Reprocessed construction and demolition waste as an adsorbent: An appraisal.

Construction and Demolition (C&D) waste is solid wastes generated from the construction, demolition, and renovation activities that constitute almost 30-40 % of globally generated solid wastes. Improper disposal and management of these materials can cause negative impacts on the environment, economy, and human health. Most research on C&D waste is limited to reduction, recycling, and reuse of the wastes. However, there is no systematic review dedicated entirely to the applicability of C&D wastes as adsorbent for waste management. This review presents the utilization of C&D wastes-based adsorbents for removing contaminants from environmental matrices covering triple edge benefits in the viewpoints of waste treatment, solid waste management, and disposal. The properties, the capability of C&D waste adsorbents on contaminant removal, and the influence of various factors on the adsorptive removal is detailed. Further, the mechanisms involved in contaminant removal by C&D waste are summarized. The review revealed that, chemisorption is the prominent mechanism of contaminant removal by most C&D wastes. Among the three types of C&D waste reviewed; concrete-based adsorbents were the most efficient for contaminant removal. Limited studies are avaiable in the literature on binary and multiple contaminant systems, reusability studies, and high dependence on solution pH, therefore further studies are warrated. As C&D waste contain trace concentration of heavy metals and contaminants, its leaching potential at different pH levels and adsorbate concentration need to be conducted, which has been hitherto neglected. Finally, the approaches, obstacles, and potential solutions to build an industrially and economically efficient C&D adsorbent are discussed.

Soil acidification and the liming potential of biochar.

Soil acidification in managed ecosystems such as agricultural lands principally results from the increased releasing of protons (H+) from the transformation reactions of carbon (C), nitrogen (N) and sulphur (S) containing compounds. The incorporation of liming materials can neutralize the protons released, hence reducing soil acidity and its adverse impacts to the soil environment, food security, and human health. Biochar derived from organic residues is becoming a source of carbon input to soil and provides multifunctional values. Biochar can be alkaline in nature, with the level of alkalinity dependent upon the feedstock and processing conditions. This review covers the fundamental aspects of soil acidification and of the use of biochar to address constraints related to acidic soil. Biochar is increasingly considered as an effective soil amendment for reducing soil acidity owing to its liming potential, thereby enhancing soil fertility and productivity in acid soils. The ameliorant effect on acid soils is mainly because of the dissolution of carbonates, (hydro)-oxides of the ash fraction of biochar and potential use by microorganisms.

Seepage characteristics of three-layered landfill cover system constituting fly-ash under extreme ponding condition.

A multi-layered final cover system is constructed over the landfill after it reaches its full capacity to minimize water ingress into the underlying hazardous waste. Three layered landfill cover are designed for areas experiencing very humid climatic conditions. Under the effects of climate change, the occurrences of extreme rainfall events become more frequent and this has resulted in catastrophic floods and hence extreme ponding. This study investigates the seepage characteristics of three-layered capillary barrier cover systems under an extreme ponding condition of 1.5 m water head, through detailed laboratory column tests and finite-element seepage analysis. Four 1.2 m-tall columns having different configurations (C1-C4) were studied. Fly ash (FA) was used to amend the surface and barrier layers in columns C2 and C4, in line with the novel concept of "waste protect waste". Spatiotemporal variations of volumetric water content of the four columns were monitored for three years continuously. With FA amendment in the surface layer and an inclusion of a 0.01 m thick geosynthetic clay liner between the drainage and barrier layers, the onset of basal percolation was significantly delayed until 700 days of ponding, compared to 115 days without FA amendment. Capillary flow dominated the gravitational flow and perched water table was formed as waterfront advanced from the drainage to barrier layers. Further seepage analysis considering a realistic humid climate boundary condition showed that all four configurations were successful in preventing basal percolation for 800 days.

A relook into plant wilting: observational evidence based on unsaturated soil-plant-photosynthesis interaction.

Permanent wilting point (PWP) is generally used to ascertain plant resistance against abiotic drought stress and designated as the soil water content (θ) corresponding to soil suction (ψ) at 1500 kPa obtained from the soil water retention curve. Determination of PWP based on only pre-assumed ψ may not represent true wilting condition for soils with contrasting water retention abilities. In addition to ψ, there is a need to explore significance of additional plant parameters (i.e., stomatal conductance and photosynthetic status) in determining PWP. This study introduces a new framework for determining PWP by integrating plant leaf response and ψ during drought. Axonopus compressus were grown in two distinct textured soils (clayey loam and silty sand), after which drought was initiated till wilting. Thereafter, ψ and θ within the root zone were measured along with corresponding leaf stomatal conductance and photosynthetic status. It was found that coarse textured silty sand causes wilting at much lower ψ (≈ 300 kPa) than clayey loam (≈ 1600 kPa). Plant response to drought was dependent on the relative porosity and mineralogy of the soil, which governs the ease at which roots can grow, assimilate soil O2, and uptake water. For clay loam, the held water within the soil matrix does not facilitate easy root water uptake by relatively coarse root morphology. Contrastingly, fine root hair formation in silty sand facilitated higher plant water uptake and doubled the plant survival time.

A feasibility study of Indian fly ash-bentonite as an alternative adsorbent composite to sand-bentonite mixes in landfill liner.

Multi-layered engineered landfill consists of the bottom liner layer (mainly bentonite clay (B)) upon which the hazardous wastes are dumped. In current practice, sand (S) is mixed with bentonite to mitigate the adverse effects of using bentonite alone in the liner layer. Incorporation of waste and unutilized fly ash (FA) as an amendment material to B has been explored in terms of its hydro-mechanical properties, but not gauged its adsorption potential. Indian subcontinent primarily relies on the thermal power source, and FA dumps have already reached its full capacity. The objective of this study is to explore the adsorption characteristics of four B-FA composite mixes sourced within India, considering Pb2+ as a model contaminant. The effect of fly ash type, fly ash amendment rate and adsorbate concentration was explored in the current study and juxtaposed with B-S mixes, based on 960 batch adsorption tests. Both B-FA and B-S mixes reached equilibrium adsorption capacity within 65 min. At higher adsorbate concentrations (commonly observed in the liner), B-FA mixes exhibited superior adsorption capacity, mainly one mixed with Neyvelli fly ash (NFA). The effect of higher amendment rate had little impact on the adsorption capacity at different concentration, but gradually decreased the percentage removal of Pb2+. The B-S mix showed a drastic decrease in percentage removal at higher adsorbate concentration among all tested mixes. Systematic characterization including geotechnical properties, microstructure and chemical analysis was also done to interpret the obtained results. Both Freundlich and Langmuir models fitted the isotherm data well for all B-FA mixes. The maximum adsorption capacity from the isotherm was correlated to easily measurable Atterberg limits by two empirical relationships.

Soil desiccation cracking and its characterization in vegetated soil: A perspective review.

The formation and propagation of surface desiccation cracks in vegetated infrastructures involve coupled factors including unsaturated soil mechanics, atmospheric conditions and vegetation parameters. Vegetation induces a "Love-hate" relationship in the development of desiccation cracks due to plant induced suction as well as root reinforcement. The objective of the paper is to provide a state-of-the-art that comprehensively reviews the desiccation process in context of the soil-water-plant interaction together. At first, basic theories of crack initiation and propagation in literature are discussed in the context of unsaturated soil mechanics. Thereafter, influence of vegetation on soil cracking is discussed systematically based on transpiration induced suction, root reinforcement, plantation strategy, root exudate and basic plant traits. Intrusive and non-intrusive measurement approaches of desiccation cracks including lab and field studies are put forward. Various schools of desiccation models have been briefly touched upon. More than 150 studies on desiccation cracks have been tabulated in this review, considering soil types, vegetation cover, drying-wetting cycles, approaches of characterizing cracks, sample size, crack pattern, hydraulic conductivity and water retention. Future scopes involving measurement considerations, usage of geotechnical centrifuge modelling, bio-amendments and plant effects on desiccation cracking have been put forward.

Adsorption characteristics of Barmer bentonite for hazardous waste containment application.

Low hydraulic conductivity and high chemical immobilization are the two characteristics that make bentonite a mandatory construction material for hazardous waste containment applications. We performed a comprehensive batch sorption study on Barmer bentonite (BB), an exclusive construction clay mined in India, using lead (Pb2+) as a model contaminant. The maximum adsorption capacity of BB was obtained as 55 mg g-1 at pH 5 and 27 ±â€¯2℃. Adsorption was extremely rapid, with equilibrium attained <5 min for the BB. Increased adsorbent dosage resulted in higher Pb2+ percentage removal, while adsorption capacity decreased. Ionic strength, salt concentration, valency and ionic radius played a critical role in suppressing the adsorption of Pb2+. Clay fabric change was observed to be dispersed at low ionic strength and gradually attained aggregated face-to-face structures at high ionic strength. The simultaneous presence of other metals/salts strongly influenced Pb2+ removal by BB, while divalent salt exhibited high suppression of adsorptive reaction at low concentrations. Sorption isotherm and kinetic modeling results indicated the possibility of chemisorption of Pb2+ on BB. Based on the thermodynamic analysis, it was noted that Pb2+ adsorption on BB is exothermic, spontaneous and adsorption reaction is less favorable at a higher temperature.

Two-year evaluation of hydraulic properties of biochar-amended vegetated soil for application in landfill cover system.

Landfill cover should ideally have a medium with high water retention ability and low hydraulic conductivity to prevent rainfall infiltrating into the hazardous waste layer. Even though biochar amended soil (BAS) is advocated as cover medium, the interactions between biochar and plant, as well as the effects of biochar aging and plant growth on soil hydraulic properties are still not clear. This study aims to investigate the effects of grass (Cynodon dactylon) growth in BAS on soil water retention and saturated hydraulic conductivity (ks) over a two-year period. Four ground conditions were tested, namely bare silty sand with and without biochar, vegetated silty sand with and without biochar. The biochar content was kept at 10% (v/v). During the first 6 months, soil water content corresponding to field capacity (FC) and permanent wilting point (PWP) in grassed soil increased by 17% and 27%, respectively. With biochar inclusion, 43% and 57% additional increases in FC and PWP respectively were observed. Moreover, ks in biochar-amended grassed soil decreased by 50%. Furthermore, grass growth from 6 to 24 months reduced FC by 32%, PWP by 40% but caused 20 times increase in ks of grassed soil. With the presence of biochar, FC and PWP decreased by only 6% and 8%, respectively, and ks increased by 200% due to the enhanced plant growth (specifically root growth) by biochar. After two years, ks of grassed soil with biochar was 16 times smaller than that without biochar. This study demonstrated the effectiveness of biochar in maintaining the enhanced soil water retention ability and reduced ks in vegetated soil over a two-year study period.

Long-term hydraulic performance of landfill cover system in extreme humid region: Field monitoring and numerical approach.

Hazardous wastes disposed of in engineered landfills interact with rainwater, generate harmful leachate and may contaminate groundwater. To minimize this, a suitable multi-layered cover system (MLCS) is constructed over the buried waste. Field assessment of complex moisture dynamics in unsaturated MLCS and its long-term hydraulic efficiency has not been investigated in detail for extremely humid conditions (annual rainfall >1000 mm). Therefore, the overarching purpose of this study was to investigate the long-term hydraulic performance of a three-layered hydraulic barrier cover system under humid Indian conditions. The field cover setup was exposed to natural weather condition in the Northeast Indian state of Assam, for 800 days. The MLCS was instrumented to measure continuous variation of volumetric water content and matric suction as function of time and depth. The field measurements were used to determine the appropriate input hydraulic parameters and evapotranspiration model that can be used for numerical modeling. The results showed that simulation using drying van Genuchten soil-water characteristic curve parameters and Penman-Monteith evapotranspiration model matched the field observations. Events of the highest precipitation and extreme drought (cause for desiccation) did not lead to percolation in the drainage layer (60 cm) and barrier layer (100 cm). Numerical analyses performed for 87 years by considering the climate data of two different humid locations (Eastern and Western part) of India revealed that the progressive saturation of barrier layer occurred within 18 to 20 years. However, when geosynthetic clay liner was incorporated as additional barrier material, the saturation time increased by two-fold (42 to 44 years).

Erodibility assessment of compacted biochar amended soil for geo-environmental applications.

Biochar amended soil (BAS) has been explored as a cover material for geo-environmental applications such as landfill cover due to its vegetation potential. Soil erosion in these infrastructures can progressively lead to failure and hamper the workability of the system. BAS is compacted for geo-environmental applications, unlike agricultural soil, which are loose in nature. Furthermore, the love-hate relationship of biochar with water can potentially affect the functioning of compacted cover system. Thus, the performance of compacted BAS in the context of erosion potential is not well understood. The major objective of this technical note was to explore the erosion potential of compacted BAS sourced from four distinct biochars. Biochar were produced in-house and mixed with soil at 5% and 10% by weight. In total, 81 pinhole erosion tests were performed to gauge the erosion rate of bare soil and BAS at three different compaction states at same compaction energy. It was revealed that the erosion rate decreased with gradual increment in water content for BAS, which was mainly attributed to the change of particle orientation from flocculated to dispersed along the compaction curve. Addition of biochar to soil resulted in decrease of erosion along the dry state whereas the opposite was observed for wet state. This was attributed to the surface functional groups as well as particle gradation of biochar. Erodibility coefficient and critical shear stress plot of soil and BAS revealed that addition of biochar had minimal effect on erosion of compacted silty sand.

Investigation of cracking and water availability of soil-biochar composite synthesized from invasive weed water hyacinth.

Water hyacinth (WH), is one of the world's most intractable and invasive weed species. Recent studies explored the efficacy of this species as a biochar (BC) in improving soil fertility and metal adsorption. However, the soil water retention (SWR) property and crack potential of soil-WH biochar composite has still not been studied. The major objective of this study is to investigate the SWR property and corresponding crack intensity factor (CIF) for compacted soil-WH BC composites. Soil-WH BC composites at five percentages (0, 2, 5, 10 and 15) was compacted and soil parameters such as suction (ψ), water content and CIF were simultaneously monitored for 63 days (including 9 drying-wetting cycles). Results showed that soil-WH BC composite at all percentages retains more water (max. 19% and min. 6.53%) than bare soil at both saturated and drought conditions. Gradual inclusion of WH BC to soil decreases the CIF potential from 7% to 2.8%.