Sustainable fertiliser mesh, ‘4D’-precision engineered by flow chemistry: minimising agrochemical pollution

Abstract

Flow chemistry is best when opening “novel process windows”, and one of six NPW paths is “new chemical transformations”. Flow chemistry has a large impact on syntheses which are particularly mixing-sensitive. Here we exploit the NPW offered by flow chemistry to generate a new kind of core–shell necklaced fertiliser which can be woven and used as a localised point source with low leachate rate to the surrounding environment. We report the flow synthesis of fertiliser capsules with a chitosan coating around a chitosan/citric acid matrix that comprises dispersed phosphorus (P) particles (apatite). These capsules are connected via a chitosan string, resembling necklaces, adding a 3^rd hierarchy in scale (3D) to the two hierarchies of coating and matrix. The strings of necklaces were woven into a mesh-like structure, adding a 4^th hierarchy scale (4D). A stack of those meshes has potential to enable precise and uniform fertiliser release over a soil volume – control both over the temporal and spatial domain. Controlled-release fertilisers of macronutrients as P are critical for agricultural efficiency to reduce non-point source pollution causing by nutrient runoff/leaching. Using combined sheath and segmented flow in a water/oil system, the capsules had a consistent composition and morphology with an average content for phosphate and potassium of 24.5 wt% and 7 wt%, respectively. The resulting fertiliser exhibited slower nutrient release and extended lifetimes in aqueous media with 62 days in mild acidic solution, against 2422 days in deionised water. Soil column experiments demonstrated a low leaching loss for the sheath-flow made P fertiliser (1.2%), which was 60 times less than for a commercial fertiliser.