The impact of temperature on soil dynamics has long been a topic of widespread interest. However, the effects of high-temperature environments caused by phenomena such as wildfires and tunnel fires on soil remain poorly understood. This study, using purple soil from Chongqing, China as a representative, investigates the effects of high-temperature conditions on the mechanical properties and microstructure of this soil type. The results show that unconfined compression strength, deformation modulus, and strain energy density at peak of purple soil tend to increase with the increase of the treatment temperature from 20 degrees C to 1000 degrees C. This enhancement becomes pronounced when the temperature exceeds 600 degrees C. The physical and chemical changes are employed to elucidate the evolution of mechanical properties, and significant reinforcement effect primarily attributed to the 'welding action' of clay minerals. The variation in pore size distribution becomes significant when the treatment temperature approaches 800-1000 degrees C, and soil samples become vesicular structure at 1000 degrees C. These transformations depend on the decomposition of CaCO3, as well as the redistribution and confining effects of melted illite. Therefore, following high-temperature treatment, purple soil exhibits the capacity to alleviate environmental degradation from the perspective of mechanical properties. Purple soil exposed to temperatures between 800 and 1000 degrees C exhibits properties akin to those of clay bricks, making it a viable material for construction purposes. This research holds substantial significance for environmental engineering, geological engineering, and the development of construction materials following thermal treatment.

As a renewable energy source, biomass has the potential to replace non-renewable, fossil fuels. However, the disposal of the waste biomass ash (generated during energy generation) needs to be studied. While prior studies attempted to utilise composite additives containing biomass ash for soil, the introduction of other additives, such as cement, was an environmental burden. By employing biomass ash composition as the sole additive for strengthening purple soil under various curing conditions using high-temperature treatment, this study maximised its utilisation. The results showed that the unconfined compressive strength (UCS) varied across different curing conditions as the biomass ash content increased. After high temperature treatment at 800 degrees C, the biomass ash consistently reinforced purple soil under all the curing conditions. However, the biomass ash stabilisation mechanism differed between dry and humid curing conditions. Under dry curing conditions, the UCS increase depended on the cementing effect of soluble salt and/or insoluble calcite; under humid curing conditions, the UCS change was attributed to the damage to clay minerals, contact mode, and cementing effects of multiple components. Therefore, the 800 degrees C temperature-treated biomass ash can be used alone to reinforce purple soil, inhibiting the soil-water loss. This study presents a novel avenue for utilising waste, biomass ash, with considerable implications for environmental protection and soil stabilisation.

Both acid and alkaline purple soils in China are increasingly affected by Cd contamination. The selection of fastgrowing trees suitable for remediating different soil types is urgent, yet there is a severe lack of relevant knowledge. In this study, we conducted a controlled pot experiment to compare the growth, physiology, and Cd accumulation efficiency of two widely recognized poplar species, namely Populus deltoides and P. x canadensis, under Cd contamination (1 mg kg-1) in acid and alkaline purple soils. The objective was to determine which poplar species is best suited for remediating different soil types. Our findings are as follows: (1) the total biomass of both poplars remained largely unaffected by Cd pollution in both soil types. Notably, under Cd pollution, the total biomass of P. deltoides in acid purple soil was 1.53 times greater than that in alkaline purple soil. (2) Cd pollution did not significantly induce oxidative damage in the leaves of either poplar species in both soil types. However, in acid purple soil, Cd contamination led to a 21% increase in NO3- concentration and a 44% increase in NH4+ concentration in P. x canadensis leaves, whereas in alkaline purple soil, it led to a 59% increase in NH4+ concentration in P. deltoides leaves. (3) Cd concentrations in all root orders of P. x canadensis were significantly higher than those in P. deltoides, especially in the first three root orders, under alkaline purple soil. The total Cd accumulation by P. x canadensis in Cd-polluted alkaline purple soil was 2.18 times higher than that in Cdpolluted acid purple soil, a difference not observed in P. deltoides. (4) redundancy analysis indicated that the sequestration effect of higher soil organic matter on Cd availability in acid purple soil was more pronounced than the release effects caused by lower pH. In conclusion, P. x canadensis is better suited for remediating alkaline purple soil due to its higher capacity for Cd uptake, while P. deltoides is more suitable for remediating Cdcontaminated acid purple soil due to its better growth conditions and greater Cd enrichment capability.