Marine biomass-derived, hygroscopic and temperature-responsive hydrogel beads for atmospheric water harvesting and solar-powered irrigation

Agriculture is a major user of ground and surface water, accounting for ∼70% of the world's freshwater withdrawals. The rising agricultural water demand potentially leads to a conflict with water use among urban, industrial and agricultural sectors. It also threatens food security, human life and the environment worldwide. The urgent need for alternative water resources motivates the exploration of atmospheric water as an abundant and untapped source of freshwater for irrigated agriculture. Herein, kelp-derived hydrogel beads with self-contained properties (i.e., hygroscopic, photothermal, temperature-responsive and durable) are developed via shape-controlled and mass fabrication. The hygroscopic beads attain maximum water uptake of over 5.0 g g−1 under 90% relative humidity (RH). Synergistic photothermal heating and temperature-driven phase transition afford multi-modal water desorption and efficient water release. Over 95% of absorbed water can be rapidly released under a broad solar intensity of 0.6-1 sun and temperature range of 40-60 °C. A solar-powered and sorption-based seed propagator is developed and demonstrated to sustain the plant germination and growth, taking the advantages of the hydrogel beads' reversible moisture sorption/desorption in night/day cycles, the high quality of irrigation water, net-zero energy consumption and thermal management in a system-level design. This work provides a perspective on controlled and mass fabrication of hygroscopic hydrogels. It highlights the solar-powered atmospheric water irrigation for electricity-free and sustainable agriculture regardless of varied geographical and hydrologic conditions. © 2022 The Royal Society of Chemistry.