Bioplastics to eliminate environmental pollution from agricultural plastic waste
Petroleum-based plastics are largely used in agriculture as plastic films for crop protection, soil mulching, pipes, containers for transplanting seedlings, and pots. After the cultivation period is complete, however, agricultural plastic waste is coated with soil, organic matter, and agro-chemicals and must therefore undergo the correct collection, disposal, and recycling processes. One sustainable solution to the serious problem of environmental pollution is to employ biodegradable materials in agriculture.1 Such materials can be integrated directly in the soil at the end of their lifetime where the bacterial flora can transform them into water, biomass, and carbon dioxide or methane.2
Due to increasing environmental awareness, researchers continue to seek new materials that can be used as ecologically friendly alternatives to agricultural materials based on synthetic petrochemical polymers. Biodegradable mulches can be fabricated by employing thermo-plasticizing, casting, and spraying processes1 using renewable and biodegradable raw materials such as starch,3 cellulose,4 chitosan, hydrolyzed proteins,5 alginate,6 and glucomannan.7 All of these approaches suffer from shortcomings, however. For example, starch-based extruded films have been widely tested in recent years,1–3, 7 but their commercialization has been hampered mainly by their cost. Further, sodium alginate, glucomannan, chitosan, cellulose, and hydrolyzed proteins are raw materials that come from potentially polluting residues of marine or agricultural origin. For these reasons, mulching spray-coatings based on such polysaccharides require a number of improvements before they can be considered suitable for such applications, particularly in terms of their mechanical properties.
We have developed biodegradable polymeric materials—for spray-mulching coatings and plant pots—by using protein hydrolysates that are derived from waste products of the leather industry and functional poly(ethylene glycol) as a crosslinking agent.8, 9 Protein-based waste materials are especially suited for this purpose because they have intrinsic agronomic values for soil fertilization due to their high nitrogen content.
In previous work, we carried out several experimental tests to prove the feasibility of our novel spray mulching coatings. We investigated their functionality as well as their mechanical and physical behaviors in standard and controlled experimental conditions.8, 9 To assess whether or not water suspensions would be capable of achieving a consistent coating when sprayed directly onto soil, we chose poly(ethylene glycol) diglycidyl ether (PEGDE) and protein hydrolysates as starting materials. We prepared the novel derivatives in water solutions following a synthetic procedure based on the reaction between protein hydrolysate amino groups and functional end groups of PEGDE. We also added wood-cellulose microfibers (up to a final 18wt%) to enhance the mechanical properties of the composite, and carbon black to obtain black films (and thus prevent photo-oxidation and weed photosynthesis).
We then distributed our bioplastic solutions with an airbrush using a spray machine. In this way, we were able to completely cover the growing substrate around the plants with a thick mulching coating that dried to a hard consistency: see Figure 1(a). The biodegradable coatings maintained their mulching effect for a period ranging from one to nine months, and although irregularities appeared on the surface—see Figure 1(b)—weed suppression was nevertheless achieved.8, 9
The lifespan of the coating depends on its thickness as well as the temperature and moisture content of the soil, but is mainly dependant on the structure and morphology of the material. Morphological analysis performed on a sample that was directly sprayed onto the soil and exposed for six months—see Figure 2(a)—shows a different pattern depending on its exposure. The side exposed to the light does not differ significantly from the original film, and there is no indication of degradation: see Figure 2(c). The surface facing the soil, however—see Figure 2(b)—consists almost exclusively of fibers, thus indicating that degradation begins in the polymeric component of the material. These results show that the biodegradation process occurs more rapidly where there is direct contact between the film and micro-organism and the remaining fibers act as a barrier, modulating the environmental duration of the blend.
We have more recently carried out tests using novel biodegradable containers for seedlings.10 The objectives of this research are to develop new biodegradable materials starting from renewable bio-based raw products, and to engineer the properties of these materials so that they can be used to produce biodegradable plant pots that guarantee no damage to roots, no transplant shock, an enhancement to plant growth, and the slow release of fertilizing protein-based compounds during their degradation.
The preparation of these novel biodegradable polymeric materials began from an aqueous solution of protein hydrolysate (derived from waste products of the leather industry), PEGDE, and natural fillers (i.e., sawdust or wood flour). We then performed compounding in a Brabender mixer at 60°C and subsequently prepared the pots by compression molding the biocomposites (which were the consistency of a paste) and drying them at 70°C: see Figure 3(a). We found that the biodegradable containers for seedlings showed good resistance during the first stage of use (i.e., when the seedlings were grown from seed, before transplanting): see Figure 3(b). After the transplant, the containers (which were buried in soil) degraded in roughly two weeks, allowing the roots to pass through the container walls and thus enabling the overall growth of the plants: see Figure 3(c). As a result of the slow release of proteinaceous material, the containers showed a soil-positive fertilizing effect. To test the efficacy of this approach, we implemented them in the cultivation of pepper plants. At harvest, the mean height of the pepper plants grown inside the biodegradable pots was 0.94m. In contrast, the control plants (grown in non-biodegradable containers) were characterized by a mean height of 0.67m.10
In summary, our biodegradable sprayable mulches and plant pots (for transplanting seedlings) could promote valid ecologically sustainable cultivations, enhance the protection of the landscape against pollution in rural areas, and increase the use of renewable non-oil raw materials. We are currently experimenting with these approaches by applying them to different cultivations. We hope to prove the feasibility of our novel biodegradable materials by investigating their functionality as well as their physicochemical and mechanical behavior in standard and controlled experimental field conditions, and by following their biodegradation process during plant cultivation.
- M. Malinconico, B. Immirzi, G. Santagata, E. Schettini, G. Vox and G. Scarascia Mugnozza, Chapter 3: An overview on innovative biodegradable materials for agricultural applications, Progress in Polymer Degradation and Stability Research, Nova Science Publishers, Inc., 2008.
- A. Kapanen, E. Schettini, G. Vox and M. Itävaara, Performance and environmental impact of biodegradable films in agriculture: a field study on protected cultivation, J. Polym. Environ. 16, pp. 109-122, 2008.
- G. Scarascia-Mugnozza, E. Schettini, G. Vox, M. Malinconico, B. Immirzi and S. Pagliara, Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment, Polym. Degrad. Stabil. 91, pp. 2801-2808, 2006.
- L. Sartore, A. D'Amore and L. Di Landro, Ethylene vinyl acetate blends with cellulosic fillers and reinforcements, Polym. Compos. 36, pp. 980-986, 2015.
- L. Sartore, F. Bignotti, S. Pandini, A. D'Amore and L. Di Landro, Green composites and blends from leather industry waste, Polym. Compos. 37, pp. 3416-3422, 2016.
- B. Immirzi, G. Santagata, G. Vox and E. Schettini, Preparation, characterisation, and field testing of a biodegradable sodium alginate-based spray mulch, Biosyst. Eng. 102, pp. 461-472, 2009.
- G. Vox and E. Schettini, Evaluation of the radiometric properties of starch-based biodegradable films for crop protection, Polym. Test. 26, pp. 639-651, 2007.
- L. Sartore, G. Vox and E. Schettini, Preparation and performance of novel biodegradable polymeric materials based on hydrolyzed proteins for agricultural application, J. Polym. Environ. 21, pp. 718-725, 2013.
- E. Schettini, L. Sartore, M. Barbaglio and G. Vox, Hydrolyzed protein based materials for biodegradable spray mulching coatings, Acta Hortic. 952, pp. 359-366, 2012.
- L. Sartore, E. Schettini, F. Bignotti, S. Pandini and G. Vox, Biodegradable plant nursery containers from leather industry wastes, Polym. Compos., 2016.