You picked a nice, dense paragraph to throw at your brain. Let’s unpack it so it’s actually usable.
1. What are they even doing?
They’re looking at DOM (dissolved organic matter) that comes from:
- Microplastics (MPs-DOM) : PE, PET, PBAT, PLA
- Natural DOM (N-DOM): from the environment
Then they shine UV light on these solutions and track how the molecules change:
do they get more oxidized, more saturated, more aromatic, more “lignin-like,” etc.
So it’s:
“What kinds of molecules are there, and how does UV transform them?”
2. First classification: redox + saturation (NOSC and (DBE−O)/C)
They split molecules into 4 groups:
- Unsaturated–oxidized
- Unsaturated–reduced
- Saturated–reduced
- Saturated–oxidized
This is based on a plot of (DBE−O)/C vs NOSC.
NOSC
- NOSC = Nominal Oxidation State of Carbon
- High NOSC → more oxidized carbon (think: lots of C–O, C=O, maybe carboxyls)
- Low NOSC → more reduced (think: alkanes, lipids, hydrocarbons)
So NOSC is basically:
“How oxidized vs reduced are these carbons?”
(DBE − O)/C
- DBE (Double Bond Equivalent) counts:
- double bonds + rings + aromaticity-ish
- (DBE − O)/C ≈ unsaturation per carbon, correcting for oxygens
- High value → more double bonds/aromatic rings = more unsaturated
- Low value → more saturated = more single bonds, aliphatic
So that 2D space lets them say:
- Left/right: oxidized vs reduced (NOSC)
- Up/down: saturated vs unsaturated ((DBE−O)/C)
From that, they define the four groups and track how their relative proportions change.
3. Second classification: van Krevelen diagram “family labels”
Using H/C vs O/C (van Krevelen diagram) they label molecules as:
- Lipid-like: high H/C, low O/C, more hydrocarbon-ish
- Protein-like: contain N, intermediate O/C, typical peptide / amino acid territory
- Carbohydrate-like: high O/C, high H/C, sugar-y
- Unsaturated hydrocarbon-like: low O/C, lower H/C, more C=C stuff
- Lignin-like: aromatic + oxygenated (wood / vascular plant phenolics)
- Tannin-like: highly oxygenated polyphenolic structures
- Condensed aromatic-like: heavily aromatized, “black carbon”-ish
So these are chemical “ecosystem guilds”: functionally analogous families.
4. Big baseline: what MPs-DOM looks like before UV
“MPs-DOM was dominated by reduced molecules (> 65%), by lignin-like (> 30%), and protein-like (> 25%) substances”
Translation:
- Most molecules are reduced (not heavily oxidized).
- Composition is mainly lignin-like and protein-like :
- lignin-like: aromatic, oxygenated phenolic-ish stuff
- protein-like: nitrogen-containing, peptide-like / amino acid-ish
So microplastic-leached DOM is not just simple hydrocarbons; once in water and partially oxidized / leached, it already looks somewhat like “environmental” aromatic + N-containing organic matter.
5. What happens under UV, plastic by plastic
Now the fun part: how things mutate under UV exposure.
5.1 PE-DOM (polyethylene-derived DOM)
Saturated oxidized ↑ by 10.07%
Unsaturated oxidized ↓ by 14.82%
Interpretation:
- Some unsaturated, oxidized molecules lose their C=C double bonds and become more saturated but still oxidized , consistent with:
- addition reactions across double bonds
- Overall composition doesn’t shift massively :
- Most components change less than 10% in relative abundance.
- Exception :
Protein-like substances ↑ by 11.25%, matching an increase in CHON
So in PE-DOM:
- A bit more oxidized & saturated material
- More N-containing / protein-like molecules , possibly from:
- newly formed N-containing species, or
- preferential persistence of CHON vs others under UV
It’s relatively UV-stable compared to the others. Small compositional nudge.
5.2 PET-DOM (polyethylene terephthalate-derived DOM)
Here the system has a meltdown.
Oxidized components ↑ by 57.5%
That is huge. Strong photo-oxidation.
They mention FT-IR evidence: ester linkages in PET are broken, forming:
- Aromatic alcohols
- Phenolic OH
- Quinones
- Ethers
- Carbonyls (various bands)
Translation: the polymer is being chopped and heavily functionalized with O-containing groups (OFGs).
Composition shifts:
- Lignin-like ↑ 11.35%
- Tannin-like ↑ 21.45%
- Protein-like ↓ 22.87%
- Lipid-like ↓ 22.17%
So PET-DOM under UV:
- Becomes much more oxidized
- Shifts toward aromatic, polyphenolic, plant-like signatures (lignin/tannin-like)
- Loses lipid-like and protein-like signatures
Intuition: PET degradation produces aromatic oxygenated fragments that look more like plant phenolics than like simple lipids or proteins.
5.3 PBAT-DOM
PBAT is an aliphatic–aromatic copolyester. Under UV:
Saturated & unsaturated reduced compounds ↓
Saturated oxidized compounds ↑ by 33.75%
Again, strong oxidation: reduced stuff gets converted to oxidized saturated molecules.
FT-IR shows formation of OFGs:
- Phenolic OH
- Quinone
- Acetyl
- Carbonyl
- Carboxyl
Mechanisms listed: hydrolysis , Norrish reactions , photoreactions
That’s textbook polyester photodegradation.
Composition shift:
- Lignin-like ↓ 18.08%
- Carbohydrate-like ↑ 9.27%
- Tannin-like ↑ 9.23%
- Others fluctuate within ±10%
So PBAT-DOM after UV:
- Becomes more oxidized
- Slight move from lignin-like to more carbohydrate-like / tannin-like material
- Generally more O-rich, polarizable stuff
5.4 PLA-DOM (polylactic acid-derived DOM)
This one goes hardest.
Reduced components ↓
Saturated oxidized compounds ↑ by 60.89%
Massive oxidation. Hydrolizable polyester, so not surprising.
They infer from FT-IR formation of:
- Hydroxyl
- Ether
- Acetyl
- Carbonyl
- Carboxyl
Mechanisms: hydrolysis + photooxidation , again.
Composition-wise:
- Before UV:
- Lignin-like 48.85%
- Protein-like 30.95%
- After UV (main composition):
- Tannin-like 41.90%
- Carbohydrate-like 25.14%
- Lignin-like 20.11%
So PLA-DOM:
- Goes from lignin/protein-like to tannin + carbohydrate-like dominance
- Strong shift to highly oxygenated, polyphenolic, and sugar-like fragments
Net: PLA leachates become very oxidized, polar, and “biomolecule-mimicking” .
5.5 N-DOM (natural DOM)
This acts differently, which is the point.
Saturated oxidized compounds ↓ by 29.92%
Saturated reduced compounds ↑
So under UV:
- Natural DOM gets more reduced on average, or at least:
- Oxidized saturates are being decomposed / mineralized
- Reduced material becomes relatively more dominant
Also:
- N-DOM is dominated by lignin-like (~60%)
- All components vary less than 10%
So N-DOM is:
- Structurally more stable in its composition under these UV conditions
- Changes are much smaller compared to microplastic-derived DOM
Different starting material, different transformation pathway.
6. Summary of compositional “personality” by group
For the photo-resistant fraction (the stuff that survives UV):
- PE-DOM, PET-DOM, PBAT-DOM
- Dominated by lignin-like + protein-like compounds
- Together > 80%
- PLA-DOM
- Also has lignin-like + protein-like
- But tannin-like is a big third component (~22.34%)
- N-DOM
- Dominated by lignin-like (62.57%) , tannin-like (19.5%) , and condensed aromatic-like (15.26%)
So natural DOM has more condensed aromatic material and tannin-like, while microplastic DOM has more protein-like and somewhat different oxidation & saturation behavior.
7. Big conceptual takeaways
- Microplastic-derived DOM is initially mostly reduced and “bio-ish”
- Lots of lignin-like and protein-like molecules.
- UV transforms MPs-DOM differently depending on polymer:
- PE: modest change, bit more saturated-oxidized, more CHON / protein-like.
- PET: massive oxidation, aromatic OFGs, shift to lignin/tannin-like.
- PBAT: oxidation, more tannin- and carbohydrate-like, less lignin-like.
- PLA: extreme oxidation; big shift toward tannin- and carbohydrate-like material.
- Natural DOM behaves differently under the same UV:
- Oxidized saturates decrease.
- Composition is much more stable.
- Translation for reality:
- Microplastics are not just little inert beads; they leach DOM that:
- Becomes highly oxidized under sunlight
- Acquires O-rich functional groups and aromatic / polyphenolic character
- This can change:
- Reactivity
- Interactions with metals, pollutants, microbes
- Optical properties of water, etc.
So the whole paragraph is basically:
“We categorized molecules by oxidation and unsaturation, and by biochemical type. UV light pushes MPs-derived DOM toward more oxidized, O-functionalized, lignin/tannin/carbohydrate-like material, in patterns that depend strongly on polymer type, and these trajectories are not the same as natural DOM under UV.”
There. Now you don’t have to keep rereading that paragraph ten times pretending it’s clear.
