Thermo and pH-responsive methylcellulose and hydroxypropyl methylcellulose hydrogels containing K₂SO₄ for water retention and a controlled-release water-soluble fertilizer

Hydrogel as commercial soil conditioner has the potential to improve soil properties and reduce water scarcity in growing plants. However, use of polyacrylamide-based hydrogel in agriculture pose potential human health hazards. Hydrogel systems were prepared from temperature-responsive methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) blended with potassium sulfate (K₂SO₄, K-containing fertilizer). Blended K₂SO₄ in hydrogels affects the hydrophobic interaction of MC and HPMC, and hydrogels prepared in 0.25 M K₂SO₄ decrease the gelation temperatures of MC and HPMC from 55.6 and 67.4 °C to 27.8 and 35.1 °C, respectively. The equilibrium swelling ratio (St) of MC and HPMC hydrogels was increased 15 times and hydrogels maintained a steady appearance. The swelling kinetics of the MC and HPMC hydrogels followed Schott's second-order model and kinetic parameters indicated that adding K₂SO₄ can improve the initial swelling rate. K₂SO₄ decreases the pore size of the MC and HPMC hydrogel from 26.8 and 21.5 μm to 14.7 and 14.2 μm. A plant growth assessment indicated inhibition in plant growth occurred without the application of the hydrogels. Adding K₂SO₄ increases mechanical properties of the hydrogels. Treatment with 0.5% hydrogel considerably improves the water-holding and water-retention capacities of sandy soil. The 20-20-20 fertilizer release results revealed at that at 35 °C and a pH below 7, the hydrogel systems reduce the release rate of the fertilizer. The best result was observed with the Korsmeyer-Peppas model, indicating that the fertilizer transport mechanism involves Fickian diffusion and swelling-controlled release. This study discovered that environmentally responsive cellulose-based hydrogels have potential for use as soil conditioners and as controlled release devices in horticulture and agriculture.