Recovery of Rhodium(III) from Nitric Acid Solutions Using Adsorbent Functionalized with N,N,N-Trimethylglycine

To understand Rh(III) adsorption behaviors of styrene–divinylbenzene copolymer functionalized with N,N,N-trimethylglycine (AMP03), we measured the distribution coefficients (Kd) of Rh(III) and the adsorbed amounts of HNO3 by varying the HNO3 concentration. It was found that the Kd value of Rh(III) decreased with increasing [HNO3], whereas the amount of adsorbed HNO3 increased. To examine the Rh(III) adsorption capabilities of AMP03 in detail, we performed adsorption experiments while varying H+ and NO3− concentrations. The results showed that the Kd value of Rh(III) increased with decreasing [H+] and increasing [NO3−]. Furthermore, we performed Rh(III) adsorption experiments using HNO3 solutions containing amine compounds such as triethylamine (TEA), ethylenediamine (EDA), or tris(2-aminoethyl)amine (Tren). In all systems, drastic increases in Kd were observed with the addition of amine compounds. The amine compounds have relatively high basicity (TEA: pKa = 10.75, EDA: pKa = 6.85 and 9.93, and Tren: pKa = 8.42, 9.44, and 10.1). The Kd increases were attributed to a decrease in [H+] with the protonation of amine compounds. The stoichiometry of the adsorbed Rh(III) species was determined with slope analysis based on ion-exchange equilibria for Rh3+ and H+. Rh3+ was adsorbed with two betaine groups and three NO3−. In contrast, H+ adsorption reactions occurred competitively with those of Rh3+. Our results show that AMP03 effectively adsorbs Rh(III) from HNO3 solutions with low [H+] and high [NO3−].