Sorption behaviour of heavy metals on a dithizone-impregnated polymer resin

Abstract

Amberchrom CG-300m, a styrene acrylic ester polymer resin, was studied for the first time as sorbent for metal ion sorption in a solid-phase extraction system. The polymer sorbent was modified via impregnation with dithizone, a chelating ligand, to improve its efficiency and selectivity. The loading capacity of the resin is 3.2 mg dithizone per gram of sorbent. The physico-chemical characterisation of the sorbent before and after impregnation was done by Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy. Surface morphology studies show a significant decrease in resin pore size after impregnation. The appearance of a broad band at around 1360 cm-1 in the FTIR spectrum of the impregnated resin signifies the introduction of the thiol functional group of dithizone into the resin backbone. This study reports on the sorption behaviour of ten (10) heavy metal ions – Ag+, Bi3+, Cd2+, Co2+, Cu2+, In3+, Mn2+, Ni2+, Pb2+ and Zn2+ – onto the dithizone-impregnated resin. The sorption behaviour of the metal ions was studied in batch and column experiments. The sorption parameters investigated were initial metal ion concentration, pH, V/m ratio, time and temperature. Sorption generally increased with initial concentration and pH, but most metals hydrolysed at higher pH, even in the presence of the tartrate ion. Cu (pH < 5.5), Ag (pH < 3) and Bi (pH < 2) were sorbed quantitatively (> 95 %) in moderate to strongly acidic medium. Mn(II) was sparsely sorbed in the pH range 4.57 – 8.54. Most metals were quantitatively sorbed at low initial concentration. However, Ag and Bi were quantitatively sorbed at relatively high initial concentration of 4.104 mg/L and 3.30 mg/L, respectively. Sorption capacity of the metal ions studied decreased in the order: Ag+ > Bi3+ > Pb2+ > Co2+ > Cu2+ > Zn2+ > Ni2+ > Cd2+ > Mn2+. Ag+ and Bi3+ were completely sorbed in the temperature range 283 K – 298 K. Thermodynamics of Mn sorption was rendered futile as it remained unsorbed in the same temperature range. Positive values of H0 suggest sorption of Cd, Co, Cu, Pb and Ni was endothermic, whereas Zn was the only metal whose sorption was exothermic (H0 < 0). H values were in the range 25 – 60 kJ suggesting sorption of heavy metal ions onto the impregnated resin could be attributed to a physico-chemical sorption process, rather than pure physi- or chemisorption. Apart from Ni, sorption was favourable over the entire temperature range (G0 < 0); sorption of Zn was favourable at T > 288 K only. The positive S0 values associated with sorption of all metals (except Zn) meant sorption was accompanied by increased disorder at the solid/liquid interface. The efficiency and reliability of the dynamic method viz-à-viz the static method was undeniable, and quantitatively proven by kinetic studies of Pb. Sorption kinetics during all column experiments was sufficiently fast so that steady-state approximation was established within the first three min. Langmuir, Freundlich, Temkin and Dubinin-Rudeshkivich sorption isotherms were investigated at 294 K. Except for Cu, the Langmuir isotherm provided the best fit to sorption data. Temkin, Langmuir and Freundlich isotherms modelled sorption of Cu best (r2 > 0.9990). The Freundlich isotherm proved a credible model for sorption of all metals except Cd. Sorption of all metal ions followed pseudo-second order reaction kinetics. Sorption of Cu onto dry, impregnated resin followed pseudo-first order kinetics though. Diffusion kinetics was successfully modelled by Weber-Morris and Homogeneous Particle Diffusion models. Weber-Morris plots indicate sorption of all metal ions were controlled by both pore and film diffusion mechanisms. Cu(II) and Co(II) diffusion were seemingly independent of resin pore size. The dithizone-impregnated resin was successfully applied to the qualitative separation of Cd2+-Sn4+, Mn2+-Co2+-Ni2+ and Bi3+-Pb2+ ion mixtures. Separation was achieved quantitatively within 5 min.