How Citrus Pulp Transforms the Circular Economy: From Bioplastics to Eco-friendly Fabrics to Fertilizers

Citrus pulp is a byproduct of the industrial processing of citrus fruits, such as oranges, lemons, grapefruits, and mandarins.

This residual material primarily forms during the production of fruit juices, when the pulp, peel, seeds, and other unused parts of the fruit are separated from the juice.

The process begins with the harvesting and selection of citrus fruits, followed by their cleaning and cutting. During the juice extraction, the solid components are mechanically separated from the liquid.

The result of this separation is the pulp, which includes a mixture of peel (flavedo and albedo), pulp, seeds, and sometimes small amounts of residual juice.

The innovation in reusing citrus pulp as raw material in various industrial sectors is a prime example of how the circular economy can transform waste into valuable resources, significantly contributing to environmental sustainability.

This transformation involves complex technical processes and innovative approaches that deserve detailed analysis.

Transformation of Citrus Pulp into Textile Fiber

The transformation of citrus pulp into textile fiber is a process that requires precision and technological innovation.

The first phase starts with the collection and drying of the pulp, which must be freed from excess moisture to facilitate the extraction of cellulose.

Subsequently, through a chemical process, the cellulose is isolated from the pulp using non-toxic solvents. This step is crucial to ensure that the resulting fiber is eco-friendly and safe for use in textiles intended for skin contact.

Cellulose Separation Process

Pretreatment: Citrus pulp is initially subjected to a pretreatment to remove impurities and non-cellulosic substances.

This may include water washes to eliminate residual sugars and organic acids, as well as a thermal or chemical treatment to facilitate the breakdown of cell walls.

Delignification: Delignification is the next step, necessary to remove lignin, a complex polymer that provides rigidity and impermeability to plant cell walls.

This process can be achieved through chemical treatments, such as the use of alkaline solutions (for example, sodium hydroxide) that solubilize lignin without significantly degrading cellulose.

Bleaching: After lignin removal, the residual material can be further treated with bleaching agents to remove the last traces of coloration and impurities, improving the purity of the cellulose.

This step is particularly important when the cellulose is intended for use in the textile industry or other applications where whiteness and purity are essential.

Cellulose Extraction: At this point, the purified cellulose is ready to be extracted from the mixture. This can be done through filtration and centrifugation processes, followed by drying the material to obtain cellulose in solid form or flakes.

Type of Cellulose Obtained

The cellulose extracted from citrus pulp is a type of regenerated cellulose, similar in some characteristics to the cellulose used to produce viscose or rayon.

However, due to the specific sources and treatment methods, this cellulose can exhibit unique properties. In particular:

High Purity: The cellulose obtained from citrus pulp, after the bleaching process, tends to have high purity, making it suitable for applications requiring characteristics of strength and shine, such as high-quality textiles.

Sustainability: Unlike cellulose extracted from wood or cotton, that derived from citrus pulp is considered more sustainable, as it comes from a byproduct of the food industry, reducing the need for dedicated agricultural resources and minimizing waste.

The cellulose obtained can be transformed into continuous filaments or cut fiber, which can then be transformed into yarn and fabric.

These materials are used not only in the textile industry but also in the production of compostable and biodegradable materials, such as eco-friendly packaging and non-wovens for medical or sanitary applications, demonstrating the versatility and added value that the recovery of citrus pulp can bring to the circular economy.

Once extracted, the cellulose undergoes treatment to be transformed into a viscous solution, which is then forced through dies to form the fibers.

These fibers are subsequently treated through washing, stretching, and drying processes to stabilize them and make them ready for spinning.

The resulting yarn can be used to weave or knit fabrics with characteristics similar to silk, known for their lightness, strength, and comfort.

Potential for Citrus Pulp Production and Environmental Impact

The amount of citrus pulp produced annually on a global scale is significant, with the citrus industry generating millions of tons of this byproduct.

For example, in Italy alone, one of the leading citrus producers in Europe, it is estimated that pulp production can exceed 700,000 tons per year.

Converting a fraction of this pulp into textile fiber could potentially produce thousands of tons of fabric, reducing dependence on synthetic fibers derived from petroleum and on intensive crops like cotton, which have a significantly greater environmental impact in terms of water use and pesticides.

Use of Citrus Pulp as Fertilizer

The use of citrus pulp as an organic fertilizer requires careful management to ensure that the material is adequately composted before use.

Composting is a biological process that transforms organic waste into a stabilized product, rich in humus and nutrients, ideal for improving soil fertility.

The process of transforming pulp into fertilizer involves specific techniques aimed at ensuring that the final product is safe, effective and of high quality for agricultural use.

These techniques are based on principles of composting, fermentation and heat treatment.

Composting

Composting is one of the most widespread techniques for transforming citrus fruit pulp into fertilizer. This aerobic biological process decomposes organic matter through the action of microorganisms, such as bacteria, fungi and protozoa, transforming it into humus, a soil improver rich in nutrients.

Preparation of the Material: The pulp is mixed with other organic materials, such as manure and plant residues, to balance the carbon/nitrogen (C/N) ratio, which is fundamental for an effective composting process.

Condition Control: During composting, it is crucial to maintain adequate levels of humidity and aeration to support the activity of microorganisms. The material can be periodically turned to ensure uniform distribution of oxygen and temperature.

Maturation: After several weeks or months, depending on the environmental conditions and the composition of the material, the compost reaches a maturation phase, in which the microbial activity decreases and the stabilized product becomes ready for use.

Anaerobic fermentation

Anaerobic fermentation, or anaerobic digestion, is another method of turning pulp into a nutrient-rich fertilizer.

This process occurs in the absence of oxygen and produces, in addition to digestate (which can be used as a fertilizer), also biogas, a mixture of methane and CO2 which can be used for energy production. Let's see the main steps:

Anaerobic Digesters: The pulp is inserted into anaerobic digesters, where specific microorganisms degrade the organic matter.

Condition Control: The temperature, pH and humidity inside the digester are carefully controlled to optimize the process and maximize biogas production.

Digestate collection: At the end of the process, the digestate is collected. It may require further treatment, such as separating solids from liquids, before being used as a fertilizer.

Heat treatment

Thermal treatment, such as pyrolysis or gasification, is a less common but effective method for turning pulp into a soil conditioner and energy.

These processes involve exposing the material to high temperatures in the absence of oxygen (pyrolysis) or in the presence of a limited amount of oxygen (gasification).

Biochar Production: Pyrolysis produces biochar, a type of carbon-rich char that can improve soil structure, water holding capacity and nutrient availability.

Energy from Gasification: Gasification transforms the pulp into a synthetic gas that can be used to generate energy, while the solid residue can be used as fertilizer.

Transformation of Pastazzo into Bioplastic

The transformation of citrus fruit pulp into bioplastic represents an excellent example of a circular economy, where an industrial by-product is valorised as a resource for the production of innovative and sustainable materials.

The process of converting pulp into bioplastic follows several key steps that involve the extraction of useful components, the polymerization of these components into a plastic matrix, and finally the formation of the final product.

Below, a general process is described that can be adapted depending on the technical specifications and requirements of the finished product:

1. Harvesting and Preparation of Pastazzo

The process begins with the collection of citrus fruit pulp, which is then dried and ground to obtain a fine powder.

This powder contains cellulose, pectin and limonene, components that can be transformed into bioplastics.

2. Component Extraction

Extraction of Cellulose and Pectin: Cellulose and pectin, natural polymers present in pulp, can be extracted through processes that include treatments with alkaline or acid solutions.

These substances serve as a base material for the production of bioplastics due to their ability to form plastic films and structures.

Limonene Extraction: Limonene, a terpene found in citrus peel, can be extracted and used as a natural plasticizer to improve the flexibility and mechanical properties of bioplastics.

3. Polymerization

Bioplastics can be produced through various polymerization methods, including:

Direct polymerization: Taking advantage of the natural properties of cellulose and pectin, which can form polymer networks through thermal or chemical treatments.

Synthesis of Polyesters: Converting pulp-derived monomers, such as ferulic acid, into polyesters through polycondensation processes. These polymers can offer biodegradable properties and are suitable for specific applications.

4. Addition of Additives

To improve the properties of bioplastics, various additives can be added to the polymer composite, including natural plasticizers such as limonene, UV stabilizers, natural dyes, and other additives to optimize the workability, strength, and durability of the material.

5. Formation of the Final Product

The polymer material is then transformed into the desired final product through standard plastic processing techniques, such as extrusion, injection molding, or blow molding.

This step determines the shape, size and specific use of the bioplastic produced.

Conclusion

The valorisation of citrus pulp through its transformation into raw materials for the textile industry, agriculture and the production of bioplastics represents a concrete example of how innovation and technology can contribute to a more sustainable and circular economy.

These approaches not only reduce the environmental impact associated with waste management and the production of new materials but also offer economic opportunities for the industries involved, promoting the development of new markets and the creation of green jobs.

The challenge for the future will be to improve these technologies to maximize their positive impact on the environment and society.