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

Abstract

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.

Valorization of hemp fibers into biocomposites via one‐step pectin‐based green fabrication process

“…93 As well, in our previously reported study, we demonstrated the use of hemp fiber composite as plant growth media for hydroponics 94 (one type of soilless farming). 45 These aforementioned examples illustrate that hemp fiber composites have an exciting prospect due to their diverse and large arrays of practical applications. Hence, we expect that, due to its generalizability and applicability, this one-pot pectin-based fabrication methodology could serve as a benchmark for developing other technologies to accommodate a wide range of sustainable applications and provide eco-friendly alternatives to synthetic and nonbiodegradable petroleum-based products.…”

“…70 Another successful strategy reported in several studies is the polymerization reaction involving lignin with bifunctional chemical linkers. 45 Research teams have also reported different types of lignin modifications using diazonium salts to obtain better adhesion to the polymer, which allow the creation of robust composites with better mechanical and thermal properties. [44][45][46] In addition, the phenolic hydroxyl groups of the lignin are good nucleophiles when compared to other possible hydroxyl groups available on the hemp fibers.…”

“…45 Research teams have also reported different types of lignin modifications using diazonium salts to obtain better adhesion to the polymer, which allow the creation of robust composites with better mechanical and thermal properties. [44][45][46] In addition, the phenolic hydroxyl groups of the lignin are good nucleophiles when compared to other possible hydroxyl groups available on the hemp fibers. Thus, we expect that the majority of the crosslinking would happen between pectin and lignin.…”

“…43 Reported studies have described different types of chemical processes for lignin modification by using the basic principles of diazonium chemistry to attain strong binding of the natural fibers towards the polymer matrix and to culminate in composite materials with enhanced physical strength and better thermal properties. [44][45][46] However, these fabrication processes were generally carried out in two steps and the pH of the reaction mixture had to be monitored during the reactions. These two factors make the composite fabrication process less convenient for implementation at a large scale in terms of logistics and overall economic feasibility.…”

“…It encompasses the hemicellulose‐coated cellulose strands and provides structural integrity, rigidity, and physical strength to the hemp fiber (Figure 3). 43 Reported studies have described different types of chemical processes for lignin modification by using the basic principles of diazonium chemistry to attain strong binding of the natural fibers towards the polymer matrix and to culminate in composite materials with enhanced physical strength and better thermal properties 44–46 . However, these fabrication processes were generally carried out in two steps and the pH of the reaction mixture had to be monitored during the reactions.…”