO 3 ). Sorption was performed at temperatures T, equal to 298 and 343 K in the interval of ytterbium concentrations 0.1082-0.0056 mol/kg.
As an ion exchange resin, the authors utilized anionite D-403 in a nitrate form (China), which is a polystyrene chelate anionite with a divinylbenzene-styrol matrix and a functional group in the form of tertiary nitrogen atom with oxyhydrilic groups in positions β, γ and δ, which decrease the mobility of a lone-pair nitrogen electrons as a result of negative inductive effect [9]: 3 OH OH OH OH.
The solution and anionite were mixed at the rate of 120 rpm in a GFL thermostated cabinet (Germany) for 5-6 h until it reached equilibrium. Ytterbium concentration in the initial and equilibrium solutions was measured using spectrophotometric method with arsenazo (III) and X-ray fluorescence method on Epsilon 3 spectrometer (produced by PANalytical).
Dynamic sorption examination. In order to confirm the values of limiting sorption, saturation of D-403 anionite with EDTA ytterbiate ions was performed under dynamic conditions. The experiment took place in a sorption column with diameter of 1.2 cm and anionite layer height of 31.5 cm, the mass of anionite was 11.3 g. Solution containing 0.0398 mol/l of EDTA ytterbiate ions was passed through the column at a rate of 1 ml/min using peristaltic pump LOIP LS-301 (JSC "Laboratory Equipment & Instruments", Russia). On column exit the solution was sampled by 5 ml until the breakthrough and by 10 ml after the breakthrough until the initial concentration was reached.
Experiment results. Estimation of equilibrium constant, Gibbs free energy of ion exchange and total anionite capacity. Data obtained in the process of sorption at temperatures 298 and 343 K are presented in Table 1. Sorption value was calculated using formula where С 0 and С ∞ are initial and equilibrium concentrations of complex ions in the solution, mol/kg; V is volume, ml; m is dry anionite mass, g.  is y-axis of inverse sorption, γ ± are average ion coefficients of electrolyte activity, expressed through activity coefficients of individual ions: Average ion activity coefficients were estimated using reference data and mathematical description of dependence between activity coefficients and ionic strength of the solutions. Under given ionic strength, for NaYbTr compound,   was considered equal to CH 3 COONa due to strong dependency of activity coefficients upon ion charge and ionic strength of the solution, as well as weak dependency on anion individual character, including the complex ion. It is established that complex ions do not demonstrate any specific features in terms of activity coefficients as compared to simple electrolytes [4].
Taking into account the impact of temperature, activity coefficients were re-estimated using formula 298 , 298  Linear forms of sorption isotherms are presented in Fig.2.
Using equations of linear form approximation (Fig.2), values for equilibrium constants, and Gibbs free energy, total anionite capacity were calculated, as well as limiting sorption of a complex EDTA ytterbiate ion (Fig.3).
Thermodynamic and sorption characteristics are presented below: Estimation of sorption enthalpy and entropy. Basing on obtained values of equilibrium constants in the temperature interval 298-343 K, enthalpy of ion exchange was calculated using isobar equation where Т 1 and Т 2 are temperatures of sorption process, K 1 and K 2 are constants of ion exchange equilibrium at respective temperatures. Likewise, ion exchange entropy was estimated: The following values were obtained: 13.88 ± 6.94 kJ/mol, 85.08 ± 4.25 J/mol·K, 91.23 ± 4.56 J/mol·K.
Estimation of total dynamic exchange capacity (TDEC) and dynamic exchange capacity (DEC). According to results, presented in Fig.2, limiting sorption of a complex ion under static conditions at 298 K was estimated at 0.279 ± 0.014 mol/kg. Sorption curve for dynamic conditions is presented in Fig.3.
TDEC and DEC values were calculated using formulas: where m is sorbent mass, g; С 0 and С n are initial and current cerium concentrations on column exit, mg/ml; V n + 1 , V n are volumes of solution, corresponding to cerium concentrations on column exit С n + 1 and С n , ml; V br is the volume of solution before the breakthrough, ml. TDEC and DEC were estimated at 0.12 ± 0.01 and 0.09 ± 0.01 mol/kg, respectively. It happens due to the specific character of ligand coordination against REE ion of light and heavy groups [2]. The inner sphere of [YbTr]and [ErTr]complexes is composed of two nitrogen atoms and four carboxyl groups, whereas [CeTr]and [YTr]complexes contain two nitrogen atoms and only three carboxyl groups (Fig.4). An increase in effective negative charge of complex erbium and ytterbium ions leads to an increase in electrostatic interaction with the matrix of ion exchange resin. As a result, with diminishing atomic radius and elevating effective charge of the complex, chemical affinity to the solid phase of anion exchange resin increases.  Decreasing Gibbs free energy in the array 'cerium-erbium-ytterbium' correlates with lanthanide contraction, diminishing atomic radius and elevating effective charge of the complex, which leads to greater affinity, intensification of electrostatic interaction with anion exchange resin and higher selectivity.
Limiting sorption value of complex ion agrees with the values of total anionite capacity at both temperatures. With rising temperature the sorption value increases, as the equilibrium shifts in the direction of nitrate ion displacement from the matrix of ion exchange resin by EDTA ytterbiate ions.
Enthalpy value is positive, but the defining factor of decreasing Gibbs energy 0 0   T G in the course of ion exchange with rising temperature is an entropy criterion. Obtained thermodynamic data characterize selectivity of ion exchange between EDTA ytterbiate ions in the REE array and  3 NO on the surface of ion exchanger. The fact that complex ion sorption does not reach the value of limiting sorption is explained by the large geometric size of EDTA ytterbiate ion.
Conclusion. The paper examined ytterbium sorption in the form of anionic complexes with Trilon B from nitrate solutions at constant ionic strength of 1 mol/kg, рН = 3, L/S ratio =5 and temperatures 298 and 343 K on weakly basic anion exchange resin D-403.
Mathematical transformation of mass action law was performed, as it was converted to a linear form similar to Langmuir equation, the graphic solution of which allowed to calculate thermodynamic parameters of ion exchange process.
Equilibrium constants and Gibbs free energy of ion exchange at temperatures 298 and 343 K were K 298 = 103.02 ± 5.15;