Bioresource Technology 192 (2015) 312–320

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Effect of pyrolysis temperature on polycyclic aromatic hydrocarbons toxicity and sorption behaviour of biochars prepared by pyrolysis of paper mill effluent treatment plant sludge Parmila Devi, Anil K. Saroha ⇑ Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India

h i g h l i g h t s  Paper mill sludge biochars were used for pentachlorophenol (PCP) removal.  Sorption behaviour of biochars was determined based on degree of carbonization.  Fractions of adsorption and partition in total PCP removal were determined.  Adsorption and partition parameters were correlated with biochar properties.  Risk assessment for 16 priority EPA PAHs in biochar matrix was performed.

a r t i c l e

i n f o

Article history: Received 6 April 2015 Received in revised form 22 May 2015 Accepted 23 May 2015 Available online 28 May 2015 Keywords: Biochar Pyrolysis Sludge Polycyclic aromatic hydrocarbons Adsorption

a b s t r a c t The polycyclic aromatic hydrocarbons (PAHs) toxicity and sorption behaviour of biochars prepared from pyrolysis of paper mill effluent treatment plant (ETP) sludge in temperature range 200–700 °C was studied. The sorption behaviour was found to depend on the degree of carbonization where the fractions of carbonized and uncarbonized organic content in the biochar act as an adsorption media and partition media, respectively. The sorption and partition fractions were quantified by isotherm separation method and isotherm parameters were correlated with biochar properties (aromaticity, polarity, surface area, pore volume and ash content). The risk assessment for the 16 priority EPA PAHs present in the biochar matrix was performed and it was found that the concentrations of the PAHs in the biochar were within the permissible limits prescribed by US EPA (except BC400 and BC500 for high molecular weight PAHs). Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The conversion of sludge into biochar is a promising method for the sludge management in comparison to traditional methods like landfilling and agricultural use. Most of the studies reported in literature have used wood and agricultural waste as raw materials for biochar preparation. These biochar are rich in carbonaceous fraction with high surface area due to abundance of cellulose, hemicellulose and other organic compound in raw materials. Recently, large number of studies have been reported in the literature on the environmental benefits by use of sludge based biochar in various applications like restoration of degraded land, increase in crop yield, adsorption of carbon dioxide and various contaminates from the environment (Tan et al., 2014; Devi and Saroha, 2014a). The usage of sludge based biochar as an adsorbent is emerging as an ⇑ Corresponding author. Tel.: +91 1126591032; fax: +91 1126581020. E-mail address: [email protected] (A.K. Saroha). http://dx.doi.org/10.1016/j.biortech.2015.05.084 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

attractive option. Although the surface area and porosity of the sludge based biochar is much smaller than the commercial activated carbon, the adsorption capacity of the biochar for organic contaminant removal is almost equal or higher than the commercial activated carbon due to presence of the mineral rich carbon phase in sludge based biochar (Wang and Xing, 2007; Luo et al., 2011). The sludge based biochar is abundant in metal oxides because the precursor sludge contains significant amount of minerals, organic compounds and heavy metals. Several studies have been reported in literature on mobility and bioavailability of heavy metals in biochar to assess the environmental impact of usage of sludge based biochar. It has been reported that pyrolysis of paper mill sludge immobilize the heavy metals in biochar matrix resulting in decrease in bioavailability and leaching potential of heavy metals (Devi and Saroha, 2014b). In order to use biochar as an adsorbent, it is significant to understand the key parameters that affect the physicochemical properties (surface area, porosity, moisture and ash content,

P. Devi, A.K. Saroha / Bioresource Technology 192 (2015) 312–320

surface functionality, elemental composition) of biochar. The pyrolysis parameters such as pyrolysis temperature and time, heating rate and feedstock particle size have significant effect on the quality as well as quantity of resultant biochar. Among these parameters, pyrolysis temperature has significant effect on the biochar properties as the sludge undergoes a number of chemical and physical changes which results in variation in H/C, O/C and (N + O)/C ratio, porosity, surface functionality and ash content of the resultant biochar (Keiluweit et al., 2010; Agrafioti et al., 2013). The bulk and surface properties of the biochar are found to strongly affect the adsorption capacity of the biochar. Therefore, it is important to characterize the biochar pyrolyzed at different temperatures to determine its physicochemical properties. A number of studies have been reported in literature to determine the effect of pyrolysis temperature on structural changes of the biochar (Keiluweit et al., 2010; Agrafioti et al., 2013; Lattao et al., 2014). However, to the best of our knowledge, there is no study reported in literature for paper mill sludge biochar correlating its properties and sorption behaviour with the degree of carbonization. Further, there is a possibility of formation of polycyclic aromatic hydrocarbons (PAHs) during pyrolysis of sludge. Since, the presence of PAHs compounds in the biochar is a matter of concern due to their potential carcinogenic and mutagenic characteristics, therefore, it is necessary to determine the concentration of PAHs in biochar matrix and risk associated with its environmental applications. In the present study, the biochar prepared from the pyrolysis of paper mill effluent treatment plant sludge in the temperature range 200–700 °C have been used as a potential adsorbent for removal of pentachlorophenol (PCP) from synthetic solution. PCP was chosen as a pollutant as it is carcinogenic, persistent and low-biodegradable organochlorine compound and is listed in the category of priority toxic pollutants by the US EPA. The effect of degree of carbonization on sorption behaviour of biochar was studied and adsorption parameters were correlated with the biochar properties like elemental composition and surface properties. The effect of pyrolysis temperature on PAHs content in biochar was investigated and risk analysis of PAHs was also determined.

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The elements composition, carbon (C), nitrogen (N), hydrogen (H) and sulphur (S) content, of the paper mill sludge and biochars were determined by elemental analyzer (Thermo Fisher Scientific Inc., USA). The oxygen (O) content was calculated by mass balance i.e. (100  (C + H + N + ash content). The H/C and (N + O)/C ratios were calculated to determine the aromaticity and polarity index of the biochars. The pH of the biochar samples was determined by preparing a suspension of biochar and distilled water in 1:10 ratio. The suspension was agitated for 1 h and then the pH was measured by pH meter (Eutech Instruments). The paper mill sludge and biochars (BC200–BC700) were characterized for the surface area, porosity and surface functional groups. The BET surface area and porosity were determined using nitrogen adsorption–desorption isotherm at 196 °C using Micromeritics ASAP 2010 apparatus. The surface functional groups were analyzed using FTIR spectrometer where the dried sludge or biochar sample was mixed with the dried KBr in a ratio of 1:30 and FTIR spectra was recorded at a resolution of 4 cm1 in the region 4000–400 cm1. 2.2. Adsorption studies The batch adsorption experiments were performed for the adsorption of PCP from the synthetic solution using biochar as an adsorbent. The stock solution of PCP (100 mg/L) was prepared by dissolving analytical grade PCP in distilled water. The working solution (10 mg/L) was prepared by dilution of the stock solution with distilled water. The batch equilibrium experiments were conducted in 250 mL conical flask by mixing 25 mg of biochar in 50 mL of PCP solution (Co; 5–50 mg/L). The mixture was then agitated in incubator shaker maintained at room temperature (25 ± 1 °C) at 120 rpm. The samples were withdrawn at regular time intervals to analyze the residual concentration of PCP in the solution using UV–vis spectrophotometer at wavelength of 320 nm. The reproducibility of the experimental results was checked and the error in the experimental results was found to be ±2%. 2.3. Data analysis

2. Methods The biochars were prepared from paper mill effluent treatment plant sludge by pyrolysis in the temperature range 200–700 °C and the biochars were assigned as BC200, BC300, BC400, BC500, BC600 and BC700 according to the pyrolysis temperature. 2.1. Characterization of biochar Thermogravimetric analysis of paper mill sludge and biochars was performed in the temperature range of ambient to 900 °C in the presence of nitrogen at the heating rate of 10 °C/min to determine the mass loss (%). The proximate analysis of biochar was conducted to determine the moisture content, volatile matter, fixed carbon and ash content. The biochar sample was heated up to 900 °C in presence of nitrogen at the heating rate of 10 °C/min. The moisture content of the sample was determined by calculating the weight loss of the sample on heating from ambient to 105 °C. The volatile content was determined by calculating the weight loss of the sample on heating from 105 to 900 °C. The nitrogen supply was stopped at 900 °C and air was introduced in the system to initiate the combustion of carbon present in the sample until a constant weight was achieved. The ash content was determined from the final weight of the sample. The fixed carbon content was determined from mass balance i.e. [100  (volatile content + ash content + moisture content)].

The equilibrium sorption data was analyzed with the help of Langmuir isotherm, linear partition isotherm and dual-mode sorption model (DMM) (Eqs. (1–3)) (Chen et al., 2012):

qe ¼

qmax bC e 1 þ bC e

ð1Þ

qe ¼ K p C e qe ¼ kp C e þ

ð2Þ n X q i¼1

max bC e 1 þ bC e

ð3Þ

where, qe (mg/g) is the equilibrium adsorption capacity; Ce (mg/L) is equilibrium concentration of the PCP in the solution, qmax (mg/g) is the maximum adsorption capacity, b (L/mg) is Langmuir isotherm constant and kp (L/g) is partition coefficient. The first term in Eq. (3) represents the partition while the second term represents the adsorption. 2.4. PAHs extraction The biochar samples were oven dried at 90 °C for 24 h and dried biochar samples were extracted using soxhlet extraction procedure. Toluene was used as an extraction solvent as toluene is reported as best extractant for PAHs extraction from biochar (Hilber et al., 2012). For extraction, 1 g of the biochar was weighed

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and extracted using 100 mL toluene in soxhlet extraction unit for 24 h. The extracted samples were cleaned as per the USEPA method 3630C (USEPA, 1996) with slight modifications. The packed silica column with top thick layer (1 cm) of anhydrous sodium sulphate (Na2SO4) was used for cleaning the extractant. The packed silica column was rinsed with 30 mL dichloromethane (DCM) to clean the impurities and then 30 mL of extractant was added into the column. The column was rinsed thrice with DCM to blow down the extractant. All the rinsed fractions were collected into 250 mL round bottom flask and concentrated to 1 mL using rotary evaporator and finally dried under nitrogen stream. The samples were diluted with 1 mL n-hexane prior to the analysis. 2.5. GC–MS analysis The PAHs samples were analyzed using gas chromatograph mass spectrophotometer (GC–MS, Agilent technologies) equipped with HP-5 (30 m  0.32 mm internal diameter  0.25 lm film thickness) capillary column, using helium as carrier gas. The samples were injected in splitless mode and the ionization was performed in the electron impact (EI) mode (70 eV). The temperature of ionization port was maintained at 250 °C. The samples were analyzed by external standard method using standard EPA PAHs mixture as reference for the preparation of calibration curve. 3. Results and discussion 3.1. Thermal degradation of paper mill sludge The thermal degradation profile of the paper mill sludge is shown in Fig. 1. An initial mass loss was observed at temperature 6100 °C due to the evaporation of moisture from the sludge. Another mass loss (15.37%, this includes the loss in the moisture content also) of the paper mill sludge was observed in the temperature range 100–340 °C due to the thermal degradation of the hemicellulose and cellulose present in the paper mill sludge. Further, a steep mass loss of 33.95% was observed over a temperature range 340–760 °C which might be due to a number of complex chain reactions occurring during the thermal degradation of the lignin and lignin like compounds (Mimmo et al., 2014). No further loss in the mass of the sludge was observed beyond 760 °C.

The results are in agreement with those reported in the literature that the thermal degradation of lignin is comparatively slow and occur over the broad temperature range 100–900 °C (Méndez et al., 2013; Mimmo et al., 2014). On the basis of thermal degradation profile of paper mill sludge, it was observed that major physicochemical changes are occurring in the temperature range 200– 700 °C, Therefore it was decided to pyrolyze the paper mill sludge in the temperature range 200–700 °C. 3.2. Effect of pyrolysis temperature on biochar properties The biochars obtained after pyrolysis (200–700 °C) were characterized by various analytical techniques to determine the effect of pyrolysis temperature on biochars properties. 3.2.1. Proximate and elemental analysis The proximate and elemental analysis of the biochars was performed and the results are shown in Table 1. It can be noticed that the volatile content in the biochars was found to decrease with an increase in the pyrolysis temperature from 400 to 700 °C, while the fixed carbon content was found to increase with an increase in pyrolysis temperature. At pyrolysis temperatures 6400 °C, the thermal transformation of lignin and cellulose lead to the formation of an amorphous biochar phase comprised of volatile, low-molecular weight aliphatic and aromatic compounds resulting in higher volatile content. However, at pyrolysis temperatures >400 °C, this amorphous phase (or volatile matter) is removed from the biochar matrix due to the formation of more thermally stable biochar resulting in a decrease in fraction of volatile matter in high temperature pyrolyzed biochar. The ash content in the biochar was found to increase with an increase in pyrolysis temperature up to 500 °C but no further substantial increase in the ash content was noticed from 500 to 700 °C. It can be concluded that the ash content of the biochar is positively correlated with pyrolysis temperature in the range 200–500 °C. The biochar pyrolyzed at higher temperature has high ash content due to the catalytic volatilization of organic matter in presence of inorganic minerals like Ca2+ and Mg2+ present in paper mill sludge. The elemental analysis results show that the oxygen (O) and hydrogen (H) content in biochar decreased with an increase in pyrolysis temperature, whereas the carbon (C) content increased with an increase in pyrolysis temperature. The nitrogen (N)

Fig. 1. Thermal degradation profile of paper mill sludge.

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P. Devi, A.K. Saroha / Bioresource Technology 192 (2015) 312–320 Table 1 Proximate and elemental analysis of paper mill sludge and biochars. Characteristics

Sludge

BC200

BC300

BC400

BC500

BC600

BC700

Yield (%)



82.83

70.79

56.14

38.02

35.91

32.48

Proximate analysis (wt%) Moisture content Ash content Volatile content Fixed carbon

6.59 40.01 42.52 10.88

5.81 42.34 40.11 11.74

4.52 47.15 32.78 15.55

4.20 48.76 27.7 19.34

3.49 53.61 20.96 21.94

3.46 55.45 18.26 22.83

3.46 56.13 15.74 24.67

Elemental analysis (wt%) Carbon (C) Hydrogen (H) Nitrogen (N) Oxygen (O) Sulphur (S)

19.31 3.64 0.28 32.79 0.46

20.72 2.74 0.28 31.23 0.35

21.14 2.57 0.24 29.26 0.33

23.37 1.93 0.22 25.81 0.32

24.98 1.24 0.23 19.97 0.977

25.13 0.59 0.19 19.30 0.411

25.66 0.44 0.18 18.45 0.40

content remained almost stable throughout the temperature range 200–700 °C (Table 1). It was found that the H/C, O/C and [(O + N)/C] ratio in the biochar decreases with an increase in pyrolysis temperature due to the demethylation (CH3) and decarboxylation (CO2 loss) reactions (Zhu et al., 2014). The lower value of H/C ratio for BC700 suggests the formation of highly carbonized and aromatic structures. On the other hand, the higher value of H/C ratio for BC200, BC300 and BC400 indicate that these biochars contains a good amount of organic compounds like cellulose, lignin and other polymeric compounds originally present in sludge (Chen et al., 2008). The high O/C ratio in paper mill sludge, BC200 and BC300 indicates the low degree of carbonization and high reactivity due to the presence of volatile matter. These results are in agreement with the proximate analysis results showing high volatile matter content in biochars obtained by pyrolysis in the temperature range 200–400 °C. The higher value of polarity index [(O + N)/C] in the temperature range 200–400 °C indicate the presence of oxygen containing polar function group in low temperature biochars. However the value of polarity index decreased in biochars pyrolyzed at temperature >400 °C due to the reduction of polar functional groups. 3.2.2. Surface area and porosity The surface area and porosity of the biochars was correlated with the pyrolysis temperature and the results are shown in Table 2. It can be noticed that the surface area and total porosity of the biochars increased with an increase in pyrolysis temperature due to the increase in the degree of carbonization which favours the generation of surface area. The maximum surface area and total pore volume was obtained for BC700 due to the progressive destruction of aliphatic alkyl and ester groups which might hide the aromatic core (Chen et al., 2008). 3.2.3. Surface functionalities of the biochars The FTIR spectrum is an easy and convenient method to find out the effect of pyrolysis temperature on surface functional groups of

Table 2 Surface area, porosity and pH of paper mill sludge and biochars. Samples

Surface area (m2/g)

Total pore volume (cm3/g)

Micropore volume (cm3/g)

Pore diameter (Å)

pH

Sludge BC200 BC300 BC400 BC500 BC600 BC700

4.8 6.19 15.37 33.56 47.42 50.44 67.0

0.024 0.029 0.037 0.051 0.063 0.074 0.083

0.012 0.009 0.009 0.015 0.018 0.026 0.026

124.70 104.19 91.02 73.66 54.74 40.42 31.71

6.26 6.75 7.09 8.23 8.78 9.17 9.37

the biochars. The spectra of paper mill sludge and biochars showed four major bands in the wavelength range 2900–3500 cm1, 1300– 1750 cm1, 800–1150 cm1 and 710 cm1. The wide band at 3400 cm1 assigned to the OAH stretching of hydroxyl functional group due to the adsorption of water molecules. Similarly, the peaks in the region of 3600 cm1 is due to OAH stretch of alcohols and phenols. The intensity of these peaks decreased with an increase in the pyrolysis temperature indicating the ignition loss of AOH group due to dehydration and dehydrogenation reactions. The peak in the region of 2923 cm1 is associated with asymmetric stretching of aliphatic functional group (CAH). The peaks in the region of 1300–1750 cm1 was due to the presence of ammonia and primary amines present in the paper mill sludge (Devi and Saroha, 2013). The peaks at 1795 cm1 and 1015 cm1 associated with the [email protected] and CAOAC stretch of the cellulosic ethers. There is no change in the intensity of these peaks observed in the biochars pyrolyzed in the temperature range 200– 500 °C, however the intensity of these peaks decreased in biochars pyrolyzed in the temperature range 600–700 °C. The sharp peak at 870 cm1 is associated with carbonate stretch of calcium carbonate (CaCO3) and the intensity of this peak decreased at temperatures >400 °C due to the conversion of CaCO3 into calcium oxide (CaO) (Calisto et al., 2014; Devi and Saroha, 2014a). The peaks at the 670 cm1, 530 cm1 and 460 cm1 are associated with the in-plane and out-of-plane aromatic ring deformation vibrations. These spectra also suggest that the alkaline groups of cyclic ketones and their derivatives increased during pyrolysis. It was observed that the FTIR spectra of paper mill sludge contain large of amount of oxygen containing functional groups (ACOO, ACOH, AOH and CO2 3 ). The intensity of peaks due to oxygen containing functional groups decreased with an increase in pyrolysis temperature, whereas the intensity of peaks due to other functional groups like ACH2, [email protected] and [email protected] remain unchanged. 3.3. Effect of pyrolysis temperature on adsorption behaviour The biochars were used as an adsorbent for the removal of PCP from synthetic solution. The experimental data was fitted into dual mode sorption isotherm and isotherm separation method was used to calculate the individual contribution of adsorption and partition in PCP removal. The calculated sorption isotherm parameters were correlated with the biochar bulk and surface properties to study the effect of degree of carbonization on sorption behaviour. 3.3.1. Sorption isotherm studies Experiments were performed for the removal of PCP from synthetic solution using paper mill sludge biochars pyrolyzed at different temperatures (200–700 °C) as an adsorbent. The adsorption experiments were performed at different initial concentrations of

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PCP in the synthetic solution (5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 mg/L) for each adsorbent and results are shown in Fig. 2a. It can be noticed from Fig. 2a that all the biochars exhibit different sorption behaviour for PCP. The sorption behaviour of PCP was linear to moderately linear for the biochars pyrolyzed in temperature range 200–500 °C (BC200, BC300, BC400, BC500) but the adsorption behaviour became highly non-linear for the biochars pyrolyzed in the temperature range 600–700 °C (BC600 and BC700).

This indicates that the pyrolysis temperature strongly influence the adsorption behaviour due to variation in degree of carbonization. The biochar pyrolyzed at temperature higher than 500 °C contains large quantity of carbonized fraction while the biochar pyrolyzed at temperature lower than 500 °C are not fully carbonized and contains both carbonized and uncarbonized fractions. It is reported in the literature that the removal of PCP (either by adsorption or partition) is strongly dependent upon the fraction of carbonized and uncarbonized fractions in the biochar matrix (Chen et al., 2008; Lattao et al., 2014). The carbonized fraction is porous in nature and provides large surface area for the adsorption of PCP, whereas no pore development occurs in uncarbonized fraction resulting in the partition of PCP. The experimental data was fitted into sorption isotherm and it was found that the sorption behaviour of PCP on the biochars pyrolyzed at temperatures P600 °C was well described by Langmuir isotherm (Fig. 2a). The biochars pyrolyzed at 300–500 °C (BC300–BC500) showed mixed sorption behaviour (moderately linear behaviour at lower initial PCP concentrations and linear behaviour at higher initial PCP concentrations) and the sorption behaviour was well described by dual mode adsorption model (Fig. 2b and c). The sorption behaviour was linear for biochar pyrolyzed at 200 °C (BC200) and well described by linear partition isotherm (Fig. 2c). This might be due to the reason that the biochar obtained by pyrolysis at 200 °C temperature contains significant amount of non-carbonized organic materials like cellulose, hemicelluloses and lignin, which facilitate the partition of PCP on biochar surface by loosening the structure of organic matter present in contact with water (Chen et al., 2012) resulting in linear sorption behaviour. Further increase in pyrolysis temperature (>300 °C) causes partial destruction of hemicellulose and cellulose resulting in decrease in polarity of partition medium. Therefore, a condensed medium develops that slows down the partition of PCP. In the temperature range 400–500 °C, the moderately linear behaviour was observed at low PCP concentrations but the linear adsorption was attained at higher PCP concentrations. The moderately linear behaviour at lower PCP concentrations is due to adsorption of PCP on available adsorption sites. At higher PCP concentrations, these active adsorption sites became saturated and further removal of PCP occur by partition in uncarbonized fraction resulting in linear sorption behaviour. The non-linear behaviour above 600 °C is due to the complete carbonization of organic matter that leads to the development of surface area and porosity in the biochar which enhance the adsorption of PCP on the biochar. The contribution of adsorption and partition in removal of PCP by biochar pyrolyzed at different temperatures was determined by isotherm separation method (Eqs. (4–7)):

qsorption ¼ qm;adsorption þ qm;partition

ð4Þ

qm;partition ¼ kp C e

ð5Þ

qsorption ¼ qm;adsorption þ kp C e

ð6Þ

The fraction of adsorption was calculated by subtracting fraction of PCP removed by partition (qm,partition) from total PCP removed by sorption (qsorption):

qsorption  qm;partition ¼ qm;adsorption ¼

Fig. 2. Isotherm plot of PCP adsorption for different biochars (a) Langmuir isotherm fit; (b) isotherm separation and DMM model fit for BC500 and BC400; (c) isotherm separation and DMM model fit for BC300 and BC200.

qmax bC e 1 þ bC e

ð7Þ

where, qsorption (mg/g) is total amount of PCP adsorbed at equilibrium; qm,partition (mg/g) is the amount of PCP removed by partition; qm,adsorption (mg/g) is the amount of PCP removed by adsorption; Ce (mg/L) is equilibrium concentration of the PCP in the solution. qmax (mg/g) is the maximum adsorption capacity, b (L/mg) is Langmuir isotherm constant and kp (L/g) is partition coefficient.

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The fraction of adsorption and partition was determined according to Eqs. (4–7) and results are shown in Fig. 2b and c. It can be observed from Fig. 2b and c that the fraction of adsorption increased with an increase in pyrolysis temperature. This is due to the fact that with an increase in the pyrolysis temperature the carbonized fraction increases which acts as an adsorption phase. In contrast, the fraction of PCP partition decreased with an increase in pyrolysis temperature due to decrease in the degree of carbonization as the uncarbonized fraction in the biochar acts as partition phase. The adsorption isotherm parameters were determined by non-linear regression using solver add-in in Microsoft’s Excel and the values of isotherm parameters are shown in Table 3. The adsorption equilibrium data of the biochars obtained by pyrolysis at 600–700 °C well fitted to Langmuir isotherm, as indicated by the higher value of correlation coefficient (R2 – 0.99) and the values of qmax obtained from the isotherm model, which are close to experimental results. The linear sorption behaviour of biochars obtained by pyrolysis in the temperature range 300–500 °C was well described by dual mode model where the removal of PCP occurred by both adsorption and partition phenomena. The value of qm,adsorption increased with an increase in pyrolysis temperature and was maximum for BC700. The surface area normalized adsorption capacity (qm,SA; lmol/m2) of the biochars was calculated to get insight of possible adsorption mechanism. The higher value of qmax in comparison to qm,SA suggest the involvement of other surface interactions rather than solely by surface area. Further, biochar properties were correlated with adsorption parameters (qm,adsorption and kp; isotherm separation) to find out the possible sorption mechanism. 3.3.2. Correlation of biochars properties with adsorption and partition parameters The adsorption of PCP on paper mill sludge biochar is a function of biochar properties. The properties like aromaticity, polarity, surface area, pore volume and ash content have direct effect on the adsorption capacity of the biochar. The biochar properties namely H/C ratio, O/C ratio, (O + N)/C ratio, surface area and total pore volume were plotted against its adsorption capacity (qm,adsorption) to study the correlation of biochar properties with its adsorption capacity and the results are shown in Fig. 3(a–c). It can be observed from Fig. 3(a and b) that the qm,adsorption of the biochar linearly decreased with an increase in H/C, O/C and (O + N)/C ratios. The biochar BC700 with lower H/C ratio showed higher affinity for PCP in comparison to the biochars with higher H/C ratio, suggesting the impact of structural arrangement of aromatic moieties in the biochar adsorption capacity. Similarly, biochars with low O/C and (N + O)/C ratios showed better adsorption affinity in comparison to higher O/C and (N + O)/C ratios. The biochars pyrolyzed in the temperature range 200–400 °C (BC200, BC300 and BC400) were found to be polar in nature due to the presence of abundant oxygen containing polar functional groups. These polar functional

groups form larger and denser water cluster through hydrogen bonding and inhibit adsorption of PCP resulting in lower adsorption capacity. The oxygen containing polar functional groups (carboxyls and lactones) have been reported to affect the adsorption of pollutants by the p–p interactions, as well as the intermolecular hydrogen bonding (Lou et al., 2011). However, the biochars pyrolyzed at temperature >400 °C (BC500, BC600 and BC700) contain less polar functional groups resulting in high PCP adsorption capacity. Further, qm,adsorption showed positive correlation with biochar surface properties and was found to increase linearly with an increase in surface area and total pore volume of biochar (Fig. 3c). The high surface area of the biochar help in PCP sorption by full surface coverage and multilayer adsorption, while high pore volume help in condensation of PCP in capillary pore space (Lou et al., 2011). The total pore volume of the biochars showed best correlation with qm,adsorption. This might be due to the reason that high temperature treatment leads to the development of better pore structure resulting in high adsorption affinity due to easy accessibility of mesopores and micopores for PCP adsorption (Monsalvo et al., 2012). Moreover, a positive correlation between ash content of the biochars and qm,adsorption was observed suggesting that the minerals Ca and Mg ions present in the biochar matrix might facilitate the adsorption of PCP through ion exchange mechanism (Fig. 3a). Since the uncarbonized matter is mainly found to involve in partition of PCP, therefore, the partition coefficient was correlated with aromaticity (H/C) and polarity index [(N + O)/C)] of the biochars. It can be observed that kp value of biochars pyrolyzed in the temperature range 200–400 °C increase with decrease in polarity index and H/C ratio of the biochars and was maximum at 400 °C (BC400). This suggests the presence of uncarbonized aliphatic organic matter in the biochars pyrolyzed at temperature 6400 °C, which allow the partition of PCP in biochar matrix. Further increase in pyrolysis temperature above 400 °C causes decomposition of aliphatic fraction and leads to the formation of more condensed aromatic fraction resulting in decrease in value of kp. The higher value of kp for BC700 is not according to pattern as expected because biochar pyrolyzed at 700 °C is expected to contain negligible amount of uncarbonized organic matter. The higher value of kp for BC700 is might be due to the pore filling mechanism (Chen et al., 2008). 3.4. PAHs concentration in biochars The concentration of 16 PAHs in biochars pyrolyzed at different temperatures (200–700 °C) was determined and the results were expressed as lg of extractable PAHs per kg of sludge/biochar (Table 4). The 16 EPA PAHs are classified into three types on the basis of number of rings: (i) high molar weight (HMW) PAHs with five and six rings (BbF, BkF, BaP, IND, DAB, BghiP), (ii) middle molar weight (MMW) PAHs with four rings (Fla, Pyr, BaA, CHR), and (iii)

Table 3 Estimated values of sorption isotherm parameters. Biochars

BC200 BC300 BC400 BC500 BC600 BC700 a b

Dual mode model (DMM)

Langmuir isothermb

Isotherm separation

kp (mL/g)

qmax (mg/g)

b (L/mg)

R2

kp (mL/g)

qm,adsorption (mg/g)

qm,SAa (lmol/m2)

qmax mg/g)

b (L/mg)

R2

0.26 0.33 0.30 0.20 0.30 0.46

0.17 6.11 20.61 30.23 41.68 45.42

0.23 0.84 0.20 0.34 0.58 1.24

0.99 0.99 0.99 0.98 0.97 0.99

0.26 0.40 0.78 0.67 0.56 0.83

0.11 4.22 10.15 17.88 24.83 28.40

0.06 1.03 1.13 1.41 1.84 1.59

– – – – 35.89 41.29

– – – – 0.71 1.47

– – – – 0.99 0.99

qm,SA (lmol/m2) is surface area normalized qm,adsorption; qm,SA (lmol/m2) = (qm,adsorption (mg/g)1000)/surface area (m2/g)/molecular weight of PCP (mg/mmol). Blank spaces indicating the non-fit of experimental data.

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322 lg/kg to 16,916 lg/kg in the biochar samples pyrolyzed in the temperature range 200–700 °C. The concentrations of the 16 PAHs as a function of pyrolysis temperature (200–700 °C) is shown in Fig. 4. It can be noticed from Fig. 4 that the concentration of total PAHs in the paper mill sludge biochar initially increased slowly up to 300 °C, thereafter a sharp increase in total PAHs concentration was observed and attained maximum value at 500 °C. The total concentration of PAHs in paper mill sludge biochar was 16,916 lg/kg at 500 °C, which was significantly higher than the raw paper mill sludge (Fig. 5). The formation of PAHs during the pyrolysis of sludge occurs due to the temperature induced dehydration, decarboxylation and dehydrogenation reactions. The increase in pyrolysis temperature results in the loss of C, H, O and N from sludge, which leads in the formation of aromatic compounds that eventually polycondense into aromatic structures (Wang et al., 2013; Dai et al., 2014). The amount of total PAHs was found to be decrease with further increase in the pyrolysis temperature above 500 °C due to the volatilization of the amorphous phase, nuclear condensation of aromatic compounds in non-extractable sheet like structures and strong sorptive retention in the biochar matrix (Keiluweit et al., 2012). It was observed that Phe was the most abundant PAHs in the paper mill sludge and BaP, IND, DBA and Bghip were not present in detectable concentrations (Fig. 4). After pyrolysis, CHR and Phe, CHR and Pyr, Phe and CHR were found to be most abundant in biochars pyrolyzed at 200 °C, 300 °C and 400 °C respectively. It was found that the carcinogenic and mutagenic PAHs (BbF, BkF, IND, DBA, Bghip) were present in significant amount in biochar pyrolyzed at 500 °C, while the concentration of these compounds significantly decreased at 600 °C and were not in detectable concentrations in biochar pyrolyzed at 700 °C due to the nuclear condensation of aromatic compounds in non-extractable sheet like structures in high temperature pyrolyzed biochars. It was observed that the concentration of 16 EPA PAHs was highest in the biochar pyrolyzed at 500 °C in comparison to the biochar pyrolyzed at higher and lower temperatures. This suggests that PAHs production occurs in the narrow temperature range. The total concentration of PAHs in paper mill sludge biochar will help to decide its suitability for use as an adsorbent. The concentration of the total PAHs in paper mill sludge and biochar was compared with USEPA standards for soil quality (Table 4) and it was at found that the total PAHs concentration in all the biochar samples (except BC400 and BC500, especially for HMW PAHs) were within the prescribed limits. 3.4.1. Risk assessment The toxic equivalent (TEQ) determines the carcinogenic potential of the biochar containing PAHs compounds. The individual PAH compound is assigned with a toxic equivalent factor (TEF), which characterizes the carcinogenic properties of individual PAH in comparison to BaP, which has the highest value of TEF (Table 4). The TEQ value can be determined using TEF to characterize the toxicity of biochar pyrolyzed at different temperatures (200–700 °C):

Fig. 3. Linear correlation of biochar properties with sorption parameter (qm,adsorption) (a) H/C atomic ratio and ash content; (b) O/C and (N + O)/C atomic ratio; (c) surface area and total pore volume.

low molar weight (LMW) PAHs with two and three rings (Nap, Acpy, Acp, Flu, Phe, Ant) (Hu et al., 2014). The total PAHs concentration in paper mill sludge was 73 lg/kg and it varied from

TEQ i ¼ PAHi  TEFi

ð8Þ

X TEQ ¼ ðTEQ i Þ

ð9Þ

TEQi is the toxic equivalency of individual PAH based on BaP, PAHi is toxic equivalency factor of individual PAH and TEQ is toxic equivalency of complex PAH mixture based on BaP (Tsai et al., 2009). In PAHs TEQ distribution profile (Fig. 5), Phe and CHR dominated TEQ in biochar pyrolyzed at temperatures 400 °C. The aromaticity and polarity indexes were negatively correlated with maximum adsorption capacity, whereas surface area and porosity were positively correlated. The concentration of PAHs in biochar was within permissible limits prescribed by USEPA suggesting that no environmental implication of usage of biochar as an adsorbent.

Acknowledgement The authors wish to acknowledge the funding received for the project from Council of Scientific & Industrial Research, New Delhi, India.

Fig. 5. TPAHs and TEQ profile of biochars pyrolyzed at different temperatures.

4. Conclusions The effect of pyrolysis temperature on properties and adsorptive behaviour of paper mill sludge biochar was studied. The linear sorption of PCP occur due to partition on uncarbonized organic

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Effect of pyrolysis temperature on polycyclic aromatic hydrocarbons toxicity and sorption behaviour of biochars prepared by pyrolysis of paper mill effluent treatment plant sludge.

The polycyclic aromatic hydrocarbons (PAHs) toxicity and sorption behaviour of biochars prepared from pyrolysis of paper mill effluent treatment plant...
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