REPOSITORY > RESULTS

Abstract

A large body of data now exists demonstrating the influence of micro and nano plastics
(MNPs) on living systems, and their detection has been documented across various
ecosystems, including organisms that form part of the human diet. Nevertheless, up till
now, a substantial portion of that research has been confined to aquatic environments.
This limitation underscores the necessity for more comprehensive studies to elucidate the
full extent of the implications of MNPs across diverse ecological settings. In
this context,
the widespread presence of MNPs in the terrestrial domain has been observed, specifically
in the agricultural ecosystem, raising concerns regarding their impact on crops.
This study investigates the interactions between microplastics generated from
commercial nonbiodegradable polyethylene (PE) and biodegradable polybutylene adipate
terephthalate (PBAT) agricultural mulch films and common agricultural pesticide residues.
It also investigates the sorption/desorption process on microplastics and
the factors influencing this complex phenomenon and the uptake, bioaccumulation, and translocation
of 100 nm polystyrene (PS) nanoplastics within the edible herb Lepidium sativum to
address concerns regarding their entry into the human food chain and impact on crop
health. Moreover, a novel approach for quantifying bioaccumulated nanoplastics is
introduced, utilizing confocal laser scanning microscopy (CLSM) in conjunction with image
analysis.
The study findings show that sorption levels were up to 90% higher when considering
microplastics originating from PE mulch films than pure PE microspheres. In PE
microplastics sourced from mulch films, the percentages of sorption for pesticides in
solutions containing CaCl2 were as follows: for pyridate, 76% and 52%; for fenazaquin, 48
% and 32%; for pyridaben, 45% and 56%; for bifenthrin, 74% and 25%; for etofenprox, 82%
and 54% and pyridalyl, 97% and 29% at concentrations of 5 μg/L and 200 μg/L,
respectively. The process of sorption was notably impacted by the octanol-water partition
coefficient (log Kow) as well as the ionic strength of the medium.
Sorption on PE microplastics followed a pseudo-first-order kinetic model, with R2
values between 0.90 and 0.98, while the best-fit isotherm model was the
Dubinin-Radushkevich model, with R2 values between 0.92 and 0.99. The findings suggest physisorption on the surface through
the filling of micropores, highlighting the significant role
of hydrophobic and electrostatic forces.
The study found that PE mulch films exhibited lower levels of pesticide residue
retention and displayed a higher desorption/release rate [with a median desorption of 71.86
μg/L, approximately 50%] compared to PBAT mulch films. Conversely, PBAT mulch films
retained higher quantities of pesticide residues on their surface and exhibited a significantly
lower desorption rate [median desorption = 24.27 μg/L, around 17%] after application.
The impact of elevated ambient temperatures did not show any notable impact on the final
desorption levels from both PE [median = 65.27 μg/L at 20°C and 74.23 μg/L at 40°C]
and PBAT [median = 24.26 μg/L at 20°C and 24.78 μg/L at 40°C] mulch films.
Nonetheless, it did promote a faster desorption rate in PE films. The desorption process in
PBAT and PE plastic categories exhibited a strong correlation with the logarithm of the
octanol-water partition coefficient (log Kow) value [Spearman’s correlation: 0.857 and 0.837,
respectively, p < 0.05]. Conversely, only a moderate correlation with the acid dissociation
constant (pKa) was noted for PBAT [Spearman’s correlation: 0.478, p <0.05], with no
significant correlation observed for PE. The adsorption of pesticides onto biodegradable
PBAT microplastics was most effectively described by Elovich [R2: 0.937 – 0.959] and
pseudo-second-order kinetics [R2: 0.942–0.987], indicating the presence of chemisorption.
Moreover, Weber Morris plots indicated a multi-step process, while Boyd plots suggest
that film diffusion or chemical bond formation represents the rate-limiting step governing
this phenomenon.
This thesis also confirmed the uptake, translocation, and bioaccumulation of
PS nanoplastics (100 nm) within various parts of the plant Lepidium sativum. The exposure
concentrations ranged from 10 μg/L, considered environmentally realistic, to 100 mg/L.
The accumulation pattern observed in the plant tissues was characterized by the
aggregation of nanoplastics within the intercellular spaces, leading to a heterogeneous
distribution throughout the plant. Nanoplastics were detected in multiple plant regions,
including the root tips, root surface, stele, lateral roots, root hairs, stem vascular bundles,
leaf veins, mesophyll, and even the leaf epidermis, including stomatal sit
es. Analysis of the quantification data revealed that most of the nanoplastic particles were retained within the
plant roots, with the accumulation in stems and leaves representing only 13% to 18% of
the median value observed in roots.
The study findings also indicated a significant reduction in various plant parameters under higher exposure concentrations
(≥50 mg/L), including a decrease in the germination rate (38%), fresh weight (55%), root weight (80%),
root length (60%), shoot weight (51% and 78%), and in the number of lateral roots (28%).
Conversely, exposure to lower environmental concentrations did not significantly impact
plant health.