Modified iron phosphate/polyvinyl alcohol composite film for controlled-release fertilisers

Traditional soluble phosphorous (P) fertilisers can be easily leached to pollute water systems, resulting in water eutrophication, a major environmental problem from the oversupply of unused nutrients. One innovative solution is to control the release of P upon demands of the plants. This study established a new concept of controlled-release P fertiliser via incorporation of ferric phosphate (FePO₄) as a P source in polyvinyl alcohol (PVA) films, which can immobilise the FePO₄ particles and stimuli-responsively accelerate their release rate in the presence of citric acid. More importantly, FePO₄ used in this work originated from steelmaking slag as a potential waste reuse. Due to the low solubility of FePO₄, diethylamine was introduced to modify FePO₄ particles to facilitate the release of P before incorporating with PVA. The effects of diethylamine modification and the properties of FePO₄/PVA films were systematically investigated through microscopic and spectroscopic methods. The release of P from particles and films was examined in both deionised water and citric acid solution for 30 days. The results showed a tenfold increase of the release rate of modified FePO₄/PVA in citric acid solution compared with that in deionised water, and also a doubled release rate of the modified FePO₄/PVA compared to that of FePO₄/PVA in citric acid. The improved performance suggests that PVA can maintain the phosphorous content with exposure to water and expedite release in citric acid upon the demand of plants. This composite film offers a new opportunity for the application of insoluble phosphate as a phosphorous fertiliser.