Long term leaching behavior of arsenic from cemented paste backfill made of construction and demolition waste: Experimental and numerical simulation studies

Investigation on the leaching behavior of natural aggregates using percolation test and total content

In the construction industry, environmental behavior of aggregates has been monitored thanks to leaching tests, especially for alternative aggregates obtained from waste (e.g., construction and demolition waste, MSWI). Few studies were carried on the leaching behavior of natural aggregates, which are often not regulated for their substance release in most EU member states (as France). Leachable content of some heavy metals, halides, and sulfates on natural aggregates was investigated using up-flow percolation test EN 16637-3 and compared to threshold values. Only three samples (NS2, NG1, and NG8) show one element which exceeded threshold values (As, Zn, As, respectively), among the 19 natural aggregates tested for leaching. In this study, three natural aggregates (NG1, NS1, NS2) have been chosen because of their measurable leaching values. Total content was obtained through acid digestion. Influence of grain size on leaching results was investigated. Predominant release mechanisms were determined using EN 16637-3 - Annex D, based on percolation results such as pH, electrical conductivity, and leached content, and were then discussed. Detailed results for releases of As, Ba, Ni, Zn, SO42-, and F- were investigated. EN 16637-3 - Annex D shows some limits, especially for trace elements. The pH was found to be one of the most important factors influencing leaching release of most elements, being more important than grain size. By comparing total content with released quantities, it has been shown that As and Mo in NS2 are easily leached, hence present in a very soluble chemical form. Determining release mechanisms accurately in this study seems only possible for elements present in significant amounts.

Research on long-term migration behaviors of heavy metals after close-distance coal seam backfill mining

The backfill mining of coal-based solid waste in goaf poses a potential risk of heavy metal pollution to the groundwater environment, and the migration behavior of heavy metals differs significantly under the disturbance of backfill mining in close-distance multi-layer coal seams and single-layer coal seams. In this study, a migration model of heavy metals after solid backfilling in the goaf of shallow-buried close-distance thick coal seams was established, and the impact of the overburden damage and the layered distribution of the filling body on the long-term migration behavior of heavy metals were analyzed. The results show that the migration of heavy metals after close-distance coal seam backfill mining exhibits a higher risk of heavy metal pollution. The peak permeability of overburden after close-distance coal seam backfill mining is about 600 × 10-19 m2 higher than that after single-layer coal seam backfill mining. The migration distance of heavy metals in the floor after backfill mining of close-distance coal seams is 7.41 m farther than that of single-layer coal seam backfill mining, and its migration time of heavy metals to the surface is 27 a earlier than that of single-layer coal seam. This research provides theoretical and empirical support for the ecological risk assessment and heavy metal pollution control in close-distance coal seam backfill mining. ENVIRONMENTAL IMPLICATION: The main filling material of close-distance coal seams backfill mining is coal gangue. Heavy metal elements such as Mn and Cr will be released in the underground environment for a long time, and the migration behavior of heavy metal elements will have an impact on the groundwater environment for more than 1000 years. This research provides theoretical and empirical support for the ecological risk assessment of close-distance coal seam backfill mining and the mitigation of heavy metal pollution.

Long-term leaching characteristics of heavy metals from bauxite tailing slurry-based geopolymer backfill: experimental and numerical simulation studies

This study aimed to evaluate the potential of replacing fly ash (FA) with bauxite tailing (BT) slurry for geopolymer synthesis and investigate the long-term leaching behaviour of BT slurry/FA geopolymers (BFGs) for heavy metal immobilisation. The mechanical properties and heavy metal immobilisation efficiency of BFGs were tested, and numerical simulations were conducted to assess their environmental impact as a backfill material. The results showed that the incorporation of 5 Wt.% BT increased the early compressive strength of the geopolymer without any additional treatment. A small quantity of Cu2+ improved the mechanical strength, while excess heavy metals harmed the geopolymer. Heavy metal immobilisation efficiency decreased with increased heavy metal addition and exceeded 99.9% for Pb2+ and Cu2+ when simulating acid rain leachate. The modified Elovich equation described the leaching kinetics of Cu2+ well, and the leaching rate decreased with time. Numerical analysis indicated that Cu2+ leaching from landfill leachate occurred in three phases, with an initial increase followed by a gradual decrease, stabilisation, and diffusion into the surrounding soil layer. This study provides insight into the material's long-term stability and environmental performance, offering a scientific basis for relevant engineering applications.HighlightsDirect utilisation of unprocessed tailing slurry to synthesise geopolymer.The leaching pattern of Pb2+ and Cu2+ under acidic conditions was explored.The modified Elovich equation effectively describes the leaching kinetics of Cu2+.The environmental impact of bauxite tailings slurry-based geopolymers was evaluated.

Preventing water inrush hazards in coal mines by coal gangue backfilling in gobs: influences of the particle size and stress on seepage characteristics

The backfilling mining method that fills gobs with coal gangue can prevent water inrush hazards, protect groundwater resources, and protect the ecological environment of the mining area. However, initial conditions including the particle size distribution of gangue and the stress environment may affect the seepage characteristics of gangue backfill and inrush prevention ability. Taking the particle size and stress as main controlling factors, the seepage tests were designed for gangue to evaluate influences of the particle size and stress on the void ratio, permeability, and non-Darcian flow factor of gangue. In the meantime, the four stages in dynamic changes of seepage channels were studied and the impervious envelope lines of gangue backfill materials were provided. The results show that the larger the particle sizes, the stronger the crushing resistance of particles; under high stress (> 6.67 MPa), seepage channels in small gangue particles (< 5 mm) change in a more complex manner, and the non-Darcian flow phenomena become more significant. The particle size and stress exert significant influences on the seepage characteristics. Therefore, when reducing water inrush hazards by gangue backfilling in gobs, the particle size distribution should be optimized by combining the stress and water pressure conditions. Seepage channels in gangue backfill materials vary with changes in the particle size and stress. Their variation can be divided into four stages: shrinkage of seepage channels, reconstruction of seepage channels, dynamic equilibrium between slight expansion and shrinkage, and persistence of the impervious effect. After the first and second stages have been fully developed, the preliminary impervious conditions are met; after full development of the fourth stage, the gangue backfill materials reach an impervious state.

Characterization of contaminant leaching from asphalt pavements: A critical review of measurement methods, reclaimed asphalt pavement, porous asphalt, and waste-modified asphalt mixtures

In recent years, the pavement industry has been seeking sustainable development through recycling reclaimed asphalt pavement and reusing other waste materials as replacements for asphalt mixture constituents. Incorporating waste material into asphalt mixture and the presence of pollutants such as exhaust fumes and gasoline due to vehicle traffic may lead to contaminants leaching from asphalt pavements to underlying soil layers and groundwater aquifers, posing serious risks to ecosystems and the environment. To cast light on contaminant leaching from asphalt pavements, this article presents a comprehensive review of the literature that is divided into four research areas: evaluation of leaching measurement methods, leaching from recycled asphalt materials, leaching characteristics of porous asphalt pavements, and waste-modified asphalt mixtures. Moreover, a critical discussion of bibliometric data, literature content and knowledge gaps in this domain is provided to help highway agencies and environmental scientists address contaminant leaching from asphalt pavements. Finally, some potential research directions are suggested for future research works.

The Energy Dissipation Mechanism and Damage Constitutive Model of Roof–CPB–Floor (RCF) Layered Composite Materials

The stability of composite material that is composed of roof rock, cemented paste backfill (CPB), and floor rock has an important impact on safe mining within metal mines. In order to explore the mechanical properties, acoustic emission (AE), energy dissipation, and damage evolution of roof–CPB–floor (RCF) layered composite materials, uniaxial compression (loading rate 0.02 mm/min) AE tests on RCF materials with different CPB height ratios were performed. The test results show that: (1) the uniaxial compressive strength (UCS) and elastic modulus (ER) of the RCF material were lower than those of the roof or floor rock and higher than that of the CPB. With the increase in the CPB’s height ratio from 0.2 to 0.7, the UCS and the ER decreased from 18.42 MPa to 10.08 MPa and 3.15 GPa to 1.79 GPa, respectively, and the peak strain first decreased from 0.695 to 0.510 and then increased from 0.510 to 0.595. The UCS increased as a polynomial function with the increase in the ER. (2) The AE ring count first increased slowly, then increased rapidly, and finally maintained a high-speed increase. The AE cumulative ring count at the peak point decreased with the increase in the CPB height ratio. The energy dissipation showed that the elastic energy UE accumulated slowly at first, then the dissipated energy UD increased, and finally the UE decreased and the UD increased almost linearly. The UT, UE, UD, UE–UT ratio and UD–UT ratio showed a decreasing trend, and the UE–UD ratio showed an increasing trend at the peak point with the increase in the CPB height ratio. (3) Two damage constitutive models were established based on the AE ring count and energy principle. The damage evolution process of RCF materials can be divided into three stages: the slow damage accumulation stage, stable damage growth stage, and rapid damage accumulation stage.