Competitive and cooperative sorption between triclosan and methyl triclosan on microplastics and soil.

The sorption behavior of single contaminant on microplastics (MPs) has been extensively studied; however, little is known about that in the more actual scenario containing multiple contaminants. In this study, the interaction between triclosan (TCS) and its primary metabolite, methyl triclosan (MTCS) on polyethylene (PE), polystyrene (PS), and soil was investigated. Results indicate that the more hydrophobic MTCS had much higher sorption capacity and affinity than TCS. Competitive sorption between them occurred in most cases and appeared to be concentration-dependent (in the range of 0.1-5 mg TCS/L and 0.01-≤0.05 mg MTCS/L of primary solutes, respectively): more pronounced at low concentrations of primary solute, while progressively weaker with the increase of concentrations. Among the sorbents, MTCS exhibited strong antagonistic effect on TCS sorption for MPs, especially PS, while significant suppression of MTCS sorption by TCS took place for soil and PS rather than PE. Additionally, it is interesting to observe that the presence of TCS substantially facilitated the sorption of MTCS exclusively at high concentrations on both PS and soil, presumably attributed to the solute-multilayer formation. Furthermore, the magnitude of the two effects varied with solution pH: TCS sorption at alkaline pH was the most suppressed by MTCS because the less hydrophobic dissociated TCS tended to be displaced, and the highest cooperative sorption of MTCS with TCS occurred at acidic pH because neutral TCS preferentially adsorbed on sorbent surface could provide additional sorption sites for MTCS. Both competitive and cooperative effects between multiple contaminants may affect their fate and transport, thereby these findings are helpful for assessing the environmental risk of MPs and TCS in soil.

Comparison of adsorption and desorption of triclosan between microplastics and soil particles.

Microplastic (MP) pollution in soil has been becoming an emerging environmental hot spot, but little is known about the interaction between MPs and chemical contaminants in soil. In this study, batch experiments were performed to study adsorption-desorption behavior and mechanism of triclosan (TCS) on MPs, polyethylene (PE) and polystyrene (PS), and soil particles. PE showed the highest adsorption rate (29.3 mg µg-1 h-1) and equilibrium capacity (1248 µg g-1), while the similar profiles between PS (0.27 mg µg-1 h-1 and 1033 µg g-1, respectively) and soil (0.60 mg µg-1 h-1 and 961 µg g-1, respectively). Two adsorption stages, representing liquid-film and intra-particle diffusion were observed obviously for PE. Adsorption isotherm results revealed that the interaction between MPs and TCS was relatively weak. The sorption potential of soil was lower than that of MPs especially at high concentrations. PE addition induced TCS sorption increase in soil, while PS had no significant (P > 0.05) influence. For MP-soil systems, TCS preferred to adsorb on MPs, which was more pronounced for PE than PS. The desorption rate of TCS was the highest for soil, followed by PE and PS, while equilibrium release amount ranked: PE > PS > soil. Moreover, soil solution better facilitated the desorption, with the amount increasing by 38% for PE compared with 0.01 M CaCl2 solution. Therefore, MPs, especially PE with high adsorption and desorption potentials may serve as a source and carrier to TCS, and its amendment can change TCS environmental behavior and further risk in soil.

Dissipation, transformation and accumulation of triclosan in soil-earthworm system and effects of biosolids application.

Triclosan (TCS), widely used as an antimicrobial ingredient, is usually introduced into soil by biosolids application, and has presented potential risk in agro-ecosystem. The dissipation pathways of TCS in soil were analyzed in the presence and absence of earthworms (including Metaphire guillelmi and Eisenia fetida). Meanwhile the accumulation and transformation potentials of TCS in the two earthworms were evaluated. Results indicated that about 44% of initial TCS amount dissipated in sterile soil after 56-day incubation, which may mainly result from the bound-residues formation. In contrast, TCS in non-sterile soil dissipated more quickly with a t1/2 of 12 days, suggesting that microbial degradation was responsible for TCS dissipation. Triclosan was methylated to methyl triclosan (MTCS) in soil, which however contributed little for TCS dissipation. The presence of M. guillelmi accelerated TCS dissipation with the reduced t1/2 to 8 days, and inhibited MTCS formation in soil, while E. fetida had no significant (P > 0.05) effects on the fate of TCS. E. fetida accumulated more TCS than M. guillelmi, with bioaccumulation factors up to 11 vs. 0.6. It was also proved that methylation metabolism occurred in earthworms (including gut microorganisms), and M. guillelmi had higher metabolic efficiency compared to E. fetida. Even though eliminations of TCS and MTCS were rapid (except for TCS in M. guillelmi), the residues of the two compounds in both earthworms remained at high levels, having the potential to transfer in the terrestrial food web. In addition, results showed that biosolids application changed TCS persistence, as well as bioavailability dependent on earthworm species. When biosolids at 1% added, more residual TCS and MTCS in soil were observed, while TCS accumulation in E. fetida decreased, however, methylation metabolism in both earthworm species was not affected. The findings provide important information for a more precise risk assessment of biosolids land-application. CAPSULE: Triclosan dissipation, methylation and bioavailability in soils were affected by biosolids amendment and dependent on earthworm species with different accumulation and metabolic potentials.