Single metal isotherm study of the ion exchange removal of Cu(II), Fe(II), Pb(II) and Zn(II) from synthetic acetic acid leachate

Abstract

A large predicted increase in population growth and dwindling phosphate resources has led to sewage sludge being an attractive alternative to commercial fertilisers. Among other contaminants, heavy metals are a barrier to unrestricted use of sewage sludge or sewage sludge ash as a phosphate source. This study will focus on the equilibrium isotherm behaviours of Cu2+, Fe2+, Pb2+ and Zn2+ towards C107E, MTS9301 and TP214 resins within acetic acid media, fit to the two parameter Freundlich, Langmuir, Temkin and Dubinin-Radushkevich (D-R) isotherm models. C107E and MTS9301 were both found to have comparable monolayer capacities for Cu2+ and Fe2+ (5 ± 1 vs 4.3 ± 0.7 mmol·g−1 and 2.1 ± 0.8 vs 2.3 ± 0.8 mmol·g−1 for Cu2+ and Fe2+, respectively). The Freundlich model implied heterogeneous binding for Pb2+ and Zn2+ adsorption to C107E and MTS9301. The monolayer capacities of MTS9301 for lead and zinc were calculated as 2.1 ± 0.2 mmol·g−1 and 3 ± 1 mmol·g−1, respectively. MTS9301 returned larger D-R free energy values than C107E, with the largest difference being Zn2+, displaying energies of 14.0 and 5.5 kJ·mol−1, respectively. TP214 displayed the lowest capacity for metals with Fe2+, Pb2+ and Zn2+ returning D-R energy values closer to physisorption mechanisms (6.0 ± 0.5, 7.1 ± 0.4 and 7.8 ± 0.5 kJ·mol−1, respectively), with copper returning a D-R energy relating to chemisorption (17 ± 1 kJ·mol−1). Overall, it was observed that the similarity of the C107E functionality to the free acetate anion led to the highest level of hindrance, seconded by the interaction between Fe2+, Pb2+ and Zn2+ to TP214, while copper displayed strong interaction with TP214 and MTS9301 displayed little or no hindrance by the acetate complexes in solution, with solution phase complexes affecting the homogeneity of binding within any ion exchange reaction.