Thermodynamic and Kinetic Adsorption Studies of a Multi-Metal System within Acetate Media, with a View to Sustainable Phosphate Recovery

Phosphate shortages and the ensuing pressures on food security have led to an interest in processed sewage sludge as a substitute for commercial fertilisers. However, the presence of heavy metals in this nutrient source causes concerns around environmental release and pollution. This work builds towards a resin-in-pulp sludge detoxification process. It showcases the kinetic and thermodynamic adsorption capabilities of the ion-exchange resins C107E (carboxylic acid functionality), MTS9301 (iminodiacetic acid) and TP214 (thiourea), with respect to Cu(II), Fe(II), Pb(II) and Zn(II), within a simulated sewage sludge weak acid (acetate) leachate. Mixed metal isotherm data were fit to common two-parameter isotherm models and also a novel modified Langmuir model, which better accounted for the effects of desorption and competition within the system. Kinetic data were also fit to common twoparameter models; results suggesting the system was likely film diffusion-controlled and followed pseudo-second-order kinetics. C107E displayed rapid adsorption of lead (t 1/2 = 26±3min), and significant uptake of all metals. MTS9301 showed high affinity for copper ions, with concurrent desorption of all the other metals, and also displayed the fastest kinetics (t 1/2 = 14.1±0.9, 130±20, 25±5 and 49±6 min for copper, iron(II), lead and zinc, respectively). C107E and MTS9301 showed far slower adsorption for iron(II) than the other three metals, which invited the possibility of kinetic separations. TP214 had reasonable effectiveness in removal of copper, but displayed poor affinity for all other metals. In general, the greatest difficulty in modelling the multi-metal system was observed to be the two-stage trends observed as metal-proton exchanges become metal-metal exchanges. While not having the highest capacity, MTS9301 was recommended as the most appropriate resin for rapid and efficient removal of Cu, Pb and Zn from the acetate medium.