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

Abstract

Sustainability spotlight The global paper industry, vital for economic growth, faces the significant challenge of managing paper mill sludge (PMS). Our research addresses this challenge by valorizing PMS in an energy-efficient manner to create cellulose-based hydrogels. Incorporating biochar from forestry biomass into the hydrogel matrix enhances water retention, thermal stability, and biodegradability. These hydrogel composites offer a circular solution that reduces landfill dependency, improves water retention, and fosters a bioeconomy, aligning with the UN SDGs. This eco-friendly approach not only addresses the challenge of PMS disposal but also contributes to sustainable agriculture. By utilizing renewable resources and minimizing environmental impacts, our approach aligns with SDG 12 (Responsible Consumption and Production), while also supporting SDG 2 (Zero Hunger) and SDG 3 (Good Health and Well-being). The transformation of waste papermill 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.

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