Synthesization and characterizations of coal fly ash-coffee grounds-based composite as super-absorbent for application in soil

To recycle and utilize two types of harmful solid waste, coffee grounds (CG) and coal fly ash (CFA), a novel and low-cost superabsorbent composite (MCG-PAA/CFA) was synthesized by aqueous solution polymerization with modified coffee grounds (MCG), acrylic acid (AA) and CFA as raw materials, and it was applied to soil to improve its drought resistance. Various reaction conditions were comprehensively investigated and analyzed to assess their influence on the water absorbency of the superabsorbent composite (SAC). After optimization, the MCG-PAA/CFA exhibited water absorbency capacities of 1260(±10.6) g/g and 82(±1.4) g/g in deionized water and physiological saline, respectively. After adding 3 wt% MCG, the water absorption of SAC was improved from 415 to 746 g/g. After further introduction of 2 wt% CFA, the water absorption of SAC increased from 746 to 1260 g/g. Fourier Transform Infrared (FTIR) analysis confirmed that the grafting reaction was successful and that CFA participated in the reaction, while scanning electron microscope (SEM) and thermogravimetric analysis (TGA) results revealed that the grafting reaction and the introduction of CFA improved the surface morphology and thermal stability of the SAC. Kinetic analysis was conducted to investigate how the grafting reaction and the introduction of CFA affected the swelling and water retention kinetics of the superabsorbent composite. In the soil experiment, adding only 0.1 wt% MCG-PAA/CFA can improve the water holding capacity of sandy soil, loam soil and clay soil by 6.65%, 4.42%, and 3.76% respectively This SAC composite has great potential in soil drought resistance.