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Review

Bioavailability Assessment of Heavy Metals and Organic Pollutants in Water and Soil Using DGT: A Review

by
Qing Zhu
1,2,
Jing Ji
1,2,
Xuejiao Tang
1,2,
Cuiping Wang
1,2,* and
Hongwen Sun
1,2
1
Key Laboratory of Pollution Processes and Environmental Criteria, Ministry of Education, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China
2
Tianjin Engineering Center of Environmental Diagnosis and Contamination Remediation, Tianjin 300071, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(17), 9760; https://doi.org/10.3390/app13179760
Submission received: 19 June 2023 / Revised: 23 August 2023 / Accepted: 26 August 2023 / Published: 29 August 2023
(This article belongs to the Special Issue Current Status of Agricultural Soil Pollution)
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Abstract

:
In recent years, the diffusive gradients in the thin films (DGT) technique has also been increasingly applied to assess the bioavailability of heavy metals and organic pollutants in the soil. The combination of binding and diffusion phases made from different materials allows for the targeted determination of different target substances. This review briefly introduces the compositions and development of the DGT technique and analyzes the composition structure of DGT and the impact of environmental factors, such as pH, ion strength (IS), and dissolved organic matter (DOM), on the bioavailability evaluation of heavy metals and organic pollutants in soil. Finally, the application potential and broad application prospects of the DGT technique were expected. In addition, standardized DGT technique methods and calibration procedures are conducive to the establishment of a more stable and reliable measurement system to enhance the robustness of the DGT technique application in the soil.

1. Introduction

Soil is the most active reservoir, not only the environmental elements that provide human resources, but also many contaminants, including heavy metals and organic pollutants in the environment. Once the pollutants enter the soil, it is difficult to diffuse and remove them because they were easily adsorbed by soil organic matter (SOM) and minerals [1,2]. The heavy metals and organic pollutants will harm microorganism community structures, soil environmental quality, and soil ecological balance, causing crop growth and plant physiological function disorder and nutritional disorders, finally endangering human beings’ health through the food chain [3,4]. Hence, to decrease the toxicity of the pollutants to the soil environment or microorganisms/plants, different technologies were used to remediate the soil. However, most of the persistent organic pollutants have the characteristics of stable chemical properties and high residues and hydrophobicity, and are not easy to be completely degraded, whereas heavy metals cannot be degraded, and finally always remained in the soil. Thus, to reduce the environment risks of heavy metals, some technologies were applied to decrease the available heavy metals through altering the heavy metal speciation. A large number of studies have shown that only considering the total amount of pollutants in the soil tends to overestimate the toxicity of pollutants to living beings [5,6]. Therefore, it is important for pollutant control, especially, the bioavailability of pollutants was accurately estimated, if necessary, it was worth considering the impact of soil properties on the assessment of pollutant bioavailability, or, resulting in over-protection of soils [7,8,9].
Bioavailability refers to the assimilation and potential toxicity of chemical substances to organisms [5]. The capabilities that pollutants have to enter organisms is closely related to the types of organisms, exposure route, exposure time, and soil properties [10]. Bioavailability is a key parameter to measure the mobility and ecological impact of metal elements, and an important indicator for assessing environmental risks. Generally, methods of assessing bioavailability are classified into two main categories, including biological evaluation methods and abiotic prediction techniques. The main biological evaluation methods commonly used are the biological index method, the plant detection method, the microbial indicator method, and the soil indicator animal monitoring method [11,12]. The biological evaluation method is an economical, simple, and reliable method to determine the biological effectiveness of heavy metals in contaminated soil, but there are some disadvantages of long test cycles, susceptible to a variety of environmental factors and not easy to control, leading to its application being somewhat limited. The abiotic prediction techniques are more widely used and include total prediction, chemical extraction, soil pore water, free ion activity, etc. [13,14,15]. However, traditionally, physical–chemical methods often do not consider the migration process of heavy metals between solid and liquid two phases in the soil, and cannot accurately and effectively predict the toxicity, bioavailability, existence state, and migration ability of pollutants. The diffusive gradients in thin films (DGT) technique [16], as a new type of in situ passive sampling technique, can realize the in situ measurement of trace elements in the environment through the gradient diffusion of the target in the diffusion layer and its buffering kinetics [13] by the principle of free diffusion (Fick’s first diffusion law). The DGT technique, as an important method to study the bioavailable content of heavy metals in the soil, was first proposed by Davison and Zhang [16].
It utilizes the property that heavy metal ions can penetrate into the film/hydrogel, and controls the exchange process of heavy metal ions through separating the ion exchange resin from the solution. The DGT device is mainly composed of a fixed layer (fixed membrane) and a diffusion layer (diffusion membrane and filter membrane). The target ions pass through the diffusion layer and are then captured by the fixed layer. At the same time, a linear gradient distribution of target ions is formed in the diffusion layer (Figure 1).
The diffusive flux of the DGT device to the target (FDGT) can be calculated using the following formula:
F DGT = M At
F DGT = D C DGT ∆ g
where t is the placement time of the DGT device; M the accumulation of target ions by the fixed membrane during the placement time of the DGT device; A the exposed window area of the DGT device; Δg the thickness of the diffusion layer, cm, and the diffusion coefficient of the target ion in the diffusion layer, D. CDGT represents the concentration of the linear gradient of the diffusion layer near the end of the ambient medium.
From the above formula, the calculation formula of CDGT can be obtained as
C DGT = M ∆ g DAt
Solvent extraction was generally used to determine the mass (M) of target ions accumulated in the fixed membrane.
M = C e ( V e + V g ) f e
where Ce represents the concentration of the extract; Ve is the volume of the extractant; Vg is the volume of the fixed membrane; and the extraction rate of the target ions on the fixed membrane by the extractant, fe.
In the early days of DGT devices, a solid binding phase was used (as shown in Figure 2a), and the device was mainly composed of a plastic shell, a cellulose film, and a diffusion layer. The main function of the cellulose film was to limit the size of the particles passing through, while protecting the internal diffusion layer and binding phase from contamination. The function of the diffusion layer was to limit the passing speed of the particles, which was proportional to the time, the external solution, and the internal ion concentration. The main role of the binding phase is to combine the diffused ions, so that the concentration difference between the diffusion phase and the binding phase is reduced to a minimum.
With the development of DGT technology, DGT devices with a liquid binding phase had appeared, which solved the disadvantages of the poor mechanical strength and elution efficiency of solid binding phase DGT devices [17]. The DGT device of the liquid combined phase innovatively used the dialysis membrane as the diffusion layer, which was the biggest difference from the solid DGT device. Moreover, the basic part of the liquid binding phase DGT device, which played the role of holding the binding phase and fixing the diffusion layer, was made of polyethylene. There was a 2 cm diameter front opening window in front of the device, which was the only channel for the exchange of internal and external substances. A DGT device using a liquid-bound phase is shown in Figure 2b. Although the liquid binding phase had the advantages of simple operation, easy handling, no elution, and high accuracy, which had been further studied in recent years, it was still difficult to prepare compared to the solid binding phase. Therefore, most of the current research was focused on the solid binding phase.
The DGT technique had achieved a rapid development since it is widely used in evaluating metal bioavailability in natural water bodies, sediment and soil, etc. [18,19]. With the continuous development of the DGT technique, its measurable metals gradually covered various types, such as conventional metal cations, precious metals, oxidizing anions, radioactive elements, and rare earth elements [20,21,22,23,24]. The DGT technique was commonly used to predict the concentration, species, migration, and potential environmental risks of metals in the environmental media [25,26,27,28]. Especially when predicting the bioavailability of metals in soil, the DGT technique provided accurate and reliable results [29,30,31]. Moreover, the combination of the DGT technique and high-resolution 2D imaging, combined with dialysis samplers, fluorescence spectroscopy, computer-imaging densitometry, etc., has deepened the understanding of metal behaviors in environmental media [32,33,34].

2. Bioavailability Assessment of Heavy Metals in the Soil Using DGT

Inorganic pollutants (especially trace metals and quasi-metals) enter organisms through their environmental behaviors, and if they are enriched in large quantities, they will cause damage to organisms, toxic effects, and even threaten life and health. Heavy metals can enter the human body, and heavy metals proteins react with proteins, ribose, vitamins, hormones, and other substances in the body to form metal complexes or chelates, causing their original biochemical functions’ disorder, which damages the kidney, liver, bone, immune system, reproductive system, and nervous system, even in pathological changes, and even death [35,36]. Therefore, measures must be taken to reduce the exposure dose of heavy metals in the population to reduce their health risks to humans.
The stress of most metals in the soil can reduce soil pH, bulk density, and cation exchange capacity (CEC), and also cause the lack of available potassium, alkali-hydrolyzed nitrogen, phosphorus, and other nutrients and their effectiveness [37]. The soil ability to provide nutrients for crops is weakened [38]. At the same time, the increase in the degree of metal pollution will destroy the active groups and spatial structure of enzymes, reduce the biosynthesis of enzymes, cause the reduction of enzyme activities [39], and inhibit the growth of microorganisms, resulting in a significant reduction in the number of soil microorganism populations and community diversity. Heavy metal intrusion made gradually decreased soil ammoniation, nitrification, and self-respiration inhibited.
For different types of heavy metals, due to the differences in their physicochemical behavior and bioavailability, the migration and transformation rules in soil were obviously different. The complex pollution formed by heavy metals made the migration and transformation of heavy metals in the soil more complicated. Moreover, due to differences in experimental conditions and heavy metal types, different scholars often drew different conclusions, causing differences in the bioavailability assessment of metals. Hence, it was necessary to explore a stable, accurate, and efficient DGT technique for the determination of bioavailability.
In 1998, DGT was used for the first time to measure the bioavailability of heavy metals in soil, then extending DGT technology to the field of soil [40]. During deployment of the DGT device, the average flux of available species in the soil transferred to the device can be directly measured, enabling the characterization of bioavailable metal concentrations in the soil. Therefore, in the early stage of the DGT technique, it was mainly used to evaluate the metal concentration in soil, and to investigate the correlation between it and the metal concentration content measured by other chemical analysis methods, such as CaCl2 aqueous solution, soil pore water, BCR methods, etc. [40,41,42,43,44].
Changes in soil metal concentrations measured by DGT over time scales also reflected the desorption of metal elements from the soil solid-phase composition and release to the liquid-phase composition, as well as changes in soil metal species. This promoted the research on the migration and transformation of metal elements in soil. A study using DGT found that Cadmium (Cd), Copper (Cu), and Zinc (Zn) in soil under plastic sheds had relatively high bioavailability, and Cd exhibited the highest migration risk due to its largest available pool and stronger desorption capacity, which better reflected the migration of metals from soil to vegetables [45]. However, some studies had shown that although DGT was sensitive to changes in soil properties and the rhizosphere, and could better show the increase in metal mobility and availability, it could not successfully simulate the uptake of Cu by the aerial parts of tolerant plants [46]. In Cu-contaminated soils, the resupply of Cu from the soil solid phase to the solution was extremely slow, and the kinetics of Cu adsorption was also slow.
Under different sampling conditions, DGT with different binding phases can determine various elements from the diverse environments (Table 1). In order to accurately evaluate the bioavailability of cationic heavy metals in soil, DGT with Chelex-100 resin as the binding phase and polyacrylamide (PAM) gel as the diffusion phase was developed, which was also the most commonly used DGT [47,48,49,50,51,52]. With the continuous application and optimization of DGT technology, agar gels, dialysis membranes, and chromatographic paper appear as diffusion phases, and good measurement results had been obtained in practical applications. The molecular structure of the binding phase of DGT technology contains some functional groups (such as hydroxyl, amino, and carboxyl) that can provide coordination electron pairs for coordination reaction with the substance to be tested. DGT was widely used in the determination of available heavy metal content in heavy metal-contaminated soil, urban soil, and agricultural soil [44,48,49,51]. The environmental risk of metalloid arsenic (As) in soil, which mainly exists in the form of anions, cannot be ignored. Therefore, DGT of ferrihydrite, Fe-oxide, Zr-oxide, and other binding phases was developed, and the bioavailability of As in soil was successfully measured [42,53,54,55]. Phytoremediation is one of the commonly used remediation technologies for soils contaminated with heavy metal, and the typical plant is hyperaccumulators. In the soil remediation, DGT could be used as a monitoring tool to evaluate plant extraction efficiency and soil heavy metal bioavailability [54,56,57,58]. Similarly, DGT could also be used to infer the migration process of heavy metals between crops and soil and evaluate the bioavailability of heavy metals in soil [42,53,55,59,60,61,62,63,64,65,66]. In order to improve the bioavailability of various heavy metals in soil determined by DGT, different types of DGT, such as diethylenetriaminepentaacetic acid-modified (DTPA-modified) layered double hydroxides (LDHs) nanomaterials [59], Mg-Al-DTPA-LDHs [61], Zirconium hydroxide and iminodiacetate [63], and Titanium dioxide (TiO2) [66], have sprung up. Furthermore, DGT also successfully predicted mercury (Hg) bioavailability in earthworms in different soil types [67,68].

3. DGT Assessment Influenced by Environmental Factors

There are many factors that affect the migration and transformation of metals in the soil, mainly including the physical and chemical properties of the soil, the type and concentration of heavy metals and their existing forms in the soil, and complexed pollution forms. The physical and chemical properties of soil mainly affect the bioavailability of heavy metals by altering the existing forms of heavy metals in the soil. The physical and chemical properties of soil mainly include pH, soil texture, soil redox potential (Eh), SOM content, CEC, etc.
The pH of the soil solution not only determines the solubility of some minerals in the soil, but also affects the degree of sorption of various heavy metal ions in the soil solution on the solid phase [69]. pH is generally the main determining factor for the concentration of soluble or labile trace element fractions. The application of organic amendments to acid soil increases pH values and reduces soluble and exchangeable Al concentrations. Chelex-100 binds metals less effectively at low pH due to the presence of hydrogen ions in solution, which compete with metal ions [41]. Cd and Ni available content in soil correlated closely with pH, SOM, and the total heavy metals content. With the increase in pH, there was an increase in the quantity of negative charges on the surface of the clay minerals, hydrous oxides, and organic matter. Therefore, the sorption capacities of Cd and Ni were enhanced. The presence of a high pH is also beneficial for increasing the metal hydroxyl compound, decreasing the average charge of ions, and decreasing the energy barrier of the sorption reaction. These were beneficial to the specific sorption of heavy metals in soil. The increase of OH− can weaken the competition of H+ on the exchange sites, improve the stability of SOM and metal complexes, and reduce the available content of heavy metals [49]. The soil total metal concentration and pHs were both important factors in determining the biological absorption of Cd, and the bioavailability of Cd decreased linearly from pH 5.5 to pH 7.5, but was not affected by pH at pH < 5.5 or pH > 7.5. Based on the segmented evaluation of Cdtotal and pH value, it was found that the bioavailability of Cd was different. In the case of Cdtotal < 0.30 mg·kg−1, only CaCl2 and DGT-extractable Cd were correlated well with Cdshoot [56]. As calcium carbonate content increases in higher-pH soils, weak Hg(II)-carbonate complexation may replace strong Hg(II)-reduced sulfur complexation on soils, resulting in an increased Hg uptake rate constant for DGT and earthworm. Furthermore, DGT and earthworm surfaces are more likely to have hydroxyl groups at higher pH, thus increasing the affinity for Hg(II) [68].
Studies had shown that higher pH reduced Cd concentration, while higher soil Eh increased Cd concentration in brown rice [60]. There was a significant correlation between Cd levels in cereals and soil Eh. The Eh of soil changed the chemical behavior, migration, and bioavailability of heavy metals in the soil through changing the existing forms of heavy metals. In general, when the soil was under the reducing conditions, many heavy metals tended to form insoluble sulfides. Under the oxidizing conditions, the content of dissolved and exchanged forms increased. CEC had a negative effect on the dissociation of heavy metals in soil. CEC reflects the negative charge of soil colloids. The higher the soil CEC, the higher the negative charge, and the more Cd ions adsorbed on the surface of soil particles by electrostatic adsorption [60].
To combine the calculation of the dissociation rate constant (K−1) of metal–organic complexes (MLs) and the concentration of free ionic metals (CM) in soil, Liu et al. [49] developed a new theoretical model. The fitting results showed that CEC was the main soil environmental factor affecting the K−1 value of Cd, and SOM was the main soil environmental factor affecting the K−1 value of Ni. The correlation analysis showed the negative correlation of the SOM and the K−1 value of Ni. The functional groups of SOM, such as hydroxyl groups, phenols, amino groups, carbonyl groups, and hydrogen sulfide groups, as the Lewis hard bases can react easily with Lewis hard acid in the metal bonding reaction. Compare to the soft acid Cd, the Ni ion as the Lewis boundary acid is more able to bind to the organic group [70]. Soil DOM, as an important component of the soil solid phase, is a highly reactive and unstable component that can form strong complexes with heavy metals and change the mobility and bioavailability of heavy metals in soil [60,71]. Luo et al. [60] found that soil DOM reduced the availability of Cd to rice grains. Spark et al. [72] found that the sorption of heavy metals in soil would change when humic acid was added, and factors such as the solubility of heavy metal–humic acid complexes and the sorption of humic acid on the solid phase would affect the change in sorption. The instability of metal–DOM complexes was tested using the adapted DGT method [73]. The emerging metal–DOM complexes were more unstable than those in peat, and the aromaticity of DOM appears to be an important factor in determining the instability of metal complexes [51,73]. Moreover, a study using DGT also confirmed that the low-molecular-weight DOM-Se fraction in the effective fraction can reflect the bioavailability of selenium in the plant–soil system [74].
In addition, iron minerals that are ubiquitous in soil will also complex with heavy metal ions to form relatively stable complexes, thereby reducing the biological availability of heavy metals. For example, a higher degree of stable binding between Sb(V) and organic matter and Fe/Al minerals is expected to reduce the ecotoxicity, bioavailability, and health risks of Sb(V) [75]. However, a study had shown that As and Sb exhibited contrasting results in soils and white icicle radish (R. sativus) [66]. Sb was most likely predominantly associated with sulfide minerals, silicates, and organic matter, while As was mainly associated with Fe oxides. Fe and Al oxides play an important role in As retention in contaminated soil due to the formation of inner spherical, monodentate and mononuclear, and bidentate binuclear complexes with soil Fe oxides. Numerous studies have shown that the addition of soil amendments can effectively reduce the bioavailability of heavy metals, which is closely related to the characteristics of soil DOM and Fe plaque [65,76,77]. Fe plaque usually exists on the surface of plant roots, mainly iron oxides or hydroxides formed by O2 secreted by roots, which chelate heavy metals through co-precipitation and adsorption, restricting the migration of heavy metals from soil to plants, resulting in the reduced bioavailability of heavy metals [65,76]. Sun et al. [48] assessed the human health risks caused by lead in urban park soils by using in vitro methods, such as simulated human gastrointestinal tract extraction and DGT, and found that Pb bioaccessibility in the gastric phase decreased with the increasing Fe, Mg, and clay content in park soils. In fact, the DGT and DIFS model can be used to assess the risk of heavy metals release from soils. While Liu et al. [47] analyzed the effect of soil chemistry on the correlation between Pb availability and DGT-Pb bioaccessibility, they found that total Fe, total Mn, and total organic carbon (TOC) had no effects on the correlation coefficients. Meanwhile, DGT measurement was less affected by different soil properties than UBM (Unified bioaccessibility research group of Europe (BARGE) Method). Compared to traditional chemical extraction methods, the DGT technique can not only simulate the root environment of plants and measure the available concentration of heavy metals in soil [78], but also shows a high correlation with the concentration of heavy metals in earthworms [68,79], which is an effective method for predicting and evaluating the bioavailability of heavy metals in different types of soil.

4. Bioavailability Assessment of Organic Pollutants Using DGT

Organic pollutants with different molecular weights were a type of different chain and ring, saturated and unsaturated keys, or many functional groups with specific location and reaction characteristics and unique space structure, which determined their different biological effectiveness, including water solubility, lipotropy, dissociation constant, molecular weight, space configuration, etc.

4.1. Determination of Hydrophobic Organic Matter Using DGT

A large number of studies have shown that the bioavailability of organic pollutants is related to their hydrophobicity. Wan et al. [80] found that the higher the hydrophobicity of the organophosphate, the easier it is to be absorbed by binding to the important specific ester transferase in wheat roots. But, it is not that the greater the hydrophobicity, the greater the bioavailability. The results reported by Yu et al. [81] indicated that the bioavailability of polybrominated diphenyl ethers (PBDEs) in mice decreased with increasing LogKow, which was related to the uptake and diffusion capacity of PBDEs with different degrees of bromine substitution across the cell membrane. As the main components of biofilms were non-polar lipids, it was often easier to adsorb lipophilic compounds. Furthermore, increasing the hydrophobicity of organic contaminants also increased the sorption capacity of soil organic matter for contaminants and reduced the bioavailability of contaminants [82]. The degree of accumulation of different pollutants in the organisms was different. Navarro et al. [83] found that the bioaccumulation factors (BAFs) value of halogenated flame retardants (HFRs) in earthworms was lower than that of perfluorinated alkyl substances (PFAS), and pointed out that this may be due to the higher protein affinity and lower lipophilicity of PFAS, which were more likely to accumulate in earthworms with higher protein content. Dissociable organic pollutants were often more readily uptaken by the organisms. Under certain pH conditions, the pKa value of organic pollutants determined their molecular states, which further influenced the degree of dissociation. The cell membrane of living things was usually negatively charged, and the cationic forms of pollutants were more easily adsorbed by plant roots, while anionic pollutants were repelled by the cell membrane; thus, it was difficult for anionic pollutants to pass through the cell membrane [84].
The molecular weight, structure, and spatial configuration of organic matter of organic pollutants were also important factors affecting their bioavailability. Polychlorinated biphenyls (PCBs) were composed of hydrogen atoms on biphenyls substituted by different numbers of chlorine atoms. The molecular volume and molecular polarizability affected their enrichment in earthworms. The process of PCBs with large molecular volume across biofilms might be limited, resulting in a longer time to reach equilibrium in vitro and in vivo. Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbons formed by condensing two or more benzene rings with dense π electrons. Quantitative structure–biodegradability relationship (QSBR) models of PAHs had shown that the biodegradability of PAHs was mainly influenced by spatial structure parameters [85]. The more complex the spatial structure, the more difficult it was for the PAHs molecule to react with the enzyme active sites of the microorganisms.
DGT suitable for studying the transport and transformation patterns, toxicity, and potential bioavailability of organic contaminants in the environment is often referred to as an o-DGT. In 2012, the first o-DGT was designed and applied to the tracking of organic pollutants for the first time [86]. Like heavy metals, the application of o-DGT also starts from water bodies. Compared with the soil environment, the water environment is relatively simple, which is conducive to the exploration of the effects of pH, ionic strength (IS), DOM, and other environmental conditions in the early stage of o-DGT. Fernández-Gómez et al. measured o-DGT (A-DGT and P-DGT) prepared by two diffusion membranes of agarose (0.76 mm) and PAM (0.40 mm) by 3-mercaptopropyl-functionalized silica gel embedded in a PAM gel as the binding phase [87]. The instability of methylmercury in fresh water provides a basis for assessing the bioavailability of organometallic compounds and understanding their potential risks and toxicity. The diffusion coefficient of MeHg+ in the agarose diffusion layer is higher than that of polyacrylamide, but the absorption capacity of P-DGT is slightly higher, which is a better choice for monitoring extremely low concentrations of MeHg+ in eutrophic waters. The ocean is an important sink of major pollutants, and a variety of antibiotics have also been detected in China’s coastal waters [88]. Xie et al. used o-DGT with XDA-1 resin as the binding phase to determine the antibiotics in seawater. The results showed it has a high adsorption capacity for antibiotics and can adapt to fluctuating seawater conditions (pH 7.9–8.2, IS 0.7 M) [89]. The hydrophilic–lipophilic-balanced (HLB) resin is another of the most commonly used binding phase materials. In 2017, Challis et al. [90] systematically evaluated the frozen storage stability of o-DGT prepared by HLB resin for 30 pesticides and insecticides, indicating that the stability of analytes on o-DGT is feasible for at least 1 year. If it cannot be frozen immediately, it is a better choice to temporarily refrigerate the sampler or the binding phase gel in a suitable storage solvent at 4 °C, and it should be determined as soon as possible, if possible. If it cannot be frozen immediately, it is a better choice to temporarily refrigerate the sampler or the binding phase gel in a suitable storage solvent at 4 °C, and it should be determined as soon as possible, if possible [91]. The o-DGT technique has been increasingly used to provide time-weighted average concentrations of biologically relevant fractions of organic pollutants in water, with a view to obtaining high spatial and temporal resolution. Targeted methods have been developed for a wide range of compounds, including pharmaceuticals, chemicals in the household and personal care products (HPCPs), pesticides, hormones, endocrine disrupting chemicals (EDCs), organophosphorus flame retardants, bisphenol, nitrophenol, perfluoroalkyl substances, and illicit drugs [92,93,94,95,96,97,98]. Except for the above binding phase materials, nano-sized zero-valent iron-assisted biochar (nZVI@biochar) binding gel exhibits high capacity and fast absorption of phenol [99]; hyper-cross-linked β-cyclodextrin polymers can stably and effectively monitor phthalate esters (PAEs) concentration [100]; and even activated carbon [101], TiO2 [102], and commercialized solid-phase extraction (SPE) packings [103] can be used as adsorbents in bound-phase gels. In addition, although biofouling adhered to the surface of the DGT sampler, the filter membrane played a strong protective role in the diffusion and binding membranes, which was also confirmed in the wastewater matrix [91].
Soil is an important sink for organic chemicals. After organic pollutants enter the soil, they will interact with soil components. Due to the complex and highly heterogeneous composition of soil components, the occurrence state of pollutants changes and differentiates greatly. As a result, the mobility, bioavailability, and even chemical reactivity of pollutants are reduced to varying degrees, which has a significant impact on their environmental ecological risks and restoration efficiency. The application of o-DGT in soil started a little later than in water, but developed equally rapidly [97]. O-DGT has been performed for various types of organic compounds (e.g., pesticides, pharmaceuticals, hormones, EDCs, HPCP). The DGT-DIFS method provides kinetic information, such as distribution coefficients, response times, and desorption/adsorption rates of organic pollutants in soil. The effectiveness of atrazine in the non-rhizosphere and rhizosphere was evaluated by DGT, and the relationship between atrazine morphology and rhizosphere soil properties was further analyzed. The influence mechanism of soil pH value and organic matter on the bioavailability of atrazine was clarified [104]. Li et al. [105] targeted the systematic development of DGT technology for nine representative pesticides using o-DGT with two types of resins (HLB and XAD18) as adhesive-layer materials. The pesticide mass accumulated by the DGT device is proportional to the deployment time and inversely proportional to the thickness of the diffusion layer, which is consistent with the theoretical prediction of DGT. By comparing the bioavailability and metabolism of pesticides in soil crop systems by DGT and traditional extraction techniques, it was proved that o-DGT is an effective method to measure the bioavailability of pesticides in soil [106]. Furthermore, an in situ monitoring technique for DGT based on porous β-cyclodextrin polymer as a binding material (CDP-DGT), stably and reliably determined the distribution of chlorophenols (CPs) in red soil in Kunming and paddy soil in Yixing [107].

4.2. Determination of Hydrophilic Organics Using DGT

Hydrophilic organic pollutants are widely found in water and soil, such as organophosphorus pesticides, antibiotics, and EDCs. These contaminants are often biotoxic and affect the survival and reproduction of organisms in the environment. They can cause long-term health effects, such as carcinogenesis, teratogenesis, and mutagenesis, when they enter the human body through the food chain and drinking water.
The effective glyphosate of plants in soil was evaluated and compared by o-DGT. The results showed that the diffusion gradient in o-DGT can effectively measure the available glyphosate under typical soil environmental boundary conditions [108]. Sulfonamide antibiotics are mostly insoluble in water, but have strong hydrophilicity. Its molecular structure has both an aniline group (-NH2) that can be protonated and a sulfonamide group (-SO2-NH-) that is easy to lose protons, which is a typical amphoteric compound. Chen et al. [109,110] used o-DGT to evaluate the desorption kinetics and bioavailability of sulfonamide and trimethoprim antibiotics in soil, demonstrating its feasibility for polar organic compounds in soil. The antibiotic sulfamethoxazole (SMX) was used as a model compound, and XAD18 resin was used as a novel binding phase material. The uptake of SMX by o-DGT mimicked its bioavailability in soil [111]. The desorption kinetics of tetracyclines (TCs) in agricultural soils can also be assessed by o-DGT [112]. In recent years, DGT has been widely used in the monitoring of EDCs in aquatic environment. Compared with traditional methods, it is easy to perform multi-site deployment at the same time, pre-concentration of in situ analytes, and reduce matrix interference [95]. Compared with the Chemcatcher sampler, the DGT is less affected by the diffusion boundary layer and can provide higher measurements [113]. Moreover, the absorption of EDCs by DGT is independent of the water flow rate, so the field concentration can be deduced by the calculation method [95]. Through the combination of high-performance liquid chromatography/mass spectrometry (HPLC/MS) and yeast estrogen screening (YES) techniques [114], as well as the application of toxic equivalents and the ecological risk quotient (RQ) [115], DGT can show the pollution status of freshwater water bodies, indicating the threat of pollutants to aquatic organisms. In addition, DGT has also been well applied and studied in the measurement of EDCs in seawater [116].

4.3. Influence Factors

The diffusion coefficient is one of the key parameters for predicting the kinetics of pollutant sorption in DGT samplers. Currently, the widely used hydrogel diffusion layer determined the diffusion coefficients of PFAS, currently used pesticides, pharmaceuticals, and personal care products (PPCPs) [117]. In addition, there are also studies reporting the effect of external filter membranes on the accuracy of DGT concentration determination [118]. Although there are DGT devices that do not use external filter membranes, biofouling is a problem that needs to be faced [91]. The decrease in mechanical strength will limit its application in soil, and increasing the thickness of the diffusion film requires remeasurement of the diffusion coefficient, and the change in the thickness of the diffusion boundary layer is also a problem that needs attention [119].
Whether external environmental factors affect the stability of the performance of the o-DGT technique is a key issue for its application. Especially in water bodies and complex soil environments, it is more necessary to determine the robustness of the measurement system. The ratio of the concentration of the target (CDGT) measured by DGT to the concentration of the native solution (Csolution) (i.e., CDGT/Csolution) is commonly used to evaluate the stability of the DGT technique in water [106,107]. pH plays a decisive role in the morphology of organic compounds in environmental media, and organic matter at a different pH may become neutral or anionic/cationic forms, which significantly affects its bioavailability. For many of the organic compounds studied so far, pH had no significant effect on the bioavailability content measured by o-DGT [120]. O-DGT showed good stability in measuring PAEs from the lake water environment (pH 3–9, IS 0.001–0.5 M, and DOM 0–20 mg/L) [100]. Considering the different pKa values of different compounds, pH can be used as one of the parameters to correct the measurement results in the future.
IS can affect sampling efficiency through a ‘salting-out’ effect that reduces analyte solubility [97]. At low IS (0.001–0.5 M), o-DGT measurements of compounds are generally independent of IS [97,120]. With the increase in IS, the diffusion of target ions to DGT is inhibited to a certain extent, resulting in the decrease in sorption efficiency. However, the extent of this influence is limited, and there is no study to quantify the effect of IS; hence, the calibration of parameters under different IS conditions is a matter of concern.
DOM can inhibit the sorption of the target compounds onto O-DGT through competitive adsorption. Therefore, in the presence of DOM, the diffusion coefficient of the target pollutant in DGT is slightly lower than that without the influence of DOM, and the corresponding amount of the target absorbed onto DGT is reduced [87,97]. However, the CDGT/Csolution values were in the range of 0.9–1.1, and DOM had no significant effect on o-DGT [100].
The o-DGT exhibits stable performance under a wide range of environmental conditions and has broad application prospects. At present, the main studies focus on measuring the content of pollutants in water and soil, especially for organic pollutants (Figure 3); thus, it is necessary to establish a general link with model organisms’ (animal/plant) bioavailability methods, so as to provide the basis and technology for bioavailability assessment support.

5. Assessment of Human Health Risk of Environmental Pollutants Using DGT

Environmental pollutants can enter the human digestive and circulatory systems through environmental media and food chains. It is necessary to more reasonably assess the human health risk of exposure to environmental pollutants consumed by the human body; the bioavailability of environmental pollutants is mainly evaluated through the in vivo test and in vitro test. In vivo methods generally used mice, minipigs, and other animals to directly detect the concentration of the targets in the blood after the animal ingests the pollutants and calculate the concentration of the target entering the blood [121,122]. The in vivo method is relatively reliable, but the methods have some disadvantages, such as high experimental cost, long experimental period, and differences among different individuals. In different in vitro methods, there are great differences among the measured results due to the different physiological parameters of the simulated gastrointestinal tract. DGT can be used to evaluate the effectiveness and toxicity of organic pollutants and heavy metals in soil or aquatic systems [104,123], and it has also confirmed the possibility of its application in human health risk assessment. The health risk index (HI) and ecological RQ are commonly used environmental risk assessment methods [100]. By combining the effective concentration of pollutants monitored by DGT methods, the toxic effects and potential risks of pollutants in environmental media can be accurately characterized to assess the environmental risk. At present, there have been reports on the monitoring of PAEs [100] and three antiviral corona virus disease 2019 (COVID-19) drugs [124] in water and the risk assessment of human health and the ecological environment using DGT, while the environmental risk assessment in soil mainly focuses on heavy metals [48]. The concentration and toxicity of heavy metals in contaminated soil [125], the accumulation in crops [45,126] and earthworms [79], and even human health risks [127] were accurately and effectively evaluated using DGT technology, the DIFS model [128] or combined with BCR [62], biogeochemical fractionation [129], whole-cell bioreporter [125], and physiological-based extraction tests [127]. Thus, the risk assessment of organic pollutants in soil using DGT needs to be further explored.

6. Conclusions

This review investigated the application of DGT in the environment, focusing on the reliability and stability of DGT in the bioavailability and potential risk assessment of target pollutants in soil environments. The combination of the binding phase and diffusion phase of different materials is used to assess different heavy metals and organic pollutants, while the impact of different pH, IS, DOM, and other environmental factors on the bioavailability of pollutants assessed by DGT, and the correlation and feedback mechanism between DGT and model biological (animal/plant) bioavailability methods all need to be further studied. The standardized methods and calibration procedures are conducive to the establishment of a more stable and reliable measurement system mechanism to enhance the robustness of DGT.

Author Contributions

Conceptualization, Q.Z. and J.J.; validation, Q.Z. and J.J.; writing, Q.Z.; review and editing, C.W. and H.S.; visualization, Q.Z. and C.W.; supervision, C.W. and X.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology of China (2018YFC1801003), Major Scientific and Technological Innovation Project of Shandong Province (2021CXGC011206), and Fundamental Research Funds for the Central Universities and 111 program, Ministry of Education, China (T2017002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of concentration gradient in DGT technique. (Δr, Δg, δ represent the thicknesses of the binding gel, diffusion gel, and diffusion boundary layer, respectively).
Figure 1. Schematic of concentration gradient in DGT technique. (Δr, Δg, δ represent the thicknesses of the binding gel, diffusion gel, and diffusion boundary layer, respectively).
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Figure 2. Solid binding phase DGT device (a) and liquid binding phase DGT device (b).
Figure 2. Solid binding phase DGT device (a) and liquid binding phase DGT device (b).
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Figure 3. Determination of organic pollutants in environmental media by DGT.
Figure 3. Determination of organic pollutants in environmental media by DGT.
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Table 1. Examples of recent studies on availability assessment of soil metals using DGT.
Table 1. Examples of recent studies on availability assessment of soil metals using DGT.
ElementSpecies/SoilsFindingsBinding Phase MaterialRef.
Lead (Pb)Pb-contaminated soilsDGT can be used as a good indicator of the actual situation of the potential risk of soil contamination to humans.Chelex-100[47]
PbBeijing’s parks soilsDGT was another method widely used to determine Pb bioavailability in addition to gastrointestinal extraction models and is highly correlated with accumulation in plants.Chelex-100[48]
Cd, Nickel (Ni)Soils contaminated with heavy metalsDGT can accurately reflect the contribution of dissociated metal complexes to the total concentration of unstable components.Chelex-100[49]
Cd, SeCd-contaminated seleniferous soilsDGT may more accurately reflect the availability of Cd and Se in the soil environment.Chelex-100[50]
Cd, Cu, ZnSoil under plastic shedDGT method can better reflect the transfer of heavy metals from soil to vegetables.Zr-Chelex[45]
Cd, Zn, PbContaminated agricultural soilThe CDGT value for Cd and Zn decreased with an increase in the soil temperature, but the C DGT values for Pb were unexpectedly high.Chelex-100[51]
Cd, Cu, Ni, Pb, ZnAgricultural soilsDGT can assess the available concentrations of metals in soil.Unknown[44]
CdPhenanthrene-contaminated soilsDGT flux can be most easily interpreted as a measure of Cd resupply from the soil solid phase to the solution phase.Chelex-100[52]
As, PbSpiked soilsDGT measurements reflected both non-specifically and specifically adsorbed As and exchangeable Pb in spiked soil.Ferrihydrite; Chelex-100[53]
CdHyperaccumulator (Sedum plumbizincicola)DGT-extractable Cd showed good correlations with Cdshoot. DGT was the best method for assessing Cd bioavailability.Chelex-100[56]
Cd, NiNoccaea caerulescens; Thlaspi goesingenseDGT measurements combined with the DIFS (DGT induced fluxes in soils) model are more useful than other chemical methods for assessing phytoremediation efficiency.Chelex-100[57]
Cd, ZnHyperaccumulator (Sedum plumbizincicola)The dynamic DIFS model of the soil–DGT system was used to obtain information on the changes in metal desorption kinetics during repeated phytoextraction.Chelex-100[58]
AsFern (Pteris vittata L.)The decrease in As flux measured by DGT suggested that As hyperaccumulating ferns are promising for phytoremediation of As-contaminated soils.Fe-oxide[54]
AsPaddy soilsDGT was used to assess the porewater and solid-phase As.Fe-oxide[42]
AsPaddy rice rhizosphere soilDGT can measure the bioavailable As in rice rhizosphere soil and obtain the dynamic curve of soil bioavailable As.Zr-oxide[55]
Cd, Fe (II), Mn (II), S (-II)Paddy soilsLDHs-DGT can extract labile Cd, Fe (II), Mn (II), and S (-II) at the same time.DTPA-modified LDHs nanomaterials[59]
CdRice (Oryza sativa L.)DGT-labile Cd levels were highly correlated with rice grain Cd levels, which showed a slight or no positive correlation with conventionally extracted Cd.Chelex-100[60]
CdRice (Oryza sativa L.)Cd measurements obtained by DGT had a higher correlation with the Cd in rice grains. DGT technique was a reliable method for the assessment of Cd bioavailability in soils.Mg-Al-DTPA-LDHs[61]
CdMaizeDGT measured bioavailable Cd well correlated with Cd in maize grains.Chelex-100[62]
Mo, Cr, Zn, Cu, Co, AsBean (P. vulgaris)The intensity of exposure to trace elements in soil was assessed by DGT probes.Chelex-100[43]
As, CuLupinus albus L.The distribution of As, Cu, P, and Fe in the lupin rhizosphere was evaluated with chemical images obtained by laser ablation-ICP-MS analysis of DGT gels.Zirconium hydroxide and iminodiacetate[63]
Cd, Cu, Ni, Pb, ZnChinese cabbage (Brassica pekinensis L.)DGT determined different soil bioavailable metal components and significantly improved prediction of metal uptake by Chinese cabbage.Unknown[64]
P, Cd, Cu, Pb, ZnZizania latifolia, Myriophyllum verticiilaturnDGT-rhizobox-DIFS should be a reliable tool to research phytoremediation mechanism of macrophytes.Ferrihydrite; Chelex-100[65]
As, SbWhite icicle radish (Raphanus sativus)DGT provided estimates of the potential kinetic recharge of As and Sb and was an effective tool for assessing and predicting bioavailable As and Sb in contaminated soils.TiO2[66]
HgEarthworm (Eisenia fetida)DGT-Hg flux was a useful tool for simulating bioavailable Hg concentrations in contaminated soils.3-mercaptopropyl-functionalized silica[67]
HgEarthworm (Eisenia fetida)DGT-measured Hg flux can predict Hg bioavailability for earthworms inhabiting diverse types of soil.3-mercaptopropyl-functionalized silica[68]
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Zhu, Q.; Ji, J.; Tang, X.; Wang, C.; Sun, H. Bioavailability Assessment of Heavy Metals and Organic Pollutants in Water and Soil Using DGT: A Review. Appl. Sci. 2023, 13, 9760. https://doi.org/10.3390/app13179760

AMA Style

Zhu Q, Ji J, Tang X, Wang C, Sun H. Bioavailability Assessment of Heavy Metals and Organic Pollutants in Water and Soil Using DGT: A Review. Applied Sciences. 2023; 13(17):9760. https://doi.org/10.3390/app13179760

Chicago/Turabian Style

Zhu, Qing, Jing Ji, Xuejiao Tang, Cuiping Wang, and Hongwen Sun. 2023. "Bioavailability Assessment of Heavy Metals and Organic Pollutants in Water and Soil Using DGT: A Review" Applied Sciences 13, no. 17: 9760. https://doi.org/10.3390/app13179760

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