Table 1 Types of NER, related properties, and methods for identification
NER types | |||
|---|---|---|---|
Properties | Strongly sorbed, entrapped (both = sequestered) | Covalently bound | Label incorporation into biomass |
Formation from | Parent substance + transformation products | Parent substance + transformation products | Ultimate degradation and mineralisation |
Evidencing | Identification of parent substance + transformation products | Identification of cleavage products, (highly difficult) | Label in biomarkers |
Formation processes | van der Waals, hydrophobic interactions, ionic/ion–dipole/dipole–dipole forces, π–π interactions, ligand exchange, charge transfer, H-bonds | C–C, C–N, C–O–C, ester bonds (covalent bonds in general) | Microbial degradation |
Stability | Low–high | High | Not relevant |
Release probability of parent or transformation/degradation products | Low–high | Low | Not relevant |
Extraction methods | ‘Mild’–‘harsh’ for parent and transformation products | Cleavage methods for bonds of parent substances and transformation products | Biomarker: PLFA, amino acids, amino sugars |
Analytical method | Typical methods used with radioisotope labelled derivatives include radio-HPLC–UV, radio-TLC, LSC, TopCount, radio-HPLC–MS, oxidative combustion followed by LSC; for additional, structural information, size exclusion chromatography or spectroscopic methods like NMR can be applied. Alternatively to radioisotopes, also stable isotope labeling can be applied with subsequent GC–MS or LC–MS analyses | GC–MS, HPLC–MS, 2D-thin layer chromatography | |
Cleavage methods | Silylation | Hydrolysis by KOH (ester-bonds), BBr3 (ether bonds), RuO4, (C–C bonds), TMAH (tetramethyl-ammonium hydroxide) | Not relevant |
Assessment of remobilization potentials | For both types of NER Physical treatments: simulation of heavy rain events, hot water extraction, freeze/thaw cycling, grinding, wet/dry cycling Chemical treatments: pH changes, long-term Tenax extraction, changes in ionic strength, hydrolysis in the presence of Na18OH or H 182 O Biological treatments: application of oxidative and other enzymes with release potential like peroxidases, laccases, and gluthathione-S-transferases, treatment with soil feeding organisms | Not relevant | |