Lignin macromolecule's implication in slowing the biodegradability of urea‐crosslinked starch films applied as slow‐release fertilizer

Abstract

High biodegradability of starch is a major limitation for its commercial usage in developing urea‐crosslinked starch (UcS) film as slow‐release fertilizer. For solving this problem, UcS films were reinforced with 5–20% kraft lignin. Implication of lignin as a macromolecule was tested for slowing the biodegradability of UcS films. These films were biodegraded and characterized in an aerobic soil burial test up to the 60th day. The results were drawn for biodegraded lignin‐reinforced films through comparison made with biodegraded control film, which received 0% lignin. The Fourier transform infrared spectroscopy peaks at 1625 and 1665 cm−1 corresponded to UcS and were found to be more conspicuous in biodegraded lignin‐reinforced films. Thermogravimetric analysis of biodegraded lignin‐reinforced films showed higher thermal stability. This was inferred from the decrease of ∼85.45°C in the thermal decomposition temperature at 5% weight loss (onset temperature), the increase of ∼31.69°C in the thermal decomposition temperature at maximum weight loss, and the increase of ∼12.90% in char. The molecular weight distribution of the biodegraded lignin‐reinforced films reduced not more than ∼1% and the polydispersity index was conserved to 1.4. Light microscopy of the biodegraded lignin—reinforced films reduced not more than ∼1% and polydispersity index was conserved to 1.4. Light microscopy of biodegraded lignin‐reinforced films showed the shape of starch particles was oblong, less disrupted, and wrinkled. Field‐emission electron microscopy showed lignin addition favored more the fungal growth beside formation of cavities. Atomic force microscopy showed the average surface roughness increased 2.00–7.32 times more as a result of residual lignin's accumulation in biodegraded lignin‐reinforced films. Based on the understanding of biodegradability in UcS films, a theoretical framework has also been proposed for biodegradability‐driven urea‐nitrogen release in soil.

Lignin improves release behavior of slow‐release fertilizers with high content of urea

“…It was noted that in the L/M/U composites, all of the used starting materials (except the urea) did not include any synthetic polymers like poly(lactic acid) but only two natural polymers employed: the lignin and the Mt. So the prepared L/M/U composites can be thought to be a kind of reassembly of natural materials or green resources materials or sustainable green products to realize a smart function‐slow release of urea fertilizer molecules . Even in this aggressive condition, the urea release of the L/M/U 2:1:4 is comparable to other controlled release systems with synthetic polymeric coatings applied in urea granules, denoting that the L/M/U 2:1:4 composite is a promising novel slow‐release fertilizer.…”

Current and emerging biodegradable mulch films based on polysaccharide bio-composites. A review

“…Starch biodegradation in a lignin-modified slow-release urea depended on the film thickness. Interestingly, the slow-release fertilizers by slowing biodegradability of ureacross-linked starch mediated by the lignin reinforcement could counteract the high biodegradability of starch (Majeed et al 2017). These studies on the effects of natural plasticizers in starch composite mulch films are very interesting but apparently, their effects on productive crops remain to be evaluated.…”

Biodegradability of bio‐based and synthetic hydrogels as sustainable soil amendments: A review

“…69 Additionally, lignin is a highly hydrophobic molecule, which reduces its availability to soil microorganisms and further contributes to its recalcitrance. 70…”