Sulfur enriched slow-release coated urea produced from inverse vulcanized copolymer

Slow-release fertilizers are developed to enhance the nutrient use efficiency (NUE), by coating urea with less water soluble or hydrophobic material. Diverse range of materials have been utilized to coat urea, however, their inherit non-biodegradability, hydrophilicity, crystallinity, and high synthesis cost limits their scalability. Herein, we reported the preparation of a novel slow-release sulfur enriched urea fertilizers using sustainable hydrophobic, biodegradable, crosslinked copolymer made from sulfur and rubber seed oil (Poly(S-RSO)) through the use of dip coating method. Scanning electron microscopy (SEM) was employed to study the fertilizers morphology and estimate the coating film thickness. A nitrogen release test was carried out in distilled water, which revealed that the coated fertilizers with a coating thickness of 165 μm, 254 μm and 264 μm released only 65 % of its total nutrient content after 2, 19 and 43 days of incubation, respectively: hence, showing an excellent slow-release property. In soil, fertilizer with 264 μm coating thickness released only 17 % nitrogen after 20 days of incubation, in line with the European standard (EN 13266, 2001). The release kinetic data best fits the Ritger-Peppas model with a R² value of 0.99 and the n value of 0.65 indicated the release was mainly due to diffusion. Submerged cultivation (SmC) demonstrated the potential of poly(S-RSO) to enhance sulfur oxidation; it was observed that the copolymer oxidation was 50 % greater than that of elemental sulfur. A comparison between the newly developed fertilizers and existing coated fertilizers was also presented. On the whole, the results demonstrated outstanding slow-release characteristics and improved sulfur oxidation.