📄 chemistry

Insights into Kinetics, Equilibrium, and Thermodynamics of Uranium Uptake onto Fir Sawdust

This study demonstrates that fir sawdust is an effective biosorbent for removing uranium from wastewater via chemical adsorption and ion exchange mechanisms, achieving optimal removal at pH 4.5 with a maximum monolayer capacity of 148.81 mg g⁻¹.

Original authors: Zhengji Yi, Jinfeng Fu, Simin Zeng, Haifan Wang, Jian Liu, Wenjin Ni

Published 2026-09-18

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CC BY 4.0

Original authors: Zhengji Yi, Jinfeng Fu, Simin Zeng, Haifan Wang, Jian Liu, Wenjin Ni

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Uranium, a heavy metal used in energy production, poses a dual threat when it escapes into the environment: it is both chemically toxic and radioactive. When it finds its way into wastewater from mining or industrial processes, it can contaminate soil and water, eventually entering the food chain and harming human organs. Traditional methods to clean this water often rely on expensive chemicals or complex machinery that generate their own waste. In the search for a simpler, greener solution, scientists have turned to nature's own byproducts. One promising avenue involves using agricultural and forestry waste, such as sawdust, which is naturally rich in tiny chemical groups capable of grabbing onto metal ions. Unlike synthetic materials, this waste is abundant, cheap, and requires no complex chemical modification to work, offering a way to treat dangerous water while simultaneously recycling discarded wood.

A team of researchers at Hengyang Normal University in China investigated whether ordinary fir sawdust could effectively pull uranium out of water. They took raw sawdust, cleaned it, and dried it, then mixed it with water containing uranium to see how well it worked under different conditions. Their experiments revealed that the sawdust is highly effective, but only if the water has the right acidity. The material performed best when the water's pH was set to 4.5, a slightly acidic level. At this specific point, the sawdust could remove the maximum amount of uranium. If the water was too acidic, the hydrogen ions in the water crowded out the uranium, preventing it from sticking. If the water was too neutral or basic, the uranium changed its chemical shape and clumped together, making it harder for the sawdust to catch. Once the conditions were right, the sawdust worked quickly, reaching its maximum cleaning capacity within two hours.

The researchers wanted to understand exactly how the sawdust held onto the uranium, so they looked at the process through the lens of time and temperature. They found that the speed at which the uranium disappeared from the water followed a specific pattern known as chemical adsorption. This means the uranium didn't just sit loosely on the surface; it formed a strong chemical bond with the wood, much like a magnet snapping firmly into place. This bonding process was the main factor controlling how fast the cleaning happened. Furthermore, the team discovered that the process worked better in warmer water. As the temperature rose, the sawdust became even more eager to grab the uranium, suggesting that the reaction absorbs heat from its surroundings to proceed.

To see what was happening on a microscopic level, the scientists examined the sawdust before and after it had soaked up the uranium. Before treatment, the wood surface was rough and covered with natural fibers, but it contained no uranium. After the experiment, the surface was dotted with tiny, spherical particles, and chemical analysis confirmed that uranium was now firmly attached to the wood. The study identified the specific parts of the wood responsible for this capture: oxygen-rich groups like hydroxyls, carboxyls, and carbonyls. These groups act as tiny hands that reach out and grab the uranium ions. The process involves two main actions working together: the wood swaps its own hydrogen atoms for uranium atoms, and then the oxygen atoms in the wood form a stable, cage-like grip around the uranium. This dual mechanism allows the sawdust to hold onto the heavy metal securely without breaking down its own structure.

The capacity of this natural material is impressive. The researchers calculated that one gram of fir sawdust could hold up to 148.81 milligrams of uranium. This performance rivals or exceeds many other biomass materials tested for the same purpose, such as rice husks or biochar made from other plants. The study suggests that the uranium sits in a single, uniform layer on the surface of the wood, filling up the available spots until the material is saturated. Because the sawdust is unmodified and requires no harsh chemicals to prepare, it represents a practical, low-cost option for treating radioactive wastewater. By turning a common waste product into a tool for environmental cleanup, this research highlights a path toward sustainable water treatment that respects both the economy and the ecosystem.

Technical Summary: Insights into Kinetics, Equilibrium, and Thermodynamics of Uranium Uptake onto Fir Sawdust

Problem Statement
The discharge of uranium-bearing wastewater from mining, hydrometallurgy, and nuclear industries poses severe risks to aquatic ecosystems and human health due to the radiological and chemical toxicity of hexavalent uranium (U(VI)). Conventional treatment methods, such as chemical precipitation, solvent extraction, and activated carbon adsorption, often suffer from high operational costs, significant chemical consumption, and difficulties in regeneration. Consequently, there is an urgent need for low-cost, green, and efficient adsorbents. While agricultural and forestry wastes are promising candidates, systematic research on the direct application of unmodified fir sawdust for uranyl ion removal remains scarce.

Methodology
This study utilized raw, unmodified fir sawdust as a biosorbent for U(VI) removal from aqueous solutions. The sawdust was prepared by washing, drying at 80°C, and sieving to 100 mesh. Batch adsorption experiments were conducted to evaluate the effects of contact time, solution pH (2.0–7.0), temperature (298–318 K), and initial U(VI) concentration.

The research employed a multi-faceted analytical approach:

Kinetic Modeling: Data were fitted to pseudo-first-order, pseudo-second-order, and intraparticle diffusion models to determine rate-controlling steps.
Isotherm Modeling: Langmuir, Freundlich, and Temkin models were applied to describe equilibrium behavior.
Thermodynamic Analysis: Standard Gibbs free energy ( $\Delta G^\circ$ ), enthalpy ( $\Delta H^\circ$ ), and entropy ( $\Delta S^\circ$ ) were calculated to assess spontaneity and heat effects.
Characterization: Fourier-transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), and X-ray Photoelectron Spectroscopy (XPS) were used to analyze surface morphology, elemental composition, and functional group changes before and after adsorption.

Key Results

Optimal Conditions: The adsorption process reached equilibrium within 120 minutes. The optimal pH for U(VI) removal was determined to be 4.5. Below pH 4.5, competition from protons ( $H^+$ ) for active sites inhibited adsorption; above pH 5.0, the formation of polynuclear uranyl complexes and precipitation reduced uptake efficiency.
Kinetics: The adsorption data aligned best with the pseudo-second-order kinetic model ( $R^2 = 0.998$ ), suggesting that chemisorption is the rate-limiting step. The intraparticle diffusion model showed a poor fit ( $R^2 = 0.656$ ), indicating that diffusion within the pores is not the primary mechanism.
Isotherms: The Langmuir isotherm model provided the best fit ( $R^2 = 0.993$ ), indicating monolayer adsorption on a homogeneous surface. The maximum adsorption capacity ( $Q_{max}$ ) was calculated to be 148.81 mg/g, a value comparable to or exceeding many other biomass adsorbents reported in the literature.
Thermodynamics: The process was found to be spontaneous ( $\Delta G^\circ < 0$ ) and endothermic ( $\Delta H^\circ > 0$ ). The increase in the absolute value of $\Delta G^\circ$ with rising temperature suggests that higher temperatures enhance the spontaneity of the adsorption process.
Mechanism: Spectroscopic analyses revealed that oxygen-containing functional groups—specifically hydroxyl (-OH), carboxyl (-COOH), and carbonyl (C=O)—are the primary active sites. FTIR showed shifts in peak positions and intensities corresponding to these groups upon U(VI) binding. XPS and SEM-EDS confirmed the presence of uranium on the surface and the formation of spherical nanoparticles, likely chelating deposits. The mechanism is attributed to a synergistic effect of ion exchange (proton release from functional groups) and surface complexation (coordination between uranyl ions and oxygen atoms).

Significance and Claims
The authors conclude that unmodified fir sawdust is a highly effective, low-cost biosorbent for uranium removal. The study demonstrates that this material can achieve high removal efficiency without the need for complex chemical modifications, thereby avoiding secondary contamination from chemical agents. By utilizing a waste product of the timber industry, the approach simultaneously addresses two environmental challenges: the remediation of radioactive wastewater and the resourceful recycling of agricultural/forestry biomass. The findings provide a theoretical and experimental basis for the potential application of fir sawdust in the resource-based treatment of uranium-contaminated water.

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