A new DGT technique comprised in a hybrid sensor for the simultaneous measurement of ammonium, nitrate, phosphorus and dissolved oxygen
Graphical abstract
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).
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2022, TalantaCitation Excerpt :The analyte mass accumulated on the binding gel is then eluted and analysed by instrumental technique [9,10]. In addition to the original purpose of the DGT technique as a tool for measuring trace metals in the aquatic environment [6,10–12], it is also used for the determination of nutrients [13–16], radioactive elements [17–22], platinum group elements [23–25], rare-earth elements [26,27], and various organic compounds (such as phenolic compounds [28], antibiotics [29], or pesticides [30] as summarized in work of Guibal et al. [31] and references cited therein) in water, sediment or soil, as well as other matrices such as food [32,33]. The advantages of using the DGT technique are: the pre-concentration of analytes that allows for measurements at low environmental concentrations, the ability to obtain time-integrated concentrations of analytes, and the possibility of simultaneous deployments of DGTs in the water and sediment in the studied area, thus obtaining more complex information about the geochemistry of the investigated analyte [34].