A new DGT technique comprised in a hybrid sensor for the simultaneous measurement of ammonium, nitrate, phosphorus and dissolved oxygen

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Highlights

  • A novel DGT can measure NO3-N, NH4-N and PO4-P simultaneously.

  • The new technique has relatively high DGT capacities for three nutrients.

  • Rinsing DGT device at 3-day intervals prevents probe biofouling.

  • Hybrid PO-DGT sensor allows simultaneous measurement of O2 and nutrients.

Abstract

A new diffusive gradients in thin films technique (ZrO-AT DGT) with zirconium oxide, A-62 MP and T-42H resins containing in a single binding gel was developed for simultaneous measurement of nitrate (NO3-N), ammonium (NH4-N) and phosphate (PO4-P). The DGT uptake was found to be independent of pH variation from 3.2–8.7. Ionic strengths below 5, 10 and 750 mmol·L−1 NaCl did not affect DGT uptake of NH4-N, NO3-N and PO4-P, respectively. This new DGT was deployed in natural freshwater environments, with in situ measurements of the three nutrients found to be accurate. It ensured that rinsing the exposed surface of the DGT device at 3-day intervals can prevent biofouling. Additionally, a hybrid sensor comprising the novel DGT binding layer overlying an O2 planar optrode was tested in sediments to evaluate the dynamics of O2 and the three nutrients. Results showed that PO4-P and NO3-N fluxes decreased while fluxes of NH4-N increased under aerobic conditions. Nearly simultaneous variation in O2 and NO3-N was observed at the sediment-water interface (SWI) and transformation of NO3-N and PO4-P was found to be sensitively influenced by O2 dynamics.

Introduction

Eutrophication is widely recognized as one of the most important environmental problems worldwide (Le Moal et al., 2019; Yang et al., 2017). Nitrogen (N) and phosphorus (P) are the major nutrients required for algal growth (Gao et al., 2018; Small et al., 2016; Stiles et al., 2018). Nitrate (NO3-N) and ammonium (NH4-N) are the dominant inorganic N forms present in aquatic systems (Liu et al., 2018). Of all forms of P, only phosphate (PO4-P) can be directly utilized by algae in aquatic environments (Reynolds and Davies, 2001). The combined effects of N and P have accelerated eutrophication globally (Gao et al., 2018) and therefore, simultaneous determination of NO3-N, NH4-N and PO4-P is desirable for improved understanding of the process of eutrophication.

Traditional sampling methods may alter nutrient speciation during sample collection, transportation, storage and treatment. In recent years, in situ passive sampling methods such as the diffusive gradients in thin films technique (DGT) (Davison and Zhang, 1994) have been widely used for the measurement of organic/inorganic elements and compounds in varying environments (Li et al., 2019a). A DGT device mainly includes three parts: a binding gel, a diffusive gel and a filter membrane. Binding gels contain a single binding agent such as PrCH resin (Huang et al., 2016b), CMI-7000 anion exchange membrane (Huang et al., 2016a) and zeolite (Feng et al., 2015) have been reportedly used for the measurement of NH4-N; A520E resin (Huang et al., 2016c) has been used for the measurement of NO3-N. Zirconium oxide (Zr-oxide) has been widely used for P measurements (Ding et al., 2015; Li et al., 2019a). A single gel composed of a mixture of binding agents has been reported for the simultaneous measurement of NO3-N and NH4-N (Huang et al., 2017) or NH4-N and PO4-P (Feng et al., 2018). In addition, a previous study reported the use of two binding gels stacked within a DGT device to simultaneously measure NO3-N, NH4-N and PO4-P (Huang et al., 2017). Nevertheless, overlaying DGT binding gels results in increased diffusive path lengths and prolonged retention of target solutes diffusing towards inner binding gels (Huang et al., 2017). Accordingly, a single gel with standard DGT assembly which can simultaneously measure NO3-N, NH4-N and PO4-P is desirable.

Transformation of nutrients in sediments can be significantly influenced by the trophic status of a waterbody (Ding et al., 2018; Porubsky et al., 2009). It has been previously reported that O2 dynamics control P release from sediments into overlying water (Cesbron et al., 2014), with transformation of N in sediments showing a close correlation with dissolved oxygen (DO), which may change the internal nitrogen load in aquatic ecosystems (Liu et al., 2018). As sediments show high spatial heterogeneity, especially at the sediment-water interface (SWI), high resolution measurements of O2 and nutrients are needed. Recently, the planar optrode (PO) method has been widely applied for measurement of the two-dimensional (2-D) distribution and dynamics of CO2, O2 and pH (Li et al., 2019b). In addition, a sensor combining DGT and PO techniques has been developed for multiple element measurements (Hoefer et al., 2017; Stahl et al., 2012). Nevertheless, to the best of our knowledge, a hybrid sensor for the simultaneous measurement of NO3-N, NH4-N, PO4-P and O2 has not yet been reported.

In this study, a novel DGT technique was developed for the simultaneous measurement of NO3-N, NH4-N and PO4-P using a novel single binding gel containing three different binding agents. Laboratory experiments were conducted to validate the DGT technique and the novel DGT device was deployed in natural freshwater environments to test its feasibility. Finally, a hybrid sensor comprising a DGT binding layer and an O2 planar optrode was tested in sediment environments to evaluate the dynamics of O2 and the three nutrients at a high-resolution.

Section snippets

Materials and reagents

20% (v/v) HCl was used to clean glass containers and quartz 96-well plates. Chemicals were provided by Zhongke Quality Inspection Biotechnology Ltd. (Beijing, China), unless otherwise stated. Working solutions of NO3-N, NH4-N and PO4-P were diluted from standard solutions (1000 mg·L−1 NH4Cl, KNO3 and KH2PO4). Diffusive gel and binding gel were prepared using agarose (Bio-Rad, USA). Platinum (II) 2, 3, 7, 8, 12, 13, 17, 18-octaethyl-21H, 23H-porphyrin (PtOEP) (Frontier Scientific Inc., USA),

Uptake and elution efficiencies of binding gels

Binding gels were immersed in mixed solutions containing three nutrients and then eluted using 1.0 mol·L−1 NaCl and 1.0 mol·L−1 NaOH successively. The uptake efficiency of the binding gels for NO3-N, NH4-N and PO4-P were 91.6 ± 0.6%, 93.0 ± 0.8% and 95.6 ± 0.3%, respectively. The elution efficiency of 1.0 mol·L−1 NaCl for NO3-N and NH4-N were 100.5 ± 1.6% and 84.3 ± 1.5%, respectively. Elution efficiency of 1.0 mol·L−1 NaOH for PO4-P was 95.5 ± 0.7%. The binding and elution efficiencies of the

Conclusions

The established ZrO-AT DGT can simultaneously measure NO3-N, NH4-N and PO4-P concentrations. The ZrO-AT gel adsorption rates for each analyte were rapid enough for DGT measurement. The three nutrients were quantitatively eluted via a two-step procedure. Variation within a pH range of 3.2–8.7 and ionic strengths below 5, 10 and 750 mmol·L−1 NaCl, did not affect DGT device uptake of NH4-N, NO3-N and PO4-P, respectively. Field experiments further confirmed that use the novel DGT for simultaneous

CRediT authorship contribution statement

Mingyi Ren: Data curation, Writing - original draft. Shiming Ding: Conceptualization, Methodology. Dan Shi: Investigation. Zhilin Zhong: Software, Validation. Jingxin Cao: Software, Validation. Liyuan Yang: Writing - review & editing. Daniel C.W. Tsang: Writing - review & editing. Dan Wang: Funding acquisition. Donghua Zhao: Funding acquisition. Yan Wang: Software, Validation.

Declaration of competing interest

No conflicts of interest exists in the submission of this manuscript, the publication of which has been approved by all authors.

Acknowledgments

This research work was financially supported by the National Key Research and Development Plan (2018YFA0903003), National Natural Science Foundation of China (41621002, 41701570, 41877492 and 41701568), Research instrument and equipment, and Development Project of the Chinese Academy of Sciences (YJKYYQ20170016), and “One-Three-Five” Strategic Planning of Nanjing Institute of Geography and Limnology (NIGLAS2017GH05).

References (39)

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