Influence of pore fluid salinity on shrinkage and water retention characteristics of biochar amended kaolin for landfill liner application

Flow and contaminant transport dynamics in clay-amended barriers through flushing experiments and multi-porosity-based modeling

Clay-amended barriers are widely used to prevent hazardous leachate percolation from landfill to subsurface. The performance of these barriers is mostly evaluated through numerical simulations with limited experimental investigation through leachate flushing experiments. To bridge this gap, contaminant loading and its flushing experiments were carried out to assess the performance of clay-amended composite materials as landfill liners. River sand (Sa), loamy soil (Ns), and alternative waste materials like fly ash (Fa) and flushed silt (Si) were used to prepare the composites. Composites fulfilling the hydraulic conductivity (<10-7 cm/s) and compressive strength (200 kPa) criteria were selected for contaminant loading and its flushing experiments to understand the fate of fluoride ions. The experimentally determined hydraulic conductivity (Ks) values for all the composites were in the order of 10-8 cm/s. The experimental breakthrough curves exhibited skewed shape, long tailing, and dual peaks. Dual porosity and dual permeability with immobile water models were employed to simulate these curves, revealing that preferential flow pathways and random chemical sorption sites significantly affect solute transport in clay-amended barriers. Further, scanning electron microscopy and energy-dispersive X-ray spectroscopy were employed to trace the preferred path of fluoride ions through the barrier. The removal efficiency and temporal moments were used to determine the percentage mass retained, mean arrival time, and spreading within the barrier. The highest solute mass was retained by sand-clay barrier (SaB30) (91%), followed by loam-clay barrier (NsB30) (59%), fly ash-clay barrier (FaB30) (38%), and silt-clay barrier (SiB30) (4%) with the least mass. The lowest mean arrival time was calculated for NsB30 (269 h) and the highest for SaB30 (990 h), with FaB30 (384 h) and SiB30 (512 h) having values in between. This study concludes that validating the design hypothesis of clay-amended barriers through contaminant loading and its flushing studies leads to an effective and sustainable design.

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.

Shrinkage behavior of compacted bentonite considering physicochemical effects

Understanding the shrinkage behavior of bentonite considering physicochemical effects is important to assess the efficiency of buffer barriers in environmental geotechnical engineering. In this paper, shrinkage experiments were conducted on Na-bentonite specimens prepared with salt solutions at various concentrations. NMR and SEM tests were conducted to study the moisture distribution and structural evolution of specimens during the evaporation of water. After sample saturation, the porosity decreases as the pore water salinity increases due to the decreasing swelling deformation with pore water concentration during the saturation process. During drying, the shrinkage deformation of compacted bentonite is anisotropic, with larger axial strains than radial strains. At the fully dried state, the bentonite specimen prepared with distilled water is the densest due to the least crystalline salts in the specimen. At the microscale, as pore water salinity increases, pore water is distributed to smaller pores, and the microstructure is more aggregated. The saline effect on water retention and distribution is weakened as pore water evaporates, originating from physicochemical effects. The structure is also more aggregated after evaporation of pore water. Theoretically, the shrinkage behavior of Na-bentonite considering the influence of water salinity is well described from the perspective of an effective stress-based constitutive relationship.

Biochar amendment combined with partial root-zone drying irrigation alleviates salinity stress and improves root morphology and water use efficiency in cotton plant

An adsorption experiment and a pot experiment were executed in order to explore the mechanisms by which biochar amendment in combination with reduced irrigation affects sodium and potassium uptake, root morphology, water use efficiency, and salinity tolerance of cotton plants. In the adsorption experiment, ten NaCl concentration gradients (0, 50, 100, 150, 200, 250, 300, 350, 400, and 500 mM) were set for testing isotherm adsorption of Na+ by biochar. It was found that the isotherms of Na+ adsorption by wheat straw biochar (WSP) and softwood biochar (SWP) were in accordance with the Langmuir isotherm model, and the Na+ adsorption ability of WSP (55.20 mg g-1) was superior to that of SWP (47.38 mg g-1). The pot experiment consisted three factors, viz., three biochar amendments (no biochar, WSP, and SWP), three irrigation strategies (deficit irrigation, partial root-zone drying irrigation - PRD, full irrigation), and two NaCl concentrations gradients (0 mM and 200 mM). The findings indicated that salinity stress lowered K+ concentration, root length, root surface area, and root volume (RV), but increased Na+ concentration, root average diameter, and root tissue density. However, biochar amendment decreased Na+ concentration, increased K+ concentration, and improved root morphology. In particular, the combination of WSP and PRD increased K+/Na+ ratio, RV, root weight density, root surface area density, water use efficiency, and partial factor productivity under salt stress, which can be a promising strategy to cope with drought and salinity stress in cotton production.

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

The influences of microplastics (MPs) contamination on soils have been extensively studied recently. Most of previous studies focus on saturated hydraulic conductivities and water retention of loose soils under laboratory conditions. The effects of MPs on the hydraulic properties of compacted soils for engineering purposes have not been well understood. This paper presents the laboratory investigation of water retention capacity, saturated (ksat) and unsaturated (kθ) hydraulic conductivities of a compacted natural soil contaminated by MPs and exposed to fresh, medium-aged, and stabilized leachates. The saturated (kg) and unsaturated air conductivities (kgθ) are calculated. The MPs with maximum particle sizes of 500, 150 and 50 μm were added to soils to obtain samples with mass ratios of 0.5, 1.0, 2.0, and 5.0 %, respectively. Under similar ranges of dry densities, permeation of fresh leachates decreases ksat of the compacted soils by 30 % while exposure to stabilized leachates increases ksat by 10 %, due to the viscosities of liquids. The flow channel properties of the compacted soils contaminated with different sizes and concentrations of MPs vary. The most complex flow channel can be found in samples with 5 % 50 μm MPs. The inclusions of MPs decrease residual moisture contents of the compacted soils regardless of MP sizes and percentages. The effects of MPs on air-entry pressures and parameter n depend on the sizes of MPs. The kθ (kgθ) of compacted soils with MPs depend on the combined effects of ksat (kg) and tortuosity parameter (l). Though l ranges from -0.85 to 2.12 with different levels of MP exposures, it does not have a significant influence on the relative hydraulic (kθ/ksat) and air conductivities (kgθ/kg) of the compacted soils. Future studies can focus on the long-term hydraulic properties of soils under MP contamination.