Yttrium Residues in MWCNT Enable Assessment of MWCNT Removal during Wastewater Treatment

Abstract

Many analytical techniques have limited sensitivity to quantify multi-walled carbon nanotubes (MWCNTs) at environmentally relevant exposure concentrations in wastewaters. We found that trace metals (e.g., Y, Co, Fe) used in MWCNT synthesis correlated with MWCNT concentrations. Because of low background yttrium (Y) concentrations in wastewater, Y was used to track MWCNT removal by wastewater biomass. Transmission electron microscopy (TEM) imaging and dissolution studies indicated that the residual trace metals were strongly embedded within the MWCNTs. For our specific MWCNT, Y concentration in MWCNTs was 76 µg g^-1, and single particle mode inductively coupled plasma mass spectrometry (spICP-MS) was shown viable to detect Y-associated MWCNTs. The detection limit of the specific MWCNTs was 0.82 µg L^-1 using Y as a surrogate, compared with >100 µg L^-1 for other techniques applied for MWCNT quantification in wastewater biomass. MWCNT removal at wastewater treatment plants (WWTPs) was assessed by dosing MWCNTs (100 µg L^-1) in water containing a range of biomass concentrations obtained from wastewater return activated sludge (RAS) collected from a local WWTP. Using high volume to surface area reactors (to limit artifacts of MWCNT loss due to adsorption to vessel walls) and adding 5 g L^-1 of total suspended solids (TSS) of RAS (3-h mixing) reduced the MWCNT concentrations from 100 µg L^-1 to 2 µg L^-1. The results provide an environmentally relevant insight into the fate of MWCNTs across their end of life cycle and aid in regulatory permits that require estimates of engineered nanomaterial removal at WWTPs upon accidental release into sewers from manufacturing facilities.

Keywords: ICP-MS; MWCNTs; RAS; spICP-MS; wastewater treatment; yttrium.

Figure 1

Representative linear sensitivity of trace metal residuals (Y, Zr, Co, Ni, Mo, Zn) using ICP-MS for a range of MWCNT concentrations dosed into water. The ratio of net signal to concentration is the sensitivity of each element (see Table 1).

Figure 2

Transmission electron microscopy (TEM) imaging of 5% MWCNT stock solution and corresponding energy dispersive X-ray spectroscopy (EDAX) of highlighted TEM images. Dense (darker) regions on MWCNTs are residual trace metals remaining after MWCNT synthesis.

Figure 3

Yttrium single particle mode inductively coupled plasma mass spectrometry (spICP-MS) data of an (A) undissolved initial MWCNT sample, (B) undissolved MWCNT permeate, and (C) 2% HNO₃ dissolved MWCNT permeate passed through a 30 kDa ultrafilter, showing that Y did not pass through the ultrafilter unless acidified and dissolved by 2% HNO₃.

Figure 4

(A) TEM image of MWCNT in 1 mM NaHCO₃ matrix solution after initial spike (<1 h), (B) TEM image of MWCNT in 2% HNO₃ matrix solution after initial spike (<1 h), (C) EDAX of MWCNT from image B, (D) TEM image of MWCNT in 1 mM NaHCO₃ matrix solution after 24 h, (E) TEM image of MWCNT in 2% HNO₃ matrix solution after 24 h, (F) EDAX of MWCNT from Image E.

Figure 5

Single particle ICP-MS detection using ⁸⁹Y of MWCNTs in a solution across a range of concentrations plus a control (blank) sample.

Figure 6

Effect of different vessels on MWCNTs removal by return activated sludge.