Fixed-Bed Column Treatment of Textile Dye Wastewater by Silicon Nitride Derived From Sand, Coffee Husk and Macadamia Nutshells
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Date
2025-11
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Kenyatta University
Abstract
To address the challenge of increased demand for clean water globally, there is a need to
treat the available wastewater. Textile dyes are carcinogenic and their discharge to the
environment has been reported to degrade the esthetic properties of water bodies. It is
imperative to assess textile dye levels in wastewater as well as decontamination to
ascertain its safety. Conventional purification processes suffer from limitations in terms
of efficiency, cost effectiveness and environmental effect. Attention has recently shifted
to adsorption as an economical technique for textile dyes removal using low-cost
adsorbents. Silicon nitride contains positively and negatively charged surface capable of
adsorbing cationic and anionic pollutants from water. This research studied the use of
silicon nitride adsorbent prepared from extracted silica from sand and biochars from
macadamia nutshell and coffee husk wastes in textile dye wastewater treatment. The
silica extraction from the raw sand was carried out via alkali fusion process. The biochars
were obtained by pyrolytic treatment of their raw biomass materials at 300 °C for 5 hours
then acid leached. Both the biochar and extracted silica materials were used in the silicon
nitride (Si3N4) adsorbents synthesis in an ammonia (NH3) environment. The prepared
materials were characterized by the use of X-Ray Diffractometer (XRD),
Thermogravimetric Analyzer (TGA), Scanning Electron Microscope (SEM), Fourier
Transform Infrared (FT-IR), X-Ray Fluorescence (XRF) and Energy Dispersive X-Ray
(EDX). The findings indicated that the raw sand contain quartz (SiO2), microcline (K (Al,
Fe) Si3O8), calcite (CaCO3) and clinochlore ((Mg, Fe)6(Si, Al)4O10(OH)8)). The extracted
silica (ES) results showed a pure and amorphous composites with siloxane (Si-O-Si) and
silanol (Si-OH) groups. The acid treatment and alkali fusion improved the silica (SiO2)
content to > 80 % with other oxide impurities decreased to < 1 %. The biochar results
showed an amorphous, pure and porous carbon structure with aromatic bonds (C=C). The
spectroscopic analysis of silicon nitride (Si3N4) adsorbents showed porous and thermally
stable adsorbents with α-Si3N4 and β- Si3N4 phases, functional groups of silicon-nitrogen
(Si-N), silanol (Si-OH) and silicon-nitrogen-silicon (Si-N-Si) which were hydroxylated in
an aqueous media to give silazane (Si2=NH2+) and silanolate (Si-O-) adsorbent sites for
congo red (CR), methylene blue (MB) and methyl orange (MO) dyes removal.
Adsorption capacity ranged from 26.93 ± 0.03 mg/g to 31.77 ± 0.15 mg/g at optimal
conditions for the dyes removal. The equilibrium and kinetic plots indicated that
Langmuir, Dubinin-Radushkevich and Pseudo-second-order models (R2 > 0.9) were more
suitable in explaining the chemisorption process. The adsorption-desorption studies using
1.0 × 10-1 M NaOH and 1.0 × 10-1 M HCl showed that the SN-5 adsorbent can be re-used
repeatedly with minimal loss. The column sorption studies gave a maximum column
capacity of 50.76 ± 0.17 mg/g (MO), 47.01 ± 0.14 mg/g (MB) and 52.56 ± 0.04 mg/g
(CR) at 4 mL/min, 7 cm and 50 mg/L optimal values. The column data obtained from the
breakthrough curves conformed with Yoon-Nelson and Thomas models (R2 > 0.9) in
describing fixed-bed operations. The column performance showed that SN-5 had a lower
efficacy using spiked water samples than using model solutions with column adsorption
capacity ranging from 38.92 ± 0.08 mg/g to 46.38 ± 0.05 mg/g. The column regeneration
showed a good reusability of the SN-5 adsorbent in textile dye wastewater treatment. The
research findings report silicon nitride as a potential adsorbent that can be applied in
purification of water to safe levels both at household and industrial scale
Description
A Research Thesis Submitted in Fulfillment of the Requirements for the Award of the Degree of Doctor of Philosophy in Chemistry in the School of Pure and Applied Sciences of Kenyatta University. November, 2025
supervisors
Ruth N. Wanjau
Esther W. Nthiga