Water Air Soil Pollut (2015) 226: 245 DOI 10.1007/s11270-015-2508-1
The Effects of Modified Flue Gas Desulfurization Residue on Growth of Sweet Potato and Soil Amelioration Yi Wang & Lin Shi
Received: 7 January 2015 / Accepted: 22 June 2015 / Published online: 8 July 2015 # Springer International Publishing Switzerland 2015
Abstract We report on treatment and disposal of flue gas desulfurization (FGD) as a solid and hazardous waste. The effects of modified flue gas desulfurization residue (MFGDR) prepared by calcining a mixture of dry/semi-dry FGD residue, potassium feldspar, and/or limestone power on growth of plant and soil amelioration are investigated. The effect of MFGDR on the sweet potato was evaluated by analyzing the soil physiochemical properties and heavy metal speciation in the soil, and the yield, quality, and heavy metal concentrations of the sweet potato. The results indicated that applying MFGDR as soil ameliorant increased total yield by 53.38 %, safety, and the quality of sweet potato. The concentrations of Cd, Cr, Cu, Pb, and As in the sweet potato reduced by 31.34, 70.57, 22.17, 79.49, and 100 %, respectively. The improvements were attributed to enhancement of soil mineral composition contained in MFGDR. The MFGDR could also improve the soil physicochemical properties and decreased phytoavailability of heavy metals. The application of MFGDR in agriculture not only was a potential and useful technique for recycling and utilization of FGD Y. Wang : L. Shi (*) College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, People’s Republic of China e-mail:
[email protected] L. Shi The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China University of Technology, Guangzhou 510006, People’s Republic of China
residue, but also had potential benefits for soil amelioration, plant growth, and decrease of heavy metals in grown products. Keywords Modified flue gas desulfurization residue . Soil ameliorant . Speciation of heavy metals . Sweet potato . Soil physicochemical properties
1 Introduction Flue gas desulfurization (FGD) residue is generated by SO2 scrubbing process to meet the clean air regulations from coal-fired power plants (Wang et al. 2008). In China, with the associated rapid development of the energy and power industries, the installed capacity of power plants will reach up to 530 GW by 2020 as compared with 53 GW in 2005 and 200 GW in 2010 (Wang et al. 2008). Meanwhile, the annual production of FGD by-product increases rapidly and is predicted to reach 90 million tons by 2020 as compared with 6.5 million tons in 2005 and 40 million tons in 2010 (Wang et al. 2008). A significant quantity of FGD residue without any unitization or treatment may lead to a large land occupation and environmental pollution. The dry/ semi-dry FGD generally contains CaSO4·H2O (main component), CaSO3·1/2H2O, CaCO3, CaMg(CO3)2, Ca(OH)2, and a little coal fly ash (Bigham et al. 2005). It is usually alkaline (pH=7.7–10.03) because it also contains about 10 % alkali material (Wang et al. 2008), which may be suitable to serve as ameliorant for soil; FGD residue also contains some nutrients required for
245 Page 2 of 13
plant growth, such as Ca, Mg, and S (Shi et al. 2011), which are able to serve as supplement of nutrition components in soil. The applications of FGD residue in agriculture and soil have been extensively studied (He and Shi 2012; Punshon et al. 2001; Chen et al. 2001; Shi et al. 2011), since it represents a more efficient and useful technique for soil amelioration and management of FGD residue (Clark et al. 2001). Meanwhile, there are the largest insoluble K-feldspar reserves in China, with total storage reach up to 920 million tons (K2O) (Hu et al. 2005). However, the distinct situation of lacking potassium fertilizer exists around China due to the difficulty in translating K-feldspar into potassium fertilizer technologically and economically (Chai et al. 2010). A type of soil ameliorant, called modified FGD residue (MFGDR), is prepared by adding K-feldspar and/or limestone power into dry/semi-dry FGD residue, and the mixture is calcined at a certain temperature under oxidized circumstance. It may be a promising and useful technique for recycling and utilization of FGD residue, when the MFGDR was applied to agricultural use. In this paper, MFGDR was prepared by the method reported before (He and Shi 2012; Shi et al. 2011). MFGDR is rich in soil mineral composition and nutrient elements, which are the medium and trace elements required for plant growth and lacking in soil. Additionally, its mild alkaline may be suitable for soil amelioration. As a main staple food in many communities (Ila'Ava et al. 2000), sweet potato was applied as the model plant. In the test, the sweet potato was planted on the soil ameliorated with MFGDR. The growth of the sweet potato and soil amelioration were detected through experimental sequences. The measured data include (1) yield, quality, and concentrations of heavy metals of the sweet potato; (2) variations of the physicochemical properties of soil; and (3) variations of speciation of heavy metals. This paper therefore investigates the feasibility of using MFGDR as soil ameliorant and its effect on the growth of sweet potato and soil amelioration.
2 Materials and Methods 2.1 The Dry/Semi-Dry FGD Residue Sample and the Characteristic of the MFGDR Dry/semi-dry FGD residue was collected from Hengyun Power Plant, Guangzhou, People’s Republic of China,
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using a semi-dry desulfurization technology called spray drying absorption (SDA) system. This Kfeldspar was derived from Shandong Province, People’s Republic of China, which was crushed to about 0.5 mm in diameter, and some impurities such as quartz, magnetite, and hematite were manually picked out of the ore under a microscope, and the sorted K-feldspar was ground to 200 meshes (0.74 μm in diameter). The major content of the SDA residue and the K-feldspar power are present in Table 1. Other chemical limestone was also sieved to 200 meshes. In the dry/semi-dry FGD residue, the calcium sulfite hemihydrate (2CaSO3·1/ 2H2O) as main component could be oxidized to gypsum (CaSO4·2H2O) and finally translated into anhydrite (CaSO4) under the oxidizing circumstance. The limestone (CaCO3) could also be decomposed to lime power (CaO) at 837 °C. Finally, the dry/semi-dry FGD residue will be changed into a combination of CaSO4+CaO. The MFGDR was prepared with addition of K-feldspar according to Eqs. (1)–(5) (Shi et al. 2011). The concentrations of heavy metals and the nutrient content of MFGDR are presented in Tables 2 and 3. 1 2CaSO3 ⋅ H2 O þ O2 þ 3H2 O ¼ 2CaSO4 ⋅2H2 O 2
ð1Þ
2CaSO4 ⋅2H2 O ¼ 2CaSO4 þ 4H2 O↑
ð2Þ
CaCO3 ¼ CaO þ CO2 ↑
ð3Þ
Table 1 The major content of the SDA residue and the K-feldspar power (g/kg)
Elements
SDA residue
K-feldspar power
SO42−-S
16.4
–
SO32−-S
86.0
–
Ca
268
0.07
Al
41.4
81.6
Si
46.2
328
Fe
7.27
0.98
Mg
33.0
0.66
K
1.33
107
Na
1.63
6.31
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Table 2 Comparison between control standards of concentrations of heavy metals in fly ash for agricultural use (GB8173-87) and the measured value in the SDA residue (μg/kg) Elements
Control standards in soil
Measured value in the SDA residue
Total cadmium
5
0.12
Total arsenic
75
1.30
Total molybdenum
10
1.20
Total selenium
15
2.55
Total nickel
200
5.87
Total chromium
250
6.40
Total copper
250
12.6
Total lead
250
20.4
pH value
10
10.6
CaðOHÞ2 ¼ CaO þ H2 O↑
potato (Guangshu 111) was planted on the 29th July 2013 and harvested after 123 days. 2.3 The Sweet Potato Samples and Test Methods The average weight of sweet potatoes and weight of sweet potato per plant were determined by weighing 20 sweet potatoes and 5 plants selected from every plot at random after harvest, respectively. And then, the total yields were determined by weighing all sweet potatoes of each plot and the units were in ton per hectare. The yield of dry matter of each sample was determined by weighing the sweet potato (only root) dried at 70 °C for 48 h. The quality and concentrations of heavy metals in the sweet potato were determined according to the national standards (China) and industry standard (China) presented in Table 4.
ð4Þ 2.4 Soil and Test Methods
2KAlSi3 O8 þ CaSO4 þ 14CaO ¼ K2 SO4 þ Ca3 Al2 O6 þ 6Ca2 SiO4
ð5Þ
2.2 Design of Field Experiment The field experiments were conducted in the cropland field located at Shaoguan Institute for Agricultural Sciences (latitude 24° 47′ 51.05″, longitude 113° 31′ 57.73″ E), Guangdong Province, China. The type of soil in this experiment, known as crimson soil, is distributed widely in the southern China. Three treatments with three replicates were planted in randomized complete layout. Each plot was 40 m2 (2 m×20 m). Three treatments were compared in this study: (1) treated without any soil ameliorant (CK), (2) treated with a 750 kg/ha dosage of MFGDR in soil (T1), and (3) treated with a 1500 kg/ha dosage of MFGDR in soil (T2). The sweet
The physicochemical properties of the soil are presented in Table 5. The soil samples (0–30 cm) collected were characterized using a variety of analytical instruments. These sample concluded (1) before planting (CK-1) and after harvest (CK-2) without any soil ameliorant, (2) after 8 days (T1-1) and after harvest (T1-2) with a 750 kg/ha dosage of MFGDR in soil, and (3) after 8 days (T2-1) and after harvest (T2-2) with a 1500 kg/ha dosage of MFGDR in soil. The field water capacity was measured by Wilcox. The content of active aluminum dissolved in the soil was determined using the methods of chemical analysis to measure forest soil water content (LY/T1257-1999). The content of organic matters in soil was determined by means of potassium dichromate titration. The nutrient availabilities of N, P, and K were also measured using Stal-2 type soil analyzer. The available Ca, Mg, and Si were determined followed by Table 4 The national standards (China) and industry standard (China) of determination of qualities and heavy metals of foods
Table 3 The nutrient content of the MFGDR (wt.%) Elements
Water solubility
Citrus solubility (0.1 mol/L)
K2O
3.57
4.63
Elements
Standard
Elements
Standard
Cd
GB/T 5009.15-2003
Reducing sugars
GB/T 5009.7-2008
Total As
GB/T 5009.11-2003
Soluble sugar
NY/T 1278-2007 GB 5009.5-2010
CaO
2.83
35.7
Hg
GB/T 5009.17-2003
Protein
SiO2
0.23
22.7
Pb
GB/T 5009.12-2010
Starch
GB/T 5009.9-2008
MgO
0.12
7.46
Cr
GB/T 5009.123-2003
β-carotene
GB/T 5009.83-2003
SO42−-S
2.02
2.07
Cu
GB/T 5009.13-2003
Vitamin C
GB/T 5009.82-2003
245 Page 4 of 13 Table 5 The physicochemical properties of the soil
Water Air Soil Pollut (2015) 226: 245
Parameters
Value
pH
7.31
Organic matter (%)
2.35
(mg/kg) Available N
145.92
Available P
45.79
Available K
149.00
Available Mg
6265.00
Available Ca
109.75
Available Si
163.60
Activated Al
534.39
leaching procedures based on the previous study (Tessier et al. 1979) are presented in Table 7. Between each successive extraction, separation was conducted by centrifuging (Anke TDL-40B) at 4000 rpm for 30 min. The supernatant was removed with a pipet and filtered with 0.45-μm filter paper. And then the concentration of the heavy metals was determined by atomic absorption spectrophotometry (HITACHI Z-2000) involving direct aspiration of the aqueous solution into an air-acetylene flame and graphite furnace atomic absorption spectrometry (HITACHI Z-2000). 2.6 Statistical Analysis
Chinese agriculture standards: NY/T 2272–2012 and NY/T 2273–2012. The pH value was determined at water:soil ratio of 2.5:1 with PHS-25 type acidity instrument. The soil was digested with HNO3, HF, and HClO4, and then the concentrations of heavy metals in the digested solutions were determined by flame atomic absorption spectrophotometry (HITACHI Z-2000) and graphite furnace atomic absorption spectrometry (HITACHI Z-2000). The concentrations of Pb, Ni, Cu, Cr, and Cd of soil are presented in Table 6.
All the data in the study were statistically analyzed using ANOVA at a significance level of p<0.05 with SPSS version 21.0. Duncan’s test was used to determine the significant difference between the mean of different treatments.
3 Results and Discussion 3.1 Effect of MFGDR on the Growth of Sweet Potato
2.5 The Extraction of Speciation of Heavy Metals and Test Methods The air-dried soil was ground to 100 meshes (150 μm in diameter) for the extraction of speciation of heavy metals (Cd, Cr, Pb, Ni, Zn, and Cu). The speciation was divided into five fractions as followed: (1) exchangeable, (2) bound to carbonates, (3) bound to FeMn oxides, (4) bound to organic matter, and (5) residual (Tessier et al. 1979). The sum percentage of exchangeable fraction and bound to carbonates fraction was used for evaluation of bioavailability. Two grams of the original sample was used for the initial extraction. The reagents and operation conditions in each step of Table 6 The concentrations of heavy metals in soil
Heavy metals
Concentration (mg/kg)
Pb
41.76
Ni
18.52
Cu
20.29
Cr
46.41
Cd
0.40
3.1.1 The Yield of Sweet Potato The application of MFGDR had significant effects on the yield of sweet potato. Table 8 presents the growth and yield of sweet potato with the CK, T1, and T2 treatments. On the growth of sweet potato, it was evident that the average weight of the sweet potato, the weight of sweet potato per plant, and the total yield increased by 45.65–63.04, 50.00–61.90, and 40.09– 53.38 %, respectively, with increasing MFGDR, compared with the CK treatment. There was a significant content of nutrient elements contained in the MFGDR, which can promote to the plant growth and increase the yield (Chai et al. 2010; Wang et al. 2008; He and Shi 2012; Shi et al. 2011). The total yield of dry matter increased by 30.00 % (T1) and 48.92 % (T2); however, the dry matter ratio was reduced by almost 2 % in the soil with the T1 treatment and the T2 treatment dropped a little. The previous study showed a negative correlation between the potassium and dry matter ratio and suggests that the Bdilution^ effect caused by the potassium, which can promote enlargement of root, leads to a reduction in the dry matter ratio (Tang et al. 2011).
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Table 7 Leaching procedures and reagents Fraction
Reagent
Operating conditions
Exchangeable
16 mL 1 M NaOAc (pH=8.2 adjust with HOAc) Agitating continually (250 rpm) at room temperature for 1 h
Bound to carbonates
16 mL 1 M NaOAc (pH=5.0 adjust with HOAc) Agitating continually (250 rpm) at room temperature for 5 h
Bound to Fe-Mn oxides 40 mL 0.04 M HONH3Cl (in 25 %v/v HOAc, pH=2.0 adjust with NaOH)
Heating in water bath at 96±3 °C for 6 h (agitate once every half an hour)
Bound to organic matter 1) 6 mL 0.02 M HNO3 and 10 mL 30 % H2O2 (pH=2.0 adjust with HNO3); 2) 6 mL 30 % H2O2 (pH=2.0); 3) 10 mL 3.2 M NH4OAc (in v/v 20 % HNO3)
1) Heating in water bath at 85±3 °C for 2 h (agitate once every half an hour); 2) Heating in water bath at 85±3 °C for 3 h (agitate once every half an hour); 3) Diluting to 40 mL with deionized water and agitating continually (250 rpm) at room temperature for 30 min.
Residual
Digest at 180 °C until sediment disappeared and heating at 150 °C until the white smoke disappeared
20 mL HNO3, 20 mL HF, and 12 mL HClO3
Figure 1 presents the size distribution of the sweet potato (20 sweet potatoes were selected at random). According to the previous study (Zhou et al. 2011), the sizes of the sweet potatoes are distributed into 3 groups: 0–100 g (small), 100– 250 g (medium), and >250 g (large). It showed a distinct rising trend that the percentage of the medium-sized sweet potatoes increased to 40.0 % (T1) and 50.0 % (T2), as compared with 35.0 % without MFGDR treatment (CK), and the largesized ones increased to 22.5 % (T1) and 25.0 % (T2), as compared with 5.0 % (CK). The percentage of the small-sized ones decreased from 60.0 % (CK) to 37.5 % (T1) and 25.0 % (T2). It may indicate that increase of the total yield may be attributed to the enhancement of the size of the sweet potato rather than the total number of sweet potato. This may be attributed to the high content of the available potassium in MFGDR, which can promote the carbohydrate synthesis by photosynthesis in the leaf (Tang et al. 2011) and then transport them to the root leading to the development and enlargement of the root.
3.1.2 The Quality of Sweet Potato Application of MFGDR to the soil not only increases the yield of sweet potato, but also promotes the quality of sweet potato. The nutritional structure of sweet potato may be improved by increasing the medium and trace elements in soil due to addition of the MFGDR, which improve the quality of sweet potato. The raise in quality is able to enhance taste and provide more nutrients for human health. It could be observed that the average concentrations and the total yields per hectare of reducing sugars, soluble sugar, protein, starch, β-carotene, and vitamin C of sweet potato increased in different degrees with application of the MFGDR as summarized in Tables 9 and 10. It could be observed that, with the application of MFGDR, the β-carotene increased by 43.13–58.78 %, which is an indispensable micronutrient for human health, and the reducing sugars and soluble sugar increased slightly as well; whereas, the other elements slightly decreased due to the restriction of the nutrient uptake ability for plant growth by root and the Bdilution effect^ due to the rise in the total yield and size of sweet
Table 8 The effects of MFGDR on yield of sweet potato in different treatments Treatment Average weight of sweet Weight of sweet potato per Total yield (t/ha) Total yield of dry matter (t/ha) Dry matter ratio (%) potatoes (kg) plant (kg) CK
0.1150±0.02120b a
0.42±0.00b a
8.43±0.81b
2.265±0.0086c a
26.86±0.09a b
T1
0.1675±0.0035
0.63±0.07
11.81±2.35
2.9446±0.0699
24.93±0.54b
T2
0.1875±0.00353a
0.68±0.00a
12.93±1.14a
3.373±0.0533a
26.08±0.37ab
The same superscript letters in the table indicate no significant difference at a p<0.05 level
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Water Air Soil Pollut (2015) 226: 245
Fig. 1 The effects of MFGDR on size distributions of sweet potato in different treatments. The mean marked with the same letters indicates no significant difference at a p<0.05 level
70
Small Medium Large
a
60
b
b
50 a
b
40 30
b
b
b
20 10
a
0
potato (Madejón et al. 2006). However, the total contents of reducing sugars, soluble sugar, protein, starch, β-carotene, and vitamin C increased by 36.27, 56.61, 49.82, 36.60, 143.70, and 23.90 %, respectively, with the T2 treatment, as compared with CK treatments. It may indicate that MFGDR promotes the sweet potato to synthesize and generate more nutritional components, although the average concentrations of nutritional components decline slightly in sweet potato. 3.1.3 The Concentrations of Heavy Metals in the Sweet Potato The concentrations of heavy metals contained in sweet potato, presented in Fig. 2, decreased in different degrees as MFGDR increased, which was expected. Particularly, there almost was no As existed in the sweet potato with the T1 and T2 treatments as compared with 12.5 ppb without MFGDR treatment. And, the concentrations of Cd,
Cr, Cu, and Pb reduced by 31.34, 70.57, 22.17, and 79.49 %, respectively, with the T2 treatment, compared with the CK treatment. The concentration of Hg increased from 0.586 ppb (CK) to 0.872 ppb (T1), but it reduced by 79.01 % to 0.123 ppb (T2). MFGDR with a large amount of available silicon may increase Si content in root and reduce root absorption of heavy metals. It was reported that heavy metal deposition of silica in the vicinity of root endodermis might partially physically block the apoplast bypass flow across the roots (Shi et al. 2005). The heavy metal sequestration and detoxification are attributed to the chemical and physical effects of Si on forming co-precipitation with heavy metals and blocking metal transfer in plants (Gu et al. 2011). Si may play a significant role in the sweet potato tolerance to the heavy metal stress. In addition, the concentrations of heavy metals in the sweet potato may be diluted due to the increased biomass production (Hodge 2004).
Table 9 The average concentrations of qualities of sweet potato in different treatments Treatment Reducing sugars (g/kg)
Soluble sugar (g/kg)
Protein (g/kg)
Starch (g/kg)
β-Carotene (mg/kg)
Vitamin C (mg/kg)
CK
5.80±0.85a
57.75±0.92a
16.90±0.99a
187.00±12.73a
13.10±2.12a
1.38±0.21a
T1
6.00±1.41a
61.95±5.44a
12.30±0.00b
171.50±7.78a
18.75±2.83a
1.15±0.04a
T2
a
a
a
a
1.12±0.03a
5.15±0.21
58.95±2.33
16.50±0.00
a
166.50±14.85
The same superscript letters in the table indicate no significant difference at a p<0.05 level
20.80±0.21
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Table 10 The total content of qualities of sweet potato in different treatments Treatment
Reducing sugars (kg/ha)
Soluble sugar (kg/ha)
Protein (kg/ha)
Starch (kg/ha)
β-Carotene (g/ha)
Vitamin C (g/ha)
CK
48.8±7.1a
486.5±7.7b
142.3±8.3b
1575.4±107.2b
110.3±17.8b
11.63±1.7a
ab
13.58±0.3a
a
14.41±1.1a
a
T1
70.8±16.7
T2
a
66.5±2.7
a
731.5±64.2
a
761.9±30.1
b
145.2±0.0
a
213.2±0.0
a
2025.1±91.8
221.4±2.5 a
2152.0±191.9
268.8±65.8
The same superscript letters in the table indicate no significant difference at a p<0.05 level
3.2 Soil Amelioration The physicochemical properties of the soil after 8 days and after harvest with CK, T1, and T2 treatments are presented in Fig. 3 and Table 11. After harvest, the water field capacity increased from 21.87 % (CK-2) to 23.89 % (T1-2) and 22.79 % (T2-2). Generally, higher field water capacity can be conducive to plant growth and improve the production of the plant (Shi et al. 2011). After adding MFGDR for 8 days, soil pH could be considered fairly consistent in all cases. After the harvest, the soil pH value increased from 7.26 (T1-1) and 7.16 (T1-2) to 8.05 (T2-1) and 8.16 (T2-2) with application of MFGDR even though the initial soil was neutral. The CK treatment changed slightly. The addition of MFGDR caused a significant increase of soil pH due to the alkalinity of MFGDR (pH=9–11), and similar results were reported (He and Shi 2012; Shi et al. 2011). With application of MFGDR, the content of organic matter increased after adding MFGDR for 8 days and almost remained the same after harvest. It may indicate that the MFGDR improves the physicochemical properties and environment to the benefit of microorganism. The content of available elements in soil also increased significantly after adding MFGDR for 8 days. This result may be attributed to the plenty of nutrients K, Si, Mg, and S in the MFGDR and the enhancement of soil pH value. However, after harvest, the content of available elements decreased significantly as compared with the T1-1 and T1-2 treatments after adding MFGDR for 8 days, while the CK treatment showed an opposite trend that it increased a little except for the Si element. It may indicate that the MFGDR promotes the plant root absorption of more nutrient elements for meeting the increased total yield of the sweet potato leading to the reduction in the content of nutrient elements in soil, while the same amount of required chemical fertilizers are applied to the soil for cultivation of sweet potato in three treatments. In addition, the reduction of alkaline-N may also be due to the increase in soil pH value after
amelioration (Shi et al. 2011). The previous studies (ElBaky et al. 2010; George et al. 2002) suggested the increases in the β-carotene content with potassium application; therefore, the β-carotene in sweet potato increases significantly (Table 9) because the MFGDR increased the available potassium in the soil. Especially, Si is beneficial and likely essential to the crop healthy growth. It may help increase the resistance to toxic metals due to stimulation of antioxidant systems, alleviation of inhibition to photosynthesis, and complexation of heavy metals with silicate (Gu et al. 2011). However, the mechanisms involved in the effects of Si on crop growth are not so clear. It can also improve photosynthetic efficiency and promote plant root growth and also make the plant stronger to resist disease and pests (Liu and Zhang 2001; Jones and Handreck 1967; Ding et al. 2005). Therefore, it was critical to the production and quality of sweet potato (Tables 8 and 10 and Fig. 2). The data showed that the available Si in soil treated by MFGDR increased slightly after adding MFGDR for 8 days and increased significantly by 16.12–22.29 % after harvest as compared with the CK treatment, while the soil without MFGDR treatment decreased from 163.6 mg/kg (CK-1) to 148.78 mg/kg (CK-2). The results demonstrate that the MFGDR can meet the silicon element uptake of plant and enhance the silicon content level in the soil further. In conclusion, the physicochemical properties of soil have been improved significantly after MFGDR amelioration in this neutral soil and the data also shows that MFGDR can promote nutrient element uptake of plants from the soil. The toxic Al injuries the root meristem and inhibits the root growth and nutrient uptake (Wendell and Ritchey 1996), and it may cause the root defection and reduce the production of crop. It was decreased by 2.17–3.25 % and 8.66–9.47 % after adding MFGDR for 8 days and harvested as compared with the CK1-1 treatment; therefore, the activation and toxic effects of Al on the plant growth were alleviated. This effect is mainly attributed to the enhancement of pH value and SO42+ content upon
245 Page 8 of 13
Water Air Soil Pollut (2015) 226: 245
a 4
a a
Cd (ppb)
10
3
a
2
5 1 b
b
0
0 CK
T1
T2
CK
T1
T2
a
a
T1
T2
200 a
1.5
a
100
Cu (ppm)
150
b b
1.0
50
0.5
0
0.0 CK
T1
T2
CK a
a 0.9
150
Pb (ppb)
b 0.6
0.3
100 b 50
c 0.0
c
0
Fig. 2 The effects of MFGDR on concentrations of heavy metals in the sweet potato in different treatments. The mean marked with the same letters indicates no significant difference at a p<0.05 level
the addition of the MFGDR forming AlSO4+ (Wendell and Ritchey 1996; Shi et al. 2011). However, the activated Al in soil without application of MFGDR also decreased from 534.39 mg/kg (CK1-1) to 472.22 mg/kg (CK1-2), which was lower than others in any case. Therefore, it appears that more activated Al without
amelioration may be absorbed by the plant root in soil as compared with soil improved by applying MFGDR and results in the reduction of activated Al in soil, suggesting that the MFGDR also inhibits the plant root absorption of activated Al and enhances the plant tolerance to the Al toxic stress further.
Water Air Soil Pollut (2015) 226: 245
25
Page 9 of 13 245
a
b
c
20 15 10 5 0 Fig. 3 The effects of MFGDR on water field capacity in different treatments. The mean marked with the same letters indicates no significant difference at a p<0.05 level
3.3 Speciation Analysis on Heavy Metals in Soil The addition of MFGDR caused the redistribution of speciation of heavy metals in soil, resulting in the transformation of heavy metals towards more stable fraction. The sequential extraction for the speciation of heavy metals is used as measures of metal phytoavailability and readily labile metals in soils. The heavy metals, particularly the exchanged fraction, can be absorbed directly by the root. The carbonate fraction can also be absorbed by the root after dissolution with acid, which is secreted by the root (Li et al. 2009). Therefore, the phytoavailability of heavy metals, contained with the exchangeable, bound to carbonates, and evaluated by mobility factor (MF) (Wu 2010), was determined according to Eq. (6) and the content of residual fraction. F1, F2,
F3, F4, and F5 represent the fraction of exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual, respectively. Sequential extraction was used to investigate the distribution of speciation of heavy metals, which were associated with MFGDR in soil as shown in Fig. 1. The MFGDR had different effects on the reduction of the mobility and phytoavailability of heavy metals to decrease the concentrations of heavy metals in the sweet potato (Fig. 2). In general, the immobilization of heavy metals decreases their phytoavailability by the methods as follows: (1) enhancing pH value (Tica et al. 2011); (2) the mobile metals were mainly deposited as their silicates, phosphates, and hydroxides in amended treatments (Gu et al. 2011) and forming the precipitation as salts and coprecipitation (Kumpiene et al. 2008); and (3) the increased organic matter after the application of MFGDR may reduce metal availability and mobility through the redistribution of heavy metals (Shuman 1999) and formation of stable complexes with organic ligands (Kumpiene et al. 2008). The distributions of speciation of Cd, Pb, Cr, Ni, and Cu, presented in Fig. 4, were analyzed as follows: MF ¼
F1 þ F2 F1 þ F2 þ F3 þ F4 þ F5
ð6Þ
The majority of Cd was formed as carbonate-bound and Fe-Mn oxide-bound fraction, without exchanged fraction. The MF reduced to 34.20 % (T1-1) and 32.15 % (T2-1) after adding MFGDR for 8 days, as compared with the CK treatment (36.63 %). Similar trend could be observed after harvest. With application of MFGDR, the MF presented at 34.60 % (T1-2) and 32.75 % (T2-2) could be
Table 11 The effects of different treatments on the physicochemical properties of soil after 8 days and after harvest of adding MFGDR Elements
After adding MFGDR for 8 days CK1-1
pH Organic matter (%)
After harvest
T1-1
T2-1
CK1-2
7.30±0.01c
7.26±0.00c
7.16±0.05e
c
b
a
2.35±0.00
2.44±0.06
T1-2
7.16±0.04d bc
2.60±0.02
2.41±0.02
T2-2
8.05±0.00b
8.16±0.00a
b
2.65±0.01a
2.44±0.04
(mg/kg) Available N Available P Available K
145.92±0.49c d
179.00±0.98b c
45.79±2.98
63.99±3.79
180.05±0.49b
224.97±0.98a
122.58±1.97d
115.97±3.45e
b
a
cd
52.88±0.00cd
d
160.00±0.00d
d
113.10±3.32d
107.97±8.84
362.50±10.61
500.00±14.14
d
c
b
149.00±1.14
c
b
Available Mg
109.75±5.16
124.55±0.57
138.45±0.49
Available Si
163.60±3.99c
168.54±1.00bc
Activated Al
130.76±8.81
d
a
534.39±6.13
a
517.04±6.13
a
200.00±0.00
a
56.14±1.35
142.50±2.12
181.00±3.18
115.83±2.51
169.95±1.00bc
148.78±2.99d
172.77±1.00b
a
b
522.82±14.31
472.22±0.00
The same superscript letters in the table indicate no significant difference at a p<0.05 level
181.94±5.99a b
483.78±20.45
488.12±6.13b
245 Page 10 of 13
Water Air Soil Pollut (2015) 226: 245 Carbonates
Fe-Mn
100
100
80
80
60
60
Pb (%)
Cd (%)
Exchangeable
40 20
Residual
40 20
0
0 CK-1
T1-1
T2-1
CK-2
T1-2
T2-2
100
100
80
80
60
60
Ni (%)
Cr (%)
Organic
40 20
CK-1
T1-1
T2-1
CK-2
T1-2
T2-2
CK-1
T1-1
T2-1
CK-2
T1-2
T2-2
40 20
0
0 CK-1
T1-1
T2-1
CK-2
T1-2
T2-2
CK-1
T1-1
T2-1
CK-2
T1-2
T2-2
100
Cu (%)
80 60 40 20 0
Fig. 4 The effects of different treatments on distribution of heavy metals after 8 days and after harvest of adding MFGDR. The mean marked with the same letters indicates no significant difference at a p<0.05 level
stable, compared with CK treatment raised to 37.66 %. It indicates that MFGDR causes the conversion of Cd from carbonate-bound fraction into the Fe-Mn oxide-bound fraction leading to the reduction in the MF of Cd, and it also ensures long-term effectiveness in soil. As the pH of soil has risen over the point of zero charge of Fe-Mn oxide colloid, the surface of colloid changes from positively charged to negatively charged and it improves the
adsorption capacity of colloid (Ding et al. 2001). Consequently, it caused the Fe-Mn oxide-bound fraction of Cd increased in soil. Si contained in MFGDR also causes co-precipitation of Cd and Si in soil and coprecipitation of Si and Cd at root surfaces to decline the phytoavailability of Cd (Sarwar et al. 2010). The data also showed that the organic matter-bound and residual fraction could be considered fairly consistent in all cases.
Water Air Soil Pollut (2015) 226: 245
In terms of Pb, with the application of MFGDR to the soil, the Pb distribution was shifted from the carbonatebound, Fe-Mn oxide-bound, and organic matter-bound fractions in different degrees to less mobile and available chemical forms measured in residual fraction after adding MFGDR for 8 days. For example, after adding MFGDR for 8 days, the proportion of fraction bound to Fe-Mn oxides decreased from 35.64 % (CK-1) to 33.30 % (T11) and 28.88 % (T2-1) and the proportion of residual fraction increased from 47.36 % (CK-2) to 50.74 % (T12) and 57.12 % (T2-2), while the MF slightly declined with 2.80 % (CK), 2.34 % (T1-1), and 2.06 % (T2-1). The MFGDR may increase the available P and then result in the generation of residual fraction, since the P causes precipitation of a new discrete Pb mineral phase, which is stable and typically found in arable soils (Brown et al. 2005). However, part of residual fraction in T1 and T2 treatments was converted into Fe-Mn oxide-bound and organic matter-bound fraction after harvest, since high alkaline condition may cause a reverse trends on Pb stability due to the amphoteric nature of Pb (Heasman et al. 1997; Garcı et al. 2004), and Geebelen et al. (2003) suggests that the pH value of soil at 8.1 and below can increase the Pb leaching. Cr and Ni were mainly in the residual fraction form, without exchangeable and carbonate-bound fraction (MF=0). As a consequence, the Cr and Ni in the soil had little or no effect on the growth of sweet potato because of the stability of residual fraction and are unlikely to be adsorbed by the root. Cr stabilization is likely attributed to the low reactivity of its usual species found in soil—hydrous chromium oxide (Jiang et al. 2013; Kumpiene et al. 2008); however, as an alkaline material, the MFGDR increases the soil pH above neutral favoring the oxidation of Cr(III) to Cr(VI) and leading to enhancement of toxicity (Jiang et al. 2013, 2014; Seaman et al. 2001; Porter et al. 2004). After adding MFGDR for 8 days, the residual fraction of Cr decreased from 91.56 % (CK-1) to 91.30 % and 90.70 % (T2-1 and T1-2); however, after harvest, CK-2 treatment reduced to 91.10 % and the T1-2 and T2-2 was up to 91.54 and 91.50 %, respectively. The organic matterbound fraction presented an opposite trend to residual fraction. The other fractions remained relatively constant with increasing MFGDR. In the case of Ni, as the amount of MFGDR increased, it showed a moderate trend that the residual fraction, decreased from 84.41 % (CK) to 83.68 % (T1) and 82.16 % (T2), was converted partly into the Fe-Mn oxide-bound and
Page 11 of 13 245
organic matter-bound fraction after adding MFGDR for 8 days, but it was on the contrary after harvest and the residual fraction increased from 83.64 % (CK) to above 85 % (T1-2 and T2-2). It indicates that the enhancement of pH value after applying MFGDR may lead to increase the content of residual fraction of Ni. With increasing the usage of MFGDR, it showed a trend that the MF of Cu decreased slightly from 1.47 (CK) to 1.07 % (T1-2) and 0.69 % (T2-2) after harvest leading to a decline in phytoavailability of Cu in soil. According to the Kumpiene et al. (2008), the decreased phytoavailability of Cu may be attributed to the precipitation of Cu carbonates and oxy-hydroxides, ion exchange, and formation of ternary cation-anion (SO42−, PO43−) complexes on the surface of Fe and Al oxyhydroxides. Jackson and Miller (2000) found that addition of coal fly ash to soil enhanced soil pH and carbonates and was an efficient method to control mobility of Cu. The application of MFGDR also increased slightly carbonate-bound fraction from 0.90 % (CK-1) to 1.02 % (T1-2), and it decreased to 0.69 % (T2-2) after harvest. However, bits of residual fraction converted to organic matter-bound fraction after harvest as compared with the case after adding MFGDR for 8 days and the other fractions could be considered fairly consistent.
4 Conclusions In this study, as a new type of soil ameliorant, MFGDR contained more nutrient components and presented mild alkaline, which significantly affects the soil physicochemical properties and of heavy metal speciation in the soil and also has beneficial effects on the growth of sweet potato. The main conclusions are drawn as follows: (1) MFGDR had significant effects on the yield, quality, and concentrations of heavy metals in the sweet potato. It increased the average weight of the sweet potato, the sweet potato weight per plant, and the total yield by 45.65–63.04 %, 50.00–61.90 %, and 40.09–53.38 %, respectively. The total yield of dry matter per hectare was increased by 30.00–48.92 %. The size of the sweet potato was also enhanced, the proportion of medium-sized sweet potato increased from 35 to 45–50 %, and the large-sized ones increased from 5 to 22.5–25 % leading to the rise in the total yield. With the dosage of 1500 kg/ha MFGDR, the average concentration of β-carotene
245 Page 12 of 13
increased by 58.78 % and the other nutrients increased slightly or declined due to the Bdilution^ effect. However, the total content of reducing sugars, soluble sugar, protein, starch, β-carotene, and vitamin C per hectare were also increased by 36.27, 56.61, 49.82, 36.60, 143.70, and 23.90 %, respectively. The concentrations of Cd, Cr, Cu, Pb, and As in the sweet potato reduced by 31.34, 70.57, 22.17, 79.49, and 100 %, respectively. (2) The soil environment was improved by the enhancement of soil mineral composition contained in MFGDR, leading to variation of soil physicochemical properties, reduction in element loss, and increase of nutrient elements. The dosage of 1500 kg/ha MFGDR showed the best performance in soil amelioration among all the treatments. The pH and content of organic matter increased from 2.35 to 2.60 % and 7.36 to 8.16 %, respectively, and the field water capacity increased from 21.87 to 22.79 %. The content of available N, P, K, and Mg in soil increased significantly by 23.3, 107.97, 235.57, and 26.15 % after adding MFGDR for 8 days and decreased after harvest, which were attributed to the absorption of more nutrient elements by root and synthesis of more nutrient components in plant. Especially, the plant resistance to toxicity of heavy metals was improved due to Si, which was increased by 22.29 % after harvest due to addition of MFGDR. The Si may decrease intake of heavy metals by sweet potatoes. The activated Al in soil decreased by 2.17–3.25 % and 8.66–9.47 % after adding MFGDR for 8 days and after harvest as compared with the initial soil, and the toxic effects of Al on the plant growth were alleviated due to the enhancement of pH value and SO42+ content upon the addition of MFGDR forming AlSO4+. (3) MFGDR significantly affected the speciation and the phytoavailability of heavy metals and its effects evaluated by the MF and the content of residual fraction. The MF of Cd reduced from 36.63 to 32.15–34.20 % leading to the decline in the phytoavailability after adding MFGDR for 8 days and maintain stable after harvest due to enhancement of pH leading to the improvement in the adsorption capacity of Fe-Mn oxide colloid and silicon contained in the MFGDR. In terms of Pb, after adding MFGDR at 1500 kg/ha for 8 days, the residual fraction increased from 47.36 to 57.12 % and the slight decline of MF may be attributed to
Water Air Soil Pollut (2015) 226: 245
the enhancement of available P. Cr and Ni were mainly in the residual fraction form, and the MFGDR almost had little effect on Cr. The MFGDR increased the residual fraction of Ni to above 85 % compared with 84.41 % without addition of MFGDR after harvest. The MFGDR decreased slightly the MF of Cu from 1.47 to 0.69– 1.07 % due to the rise in pH after harvest. The phytoavailability of heavy metals was decreased due to addition of MFGDR. (4) It was most effective to apply 1500 kg/ha MFGDR as soil ameliorant. It can be observed that the improvement of soil environment due to addition of MFGDR contained with mineral composition not only improves the soil physicochemical properties and nutrition elements and declines the phytoavailability of heavy metals in soil, but also enhances the yield and quality of the sweet potato and decreases the concentrations of heavy metals in the sweet potato. The study indicates that MFGDR has potential benefits, when it is applied in agricultural use, and is useful in recycling and utilization of FGD residue.
Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Sciences Foundation of China (Project U1301231), Natural Sciences Foundation of Guangdong Province (Project S2011020005187), and Science and Technology Support Plan of Guangzhou (Projects 11A92081308).
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