Towards phosphate loop closure by sustainable remediation of sewage sludge: Removal of divalent metals from citric acid leachate with a chelating iminodiacetic acid resin

There is an urgent global need to work toward closure of the phosphate recycling loop. One possible pathway is investigation of use of sewage sludge in lieu of commercial fertilisers. This necessitates removal of the problematic heavy metals therein. This study investigates abatement of copper, iron(II), lead and zinc, after leaching with citric acid, with a commercially-available chelating ion-exchange resin (Purolite MTS9301). Key parameters investigated include citric acid concentration, equilibrium metal uptake, kinetic metal uptake and aqueous speciation (predicted by the HySS computer programme). Equilibrium data were fitted to a number of common isotherm models, including a version of the Langmuir, modified to account for desorption in a competitive system. This model proved most capable of fitting the data and the derived maximal adsorption capacities in the multi-metal system were 1.81 ± 0.06, 0.8 ± 0.1 and 0.44 ± 0.02 mmol g-1 for Cu, Pb and Zn respectively. Kinetic data were likewise fitted to widely-used models and the pseudo second-order (PSO) model most adequately described the data. The adsorption half-lives were calculated as 12.7 ± 0.7, 32 ± 4 and 31 ± 2 min for Cu, Pb and Zn. Fe(II) data could not be reliably modelled, due to the very low affinity for the resin. The order of selectivity across all experiments was clearly established as Cu > Pb ∼ Zn ≫ Fe. Generally, quantities and rapidity of uptake was superior in the citric acid leachate than for the same resin in the equivalent acetic acid leachate. Further advantages are lower required lixiviant concentration, reduced toxicity and more sustainable manufacture. Overall, the citric acid/ MTS9301 combination showed considerable potential in the management of three key heavy metals in sewage sludge.