Synthesize, swelling and morphological properties of tea wased-based hydrogel as a slow-release NPK fertilizer applied to mini eggplant seeds growth

Incorporating tea waste (TW) into hydrogel promotes sustainability and supports the principles of circular economy. Tea waste-grafted-polyacrylic acid containing NPK fertilizer (TW-g-PAA/NPK) was developed as potential slow-release fertilizer hydrogel (SRFH). Its properties were compared to those of synthesized PAA superabsorbent polymer (SAP) and TW-g-PAA superabsorbent polymer composite (SAPC). Chemical, morphological and elemental properties of SAP, SAPC and SRFH were analyzed using FTIR spectroscopy, SEM microscopy and SEM-EDX spectrometer. The grafting reactions were proven by the appearance of peaks at 1046 cm-1 in SAPC and SRFH spectra, indicating that -COH bonds of cellulose were broken to form -COC- bonds. SEM images showed that, TW fiber was grafted with PAA hydrogel and SRFH had the smallest pore sizes. Furthermore, EDX images proved the presence of nitrogen (N), phosphorus (P) and potassium (K) elements in SRFH. Swelling properties indicated that SRFH has the lowest WAC, resulting in slowest release rate. Re-swelling performance showed that SRFH and SAPC has no vital change in WAC even after five cycles of swelling. Germination and growth of mini eggplants seeds in black soil treated with SRFH showed faster germination and the best development. The developed TW-g-PAA/NPK provide an innovative approach as slow-release fertilizer and water retention in agriculture fields.