The production of steel and concrete is responsible for approximately 13½ percent of global carbon emissions, according to a 2022 article in the journal Nature. After clean water, cement (a key ingredient in concrete) is the most-used product on the planet. In an effort to decrease reliance on concrete and steel and to make the construction industry greener, researchers have been looking at more sustainable alternatives, including modifying the structural composition of wood. Wood is a renewable resource that combines low weight with impressive strength through a honeycomb network of hollow cells and sturdy polymers. Wood does not, however, have the strength required to be used alone as a load-bearing material in large structures such as high-rise buildings.
In an article published in March in the journal ACS Applied Materials and Interfaces, a team of scientists from Florida Atlantic University and partner institutions explored a novel way to make wood stronger: by adding iron to the wood’s cell walls.
The researchers used northern red oak (Quercus rubra) for this work. Hardwoods, including red oak, tend to be denser and more durable than softwoods, and they generally contain thick, short fibers that provide strong mechanical support. To reinforce the wood’s strength, the authors treated cubes of red oak with ferric nitrate and potassium hydroxide. This treatment resulted in the production of ferrihydrite nanoparticles inside the wood cell walls. Ferrihydrite is a widespread, naturally occurring iron oxide that exists in some soils, as well as in the ultrahard teeth of marine chitons and limpets.
Prior research by the group had shown that this technique resulted in ferrihydrite mineralization of all cell wall layers of the wood. In the new study, the team examined the impact of this treatment on the mechanical properties of the wood at multiple scales. They found that at the nanoscopic and microscopic scales, the mineralized wood was significantly harder and stiffer than the untreated wood, without significantly increasing the wood’s weight. In other words, the mineral-infused layers bore loads more effectively and resisted wear and tear far better than their natural counterparts.
These improvements at the subcellular level, however, did not translate to improved performance in the bulk mechanical testing, which evaluates stiffness, hardness, and failure strength of wood at the macroscopic scale. This test involved applying force to a wood beam until it bent or broke. The mineral treatment slightly increased the wood’s stiffness (by about 10 percent) but did not significantly improve its resistance to breaking. In fact, the treated wood’s strength decreased slightly, by approximately 7 percent. Neither the increased stiffness nor the reduced strength was statistically significant, but the findings suggest that while the ferrihydrite strengthened individual cell walls, the experimental treatment (including the use of chemicals to pretreat the wood) may have weakened the connections between cells.
Despite the lack of improved performance in the bulk testing, this work highlights the promise of bioengineering wood using low-cost, simple, and environmentally benign materials and processes. Future efforts may focus on gentler chemicals or alternative minerals to preserve cell-to-cell adhesion in the pursuit of more sustainable construction and manufacturing products.