HIGHLIGHTS
• Mechanical properties of wood, such as stiffness, were improved through the incorporation of the mineral ferrihydrite, leading to the potential for using this sustainable material in structural applications.
• Ferrihydrite was added by immersing red oak samples of various sizes and directions in a solution containing ferric nitrate and potassium hydroxide.
• X-ray microtomography was used to determine that the nano-iron particles became embedded within the cell walls of the wood.

Structural materials based on metals and concrete are typically produced using energy intensive processes. Efforts have been underway to determine how to take advantage of the characteristics of wood, a renewable material, in the preparation of advanced high-performance materials.

In a previous TLT article,¹ researchers produced a synthetic material with the physical properties of wood by electrodeposition of nickel into the voids of polystyrene spheres. The resulting material contains interconnected spherical pores in a face-centered cubic orientation. Exceptionally high strength was achieved due to the presence of nanopillars. This material was designated as a “metallic wood” due to its porous structure. Mechanical properties were found to be close to the theoretical strength of nickel due to the presence of nanopillars. The researchers considered the nickel-based metallic wood to exhibit the strength of titanium and the density of water. 

Wood or lignocellulose demonstrates mechanical strength due to the presence of three structural polymers: cellulose, hemicellulose and lignin. This characteristic is combined with high porosity to enable wood to exhibit strong mechanical stability at a low specific density.

Dr. Vivian Merk, assistant professor in the department of chemistry and biochemistry and the department of ocean and mechanical engineering at Florida Atlantic University in Boca Raton, Fla., says, “On the macroscale, wood consists of a honeycomb network of hollow cells joined by a compound middle lamella. In moving to the microscale, wood cell walls contain oriented microfibrils that are the building blocks of this material. They contain crystalline cellulose that is embedded within a matrix of hemicellulose and lignin. The angle of microfibrils oriented roughly parallel to the tree axis, known as the microfibril angle, results in the strong mechanical features of wood.”

To demonstrate performance comparable to existing structural materials, wood needs improvement in two areas. Merk says, “We strived to improve the mechanical properties of wood, while keeping this material’s unique properties, such as its low density. In addition, the resulting material must not be too expensive or harmful to the environment.”

One clue for how to approach this challenge is to understand the source of the hard teeth grown by molluscs such as limpet and chiton. Merk says, “Both of these animals take advantage of iron oxide minerals such as ferrihydrite in their environments to strengthen organic chitin scaffolds into ultrahard teeth.”

This mineralization effect was used by Merk and her colleagues to improve the mechanical properties of wood.


The researchers added ferrihydrite to red oak wood because of its unique structure, including thick, short fibers that provide mechanical support and large, interconnected vessels.

Nano-iron hydroxide
Modification of wood to improve its properties was achieved using the mineral ferrihydrite, which is poorly crystalline hydrous ferric oxyhydroxide. Merk says, “Ferrihydrite is a naturally occurring mineral that is part of soil and sediments. We synthesized ferrihydrite through the use of ferric nitrate and potassium hydroxide. The reaction is sustainable because no organic solvents were required, and the process took place at room temperature.”

The researchers added ferrihydrite to red oak wood. Merk says, “Red oak is a ring-porous hardwood, which is typically denser and more durable than softwoods. The reason for testing the modification of hardwood, which is often used in furniture or flooring, was its unique structure, including thick, short fibers that provide mechanical support and large, interconnected vessels that facilitate water conduction.”

Ferrihydrite was introduced by immersing wood samples of various sizes and directions in a solution containing ferric nitrate and potassium hydroxide. Vacuum impregnation was used to impregnate the wood structure with the chemical solutions. 

X-ray microtomography was utilized to demonstrate that nano-iron hydroxide particles were present in the wood cell wall. The distribution of the nano-iron hydroxide particles can be seen in two images in Figure 2.


Figure 2. Two X-ray microtomography images show the distribution of nano-iron particles in the wood cell wall. The images were produced in turquoise. Figure courtesy of Florida Atlantic University.

Merk says, “The nano-iron hydroxide particles become embedded within the cell walls of the wood and not in the hollow spaces. We believe that the pH of the process was significant in getting this result. Reacting ferric nitrate and potassium hydroxide at a pH between 8 and 9 led to the formation of ferrihydrite in the form of nano-iron oxide particles. This leads to an improvement in mechanical stability.”

A series of mechanical tests were conducted to evaluate the performance of nano-iron impregnated wood. Hardness and rigidity properties were determined by mounting a nanoindenter inside a scanning electron microscope. The values for stiffness (measured by Young’s modulus) and hardness were found to increase significantly as compared to untreated wood.

The elastic and viscoelastic properties of the nano-iron impregnated wood were measured using functional atomic microscopy. The researchers found a slightly wider spread of stiffness values in modified wood leading to the belief that there was a heterogeneous distribution of nano-iron across the cell wall. Young’s modulus values were also significantly higher, which is consistent with findings from the nanoindentation testing.

Bulk mechanical testing was carried out to determine flexural modulus. In this phase of the study, no significant difference was found between nano-iron impregnated wood and untreated wood. Merk says, “We believe the results from the bulk testing are due to the weaker adhesion between adjacent cells.”

This study shows the potential for using minerals to improve the mechanical properties of wood to make it suitable for use in structural applications. Additional information can be found in a recent paper² or by contacting Merk at vmerk@fau.edu. 

REFERENCES
1. Canter, N. (2019), “New material with strength of titanium and density of water,” TLT, 75 (5), pp. 16-17. Available at www.stle.org/files/TLTArchives/2019/05_May/Tech_Beat_I.aspx.
2. Soini, S., Lalani, I., Maron, M., Gonzalez, D., Mahfuz, H., Marimon, N. and Merk, V. (2025), “Multiscale mechanical characterization of mineral-reinforced wood cell walls,” ACS Applied Materials and Interfaces, 17 (12), pp. 18887-18896.