Sorption of antimony onto hydroxyapatite

Antimony contamination in water sources remains a challenge due to its toxicity and mobility. Existing methods for removing antimony often require high energy input or produce secondary waste. Hydroxyapatite has shown promise in removing various heavy metals, but its effectiveness for trivalent antimony is not well established. Prior research has shown that hydroxyapatite can interact with metal ions through surface adsorption and ion exchange. However, the specific mechanisms for Sb(III) removal using HAP are not fully understood. This gap motivated the current investigation into the sorption behavior of Sb(III) onto hydroxyapatite. No prior work had resolved the kinetics and equilibrium parameters for Sb(III) sorption under controlled pH and ionic strength conditions. The study aimed to clarify the role of surface charge and solution chemistry in the process. Understanding these factors is essential for optimizing HAP as a sorbent in water treatment systems.

The study aimed to evaluate the capacity of synthetic hydroxyapatite to remove trivalent antimony from aqueous solutions. Researchers focused on the sorption kinetics and equilibrium behavior of Sb(III) onto HAP under controlled experimental conditions. The goal was to quantify how quickly and how much Sb(III) could be removed by the material. They also sought to understand the surface chemistry changes during the process. The investigation considered the influence of pH and ionic strength on sorption efficiency. The researchers aimed to determine the maximum sorption capacity and the adsorption constant. They also wanted to identify the likely surface reactions between Sb(III) and HAP. This work contributes to the development of low-cost, efficient water treatment methods.

The researchers synthesized hydroxyapatite using a chemical precipitation method. They characterized the material using X-ray diffraction to confirm its crystalline structure. Scanning electron microscopy provided information about the particle morphology. X-ray energy dispersive spectroscopy was used to analyze elemental composition. X-ray photoelectron spectroscopy helped determine surface chemical states. Infrared spectroscopy was applied to investigate functional groups. The sorption experiments were conducted in closed vessels to prevent atmospheric interference. The researchers varied the Sb(III) concentration and measured the adsorption over time and at equilibrium.

The study found that more than 95% of Sb(III) was removed from solution within 30 minutes. The equilibrium data fit a Langmuir model with a maximum sorption capacity of 6.7 ± 0.1 × 10⁻⁸ mol m⁻². The adsorption constant was 1.5 ± 0.2 × 10³ dm³ mol⁻¹. The isoelectric point of the HAP suspensions decreased from 7.7 to 6.9 during Sb(III) adsorption. This shift suggests an increase in the negative surface potential of HAP. The researchers observed that Ca was enriched in the HAP particles after dissolution. The decline in pH(iep) supports the idea that Sb(III) adsorbed as a charged species. The results indicate that surface reactions likely involve the exchange of phosphate and hydroxide ions.

The study supports the use of hydroxyapatite as an effective sorbent for trivalent antimony in water. The high removal efficiency within 30 minutes suggests a rapid adsorption process. The Langmuir model parameters indicate a monolayer adsorption mechanism. The observed decrease in pH(iep) supports the hypothesis that Sb(III) interacts with HAP through surface charge changes. The presence of Ca enrichment suggests that incongruent dissolution plays a role in the process. The surface reactions likely involve the exchange of phosphate and hydroxide ions. The results suggest that HAP can be a viable material for water treatment applications. The findings align with the authors' claim that phosphate enrichment enhances Sb removal efficiency.