## 4 Future Perspectives
### The Use of Eggshell Waste in Slow-Release Fertilizer (SRF) System  
The ability to provide a continuous supply of fertilizer for ensuring good crop yield is necessary to meet the demands for addressing food security for the world's growing population. In the case of non-uniform application of fertilizer, inefficiency in fertilizer uptake by plants are known as a source of pollution to land, air, and water due to vaporization into the atmosphere, leaching, and surface run-off. In a bid to circumvent the various limitations and challenges faced in the controlled application of fertilizer to agricultural fields, slow-release delivery systems have the potential to enable more efficient, cost-effective, and sustainable uptake of fertilizer. The potential utility of ES-based substrates as a viable  
Figure 11: Application of ES as a photocatalyst in water treatment. Copied and redrawn with permission [59].  
Figure 10: Application of eggshell waste in catalysis.  
support for SRF was reported by [PERSON] et al. [195], where the ES-SRF system and its preparation are outlined in Figure 12.  
In this work, an investigation of the use of ES as a support to supplement nutrients in soils was reported, where soils with ES-SRF had greater plant growth (height and root length) when compared to the soils treated with pristine ES or without any treatment (ES or ES-SRF). The germination rate of the tested crops, i.e., cucumber and tomato, increased by 57.7 and 76.0%, respectively; moreover, the application of ES-SRF led to improved water holding and water retention capacities of the soils (cf. Figure 13). It was inferred that ES-SRF serves as a reservoir of nutrients that was capable of providing essential nutrients to plants throughout the growth period.  
The production of K\({}_{3}\)CaH(PO\({}_{4}\))\({}_{2}\) and CaKPO\({}_{4}\) was reported by using a mechanochemical process between eggshell and KH\({}_{2}\)PO\({}_{4}\)[196]. There is a better management of P, K, and Ca when the produced K\({}_{3}\)CaH(PO\({}_{4}\))\({}_{2}\) and CaKPO\({}_{4}\) are applied to soil systems. The result showed an increase in phosphorous (P) release from 0 to 25 mg/kg after 3 days and 45 mg/kg at 30 days, indicating that a longer delay in P release was realized.  
Figure 12: Application of eggshell as a slow-release fertilizer system. Adapted with permission [195].  
As shown in Figure 14, a granular adsorbent made from torrefied wheat straw, eggshells, and chitosan was used for orthophosphate adsorption studies, where the results revealed that the granular adsorbent was capable of adsorbing 23-30 mg/g orthophosphate at pH 4.5, and between 9-12 mg/g at pH 8.5 [16]. This study highlights the role of closed loop-processes, where one loop for the design of a suitable adsorbent from waste biomass (e.g., ESP and wheat straw); whereas a second loop demonstrates the utility of adsorbed phosphate as a SRF system can be applied for agricultural crop production.  
The utilization of ES substrate for the preparation of composite adsorbents represents a target material for the valorization and utilization of ES waste. The utility of ES-based materials is further illustrated in their application for the removal of pollutants from wastewater, as described in Section 4.3.  
Figure 14: The use of granular ternary agro-waste adsorbent for orthophosphate uptake at pH 4.5 and 8.5. Copied with permission [16].  
Figure 13: (**a**) Water holding capacity and (**b**) water retention capacity of soil with and without ES and ES-SRF. Copied and modified with permission [195].