Adsorption-desorption of phosphate on airborne dust and riverborne particulates in East Mediterranean seawater

The potential importance of adsorption-desorption behavior of phosphorus (P) on the East Mediterranean (E. Med) P cycle was investigated. Contrasting adsorption behavior between Saharan dust (SD) and Nile particulate matter (Nile PM) was observed. SD was a source of P to the region, which released an average of 3.3±0.3 μmolP/g into the surface seawater and showed no adsorption ability under the conditions close to the E. Med deep water. Saharan dust is therefore unlikely to be the reason for P limitation in the region. By contrast, Nile PM acted dual roles of a sink and source of P in different waters (surface seawater, deep seawater, and river water). A new crossover-type adsorption-desorption model explained the contrasting adsorption behavior and the dual nature of natural particles. The model indicates that when natural particles are transported between different waters, they can be a sink (adsorption) or a source (desorption) of phosphorus depending on the "specific concentration (λ)", which is the ratio between the aqueous P concentration and the zero equilibrium P concentration (EPC₀). EPC₀ refers to the solute concentration value where the adsorption isotherm crosses over the aqueous concentration axis. When λ > 1, adsorption occurs, whereas when λ < 1, desorption occurs. The model added a general development to the methodology of adsorption isotherm, where, for the first time, effects of solute concentration, solid concentration, and aqueous medium (EPC₀) on the adsorption and desorption of P in natural waters were simultaneously described by a single equation. Using the model, it was quantitatively reconstructed that particles emitted during the pre-1964 Nile floods could be a major source of P to Egyptian coastal waters (up to 4800 tonsP/yr), greater than the dissolved P flux (∼3200 tonsP/yr), but a trapper of dissolved phosphate in E. Med deep waters.