Accepted Manuscript Risk analysis of pyrolyzed biochar made from paper mill effluent treatment plant sludge for bioavailability & eco-toxicity of heavy metals Parmila Devi, Anil K. Saroha PII: DOI: Reference:
S0960-8524(14)00398-8 http://dx.doi.org/10.1016/j.biortech.2014.03.093 BITE 13224
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
6 February 2014 13 March 2014 18 March 2014
Please cite this article as: Devi, P., Saroha, A.K., Risk analysis of pyrolyzed biochar made from paper mill effluent treatment plant sludge for bioavailability & eco-toxicity of heavy metals, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.03.093
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Risk analysis of pyrolyzed biochar made from paper mill effluent
treatment plant sludge for bioavailability & eco-toxicity of heavy metals Parmila Devi and Anil K. Saroha* Department of Chemical Engineering, Indian Institute of Technology, Delhi Hauz Khas, New Delhi – 110016, India Abstract The risk analysis was performed to study the bioavailability and eco-toxicity of heavy metals in biochar obtained from pyrolysis of sludge of pulp and paper mill effluent treatment plant. The sludge was pyrolyzed at different temperatures (200 – 700 oC) and the resultant biochar were analyzed for fractionation of heavy metals by sequential extraction procedure. It was observed that all the heavy metals get enriched in biochar matrix after pyrolysis, but the bioavailability and eco-toxicity of the heavy metals in biochar were significantly reduced as the mobile and bioavailable heavy metal fractions were transformed into the relatively stable fractions. Moreover, it was observed that the leaching potential of heavy metals decreased after pyrolysis and the best results were obtained for biochar pyrolyzed at 700oC. Keywords: Sludge, Biochar, Pyrolysis, Heavy Metal, Leaching *Corresponding author Email id: [email protected]
Tel: +911126591032; FAX: +911126581020
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
1. Introduction Sludge handling and disposal is a universal problem as it contains significant amount of heavy metals, organic toxins and pathogenic microorganisms (Fang et al., 2012; Ren et al., 2011, Azeez et al., 2011). The contaminated sludge can have direct toxic effects on aquatic
ecosystem, and through the bioaccumulation of toxic contaminants in the food chain, which can cause an indirect risk to the humans (Nzihou and Stanmore, 2013). Accumulation and
bioavailability of heavy metals is the limiting factor for disposal and utilization of sludge. The recycling of sludge is only possible if the toxic dioxins and heavy metals are removed or stabilized, thus allowing the treated sludge to recycle into the materials cycle.
In order to reduce the toxicity of heavy metals in the sludge, two main approaches can be applied, i.e. removal of heavy metals from the sludge or heavy metal immobilization inside the sludge (Shi et al., 2013). Various methods like chemical extraction (Silva et al., 2005),
bioleaching (Pathak et al., 2008), and bioremediation (Gaur et al., 2014) are used for heavy
metal removal from the sludge. But these methods are time consuming and it is very difficult to control the heavy metal removal efficiency. Heavy metal immobilization is widely used in sludge remediation due to its simplicity and cost effective management. Pyrolysis is an effective technique to immobilize the heavy metals in the pyrolysis residue (biochar). Immobilization of heavy metals decreases the direct toxicity or leachable fraction of heavy metals, resulting in significant reduction in the environmental risks. Various studies have been reported in the literature on the mobility of heavy metals in the biochar produced from sewage sludge. Kistler et al. (1987) studied the behavior of heavy metals Cr, Ni, Cu, Zn, Cd, Pb, and Hg during the pyrolysis of sewage sludge and found that the heavy metals were highly immobile in the char due to its alkaline properties. Debela et al. (2012) tested the optimum combination of high heating temperature (HHT) and heating time to effectively
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
immobilize heavy metals in the biochar. Similar reductions in metal leaching from dredged sediments after pyrolysis have been reported by Zhang et al. (2009). The content and mobility of heavy metals in the biochar matrix should be determined to assess the risk to the environment by the recycling of biochar. The total heavy metal concentration is a useful pollution indicator, but it provides no information on the mobility of heavy metals, which depends on their chemical form (Su and Wong, 2003). Chemical speciation or sequential extraction of heavy metals from the sludge is a useful technique for determining the chemical forms in which the heavy metals are present in the sludge and the biochar. Such information is valuable for determining the mobility, bioavailability and leaching potential of heavy metals in the sludge and the biochar (Flyhammar, 1998). Therefore, the evaluation of sludge toxicity by chemical fractionation is important in deciding the suitability as well as the optimum use rate of sludge in recycling. Pulp and paper industry is capital-intensive with high consumption of raw materials,
chemicals and utilities. Huge amount of effluent (20–250 cubic meters per metric ton (m3/t)
of air dried pulp) is generated and the disposal of waste (liquid, solid and suspended matter)
generated (10-400 kg/ton of paper produced) during the paper manufacturing process
contributes to a very high impact on the environment (Pokhrel and Viraraghavan, 2004). To
the best of our knowledge, there is no study reported in the literature investigating the
bioavailability and eco-toxicity of the sludge generated from the effluent treatment plant in
the pulp and paper mill. The lack of the information limits the use of the sludge in various
applications. Therefore, efforts have been made in the present study to immobilize the heavy
metals present in the sludge using pyrolysis to explore the potential of reutilization of the
biochar in agricultural and other purposes like brick formation, use as low-cost adsorbents
etc. The pyrolysis of the sludge leads to the production of biogas, bio-oil and biochar and the heavy metals present in the sludge can partition in any of the products depending on the 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
pyrolysis temperature. The effect of pyrolysis temperature on partitioning and mobility of the six heavy metals (Cr, Cu, Ni, Zn, Pb, and Cd) was studied. The speciation and fractionation of the heavy metals in the sludge and biochar was analyzed. The toxicity and bioavailability of the heavy metals in the sludge and biochar was examined. The leachability of the heavy metals and the effect of solution pH on the leachability of the heavy metals were investigated. 2. Materials and methods 2.1 Pyrolysis of paper mill sludge Paper mill sludge (PMS), collected from the effluent treatment plant of a pulp and paper mill,
and was used as a raw material for biochar production. The PMS was characterized and
results are given in the Table S1 (supplementary materials). Pyrolysis of PMS was carried out according to the procedure reported elsewhere (Devi and Saroha, 2013). The pyrolysis was performed in the temperature range 200-700oC and the resultant biochar samples were assigned codes BC200, BC300, BC 400, BC500, BC600 and BC700 depending on their
pyrolysis temperature. The produced biochars were stored in air-tight containers for further use. 2.2 Acid digestion Heavy metal concentration in PMS, biochar and bio-oil were determined by acid digestion
method (Ramteke and Moghe, 1988; Rodriguez et al., 2009). Initially, 1 g of the solid sample (biochar or PMS; 1 mL in case of bio-oil) was added in 25 mL of HNO3-HClO4 (3:1) and was heated gently for 4 h. A colorless solution was obtained which was evaporated to near dryness. After completion of the digestion and adequate cooling of residues, solutions were made up to 25 mL by adding 0.04 N HNO3. 2.3 Sequential extraction
4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
The PMS and biochar samples were sequentially extracted as per the procedure shown in Fig. 1. The procedure categorizes the heavy metals in five fractions (F1, F2, F3, F4 and F5), according to their nature and mode of occurrence. Similar procedure for sequential extraction has been reported by Poykio et al. (2006). After each extraction, the supernatant was collected by centrifugation at 5000 rpm for 10 min in a centrifuge and filtered through a 0.45-µm nylon filter. The resultant filtrate was diluted to the desired volume (25 mL) by adding 2 % HNO3. All the extractants were analysed using atomic absorption spectrophotometer (AAS) for the six heavy metals. The heavy metal concentration was determined using the following equation 1:
∗ !
2.4 Leaching tests Leaching behaviour of the heavy metals was analysed using the toxicity characteristic leaching procedures (TCLP). The US EPA TCLP procedure involves adding 10 g powder sample in 100 mL of 0.1 M acetic acid with a liquid/solid ratio of 10:1 and shaking for 18 h in a rotary incubator shaker at 32 rpm. The resultant mixture was filtered through a 0.45 µm nylon filter and the concentration of the heavy metals in the leachate was determined using AAS. The leaching potential of a heavy metal was determined by dividing its concentration in the leachate with its initial concentration in the solid. 2.5 Effect of pH on heavy metal leaching Experiments were conducted to study the effect of pH of the solution (pH 3-13) on heavy metals leaching from PMS and biochar samples. The solid sample (5 g) was mixed with 50 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
mL of distilled water and the pH of the solution was adjusted to the desired value by addition of 0.1 N HCl/NaOH solutions. The solution was kept for 24 h and was filtered using 0.45 µm nylon filter. The resultant filtrate was analysed for heavy metals using AAS. The reproducibility of the experimental results was checked and the error in the experimental results was found to be ± 2 %. 3. Results and discussion In the present study, pyrolysis of the sludge collected from the effluent treatment plant of a
pulp and paper mill was carried out to explore the potential of reutilization of the biochar. The effect of pyrolysis temperature on the partitioning and mobility of the heavy metals was investigated. 3.1 Effect of pyrolysis temperature on heavy metal partitioning The partitioning of heavy metals in the pyrolysis products was studied by performing the pyrolysis of PMS in the temperature range 200oC - 700oC and the results are shown in Fig. 2. It can be observed that Zn and Cu were present in significant quantities in the PMS compared to other heavy metals and their concentration was found to be 193.48 mg/kg and 83.98 mg/kg respectively. The heavy metals partitioned into the solid (biochar), liquid (bio-oil) and gaseous products (biogas) on pyrolysis, depending upon the pyrolysis temperature and speciation of the heavy metals. It can be noticed from Fig. 2 that the major amount of the heavy metals were partitioned in the biochar, and the heavy metals concentration (mass basis, mg of heavy metal/kg of the sample) was not significant in the bio-oil and biogas samples
(Table S2; supplementary materials). The concentrations of the heavy metals in the PMS and the biochar was found in the order Zn > Cu > Pb > Ni >Cr > Cd.
6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
The concentration of the heavy metals in the biochar was found to be higher compared to their concentration in the PMS except for Cd. This is due to the fact that decomposition of the organic matter present in the sludge causes the release of heavy metals bound to the organic matter. The heavy metals precipitated in the biochar matrix and resulted in the partitioning of large fraction of heavy metals into the biochar (Bo et al., 2009). Further, there was a loss in the weight of the sludge due to the decomposition of the organic matter during pyrolysis. But the loss in the weight of the heavy metals was lower compared to the loss in weight of the organic matter during pyrolysis, resulting in the enrichment of heavy metals in the biochar matrix (Yeiwei et al., 2008). The concentration of Cd in biochar was found to be lower as it
may have volatilized into the biogas during pyrolysis (Shi et al., 2013). The partitioning of the heavy metals in biochar was quantified in terms of enrichment factor (Ef) which is defined as the ratio of heavy metal concentration in the biochar to its concentration in the PMS (Yeiwei et al., 2008). The value of the enrichment factor of the heavy metals in biochar for different pyrolysis temperatures is shown in Table 1. It can be noticed from Table 1 that the enrichment factor was found to increase with an increase in the pyrolysis temperature for Cu, Zn and Pb. No significant effect of pyrolysis temperature on the enrichment factor for Cr was observed. The enrichment factor for Ni and Cd were found to decrease with an increase in the pyrolysis temperature since Ni and Cd may have volatilized to the gas stream at higher
temperatures (Shi et al., 2013). 3.2 Fractionation and bioavailability of heavy metals Heavy metals can be divided into three categories on the basis of their bioavailability: a)
bioavailable (Cbio), b) potentially bioavailable (Cpbio), and c) non-bioavailable heavy metals (Cnon-bio). Bio-available category includes the water soluble (F1) and exchangeable (F2)
fraction of the heavy metals, which are easily prone to leaching. Potentially bioavailable category includes reducible (F3) and oxidizable (F4) fractions which undergo degradation 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
and leaching under very rigorous conditions (highly acidic conditions and oxidising
atmosphere). Non bioavailable category includes residual (F5) fraction, which is not prone to
leaching and degradation. The fractionation of the heavy metals in the PMS and biochar was analysed using sequential extraction procedure and the results are shown in Fig. 3 a-f. It was observed that pyrolysis could immobilize heavy metals by converting a part of unstable fractions (F1, F2, F3 and F4) into stable fraction (F5). The total heavy metals concentration obtained by sequential extraction procedure (i.e. sum of all the five fractions) must be equal to the heavy metals concentration determined by acid digestion method. The comparison between the two heavy metal concentrations obtained by sequential extraction procedure and acid digestion method highlights the complete sequential extraction of the heavy metal from the solid and the agreement is reported in terms of Recovery. The result for the evaluation of the procedures is shown in Table 2 and a good
agreement (±4%) between the two heavy metals concentrations can be noticed. A deviation
of ± 1- 8 % has been reported in the literature (Filgueiras et al., 2002). The fractionations of Cr in PMS and biochar are shown in Fig. 3a. It can be noticed that the concentration of Cr in the water soluble and exchangeable fractions was reduced from 20 % in PMS to 13.57 % in biochar after pyrolsis at 700oC (BC700). In contrast, the concentration of Cr in the residual fraction was found to increase from 30.81% in PMS to 47.51% in biochar BC700. This could be due to the formation of free CaO during pyrolysis which can immobilize Cr by forming a stable complex (Hu et al., 2013). The XRD analysis of the PMS
showed the presence of CaCO3 in significant quantity (Devi and Saroha, 2013). The chemical
speciation of Cu changed significantly when PMS was pyrolyzed at different temperatures
(Fig. 3b). The major amount of the Cu existed in the potentially bioavailable form (Cpbio ~ 60
%), in the PMS and biochar. The bioavailable fraction (F1+F2) of the Cu decreased
significantly from 22.02 % (PMS) to 1.70 % (BC700) after the pyrolysis. The large fraction 8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
of the Ni (70 %) existed in non bioavailable category in the PMS and biochar and the fraction of Ni in bioavailable category was not significant (Fig. 3c). The Ni content in the residual phase was found to increase with an increase in the pyrolysis temperature.
The Zn content in the bioavailable category in PMS was found to be 22.02 % which was
reduced to 0.89 % in BC700 after pyrolysis (Fig 3d). The Zn content in the non bioavailable category, 18.36 % in the PMS was found to increase to 45.32 % in biochar BC700 after the pyrolysis. This may be due to the fact that CaCO3, which is present in significant amount in PMS, may act as a strong adsorbent for Zn and could lead to the formation of complex salts like CaCO3.ZnCO3 during precipitation of Zn (Banerjee, 2003). The major content of Pb was found in the reducible (F3) and oxidizable (F4) fractions in PMS. The Pb content in water soluble (F1) and exchangeable (F2) fractions were not significant and were found to decrease in the biochar after pyrolysis (Fig. 3e). The Pb content in residual fraction (F5) in PMS was found to increase significantly in the biochar after pyrolysis. This may be due to the fact that at higher temperature biochar formed is alkaline in
nature (Hossain et al., 2011), which leads to the formation of lead hydroxide. The lead hydroxide eventually results in the formation of insoluble PbO during precipitation, which eventually gets fixed as residual fraction in the biochar matrix (Song et al., 2013). The speciation behaviour of Cd is slightly different compared to other five heavy metals (Fig.
3f). It can be noticed from Fig. 3f that about 30 % of the Cd in PMS is present in bioavailable form (Cbio), which is easily prone to leaching. This may be due to the presence of Cd as their chlorides or as free metal (Abanades et al., 2002).It can be further noticed from Fig. 3f that Cd is mainly associated with the oxidizable fraction in the biochar. Similar results have reported in the literature for other types of sludge (Jamali et al., 2007; He et al., 2010).
9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
It can be concluded from the Fig. 3 (a-f) that the distribution pattern in the five fractions (F1 –F5) differs with each heavy metal. The bioavailable fractions (F1 and F2) of the heavy metals were found to decrease on pyrolysis of PMS to biochar whereas the non bioavailable fractions (F5) for the heavy metals were found to increase significantly on pyrolysis. Since the residual fraction is the most stable form of the heavy metals, the pyrolysis temperature is a decisive factor for immobilization of heavy metals in the biochar. 3.3 Leaching studies The heavy metals present in the sludge are major restrictive factor for reutilisation of PMS and PMS derived biochar. The environmental behaviour of heavy metals depends strongly on their specific chemical fractions or binding characteristics in PMS and biochar. The leaching characteristics of PMS and biochar were studied by performing the experiments as per toxicity characteristic leaching procedure (TCLP) recommended by USEPA. The effect of the pH of the solution on the leaching behaviour of the heavy metals was also studied. The results of the TCLP test for the six heavy metals in the PMS and biochar are shown in the Fig. 4. In case of PMS, the leaching potential of the six heavy metals followed the following sequence: Zn > Cu > Cd > Cr > Ni > Pb. The leaching potential of the heavy metals in the biochar was found to be lower compared with that in the PMS and decreased with an increase in the pyrolysis temperature from 200oC to 700oC. The result is in agreement with the fractionation pattern of the heavy metals where the bioavailable fraction (F1 + F2) was found to be lower in biochar and decreased with an increase in the pyrolysis temperature. The leaching potential of the heavy metals in BC700 was found to follow the sequence Zn > Cr > Cd > Ni > Cu > Pb. It was observed that the leaching of the heavy metals in the biochar was within the limits as their concentrations in the leachate of TCLP test were lower than the statuary limits 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
prescribed by Indian Standards for Industrial and Sewage Effluents Discharge (inland surface
water) (Ramteke and Moghe, 1988) Table 3. 3.4 Effect of solution pH on heavy metals leaching The solution pH is an important parameter affecting the leachability of the heavy metals. Experiments were carried out at different solution pH (3-13) to study the effect of solution pH on the leaching potential of the heavy metals from the PMS and biochar (BC700) and the results are shown in Fig. 5. It can be noticed that all the heavy metals present in the PMS and biochar showed maximum leaching potential at pH 3 as low pH conditions generally enhance the metal dissolution. The leaching of the heavy metals was found to decrease with an increase in solution pH from 3 to 7. Further increase in the solution pH from 7 to 13 resulted in an increase in the leaching of the heavy metals. The leaching potential was significant in case of Cr and Cd. It can be further noticed that except for Cr, the leaching potential of the heavy metals in the biochar was lower compared to that in the PMS. This may be due to the fact that the biochar obtained after pyrolysis at elevated temperatures is relatively alkaline in nature compared to the PMS. The decomposition of carbonate form of alkaline earth metals at the higher temperature leads to the formation of metal oxides which are not prone to leaching (Zhang et al., 2009). In case of Cr, the availability of CaO after the decomposition of carbonates leads to the formation of leachable CaCrO4 (Zheng et al., 2010). 3.5 Ecological risk assessment Potential ecological risk index (RI) was used to assess the degree of potential risk of heavy metal pollution in PMS and biochar using the equations (2-4) proposed by Hakanson (1980). " # ⁄ %
(2)
&' (' "
(3) 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
)* ∑ &'
(4)
where, " is the contamination factor; Ci and Cn are the mobile and stable fractions of the heavy metals respectively, &' is the potential ecological index for individual heavy metal; (' is toxic factor of the individual heavy metal; and RI is the potential ecological risk index. The (' values used for calculation of potential ecological index for individual metal are Cr (2), Cu (5), Ni (6), Zn (1), Pb (5) and Cd (30) (Hakanson, 1980; Chabukdhara and Nema, 2012). Contamination factor (Cf ) of a heavy metal is the ratio of the sum of the concentrations of the heavy metal extracted in the first four fractions of the sequential extraction (F1+F2+F3+F4) to the concentration of the heavy metal in the residual fraction (F5) (Jamali et al., 2007). The value of the contamination factor of a heavy metal is inversely proportional to the leaching potential of the heavy metal. The potential ecological index of a heavy metal (Er) is obtained by multiplying the contamination factor of the heavy metal with the toxic factor (Tr) of the heavy metal. The potential ecological risk index (RI) of the solid (PMS/biochar) is obtained by adding the potential ecological index of each heavy metal present in the solid. The significance of the Cf, Er and RI along with their risk potential is shown in Table 4. The values of Cf, Er and RI were determined to find out the risk level of the heavy metals in the PMS and biochar (BC700) and are tabulated in Table 5. It can be noticed that the values of Cf for Pb and Cd in the PMS are13.19 and 18.22 respectively indicating a high metal contamination. The values of Cf for Pb and Cd in the biochar (BC700) are 0.77 and 1.62 respectively, indicating a significant reduction in the metal contamination after pyrolysis. Similarly, the value of RI of PMS was found to be 632.26 indicating a high degree of contamination by the heavy metals. The value of RI for the biochar (BC700) was 65.19 indicating a significant reduction in the degree of potential risk of heavy metal pollution.
12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Conclusions
The risk analysis of pyrolyzed biochar made from pulp and paper mill effluent treatment
plant sludge was carried out for bioavailability and eco-toxicity of heavy metals. Most of the
heavy metals were found to exist in the residual form in the biochar pyrolyzed at 700oC. It
was found that the pyrolysis is a promising sludge treatment method for heavy metals
immobilization resulting in significant reduction in the bioavailability and leaching potential
of the heavy metals in the biochar. The eco-toxicity of the heavy metals reduced significantly
after the pyrolysis, resulting in a decrease in the environmental risk of biochar utilisation. Acknowledgement: The authors wish to acknowledge the funding received for the project from CSIR, New Delhi. References 1. Abanades, S., Flamant, G., Gagnepain, B., Gauthier, D., 2002. Fate of heavy metals during municipal solid waste incineration. Waste Manage. Res. 20, 55-68. 2. Azeez, A.M., Meier, D., Odermatt, J., 2011. Temperature dependence of fast pyrolysis volatile products from European and African biomasses. J. Anal. Appl. Pyrolysis 90, 81–92. 3. Banerjee, A.D.K., 2003. Heavy metal levels and solid phase speciation in street dusts of Delhi, India. Environ. Pollut. 123, 95–105. 4. Bo, D., Zhang, F.S., Zhao, L., 2009. Influence of supercritical water treatment on heavy metals in medical waste incinerator fly ash. J. Hazard. Mater. 170, 66-71. 5. Chabukdhara, M., Nema, A.K., 2012. Heavy metals in water, sediments, and aquatic macrophytes: river Hindon, India. J. Hazard. Toxic Radioact. Waste 16, 273-281.
13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
6. Debela, F., Thring, R.W., Arocena, J.M., 2012. Immobilization of heavy metals by co-pyrolysis of contaminated soil with woody biomass, Water Air Soil Pollut. 223, 1161–1170. 7. Devi, P., Saroha, A.K., 2013. Effect of temperature on biochar properties during paper mill sludge pyrolysis. Int. J. ChemTech Res. 5, 682-687. 8. Fang, L., Yuan, N., Wu, Y., Zhao, X., Sun, H., 2012. Evolution of heavy metals leachability and speciation in residues of sewage sludge treated by microwave assisted pyrolysis. Appl. Mech. Mater. 178-181, 833-837.
9. Filgueiras, A.V., Lavilla, I., Bendicho, C., 2002. Chemical sequential extraction for
heavy metal partitioning in environmental solid samples. J. Environ. Monit. 4, 823-
857. 10. Flyhammar, P., 1998. Use of sequential extraction on anaerobically degraded municipal solid waste. Sci. Total Environ. 212, 203-215. 11. Gaur, N., Flora, G., Yadav, M., Tiwari, A., 2014. A review with recent advancements on bioremediation-based abolition of heavy metals. Environ. Sci. Process. Impacts 16, 180-193. 12. Hakanson, L., 1980. An ecological risk index for aquatic pollution control: A sedimentological approach. Water Res. 14, 975–1001. 13. He, Y.D., Zhai, Y.B., Li, C.T., Yang, F., Chen, L., Fan, X.P., Peng, W.F., Fu, Z.M., 2010. The fate of Cu, Zn, Pb and Cd during the pyrolysis of sewage sludge at different temperatures. Environ. Technol. 31, 567-574
14. Hossain, M.K., Strezov, V., Chan, K.Y., Ziolkowski, A., Nelson, P.F., 2011.
Influence of pyrolysis temperature on production and nutrient properties of
wastewater sludge biochar. J. Environ. Manage. 92, 223-228.
14
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
15. Hu, H.Y., Liu, H., Shen, W.Q., Luo, G.Q., Li, A.J., Lu, Z.L., Yao, H., 2013. Comparison of CaO's effect on the fate of heavy metals during thermal treatment of two typical types of MSWI fly ashes in China. Chemosphere 93, 590-596. 16. Jamali, M.K., Kazi, T.G., Afridi, H.I., Arain, M.B., Jalbani, N., Memon, A.R., 2007. Speciation of heavy metals in untreated domestic wastewater sludge by time saving BCR sequential extraction method. J. Environ. Sci. Health A 42, 649–659. 17. Kistler, R.C., Widmer, F., 1987. Behaviour of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge. Environ. Sci. Technol. 21, 704-708. 18. Nzihou, A., Stanmore, B., 2013. The fate of heavy metals during combustion and gasification of contaminated biomass—A brief review. J. Hazard. Mater. 256-257, 56– 66. 19. Pathak, A., Dastidar, M.G., Sreekrishnan, T.R., 2008. Bioleaching of heavy metals
from
anaerobically
digested
sewage
sludge.
J.
Environ.
Sci. Health A
Tox. Hazard. Subst. Environ. Eng. 43, 402-411.
20. Pokhrel, D., Viraraghavan, T., 2004. Treatment of pulp and paper mill wastewater - a
review. Sci. Total Environ. 333, 37– 58. 21. Poykio, R., Nurmesniemi, H., Kuokkanen, T., Peramaki, P., 2006. The use of sequential leaching procedure for assessing the heavy metal leachability in lime waste from the lime kiln at a caustizicing process of a pulp mill. Chemosphere. 65, 21222129.
22. Ramteke, D.S., Moghe, C.A., 1988. Manual on water and wastewater analysis,
National Environmental Engineering Research Institute (NEERI), Nagpur, India.
15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
23. Ren, X., Liang, B., Liu, M., Xu, X., Cui, M., 2012. Effects of pyrolysis temperature, time and leaf litter and powder coal ash addition on sludge-derived adsorbents for nitrogen oxide. Biores. Technol. 125, 300–304.
24. Rodriguez, L., Ruiz, E., Azcarate, J.A., Rincon, J., 2009. Heavy metal distribution and
chemical speciation in tailings and soils around a Pb–Zn mine in Spain. J. Environ.
Manage. 90, 1106–1116. 25. Shi, W., Liy, C., Ding, D., Lei, Z., Yang, Y., Feng, C., Zhang, Z., 2013. Immobilization of heavy metals in sewage sludge by using subcritical water technology. Biores. Technol. 137, 18-24. 26. Silva, J.E., Paiva, A.P., Soares, D., Labrincha, A., Castro, F., 2005. Solvent extraction applied to the recovery of heavy metals from galvanic sludge. J. Hazard. Mater. B120, 113–118. 27. Song, F., Gu, L., Zhu, N., Yuan, H., 2013. Leaching behavior of heavy metals from sewage sludge solidified by cement-based binders. Chemosphere 92, 344-350. 28. Su, D.C., Wong, J.W.C., 2003. Chemical speciation and phytoavailability of Zn, Cu, Ni and Cd in soil amended with fly ash-stabilized sewage sludge. Environ. Int. 29, 895–900. 29. Yeiwei, C., Guijian, L., Lei, W., Yu, K., Jianli, Y., 2008. Occurrence and fate of some trace elements during pyrolysis of yima coal, China. Energ. Fuel. 22, 38773882. 30. Zhang, D., He, S., Dai, L., Xie, Y., Wu, D., Bu, G., Peng, K., Kong, H., 2009. Impact of pyrolysis process on the chromium behavior of COPR. J. Hazard. Mater. 172, 1597–1601. 31. Zhang, D., Kong, H., Wu, D., He, S., Hu, Z., Dai, L., 2009. Impact of pyrolysis treatment on heavy metals in sediment. Soil Sediment Contam. 18, 754–765.
16
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
32. Zheng, L., Wang, W., Shi, Y.C., 2010. The effects of alkaline dosage and Si/Al ratio on the immobilization of heavy metals in municipal solid waste incineration fly ashbased geopolymer. Chemosphere 79, 665–671.
17
Fig. 1 Sequential extraction procedure of the heavy metals in PMS and biochar Fig. 2 Partitioning of the heavy metals in biochar pyrolyzed at different temperatures Fig. 3 Fractionation and bioavailability of the heavy metals in PMS and biochar (a) Cr; (b) Cu; (c) Ni; (d) Zn; (e) Pb; (f) Cd F1= Water soluble fraction; F2= Exchangeable fraction; F3= Reducible fraction; F4= Oxidizable fraction; and F5= Residual fraction Fig. 4 Leaching potential of the heavy metals Fig. 5 Effect of pH on leachability of heavy metals from PMS and biochar (S= PMS; BC= Biochar pyrolyzed at 700 oC)
18
Table 1 Enrichment of heavy metals in biochar pyrolyzed at different temperatures Temperature (oC)
Enrichment Factor (Ef) Chromium
Copper
Nickel
Zinc
Lead
Cadmium
(Cr)
(Cu)
(Ni)
(Zn)
(Pb)
(Cd)
200
1.18
1.09
1.55
1.04
1.28
0.77
300
1.23
1.14
1.481
1.14
1.53
1.07
400
1.24
1.17
1.32
1.19
1.90
0.73
500
1.26
1.44
1.14
1.26
2.01
0.71
600
1.28
1.55
1.01
1.49
2.09
0.59
700
1.29
1.753
0.87
1.72
2.22
0.53
19
Table 2 Percentage recovery of heavy metals calculated by comparison of analytical procedures Samples
% Recovery Cr
Cu
Ni
Zn
Pb
Cd
Sludge
97.82
97.86
99.55
98.50
92.64
101.16
BC 200
98.83
100.0
98.73
99.19
98.72
101.01
BC 300
98.78
100.0
99.42
98.02
97.92
101.01
BC 400
99.82
100.0
98.33
99.63
98.53
99.76
BC 500
98.37
99.65
101.01
99.46
102.36
99.55
BC 600
98.41
97.15
97.02
101.41
104.51
98.23
BC 700
99.60
99.95
99.49
98.83
102.71
100.82
20
Table 3 Concentration of different heavy metals (mg/L) in leachate (TCLP) Samples
Sludge
BC200
BC300
BC400
BC500
BC600
BC700
Permissible limits
Cr
1.71
2.75
2.72
2.59
2.39
2.11
1.80
2.0
Cu
23.03
16.67
3.72
2.71
4.00
3.41
1.97
3.0
Ni
1.77
1.21
1.81
1.20
0.88
0.69
0.54
3.0
Zn
28.05
16.20
7.98
4.18
3.05
2.71
1.42
5.0
Pb
0.84
0.76
0.72
0.80
0.74
0.57
0.20
0.1
Cd
0.32
0.22
0.25
0.13
0.11
0.11
0.14
2.0
21
Table 4 Indices for the ecological risk assessment Cf
Metal
Er
Potential
Contamination
RI
ecological
Sludge/Biochar contamination
risk Cf < 1
Clean
Er < 40
Low
RI < 50
Low
1 < Cf < 3
Low
40 ≤ Er
S0960-8524(14)00398-8 http://dx.doi.org/10.1016/j.biortech.2014.03.093 BITE 13224
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
6 February 2014 13 March 2014 18 March 2014
Please cite this article as: Devi, P., Saroha, A.K., Risk analysis of pyrolyzed biochar made from paper mill effluent treatment plant sludge for bioavailability & eco-toxicity of heavy metals, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.03.093
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Risk analysis of pyrolyzed biochar made from paper mill effluent
treatment plant sludge for bioavailability & eco-toxicity of heavy metals Parmila Devi and Anil K. Saroha* Department of Chemical Engineering, Indian Institute of Technology, Delhi Hauz Khas, New Delhi – 110016, India Abstract The risk analysis was performed to study the bioavailability and eco-toxicity of heavy metals in biochar obtained from pyrolysis of sludge of pulp and paper mill effluent treatment plant. The sludge was pyrolyzed at different temperatures (200 – 700 oC) and the resultant biochar were analyzed for fractionation of heavy metals by sequential extraction procedure. It was observed that all the heavy metals get enriched in biochar matrix after pyrolysis, but the bioavailability and eco-toxicity of the heavy metals in biochar were significantly reduced as the mobile and bioavailable heavy metal fractions were transformed into the relatively stable fractions. Moreover, it was observed that the leaching potential of heavy metals decreased after pyrolysis and the best results were obtained for biochar pyrolyzed at 700oC. Keywords: Sludge, Biochar, Pyrolysis, Heavy Metal, Leaching *Corresponding author Email id: [email protected]
Tel: +911126591032; FAX: +911126581020
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
1. Introduction Sludge handling and disposal is a universal problem as it contains significant amount of heavy metals, organic toxins and pathogenic microorganisms (Fang et al., 2012; Ren et al., 2011, Azeez et al., 2011). The contaminated sludge can have direct toxic effects on aquatic
ecosystem, and through the bioaccumulation of toxic contaminants in the food chain, which can cause an indirect risk to the humans (Nzihou and Stanmore, 2013). Accumulation and
bioavailability of heavy metals is the limiting factor for disposal and utilization of sludge. The recycling of sludge is only possible if the toxic dioxins and heavy metals are removed or stabilized, thus allowing the treated sludge to recycle into the materials cycle.
In order to reduce the toxicity of heavy metals in the sludge, two main approaches can be applied, i.e. removal of heavy metals from the sludge or heavy metal immobilization inside the sludge (Shi et al., 2013). Various methods like chemical extraction (Silva et al., 2005),
bioleaching (Pathak et al., 2008), and bioremediation (Gaur et al., 2014) are used for heavy
metal removal from the sludge. But these methods are time consuming and it is very difficult to control the heavy metal removal efficiency. Heavy metal immobilization is widely used in sludge remediation due to its simplicity and cost effective management. Pyrolysis is an effective technique to immobilize the heavy metals in the pyrolysis residue (biochar). Immobilization of heavy metals decreases the direct toxicity or leachable fraction of heavy metals, resulting in significant reduction in the environmental risks. Various studies have been reported in the literature on the mobility of heavy metals in the biochar produced from sewage sludge. Kistler et al. (1987) studied the behavior of heavy metals Cr, Ni, Cu, Zn, Cd, Pb, and Hg during the pyrolysis of sewage sludge and found that the heavy metals were highly immobile in the char due to its alkaline properties. Debela et al. (2012) tested the optimum combination of high heating temperature (HHT) and heating time to effectively
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
immobilize heavy metals in the biochar. Similar reductions in metal leaching from dredged sediments after pyrolysis have been reported by Zhang et al. (2009). The content and mobility of heavy metals in the biochar matrix should be determined to assess the risk to the environment by the recycling of biochar. The total heavy metal concentration is a useful pollution indicator, but it provides no information on the mobility of heavy metals, which depends on their chemical form (Su and Wong, 2003). Chemical speciation or sequential extraction of heavy metals from the sludge is a useful technique for determining the chemical forms in which the heavy metals are present in the sludge and the biochar. Such information is valuable for determining the mobility, bioavailability and leaching potential of heavy metals in the sludge and the biochar (Flyhammar, 1998). Therefore, the evaluation of sludge toxicity by chemical fractionation is important in deciding the suitability as well as the optimum use rate of sludge in recycling. Pulp and paper industry is capital-intensive with high consumption of raw materials,
chemicals and utilities. Huge amount of effluent (20–250 cubic meters per metric ton (m3/t)
of air dried pulp) is generated and the disposal of waste (liquid, solid and suspended matter)
generated (10-400 kg/ton of paper produced) during the paper manufacturing process
contributes to a very high impact on the environment (Pokhrel and Viraraghavan, 2004). To
the best of our knowledge, there is no study reported in the literature investigating the
bioavailability and eco-toxicity of the sludge generated from the effluent treatment plant in
the pulp and paper mill. The lack of the information limits the use of the sludge in various
applications. Therefore, efforts have been made in the present study to immobilize the heavy
metals present in the sludge using pyrolysis to explore the potential of reutilization of the
biochar in agricultural and other purposes like brick formation, use as low-cost adsorbents
etc. The pyrolysis of the sludge leads to the production of biogas, bio-oil and biochar and the heavy metals present in the sludge can partition in any of the products depending on the 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
pyrolysis temperature. The effect of pyrolysis temperature on partitioning and mobility of the six heavy metals (Cr, Cu, Ni, Zn, Pb, and Cd) was studied. The speciation and fractionation of the heavy metals in the sludge and biochar was analyzed. The toxicity and bioavailability of the heavy metals in the sludge and biochar was examined. The leachability of the heavy metals and the effect of solution pH on the leachability of the heavy metals were investigated. 2. Materials and methods 2.1 Pyrolysis of paper mill sludge Paper mill sludge (PMS), collected from the effluent treatment plant of a pulp and paper mill,
and was used as a raw material for biochar production. The PMS was characterized and
results are given in the Table S1 (supplementary materials). Pyrolysis of PMS was carried out according to the procedure reported elsewhere (Devi and Saroha, 2013). The pyrolysis was performed in the temperature range 200-700oC and the resultant biochar samples were assigned codes BC200, BC300, BC 400, BC500, BC600 and BC700 depending on their
pyrolysis temperature. The produced biochars were stored in air-tight containers for further use. 2.2 Acid digestion Heavy metal concentration in PMS, biochar and bio-oil were determined by acid digestion
method (Ramteke and Moghe, 1988; Rodriguez et al., 2009). Initially, 1 g of the solid sample (biochar or PMS; 1 mL in case of bio-oil) was added in 25 mL of HNO3-HClO4 (3:1) and was heated gently for 4 h. A colorless solution was obtained which was evaporated to near dryness. After completion of the digestion and adequate cooling of residues, solutions were made up to 25 mL by adding 0.04 N HNO3. 2.3 Sequential extraction
4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
The PMS and biochar samples were sequentially extracted as per the procedure shown in Fig. 1. The procedure categorizes the heavy metals in five fractions (F1, F2, F3, F4 and F5), according to their nature and mode of occurrence. Similar procedure for sequential extraction has been reported by Poykio et al. (2006). After each extraction, the supernatant was collected by centrifugation at 5000 rpm for 10 min in a centrifuge and filtered through a 0.45-µm nylon filter. The resultant filtrate was diluted to the desired volume (25 mL) by adding 2 % HNO3. All the extractants were analysed using atomic absorption spectrophotometer (AAS) for the six heavy metals. The heavy metal concentration was determined using the following equation 1:
∗ !
2.4 Leaching tests Leaching behaviour of the heavy metals was analysed using the toxicity characteristic leaching procedures (TCLP). The US EPA TCLP procedure involves adding 10 g powder sample in 100 mL of 0.1 M acetic acid with a liquid/solid ratio of 10:1 and shaking for 18 h in a rotary incubator shaker at 32 rpm. The resultant mixture was filtered through a 0.45 µm nylon filter and the concentration of the heavy metals in the leachate was determined using AAS. The leaching potential of a heavy metal was determined by dividing its concentration in the leachate with its initial concentration in the solid. 2.5 Effect of pH on heavy metal leaching Experiments were conducted to study the effect of pH of the solution (pH 3-13) on heavy metals leaching from PMS and biochar samples. The solid sample (5 g) was mixed with 50 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
mL of distilled water and the pH of the solution was adjusted to the desired value by addition of 0.1 N HCl/NaOH solutions. The solution was kept for 24 h and was filtered using 0.45 µm nylon filter. The resultant filtrate was analysed for heavy metals using AAS. The reproducibility of the experimental results was checked and the error in the experimental results was found to be ± 2 %. 3. Results and discussion In the present study, pyrolysis of the sludge collected from the effluent treatment plant of a
pulp and paper mill was carried out to explore the potential of reutilization of the biochar. The effect of pyrolysis temperature on the partitioning and mobility of the heavy metals was investigated. 3.1 Effect of pyrolysis temperature on heavy metal partitioning The partitioning of heavy metals in the pyrolysis products was studied by performing the pyrolysis of PMS in the temperature range 200oC - 700oC and the results are shown in Fig. 2. It can be observed that Zn and Cu were present in significant quantities in the PMS compared to other heavy metals and their concentration was found to be 193.48 mg/kg and 83.98 mg/kg respectively. The heavy metals partitioned into the solid (biochar), liquid (bio-oil) and gaseous products (biogas) on pyrolysis, depending upon the pyrolysis temperature and speciation of the heavy metals. It can be noticed from Fig. 2 that the major amount of the heavy metals were partitioned in the biochar, and the heavy metals concentration (mass basis, mg of heavy metal/kg of the sample) was not significant in the bio-oil and biogas samples
(Table S2; supplementary materials). The concentrations of the heavy metals in the PMS and the biochar was found in the order Zn > Cu > Pb > Ni >Cr > Cd.
6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
The concentration of the heavy metals in the biochar was found to be higher compared to their concentration in the PMS except for Cd. This is due to the fact that decomposition of the organic matter present in the sludge causes the release of heavy metals bound to the organic matter. The heavy metals precipitated in the biochar matrix and resulted in the partitioning of large fraction of heavy metals into the biochar (Bo et al., 2009). Further, there was a loss in the weight of the sludge due to the decomposition of the organic matter during pyrolysis. But the loss in the weight of the heavy metals was lower compared to the loss in weight of the organic matter during pyrolysis, resulting in the enrichment of heavy metals in the biochar matrix (Yeiwei et al., 2008). The concentration of Cd in biochar was found to be lower as it
may have volatilized into the biogas during pyrolysis (Shi et al., 2013). The partitioning of the heavy metals in biochar was quantified in terms of enrichment factor (Ef) which is defined as the ratio of heavy metal concentration in the biochar to its concentration in the PMS (Yeiwei et al., 2008). The value of the enrichment factor of the heavy metals in biochar for different pyrolysis temperatures is shown in Table 1. It can be noticed from Table 1 that the enrichment factor was found to increase with an increase in the pyrolysis temperature for Cu, Zn and Pb. No significant effect of pyrolysis temperature on the enrichment factor for Cr was observed. The enrichment factor for Ni and Cd were found to decrease with an increase in the pyrolysis temperature since Ni and Cd may have volatilized to the gas stream at higher
temperatures (Shi et al., 2013). 3.2 Fractionation and bioavailability of heavy metals Heavy metals can be divided into three categories on the basis of their bioavailability: a)
bioavailable (Cbio), b) potentially bioavailable (Cpbio), and c) non-bioavailable heavy metals (Cnon-bio). Bio-available category includes the water soluble (F1) and exchangeable (F2)
fraction of the heavy metals, which are easily prone to leaching. Potentially bioavailable category includes reducible (F3) and oxidizable (F4) fractions which undergo degradation 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
and leaching under very rigorous conditions (highly acidic conditions and oxidising
atmosphere). Non bioavailable category includes residual (F5) fraction, which is not prone to
leaching and degradation. The fractionation of the heavy metals in the PMS and biochar was analysed using sequential extraction procedure and the results are shown in Fig. 3 a-f. It was observed that pyrolysis could immobilize heavy metals by converting a part of unstable fractions (F1, F2, F3 and F4) into stable fraction (F5). The total heavy metals concentration obtained by sequential extraction procedure (i.e. sum of all the five fractions) must be equal to the heavy metals concentration determined by acid digestion method. The comparison between the two heavy metal concentrations obtained by sequential extraction procedure and acid digestion method highlights the complete sequential extraction of the heavy metal from the solid and the agreement is reported in terms of Recovery. The result for the evaluation of the procedures is shown in Table 2 and a good
agreement (±4%) between the two heavy metals concentrations can be noticed. A deviation
of ± 1- 8 % has been reported in the literature (Filgueiras et al., 2002). The fractionations of Cr in PMS and biochar are shown in Fig. 3a. It can be noticed that the concentration of Cr in the water soluble and exchangeable fractions was reduced from 20 % in PMS to 13.57 % in biochar after pyrolsis at 700oC (BC700). In contrast, the concentration of Cr in the residual fraction was found to increase from 30.81% in PMS to 47.51% in biochar BC700. This could be due to the formation of free CaO during pyrolysis which can immobilize Cr by forming a stable complex (Hu et al., 2013). The XRD analysis of the PMS
showed the presence of CaCO3 in significant quantity (Devi and Saroha, 2013). The chemical
speciation of Cu changed significantly when PMS was pyrolyzed at different temperatures
(Fig. 3b). The major amount of the Cu existed in the potentially bioavailable form (Cpbio ~ 60
%), in the PMS and biochar. The bioavailable fraction (F1+F2) of the Cu decreased
significantly from 22.02 % (PMS) to 1.70 % (BC700) after the pyrolysis. The large fraction 8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
of the Ni (70 %) existed in non bioavailable category in the PMS and biochar and the fraction of Ni in bioavailable category was not significant (Fig. 3c). The Ni content in the residual phase was found to increase with an increase in the pyrolysis temperature.
The Zn content in the bioavailable category in PMS was found to be 22.02 % which was
reduced to 0.89 % in BC700 after pyrolysis (Fig 3d). The Zn content in the non bioavailable category, 18.36 % in the PMS was found to increase to 45.32 % in biochar BC700 after the pyrolysis. This may be due to the fact that CaCO3, which is present in significant amount in PMS, may act as a strong adsorbent for Zn and could lead to the formation of complex salts like CaCO3.ZnCO3 during precipitation of Zn (Banerjee, 2003). The major content of Pb was found in the reducible (F3) and oxidizable (F4) fractions in PMS. The Pb content in water soluble (F1) and exchangeable (F2) fractions were not significant and were found to decrease in the biochar after pyrolysis (Fig. 3e). The Pb content in residual fraction (F5) in PMS was found to increase significantly in the biochar after pyrolysis. This may be due to the fact that at higher temperature biochar formed is alkaline in
nature (Hossain et al., 2011), which leads to the formation of lead hydroxide. The lead hydroxide eventually results in the formation of insoluble PbO during precipitation, which eventually gets fixed as residual fraction in the biochar matrix (Song et al., 2013). The speciation behaviour of Cd is slightly different compared to other five heavy metals (Fig.
3f). It can be noticed from Fig. 3f that about 30 % of the Cd in PMS is present in bioavailable form (Cbio), which is easily prone to leaching. This may be due to the presence of Cd as their chlorides or as free metal (Abanades et al., 2002).It can be further noticed from Fig. 3f that Cd is mainly associated with the oxidizable fraction in the biochar. Similar results have reported in the literature for other types of sludge (Jamali et al., 2007; He et al., 2010).
9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
It can be concluded from the Fig. 3 (a-f) that the distribution pattern in the five fractions (F1 –F5) differs with each heavy metal. The bioavailable fractions (F1 and F2) of the heavy metals were found to decrease on pyrolysis of PMS to biochar whereas the non bioavailable fractions (F5) for the heavy metals were found to increase significantly on pyrolysis. Since the residual fraction is the most stable form of the heavy metals, the pyrolysis temperature is a decisive factor for immobilization of heavy metals in the biochar. 3.3 Leaching studies The heavy metals present in the sludge are major restrictive factor for reutilisation of PMS and PMS derived biochar. The environmental behaviour of heavy metals depends strongly on their specific chemical fractions or binding characteristics in PMS and biochar. The leaching characteristics of PMS and biochar were studied by performing the experiments as per toxicity characteristic leaching procedure (TCLP) recommended by USEPA. The effect of the pH of the solution on the leaching behaviour of the heavy metals was also studied. The results of the TCLP test for the six heavy metals in the PMS and biochar are shown in the Fig. 4. In case of PMS, the leaching potential of the six heavy metals followed the following sequence: Zn > Cu > Cd > Cr > Ni > Pb. The leaching potential of the heavy metals in the biochar was found to be lower compared with that in the PMS and decreased with an increase in the pyrolysis temperature from 200oC to 700oC. The result is in agreement with the fractionation pattern of the heavy metals where the bioavailable fraction (F1 + F2) was found to be lower in biochar and decreased with an increase in the pyrolysis temperature. The leaching potential of the heavy metals in BC700 was found to follow the sequence Zn > Cr > Cd > Ni > Cu > Pb. It was observed that the leaching of the heavy metals in the biochar was within the limits as their concentrations in the leachate of TCLP test were lower than the statuary limits 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
prescribed by Indian Standards for Industrial and Sewage Effluents Discharge (inland surface
water) (Ramteke and Moghe, 1988) Table 3. 3.4 Effect of solution pH on heavy metals leaching The solution pH is an important parameter affecting the leachability of the heavy metals. Experiments were carried out at different solution pH (3-13) to study the effect of solution pH on the leaching potential of the heavy metals from the PMS and biochar (BC700) and the results are shown in Fig. 5. It can be noticed that all the heavy metals present in the PMS and biochar showed maximum leaching potential at pH 3 as low pH conditions generally enhance the metal dissolution. The leaching of the heavy metals was found to decrease with an increase in solution pH from 3 to 7. Further increase in the solution pH from 7 to 13 resulted in an increase in the leaching of the heavy metals. The leaching potential was significant in case of Cr and Cd. It can be further noticed that except for Cr, the leaching potential of the heavy metals in the biochar was lower compared to that in the PMS. This may be due to the fact that the biochar obtained after pyrolysis at elevated temperatures is relatively alkaline in nature compared to the PMS. The decomposition of carbonate form of alkaline earth metals at the higher temperature leads to the formation of metal oxides which are not prone to leaching (Zhang et al., 2009). In case of Cr, the availability of CaO after the decomposition of carbonates leads to the formation of leachable CaCrO4 (Zheng et al., 2010). 3.5 Ecological risk assessment Potential ecological risk index (RI) was used to assess the degree of potential risk of heavy metal pollution in PMS and biochar using the equations (2-4) proposed by Hakanson (1980). " # ⁄ %
(2)
&' (' "
(3) 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
)* ∑ &'
(4)
where, " is the contamination factor; Ci and Cn are the mobile and stable fractions of the heavy metals respectively, &' is the potential ecological index for individual heavy metal; (' is toxic factor of the individual heavy metal; and RI is the potential ecological risk index. The (' values used for calculation of potential ecological index for individual metal are Cr (2), Cu (5), Ni (6), Zn (1), Pb (5) and Cd (30) (Hakanson, 1980; Chabukdhara and Nema, 2012). Contamination factor (Cf ) of a heavy metal is the ratio of the sum of the concentrations of the heavy metal extracted in the first four fractions of the sequential extraction (F1+F2+F3+F4) to the concentration of the heavy metal in the residual fraction (F5) (Jamali et al., 2007). The value of the contamination factor of a heavy metal is inversely proportional to the leaching potential of the heavy metal. The potential ecological index of a heavy metal (Er) is obtained by multiplying the contamination factor of the heavy metal with the toxic factor (Tr) of the heavy metal. The potential ecological risk index (RI) of the solid (PMS/biochar) is obtained by adding the potential ecological index of each heavy metal present in the solid. The significance of the Cf, Er and RI along with their risk potential is shown in Table 4. The values of Cf, Er and RI were determined to find out the risk level of the heavy metals in the PMS and biochar (BC700) and are tabulated in Table 5. It can be noticed that the values of Cf for Pb and Cd in the PMS are13.19 and 18.22 respectively indicating a high metal contamination. The values of Cf for Pb and Cd in the biochar (BC700) are 0.77 and 1.62 respectively, indicating a significant reduction in the metal contamination after pyrolysis. Similarly, the value of RI of PMS was found to be 632.26 indicating a high degree of contamination by the heavy metals. The value of RI for the biochar (BC700) was 65.19 indicating a significant reduction in the degree of potential risk of heavy metal pollution.
12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Conclusions
The risk analysis of pyrolyzed biochar made from pulp and paper mill effluent treatment
plant sludge was carried out for bioavailability and eco-toxicity of heavy metals. Most of the
heavy metals were found to exist in the residual form in the biochar pyrolyzed at 700oC. It
was found that the pyrolysis is a promising sludge treatment method for heavy metals
immobilization resulting in significant reduction in the bioavailability and leaching potential
of the heavy metals in the biochar. The eco-toxicity of the heavy metals reduced significantly
after the pyrolysis, resulting in a decrease in the environmental risk of biochar utilisation. Acknowledgement: The authors wish to acknowledge the funding received for the project from CSIR, New Delhi. References 1. Abanades, S., Flamant, G., Gagnepain, B., Gauthier, D., 2002. Fate of heavy metals during municipal solid waste incineration. Waste Manage. Res. 20, 55-68. 2. Azeez, A.M., Meier, D., Odermatt, J., 2011. Temperature dependence of fast pyrolysis volatile products from European and African biomasses. J. Anal. Appl. Pyrolysis 90, 81–92. 3. Banerjee, A.D.K., 2003. Heavy metal levels and solid phase speciation in street dusts of Delhi, India. Environ. Pollut. 123, 95–105. 4. Bo, D., Zhang, F.S., Zhao, L., 2009. Influence of supercritical water treatment on heavy metals in medical waste incinerator fly ash. J. Hazard. Mater. 170, 66-71. 5. Chabukdhara, M., Nema, A.K., 2012. Heavy metals in water, sediments, and aquatic macrophytes: river Hindon, India. J. Hazard. Toxic Radioact. Waste 16, 273-281.
13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
6. Debela, F., Thring, R.W., Arocena, J.M., 2012. Immobilization of heavy metals by co-pyrolysis of contaminated soil with woody biomass, Water Air Soil Pollut. 223, 1161–1170. 7. Devi, P., Saroha, A.K., 2013. Effect of temperature on biochar properties during paper mill sludge pyrolysis. Int. J. ChemTech Res. 5, 682-687. 8. Fang, L., Yuan, N., Wu, Y., Zhao, X., Sun, H., 2012. Evolution of heavy metals leachability and speciation in residues of sewage sludge treated by microwave assisted pyrolysis. Appl. Mech. Mater. 178-181, 833-837.
9. Filgueiras, A.V., Lavilla, I., Bendicho, C., 2002. Chemical sequential extraction for
heavy metal partitioning in environmental solid samples. J. Environ. Monit. 4, 823-
857. 10. Flyhammar, P., 1998. Use of sequential extraction on anaerobically degraded municipal solid waste. Sci. Total Environ. 212, 203-215. 11. Gaur, N., Flora, G., Yadav, M., Tiwari, A., 2014. A review with recent advancements on bioremediation-based abolition of heavy metals. Environ. Sci. Process. Impacts 16, 180-193. 12. Hakanson, L., 1980. An ecological risk index for aquatic pollution control: A sedimentological approach. Water Res. 14, 975–1001. 13. He, Y.D., Zhai, Y.B., Li, C.T., Yang, F., Chen, L., Fan, X.P., Peng, W.F., Fu, Z.M., 2010. The fate of Cu, Zn, Pb and Cd during the pyrolysis of sewage sludge at different temperatures. Environ. Technol. 31, 567-574
14. Hossain, M.K., Strezov, V., Chan, K.Y., Ziolkowski, A., Nelson, P.F., 2011.
Influence of pyrolysis temperature on production and nutrient properties of
wastewater sludge biochar. J. Environ. Manage. 92, 223-228.
14
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
15. Hu, H.Y., Liu, H., Shen, W.Q., Luo, G.Q., Li, A.J., Lu, Z.L., Yao, H., 2013. Comparison of CaO's effect on the fate of heavy metals during thermal treatment of two typical types of MSWI fly ashes in China. Chemosphere 93, 590-596. 16. Jamali, M.K., Kazi, T.G., Afridi, H.I., Arain, M.B., Jalbani, N., Memon, A.R., 2007. Speciation of heavy metals in untreated domestic wastewater sludge by time saving BCR sequential extraction method. J. Environ. Sci. Health A 42, 649–659. 17. Kistler, R.C., Widmer, F., 1987. Behaviour of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge. Environ. Sci. Technol. 21, 704-708. 18. Nzihou, A., Stanmore, B., 2013. The fate of heavy metals during combustion and gasification of contaminated biomass—A brief review. J. Hazard. Mater. 256-257, 56– 66. 19. Pathak, A., Dastidar, M.G., Sreekrishnan, T.R., 2008. Bioleaching of heavy metals
from
anaerobically
digested
sewage
sludge.
J.
Environ.
Sci. Health A
Tox. Hazard. Subst. Environ. Eng. 43, 402-411.
20. Pokhrel, D., Viraraghavan, T., 2004. Treatment of pulp and paper mill wastewater - a
review. Sci. Total Environ. 333, 37– 58. 21. Poykio, R., Nurmesniemi, H., Kuokkanen, T., Peramaki, P., 2006. The use of sequential leaching procedure for assessing the heavy metal leachability in lime waste from the lime kiln at a caustizicing process of a pulp mill. Chemosphere. 65, 21222129.
22. Ramteke, D.S., Moghe, C.A., 1988. Manual on water and wastewater analysis,
National Environmental Engineering Research Institute (NEERI), Nagpur, India.
15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
23. Ren, X., Liang, B., Liu, M., Xu, X., Cui, M., 2012. Effects of pyrolysis temperature, time and leaf litter and powder coal ash addition on sludge-derived adsorbents for nitrogen oxide. Biores. Technol. 125, 300–304.
24. Rodriguez, L., Ruiz, E., Azcarate, J.A., Rincon, J., 2009. Heavy metal distribution and
chemical speciation in tailings and soils around a Pb–Zn mine in Spain. J. Environ.
Manage. 90, 1106–1116. 25. Shi, W., Liy, C., Ding, D., Lei, Z., Yang, Y., Feng, C., Zhang, Z., 2013. Immobilization of heavy metals in sewage sludge by using subcritical water technology. Biores. Technol. 137, 18-24. 26. Silva, J.E., Paiva, A.P., Soares, D., Labrincha, A., Castro, F., 2005. Solvent extraction applied to the recovery of heavy metals from galvanic sludge. J. Hazard. Mater. B120, 113–118. 27. Song, F., Gu, L., Zhu, N., Yuan, H., 2013. Leaching behavior of heavy metals from sewage sludge solidified by cement-based binders. Chemosphere 92, 344-350. 28. Su, D.C., Wong, J.W.C., 2003. Chemical speciation and phytoavailability of Zn, Cu, Ni and Cd in soil amended with fly ash-stabilized sewage sludge. Environ. Int. 29, 895–900. 29. Yeiwei, C., Guijian, L., Lei, W., Yu, K., Jianli, Y., 2008. Occurrence and fate of some trace elements during pyrolysis of yima coal, China. Energ. Fuel. 22, 38773882. 30. Zhang, D., He, S., Dai, L., Xie, Y., Wu, D., Bu, G., Peng, K., Kong, H., 2009. Impact of pyrolysis process on the chromium behavior of COPR. J. Hazard. Mater. 172, 1597–1601. 31. Zhang, D., Kong, H., Wu, D., He, S., Hu, Z., Dai, L., 2009. Impact of pyrolysis treatment on heavy metals in sediment. Soil Sediment Contam. 18, 754–765.
16
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
32. Zheng, L., Wang, W., Shi, Y.C., 2010. The effects of alkaline dosage and Si/Al ratio on the immobilization of heavy metals in municipal solid waste incineration fly ashbased geopolymer. Chemosphere 79, 665–671.
17
Fig. 1 Sequential extraction procedure of the heavy metals in PMS and biochar Fig. 2 Partitioning of the heavy metals in biochar pyrolyzed at different temperatures Fig. 3 Fractionation and bioavailability of the heavy metals in PMS and biochar (a) Cr; (b) Cu; (c) Ni; (d) Zn; (e) Pb; (f) Cd F1= Water soluble fraction; F2= Exchangeable fraction; F3= Reducible fraction; F4= Oxidizable fraction; and F5= Residual fraction Fig. 4 Leaching potential of the heavy metals Fig. 5 Effect of pH on leachability of heavy metals from PMS and biochar (S= PMS; BC= Biochar pyrolyzed at 700 oC)
18
Table 1 Enrichment of heavy metals in biochar pyrolyzed at different temperatures Temperature (oC)
Enrichment Factor (Ef) Chromium
Copper
Nickel
Zinc
Lead
Cadmium
(Cr)
(Cu)
(Ni)
(Zn)
(Pb)
(Cd)
200
1.18
1.09
1.55
1.04
1.28
0.77
300
1.23
1.14
1.481
1.14
1.53
1.07
400
1.24
1.17
1.32
1.19
1.90
0.73
500
1.26
1.44
1.14
1.26
2.01
0.71
600
1.28
1.55
1.01
1.49
2.09
0.59
700
1.29
1.753
0.87
1.72
2.22
0.53
19
Table 2 Percentage recovery of heavy metals calculated by comparison of analytical procedures Samples
% Recovery Cr
Cu
Ni
Zn
Pb
Cd
Sludge
97.82
97.86
99.55
98.50
92.64
101.16
BC 200
98.83
100.0
98.73
99.19
98.72
101.01
BC 300
98.78
100.0
99.42
98.02
97.92
101.01
BC 400
99.82
100.0
98.33
99.63
98.53
99.76
BC 500
98.37
99.65
101.01
99.46
102.36
99.55
BC 600
98.41
97.15
97.02
101.41
104.51
98.23
BC 700
99.60
99.95
99.49
98.83
102.71
100.82
20
Table 3 Concentration of different heavy metals (mg/L) in leachate (TCLP) Samples
Sludge
BC200
BC300
BC400
BC500
BC600
BC700
Permissible limits
Cr
1.71
2.75
2.72
2.59
2.39
2.11
1.80
2.0
Cu
23.03
16.67
3.72
2.71
4.00
3.41
1.97
3.0
Ni
1.77
1.21
1.81
1.20
0.88
0.69
0.54
3.0
Zn
28.05
16.20
7.98
4.18
3.05
2.71
1.42
5.0
Pb
0.84
0.76
0.72
0.80
0.74
0.57
0.20
0.1
Cd
0.32
0.22
0.25
0.13
0.11
0.11
0.14
2.0
21
Table 4 Indices for the ecological risk assessment Cf
Metal
Er
Potential
Contamination
RI
ecological
Sludge/Biochar contamination
risk Cf < 1
Clean
Er < 40
Low
RI < 50
Low
1 < Cf < 3
Low
40 ≤ Er
