New Controlled-Release Fertilizers with Keratin-Based Coating from Chicken Waste Feathers ^†

Circular economy systems employ minimizing the use of resource inputs and generation of waste, pollution and CO₂ emissions. The new EU regulation [1] advises on the use of urea-based fertilizers and allows only biodegradable coatings in order to be eco-friendly. In this work, we developed an NPK fertilizer coated with keratin from chicken feather waste. 1. Keratin extraction: several methods [2] for obtaining keratin were investigated: (i) solubilisation in natural deep eutectic solvents (NaDESs), (ii) microwave extraction, (iii) extreme pH, (iv) aqueous thermal hydrolysis and (v) enzymatic hydrolysis. 2. Fertilizer formulation: a granular (2–4 mm) NPK-type fertilizer was prepared with a longitudinal oscillator mixer with variable amplitude and granulated using a lab-scale rotating pan granulator. 3. Keratin application: two types of keratin application were tried: using the Wurster method [3] on a fluidized bed granulator and other trials using the same granulator. 4. Leaching tests: the last part was the leaching testing using a Behr system, adapted after [4,5]. The working conditions were 1680 h, 3 mL/h. The eluent was pumped into the soil cartridge, and the liquid eluate was analysed for N, P and K content (%). The classical methods for keratin extraction—extreme pH and aqueous thermal hydrolysis—had much higher extraction yields than the greener ones (NaDES and microwave extractions), at least 100% higher. At 200 °C, keratin from feathers was over 90% extracted in an aqueous solution. It was observed that the coated fertilizers had increased nitrogen content, comparative to the untreated sample. The first fertilizer formulation, N/P/K 1/1/1 with keratin coating, released the highest N content, 0.3%, almost three times higher than the uncoated fertilizer. During the first half of the leaching period, the N and K concentrations were at the highest values, with higher values for the coated fertilizers, indicating controlled macronutrient release. Higher concentrations of N and K were released from the coated fertilizers, P concentration remaining the same. The obtained fertilizer could be used for efficient controlled macronutrient release.