Effects of mineral inputs on organic phosphorus (Adenosine 5′-monophosphate) sorption and implications for its biogeochemical behavior on marine sediments

The runoff input of mineral particles in coastal areas affects the migration and transformation of nutrients. Organic phosphorus (OP) has attracted much attention due to coastal eutrophication, while the migration under the influence of mineral inputs has not been understood deeply. This study investigated the sorption behavior of an OP compound (Adenosine 5’-monophosphate, AMP) on goethite, kaolin, and two mixtures of the minerals with natural sediment. FTIR spectra and surface acid-base characteristics revealed interactions presented within the mixtures. Sorption kinetics and thermodynamics experiments were carried out through batch methods. The kinetic data were described using a two-compartment first-order equation, and the sorption isotherms were well-fitted by both the Freundlich and Langmuir models. Goethite exhibited a higher sorption ability than kaolin, and the sorption was more sensitive to salinity. The sorption capacity of mineral-added sediments was significantly lower than predicted, particularly for sediments mixed with kaolin, which decreased by 57.4 %. The analysis of OP fractions during sorption revealed an obvious increase in exchangeable or loosely sorbed organic phosphorus (Ex-Po) and iron/aluminium-bound organic phosphorus (NaOH-Po), with NaOH-Po dominating. The sum of Ex-Po and NaOH-Po, as the bioavailable OP, in goethite and kaolin was estimated to be around half of the total sorption amount. Compared to the discrete original ones, the Ex-Po in the sediment with the mineral addition increased while NaOH-Po decreased after sorption, suggesting that the presence of interactions between the minerals and sediment matrix reduced their ability to retain AMP. These findings highlight the significant role of mineral-sediment interactions in altering OP sorption characteristics, with important implications for their biogeochemical fate.