Capture and Release Mechanism of Ni and La Ions via Solid/Liquid Process: Use of Polymer-Modified Clay and Activated Carbons

Abstract

This study is a starting point for the development of an efficient method for rare earths (REs) and transition metals (TMs) recovery from waste electrical and electronic equipment (WEEE) via a hydrometallurgical process. The capture and release capability of mineral clays (STx) and activated carbons (AC), pristine and modified (STx-L6 and AC-L6) with a linear penta-ethylene-hexamine (L6), towards solutions representative of the process, are assessed in the lab-scale. The solids were contacted with synthetic mono- and bi-ionic solutions containing Ni(II) and La(III) in a liquid/solid adsorption process. Contacting experiments were carried out at room temperature for 90 min by fixing a La concentration at 19 mM and varying the Ni one in the range of 19–100 mM. The four solids were able to capture Ni(II) and La(III), both in single- and bi-ionic solutions; however, the presence of the polyamine always results in a large improvement in the capture capability of the pristine sorbents. For all the four solids, capture behaviour is ascribable to an adsorption or ion-sorbent interaction process, because no formation of aquo- and hydroxy-Ni or La can be formed. The polyamine, able to capture Ni ions via coordination, allowed to differentiate ion capture behaviour, thus bypassing the direct competition between Ni and La ions for the capture sites found in the pristine solids. Release values in the 30–100% range were found upon one-step treatment with concentrated HNO3 solution. However, also, in this case, different metals recovery was found depending on both the sorbent and the ions, suggesting a possible selective recovery.

Analysis of the Adsorption-Release Isotherms of Pentaethylenehexamine-Modified Sorbents for Rare Earth Elements (Y, Nd, La)

“…According to the results discussed in previous works [ 20 , 23 , 39 , 51 ], the improvement in adsorption capacity should be ascribed to the presence of the intercalated PEHA chains. As proposed in Section 3 , the amino groups (−NH, −NH 2 ) operated as coordination sites thanks to their electron lone pairs on the nitrogen atoms.…”

“…It was characterized by almost equivalent Q res values, resembling a sort of plateau of desorbed cations. An accentuated release from weakly interacting sites was hypothesized [ 39 ]. The second region involved higher C rel values.…”

“…The prevision provided by the Langmuir isotherm was not able to accurately approximate the experimental data, as it can be ascertained from Figure 13 b. No clear trend was deduced from the Q res vs. C rel plot, and such an outcome was ascribed to the existence of heterogeneous electronic interactions during the desorption process [ 39 ]. Consequently, cation exchange mechanism was not regarded as sufficient to explain the release behavior of La from MAC.…”

“…The authors’ research group dealt with the investigation of innovative sorbents such as mineral clays [ 17 , 37 ], organoclays [ 37 , 38 ] and activated carbon [ 20 , 39 ] in multiple works. Activated carbon (AC) is widely used for the adsorption of metallic elements from aqueous solutions due to its inherent porosity, high specific surface area, peculiar surface chemistry, and cheapness [ 31 , 35 , 40 ].…”

“…Within such context, this work intends to be a step further toward the comprehension of the adsorption and release mechanisms of three REEs, namely yttrium (Y), neodymium (Nd), and lanthanum (La), on both AC and its modified derivative (MAC). In previous works, only the capture and release capability of both pristine and PEHA-modified clays and activated carbons (AC) towards Ni and La were evaluated [ 20 , 39 ], and AC and MAC were characterized morphologically and micro-structurally upon La adsorption [ 20 ], which was taken as representative of REEs. Moreover, only a preliminary thermodynamic analysis on La capture was discussed.…”