Introduction
Mercury cycling in the environment
Mercury (Hg) is one of the top ten concern elements listed by WHO 2019 Hg presents ubiquitously as a contaminant at low concentration in all the compartments of the environment (Driscoll et al 2013, Xu et al 2015, Yang et al 2020, Clarkson et al 2003) Hg releases into the atmosphere, aquatic, and terrestrial ecosystems, derived from various natural (i.e., volcano eruption, forest fires, cinnabar) and anthropogenic activities (i.e., fuel combustion, industrial processes, and the chlor-alkali industry) (Obrist et al 2018, Amos et al 2015, Streets et al 2018, Gworek et al 2020) The expansion of worldwide industrialization after World War II drove a significant increase of the emission of Hg to the environment (Streets et al 2017) In the recent update by UNEP (2003), approximately 30% of direct Hg emission to the atmosphere is from anthropogenic activities, while 10% the emission is related to natural sources The reemission of legacy Hg accounts for 60% of total emission, and it comes from previously anthropogenic sources built up for a long-term period on the surface of soil and ocean that return to the atmosphere Once entering to the environment, Hg can be transported in the atmosphere over very long distances due to the long resident time of Hg, which is fundamental to Hg global cycling (Schroeder et al 1991, O'Connor et al 2019, Kim &Zoh 2012) Mercury cycles in the terrestrial, aquatic, and atmospheric ecosystem, until it moved out of the system and buried in soil and sediment through entrapment in stable mineral compounds (Obrist et al 2018, Bishop et al 2020, Selin 2009) The estimation of global anthropogenic Hg emission to the atmosphere was approximately 2220 tons per year,
2 including re-emission (UNEP 2019) UNEP (2008) estimated the sum of natural emission and re- emission to be in the range of 1,800 – 4,800 tons yr -1 The amount of Hg mass accumulated in soils is very large, assumed to be in the range of 250 – 1000 Gg (Obrist et al 2018) A significant proportion of Hg in soils is attributed to anthropogenic influences, with an estimated 86 Gg of anthropogenic Hg is now accumulated in surface soils (Futsaeter &Wilson 2013)
Mercury could be found in the environment as Hg0 (elemental mercury), Hg +1 (mercurous) and Hg +2 (mercuric)( Bigham et al 1964, Park &Zheng 2012, Bernhoft 2012) In soils, Hg 0 and
Hg +2 compounds are stable in typical conditions, of these, Hg +2 is the predominant oxidation state of mercury compounds, while Hg (0) is very susceptible to oxidation Hg(0) volatilizes easily to the atmosphere (Teng et al 2020, Gai et al 2016, Jagtap &Maher 2015) In the atmosphere, the elemental Hg, (Hg(0) account for 98% of the total species (Cohen et al 2004, Bloom &Fitzgerald
1988, Morel et al 1998), while in the aquatic ecosystem, the Hg(0) concentration is high variable depend on the depth, location, season, bacteria, and plankton (Morel et al 1998, Jiang et al 2018)
Hg (0) was discovered in higher concentrations closer to the air-water interface whereas total Hg and methylmercury (MeHg) exist in greater concentrations near the sediment-water interface (Morel et al 1998,Gill et al 1999) In sediments, Hg is mainly bound to sulphur as well as organic matter and inorganic particles (Morel et al 1998, Hesterberg et al 2001, Zhong &Wang 2009).
Hg in soil ecosystem
High variability of Hg concentration in soils has been reported worldwide In southern Amazon Brazil, Lacerda et al (2004) identified Hg concentrations in the range of 15–248 μg kg −1 in forest soils and 10–74 μg kg −1 in pasture soils In another site in the Amazon, Teixeira et al (2018) reported Hg concentrations almost up to 1000 mg kg −1 Studies conducted in soils from
3 industrial areas in Spain reported concentrations of Hg roughly 152 ± 47 μg kg −1 (Gonzalez- Fernandez et al 2014) In mining area in Almadén, Spain, high concentrations was detected around
9000 mg kg −1 (Higueras et al 2003) In soil in state of Virginia, America, Hg was found at 0.1–94 μg kg −1 in an industrial area (Bigham et al 2015) The high mercury concentrations are related to the proximity to natural or anthropogenic Hg sources
Soil is the largest terrestrial Hg pool and play a critical role in the global Hg cycling as a sink for atmospheric Hg deposition and a re-emission source (Jiskra et al 2015, Wang et al 2019) Several pathways lead to atmospheric Hg deposition to terrestrial surfaces Gaseous Hg(0) is oxidized to Hg(II) in the atmosphere and deposited to terrestrial surfaces directly through precipitation (Jiskra et al 2015, Yin et al 2014b) Once taken up by plant, Hg(II) can be photo- reduced to Hg(0) and re-emit to atmosphere (Jiskra et al 2015, Selin 2009) Under typical soil conditions, Hg(II) occur abundantly in the forms of inorganic mercuric salts (i.e., HgCl2, HgO, HgS) and minerals (primarily cinnabar, metacinnabar) (O'Connor et al 2019, Yin et al 2016) Hg(II) can transform under the influence of soil redox potential and exist dominantly in Hg-S compounds in oxidizing conditions Under the reducing condition, biological and chemical processes transform Hg +2 to organic mercury species mainly methylmercury (CH3Hg) which is the most toxic form of Hg In organic soils, the dominant Hg form is Hg(II), mainly bound to reduced sulfur groups of natural organic matter (NOM) In mineral soils, Hg(II) is primarily adsorbed to mineral surfaces such as Fe, Al, Mn oxides or clay minerals (references are needed here).
Bioavailability and toxicity of Hg in soil
Bioavailability of Hg in soil is related to labile Hg species, as it controls the uptake fraction of Hg by living biota as well as Hg leaching through the soil (Wang et al 2014a) The concentration
4 of labile Hg means available Hg in the soil solution and resupplied Hg from the soil particles (Reis et al 2015, Shetaya et al 2017) Bioavailability of Hg defines the correlation between the total Hg concentration in the soil and the level of Hg uptake into the biota (Fang et al 2011, Araujo et al 2019) Plant or terrestrial animals such as earthworm have been frequently used to evaluate the bioavailable pool of Hg in soil Wang et al (2014) revealed that the fraction of Hg in soil that can be uptaken by plants directly reflects bioavailable Hg Strong and significant correlations were found between the Hg amounts in vegetables and the total Hg concentrations in farmland soils near to fluorescent lamp factories in China (Shao et al 2012) Meanwhile, earthworm is the organism that has been widely used for investigating the bioavailability of Hg in terrestrial ecosystem, due to its important as a major soil ecosystem component and a food source for variety of organisms Multiple field studies have been conducted to examine the bioaccumulation of Hg by individual earthworm species.
DGT technique for Hg measurement
The diffusive gradients in thin films (DGT) is an in-situ sampling technique that was first applied by Davison &Zhang (1994) for the measurement of heavy metal in freshwater This technique was later applied to sediment and soil Piston and plate are the two common DGT device types that have been reported Piston type DGT was used for solution and soil application, while the plate-type DGT was used for sediment application (Ding et al 2016) The DGT device typically comprises a binding hydrogel layer embedded with specific adsorbent, a diffusion layer composed of a hydrogel, and a filter membrane The binding gel, diffusive gel, and filter membrane stack were laid together and sealed tightly by a plastic base and cap The difference in concentration between binding layer and DGT surface generates a concentration gradient, which
5 is a driving force that allow solutes to pass through the diffusive layer and accumulate in binding layer Initially, the application of the technique to Hg was failed by the fact that Hg accumulates in amide groups of polyacrylamide gel working as the diffusive layer in original method The DGT method was modified by replacing the polyacrylamide gel by agarose (Divis et al 2005), which allow Hg to pass freely through with a diffusion coefficient close to water (Docekalova &Divis 2005) Variety of binding agents have been applied for Hg depending on the method and target species, such as Spheron-Thiol, 3-mercaptopropyl-functionalized silica gel (3-MFSG), Duolite GT73, Iontosorb AV-MP, and Ambersep GT74
According to the principles of the technique, analyte concentration in the solution can be determined by the equation below:
𝐷 × 𝑡 × 𝐴 (1) where Cb is the free or labile concentration of metals in the deployment media, M is the recovered mass of the analyte, g is the diffusive gel thickness, D is the diffusion coefficient of the analyte in the gel, t is the deployment time, and A is the exposure window area Temperature dependent D can be calculated using Equation (2)
298 (2) where D 25 g is the diffusion coefficient of metal in a diffusive gel at 25 °C
Considering the thickness of the DBL (δ) and filter layer (Δf) (Garmo et al 2006; Kreuzeder et al 2015), Equation (1) can also be expressed as:
In equation (1), the accumulated mass M can be measured directly in the binding-gel layer by combustion method (using AMA-254) or using a beam technique such as laser ablation ICPMS
6 or, in the case of radionuclides, by direct counting It can be also obtained via elution of the ion from the binding gel into solution for routine measurements to minimize the cost The accumulated mass, M, can be calculated using the following Eq 4
𝑓 (4) where Ce is the concentration of the ion in the elution solution, Ve is the volume of the elution solution, Vg is the volume of the gel and ƒe is the elution efficiency.
DGT technique to evaluate lability of Hg in soil
DGT application for labile Hg measurement in soil
Mercury speciation and its interaction with environmental components to form water- soluble, colloidal, and Hg insoluble complexes affects to the mobility and bioavailability to organism The DGT method is advantageous for in-situ measurement of Hg speciation, because DGT preserves stability and distribution of the Hg species, while it also preconcentrates target analyte during the deployment Cattani et al (2008) evaluated the rhizosphere Hg bioavailability from contaminated soil by combine DGT and HPLC-ICP-MS methods The results indicated that both of the methods are appropriate for the estimation of Hg root availability from contaminated soils Cattani et al (2009) investigated the influence of humic acid (HA) on Hg mobility and bioavailability in contaminated soils by DGT method In soil, Hg can be mobilized or stabilized in binding sites of humic substances By using both ultrafiltration and DGT methods for assessing the lability of Hg-HA complexes, the author demonstrated that DGT can be used to estimate the labile fractions of Hg-HA complexes DGT technique is capable of measuring free Hg ions in soil solution, Hg from the dissociation of the complexes with organic and inorganic ligands and Hg
7 complexes released from the solid phase The DGT results showed a significant enhancement of
Hg availability in the presence of HA, which extracted Hg from soil particle to soil solution Turull et al (2019b) successfully applied DGT to quantify the inorganic and organic Hg complexes which is in the forms with humic and fulvic acids The author has used two diffusive layers with different pore size to distinguish the inorganic and organic bioavailable Hg species in soil The large-pore size (>5 nm) diffusive layer which is called open diffusive layer (ODL) could allow both of inorganic and organic labile Hg species to pass through, while small-pore size (sodium pyrophosphate (Fep) (average 0.21 ± 0.11%) Fed was found to be similar concentration in uncontaminated forest, YSR and agricultural soils (1.1 ± 0.3, 1.1 ±0.4 and 1.3 ± 0.2 %, respectively), and significant high concentration in contaminated HSR soils (3 ± 2%) Content of Fe in form of organic complexes (Fep) was the lowest in HSR soils (0.08 ± 0.04%), and not significant differences among YSR, forest, and agricultural soils (0.3 ± 0.1, 0.2 ± 0.1, and 0.2 ± 0.1%, respectively) Amorphous Fe complexes (Feh) was higher concentration in YSR and HSR soils (1 ± 0.3 and 1.1 ± 0.3%, respectively) Al dominant in amorphous form (0.3 ± 0.2%), followed by crystalline and pyrophosphate forms (0.2 ± 0.1 and 0.1 ± 0.1, respectively)
Ald, Alp and Alh were all found to be the highest values in forest soils (0.3 ± 0.1%, 0.2 ± 0.1 and 0.5 ± 0.2%, respectively)
4.3.2 Correlation analysis among soil properties
The result of the Pearson’s correlation analysis between soil properties was presented in table 4.3 Both of soil 𝑝𝐻 and 𝑝𝐻 showed significant negative correlation to ORP and Alp
(p