Facile synthesis and characterization of multi-walled carbon nanotubes decorated with hydroxyapatite from cattle horns for adsorptive removal of fluoride

Walter Ojok; James P Bolender; John Wasswa; Emmanuel Ntambi; William Wanasolo; Brenda Moodley

doi:10.1016/j.heliyon.2023.e14341

Facile synthesis and characterization of multi-walled carbon nanotubes decorated with hydroxyapatite from cattle horns for adsorptive removal of fluoride

Heliyon. 2023 Mar 9;9(3):e14341. doi: 10.1016/j.heliyon.2023.e14341. eCollection 2023 Mar.

Authors

Walter Ojok^{1

2

3}, James P Bolender⁴, John Wasswa⁵, Emmanuel Ntambi¹, William Wanasolo⁶, Brenda Moodley³

Affiliations

¹ Department of Chemistry, Faculty of Science, Mbarara University of Science and Technology, P.O Box 1410, Mbarara, Uganda.
² Department of Chemistry, Faculty of Science, Muni University, P.O Box 725, Arua, Uganda.
³ School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, 4000, South Africa.
⁴ Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, USA.
⁵ Department of Chemistry, College of Natural Sciences, Makerere University, P.O Box 7062, Kampala, Uganda.
⁶ Department of Chemistry, Faculty of Science, Kyambogo University, P.O Box 1 Kyambogo, Kampala, Uganda.

Abstract

Developing a new adsorbent for fluoride removal from cattle horn waste materials by a facile chemical method has shown great potential for fluoride removal. This paper reports the synthesis of multi-walled carbon nanotubes decorated with hydroxyapatite from cattle horns (MWCNT-CH) using a facile chemical method. Characterization studies using standard techniques showed that the composite is mesoporous with a rough morphology and contained MWCNTs uniformly encapsulated by the hydroxyapatite forming a crystalline MWCNT-CH composite. Optimization of fluoride adsorption by the as-synthesized composite using Response Surface Methodology (RSM) showed that a maximum fluoride removal efficiency of 80.21% can be attained at initial fluoride concentration = 10 mg/L, pH = 5.25, adsorbent dose = 0.5 g and a contact time of 78 min. ANOVA indicates contribution of the process variables in descending order as pH > contact time > adsorbent dose > initial fluoride concentration. Langmuir isotherm (R² = 0.9991) best described the process, and the maximum adsorption capacity of fluoride onto the as-synthesized MWCNT-CH composite was 41.7 mg/g. Adsorption kinetics data were best fitted in the pseudo-second-order kinetic model (R² = 0.9969), indicating chemisorption. The thermodynamic parameter ( $Δ$ H = 13.95 J/mol and $Δ$ S = 65.76 J/mol/K) showed that fluoride adsorption onto the MWCNT-CH composite was a spontaneous, endothermic, and entropy-driving process. Moreover, the adsorption mechanism involves ion exchange, electrostatic interaction, and hydrogen bonding. Fluoride was successfully desorbed (using 0.1 M NaOH) from the composite in four cycles, retaining fluoride removal efficiency in the fourth cycle of 57.3%.

Keywords: Cattle horn; Fluoride adsorption; Hydroxyapatite; Kinetics; Multi-walled carbon nanotubes; Response surface methodology.