Coupled effects of sedimentary iron oxides and organic matter on geogenic phosphorus mobilization in alluvial-lacustrine aquifers

The organic matter (OM) biodegradation and reductive dissolution of iron oxides have been acknowledged as key fac-tors in the release of geogenic phosphorus (P) to groundwater. However, the coupled effects of natural OM with iron oxides on the mobilization of geogenic P remain unclear. Groundwater with high and low P concentrations has been observed in two boreholes in the alluvial-lacustrine aquifer system of the Central Yangtze River Basin. Sediment sam-ples from these boreholes were examined for their P and Fe species as well as their OM properties. The results show that sediments from borehole S1 with high P levels contain more bioavailable P, particularly iron oxide bound P (Fe-P) and organic P (OP) than those from borehole S2 with low P levels. Regarding borehole S2, Fe-P and OP show positive correlations with total organic carbon as well as amorphous iron oxides (FeOX1), which indicate the presence of Fe-OM-P ternary complexes, further evidenced by FTIR results. In a reducing environment, the protein-like compo-nent (C3) and terrestrial humic-like component (C2) will biodegrade. In the process of C3 biodegradation, FeOX1 will act as electron acceptors and then undergo reductive dissolution. In the process of C2 biodegradation, FeOX1 and crys-talline iron oxides (FeOX2) will act as electron acceptors. FeOX2 will also act as conduits in the microbial utilization path-way. However, the formation of stable P-Fe-OM ternary complexes will inhibit the reductive dissolution of iron oxides and OM biodegradation, thus inhibiting the mobilization of P. This study provides new insights into the enrichment and mobilization of P in alluvial-lacustrine aquifer systems.

Automated summary

The biodegradation of organic matter and the dissolution of iron oxides play important roles in the release of geogenic phosphorus to groundwater. However, the combined effects of natural organic matter and iron oxides on phosphorus mobilization are not well understood. This study investigates sediment samples from boreholes in the Central Yangtze River Basin and finds that sediments with high phosphorus levels contain more bioavailable phosphorus, particularly iron oxide bound phosphorus and organic phosphorus, compared to sediments with low phosphorus levels. The study also reveals the formation of stable phosphorus-iron-organic matter complexes that inhibit the dissolution of iron oxides and the degradation of organic matter, thereby limiting phosphorus mobilization.