Application of graphene based composites in agriculture

Abstract

The main challenge faced by agricultural research is to produce high quantity and quality food to feed a constantly growing world population. Fertilizers are an essential component of productive agricultural systems, but their efficiency of use is low due to losses to the atmosphere, soil and waters, which consequently can cause environmental damage. In addition, reaction of nutrients with soil components reduces their availability to plants and thus they may accumulate in soil. Nutrients may also be leached from soil and end up in rivers, lakes and the ocean. Improving fertilizer use efficiency is therefore a global goal and new engineering approaches are needed to design more effective nutrient delivery systems to crops which minimize losses to the environment. Recent strategies to address these problems are based on designing slow-release fertilizers using porous materials or polymer-coating of conventional fertilizers, which have seen some success but are severely limited by their cost. Graphene (GN) and its derivatives may offer a path-way to develop more efficient fertilizers due to the outstanding physicochemical properties of GN. During the relatively short time since the discovery of GN in 2004, its unique properties have attracted great interest in multiple fields including chemistry, physics, materials science, biology and engineering. The two dimensional structure of GN, in addition to its high surface area, makes this material very attractive for the delivery of drugs or genetic material and there is also potential for application as a nutrient carrier in agriculture. Despite being one atom thick, GN is the strongest material ever tested and its unique mechanical properties made it a favourable candidate to be used as a reinforcement material to enhance the toughness of different composites and therefore a potential application to enhance the mechanical properties of fertilizer granules. Therefore, considering the excellent properties of GN-based materials, including a two-dimensional (2D) structure, a high specific surface area, a tailorable surface chemistry and a high mechanical strength, this thesis examined the potential for use of GN-based materials to improve thenutrient delivery to crops, to enhance fertiliser use efficiency as well as the mechanical properties of granular fertilizers. The following four concepts were developed and explored in this thesis: The first part of the thesis focuses on the development of a new carrier platform for delivery of plant nutrients based on graphene oxide (GO) sheets. To prove this concept, the micronutrients zinc (Zn) and copper (Cu) were loaded onto GO sheets. The GO sheets provided a high loading capacity for Zn and Cu (14% and 10% by weight, respectively) with slow release properties. The GO-based fertilizers displayed a biphasic release behaviour with a portion of the micronutrients released quickly, and a portion having a slow release behaviour. This was likely due to 2D structure of GO as well as the tight coordination of nutrients with oxygen functional groups of the GO sheets. A visualization method was used to assess the release and diffusion of Cu and Zn in soil from these GO-based fertilizers and demonstrated the advantages of GO carriers compared to commercial fertilizers. A pot trial demonstrated that Zn and Cu uptake by wheat was higher when using GO-based fertilizers compared to commercial zinc or copper salts. This is the first report on the agronomic performance of GO-based slow-release fertilizers and demonstrated their capability to be used as a generic platform for micronutrient delivery. In the second part of this thesis, different formulations of GO-based micronutrient fertilizers were assessed for their ability to supply micronutrients to wheat, compared with commercial fertilizers. Both granular versus fluid forms of fertilizer, and fertilizer placement (banded versus mixed), were investigated in this study. Fluid (suspension) forms of the GO-based fertilizers were more effective than the granular forms, and the GO-based formulations were more effective than equivalent fluid and granular commercial zinc sulphate products. The third part of this thesis utilized GN and GO as hardening agents to enhance the physical properties of granular monoammonium phosphate (MAP) fertilizers. Co-granulation of 0.5% w/w GN sheets in MAP granules (MAP-GN) significantly enhanced the mechanical strength of MAP granules while inclusion of the same amounts of GO sheets (MAP-GO) improved the strength to a lesser extent (18 times improvement versus 8 times). The abrasion of MAP-GN was 70% less than the unamended MAP granules, while the impact resistance of MAP-GN was 75% greater than unamended MAP. The inclusion of GN not only improved the physical properties of granules but also slightly slowed the release of phosphorus to soil. The advantages of GN and GO sheets in improving the physical properties of MAP granules were explained by their high specific area and high mechanical properties in addition to their 2D geometry. These results indicate the potential for GN/GO additives to improve the physical properties of granular fertilizers. The fourth and final part of this thesis investigated the concentration dependence of GN addition to fertilizer (MAP and diammonium phosphate (DAP)) granules in improving fertilizer physical quality. The optimum concentration of GN for MAP and DAP were 0.5 w% and 0.05w%, respectively and adding greater amounts of GN decreased the crushing strength rather than increasing it. It was also observed that the improved crushing strength of GNamended granules depended on the initial hardness of fertilizers - the crushing strength of softer granules such as MAP was enhanced almost 15 fold, while harder granules such as DAP had much smaller improvements in crushing strength. Furthermore, this work investigated whether GN made by different methods, and therefore having different properties (level of deoxygenation, specific surface area (SSA) and sheet size), had similar effects on the physical quality of fertilizers. Graphene with a higher degree of deoxygenation and SSA, and lower particle size, was more effective in improving the crushing strength of MAP. However, there was little effect of GN properties on DAP granules amended with GN, likely due to the higher initial crushing strength of DAP.