Xing Zeng

The one-dimensional convection–dispersion equation has been widely used to describe the migration process of contaminant leachate through barriers. However, most of the existing solutions are limited to simple conditions. In this study, a one-dimensional convection–dispersion model with time-dependent velocity was established while considering the change in the permeability coefficient. The analytical solution of the model was obtained by using the integral transformation method. Based on the analytical model, three special conditions were assumed for comparison. The results showed that the concentration levels of pollutants inside the barrier would significantly increase with the increase in the flow velocity, and the pollutant concentrations inside the barrier would be increased by four times compared with the normal flow velocity when the flow rate increased by two times. The transports of heavy metal ions with variable velocities through soil–bentonite and soil–attapulgite barriers were predicted under field conditions. The predicted results showed that the breakthrough time would be reduced by as much as two times. In engineering practice, a barrier’s service performance can be improved by controlling the temperature of the seepage field and improving the chemical compatibility of the barrier materials.

The 2 m thick compacted clay liner with a permeability coefficient of 1 × 10⁻⁷ cm/s is required in the Chinese technical specifications about landfills. The processes of chloride ion completely breakthrough low permeability barriers (k ≤ 1 × 10⁻⁷ cm/s) were modeled at 50 g in a geo-centrifuge. A measuring system was used to monitor solute velocity and conductivity. The entire process of chloride ion completely breaking through 2 m Kaolin clay liner was modeled successfully, which provided a valuable testing technology for centrifuge modeling of contaminant transport through low-permeable clay. The analyses results indicated the breakthrough time of conservative pollutant for the 2 m clay liner with a hydraulic conductivity of 1.0 × 10⁻⁹ m/s under Δh_w of 40 m was 1.6 years. As for strongly adsorptive pollutants, the breakthrough time t_0.1 increased by 9 times when R_d increased from 1 to 10, which indicates that the effect of R_d on the performance of the liner was significant.

The concentration profiles and breakthrough curves of the 2 m thick compacted clay liner (CCL) given in the specification were compared, considering three different adsorption isotherms (upper convex, linear, and lower concave). In addition, the effects of transport parameters, sorption isotherms, and source concentrations on pollutant migration were analyzed. The results showed that the dimensionless breakthrough curves of different source concentrations considering the linear adsorption isotherm coincided with each other, as the partition coefficient of the linear adsorption isotherm was constant. For the lower concave isotherm, the migration of a large source concentration was slowest, because the partition coefficient of the lower concave isotherm increased with an increase in concentration. For the upper convex isotherm, the migration of a large source concentration was fastest, because the partition coefficient decreased with an increase in concentration. The effects of the nonlinear isotherms on the shape of the outflow curve were similar to the effects of a change in the hydrodynamic dispersion (D_h): the concentration front of the upper convex isotherm was narrower, which was similar to the effect of a reduction in D_h (i.e., P_L), and the concentration front of the lower concave isotherm was wider and similar to the effect of an increase D_h (i.e., P_L). Therefore, the diffusion and adsorption parameters were fitted separately in the study, in case the nonlinear adsorption behavior was mistakenly defined as linear adsorption.

Phosphate rock has great potential value in the application of cut-off walls for heavy metal contaminated sites. The effects of initial concentration of Pb2+, contact time, temperature and pH on the adsorption performance of phosphate rock modified soil-bentonite (PSB) were analyzed by Batch tests, and the microstructure and physicochemical properties of the adsorbent were characterized and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The results showed that the incorporation of phosphate rock improves the adsorption performance of the mixed material for Pb2+ in an acidic environment, and the maximum adsorption amount of PSB can be 154% greater than that of soil-bentonite (SB). Kinetic analysis showed that intraparticle diffusion and liquid film diffusion both participate in control of the adsorption process, and the Langmuir model fitted the isothermal adsorption data better. The adsorption process of PSB to Pb2+ is an exothermic reaction process, and a low temperature environment is beneficial to the adsorption of Pb2+. Studies on the microscopic mechanism show that PO43− reacts with Pb2+ in solution to form phosphate precipitation, and –OH and PO43− are on the surface or interlayer of the adsorbent participate in the adsorption process of Pb2+.

Environmental pollution caused by excessive Sb(III) in the water environment is a global issue. We investigated the effect of processing parameters, their interaction and mechanistic details for the removal of Sb(III) using an iron salt-modified biosorbent (Fe(III)-modified Proteus cibarius (FMPAs)). Our study evaluated the optimisation of the adsorption time, adsorbent dose, pH, temperature and the initial concentration of Sb(III). We use response surface methodology to optimize this process, determining optimal processing conditions and the adsorption mechanism evaluated based on isotherm model and adsorption kinetics. The results showed that—(1) the optimal conditions for the adsorption of Sb(III) by FMPAs were an adsorption time of 2.2 h, adsorbent dose of 3430 mg/L, at pH 6.0 and temperature 44.0 °C. For the optimum initial concentration of Sb(III) 27.70 mg/L, the removal efficiency of Sb(III) reached 97.60%. (2) The adsorption process for Sb(III) removal by FMPAs conforms to the Langmuir adsorption isotherm model, and its maximum adsorption capacity (q_max) is as high as 30.612 mg/g. A pseudo-first-order kinetic model provided the best fit to the adsorption process, classified as single layer adsorption and chemisorption mechanism. (3) The adsorption of Sb(III) takes place via the hydroxyl group in Fe–O–OH and EPS–Polyose–O–Fe(OH)₂, which forms a new complex Fe–O–Sb and X≡Fe–OH. The study showed that FMPAs have higher adsorption capacity for Sb(III) than other previously studied sorbents and with low environmental impact, it has a great potential as a green adsorbent for Sb(III) in water.

The determination of liquid limit is of great significance in the engineering classification of soil and the selection of the control standard of highway subgrade packing and compactness. Based on the research achievements of many scholars on the relationship between the shear strength and the depth of cone penetration in fall cone tests in the liquid limit tests, the process of penetration was analyzed according to the Law of Conservation of Energy, and the expression of K was derived. Then, the expression was verified by the experimental data of different scholars and the existing data of various countries and institutions. And, the results were also compared with those obtained by the previous scholars using limit equilibrium theory. The results indicate that the new expression is in good agreement with the experimental results. K can be predicted well. And so, the shear strength can be calculated based on the depth of cone penetration, and then, the experimental value of the shear strength can be compared with the calculated value through laboratory tests. The influence of cone weight and cone angle on the shear strength calculation was analyzed. In addition, the liquid limit index of different standards was discussed based on the expression of K, and suggestions on how to achieve relative combination of different codes and standards were raised, which was very helpful for international communication.

As the 2 m thick compacted clay liner with permeability coefficient of 1 × 10⁻⁷ cm/s is required in the Chinese technical specifications about landfill, the performance of this compacted clay liner was analyzed considering three different adsorption isotherms (convex, straight, and concave). The effects of source concentration, adsorption mode, and waterhead on the breakthrough curve and breakthrough time of Pb²⁺ were discussed. The results indicate that reducing the concentration of pollution sources is beneficial to prolonging the breakthrough time. With the waterhead of 10 m, the absolute breakthrough time, respectively, increased from 2.77 to 3.7 years (concave type isotherm), from 17.63 to 26.58 years (straight type isotherm), and from 35.43 to 59.6 years (convex type isotherm), as the source concentration decreased from 1000 mg/L to 10 mg/L. The effect of adsorption isotherm type on the performance of the barrier is very obvious: with the waterhead of 10 m, the absolute breakthrough time corresponding to the convex isotherm is more than twice that of the straight adsorption isotherm, and more than 12.8 times that of the concave isotherm. The absolute breakthrough time corresponding to 0.3 m waterhead is more than 4 times that of 10 m, and reducing the waterhead can effectively increase the breakthrough time.