Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation.

Elucidating the mechanisms of heavy metal (HM) adsorption on clay minerals is key to solving HM pollution in soil. In this study, the adsorption of four HM atoms (As, Cd, Cr, and Hg) on the illite(001) surface was investigated using density functional theory calculations. Different adsorption configurations were investigated and the electronic properties (i.e., adsorption energy (Ead) and electron transfer) were analyzed. The Ead values of the four HM atoms on the illite(001) surface were found to be As > Cr > Cd > Hg. The Ead values for the most stable adsorption configurations of As, Cr, Cd, and Hg were -1.8554, -0.7982, -0.3358, and -0.2678 eV, respectively. The As atoms show effective chemisorption at all six adsorption sites, while Cd, Cr, and Hg atoms mainly exhibited physisorption. The hollow and top (O) sites were more favorable than the top (K) sites for the adsorption of HM atoms. The Gibbs free energy results show that the illite(001) surface was energetically favorable for the adsorption of As and Cr atoms under the influence of 298 K and 1 atm. After adsorption, there was a redistribution of positions and reconfiguration of the chemical bonding of the surface atoms, with a non-negligible influence around the upper surface atoms. Bader charge analysis shows electrons were transferred from the surface to the HM atoms, and a strong correlation between the valence electron variations and the adsorption energy was observed. HM atoms had a high electronic state overlap with the surface O atoms near the Fermi energy level, indicating that the surface O atoms, though not the topmost atoms around the surface, significantly influence HM adsorption. The above results show illite(001) preferentially adsorbed As among all four investigated HM atoms, indicating that soils containing a high proportion of illite might be more prone to As pollution.

Toward a more sustainable mining future with electrokinetic in situ leaching.

Metals are currently almost exclusively extracted from their ore via physical excavation. This energy-intensive process dictates that metal mining remains among the foremost CO2 emitters and mine waste is the single largest waste form by mass. We propose a new approach, electrokinetic in situ leaching (EK-ISL), and demonstrate its applicability for a Cu-bearing sulfidic porphyry ore. In laboratory-scale experiments, Cu recovery was rapid (up to 57 weight % after 94 days) despite low ore hydraulic conductivity (permeability = 6.1 mD; porosity = 10.6%). Multiphysics numerical model simulations confirm the feasibility of EK-ISL at the field scale. This new approach to mining is therefore poised to spearhead a new paradigm of metal recovery from currently inaccessible ore bodies with a markedly reduced environmental footprint.

Particle shape analysis of tailings using digital image processing.

The physical and mechanical properties of the dielectric materials mainly depend on shapes of particles in granular media. In order to reveal the differences of physical and mechanical properties between tailings and natural sands from the microscopic view, the usage of digital image processing techniques contributes to the quantification of shape descriptors (elongation, sphericity, convexity, and roughness) describing the shapes of particles. The comparison between four tailings (gold, tin, copper, and iron) and two natural sands (river sand and sea sand) is made in the current study. Results show that particle shape descriptors have great relationship with particle size. The decrement of particle size, on one hand, leads to the increase of the elongation of tailings and sea sand, and thus forming the needle-like or columnar shape of particles. The sphericity of tailings and river sand also increases and generates spherical shapes of particles. On the other hand, both of the convexity and roughness of tailings and sea sand grow with larger particle size. The remarkable difference can be observed on surface texture of particles between tailings and sea sand. Much higher angularity of tailings is also represented by comparing with that of sea sand and river sand.

Utilization of phosphogypsum and phosphate tailings for cemented paste backfill.

This research is an investigation of the feasibility of utilizing phosphogypsum (PG) and phosphate tailings (PTS) for cemented paste backfill. Some experiments were conducted with various combinations of PTS and PG as aggregates, along with slags and/or Portland cement as binders and CaO as an additive. The influence of the PG's ageing time on the consolidation of backfill was also explored. The unconfined compressive strength (UCS), the generated gases and the scanning electron microscope (SEM) were all tested and used in the analysis of backfill characteristics. The results show that (i) the highest UCS of backfill prepared by PG and PTS after curing for either 7 days or 28 days is still less than 1.0 MPa, with a large amount of CO2 and SO2 generated; (ii) the slags can improve the UCS by a factor of three, but not without a vast generation of CO2, SO2, and H2S. However, the gases were not produced when CaO was added, but the UCS decreases suddenly to 0.2 or 0.4 MPa after curing for 7 days or 28 days, respectively; (iii) the UCS of backfill increases linearly with increasing cement content. When the CaO was added at 2%, the UCS reached 3.36 MPa after curing for 7 days and 4.44 MPa after curing for 28 days, and no gases were generated; (iv) the influence of the PG's ageing time on the UCS is negligible after 4 days of aging. Based on these results, it was concluded that PG and PTS can be utilized as backfill materials when Portland cement is used as a binder and CaO is used as an additive.

Electrokinetic in situ oxidation remediation: assessment of parameter sensitivities and the influence of aquifer heterogeneity on remediation efficiency.

A newly developed groundwater and electrokinetic (EK) flow and reactive transport numerical model was applied to simulate electrokinetic in situ chemical oxidation (EK-ISCO) remediation. Scenario simulations that considered the oxidation of a typical organic contaminant (tetrachloroethene) by permanganate were used to gain a better understanding of the key processes and parameters that control remediation efficiency. In a first step a sensitivity analysis was carried out to investigate a range of EK, hydraulic and engineering parameters on the performance of EK-ISCO. While all investigated parameters affected the remediation process to some extent, the duration and energy required for remediation were shown to be most dependent upon the applied voltage gradient, the natural oxidant demand and the concentration of the injected oxidant. Secondly, the efficacy of EK-induced oxidant transport was further examined for a heterogeneous aquifer system with random permeability fields. Oxidant migration under EK was slower in low-permeability media due to the increased oxidant consumption of competing reductants. Instead of injecting oxidant only at the cathode, locating injection wells between the electrodes greatly increased the contaminant degradation by decreasing the distance the amendment had to migrate before reaching the contaminant.

The effect of using a geotextile in a monolithic (evapotranspiration) alternative landfill cover on the resulting water balance.

This paper examines the potential effects of a geotextile layer used in a lysimeter pan experiment conducted in a monolithic (evapotranspiration) soil cover trial on its resulting water balance performance. The geotextile was added to the base of the lysimeter to serve as a plant root barrier in order to delineate the root zone depth. Both laboratory data and numerical modelling results indicated that the geotextile creates a capillary barrier under certain conditions and retains more water in the soil above the soil/geotextile interface than occurs without a geotextile. The numerical modelling results also suggested that the water balance of the soil cover could be affected by an increase in plant transpiration taking up this extra water retained above the soil/geotextile interface. This finding has a practical implication on the full-scale monolithic cover design, as the absence of the geotextile in the full-scale cover may affect the associated water balance and hence cover performance. Proper consideration is therefore required to assess the final monolithic cover water balance performance if its design is based on the lysimeter results.