(25c) Green Processes to Use Extract from Citrus Peel Waste for Novel Applications (Direct Polystyrene Recycling to Natural Solvent to Source of Carbon)

Authors: 
Sharma, C. S., Indian Institute of Technology, Hyderabad
Recycling and reuse through green engineering is always an open challenge for sustainable environment. Here, we present some innovative ways to recycle and reuse two different kinds of waste: plastic (polystyrene) waste and agriculture waste (citrus peel). Worldwide, Citrus fruits production is more than 31 million tons annually out of which nearly 50% is waste in terms of citrus peel. This amounts to a large quantity of citrus peel waste as a by-product in fruit juice industry. Orange contribute nearly half of this citrus peel waste. Although orange peel waste is further processed to yield some chemicals such as limonene and pectin which are used a fragrance, cosmetics, cleansing agents and sometimes as solvents, these processes (distillation, microwave heating) are highly energy consuming and uneconomical and therefore have limited industrial acceptability. We have developed cost effective, low-energy novel and green approaches to directly use the orange peel extract for a variety of applications such as in print transfer and natural solvent for biopolymers. Print transfer technique is used to prepare beautiful patterns or artistic designs on cloth, fabric, paper, walls etc. in order to enhance the appearance of respective objects creatively. However, conventionally used solutions for print transfer such as alcohols, xylene, acetone, acrylic mediums or Citra-solv are known to be toxic and harmful, if concentrated or overused. Therefore, the use of orange peel extract as the transfer solution is not only advantageous because it is natural, non-toxic, and inexpensive. In this process, orange peel extract facilitates in deteriorating the adhesive forces between the ink and the paper and thus transfers the ink on to the other substrates. Interestingly, this technique of print transfer is not limited to any particular choice of substrate.

Further, we centrifuged orange peel extract to yield three distinct layers. Bottom layer was then used as a solvent to prepare the solution for biopolymers such as gelatin. This gelatin solution made by bottom layer of orange peel extract as a solvent was then electrospun to prepare gelatin nanofibers followed by its successful demonstration in drug delivery. Again, the use of natural solvent in these biopolymer formulations minimizes any toxic effect associated with conventionally used organic solvents especially for healthcare applications. The middle part of the orange peel extract was comprised of small sized cellulosic flakes which can be pyrolyzed into nano-structured hard carbon and thus may potentially be used as an electrode material for rechargeable batteries.

The top layer of the orange peel extract was used for direct recycling of Polystyrene waste. Polystyrene is one of the most widely used thermoplastic polymer. It is used in various forms like solid in applications such as electronics, construction, house and medical ware, disposal food services etc. whereas, for protective packaging, in electrical, pharmaceutical and retail industries etc., it is used in form of foam, because of its light weight, shock resistance, cushioning properties, and flexibility in design possibilities. Obviously due to huge applications, polystyrene is one of the major solid waste. It is major component of plastic debris in ocean, where it becomes hazardous to marine life. Also, Styrofoam (another form of polystyrene) blows in wind and floats on water, making it difficult to collect and control the waste. Polystyrene packaging products are usually discarded in dumps, landfills or simple litter after their useful application. As the waste plastic material has become a serious problem, recycling is getting attention to save environment and resource recovery. The three main alternatives for PS recycling are, mechanical, chemical and thermal recycling. Mechanical recycling involves relatively simple technologies for converting scrap polystyrene into new product, by compressing and melting. However it is quite labour- or energy-intensive, depending on whether the process is manual or automated. Chemical recycling can have high capital cost such as in catalytic degradation of polystyrene whereas, toxic emission in thermal recycling refrains its use. All recycling methods involves energy input as the preliminary step either to crush PS objects into granules or thermally degrade them. Therefore, it is not surprising that recycling of polystyrene waste gets the worst rating. Recycled plastic is always a lower grade than the "virgin" material, so some applications, such as that for food packaging, cannot use recycled plastic. According to a 2004 study by the California Integrated Waste Management Board, of the 377,580 tons of polystyrene produced in the state, only 0.8% is recycled. Of that, only 0.2% (310 tons) of polystyrene food service packaging is recycled.

As stated above, we present a novel and innovative way to recycle objects made up of polystyrene directly into sub-micron, aligned fibers using citrus fruit’s peel extract (top layer). Recycled PS fibers were then structurally characterized and were found to be hydrophobic as well as lyophilic. Based on this property, these fibers were then tested as a sorbent material for oil in terms of absorption capacity and retention capacity. Furthermore, we also figured out that same fabrics can be re-used at least for five time with half of their original absorption capacity and therefore these fabrics are also useful not only for oil spills cleaning but also for oil recovery. We also measured buoyancy properties of as-fabricated recycled Ps fabrics in static as well as in turbulent conditions as it is important for their practical applications in oil spills remediation in large water bodies. In summary, the novel processes developed here in this work are perfect example of green engineering to make use of waste material into useful products.