Effects of enzyme-induced carbonate precipitation technique on multiple heavy metals immobilization and unconfined compressive strength improvement of contaminated sand.

Enzyme-induced carbonate precipitation (EICP) has been studied in remediation of heavy metal contaminated water or soil in recent years. This paper aims to investigate the immobilization mechanism of Zn2+, Ni2+, and Cr(VI) in contaminated sand, as well as strength enhancement of sand specimens by using EICP method with crude sword bean urease extracts. A series of liquid batch tests and artificially contaminated sand remediation experiments were conducted to explore the heavy metal immobilization efficacy and mechanisms. Results showed that the urea hydrolysis completion efficiency decreased as the Ca2+ concentration increased and the heavy metal immobilization percentage increased with the concentration of Ca2+ and treatment cycles in contaminated sand. After four treatment cycles with 0.5 mol/L Ca2+ added, the immobilization percentage of Zn2+, Ni2+, and Cr(VI) were 99.99 %, 86.38 %, and 75.18 %, respectively. The microscale analysis results presented that carbonate precipitates and metallic oxide such as CaCO3, ZnCO3, NiCO3, Zn(OH)2, and CrO(OH) were generated in liquid batch tests and sand remediation experiments. The SEM-EDS and FTIR results also showed that organic molecules and CaCO3 may adsorb or complex heavy metal ions. Thus, the immobilization mechanism of EICP method with crude sword bean urease can be considered as biomineralization, as well as adsorption and complexation by organic matter and calcium carbonate. The unconfined compressive strength of EICP-treated contaminated sand specimens demonstrated a positive correlation with the increased generation of carbonate precipitates, being up to 306 kPa after four treatment cycles with shear failure mode. Crude sword bean urease with 0.5 mol/L Ca2+ added is recommended to immobilize multiple heavy metal ions and enhance soil strength.

Nonlinear Optical Saturable Absorption Properties of 2D VP Nanosheets and Application as SA in a Passively Q-Switched Nd:YVO4 Laser.

Two-dimensional (2D) violet phosphorus (VP) plays a significant role in the applications of photonic and optoelectronic devices due to its unique optical and electrical properties. The ultrafast carrier dynamics and nonlinear optical absorption properties were systematically investigated here. The intra- and inter-band ultrafast relaxation times of 2D VP nanosheets were measured to be ~6.83 ps and ~62.91 ps using the pump-probe method with a probe laser operating at 1.03 µm. The nonlinear absorption coefficient ßeff, the saturation intensity Is, the modulation depth ΔR, and the nonsaturable loss were determined to be -2.18 × 104 cm/MW, 329 kW/cm2, 6.3%, and 9.8%, respectively, by using the Z-scan and I-scan methods, indicating the tremendous saturable absorption property of 2D VP nanosheets. Furthermore, the passively Q-switched Nd:YVO4 laser was realized with the 2D VP nanosheet-based SA, in which the average output power of 700 mW and the pulse duration of 478 ns were obtained. These results effectively reveal the nonlinear optical absorption characteristics of VP nanosheets, demonstrating their outstanding light-manipulating capabilities and providing a basis for the applications of ultrafast optical devices. Our results verify the excellent saturable absorption properties of 2D VP, paving the way for its applications in pulsed laser generation.

Life cycle analysis of common landfill final cover systems focusing on carbon neutrality.

Carbon emissions from landfill construction and management have become a global concern. Life cycle analysis (LCA) has been widely used to assess the environmental impacts of engineered infrastructures over their lifetimes. LCA has also been applied to landfill leachate and gas management, but rarely to landfill final cover systems. This paper reports the results of an LCA of the following landfill final cover systems: compacted clay cover, geomembrane cover, cover with capillary effects (CCBE), dual capillary barrier cover, three-layer landfill cover system using natural soils, three-layer cover using recycled concrete aggregate (RCA) and biochar-amended three-layer landfill cover system using RCA. The LCA assessment of landfill cover considers the cost, carbon emissions and carbon sequestration during the production, construction and operation phases. The effects of landfill cover on global warming, freshwater eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity and fossil resource scarcity are also evaluated. In addition, the sensitivities of cost and carbon emission to the use of electric-powered machines and transportation distance are analysed. It is revealed that the three-layer cover system using RCA and biochar has the lowest unit cost and carbon emission of all of the covers, up to 88 % and 66 % lower, respectively, than those of the other six covers. In addition, this cover system has the highest carbon sequestration rate, with a value of 47.9 kg CO2/(y·m2), four times higher than that of the compacted clay cover. Finally, this sustainable cover mitigates global warming and reduces adverse environmental impacts by up to 82 %. Therefore, the biochar amended three-layer cover system using RCA without geomembrane offers the greatest economic benefits, performs effectively in terms of the pursuit of carbon neutrality and promotes sustainable development.

Effects of biochar on plant growth and hydro-chemical properties of recycled concrete aggregate.

Biochar has been used as a sustainable amendment to mitigate environmental risks, improve plant growth and soil properties. This study conducted laboratory column tests to investigate the effects of plant-biochar interactions on shrub growth, hydraulic properties and nutrient contents of recycled concrete aggregates (RCAs). In total, three test conditions, namely, vegetated RCA without biochar (R), with 5 % biochar (R5) and 10 % biochar (R10) were subject to drying. With biochar application, total N, P and K of RCA increased by >100 %, 200 % and 31 %, respectively, while pH reduced to 8.3. With shrub growth, the lowest RCA pH was reduced to 7.8. The leaf area index (LAI) of shrub increased by 51 % due to biochar amendment, while the differences in shoot height were insignificant. The water retention capacity of RCA was enhanced by improving the saturated water content and air-entry value by 27 % and 100 %. The slope of the soil suction-LAI correlation for biochar amend cases was 1.6 times lower than R. This indicates that biochar may limit the increase of matric suction and prevent excessive water loss during drying. However, the differences between R5 and R10 were not significant. Therefore, 5 % biochar amendment is highly suggested as it can substantially improve plant growth and soil hydraulic properties during drying.

High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber.

Atomically thin Bi2O2Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi2O2Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi2O2Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar+ plasma treatments are employed to precisely regulate the O and Se defect states in Bi2O2Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm-2 after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.