Heterogeneous reaction of NO2 with hydroxyapatite particles: Effects on phosphorus water solubility

Model simulations indicate that atmospheric acidification increases the solubility of phosphorus (P) in mineral dust, thereby altering primary productivity in many P-limited ecosystems. However, results from laboratory simulation studies are still lacking. We investigated the heterogeneous reaction between nitrogen dioxide (NO 2 ) and hydroxyapatite (HAP) using a flow reaction system. The results reveal the formation of nitrate products and an increase in the water solubility of P in HAP. Under conditions of 50 ppm NO 2 and 50% relative humidity, after 8 h of reaction, the reactive uptake coefficients calculated using geometric surface area and Brunauer–Emmett–Teller surface area were 1.61 × 10 −5 and 1.26 × 10 −9 , respectively. After 10 h of reaction, the water solubility of P was 3.61 times its pre-reaction value. Longer reaction times and higher NO 2 concentrations promote both greater NO 3 − formation and increased P dissolution. The mechanism for increased water solubility involves hydrogen ions (H + ) weakening calcium bonds, thereby enhancing the release of P from the HAP crystal surface. This study demonstrates that atmospheric heterogeneous reactions between HAP in mineral dust and acidic gases can enhance the water solubility of P, thus increasing its bioavailability. This enhancement partially alleviates P limitation in tropical rainforests and open-ocean ecosystems, boosting primary productivity in P-limited ecosystems. Primary production processes absorb and fix atmospheric CO 2 , which in turn influences the carbon cycle in P-limited ecosystems.