Synthesis of slow release nanofertilizers of zinc and copper and their effect on the availability of these elements in lettuce

Background an Objectives
The utilization of traditional micronutrients faces certain constraints, such as the significant retention of micronutrients in the soil. This can occur due to robust adsorption reactions to clays and organic matter, or the formation of insoluble compounds in the soil. These factors greatly diminish the effectiveness of micronutrient fertilizers. The environmental concerns and limited effectiveness linked to conventional fertilizers motivate the pursuit of innovative fertilizers with delayed release and enhanced performance. A possible solution that has been explored for these issues is the utilization of slow-release fertilizers (SRFs) or controlled-release fertilizers (CRFs). These fertilizers deliver vital nutrients to plants at a slower pace compared to conventional fertilizers. The release mechanisms of these CRFs are dependent on either diffusion through their coating or gradual hydrolysis. Yet, soil characteristics like moisture level, pH, ion concentration, and temperature also play a role in influencing nutrient release through hydrolysis or diffusion. Hence, there exists a potential disparity between the speed at which micronutrients are released into the soil and the necessary pace of nutrient absorption by crops. Graphene oxide (GO) stands out as a common material for crafting slow-release fertilizers (SRFs). Given GO's strong affinity for metal ions and its role in ferrying therapeutic compounds, it's logical to envision GO being effectively employed as a transporter for plant micronutrients. In this study, we introduce an innovative approach to tackle these constraints, showcasing the utility of GO sheets as novel carriers for efficiently loading plant micronutrients. This technique finds application in creating advanced fertilizers that enable gradual and sustained release.
Methodology
In this document, we present the creation of a novel carrier system using sheets of graphene oxide (GO). These sheets are capable of efficiently carrying plant micronutrients, allowing for a controlled and gradual release. To demonstrate this idea, we utilized zinc (Zn) and copper (Cu) as examples of micronutrients, loading them onto the GO sheets to formulate a fertilizer based on GO. We verified the chemical composition and successful loading of both nutrients onto the GO sheets using techniques such as X-ray photoelectron spectroscopy and thermogravimetric analysis.
Findings
The prepared Zn-graphene oxide (Zn−GO) and Cu-graphene oxide (Cu−GO) fertilizers showed a biphasic dissolution behavior compared to that of commercial zinc sulfate and copper sulfate fertilizer granules, displaying desirable fast and slow micronutrient release. A visualization method and chemical analysis were used to assess the release and diffusion of Cu and Zn in soil from GO-based fertilizers compared with commercial soluble fertilizers to demonstrate the advantages of GO carriers and show their capability to be used as a generic platform for macro- and micronutrients delivery. The results of kinetic rate of adsorption show a significant increase in adsorption of both ions at the beginning of the process (first 10 min), with slow increase after 10−20 min and reaching the maximum after 120 min. In the case of ZnSO4 and Zn−GO granules, similar amounts of Zn were recovered at >9 mm from the granules, 28 and 25%, respectively. In soil with CuSO4 granules, 29 % of the fertilizer Cu was recovered at >9 mm, whereas in soil with Cu−GO granules, 18 % of the fertilizer Cu was recovered in this zone. A pot trial demonstrated that Zn and Cu uptake by lettuce was higher when using GO-based fertilizers compared to that when using standard zinc or copper salts.
Conclusion
The findings revealed that solid pellet forms of micronutrient fertilizers (Zn−GO and Cu−GO) exhibited a notable capacity for nutrient retention (over 10%). This capacity is attributed to GO's substantial surface area and numerous oxygen binding sites on its surface and edges, facilitating the binding of micronutrient ions. The carrier based on GO exhibited a two-phase nutrient release profile, enabling the supply of micronutrients through both rapid release (approximately 40% within 5 hours) and gradual sustained release. This release pattern is highly advantageous for crops, as it meets the high nutrient demands during seedling establishment and provides a slower, continuous release during later growth stages. The impressive nutrient retention and favorable release properties of GO-based carriers make them an attractive choice for loading various nutrients (both macro and micro) and their combinations. Consequently, they have the potential to serve as versatile carriers, ushering in a new era of advanced Slow-Release Fertilizers (SRFs).