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 (K-d) of Rh(III) and the adsorbed amounts of HNO3 by varying the HNO3 concentration. It was found that the K-d 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 K-d 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 K-d were observed with the addition of amine compounds. The amine compounds have relatively high basicity (TEA: pK(a) = 10.75, EDA: pK(a) = 6.85 and 9.93, and Tren: pK(a) = 8.42, 9.44, and 10.1). The K-d 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-].