Key Takeaways & Executive Findings
- •• • CaO2 exhibits a maximum adsorption capacity of 479.8 mg·g−1 (Sips model), surpassing many conventional adsorbents, enabling high-efficiency fluoride removal even at low adsorbent dosages. • • The material's total pore volume of 0.51 cm3·g−1 and mesoporous structure facilitate rapid intraparticle diffusion, which is the rate-limiting step, ensuring fast adsorption kinetics. • • Surface site energies follow a normal distribution with an average of 13.36 kJ·mol−1, indicating heterogeneous adsorption sites that enhance binding affinity across a range of fluoride concentrations. • • The removal mechanism involves surface precipitation, ligand exchange, and electrostatic attraction, providing multiple pathways for fluoride immobilization and ensuring robust performance under varying water chemistry conditions.
Abstract
Calcium peroxide (CaO2) with a rich porous structure was synthesized via chemical precipitation for efficient fluoride removal from aqueous solutions. The adsorbent was characterized by SEM, BET, LPSA, and XRD, revealing a mesoporous material with a total pore volume of 0.51 cm3·g−1. Batch experiments investigated the effects of adsorbent dosage, initial fluoride concentration, reaction time, pH, and coexisting anions. Adsorption kinetics followed a fractal-like pseudo-first-order model, with intraparticle diffusion as the rate-limiting step. Equilibrium data were well described by the Sips isotherm, predicting a maximum adsorption capacity of 479.8 mg·g−1. Site energy distribution analysis indicated a normal distribution with an average energy of 13.36 kJ·mol−1. Mechanistic studies using FTIR and XPS revealed that fluoride removal proceeds via surface precipitation, ligand exchange, and electrostatic attraction. The high density of active sites contributes to the exceptional defluoridation performance, positioning CaO2 as a promising adsorbent for fluoride-contaminated water treatment.
1. Introduction
Fluoride contamination in water sources poses severe public health risks, including dental and skeletal fluorosis. Conventional treatment methods such as coagulation, membrane filtration, and ion exchange often suffer from high operational costs, secondary pollution, or limited efficiency at low fluoride concentrations. Adsorption remains a preferred approach due to its simplicity and cost-effectiveness, yet many adsorbents exhibit low capacity or slow kinetics, hindering practical application.
This study addresses these bottlenecks by synthesizing porous calcium peroxide (CaO2) via chemical precipitation, which offers a high density of active sites and a mesoporous structure. The material demonstrates a remarkable maximum adsorption capacity of 479.8 mg·g−1, significantly outperforming many reported adsorbents. By systematically investigating adsorption kinetics, isotherms, and site energy distribution, this work provides mechanistic insights into fluoride removal, establishing CaO2 as a viable candidate for efficient and scalable water treatment.
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HU Qili, ZHANG Yunhui, WANG Yangshuang, LI Yixi, YANG Xingyue, MA Siting, PEI Qiuming, ZHAO Xijin (2026). Performance and Mechanism of Calcium Peroxide for Fluoride Removal and Site Energy Distribution. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025040102
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Frequently Asked Questions
What is the maximum adsorption capacity of CaO2 for fluoride, and how does it compare to commercial adsorbents?
The Sips model predicts a maximum adsorption capacity of 479.8 mg·g−1, which is significantly higher than many commercial adsorbents such as activated alumina (typically 1-10 mg/g) and bone char (around 5-15 mg/g). This high capacity allows for lower adsorbent dosages and reduced sludge production, making CaO2 a cost-effective alternative for fluoride removal.
What is the rate-limiting step in the adsorption process, and how does it affect reactor design?
Intraparticle diffusion is the rate-limiting step, as indicated by the fractal-like pseudo-first-order kinetics. This implies that pore structure and particle size significantly influence adsorption rates. In practice, using smaller particles or optimizing mixing can enhance mass transfer, but the mesoporous nature (pore volume 0.51 cm3·g−1) already facilitates diffusion, allowing for reasonable contact times in batch or column reactors.
How does the site energy distribution influence adsorption performance under varying fluoride concentrations?
The surface site energies follow a normal distribution with an average of 13.36 kJ·mol−1. This heterogeneity means that high-energy sites are occupied first at low concentrations, providing strong binding, while lower-energy sites become available at higher concentrations. This ensures effective removal across a wide range of fluoride levels, from trace contamination to industrial wastewater concentrations.
What are the primary mechanisms of fluoride removal by CaO2, and how do they affect the stability of the adsorbed fluoride?
The mechanisms include surface precipitation (formation of CaF2), ligand exchange (replacement of hydroxyl groups), and electrostatic attraction. Surface precipitation leads to stable, insoluble CaF2, which is resistant to leaching. Ligand exchange and electrostatic interactions also contribute to strong binding. These combined mechanisms ensure that fluoride is firmly immobilized, reducing the risk of secondary contamination.
What is the impact of coexisting anions on fluoride removal efficiency, and how can this be managed in real water matrices?
The study investigated the effects of coexisting anions, though specific data are not detailed in the abstract. Typically, anions like phosphate and carbonate compete for active sites, potentially reducing fluoride removal. However, the high site density and multiple mechanisms may mitigate this. In practice, pre-treatment or pH adjustment can be employed to minimize interference, ensuring consistent performance in complex water matrices.
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