Chemical, Leaching, and Toxicity Characteristics of ...

Water Air Soil Pollut (2015) 226:312 DOI 10.1007/s11270-015-2367-9

Chemical, Leaching, and Toxicity Characteristics of Coal Ashes from Circulating Fluidized Bed of a Philippine Coal-Fired Power Plant Susan Gallardo & Eric D. van Hullebusch & Denvert Pangayao & Beatice Mari Salido & Ria Ronquillo Received: 4 September 2014 / Accepted: 27 February 2015 # Springer International Publishing Switzerland 2015

Abstract Characterization of the coal ash from a typical coal-fired circulating fluidized bed (CFB) power plant in the Philippines was done by studying physical and chemical properties as well as toxic elements content from Semirara and Indonesian fly and bottom ashes. Laboratory-scale experiment was carried out using serial batch leaching procedure (SBLP) to determine the leaching behavior of toxic elements from coal ashes and to mimic the environmental condition using sulfuric acid. From the X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses, the main compound present is CaO which makes the coal ash alkaline in nature. Moreover, SiO2, Fe3O4, and other trace minerals are also present. Also, toxicity characterization leaching procedure (TCLP) shows that more than 99 % of chromium and arsenic remain in the coal ashes matrix. The results of the chemical analysis of eluates deduced by the application of standard leaching tests according to TCLP method indicated that hazardous elements such as heavy metals and metalloids contained in fly and bottom ashes could potentially be transferred to the liquid phase. According to the Microtox analysis, the bottom ashes are less toxic than fly ashes due to the vaporization of toxic elements during the combustion S. Gallardo (*) : D. Pangayao : B. M. Salido : R. Ronquillo Chemical Engineering Department, De La Salle University, 2401 Taft Avenue, Manila, Philippines e-mail: [email protected] E. D. van Hullebusch Université Paris-Est, Laboratoire Géomatériaux et Environnement (EA 4508), UPEM, 77454 Marne-la-Vallée, France

and their subsequent adsorption on the surface of fly ashes. Furthermore, leaching of chromium from the coal ash samples was significantly affected by initial pH of the leachant adjusted with sulfuric acid. The highest leaching rate was reached using the combined condition of pH of 8, contact time of 8 h, and L/S of 5. With these conditions, the leaching rate of chromium from SBA is 0.059, from SFA is 0.070, from IBA is 0.054, and from IFA is 0.06 g Cr/g of ash per hour. Based on literature, the results are relatively comparable. Keywords Coal ash . CFB . Toxicity test . Leaching . Chromium

1 Introduction Coal ash is a by-product of coal combustion for the generation of electricity. Typical coal ash consists of fly ash, bottom ash, and some other by-products (Hassett et al. 2005). One of the power plants in the Philippines uses a circulating fluidized bed (CFB) boiler which is a new and green technology for coal combustion. After combustion, fly ash which consists of fine particles are collected in an electrostatic precipitator while the bottom ash which consists of large particle exits at the bottom of the boiler (Skodras et al. 2009; Koukouzas et al. 2011). Coal ash, both fly, and bottom ashes contains toxic elements (heavy metals and metalloids) and various mineral phases. In the study of Meawad et al. (2010), typical concentration ranges of toxic elements for coal

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ash are reported in ppm: for fly ash, arsenic (0.0003– 391), cadmium (0.01–76), chromium (3.6–437), lead (0.02–273), mercury (0.013–49.5), and zinc (0.28– 2200), and for bottom ash, arsenic (0.8–36.5), cadmium (0.05–5.5), chromium (3.4–350), lead (0.86–813), mercury (0.003–0.04), and zinc (3.8–717). Typical minerals phase harbored by coal ashes are CaO, Fe3O4, SiO2, SO3, and TiO2 (Ahmaruzzaman 2010; Koukouzas et al. 2011; Lokeshappa and Dikshit 2012; Akar et al. 2012). Improper coal ash disposal may have adverse effects to the environment (Sushil and Batra 2006; Bhattacharyya et al. 2009; Gottlieb et al. 2010). There are different ways to manage and dispose coal ash, namely, disposal in abandoned mines, surface impoundments, off-site landfills, co-disposal with wastes, soil conditioner for pH adjustment, and recycling for agricultural and engineering applications. Drawbacks of these methods are the aerial dissemination of fine particle to the surrounding community or percolation to the soil and groundwater (Demir et al. 2008; Meawad et al. 2010; Gottlieb et al. 2010; Ahmaruzzaman 2010; Izquierdo and Querol 2012; Jayaranjan et al. 2014). When coal ash is mixed with water, its toxic components can leach or dissolve out from the ash and percolate through water (Kim and Hesbach 2009; Ward et al. 2009; Akar et al. 2012). This is also known as leaching. Toxic elements contained in coal ash may leach out in the environment, which may result to potential threats to human health such as cancer, heart, lung, and kidney disease, reproductive problems and birth defect, nervous system impacts, and behavioral problems if prolonged exposure (Gottlieb et al. 2010). There is a need to assess the toxic element content of such materials since it may lead to heavy metal contamination; the extent of such event could be alarming (Dutta et al. 2009; Gottlieb et al. 2010; Tsiridis et al. 2012; Akar et al. 2012). Based on an initial study conducted by the authors regarding the leachability of chromium, arsenic, lead, mercury, and cadmium, only chromium significantly leached out using sulfuric acid. Chromium(IV) is highly toxic when inhaled or ingested over a long period of time. If ingested via contaminated water, it may result to small intestine ulcer, stomach cancer, and anemia. If inhaled in large amount, it can cause lung cancer and breathing problems such as asthma, wheezing, and nose ulcers (Gottlieb et al. 2010). Chromium has two oxidation states that may occur + 6 and +3. In aqueous solution the +3 state is most stable, followed by the +2 state. The +6 state is unstable in acid

Water Air Soil Pollut (2015) 226:312

solution and goes to the +3 state. Furthermore, chromium in the +2 state is a good reducing agent, while in the +6 state, it is a powerful oxidizing agent (Shupack 1991). The conversion of chromium complex can be formed via aquation, hydration, oxolation, anion penetration, and complete and partial de-olation depending on the leaching agent (Mandich 1995). Chromium(VI) is highly toxic when inhaled or ingested over a long period of time. If ingested via contaminated water, it may result to small intestine ulcer, stomach cancer, and anemia. If inhaled in large amount, it can cause lung cancer and breathing problems such as asthma, wheezing, and nose ulcers (Gottlieb et al. 2010). Although there are already many leaching methods used for different purposes by various research groups, industries, and regulators, there is still a need for a simple yet comprehensive approach to assess potential release of heavy metals from coal ash when in contact with natural fluids. Common leaching test are synthetic precipitation leaching procedure (SPLP), ASTM D3987, mine water leaching procedure (MWLP), synthetic groundwater leaching procedure long-term leaching (SGLP/PLTL), toxicity characteristic leaching procedure (TCLP), and serial batch leaching procedure (SBLP). The advantages of these methods are the ease of performance, determination of long-term leaching behavior, appropriate for specific environmental situation and specific conditions. However, most of the tests are not yet standardized, lacks long-term leaching components for reactive ash, and are designed for specific site conditions (Hassett et al. 2005). The SBLP, developed by National Energy Technology Laboratory (NETL), is a method that provides an estimate of the total metals being released under varying pH conditions and increasing liquid to solid ratios which determine the rate of release of the metal (Kim and Hesbach 2009). SBLP is a simple yet comprehensive method of estimating the heavy metal release from coal ash when associated with natural fluids like acid rain (Praharaj et al. 2002). Also, SBLP is applicable to evaluate alkaline coal combustion by-product and provides multiple liquid to solid ratio (Hassett et al. 2005). Moreover, TCLP could be also used to determine the hazardousness of the material and to simulate the effect of biogenic organic acids on toxic elements leaching in landfill conditions (Hassett et al. 2005; Jones et al. 2012). The general objective of this study is to determine the physical and chemical characteristics such as surface


area, particle size and pore size distribution, mineral and elemental composition, toxicological status, and toxic element content from both fly ash and bottom ash obtained from a local coal-fired power plant in the Philippines. In line with this, the specific objectives are to determine the rate of leaching of chromium of the ash samples used in the study, to compare the leaching rate of chromium from coal ash derived from local and imported coals obtained from the local coal-fired power plant. Lastly, to determine the effect of the different parameters such as pH, liquid to solid ratios of and contact time to the rate of leaching of chromium from fly ash and bottom ash.

2 Materials and Methods 2.1 Materials The partner power plant uses a single drum CFB boiler with a capacity of 300 t of coal per hour, producing 82 MW of electricity. The power plant has a coal consumption of approximately 900 to 1084 t per day. Its combustion process takes place at 860 °C and uses coal as its main fuel. Coal is crushed through the coal crusher and is fed to the lower furnace together with sorbents which are limestone and sand. Seventy to 80 t per day of limestone and 18 t (during start-up) and additional of 2 t per week of sands are added to the furnace for the reason of using it as a lining for the furnace. From the CFB boiler, bottom ash and fly ash are removed. Bottom ash, which basically consists up of larger particles exits at the bottom of the boiler into a cooler where finer particles are removed and is redirected back to the boiler. Fly ash, which consists of finer particles on the other hand exits at the top of the boiler together with the flue gas and travels through an economizer and then to an air pre-heater. The boiler uses an electrostatic precipitator to control the emission of the particulate matter. Through a conveyor system, the ash is afterward sent to concrete storage silos for recycling or disposal. The partner coal-fired power plant produces 288 to 336 t per day of fly ash and 24 t per day of bottom ash. The coal ash is being disposed in an ash pond covered with high-density polyethylene (HDPE) plastic lining. For the purpose of the present study, four ash samples were collected from a coal power plant located in the Philippines which uses both Semirara (sub-bituminous)

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and Indonesian (bituminous) coals. The combustion products of the coal-fired power plant obtained consist of Indonesian fly ash (IFA), Indonesian bottom ash (IBA), Semirara fly ash (SFA), and Semirara bottom ash (SBA). 2.1.1 Coal Ashes Physicochemical Characterization Particle Size Distribution Analysis The particle size analyses of coal ashes were carried out in accordance with Standard Test Method for Particle Size Analysis of Soils, known as ASTM D422. It determined the quantitative distribution of particle sizes of fly and bottom ash. To obtain this, sieve analyses were done for particle sizes larger than 75 μm (or those retained at no. 200 sieve). Surface Area and Pore Size Determination A Quantachrome Autosorb-1 chemisorption pore size analyzer was used to perform three-point BrunauerEmmett-Teller (BET) method surface area, pore volume, and pore size distribution analysis on each of the four sample ashes with a fixed particle size. Approximately 25 mg of each sample was degassed under vacuum at 300 °C for 2 h for the sample preparation prior to analysis using N2 as the analysis gas. X-Ray D iffraction and X-Ray Fluorescence Analyses X-ray diffraction (XRD) analysis was performed on a RigakuMultiflex XRD analyzer equipped with NaI, photomultiplier with preamplifier. The acquisition was recorded between 4° and 79° with 0.002° scan step and 2-s step time. On the other hand, X-ray fluorescence (XRF) analysis was performed using PANalyticalX fluorescence spectrometer equipped with an Energy Dispersive Minipal 4 (Rh X Ray tube-30 kV9 W) at a resolution of 150 eV (MnKa). Samples were previously vacuum-dried at room temperature. Atomic Absorption Spectrometer Analyses Atomic absorption spectrometer analysis was done using Perkin Elmer AAnalyst 400 to determine the elemental concentration present in the ash. The equipment has high efficiency, segmented solid-state detector, and is equipped with Echelle monochromator, focal length of 300 mm, wavelength of 189–900 nm, spectral bandpass of 0.15 nm at 200 nm, and reciprocal linear dispersion of 2.4 nm/mm. The liquid samples were acidified by 20 % nitric acid to pH 2.

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2.1.2 Standardized Leaching Test Procedure and Toxicity Analysis

Table 1 Design of experiment using Taguchi Method Factors

Levels

The USEPA SW 864 method 1311 (US Environmental Protection Agency 1992) was used for the testing the leachability of coal ash samples. TCLP aims to estimate the potential hazard that chemicals in solid wastes represent to public health and the environment. To assess the toxicity level, several tests with microorganisms, invertebrates, plants, and fish have been developed. However, the most common one is the Vibrio fischeri bioluminescence inhibition assay (Skodras et al. 2009). In the aim to get data comparable to other research papers, toxicity assays of the present study were performed by using Microtox® standard method (ISO 11348-3) with marine bacteria V. fischeri from LUMIStock LCK-487 (Hach Lange). A BERTHOLD Autolumat Plus LB 953 equipment was used. Twentytwo percent of NaCl was added in each sample to insure an osmotic protection for bacteria. Before each toxicity measurement, all the samples were adjusted to circumneutral pH and samples from TCLP experiments were filtered with RC filter (0.2 μm) to remove particles (Dirany et al. 2011). In each batch test, the inhibition percentage of a blank (sample without the compound studied) was measured and used for percentage of inhibition calculation based on 15 min of exposure. For TCLP, arsenic, chromium, cadmium, lead, and mercury were monitored due to its probable adverse impact to the environment and are originally present in the coal.

pH

2

5

Contact time (h)

8

16

24

L/S ratio

2

3.5

5

2.2 Experimental Design 2.2.1 Experimental Design Using Taguchi Method Three levels of each factor, namely, pH which has a direct control on the mobility of heavy metals, contact time which is dependent on the amount of the heavy metal to be leached, and liquid to solid (L/S) ratio, also control the metal leachability. These parameters were chosen due to the direct effect in the leachability of metals in coal ash. The experiment has a total of nine runs to be done for each type of ash. The summary of the parameters used in the experiment can be seen at Table 1. The response, Y, is the rate of leaching expressed in ppm Cr per gram of ash per hour. Taguchi method is designed to have a systematic and efficient technique for experimental run with the objective of selecting the best combination of variable

8

parameter to obtain the optimum result with respect to the noise factor. Also, it utilizes orthogonal arrays of design of experiments to study large number of variables with small number of experimental runs (Ulan and Dean 1991). Moreover, it emphasizes a mean performance characteristic value that is close to the target value with certain limitation. Taguchi method is a straightforward and easy to use, making it powerful yet simple tool (Esme 2009).

2.2.2 Serial Batch Leaching Procedure This fraction was sieved using no. 200 sieve mesh. The ash was mixed in a 15-ml centrifuge vial with 10 ml of the leachant needed. The leaching solution was prepared using concentrated sulfuric acid in deionized water using sequential dilution process to achieve the desired pH which is 2 and 5. For pH 8, 0.115 M sodium hydroxide was used to adjust the pH of the solution. Moreover, the L/S ratio of 2, 3.5, and 5 were used during the experiment for all four ash samples. Using the rotary shaker, each of the sample solution was mixed at 200 rpm for contact time of 8, 16, and 24 h respectively at a room temperature of 28 °C. After which, the supernatant solution was separated using a centrifuge for 20 min. Using a 20 % nitric acid solution, the leachates were acidified to a pH of 2 and were stored at 4 °C for preservation. Finally, the samples were subjected to Atomic Absorption Spectroscopy (AAS) for analysis. The experiment was done in duplicate runs. SBLP was performed in a 15-ml centrifuge vial with the ash and leaching solution at 200 rpm and 28 °C in a rotary shaker. The solution had a contact time of 8, 16, and 24 h with varying pH values and L/S ratio. After the leaching procedure, the solution was placed in a centrifuge for 20 min and filtered using no. 20 Whatman filter paper. The filtered solution was acidified to pH 2 using 20 % nitric acid and stored at 4 °C prior to AAS analysis. The experiment was done in duplicate runs.


Page 5 of 11 312 Table 3 XRF analysis of ash samples (% by weight)

3 Results and Discussion 3.1 Particle Size and BET Table 2 shows the mean diameter and specific surface area of Semirara and Indonesian fly and bottom ashes determined by BET analyzer. The surface area of the four samples varies between 1.16 and 2.92 m2/g. Smaller particle size have larger surface area which make them amenable for contact with the leachant and gives faster leaching kinetics (Haddadin et al. 1995; Bosecker 1997; Iyer 2002). The pore size diameter ranges from 0.9 to 1.05 nm, classifying them as micropores. In comparison to other data published by Dutta et al. (2009), the average specific area of the fly ash is ranging from 2.978 to 6.04 m2/g and the mean diameter is ranging from 0.21 to 0.53 μm. Also, Meawad et al. (2010) shows an average mean diameter for fly ash of 0.02–0.11 μm. On the other hand, Akar et al. (2012) and Neupane and Donahoe (2013) shows an average specific surface area of 0.175 and 2.11–4.43 m2/g of fly ash, respectively. The difference in mean diameter and specific surface area is strongly dependent on coal origin as well on the coal pretreatment and boiler design and operation conditions. 3.2 Major and Trace Elements Composition—XRF Chemical analysis of the ash samples was performed to determine the composition of the material. Table 3 shows that the major component in four ash samples was CaO; this may be originated on the limestone that was added during the combustion process. Other components are Fe2O3, SiO2, SO3, and Al2O3. Moreover, according to Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete (ASTM C 618), the four ashes are classified under class C due to its high calcium content. Also, class C fly ash is normally produced by burning low-ranked coals such as lignite or sub-bituminous, and Table 2 Pore size and pore surface area Sample

Mean diameter (nm)

Specific surface area (m2/g)

SBA

1.0509

2.6260

SFA

0.9006

1.1560

IBA

1.0082

2.8510

IFA

1.0058

2.9239

Element

IBA

SBA

IFA

SFA

CaO

74.245

78.367

30.186

32.166

SiO2

8.137

2.782

34.551

34.498

Fe2O3

4.978

3.003

8.750

6.525

SO3

6.344

13.018

5.256

6.527

Al2O3

3.404

1.183

12.972

13.020

MgO

1.255

0.952

2.614

2.049

TiO2

0.751

0.322

1.554

1.525 1.232

K2O

0.340

0.184

1.293

SrO

0.003

0.002

0.005

0.005

MnO

0.030

0.026

0.036

0.028

ZrO2

0.002

0.001

0.002

0.002

Cr2O3

0.024

0.014

0.027

0.026

ZnO

0.017

0.008

0.016

0.012

CuO

0.008

0.006

0.012

0.011

class F fly ash is normally produced by burning higher rank coals such as bituminous coal or anthracite (Ahmaruzzaman 2010). However, Indonesian fly ash (came from a bituminous coal) was classified as class C because CFB ashes contains higher amount of calcium as an oxide and sulfate because of the addition of limestone as sorbent but has a lower content of silica and alumina than ashes generated from pulverized coal boilers (Skodras et al. 2009). Furthermore, the chemical characteristic of coal ashes may change depending on the mineralogy and composition of the feed coal and the combustion process that the power plant is using. Thus, no individual coal-fired power plant has same chemical ash characteristic (Bhattacharyya et al. 2009; Bhangare et al. 2011). The major and minor elemental compositions coincide with the results of the studies conducted by the following authors: Ahmaruzzaman (2010), Koukouzas et al. (2011), Lokeshappa and Dikshit (2012), and Akar et al. (2012) wherein CaO, Fe3O4, SiO2, SO3, and Al2O3 are some of the composition of coal ashes. High amount of calcium oxide contributes to ashes alkalinity (Steenari et al. 1999; Skodras et al. 2009; Akar et al. 2012).

3.3 Mineralogical Data—XRD Figures 1 and 2 show the XRD analysis of the fly ash and bottom ash, respectively. The compounds identified in coal ash are quartz (SiO 2 ), lime (CaO), mullite

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Table 4 Total trace elements composition of coal ashes Element (mg/kg)

As

Cd

Cr 0.60

Pb

Hg

MDL (mg/kg)

0.16

0.20

1.40

0.10

SFA

1.10

ND

46

ND

ND

IFA

0.73

ND

41

ND

ND

SBA

0.36

ND

22

ND

ND

IBA

0.55

ND

20

ND

ND

MDL minimum detection limit, ND not detected (below detection limit)

Fig. 1 XRD analysis for fly ash (a, quartz-SiO2; b, melilite; c, anhydrite; d, tricalcium aluminate; f, lime-CaO; m, mullite)

(3Al 2 O 3 SiO 2 ), melilite ((Ca,Na) 2 (Al,Mg,Fe 2+ ) [(Al,Si)SiO7]), periclase (MgO), rutile (TiO), and anhydrite (CaSO4). These minerals were also identified by several other authors (Demir et al. 2008; Skodras et al. 2009; Ahmaruzzaman 2010; Hareeparsad et al. 2011; Akar et al. 2012; Neupane and Donahoe 2013). The minerals identified by XRD were in accordance with the results obtained by XRF analysis (Table 3). 3.4 TCLP and Microtox Test Table 4 shows the total trace elements concentration. It is shown that the four ash samples showed only arsenic and chromium within the detectable limit of AAS. Also, fly ashes display higher arsenic and chromium

concentration than the bottom ashes. This observation is in agreement with Lam et al. (2010) who showed that is due to the vaporization of metals during the combustion and the process of metal adsorption on the surface of fly ash particles. TCLP test results showed that the four ash samples are not hazardous to Republic Act 6969 (Toxic Substances, Hazardous and Nuclear Waste Control Act of 1990). Based on TCLP result, cadmium, lead, and mercury were not detected by AAS. However, chromium and arsenic were detected but in lower concentrations than the regulatory limits. Chromium leached was 0.1956, 0.1951, 0.3182, and 0.35 % for SFA, IFA, SBA, and IBA, respectively, while arsenic leached was 0.6364, 0.6849, 0.5556, and 0.5455 % for SFA, IFA, SBA, and IBA, respectively. More than 99 % of chromium and arsenic remains in the ash matrix after the TCLP leaching. The results of the chemical analysis of eluates deduced by the application of standard leaching tests according to TCLP method indicated that the compounds contained in fly and bottom ashes could potentially be transferred to the liquid phase (Table 5). The results are in agreement with Shah et al. (2012) with average total Table 5 TCLP analysis for coal ashes Elements (mg/L)

As

Cd

Cr

Pb

Hg

MDL (mg/L)

0.010 0.010 0.030 0.070 0.0001

SFA

0.007 ND

0.090 ND

ND

IFA

0.005 ND

0.080 ND

ND

SBA

0.002 ND

0.070 ND

ND

IBA

0.003 ND

0.070 ND

ND

Regulation limitsa (mg/L) 5.000 1.000 5.000 5.000 0.200

Fig. 2 Mineral analysis for bottom ash (a, quartz-SiO2; b, melilite; c, anhydrite; f, lime-CaO; g, periclase; m, mullite; r, rutile)

MDL minimum detection limit, ND not detected (below detection limit) a

Regulation limit set in the Philippines (RA 6969)


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dissolved chromium for fly ash of 109.425 ppm and for bottom ash of 47.935 ppm. Also, Nelson et al. (2010) shows average Cr3+ of 94 and 43.4 mg/kg for fly ash and bottom ash, respectively. Levandowski and Kalkreuth (2009) shows average total Cr of 116.17, 78.83, and 53.07 ppm for fly ash, fly ash from cyclone, and bottom ash, respectively. In addition, Indonesian ashes were shown to leach significantly more chromium than Semirara ashes. Based on the Microtox analysis, Indonesian fly ash leachate has the highest toxicity, followed by Semirara fly ash leachate, then Indonesian bottom ash leachate and lastly Semirara bottom ash leachate (Fig. 3). Toxicity of the ash leachate increases with lower particle size and with higher heavy metal concentrations (Skodras et al. 2009). Table 6 displays data from different toxicological assessment studies of coal ashes using Microtox. Palumbo et al. (2007) studied class F and class C fly ash and indicated a little potential toxicity in leachate except for the fly ash with the highest pH of 12.4. Also, Skodras et al. (2009) studied South African and Columbian fly ash and bottom ash. The results show that heavy metals were more concentrated in fly ash compared to bottom ash. Thus, fly ash displays higher toxicity than bottom ash. Tsiridis et al. (2012) studied five coal fly ashes and one lignite fly ash that were collected from different coal-fired power plants in Europe. Results show that the bioluminescence inhibition varied between 18.3 and 40.0 % indicating low toxic response. Also, Darakas et al. (2013) studied a lignite fly

% Luminescence compared to control

100

Ind. Fly ash Ind. Bottom ash Semirara Fly ash Semirara Bottom ash

80

60

40

20

0 5

15

Fig. 3 Microtox analysis of TCLP leachate. The results are expressed in percentage residual luminescence compared to blank measurements after 5 and 15 min of incubations

ash and show that after 5-min exposure, bioluminescence inhibition varied from 63.3 at L/S=10 L/kg to 13 % stimulation at L/S=50 L/kg. The inhibition for exposure time of 30 min was significantly lower than for exposure time of 5 min. 3.5 SBLP: Chromium and Arsenic Leaching Preliminary leaching tests were conducted to determine if chromium and arsenic would leached significantly out from the four ash samples (see Sect. 3.4). Due to the AAS detection limit of 0.01 ppm, only chromium was detectable after 24-h contact time. Thus, the focus of this sub-section is on the leaching of chromium. Taguchi analysis was done in nine combinations of parameters for the Semirara and Indonesian bottom and fly ashes to observe the effect of parameters on the leachability of chromium as shown Table 7. Figure 4 shows the linear graphs for the main effect plots for mean for the different ashes. In order to determine the most significant factor influencing Cr leaching rate of leaching, main effect plots for mean is used in the Taguchi analysis. The leachability of all the ashes displays the same trend in terms of the parameters pH, contact time, and L/S ratio. The highest value of mean is the most significant variable according to the experimental conditions investigated. Furthermore, from Fig. 4, the optimum parameters are pH 8, 8 h for contact time, and L/S ratio of 5 which are all the same in the four ash samples. All four samples have an optimum leaching (highest amount of chromium leached at a given parameter) of chromium at pH 8. According to Izquierdo and Querol (2012), the leaching pattern of chromium is pH dependent. The lowest leaching values are near neutral and show a leachability plateau from pH 8 to 12. It is also reported that there are higher mobility for alkaline ash and increases with increasing pH (Narukawa et al. 2007; Izquierdo and Querol 2012; Dhal et al. 2013). The effect of pH was found to be consistent with the review of Izquierdo and Querol (2012), who reported that the leaching pattern of Cr is usually displaying a marked pH dependence. Cr leaching is usually reaching the lowest values at near neutral pH (in average 0.02 mg/kg) and showing a leachability plateau from pH 8 to 12 (typically around 5 mg/kg). Most of the data in the literature reports predominant where maximum leachability of chromium is stated to be lowest at normal environmental conditions (Huggins et al. 1999; Stam et al. 2011; Izquierdo and Querol 2012). The experiment

Materials

Leaching Test

Indonesian and Semirara fly and bottom TCLP ashes (circulating fluidized bed boiler)

This study

Column leach test (NEN 7341)

Fly ash (thermal power plant)

Darakas et al. (2013)

Only class C fly ash with pH 12.4 exhibited toxicity on the marine bacteria Fly ash leachate has higher toxic effect compared to bottom ash in marine bacteria

Vibrio fischeri

V. fischeri

Microorganism used Results

Indonesian fly ash leachate has the highest toxicity, followed by Semirara fly ash leachate then Indonesian bottom ash leachate and lastly Semirara bottom ash leachate

For V. fischeri, 30 min have lower inhibition than 5 min For D. magna, there is a decrease in toxicity with increasing L/S ratio

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V. fischeri

V. fischeri Daphnia magna

Batch leaching test (EN 12457-2) V. fischeri Less than 10 % inhibition for V. fischeri Daphnia magna 70 % higher inhibition for D. magna and P. subcapitata Pseudokirchneriella Two-stage bioleaching The inhibition of D. magna and P. subcapitata depends on subcapitata (EN 12457-3) the ash used for 1st and 2nd stage bioleaching

Fly ash (power plants in Europe)

Tsiridis et al. (2012)

Palumbo et al. (2007) Class F and class C fly ash (power plant) Nitric acid extraction - Batch - Column Skodras et al. (2009) South African and Columbian coal ash Toxicity characteristic leaching (circulating fluidized bed boiler) procedure (TCLP)

Authors

Table 6 Summary of toxicity test using Microtox

312 Water Air Soil Pollut (2015) 226:312


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Table 7 Results of Taguchi experiments Run

Factors pH

Observable values (g Cr/g ash/h) Contact time (h)

L/S ratio

SBA

SFA

IBA

IFA

1

2

8

2

0.0137

0.0187

0.0111

0.0169

2

2

16

3.5

0.0108

0.0138

0.0084

0.0134

3

2

24

5

0.0067

0.0120

0.0045

0.0117

4

5

8

3.5

0.0056

0.0092

0.0052

0.0079

5

5

16

5

0.0035

0.0059

0.0034

0.0045

6

5

24

2

0.0022

0.0024

0.0021

0.0023

7

8

8

5

0.0591

0.0698

0.0542

0.0606

8

8

16

2

0.0169

0.0178

0.0151

0.0168

9

8

24

3.5

0.0158

0.0189

0.0141

0.0175

The values in italics shows the highest amount of chromium extracted from different coal ash samples

shows that leachability chromium from coal ash is possible using sulfuric acid. pH

However, several authors reported that Cr(VI) might represent up to 30 % of total chromium of coal fly ash

Time

pH

Time

0.025

0.03

0.020 0.015

0.01 2

5 L/S

8

8

16

24

0.03

Mean of Means

Mean of Means

0.02

0.010 0.005 2

5 L/S

8

2.0

3.5

5.0

8

16

24

0.025 0.020 0.015

0.02

0.010

0.01

0.005

2.0

3.5

5.0

Semirara Bottom Ash Time

pH

0.025

0.025

0.020

0.020

0.015

0.015

0.010

0.010

0.005 2

5 L/S

8

0.025

8

16

24

Mean of Means

Mean of Means

pH

Semirara Fly Ash

0.005 2

5 L/S

8

2.0

3.5

5.0

0.025

0.020

0.020

0.015

0.015

0.010

0.010

0.005

Time

0.005 2.0

3.5

5.0

Fig. 4 Main effects plot for means

8

16

24

312

Page 10 of 11

and bottom ash (Huggins et al. 1999; Stam et al. 2011). Narukawa et al. (2007) showed that only Cr(IV) is leached from coal fly ash at alkaline pH. We might therefore expect that chromium has a mixed redox state (i.e., Cr(III) and Cr(IV)) in the coal ashes investigated in the present study. On the other hand, a L/S ratio of 5 and a contact time of 8 h give the optimum leaching for chromium. Higher L/S ratio indicates that there is more chromium that can be leached in the sample. However, the decreasing concentration of an element with increasing L/S ratio indicates that, with time, the concentration of that particular element in the leachate would reduce and saturation point might be achieved (Praharaj et al. 2002; Tsiridis et al. 2012).

4 Conclusion The four ash samples investigated from the CFB boiler power plant contain arsenic, lead, mercury, cadmium, and chromium, but all of them are nonhazardous based on TCLP leaching data and only moderately toxic according to Microtox analysis. The ash samples contain considerable quantity of CaO causing alkalinity in the leaching water. The chromium leaching rate is highest at pH of 8, L/S ratio of 5, and contact time of 8 h. Semirara fly ash was seen to have the highest Cr leaching rate with a value of 0.0698 g Cr/g ash/h, followed by Indonesian fly ash with 0.0606 g Cr/g ash/h, then Semirara bottom ash with 0.0591 g Cr/g ash/h and Indonesian bottom ash with 0.0542 g Cr/g ash/h. Based on the Taguchi design of experiment, pH is the most significant factor influencing chromium leaching. Acknowledgments This research was financially supported by the De la Salle University Research and Coordination Office, the Science Foundation of De La Salle University and the Asian Regional Research Program on Environmental Technology. It was also technically supported by the Tokyo Institute of Technology, Global Business Power Corporation, and Université ParisEst. The authors would like to thank Dr. Yann Sivry (IPGP, Paris, France) for his help in XRF analysis.

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