Coarser biochar proves more effective for willow-based heavy metal cleanup in clay soil

The Challenge of Contaminated Soils

Soils contaminated with heavy metals from industrial activities like mining pose a substantial threat to environmental and human health. Phytoremediation, a method that uses fast-growing plants like willows to absorb and remove these toxins, offers an eco-friendly and cost-effective cleanup strategy. However, the harsh conditions of contaminated soils—high acidity, low nutrients, and metal toxicity—often inhibit plant survival and growth, limiting the effectiveness of this approach.

Improving Soil with Biochar

To overcome these obstacles, scientists use soil amendments to improve conditions for plant life. Biochar, a charcoal-like substance made from pyrolyzing biomass, is a popular choice. In a recent study, a team of researchers led by Jiang Xiao and Guangcai Chen from the Research Institute of Subtropical Forestry, Chinese Academy of Forestry, investigated how bone-derived biochar could aid the phytoremediation of acidic clay soil contaminated with copper, lead, cadmium, and manganese.

An Experimental Comparison

The researchers created two types of bone biochar: a standard version called BC and a micro-nano version called MBC, which was produced by ball-milling the standard biochar into much finer particles. They mixed these into the contaminated soil at various concentrations and planted willow cuttings, a species known as Salix jiangsuensis '172'. The experiment ran for 150 days in a greenhouse setting to observe the effects on soil properties, plant survival, growth, and metal uptake.

Biochar Ensures Plant Survival

The results first confirmed the value of biochar as a soil amendment. In the untreated contaminated soil, none of the willow cuttings survived. In contrast, when either BC or MBC was added at rates of 1.0 percent or higher, the survival rate of the willows jumped to 100 percent. Both forms of biochar successfully reduced the bioavailability of heavy metals and improved soil nutrient levels, making the environment habitable for the plants.

Size Determines Remediation Success

A notable difference emerged between the two biochar types. The finer, nano-sized MBC was more effective at immobilizing heavy metals within the soil, locking them in place and making them less available. Surprisingly, the coarser BC promoted substantially better plant growth. Willows grown in soil with standard BC were taller, had thicker stems, and produced significantly more total biomass compared to those grown with MBC. Researchers suggest the tiny MBC particles may have negatively altered the physical structure of the clay soil, hindering water retention and root respiration.

A Tale of Two Strategies

This difference in plant growth directly influenced the overall cleanup efficiency. Because the willows grew much larger and healthier with the coarser BC, they were able to absorb a far greater total quantity of heavy metals from the soil. The total accumulation of cadmium, lead, manganese, and copper was significantly higher in plants amended with standard BC. The study concludes that for phytoremediation aimed at extracting and removing metals, standard-sized biochar is the superior choice for clay soils. Conversely, nano-sized biochar is better suited for phytostabilization, where the goal is to contain contaminants in the soil. This research provides valuable direction for tailoring remediation strategies to specific environmental goals and soil conditions.

Corresponding Author:

Guangcai Chen

Original Source:

https://doi.org/10.1007/s44246-023-00053-5

Contributions:

Jiang Xiao: Conceptualization, Investigation, Writing-original draft, Writing - review & editing, Funding acquisition; Xiaogang Li: Investigation, review & editing; Yini Cao: Investigation, review & editing; Guangcai Chen: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition. The authors read and approved the final manuscript.