Exploring implication of variation in biochar production on geotechnical properties of soil

Biochar produced from the pyrolysis of plant based feedstock has been advocated as an alternative soil amendment for landfill cover. Previous literature indicated that the pyrolysis temperature influences the intra-pore distribution and surface functional groups (especially hydroxyl groups), resulting in “love-hate relationship” of the biochar amended soil (BAS) with water. From the purview of geotechnical engineering, the effect of pyrolysis temperature on geotechnical properties are rarely investigated. In total, three biochar rates (0, 5 and 10%) were considered for a set of geotechnical experiments in sand clay mixture soil with biochar produced at 350 ℃ and 550 ℃. Test results show that biochar addition in soil, in general regardless of pyrolysis temperature, increased the optimum moisture content (OMC), plasticity index, soil water retention characteristics (SWRC) and decreased the maximum dry density (MDD), shear strength parameters (cohesion, friction), erosion rates. Whilst comparing the pyrolysis temperature effects on two biochar amended soils, only marginal effects (in terms of magnitude) on SWRC were observed. The most significant decrease of MDD (or increase of OMC) for 5% (w/w) and 10% (w/w) biochar additions occurred at pyrolysis temperatures of 550 ℃ and 350 ℃, respectively. In addition, biochar produced at lower pyrolysis temperature (350 0C) was more effective in reducing cracks and enhancing shrinkage area ratio. 10% biochar addition with pyrolysis temperature of 350 0C was the optimum combination in resisting soil erosion. The study provides evidence that the geotechnical properties of biochar amended soils for landfill cover soil applications could be tailor made by controlling the pyrolysis temperature

Tree morphology dependent transpiration reduction function of Schefflera arboricola for landfill cover restoration

Changes in hydrological processes and water resources required to sustain vegetation for ecological restoration of landfill covers and post mining sites in arid environments pose challenges in the context of extended droughts. Knowledge of actual threshold and wilting suction values based on tree morphological feature or plant age is essential for understanding the variation of root water uptake with drought stress and numerically predict the pore water pressure profile in root zone. The objective of this study is to quantify the transpiration reduction function (TRF; in terms of stomatal conductance (SC) and xylem sap flow (SF)) of Schefflera arboricola, considering the effects of tree morphology. Continuous drought condition was applied on the plant quantified with leaf area index (LAI) values at 0.5, 2 and 3.5, wherein each LAI represent tree age. The soil matric suction (ψ) and volumetric water content were measured by embedded sensors in the root zone. Based on the TRF obtained from SC values, a unique threshold suction (ψNTRt) ranging from 30 to 50 kPa was identified. Beyond this ψNTRt, measured leaf abscisic acid concentration increased up to 35 ng/mL, indicating the start of water stress avoidance mechanism. It is evident that ψNTRt is independent of tree morphological parameter- leaf area to root length ratio (LA/RL). On the contrary, a threshold suction (ψSAPt), depending on LA/RL ratio, can be determined, indicating the start of xylem cavitation. This ψSAPt values ranging from 80 to 500 kPa depending on the LA/RL value, imply that the plant could significantly resist xylem embolization at higher LA/RL. In contrast, the plant with low LA/RL values have less tolerance of drought stress and hence low survivability. The results from this research study can be vital for devising and predicting plant available water in water scarce arid environments by a flux-based approach which is dependent on the tree age

Hydraulic conductivity variation in compacted bentonite-fly ash mixes under constant volume and free swelling flow conditions

Compacted bentonite-sand (B-S) and bentonite-fly ash (B-FA) are established combinations for the construction of landfill liners. This study determined the upper and lower bounds of equilibrium hydraulic conductivity (keq) of amended bentonite under extended duration of flow. The keq for constant volume flow condition differed from free swelling condition by more than two orders of magnitude due to the difference in geomaterial interaction, microstructural changes, and mineralization. Considering constant volume and free swelling condition, B-FA mix with class F and class C fulfilled the hydraulic conductivity criterion up to 70 % and 30 % amendment, respectively. The higher keq observed for the B mixed with class C FA was attributed to the formation of porous calcium aluminium silicate hydrate gel and ettringite needle type minerals. The time taken to achieve equilibrium was inversely related to keq by a power relationship. The data from this study were used to propose empirical relationships for estimating keq (long-term) based on k obtained at 48 hours (short-term), plasticity and geomaterial type. The study reveals that FA can be used as an alternate for S as amendment material and keq based on free swelling condition should be used for designing the liner.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Influence of plasticity and porewater salinity on shrinkage and water retention characteristics of biochar-engineered clays

Funding Information: The first author is grateful for the support by USDOE (United States Department of Education) GAANN (Graduate Assistance in Areas of National Need). The corresponding author acknowledges the startup research grant provided by the School of Engineering, Aalto University for supporting this research work. Publisher Copyright: © 2023 The Authors. Soil Science Society of America Journal published by Wiley Periodicals LLC on behalf of Soil Science Society of America.Clay-engineered barriers might be subjected to soil salinization issues under climate change. A recently emerged desalinization method is achieved by modifying clays using biochar. However, unsaturated soil responses of biochar-engineered clays in saline environments under drought conditions remain unknown. This study aims to investigate soil shrinkage and water retention characteristics of biochar-amended kaolin and bentonite under saline conditions. Soil shrinkage and water retention tests were conducted on clays (with and without biochar addition) with various porewater salinity (i.e., 0%–10%). Physiochemical properties (including zeta potential and porewater pH) were measured to interpret particle–fluid interactions. Shrinkage characteristics of kaolin and bentonite exhibited sensitivity and insensitivity to the porewater salinity, respectively. This phenomenon was explained by hydrogen-sodium ion exchange and deprotonation phenomenon occurring on kaolin and bentonite, respectively. Biochar significantly alleviated the salinity-induced shrinkage of clays by increasing the shrinkage limit of kaolin and bentonite by 6%–14% and 50%–107%, respectively (p < 0.05). This was attributed to the porous structure and hydrophilic functionality of biochar that immobilized sodium ions through ion exchange and protonation reactions. The air entry value of clays significantly increased with porewater salinity and biochar addition due to the reduction of void ratio and enhanced capillarity, respectively. An empirical equation was proposed to predict the shrinkage limit of clay in various saline conditions. It highlighted that the application of biochar-engineered clays could contribute to the desalination and the improvement of resistance to shrinkage damage in hydro-chemical barriers.Peer reviewe

Investigation of mechanical factor of soil reinforced with four types of fibers: An integrated experimental and extreme learning machine approach

This study investigates and compares mechanical factor (a dimensionless parameter and defined as the ratio of the compressive strength of fiber reinforced soil to that of unreinforced soil) for soils reinforced with four different fibers (three natural fibers and one synthetic fiber). An integrated methodology was utilized, including 351 laboratory experiments for obtaining data and Extreme Learning Machine (ELM) technique for developing functional relationships between mechanical factor and soil and fiber parameters. Soils reinforced with synthetic fiber (Polypropylene) and with natural fibers exhibited different characteristics when subjected to the same variation in soil parameters. This phenomenon can be attributed to the differences in surface morphology and water absorption capability of Polypropylene comparative to other natural fibers. Polypropylene–soil composite shows the maximum sensitivity to the soil moisture. It also shows the least sensitivity toward soil density and fiber content among all tested fiber–soil composites

Microplastics in soil: Current status and evaluation of the greenness of various analytical methods of identification

Microplastics (MPs) have received increasing attention because of their ubiquitous and persisting nature and high degree of resistance to degradation, and bioaccumulation. There has been growing evidence of their direct and indirect impacts on the ecosystem's health. Although the extensive research has been carried out to assess the impact of MPs on aquatic environment, terrestrial environment is a significant research hotspot that needs to be focused more as it is highly polluted by anthropic plastic usage. The overarching aim of this study was to assess the current research trends on MPs analysis and the evaluation of the greenness of existing quantification methods, particularly in soil samples. In the last three years the trend in the quantification of MPs in soil media is found to be exponential and few studies have reported significant concentration of MPs of about 85,000 particles per kg dry weight of soil samples, raising an alarm to the policy makers to implement stringent policies. It has been observed that the existing quantification methodologies are not following the sustainable principles of green analytical chemistry due to their low score obtained from AGREE method. In this context, the electrostatic method of separation of MPs can be considered green, while the magnetic and Olive oil-based separation methods are classified as worse based on the greenness parameter. This paper attempts to provide some new information on the implementation of sustainable methods of MP quantification