Adsorption of palladium and rhodium onto polyphenol-formaldehyde resins

In this study, polyphenols such as valonia tannin, gallic acid and pyrogallol, and formaldehyde condensation reaction has been investigated. Polyphenol resins were prepared by NH3 solution (13.3 N) and formaldehyde (37 %wt). Polyphenol resins, obtained from condensation reaction, were characterized by FTIR spectroscopy. There are two possible condensation reaction paths for the reaction of polyphenols and formaldehyde, leading to the formation of a methylene bridge. The first step for two mechanisms, methylolation, is an electrophilic aromatic substitution reaction. The second step is a condensation reaction. The adsorption of palladium and rhodium onto polyphenol resins were studied using a batch adsorber. The isotherms were determined by mixing 1 g polyphenol resin, with 1 L of metal solution of initial concentrations from 20 to 150 mg/L for palladium and from 15 to 90 mg/L for rhodium. The effects of contact time, initial pH, initial Cl- concentration, adsorbent dose, temperature and initial metal concentration on the palladium and rhodium adsorption by the polyphenol resin have been studied. A contact time of 120 min was required to achieve equilibrium. The experimental isotherm data were analyzed using the Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich and Redlich-Peterson equations. Adsorption of palladium and rhodium onto polyphenol resin followed the Langmuir isotherm. The monolayer adsorption capacities of each resin were determined and discussed. The thermodynamic parameters, such as ? Ho, ? So and ? Go, were also determined and evaluated. The experimental data were analyzed using four adsorption kinetic models - the pseudo first- and second-order equations, the Elovich equation and intraparticle diffusion equation ? to determine the best fit equation for the adsorption of palladium and rhodium onto polyphenol resins. The characteristic parameters for each kinetic models have been determined and the correlation coefficients have been calculated in order to assess which model provides the best fit predicted data with experimental results. Results show that the pseudo second-order equation provides the best correlation for the adsorption process. Adsorption mechanism was also proposed for the adsorption of palladium and rhodium onto polyphenol resins. It was found that Pd (II) and Rh (III) was adsorbed onto the polyphenol particles as a reduced metallic Pd and Rh through redoxreaction mechanism: chloropalladium (II) and aqua chloro rhodium (III) species were reduced to Pd (0), while hydroxyl groups of polyphenol resin were oxidized during the adsorption. Additionally, it was observed that Pd (II) species containing fewer Cl, such as PdCl2(H2O)2 and PdCl(H2O)3+, were more favorable for the adsorption than PdCl3(H2O)- and PdCl42-. Rh (III) species [RhCl_5 H_2 O]^(2-) and [RhCl_2 (H_2 O)_4 ]^- adsorbed onto polyphenol resin surface. By utilizing such characteristics of polyphenol resin particles, it is expected that they can be applied to recover Pd (II) and Rh (III) efficiently and simply with low cost. A single stage batch adsorber was designed for different adsorbent mass/treated effluent volume ratios using the Langmuir isotherm.