One-step green construction of granular composite hydrogels for ammonia nitrogen recovery from wastewater for crop growth promotion

The great application potential of hydrogel adsorbent in water treatment has been widely reported in recent years. However, practical application is severely restricted due to the tedious preparation procedures, high energy consumption, and potential environmental threat of spent adsorbent after adsorption. To address these challenges, a new method was proposed in this study to prepare granular hydrogel composites through one-step polymerization induced phase separation (PIPS), initiated via a redox reaction consisting with calcined oil shale semi-coke (COSSC) and active ingredients of in green tea (GT) at 40 oC. Then the granular hydrogel was applied to remove NH4-N in wastewater, and the spent adsorbent after adsorbing was used as the medium for plant growth. The granular hydrogel exhibited a favorable swelling capacity of 205 g/g and excellent adsorption capacity of 40.2 mg/g for NH4-N. Remarkably, the adsorption process achieved equilibrium only within 30 min. To avoid the influence of soil and other factors, we conducted a soilless cultivation experiment, and the results showed that the total seed biomass of mung bean increased by 33.69% with adding 1% of spent hydrogels into the culture medium. Based on above excellent performance, this study presented a feasible idea for the green preparation granular hydrogel composites and offered a promising strategy for the sustainable utilization of spent adsorbent.