Chitosan: A Sustainable Solution for Heavy Metal Remediation in Contaminated Soil

Sustainable soil regeneration through chitosan-based remediation

Introduction

Heavy metal contamination in soil poses a significant global environmental challenge, threatening ecosystem health, agricultural productivity, and human well-being. Industrial activities, agricultural practices, and improper waste disposal have led to the accumulation of toxic heavy metals such as cadmium (Cd), arsenic (As), lead (Pb), and mercury (Hg) in soil, rendering vast areas unsuitable for cultivation and posing severe health risks through the food chain (Zhao et al., 2022). Traditional remediation methods often fall short due to their high cost, limited effectiveness, or potential for secondary pollution. Consequently, there is an urgent need for innovative, sustainable, and environmentally friendly solutions to address this pervasive problem.

Chitosan, a natural biopolymer derived from chitin, the second most abundant polysaccharide in nature, has emerged as a highly promising candidate for heavy metal remediation. Its unique chemical structure, characterised by abundant amino and hydroxyl functional groups, grants it exceptional chelating and adsorptive properties, making it highly effective in binding and removing heavy metal ions from various matrices, including contaminated soil. This article delves into the properties of chitosan that make it an effective agent for heavy metal removal, citing scientific studies and highlighting its potential as a sustainable solution for soil remediation. We will explore the mechanisms by which chitosan interacts with heavy metals, examine its application in contaminated soil, and discuss the future prospects of this remarkable biopolymer in environmental restoration.

The Pervasive Threat of Heavy Metal Contamination in Soil

Heavy metals are naturally occurring elements with high atomic weights and densities. While some are essential micronutrients for plant growth, their accumulation above certain thresholds becomes toxic. Sources of heavy metal contamination are diverse and include:

Industrial activity is a major contributor to heavy metal contamination in soil

• Industrial Activities: Mining, smelting, manufacturing, and industrial waste discharge are major contributors to heavy metal pollution. For instance, sulfide mining sites are often co-contaminated with cadmium and arsenic (Ullah et al., 2023).

• Agricultural Practices: The long-term use of certain fertilisers, pesticides, and sewage sludge can introduce heavy metals into agricultural soils (Wuana et al., 2011).

• Waste Disposal: Improper disposal of municipal and industrial waste can lead to the leaching of heavy metals into the surrounding soil and groundwater.

The consequences of heavy metal contamination are far-reaching:

• Environmental Degradation: Heavy metals can inhibit the biodegradation of organic contaminants, disrupt soil microbial communities, and impair vital ecosystem services (Priya et al., 2023). They can also leach into groundwater and surface water, contaminating aquatic ecosystems.

• Impact on Plant Health: Elevated levels of heavy metals in soil can negatively affect crop health and productivity, leading to reduced yields and stunted growth. Plants can absorb these metals, accumulating them in their tissues (Rashid et al., 2023).

• Human Health Risks: Through the consumption of contaminated crops and water, heavy metals enter the human food chain, posing severe health risks. Exposure to heavy metals can lead to neurological disorders, organ damage, and various cancers. For example, arsenic is a known carcinogen and can cause arsenic poisoning, while cadmium can damage kidneys and bones (Envireaugence, 2025).

The urgent need for effective and sustainable remediation strategies is underscored by the persistent nature of heavy metals in the environment and their profound impact on ecological and human health.

Chitosan: Properties and Mechanisms for Heavy Metal Remediation

Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It is derived from chitin through a process called deacetylation. The remarkable ability of chitosan to bind heavy metals is primarily attributed to its unique chemical structure, particularly the presence of abundant reactive functional groups (Upadhyay et al., 2021).

Key Properties Enabling Heavy Metal Removal:

1. Amino (-NH2) and Hydroxyl (-OH) Groups: The most crucial functional groups in chitosan are the primary amino groups and the hydroxyl groups. These groups act as active sites for the adsorption and complexation of heavy metal ions. The lone pair of electrons on the nitrogen atom in the amino group and the oxygen atoms in the hydroxyl groups can readily form coordinate bonds with metal ions (Upadhyay et al., 2021; Kaczorowska et al., 2024).

2. Cationic Nature: In acidic conditions (typically below pH 6.5), the amino groups in chitosan become protonated, acquiring a positive charge (-NH3+). This cationic nature allows chitosan to effectively attract and bind negatively charged heavy metal oxyanions (e.g., arsenate, chromate) through electrostatic attraction (Ullah et al., 2023; Xu et al., 2024).

3. Chelating Ability: Chitosan can act as a chelating agent, forming stable complexes with heavy metal ions. The amino and hydroxyl groups can collectively bind to a single metal ion, forming a stable ring-like structure. This chelation process effectively sequesters the metal ions, reducing their mobility and bioavailability in the soil (Upadhyay et al., 2021; Abdel-Raouf et al., 2023).

4. Biodegradability and Biocompatibility: Unlike synthetic polymers, chitosan is biodegradable and biocompatible, meaning it can naturally break down in the environment without forming harmful byproducts and is non-toxic to living organisms. This makes it an environmentally friendly alternative for remediation applications (Pal et al., 2021).

5. Film-forming Properties: Chitosan can form films or coatings, which can be utilised to encapsulate contaminated soil particles or to create permeable reactive barriers that immobilise heavy metals (Edward et al., 2024).

   

   Chemical structure of Chitin and Chitosan. Chitosan is derived from Chitin by the process of deacetylation

Mechanisms of Heavy Metal Binding:

The interaction between chitosan and heavy metal ions involves several mechanisms, often occurring simultaneously:

• Adsorption: This is the primary mechanism, where heavy metal ions adhere to the surface of chitosan through various forces, including electrostatic attraction, hydrogen bonding, and van der Waals forces (Ullah et al., 2023).

• Complexation and Chelation: As mentioned, the amino and hydroxyl groups on the chitosan backbone can form stable complexes and chelates with metal ions. This is a strong and specific interaction that effectively removes metal ions from the solution or soil matrix (Upadhyay et al., 2021; Abdel-Raouf et al., 2023).

• Ion Exchange: The protonated amino groups can exchange with metal cations in the solution, leading to the uptake of heavy metals by chitosan (Xu et al., 2024).

• Precipitation: In some cases, chitosan can induce the precipitation of heavy metal hydroxides or other insoluble metal compounds, thereby immobilising them in the soil (Ullah et al., 2023).

Enhancing Chitosan's Efficacy: Modified Chitosan Derivatives

While native chitosan possesses excellent heavy metal binding capabilities, its practical application can be limited by factors such as low acid stability, inadequate mechanical strength, and relatively low adsorption capacity in certain conditions (Ullah et al., 2023). To overcome these limitations, researchers have developed various modified chitosan derivatives (Chitosan-based Composites, CSCs) that exhibit enhanced performance in heavy metal remediation. These modifications aim to increase the number of active binding sites, improve stability, and enhance selectivity for specific heavy metals. Some notable modifications include:

• Cross-linked Chitosan: Cross-linking agents (e.g., glutaraldehyde, epichlorohydrin, tripolyphosphate) are used to create a three-dimensional network structure within the chitosan matrix. This enhances mechanical strength, stability, and significantly improves the adsorption capacity for heavy metals (Ullah et al., 2023; Wang et al., 2023).

• Carboxymethyl Chitosan: The introduction of carboxymethyl groups onto the chitosan backbone increases the number of carboxyl functional groups, leading to stronger complexation with metal ions and improved solubility and stability across a wider pH range (Ullah et al., 2023; Ariani et al., 2023).

• Thiolated Chitosan: Incorporating thiol (-SH) groups enhances the affinity for heavy metals, particularly those that form strong bonds with sulfur (e.g., mercury, lead, cadmium). This modification boosts selectivity and efficiency (Ullah et al., 2023; Azhar et al., 2023).

• Quaternised Chitosan: Quaternisation introduces quaternary ammonium groups, resulting in improved adsorption capacity through enhanced ion exchange and electrostatic attraction (Ullah et al., 2023; Huang et al., 2023).

• Chitosan-Inorganic Composites: Combining chitosan with inorganic materials like iron filings, biochar, or clay can create synergistic effects, improving adsorption capacity, stability, and reusability. For instance, chitosan-modified iron filings have shown promising results in stabilising arsenic-contaminated soil through surface complexation, hydrogen bonding, precipitation, and electrostatic attraction (Xu et al., 2024).

These modifications expand the applicability of chitosan, making it a versatile and powerful tool for addressing diverse heavy metal contamination scenarios in soil.

Chitosan in Action: Application in Contaminated Soil Remediation

The application of chitosan for heavy metal remediation in contaminated soil is a multifaceted approach, leveraging its unique properties to either remove or immobilise pollutants. The choice of application method often depends on the type and concentration of heavy metals, soil characteristics, and the desired remediation outcome.

In-situ Remediation Strategies:

• Immobilisation: Chitosan can be directly applied to contaminated soil to immobilise heavy metals, reducing their mobility and bioavailability. This is particularly useful for preventing the uptake of heavy metals by plants and their subsequent entry into the food chain. The chelating and adsorptive properties of chitosan bind the metal ions, forming stable complexes that are less prone to leaching (Kamari & Yusoff, 2011). Studies have shown that chitosan, and its composites with materials like zeolite and bentonite, can effectively increase soil pH and reduce the biological effectiveness of heavy metals (Yi & Li, 2019).

• Phytoremediation Enhancement: Chitosan can be used to enhance phytoremediation, a green technology that uses plants to extract or stabilise pollutants from soil. Water-soluble chitosan, for example, has been shown to promote the remediation of lead (Pb)-contaminated soil by increasing the accumulation of Pb in hyperaccumulator plants like Hylotelephium spectabile in field trials (Guo et al., 2024). This approach not only cleans the soil but also offers a sustainable and aesthetically pleasing solution.

• Soil Washing: Chitosan can be incorporated into soil washing solutions to extract heavy metals. The chitosan-based solution binds to the metal ions, which are then removed from the soil. This method is effective for highly contaminated soils and can be combined with other techniques for optimal results (Hu et al., 2021).

Ex-situ Remediation Strategies:

While primarily focused on in-situ applications, chitosan can also play a role in ex-situ remediation, where contaminated soil is excavated and treated off-site. Chitosan-based adsorbents can be used in packed columns or stirred tanks to remove heavy metals from extracted soil solutions or leachate (Gamage & Wijesekara, 2023).

Factors Influencing Application Effectiveness:

Several factors influence the effectiveness of chitosan-based remediation in soil:

• pH: The pH of the soil significantly affects the protonation of chitosan's amino groups and the speciation of heavy metals, thereby influencing adsorption efficiency. Optimal pH ranges vary depending on the specific heavy metal and chitosan modification (Ullah et al., 2023; Xu et al., 2024).

• Chitosan Concentration and Dosage: The amount of chitosan applied directly impacts the number of available binding sites for heavy metals. Determining the optimal dosage is crucial for cost-effectiveness and efficient remediation (Ullah et al., 2023).

• Contact Time: Sufficient contact time between chitosan and the contaminated soil is necessary for effective adsorption and immobilisation of heavy metals (Ullah et al., 2023).

• Presence of Competing Ions: Other ions present in the soil can compete with heavy metals for binding sites on chitosan. However, chitosan often demonstrates selective adsorption for heavy metals even in the presence of competing ions (Ullah et al., 2023).

• Soil Characteristics: Soil texture, organic matter content, and cation exchange capacity can influence the distribution and effectiveness of chitosan in the soil matrix.

Case Studies and Field Trials

Ongoing laboratory research continues to refine chitosan-based technologies for safer, more efficient heavy metal remediation.

While much of the research on chitosan for heavy metal removal has been conducted in laboratory settings, a growing number of studies are exploring its efficacy in real-world scenarios. The field trial demonstrating water-soluble chitosan's ability to enhance lead accumulation in Hylotelephium spectabile in contaminated soil is a significant step towards practical application (Guo et al., 2024). Another study highlighted the use of chitosan-modified iron filings for stabilising arsenic-contaminated soil, providing a new direction for green and efficient stabilising materials (Xu et al., 2024). These studies underscore the potential of chitosan to be a viable and sustainable solution for large-scale soil remediation projects.

The Future of Chitosan in Environmental Remediation

Chitosan's role in environmental remediation is continuously expanding, driven by ongoing research and a growing demand for sustainable solutions. Future developments are likely to focus on:

• Novel Chitosan Composites: Further innovation in creating advanced chitosan-based composites with enhanced selectivity, adsorption capacity, and stability for a wider range of heavy metals and environmental conditions.

• Nanotechnology: The development of nano-chitosan and chitosan-based nanomaterials offers increased surface area and reactivity, leading to improved remediation efficiency (Rahman & Islam, 2025).

• Integrated Remediation Systems: Combining chitosan-based approaches with other remediation technologies (e.g., bioremediation, electrokinetic remediation) to create more comprehensive and efficient treatment systems.

• Sustainable Sourcing: Continued exploration of alternative and more sustainable sources of chitin, such as insect exoskeletons, to reduce reliance on traditional marine sources and promote circular economy principles (Siddiqui & Khan, 2022).

• Policy and Regulatory Frameworks: The establishment of standardised guidelines and regulatory frameworks for the use of biopolymers like chitosan in environmental remediation will be crucial for widespread adoption and commercialisation.

As the world grapples with the escalating problem of heavy metal contamination, chitosan stands out as a beacon of hope – a natural, biodegradable, and highly effective biopolymer with the potential to transform contaminated landscapes into healthy, productive ecosystems.

Conclusion

The pervasive challenge of heavy metal contamination in our soils demands innovative, sustainable, and effective solutions. Chitosan, with its remarkable properties as a natural, biodegradable, and highly efficient adsorbent, offers a compelling answer to this critical environmental issue. Its ability to bind, immobilise, and facilitate the removal of a wide spectrum of heavy metals, coupled with ongoing advancements in chitosan modification and application techniques, positions it as a cornerstone for future soil remediation efforts.

At Entoplast, we are at the forefront of harnessing the power of this extraordinary biopolymer. As a leading UK-based manufacturer of high-quality chitin and chitosan, we are committed to providing the foundational ingredients that drive sustainable innovation across various industries, including agriculture and environmental remediation. Our dedication to research, quality, and responsible sourcing ensures that our chitin and chitosan products are not only effective but also align with the principles of a circular economy.

We invite academics, scientists, and potential investors to join us in this vital mission. By incorporating Entoplast's premium chitin and chitosan into your product lines or research initiatives, you can contribute to a cleaner, healthier planet and unlock new possibilities for sustainable development. Partner with Entoplast, and together, let's cultivate a future where contaminated soils are a challenge of the past, and environmental stewardship is a shared success. For more information on our products, capabilities, and partnership opportunities, feel free to contact us at hello@entoplast.com or by using the form below. Together, we can cultivate a healthier and more sustainable future.