Green Chemical Approach to Fabricate Hemp Fiber Composites for Making Sustainable Hydroponic Growth Media

We report a water-based approach for making chemically reactive forms of hemp fibers and employing these materials to fabricate biocomposites for hydroponic applications. Our chemical strategy focused on coupling the lignin of hemp fibers and a bifunctional linker molecule (2-[(4-aminophenyl)­sulfonyl]­ethyl hydrogen sulfate) that contains an aromatic amine and a protected vinyl sulfone. The synthetic process started with converting the amine of this linker to an electrophilic diazonium ion, which then reacted with the electron-rich aromatic rings of the lignin within the hemp fibers to form the chemically reactive lignin. The other functional group of the linker was activated under basic conditions to yield the intermediate vinyl sulfone, which reacted with poly­(vinyl alcohol) via the Michael addition to yield the cross-linked hemp fiber composites in a 1 in. diameter quartz tube mold. The overall fabrication process was ecofriendly because only water was used as the solvent, and harmless inorganic salts were the only major byproducts. These hemp composites were durable and did not easily crumble under compressive mechanical tests. Fabrication experiments were performed with different weight ratios of bifunctional linkers to hemp fibers to evaluate the effect of this factor on the mechanical strength of resulting hemp composites. The compressive strength of these dry hemp composites was measured to increase from 0.91 to 1.81 MPa when this weight ratio was raised from 3: 40 to 3: 5. The hemp fiber composites fabricated using an intermediate weight ratio (3: 10) of the bifunctional linkers to hemp fibers were evaluated as a hydroponic growth medium. Properties of the hemp fiber composites, such as water holding capacity, carbon/nitrogen ratio, salinity, and acidity, were also evaluated to determine their suitability as plant growth media for hydroponic applications. The hemp fiber composites were demonstrated to be effective hydroponic growth media for Daikon radish and green peas in a 14-day growth period