Characterization of flue gas desulphurized (FGD) gypsum of a coal-fired plant and its relevant risk of associated potential toxic elements in sodic soil reclamation

Thermal Power Plant generates FGD gypsum as by-product during coal combustion. This study evaluates the characterization (spectroscopic and elemental), potentially toxic elements (PTEs) distribution, and environmental risk assessment of FGD gypsum for safe and sustainable use in agriculture. The XRD and SEM analysis confirmed the dominance of crystalline CaSO4·2H2O in FGD gypsum. The order of concentrations of PTEs in FGD gypsum was Fe > Al > Mn > Zn > Ni > Co. The residual fraction was the dominant pool, sharing 80–90% of the total PTEs. The heavy metals (HMs) were below the toxic range in the leachates. The Co, Ni, Al, Fe Mn, Zn had low (< 10%) risk assessment code and the ecotoxicity was in the range of 0.0–7.46%. The contamination factor was also low (0.0–0.16) at the normal recommended doses of FGD gypsum application for sodicity reclamation. The enrichment factor was in the order of Al < Mn < Co < Zn < Ni. Mn [enrichment factor (Ef) 1.2–2.0] and Co (Ef 1.7–2.8) showed negligible enrichment of metals, whereas Ni (Ef 4.3–5.2) and Zn (Ef 4.5–5.6) reported moderate accumulation in soil. The application of FGD gypsum @ 10 t ha−1 for sodicity reclamation will develop a geo-accumulation index below the critical values indicating its safe and sustainable use to achieve land degradation neutrality (LDN) and UN’s Sustainable Development Goals.


XRD, SEM and elemental characterization of FGD gypsum
X-ray diffraction analysis revealed that the characteristic peaks at 2θ value of 11.8, 20.8 and 23.5 (Fig. 2a) of the FGD gypsum 1 suggested that crystalline (CaSO 4 •2H 2 O) gypsum was the dominant mineral present in the FGD gypsum by-product.Some traces of quartz were also observed from the peaks at 2θ value of 31.2.Fu et al. 18 also mentioned the presence of quartz in FGD gypsum produced from the FGD system of a coal power plant in China.Traces of calcite were also observed in FGD gypsum 1 at 2θ value of 29.2 and 42.2.Similarly, for FGD gypsum 2 the samples showed the characteristics peaks of gypsum at 2θ value of 14.8, 25.7, and 49.3 (Fig. 2b).Few traces of calcite appeared with characteristic peaks of 2θ value of 29.8 and 42.3, while the traces of quartz were observed at characteristics peak at 2θ value of 31.9 and 44.6.Similar characteristics peaks of gypsum, quartz, and calcite in FGD gypsum by-products were also reported by Fu et al. 18 , Hao et al. 21.The traces of quartz and calcite present were the impurities incorporated during FGD gypsum generation from the coal plant.The source of these impurities could be the raw coal used or the limestone used in the desulfurization process.However, a higher amount of quartz and calcite will interfere with the functioning of FGD gypsum in sodic soil reclamation process 29,30 .
The SEM images of FGD gypsum 1 (Fig. 3a) and FGD gypsum 2 (Fig. 3b) showed the presence of a clear flaky crystal-like structure of gypsum in both the FGD gypsums 18,25 .Ca and S are the dominant elements found in FGD gypsums with concentrations of 276.2 and 186.5 g kg −1 and other trace elements like Si and Mg with concentrations of 1.94 and 8.7 g kg −1 were observed in elemental analysis of FGD gypsum (Table 1).Elemental analysis of FGD gypsum performed by Fu et al. 18 , Li et al. (2015)  31 also indicated the dominance of Ca and S and the presence of Si and Mg as trace (Table 1).Since, there was no such significant difference between the three FGD gypsum samples in surface morphology through SEM, and diffraction angle and peaks through XRD analysis, (1) therefore, XRD and SEM analysis were conducted only for two samples i.e.FGD gypsum 1 and FGD gypsum 2. The FGD gypsum samples had an average pH value of 8.73 and EC 4.87 dS m −1 .The calcium carbonate percentage ranged from 15 to 17% for different FGD gypsum samples with a moisture content of 16.41% (Table 1).

Total concentration of HMs in FGD gypsum
The concentration of twenty-five potential elements was analyzed.Out of these Al, Co, Fe, Mn, Ni, Zn, Mg, S, NA, K, P, Ca were detected in different samples.As, B, Ba, Cd, Cr, Cu, Hg, Li, Mo, Pb, S, Sb, and V were not detected through ICP-OES estimation.The total concentration of detected PTEs was in the ranges of 1.4-1.7 mg kg −1 , 7.0-7.2mg kg −1 and 23.4-24.7 mg kg −1 for Co, Ni, and Zn, respectively (Table 2).The total concentrations of Mn, Fe, and Al varied from 68.2 to 76.9 mg kg −1 , 2456.5 to 2697.1 mg kg −1 and 1489.4 to 1863.5 mg kg −1 , respectively.Among the three FGD gypsum samples, only Al in FGD gypsum 3 was significantly (p < 0.05) greater than FGD gypsum 1 and FGD gypsum 2. The study of metals in FGD gypsum from different coal-fired power plants have been extensively done in China 18,28,32 .The presence of HMs in coal and lime are the source of metals present in the FGD gypsum and other byproducts 33,[32][33][34][35] .The coal samples are heterogenous in nature having varied compositions of elements 35 .Therefore, spatial and temporal study of coal material is important to estimate the variability in metal percentage in the different by-products of thermal power plants.Bhangare et al. 36 studied the distribution of different trace elements in the coal and combustion residues (fly ash and bottom ash) from the five thermal power plants in India.The characterization of coal samples from the Vindhyachal thermal power plant is presented in supplementary Table S4.The furnace temperature contributes to the release of HMs during combustion (Attalla et al.,  2004)  37 .The devices installed for pollution control in power plants and their operational environments also affect the partitioning of the HMs in different components of coal as well as gases (Hermine et al., 2012)  38 .The concentration of Al remained ~ 5200 mg kg −1 in the gas desulphurization system in NTPC when limestone was sprayed.Therefore, systematic investigation of used materials such as coal and limestone; operation of pollution control devices as well as combustion techniques will help in understanding the distribution of HMs in the FGD gypsums.The production of FGD gypsum is expected to be around 10-14 million metric tonnes per annum from 2024 to 2025 (https:// cpcb.nic.in/ uploa ds/ hwmd/ Guide lines_ HW_5.pdf).Therefore, the huge production of FGD gypsums may be utilized as an alternative amendment to mineral gypsum for sodic land reclamation.

Chemical speciation of metals
The estimation of total HMs in the by-products defines the level of contamination that affects the environment.However, the extent of the toxicity to the surrounding can only be expressed by the study of the behaviour of metal with respect to mobility, bioavailability, accumulation, or change from one form to another through a sequential extraction method.This methodology differentiates the metals into different behavioural groups viz., the acid-soluble fraction (F acid sol ), the reducible fraction (F red ), the oxidizable fraction (F oxi ), and the residual fraction (F res ).The acid-soluble fraction is readily mobile and largely available to the environment, while F red and F oxi are only released under the presence of reduced/oxidized environment, and F res form is considered the most stable form 21,39 .The sum of F acid sol , F red and F oxi refers to the mobile fraction.The HMs studied for the sequential extraction were Al, Co, Fe, Mn, Ni, and Zn for different FGD gypsums (Fig. 4).The reducible fraction (F res ) contributed to almost 84-90% of total Al and this amount was consistent for all the collected FGD gypsums.Therefore, Al concentration was low in F acid sol , F red and F oxi fractions.A maximum percent of Cd resided in the Table 2. Distribution of heavy metals (mg kg −1 ) in different fractions of FGD gypsum according to the sequential extraction procedure.nd, not detected; Values with different uppercase letters (A-B) in column are significantly different (p < 0.05) for bulk analysis of FGDG; values with different lowercase letters (a-b) in columns are significantly different (p < 0.05) for different fraction of FGDG samples.R SCE (%), Recovery of sequential chemical extraction divided by bulk analysis results.Acid-soluble fraction (F acid sol ), reducible fraction (F red ), oxidizable fraction (F oxi ), and residual fraction (F res ).As, B, Ba, Cd, Cr, Cu, Hg, Li, Mo, Pb, S, Sb, V were below the detectable limits.Indian std: Indian standards Awashthi (2000) www.nature.com/scientificreports/residual phase.However, the release of Cd depends on soil reaction and the suitability of the environment 40 .
The maximum portion of Ni was found as F res phase, which accounted for more than 95% of total Ni.Therefore, a negligible amount of Ni stayed as F red, and no Ni was found in the F acid sol and F oxi phases in FGD gypsum samples.Around 50-76% of Ni of the combustion mixture of coal resided in the residual phase 41,42 .However, Hao et al. 21reported 46.7-91.0% of the total Ni in the F red and F oxi phases.This variability may depend upon the natural mineralogy of the native coal and combustion technique adopted in different parts of the globe.The larger portion of Fe remained as the residual phase.The F acid sol , F red and F oxi fractions contained only 1-3% of total Fe.Similarly, about 90% of Mn and Zn remained in the residual form.In other phases, F acid sol , F red and F oxi carried around 7-16% of the total for both Mn and Zn.The association of Zn with Fe-Mn oxides of the combustion wastes has been recognized 18 .Mn is believed to occur in carbonate and residual bound form extracted from coal or the limestone used in desulphurization process and transfer into the by-product in the form of gypsum 24,35,43 .The Co was not detected in F acid sol , F red and F oxi phases.It was only detected in the residual phase.
Other researchers have reported the presence of Cd, Cr, Pb, As, and Cu in samples of FGD gypsum and fly ash from different power plants in China 18,21,24 .The study of heavy metals from 31 power plants in China reported cadmium content (0.01-2.10 mg kg −1 ) in the FGD gypsum samples higher than the soil quality standards of China 44 .Several other trace metals like Mo, Cr, Ni and Cd are also reported in a study carried out on reclamation of sodic soils through FGD gypsum application 45 .
The sequential extraction of FGD gypsum reported a larger concentration of HMs in the F res phase (80-90%).The higher association with the residual fraction indicated its low bioavailability to biota.This observation supported the low risk of these metals to the environment.However, the speciation of metals present in FGD gypsum will depend upon the factors, such as combustion temperature, chemical characterization of flue gas, and operational parameters of the gas desulphurization process, causing various metal speciation and distribution in FGD gypsum from coal-fired power plants of different locations 21 .

Leaching characteristics of HMs metals in the FGD gypsum
The organic and inorganic components present in the solid materials when exposed to the environment on weathering, rainfall, microbial action, or other related activities may cause environmental toxicity.The concentration of HMs in the deionized water/acid leachate from the three FGD gypsum samples collected from power plants is shown in Table 3.The different metals analyzed under leaching toxicity were Fe, Mn, Zn, Cd, Ni, Pb, As, Cr, Cu, and Mo.Among the different elements studied, only Mn, Ba and Cu were detected.Mn leached through the acid solution and was absent in the leachates of deionized water; while the leachates of the acid solution showed a negligible amount of Cu in FGD gypsum 1.Similarly, a negligible amount of Cu was detected in water soluble leachate of FGD gypsum 2. However, Cu was absent in leachates of FGD gypsum 3. Barium leached through both the SPLP solution/ deionized water.However, the concentration remained negligible according to the Toxicity Characteristic Leaching Procedure (TCLP) Regulatory Levels of the Resource Conservation and

Risk assessment code (RAC)
The risk assessment code evaluates the movement of HMs from acid-soluble fraction (F acid sol ) from FGD gypsum samples into the environment 36 .The results of RAC of different metals present in the FGD gypsum are displayed in Fig. 5 showing the level of risk to the environment.Neither Co and Ni were detected in the acid/water soluble fractions, nor had a risk to the environment.For Al, the three samples fell into the low-risk category.The risk levels of Mn, Fe, and Zn ranged from 4.6 to 7.9, 1.2 to 1.3, and 3.2 to 7.5.The RAC analysis of all the metals showed a low level of eco-toxicity.Therefore, the results depicted that the FGD gypsum will not pose any significant harmful effects on the health of the organisms in the ecosystem.

Pollution indices for environmental risk assessment
The toxicity of trace metals from HMs accumulation in FGD gypsum differs from the total estimation as it is dependent on its availability, mobility, and transformation subjected to environmental conditions.FGD gypsum is an emerging amendment source for sodic soil reclamation showing negligible levels of risk and concern for the environment 32,46 .However, it is necessary to assess the soil contamination level as well as the ecotoxicological impacts of FGD gypsum when applied to soil for sodic soil reclamation.Nevertheless, the application of FGD gypsum depends upon the presence of alkalinity (CO 3 2− /HCO 3 − ) and degree of soil sodicity i.e. the presence of Na + in soil solution and exchange phase 47,48 .The standard rate of application of gypsum for sodic soil reclamation is 10 tonnes per hectare 49 .Therefore, the chances of possible contamination of soil through the prescribed application rate of FGD gypsum for the reclamation of sodic soils were estimated to extrapolate the extent of risk to soil system.The contamination factor calculated for metals present in different FGD gypsum samples showed no contamination (C f 0.0-0.2) transfer in the soil through the application of FGD gypsum @ 10 t ha −1 (Table 4).The enrichment factor is another index used to assess the toxicity of metals in the soil.The enrichment of different metals in FGD gypsum remained in order: Al < Mn < Co < Zn < Ni.Metals such as Ni (E f 4.3-5.2),Zn (E f 4.5-5.6),Mn (E f 1.2-2.0),and Co (E f 1.7-2.8)were below the national and internal standard limits 50,51 and it will  www.nature.com/scientificreports/cause low enrichment of metals into the soil upon application (Table 5).However, the sodic soils are reported to be deficient in Zn 52 .The enrichment of Zn will help in the Zn-fertilization of the soil.The geo-accumulation values (I geo < 1) of FGD gypsum reported that its application FGD gypsum in sodicity reclamation would not add any toxic level concentration of heavy metals to soil (Table 3).

Changes in soil pH s , EC e, and SAR e after incubation
Amending soil with FGD gypsum (50GR and 100 GR) significantly decreased the soil pH s (pH of soil water saturation paste) up to 1.09-1.22(P > 0.05).The EC e (electrical conductivity of soil water saturation paste extract) was reduced by 1.35-1.92units in unamended and treated soils (Table 6).The SAR e (SAR of soil water saturation paste extract) of the soil was significantly reduced with the application of FGD gypsum.There was 26 percent decrease in the total alkalinity of the soil with the application of 100GR FGD gypsum compared to the unamended soil (Fig. 6).The soil reclamation with the application of FGD gypsum showed a significant reduction in pH and water-soluble Na + , Cl − , and CO 3 2− + HCO 3 − of the sodic soils in China 41,53,54 .

Conclusion
The FGD gypsum, a by-product of the coal industry was characterized and evaluated for heavy metal toxicity to use as a futuristic alternative amendment for sodic soil reclamation.The XRD, SEM, and elemental characterization confirmed the presence of the crystalline (CaSO 4 •2H 2 O) gypsum as the dominant mineral present in the FGD gypsum by-product, other impurities like Si and Mg corresponded to the presence of quartz and calcite.The presence of calcium in the FGD gypsum significantly reduced the pH s and SAR s of the sodic soil.The total heavy metal concentration followed the order of Fe > Al > Mn > Zn > Ni > Co.The maximum percentage of metals studied under sequential extraction remained in a more stable form (F res phase ~ 80-90%) which are considered hard to release unless in adverse weathering conditions.Leaching toxicity showed no toxicity of metals while, RAC analysis showed a low level of eco-toxicity of Mn, Fe, and Zn.The results of the environmental indices further ascertained no contamination (C f 0.0-0.16) of FGD gypsum to the environment.However, Mn and Co showed minor enrichment and Ni and Zn showed moderate enrichment in the soil which might be good for improving the micronutrient concentration in deficient sodic soil.The low geo-accumulation values (I geo < 1) of FGD gypsum indicated no addition of any toxic metal to the soil upon application FGD gypsum for soil reclamation and thereby, transferred to humans through edible crops.This study revealed the possibility of FGD gypsum as a safe and environmentally sustainable alternative amendment for the reclamation of sodic soil.

Sample collection from the power plant
The FGD gypsum as by-product of the FGD system was received from the wet FGD system of the coal power plant of NTPC, Vindhyachal, Singrauli, Madhya Pradesh, India.The FGD gypsum samples were collected at different time intervals.The first sample was collected in May 2020 (FGD gypsum 1), the second sample (FGD gypsum 2) was collected in August 2020, third sample (FGD gypsum 3) in June 2021.The three samples as received (without purification) were further used for mineral, elemental, and heavy metals characterization and other experimental works.

Physico-chemical analysis
The pH and EC of FGD gypsum were measured in a 1:2 material-water suspension using a glass electrode and conductivity meter, respectively.The CaCO 3 percentage was calculated following the manometric method using Collin's calcimeter method of Allison and Moodie 55 .For moisture content estimation, the FGD gypsum samples were weighed and dried in a hot air oven at 105 °C for 48 h, and volumetric gypsum moisture content was expressed as percent weight loss on a volume basis (Table 1).

X-ray diffraction analysis
X-ray diffraction analysis of the powdered FGD gypsum samples was performed using Phillips diffractometer with Ni-filtered Cu Kα (λ = 1.5418Å) source operating at 40 kV and 20 mA.The diffraction pattern was recorded at a scanning speed of 2°2θ min −1 in the 2θ range between 5° and 90°.

Scanning electron microscopy
A VEGA3 LM scanning electron microscope (SEM) (Tescan Orsay Holding Instrument, Czech Republic) having backscattered electron (BSE) and secondary electron (SE) detectors were used to acquire the SEM images of the FGD gypsum samples to analyze the surface morphology.

Bulk analysis
The FGD gypsum samples collected from the NTPC unit were grounded and sieved with a 2 mm sieve for bulk analysis.Approximately 0.5 g of the sample was digested adding 10 mL of concentrated HNO 3 , 5 mL HClO 4 , and 10 mL HF acid at 135 °C.The digestion process was repeated with an acid mixture till the dissolution of the FGD gypsum samples 21 .The solution was filtered through Whatmann no.42 after the process completion and diluted to a standard volume of 50 mL with distilled water.The elemental composition of FGD gypsum was carried out using Inductively Coupled Plasma Emission Spectroscopy (ICP-OES) (ICPE-9000, Shimadzu, Japan).Meanwhile, the precision of the process was ensured by analysis of HMs in the certified material, i.e., Periodic

Sequential chemical extraction
The chemical speciation of the trace elements was done by the selective sequential extraction (SSE) procedure described by Rauret et al. 36 .This process categorizes the sample components into different behavioural classes.The description of the extraction procedure is displayed in Fig. 1.The extracted fractions (leachate) collected from each step were centrifuged at 3000 rpm for 20 min.and the supernatant separated was filtered with a 0.45 μm cellulose acetate membrane filter, and stored at 4 °C before determination of elemental concentration by ICP-OES.

Leaching toxicity
The

Figure 4 .
Figure 4.Chemical speciation percentage of heavy metals in the three FGD gypsums from selective sequential extraction procedure.

Figure 5 .
Figure 5. Risk assessment code of heavy metals in the heavy metals FGD gypsums.
leaching test of the HMs present in FGD gypsum samples was done following the US EPA SPLP standard to extract the acid-soluble fraction (USEPA, Method 1312, 1994) and European Standard leaching test EN 12457-2 (2002) for water-soluble fraction.The extraction fluid was prepared by mixing concentrated sulfuric acid with nitric acid (mass ratio 2:1) with the pH value adjusted to 3.20 ± 0.05.The solution-to-sample ratio

Table 3 .
28aching characteristics of heavy metals (mg kg −1 ) in different samples of FGD gypsum according to leaching tests.nd,notdetected;Fe, Zn, Cd, Ni, Cr were not detected in toxicity leaching test.Recovery Act (EPA, 2014) (www.epa.gov).Researchers compared the leaching toxicity results with the standard limits of the Hazardous Waste-identification for extraction toxicity for the sewage leaching from the domestic waste landfills and the limit values of the leaching of inert waste landfills in European Community28.

Table 4 .
Contamination factor (C f ) and geo-accumulation index (I geo ) values of elements present in the FGD gypsum samples.

Table 5 .
Enrichment factor (E f ) values of elements present in the FGD gypsum samples.

Table 6 .
Improvement in soil properties on application of FGD gypsum and leaching in sodic soil.LSD, Least significant difference; values with different lowercase letters (a-b) in columns are significantly different (p < 0.05).
Figure 6.Utilization of FGD gypsum in reclamation of sodic soil.