Article Cite This: J. Agric. Food Chem. 2019, 67, 6397−6406
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Development of a Passive Sampling Technique for Measuring Pesticides in Waters and Soils Yanying Li,† Chang-Er L. Chen,†,∧ Wei Chen,†,# Jingwen Chen,‡ Xiyun Cai,‡ Kevin C. Jones,*,† and Hao Zhang*,† †
Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, U.K. Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China
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ABSTRACT: It is essential to monitor pesticides in the environment to help ensure water and soil quality. The diffusive gradients in thin-films (DGT) technique can measure quantitative in situ labile (available) concentrations of chemicals in water, soil, and sediments. This study describes the systematic development of the DGT technique for nine current pesticides, selected to be representative of different classes with a wide range of properties, with two types of resins (HLB (hydrophilic-lipophilicbalanced) and XAD 18) as binding layer materials. The masses of pesticides accumulated by DGT devices were proportional to the deployment time and in inverse proportion to the thickness of the diffusive layer, in line with DGT theoretical predictions. DGT with both resin gels were tested in the laboratory for the effects of typical environmental factors on the DGT measurements. DGT performance was independent of the following: pH in the range of 4.7−8.2; dissolved organic matter concentrations 99% remaining in soils.6 This may cause unintended environmental effects as pesticides can be hazardous to the indigenous microorganisms, including beneficial competitors, predators, and parasites of target pest insects.7 Studies have shown that pesticides inhibit soil microbial diversity and activities,8,9 adversely influence soil biochemical processes, and disturb soil ecosystems.10 In recent decades, there has been increasing concern that pesticides constitute a risk to humans by entering the food chain,11 through direct contact with soil, inhalation of volatile pesticides,12 and through groundwater contamination by pesticides leaching from soils. It is clear that measurements of pesticides in soils are needed to understand their fate and dissipation. These are usually performed using various extraction methods,13 which can be complicated, expensive, laborious, and time-consuming.14 These extraction methods usually focus on the “total © 2019 American Chemical Society
concentration”, although some of them could be related to the bioavailable fraction, which is more relevant in risk assessment.15 However, they cannot provide any kinetic parameters of in situ soil processes of pesticides, such as (i) exchange between soil solution and solid phase and (ii) resupply kinetics in response to biological uptake. Therefore, a technique which considers kinetic aspects and bioavailability would be of great benefit. Pesticides can enter surface waters through diffuse pollution and leaching.16 There are requirements to monitor pesticides to assess water quality. Grab sampling, which is widely used in water monitoring, is an effective way to measure the occurrence of organic contaminants in aquatic systems, but it only provides snapshot information at the time of sample collection; episodic contaminant events may be missed.17,18 The development of passive sampling approaches, which can give time-weighted average (TWA) concentrations, has therefore increased in recent years, as summarized below. Passive samplers are able to retain trace analytes by preconcentration; the in situ sampling does not affect the environment.19 Passive samplers also limit the degradation of trapped chemicals during transport and storage.20 Techniques Received: Revised: Accepted: Published: 6397
January 2, 2019 April 14, 2019 May 8, 2019 May 8, 2019 DOI: 10.1021/acs.jafc.9b00040 J. Agric. Food Chem. 2019, 67, 6397−6406
Article
Journal of Agricultural and Food Chemistry
developed for many important and widely used pesticides, nor has it solved some essential technical issues, notably the choice of filter membrane, diffusive and resin gels. DGT devices in these two recent papers were deployed without a filter membrane because of significant adsorption of the target chemicals on to the filter, which can affect the accuracy of the measurements. The filter has been a key part of the DGT design since its invention,31 its purpose being to protect the hydrogel from direct exposure in waters, soils, and sediments and prevent particulates/microbes from becoming embedded in it. There is an extensive literature from work on inorganics supporting the key role of the filter.27 As DGT is now being extended for trace organics, some compound classes with high partition coefficients are prone to retention/retardation of diffusion on the filter. Challis et al. therefore recently conducted field deployments for trace organics without filter protection; they reported that hydrogel degradation was rare and site specific.46 Other studies have been conducted to quantify, model, and correct for any retardation on the filter and to expand the range of filter types available.44,45,47,48 Systematic tests are therefore needed because DGT needs to be robust and reliable across a wide range of environments (e.g. at the most extreme in warm tropical waters and wastewater treatment plants). The aim of this study was therefore to develop the DGT technique to measure the available concentration of a wide range of pesticides in waters and soils. In evaluating the performance characteristics of the new DGT device, nine pesticides were selected as test chemicals, and two kinds of binding materials were tested. The binding kinetics and capacity of the binding gels were determined, and the effects of deployment time, diffusive gel thickness, pH, ionic strength, and organic matter concentration were studied. A field study deploying DGT in waters and the application of DGT in a defined soil/water ratio were also undertaken to demonstrate the performance and applicability of the technique. The nine target chemicals were selected from various pesticides in use in the UK and China and chosen to cover a range of different classifications (pesticides, insecticides, and fungicides) and different functional groups (detailed properties are listed in Table S1). They represent most of the classes of polar pesticides in use.49 The method was also tested for some of the metabolites of atrazine, to demonstrate its utility for fate studies.
such as POCIS (polar organic chemical integrative sampler), Chemcatcher,21,22 ceramic dosimeters,23 and microporous samplers24 are currently used for the measurement of pesticides in waters. However, they are dependent on hydrodynamic conditions during field deployment and/or rely on a laboratory calibration and losses of performance reference compounds to estimate sampling rates.25 DGT (diffusive gradients in thin-films) is a passive sampling technique which can be used for field deployment without the need for field calibration in a solution of similar matrix.26 It just requires that the diffusion coefficient is known for the compound of interest in the DGT diffusive gel. The thick diffusion gel layer which controls the uptake of analytes into the receiving phase limits the influence of hydrodynamic conditions by minimizing the effect of the diffusive boundary layer (DBL).27 It is also a “dynamic” technique that has been shown to sample the bioavailable fraction of trace metals and nutrients in soils.28−30 The development and use of DGT for inorganics has a long and well-published pedigree. The principles were first published in 1994 in Nature,31 and now over 800 peerreviewed papers have been published on developing, testing and applying the technique in different environmental media, such as waters,32,33 soils,34 and sediments.35 Until recently, the focus has been on metals, nutrients, and radionuclides. DGT typically utilizes a three-layer system: a resin-impregnated hydrogel layer, a hydrogel diffusion-layer, and a filter membrane. Uptake and preconcentration is balanced against exposure time to yield sufficient time-intergated mass of analytes(s) for detection.31 The principle of DGT is based on Fick’s first law,31 such that the DGT measured-concentration (CDGT) of target chemicals in solution can be calculated using eq 1: C DGT =
M(Δg + δ) DeAt
(1)
where M is the mass of analyte accumulated in the binding gel, t is the exposure time, De is the diffusion coefficient of the analyte in the diffusive layer, A represents the sampling area of DGT, Δg is the diffusion length through which the analyte passes before being taken up by the binding phase, and δ is the thickness of the DBL. The first application of DGT to organic compounds was published in 2012 by Chen et al.26 They investigated the performance characteristics of DGT for quantifying polar organic compounds (with logKow value 80% of Maximum Water Holding Capacity) before deployment. The details of soil properties, soil collection and treatments, and DGT deployment in soils are listed in SI and Table S5. Quality Assurance/Control (QA/QC). All DGT deployments in the laboratory and field were carried out in triplicate, and the results are expressed as the average ± standard deviation (SD). Three DGT devices were retieved prior to each deployment as blank samples. Control samples (test solution without DGT devices) were performed in each experiment to prevent possible interference during the experiment. All the SPE samples were replicated, no target compounds were found in the blank SPE samples.
RESULTS AND DISCUSSION Sorption by DGT Holder, Filter Membrane, and Diffusive Gels. There was no appreciable sorption of target compounds on the two types of diffusive gels or DGT moldings as shown in Figure S1a. However, compounds were sorbed substantially by PES, NL, OE, and ME filter membranes (Figure S1b). Sorption to the PES filters was marked (>50%); this filter type has been used for POCIS19 and Chemcatcher.22 Loss on the ME filter was also considerable. The PES filters were also used in DGT devices for other medium polar chemicals in other studies, and the adsorption effect was negligible. PC and GHP showed little sorption of the compounds; PC membrane performed the best, with
