Functionalized chitosan-silica hybrid materials for the recovery of critical metals from diluted aqueous waste streams.

Critical metals are defined as economically important metals with a high supply risk. On one hand, their availability is essential for the production of high-tech and green-tech applications, in turn crucial to enable the transition towards a low-carbon economy. On the other hand, their supply is vulnerable to geopolitically or economically driven fluctuations, since the market of technological metals is dominated by only a handful of countries (notably China). In order to ensure a self-sufficient supply of critical metals in a sustainable way, alternatives for primary production are fully explored. A circular economy aims at a more efficient use of resources by closing the materials loop and reusing "waste" as a secondary raw material source. This demands new technologies and methods. For instance, in the case of dilute waste streams from industrial processes, the challenge is to efficiently recover very low concentrations of valuable metals from huge volumes of waste water. Adsorption is considered to be the key technology to do so.

Biopolymers, such as chitosan and alginate, were examined for adsorption of critical metals from dilute aqueous solutions. Their use holds several advantages. They have interesting physicochemical characteristics and a high adsorption capacity. Biopolymers are renewable and biodegradable, given their natural origin, which also makes them easily available, in any quantity and at low cost. Chitosan, for example, is derived from crustacean waste produced by the (sea)food industry. Although biopolymers appear to be excellent sorbent materials, they do not always show the desired mechanical properties. They are non-porous, exhibit strong, solvent-dependent swelling behavior and have an elastic character. By sol-gel chemistry with silica precursors, chitosan-silica hybrid materials were obtained, with covalent bonds between the entangled networks of the organic material and the inorganic carrier. In these hybrid materials, the advantages of both components were combined, thus obtaining superior properties in terms of porosity, surface area, rigidity and chemical resistance.

Another feature of biopolymers is the high availability of functional groups. These enable chemical modification of the biopolymers. The abundant amino groups on chitosan were used for direct immobilization of various organic ligands with a high affinity for particular (critical) metals. As such, chitosan-silica was functionalized with the chelating agents ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). The adsorption performance of the resulting materials was investigated from single-element solutions of neodymium(III). During adsorption studies from a multi-element mixture, a high affinity was observed for the lanthanides as a group, but also between the mutually different lanthanides, a high selectivity was revealed. Selectivity between distinct metal ions results when the stability of the complexes formed by the organic ligand and the respective metal ions is different. The speciation of europium(III) coordinated to DTPA-chitosan-silica was studied by luminescence spectroscopy and EXAFS. These measurements indicated a hydration number of 1 and a total coordination number of 9 for the central europium(III) ion. Functionalization of chitosan-silica with ethyleneglycoltetraacetic acid (EGTA) resulted in a material with an extremely high selectivity for scandium(III), especially compared to iron(III), which normally behaves very similar. Separation by chelating ion-exchange chromatography with EGTA-chitosan-silica as the stationary phase enabled the isolation of scandium(III) from a leachate of Greek bauxite residue. By immobilization of 8-hydroxyquinoline (8-HQO) and 8-hydroxyquinaldine (8-HQA), materials with a high selectivity for gallium(III) were obtained. Proper control of the experimental parameters during the desorption step allowed the separation of low concentrations of gallium(III) from the major, low-grade aluminosilicate matrix in a highly alkaline synthetic Bayer liquor sample.

Given the aim to utilize these materials in industrial chromatography columns, attention was also paid to structural design. With spherically shaped particles based on alginate, the best way to incorporate silica was investigated. Then, the alginate spheres were simultaneously hybridized and functionalized with sulfonic acid groups by means of co-condensation reactions between tetramethyl orthosilicate (TMOS) and (3-mercaptopropyl) trimethoxysilane (MPTMS). The alginate-sulfonate-silica (ASS) particles showed a high adsorption capacity and selectivity for indium, which could be separated together with gallium from a simulated leachate of a zinc refinery residue.

In summary, high-performance adsorbent materials based on biopolymers were developed during this PhD. These materials have proven to be particularly suitable for the selective recovery and separation of critical metals from dilute waste streams of industrial processes. In time, this knowledge should allow a more efficient use of our resources in the transition towards a more sustainable future.