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

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).

Figure 1

pH_pzc of the SD, ST (a), CF, and SS (b) adsorbents (SE = ±1.11% as error bars).

Figure 2

FTIR and XRD spectra for SD, ST (a,c), CF, and SS (b,d).

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

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

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

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

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

Adsorption mechanism of OMW organic matter into amorphous and crystalline cellulose, lignin, chitin, and keratin.