Soil property controls on plasticiser, antioxidant and UV absorber additive degradation across a global soil gradient

Abstract

Additives in agricultural plastics can leach into the surrounding soil during use or improper disposal. Their subsequent degradation rates directly regulate whether they persist and accumulate to levels with ecotoxicological effects or are rendered benign. However, which soil properties primarily regulate the degradation of additives remains unclear (e.g. soil carbon, pH, available nutrients, microbial biomass and community structure). We assessed the degradation of the common plastic additives with different functionalities (DEHP (di(2-ethylhexyl) phthalate; plasticiser), 2-hydroxy-4-n-octyloxybenzophenone (benzophenone-12; BP12; UV stabiliser) and AO168 (tris(2,4-di-tert-butylphenyl) phosphite; antioxidant)) in soils under controlled moisture and temperature conditions over 21 days across contrasting agricultural soils from six countries across a global transect (Australia, Brazil, Egypt, India, Vietnam and the UK). DEHP followed zero-order degradation kinetics, with negligible degradation in soils with low microbial biomass. BP12 degraded fastest via first-order degradation kinetics via ether cleavage and hydroxyl loss. The degradation of DEHP and BP12 was correlated with soil microbial biomass and nitrate concentration. BP12 degradation products detected included benzophenone and benzoic acid. DEHP is degraded via β-oxidation of alkyl groups to dibutyl phthalate and diethyl phthalate and through ester hydrolysis to phthalic acid. AO168 degraded via abiotic oxidation and phosphate ester hydrolysis to 2,4-di-tert-butyl-phenol, and degradation was not well correlated with any measured soil variable. Overall, these results show that the components of additive mixtures leached into soils will degrade at different rates due to varying mechanisms and controls exerted by the soil microbial biomass. Plastic additives have differing potentials to persist in agricultural soils globally, with some likely to accumulate to levels that may impact soil function and pose an ecotoxicological threat to soil biota.

Keywords: Abiotic degradation; Antioxidant; Microbial turnover; Plasticiser; UV absorber.

Fig. 1

Targeted degradation products of A DEHP (Zhu et al. ; Tran et al. 2023), B BP12 (Badia-Fabregat et al. 2012) and C AO168 (Gong et al. 2021)

Fig. 2

Principal component analysis biplot of initial soil properties for soils used in additive degradation experiments. The arrows indicate the variables that had a significant contribution to the PCA analysis, defined as a loading of > 0.5, with the relative contribution indicated by arrow length. Ellipses represent 95% confidence intervals for each country, and individual points indicate replicates (n = 4)

Fig. 3

Degradation of the plastic additives DEHP, AO168 + AO168ox and BP12 over 21 days across a global range of soil types. Values are mean ± SE (n = 4). Note that DEHP degradation in the Indian soil was not determined due to high background levels of this additive at the .

Fig. 4

Degradation products of DEHP (DBP, DEP, DMP and phthalic acid), AO168 + AO168ox (24DTP) and BP12 (benzophenone, benzoic acid) after additive addition. Values are mean ± SE (n = 4). Note that DEHP degradation products in the Indian soil are not presented due to high background levels of this additive at the site, indicated by no y-axis shown. Possible degradation products not detected (outlined in Fig. 1) are also not presented

Fig. 5

Correlations of soil properties and additive degradation rates (k). A Pearson correlation matrix, B correlation network for DEHP with soil properties, and C correlation network for BP12 with soil properties. No correlation network is shown for AO168 + AO168ox as it was not significantly correlated with any soil properties. Significant (p ≤ 0.05) positive and negative correlations are indicated by blue and red circles (A) or lines (B, C), respectively. Grey lines in B and C indicate the correlation between soil parameters was not significant. In B and C, *p ≤ 0.05, **p ≤ 0.01 and ***p ≤ 0.001