Effect of sewage sludge properties on the biochar characteristic

Accepted Manuscript Title: Effect of Sewage Sludge Properties on the Biochar Characteristic Author: Anna Zieli´nska Patryk Oleszczuk Barbara Charmas Jadwiga Skubiszewska-Zi˛eba Sylwia Pasieczna-Patkowska PII: DOI: Reference:

S0165-2370(15)00037-6 http://dx.doi.org/doi:10.1016/j.jaap.2015.01.025 JAAP 3403

To appear in:

J. Anal. Appl. Pyrolysis

Received date: Revised date: Accepted date:

5-8-2014 21-1-2015 27-1-2015

Please cite this article as: Anna Zieli´nska, Patryk Oleszczuk, Barbara Charmas, Jadwiga Skubiszewska-Zi˛eba, Sylwia Pasieczna-Patkowska, Effect of Sewage Sludge Properties on the Biochar Characteristic, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2015.01.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of Sewage Sludge Properties on the Biochar Characteristic

Anna Zielińska1, Patryk Oleszczuk1*, Barbara Charmas2, Jadwiga SkubiszewskaZięba2, Sylwia Pasieczna-Patkowska3

1Department

of Environmental Chemistry, Faculty of Chemistry, 3 Maria Curie-

Skłodowska Square, 20-031 Lublin, Poland 2Department

of Chromatographic Methods, Faculty of Chemistry, 3 Maria Curie-

Skłodowska Square, 20-031 Lublin, Poland 3Department

of Chemical Technology, Faculty of Chemistry, 3 Maria Curie-

Skłodowska Square, 20-031 Lublin, Poland

Correspondence to: Patryk Oleszczuk, Department of Environmental Chemistry, University of Maria Skłodowska-Curie, pl. M. Curie-Skłodowskiej 3, 20-031 Lublin, Poland, tel. +48 81 5375515, fax +48 81 5375565; e-mail: [email protected]

1

ABSTRACT

The present study evaluated how the initial sewage sludge properties affect the characteristics and composition ofsewage sludge-based biochars. Sewage sludges of varying organic matter contentwerepyrolyzed attemperatures of 500, 600 and 700°C. The obtained materials were characterized in terms oftheir composition and physicochemical as well as their surface and thermal properties. With increasing treatment temperature,the pH,ash content and macro- and micronutrient content increased.The biochararomaticity also increased. On the other hand,the pyrolysis yield, percentages of H, N and O, molar ratios, polarityof biochars,and crystallitesize decreased. The direction of the changes in the content of elemental carbon (C) and in surface areawas dependent on the type of sewage sludge. It was found that for some properties of biocharsproduced could be determined on the basis of an analysis of the properties of the initial sewage sludge.

Keywords: sewage sludge; biochar; temperature; properties;

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1. Introduction Increasing problems associated with the need todispose of sewage sludges have been observed in recent years. Sewage sludge is a waste difficult to recycle. Due to this, new solutions are constantly sought.Biocharcan be an answer to the current problems related to sewage sludge reuse.Biocharand its use are not a new solution. This applies in particular to its use in agriculture. The renewed interest inbiocharbegan to grow in the 1980’s. It can be said that biocharis now being “discovered” anew[1]. Conversion of biomass to biocharand its use for soil amendmenthave been proposed as oneof the best methods of climate change mitigation through soil carbon sequestration [2]. Furthermore,the alkaline pH of biochar may account for the liming effect induced by biochars. In this way, they be can be effectively used forsoil deacidificationand thereby increase crop productivity. Biochars significantly differ from one to anotherin their properties.Theproperties of biocharsdepend on the type of biomass used to produce biochars, growth conditions of biomass and alsoon pyrolysis conditions[3–8]. Temperatureis one of the most important parametersof the pyrolysis processand as a consequence significantly affects the chemical and physical properties of biochar[8]. Taking into account thepotential possibilities of biocharuse in agriculture, the following should be included in its most important properties: chemical composition, specific surface area, and porosity. In terms of their chemical composition, biocharsdiffer from other types of organic matter in a much higher content ofaromatic carbon compounds. These materials alsocontain a mineral fractionconsisting of macroand micronutrients. Due to this, they are a valuable source of mineral substances (among others, calcium, magnesium, and carbonates) for soil microorganisms[9]. 3

The concept of biocharproductionfrom sewage sludgehas become more popular in recent years[4,7,8,10–16]. However, this research is predominantly focused onanalysis of biochar produced either from one type of sewage sludgeor within a limited range of temperatures. In such case, the sewage sludge characteristics are not complete, either. Sewage sludge is such a diverse material in terms of its physico-chemical propertiesthat the simplification of results based on only one biochar(and this has been done so far) may lead to wrong conclusions. In the literature, there is no information on systematic evaluation of the properties of biochars produced from sewage sludges of varying properties and at different temperatures. Thus, to know the answer to the question “How do the initial properties of sewage sludgesdetermine the properties of biocharderived from them?” is of key importance in a situation of growing interest in the use of sewage sludge for producing biochars. The aim of the present studywas to determine how the sewage sludge properties determine the properties of sewage sludge-basedbiochars, using advanced analytical methods. The detailed characteristics of materials of varying properties before and after pyrolysis at different temperatureswill allow the properties of biocharproduced to be predicted to a certain degree. Furthermore, this will enable its parameters to be adapted with a view to producing materials that will best perform their functions related to soil amendment or the removal of contaminants from the soil.

2. Materials and methods

2.1. Sewage sludges Four different sewage sludges (SS)were used in the present study. The sewage sludges were obtained from municipal (mechanical - biological) wastewater treatment plants 4

(WTTPs) located in different regions of Poland: Koszalin (KN, 54°11′25″N 16°10′54″ E), Kalisz (KZ, 51°45′45″N 18°05′23″E), Chełm (CM, 51°07′56″N 23°28′40″E) and Suwałki (SI, 54°06′04″N 22°55′57″E). All the WTTPs use an anaerobic digestion process and dewatering. The test samples were selected from a larger group of sludges, characterized by diverse properties.As the main criterion for the sludge selection, their content of total organic carbon (TOC) was adopted as a factor that potentially in the greatest extent may determine properties of biochars. Sewage sludges were collected during summer 2012, at the end point, after the sewage sludge digestion process.A few representative subsamples were taken for the present experiments. Samples were mixed, dried in air (about 25oC for few weeks) in the dark, ground and passed through a 2 mm sieve. Such pre-prepared sewage sludge samples were stored in glass containers, and then subjected to the process of pyrolysis.The sewage sludge samplesbefore pyrolysis were characterized by moisture content, which was 4.9 (SSKN), 4.3 (SSKZ), 4.6 (SSCM) and 4.4% (SSSI).

2.2. Biochar preparation The sewage sludge samples (about 75-150 g) have been subjected tothe process of pyrolysis in the furnace of own construction. Biochars (BC)were prepared at 500, 600 and 700oC. The pyrolysis heating rate was employed at 25°C min-1.The temperature was measured with a thermocouple and was held for 5 h (slow pyrolysis).The selection of pyrolysis time was based on our preliminary studies and research of other authors [4,7,8,10–14,16,17].These conditions help to produce the biochar well pyrolyzed[18]. The samples of sewage sludge were placed in thepre-heated quartz tube (about 300 mL). The quartz tube with sewage sludge was inserted into already heated furnace. It allows to keep required temperature (decrease in the temperature was not greater than 5

10°C relative to the required temperature of pyrolysis). The most intensive stage of pyrolysis lasted up to60 minutes depending on the temperature. As evidence, the intense fumes emission was observed during this stage of the process.During the pyrolysis process thegases and bio-oilproduction havenot been examined. An oxygenfree atmosphere during the process of pyrolysis was maintainedbythe constant flow ofnitrogen, whichwas controlledat the mass flow controller(BETA-ERG, Poland). The nitrogen gas was dosed straight from the tank and injected at a rate 630 mL N2 min-1 without preliminary pre-heating.

Sewage sludge-derived biochars were produced five times (subsamples) and the subsamples were mixed together and homogenized to prepare a composite sample for chemical analysis.

2.3. pH, elemental composition, ash content and pyrolysis yield A 1-gram sample of a dry sewage sludge or sewage sludge-derived biochar was dispersed into 10 mL of deionized water. The pH of the suspensions was measured using a digital pH meter[19]. Hydrogen (H) and nitrogen (N) were determined using CHN 2400 series (Perkin-Elmer, 1997) equipment via combustion at 950oC. Prior to analysis, the samples were additionally homogenized by grinding. Total O was determined by subtraction as follows [5]:

Ash content of sewage sludge and biochar samples was estimated following the ASTM D 3176 standard method by combustion of dry samples at 760oC for 6 h and measured as the residue remained after heating[20]. The ash content was determined according the following equation (2):

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The pyrolysis yield (PY) was determined as the ratio of the produced biochar weight (mBC) to the dry weight of sewage sludge subjected to pyrolysis (mSS):

2.4. Mineral fraction The X-ray fluorescence (XRF) spectroscopy technique was used for the characterization of the inorganic constituents of sewage sludge and biochar samples. The analysis was used to identify metals in the samples with atomic numbers in the range from uranium to sodium (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Sb, Ba, Pb and Bi) and to measurement these elements content. The AxiosmAX (PANalytical, Netherlands, 2012) X-ray fluorescence spectrometer was used, operating on the basis of the measurement of the wavelength dispersion - WDXRF (Wavelength Dispersive X-Ray Fluorescence), using SuperQ software (version 5.0). The samples for measurement were prepared in the form of compressed tablets. They were excited ceramic X-ray tube Rh SST-mAX equipped with an ancestral anode with power of 4 kW.

2.5. Crystallographic structures The existence and crystalline form of structures present in sewage sludge and biochar samples was checked using Empyrean (PANalytical, 2012) X-ray diffractometer. XRD measurements were conducted using standard powder diffraction procedures[14].

2.6. Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) FTIR-PAS spectra of the biochar samples were recorded by means of the Bio-Rad Excalibur 3000 MX spectrometer equipped with photoacoustic detector MTEC300 (in 7

the helium atmosphere in a detector) at RT over the 4000-400 cm−1 range at the resolution of 4 cm−1 and maximum source aperture. The spectrum was normalized by computing the ratio of a sample spectrum to the spectrum of a MTEC carbon black standard. A stainless steel cup (diameter 10 mm) was filled with biochar sample (thickness <6 mm). Interferograms of 1024 scans were averaged for the spectrum[15].

2.7. Raman spectroscopy In order to determine both the crystalline and amorphous nature of the carbon in the biochar samples ainVia (Renishaw, UK, 2006) Raman microscope equipped with 514 nm laser diode was used. The spectral resolution was 4 cm−1 with 50% laser power and a total acquisition of 15 was considered. The Raman spectra in the range of 150–3200 cm−1 were curve-fitted using the WiRE Raman software (version 3.2).

2.8. Surface properties The parameters of the sewage sludge and biochar porous structure were determined from low-temperature (77.4 K) nitrogen adsorption-desorption isotherms obtained from an ASAP 2420 (Micromeritics, USA) surface area and porosity analyzer. Before adsorption measurements, the samples were out gassed at 200oC under vacuum. From the adsorption branch date, using linear form of the BET equation the specific surface area (SBET) was calculated. Total pore volume (Vt) was calculated from the last point of isotherm (p/p0 ≈ 1) and mean pore size (d) was obtained using the equation (4), assuming their cylindrical shape:

Volume of micropores (Vmicro)and their specific surface area (Smicro) was evaluated by the t-plot method. Volume of mesopores (Vmeso) was calculated as a difference 8

between the total pore volume and the volume of micropores[17].

2.9. SEM Surface morphology of biochars was studied by scanning electron microscopy (SEM). SEM images of biocharswere obtained using a scanning electron microscope (SEM) Quanta 3D, FEG, FEI with an accelerating voltage of 2.00 kV in high vacuum mode [14].

2.10. Thermal analysis Thermal analysis of sewage sludge and biochar samples was carried out using Derivatograph C (Paulik, Paulik&Erdey, MOM, Hungary). The test samples were placed in ceramic crucibles, using alumina as a reference. The sample, weighting approximately 10 mg, was heated from 20 to 1200oC with a heating rate of either 10oC min-1 under an air atmosphere. TG and DTA curves were recorded. The differential thermogravimetric (DTG) curves were obtained by numerical derivation of the TG curves[14].

3. Results and discussion

3.1. pH of sewage sludges and biochars The sewage sludges used in thisstudy were characterized by neutral pH (Table 1). Sludge pyrolysis had an effect on the change inthe pH of the biocharsproduced, but a significant difference was found only at temperatures ≥600oC. The biocharsproduced at the lowest temperature (500oC) were characterized by similar pH to sewage sludges. An increase inpyrolysis temperatureup to 600oC caused a significant increase in pH (up 9

to 11.0). Only the biocharBCSI600 was an exception, since in its case the increase in pH was not as significant as the one observed for the otherbiochars. The biocharsproduced at the highest temperature (700oC) were characterized by a pH ranging from 12.0to 13.0. An increase inthe pH with increasing temperatureis a typical phenomenon observedforbiocharsderived from both sewage sludge[4,8,10]and other feedstock [5,6,19]. This process is associated with the polymerization/condensation reactions of aliphatic compounds (at lowertemperatures – minimal) and with the effect of dehydrationof the feedstockassociated with a decrease in the amount ofacidic surface groupsduring thermal treatment[4]. Moreover,the increasein pH could have been due to the concentration of inorganic constituentsin the biochar[5]as a result of the separation ofalkali metal salts from theorganic matrix with increasingtemperature [21,22]. Based on the available literature and the obtained results,it can be stated that the pH of the biocharproduced at a low temperature (≤500oC) largely depends on the sludgepH.Hossain et al. [10] observed that a strongly acidic sludge pH (<4.5) at 500oCresults in the formation ofbiocharwith a neutral pH (≤7.3). In turn, the other authors [4,8] indicated thatthepH of the sludgepyrolyzed at≤450oC in the range of 5.87.8 produces biocharwith a neutral or alkaline pH.It should be noted that the sewage sludgespyrolyzed in this study were characterized by neutral pH and at a temperature of 500oC this also resulted in producing biocharswith a neutral pH (Table 1). The highertemperaturesof pyrolysis (≥550oC), on the other hand,promote the formation ofbiocharwith an alkaline pH, regardless of the feedstock pH[4,8,10,14,16], what was also confirmed byour results.

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3.2. The elemental composition The percentage of C in the investigated sewage sludges ranged from 21.6 to 26.2% (Table 1). The SSSI sludge was characterized by the highest C content, whereas the lowest percentage was found for SSKN. The sludges were also characterized by a relatively low percentage of H (from 3.8 to 5.1%) (Table 1). The content of N was at a similar level (from 3.5 to 5.7%) (Table 1). The highest percentages of H and N were found for the SSSI sludge which, at the same time, was distinguished by the highest C content. Basically, however, no significant variations were observed in the percentages of individual elements between the sludges. An exception was the percentage of O which ranged from 4.4 to 14.8%, depending on the sludge (Table 1). Generally, sludge pyrolysis did not have a significant effect on the change in the percentage of C in the biocharsproduced (Table 1). An exception was the SSKN sludge whose pyrolysis caused a significant decrease C contribution by about 12% to 16%, depending on the temperature. However, a slight, though statistically significant increase (6%) in the C was observed for the SSSI sludge pyrolyzed at 600 and 700oC. The content of H, N and O decreased significantly after pyrolysis in all biochars compared to the initial sewage sludges (Table 1). The greatest changes were observed in the percentages of O and H. For example, in the BCKN700 biochar the proportion of Odecreased by 95.4%, while for H by 94.3% relative to the initial sludge. The percentage of O and H was reduced by 94.6 and 92.9%, respectively, for the BCKZ700 biochar. After pyrolysis, the N content did not change drastically, as observed for H and O. For N, the highest losses were found for the biochars produced at 700oC and they were from 34% (BCCM) to 52% (BCKZ). Generally for all the sludges, the greatest changes in the content of O, H and N were recorded at the highest pyrolysis temperature. 11

While an increase in C content in a biochar relative to the feedstock is a typical phenomenon in most pyrolyzed organic materials [3,5,6,19,23], but for sewage sludges a decline of this parameter is usually observed [11,13,14,16], though exceptions happen [4,13]. For the tested materials both the increase and decrease in C content as a result of pyrolysis of the sewage sludgewere noticed. In the present study, an increase of C content was only observed for the SSSI sludge (Table 1). This is most probably associated with the loss of –OH surface functional groups as a result of dehydration, which is confirmed by the FTIR-PAS data (discussed further down in this paper). At higher temperatures, carbon-bound O and H atoms are also lost due to the cleavage of weaker bonds [3,19]. Under such conditions, the concentration of C increases as a result of the removal of volatile products of decomposition of both organic and inorganic matter. Agrafioti et al. [13] found that the direction of changes in C content was dependent on the pyrolysis temperature of sewage sludge. On the other hand, Gasco et al. [4] indicate the type of sludge to be the main factor affecting the C content. In presented research, both pyrolysis temperature and the sludge properties played an important role for the percentage contribution of C to the final product. Most studies on sewage sludge pyrolysis show a reduction in the percentage of C in biochar produced relative to the feedstock. In the present research, this was observed only for the SSKN sludge. The losses in C are most probably attributable to increased volatility of C during the process of increasing pyrolysis temperature. This supposition is confirmed by the X-ray diffraction (XRD) data (discussed further down in this paper). ForBCKZ and BCCM, no significant difference was found (with the exception of BCKZ700 and BCCM700) and for BCSI the increase of C was observed.

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3.3. Ash content Both the investigated sewage sludges and biochars were characterized by high ash content, which ranged from 55.8 to 61.3% in the sewage sludges and from 64.1 to 79.1% for biochars. It shows an increase in ash content compared to the feedstock. Moreover, with increasing pyrolysis temperature the value of this parameter increased. The BCKN biochar (73.56-79.08%), whose feedstock contained the lowest amount of ash (55.8%), was characterized by its highest content. An increase in ash content with increasing pyrolysis temperature is a typical tendency for biochars derived from both sewage sludge [4,8,10] and other materials [5,19]. This is associated with the concentration of non-volatile mineral constituents forming ash [24] and also with the removal of volatile organic decomposition products.This is confirmed by the fact that mineral fractions were a dominant fraction both in the initial sewage sludges and in the biochars. The ash content in sewage sludge-based biochar is much higher compared to biochars derived from other materials [5]. This results from the complexity of sewage sludge and from the diversity of components contained in it. Sewage sludges content very high concentration of silica (19-58%in sewage sludge before pyrolysis), discussed further down in this paper, is of additional importance. Based on the proportions of mineral components in sewage sludge, it is however difficult to predict exactly the ash content in biochars produced from it. Nevertheless, it is known that, regardless of pyrolysis temperature, the percentage of ash in biochar will be higher than in the feedstock and moreover an increase in temperature will cause an increase in the percentage of ash in pyrolyzed sewage sludge.

3.4. Mineral fraction 13

Table 2 includes the elemental composition of the mineral part of the initial sewage sludges and biochars. The investigated sewage sludges contained large amounts of Ca (5.1-7.4%), Si (2.5-5.8%,), and P (3.4-4.9%), while in the samples of SSKZ and SSKN large amounts of Fe (respectively, 5.2 and 6.8%) and significant amounts of S (respectively, 3.8 and 2.8%) were additionally identified. The analysis of these data reveals that the sewage sludge pyrolysis caused an increase in the content of all elements in the biochars produced. The increase in pyrolysis temperature also resulted in an increasing trend in the percentage of the elements analyzed (Table 2). This related in particular to the biochars derived from the SSKN sludge. The highest percentage increases of the respective elements (except of P, Al and Mg), compared to the initial sewage sludge, were found for the BCSI biochar. In turn, the highest increase in the content of Al and S was found for the BCKN700 biochar (by 77.1 and 61.1%, respectively). The only element whose amount decreased in relation to the initial sludge was sulfur (S) for BCSI. As a result of pyrolysis, sulfur-containing compounds are degraded to volatile SO3 and hence the decrease in the content of this element in the biochars relative to the sludge feedstock. The increase in the concentration of the other elements during sewage sludge pyrolysis is typical. This is caused by the increased concentration of elements in the biochar samples due to the gradual loss of C, H and O [19]. These elements (Al, P, Ca, Mg, Na and K) cannot be lost, since their oxides are not volatile, except for the above-mentioned sulfur. The increase in the content of alkali elements (Ca, Mg and K) may be responsible for the liming effect induced by the biocharspyrolyzed at high temperatures [19]. Due to their high content of alkali elements, such biochars can be effectively used for soil deacidification.

3.5. Crystallographic structures 14

The crystallographic structures in the sewage sludges and sewage sludgederivedbiochars are shown in Table 3. The presented data relate only to compounds in crystalline form. SiO2 had a major contribution in most sewage sludges (SSKN, SSKZ and SSCM) ranged from 35.8 to 58.1% of all crystallographic structures (Table 3). The presence of SiO2 in the sludges is associated with the presence of sand removed from sewage as a result of mechanical pre-treatment. The next crystalline phase found in large amounts wasdihydrate calcium sulfate – CaSO4∙2H2O (from 19.1 to 34.7%of all crystallographic structures). Moreover, hydrated calcium hydroxyphosphate (CaPO3(OH)∙2H2O) was identified in the samples of SSCM and SSKN. The presence of this compound is probably caused by liming of sewage to remove phosphorus from it. Forsewage sludges SSKN and SSKZ, titanium (IV) oxide, which is one of ingredients of cosmetics and toothpaste, occurs in small amounts. The SSSI sludge was characterized by a different composition than the other sludges. Magnesium ammonium monophosophate (NH4MgPO4∙6H2O) (struvite) was a dominant compound in this sludge, which was associated with a different sewage treatment system compared to the other wastewater treatment plants. After the pyrolysis process, silica continues to have a significant proportion in biochars. Its percentage content decreased minimally compared to the initial feedstock in favor of new compounds formed as a result of thermal treatment. CaS appeared in the biochars, which can be a product of a reaction of anhydrite with carbon contained in the biochars produced according to the following reaction (1): CaSO4

+

2

C

→

CaS

+

2

CO2↑

(1) This reaction could have occurred during pyrolysis of the SSKN sludge and it may be responsible for the losses in C, volatalized as CO2, which were observed in this sludge. 15

To be more precise, this reaction occurred in the following way: during pyrolysis of SSKN, CaSO4∙2H2O contained in this sludge gradually lost successive water particles with increasing temperature until it was completely dehydrated to the form of anhydrite (CaSO4). This is confirmed by the data presented in Table 3 (the BCKN700 sample). The decrease in the content of elemental carbon in the biochar is accompanied by the appearance of CaS in it. During sewage sludge pyrolysis, it could also be observed that with increasing pyrolysis temperature the content of calcium sulfide (CaS) in the biochar also increased. At the same time, the amount of CaSO4 was observed to decrease. In the biochars in which CaSO4∙2H2O was not dehydrated completely (BCCM500, BCCM600, BCSI500, BCSI600, BCSI700), calcium sulfide does not appear. This suggests that the pattern of this reaction may be dependent on the amount of CaSO4 in the biochar. Hence, an intermediate form associated with gradual loss of water, namely CaSO4∙0.5H2O, can also be observed in the biochars. In turn, berlinite (AlPO4) identified in the sludges SSSI and SSCM is probably responsible for the atypical properties of the biochars derived from these sludges which are attributable to the decreasing specific surface area. On the other hand, potassium alum (KAl(SO4)2∙12H2O) identified in the sludges SSSI, SSCM and SSKZ does not appear in any biochar. It should also be stressed that titanium (IV) oxide identified in the sewage sludges did not change in the biochars produced, which is due to its thermal resistance.

3.5. FTIR-PAS Interpretation of theIRspectraof all carbonaceous materials, including biochars,is complicated because eachof the functional groups visible on the IR spectrum may be 16

responsible forthe appearance ofmultiplebandsin a wide rangeof wavenumbers, so that each bandmay have acontribution of manyfunctional groupspresent on thesurface of the sample. This is notthe only oneinconvenience. Biocharobtained fromsewage sludgecontains also admixturesof minerals - IR bands then oftenoverlap with the bands of oxygenfunctional groups. Such a situationoccurs in the caseof examinedbiochars. Spectra of examined biochars have several bands in the 4000-2500 cm-1range (Fig. 1). The broad band centered at about 3400 cm-1 may be attributed to the stretching vibration of hydrogen-bonded hydroxyl groups of water or to phenolic C-OH stretching [25]. The bands at 3650 may be attributed to the “free” O-H groups of alcohols [26,27]. The band at 3350 cm-1 suggests the presence of the OH-ether hydrogen bonds [28]. However, bands at ~3440 and 3345 cm-1 are also observed in the spectra of some inorganic compounds, namely carbonates, sulphates and phosphates [27,29]. These spectroscopic results are in a good agreement with the data included in Table 2, what may confirm the presence of inorganic matter in all biochars. In the FTIR-PASspectra of all starting materials (before pyrolysis) there are bands due to aliphatic C-H stretching within 2961-2855 cm-1range (Fig. 1). Increase in pyrolysis temperature results in decrease of stretching intensity of these groups, but only in the case of BCCM (Fig. 1C) and BCSI (Fig. 1D). No absorption bands due to aliphatic CH bonds within 2975-2840 cm-1range were found in BCKN (Fig. 1A) and BCKZ spectra (Fig. 1B), obtained even at relatively low temperature (600C for BCKN, 500C for BCKZ). Increase in pyrolysis temperature results also in a decrease in O-H stretching for all biochar samples. In the spectra of all biochars within 4000-2500 cm1range

there are stillbands (3400, 3345 cm-1) due to the presenceof rather inorganic

compounds, than -OH groups in carbon oxygen structures, becausethe presenceof the lattercannot bedefinitelyconfirmedby analyzing the2000-600cm-1spectral range for the 17

samples after pyrolysis (Fig. 1). In the FTIR-PASspectra of starting materials (before pyrolysis) in the 2000-600 cm-1 range there are bands at ~1730 cm-1 (C=O), 1620 cm-1 (C-OH, C=O and/or C=C), 1540 cm-1 (carboxylate anion), 1460 cm-1 (-OH in carboxyl structures and/or –CH bending) and in the 1310-1110 cm-1 range (C-O) which may indicate the presence ofoxygenfunctional groupson thesurface. Some of them disappear in the IR spectraof pyrolyzed samples, even for biochars obtained at 500C (Fig. 1). However, in the spectra of all biochars, bands at 1620, 1400, 1155, 1110, 1055, 1000 cm-1 still remain, regardless of the pyrolysis temperature. These bandsindicate the presenceof inorganic compounds, such as carbonates, sulphates, phosphates and silica compounds [27,29] rather than any of oxygen containing groups on biochars surface. Suchargumentcan be confirmedwhile analyzing thedata in the Table 3. The band at ~1400 cm-1 may indicate both carboxyl-carbonate structures and aliphatic C-H stretching. The latter cannot be certainly confirmed when analyzing2961-2855 cm-1 range (Fig. 1). Additionally, the band centered at about 875 cm-1 may be indeed the result of C-H bending in aromatic structures or rather the out-of-plane bending for carbonate structures. The latter may be confirmed by the presence of the band at 1100 cm-1[22].The bands at 980, 780, 680 cm-1 can be additionally assigned to Si-O vibrations of inorganic materials in biochar [30]. While comparing IR spectra of BCKN (Fig. 1A), BCKZ (Fig. 1B) biochars and BCCM (Fig. 1C), BCSI (Fig. 1D) biochars in the 4000-2500 cm-1range, some differences can be seen. As it was already mentioned, the C-H stretching vibrations are not observed in the spectra of BCKN and BCKZ biochars, probably because of the formation of carbon structures akin to graphite, while these bands are still present in the spectra of BCCM, BCSI biochars. This may indicatethatdifferent formsof carbon are present in 18

the case of BCCM, BCSI (more aliphatic character) and in the case of BCKN and BCKZ biochars (more graphite-like structures).

3.6. Molar ratios To characterize the relationships between the pyrolysis temperature of the respective sewage sludge and the degree of biochar hydrophobicity/polarity, the molar ratios between particular elements, O, C, H and N, were calculated for the sewage sludges and biochars (Table 1). For sewage sludges, the values of the O/C ratio ranged from 0.13 (SSSI) to 0.52 (SSKN). The H/C values for SSKN, SSKZ and SSSI were practically at the same level (2.33-2.35). A slightly lower value (1.99) was only obtained for SSCM. Much greater differences were however found for the (O + N)/C ratio. Depending on the sludge, this parameter ranged from 0.32 to 0.66 (Table 1). After the pyrolysis, all the ratios were found to significantly decrease relative to the values obtained for the initial sewage sludges. It should also be stressed that the higher the pyrolysis temperature, the lower this ratio was (Table 1). Relative to the sewage sludge, the largest changes were found for the O/C ratio for biochars BCKN600/700 (a decrease of 90.4 and 94.2%, respectively) and BCKZ700 (a decrease of 94.4%) as well as for the H/C ratio for BCKN600/700 (a decrease of 89.3 and 93.2%, respectively) and BCKZ600/700 (a decrease of 90.6 and 92.8%, respectively). These ratios also significantly changed for the BCSI700 biochar: the O/C parameter decreased by 84.6%, while the H/C ratio by 91.0%. As regards the other biochars, the values of these ratios declined less (at the maximum 69.2% for O/C and 84.5% for H/C). The changes in the (O + N)/C ratio were greater compared to O/C and smaller relative to H/C. The greatest change in the parameter in question, 19

compared to the sewage sludge, was found for the BCKN biochars. The obtained results show that an increase in pyrolysis temperature caused a reduction in the values of all the ratios for the elements analyzed. The largest changes were found for the O/C parameter. On the other hand, the changes in the values of H/C and (O + N)/C were at a similar level. It turns out that the observed reduction in the values of the particular ratios of the elements, both in relation to the initial feedstock and with increasing temperature, is typical for pyrolyzed organic materials [5,6,19,23]. The decrease in the value of the O/C parameter with increasing temperature results from the dehydration reaction and corresponds to the less and less hydrophilic biochar surface [6]. This is confirmed by theFTIR-PAS spectra, or to be more accurate, by the reduced intensity of the 3400 cm-1 band corresponding to stretching vibrations of hydroxyl groups bound by hydrogen bonds in the water particles. Therefore, the hydrophobicity of biochars increases due to a substantial removal of O. The reduction in O content at high temperatures is also an effect of the removal of acidic functional groups, causing the biochar surface to become more alkaline [31]. This supposition is confirmed by the higher pH values for the biochars produced at the higher temperatures (Table 1). The decrease in the H/C ratio value indicates an increased level of biochars carbonization [6,19]. The reduction in this parameter observed in the investigated materials is primarily due to the decrease in the proportion of H. The low H/C ratio, due to a high degasification of the samples, suggests that these biochars are highly thermally modified and have a large content of unsaturated structures. The decrease in H content with increasing temperature and the increase in the proportion of C in some biochars indicate the higher aromaticity of the biochars produced at 700oC compared 20

to the biochars obtained at the other temperatures, in particular at the lowest one (500oC). Biochars with a high content of aromatic carbon compounds are resistant to microbiological decomposition, which ensures their long half-life time in soil. It can be presumed that they will perform well for long-term soil C sequestration [32]. The high values of the H/C ratio for the biochars BCSI500 (0.49) and BCKN500 (0.46) demonstrate that these biochars contain large amounts of primary organic matter (residues). This is confirmed by the FTIR-PAS spectra, or to be more accurate, by the reduced intensity of the bands corresponding to C-H stretching vibrations of the aliphatic groups with increasing temperature for biochars BCCM and BCSI (Figs1C, 1D) and by their absence for BCKN600 and BCKN700 (Fig. 1A) as well as BCKZ (Fig.1B). Analyzing the spectra of the biochars produced at 700oC (Fig. 1) and the values of the H/C ratios, it can be confirmed that the biochars produced at higher temperatures exhibit higher aromaticity – in particular BCKN (Fig. 1A) and BCKZ (Fig. 1B). The highest values of the (O + N)/C ratio for the biochars produced at 500oC indicate that the samples of these biochars contain polar functional groups, that is, they can potentially interact with water. The amount of these groups decreases with increasing temperature. Due to this, the biochars produced at the highest temperature contain a higher amount of aromatic carbon compounds and their surface is less polar, which is also confirmed by the O/C value. This shows that the biochars produced at the lowest temperature will most interact with water.

3.7. Raman spectroscopy The data included in Table 1 show that the size of graphite crystallites (Lα)in the 21

investigated adsorbents ranged between 17.86 and 24.28 nm. Their sizes were observed to decrease with increasing pyrolysis temperature. At the same time, the intensity ratios of the ID/IG bands were found to increase (Table 1). It can be concluded that with increasing pyrolysis temperature more regular carbon rings occur in the biochars and they form smaller and smaller crystallites (Table 1). This is confirmed by the increase in the D band intensity. Due to the fact that the G band does not change its height (data not presented), integral intensity or width, one can conclude about the disordered nature of carbon found in crystallites. Moreover, this disorder decreases with increasing temperature. This is also confirmed by the fact that in the case of disordered carbons the Raman spectra are dominated by sharp D and G bands describing the sp2 carbon phases, just as in the case of the biochars studied. However, the analyzed materials lack amorphous carbon phases (data not presented) which are manifested by the presence of a wide band (GR) in the region 1500-1550 cm-1.

3.8. Surface properties The N2adsorption/desorption isotherms (Fig. 2) for the initial sewage sludgesand sewage sludge derived biochars reflect the changes in the porous structure of the adsorbents in question. According to the IUPAC classification,these isotherms can be classified as Type IV corresponding to physical adsorption[33]. The analysis of their shape reveals that the initial sludges are characterized byvery poorly developed porosity andslight adsorption is observedin theentire relative pressure range. In theisotherms of the biochars, on the other hand, we observe an increase inadsorption at the low values of p/p0, which is evidence of the development ofmicroporosity (especiallyin the case ofthe biocharsBCKN700, BCKZ700, BCCM500 and BCSI500),and also well-developedhysteresis loopswhich testify to the development of 22

the mesopore structure. According to the IUPAC classification, these loops belong tothe H3 type evidencingthe slit-like pore shape, which is a characteristic feature of carbon adsorbents. It should however be noted thatin all the isotherms hysteresis is also present in the low relative pressure range at p/p0<0.4, that is, belowthe beginning ofthe usual capillary condensation. It is known from the literature[34]that one of the reasons for the occurrence of hysteresis at low relative pressures can be swelling of material particlesduring the adsorption process. Swellingloosens the structure, e.g. as a result ofthe breaking of weaker bondsof primary particles, and allows accessfor the adsorbateto previously inaccessible areas.Since the deformation of the structure is not completely irreversible, some particles of the adsorbatecan be trapped and desorb very slowly or do not desorb at all. For obtained biochars,we have to do with the slit pore structure. It is therefore quite probable that under the influence of a lowtemperature (196oC) nitrogen condensed in the slits of the poresof the carbon phase of thebiocharscaused the walls of these pores to crack, thus loosening the initial structure. Table 1 includes the parametersof the porous structure of theinvestigated adsorbents. We can see here that the initial sewage sludges are in practicenonporous. As a result of theirpyrolysis,a material with a more developed surface was obtained relative to the feedstock. The highest increase in surface area was found for the biocharsBCSI500 (38 times) and BCCM500 (31 times), whereas the lowest one for the samples ofBCKN600 (6 times) and BCKZ600 (7 times). It was difficult todefine unambiguously the relationships between the specific surface area ofthe initial sewage sludges and the surface area ofthe biocharsproduced. It was only noted that the sludges characterized by larger surface area producedbiocharsampleswith a higher value of thisparameter. Inthe biocharsproduced at a temperature of 600oC,there was a decrease inthe SBETsurface compared to the samples pyrolyzedat 500oC. Analyzing the structural 23

parameters ofthe biocharsproduced at the highest temperature (700oC),on the other hand, two groups can be distinguished. The first group includes the samples of BCCM and BCSIin which a further decrease in surface area was observed, whilein the other groupincluding the biocharsBCKN and BCKZ this parameter was found to increase up to a value that significantly exceeded the one obtainedfor the samplepyrolyzed at 500oC. The increase in surface areawith increasingtemperatureup to600oCand then its decreaseat 700oC, which were observed in the present study,have also been recorded by other authors[7].Lu et al. [17]showthat the higher thetemperature and pyrolysis time, the stronger thesintering processes are. These processes can leadto sealing of the pores in biochar, which probably took place in the present studyforBCSI and BCCM. Furthermore,a decrease in surface area with increasing temperaturecan also be due to an increase in the proportion of ash. The negative effect of ash on the surface area value may be caused by filling or blocking the pores by inorganic compounds[5]. In present research, such a relationship between ash content and surface area was observed only forthe BCCM biochar(r2 = 0.888),but such a relationship was not found for other biochars. Probably, in this case the sintering processes could have been responsible for the decrease in surface area. In turn, the increase in surface area observedin the case ofthe BCKN and BCKZ series samples at the highestpyrolysis temperaturemay be associated with the removalof volatile organic matter, resulting in unblocking themicropores[13,15,17].

3.9. SEM Figure3 showsexample SEM images of two sample seriesof the investigated biochars, i.e. BCKZ and BCSI. Ascan be seen, the surface morphology of these materials is 24

extremely rich (Figs3B, 3D). We can see herethe amorphous carbon phase relative to theinorganic condensed phase. A wide range of minerals and organic matter is present in the biocharstructure.Moreover, porous structures can be seen anddifferent forms can be observed, among others, elongated forms, forms resembling honeycombs, fluffy sponges, balls, or simply small formless particles. At higher magnification (50000x),the micropores and even wider mesopores are seen quite clearly (Figs3A, 3C). When comparing the images of the biochars BCKN (Fig. 3B) and BCSI (Fig. 3D),it can notice the differences in their morphology related to the chemical composition. However, taking into account thepyrolysis conditions, it can be clearly seen that the higher the process temperature, the more defined the biochar surface morphology is. The porous structure becomes more exposed, making the pores better accessible for the adsorbate particles (Figs3A, 3B).

3.10. Thermal analysis Figure4presents the resultsof the thermal analysis.Analyzing the TG (Fig. 4a), DTG (Fig. 4b) and DTA (Fig. 4c) curves generated for the sewage sludges, three major areas can be identified. The initial area (20-200oC) is associated with the loss ofphysically adsorbed water particles. Then,in the temperaturerange from 200 to 800oC, whilefor SSSI sample up to 900oC, there is an area associated with thedegasification and removalof volatile substancesand with the degradation ofthe main sludge components[15]. Up to 1200oC,a third area can be distinguished which is related to the decomposition of organic matter such as. e.g.,calcium carbonate [35,36]. Forbiochars,the analysis ofthe TG curve patterns (Fig. 4a) shows that the studied materials have varying thermal resistance. The patternof the analyzed TG curvesreveals a systematic slow decrease in the mass of allbiocharsup to800-900oC. After 25

thistemperature is reached,we observe only a slight reduction in the mass associated with the decomposition of the mineral part. We can see here that thebiocharsBCKN and BCKZ are stable,depending on their production temperature,up to about 320325oC (BC500) or about 375-400oC (BC600/700). But thebiocharsBCCM and BCSI are less thermally resistant and lose their mass alreadyat a temperatureof 240oC (BCCM500),330oC (BCCM600/700), 345oC (BCSI500/600), and 365oC (BCSI700). After these temperatures are exceeded,the burning of carbon matter and the migration of volatile decomposition products probably begin. For allthe biochars, the temperatureat which burningbegins is shifted towards higher values compared to the initial sewage sludge. This means that the biocharsproduced are thermally more stable compared to the feedstocks. The biocharsproducedat highertemperatures(600, 700oC) prove to be more stable already in the first temperature range compared to their equivalents obtained at 500oC. Furthermore,in the case ofthe biocharsBCKN700, BCKZ700, and BCSI700 the next curvature of the TG curvesis also moved towardsthe higher temperaturesin relation to both the sewage feedstocks andbiocharsproduced at the lowertemperatures. The various thermal resistanceof thebiocharsproduced results from the differences in the nature of the sewage sludges.Sewage sludge is a material of varying properties, containing both mineraland organic parts. Analyzing the obtained results, it can be saidthat the lower the pyrolysis temperature, the morenon-pyrolyzed compounds, which are degraded under the influence oftemperature, the biocharcontains. These observations are confirmed by the analysis of the DTG and DTA curves (Figs4b and c) in which several peaks can be distinguished in different temperature ranges, i.e. 20200oC, 200-450oC, 450-800oC, 800-1200oC,which reflectthe multi-stage nature of thermal decomposition. 26

3.11. Pyrolysis yield Table 1 presents the percentagepyrolysis yield forthe biocharsproduced from the investigated sewage sludges at differenttemperatures. In the case ofall the samples,this processwas characterized by a high percentage yield which, depending on thetemperature,rangedfrom 40.2 to 54.5%. The highestvalues were recorded for thebiocharsderived from the sludges SSCM (49.5-54.5%) and SSKN (48.7-54.3%). A relatively low yieldwas found for the SSSI-basedbiochars. In the case ofall the sewage sludges, the process yielddecreased with increasingtemperatureand the highest decline in yield was observed for BCKZ. The pyrolysisyields obtained for the sewage sludges were distinctly higher than those recorded for plant biomass [3,5,6,19]and pyrolyzed sewage sludges by other authors [7,13,15]. A high yield is attributable to the higher inorganic content in sewage sludge (Table 2) than in traditionally used biomass. This is confirmed by the relatively high ash content obtained afterincineration of the sewage sludges studied (Table 1). A decrease inpyrolysisyield with increasingtemperatureis a typical processin the case ofsewage sludge[7,8,10,13,15] and it is probably associated with more intensivepyrolytic conversion of sewage sludge. This may correspond to more effective primary decompositionof the initial feedstockor secondary reactions ofthe stable residue [23]. This is evidenced by the reduction in the content of major elements (C, H, N and O) with increasingtemperatureand additionally by their lower contentin the biocharsthan in the initial sewage sludge. This applies to the samples of BCKN, BCKZ and BCCM (Table 1). In the case ofthe BCSI series biochars,on the other hand, a decrease was found for H, N and Oboth in relation to the initial sludge and with increasing temperature (Table 1). The C contentincreasedboth relative to the feedstock and with 27

increasing temperature,whereas the sulfur content dropped (Table 2). The SSSI-derived biocharswere characterized by the relatively poorest yield (40.245.1%). This may be associated with the presence ofmagnesium ammonium phosphate hexahydrate (NH4MgPO4∙6H2O) in large amounts (Table 3) in the initial sewage sludge.As a result of thepyrolysisprocess, this compound can be degraded to volatile Mg compounds and gaseous ammonia. Besides, theSSSI sludge was also characterized by the highestpercentage of C, which also had an effect on its lower yield. The decomposition of dolomite (CaMg(CO3)2)may also be responsible for such a low yield. As reported in the literature, it begins already at480oC, which is confirmed by the high content of CaCO3in the sample ofBCSI500 (an increase relative to the content of CaCO3in SSSI), being a product of thermal dissociation of this compound. Apart from it, MgO and CO2 should also be formed. The decomposition of this mineral is twostage and the above-mentioned compounds are formed during the first stage. This process is irreversible, since magnesium oxideis recombined with CO2with difficulty. During the second phase, CaCO3is degraded toCaO and CO2. The thermal decomposition of dolomite usuallyends in the 900-1000oC temperature rangeand that is why itscrystallographic structures can also be seen in BCSI600 and BCSI700.With increasingtemperature,in the case of BCSI600 and BCSI700 the contentof CaCO3decreases, which is caused by its degradationat highertemperatures, in particular at 700oC, to CaO and CO2. The formation of larger and larger amounts of CO2 may be responsible for thedecrease in pyrolysis yield with increasingtemperature, in particular in the case of BCSI700 (PY= 40.2%)where we have to do with practically complete decomposition of CaCO3 originating from the thermal decomposition of dolomite. On the basis of the minerals discussed in this paper, it can be clearly seen what crystalline phases present in the initial sewage sludge mayhave an effect on the 28

formation of a smaller amount of biochar. For example, the presence of struvite in the sewage sludge may significantly reducethe pyrolysis yield.

4. Conclusions

In this study, it was found that the properties of sewage sludge-derived biochar depend to a significant degree on the properties of sewage sludge. However it is difficult on the basis of the known properties of the initial material predict the properties of the biochar. Only the presence in sewage sludge of particular crystallites may provide information about some of the biochar properties (e.g. surface area). The results showed that not only the properties of sewage sludge but also the temperature of pyrolysis plays an important role in controlling the properties of biochars.Mixing these two parameters is crucial and determines the final sewage sludge-derived biochars properties.The pyrolysis of sewage sludges with a neutral pH at temperature of 500°C produces biochar with the same pH. However, higher pyrolysis temperatures (≥600oC) favor formation of biochar with alkaline pH. It was also observed that the content of ash in biochars will be higher in relation to the sewage sludges, and increase the temperature of pyrolysis from 500oC to 700oC will further increase the ash quantity. In addition, it was found that higher pyrolysis temperatures will promote the formation of biochar with a higher contribution of nutrients.On the basis of the surface properties of sewage sludges is not possible to predict the surface area of biochars butit may be concluded that the higher surface area of the sewage sludges will be transformed into more developed surface area of biochars.Moreover, the presence of certain crystalline phases in the sewage sludge will reduceof the biochar surface area.

29

Acknowledgements The project was funded by the National Science Centre granted on the basis of the decision number DEC-2012/07/E/ST10/00572.

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34

B

C

D

Fig. 1. FTIR-PAS spectra of SSKN and BCKN (A), SSKZ and BCKZ (B), SSCM and BCCM (C) and SSSI and BCSI (D) sewage sludge and biochar samples.

35

Fig. 2

36

37

Fig. 3

500oC

600oC

700oC

A

B

C

D

Fig. 3. SEM micrographs of biochars: (A, B) BCKN and (C, D) BCSI produced at 500, 600 and 700oC. A and C magnification x50000; B and D magnification x10000. 38

Fig. 4

Fig. 4. TG (A), DTG (B) and DTA (C) curves of sewage sludges (SS) and sewage sludgederived biochars (BC) produced in different temperatures – 500, 600 and 700oC.

39