Industrial wastes: Fly ash, steel slag and phosphogypsum- potential candidates to mitigate greenhouse gas emissions from paddy fields

Chemosphere 241 (2020) 124824

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Review

Industrial wastes: Fly ash, steel slag and phosphogypsum- potential candidates to mitigate greenhouse gas emissions from paddy fields Smita S. Kumar a, 1, Amit Kumar b, 1, Swati Singh c, Sandeep K. Malyan d, Shahar Baram d, Jyoti Sharma a, Rajesh Singh e, Arivalagan Pugazhendhi f, * a

Center for Rural Development & Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India Department of Botany, Dayalbagh Educational Institute (Dayalbagh Educational Institute Deemed University), Agra, 282005, Uttar Pradesh, India Department of Environmental Science, Chaudhary Charan Singh University, Meerut, 250001, Uttar Pradesh, India d Institute for Soil, Water, and Environmental Sciences, The Volcani Center, Agricultural Research Organization (ARO), Rishon LeZion, 7505101, Israel e Environmental Hydrology Division, National Institute of Hydrology, Roorkee, 247667, Uttarakhand, India f Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Methane and nitrous oxide are two major greenhouses emitted from rice soil.
In this study, industrial byproducts such as steel slag, phosphogypsum, and fly ash were analyzed.
Steel slag has mitigation potential up to 60% in subtropical rice.
Steel slag has negative N2O emission from rice soil.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2019 Received in revised form 3 September 2019 Accepted 9 September 2019 Available online 9 September 2019

Waste management and global warming are the two challenging issues of the present global scenario. Increased human population has set the platform for rapid industrialization and modern agriculture. The industries such as energy, steel, and fertilizers play a significant role in improving the social, and economic status of human beings. The industrial production of energy (that involves combustion of coal), production of steel items and diammonium ammonium fertilizer generate a huge amount of wastes such as fly ash (FA), steel slag (SS) and phosphogypsum (PG), respectively. Inappropriate dumping of any kind of waste poses a threat to the environment, therefore, scientific management of waste is required to reduce associated environmental risks. These wastes i.e. SS, FA, and PG being rich sources of oxides of calcium (CaO), silicon (SiO2), iron (FeO), and aluminum (Al2O3), etc. may affect the release of greenhouse

Handling Editor: X. Cao

* Corresponding author. Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety Ton Duc Thang University, Ho Chi Minh City, Viet Nam. E-mail address: [email protected] (A. Pugazhendhi). 1 The authors contributed equally as the first author to this work. https://doi.org/10.1016/j.chemosphere.2019.124824 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

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S.S. Kumar et al. / Chemosphere 241 (2020) 124824

Keywords: Rice Methane Nitrous oxide Steel slag Phosphogypsum Fly ash

gases from the soil. The information associated with the application of FA, SS, and PG onto the paddy fields and their impacts on methane and nitrous oxide emissions are highly fragmented and scarce. The present review extensively and critically explores the available information with respect to the effective utilization of FA, SS, and PG in paddy cultivation, their potential to mitigate greenhouse gases emission and their associated mechanisms. The fine grid assessment of these waste management provides new insight into the next level research and future policy options for industries and farmers. © 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 GHG mitigation potential of steel slag and the mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 GHG mitigation potential of phosphogypsum and the mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 GHG mitigation potential of fly ash and the mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Future prospects industrial byproducts for GHG mitigation in paddy soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1. Introduction The global emission of anthropogenic greenhouse gases (GHGs) has been amplified from 27 to 49 gigatons carbon dioxide (CO2) equivalent emission year1 between 1970 and 2010 (Fig. 1a). The total annual anthropogenic emission in 2010 has been 81.48% more than that in 1970 (Pachauri et al., 2014; Edenhofer, 2015). In 2010, the globally emitted anthropogenic GHGs were comprised of 76% CO2, 16% methane, 6% nitrous oxide and 2% fluorinated gases (Fig. 1b). Almost 65% of CO2 emissions were recorded from fossil fuels and industrial processes, and 11% from forestry and other land use. Combustion of fossil fuels and chemical reactions necessary to produce goods from raw materials are the primary sources of CO2 to the atmosphere from the industrial sectors. CO2 is the GHG and it is 60% denser than dry air, therefore, it mainly exists in the biosphere of the atmosphere. The atmospheric concentration of CO2 is constantly rising since industrialization and it is considered as one the biggest environmental risks to climate and human health (WHO, 2016; Perera and health, 2017; Landrigan et al., 2018). To reduce climate changes and human health risks due to rising atmospheric CO2 concentrations, significant researches on food production, production and use of renewable energy, carbon sequestration, invention of fuel efficient machines, etc. are going on globally (Khan et al., 2018, 2019; Pugazhendhi et al., 2019). GHG emission from agriculture sector contributes significantly to climate changes. Paddy cultivation contributes to 10% of total methane (CH4) emission worldwide (Zhang et al., 2016). Methane and nitrous oxide (N2O) are the two major GHGs emitted from paddy soil (Gupta et al., 2016; Malyan et al., 2016a; Xu et al., 2017). Generally, a huge quantity of CO2 is exchanged between the atmosphere and agricultural lands, despite, the net CO2 flux is estimated to be approximately balanced (Gupta et al., 2016). Soil microbial respiration is the major source of CO2 to the atmosphere (Yadav et al., 2019) while consumption of CO2 by crops and agricultural orchard trees during photosynthesis is the main sink of CO2 in the agriculture system (Bhatia et al., 2013; Mina et al., 2017). In paddy soil, CH4 is produced from organic matter degradation by methanogens under anaerobic conditions (Majumdar, 2003; Hussain et al., 2015; Kumar and Malyan, 2016). Organic carbon enters into the soil via deposition by the root portion of plants, plant litter, paddy stubbles, organic fertilizer, algal biomass, dead

microbial biomass and aquatic animals (Gougoulias et al., 2014; Pausch and Kuzyakov, 2018). CH4 production is a complex processes in which bacteria (methanogens) degrade complex organic matter to CH4 via four steps namely, hydrolysis, acidogenesis, acetogenesis and methanogenesis (Fig. 2). About 70% of N2O in the atmosphere is emitted by soil and agricultural soil is the largest source of N2O (6.8 Tg N2O-N year1), worldwide (Baggs, 2011). Nitrogen addition to agricultural soil, manure management, and biomass burning contributes 4.2 (0.6e14.8), 2.1 (0.6e3.1) and 0.5 (0.2e1.0) Tg N2O-N year1, respectively (Smith et al., 2009; Baggs, 2011; Fagodiya et al., 2017). In paddy soil, N2O is produced by microbial processes viz. nitrification and denitrification, which in turn is controlled by several factors such as available N and variable carbon contents, soil moisture, temperature, and pH, etc., (Kurukulasuriya and Rosenthal, 2013; Lai et al., 2019; Mohanty et al., 2019). Nitrification and denitrification are the primary biological processes that utilize the inorganic nitrogen compounds and these two primary processes contribute approximately 70% to the global N2O emission (Butterbach-Bahl et al., 2013). Under aerobic and anaerobic soil conditions, N2O emission from soil is the result of the nitrification and denitrification processes, which occur simultaneously (Braker and Conrad, 2011). Heterotrophic nitrification and co-denitrification are the other microbial processes, which play a significant role in biological N2O production, after nitrification and denitrification (Fig. 3). On the whole, the cumulative N2O emission is very low as compared to the emission of CH4 but global warming potential of N2O (298) is 11.92 times higher than that of CH4 (25), making it more significant while considering mitigation of GHGs (Malyan et al., 2016b). Currently, a consortium of techniques is being used to deal with the mitigation of GHGs such as proper management of soil, fertilizers, water, and changes in agronomic practices (Hussain et al., 2015). Moreover, research related to the use of industrial byproducts are being carried out by various researchers around the globe (Kowshika et al., 2017; Gwon et al., 2018). Steel, fertilizer and power industries play significant roles in the cultural and economic shifts as they lead to the generation of a wide array of byproducts such as steel slag (steel production), phosphogypsum (DAP production) and fly ash (coal burning). These three byproducts stimulate the identification of effective solutions due to rising environmental concerns. Steel Slag (SS) is produced from the pyrometallurgical processing of various

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Fig. 1. (a) Greenhouses gas emission from 1970 to 2010 at global level (Source: IPCC, 2014); (b) Percentage contribution of different greenhouses gases emission in 2010 at the global level in 2010 (Source: IPCC, 2014) FOLU- Forestry and Other Land Use; FFIP- Fossil fuel and Industrial Processes.

ores (Piatak et al., 2015). Crude steel production has been accelerated from 189 MT in 1950 to 1665 million tons (MT) during 2014. India is the fourth-largest steel producer following China, Japan, and the USA. By 2050, steel use has been projected to increase by 1.5 folds from the current levels in order to meet the needs of the growing population (Xuan et al., 2016; Kuramochi, 2017; Hernandez et al., 2018). The worldwide annual SS output in 2014 was about 170e250 million tons based on typical ratios of slag to steel output (Yüksel and sustainability, 2017). About 100e150 kg SS is co-produced with 1000 kg steel and a huge amount of slag is generated annually, worldwide. Phosphogypsum (PG; CaSO4. 2H2O) is a byproduct of the production of phosphoric acid. The annual phosphogypsum (PG) production in India has been 6.5 million tons and about 4.5e5.0 tons (dry basis) of PG is generated per ton of phosphoric acid production. Fly ash (FA; an amorphous mixture of ferroaluminosilicate minerals) has been produced in a huge quantity of the
nchez et al., 2015) order of 600 million tons worldwide (García-Sa with varying elemental compositions (both nutrient and toxic elements) depending on the type and nature of the coal used for combustion and the method of production (Camberato et al., 1997).

The waste load per unit area is also increasing with the everincreasing industrial growth rate. The increasing industrial waste generation is the burning problem at present and will be intensified in the near future; thus, warrants special attention in terms of management and effective utilization. Global climate changes and industrial waste management are the two major environmental issues being faced in the 21st century. Therefore, this is the high time to explore and evaluate promising techniques to mitigate the GHG emission, simultaneously with industrial waste management. In the present study, the methods to efficiently deal with the three important industrial wastes viz. SS, PG, and FA, and their potential applications to mitigate CH4 and N2O emission from the paddy soil along with some additional benefits have been explored. 2. GHG mitigation potential of steel slag and the mechanism Few scientific studies revealed the potential impact of Steel slag (SS) on the GHG emissions from the agricultural soils (Singla et al., 2015; Gwon et al., 2018; Wang et al., 2018a; Das et al., 2019) CH4 and N2O emissions are strongly managed by the rate of SS application. SS contains a high concentration of free and active oxides of

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Fig. 2. Schematic representation of methane production and emission from rice soil (Source: (Malyan et al., 2016a)

Fig. 3. The different processes involved in the biological production of N2O in soil.

silica, iron, and calcium. Iron (Fe3þ) free oxides play a major role in the mitigation of CH4 emissions from paddy soil (Wang et al., 2015). Fe3þ ions could be thermodynamically highly favorable electron acceptors than H2/acetate for non-aerobic degradation of organic matter, thereby suppressing methanogens to produce CH4 from paddy cultivation (Alpana et al., 2017; Wang et al., 2018a). Therefore, Fe3þ ions ultimately reduce CH4 emission via competition with the methanogens for electron donors and the sequential inhibitory effects (Ali et al., 2008). Silica oxide present in SS mitigates CH4 €ckel et al., 2005; Ali emission by the enlargement of aerenchyma (Ja et al., 2012), which increases oxygen transportation from the atmosphere to the root region of the plant (Fig. 4), thus enhancing rhizospheric CH4 oxidation (Wang et al., 2015; Gwon et al., 2018; Bhattacharyya et al., 2019). Recently, a field study was conducted by Wang et al. (2018a) in the Wufeng Agronomy Field of Fujian Province, South Eastern China. They reported SS at the rate of 8 t ha1 application in

subtropical rice, which reduced 53.84% cumulative GWP as compared to the control (Table 1). The addition of SS increased the soil redox potential, which resulted in a lower cumulative CH4 emission (Wang et al., 2018b). In another study, Wang et al. observed that the addition of steel slag at the levels of 2, 4 and 8 t ha1 supplied additional Fe at the rate of 67.2, 134 and 269 kg ha1 respectively, which aided in suppressing the activity of CH4 producing bacteria (Wang et al., 2015). Application of SS at the levels of 2, 4 and 8 t ha1 reduced CH4 emission by 36.32%, 52.14%, and 55.98%, respectively as compared to control (0 t ha1). High iron in soil has increased the population of iron-reducing bacteria, which were in competition with the methanogens for electrons, consequently suppressing CH4 emission (Malyan et al., 2016a; Gwon et al., 2018). In one recent incubation study, Gown et al. found that the application of SS does not pose an environmental risk and its application at an optimum rate reduced CH4 production significantly (Gwon et al., 2018). Singla and Inubushi used two different types of slag fertilizers (Agripower and Minakari byproduct) at the levels of 1 and 2 t ha1 into paddy soil and observed that the application at the rate of 2 t ha1 reduced CH4 emission by 27.54% as compared to control (Singla et al., 2015); however, the addition of SS at the level of 1 t ha1 enhanced CH4 emission by 7.98% as compared to control (Wang et al., 2015) (Table 1). Ali et al. have reported the reduction in the total cumulative CH4 emission from paddy soil following the application of SS with NPK in the Republic of Korea (33.74%), Japan (14.68%) and Bangladesh (18.35%) over control (NPK alone) (Ali et al., 2015). SS applications in paddy soil were found to be very effective for CH4 mitigation from different paddy growing regions. However, SS applications did not influence the pattern of CH4 fluxes. SS addition at a low rate (1 t ha1) did not reduce CH4 emission in dry season while the applications at the levels of 1 and 2 t ha1 reduced CH4 emission in the rainy season than the non-amended soil, which might have been due to the presence of higher amounts of iron. Therefore, applications of SS in soil saturation level or in the

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Fig. 4. Role of steel slag, phosphogypsum, and fly ash in paddy soil.

flooded soil for CH4 mitigation emission shows more signification (Singla et al., 2015; Susilawati et al., 2015). Recently, the potential of SS for GHGs mitigation has been explored in several fields and pot experiments were conducted at different geographical regions of the world such as Bangladesh, Japan, Republic of Korea, China and Indonesia to mitigate GHG i.e. N2O (Wang et al., 2014; Ali et al., 2015; Susilawati et al., 2015; Gwon et al., 2018). Ali et al. reported that the addition of SS with urea to the soil reduced N2O as compared to control (NPK) at all study sites namely, Republic of Korea (5.74%), Bangladesh (14.18%), and Japan (17.65%) (Ali et al., 2015). On the application of SS, the highest mitigation was achieved in Japan followed by Bangladesh and the Republic of Korea. Soil amendment with SS at 2, 4 and 8 t ha1 reduced N2O from 32.43 mg m2 h1 (0 level SS) to 18.62, 19.04 and 0.41 mg m2 h1, respectively in Chinese paddy soil (Wang et al., 2015). The high iron oxide concentrations in SS could have mitigated the total N2O emission during the season by suppressing the microbial activities (Noubactep, 2011). In SS amended paddy soils, higher C/N ratios, and inhibitions of nitrifications slowed down the N2 cycling; thus, affecting N2O emissions. Besides, the high amounts of ferric iron might have also stimulated the reduction of nitrite to N2O (Ali et al., 2015). Wang et al. evinced that soil could act as a sink for N2O at the rate of 8 t ha1 SS The N2O reductions have been directly proportional to SS doses and reported that the lowest and highest N2O emissions were at 8 and 0 t ha1 of SS, respectively (Wang et al., 2015) (Table 1). Susilawati et al. observed that the addition of SS could be an effective tool for the reduction of N2O emissions both in dry and rainy seasons from Indonesian paddy soils. Application of SS at the rate of 1 t ha1 in dry season reduced the total seasonal N2O emission by 21.11% as compared to control and in the rainy season, 1 and 2 t ha1 SS reduced CH4 emission by 28.33 and 33.82%, respectively over control (Susilawati et al., 2015). Soil amendment via SS also resulted in the evolution of an early N2O peak as compared to the non-amended soil, which might be due to the stimulation by iron (Huang et al., 2009; Singla et al., 2015). Singla and Inubushi reported that SS fertilizer mitigated N2O emission from 619 mg N m2 (0 t ha1) to 380 mg N m2 (1 t ha1) and 430 mg N m2 (2 t ha1), respectively in paddy soil

(Singla et al., 2015). Nitrite can be reduced to N2O and NO in the presence of iron oxide at a near-neutral pH (Van Cleemput, 1998). Nitrite reduction will affect the global production of NOx, especially nitric oxide (NO) and N2O. Kampschreur et al. (2011) described the chemical conversions of nitrite by Fe into nitric and nitrous oxides as follows: 2þ þ 3þ NO 2 þFe þ2H /Fe þNO þ H2 O

NO þ Fe2þ þ 1H/Fe3þ þ 0:5 N2 O þ 0:5 H2 O Under anoxic conditions, Fe (II)/Fe (III) oxidation may induce biological and chemical reductions of nitrate and nitrite to NO and N2O, respectively. Biological Fe oxidation releases smaller N2O from nitrate, and under a strong anaerobic environment, N2O is further reduced to N2 (end product) (Kampschreur et al., 2011). 3. GHG mitigation potential of phosphogypsum and the mechanism Phosphogypsum (PG) is a rich source of sulfur and contains about 90% of sulfur in the form of calcium sulfate (CaSO4.2H2O) (Fig. 4). On the application of PG water-soluble sulfate concentration in the soil increases which stimulate the population sulfurreducing bacteria (SRB) in paddy soil (Fig. 5). Soil organic carbon is the major source of carbon for both SRB and methanogenic archaea in agricultural soils (Ozuolmez et al., 2015; Malyan et al., 2016a; Awasthi et al., 2019). Due to this homogeneous substrate preference, there is a strong competition between SRB and methanogenic archaea under the common niche environment. The competition between SRB and methanogenic archaea reduced the availability of substrate for methanogens processes, which resulted in lower rate CH4 production as compared to the environment in which no SRB were prevailing (Malyan, 2017; Sun et al., 2018) (Fig. 5). PG application at the rate of 2e20 t/ha1 resulted in a decrease in total CH4 emission by 9e35% from paddy soil (Ali et al., 2009, 2015). Lindau et al. also observed a reduction in cumulative CH4 emission by 47%, 46%, and 51% at the levels of 2.5, 5.0 and

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Table 1 Influences of various industrial wastes on GHG emissions from paddy cultivation. Reference

Treatment and dose

CH4 flux

N2O flux

GWPa (kg CO2 eq. ha1)

Mitigation (%)

Wang et al. (2018a)

Control Steel slag (8 t ha1) Biochar (BC) (8 t ha1) þ (8 t ha1) Steel slag (8 t ha1) þ BC (8 t ha1) Control (0 t ha1) Steel slag (8 t ha1) Shell slag (8 t ha1) Gypsum slag (8 t ha1) NPK NPK þ Fly ash (20 t ha1) Steel slag (0 t ha1 Steel slag (2 t ha1) Steel slag (4 t ha1) Steel slag (8 t ha1) NPK NPK þ Fly ash (2 t ha1) NPK þ Silicate slag (SS) (2 t ha1) NPK þ Phosphogypsum (PG) (2 t ha1) NPK þ Revolving furnace slag (RFS) (2 t ha1) NPK þ Blast furnace slag (BFS) (2 t ha1) NPK þ BFS(1 t ha1) þ RFS(1t ha1) NPK NPK þ Biochar (BC) (2 t ha1) NPK þ SS (2 t ha1) NPK þ PG (2 t ha1) NPK þ BC (2 t ha1) þ Azolla-BGA NPK þ SS (2 t ha1) þ Azolla-BGA NPK þ PG (1 t ha1) þ Azolla-BGA þ RFS (1 t ha1) NPK NPK þ BC (BC) (2 t ha1) NPK þ SS (2 t ha1) NPK þ PG (2 t ha1) NPK þ BC (2 t ha1) þ Azolla-BGA NPK þ SS þ Azolla-BGA NPK þ PG(1 t ha1) þ Azolla-BGA þ RFS (1 t ha1) Control (0 t ha1) Slag-type fertilizer (Agripower) (1 t ha1) Slag-type fertilizer (Minekaru) (2 t ha1) DS-Steel slag (0 Mg ha1) DS-Steel slag (1 Mg ha1) RS-Steel slag (0 Mg ha1) RS-Steel slag (1 Mg ha1) RS-Steel slag (2 Mg ha1) Steel slag (0 Mg ha1) Steel slag (2 Mg ha1) Steel slag (4 Mg ha1) Steel slag (8 Mg ha1) Urea alone (250 kg ha1) Urea þ coal ash (1 t ha1) Urea þ PG (90 kg ha1) Urea þ Silicate slag (SS) (150 kg ha1) Ammonium sulfate (AS) (400 kg ha1) AS (400 kg ha1) þ SS (150 kg ha1) Urea(190 kg ha1) þ Azolla-BGA(1 t ha1) Urea alone (250 kg ha1) Urea þ coal ash (1 t ha1) Urea þ PG (90 kg ha1) Urea þ silicate slag (SS) (150 kg ha1) Ammonium sulfate (AS) (400 kg ha1) AS (400 kg ha1) þ SS (150 kg ha1) Urea (190 kg ha1) þ Azolla-BGA(1 t ha1) PG (0 Mg ha1) PG (2 Mg ha1) PG (10 Mg ha1) PG (20 Mg ha1) Fly ash (0 Mg ha1) Fly ash (2 Mg ha1) Fly ash (10 Mg ha1) Fly ash (20 Mg ha1)

243.1 kg CH4 ha1 209.8 kg CH4 ha1 170.3 kg CH4 ha1 161.4 kg CH4 ha1 158.38 kg CH4 ha1 73.24 kg CH4 ha1 59.03 kg CH4 ha1 29.27 kg CH4 ha1 54.9 kg ha1 40.5 kg ha1 2.34 mg m2 h1 1.49 mg m2 h1 1.12 mg m2 h1 1.03 mg m2 h1 16.3 g CH4 m2 16.9 g CH4 m2 10.8 g CH4 m2 11.1 g CH4 m2 11.9 g CH4 m2 14.7 g CH4 m2 12.3 g CH4 m2 14.65 g CH4 m2 16.7 g CH4 m2 12.5 g CH4 m2 11.7 g CH4 m2 13.5 g CH4 m2 11.37 g CH4 m2 10.9 g CH4 m2 15.8 g CH4 m2 17.3 g CH4 m2 12.9 g CH4 m2 11.5 g CH4 m2 14.3 g CH4 m2 11.6 g CH4 m2 11.3 g CH4 m2 50.1 g C m2 54.1 g C m2 36.3 g C m2 135 kg C ha1 149 kg C ha1 335 kg C ha1 304 kg C ha1 299 kg C ha1 3.11 mg m2 h1 2.29 mg m2 h1 1.76 mg m2 h1 1.59 mg m2 h1 129.0 kg CH4 ha1 112.0 kg CH4 ha1 105.0 kg CH4 ha1 98.6 kg CH4 ha1 102.0 kg CH4 ha1 95.5 kg CH4 ha1 114.5 kg CH4 ha1 105.5 kg CH4 ha1 93.0 kg CH4 ha1 90.0 kg CH4 ha1 86.5 kg CH4 ha1 89.0 kg CH4 ha1 83.5 kg CH4 ha1 91.3 kg CH4 ha1 236 g CH4 m2 215 g CH4 m2 169 g CH4 m2 161 g CH4 m2 236 g CH4 m2 222 g CH4 m2 189 g CH4 m2 168 g CH4 m2

2.73 kg N2O ha1 1.98 kg N2O ha1 2.31 kg N2O ha1 2.17 kg N2O ha1 0.553 kg N2O ha1 0.241 kg N2O ha1 0.038 kg N2O ha1 0.544 kg N2O ha1 Not mention (NM) NM 32.43 mg m2 h1 18.62 mg m2 h1 19.04 mg m2 h1 0.41 mg m2 h1 45.3 mg N2O m2 43.7 mg N2O m2 42.7 mg N2O m2 39.6 mg N2O m2 41.8 mg N2O m2 41.3 mg N2O m2 40.5 mg N2O m2 28.9 mg N2O m2 19.7 mg N2O m2 23.8 mg N2O m2 24.6 mg N2O m2 21.3 mg N2O m2 24.5 mg N2O m2 25.7 mg N2O m2 39.5 mg N2O m2 31.6 mg N2O m2 33.9 mg N2O m2 35.6 mg N2O m2 29.6 mg N2O m2 34.7 mg N2O m2 36.6 mg N2O m2 619 (mg N m2) 380 (mg N m2) 430 (mg N m2) 38.75 g N ha1 30.57 g N ha1 19.31 g N ha1 13.84 g N ha1 12.78 g N ha1 NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM

9077 7723 6477 6134 5550 2562 1996 833 1867 1377 89.22b 56.21 43.75 35.14 568c 588 380 389 417 512 430 507c 574 432 405 465 394 378 549c 598 449 402 495 405 395 2561c 2630 1847 6138 6769 15196 13788 13562 105.7b 77.86 59.84 54.06 4386 3808 3570 3352 3468 3247 3893 3587 3162 3060 2941 3026 2839 3104 8024c 7310 5746 5474 8024c 7548 6426 5712

Control (C) 14.92% 28.64% 32.42% C 53.84% 64.04% 84.99% C 26.23% C 37.00% 50.96 60.61 C 3.51 33.08 31.44 26.54 9.79 24.21 C 13.21 14.73 20.05 8.16 22.27 25.35 C 8.86 18.26 26.84 9.83 26.27 28.03 C 2.71 27.88 C 10.28 C 9.27 10.76 C 26.37 43.41 48.87 C 13.18 18.60 23.57 20.93 25.97 11.25 C 11.85 14.69 18.01 15.64 20.85 13.46 C 8.90 28.39 31.78 C 5.93 19.92 28.81

Wang et al. (2018a)

Kowshika et al. (2017) Wang et al. (2015)

Ali et al. (2015)

Ali et al. (2015)

Ali et al. (2015)

Singla et al. (2015)

Susilawati et al. (2015)

Wang et al. (2014)

Ali et al. (2012)

Ali et al. (2012)

Ali et al. (2007a)

Ali et al. (2007b)

DS - Dry Season, RS-Rainy Season. a GWP (kg CO2 equivalent ha1) ¼ seasonal CH4 emission (kg CH4 ha1)* 34 þ seasonal N2O emission (kg N2O ha1)* 298 (Source: IPCC, 2013). b GWP in terms of mg CO2 equivalent h1. c GWP in terms of g CO2 equivalent m2.

S.S. Kumar et al. / Chemosphere 241 (2020) 124824

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Fig. 5. General flow diagram for methane mitigation by PG application in rice soil.

10.0 t/ha1 PG application in Louisiana paddy soil, respectively (Lindau et al., 1991). Ali et al. reported that amending the soil with PG at the levels of 2, 10 and 20 t ha1 reduced CH4 by 9, 24 and 32%, respectively over control (without PG). The PG (90 t/ha1) application along with urea reduced cumulative CH4 emission by 105.5 to 90.0 kg ha1 and 129 to 105 kg ha1 over urea alone, respectively in upland and lowland paddy, respectively (Ali et al., 2007a). Ali et al. reported PG application along with Azolla and cyanobacteria, which reduced methane emission to about 29.11% and 25.6% while PG with NPK reduced 27.22% and 25.14%, respectively as compared to NPK application alone in Bangladesh and Japan, respectively (Ali et al., 2015). The higher CH4 mitigation for PG along with Azolla and cyanobacteria was due to the synergistic effects of cyanobacterial photosynthetic activity (which enhanced oxygen in paddy soil near the rhizospheric zone) with free Fe oxides and sulfate ions (which acted as electron acceptors). The liberation of oxygen in the rhizosphere zone also improved soil redox potential. Therefore, suppression of methanogenesis might be due to the conversion of anaerobic conditions to aerobic in the rhizosphere zone, which €ckel et al., 2001; ultimately decreased cumulative CH4 emission (Ja Ali et al., 2015). Thus, application of PG along with Azolla and cyanobacteria can be recommended for paddy soils to reduce CH4 emission. Ali et al. also reported that amendment of soil with PG along with NPK reduced total seasonal N2O from 45.3 to 39.6 mg N2O m2 (NPK) and total CH4 emissions from 16.3 to 11.1 gm2, respectively over control in paddy soil in the Republic of Korea (Ali et al., 2015). In another pot study, conducted in Japan and Bangladesh, Ali et al. found that PG along with Azolla þ cyanobacteria (27.7 mg m2 and 36.6 mg m2) was more effective for mitigating N2O emission in paddy followed by PG along with NPK (24.6 mg m2 and 35.6 mg m2) and NPK (28.9 mg m2 and 39.5 mg m2), respectively (Ali et al., 2012) (Table 1). It revealed that PG had good potential for mitigating both greenhouse gases from paddy fields. However, before making the final statement much more field studies should be carried out to confirm the findings obtained in pot studies. 4. GHG mitigation potential of fly ash and the mechanism Fly ash (FA) consists of low to medium bulk density particles with a diameter <10 mm, high surface area and light texture. FA is rich in essential elements (S, P, K, Na, Ca, Fe, Al, Si, Mg, Cu, Mn and Zn) except organic carbon and nitrogen and thus, promotes plant growth by supplying these nutrients while decreasing their mobility (Pandey et al., 2010; Singh et al., 2011, 2013; Gorai, 2018).

FA also neutralizes soil acidity due to its alkaline nature and hence, can be used for buffering the soil pH (Fig. 4). FA can be considered physiologically equivalent to approximately 20% reagent grade CaCO3. The high levels of calcium oxide and hydroxide present in FA play significant roles to reduce the soil acidity (Pandey et al., 2010; Singh et al., 2011, 2013). Other beneficial effects of FA, which make it useful in agriculture, include improved soil texture, soil aeration, water percolation, and water retention in the treated zone, and reduction in soil bulk density (Pandey et al., 2010; Singh et al., 2013; Gorai, 2018). FA also increases consumption of other soil amelioration agents and functions as an insecticide due to the presence of abrasive silica. Kowshika et al. (conducted the field experiment and they observed that on the application FA at the rate of 20 t/ha reduced the average CH4 emission from 54.9 kg CH4 ha1 to 40.5 kg CH4 ha1. In another study, Ali et al. observed that the application of FA at the rate of 2, 10 and 20 t ha1 mitigated CH4 emissions by 5.94%, 19.92%, and 28.81%, respectively as compared to control due to its high Fe content, which suppressed methanogenesis in paddy soil (Ali et al., 2007b). In another study, Ali et al. observed a 20% reduction in total seasonal CH4 emissions by FA application of 10 t ha1. FA application with urea reduced total seasonal CH4 emissions from 105.5 to 93.0 kg ha1 and 129 to 112 kg ha1 over control (urea alone) in upland and lowland areas of Bangladesh, respectively. FA enhanced CH4 emission from 16.3 to 16.6 g CH4 m2 while mitigating N2O emission from 45.3 mg N2O m2 to 43.7 mg N2O m2, respectively (Ali et al., 2009, 2015) (Table 1). The reason of this CH4 increase was not elucidated by the author of the study, however, other studies indicated a reduction in CH4 emission. As the study was not supported by any theoretical basis or evidence, no conclusions could be drawn and hence, more studies are required to comprehend the role of FA in CH4 mitigation. However, taking into consideration the fact that cumulative reduction of CH4 and N2O, was between 0.453 and 0.435 kg CO2 equivalent m2. It can be said that FA has some potential to combat global warming (Ali et al., 2015). FA positively influences the soil characteristics except for bulk density and the magnitude of the beneficial impacts are enhanced when other organic amendments such as farmyard manure are added owing to the additional support of carbon and nitrogen. These increases were the maximum in FA þ Farmyard manure (FYM) treatment, which indicated the preferential uptake of available-N as inorganic NHþ 4 by the paddy plants (Singh et al., 2011). A short-term study also indicated that the addition of FA to soils increased the N-mineralization and nitrification processes (Cervelli et al., 1986; Singh et al., 2011). Therefore, the effects of FA

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application with an organic amendment on nitrous oxide and methane emissions require further experimental studies to perceive the net effect of the amended paddy soil in different paddy growing regions on global warming potential. 5. Future prospects industrial byproducts for GHG mitigation in paddy soil In this line, the future prospects of the applications of the industrial byproducts in paddy soil need to be evaluated for their effective utilization to control the greenhouse gas emissions and finally, climate changes. Due to different types of paddy varieties, cultivation practices, soil characteristics, optimization of the dosage are highly required before recommending these byproducts to the farmers. Paddy field is one of the important emitters of methane; the spatial and temporal variation methane efflux under the application of the optimized dose to paddy field will support to understand the behaviours of these byproducts under various paddy regimes. The effects on the physical, chemical and biological properties of the soil of these industrial byproducts are available in different long term experiments, however, the long term effects on GHG efflux under field conditions are rarely available, especially, nitrous oxide and methane. Thus, the integration of GHG measurements with these long term experiments should be lined (if possible). The field based acceptance and recommendation of these industrial byproducts to the farmers warrant more studies to reach the final conclusion. To reduce the environmental load of these byproducts and effective utilization of the resources, some other options need to be evaluated for example, fertilizers/biofertilizers, biogas production potential of steel slag, and phosphate and N based fertilizers. Silicate and phosphogypsum based N fertilizers can also be potential options to utilize these products in the farmer fields. Application of fly ash along with organic amendments can also be explored for their GHG mitigation potential. 6. Conclusion Global warming and industrial waste management are important challenges at the present time. Steel slag, phosphogypsum, and fly ash can serve a key role to mitigate the methane and nitrous oxide emissions from the paddy fields. Besides, some of the nutrients present in these wastes can also provide additional benefits to paddy soils. The steel slag, a potential industrial waste, has been studied specifically in various paddy growing regions of the world such as China, Korea, Japan, and Bangladesh followed by phosphogypsum and fly ash. These industrial wastes can be promising options to be utilized in the paddy fields for GHG mitigation; however, the mechanistic understanding and comprehensive evaluation of the recommended doses, application time, and hydrological processes are still under infancy and few more long-term field experiments are required to conclude their regular usage. Acknowledgments We are thankful to the Science & Engineering Research Board (SERB) for awarding National Post-doctoral Fellowship (NPDF) to Dr. Smita vide letter no PDF/2017/002783. The authors would like to thank IGPRED (www.igpred.com) for providing insight and expertise on the research topic and for the assistance that greatly improved the manuscript. References Ali, M., Farouque, M., Haque, M., ul Kabir, A.J., 2012. Influence of soil amendments on mitigating methane emissions and sustaining rice productivity in paddy soil

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