Content of water-soluble mercury
It is assumed that the water-soluble mercury compounds are mainly salts—e.g. HgCl
2, HgBr
2, Hg(CN)
2, Hg
2(C
2H
3O
2)
2 (Barnett and Turner
2001; Han et al.
2003; Schroeder and Munthe
1998), and also low-molecular-weight organic Hg compounds (Wallschläger et al.
1998a). It should, however, be noted that the solubility of mercury compounds depends to a great extent on the composition and properties of the soil solution. For example, cinnabar (HgS), which is sparingly soluble, may dissolve in solutions containing dissolved organic matter (fulvic acids) or compounds with thiol ligands (Jacobson et al.
2005). The content of soluble fractions in soils does not usually exceed 4 % of the total organic matter content (Henderson et al.
1998).
Table 1
Determined components and properties of studied soil profiles (Różański
2009)
Haplic Luvisol
|
1 | Ap | 0–22 | 15 | 9.7 | 6.5 | 5.9 | 5.2 | 68.2 | 12.93 | 4.3 | 1.4 |
Bt | 22–61 | 22 | 3.0 | 6.3 | 4.9 | 5.0 | 116.0 | 24.55 | 8.0 | 1.7 |
Ck1 | 61–95 | 17 | 1.2 | 8.0 | 7.3 | 0.8 | 100.7 | 17.53 | 4.5 | 0.7 |
2Ck2 | <95 | 26 | 1.0 | 8.0 | 7.3 | 0.8 | 81.5 | 20.02 | 6.0 | 0.6 |
Haplic Luvisol
|
2 | Ap | 0–26 | 7 | 6.4 | 5.7 | 4.7 | 13.5 | 43.9 | 7.91 | 2.1 | 0.8 |
E | 26–36 | 5 | 5.5 | 5.8 | 4.6 | 6.9 | 39.1 | 8.58 | 1.8 | 0.5 |
E/B | 36–57 | 18 | 1.3 | 6.3 | 4.9 | 4.8 | 82.6 | 18.19 | 4.5 | 1.0 |
Bt1 | 57–90 | 22 | 1.4 | 6.7 | 5.2 | 4.5 | 112.1 | 25.43 | 6.9 | 1.3 |
Bt2 | 90–120 | 23 | 0.7 | 7.1 | 5.9 | 3.4 | 124.5 | 22.78 | 6.2 | 1.0 |
Ck | <120 | 17 | 0.7 | 8.0 | 7.3 | 0.7 | 103.5 | 18.07 | 4.2 | 0.5 |
Haplic Luvisol
|
3 | Ap | 0–27 | 4 | 3.8 | 6.8 | 6.6 | 1.8 | 28.0 | 7.17 | 2.1 | 0.9 |
Eg | 27–40 | 1 | 1.1 | 6.6 | 5.6 | 2.8 | 25.0 | 7.66 | 1.7 | 0.8 |
Btg1 | 40–76 | 20 | 1.4 | 6.9 | 5.3 | 5.4 | 145.4 | 29.03 | 7.6 | 2.6 |
Bt2 | 76–105 | 18 | 1.0 | 6.5 | 5.0 | 4.5 | 107.0 | 21.88 | 6.1 | 1.4 |
BC | 105–135 | 16 | 0.5 | 7.3 | 6.5 | 2.2 | 93.2 | 19.20 | 5.3 | 1.2 |
Ck | <135 | 13 | 0.4 | 8.0 | 7.4 | 0.6 | 84.6 | 16.45 | 4.6 | 0.4 |
Eutric Fluvisol
|
4 | Ap | 0–15 | 7 | 18.0 | 7.0 | 6.7 | 3.2 | 93.3 | 9.65 | 3.7 | 2.0 |
C1 | 15–55 | 10 | 8.7 | 7.4 | 7.2 | 1.7 | 98.6 | 12.47 | 4.4 | 2.5 |
C2 | 55–73 | 15 | 6.9 | 7.6 | 7.2 | 1.6 | 153.1 | 18.07 | 7.0 | 3.8 |
C3 | 73–90 | 13 | 9.9 | 7.7 | 7.3 | 1.4 | 139.7 | 16.72 | 6.1 | 3.4 |
C4 | <90 | 2 | 7.3 | 7.8 | 7.4 | 2.0 | 19.6 | 2.34 | 0.9 | 0.4 |
Endogleyic Fluvisol
|
5 | Ap | 0–20 | 42 | 23.5 | 7.5 | 7.1 | 2.3 | 317.2 | 39.59 | 18.9 | 8.1 |
AC | 20–45 | 29 | 20.4 | 7.6 | 7.0 | 1.8 | 252.1 | 33.65 | 15.1 | 6.4 |
Cg1 | 45–70 | 26 | 14.1 | 7.7 | 7.1 | 1.6 | 236.7 | 30.22 | 13.5 | 5.9 |
Cg2 | 70–100 | 21 | 11.7 | 7.8 | 7.2 | 1.9 | 198.6 | 24.03 | 10.6 | 4.7 |
Cg3 | <100 | 20 | 11.7 | 7.8 | 7.2 | 1.6 | 196.9 | 24.48 | 10.1 | 3.4 |
Endogleyic Phaeozem
|
6 | Ap | 0–35 | 8 | 18.8 | 6.2 | 5.8 | 7.5 | 138.3 | 7.17 | 1.8 | 0.7 |
AC | 35–48 | 23 | 4.9 | 7.0 | 5.8 | 4.2 | 150.9 | 15.63 | 2.3 | 0.3 |
Cg1 | 48–95 | 22 | 2.0 | 7.5 | 6.4 | 2.1 | 132.1 | 23.17 | 10.5 | 0.5 |
Cg2 | 95–140 | 20 | 0.1 | 7.6 | 6.5 | 2.3 | 107.0 | 18.76 | 5.3 | 0.4 |
Cgk3 | <140 | 19 | 0.3 | 7.9 | 7.3 | 1.4 | 106.8 | 18.33 | 5.3 | 0.6 |
Brunic Arenosol
|
7 | Ap | 0–29 | 3 | 8.5 | 4.8 | 4.1 | 20.9 | 30.3 | 5.90 | 2.3 | 1.1 |
A/B | 29–37 | 4 | 4.9 | 5.3 | 4.5 | 12.0 | 18.6 | 5.21 | 2.2 | 1.6 |
Bs | 37–65 | 5 | 2.9 | 5.7 | 4.8 | 13.4 | 25.3 | 5.15 | 2.3 | 1.4 |
BC | 65–77 | 2 | 1.3 | 6.3 | 5.1 | 7.3 | 20.2 | 5.12 | 1.4 | 0.6 |
C | <77 | 3 | 0.4 | 6.6 | 5.2 | 6.1 | 21.7 | 3.94 | 1.1 | 0.4 |
Albic Podzol
|
8 | Oi | 10–9 | – | 448.2 | 4.7 | 4.3 | 104.3 | 852.5 | 1.94 | 1.0 | 0.4 |
Oe | 9–3 | – | 485.2 | 4.3 | 3.7 | 189.0 | 816.9 | 5.17 | 2.7 | 1.4 |
Oa | 3–0 | – | 216.9 | 4.2 | 3.5 | 144.0 | 515.3 | 5.58 | 3.5 | 1.8 |
AE | 0–12 | 5 | 16.6 | 4.4 | 3.9 | 48.8 | 55.1 | 2.82 | 1.4 | 0.9 |
Bh | 12–18 | 6 | 9.5 | 4.7 | 4.3 | 25.2 | 27.4 | 2.82 | 1.5 | 1.2 |
Bs | 18–36 | 8 | 6.7 | 4.6 | 4.5 | 18.3 | 19.8 | 3.10 | 1.7 | 1.4 |
B/C | 36–84 | 4 | 1.6 | 4.8 | 4.6 | 12.3 | 14.4 | 2.59 | 0.8 | 0.4 |
C1 | 84–125 | 4 | 0.3 | 4.8 | 4.7 | 7.5 | 13.0 | 2.99 | 0.8 | 0.4 |
C2 | <125 | 3 | 1.4 | 5.1 | 4.7 | 6.7 | 14.0 | 2.77 | 0.4 | 0.1 |
The content of mobile, water-soluble mercury forms was very low in the examined soils and ranged between 0 and 0.82 µg·kg
−1 (Table
2). The percentage of Hg
H2O in the total content of this metal was on average 0.28 %. Low share of readily soluble Hg compounds indicates that their migration into the deeper soil horizons is low. The highest content of Hg
H2O was found in surface horizons, especially organic horizons of
Albic Podzol (profile 8). Slightly different results were found for the polluted soils of Europe, where the highest content of mobile mercury forms was found in subsurface horizons (below 20 cm). This was linked with Hg being washed together with humus acids from surface horizons to subsurface horizons, where these were sorbed by mineral components of soil sorption complex (Biester et al.
2002a,
b).
Table 2
Total mercury content and its forms
Haplic Luvisol
|
1 | Ap | 0–22 | 48.62 | 0.23 | 0.27 | 35.83 | 0.47 | 0.56 | 73.70 |
Bt | 22–61 | 36.26 | 0.06 | 0.13 | 16.68 | 0.17 | 0.36 | 46.01 |
Ck1 | 61–95 | 19.07 | 0.02 | 0.15 | 8.99 | 0.10 | 0.78 | 47.13 |
2Ck2 | <95 | 19.50 | 0.05 | 0.12 | 7.86 | 0.26 | 0.64 | 40.31 |
Haplic Luvisol
|
2 | Ap | 0–26 | 26.93 | 0.13 | 0.61 | 15.62 | 0.48 | 2.26 | 57.99 |
E | 26–36 | 16.01 | 0.08 | 0.28 | 6.51 | 0.50 | 1.76 | 40.68 |
E/B | 36–57 | 21.60 | 0.04 | 0.21 | 9.06 | 0.18 | 0.97 | 41.96 |
Bt1 | 57–90 | 22.15 | 0.04 | 0.10 | 7.32 | 0.18 | 0.47 | 33.04 |
Bt2 | 90–120 | 33.04 | 0.01 | 0.09 | 8.77 | 0.03 | 0.26 | 26.54 |
Ck | <120 | 17.67 | 0.01 | 0.11 | 4.64 | 0.06 | 0.62 | 26.26 |
Haplic Luvisol
|
3 | Ap | 0–27 | 26.61 | 0.09 | 0.30 | 14.01 | 0.34 | 1.13 | 52.66 |
Eg | 27–40 | 14.19 | 0.10 | 0.34 | 4.90 | 0.70 | 2.36 | 34.53 |
Btg1 | 40–76 | 36.85 | 0.08 | 0.08 | 10.79 | 0.22 | 0.22 | 29.28 |
Bt2 | 76–105 | 29.38 | 0.09 | 0.09 | 6.68 | 0.31 | 0.31 | 22.74 |
BC | 105–135 | 31.30 | 0.02 | 0.09 | 7.73 | 0.06 | 0.29 | 24.71 |
Ck | <135 | 17.08 | 0.02 | 0.16 | 4.02 | 0.12 | 0.94 | 23.52 |
Eutric Fluvisol
|
4 | Ap | 0–15 | 142.90 | 0.42 | 0.49 | 76.00 | 0.29 | 0.35 | 53.18 |
C1 | 15–55 | 80.60 | 0.08 | 0.12 | 43.97 | 0.10 | 0.15 | 54.56 |
C2 | 55–73 | 88.93 | 0.04 | 0.12 | 48.36 | 0.04 | 0.14 | 54.38 |
C3 | 73–90 | 71.90 | 0.03 | 0.14 | 35.23 | 0.04 | 0.19 | 49.00 |
C4 | <90 | 3.73 | 0.04 | 0.38 | 1.18 | 1.07 | 10.19 | 31.59 |
Endogleyic Fluvisol
|
5 | Ap | 0–20 | 1438.00 | 0.18 | 0.20 | 33.70 | 0.01 | 0.01 | 2.34 |
AC | 20–45 | 280.50 | 0.05 | 0.21 | 113.82 | 0.02 | 0.07 | 40.58 |
Cg1 | 45–70 | 384.70 | 0.08 | 0.19 | 159.37 | 0.02 | 0.05 | 41.43 |
Cg2 | 70–100 | 292.00 | 0.04 | 0.24 | 142.49 | 0.01 | 0.08 | 48.80 |
Cg3 | <100 | 238.40 | 0.04 | 0.24 | 96.70 | 0.02 | 0.10 | 40.56 |
Endogleyic Phaeozem
|
6 | Ap | 0–35 | 36.66 | 0.20 | 0.39 | 23.46 | 0.54 | 1.07 | 63.99 |
AC | 35–48 | 21.66 | 0.09 | 0.28 | 8.83 | 0.41 | 1.29 | 40.77 |
Cg1 | 48–95 | 23.26 |
bdl
| 0.31 | 9.46 |
bdl
| 1.32 | 40.65 |
Cg2 | 95–140 | 17.60 | 0.01 | 0.44 | 6.43 | 0.06 | 2.52 | 36.51 |
Cgk3 | <140 | 18.13 | 0.02 | 0.32 | 3.82 | 0.11 | 1.74 | 21.07 |
Brunic Arenosol
|
7 | Ap | 0–29 | 24.64 | 0.09 | 2.20 | 14.36 | 0.37 | 8.92 | 58.26 |
A/B | 29–37 | 22.20 | 0.11 | 0.53 | 13.03 | 0.50 | 2.40 | 58.71 |
Bs | 37–65 | 21.71 | 0.05 | 0.36 | 11.55 | 0.23 | 1.64 | 53.19 |
BC | 65–77 | 8.95 | 0.09 | 0.29 | 2.75 | 1.00 | 3.21 | 30.68 |
C | <77 | 6.98 | 0.10 | 0.28 | 0.78 | 1.43 | 3.96 | 11.22 |
Albic Podzol
|
8 | Oi | 10–9 | 126.50 | 0.59 | 5.45 | 58.40 | 0.46 | 4.31 | 46.17 |
Oe | 9–3 | 266.70 | 0.38 | 1.21 | 85.86 | 0.14 | 0.46 | 32.19 |
Oa | 3–0 | 322.00 | 0.82 | 6.83 | 76.06 | 0.25 | 2.12 | 23.62 |
AE | 0–12 | 21.12 | 0.09 | 1.46 | 9.93 | 0.43 | 6.93 | 47.00 |
Bh | 12–18 | 19.56 | 0.01 | 0.99 | 8.82 | 0.05 | 5.08 | 45.11 |
Bs | 18–36 | 23.11 | 0.02 | 0.73 | 12.57 | 0.09 | 3.14 | 54.38 |
B/C | 36–84 | 7.40 |
bdl
| 0.98 | 1.81 |
bdl
| 13.26 | 24.52 |
C1 | 84–125 | 6.88 |
bdl
| 0.74 | 1.40 |
bdl
| 10.79 | 20.39 |
C2 | <125 | 5.56 | 0.01 | 0.61 | 0.85 | 0.18 | 10.98 | 15.34 |
Forest soils of northern Poland were characterized by higher content of Hg than the content found in this research (Malczyk
2000), while in the mercury-polluted soils of Great Britain, the concentration of Hg
H2O was so low that it did not exceed the detection level in the analytic method used (Panyametheekul
2004). In the soils of central Spain, the content of Hg
H2O did not exceed the level of 0.025 µg·kg
−1 (using both the AMA and ICP-MS method) (Sánchez et al.
2005).
The content of readily soluble mercury compounds was positively correlated with the content of organic carbon (
r = 0.51;
p < 0.05, Table
3). The highest content of Hg
H2O was determined in surface horizons of the analysed soils, rich in organic matter (except profile 3 and 7, Tables
1,
2). The greatest percentage of water-soluble forms in the total content of Hg was found in horizons with considerably low content of organic matter and poor in clay fraction (especially in
E horizons of
Haplic Luvisols, profiles 2 and 3). Such results indicate considerable influence of both organic matter and clay fraction on the content of Hg
H2O.
Table 3
Statistically significant relationship between mercury forms and main soil properties
HgH2O
| | 0.51 | | | | | | | | |
HgDTPA
| | | −0.72 | −0.58 | 0.81 | −0.59 | | | | −0.49 |
HgNaOH
| 0.90 | 0.77 | | | | | 0.44 | 0.74 | 0.40 | 0.66 |
These results indicate that the amount of readily soluble Hg forms was relatively low in relatively fine-textured soil, rich in clay minerals (profile 5). When comparing the content of Hg
H2O in surface horizons in both analysed
Fluvisols (profiles 4 and 5), it could be concluded that the percentage of these Hg forms, despite comparable content of organic carbon, and considerably high total content of mercury in fine-textured
Endogleyic Fluvisol (profile 5), was significantly higher in coarse-textured
Eutric Fluvisol (profile 4, Tables
1,
2). Therefore, it seems that clay fraction was very influential for binding mercury in these soils. Moreover, the content of the analysed
Fluvisols suggests that in soils enriched in humus and clay minerals, as a result of adsorption of positively charged mercury cations on the negatively charged surface of humus compounds and clay minerals, sparingly soluble in water complexes Hg-humus-clay minerals may be formed. Mercury is hence bound to the solid phase of the soil. This could account for a relatively high content of Hg
H2O in organic horizons (
Oi,
Oe, and
Oa) of the
Albic Podzol (profile 8), in which no clay minerals were found. The above hypothesis stating the role of clay minerals in binding mercury in soil has also been confirmed by other authors (Biester et al.
2002a; Boszke et al.
2004; Inácio et al.
1998).
Content of bioavailable mercury
The content of DTPA-extractable metals in soils are considered as fractions available for plants (Kabata-Pendias and Pendias
2000; Lindsay and Norvell
1978). The percentage of Hg
DTPA in the total content of mercury was low and ranged between 0.01 and 13.26 % (2.45 % on average, Table
2). A comparable participation of available forms was found by Barnett and Turner (
2001) in soils polluted by this metal (0.3–14 %, 3.2 % on average). However, these cited authors found that higher available mercury values were in the subsurface horizons, which contrasts with the results of this study since in this case the highest values of available mercury appeared mostly associated with both surface (profiles 1, 2, 4, 7 and 8) and subsurface (usually the parent material horizons—profiles, 3, 5, and 6) horizons.
The concentration of bioavailable forms of Hg in the examined soils ranged between 0.09 and 2.20 µg kg
−1 in mineral horizons (0.39 µg kg
−1 on average) and from 1.21 to 6.83 µg kg
−1 in organic horizons (4.50 µg kg
−1 on average, Table
2). The profile distribution for Hg
DTPA was not homogeneous. No statistically significant correlation was determined between the content of this mercury fraction and total content of the metal. Significant correlations were stated for Hg
DTPA with acidity (pH in H
2O and in KCl), total content of exchangeable H
+ cations, total content of Fe and content of the clay (
r = −0.72,
r = −0.58,
r = 0.81,
r = −0.59,
r = −0.49;
p < 0.05, respectively, Table
3).
In the study of the content of available, DTPA-extractable, mercury forms in soils with acid reaction (
Brunic Arenosol and
Albic Podzol, profiles 7 and 8, Table
1), the content of these forms was higher in comparison with soils with neutral reaction (
Fluvisols and
Endogleyic Phaeozem, profiles 4, 5, and 6). Higher content of Hg
DTPA in these acidic soils may be the result of the release of mercury from Fe and Al complexes (Schlüter
1997). Regarding this, it must be taken into account that the acid reaction increases the content of soluble, low-molecular-weight fulvic acids, mainly responsible for binding of Hg in the soil solution (Biester et al.
2002a; Wallschläger et al.
1998a). Another important fact in this regard is the important role of pH fluctuations, which influence the methyl mercury sorption more than Hg
2+ sorption (Boszke et al.
2003).
A relatively low content of DTPA-extractable mercury should be noted in
Fluvisols and
Bt horizons of
Luvisols (profiles 1–3). These soils were characterized by considerably high content of clay and amorphous Fe oxides in comparison with other examined samples (Table
1). High content of clay fraction and Fe
o, which forms soil sorption complex, leads to binding of mercury by the solid phase of the soil (Boszke et al.
2003; Dreher and Follmer
2004). This fact does not limit mercury bioavailability when soil shows low amount of organic matter (Biester et al.
2002a; Wang et al.
1997). The process of mercury binding by clay minerals has been observed in soils containing a minimum of 15 % of clay (Wang et al.
1997).
Toxicity of mercury depends on formation of compounds with alkyl groups (mostly methyl group), and therefore alkylation is a very important process from this point of view. It usually takes place in alluvial soils, in which this process, due to high content of organic matter, high level of ground water and seasonal floods, may occur quite fast (Montgomery et al.
2000). Such a direction in transformation of mercury compounds may have influenced the distribution of DTPA-extractable mercury forms in the profiles of
Fluvisols and
Endogleyic Phaeozem (profiles 4, 5, and 6). In horizons of gleyic characteristics (
g) in
Endogleyic Phaeozem (profile 6), fine-textured
Endogleyic Fluvisol (profile 5) and in
Haplic Luvisol (profile 4), the percentage of Hg
DTPA forms was greater in the entire profile. This could be the result of in situ formed alkyl mercury compounds, and the migration of mobile mercury forms from higher horizons (Barnett and Turner
2001; Montgomery et al.
2000). The consequence of the occurrence of favourable alkylation conditions may be the increase in the content of especially toxic methyl and ethyl mercury, which as bioavailable forms constitute greater danger for all living organisms (Boening
2000; Han et al.
2003; Tsiros and Ambrose
1999). According to Gilmour and Henry (
1991) and Paterson et al. (
1990) even in such conditions, the share of alkyl mercury forms usually does not exceed 3 % of the total content of this metal in soils.
Mercury bound to organic matter
According to some authors (Biester et al.
2002a; Dmytriw et al.
1995), the forms of mercury extractable by NaOH are mainly typical humus compounds.
The concentration of Hg
NaOH in the analysed soils ranged between 0.78 and 159.37 µg kg
−1, and the profile distribution was proportionally close to the distribution of total Hg content (Table
2). It confirms the positive significant correlation coefficient between Hg
tot. and Hg
NaOH (
r = 0.90;
p < 0.05, Table
3). Similar results were determined by Malczyk (
2000) in unpolluted forest soils. This indicates that organic matter plays a dominant role in binding of mercury in soils. Mercury often forms stable complexes with organic ligands with a stability constant ranging from 18.4 to 22.1 (Stein et al.
1996).
Due to the complexity of the organic matter transformations in soil, scientific reports give contradictory information in this aspect. Wang et al. (
1997) observed that the increase of humus in soils affects the decrease of Hg content in plants, which could indicate strong binding of this element by organic matter. Montgomery et al. (
2000), however, found relatively high concentration of mobile and available mercury forms in soils with comparatively high amount of organic matter. Furthermore, the influence of soil humus on binding of mercury is dependent on the clay content, and whether clay is high may even play the dominant role in this process (Inácio et al.
1998; Wang et al.
1997).
The proved lack of pH influence on the content of Hg
NaOH in the analysed soils (no significant correlation coefficient) may be caused by either the binding of mercury by organic matter regardless of pH value, or by the fact that determined pH range favoured such complexation. Gabriel and Williamson (
2004) noticed the dominant influence of organic matter on binding mercury in soils, in which pH was lower than 7. This process occurred in both aerobic and anaerobic conditions.
The percentage of Hg
NaOH forms in the examined soils ranged from 2.34 to 73.70 % of the total content of mercury (Table
2). Other authors indicated that in some soils the organic matter binds about 30 % (Munthe et al.
2001), and even 80–85 % of total mercury content (Dmytriw et al.
1995; Henderson et al.
1998; Renneberg and Dudas
2001).
The highest percentage of Hg
NaOH in the total content of mercury was determined in surface horizons of the studied soils. The share decreased with the depth in the soil profiles. Such Hg
NaOH distribution was mainly connected with the content of organic carbon in profiles of soil horizon, which is confirmed by a significant positive correlation coefficient between this form of mercury and total organic carbon content (
r = 0.77;
p < 0.05, Table
3). A rather low percentage content of mercury bound with organic matter in enrichment horizons (Bt) of
Luvisols (profiles 1, 2 and 3) could be a result of formation of Fe-clay complexes responsible for binding mercury in these horizons, and not of greater Fe-humus-clay or humus-clay complexes (Dmytriw et al.
1995; Schlüter
1997).
In
Fluvisols, the percentage of Hg
NaOH in the total content of mercury in separate layers, despite its varied concentration, was relatively even. An exception was the surface horizon of
Endogleyic Fluvisol (profile 5), in which the percentage of this mercury form (2.34 %) was the lowest in all of the horizons of the examined soils, showing, however, high values in the remaining subsurface horizons (40.56–48.80 %). Also, a considerably lower percentage of Hg
NaOH as compared with the remaining part of the profile was determined in the deepest horizon of the
Eutric Fluvisol (31.59 %), with content in the remaining part of the profile of 49.0–54.56 % (profile 4). This was most probably due to different texture of these horizons in comparison with the rest of the profile (especially varied content of clay fraction, Table
1), or the difference in humus composition (Boszke et al.
2004; Wallschläger et al.
1998b).
After statistical analysis, correlations between the content of Hg
NaOH and the content of clay as well as free (Fe
d) and amorphous (Fe
o) ferric oxides (
r = 0.40,
r = 0.44,
r = 0.74;
p < 0.05, respectively, Table
3), has been confirmed. This may be due to the fact that during extraction procedure (1 M NaOH), the solution contained exchangeable forms of mercury, bound to these soil elements (Wang et al.
1997). Even taking this into account, their percentage in soils does not usually exceed 3 % of the total content of mercury (Panyametheekul
2004). Moreover, the application of 1 M NaOH solution, in comparison with other reagents used for extraction of typical humus fractions (e.g. Na
4P
2O
7), allows us to assess mercury bound to organic matter, obtaining by this way the fraction more similar to the actual content of this Hg form in soil (Hall and Pelchat
1997; Schnitzer and Khan
1978).
The content of Hg
NaOH forms was significantly, positively correlated with cation exchange capacity (
r = 0.66;
p < 0.05, Table
3), which is linked with high sorption capacity of organic matter (Gabriel and Williamson
2004) and high affinity of Hg to functional groups containing sulphur (Kabata-Pendias and Pendias
2000; Skyllberg et al.
2003; Xia et al.
1998) proved in their research that from 50 to 70 % of the total sulphur content in soils was included in the functional groups (mainly thiol) of organic matter, which bind metallic mercury as well as alkyl compounds very easily.