Eco-synthesis, application and enhanced water-retention properties of cellulosic hydrogels derived from waste papermill sludge as soil amendments

Poor water retention in acidic soils and prolonged droughts cause major crop losses each year. In boreal ecosystems this poses a challenge for growing food locally, adding to nutrient insecurity. Hydrogels derived from waste biomass offer a sustainable solution by improving water retention without relying on virgin, petroleum or renewable biomass-based resources, contributing to a circular bioconomy. In this study hydrogel-composites were synthesized (i) from cellulose recovered from waste papermill sludge using energy-efficient protocols, (ii) by crosslinking recovered cellulose with citric acid (iii) followed by incorporating various amounts of biochar (BC) as a biocompatible filler. Four hydrogel composites containing BC compositions of 0 g (CH), 0.5 g (BH0.5), 1 g (BH1.0) and 1.5 g (BH1.5) were synthesized and characterized by Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and swelling studies (study 1). The effects of 1% and 2% (w/w) of cellulose hydrogel (CH) and biochar-incorporated hydrogel (BH) on sandy loam and silty loam soil were evaluated using water retention characteristics and drought resistance (study 2). The energy-efficient extraction with microwave and ultrasonication recovered cellulose in a 81.5% yield with 93.8% purity. The hydrogel composites showed enhanced water retention in the order BH1.0 > CH > BH0.5 > BH1.5. The application of hydrogels significantly (p<0.05) increased field capacity (FC) in both soils. In sandy loam soil, 2% of CH and BH increased the plant available water (PAW) by 313% and 405%, respectively. In drought-simulated pot experiments, tomato plants treated with 2% CH and BH remained fresh for up to 17 days without water, compared to 5 days for untreated soil. Waste papermill sludge cellulose-derived hydrogels exhibited significant potential as agricultural amendments, enhancing water retention and mitigating crop water stress, contributing to United Nations Sustainable Development Goal 12 (Sustainable Consumption and Production).