Influence of weathering-driven mineralogical evolution on phosphorite dissolution behavior

Abstract

The phosphorite dissolution and aqueous species release play a pivotal role in the global phosphorus (P) cycle. To elucidate differences in dissolution mechanisms between phosphorite of different weathering degrees, three phosphorite samples (with high, moderate, and low weathering degrees) were collected from field environments; then, a series of mineralogical characterizations coupled with dissolution experiments were conducted. Compared to primary phosphorite, weathered phosphorite exhibited substantial depletion of carbonate minerals, while silicate minerals and phosphate minerals were relatively enriched, and secondary minerals (e.g., FePO₄ and CaF₂) were formed. Furthermore, the dissolution rates of phosphorite samples within a pH range of 3–7 were determined using a mixed-flow reactor. The results demonstrated a negative linear relationship between the logarithm of dissolution rate and pH (R² = 0.54–0.94). The P release rate constant (from 0.7 to 1.52 mol g⁻¹ s⁻¹) and kinetic reaction order (from 0.58 to 0.74) of weathered phosphorite were significantly higher than those of primary phosphorite. We modeled the co-dissolution behavior of associated minerals with fluorapatite using transition state theory. Calcite exhibited the strongest inhibitory effect on fluorapatite dissolution, whereas the effects of other minerals were relatively weak. In the column leaching experiment, compared to the primary phosphorite, the elements released from weathered phosphorite were predominantly present in ionic forms with a higher migration capability. During the leaching process, phosphate minerals in phosphorite were mainly converted to Exchangeable/loosely bound P (Ex-P), Iron-bound P (Fe–P), and Calcium-bound P (Ca–P); Ca²⁺ and CO₃²⁻ were mainly converted to dolomite, and F⁻ was mainly converted to fluorite and fluorapatite. This systematic investigation provides critical insights into weathering-driven mineralogical evolution and its mechanistic control on phosphorite dissolution.