Effect of long-term acid attack on impermeability and microstructure of compacted cement-bound soils.

Compacted cement-bound soils (CCS) are widely used as leakage barriers around solid waste landfills and contaminated sites. However, a study of its long-term reliability in an acidic environment is relatively scarce. A 3-year-long experimental study on the changes in permeability coefficient and microstructure of CCS under acid attack was conducted, from which the following conclusions can be drawn: the permeability coefficient of CCS under weak acid (pH = 5.00) attack decreased progressively with time, from 4.90 × 10-8 cm/s at the beginning to 6.70 × 10-10 cm/s after 3 years. Under strongly acidic environments with pH values of 2.65 and 3.65, the permeability coefficients of CCS initially decreased and then increased with time, reaching 6.70 × 10-5 and 9.37 × 10-8 cm/s, respectively. The degradation effect of a weak acid (pH = 5.00) on the hydration products of cement was mild as a large amount of hydration products (e.g. C-S-H shaped in short fibrous) remained in the pores of CCS after 3 years of immersion. However, strong acid caused an obvious degradation effect on the hydration products, which almost disappeared after 3 years of immersion. Based on the study data, a unified mathematical model was developed to correlate the permeability coefficient of CCS, immersion time and acidic solution pH value. Furthermore, a quantitative expression function between the service life of CCS and solution pH value was established.

Effects of plastic contamination on water evaporation and desiccation cracking in soil.

Environmental contamination of plastics is becoming an issue of concern globally. Detection of plastics, particularly microplastics, has been increasingly reported in both marine environments and inland waters. Recent work has indicated that soil in terrestrial environments has also been contaminated by plastics. Research has also shown that plastics can have adverse effects on soil biota. However, the impact of plastics on soil physical properties is still unclear. In this work, effects of plastic film of different sizes at environmental relevant concentrations on water evaporation and desiccation cracking in two clay soils were studied. The results showed that the presence of plastics in soil significantly increased the rate of soil water evaporation by creating channels for water movement. The effect was more pronounced in soils treated with 2 mm plastics than in soils treated with 5 and 10 mm plastics, and increased with increasing plastic content. Desiccation cracking was observed on the surface of soil treated with 5 and 10 mm plastics likely due to the destruction of soil structural integrity. While 2 mm plastics increased the rate of desiccation shrinkage, the shrinkage ratio was reduced at the residual stage. Results from this work suggest that plastic contamination can alter the water cycle in soils, which may exacerbate soil water shortages and affect the vertical transport of pollutants. Further work is required to study the effects of plastics of other shapes, and laboratory observations should be tested at field scale.

Coupling model of aerobic waste degradation considering temperature, initial moisture content and air injection volume.

A quantitative description of aerobic waste degradation is important in evaluating landfill waste stability and economic management. This research aimed to develop a coupling model to predict the degree of aerobic waste degradation. On the basis of the first-order kinetic equation and the law of conservation of mass, we first developed the coupling model of aerobic waste degradation that considered temperature, initial moisture content and air injection volume to simulate and predict the chemical oxygen demand in the leachate. Three different laboratory experiments on aerobic waste degradation were simulated to test the model applicability. Parameter sensitivity analyses were conducted to evaluate the reliability of parameters. The coupling model can simulate aerobic waste degradation, and the obtained simulation agreed with the corresponding results of the experiment. Comparison of the experiment and simulation demonstrated that the coupling model is a new approach to predict aerobic waste degradation and can be considered as the basis for selecting the economic air injection volume and appropriate management in the future.