Leaching behaviour and ecotoxicity evaluation of chars from the pyrolysis of forestry biomass and polymeric materials

Ecotoxicology and Environmental Safety 107 (2014) 9–15

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Leaching behaviour and ecotoxicity evaluation of chars from the pyrolysis of forestry biomass and polymeric materials M. Bernardo a,n, S. Mendes a, N. Lapa a,b, M. Gonçalves a, B. Mendes a, F. Pinto b, H. Lopes b a

Unidade de Biotecnologia Ambiental, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal Unidade de Tecnologias de Conversão e Armazenamento de Energia, Laboratório Nacional de Energia e Geologia, Ed. J, Estrada do Paço do Lumiar 22, 1649038 Lisboa, Portugal

b

art ic l e i nf o

a b s t r a c t

Article history: Received 6 December 2013 Received in revised form 9 May 2014 Accepted 12 May 2014

The main objective of this study was to assess the environmental risk of chars derived from the pyrolysis of mixtures of pine, plastics, and scrap tires, by studying their leaching potential and ecotoxicity. Relationships between chemical composition and ecotoxicity were established to identify contaminants responsible for toxicity. Since metallic contaminants were the focus of the present study, an EDTA washing step was applied to the chars to selectively remove metals that can be responsible for the observed toxicity. The results indicated that the introduction of biomass to the pyrolysis feedstock enhanced the acidity of chars and promote the mobilisation of inorganic compounds. Chars resulting from the pyrolysis of blends of pine and plastics did not produce ecotoxic eluates. A relationship between zinc concentrations in eluates and their ecotoxicity was found for chars obtained from mixtures with tires. A significant reduction in ecotoxicity was found when the chars were treated with EDTA, which was due to a significant reduction in zinc in chars after EDTA washing. & 2014 Elsevier Inc. All rights reserved.

Keywords: Pyrolysis Chars Leaching Ecotoxicity Environmental risk

1. Introduction The European Waste Framework Directive (EU, 2008) established the waste management hierarchy that should be respected and applied in the member states of the European community. Recycling and other hierarchical recovery activities such as the energy recovery from wastes are considered key parts of this classification. According to this directive, the thermochemical treatment of wastes without air supply, commonly known as pyrolysis, can be considered a recycling operation since waste materials are reprocessed into three types of products: chars, gases, and heavy compounds that condense as oils when cooled down (tars). The gaseous and liquid fractions can be used as a basic chemical feedstock in the petrochemical and refining industries (Vamvuka, 2011; Al-Salem et al., 2010; Quek and Balasubramanian, 2013). The char can be processed further to be used as adsorbent (González et al., 2009; Méndez-Liñán et al., 2010; Hale et al., 2013), for catalytic applications (Kastner et al., 2009; Zhang et al., 2011), for soil amendment (Uchimiya et al., 2010; Manyá, 2012), and as a metallurgical reducing agent (Griessacher et al., 2012; Kantarelis et al., 2010) among other uses. Alternatively, pyrolysis can also be considered an energy recovery operation, as the three products

n

Corresponding author. Fax: þ 351 212948543. E-mail address: [email protected] (M. Bernardo).

http://dx.doi.org/10.1016/j.ecoenv.2014.05.007 0147-6513/& 2014 Elsevier Inc. All rights reserved.

obtained can be used as fuels because of their calorific content (Vamvuka, 2011; Al-Salem et al., 2010). The recycling attribute of pyrolysis is one step beyond incineration in the waste hierarchy and for this reason it should be preferred as a thermal treatment. In recent years, pyrolysis have received a great deal of attention from the scientific community and started to be commercially applied (Vamvuka, 2011; Al-Salem et al., 2010; Bosmans et al., 2013) because it can provide the same advantages of incineration (waste reduction by volume and weight) with additional advantages such as reduced gas emissions in volume and toxicity (Zaman, 2010). Therefore, it is expected that waste treatment by pyrolysis will grow in importance in the near future, being anticipated that large amounts of pyrolytic chars will be available as by-products or as main products. It is then important to study the properties, composition, and risk assessment of chars, in order to avoid risks to environmental compartments and to exploit the potential value of these pyrolysis products. In particular, applications of these materials involving contact with or potential exposition to water, require the knowledge of their leaching behaviour and ecotoxicological properties. Chars are essentially carbon materials that retain the mineral matter initially present in the wastes and may contain significant amounts of pyrolytic tars. Therefore, the release of toxic compounds from chars is a possibility that might restrict their applications, which demands the evaluation of contaminant mobility through leaching tests. Combining chemical analyses with ecotoxicological tests as an integrated strategy to characterise a given sample has the

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advantage of providing more complete information about its global toxic effects (Blasco and Pico, 2009). This strategy was applied for the first time to the characterisation of pyrolysis chars by Bernardo et al. (2009, 2010); the results underlined the need for relating chemical and ecotoxicological parameters in the hazard assessment of these carbon-based materials. Nevertheless, only very recently, a study of the ecotoxicological assessment of biochars by using both chemical and biological analyses was presented (Oleszczuk et al., 2013). Given the aforementioned, the main objective of the present work was to provide a contribution to the environmental risk assessment of chars obtained in the pyrolysis of feedstocks with pine biomass, plastic wastes, and scrap tires. To achieve this goal, the potential release of contaminants from the chars was evaluated through the application of leaching tests and the ecotoxic levels of the obtained eluates were determined by combining chemical and ecotoxicological analyses. Possible relations within chemical and ecotoxicological data were made to identify contaminants responsible for the ecotoxicity levels. Metallic contaminants and their contribution to the ecotoxicity levels of the chars are the foci. A washing step with EDTA was applied in this work to selectively remove metals that can be responsible for the ecotoxicity levels of the chars.

The high ash content of PP (17.2 per cent) is related to its origin. PP wastes were obtained from automobile fenders containing calcium as the major metal (Bernardo et al., 2009) because of the use of calcium carbonate as inorganic filler in PP for automotive applications (Moritomi et al., 2010).

2. Materials and methods

Plastics were the common raw material in the three feedstocks with the objective to improve the quantity and quality of the pyrolysis liquid fraction through the H-donor effect of plastics. This was the main objective of the previous studies (Miranda et al., 2010; Paradela et al., 2009a, 2009b). In the three experiments, plastics were a mixture of 56 per cent (w/w) PE, 27 per cent (w/w) PP, and 17 per cent (w/w) PS, simulating the average composition of the plastic fractions present in the Portuguese municipal solid wastes (MSW).

2.1. Raw materials—plastic wastes, used tires, and pine forestry biomass The feedstock submitted to the pyrolysis experiments was composed by different mixtures of three materials: plastic wastes, used tires, and pine residues. The plastics used were polyethylene (PE), polypropylene (PP), and polystyrene (PS) provided by a recycling company in the form of pellets with a diameter of approximately 2–5 mm. The scrap tires were provided by a recycling company in strips approximately 2 cm in length and 1–2 mm in diameter after the removal of metal components. Pinus pinaster or maritime pine was selected as the biomass feedstock because it is the predominant specie in the Portuguese forest. This biomass was in the form of shredded pieces similar to the pieces of scrap tires and it was obtained from a sawmill. Table 1 is a presentation of the elemental and proximate analysis of the raw materials used. Tire rubber and plastics were mainly composed by carbon and hydrogen, which was expected given their hydrocarbon character with very low heteroatom content. However, tire rubber presents significant sulphur content as the vulcanisation process of tire rubber is performed with sulphur. The high nitrogen content of PS should also be underlined being most likely associated to an additive of this plastic.

Table 1 Elemental and proximate analysis of plastics, used tires and pine biomass. Analyses performed in the materials as received. Tire

Pine

PE

PP

PS

LHV (MJ/kg daf) HHV (MJ/kg daf)

n.d. 38.5

n.d. 20.2

43.3 46.4

35.1 37.6

37.4 39.0

Proximate analysis Fixed carbona (per cent w/w) Volatiles (per cent w/w) Ash (per cent w/w) Moisture (per cent w/w)

33.5 61.6 2.9 2.0

13.6 74.5 0.3 11.6

0.1 99.8 0.1 0.0

0.1 82.6 17.2 0.1

0.2 99.5 0.0 0.3

Elemental analysis C (per cent daf) H (per cent daf) S (per cent daf) N (per cent daf) Cl (per cent daf) O (per cent daf)

86.1 7.2 1.5 0.2 – 0.1

50.6 6.4 0.2 0.2 0.07 42.5a

84.8 14.5 0.3 0.3 – –

70.5 11.6 o 0.1 0.5 – –

86.1 7.4 o 0.1 6.1 – –

LHV – low heating value; HHV – high heating value; daf – dry ash free; n.d. – not determined. a

Estimated by difference.

2.2. Pyrolysis experiments Pyrolysis experiments were carried out in stirred batch reactors of 1 and 5 l (Parr Instruments), built in Hastelloy C276, a nickel—molybdenum–chromium alloy, which were purged and pressurised to 0.41 MPa with nitrogen. Heating rates of approximately 5 1C/min were used until the desired reaction temperature of 420 1C was reached. This temperature was maintained for 15 min. At the end of this period, the reactor vessel was cooled to room temperature. Given the low heating rate and the cooling period, the total residence time of the materials inside the reactor vessel was approximately 90 min. When the temperature inside the reactor reached near room temperature, the reactor was depressurised and the volume of the gaseous fraction was measured using a gas metre (the volume of nitrogen was discounted). The gaseous compounds were then collected in a sampling bag for density determination; afterwards the gases mass yield was obtained. More information about the pyrolysis installation and experiments can be found in the previous works (Miranda et al., 2010; Paradela et al., 2009a, 2009b). Three different mixtures of the raw materials were subjected to pyrolysis: 1) Mixture 1–30 per cent (w/w) pine biomass þ30 per cent (w/w) used tire rubber þ 40 per cent (w/w) plastics; 2) Mixture 2–50 per cent (w/w) pineþ 50 per cent (w/w) plastics; 3) Mixture 3–50 per cent (w/w) used tire rubber þ 50 per cent (w/w) plastics.

2.3. Char samples The chars obtained from the pyrolysis of mixtures 1, 2, and 3 were named as chars 1, 2, and 3, respectively. Given the batch operation of the pyrolysis process, the resulting solids were a carbonized pasty residue covered with oils and tars. The high concentrations of pyrolytic liquids in the chars are of concern given their composition rich in aromatic, oxygenated, and aliphatic hydrocarbons, some of them of high toxicity and a particularly high environmental mobility. Moreover, the physical characteristics of the chars (viscous, pasty, and very smelly) make them difficult to handle. Thus, it is crucial to remove and recover these liquids, because of the environmental risks they pose and because both of these oils and tars are potential sources of valuable chemicals. The pyrolytic solids were submitted to a sequential solvent extraction with solvents of increasing polarity, namely, hexane, a mixture of 1:1 (v:v) hexane:acetone and acetone, according to an adaptation of the EPA Soxhlet method. This strategy has previously proved to be efficient for the decontamination of pyrolysis chars, allowing the removal of several organic compounds from different classes (Bernardo et al., 2012). The solvents were eliminated from the crude extract solutions using a vacuum rotary evaporator and the resulting solids were dried at a temperature of 80 1C for 24 h in a vacuum oven. The chars were then submitted to a thermal analysis that consisted of measuring the progressive weight loss associated with the combustion of samples in a muffle furnace, under an air atmosphere, from room temperature up to 750 1C with increments of 50 1C, remaining 10 min. at each temperature stage. This thermal analysis allows defining the composition of the chars in terms of the volatility of their components. It was considered that volatile compounds were those volatilised up to 250 1C; the weight loss registered between 250 1C and 350 1C was attributed to semi-volatile compounds, while the weight decrease observed from 350 1C to 600 1C was assigned to the volatilization and combustion of heavy compounds denominated as fixed residue; the residue non-combusted above 600 1C that presented a stable weight was considered to be the ashes. The chars were also submitted to an elemental analysis performed with a LECO elemental analyser by combustion technique. Carbon, hydrogen, and nitrogen were determined according to the ASTM D5373 standard and sulphur determination followed the ASTM D4239 standard (ASTM, 2002). 2.4. Metal content of chars The metal content of chars was estimated according to the following procedure: the chars were submitted to a previous digestion in porcelain crucibles performed with hydrogen peroxide 30 per cent (v/v) in a heated bath at a

M. Bernardo et al. / Ecotoxicology and Environmental Safety 107 (2014) 9–15 temperature of 95 1C and then digested with aqua regia (HCl:HNO3, 3:1, v/v) at the same temperature. Finally, a microwave acidic digestion adapted from the EPA 3015A method was used to complete the solubilisation of the inorganic components of the samples. A broad group of metals were quantified in the digested samples using Thermo Elemental Solaar atomic absorption spectrometry (AAS) equipment. Cadmium (Cd), lead (Pb), zinc (Zn), copper (Cu), nickel (Ni), potassium (K), manganese (Mn), iron (Fe), sodium (Na), and magnesium (Mg) were analysed using the air–acetylene flame technique. Calcium (Ca), aluminium (Al), chromium (Cr), molybdenum (Mo), and barium (Ba) were analysed using the acetylene– nitrous oxide flame technique. Mercury (Hg), arsenic (As), selenium (Se), and antimony (Sb) were determined using the hydride generation technique.

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the char, which was further washed several times with ultra-pure water to remove residual chelant until the pH of the washing water was neutral. The amount of metals leached with the EDTA washing solution was monitored by using the Thermo Elemental Solaar AAS equipment. Finally, the washed chars were dried in a vacuum oven at 80 1C for 24 h. The EDTA washed chars were then submitted to the leaching test according to the standard ISO/TS 21268-2 and the eluates were analysed for pH, metal content, and ecotoxicity according to the methodologies described previously.

3. Results and discussion 2.5. Leaching tests The chars were leached according to the standard ISO/TS 21268-2 that has been developed to measure the release of inorganic and non-volatile organic constituents from soil and soil materials and the ecotoxicological effects of their eluates. The ISO/TS standard prescribes as the leaching agent a neutral calcium chloride (CaCl2) solution with a concentration of 0.001 mol/L. This CaCl2 solution has an ionic strength comparable to the natural soil solutions and is adequate to ecotoxicological tests. Although the scope of ISO/TS 21268-2 covers only soil and soil materials, an adaptation of this standard was carried out in this work and applied to the chars taking into account the physical similarities between the chars and soil materials (they are both granular) and taking into account a possible aqueous or soil application of these materials. The chars, previously milled to particle sizes of less than 1 mm, were mixed with the leaching solution in a single stage batch test performed at a L/S ratio of 10 L/kg, at a constant temperature of 20 7 2 1C. The containers (borosilicate glass bottles) were shaken in a roller-rotating device at 10 rpm, for a period of 247 0.5 h. At the end of the leaching test, the mixtures were allowed to settle for 15 min. and the eluates were filtrated over fibreglass filters GF/C Whatman to minimise the sorption of organics. Blank tests, with CaCl2 0.001 mol/L and without chars, were made for each sample. The eluates were immediately analysed for pH and electrical conductivity and for total organic carbon (TOC) and inorganic carbon (IC) using a TOC analyser operating with the combustion–infrared method. The eluates were divided into sub-samples to be used in the different chemical and ecotoxicological analyses. The eluates used in chemical analyses were preserved with HNO3 to a pHo 2 and refrigerated at 4 1C. For the ecotoxicological tests, the eluates were preserved at a temperature of 4 1C in airtight vessels. 2.6. Chemical characterisation of the eluates The acidified eluates were analysed for the same group of metals determined in the char samples: Cd, Pb, Zn, Cu, Ni, K, Mn, Fe, Na, Mg Ca, Al, Cr, Mo, Ba, Hg, As, Se, and Sb. The equipment was also the Thermo Elemental Solaar AAS. 2.7. Ecotoxicological characterisation of the eluates The ecotoxicological parameter analysed was the inhibitory effect of the eluates on the light emission of the marine bacterium Vibrio fischeri (Azur Environmental Microtoxs system) according to the ISO 11348-3:1998 standard. The luminescence inhibition of V. fischeri was evaluated for an exposure period of 5, 15, and 30 min. A blank test was performed with the leaching solution (CaCl2 0.001 mol/L). The results of the ecotoxicity test were expressed as EC50 (per cent v/v) values, which represent the effective concentration of the eluate analysed that causes a reduction of 50 per cent on the V. fischeri bioluminescence. The 50 per cent threshold was adopted in the present work, which means that the light emission intensity must decrease by at least 50 per cent to be considered as toxic effect. If the reduction of the light emission intensity is below 50 per cent, then the effect is not considered as toxic. Determining the toxicity of chars' eluates to bacterial species is of great importance since bacteria are essential decomposers of organic matter and are the base for many aquatic and terrestrial food chains. Thus, adverse effects on bacteria are an indication of the presence of compounds that may have toxic effects on organisms of high trophic levels. The choice of V. fischeri as a test organism for the ecotoxicity evaluation in this work using the Microtoxs bioassay was based on its recognised advantages as a toxicity screening tool: answer speed, simplicity, reproducibility, precision, sensitivity, standardisation, cost effectiveness, and convenience (Girotti et al., 2008). 2.8. EDTA washing of chars To remove and identify the metals potentially responsible for the ecotoxicity presented by chars, a selective chelant extraction of heavy metals was performed on the chars. The chars were treated with 0.05 M disodium EDTA, at 60 1C for 2 h, with continuous stirring. The EDTA to char ratio was 100 ml/g. At the end of the washing process, the mixtures were allowed to settle and then filtrated to separate

3.1. Pyrolysis chars The product yields obtained in each of the pyrolysis experiments were the following: 1) Mixture 1–10 per cent (w/w) gases þ77 per cent (w/w) liquids þ 9 per cent (w/w) chars; 2) Mixture 2–12 per cent (w/w) gases þ68 per cent (w/w) liquids þ 8 per cent (w/w) chars; 3) Mixture 3–2 per cent (w/w) gases þ79 per cent (w/w) liquids þ 18 per cent (w/w) chars. The mass balance does not close to 100 per cent due to losses. The yield of liquids corresponds to the liquids settled from the pyrolysis reactor and the liquids (oils and tars) recovered in the sequential solvent extractions. The char yields correspond to the dried solids obtained after the sequential solvent extractions. The chars obtained are mainly originated as primary products from the thermal decomposition of pine biomass and/or rubber of tires, as the production of char from thermoplastics pyrolysis, particularly polyolefins, is quite low and resulting only from secondary tar-cracking reactions (Beyler and Hirschler, 2002; Scheirs and Kaminsky, 2006). Table 2 is a presentation of the composition of the chars based on the thermal and elemental analyses. The three chars presented residual amounts of volatiles but significant amounts of semi-volatile matter (volatility between 250 1C and 350 1C). These semi-volatiles are probably, asphaltene tars (complex polyaromatic hydrocarbons mixtures with alkyl side chains and heteroatoms side-chain functional groups) that are not soluble in the solvents used in the organic extractions, namely hexane and/or acetone, but are soluble in aromatic solvents such as toluene. However, the aromatic solvents were not used, because the goal of the sequential solvent extraction applied to the chars

Table 2 Composition of chars according to their volatility and elemental analysis. Char 1

Char 2

Char 3

Thermal analysis Volatiles (per cent w/w) Semi-volatiles (per cent w/w) Fixed residue (per cent w/w) Ashes (per cent w/w)

2.62 12.4 81.3 3.66

5.36 9.92 83.3 1.39

1.18 14.2 75.9 8.68

Elemental analysis (ar) Carbon content (per cent w/w) Hydrogen content (per cent w/w) Nitrogen content (per cent w/w) Sulphur content (per cent w/w) Oxygen content (per cent w/w)a H/C atomic ratio O/C atomic ratio

82.9 7.1 0.7 0.75 4.89 1.03 0.044

79.2 6.2 1.1 0.08 12.03 0.94 0.114

82 5.7 0.5 1.35 1.77 0.83 0.016

ar – as-received basis. a By difference (include the oxygen in the free moisture associated with the sample and errors).

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M. Bernardo et al. / Ecotoxicology and Environmental Safety 107 (2014) 9–15

was the recovery of the pyrolysis oils/tars using solvents of low toxicity. Char 2 presented a higher fixed residue (83.3 per cent) than carbon content (79.2 per cent) since the mass loss observed between 350 1C and 600 1C, assigned to fixed residue, might have included the decomposition/volatilization of mineral species. The chars obtained from mixtures with tires in their composition presented the highest ash contents. Char 3 showed the lowest H/C ratio being the most aromatic and hydrophobic char. The highest O/C ratio for char 2 suggests the presence of polar surface functional groups in this char. The highest sulphur content in chars 1 and 3 originated from the tires, which is the feedstock that presented the major content for this element.

3.2. Metal content of chars Table 3 is a presentation of the metal content determined in chars. Generally, char 3 presented higher amounts of metallic elements, which is consistent with its higher ash content (Table 2). The element dominant in chars 1 and 3 is zinc, being its source the used tires (Bernardo et al., 2012); zinc oxide is generally used as activator during the vulcanisation process of tires (Coran, 2005). In char 1, calcium is the element more abundant after zinc. Calcium is added as carbonate during tire manufacturing to act as filler (Mouri, 2001). Therefore, it could be assumed that the source of calcium in char 1 is the used tires. However, the concentration of calcium in char 3, which resulted from a mixture with a higher mass of tires, is significantly lower. There is the possibility that calcium is underestimated in char 3 or overestimated in char 1. In char 2, the prevalent metal was magnesium. The principal source of magnesium was the pine biomass (Bernardo et al., 2012). In addition, calcium, potassium, and iron are significant elements in this char. Char 2 also presented quantifiable amounts of chromium, nickel, and molybdenum, which were not detected in the raw materials (Bernardo et al., 2012). A possible explanation for the presence of these metals in char 2 is a contamination from the pyrolysis reactor, which is built with an alloy that contains these elements. The biomass pyrolysis creates an acidic environment inside the reactor that might be the cause of metal release from the reactor walls. Nevertheless, no damage in the reactor walls was observed in the plain sight. This result is significant as Table 3 Metal content of chars. Metals (mg/kg)

Char 1

Char 2

Char 3

Cd Pb Zn Cu Cr Ni Mo Ba Hg As Se Sb K Mn Fe Na Ca Al Mg

o4.78 76.6 75.9 91287 183 4.5 7 2.5 o4.5 o6.5 o8.8 o16.8 0.25 7 0.05 0.29 7 0.01 0.16 70.11 0.76 70.34 4077 61 11.5 7 0.3 1747 15 256 740 3723 7412 1807 67 341 791

o 4.78 15.8 7 4.3 99.6 7 1.9 o 4.3 36.4 7 5.4 70.0 747.2 1237 37 o 16.8 o 0.13 0.32 7 0.03 1.17 0.7 o 0.07 5707 83 22.5 7 0.1 309 7 22 8.7 7 5.4 594 7 233 o 34.0 1794 7 66

13.7 7 0.6 88.1 716.0 28,6857 1022 o 4.3 14.0 7 0.3 o 6.5 o 8.8 o 16.8 o 0.13 0.517 0.05 0.1 70.004 0.217 0.02 10117 119 3.5 7 1.5 3677 7 42.4 7 18.0 182 7 18 209 7 14 10337 238

The mean and standard deviation of duplicates are shown.

Table 4 Chemical and ecotoxicological characterization of chars' eluates. Eluate 1

Eluate 2

Eluate 3

pH Conductivity (mS/cm) TOC (mg/kg) IC (mg/kg)

6.07 451 1673 5.34

5.11 760 2885 3.57

6.68 347 204 28.1

Metals (mg/kg) Cd Pb Zn Cu Cr Ni Mo Ba Hg As Se Sb Al Mn Fe Na Ca K Mg

o 0.057 o 0.091 593 o 0.036 o 0.064 o 0.119 o 0.513 1.58 o 0.004 o 0.022 o 0.024 o 0.021 1.14 1.34 o 0.14 96.1 586 443.2 97.8

o0.057 o0.091 8.35 o0.036 o0.064 3.99 8.191 1.65 o0.004 o0.022 o0.024 o0.021 o0.869 1.09 o0.14 o0.065 879 256 156

o 0.057 o 0.091 29.54 o 0.036 o 0.064 0.244 1.517 o 0.758 o 0.004 o 0.022 o 0.024 o 0.021 o 0.869 0.152 o 0.14 o 0.065 55.8 185 21.9

EC50 (per cent v/v) 5 min 15 min 30 min

66.7 (50.9) 13.5 (90.9) 7.28 (95.9)

4100 (4.81) 4100 (11.9) 4100 (43.5)

4100 (19.8) 74.1 (64.8) 35.9 (92.5)

Values between parentheses – highest bioluminescence inhibition effect (per cent).

it shows the importance of a proper choice and design of pyrolysis reactor if a scale-up of the process will be considered. 3.3. Chemical and ecotoxicological characterisation of the eluates Table 4 shows the results of the chemical and ecotoxicological characterisation of the eluates obtained in the leaching process of chars. Eluates 1, 2, and 3 result from chars 1, 2, and 3, respectively. It can be seen that the chars obtained from mixtures with pine biomass (chars 1 and 2) gave origin to more acidic eluates, probably because of the release of acidic functionalities such as carboxylic and phenolic groups incorporated in the char matrix as a result of the incomplete thermal decomposition of the lignocellulosic matter. This hypothesis is strengthened by the higher TOC content of eluates 1 and 2. It was previously demonstrated that the pH of lignocellulosic chars is strongly dependent on pyrolysis temperature, being quite acidic for low pyrolysis temperatures (250 1C), slightly acidic for temperatures around 400 1C and become alkaline for higher pyrolytic temperatures (Mukherjee et al., 2011). Eluate 2 showed the highest conductivity which means that char 2 leached higher levels of salts that dissociated in solution. The IC of eluate 3 was significantly higher when compared to the other eluates, which puts in evidence a significant leaching of carbonates from char 3. From the concentrations of heavy metals, zinc leached in the highest amounts from the three chars, but particularly from chars 1 and 3 that were also the ones with higher concentrations of this metal. Although char 3 presented a zinc concentration 3 times higher than char 1, a linear correlation was not observed between the initial quantity of zinc in chars and the concentration of zinc eluted. There are several possible explanations for this fact: the more hydrophobic character of char 3 that creates aggregates, which expose a lower surface area than that of dispersed particles; the slightly more acidic eluate 1 may lead to higher zinc leaching;

M. Bernardo et al. / Ecotoxicology and Environmental Safety 107 (2014) 9–15

also, the speciation of this metal in both chars might have influenced its mobility. Char 3 presented a higher sulphur content, as already discussed for Table 2, and some zinc may be in the form of zinc sulphide (ZnS) originated in the sulphidation of zinc oxide, being this metallic sulphide quite insoluble for pH values above 6–7 (Lewis, 2010). In addition, lead, which was present in significant concentrations in chars 1 and 3, was not detected in the eluates, suggesting its presence in the chars is in a stable form. Lead might be present in these chars also as sulphide presenting a very low solubility for pH values above 4 (Lewis, 2010). Char 2 leached quantifiable amounts of molybdenum and nickel, but chromium was not detected. The stability and immobilisation of chromium in wood-derived chars already had been observed (Bernardo et al., 2009; Kakitani et al., 2004) by other authors that assumed the presence of chromium in the trivalent oxidation state, a very stable and insoluble form of chromium at moderate pHs. Alkali and alkaline earth metals leached more readily from chars resulting from the mixtures with biomass, which is consistent with the high conductivity presented by eluates 1 and 2. Regarding the ecotoxicological data, eluate 2 did not present toxicity to V. fischeri for all the exposure periods, although some level of effect in the undiluted leachate was observed, but not sufficient to get a measurement of EC50. Eluate 2 presented an inhibition of V. fischeri luminescence at a level quite similar to that obtained by Oleszczuk et al. (2013) with biochars made from coconut shell. Eluate 1 presented the highest ecotoxicity level, particularly at 30 min of exposure, in which a concentration of 7.28 per cent of the eluate caused 50 per cent of bioluminescence inhibition. Eluate 3 started to present ecotoxic effects only above 15 min. of exposure period; nevertheless, at the higher exposure time (30 min) a concentration of 36 per cent of eluate 3 induced 50 per cent of bioluminescence inhibition. 3.4. Relationships between the ecotoxicity of chars' eluates and their chemical composition The interpretation of results obtained from the chemical and ecotoxicological characterisation of chars' eluates allowed the assessment of possible relationships within chemical parameters and ecotoxicity data. The objective is the identification of specific contaminants responsible for the ecotoxicity levels of the aqueous soluble fractions of chars. The ecotoxicity levels of eluates 1 and 3 seemed to be mainly related with the presence of zinc in these eluates with significant concentrations, particularly in eluate 1, which was also the eluate with the highest ecotoxic effect. The hypothesis that zinc is the main metal responsible for the ecotoxicity of eluates 1 and 3 is supported by the absence or the presence in trace amounts of other potentially toxic elements in these eluates and by the presence of zinc in both eluates with concentrations able to cause toxic effects to V. fischeri. Zinc sulphate (ZnSO4) is used as inorganic reference toxicant in the Microtox test used in the present work and the EC50 values for 15 min are typically between 1.5 and 3.0 mg/L. However, it is necessary to confirm this relationship between zinc concentrations of eluates 1 and 3 and their respective ecotoxicity. Alternatively, it should not be excluded that the observed ecotoxicity of eluates 1 and 3 is because of the presence of organic compounds with toxic effects to V. fischeri. In fact, the presence of organic compounds in the eluates is confirmed by their quantifiable TOC content. Oleszczuk et al. (2013) observed that biochars with higher contents of polycyclic aromatic compounds (PAHs) were the ones with the highest inhibition of V. fischeri luminescence. Nevertheless, eluate 2 presented the higher TOC content and did not exhibit toxic effects to bacterium. This indicates a

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possible weak contribution of the leached organics for the ecotoxic response of eluates but only a detailed analysis of the organic fraction of the eluates can elucidate this possibility; however, the determination of the identity of the organics was outside the scope of the present work. Most of the organic compounds were removed from the chars during the sequential solvent extraction procedure but others still may be present. As referred in Section 3.1, the three chars presented residual amounts of volatiles but significant amounts of semi-volatile matter that are probably asphaltene tars; however these heavy tars are hardly soluble in water or aqueous solutions.

3.5. Influence of EDTA extraction on the ecotoxicity of chars Ethylene Diamine Tetraacetic Acid (EDTA) is widely recognised as a very effective chelating agent to remove heavy metal from soils and soil materials (Peters, 1999; Udovic and Lestan, 2012). From the authors' knowledge, this is the first time that the same strategy was applied to pyrolytic chars with the objective to selectively sequester metal ions that can be responsible for their ecotoxicity. The EDTA washing procedure was only applied to chars 1 and 3, as they were the ones that produced ecotoxic eluates. The choice of EDTA was because of its high affinity to zinc, the metallic element supposed to cause the ecotoxicity of eluates 1 and 3. Table 5 is a presentation of the detectable amounts of metals that were removed from the chars during the EDTA washing procedure. 27.5 Per cent of zinc was extracted from char 1 and 35.3 per cent from char 3 during the EDTA treatment. Some competition among other cations that have affinity to form chelates with EDTA was observed but these were identified mainly as being Ca, Mg, Fe, and K. The amount of calcium removed from char 3 (1517 mg/kg) is quite above the concentration that had been determined in the char (182 mg/kg) presented in Table 3, confirming the hypothesis that calcium was underestimated in char 3. Table 6 is a presentation of the results of the characterisation of eluates of the EDTA treated chars. Only zinc was monitored in the eluates. The pH difference of eluates was not significant so it can be concluded that this parameter did not influence the leachability of zinc. Alternatively, the concentrations of zinc leached from the treated chars were much lower than those leached before the EDTA treatment; the concentration of zinc leached from char 1 decreased 95.8 per cent and from char 3 decreased 87.8 per cent, which shows that after the EDTA washing the remaining zinc in chars' matrices was in a stable and more hardly-leaching form. This substantial decrease of zinc concentration in the eluates of chars was reflected in the ecotoxicological data with a corresponding significant increase in the EC50 values. Eluate 3 did not present ecotoxicity for all the exposure periods but some level of effect in the undiluted leachate was observed although not sufficient to get a measurement of EC50. Eluate 1 only presented ecotoxic effect for Table 5 Concentrations of metals extracted from chars 1 and 3 during the EDTA washing procedure. Metals (mg/kg)

Char 1 (EDTA)

Char 3 (EDTA)

Zn Cu Ni K Mn Fe Ca Mg

2509 o1.15 o2.57 156 o0.300 21.6 512 59.7

10,134 1.23 5.57 500 2.03 118 1517 196

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M. Bernardo et al. / Ecotoxicology and Environmental Safety 107 (2014) 9–15

Table 6 Chemical and ecotoxicological characterization of eluates of chars 1 and 3 after their washing with EDTA. Eluate 1

Eluate 3

pH Zn (mg/kg)

5.77 24.76

6.38 3.59

EC50 (per cent) (v/v) 5 min 15 min 30 min

4100 (24.1) 4100 (42.0) 49.1 (87.7)

4100 (25.4) 4100 (30.6) 4100 (34.0)

Values between parentheses – highest bioluminescence inhibition effect (per cent).

the longer exposure period (30 min), which is consistent with the fact that it was the eluate with the highest concentration of zinc.

4. Conclusions The leaching behaviour and the potential environmental effects of chars obtained from the pyrolysis of mixtures with plastics, used tires, and pine residues were assessed in this work. The results allowed to conclude that the composition of the feedstock submitted to pyrolysis have a considerable influence on chars characteristics: the highest is the ratio of tire rubber in feedstock; the highest is the aromatisation, hydrophobicity degree, and ash content of the resulting chars. Alternatively, the introduction of pine biomass on the feedstock composition enhances the acidic character of chars promoting the mobilisation and release of inorganic elements. Chars resulting from the pyrolysis of mixtures with pine and plastics did not produce ecotoxic eluates, which is a good indicator that this type of chars can be safely applied to environmental matrices. A relationship between zinc concentrations in eluates and their ecotoxicity behaviour was found for chars obtained from mixtures with tires in their composition, suggesting the necessity of controlling zinc mobility in these tire-derived chars. A significant ecotoxicity reduction was registered when the chars were treated with EDTA because a significant reduction in zinc leaching was also observed. The EDTA treatment allowed removing the most leachable fraction of zinc from the chars' matrices, remaining after that the more stable and hardly-leaching forms of Zn. If these chars were considered to be used in environmental applications, particularly water applications, the best condition is above pH of 6–7, since there is a significant risk of zinc mobilisation in the case of lower pH values at which this metal is highly soluble.

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