Dissolution kinetics of citrate coated CoFe2O4 nanoparticles in soil solution

Introducing nanomaterials in agriculture may help to transform the way we farm into a sustainable model by improving the efficiency of fertilizers and reducing the inputs. However, to make sure that these nanomaterials are safe for the environment and their users, the characterization of these nanomaterials and their interactions with the media are needed. Magnetic nanomaterials (such as those based on Fe, Fe oxides, Co-ferrite, etc.) have been investigated for agricultural applications due to their ability to increase the crop yield, photosynthetic rate, and plant biomass. In this context, this research addressed the dissolution rate of citrate-coated CoFe2O4 nanoparticles (NPs) at different pH values (5, 7, and 8) and short periods (to 0.25 until 168 h) in the soil solution of alkaline soil and artificial root exudates. Several equations were tested to fit the rate of dissolution, but the pseudo-second-order model was the best one to fit the release of Co (R2 between 0.80 to 0.99) under all pH soil solution conditions. The k value at pH 5, 7, and 8 was 21.56, 10.52, and −31.33 L mmol−1 h, respectively. Iron was not detected in soil solution experiments; in contrast, artificial root exudates released Fe from NPs. The best models to fit the Fe release from the NPs were the Higuchi and Korsmeyer–Peppas model (R2 >0.92 and R2 adjusted = 0.91). The k values were 4.33 × 10−5 mM h1/2 and 9.88 × 10−3 h−n, respectively. The main species formed from the elements released from the NPs by the ARE action were the complex of Fe and Co with malate (92% of Fe as Fe-malate+, 71% of Co as CoH-malate(aq), and 14% of Co as CoH-malate+). The Fe contained in those complexes may be an Fe exchangeable source for plants. So, due to the poor water solubility of citrate-coated CoFe2O4 NPs and their dissolution by the action of artificial root exudates, these may be considered an option for slow-bio-release Fe fertilizer.