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 CdII, NiII, ZnII, and PbII without altering the phosphorus content, while other toxic metals such as CrIII, CrVI and AsIII 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.

In each case, a pre-weighed quantity of acid (1 M for HCl and 5 M for H 2 SO 4 (Sigma-Aldrich)) was added to an equimolar quantity of amine (Sigma-Aldrich) inside a round bottomed-flask using a funnel. The reacting solution was cooled constantly by submerging the round-bottomed flask inside an ice-water bath. The acid:base ratio was determined using a mass balance and confirmed by titration with 1 M NaOH using S3 a G20 compact titrator (Mettler Toledo). The water content was controlled using water addition/evaporation and was confirmed using a Karl Fischer titrator (Mettler Toledo).
The formation of the IL was verified using NMR spectroscopy ( Figure SI2).

Sample Characterization
The sludge (and the treated samples) were further characterized using ultimate and proximate analysis carried out on a Vario MICRO CUBE elemental analyzer (Elementar) and Q5000 IR TGA (TA instruments), respectively. For the proximate analysis, the sludge samples were heated at a ramp rate of 60 °C min -1 under N 2 (50 ml min -1 ) to 110 °C. After maintaining this temperature for 20 minutes to remove moisture, the sample was heated to 600 °C at a ramp rate of 30 °C min -1 and kept isothermal for another 20 minutes to remove the volatiles. The gas was then switched to a mixture containing 10 vol% O 2 (balanced by N 2 ) for 30 minutes to combust the carbon.

Trace Element Analysis
The heavy metal content of the solid samples was determined by closed aqua regia microwave digestion followed by ICP-MS. For each sample, 600 μL 67-69 % HNO 3 (trace element grade, Fischer Scientific) and 200 μL of 35-37 % HCl (trace element grade, Fischer Scientific) were added to 20 mg of the solid inside 20 mL MARS Xpress PFA vessels and left at room temperature overnight before being microwaved (MARS Xpress) using an adapted 3051a protocol (for sediments and sludges) whereby the samples were heated to 180 °C and held for 10 min before cooling down. The vessels were then transferred to a fridge for 1 h to aid cooling. After cooling, the digested samples were transferred into plastic volumetric flasks and made up to 10 mL using a 2 % HNO 3 , 0.5 % HCl mix solution (prepared with ultra-pure 18 MΩ Milli-Q® water).
The flasks where then shaken to facilitate mixing and passed through a 0.45 μm Whatman syringe filter. Acid digestion of the ionic liquors were carried out with the same procedure, but with conc. HNO 3 in a 1:10 v/v ratio (100 μL of liquid to 1 mL of HNO 3 ). After dilution, the vessels and volumetric flasks were rinsed with 5 % HNO 3 , wiped with a tissue and submerged in a 5 % HNO 3 storage bath. The diluted samples S6 were analyzed using ICP-MS (Agilent Technologies) under helium and calibrated against solutions prepared from mixing and diluting single-element and multi-element standards (Inorganic Ventures) of known concentrations. The matrix effects of the dilution mixture were corrected using Sc II (for P V , Ni II , Cu II , Zn II ), Ge IV (for As V ), Y III (for Pb II ), and In III (for Cd II ) as the internal standards. A certified reference material, ERM® 144 Sewage Sludge (trace elements), was digested with the same procedure in order to validate the results. The % recovery of the reference materials is shown in Figure   SI3.