Mechanochemically Activated Halloysite Nanotube-Rich Kaolin Clay As a Carrier for Slow-Release Phosphate Fertilizer

Formulation of sustainable slow-release phosphate (SRP) fertilizers using low-cost carrier materials is a growing area of research. This fertilizer can prevent its nutrient loss caused by surface runoff or soil leaching. Here, we investigated the mechanochemical activation of halloysite-rich kaolin clay by planetary ball milling and produced an enhanced SRP fertilizing substrate. The milling process was carried out under dry (clay only and KH2PO4 solution added after milling) and wet conditions (slurry of clay and KH2PO4) over varying durations (e.g., 1–8 h). Changes in crystallinity and microstructure of materials induced by milling were characterized by X-ray diffraction and electron microscopy. The retention and release of phosphate from the water-extractable phase of the fertilizer were also analyzed. High-resolution transmission electron microscopy mapped the elemental distribution at the crystal scale. The milling method had a pronounced effect on the phosphate release behavior. Dry-ground materials (3–5 h) showed better retention and controlled release (∼40% phosphate released in the first wash followed by ∼5% in two successive washes). However, wet-ground samples released more phosphate initially (∼50%), leaving less for later release. Compared to wet milling, dry milling caused greater crystal damage, particularly halloysite tube breaks, and increased the amorphousness of the material. These affected the containment of KH2PO4 salt into halloysite lumen and the release of phosphate ions in the water phase. This provides a choice of fertilizer formulations simply by adjusting milling conditions. To move forward, we need to study the scale-up of this potentially sustainable slow-release phosphate fertilizer and test it in soil and crops. This will benefit raw mineral resources and improve the nutrient efficiency.