The impact of organic matter and iron/calcium coupling on phosphorus retention in the hyporheic zone of the Danjiangkou area tributary: Evidence from bonding recognition.

Abstract

The coupling between organic matter (OM) and minerals considerably influences the phosphorus (P) cycle within the hyporheic zone, but the role of different geological mineral-organic complexes (MOCs) on P burial during hyporheic exchange remains under-explored. This study investigates the effects of OM and iron (Fe)/calcium (Ca) coupling on P migration within the hyporheic zone of an agricultural tributary to the Danjiangkou Reservoir. These relationships were explored by measuring hyporheic flow (q), organic and inorganic P forms, and sediment PO₄-P adsorption capacity [following treatment with fulvic acid (FA), Fe-OM, or Ca-OM]. Multivariate statistical analysis, X-Ray Diffraction, Fourier-transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy were employed to elucidate the underlying mechanisms. Results indicate that upward hyporheic flow transports dissolved porewater P into surface water, contributing 11.27-12.13 % of the total P flux. MOCs associated with Fe(III)/Ca silicate minerals, along with FA and labile OM, were identified as key OM fractions influencing P migration, contributing 5-24 %, 10-11.7 %, and 6-14.9 % to the overall flux, respectively. FA and labile OM facilitate P release, whereas MOCs enhance P retention. Ca-OM is the most efficient PO₄-P adsorption [adsorption capacity (AC): 0.8980-0.9524 mg/g], followed by Fe-OM (AC: 0.5120-0.7020 mg/g), original sediment (AC: 0.4368-0.5596 mg/g), and FA (AC: 0.2657-0.2769 mg/g). Cation bridges, primarily formed by -OH and -NH₂ groups within Ca-OM (outer-sphere complexes), promote greater P adsorption than Fe-OM (inner-sphere complexes, mainly associated with -COOH). However, Fe-OM-P exhibits a more stable structure. In high P environments, P adsorption onto Ca-OM may induce the release of labile OM, temporarily retaining P through resorption onto labile OM. Hyporheic flow with higher pH and Eh values promotes MOC formation, underscoring their significant P retention capacity. Therefore, strategic MOC use within the hyporheic zone is crucial for mitigating surface water eutrophication.