# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 3 SHs Formation Methods  
SHs can be formed through (i) graft copolymerization of monomers onto the starch chains, (ii) using a chemical crosslinker, and (iii) by physical interactions between polysaccharide chains. Graft copolymerization of acrylic acid onto starch chains has been reported using ammonium persulfate (KPS) or ceric salts as polymerization initiators (Figure 2A,B). The persulfate ion acts upon heating, forming a sulfate anion-radical, which abstracts hydrogen from the starch hydroxyl groups, creating a radical on the polysaccharide chain. This provides an active center to initiate acrylic acid polymerization, resulting in the formation of the graft copolymer [63; 64]. On the other hand, ceric (Ce4+) forms a redox complex with the hydroxyl groups of starch, which is then dissociated to generate a carbon radical on the starch chains, thereby initiating graft polymerization. Starch graft copolymerization using persulfate and ceric salts has also been reported with copolymers such as acrylamide [65], styrene [66], methyl methacrylate [67], acrylonitrile [68], and others. A review of experimental conditions for starch graft copolymerization was reported by [PERSON] et al. (2017) [69]. In this section, we will focus on the characteristics and properties of the hydrogels obtained. The crosslinking material is formed after copolymerization, typically using crosslinking mediators like N,N\({}^{\prime}\)-methylenebisacrylamide (MBA) (Figure 2C). [PERSON] and [PERSON] (2020) created a starch superabsorbent hydrogel with acrylic acid using ceric ammonium nitrate (CS-g-PAA/CAN) or KPS (CS-g-PAA/KPS) as the initiator [64]. Figure 3A shows the SEM images of corn starch and the formed hydrogels, highlighting differences in morphology and pore size. The CS-g-PAA/KPS hydrogel exhibits a porous surface, with pore sizes ranging from 11 to 136 \(\upmu\)m, providing accessible spaces and channels that facilitate the hydration of hydrophilic groups, resulting in higher swelling. Another method for grafting starch with acrylic acid involves radiation. This technique is highly efficient and is initiated by hydroxyl radicals generated when water molecules  
Figure 2: **(A)** Mechanism of starch-g-poly(acrylic acid) formation initiated by K persulfate. (**B**) Mechanism of starch-g-poly(acrylic acid) formation initiated by ceric (IV). (**C**) Starch-g-poly(acrylic acid) hydrogel formation using N,N\({}^{\prime}\)-methylenebisacrylamide as a crosslinking agent (modified from [64]). are irradiated with gamma rays. The hydroxyl radicals remove hydrogen from the starch chain, thereby generating a polymer matrix [70]. Another widely studied hydrogel is starch-g-PAM (PAM stands for polyacrylamide), which is formed by the polymerization of the acrylamide monomer in the presence of an initiator such as KPS. [PERSON] et al. (2022) reported the polymerization reaction using water as the solvent at 80 \({}^{\circ}\)C for 4 h, with the addition of MBA for crosslinking [72]. Starch grafting with acrylamide can also be achieved using KPS combined with microwave irradiation followed by cooling cycles. The grafting percentage increased from 71.25% to 1237.32% as the mass of acrylamide was increased from 2 to 10 g. A similar trend was observed, where increasing the irradiation time also led to an increase in the grafting percentage [73]. However, the properties of CS-g-PAA (using cassava starch in this experiment) were affected by both lab-scale and up-scaling production processes. Specifically, the degree of grafting (Dg) and swelling ratio (SW) were influenced, with a higher Dg observed in ups-caled production (using a 150 L reactor, Figure 3B). Consequently, this resulted in a lower SW compared to the lab-scale synthesis [71]. This can be observed by tracking the SW over time, with the burst profile for both materials reaching approximately 200 and 550 g/g for lab-scale synthesis and up-scaled production, respectively (Figure 3C,D).