Demetallization of Sewage Sludge Using Low-Cost Ionic Liquids

Sludge produced from wastewater treatment has little to no value and is typically treated through volume reduction techniques, such as dewatering, thickening, or digestion. However, these methods inherently increase heavy metal concentrations, which makes the sludge unsuitable for land spreading and difficult to dispose of, owing to strict legal requirements/regulations concerning these metals. We addressed this problem, for the first time, by using recyclable low-cost protic ionic liquids to complex these toxic metals through a chemical fractionation process. Sewage sludge samples collected from wastewater plants in the UK were heated with methylimidazolium chloride ([Hmim]Cl, triethylammonium hydrogen sulfate ([TEA][HSO4]) and dimethylbutylammonium hydrogen sulfate ([DMBA][HSO4]) under various operating temperatures, times and solids loadings to separate the sludge from its metal contaminants. Analysis of the residual solid product and metal-rich ionic liquid liquor using inductively coupled plasma-emission spectrometry showed that [Hmim]Cl extracted >90% of Cd-II, Ni-II, Zn-II, and Pb-II without altering the phosphorus content, while other toxic metals such as Cr-III, Cr-VI and As-III were more readily removed (>80%) with [TEA][HSO4]. We test the recyclability of [Hmim]Cl, showing insignificant efficiency losses over 6 cycles and discuss the possibilities of using electrochemical deposition to prevent the buildup of metal in the IL. This approach opens up new avenues for sewage sludge valorization, including potential applications in emulsion fuels or fertilizer development, accessed by techno-economic analysis.

Automated summary

This study focused on utilizing low-cost ionic liquids for complexing toxic metals in sewage sludge, showing promising results in efficiently separating metal contaminants. Specifically, [Hmim]Cl exhibited a high extraction rate for Cd-II, Ni-II, Zn-II, and Pb-II, while [TEA][HSO4] was more effective in removing Cr-III, Cr-VI, and As-III. Recycling [Hmim]Cl and exploring the use of electrochemical deposition for preventing metal buildup in the IL were also discussed, offering new possibilities for sewage sludge valorization.