Bull Environ Contam Toxicol DOI 10.1007/s00128-015-1530-8
Magnesium Contamination in Soil at a Magnesite Mining Region of Liaoning Province, China Lei Wang1,2,3 • Peidong Tai1 • Chunyun Jia1 • Xiaojun Li1 • Peijun Li1 Xianzhe Xiong4
•
Received: 23 October 2014 / Accepted: 23 March 2015 Ó Springer Science+Business Media New York 2015
Abstract Magnesite is the world’s most important source material for magnesia refractory production, and Haicheng City in Liaoning Province, China has been called ‘‘the magnesium capital of the world.’’ However, magnesite mining in these areas has caused serious environmental problems. Field investigations have shown that the soil profile of many sites in the mining region are contaminated by magnesium, and the magnesium-enriched crusts that have formed on the soil surface have affected ecologically important soil functions, particularly reduced water penetration rate. Laboratory experiment revealed that anionic polyacrylamide and calcium dihydrogen phosphate can be used to improve soil condition, and have positive effects on soil function. The findings of this study are of significance in the magnetite mining areas, providing clear options for the remediation of soils that should be carried out immediately. Keywords Magnesium Contamination Magnesite Crust Distribution China
& Chunyun Jia [email protected] & Xianzhe Xiong [email protected] 1
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Shenyang Academies of Environmental Sciences, Shenyang 110016, China
4
Department of Resource Management and Geography, The University of Melbourne, Melbourne 3051, Australia
Magnesite is the world’s most important raw material source for magnesia refractory production. Magnesite reserves in China are estimated at 3.05 9 109 t, accounting for the 1/4 of the total world reserve, with a large proportion of the reserve in Liaoning Province. Unfortunately, the random approaches taken in magnesite mining and the low technologies used in magnesite areas have caused not only a great loss of magnesite material, but also an apparently intractable ecological disaster (Kautz et al. 2001; Singh et al. 2003). In the magnesite mining areas of Liaoning Province in China, the dust emissions from mining enterprises are enormous, with some of the mining entities completely ignoring environmental protection, thus bringing about significant destruction to the vegetation, landscape and water bodies. The soils in the magnesite mining areas are covered with magnesium (Mg) containing dust that in turn increases the content of magnesium in the soil. The dusts have Mg in a range of forms, some of which can also form crusts through series of sedimentary and chemical processes. In the mining area of Haicheng, Liaoning Province about 7.8 9 104 ha is covered with such crusts, of which 4.9 9 104 ha is arable land (Zhou et al. 1995). The main component of the dust in these mining areas is magnesium oxide (MgO), which can react with carbon dioxide and water in air and soil, changing into magnesium carbonate (MgCO3) and magnesium hydroxide (Mg(OH)2), and in turn causing soil pH to increase. The high concentrations of magnesium in soil profile combined with the crusts on the soil surface have a marked negative effect on the soil hydrological cycle, leading to reduce penetration and to increase rainfall runoff, in turn affecting wider ecoenvironmental processes. Environmental department in China have now attached great importance to the rehabilitation for the magnesite
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mining areas in Liaoning Province. Land remediation methods include a range of physical, biological and chemical approaches. Physical methods include earth removal, earth replacement and deep ploughing. These can effectively reduce the soil contamination, but are costly. Biological methods involve plant restoration and microbial restoration, but are quite slow, with some taking years to show a positive effect. In reality, the choice of soil remediation method depends on local conditions, and in some cases it may be most effective to use chemical methods in situ. Compared to physical and biological methods, the chemical methods can be much lower in cost and faster in effect. Consequently, in recognition of the threat posed by magnesium on soil and aquatic ecosystem health, this study was initiated to explore options for chemical amelioration of Mg-induced soil crusting. This study aims to characterize the effects of the magnesium-containing dusts on the soils impacted by the magnesite industry; to examine the composition and formation of crust in soil surface; and to examine two possible chemical remediation methods which would allow rehabilitation of the areas by reducing the soil encrustation and increasing soil permeation. The specific objectives, therefore, were to determine the physicochemical characteristics of soil profiles, to examine the process of soil encrustation and to investigate the effectiveness of two soil improvers through laboratory simulation studies for the magnesite mining in China.
Fig. 1 Location of the study area and crust on soil surface in the sapling site
20 cm of the soil at a site in an agricultural area in Haicheng. A total of six crust samples were collected. The analytical methods for soil and crust used in this study as follows: (a)
Materials and Methods This study was conducted in two magnesite mining areas: Pailou Town and Bali Town in Haicheng City, Liaoning Province, in northeast China. Haicheng City has the reputation of being the ‘‘magnesium capital of the world,’’ and is located in the south of Liaoning Province (122°180 – 123°080 E, 40°290 –41°110 N). The magnesite mines have been in operation for more than 50 years and have reserves of more than 860 million tons. The soil in Pailou town is burozem (Institute of Forestry and Soil 1980), which is classified as Hapludalfs of Udalfs in the order of Alfisols (Soil Survey Staff 1975; Xiao 1992). Around the mining area there are many calcination factories, which have been releasing a variety of environmental contaminants, especially dust to the soil surface, which has crusted and resulted in a significant amount of farmland being abandoned and crop yields being reduced. The soil and crust were sampled at P1 and P2 (in Pailou Town), and crust also was sampled at B1 (in Bali Town) at Haicheng City in Liaoning Province (Fig. 1). At P1, soil samples were taken through the profile up to a depth 40 cm; at P2, samples were taken up to a depth 100 cm. In addition a reference soil was collected from the upper
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(b)
(c)
(d)
(e)
The C, N compounds and available P were determined by national standard method NY/T1848-2010. The pH was determined by the agriculture standard method of People’s Republic of China NY/T13782007, using a Leici PHS-3B pH meter. The soil mechanical composition was determined by national standard method NY/T1121.3-2006. Morphology and microstructure of crust was observed by scanning electron microscopy (SEM). The crust sample size was made to 1 9 1 cm, as thin as the SEM required, analyzed by SEM (S-3400, Rigaku Corporation). The elemental composition of crust samples was analyzed by X-ray fluorescence spectroscopy (XRF; Model 2SX100e, Rigaku Corporation). The phase composition of crust samples was determined by using X-ray diffraction spectroscopy (XRD; Rigaku D/Max 2500pc, Rigaku Corporation, Japan). The diffraction patterns were analyzed using the software Jade 5.0 (Materials Data Incorporated, Livermore CA, USA). Figure 2 The soil improvement simulation tower (Fig. 2) was made from a plexiglass column (i.e. 5 cm; height 30 cm) fitted with a base made from a perforated plexiglass plate covered with two layers
Bull Environ Contam Toxicol
permeates up to Q (s); h: the thickness of the water in test, i.e. water head (water level difference) in cm.
Results and Discussion
Fig. 2 Design of soil improvement simulation tower
of grade 43, quantitative filter paper. The variablespeed pump delivered water at a rate of 0.32–26.17 mL min-1. In the experiment, the reference soil was drying naturally, and ground through a 2 mm sieve. For the control, reference soil 405 g is transferred into the plexiglass to form column of height 25 cm with bulk density of 1.10–1.15 g cm-3. For treatment 1, 100 g of reference soil mixed with 5.0 g LBM (light burned magnesia, containing 97 %, MgO with a particle size about 5 lm) was spread on top of 20 cm of reference soil, making the total height also was 25 cm. Treatment 1 imitates the natural conditions in the magnesite mining area. Treatments 2 and 3 had an additional 0.04 g anionic polyacrylamide (PAM) or 4 g Ca(H2PO4)2 added, respectively, to measure the effect of chemical improvers on the penetration rate. In order to simulate natural conditions, the soil column was subjected to alternating wet-dry cycles. Distilled water was added to the soil column until the entire column was saturated. After 12 h saturation, the water level was increased until there was approximately 5 cm of water above the soil surface. Thereafter, the water was allowed to drain from the column. After the first leaching, the soil columns were placed outside to dry naturally for 1 week, after which they underwent two further leaching/drying cycles. The volume of leachate was measured 17 times over 350 min for each leaching. The experiment was repeated three times. The soil penetration rate, K, was calculated according to Darcy’s law as Eq. 1: K¼
Ql Sth
ð1Þ
where Q: the flow; l: the thickness of saturated soil, i.e. penetration distance (cm); S: cross-sectional area of the soil column (cm2); t: the time required for the water volume
The soil Mg concentration was very high in the surface layer (Table 1, up to 33.8–38.4 g kg-1), which is comparable with Fu et al. (2009). Soil pH, especially in the upper 40 cm was also high up to 8.00–8.48, and even below 40 cm the pH was still close to 8.0. This high pH is consistent with Machin and Navas (2000). The total carbon distribution decreased down the soil profile. Currently there are no formal criteria that can be used to evaluate the seriousness of Mg contamination in soil; therefore we have to use the background Mg value for comparison purposes. The available information is that mean background concentration value of Mg in soil was 9.25 g kg-1 in Liaoning Province, and 10.00 g kg-1 in Haicheng City (Liu et al. 1998). In Pailou town, the soil depth only below 40 cm, the Mg contaminations close to the background concentrations, but soil layers above 40 cm in profile were polluted with Mg at up to 4 times the level observed in the normal soil. The increased Mg in soil has affected the element balance of the soil, and the content of soluble calcium and available phosphorus has decreased. Obviously, compared to the natural background, the Mg contamination in the soil in the mining area is severe. The mechanical composition in P1 at 0–20 cm soil profile, sand (2.0–0.05 mm), silt (0.05–0.002 mm) and clay (\0.002 mm) occupied 50.01 %, 35.36 % and 12.33 %, respectively (2.30 % particles lost in the treatment). In P2 the percentage of sand, silt and clay were 44.15 %, 39.91 % and 14.02 %, respectively (1.92 % particles lost in the treatment). The soil texture of P1 and P2 are classified as loam that is the one of twelve major soil texture classifications are defined by the USDA (Soil Survey Division Staff 1993). The clay minerals are dominated by quartz, containing a small amount of high mountain stone, mica and palygorskite (Juan et al. 2010). Soil crusting is widespread in the magnesite mining areas. Crust thickness varied in the 6 crust samples collected in Pailou and Bali Town, but it was generally relatively thin at less than 1 cm. The pH of crusts were 9.83–9.86; i.e. higher than the pH in soil surface. The relative content (wt%) of the different elements was calculated as the oxide form only, regardless of the actual elemental forms. For example the MgO content is 51.8–78.5 wt% (Table 2), although Mg in fact presents as MgO, MgCO3, Mg(OH)2 and indeed other forms. Magnesium was the dominant element in the crust (can up to 51.8–78.5 wt%), with Si as the second ranked element. The sedimentary crust leads to serious contamination
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Bull Environ Contam Toxicol Table 1 Basic physical and chemical properties of soil in sampling site Profile (cm)
Total (C, %)
Inorganic (C, %)
Organic (C, %)
Total (N, %)
Available (P, mg/100 g)
Surface area (BET, m2 g-1)
pH
Total (Mg, g kg-1)
P1 N: 40°430 4300 E: 122°470 1800 0–5
1.69
0.05
1.64
0.13
2.44
90.35
8.31
38.4
5–10
0.78
0.01
0.77
0.05
2.95
85.49
8.16
15.8
10–20
0.76
0.01
0.74
0.05
2.14
74.95
8.08
17.8
20–30
0.85
0.01
0.84
0.05
2.00
82.67
8.12
23.5
30–40
0.80
0.05
0.75
0.03
2.41
98.37
8.15
15.8
P2 N: 40°45 50 E: 122°50 55 0–5 1.77 0.08
0
00
0
00
1.69
0.08
2.38
108.35
8.48
33.8
5–10
1.55
0.01
1.54
0.09
2.29
97.37
8.15
21.9
10–20
0.73
0.04
0.69
0.07
2.33
84.95
8.10
14.3
20–40
0.62
0.04
0.58
0.04
2.60
82.48
8.00
7.9
40–60
0.62
0.05
0.57
0.04
2.13
78.37
7.79
9.5
60–80
0.64
0.04
0.60
0.04
2.49
78.15
7.75
10.3
80–100
0.59
0.01
0.58
0.04
2.88
76.94
7.70
9.4
Table 2 The content of compound in crust (wt%)
Compound
MgO
Al2O3
CaO
Fe2O3
wt%
51.8–78.5
10.4–35.0
3.03–7.59
3.48–4.69
2.75–5.33
Compound
K2O
SO3
P2O5
TiO2
MnO
wt% Compound
0.43–1.76 ZnO
0.23–0.73 Na2O
0.14–0.36 SrO
0.32–0.53 NiO
0.13–0.19 F
wt%
0.03–0.04
0.29–0.37
0.02
0.09
0.58
not only in the soil surface, but also the lower soil profiles. The Fig. 3 showed the SEM photographs of Mg compounds, the morphology and microstructure are same with Liu et al. (2011), Lu et al. (2011) and Zhang et al. (2009). Some of crust may be divided into two or three layers that can be distinguished by different color and textures; this layering is due to long time of its formation (Rengasamy 1983). One crust sample collected at B1 was thicker and had three layers, each with their own characteristics of chemistry and morphology shown in Table 3. The first layer of crust was pale color with looser texture. The main components are MgO and MgCO3 at
A
SiO2
B
58.83 % and 30.59 % of the weight percentage respectively. The main component of the dust powder discharged from magnesite calcination plants is MgO (Machin and Navas 2000; Kautz et al. 2001). The chemical reactivity of MgO is high; in particular MgO from light burned magnesia, and easily forms MgCO3 by absorbing CO2 in air (Birchal et al. 2000). In addition, MgCO3 (the ore) in the dust from the quarry roads can also be incorporated into wind-blown dust. The middle layer of crust was white in color with tighter textures. It has more MgCO3 up to 41.78 % shown in Table 3, because MgO dust settled down to the soil surface
C
Fig. 3 SEM images for Mg compounds by 10,000 times. a MgO with compactly platy. b MgCO3 with petaloid. c Subcarbonate with cluster of petaloid
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Bull Environ Contam Toxicol
Layer
MgO
MgCO3
4MgCO3Mg(OH)24H2O
Mg(OH)2
SiO2
Surface
58.9 ± 2.3
30.6 ± 5.8
3.1 ± 1.4
4.8 ± 1.5
2.7 ± 0.2
Middle
28.7 ± 7.1
41.8 ± 11.1
5.6 ± 2.6
21.7 ± 3.4
2.2 ± 0.5
Bottom
24.2 ± 4.8
27.2 ± 6.1
0.7 ± 0.3
45.4 ± 6.1
2.6 ± 0.6
and reacted with the CO2 in the air, forming MgCO3, which continued to migrate from the surface layer into the middle layer (Birchal et al. 2000), meanwhile, new MgO dust continued to settle down on the surface layer. The bottom layer of crust was lying immediately above the soil surface; it appears to have been affected by both the soil surface and the middle layer. It was pale color with looser texture and bigger particles. This layer of crust mainly consisted of alkaline magnesium carbonate (subcarbonate) up to 45.38 % of mean weight percentage, and with less MgO and MgCO3. In the bottom layer, the subcarbonate is formed as: 4MgCO3 þ MgðOHÞ2 þ 4H2 O ! 4MgCO3 MgðOHÞ2 4H2 O
2.2
1.8 1.6 1.4
y=2.40x -0.1019 R 2 =0.78
1.2 1.0
y=1.27x -0.0364 R 2 =0.80
0.8
ð2Þ
0
50
100
150
200
250
300
350
Time (min) Fig. 4 Penetration rate of reference (control) soil
1.8
y=2.22x -0.096 R2 =0.78
1.6
Penetration rate (mm/min)
The water required to complete the reaction was mainly derived from the soil by capillary action, although some may also have come from the middle layer. Crust formation is initially a sedimentary process, but is also accompanied by complex chemical reactions. The following is discussing the soil treatment and the column experiments. The aqueous penetration rate is one of most important functions in soil; a poor penetration rate has a markedly negative effect on soil ecology and agriculture. In the control trial there was some difference in penetration rates. For instance, in the first leaching, K went down from the 1.85 mm min-1 at the start to 1.21 mm min-1 at the end (Fig. 4); during the second leaching K was lower, but very stable. However, in the third leaching K was higher initially at the beginning (2.14 mm min-1) falling to 1.55 mm min-1 at end. Overall, in the control the K value was than 1.0 mm min-1. This result is consistent with Levy et al. (1988), Dontsova and Norton (2002), and Yu et al. (2003). The behavior of K for Treatment 1 (LBM added), first leaching has some similar to the control with the K declining from the 1.77 mm min-1 at beginning to 1.15 mm min-1 at end. However, K decreased significantly with the dry-wet cycles, being 0.78 and 0.41 mm min-1 for second and third leaching respectively (Fig. 5). The soil column experiment clearly demonstrated that the settling of magnesia dust has decreased the penetration rate significantly. The aqueous penetration rate (K) is low in the mining area, which promotes surface runoff damaging the ecoenvironment and also impacting upon agricultural production, especially during sowing and affecting seedling growth. In view of the low available phosphorus in the
First leaching Second leaching Third leaching
y=2.38x -0.0716 R 2 =0.89
2.0
Penetration rate (mm/min)
Table 3 The mean weight percentage of main components in the different layers of crust
First leaching Second leaching Third leaching
1.4 1.2
y=0.78x -0.021 R2 =0.93
0.6
y=0.42e 0.11/(x-4.35) R2 =0.63 0.4
0
50
100
150
200
250
300
350
Time (min) Fig. 5 Treatment 1 penetration rates (added LBM) of soil column experiment
magnesite mining soil of this studies (only about 2 mg/ 100 g in Table 1) and similar other report (Zhou et al. 1995), Ca(H2PO4)2 and anionic PAM were trialled as soil condition improvers (Miller 1987; Laird 1997). In Treatment 2 when 0.04 g. In Treatment 2 when 0.04 g anionic PAM was added into the mixture of the soil with LBM, although K was very
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Bull Environ Contam Toxicol
1.6
Penetration rate (mm/min)
1.4
First leaching Second leaching Third leaching
y=1.54x-0.052 R 2 =0.48
First leaching Second leaching Third leaching
1.2
y=1.06x-0.034 R2 =0.92
1.0
Penetration rate (mm/min)
low during the first leaching i.e. 1.18 mm min-1 at 5 min, and decreased sharply to 0.2 mm min-1 at 35 min in Fig. 6, but K was improved after the wetting and drying cycle with both the initial and final penetration rates increasing to more than 1.20 mm min-1 for both of second and third leaching. A similar effect of PAM was reported in Laird (1997). Even though the final penetration rate in Treatment 2 didn’t reach the penetration rate of the Control (1.85 mm min-1), there was still a marked (threefold) improvement compared with Treatment 1 (about 0.41 mm min-1 during third leaching), which strongly suggests that PAM eventually has a strong, positive effect on the soil penetration rate. In the first leaching the initial negative effect on penetration rate is explained by the anionic PAM having a negative charge that is similar to the surface of soil clays. The molecular weight of PAM is up to 8,000,000, and it has a strong cohesion to soil particles. Once the PAM is applied into the soil, the soil penetration rate slows due to swelling of the PAM as it hydrates. However, after drying out, soil particles have been pushed aside, leaving many micro channels for the water penetration. When 4 g Ca(H2PO4)2 was added into the mixture of soil with LBM, at first K was extremely variable (Fig. 7). However after the drying and wetting cycles, the K increased. The second leaching produced the best penetration rate, which stabilized at around 0.9 mm min-1; after the third leaching the final penetration rate was approximately 0.65 mm min-1. Overall, K in Treatment 3 was much better than the Treatment 1. The PAM and Ca(H2PO4)2 produced a similar improvement for K, but the functional mechanisms are
0.8
y=1.11x-0.107 R 2 =0.80
0.6
0.4
y=1.41x-0.524 R2 =0.72 0.2
0.0 0
50
100
150
200
250
300
350
400
Time (min) Fig. 7 Treatment 3: the effect of Ca(H2PO4)2 on soil penetration rate
different (Yu et al. 2003). For instance, the generally accepted explanation for Ca(H2PO4)2 is that this compound has a phosphorus atom and two oxygen atoms that share a double bond, and therefore a coordination reaction between the double bond and magnesium ion can occur in solution (Deng and Zhang 1999). In the magnesite area, the Mg contamination can lead the loss of some available elements, especially phosphorus (Zhou et al. 1995). Consequently, the application of the Ca(H2PO4)2 in the magnesite mining areas may not only increase the soil penetration rate, but also supply soil with phosphorus, which in turn may increase agricultural outputs. Acknowledgments This work was financially supported by the twelfth Five-year national science and technology support program (2012BAC13B03) and High Technology Development Project of Liaoning Provincial Industrial Special Resources.
1.2
References
y=1.21x-0.009 R 2 =0.38
1.0 0.8 0.6 0.4
y=3.94x-0.73 R 2 =0.94
0.2 0.0 0
50
100
150
200
250
300
350
Time (min) Fig. 6 Treatment 2: the effect of PAM on soil penetration rate
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Birchal VSS, Rocha SDF, Ciminelli VST (2000) The effect of magnesite calcinations conditions on magnesia hydration. Miner Eng 13:1629–1633 Deng DH, Zhang CM (1999) The formation mechanism of the hydrate phases in magnesium oxychloride cement. Cem Concr Res 29:1365–1371 Dontsova KM, Norton LD (2002) Clay dispersion, infiltration and erosion as influenced by exchangeable Ca and Mg. Soil Sci 167:184–193 Fu SS, Li PJ, Feng Q, Jia HY, Li XJ (2009) Advances in magnesium pollution and mechanisms in the soil of magnesite mine. Chin J Ecol 28:549–555 Institute of Forestry and Soil of Chinese Academic Sciences (1980) The soils in the northeast of China. Science Press, Beijing, pp 92–93
Bull Environ Contam Toxicol Juan Y, Wu ZJ, Chen LJ, Wang R, Gong L, Bao HJ, Liu Y (2010) Preliminary characterization of clay minerals from four typical soils of Northeast China. Spectrosc Spectr Anal 30(7):1918–1921 Kautz G, Zimmer M, Zach P (2001) Suppression of soil microorganisms by emissions of a magnesite plant in the Slovak Republic. Water Air Soil Pollut 125:121–132 Laird DA (1997) Bonding between polyacrylamide and clay mineral surfaces. Soil Sci 162:826–832 Levy GJ, Van Der Watt HVH, Du P (1988) Effect of Na/Mg and Na/ Ca systems on soil hydraulic conductivity and infiltration rate. Soil Sci 146:303–310 Liu Q, Mao XN, Zhao X (1998) Soil pollution and its control strategies in eastern mountain areas of Liaoning Province. Chin J Appl Ecol 9:101–106 Liu XW, Fen YL, Li HR (2011) Preparation of basic magnesium carbonate and its thermal decomposition kinetics in air. J Cent South Univ Technol 18:1865–1870 Lu CY, Chen M, Li YY, Sun ZQ, Yu JK (2011) Preparation of highpurity magnesium Carbonate from low-grade magnesite. Min Metall Eng 31(1):51–53 Machin J, Navas A (2000) Soil pH changes induced by contamination by magnesium oxides dust. Land Degrad Dev 11:37–50 Miller WP (1987) Infiltration and soil loss of three gypsum-amended Ultisols under simulated rainfall. Soil Sci Soc Am J 51:1314–1320
Rengasamy P (1983) Clay dispersion in relation to changes in the electrolyte composition of dialysed red-brown earths. Eur J Soil Sci 34(4):723–732 Singh OV, Labana S, Pandey G (2003) Phytoremediation: an overview of metallic decontamination from soil. Appl Microbiol Biotechnol 61:405–412 Soil Survey Division Staff (1993) Soil survey manual. United States Department of Agriculture, Washington DC, pp 63–65 Soil Survey Staff (1975) Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. U.S. Government Printing Office, Washington, pp 95–133 Xiao D (1992) Alfisols and closely related soils in China. Chin Geogr Sci 2(1):18–29 Yu J, Lei T, Shainberg I, Mamedov AI, Levy GJ (2003) Infiltration and erosion in soils treated with dry PAM and gypsum. Soil Sci Soc Am J 67:630–636 Zhang SG, Xu L, Liu HC, Meng SH, Yang Y, Chen JP, Liu ZM (2009) Synthesis, characterization and catalytic performance of nest-like magnesia spheres. Chin J Catal 30(6):514–518 Zhou QX, Gibson CE, Zhu YM (1995) Evaluation of phosphorus bioavailability in sediments of three contrasting lakes in China and the UK. Chemosphere 42:221–225
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Magnesium Contamination in Soil at a Magnesite Mining Region of Liaoning Province, China Lei Wang1,2,3 • Peidong Tai1 • Chunyun Jia1 • Xiaojun Li1 • Peijun Li1 Xianzhe Xiong4
•
Received: 23 October 2014 / Accepted: 23 March 2015 Ó Springer Science+Business Media New York 2015
Abstract Magnesite is the world’s most important source material for magnesia refractory production, and Haicheng City in Liaoning Province, China has been called ‘‘the magnesium capital of the world.’’ However, magnesite mining in these areas has caused serious environmental problems. Field investigations have shown that the soil profile of many sites in the mining region are contaminated by magnesium, and the magnesium-enriched crusts that have formed on the soil surface have affected ecologically important soil functions, particularly reduced water penetration rate. Laboratory experiment revealed that anionic polyacrylamide and calcium dihydrogen phosphate can be used to improve soil condition, and have positive effects on soil function. The findings of this study are of significance in the magnetite mining areas, providing clear options for the remediation of soils that should be carried out immediately. Keywords Magnesium Contamination Magnesite Crust Distribution China
& Chunyun Jia [email protected] & Xianzhe Xiong [email protected] 1
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Shenyang Academies of Environmental Sciences, Shenyang 110016, China
4
Department of Resource Management and Geography, The University of Melbourne, Melbourne 3051, Australia
Magnesite is the world’s most important raw material source for magnesia refractory production. Magnesite reserves in China are estimated at 3.05 9 109 t, accounting for the 1/4 of the total world reserve, with a large proportion of the reserve in Liaoning Province. Unfortunately, the random approaches taken in magnesite mining and the low technologies used in magnesite areas have caused not only a great loss of magnesite material, but also an apparently intractable ecological disaster (Kautz et al. 2001; Singh et al. 2003). In the magnesite mining areas of Liaoning Province in China, the dust emissions from mining enterprises are enormous, with some of the mining entities completely ignoring environmental protection, thus bringing about significant destruction to the vegetation, landscape and water bodies. The soils in the magnesite mining areas are covered with magnesium (Mg) containing dust that in turn increases the content of magnesium in the soil. The dusts have Mg in a range of forms, some of which can also form crusts through series of sedimentary and chemical processes. In the mining area of Haicheng, Liaoning Province about 7.8 9 104 ha is covered with such crusts, of which 4.9 9 104 ha is arable land (Zhou et al. 1995). The main component of the dust in these mining areas is magnesium oxide (MgO), which can react with carbon dioxide and water in air and soil, changing into magnesium carbonate (MgCO3) and magnesium hydroxide (Mg(OH)2), and in turn causing soil pH to increase. The high concentrations of magnesium in soil profile combined with the crusts on the soil surface have a marked negative effect on the soil hydrological cycle, leading to reduce penetration and to increase rainfall runoff, in turn affecting wider ecoenvironmental processes. Environmental department in China have now attached great importance to the rehabilitation for the magnesite
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mining areas in Liaoning Province. Land remediation methods include a range of physical, biological and chemical approaches. Physical methods include earth removal, earth replacement and deep ploughing. These can effectively reduce the soil contamination, but are costly. Biological methods involve plant restoration and microbial restoration, but are quite slow, with some taking years to show a positive effect. In reality, the choice of soil remediation method depends on local conditions, and in some cases it may be most effective to use chemical methods in situ. Compared to physical and biological methods, the chemical methods can be much lower in cost and faster in effect. Consequently, in recognition of the threat posed by magnesium on soil and aquatic ecosystem health, this study was initiated to explore options for chemical amelioration of Mg-induced soil crusting. This study aims to characterize the effects of the magnesium-containing dusts on the soils impacted by the magnesite industry; to examine the composition and formation of crust in soil surface; and to examine two possible chemical remediation methods which would allow rehabilitation of the areas by reducing the soil encrustation and increasing soil permeation. The specific objectives, therefore, were to determine the physicochemical characteristics of soil profiles, to examine the process of soil encrustation and to investigate the effectiveness of two soil improvers through laboratory simulation studies for the magnesite mining in China.
Fig. 1 Location of the study area and crust on soil surface in the sapling site
20 cm of the soil at a site in an agricultural area in Haicheng. A total of six crust samples were collected. The analytical methods for soil and crust used in this study as follows: (a)
Materials and Methods This study was conducted in two magnesite mining areas: Pailou Town and Bali Town in Haicheng City, Liaoning Province, in northeast China. Haicheng City has the reputation of being the ‘‘magnesium capital of the world,’’ and is located in the south of Liaoning Province (122°180 – 123°080 E, 40°290 –41°110 N). The magnesite mines have been in operation for more than 50 years and have reserves of more than 860 million tons. The soil in Pailou town is burozem (Institute of Forestry and Soil 1980), which is classified as Hapludalfs of Udalfs in the order of Alfisols (Soil Survey Staff 1975; Xiao 1992). Around the mining area there are many calcination factories, which have been releasing a variety of environmental contaminants, especially dust to the soil surface, which has crusted and resulted in a significant amount of farmland being abandoned and crop yields being reduced. The soil and crust were sampled at P1 and P2 (in Pailou Town), and crust also was sampled at B1 (in Bali Town) at Haicheng City in Liaoning Province (Fig. 1). At P1, soil samples were taken through the profile up to a depth 40 cm; at P2, samples were taken up to a depth 100 cm. In addition a reference soil was collected from the upper
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(b)
(c)
(d)
(e)
The C, N compounds and available P were determined by national standard method NY/T1848-2010. The pH was determined by the agriculture standard method of People’s Republic of China NY/T13782007, using a Leici PHS-3B pH meter. The soil mechanical composition was determined by national standard method NY/T1121.3-2006. Morphology and microstructure of crust was observed by scanning electron microscopy (SEM). The crust sample size was made to 1 9 1 cm, as thin as the SEM required, analyzed by SEM (S-3400, Rigaku Corporation). The elemental composition of crust samples was analyzed by X-ray fluorescence spectroscopy (XRF; Model 2SX100e, Rigaku Corporation). The phase composition of crust samples was determined by using X-ray diffraction spectroscopy (XRD; Rigaku D/Max 2500pc, Rigaku Corporation, Japan). The diffraction patterns were analyzed using the software Jade 5.0 (Materials Data Incorporated, Livermore CA, USA). Figure 2 The soil improvement simulation tower (Fig. 2) was made from a plexiglass column (i.e. 5 cm; height 30 cm) fitted with a base made from a perforated plexiglass plate covered with two layers
Bull Environ Contam Toxicol
permeates up to Q (s); h: the thickness of the water in test, i.e. water head (water level difference) in cm.
Results and Discussion
Fig. 2 Design of soil improvement simulation tower
of grade 43, quantitative filter paper. The variablespeed pump delivered water at a rate of 0.32–26.17 mL min-1. In the experiment, the reference soil was drying naturally, and ground through a 2 mm sieve. For the control, reference soil 405 g is transferred into the plexiglass to form column of height 25 cm with bulk density of 1.10–1.15 g cm-3. For treatment 1, 100 g of reference soil mixed with 5.0 g LBM (light burned magnesia, containing 97 %, MgO with a particle size about 5 lm) was spread on top of 20 cm of reference soil, making the total height also was 25 cm. Treatment 1 imitates the natural conditions in the magnesite mining area. Treatments 2 and 3 had an additional 0.04 g anionic polyacrylamide (PAM) or 4 g Ca(H2PO4)2 added, respectively, to measure the effect of chemical improvers on the penetration rate. In order to simulate natural conditions, the soil column was subjected to alternating wet-dry cycles. Distilled water was added to the soil column until the entire column was saturated. After 12 h saturation, the water level was increased until there was approximately 5 cm of water above the soil surface. Thereafter, the water was allowed to drain from the column. After the first leaching, the soil columns were placed outside to dry naturally for 1 week, after which they underwent two further leaching/drying cycles. The volume of leachate was measured 17 times over 350 min for each leaching. The experiment was repeated three times. The soil penetration rate, K, was calculated according to Darcy’s law as Eq. 1: K¼
Ql Sth
ð1Þ
where Q: the flow; l: the thickness of saturated soil, i.e. penetration distance (cm); S: cross-sectional area of the soil column (cm2); t: the time required for the water volume
The soil Mg concentration was very high in the surface layer (Table 1, up to 33.8–38.4 g kg-1), which is comparable with Fu et al. (2009). Soil pH, especially in the upper 40 cm was also high up to 8.00–8.48, and even below 40 cm the pH was still close to 8.0. This high pH is consistent with Machin and Navas (2000). The total carbon distribution decreased down the soil profile. Currently there are no formal criteria that can be used to evaluate the seriousness of Mg contamination in soil; therefore we have to use the background Mg value for comparison purposes. The available information is that mean background concentration value of Mg in soil was 9.25 g kg-1 in Liaoning Province, and 10.00 g kg-1 in Haicheng City (Liu et al. 1998). In Pailou town, the soil depth only below 40 cm, the Mg contaminations close to the background concentrations, but soil layers above 40 cm in profile were polluted with Mg at up to 4 times the level observed in the normal soil. The increased Mg in soil has affected the element balance of the soil, and the content of soluble calcium and available phosphorus has decreased. Obviously, compared to the natural background, the Mg contamination in the soil in the mining area is severe. The mechanical composition in P1 at 0–20 cm soil profile, sand (2.0–0.05 mm), silt (0.05–0.002 mm) and clay (\0.002 mm) occupied 50.01 %, 35.36 % and 12.33 %, respectively (2.30 % particles lost in the treatment). In P2 the percentage of sand, silt and clay were 44.15 %, 39.91 % and 14.02 %, respectively (1.92 % particles lost in the treatment). The soil texture of P1 and P2 are classified as loam that is the one of twelve major soil texture classifications are defined by the USDA (Soil Survey Division Staff 1993). The clay minerals are dominated by quartz, containing a small amount of high mountain stone, mica and palygorskite (Juan et al. 2010). Soil crusting is widespread in the magnesite mining areas. Crust thickness varied in the 6 crust samples collected in Pailou and Bali Town, but it was generally relatively thin at less than 1 cm. The pH of crusts were 9.83–9.86; i.e. higher than the pH in soil surface. The relative content (wt%) of the different elements was calculated as the oxide form only, regardless of the actual elemental forms. For example the MgO content is 51.8–78.5 wt% (Table 2), although Mg in fact presents as MgO, MgCO3, Mg(OH)2 and indeed other forms. Magnesium was the dominant element in the crust (can up to 51.8–78.5 wt%), with Si as the second ranked element. The sedimentary crust leads to serious contamination
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Bull Environ Contam Toxicol Table 1 Basic physical and chemical properties of soil in sampling site Profile (cm)
Total (C, %)
Inorganic (C, %)
Organic (C, %)
Total (N, %)
Available (P, mg/100 g)
Surface area (BET, m2 g-1)
pH
Total (Mg, g kg-1)
P1 N: 40°430 4300 E: 122°470 1800 0–5
1.69
0.05
1.64
0.13
2.44
90.35
8.31
38.4
5–10
0.78
0.01
0.77
0.05
2.95
85.49
8.16
15.8
10–20
0.76
0.01
0.74
0.05
2.14
74.95
8.08
17.8
20–30
0.85
0.01
0.84
0.05
2.00
82.67
8.12
23.5
30–40
0.80
0.05
0.75
0.03
2.41
98.37
8.15
15.8
P2 N: 40°45 50 E: 122°50 55 0–5 1.77 0.08
0
00
0
00
1.69
0.08
2.38
108.35
8.48
33.8
5–10
1.55
0.01
1.54
0.09
2.29
97.37
8.15
21.9
10–20
0.73
0.04
0.69
0.07
2.33
84.95
8.10
14.3
20–40
0.62
0.04
0.58
0.04
2.60
82.48
8.00
7.9
40–60
0.62
0.05
0.57
0.04
2.13
78.37
7.79
9.5
60–80
0.64
0.04
0.60
0.04
2.49
78.15
7.75
10.3
80–100
0.59
0.01
0.58
0.04
2.88
76.94
7.70
9.4
Table 2 The content of compound in crust (wt%)
Compound
MgO
Al2O3
CaO
Fe2O3
wt%
51.8–78.5
10.4–35.0
3.03–7.59
3.48–4.69
2.75–5.33
Compound
K2O
SO3
P2O5
TiO2
MnO
wt% Compound
0.43–1.76 ZnO
0.23–0.73 Na2O
0.14–0.36 SrO
0.32–0.53 NiO
0.13–0.19 F
wt%
0.03–0.04
0.29–0.37
0.02
0.09
0.58
not only in the soil surface, but also the lower soil profiles. The Fig. 3 showed the SEM photographs of Mg compounds, the morphology and microstructure are same with Liu et al. (2011), Lu et al. (2011) and Zhang et al. (2009). Some of crust may be divided into two or three layers that can be distinguished by different color and textures; this layering is due to long time of its formation (Rengasamy 1983). One crust sample collected at B1 was thicker and had three layers, each with their own characteristics of chemistry and morphology shown in Table 3. The first layer of crust was pale color with looser texture. The main components are MgO and MgCO3 at
A
SiO2
B
58.83 % and 30.59 % of the weight percentage respectively. The main component of the dust powder discharged from magnesite calcination plants is MgO (Machin and Navas 2000; Kautz et al. 2001). The chemical reactivity of MgO is high; in particular MgO from light burned magnesia, and easily forms MgCO3 by absorbing CO2 in air (Birchal et al. 2000). In addition, MgCO3 (the ore) in the dust from the quarry roads can also be incorporated into wind-blown dust. The middle layer of crust was white in color with tighter textures. It has more MgCO3 up to 41.78 % shown in Table 3, because MgO dust settled down to the soil surface
C
Fig. 3 SEM images for Mg compounds by 10,000 times. a MgO with compactly platy. b MgCO3 with petaloid. c Subcarbonate with cluster of petaloid
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Bull Environ Contam Toxicol
Layer
MgO
MgCO3
4MgCO3Mg(OH)24H2O
Mg(OH)2
SiO2
Surface
58.9 ± 2.3
30.6 ± 5.8
3.1 ± 1.4
4.8 ± 1.5
2.7 ± 0.2
Middle
28.7 ± 7.1
41.8 ± 11.1
5.6 ± 2.6
21.7 ± 3.4
2.2 ± 0.5
Bottom
24.2 ± 4.8
27.2 ± 6.1
0.7 ± 0.3
45.4 ± 6.1
2.6 ± 0.6
and reacted with the CO2 in the air, forming MgCO3, which continued to migrate from the surface layer into the middle layer (Birchal et al. 2000), meanwhile, new MgO dust continued to settle down on the surface layer. The bottom layer of crust was lying immediately above the soil surface; it appears to have been affected by both the soil surface and the middle layer. It was pale color with looser texture and bigger particles. This layer of crust mainly consisted of alkaline magnesium carbonate (subcarbonate) up to 45.38 % of mean weight percentage, and with less MgO and MgCO3. In the bottom layer, the subcarbonate is formed as: 4MgCO3 þ MgðOHÞ2 þ 4H2 O ! 4MgCO3 MgðOHÞ2 4H2 O
2.2
1.8 1.6 1.4
y=2.40x -0.1019 R 2 =0.78
1.2 1.0
y=1.27x -0.0364 R 2 =0.80
0.8
ð2Þ
0
50
100
150
200
250
300
350
Time (min) Fig. 4 Penetration rate of reference (control) soil
1.8
y=2.22x -0.096 R2 =0.78
1.6
Penetration rate (mm/min)
The water required to complete the reaction was mainly derived from the soil by capillary action, although some may also have come from the middle layer. Crust formation is initially a sedimentary process, but is also accompanied by complex chemical reactions. The following is discussing the soil treatment and the column experiments. The aqueous penetration rate is one of most important functions in soil; a poor penetration rate has a markedly negative effect on soil ecology and agriculture. In the control trial there was some difference in penetration rates. For instance, in the first leaching, K went down from the 1.85 mm min-1 at the start to 1.21 mm min-1 at the end (Fig. 4); during the second leaching K was lower, but very stable. However, in the third leaching K was higher initially at the beginning (2.14 mm min-1) falling to 1.55 mm min-1 at end. Overall, in the control the K value was than 1.0 mm min-1. This result is consistent with Levy et al. (1988), Dontsova and Norton (2002), and Yu et al. (2003). The behavior of K for Treatment 1 (LBM added), first leaching has some similar to the control with the K declining from the 1.77 mm min-1 at beginning to 1.15 mm min-1 at end. However, K decreased significantly with the dry-wet cycles, being 0.78 and 0.41 mm min-1 for second and third leaching respectively (Fig. 5). The soil column experiment clearly demonstrated that the settling of magnesia dust has decreased the penetration rate significantly. The aqueous penetration rate (K) is low in the mining area, which promotes surface runoff damaging the ecoenvironment and also impacting upon agricultural production, especially during sowing and affecting seedling growth. In view of the low available phosphorus in the
First leaching Second leaching Third leaching
y=2.38x -0.0716 R 2 =0.89
2.0
Penetration rate (mm/min)
Table 3 The mean weight percentage of main components in the different layers of crust
First leaching Second leaching Third leaching
1.4 1.2
y=0.78x -0.021 R2 =0.93
0.6
y=0.42e 0.11/(x-4.35) R2 =0.63 0.4
0
50
100
150
200
250
300
350
Time (min) Fig. 5 Treatment 1 penetration rates (added LBM) of soil column experiment
magnesite mining soil of this studies (only about 2 mg/ 100 g in Table 1) and similar other report (Zhou et al. 1995), Ca(H2PO4)2 and anionic PAM were trialled as soil condition improvers (Miller 1987; Laird 1997). In Treatment 2 when 0.04 g. In Treatment 2 when 0.04 g anionic PAM was added into the mixture of the soil with LBM, although K was very
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Bull Environ Contam Toxicol
1.6
Penetration rate (mm/min)
1.4
First leaching Second leaching Third leaching
y=1.54x-0.052 R 2 =0.48
First leaching Second leaching Third leaching
1.2
y=1.06x-0.034 R2 =0.92
1.0
Penetration rate (mm/min)
low during the first leaching i.e. 1.18 mm min-1 at 5 min, and decreased sharply to 0.2 mm min-1 at 35 min in Fig. 6, but K was improved after the wetting and drying cycle with both the initial and final penetration rates increasing to more than 1.20 mm min-1 for both of second and third leaching. A similar effect of PAM was reported in Laird (1997). Even though the final penetration rate in Treatment 2 didn’t reach the penetration rate of the Control (1.85 mm min-1), there was still a marked (threefold) improvement compared with Treatment 1 (about 0.41 mm min-1 during third leaching), which strongly suggests that PAM eventually has a strong, positive effect on the soil penetration rate. In the first leaching the initial negative effect on penetration rate is explained by the anionic PAM having a negative charge that is similar to the surface of soil clays. The molecular weight of PAM is up to 8,000,000, and it has a strong cohesion to soil particles. Once the PAM is applied into the soil, the soil penetration rate slows due to swelling of the PAM as it hydrates. However, after drying out, soil particles have been pushed aside, leaving many micro channels for the water penetration. When 4 g Ca(H2PO4)2 was added into the mixture of soil with LBM, at first K was extremely variable (Fig. 7). However after the drying and wetting cycles, the K increased. The second leaching produced the best penetration rate, which stabilized at around 0.9 mm min-1; after the third leaching the final penetration rate was approximately 0.65 mm min-1. Overall, K in Treatment 3 was much better than the Treatment 1. The PAM and Ca(H2PO4)2 produced a similar improvement for K, but the functional mechanisms are
0.8
y=1.11x-0.107 R 2 =0.80
0.6
0.4
y=1.41x-0.524 R2 =0.72 0.2
0.0 0
50
100
150
200
250
300
350
400
Time (min) Fig. 7 Treatment 3: the effect of Ca(H2PO4)2 on soil penetration rate
different (Yu et al. 2003). For instance, the generally accepted explanation for Ca(H2PO4)2 is that this compound has a phosphorus atom and two oxygen atoms that share a double bond, and therefore a coordination reaction between the double bond and magnesium ion can occur in solution (Deng and Zhang 1999). In the magnesite area, the Mg contamination can lead the loss of some available elements, especially phosphorus (Zhou et al. 1995). Consequently, the application of the Ca(H2PO4)2 in the magnesite mining areas may not only increase the soil penetration rate, but also supply soil with phosphorus, which in turn may increase agricultural outputs. Acknowledgments This work was financially supported by the twelfth Five-year national science and technology support program (2012BAC13B03) and High Technology Development Project of Liaoning Provincial Industrial Special Resources.
1.2
References
y=1.21x-0.009 R 2 =0.38
1.0 0.8 0.6 0.4
y=3.94x-0.73 R 2 =0.94
0.2 0.0 0
50
100
150
200
250
300
350
Time (min) Fig. 6 Treatment 2: the effect of PAM on soil penetration rate
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