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. 2025 Aug 11;26(16):7738.
doi: 10.3390/ijms26167738.

Unveiling the Adsorptive Potential of Natural Biopolymers for Olive Mill Wastewater Treatment: A Synergistic Approach Using RSM-BBD, Mixture Design, Kinetics, and Mechanistic Analysis

Sabah Elamraoui  1 , Nouhaila Asdiou  1 , Rachid El Kaim Billah  1 , Mounir El Achaby  2 , Said Kounbach  3 , Rachid Benhida  3   4 , Mounia Achak  1   3
Affiliations

Unveiling the Adsorptive Potential of Natural Biopolymers for Olive Mill Wastewater Treatment: A Synergistic Approach Using RSM-BBD, Mixture Design, Kinetics, and Mechanistic Analysis

Sabah Elamraoui et al. Int J Mol Sci. .

Abstract

This study evaluates the structural properties and adsorption capacities of four bio-based adsorbents, sawdust (SD), straw (ST), chicken feathers (CFs), and shrimp shells (SSs), for chemical oxygen demand (COD) removal from olive mill wastewater (OMW). Response Surface Methodology (RSM) with a Box-Behnken Design (BBD) was applied to optimize the operational parameters, resulting in maximum COD uptake capacities of 450 mg/g (SD), 575 mg/g (ST), 700 mg/g (CFs), and 750 mg/g (SSs). Among these materials, SSs exhibited the highest COD removal efficiency of 85% under optimal conditions (pH 8, 20 g/L, 30 °C, 5 h, 111 rpm). A mixture design approach was then used to explore the synergistic effects of combining lignocellulosic (SD and ST), chitin-based (SSs), and keratin-based (CFs) adsorbents. The optimized blend (SD 10%, ST 28.9%, SS 38.3%, and CF 22.6%) achieved a COD removal efficiency of 82%, demonstrating the advantage of using mixed biopolymer systems over individual adsorbents. Adsorption mechanisms were investigated through isotherm models (Langmuir, Freundlich, Temkin, and Redlich-Peterson) and kinetic models (pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion). Lignocellulosic adsorbents predominantly followed physisorption mechanisms, while chitin- and keratin-rich materials exhibited a combination of physisorption and chemisorption. Thermodynamic analysis confirmed the spontaneous nature of the adsorption process, with SSs showing the most favorable Gibbs free energy (ΔG = -21.29 kJ/mol). A proposed mechanism for the adsorption of organic compounds onto the bio-adsorbents involves hydrogen bonding, electrostatic interactions, π-π interactions, n-π stacking interactions, hydrophobic interactions, and van der Waals forces. These findings highlight the potential of biopolymer-based adsorbents and their optimized combinations as cost-effective and sustainable solutions for OMW treatment.

Keywords: COD; adsorption; chitin; keratin; lignocellulose; olive mill wastewater (OMW).

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Figure 1
Figure 1
pHpzc of the SD, ST (a), CF, and SS (b) adsorbents (SE = ±1.11% as error bars).
Figure 2
Figure 2
FTIR and XRD spectra for SD, ST (a,c), CF, and SS (b,d).
Figure 3
Figure 3
(a) SEM micrographs of the samples SD, ST, CFs, and SSs; (b) EDX of the samples SD, ST, CFs, and SSs.
Figure 4
Figure 4
Evaluation of COD adsorption by ST and SD under varying conditions (a) pH, (b) mass, (c) time, (d) temperature and (e) stirring speed effect using BBD (SE = ±2.25% as error bars).
Figure 5
Figure 5
Evaluation of COD adsorption by CFs under varying conditions (a) pH, (b) mass, (c) time, (d) temperature and (e) stirring speed effect using BBD (SE = ±2.25% as error bars).
Figure 6
Figure 6
Evaluation of COD adsorption by SSs under varying conditions (a) pH, (b) mass, (c) time, (d) temperature and (e) stirring speed effect using BBD (SE = ±2.25% as error bars).
Figure 7
Figure 7
Contour plot and 2D and 3D response surface variations for COD removal by adsorbent mixtures (blue color: low, green: moderate, and orange-red color: high removal).
Figure 8
Figure 8
Adsorption mechanism of OMW organic matter into amorphous and crystalline cellulose, lignin, chitin, and keratin.
See this image and copyright information in PMC

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