Air-Exposure-Driven Color and Optical Variations in Hydroxyapatite Extracted from Fish Scales

Year 2025, Volume: 29 Issue: 1, 125 - 139, 28.02.2025

H. Esma Okur

https://doi.org/10.16984/saufenbilder.1589238

Abstract

The disposal of fish scales as waste presents an environmental challenge and an untapped opportunity for resource recovery. In this study, hydroxyapatite (HAp) was extracted from European seabass (Dicentrarchus labrax) scales to explore how air exposure during calcination affects its optical and surface properties. HAp powders were prepared under two distinct calcination conditions: fully exposed to air (producing white powder) and partially shielded from air (resulting in gray powder). Rietveld refinement of X-ray powder diffraction (XRPD) data confirms that both powders crystallize in the hexagonal HAp structure, with a minor Mg-whitlockite impurity. Despite these differences in air exposure, the bulk structure of the HAp remains unchanged. The color variations are linked to surface oxidation, as subsurface layers in the partially shielded scales retains a grayish tone while the exposed surfaces turn completely white. Scanning electron microscopy reveals subtle differences in particle morphology: the white powder had a smoother surface compared to the slightly rougher gray powder. Fourier transform infrared spectra confirms the presence of characteristic phosphate and hydroxyl groups in both powders, indicating that the core chemical structure of HAp is intact in both cases. The Ca/P ratios—1.504(7) for the white powder and 1.505(7) for the gray powder obtained from the Rietveld analysis—further supports the stoichiometric integrity of the material. UV-Vis spectroscopy reveals direct bandgap values of 3.99 eV for the white powder and 3.87 eV for the gray powder. These bandgap values, which are lower than those typically reported for defect-free HAp (5–6 eV), suggest that the optical differences between the powders are driven by surface effects, such as oxygen vacancies or trace impurities. This study highlights how calcination conditions, particularly air exposure, influence surface properties and optical behavior, paving the way for potential applications of fish-scale-derived HAp in electronic and optical materials.

Keywords

Hydroxyapatite, Fish scales, Band gap, Air exposure, Rietveld refinement

Balık Pullarından Elde Edilen Hidroksiapatitin Renk ve Optik Özelliklerinde Hava Maruziyetine Bağlı Değişimler

Abstract

Balık pullarının atık olarak bertaraf edilmesi, çevresel bir sorun ve kaynak geri kazanımı için değerlendirilmeyen bir fırsat sunmaktadır. Bu çalışmada, Avrupa levreği (Dicentrarchus labrax) pullarından hidroksiapatit (HAp) elde edilerek, kalsinasyon sırasında hava maruziyetinin optik ve yüzey özellikleri üzerindeki etkisi araştırılmıştır. HAp tozları, tamamen havaya maruz bırakılan (beyaz toz üreten) ve kısmen havadan korunan (gri toz üreten) iki farklı kalsinasyon koşulunda hazırlanmıştır. X-ışını toz kırınımı (XRPD) verilerinin Rietveld analizi, her iki tozun da hekzagonal HAp yapısında kristalleştiğini ve az miktarda Mg-whitlockite safsızlığı içerdiğini doğrulamaktadır. Hava maruziyetindeki bu farklılıklara rağmen, HAp'ın kristal yapısı değişmeden kalmaktadır. Renk farklılıkları yüzey oksidasyonuna bağlanmakta, çünkü kısmen korunmuş pulların alt katmanları gri tonunu korurken, açıkta kalan yüzeyler tamamen beyaza dönmektedir. Taramalı elektron mikroskobu, beyaz tozun daha düzgün bir yüzeye, gri tozun ise hafifçe pürüzlü bir yüzeye sahip olduğunu göstermektedir. Fourier dönüşümlü kızılötesi (FTIR) spektrumları, her iki tozda da karakteristik fosfat ve hidroksil gruplarının varlığını doğrulamakta, HAp'ın çekirdek kimyasal yapısının her iki durumda da korunduğunu göstermektedir. Rietveld analizi ile elde edilen beyaz toz için 1.504(7) ve gri toz için 1.505(7) olarak belirlenen Ca/P oranları, malzemenin stokiyometrik bütünlüğünü desteklemektedir. UV-Vis spektroskopisi, beyaz toz için 3.99 eV ve gri toz için 3.87 eV bant aralığı değerlerini ortaya koymaktadır. Bu bant aralığı değerleri, kusursuz HAp için genellikle bildirilen (5–6 eV) değerlerden daha düşük olup, optik farklılıkların iç yapısındaki değişikliklerden ziyade oksijen boşlukları veya safsızlıklar gibi yüzey etkilerinden kaynaklandığını önermektedir. Bu çalışma, kalsinasyon koşullarının, özellikle hava maruziyetinin, HAp'ın yüzey özellikleri ve optik davranışını nasıl etkilediğini vurgulamakta ve balık pullarından türetilmiş HAp'ın elektronik ve optik malzemelerdeki potansiyel uygulamalarına ışık tutmaktadır.

Keywords

Hidroksiapatit, balık pulları, band boşluğu, hava mazuriyeti, Rietveld analizi

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