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The reverse of the above process of leaching results in plastics absorbing free additives and other chemicals or acting as a sink for these compounds in the ocean (see Chapter 9; Amelia et al. 2021; Liu et al. 2016; Menéndez‐Pedriza et al. 2020). In fact, the sorption of nonadditives, such as metals and HOCs, to the surface of the polymer can even affect the leaching kinetics of intended additives from a plastic (Kedzierski et al. 2018).

Field studies investigating environmental transport of additives are, therefore, confounded by additives that contain contributions from intentionally added compounds as well as those adsorbed by the plastic from water. Chen et al. (2019) found higher additive concentrations in smaller versus larger marine plastic fragments, a finding opposite to that from Tanaka et al. (2020). The two studies examined the same polymer type, highly weathered PE fragments, from the same general region, central North Pacific, but differed in the additive classes targeted. Chen et al. (2019) measured BPA and APs, which are not common additives in PE, but instead are globally distributed chemicals free from plastics, whereas Tanaka et al. (2020) measured UV stabilizers that are almost always added to PE. The findings contradict because the dominant transport mechanisms were different for the two cases. UV stabilizers were leaching out of, while the BPA and APs were sorbing to the smaller fragments. Both leaching and adsorbing, however, were enhanced by the smaller fragments’ increased surface area‐to‐volume ratio.

Table 2.5 Reported log K _ow values, water solubility, molecular weight, and LD₅₀ values of common plastic additives.

Source:

Chemical name	Abbreviation	Additive class	Log K ow	Water solubility (μg/L)	Molecular weight (g/mol)	D. magna 48 h LD50 (mg/L)
Butyl benzyl phthalate	BBP	Plasticizer	4.70	2.7	312.65	3.24
Di(2‐ethylexyl) phthalate	DEHP	Plasticizer	4.88–7.73	23–340	390.57	0.35
Diethyl phthalate	DEP	Plasticizer	2.54	1 080 000	222.24	86
Diisodecyl phthalate	DiDP	Plasticizer	9.46–10.36	0.0022	446.68	>0.02
Diisononyl phthalate	DiNP	Plasticizer	8.60–9.37	0.023	418.62	>0.06
Di‐n‐butyl phthalate	DnBP	Plasticizer	4.27	11 200 000	278.34	2.99
Di‐n‐octyl phthalate	DnOP	Plasticizer	7.73	22	390.6	>0.16
Tris‐(2‐chloropropyl) phosphate	TCPP	Plasticizer	2.59	1 600 000	327.6	81
Di‐2‐ethylhexyl adipate	DEHA	Plasticizer	8.12	0.53	370.58	>54
Acetyl tributyl citrate	ATBC	Plasticizer	4.92	1700	402.48	5.1
Hexabromocyclododecane	HBCD	Flame retardant	5.07–5.47	2.1–48.8	641.7	146
2,2′,4,4′‐Tetrabromodiphenyl ether	PBDE 47	Flame retardant	6.81	15	485.79	0.00789
2,2′,4,4′,5‐Pentabromodiphenyl ether	PBDE 99	Flame retardant	7.39	9.0	564.7	0.00261
Decabromodiphenyl ether	PBDE 209	Flame retardant	9.97	<0.1	959.2	>2.5
Tetrabromobisphenol A	TBBPA	Flame retardant	4.50	171.00	543.9	0.69
2,2‐bis(bromomethyl)‐1,3‐propanediol	BBMP	Flame retardant	0.85	38 000	261.94	653
Tris‐(2‐chloropropyl) phosphate	TCEP	Flame retardant	2.59	1600	285.48	381
2,6‐ditert‐butyl‐4‐methylphenol	BHT	Antioxidant	5.03	5.7	220.36	0.42a
Pentaerythritol tetrakis(3‐(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)propionate)	Irganox 1010	Antioxidant	19.6	0.0052	1177.67	86b
Octadecyl 3‐(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)propionate	Irganox 1076	Antioxidant	13.8	0.00004	530.9	>100
Tris(2,4‐di‐tert‐butylphenyl)phosphite	Irgafos 168	Antioxidant	15.5	0.0010	646.937	>180b
Triphenyl phosphate	TPP	Antioxidant	4.59	1900	326.3	1.7
2‐Tert‐butyl‐6‐(5‐chlorobenzotriazol‐2‐yl)‐4‐methylphenol	UV326	Light stabilizer	5.55	0.68	315.8	100b
2,4‐Di‐tert‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl)phenol	UV327	Light stabilizer	6.91	0.026	358	N/A
2‐(benzotriazol‐2‐yl)‐4,6‐bis(2‐methylbutan‐2‐yl)phenol	UV328	Light stabilizer	7.25	0.015	351.5	>0.083
Nonylphenol	NP	Multiple	4.48–4.80	4.9	220.35	0.31
Styrene	Styrene	Monomer	5.23–5.64	300	104.15	23
Bisphenol A	BPA	Monomer	3.40	120–300	228.29	11.9
Titanium dioxide	TiO2	Colorant	2.23	1.634	79.87	5.5
Carbon black	CB	Colorant	3.97–5.74	Insoluble	12.011	>5,600b
Basic Red 51	BR51	Colorant	N/A	97.91	279.77	0.10
Cadmium	Cd	Colorant, etc.	N/A	N/A	112.41	0.054
Copper	Cu	Colorant, etc.	N/A	N/A	63.55	0.10
Zinc	Zn	Heat stabilizer, etc.	N/A	N/A	65.4	0.928
Calcium carbonate	CaCO3	Filler	–2.12	1000	100.09	>>>800

Note:

^a Not experimentally derived, rather estimated from ECOSAR program.

^b 24‐hour exposure.

> means the LD50 is above the water solubility; >>> means the LD₅₀ is well above this measured concentration of water hardness in toxicity test water.Red shades indicate >5 log K _ow (concern for bioaccumulation/biomagnification) or LD₅₀ < 1 (concern for acute aquatic toxicity).

These two mechanisms influence the modeling of additive transport, leading to another comparison of seemingly conflicting results. Koelmans et al. (2016) concluded that most plastic additives in the ocean had already reached equilibrium between seawater and plastic debris. This interpretation was based on the estimate that 80–90% of plastic in the ocean has been there for two to four years, much longer than it takes free additives to reach equilibrium between the plastic and water. In contrast, Kwon et al. (2017) concluded that plastic additives intentionally added may not ever attain equilibrium between plastic and water in the marine environment. Both perspectives can be correct – certain additives especially those that have already escaped plastics (e.g. BFRs, APs, BPA) may already be at sorption equilibrium with plastics in the ocean. Incorporating both perspectives into models will make for the most accurate real‐world estimates of plastic additive mass balance and fluxes.