Specific scavenging of polluting metals by prolate particles of phosphonate functionalized mesoporous silica

Highlights

• Phosphonate-functionalized MS adsorb selectively large diameter metal ions.

• Prolate particles form upon MS functionalization with phosphonate.

• Phosphonate/bisphosphonate NMR signals are inversely shifted after metal adsorption.

• Good distribution coefficients >500 mL/g obtained for metal sorption on sorbents.

• Modification of the textural properties of the prolate particles by metal sorption.

Abstract

Phosphonate and/or bisphosphonate-functionalized mesoporous silica (MS) sorbents were developed and studied using ³¹P-MAS-NMR, SEM-EDS, TEM, BET, and batch sorption experiments before and/or after metal ion scavenging (Bi⁺³, Ba⁺², Cd⁺², U⁺⁶, Ag⁺, Pd⁺² and Zn⁺²). The amount of phosphorus inside the sorbents was ascertained by SEM-EDS, and the phosphonate vs bisphosphonate content derived from NMR. The organic matter loading was between 15 and 38 %w/w and inversely correlated with BET. Pore size (4.9–6.6 nm) and surface areas (219–473 m²/g) were derived from nitrogen sorption isotherms. The NMR signals of the phosphonates are shifted downfield by the metal cations while the bisphosphonate signals are shifted upfield for urea-derived sorbents and downfield for the amino sorbent. The sorbents are composed of prolate particles of micrometric dimensions. The prolate particles of the urea-derived sorbents are stable following the adsorption of metal cations, while they are disrupted in the case of the amino sorbent, due to the repulsion between negative charges at basic pH. Only metal cations with relatively large ionic diameters were adsorbed revealing the need for multi-point interactions inside the sorbent, which is not possible for smaller cations. For all the sorbents, the sorption data of uranyl fit the Freundlich isotherm model, which predicts a heterogeneous surface. The K and K_d values derived from this model show good sorption capabilities. K_ds for metal cation solutions (20 μg/mg) on the amino sorbent were between 500 and 35000 mL/g. Both selectivity and sorption capabilities of the sorbents allow their use in environmental analysis and remediation.

Introduction

For the purpose of developing sorbents for the scavenging and identification of heavy metal pollutants in the environment, various technologies were developed. These have been based on coordination polymers, some of them to decontaminate water reservoirs [1,2]. Metal organic frameworks (MOFs), coordination or scavenging polymers create strong interactions between metal cations and chemical functionalities bound to the polymeric chain [3]. In industry, with the aim of eliminating impure compounds from polluting metals, efforts have been made to introduce separation methods such as membrane trapping [4]. Self-assembled monolayer on mesoporous supports were used for actinide sequestrations [5]. A variety of sorbents based on mesoporous silica (MS) [[6], [7], [8]] were developed for the scavenging of heavy metal ions present in water [[9], [10], [11], [12]]. Other sorbents were based on polymers like poly(glycidylmethacrylate) [13] or poly(vinylbenzyl chloride) [14] functionalized with anionic groups like carboxylates, phosphonates, sulfonates or amidoximes [15] linked to the polymer scaffold through organic chains. The mesoporous silica scaffold is a good platform for further chemical derivatization [[6], [7], [8]]. MS by itself is quite a good sorbent [12,16], and it was shown that MS derivatives gave in general large surfaces of contact between the solution and the silica based sorbents [6,8]. On the other hand, anionic groups like carboxylates and phosphonates linked to the MS scaffold are able to attract positively charged ions, and form stable complexes. This chemical binding together with the physical adsorption by the mesoporous silica produce powerful sorbents. We were aware of new adsorbing systems, based on functionalized MS [[17], [18], [19]] with amidoxime, imide dioxime, carboxylate and particularly phosphonates MS [16,[20], [21], [22], [23], [24], [25], [26], [27], [28], [29]]. Some of them were described as excellent sorbents for a variety of metal ions, like uranyl (UO₂⁺²). The lead compound in one case was an organophosphonate-functionalized MS, named MFPh-III (same molecule as MUP, see Fig. 1) by the authors [20]. Inspired by this work, we designed new phosphonate-functionalized MS materials, MUPBP (MSU-H-urea-phosphonate and bisphosphonate), MUBP (MSU-H-urea-biphosphonate) and MAPBP (MSU-H-amino-phosphonate and bisphosphonate), in which new organic phosphonates and/or bisphosphonate (6–9, Fig. 1) were linked to the mesoporous silica scaffold. In some cases we kept the urea linker and replaced the phosphonate by a bisphosphonate (MUBP) or a mixture of both (MUPBP). In one case we replaced the urea linker by a secondary amine (MAPBP). MSPh-III was also prepared as a reference, but as will be described later, we found that our sorbent MUP (MSU-H-urea-phosphonate) did not have the same properties. These new functionalized MS were characterized by their ³¹P-MAS-NMR, SEM-EDS, TEM and nitrogen adsorption/desorption isotherms. Their properties were compared as sorbents for uranyl and other metal cations. The effects of the scavenged metals on the microstructure and on the spectroscopic properties were also evaluated using the same methods. The sorption capacity and the distribution coefficient of each sorbent were evaluated and the sorption mechanism was studied, using the Langmuir’ and Freundlich’ models [[30], [31], [32]].