Qiang Xue

The role of roots in the stability of landfill clay covers under the effect of dry–wet cycles

This study investigates the role of vegetation roots in the stability of landfill clay covers subject to dry–wet cycles. Soil shear strength is tested at different depths of bare and vegetation covers before and after 1 year of seasonal dry–wet cycles. Results show that the effect of these cycles and of vegetation roots on the soil shear strength of covers mainly alters soil cohesion. By contrast, the effect on the internal friction angle of soil is inconspicuous. After 1 year of seasonal dry–wet cycles and grass root growth, the soil cohesion of the vegetation cover increases, whereas that of the bare cover decreases. The increment in the soil cohesions of two covers tends to decline exponentially with increasing cover depth. Thus, the cover stability model is established according to upper-bound solution theory based on the change rule of cover shear strength under the effect of dry–wet cycles and vegetation roots. The theoretical analysis results indicate that the increase in cover slope safety factor that was caused by vegetation roots is significant, compare that of bare cover under dry–wet cycles. Nonetheless, the effect of roots on cover slope safety factor decreases with increasing slope length and cover slope angle.

Effect of drying-wetting cycles on leaching behavior of cement solidified lead-contaminated soil.

Lead contaminated soil was treated by different concentration of ordinary Portland cement (OPC). Solidified cylindrical samples were dried at 40°C in oven for 48 h subsequent to 24h of immersing in different solution for one drying-wetting. 10 cycles were conducted on specimens. The changes in mass loss of specimens, as well as leaching concentration and pH of filtered leachates were studied after each cycle. Results indicated that drying-wetting cycles could accelerate the leaching and deterioration of solidified specimens. The cumulative leached lead with acetic acid (pH=2.88) in this study was 109, 83 and 71 mg respectively for solidified specimens of cement-to-dry soil (C/Sd) ratios 0.2, 0.3 and 0.4, compared to 37, 30, and 25mg for a semi-dynamic leaching test. With the increase of cycle times, the cumulative mass loss of specimens increased linearly, but pH of filtered leachates decreased. The leachability and deterioration of solidified specimens increased with acidity of solution. Increases of C/Sd clearly reduced the leachability and deterioration behavior.

Fate of metals before and after chemical extraction of incinerated sewage sludge ash

Chemical extraction of incinerated sewage sludge ash (ISSA) can effectively recycle P, but it may change the speciation and mobility of the remaining metals. This study investigated the changes of the leaching potential and distribution of metals in the chemically extracted ISSA. Batch extraction experiments with different extractants, including inorganic acids, organic acids, and chelating agents, were conducted on the ISSA collected from a local sewage sludge incinerator. The extraction of Zn, Cu, Pb, Ni, Cd, Ba, Cr and As from the ISSA and the corresponding changes of the mobility and speciation were examined. The results showed that the metals in ISSA were naturally stable because large portions of metals were associated with the residual fraction. The inorganic (HNO3 and H2SO4) and organic acids (citric acid and oxalic acid) significantly co-dissolved the metals through acid dissolution, but the reduction in the total concentrations did not tally the leaching potential of the residual metals. The increase in the exchangeable fraction due to destabilization by the extractants significantly enhanced the mobility and leachability of the metals in the residual ISSA. Chelating agents (EDTA and EDTMP) only extracted a small quantity of metals and had a marginal effect on the fate of the residual metals, but they significantly reduced the Fe/Mn oxide-bound fraction. In comparison, the bioaccessibility of residual metals were reduced to varying extent. Therefore, the disposal or reuse of chemically extracted ISSA should be carefully evaluated in view of possible increase in mobility of residual metals in the environment.

Experimental study on the microstructure and mechanical behaviors of leachate-polluted compacted clay

The intent of this article is to investigate the microstructure characteristics and macroscopic mechanical behaviors of leachate-polluted compacted clays. X-ray diffraction, scanning electron microscope, and mercury intrusion porosimetry were used to observe the mineral component, microstructure of the compacted clays polluted by different concentrations of landfill leachate. One-dimensional swelling test, direct shear test, and permeability test were conducted to measure and analyze the swelling properties, shear strength, and permeability of leachate-polluted compacted clays. Results indicated that the leachate pollution caused a decrease in smectite content of compacted clays. Aggregate structure was destroyed by leachate pollution, causing an increase in the relative porosity of the smaller pores and a decrease in that of the larger pores. However, the total porosity increased with the leachate concentration because of the decrease in swelling deformation of compacted clay. Leachate pollution caused a significant increase in cohesion and a slight increase in internal friction angle of the compacted clays. Moreover, the permeability of the leachate-polluted compacted clays decreased with the increase in time and leachate concentration.

Characterization of phosphorus leaching from phosphate waste rock in the Xiangxi River watershed, Three Gorges Reservoir, China

Phosphate mining waste rocks dumped in the Xiangxi River (XXR) bay, which is the largest backwater zone of the Three Gorges Reservoir (TGR), are treated as Type I industry solid wastes by the Chinese government. To evaluate the potential pollution risk of phosphorus leaching from phosphate waste rocks, the phosphorus leaching behaviors of six phosphate waste rock samples with different weathering degrees under both neutral and acidic conditions were investigated using a series of column leaching experiments, following the Method 1314 standard of the US EPA. The results indicate that the phosphorus release mechanism is solubility-controlled. Phosphorus release from waste rocks increases as pH decreases. The phosphorus leaching concentration and cumulative phosphorus released in acidic leaching conditions were found to be one order of magnitude greater than that in neutral leaching conditions. In addition, the phosphorus was released faster during the period when environmental pH turned from weak alkalinity to slight acidity, with this accelerated release period appearing when L/S was in the range of 0.5-2.0 mL/g. In both neutral and acidic conditions, the average values of Total Phosphorus (TP), including orthophosphates, polyphosphates and organic phosphate, leaching concentration exceed the availability by regulatory (0.5 mg/L) in the whole L/S range, suggesting that the phosphate waste rocks stacked within the XXR watershed should be considered as Type II industry solid wastes. Therefore, the phosphate waste rocks deposited within the study area should be considered as phosphorus point pollution sources, which could threaten the adjacent surface-water environment.

Precipitates in landfill leachate mediated by dissolved organic matters

Clogging of landfill leachate collection system is so ubiquitous that it causes problems to landfills. Although precipitations of calcite and other minerals have been widely observed, the mechanism of precipitation remains obscure. We examined the clog composition, dissolved organic matters, leachate chemical compositions and the correlation of these variables in view of the precipitation process. It is shown that Dissolved Organic Carbon (DOC) inhibits precipitation of landfill leachate. Using the advanced NICA-Donnan model, the analysis of aqueous chemical reactions between Mg-Ca-DOC-CO2 suggests a good agreement with experimental observations. Calcite and dolomite are both found to be oversaturated in most of the landfill leachate samples. DOC is found to preferentially bind with Mg than Ca, leading to more likely precipitation of Calcite than dolomite from landfill leachate. The NICA-Donnan model gives a reasonable estimation of dolomite saturation index in a wide range of DOC. Modeling confirms the major precipitation mechanism in terms of alkaline earth metal carbonate. Uncertainties in model parameters are discussed with particular focus on DOC composition, functional group types and density concentration and the influential factors.

A Coupling Kinetics Model for Pollutant Release and Transport in the Process of Landfill Settlement

A coupling kinetics model is developed to simulate the release and transport of landfill leachate pollutants in a deformable municipal solid waste landfill by taking into account of landfill settlement, seepage of leachate water, hydrolyse of insoluble and degradable organic pollutants in solid phase, biodegradation of soluble and degradable organic pollutants in solid phase and aqueous one, growth of aerobic and anaerobic microorganism, and consumption of dissolved oxygen. The release and transport of organic pollutants and microorganisms in landfills in the process of landfill settlement was simulated by considering no hydraulic effect. Simulation results demonstrated that the interaction between landfill settlement and the release, transport and biodegradation of landfill leachate pollutants was significant. Porosity and saturated hydraulic conductivity were not constants because of the landfill settlement, which affected the release, transport and biodegradation of landfill leachate pollutants, and furthermore acted on the landfill settlement. The simulation results accorded with the practical situation, which preliminarily verified the reliability of the mathematical model and the numerical program in this paper.

Experimental Study On The Nonlinear Change Of Saturated Hydraulic Conductivity Of Waste Soil

The experiment for studying the nonlinear change law of the saturated hydraulic conductivity of municipal solid waste (MSW) was developed based the self-developed experimental instrument, the effect law of filling time and porosity on the nonlinear evolution process of saturated hydraulic conductivity was discussed, the limitations of using Freeze-Carman formula to describe the nonlinear relationship between saturated hydraulic conductivity and porosity was proved, and based on the analysis and fitting of experimental results the nonlinear constitutive models between the saturated hydraulic conductivity and the porosity at different filling time were put forward. The experimental results showed that the saturated hydraulic conductivity presented an exponential type increasing trend with porosity; the self-defined exponential function could describe the nonlinear relationship between saturated hydraulic conductivity and porosity well, and the correlation coefficient were all more than 0.92; and the effects of filling time on saturated hydraulic conductivity were significant, especially when the porosity was larger.

The Tension and Puncture Properties of HDPE Geomembrane under the Corrosion of Leachate

To investigate the gradual failure of high-density polyethylene (HDPE) geomembrane as a result of long-term corrosion, four dynamic corrosion tests were conducted at different temperatures and durations. By combining tension and puncture tests, we systematically studied the variation law of tension and puncture properties of the HDPE geomembrane under different corrosion conditions. Results showed that tension and puncture failure of the HDPE geomembrane was progressive, and tensile strength in the longitudinal grain direction was evidently better than that in the transverse direction. Punctures appeared shortly after puncture force reached the puncture strength. The tensile strength of geomembrane was in inversely proportional to the corrosion time, and the impact of corrosion was more obvious in the longitudinal direction than transverse direction. As corrosion time increased, puncture strength decreased and corresponding deformation increased. As with corrosion time, the increase of corrosion temperature induced the decrease of geomembrane tensile strength. Tensile and puncture strength were extremely sensitive to temperature. Overall, residual strength had a negative correlation with corrosion time or temperature. Elongation variation increased initially and then decreased with the increase in temperature. However, it did not show significant law with corrosion time. The reduction in puncture strength and the increase in puncture deformation had positive correlations with corrosion time or temperature. The geomembrane softened under corrosion condition. The conclusion may be applicable to the proper designing of the HDPE geomembrane in landfill barrier system.

Pollutant Transport Model and Numerical Simulation in Landfill Site

The simulation model for describing the transportation and transformation of landfill leachate pollutant in landfill, soil and underground water was established. Taking Wuhan Changshankou landfill for example, the numerical simulation was carried out. The simulation results showed that if there’s no any seepage control measure, the groundwater and soil under landfill will be polluted seriously after MSW was filled; the highest pollutant concetration in landfill was about 25000, and the one in groundwater and soil was about 20000; at the thirtieth year, the pollutant concetration in groundwater and soil still remained up to 10000 although the one in landfill was about 0. The simulation results also showed that the vertical stonewall can’t be used as a nature, and artificial seepage control system must be used.