chemical and ecotoxicological properties of size fractionated biomass ashes
TRANSCRIPT
Fuel Processing Technology 109 (2013) 124–132
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Fuel Processing Technology
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Chemical and ecotoxicological properties of size fractionated biomass ashes
Rui Barbosa a,⁎, Diogo Dias a, Nuno Lapa a, Helena Lopes b, Benilde Mendes a
a Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia, Departamento de Ciências e Tecnologia da Biomassa, Campus da Caparica. 2829–516 Caparica, Portugalb Laboratório Nacional de Energia e Geologia (LNEG), Unidade de Emissões Zero (UEZ), Ed. J., Estrada do Paço do Lumiar, 22, 1649–038 Lisboa, Portugal
⁎ Corresponding author. Tel./fax: +351 212948543.E-mail address: [email protected] (R. Barbosa).
0378-3820/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.fuproc.2012.09.048
a b s t r a c t
a r t i c l e i n f oArticle history:Received 16 July 2012Received in revised form 20 September 2012Accepted 25 September 2012Available online 15 October 2012
Keywords:Biomass ashesParticle size fractionationChemical propertiesEcotoxicological propertiesBulk contentLeachability
The main aim of this work was to study the chemical and ecotoxicological properties of ashes produced in abiomass boiler of a pulp and paper industry and evaluate possible differences depending on the particle sizeof bottom and fly ashes. This industry produces electricity by burning eucalyptus and pine bark in a bubblingfluidized bed combustor. Bottom and fly ashes and their size fractions, obtained by sieving, were analysed fora set of metals and leaching behaviour. The eluates were also submitted to ecotoxicological characterization,using five indicators. The highest concentrations of metals and metalloids were found in the lower particlesize fractions of bottom and fly ashes. However, generally, it could not be observed any specific releasing pat-tern of metals depending on the particle size, except for fly ashes in which the releasing rate of some earthand alkali-earth metals seemed to increase for lower particle size fractions. No specific pattern of theecotoxicity levels could be associated to the different particle size fractions of ashes. The fractions of bottomashes with 4,000–10,000 μm and >10,000 μm have presented the lowest ecotoxicity levels. All the sampleswere classified as ecotoxic, except the fraction of bottom ashes >10,000 μm.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Coal combustion and co-firing of low cost alternative fuels, such astires, fuel oil and sewage sludge, have been tested since several yearsago [1–6]. These studies have demonstrated that some wastes whenused as alternative fuels can contribute to higher emissions levels ofpollutants on flue gases and on the ashes produced during the ther-mal valorisation. More recently, the scientific community has startedto study other types of wastes, namely biomass residues, in order tofind fuels with lower content of pollutants. A huge fraction of the bio-mass received by pulp and paper industries is not appropriate forpulp and paper production. This residual fraction of biomass is con-sidered to be as a bio-waste that can be valorised. The most commonvalorisation route of these forest residues is their thermal valorisationthrough combustion, since the energy content is high enough forenergy recovery [7–10]. In the Portuguese pulp and paper industry,one of the largest worldwide, the thermal valorisation of forest resi-dues is widely used, contributing to improve the environmental per-formance of the energy production sector and, particularly, to reducethe emissions of greenhouse gases (GHG) [11,12].
Nevertheless, the combustion of forest residues gives rise to impor-tant quantities of ashes which requires sustainablemanagement strate-gies. The type of ashes produced depends on the type of boiler and thetreatment system of the exhaustion gases. Currently, two types of ashesare produced: bottom and fly ashes. The former are collected at the
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bottom of the boilers, while the latter are collected in the cooler zonesof the boilers or retained in the treatment systems of the exhaustiongases.
These two types of ashes usually present different physical andchemical properties, depending on the biomass used and on the com-bustion conditions (furnace temperature profile, residence time offuels and gases, biomass mixture, fuel feeding systems and gas treat-ment systems, among other factors). The average size and particlesize distribution of the ashes are also different as it was observed byseveral authors [13–16]. The hazardous nature of ashes is normallyassociated to the presence of contaminants, such as heavy metals,halogens and sulphates, among others. However, ashes are mainlycomposed by silica and salts of alkali and alkali-earth metals, whichmay influence the release of contaminants and their global toxicity.Stiernström et al. [17] revealed that the presence of non-hazardousmetals in the eluates, such as Ca and K, plays an important role onthe overall toxicity of complex ash eluates. In addition to Ca com-pounds, also the presence of Fe and Al hydrates interfere with therelease of heavy metals due to sorption [18].
Fly ashes present lower particle sizes than bottom ashes, while theconcentrations of heavy metals and other contaminants, such as Cl,are generally higher than that of bottom ashes [19–21]. Much litera-ture reveals that fly ashes present usually higher toxicity than bottomashes [22–24]. There are several studies dedicated to the leachingbehaviour of ashes. Lindberg et al. [25] have studied the leaching rateof Sb, Mo, Ba and Cr VI in this type of matrix. Sloot et al. [26] andKlemm [27] have performed a deep study of the leaching behaviour ofMo and have concluded that this element leaches as molybdate and
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the leaching rate is pH-dependent. They have also observed an associa-tion of Mo to ettringite which, under high pH levels, produce water-soluble oxyanions species. Goumans et al. [28] and Kent et al. [29]have studied the mechanisms that influence the solubility of Ba.Goumans et al. [28] have observed a high influence of sulphate in theleaching behaviour of Ba, while Kent et al. [29] have demonstrated theimportant role of chromate in the leaching behaviour of Ba. These twostudies have concluded that the solubility of Ba is influenced either bysulphate and chromate.
The study of the leaching behaviour of Cr VI was studied by severalauthors [30–33]. These authors have observed that Cr VI may leachatein the presence of dissolved organic carbon [30], associated withettringite [31] or as calcium metalates [32,33].
However studies focusing on both chemical and ecotoxicologicalproperties of size fractionated biomass ashes are scarce [13,24,34],being not possible to know if any patterns on the distribution of chem-ical species can be defined among the different size fractions of biomassashes and even any kind of ecotoxicological pattern is associated tothem.
The main aim of this study was to assess the chemical and ecotox-icological properties of size fractionated biomass ashes, which wereproduced during the combustion of forest residues in a boiler of apulp and paper industry. Several ecotoxicological indicators weretested in order to evaluate the different toxicity responses and possi-ble correlations with the chemical parameters. As there is a growinginterest in the reutilization of these ashes, e.g. in civil engineeringmaterials, thiswork intended tofind out if any specific particle size frac-tions might be less interesting for this type of valorisation due to even-tual highmetal content and high ecotoxicity levels. This studywill allowthe development of a newwork,whichwill consist in the preparation ofnew formulations of concrete constituted, partially, by selected frac-tions of biomass ashes. These new formulations of concrete will beused for coastal zone protection.
2. Material and methods
2.1. Origin of biomass ashes
The biomass ashes – fly and bottom ashes – were produced in aPortuguese biomass boiler of a pulp and paper industry that produceselectricity by burning eucalyptus and pine bark in a bubbling fluidizedbed combustor (BFBC). Bottom ashes were collected at the bottom ofthe BFBC and the fly ashes were collected in the hopper of the electro-static precipitator used for flue gas treatment. The BFBC uses sand asfluidizing agent. The ashes were stored in air-tight polypropylenecontainers and maintained at a temperature of 4±1 °C, in theabsence of light, to prevent their weathering by uptake of moistureand carbonation.
2.2. Size fractionation
Bottom and fly ashes were sieved using the following meshes(Retsch): 20, 50, 200, 500, 850, 2000, 4000 and 10,000 μm (ISO 3310),depending on the type of ash. The sieving process was performed in avibratory sieve shaker AS 200 Digit (Retsch), during 90 min and withamplitude of 1.5 mm. The significant size fractions (mass higher than5% w/w) were selected for detailed characterization.
2.3. Ash inorganic composition
The ashes were submitted to acidic and alkaline digestions todetermine the metal bulk contents. The quantification of Cr, Zn, Ni, Cu,Pb, Cd, Ba, Mo, Sb, Se, As, Hg, Mg, Al, Fe, Ca, Na and K was performedover samples submitted to an acidic digestion according to the USEPAMethod 3051A (HNO3/HCl). The digestionwas developed inmicrowaveoven (Milestone Ethos 1600) using closed vessels and with controlled
temperature (175±5 °C, 10 min). The digested samples were filteredthrough glass fiber filters (Schleicher & Schuell) and the quantificationof metals was achieved through AAS (Thermo AAS, M series). The alka-line digestion was performed following USEPA Method 3060A using amixture of 20 g NaOH+30 g Na2CO3 in 1 L of deionized water. Thedigested samples were filtered through 0.45 μm membrane filters(Schleicher & Schuell) and the final pH values were adjusted to 7.5±0.3 for quantification of Cr VI (USEPA Method 7196A). The content ofsulphur was measured in the as-received ashes and their particle sizesusing an automatic analyser Leco SC-144DR (ASTMMethod D 5016).
2.4. Impure silica content
The glass fiber filters with the filtration residues resulting from theacidic digestion were heated up to 1200 °C, during 10 min, in a micro-wave oven (CEM,modelMAS 7000). The remaining residuewasweight-ed with an analytical balance (Metler Toledo; precision ±0.0001 g) andassumed to be impure silica.
2.5. Leaching test and chemical and ecotoxicological characterizations ofaqueous eluates
The as-received ashes and the particle size fractions selected weresubmitted to leaching according to the European standard EN12457–2. The leaching test was performed with deionized water,using a liquid to solid ratio (L/S) of 10 L/kg, during 24 h, at a temper-ature of 20±2 °C. The suspensions were then filtered through0.45 μm porosity nitrate cellulose membranes (Schleicher & Schuell).The chemical characterization of the filtered eluates comprised thefollowing parameters: pH (ISO 10523, 2008), F−, Cl−, SO4
2−, DOC,total dissolved solids (APHA et al. 2005), As (EN ISO 11969), Hg(ISO 5666/1), Cd, Cu, Ni, Pb, Zn (ISO 8288), Cr (ISO 9174), Cr VI (NFT90-043, 1988), Se (ISO 9965, 1993), Ba, Mo, Sb, Mg, Fe, Al (APHA/AWWA/WPCF, 1996), Ca (ISO 7980, 1986), Na (ISO 9964–1, 1993),K (ISO 9964–2, 1993). The ecotoxicological characterization of theeluates comprised the following assays: (a) luminescence inhibitionof the bacteriumVibrio fischeri (Azur EnvironmentalMicrotox® system);(b) mobility inhibition of the freshwater micro-crustacean Daphniamagna (Daphtoxkit F magna™ of Microbiotests); (c) mobility inhibitionof the marine micro-crustacean Artemia franciscana (Artoxkit M™ ofMicrobiotests); (d) growth inhibition of the freshwater microalgaeSelenastrum capricornutum (Algaltoxkit F™ ofMicrobiotests); (e) growthinhibition of the marine microalgae Phaeodactylum tricornutum (MarineAlgaltoxkit of Microbiotests). The effective concentration (ECxx) of eacheluate was determined for each biological indicator. ECxx means the elu-ate concentration, expressed in percentage, which caused a specific ref-erence effect of xx% for each biological population tested. Each ECxx hasbeen transformed in toxicity units (TU) according to Eq. (1):
TU ¼ 100%=ECxx ð1Þ
Three types of TU were calculated for each sample: (a) a globalTU, which was based on the average of the TU of all biological indica-tors; (b) a TU based on the average values of TU of the marine biolog-ical indicators (V. fischeri, P. tricornutum and A. franciscana); (c) a TUbased on the average values of TU of the freshwater biological indica-tors (D. magna and S. capricornutum). If ECxx values were below orabove the extreme concentrations tested, the calculation of TU wasbased on the extreme concentrations (ex. when ECxxb1% it was con-sidered ECxx=1%; when ECxx>90% it was considered ECxx=90%).The eluates were not submitted to the correction of the pH levelsprior the ecotoxicological tests, since the correction of the pH maypromote changes in the solubilisation or speciation of metals.
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2.6. Ecotoxicological classification of the ashes
The evaluation of the ecotoxic properties (Hazardous property H14,according to the Council Directive 91/689/EEC) of the ashes were basedon the Criterion and Evaluation Methods for Waste Ecotoxicity(CEMWE) [35,36]. The original CEMWE methodology was adapted asit was discussed previously [35]. The ecotoxicological classification ofthe ashes was based on chemical and ecotoxicological characterizationof the eluates of the ashes. The chemical parameters comprised in theecotoxicological classification, through CEMWE, were As, Cd, Cr, CrVI,Cu, Hg, Ni, Pb, Zn and phenolic compounds. The ecotoxicological param-eters comprised in the ecotoxicological classification, through CEMWE,were D. magna and V. fischeri.
3. Results and discussion
3.1. Size fractionation
Fig. 1 shows the particle size distribution of bottom and fly ashesas cumulative undersize distributions. The horizontal lines crossingyy=10%, yy=50% and yy=90% define the parameters D10, D50 andD90, respectively.
The bottom ashes were mainly constituted by particles withdimensions falling in the size ranges of: 200–500 μm (9.2%),500–850 μm (24.1%), 850–2000 μm (34.9%), 2000–4000 μm (10.0%),4000–10,000 μm (10.8%) and >10,000 μm (8.5%). These four particlesize fractions comprised 97.5% of the total mass of bottom ashes. Thebottom ashes presented D10=307 μm, D50=980 μm and D90=6444 μm.
Fly ashes were composed by finer particles. The majority of theirmass, 94.8%, was separated in three size ranges: 20–50 μm (48.0%),50–200 μm (36.3%) and 200–500 μm (10.5%). The fly ashes presentedD10=13 μm, D50=34 μm and D90=161 μm.
These size distribution of fly ashes is similar to that found by Singhet al. [13] (D90=135 μm). The studies performed by Rajamma et al.[14] have shown that the particle sizes of fly ashes were typicallybelow 50 μm. These authors have cut the ashes at 500 μm in orderto remove the elongated particles, which can explain the lowerdimension of the particles. Esteves et al. [15] have found, in a studyrelated with fly ashes produced in a co-generation process of the pro-duction of a pulp and paper industry, a slightly lower particle dimen-sion (D50=21 μm). The works developed by Sata et al. [16] have alsoshown a lower particle dimension of the fly ashes, since their D50
were between 10 and 13 μm. The differences in the particle size canbe related with several aspects, namely the combustion system, thecharacteristics of the fluidizing agent, the system treatment of thegaseous effluents and the fuels used.
Fig. 1. Particle size distribution of biomass ashes.
3.2. Bulk content in metals
Table 1 shows the bulk inorganic composition of bottom ashes andtheir particle size fractions for a set of metals. Analyses were made induplicate and the mean values are reported. Relative standard devia-tion of duplicates (rsd%) varied between 0.5 and 19.6% for the valuesabove the quantification level, with only 6% of the duplicate analysisexceeding 15% rsd.
Bottom ashes were mainly composed by alkali and alkali-earthmetals and trace concentrations of heavy metals andmetalloids. Alka-li (Na, K) and alkali-earth elements (Ca, Mg) were the major compo-nents of biomass ashes, due to high concentrations of these elementsin forest biomass. Some of the metals and metalloids determined arealso present in forest biomass due to their role as minor nutrients[13,23,37].
Generally, the highest concentrations of alkali, alkali-earth metals,heavy metals and metalloids were found in the lowest particle sizefractions of 200–500 and 500–800 μm. Dahl et al. [23] have found intheir work related with ashes produced by the co-combustion ofbiomass-derived fuels (wood chips, sawdust, bark, and peat) similarconcentrations of Cu and Zn (3.7 mg Cu/kg and 41 mg Zn/kg), butlower concentrations of K (90 mg K/kg), Mg (2100 mg Mg/kg) and Na(100 mg Na/kg) and higher concentration of Ca (19,200 mg Ca/kg). Ina work dedicated to chemical extraction of heavy metals in bottomand fly ashes from a pulp and paper mill complex, Nurmesniemi et al.[38] have found higher values of Ca (29,300 mg/kg). These differencesmay be relatedwith the composition of the fuels orwith the combustionconditions, namely, with the temperature in the combustion reactors.
It was not possible to measure S content in the higher size frac-tions, due to the heterogeneity and dimension of the particles. Itwas observed a tendency for the increasing of the S content withthe increasing of the particle size, in the fractions higher than500–850 μm. In a review paper of Khan et al. [39] it was presentedthe composition of several biomass types, sewage sludge and bitumi-nous coal and the composition of the ashes produced by these fuels.In what concerns the S content, its concentration has ranged betweenthe detection limit and 2000 mg/kg in the biomass fuels and betweenthe detection limit and 56,000 mg S/kg in the bottom ashes producedby this fuel. The work developed by Vamvuka et al. [40] related withthe combustion of lignite, olive kernel and olive tree wood has indi-cated that the biomass fuel presented an S content much morereduced than that indicated by Khan et al. [39], namely 800 mg S/kg,for olive kernel and 300 mg S/kg for olive tree wood, which has pro-duced bottom ashes with lower S content (14,000 mg S/kg and8400 mg S/kg in the bottom ashes from the combustion of olive kerneland olive tree wood, respectively). Ingerslev et al. [41] have found an Scontent of 5260 mg/kg in the ashes they have characterized. The widerange of the S content, in the bottom ashes, found by Khan et al. [39],Vamvuka et al. [40] and Ingerslev et al. [41] may be related with thecomposition of the biomass used in the combustions assays. The con-centration of S in the as-received bottom ashes characterized in thepresent work falls in the range indicated by these authors despite it isclose to the lower limit of that range.
Table 2 shows the results of bulk content of metals present in flyashes. Mean values of duplicate analysis are reported. The rsd of du-plicate analyses of the measured values varied between 0.1 and16.1%, with only 6.6% of the rsd percentages above 15%.
These ashes were mainly composed by alkali and alkali-earth ele-ments, and vestigial concentrations of heavy metals and metalloids.Globally, the highest concentrations of metals were also determinedin the lowest particle size fraction (20–50 μm) and the lowest con-centrations were detected in the highest particle size fraction(200–500 μm). The concentrations of heavy metals were globallyhigher in fly ashes than those determined for bottom ashes. Forinstance, Pb and Ba contents were higher in the size fractions of20–50 μm and 50–200 μm, ranging from 27.9 mg Pb/kg to 306 mg
Table 1Inorganic bulk content of bottom ashes and of their particle size fractions for a set of metals (mg/kg db±SD; n=2; n.a.: not applicable).
Element Ashes as-received 200–500 μm 500–850 μm 850–2000 μm 2000–4000 μm 4000–10,000 μm >10,000 μm
As 0.80 1.48 1.7 0.93 0.65 0.64 0.084(±0.02) (±0.29) (±0.2) (±0.06) (±0.01) (±0.05) (±0.008)
Sb 0.19 0.28 0.18 0.17 0.087 0.064 0.027(±0.02) (±0.02) (±0.02) (±0.03) (±0.001) (±0.007) (±0.004)
Se 3.1 4.3 3.4 2.9 1.7 1.5 0.77(±0.1) (±0.4) (±0.2) (±0.1) (±0.1) (±0.3) (±0.06)
Hg b0.47 b0.25 b0.24 b0.24 b0.23 b0.16 b0.098(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Cd b12.6 b6.7 b6.4 b6.4 b5.9 b5.1 b2.1(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Ni b24.7 b13.2 b12.7 b12.7 b11.7 b10.1 b4.3(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Mo b38.4 b20.6 b19.8 b19.6 b18.2 b15.7 b6.6(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Zn 30.3 43.3 38.1 27.0 10.1 9.8 5.9(±2.4) (±0.3) (±1.6) (±2.9) (±1.8) (±0.9) (±0.6)
Pb b33.5 b20.9 b20.1 b20.1 b18.6 b16.0 b6.7(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Cu b16.1 b8.6 b8.3 b8.2 b7.6 b6.6 b2.8(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Ba b62.6 b32.5 b32.1 b32.2 b29.8 b25.6 b10.8(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Cr b19.6 b10.5 b10.1 b10.1 b9.3 b8.0 b3.4(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)
Cr VI 0.86 0.51 0.82 0.87 0.42 0.35 0.067(±0.08) (±0.06) (±0.07) (±0.10) (±0.05) (±0.009) (±0.001)
Ca 14945 17836 17226 10580 6070 2377 1997(±100) (±1000) (±2500) (±200) (±1130) (±200) (±10)
Mg 4627 4717 4845 3159 2155 914 186(±432) (±23) (±711) (±554) (±172) (±81) (±12)
Na 283 231 439 413 339 78 69(±15) (±37) (±24) (±58) (±19) (±9) (±7)
K 3419 4137 3016 1725 1299 474 243(±119) (±350) (±106) (±181) (±43) (±48) (±15)
Fe 3560 5349 2823 2071 678 1473 615(±340) (±125) (±534) (±358) (±39) (±34) (±15)
Al 3737 6815 4102 3145 1837 782 152(±277) (±932) (±394) (±275) (±273) (±109) (±2)
S 376 444 231 464 634 n.an n.an(±4) (±1) (±13) (±9) (±13)
db, dry base; n.an, not analyzed.
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Ba/kg. The Hg content was b0.503 mg/kg in the as-received ashes andranged between 0.408 and 0.598 mg/kg in the size fractionated sam-ples. Cu and Cr were above the quantification limits in all size frac-tions in concentrations ranging from 8.96 mg Cu/kg (200–500 μm)up to 59 mg Cr/kg (20–50 μm). The presence of these elements inthe fly ashes can be explained by the fact that some trace metals areoften volatilized during combustion and condense in the coolerparts of the exhaustion system. Homogeneous condensation or con-densation over the surface of particles that constitute fly ashes leadsto higher concentrations of heavy metals in fly ashes rather than inbottom ashes [42].
Rajamma et al. [14] have characterized two types of fly ashes. One ofthem was produced in a biomass thermal power plant dedicated toelectricity production from forest residues, while the other one wasproduced in a biomass co-generation plant located in a pulp andpaper industry. These authors have found similar concentration levels,e.g. Hg (b1.0 mg/kg), Cd (1 and 1.3 mg/kg) and Ni (27–35 mg/kg) tothose determined in the present work. The concentrations of Pb andCu were also similar to those found by Rajamma et al. [14] in the flyashes from the biomass co-generation plant (12 mg Pb/kg and 27 mgCu/kg). Nevertheless, these authors have found higher concentrationsof these two metals in fly ashes collected at the biomass thermalpower plant dedicated to electricity production (191 mg Pb/kg and99 mg Cu/kg).
Ingerslev et al. [41] have found slightly higher levels of Cd(14.6 mg/kg), Ni (18.8 mg/kg) and lower Pb (19.3 mg/kg) thanthose determined in the present study, but higher concentrations of
Cu (110 mg/kg) and Cr (159 mg/kg). Dahl et al. [23] have foundlower concentrations of Cu (22 mg/kg), but higher concentrations ofZn (370 mg/kg).
Singh et al. [13] have performed an extensive characterization of flyashes produced from different fuels. Concerning the biomass fly ashes,these authors have found similar concentrations of Hg (b0.1 mg/kg),Mo (10 mg/kg), Zn (161 mg/kg), Pb (26 mg/kg), Ba (376 mg/kg) andCr (61 mg/kg) to those determined in the presentwork. The concentra-tion of Cu was higher (113 mg/kg) and As and Se concentrationsreported were 60-fold and about 15-fold lower, respectively, thanthose found in the present work.
According to IAWG [43], the metals that may form oxyanions, As, Seand Sb, tend to form volatile compounds at relatively low temperaturesand are easily partitioned into the fly ashes and other air pollution con-trol residues. Nearly all As and Sb volatilize at temperatures above500 °C. If these elements condense in fly ash surface, they tend to beoxidized by the metal oxides and form the corresponding non-volatilemetal arsenates or antimonates [44–47]. This fact may explain thehigher concentrations of As, Se and Sb in fly ashes than in bottom ashes.
Dahl et al. [23], Ingerslev et al. [41] and Esteves et al. [15] have foundsimilar concentrations of Mg: 25,000 mg/kg, 19,860 mg/kg and17,000 mg/kg, respectively. In what concerns K, Esteves et al. [15] havefound a similar concentration (17,177 mg/kg), but the concentrationsfound by Ingerslev et al. [41] (43,500 mg/kg) and Dahl et al. [23](9700 mg/kg)were slightly different. Esteves et al. [15] have determineda similar concentration of Ca (37000 mg/kg), while Ingerslev et al. [41]and Dahl et al. [23] have found higher concentrations (129,000 mg/kg
Table 2Inorganic bulk content of fly ashes and of their particle size fractions for a set of metals(mg/kg db±SD; n=2; n.a.: not applicable).
Element Ashes as-received 20–50 μm 50–200 μm 200–500 μm
As 6.27 11.2 5.6 2.0(±0.92) (±0.1) (±0.2) (±0.2)
Sb 0.51 0.97 0.58 0.33(±0.08) (±0.11) (±0.06) (±0.04)
Se 1.6 3.2 0.57 b0.17(±0.2) (±0.4) (±0.06) (n.a.)
Hg b0.50 0.41 0.60 0.48(n.a.) (n.a.) (n.a.) (n.a.)
Cd b13.4 b6.4 b6.4 b6.0(n.a.) (n.a.) (n.a.) (n.a.)
Ni b26.4 b12.7 b12.7 b11.8(n.a.) (n.a.) (n.a.) (n.a.)
Mo b41.1 b19.7 b19.7 b18.3(n.a.) (n.a.) (n.a.) (n.a.)
Zn 142 167 117 54.3(±0.3) (±0.4) (±0.2) (±4.5)
Pb b41.9 61.7 27.9 b18.7(n.a.) (±2.3) (±0.5) (n.a.)
Cu 33.1 47.0 27.5 8.96(±2.3) (±0.4) (±1.6) (±0.2)
Ba 248 306 141 b29.9(±8) (±6) (±7) (n.a.)
Cr 48.6 59.0 43.9 10.2(±3.7) (±0.8) (±1.7) (±0.9)
Cr VI 0.69 0.95 0.87 0.51(±0.11) (±0.03) (±0.01) (±0.03)
Ca 43,576 58,550 37,202 7852(±6103) (±3896) (±5952) (±398)
Mg 22,317 28,038 9888 5197(±2128) (±385) (±920) (±4)
Na 1953 1957 1879 530(±269) (±298) (±55) (±44)
K 17,529 23,663 14,497 3752(±1372) (±1158) (±1456) (±88)
Fe 17,696 18,561 17,352 8072(±1265) (±72) (±1046) (±257)
Al 27,913 35,861 27,604 7890(±4298) (±106) (±1216) (±453)
S 3417 4140 1601 328(±9) (±4) (±9) (±5)
Table 3Content on impure silica (% SiO2 db, ±SD, n=2).
Ashes Impure silica (% SiO2 bs)
Bottom ashes As-received 92.6 (±1.3)200–500 μm 87.1 (±0.4)500–850 μm 90.9 (±3.3)850–2000 μm 96.7 (±1.6)2000–4000 μm 93.5 (±3.1)4000–10,000 μm 93.8 (±7.7)>10,000 μm 91.4 (±9.0)
Fly ashes As-received 67.8 (±0.2)20–50 μm 56.5 (±0.5)50–200 μm 79.2 (±4.4)200–500 μm 85.5 (±0.9)
128 R. Barbosa et al. / Fuel Processing Technology 109 (2013) 124–132
and 140000 mg/kg, respectively). The concentrations of Na determinedin the present study were similar to those found by Ingerslev et al. [41](5970 mg/kg) and Dahl et al. [23] (1400 mg/kg).
The S content in the as-received fly ashes was higher than in theas-received bottom ashes and has shown a tendency to decreasewith the increasing of the particle size of fly ashes, which was verymarked. Nevertheless, the S content seems low when comparedwith the results of other works. For instance, the work developed byIngerslev et al. [41], related with the properties of fly ashes from for-est biomass, has shown an S content about four times higher than thatobserved in the present work.
3.3. Impure silica content
Table 3 shows the content on impure silica for both as-receivedbottom and fly ashes and for their particle size fractions.
The content on impure silica was found to be higher in bottomashes than in fly ashes. It was not observed any pattern in the distri-bution of this parameter with the particle size increasing of bottomashes, except that the lower size fraction presented the lower con-tents. On the contrary, the concentration of impure silica in flyashes has risen with the particle size increase, which may be relatedwith the elutriation effect of sand that was used as fluidizing agent,from the BFBC bed zone.
3.4. Chemical characterization of aqueous eluates
Tables 4 and 5 show the chemical characterization of eluates ofbottom and fly ashes, respectively. The eluates of both biomassashes were highly alkaline, due to the presence of oxides that areformed during the combustion process in excess air.
All eluates have demonstrated the high mobility of chlorides andsulphates from the bottom and fly ashes, although they present a dif-ferent mobility pattern, depending on the particle size increase. Forboth ashes the concentration of chlorides has decreased with the par-ticle size increase from 200 to 500 μm up to >10,000 μm, while sul-phates have shown an increasing mobility with the increase ofparticle size distribution. The increase was more evident in the highersize fractions. Nevertheless, due to the higher concentrations of chlo-rides than sulphates in the eluates of both ashes, TDS have shown avariation similar to chlorides, i.e., a decrease of concentration withthe increase of the particle dimensions, although this variation hasbeen more pronounced in fly ashes.
The different mobility of alkali and alkali-earth metals, namely Ca,Na and K, from both bottom and fly ashes, was the most evident char-acteristic of these biomass ashes. No mobility pattern of these metalsfrom bottom ashes was evident, as their release may be highly con-trolled by the presence of other chemical species, such as sulphates.In a different way, the release patterns of Ca, K, and in a lower extentof Na, seemed to follow a decreasing leaching rate with the particlesize increase of fly ashes. This is probably due to the lower concentra-tion of controlling chemical releasing species in fly ashes, such as sul-phates, than in bottom ashes.
The eluates of both biomass ashes were characterized by low orundetectable concentrations of heavy metals and metalloids showingthe low content and low mobility of these metals in fly and bottomashes. No special mobility pattern from both biomass ashes was possi-ble to define for these groups ofmetals, due to their low concentrations.
The eluates of bottom ashes were characterized by low orundetectable concentrations of heavy metals and metalloids showingthe low content and, generally, low mobility of these metals. Never-theless, it was observed some exceptions in what concerns thesolubilisation rate of Sb, Mo, Ba and Cr VI. According to Lindberg etal. [25], these elements can form compounds such as oxides, sul-phides and sulphates, which are easily soluble.
According to a study performed by van der Sloot et al. [26] relatedwith thewater leachablemetal forms in bottomashes fromMSW inciner-ation plants, at a L/S=2 L/kg, almost all the water-soluble Mo wasreleased, in the form of molybdate (oxyanion). These authors have con-cluded that the Mo leaching from that type of ash was pH-dependent,with the highest leaching rates being observed for pH levels above 8.According to Klemm [27], it is possible to find ettringite in ashes. Thismineral may contain oxyanions of amphoteric heavy metals, whichunder high pH levels usually form water-soluble oxyanions species. Thecombination of pH level and oxyanion-substituted ettringite may explainthe high leaching levels observed for Mo.
Table 4Chemical characterization of eluates from bottom ashes (DOC: dissolved organic carbon; TDS: total dissolved solids; mg/kg db±SD, except pH in Sorensen scale and Hg, As, Sb andSe in μg/kg db; n=2; n.a.: not applicable).
Parameter Ashes as-received 200–500 μm 500–850 μm 850–2000 μm 2000–4000 μm 4000–10,000 μm >10,000 μm
pH 12.3 13.4 13.3 13.4 13.3 13.0 12.0(b0.1) (b0.1) (b0.1) (b0.1) (b0.1) (b0.1) (±0.1)
Cl− 17,998 18,487 17,027 17,865 14,243 6,782 2,828(±2508) (±1008) (±999) (±2019) (±1000) (±983) (±404)
SO42− 105 90.3 120 101 95.4 1,128 3,040
(±15) (b0.1) (±18) (±17) (±7.1) (±180) (±75)F− 1.5 55.2 1.0 1.3 1.0 6.0 22.2
(±0.1) (±7.1) (±0.1) (±0.2) (b0.1) (±1.0) (±0.4)DOC 48.3 48.7 49.0 45.9 41.2 41.0 34.8
(±5.3) (±0.6) (±5.3) (±4.6) (±2.6) (b0.1) (±2.1)TDS 26,263 31,311 28,238 29,189 29,255 23,301 24,749
(±480) (±3096) (±3134) (±60) (±91) (±1784) (±2468)As b3.2 b3.2 b3.2 b3.2 b3.2 b3.2 b3.2
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Sb 13.3 7.3 19.1 18.4 9.2 4.4 15.8
(±1.0) (±0.7) (±1.9) (±3.1) (±1.1) (±0.1) (±1.0)Se b9.1 b9.1 b9.1 b9.2 b9.1 b9.2 b9.1
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Hg b12.0 b12.0 b12.0 b12.1 b12.1 b12.1 b12.0
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Cd b0.32 b0.32 b0.32 b0.32 b0.32 b0.32 b0.32
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Ni b0.63 b0.63 b0.63 b0.64 b0.63 b0.64 b0.63
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Mo 2.0 b0.98 1.6 1.4 2.0 3.0 2.4
(±0.2) (n.a.) (±0.2) (±0.2) (±0.3) (±0.3) (±0.4)Zn b0.13 b0.13 b0.13 b0.13 b0.13 b0.13 b0.13
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Pb b1.00 b1.00 b1.00 b1.01 b1.00 b1.01 b1.00
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Cu b0.41 b0.41 b0.41 b0.41 b0.41 b0.41 b0.41
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Ba 4.8 5.0 3.6 2.4 b1.6 b1.6 b1.6
(±0.5) (±0.1) (±0.3) (±0.1) (n.a.) (n.a.) (n.a.)Cr b0.50 b0.50 b0.50 b0.50 b0.50 b0.50 b0.50
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Cr VI 0.25 0.24 0.22 0.24 0.24 0.26 0.06
(±0.02) (b0.01) (±0.01) (±0.02) (±0.02) (±0.05) (±0.01)Ca 1,303 1,505 789 928 1,429 1,531 1,517
(±42) (±190) (±7) (±74) (±130) (±210) (±70)Mg 0.50 0.10 0.06 0.32 0.16 0.45 2.7
(±0.05) (±0.01) (±0.0) (±0.03) (±0.01) (±0.01) (±0.4)Na 13.7 39.9 13.3 24.4 27.7 39.7 34.2
(±2.5) (±4.0) (±1.6) (±2.4) (±1.9) (±3.5) (±2.0)K 104 232 82.5 112 349 276 141
(±8) (±3) (±9.4) (±7) (±4) (±5) (±7)Fe b0.60 b0.60 b0.60 b0.61 b0.60 b0.60 b0.60
(n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.) (n.a.)Al 3.9 3.8 4.1 4.2 3.9 4.1 13.5
(±0.1) (±0.2) (±0.1) (±0.4) (±0.3) (±0.3) (±0.3)
129R. Barbosa et al. / Fuel Processing Technology 109 (2013) 124–132
According to Goumans et al. [28], the solubility of Ba is sulphate-dependent. The ratio between the concentrations of sulphate andBa, in the eluates of as-received bottom ashes, was 31 mmolSO4
2−.mmol−1 Ba. This means that the concentration of sulphatewas clearly higher than the concentration of Ba, which may indi-cate that Ba might be solubilized as barium sulphate. In the eluatesfrom the size fractionated bottom ashes, the ratio between the con-centrations of sulfate and Ba was 23 mmol SO4
2−.mmol−1 Ba(200–500 μm), 48 mmol SO4
2−.mmol−1 Ba (500–850 μm) and58 mmol SO4
2−.mmol−1 Ba (850–2000 μm). Once again, this mayindicate that Ba has, probably, solubilized as barium sulphate. Accordingto Kent et al. [29], Ba can also be mobilized as barium chromate(BaCrO4) at high pH levels. In the eluates of the higher particle size frac-tions, the concentrations of Ba were below the quantification limitdespite the high concentration of sulphates.
Although the sensibility of the quantificationmethods of Cr and Cr VIare different, one can deduce that at least 50% of Cr may exist in thehexavalent oxidation state. Cr VI ismoremobile than Cr III under alkalineconditions and it does not exist as free ion, but as chromate (CrO4
2−),
dichromate (Cr2O72−), or chromium trioxide (CrO3). Therefore, Cr VI
behaves as divalent anions rather than hexavalent cations [30].According to Chirenje et al. [30], the solubilisation of Cr VI may be asso-ciated with the presence of DOC, if the organic carbon compounds formcomplexes of cations, like Fe or Al. Since the concentrations of Fe in theeluates of the ashes are below the quantification limit, the mobilizationof Cr VI from the ashes may be associated to the complex DOC-Al-Cr VI.As it was indicated previously, Cr VI might have also leached as bariumchromate [29]. Perkins [31] has shown that Cr VI-ettringite precipitatedfrom Ca- and Al-containing solutions at pH values greater than 10. Athigh pH levels, when there is an excess of carbonate over the availableCa, Cr VI-ettringite is unstable due to the formation of calcite andgibbsite. Therefore, Cr VI-ettringite and related minerals are most likelyto be important in systems at high pH valueswhen there is a large excessof Ca over carbonate or when precipitation of calcium carbonate isinhibited. Allison et al. [32] and Jing et al. [33] have also observed thatthe release of CrVI and CrIII is essentially associated to calciummetalates(CaCrO4.xH2O, Ca2Cr2O5.yH2O) and the solubility of CrVI as calciummetalate is higher than the solubility of CrIII as calciummetalate. Besides
Table 5Chemical characterization of eluates from fly ashes (DOC: dissolved organic carbon;TDS: total dissolved solids; mg/kg db±SD, except pH in Sorensen scale and Hg, As, Sband Se in μg/kg db; n=2; n.a.: not applicable).
Parameter Ashesas-received
20–50 μm 50–200 μm 200–500 μm
pH 12.3 (b0.1) 12.9 (b0.1) 12.7 (b0.1) 12.2 (±0.1)Cl− 10,293 (±504) 11,253 (b1) 8,773 (±502) 4,167 (±
514)SO4
2− 80.2 (b0.1) 75.1 (±7.1) 80.1 (b0.1) 201 (±27)F− 6.5 (±0.4) 4.8 (±0.7) 7.3 (b0.1) b0.50 (n.a.)DOC 64.6 (±5.0) 67.9 (±2.0) 54.8 (±5.8) 67.0 (±3.9)TDS 28,658 (±1192) 35,385 (±
1501)16,689 (±1162)
8,154 (±170)
As 8.9 (±0.4) 3.4 (±0.3) 22.6 (±2.9) 9.4 (±0.6)Sb 6.7 (±0.6) 6.3 (±0.4) 10.9 (±1.6) 14.6 (±1.3)Se b9.1 (n.a.) b9.1 (n.a.) b9.1 (n.a.) b9.1 (n.a.)Hg b12.0 (n.a.) b12.0 (n.a.) b12.0 (n.a.) b12.1 (n.a.)Cd b0.32 (n.a.) b0.32 (n.a.) b0.32 (n.a.) b0.32 (n.a.)Ni b0.63(n.a.) b0.63 (n.a.) b0.63 (n.a.) b0.63 (n.a.)Mo b0.98 (n.a.) 2.4 (±0.2) 1.5 (±0.1) b0.98 (n.a.)Zn b0.13 (n.a.) b0.13 (n.a.) b0.13 (n.a.) b0.13 (n.a.)Pb b1.0 (n.a.) b1.0 (n.a.) b1.0 (n.a.) b1.0 (n.a.)Cu 0.99 (±0.01) 0.99 (±0.03) 0.99 (±0.01) 0.98 (±0.01)Ba 29.8 (±0.1) 37.7 (±2.9) 11.3 (±0.9) b1.61 (n.a.)Cr b0.50 (n.a.) b0.50 (n.a.) b0.50 (n.a.) b0.50 (n.a.)Cr VI 0.33 (±0.04) 0.47 (±0.03) 0.37 (±0.01) 0.27 (±0.02)Ca 7635 (±9) 9911 (±264) 6052 (±179) 2647 (±190)Mg 0.18 (±0.02) b0.03 (n.a.) 0.047 (±0.002) 0.18 (±0.04)Na 338 (±13) 578 (±40) 285 (±38) 366 (±19)K 1132 (±26) 4253 (±94) 1030 (±64) 315 (±33)Fe b0.60 (n.a.) b0.60 (n.a.) b0.60 (n.a.) b0.60 (n.a.)Al 26.5 (±3.7) 5.1 (±0.8) 14.6 (±0.1) 179 (±12)
130 R. Barbosa et al. / Fuel Processing Technology 109 (2013) 124–132
this, Cornellis et al. [48] have indicated that the most common oxidationstate of Cr in the ashes is the hexavalent state. These facts may explainthe presence of CrVI in the eluates.
The concentrations of As, Sb and Ba in fly ashes were all above thequantification limits. Nevertheless, the leaching rates were very low,especially for As and Sb. According to Cornelis et al. [48], the releaseof As and Sb is associated to Ca and Ba metalates. Nevertheless, thesolubilities of Ca and Ba metalates are relatively low, especially forpH around 12, which may explain the low leaching rates of As and Sb.
Release of sulphates was similar to what happened for the bottomashes, presenting an increase in the higher size fractions ashes. In the
Table 6Ecotoxicological characterization of eluates from bottom and fly ashes and from their parti
Ashes Marine organisms
V. fischeri EC50 (30 min) A. franciscana EC50 (24 h)
Bottom ashes As-received b1.0 81.2(n.a.) (±2.9)
200–500 μm b1.0 70.1(n.a.) (±3.8)
500–850 μm b1.0 67.1(n.a.) (±5.8)
850–2000 μm b1.0 63.5(n.a.) (±3.6)
2000–4000 μm b1.0 78.5(n.a.) (±2.5)
4000–10,000 μm 2.1 83.4(±0.1) (±3.6)
>10,000 μm >99.0 >90.0(n.a.) (n.a.)
Fly ashes As-received b1.0 >90.0(n.a.) (n.a.)
20–50 μm b1.0 86.3(n.a.) (±1.1)
50–200 μm b1.0 84.5(n.a.) (±0.7)
200–500 μm b1.0 >90.0(n.a.) (n.a.)
bottom ashes, the release of S, as sulphate, has not followed any pat-tern and has ranged between 5% and about 17%. In what concern thefly ashes, the release of S, as sulphate, has ranged between 0.6% andabout 20%. In the fly ashes it was observed a very marked tendency,since the release of S as sulphates has increased with the increasingof the particle size of fly ashes.
The concentrations of Cu in the eluates of fly ashes and in the elu-ates of the particle size fractions were similar. Cappai et al. [49] havestudied the mobility of a set of metals from fresh fly ashes of MSWcombustion plants and from fly ashes submitted to accelerated car-bonation. In what concerns the leaching behaviour of Cu, Cappai etal. [49] have concluded, for pH levels similar to those observed inthe present work (around 12–13), that the solubilisation of Cu wasslightly higher in the treated fly ashes than in the fresh ashes. Inthis study, despite the ashes have not been submitted to acceleratedcarbonation, the leaching behaviour of Cu was similar to that ob-served by Cappai et al. [49] for treated fly ashes.
Quina et al. [50] have studied the influence of pH on the leachingbehaviour of a set of metals from municipal solid waste air pollutioncontrol residues. They have concluded that the leaching level of Cuwas similar to those observed in the present work (about 1 mg Cu/kgfor pH levels of 12–13). Those authors have observed, for pH levels ofabout 12–13, concentrations levels of Pb (300–800 mg/kg), Zn(30–60 mg/kg), Cr (5–20 mg/kg) and Ni (1–4 mg/kg) higher thanthose found in the present study. These relative high differences in elu-ates, especially in what concerns Pb, may be related with the differentcomposition of fuels used.
3.5. Ecotoxicological characterization and classification of eluates
Table 6 shows the ecotoxicological characterization of eluates.Figs. 2–4 show the TU values for all biological indicators tested, formarine biological indicators, and for the freshwater biological indica-tors, respectively.
Generally, V. fischeri was found to be the most sensitive biologicalindicator and A. franciscana the least sensitive for all eluates. Exclud-ing the bacterium V. fischeri, the biological indicators from marineenvironment (P. tricornutum and A. franciscana) have shown lowersensitivity to the toxicity levels of all eluates than the organismsfrom fresh water environments (S. capricornutum and D. magna).This behaviour may be related with the fact that the marine
cle size fractions (% v/v±SD; n=2; n.a.: not applicable).
Freshwater organisms
P. tricornutum EC50 (72 h) S. capricornutum EC20 (72 h) D. magna EC50 (48 h)
34.4 5.5 8.5(±4.0) (±0.1) (±1.6)18.7 7.0 5.6(±1.6) (±1.3) (±1.0)12.3 18.1 5.8(±1.9) (±2.6) (±1.1)40.1 5.2 6.4(±6.7) (±0.5) (±1.1)26.1 5.4 8.4(±3.2) (±0.9) (±0.7)45.6 12.9 19.0(±1.9) (±1.2) (±1.4)>80.0 >80.0 >95.0(n.a.) (n.a.) (n.a.)25.4 2.4 11.6(±1.9) (±0.3) (±1.5)30.3 2.4 6.8(±1.3) (±0.4) (±0.6)61.7 1.5 13.5(±1.9) (±0.3) (±2.0)43.4 1.6 4.0(±3.1) (±0.1) (±0.1)
0
10
20
30
40
50
Toxi
city
Uni
ts(f
or m
arin
e an
d fr
eshw
ater
bio
indi
cato
rs)
Fig. 2. Toxicity units of bottom and fly ashes and of their particle size fractions for bothmarine and freshwater biological indicators.
0
10
20
30
40
50
Toxi
city
Uni
ts(f
or f
resh
wat
er b
ioin
dica
tors
)
Fig. 4. Toxicity units of bottom and fly ashes and of their particle size fractions forfreshwater biological indicators.
131R. Barbosa et al. / Fuel Processing Technology 109 (2013) 124–132
environmentmayhave promoted a reduction of the toxicity of the com-pounds, due to salt concentration. The higher degree of the toxicity leveldetermined for fresh water organisms may be also due to osmotic ef-fects on these organisms caused by high salt concentration in eluates,and not to the presence of toxic compounds.
The eluate of the particle size fraction >10,000 μm of bottomashes has presented the lowest ecotoxicological levels. Marine organ-isms were more sensitive to the eluate produced by the bottom ashesas-received than to the eluates produced by the different particle sizefractions of this ash. In what concerns the ecotoxicological levels of flyashes for marine organisms, it was not observed any significant dif-ference between the as-received samples and the size fractionatedsamples.
For freshwater organisms, it was observed a reduction in TUvalues in the particle size fraction of 500–850 μm. It was not identi-fied any reason for this behaviour.
Once again, the eluate of the particle size fraction >10,000 μm ofbottom ashes has presented the lowest ecotoxicological levels. Inwhat concerns the fly ashes it was observed that the as-receivedashes have present lower ecotoxicological levels than the size frac-tions. It was observed an increase in the TU with the particle sizeincreasing.
Table 7 shows the ecotoxicological classification of the bottom andfly ashes and their particle size fractions.
According to Table 7, all the samples were classified as ecotoxicexcept the fraction of bottom ashes >10,000 μm. The ecotoxic classifi-cationwasmainly due to the effects on V. fischeri andD. magna (bottomashes as-received and their particle size 200–500 μm, 500–850 μm,
0
10
20
30
40
50
Toxi
city
Uni
ts(f
or m
arin
e bi
oind
icat
ors)
Fig. 3. Toxicity units of bottom and fly ashes and of their particle size fractions for ma-rine biological indicators.
850–2000 μm and 2000–4000 μm and the fractions 20–50 μm and200–500 μm of fly ashes) and V. fischeri (bottom ashes fraction of4000–10,000 μm, fly ashes as-received and its fraction of 50–200 μm).
Further studies are needed to full understand the relationshipbetween the chemical and the ecotoxicological behaviour of theeluates.
4. Conclusions
The bulk characterization has shown that bottom and fly asheswere mainly composed by earth, alkali-earth metals and silica com-pounds. The concentrations of heavy metals and metalloids werefound to be vestigial or undetectable. The highest concentrations ofmetals and metalloids were found in the lower particle size fractionsof both bottom and fly ashes.
Chlorides, sulphates, Ca, K and, in a lower extent, Na were themain elements leached from all fractions of bottom and fly ashes. Itwas not observed any releasing pattern from the particle size frac-tions, except in the fly ashes for which the releasing rate of someearth and alkali-earth metals seemed to increase with the decreaseof particle size fractions.
The freshwater organisms revealed to be more sensitivity to theeluates of bottom and fly ashes and of their particle size fractionsthan marine organisms. The larger particle size fractions of bottomashes of 4000–10,000 μm and >10,000 μm have presented lowerecotoxicity levels than the other particle size fractions. All the sam-ples were classified as ecotoxic, except the fraction of bottom ashes>10,000 μm.
It was not found any relationship between the chemical and theecotoxicological behaviour of the eluate of the ashes. Further studiesare needed to comprehend the ecotoxicological levels of the ashes.
Table 7Ecotoxicological classification of bottom and fly ashes and their particle size fractions.
Sample Classification Due to
Bottomashes
As-received Ecotoxic V. fischeri, D. magna200–500 μm Ecotoxic V. fischeri, D. magna500–850 μm Ecotoxic V. fischeri, D. magna850–2000 μm Ecotoxic V. fischeri, D. magna2000–4000 μm Ecotoxic V. fischeri, D. magna4000–10,000 μm
Ecotoxic V. fischeri
>10,000 μm Without evidences of ecotoxicity –
Flyashes
As-received Ecotoxic V. fischeri20–50 μm Ecotoxic V. fischeri, D. magna50–200 μm Ecotoxic V. fischeri200–500 μm Ecotoxic V. fischeri, D. magna
132 R. Barbosa et al. / Fuel Processing Technology 109 (2013) 124–132
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