Biosorption of Hexavalent Chromium onto Luffa cylindrica: Equilibrium and Kinetic Insights

Introduction: Hexavalent chromium (Cr(VI)) is a highly toxic heavy metal frequently discharged into the environment through industrial effluents from leather tanning, electroplating, textile, and metal-processing industries. Due to its strong oxidizing properties and high solubility in water, Cr(VI) poses severe environmental hazards, including contamination of surface and groundwater, as well as significant health risks to humans and aquatic life. Exposure to Cr(VI) has been linked to carcinogenic, mutagenic, and teratogenic effects, making its removal from wastewater a critical environmental concern. Conventional treatment methods, such as chemical precipitation, ion exchange, membrane filtration, and electrochemical reduction, while effective, are often associated with high operational costs, energy consumption, and the generation of secondary pollutants that require further treatment. In recent years, biosorption has emerged as an environmentally friendly and cost-effective alternative for heavy metal removal. This process involves the use of biological materials, including algae, fungi, bacteria, and plant-based biosorbents, to adsorb and concentrate metal ions from aqueous solutions. Among these, Luffa cylindrica, a widely available and renewable natural material, has gained attention due to its high surface area, porous structure, and abundance of functional groups capable of binding Cr(VI) ions. This review focuses on studies published between 2010 and 2020 that investigate the equilibrium, kinetics, and thermodynamic aspects of Cr(VI) biosorption using L. cylindrica, highlighting its potential as a sustainable and efficient biosorbent for industrial wastewater treatment.

2. Biosorption Capacity

Several studies have investigated the biosorption capacity of L. cylindrica for Cr(VI). Notable findings include:

• Laidani et al. (2023) reported an adsorption capacity of 29.98 mg/g under optimal conditions of pH 7.7 and 343 K
• Amin et al. (2018) found a capacity of 5.91 mg/g using the diphenylcarbazide method
• Khan et al. (2012) achieved a capacity of 6.5 mg/g with activated L. cylindrica

These variations can be attributed to differences in biosorbent preparation, solution chemistry, and experimental conditions.

3. Kinetic Studies

Kinetic analyses reveal that Cr(VI) biosorption onto L. cylindrica follows pseudo-second-order kinetics, indicating chemisorption as the rate-limiting step. Key observations include:

• Laidani et al. (2023) reported rapid adsorption reaching equilibrium within 60 minutes
• Amin et al. (2018) observed a reversible process with fast equilibrium establishment

These findings suggest that the biosorption process is efficient and suitable for practical applications.

4. Isotherm Models

Isotherm studies provide insights into the adsorption mechanism:

• Langmuir Model: Describes monolayer adsorption on a surface with a finite number of identical sites.
• Freundlich Model: Applicable to heterogeneous surfaces with a non-uniform distribution of adsorption sites.
• Temkin Model: Accounts for adsorbent–adsorbate interactions.

Studies indicate that Cr(VI) adsorption onto L. cylindrica is best described by the Langmuir isotherm, suggesting a monolayer adsorption on a surface with a finite number of identical sites.

5. Thermodynamic Studies

Thermodynamic parameters provide information on the nature of the adsorption process:

• ΔH° (Enthalpy Change): Negative values indicate exothermic reactions.
• ΔS° (Entropy Change): Negative values suggest decreased randomness at the solid–solution interface.
• ΔG° (Gibbs Free Energy Change): Negative values confirm the spontaneity of the process.

Laidani et al. (2023) reported ΔH° = -11.49 kJ/mol, ΔS° = -0.033 kJ/K·mol, and ΔG° values ranging from -0.171 to -1.722 kJ/mol, indicating that Cr(VI) adsorption is exothermic and spontaneous IJCCE.

6. Factors Affecting Biosorption

Several factors influence the efficiency of Cr(VI) biosorption:

• pH: Optimal adsorption occurs at acidic pH levels.
• Contact Time: Equilibrium is reached within 60–150 minutes.
• Temperature: Higher temperatures enhance adsorption up to a certain point.
• Biosorbent Dosage: Adequate biosorbent concentration is crucial for effective removal.

These parameters must be optimized to maximize biosorption efficiency.

7. Comparative Analysis

Study	Adsorption Capacity (mg/g)	Isotherm Model	Kinetic Model	Thermodynamics
Laidani et al. (2023)	29.98	Langmuir	Pseudo-second-order	Exothermic, Spontaneous
Amin et al. (2018)	5.91	Langmuir	Pseudo-second-order	Exothermic, Spontaneous
Khan et al. (2012)	6.5	Langmuir	Pseudo-second-order	Not reported

8. Conclusion

Luffa cylindrica demonstrates significant potential as an effective biosorbent for the removal of hexavalent chromium (Cr(VI)) from aqueous solutions. Numerous studies have reported high adsorption capacities, rapid uptake rates, and favorable kinetic and thermodynamic profiles, indicating that the biosorption process is both efficient and spontaneous. The structural characteristics of L. cylindrica, including its porous surface and abundance of functional groups such as hydroxyl, carboxyl, and amino moieties, contribute to its strong affinity for Cr(VI) ions. Additionally, its natural abundance, low cost, biodegradability, and minimal environmental impact further enhance its appeal as a sustainable and eco-friendly alternative to conventional adsorbents.

Given these advantages, L. cylindrica emerges as a promising candidate for large-scale applications in wastewater treatment, particularly for industrial effluents rich in Cr(VI). However, to fully realize its potential, future research should focus on optimizing key biosorption parameters, such as pH, contact time, temperature, and biosorbent dosage, to maximize removal efficiency under real-world conditions. Investigations into regeneration and reuse of the biosorbent are also critical for improving economic feasibility and sustainability. Furthermore, studies that evaluate its performance in complex wastewater matrices, which may contain competing ions and other contaminants, are necessary to validate its practical applicability. Integrating L. cylindrica biosorption with other treatment technologies, such as membrane filtration or advanced oxidation processes, could further enhance its efficiency and broaden its utility in industrial-scale wastewater management.

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