Release dynamics, risk evolution and driving mechanisms of heavy metals in superalkaline fly ash co-disposed by MSW landfill.

Fly ash from waste incineration is growing rapidly and has become a global problem. Landfill is the main treatment method, but the release behavior of ultra-alkaline fly ash needs further study. In this study, the release pattern of heavy metals from fly ash, the long-term risk after seepage, and the main control mechanisms were explored by indoor simulation experiments and process simulation modeling. The results show that carbonation is the main control mechanism for the release rate of heavy metals from super-alkaline fly ash, and the release rate is slow at the initial stage, but the release concentration of Zn and Pb may increase tens of times with the continuous reaction between the acidic substances in the leachate and the alkaline substances in the fly ash. The heavy metals released into the leachate can cause the concentration of Zn, Cd and Pb in the groundwater to exceed the standard by 39.50, 6.70 and 5.99 times due to seepage. Furnace type is the key controlling factor for background concentrations of heavy metals in ultra-alkaline fly ash, and the exposure concentrations of Cu, Cd, Zn, and Pb in ultra-alkaline fly ash from grate furnaces as well as the GT1 facility are 4.19, 4.19, 4.14, and 37.5 times greater than those of fluidized beds, respectively, with a higher risk of long-term landfill. Regionally, the regional occupancy rate of heavy metal concentrations indicated that the risk of adequate rainfall was high in the southeastern coastal region, which was five times higher than that in the inland northwest. Therefore, the long-term dynamics and risk evolution of Zn, Cd, and Pb in the groundwater around MSWLs in the coastal area should be paid attention to after the landfilling of ultra-alkaline fly ash in order to ensure the safety of the shallow groundwater environment after landfilling.

Dynamic evolution and response strategy of demand in buffer zone between scattered groundwater sources and hazardous waste landfill.

Groundwater contamination by landfill leachate is a major concern. Ignoring the long-term increase in leakage caused by the aging of engineered materials may lead to underestimation of the buffer distance (BFD) demand of landfills. In this study, a long-term BFD prediction model was developed by coupling an engineering material aging and defect evolution module with leachate leakage and migration transformation model, and was applicated and validated. The results showed that under landfill performance degradation, the required BFD was 2400 m, i.e., 6 times higher than under undegraded conditions. With the degradation of the performance, the BFD required to attenuate the heavy metal concentrations of groundwater increases more than the BFD required to attenuate organic pollutants. For example, the BFD required for zinc (Zn) was 5 times higher than that required for undegraded conditions, while for 2,4-dichlorophenol (2,4-D), the BFD was 1 times higher. Considering the uncertainties of the model parameters and structure, the BFD should be greater than 3000 m to ensure long-term safe water use under unfavorable conditions such as large leachate production and leakage, weak degradation and fast diffusion of pollutants. If the actual BFD does not meet the demand due to landfill performance degradation, the landfill owner can reduce the reliance on the BFD by reducing the waste leaching behavior. For example, the landfill in our case study would require a BFD of 2400 m, but by reducing the leaching concentration of zinc in the waste from 120 to 55 mg/L, this requirement could be reduced to 900 m.

LDI-MVFNet: A Multi-view fusion deep network for leachate distribution imaging.

The accurate monitoring and early warning of groundwater pollution caused by the concealed leakage of landfills is a major challenge globally in the field of solid waste management and groundwater protection. Electrical resistivity tomography (ERT) represents a potential solution with advantages, owing to its fast and nondestructive characteristics. However, traditional ERT based on a single array cannot reveal the distribution and dynamics of pollution in complex underground media owing to the limited information it carries. We designed a novel deep network for multi-view fusion to invert the real resistivity distribution of the medium caused by leachate (named LDI-MVFNet) so as to infer the distribution of leachate. To support model establishment and validation, ERT instances collected from synthetic models and a salt tracer experiment were inverted. We compared the inversion results of the LDI-MVFNet with those of single arrays and found that the LDI-MVFNet performed the best overall. The average root mean square error (RMSE) of synthetic models reached 0.98, performing better than Dipole-Dipole (3.86), Wenner-Schlumberger (3.37), and Pole-Pole (6.61), which were inverted separately. The resultant inverted subsurface true resistivity data were presented in the form of two-dimensional (2D) cross sections. The imaging results of 2D cross sections showed that LDI-MVFNet was superior to others in data noise suppression and inversion accuracy. The results of this study indicate that the data fusion of multiple views can more accurately reflect the real resistivity than the inversion of a single array can.

Simulation and Optimization of a Planar-Type Micro-Hotplate with Si3N4-SiO2 Transverse Composite Dielectric Layer and Annular Heater.

Micro-hotplates (MHPs) have become widely used basic structures in many micro sensors and actuators. Based on the analysis of the general heat transfer model, we propose a new MHP design based on a transversal composite dielectric layer, consisting of different heat transfer materials. Two general proven materials with different thermal conductivity, Si3N4 and SiO2, are chosen to form the composite dielectric layer. An annular heater is designed with a plurality of concentric rings connected with each other. The relationship between MHP performance and its geometrical parameters, including temperature distribution and uniformity, thermal deformation, and power dissipation, has been fully investigated using COMSOL simulation. The results demonstrate that the new planar MHP of 2 µm thick with a Si3N4-SiO2 composite dielectric layer and annular heater can reach 300 °C at a power of 35.2 mW with a mechanical deformation of 0.132 µm, at a large heating area of about 0.5 mm2. The introduction of the composite dielectric layer effectively reduces the lateral heat conduction loss and alleviates the mechanical deformation of the planar MHP compared with a single SiO2 dielectric layer or Si3N4 dielectric layer.

Framework, method and case study for the calculation of end of life for HWL and parameter sensitivity analysis.

Mass construction and operation of hazardous waste landfill infrastructure has greatly improved China's waste management and environmental safety. However, the deterioration of engineering materials and the failure of landfill may lead to the release of untreated leachate rich in persistent toxic pollutants to the soil and shallow groundwater. Accordingly, we develop the framework and process model to predict landfill life by coupling the landfill hydrological performance model and material degradation model. We found that the decrease rate of the concentration of persistent pollutants in leachate was significantly slower than the deterioration rate of the landfill engineering materials. As a result, when the materials failed, the leachate with high concentrations of persistent pollutants continued to leak, resulting in the pollutants concentration in surrounding groundwater exceeding the acceptable concentration at around 385 a, which is the average life of a landfill. Further simulation indicated that hydrogeological conditions and the initial concentration of leachate will affect landfill lifespan. The correlation coefficients of concentration, the thickness of vadose zone and the thickness of aquifer are - 0.79, 0.99 and 0.72 respectively, so the thickness of vadose zone having the greatest impact on the life of a landfill. The results presented herein indicate hazardous waste landfill infrastructure reinvestment should be directed toward long-term monitoring and maintenance, waste second-disposal, and site restoration.

Complex resistivity characteristics of saltwater-intruded sand contaminated by heavy metal.

Different pollutants affect electrical characteristics of soil, e.g., electric resistivity and capacity. The most extensively used non-intrusive methods in mapping these physical characteristics are electrical method. To better understand the effect of different hydrogeological and environmental process on resistivity and phase of complex resistivity under water-saturated soil, we carried out a controlled laboratory experiment where the host material was simulated by sand soil and the hydrogeological and environmental processes by groundwater table rise, seawater intrusion and heavy metal contamination. The experiment measured the resistivity and phase of soil saturated and unsaturated, with different pollutants added, together with their time-lapse change in a well-controlled column. With the involvement of more measurement parameters, complex resistivity method can provide more information than resistivity method, thereby having better performance in the detection and monitoring of changes in electrical properties of complex contaminated sites. For example, it is capable of discriminating the different contamination process, in this case, e.g., seawater intrusion and heavy metal contamination. In addition, it is still sensitive to the change of pollutant concentration even in site with high added concentration. Furthermore, simulating the saltwater-intruded site contaminated by manganese, it was found that the change of resistivity (ρ) can hardly be observed, while the responses of phase (φ) are so obvious that can be clearly observed.

Long-term dynamics of leachate production, leakage from hazardous waste landfill sites and the impact on groundwater quality and human health.

The long-term dynamics of leachate leakage from hazardous waste landfills (HWLs) and its influence on the surrounding groundwater and human health urgently requires decision-making processes for long-term HWL management and risk control. This study, based on the DMFU model, which is described in the literature as simulating the performance degradation of a landfill's main functional units, constructs a comprehensive model by coupling the HELP, EPACMTP, and Dose-Effect modules to investigate the long-term emissions of leachate from HWLs and their potential influence on groundwater quality and human health. Our results showed that the leakage rate over a long time period (50-1000 years) is 10 times higher than that of either a short (0-10 years) or medium (10-50 years) period. Due to the substantial increase in leakage rate, the negative influence on regional groundwater quality and human health changes from "insignificant" in the short term to "slight but acceptable" in the medium term, and finally to "substantial and unacceptable" in the long term. Studies also reveal that the uncertainty of risk increases over time. The information gained from this research provides useful insights into the long-term dynamics of leachate leakage, its risk consequences, and associated uncertainty, which can help landfill owners or risk managers make better decisions regarding the after-closure management of landfills.

[Numerical simulation and application of electrical resistivity survey in heavy metal contaminated sites].

In order to analyze the effects of electrical resistivity in heavy metal contaminated sites, we established the resistivity model of typical contaminated sites and simulate the DC resistivity method with Wenner arrays using the finite element method. The simulation results showed that the electrical method was influenced by the contamination concentration and the location of pollution. The more serious the degree of pollution was, the more obvious the low resistivity anomaly, thus the easier the identification of the contaminated area; otherwise, if there was light pollution, Wenner array could not get obvious low resistivity anomalies, so it would be hard to judge the contaminated area. Our simulation results also showed that the closer the contaminated areas were to the surface, the more easily the pollution was detected and the low resistivity anomalies shown in the apparent resistivity diagram were influenced by the Layered medium. The actual field survey results using resistivity method also show that the resistivity method can correctly detect the area with serious pollution.

[Experiment results of conduction, spectral induced polarization and dielectric characteristics for chrome-contaminated soil].

The resistivity, complex resistivity and complex permittivity of the chrome-contaminated soil were studied. Under the different pollution concentration and water content in the soil samples conditions, the relations between the resistivity, complex resistivity and complex permittivity of the chrome-contaminated soil and water content and the concentration of pollution were analyzed. When adding chrome pollution with different concentrations and water content, the experimental results show that the resistivity and complex resistivity of all the soil samples decreased with the pollution concentration and water content increased; but the phase of complex resistivity, which reflects the soil's capacitance, decreased below the 20 kHz and increase above the 20 kHz frequency. The real part and imaginary part of complex resostivity increased with the increase of pollution concentration and water content. The concentration of chrome pollutions and water content were the two main factor to determine the soil electrical characteristics.

[Comparison of two types of double-lined simulated landfill leakage detection based on high voltage DC method].

Two types of double high density polyethylene (HDPE) liners landfill that clay or geogrid was added between the two HDPE liners. The general resistance of the second mode is 15% larger than the general resistance of the first mode in the primary HDPE liner detection, and 20% larger than that of the first one in the secondary HDPE liner detection. High voltage DC method can accomplish the leakage detection and location of these two types of landfill and the error of leakage location is less than 10cm when electrode space is 1m.

[Setup of high voltage direct circuit equivalent circuit model in leakage detection of landfill].

An equivalent circuit model is put forward through analysis of the experiment data under the conditions of applying 400V, 350V and 300V DC supply power to a small-scale simulated landfill. Due to the fact that the existence of the HDPE geomembrane liner endows the model with commutating character, the electrolytic capacity increases as the area of geomembrane liner and the supplying electric voltage increase. The contact electric resistance of the power supply electrode is in direct proportion to earth electric resistance rate, and in relation to the diameter of the electrode and the depth of the earth. Moreover, the contact resistance is mainly determined by the earth electric resistance around the electrode (the influence induced by the earth farther than 10 times electrode radius is less than 10 percent of those involved by all resistance), hence decrease of the earth electric resistance rate around the electrode may result in effective decrease of the contact electric resistance of the electrode.