Green-synthesized iron nanoparticles enhance CMC/PVA coatings for biochar‑zeolite slow‑release fertilizers

Abstract The escalating demand for sustainable agriculture necessitates innovative slow-release fertilizers (SRFs) that enhance nutrient efficiency while mitigating environmental impacts. This study presents a new green-synthesized nano-iron (tea extract iron nanoparticles, T-FeNPs) reinforced carboxymethyl cellulose/polyvinyl alcohol (CMC/PVA) composite coating for zeolite–biochar hybrid SRFs. T-FeNPs were synthesized using tea extract as a reducing agent and incorporated into CMC/PVA matrices. The nanocomposite coatings were applied to fertilizer cores comprising zeolite, nitrogen-phosphorus-potassium (NPK) compound fertilizer, and rice straw biochar (RSBC). Soil leaching tests demonstrated superior nutrient retention: CMC/PVA/0.5Fe-SRF reduced cumulative N and P release to 58.47% and 15.82%, respectively, outperforming unmodified CMC/PVA-SRF and compound NPK fertilizers. Mechanistic investigation uncovered that physical barrier enhancement via pore obstruction by T-FeNPs significantly impedes water ingress and nutrient ion diffusion. In tomato cultivation, the CMC/PVA/0.5Fe-SRF treatment maximized plant height, biomass, and water retention, attributed to sustained nutrient release. Soil analysis revealed enhanced total N, P, K, cation exchange capacity, and organic matter content. Economic analysis indicated a production cost of $562.02/ton for CMC/PVA/0.5Fe-SRF, with potential global GHG reductions of 35.69 Mt CO₂e in East Asia alone via improved nitrogen use efficiency. Overall, this work presents a scalable, eco-friendly strategy for enhancing fertilizer efficiency and soil health. Graphical Abstract