Thermal-Driven Moisture and Heavy Metal Transport in Unsaturated Soils: Unveiling Hysteretic Effects and Long-Term Contaminant Mobility

Contaminant migration in unsaturated soils is a major issue for both environmental safety and geotechnical stability, especially when it comes to heavy metal pollution such as lead (Pb²⁺) which does not break down over time like some organic pollutants do and can cause serious threat to ecosystems and human health. To manage these risks, it is important to understand how contaminants move through soil, especially in cases where they might spread and reach water sources. One of the biggest influences on contaminant movement is temperature. Heat from industrial sites, underground cables, geothermal energy systems, or nuclear waste storage can change how water and contaminants behave in soil. When the temperature rises, moisture moves through evaporation and condensation, creating a complex cycle that also affects how pollutants spread. While scientists have spent years studying how contaminants move due to water flow and chemical reactions, they have not fully studied how temperature itself can push heavy metals through the soil. A research team led by Professor Bing Bai, Dr. Tao Xu, and Dr. Pengpeng Li from Beijing Jiaotong University, along with Dr. Qingke Nie from the Hebei Research Institute of Construction and Geotechnical Investigation Co. Ltd., took a closer look at how Pb²⁺ moves in response to temperature changes. Their work, published in the International Journal of Heat and Mass Transfer, goes beyond theoretical models and includes real-world experiments to see how heat, moisture, and contaminants interact inside soil. The challenge with this type of research is that these factors do not act independently—heat influences water movement, which in turn affects how contaminants spread. Traditional models often assume the soil is uniform and behaves predictably, but in reality, things are much more complicated. One particularly tricky issue is hysteresis in soil-water characteristic curves (SWCC), which describes how soil holds onto and releases moisture depending on whether it is drying out or being rewetted. Existing models struggle to capture how cyclic temperature changes influence moisture retention, and since heavy metals tend to move with moisture, ignoring this effect can lead to inaccurate predictions about where pollutants will end up.

To better understand how heat influences the movement of water and contaminants in soil, the researchers built a custom one-dimensional soil column. This setup let them create controlled conditions that mimic real-world temperature changes while tracking how moisture and pollutants move over time. The column was made from high-temperature-resistant glass, ensuring clear measurements without outside interference. Kaolin clay was the soil of choice for these tests because of its low permeability and strong ability to trap contaminants. The soil was carefully mixed with precise moisture levels, packed in layers, and compacted to maintain consistency. To simulate a heat source, one end of the column was warmed to temperatures between 70°C and 80°C, while the other end remained cooler, between 22°C and 30°C. This temperature difference encouraged moisture to shift through the soil, much like what happens in industrial zones, near underground heat sources, or in areas affected by thermal pollution. One of the first things the team noticed was that moisture did not simply move in a straight line away from the heat source. Instead, the water initially migrated toward the cooler regions, but once temperatures dropped, some of it flowed back toward the heated side.  Moreover, the authors also monitored how Pb²⁺ ions migrated through the soil by introducing lead nitrate at the heated end of the column, they could see how the contaminant dispersed over time. The authors’ findings were clear: Pb²⁺ moved much further in soils that contained more moisture which suggest that its transport relied heavily on available water pathways. Even though kaolin clay effectively slowed down Pb²⁺ migration, it was not perfect. Over extended heating periods, the contaminant traveled further than expected, showing that long-term exposure to heat could weaken the soil’s ability to hold onto heavy metals. Another significant discovery was that Pb²⁺ migration changed depending on how long the heating lasted. In the beginning, the contaminant’s movement was closely tied to bulk moisture flow—water carried the Pb²⁺ ions as it moved. However, as heating continued, the water transport slowed, and Pb²⁺ migration became dominated by molecular diffusion, therefore, the spread of contaminants slowed down but did not stop.

The authors also examined how temperature affects Pb²⁺ adsorption to soil particles. The results showed that higher temperatures reduced adsorption efficiency, meaning more lead remained in liquid form instead of sticking to the soil. This effect was most noticeable in drier soil samples, where Pb²⁺ detached more easily from particles and traveled further compared to wetter conditions. To understand how different soil properties affect contaminant movement, the researchers adjusted key transport parameters. They found that when the dispersion coefficient was increased, Pb²⁺ moved faster and farther, while a higher deposition coefficient slowed the migration process. In conclusion, the research work of Professor Bing Bai and colleagues teach us how temperature changes affect the movement of moisture and heavy metals in unsaturated soils. Their findings are particularly valuable for industries and environmental initiatives focused on waste containment, soil cleanup, and groundwater safety. By showing that heat-driven moisture migration does not follow a straightforward pattern but instead exhibits hysteretic behavior, the study challenges the conventional models used to predict contaminant transport. This suggests that in areas near industrial facilities, underground power cables, or nuclear waste storage sites, moisture movement is far less predictable than previously believed. That unpredictability could have serious consequences for assessing the spread of pollutants like lead, possibly leading to underestimations of long-term environmental risks. One of the most significant takeaways from this study is its impact on waste containment system design and many hazardous waste sites rely on clay barriers to prevent contaminants from spreading, but as temperatures rise, these barriers lose their effectiveness. Additionally, the authors’ research highlights the importance of developing more advanced contaminant monitoring systems in thermally affected regions to help environmental agencies detect early signs of groundwater contamination before it becomes irreversible.