Fast and Inexpensive Detection of Total and Extractable Element Concentrations in Aquatic Sediments Using Near-Infrared Reflectance Spectroscopy (NIRS)

Till Kleinebecker,Alfons J P Smolders,Moni D M Poelen,Norbert Hölzel,Leon P M Lamers

doi:10.1371/journal.pone.0070517

Till Kleinebecker, Alfons J P Smolders + Show 3 more

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https://doi.org/10.1371/journal.pone.0070517

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Abstract

Adequate biogeochemical characterization and monitoring of aquatic ecosystems, both for scientific purposes and for water management, pose high demands on spatial and temporal replication of chemical analyses. Near-infrared reflectance spectroscopy (NIRS) may offer a rapid, low-cost and reproducible alternative to standard analytical sample processing (digestion or extraction) and measuring techniques used for the chemical characterization of aquatic sediments. We analyzed a total of 191 sediment samples for total and NaCl-extractable concentrations of Al, Ca, Fe, K, Mg, Mn, N, Na, P, S, Si, and Zn as well as oxalate- extractable concentrations of Al, Fe, Mn and P. Based on the NIR spectral data and the reference values, calibration models for the prediction of element concentrations in unknown samples were developed and tested with an external validation procedure. Except Mn, all prediction models of total element concentrations were found to be acceptable to excellent (ratio of performance deviation: RPD 1.8–3.1). For extractable element fractions, viable model precision could be achieved for NaCl-extractable Ca, K, Mg, NH4 +-N, S and Si (RPD 1.7–2.2) and oxalate-extractable Al, Fe and P (RPD 1.9–2.3). For those elements that showed maximum total values below 3 g kg−1 prediction models were found to become increasingly critical (RPD <2.0). Low concentrations also limited the performance of NIRS calibrations for extracted elements, with critical concentration thresholds <0.1 g kg−1 and 3.3 g kg−1 for NaCl and oxalate extractions, respectively. Thus, reliable NIRS measurements of trace metals are restricted to sediments with high metal content. Nevertheless, we demonstrated the suitability of NIRS measurements to determine a large array of chemical properties of aquatic sediments. The results indicate great potential of this fast technique as an analytical tool to better understand the large spatial and temporal variation of sediment characteristics in an economically viable way.

Highlights

Soils and sediments of natural systems show high spatial and temporal variation in physical and chemical characteristics, and in fluxes of elements among this compartment, the hydrosphere and the atmosphere [1,2]
The spectral bands found in this study are consistent with data reported in literature, where absorbance around 1400 and 1900 nm was attributed to O–H bonds and absorbance around 2200 nm was found to be related to absorption by C–H bonds [19,38]
As organic matter and soil minerals have multiple absorption bands .2100 nm, a definite assignment of the two small peaks around 2300 and 2350 nm to specific functional groups is not possible [28,39]. Both samples with high and with low organic matter content showed the same runs of the curves, but samples with high organic matter content had clearly higher NIR-optical density. This was in accordance with the study of Chang and Laird [28], who found weak reflectance of dark colored humic acids in the visible and NIR wavelength range between 400 and 2500 nm

Summary

Introduction

Soils and sediments of natural systems show high spatial and temporal variation in physical and chemical characteristics, and in fluxes of elements among this compartment, the hydrosphere and the atmosphere [1,2]. In aquatic ecosystems and wetlands, information on the distribution and retention of elements in the sediments is of crucial importance because of the strong interactions between the sediment and the surface water with respect to nutrients and contaminants [6,7,8]. One of the biogeochemical key processes causing massive eutrophication is sulfate-mediated phosphate mobilization in the sediment [10,12] In this context, knowledge about sediment phosphorus, sulfur and iron contents including differently extracted fractions add valuable information for process understanding and the selection of appropriate measures to counteract massive sulfate-mediated phosphate release. The element composition of sediments provides important information about their biogeochemical role in aquatic systems

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