Influence of hydrophilic groups and metal-ion adsorption on polymer-chain conformation of amidoxime-based uranium adsorbents

Abstract

This study focuses on the influence of hydrophilic groups and metal-ion loading on adsorbent polymer conformation, which controls access to adsorption sites and may limit adsorption capacity. Gaining a better understanding of the factors that influence conformation may yield higher-capacity adsorbents. Polyamidoxime (PAO), deuterated-PAO polyacrylic acid diblock copolymers (d-PAO-b-PAA), and randomly configured copolymers (PAO-co-PAA) were synthesized and characterized by neutron reflectometry in air and D₂O. For d-PAO-b-PAA, characterization was also performed after alkali conditioning and in simulated seawater. PAO and PAO-co-PAA, with similar molecular weight and grafting density, extended from 95-Å thickness in air to 180 and 280-Å in D₂O, respectively. This result suggests that polymer swelling may cause the additional adsorption capacity observed when polymer hydrophilicity increases. Two d-PAO-b-PAA samples, A and B, with a d-PAO thickness of 55-Å swelled to 110-Å and 140-Å, respectively, with an overall thickness increase of ∼160% in D₂O. After alkali conditioning, molecular interactions increased the density of PAA near the PAO-PAA interface, while the d-PAO thickness only decreased by ∼10 Å. The d-PAO thickness of both samples declined to ∼90-Å after adsorption in simulated seawater due to polymer-chain crosslinking. These results are expected to aid in improving adsorbent synthesis to increase uranium capacity.

Introduction

At the current rate of consumption, terrestrial uranium reserves are expected to be depleted within the next century [1]. Moreover, global demand for nuclear energy is expected to rise by as much as 56% over the next fifteen years [2]. To accommodate growing consumption and ensure that a sustainable uranium supply is available, new sources of uranium will need to be explored. One potential source is the 4.5 billion tons of oceanic uranium, which is distributed in the ocean with an approximate average concentration of 3.3 ppb [3]. This low concentration of uranium makes recovery difficult and requires a highly efficient means of capturing dissolved uranyl ions. While both active and passive adsorption techniques have been considered, low-cost, high-capacity passive braided polymer adsorbent systems are a far more realistic means of large-scale uranium recovery [3]. The development of these adsorbents requires several factors to be properly optimized such as production cost, reusability, and adsorbent capacity.

Uranium recovery from seawater has been an area of interest since the early 1950s and has covered a wide variety of materials, ranging from organic resins to more recent studies involving amidoxime. Ion-exchange resins such as Zeo-Karb 226, 8-hydroxyquinoline, and resorcinol arsenic acid were among the earliest materials considered; however, low uranium selectivity [4] and rapid aging in seawater [5] made these materials ineffective for uranium recovery. Various microorganisms [6], [7], [8], plant wastes [9], and immobilized tannins [10], [11] have also been considered as biosorbents for uranium recovery. Adsorbents such as titanic acid [12], zinc carbonate [13], galena [14], activated carbon [15], iron (III) oxide [16] and particularly titanium hydroxide [3], [13], [14], [15], [16], [17] received the most attention in the literature until the 1980s. Some more recent studies involving gold nanoparticles [18], metal sulfides [19], and ion-imprinted chelating microspheres [20] have been performed, though amidoxime adsorbents are considered more promising [3]. Japanese researchers began developing amidoxime adsorbents in the 1980s, which led to a number of marine tests during the 1990s and early 2000s [3], [21]. These tests culminated in the work of Seko and coworkers who recovered 1 kg, or approximately 2.85 mg U/g adsorbent, of uranium after 240 days of exposure to seawater using passive amidoxime adsorbents [22]. Over the last seven years, Oak Ridge National Laboratory (ORNL), with the support of the US Department of Energy, has continued to develop amidoxime materials. These studies have led to the development of amidoxime adsorbents with capacities as high as 6.56 mg U/g adsorbent after 56 days of exposure to seawater [23].

Amidoxime adsorbents with comonomers, such as vinylphosphonic acid [24] and itaconic acid [25], have a higher uranium adsorption capacity than adsorbents synthesized with just amidoxime. Since these comonomers do not directly affect uranium adsorption, their influence on polymer hydrophilicity and, by extension, polymer conformation is likely to be a contributing factor to this increase in adsorption capacity [26], [27]. Polymer conformation is primarily determined by grafting density (surface coverage), polymer chain size (molecular weight), and polymer-polymer and/or polymer-medium interactions (electrostatic interactions and hydrogen bonding). Increasing grafting density and chain length will multiply the total number of adsorption sites, however, beyond a certain point, diffusion limitations may restrict the number of accessible adsorption sites. This effect is believed to be caused by polymer crosslinking, which leads to polymer chains becoming intertwined as a result of attractive molecular interactions. These properties are particularly important because they can be manipulated to reduce the diffusion barrier, increase accessibility, and promote polymer brush stability through secondary polymer interactions.

The primary objectives of this study are to examine the influence of hydrophilic acid segments, alkali conditioning, and metal ion loading in simulated seawater on polymer conformation. Amidoxime and amidoxime acrylic acid copolymers are characterized by neutron reflectometry. Optimization of polymer conformation has the potential to produce the maximum number of effective adsorption sites and, in turn, improve the uranium adsorption capacity. The results of these experimental studies, combined with an understanding of the influence of grafting density and chain size, will aid in the optimization of graft-chain conformation.