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