Towards Base Metal Recovery from Sewage Sludge

With a need for the reuse of sewage sludge as a sustainable phosphate source, removal of contaminant metallic hazards become imperative. Success in leaching metals from sewage sludge leaves open the study of removal and recovery of mobile metals from a slurry. Careful consideration of lixiviants and solution condition controls have been considered for the total removal of specific metals of interest, and the solid phase extraction of target metals has been studied extensively. Copper, iron(II), lead and zinc extraction by C107E, TP214, MTS9100, MTS9501, MTS9301, MTS9570 were screened over the buffering regions of acetic, lactic and citric acid. With acetic acid providing the least hindrance to extraction, isotherm and kinetic characterisation determined MTS9301 as the most effective. Kinetically able to separate iron(II) and other metals (130min vs a maximum of 49min), and extracting >90% of metals without desorption or competition with a surface concentration 2.729mmol/g. Due to the large variation in sewage sludge, a resin-in-pulp was conducted utilising a modified literature surrogate. Leaching was conducted for 24h in pH 1 acetic acid and 9% H2O2, dissolving 33-36, 88-89 and 91-92% of copper, lead and zinc, respectively, before neutralisation and final addition of MTS9301. Elution determined that with 2mL resin 38.9, 37.9 and 53.6% recovery of copper, lead and zinc, respectively, was attained with a simple one stage extraction. As citric acid is not only relevant to this work, but also resource recovery in general, further study was conducted into the performance of MTS9301 within citrate. It was found that optimal extraction was obtained in 0.1mol/L citrate. Total equilibrium and kinetic performance was only slightly effected for copper, lead and zinc, with the resin displaying far lower capacity for iron(II) and a secondary desorption once peak extraction was achieved.