Speciation of Phosphorus from Suspended Sediment Studied by Bulk and Micro-XANES

• Main Authors: Qingxin Zhang, Mackenzie Wieler, David O’Connell, Laurence Gill, Qunfeng Xiao, Yongfeng Hu
• Format: Article
• Language: English
• Published: MDPI AG 2020-08-01
• Series: Soil Systems
• Subjects: phosphorus; sediment; chemical P extraction; microanalysis; X-ray absorption near-edge structure (XANES) spectroscopy
• Online Access: https://www.mdpi.com/2571-8789/4/3/51

Mobilization, transformation, and bioavailability of fluvial suspended sediment-associated particulate phosphorus (PP) plays a key role in governing the surface water quality of agricultural catchment streams. Knowledge on the molecular P speciation of suspended sediment is valuable in understanding in-stream PP cycling processes. Such information enables the design of appropriate catchment management strategies in order to protect surface water quality and mitigate eutrophication. In this study, we investigated P speciation associated with fluvial suspended sediments from two geologically contrasting agricultural catchments. Sequential chemical P extractions revealed the operationally defined P fractions for the fluvial suspended sediments, with Tintern Abbey (TA) dominated by redox-sensitive P (P_CBD), Al, and Fe oxyhydroxides P (P_NaOH) and organic P (P_Org) while Ballyboughal (BB) primarily composed of acid soluble P (P_Detr), redox-sensitive P (P_CBD), and loosely sorbed P (P_NH4Cl). The dominant calcareous (Ca) elemental characteristic of BB suspended sediment with some concurrent iron (Fe) influences was confirmed by XRF which is consistent with the catchment soil types. Ca-P sedimentary compounds were not detected using bulk P K-edge XANES, and only P K-edge µ-XANES could confirm their presence in BB sediment. Bulk P K-edge XANES is only capable of probing the average speciation and unable to resolve Ca-P as BB spectra is dominated by organic P, which may suggest the underestimation of this P fraction by sequential chemical P extractions. Notably, µ-XANES of Ca K-edge showed consistent results with P K-edge and soil geochemical characteristics of both catchments where Ca-P bonds were detected, together with calcite in BB, while in TA, Ca-P bonds were detected but mostly as organic complexed Ca. For the TA site, Fe-P is detected using bulk P K-edge, which corresponds with its soil geochemical characteristics and sequential chemical P extraction data. Overall, P concentrations were generally lower in TA, which led to difficulties in Fe-P compound detection using µ-XANES of TA. Overall, our study showed that coupling sequential chemical P extractions with progressively more advanced spectroscopic techniques provided more detailed information on P speciation, which can play a role in mobilization, transformation, and bioavailability of fluvial sediment-associated P.