Physicochemical properties of hydroxyapatite modified with vanadium ions for degradation of methylene blue

Highlights

• Hydroxyapatite modified with different additional vanadium ions has been fabricated using a precipitation method.

• Direct and indirect bandgaps achieved around 3.9 and 4 eV at x = 0.0 and decrease, reaching 3.82 and 3.94 eV at x = 1.2, respectively.

• Methylene blue removal efficiency is rising with vanadate ions attained its max at x = 1.2 recorded 97.2% after 120 min of continuous exposure to visible light radiation.

Abstract

Degradation of pollutants, including organic dyes via photocatalysis technique, has attracted great concern last decade. Thus, modified hydroxyapatite (HAP) with vanadate ions ($V{O}_4^{3-}$) has been fabricated using the co-precipitation method at different vanadate contributions. The obtained compositions have been investigated upon their structure, morphology, and optical properties. The structural calculations indicated that a-axis increased from 9.483 to 9.562 Å for the lowest and the highest additional vanadate ions. Moreover, the average roughness developed from 30.5 to 56.3 nm upon the vanadate insertion. Moreover, the direct bandgap decreased from 3.9 to 3.82 eV upon the additional vanadate ions, while the refractive index increased slightly from 1.86 to 1.87. Further, the removal of methylene blue through aqueous solutions has been examined and showed that the removal effectiveness increased from 89.3 to 97.2% after 120 min of irradiation using visible light. These modified HAP crystals could encourage their utilization for potential applications incusing water treatment, besides their usages for biomedical applications.

Introduction

Pollution is considered one of the main global sufferings, especially water pollution, reducing the accessibility of freshwater. Water pollutants include industrial by-products like dyes, chemicals, metals, lubricants etc. [1]. Several industrials types, including textiles, printing, food, and paper, are responsible for dye releasing, which is considered the main reason for water pollution [2]. The textile industry uses about ten thousand tons of dye per year [3], and around 50% of those dyes have an azo functional group in their molecular configuration [4]. The azo group in dye structures is carcinogenic, leading to health problems if they reached humans. Methylene blue (MB) dye used in the textile industry and its inhalation may cause disorders in the digestive, respiratory, and nervous systems [5]. Therefore, several techniques have been established for the degradation of these dyes [6]. Among these strategies, photocatalysis is one of the most promising dye removal methods due to its feasibility and availability. It might be reported that particle size, surface area, bandgap, and amount of tested material were the main parameters' photocatalytic process [7]. It has been observed that surface area controls dye molecules' adsorption capacity upon the photocatalyst surface [8].

Pure calcium phosphate (CP) shows sorption properties. Moreover, Hao et al. studied HAP morphology and its influence on removing Pb(II). They found that the dandelion-like shape exhibited a high adsorption capacity around 819.7 mg/g [9]. Additionally, calcium phosphates (CPs) possess adequate properties, including thermal and chemical stability, bioactivity, biocompatibility, low cytotoxicity, low fabrication cost, low solubility in water, which support their utilization as promising sorbents [10]. Among CPs phases, hydroxyapatite (HAP) is an attractive form to be used for numerous applications due to its similarity to the mineral part of hard tissue. Furthermore, modifications of HAP via doping strategy have displayed promising configurations in both medical and environmental applications.

Numerous researches estimate precise parameters to control surface, the mechanical and physicochemical performance of HAP. The lattice structure of HAP encourages ionic substitution to be done in various crystallographic sites. Mono or divalent cations such as Ag₊, K₊, Mg²⁺, Fe²⁺, and Zn²⁺can substitute Ca²⁺positions, die, or trivalent anions such as silicate, carbonate, or vanadate prefer to replace phosphate positions. Each of these ionic substitutions causes physicochemical consequences, including thermal and chemical stability, besides the biological response involving cytotoxicity and optical behaviors such as bandgap and photoluminescence approaches [11]. HAP has been examined for exceptional sorption features, which have been utilized to restrain heavy metals [12], radionuclides [13], and organic pollutants [14].

Vanadate ions act as a structured controller in optimizing some appropriate adsorbent materials such as Polyethersulfone (PES) nanofiber, which has a high surface area; besides the addition of surface hydroxyl group that enhances adsorption behavior and removal capacity [15], Vanadium pentoxide (V₂O₅) is a ceramic composition which was used for various applications such as optical devices, catalysis, solar cell, and sensors. The adsorption mechanism depends strongly on the surface charge of the solutions. Therefore surface is negatively charged, can optimally catch cationic dyes [16].

Y. Zhou et al. used the Sr doped HAP for adsorption of Cr (VI) from aqueous solutions, and the removal effectiveness reached around 443 mg/g [17]. It was rare to find previous studies were based on the ionic replacement of vanadate ($V{O}_4^{3-}$) and phosphate ($P{O}_4^{3-}$) ions. It might be reported that HAP possesses high affinity to abdorb carbonate ions from the ambient atmosphere through the synthesis stage. These ions are suggested to occupy hydroxyl (OH)⁻ or phosphate ($P{O}_4^{3-}$), or both of them simultaneously. In our case, vanadate ions are suggested to compete with carbonate ions to replace phosphate sites. This complicated process induces significant crystallographic distortion to occur. Further, the physical properties of the obtained compositions can be changed dramatically.

Having the ideas mentioned above, for successful dye removal, the microstructure, porosity shape, bandgap, and surface area must be put into consideration. This work aimed to investigate the behavior of structural modification of HAP via vanadate ions for photocatalytic behavior towards dye removal. Besides, the morphological features, the optical properties, the cytotoxicity towards human cells in vitro will be explored.