Incorporating biochar to make hydrogel composites with improved structural properties, valorized from waste-paper mill sludge and forestry residues using energy efficient protocols†

The transformation of waste-paper mill sludge into high-value materials with minimized chemical and energy consumption addresses the 12th United Nations Sustainable Development Goal, Responsible Consumption and Production. In this study, cellulose was recovered from dewatered sludge (DS), procured from a local paper mill, using energy-efficient microwave and ultrasonication techniques. Crosslinked hydrogel composites were synthesized from the recovered cellulose and citric acid, as agricultural amendments to optimize water consumption. Powdered biochar (BC) was incorporated into the crosslinked hydrogels, as a biocompatible filler to further enhance thermal stability and water retention. Four hydrogel composite samples were prepared containing BC compositions of 0 g (CH), 0.5 g (BH0.5), 1 g (BH1.0) and 1.5 g (BH1.5). The physicochemical composition, functional groups, thermal stability, water retention, gel fraction, and degradation rate of the extracted cellulose (EC) and prepared hydrogel composites were compared. The energy-efficient extraction process successfully yielded a high EC yield (81.5%) with a cellulose fraction of 93.8% compared to the raw DS at 66.6%, resulting in a conversion efficiency of 140.8%. Incorporating 1 g BC into the hydrogel matrix (BH1.0) improved water absorbency by 992% over CH. Water retention for the hydrogel composites enhanced in the order of BH1.0 > CH > BH0.5 > BH1.5. BC addition also improved the gel fraction, and the thermal stability of the composites increased by up to 60%. Biodegradation studies using the soil burial method showed that cellulose-biochar composites degraded by 40% in 50 days, exhibiting promising potential as agricultural amendments for podzolic soils in the northern boreal ecosystem.