Water retention and hydraulic properties of a natural soil subjected to microplastic contaminations and leachate exposures

Modeling microplastic transport through porous media: Challenges arising from dynamic transport behavior

Modelling microplastic transport through porous media, such as soils and aquifers, is an emerging research topic, where existing hydrogeological models for (reactive) solute and colloid transport have shown limited effectiveness thus far. This perspective article draws upon recent literature to provide a brief overview of key microplastic transport processes, with emphases on less well-understood processes, to propose potential research directions for efficiently modeling microplastic transport through the porous environment. Microplastics are particulate matter with distinct physicochemical properties. Biogeochemical processes and physical interactions with the surrounding environment cause microplastic properties such as material density, geometry, chemical composition, and DLVO interaction parameters to change dynamically, through complex webs of interactions and feedbacks that dynamically affect transport behavior. Furthermore, microplastic material densities, which cluster around that of water, distinguish microplastics from other colloids, with impactful consequences that are often underappreciated. For example, (near-)neutral material densities cause microplastic transport behavior to be highly sensitive to spatio-temporally varying environmental conditions. The dynamic nature of microplastic properties implies that at environmentally relevant large spatio-temporal scales, the complex transport behavior may be effectively intractable to direct physical modeling. Therefore, efficient modeling may require integrating reduced-complexity physics-constrained models, with stochastic or statistical analyses, supported by extensive environmental data.

Potential improvement in the mechanical performance and thermal resistance of geopolymer with appropriate microplastic incorporation: A sustainable solution for recycling and reusing microplastics

The accumulation of microplastics (MPs) has been a major threat to the natural environment and human health. However, incineration and landfilling may not be appropriate for the management of MPs. This paper evaluated the feasibility of incorporating MPs with diverse dimensions (50 to 500 μm) and contents (2.5 % to 10 %) into geopolymer cured under different temperatures (40 and 80 °C). The compressive (fc) and flexural strength (ff) after curing and thermal exposure (200 to 600 °C) were determined. When cured at 40 °C, fc and ff decreased with percentages of MPs incorporated. By contrast, when cured at 80 °C, the addition of 2.5 % MPs increased fc and ff by up to 33 % (from 52.2 to 69.4 MPa) and 18 % (from 8.2 to 9.7 MPa), depending on MPs' sizes. The XRD and TGA results suggested that the observed increases in mechanical properties can be attributed to the formation of more calcium alumino (silicate) hydrates (C-(A)-S-H gels) induced by the incorporation of a small quantity of MPs (2.5 %). The SEM images also showed better adhesion between MPs and geopolymeric products when cured under 80 °C, potentially inhibiting crack development. After being exposed to evaluated temperatures (200 and 400 °C), fc of the specimens with 2.5 % MPs and cured at 80 °C was higher than that without MPs. The fc dropped dramatically due to the degradation of MPs between 400 and 600 °C. The increase in strength and heat resistance (up to 400 °C) of MPs-incorporated geopolymer cured under 80 °C indicated the potential recycling and reuse of MPs for geopolymer materials.

Experimental and simulated microplastics transport in saturated natural sediments: Impact of grain size and particle size

Microplastics (MPs) present in terrestrial environments show potential leaching risk to deeper soil layers and aquifer systems, which threaten soil health and drinking water supply. However, little is known about the environmental fate of MPs in natural sediments. To examine the MPs transport mechanisms in natural sediments, column experiments were conducted using different natural sediments and MPs (10-150 µm) with conservative tracer. Particle breakthrough curves (BTCs) and retention profiles (RPs) were numerically interpreted in HYDRUS-1D using three different models to identify the most plausible deposition mechanism of MPs. Results show that the retention efficiency for a given particle size increased with decreasing grain size, and RPs exacerbated their hyper-exponential shape in finer sediments. Furthermore, the amounts of MPs present in the effluent increased to over 85 % as MPs size decreased to 10-20 µm in both gravel and coarse sand columns, while all larger MPs (125-150 µm) were retained in the coarse sand column. The modeling results suggested that the blocking mechanism becomes more important with increasing particle sizes. In particular, the attachment-detachment without blocking was the most suited parameterization to interpret the movement of small MPs, while a depth-dependent blocking approach was necessary to adequately describe the fate of larger particles.

Effects of microplastic contamination on the hydraulic, water retention, and desiccation crack properties of a natural clay exposed to leachate

Microplastic (MP) can significantly affect soil behaviour and the ecosystem. This paper presents an experimental study to investigate the effects of MP contamination and leachate exposure on the desiccation cracks, hydraulic conductivity, and water retention properties of the natural black clay. The leachate was from a landfill in Australia. The black clay was incorporated with up to 2.0% MPs by weight (w/w) with diverse dimensions and mixed with water/leachate. The measured properties include saturated hydraulic conductivity (ksat), soil-water characteristic curves, moisture evaporation rates, and crack intensity factors. The results suggest that the inclusion of MPs significantly increases ksat, and this increase is more obvious for soils with larger dimensions and contents of MPs, e.g., ksat of the black clay with 2.0% of 500 μm MP increases significantly by 206% (p < 0.05). The black clay exposed to leachate exhibits a slight increase in ksat due to the low viscosity of leachate. The existence of MPs decreases the residual moisture contents and air-entry pressures, and so does the water retention capacity (v/v %) of the black clay. The exposure to leachate increases the air-entry pressures by 6.0%-15.8% of the clay. The evaporation rates increase with the dimensions and concentrations of MPs. The highest evaporation rate (0.96 g/h) can be observed in samples exposed to 2.0% 500 μm MP with water addition. For all samples, the crack intensity factors increase when MP content is between 0.2% and 1.0% and decreases slightly after that. After being exposed to leachate, the evaporation rates and crack intensity factors of the black clay are decreased by 2.4%-12.6% and 3.6%-13.7%, respectively.

Effects of biochar-amended soils as intermediate covers on the physical, mechanical and biochemical behaviour of municipal solid wastes

The effects of biochar-amended soils as landfill covers have been extensively studied in terms of liquid and gas permeability. However, the influences of biochar-amended soils on the performance of municipal solid wastes (MSWs) in bioreactor landfills have not been well understood. This paper investigates the potential application of biochar-amended soils as final and intermediate covers in landfills. The MSWs with biochar-amended soils as final and intermediate covers were recirculated with mature leachate in laboratory-scale bioreactors. The pH, chemical oxygen demand, ammonia and volatile fatty acids (VFAs) concentrations of leachates, mass reduction rates, settlement, methane, and total gas generations of MSWs were investigated. The results indicate that biochar-amended soils as intermediate landfill covers can provide pH-buffer capacity, increase the pH of leachate and decrease the accumulation of VFAs in the early stage of decomposition. The concentration of ammonia in the leachate with biochar-amended soils as intermediate cover is lower than that with natural soils. The application of biochar-amended soils as intermediate and/or final covers increases the biocompression ratios and settlement of MSWs. The application of biochar-amended soils as final cover slightly decreases the methane generation potential (L0). Biochar-amended soils as intermediate covers increase L0 by 10%, and biochar-amended soils as both intermediate and final covers enhance L0 by 25%. The increase in the ammonia removal, settlement, and methane yield indicates the viability of biochar-amended soils as intermediate landfill covers. Further studies can focus on the long-term behaviour of MSWs with soil covers with different biochar amendment rates and particle sizes.