Bio-Integrated Mycoremediation of Microplastic Contamination in Subsurface Urban Soil Matrixes

Introduction

Urban environments serve as complex sinks for anthropogenic pollutants, among which microplastic contamination is increasingly prevalent in geotechnical profiles. These plastic particles, ranging from fragments to fibres, exhibit physicochemical properties that complicate degradation, leading to persistent environmental burdens upon soil biota and hydrology [1]. While macro-scale remediation strategies exist for surface contamination, the subsurface soil matrix presents a significantly more heterogeneous and less accessible target.

The introduction of fungal mycelia, particularly from white-rot fungi like Pleurotus ostreatus, offers a powerful biological tool due to its extracellular enzymatic machinery capable of degrading recalcitrant organic polymers. Our central research question was whether fungal colonization could accelerate the sequestration or degradation of microplastics embedded within dense, structurally compromised urban topsoil.

Research Objectives

Our primary objectives were threefold: 1) To establish optimal fungal strain viability within compacted soil microcosms; 2) To quantify the mass loss or alteration of $\text{PET}$ fragments over time following fungal inoculation; and 3) To model the kinetic rate of biodegradation relative to soil porosity.

Theoretical Framework and Experimental Design

Bio-remediation hinges on the microbial ability to catalyze undesirable chemical reactions. In this system, the fungal degradation process ($\text{BioDeg}$) involves enzymatic attack on the polymer chains ($\text{P}$):

$$\frac{\text{d}[\text{MP}]}{\text{dt}} = -k_{\text{bio}} [\text{MP}]^n$$

Where $k_{\text{bio}}$ is the pseudo-first-order biodegradation rate constant, and $[\text{MP}]$ represents the concentration of intact microplastic particles.

We established triplicate microcosm units, each containing $1 \text{kg}$ of aged urban topsoil ($\approx 40\% \text{ compaction}$). These microcosms were colonized with cultures of P. ostreatus strains (Strain $\text{XO}-7$ and Strain $\text{YF}-2$). Contamination load was standardized to $0.5\% \text{ w/w}$ $\text{PET}$ fragments ($\text{size range}: 100-500\ \mu\text{m}$). The treatment groups included: Control (No fungus), Fungus Only, and Combined Treatment.

Table 1 details the initial conditions monitored across the treatment groups:

Table 1. Initial Microcosm Conditions

Monitoring was conducted bi-weekly for 90 days, tracking moisture content, respiration rate ($\text{CO}_2$ efflux), and mechanical integrity of the plastic particles via Fourier-Transform Infrared Spectroscopy ($\text{FTIR}$) analysis after extraction.

Results and Observations

Significant differences emerged between the treatment groups, particularly in the Combined Treatment microcosm. The fungal colonization reached near-maximal density within 14 days, indicated by sustained high $\text{CO}_2$ efflux attributable to fungal respiration ($\text{Respiration Rate} \propto \frac{\text{d}[\text{CO}_2]}{\text{dt}}$).

FTIR spectral analysis of the $\text{PET}$ particles showed a statistically significant reduction in characteristic carbonyl ($\text{C}=\text{O}$) stretching peaks and an increase in surface pitting uniformity in the treated samples compared to controls. This suggested hydrolytic or oxidative attack on the polymer backbone, rather than mere physical burial.

The kinetics of mass reduction ($\text{R}$) in the Combined Treatment group followed an approximate pseudo-first-order decay profile, suggesting enzymatic activity was rate-limiting. The fitted expression for the degradation percentage ($\% \text{Deg}$) observed after 90 days was:

$$\% \text{Deg} = 15.4 \pm 2.1\%$$

This translates to a kinetic constant $k_{\text{obs}} = 0.018 \text{ day}^{-1}$ for the most active mycelial strain ($\text{Strain } \text{XO}-7$).

Discussion of Bio-Interface Dynamics

Our findings suggest that fungal mycelia do not merely encapsulate the microplastics; they actively engage with them. The mechanical pressure exerted by growing mycelial hyphae aids in fracturing larger fragments, increasing the effective surface area accessible to hydrolytic enzymes secreted into the soil pore water. This concept of a ‘bio-physical enhancement’ is crucial for scaling up this technology [2].

Soil porosity ($\epsilon$) plays a vital role in mass transfer. In the dense urban soil ($\epsilon_{\text{initial}} = 0.32$), contaminant bioavailability is limited. However, the introduction of fungal biomass contributes to localized biomechanical loosening, effectively transiently increasing $\epsilon_{\text{effective}}$ around the contaminant binding sites. This dynamic shift in pore structure appears to be a prerequisite for successful bio-remediation, as quantified by the relationship: $\text{Mass Transfer Rate} \propto f(\epsilon_{\text{fungal}})$.

Table 2 summarizes the comparison of degradation indicators:

Table 2. Comparison of Degradation Indicators After 90 Days

Conclusion and Future Directions

The synergistic interaction between P. ostreatus mycelial activity and contaminated urban soil provides a viable, albeit slow, pathway for mitigating microplastic accumulation in subsurface profiles. While $15\%$ degradation is modest, it represents a significant initial success indicating chemical weathering initiation rather than simple physical sequestration. Future work must focus on optimizing fungal consortia and nutrient amendments to accelerate $k_{\text{bio}}$ into a practical remediation timeframe, moving towards quantitative models that predict recovery time based on initial contamination load and soil matrix characteristics [3].