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research-article2014
WMR0010.1177/0734242X14565210Waste Management & ResearchKhanmohammadi et al.
Original Article
Effect of pyrolysis temperature on chemical and physical properties of sewage sludge biochar
Waste Management & Research 1–9 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14565210 wmr.sagepub.com
Zahra Khanmohammadi, Majid Afyuni and Mohammad Reza Mosaddeghi
Abstract The objective of this study was to evaluate the effects of pyrolysis temperatures (300, 400, 500, 600 and 700°C) on properties of biochar produced from an urban sewage sludge. Biochar yield significantly decreased from 72.5% at 300°C to 52.9% at 700°C, whereas an increase in temperature increased the gas yield. Biochar pH and electrical conductivity increased by 3.8 and 1.4 dS m–1, proportionally to the increment of temperature. Biochar produced at low temperatures had higher total nitrogen and total organic carbon content but a lower C/N ratio, calcium carbonate equivalent, and total P, K and Na contents. Total and diethylene triamine penta acetic acid (DTPA)-extractable concentrations of Fe, Zn, Cu, Mn, Ni, Cr and Pb increased with increment of temperature. Lower DTPA-extractable concentrations of Fe, Zn, Cu, Mn, Ni and Pb were found in biochars compared to the sewage sludge. Pyrolysis decreased bulk density, whereas particle density and porosity increment was observed upon pyrolysis with increment of temperature. Sewage sludge saturated water content (θs) was 130.4 g 100g–1 and significantly greater than biochar, but biochar θs significantly increased with temperature (95.7 versus 105.4 g 100g–1 at 300 and 700°C, respectively). Pyrolysis decreased the biochar’s water repellency, assessed by molarity of ethanol droplet (MED), compared to the sewage sludge. The lowest MED of 0.2 and water repellency rating of 3 were found for the biochar produced at 700°C. Based on our results and considering the energy consumption, pyrolysis temperature in the range of 300–400°C may be suggested for sewage sludge pyrolysis. Keywords Pyrolysis, sewage sludge biochar, heavy metals, bulk density, water repellency, molarity of ethanol droplet
Introduction Population growth and economic developments have resulted in increasing sewage sludge production. The role of sewage sludge utilization on agricultural lands as a fertilizer is known to result in the recycling of valuable components, including organic matter, N, P, K and micronutrients (Singh and Agrawal, 2007). However, high concentrations of heavy metals (HMs), such as Zn, Cu, Cr, Ni, Cd, and high amounts of pathogens in sewage sludge led to concerns regarding soil and groundwater pollution (Gascó et al., 2005; Wang et al., 2008). In addition, high concentrations of N and P in sewage sludge have adverse environmental effects (Sumner, 2000). Sewage sludge effect on soil organic matter (SOM) and cation exchange capacity (CEC), improving plant water availability and uptake of micronutrients such as Zn and Cu in calcareous soils have been reported (Karami et al., 2008; Yeganeh et al., 2010). Pyrolysis of organic wastes, such as sewage sludge, crop residues or wood, involves the thermal transformation of biomass under partial or no oxygen supply to solid (charcoal or biochar), liquid (bio-oil) and gas phases (Laird, 2008; Lehmann, 2007). When biochar is applied to the soils, it remains in an essentially permanent form and leads to net removal of carbon from the
atmosphere (Glaser et al., 2009; Laird, 2008; Lehmann, 2007). In fact, the polycyclic aromatic structure of biochar is the reason for its stability and carbon sequestration in the soil (Verheijen et al., 2010). In addition, incorporation of biochar can improve soil physical, chemical and biological properties (Lehmann and Joseph, 2009). Biochar application to the soil increases the soil aeration (Laird, 2008), porosity and specific surface area (Laird et al., 2010; Steiner et al., 2007), water-holding capacity and drainage condition (Ibrahim et al., 2013; Steiner et al., 2007), soil fertility and following plant yield responses (Major et al., 2010; Park et al., 2011). Pyrolysis conditions and feedstock characteristics largely control the physical and chemical properties (e.g. composition, particle and pore size distribution) of the biochar, which in turn Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Corresponding author: Zahra Khanmohammadi, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran. Emails: [email protected]
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Figure 1. Schematic diagram of pyrolysis apparatus used in this study.
determine its suitability for a given application (Chan and Xu, 2009). Fu et al. (2011) reported that increasing pyrolysis temperature decreases biochar and liquid yield but raises gas yield. Yuan et al. (2011) studied the forms of alkalis of the biochars produced from the straws of canola, corn, soybean and peanut at different pyrolysis temperatures. They reported that the alkalinity and pH of biochars increased with raising pyrolysis temperature. Hossain et al. (2011) found that biochar yield of wastewater sludge decreased from 72.3% at 300°C to 52.4% at 700°C. Besides, pH of the produced biochars increased with increasing pyrolysis temperature from 300 to 700°C but the electrical conductivity (EC) of the biochars had a decreasing trend with temperature. Liu et al. (2014) investigated the nutrients and HMs in biochar produced at 450°C from sewage sludge. They found that total and available N contents of biochar were lower, whereas total and available P and K contents were higher when compared to sewage sludge. Total concentrations of HMs were greater in the biochar than in sewage sludge, but their diethylene triamine penta acetic acid (DTPA)-extractable concentrations were lower in the biochar. In a recent study, Claoston et al. (2014) reported that porosity and surface areas (determined by the Brunauer–Emmett–Teller method) of empty fruit bunch and rice husk biochars increased with pyrolysis temperature increment. Therefore, sewage sludge pyrolysis can be an acceptable method for its management (Hwang et al., 2007). Pyrolysis removes sewage sludge pathogens, reduces its volume and transport costs (Caballero et al., 1997), and has positive effects on soil properties and agricultural outputs. Various studies have been conducted on the pyrolysis of biomass as a potential carbon sequestrator (Glaser et al., 2009; Laird, 2008; Verheijen et al., 2010). Although some chemical and physical properties of sewage sludge biochar, including nutrient, HMs, specific surface area, pore volume and its morphology, have been studied, less attention has been given to the effect of pyrolysis temperatures
on the water repellency and water-holding capacity of sewage sludge biochar. The objective of this study was to investigate the effects of pyrolysis temperature on chemical and physical properties of urban sewage sludge’s biochar with emphasis on physical properties, such as water repellency and water-holding capacity.
Materials and methods Pyrolysis experimental setup and procedure A secondary anaerobically digested sewage sludge was collected from Isfahan wastewater treatment plant (32° 48’ 21” N 51° 35’ 21” E). The sewage sludge was oven-dried at 40°C for 24 h. The sewage sludge was then grinded and sieved to obtain particle sizes less than 2 mm, then stored in air-tight plastic bags until it was pyrolized. The sample was pyrolized under controlled conditions to ensure uniform heating. A modified electrical furnace was used to produce the limiting-oxygen condition of pyrolysis (Figure 1). A cubic iron encasement with dimensions of 16 cm in diameter and 45 cm in length with three shelves was designed and put in the electrical furnace. The door of the enclosure had an inlet for inert gas and an outlet to pass the produced liquid phase and gases. Sewage sludge samples were placed on the encasement shelves, and then the encasement was put in the chamber of the electrical furnace (Figure 1). Sewage sludge pyrolysis was done at 300, 400, 500, 600 and 700°C at a rate of 3°C min–1 and using a continuous inflow of argon (inert gas) in three replicates. Therefore, 15 samples were pyrolized (5 temperatures × 3 replicates). Approximately 1.4 kg sewage sludge was used for pyrolysis for each temperature and replicate. The argon inflow pushed the air out of the encasement to maintain the limiting-oxygen condition during pyrolysis. The final temperature was maintained for 2 h. Then the furnace was
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Khanmohammadi et al. cooled down to 100°C slowly. As shown in Figure 1, the liquid phase (effluent) flowing through the outlet tube was collected in a container. The percentage of biochar yield at various temperatures was calculated using Equation (1): mass of Biochar yield = biochar
loaded samples (BD100kPa) was also determined, which is better comparable among the treatments due to the well-defined compaction condition. The porosity (ƒ) of samples was calculated at loose (ƒloose) and 100 kPa (ƒ100kPa) conditions as follows:
−1
mass of original ×100 (1) × sewage sludge
Liquid phase yield was calculated similar to the biochar yield. The gas yield percentage was obtained with subtraction of biochar and liquid yields from 100.
Chemical analysis The pH and EC of sewage sludge and biochars were measured in 0.01M CaCl2 solution (1:10 ratio) and 1:10 biochar:deionized water ratio, respectively, based on the method of Blakemore et al. (1987). The pH and EC in the liquid phase were measured directly in the effluent by a pH meter (Metrohm) and an EC meter (Metrohm), respectively. Total organic carbon (TOC) and total nitrogen (TN) were determined with a TOC analyser (CS22 SKALAR) and the Kjeldahl method, respectively. To determine total P, K and Na, sewage sludge and biochars were first combusted at 550°C and the resulting ash was extracted with 2 M HCl. Then the total P in the solution was determined using ascorbic acid–NH4–molybdate blue colorimetry at 880 nm. Total K and Na were measured by flame photometry. Calcium carbonate equivalent (CCE) was determined by the back-titration method with NaOH (Sims, 1996). Total concentration of Fe, Zn, Cu, Mn, Ni, Cr, Pb, Co, and Cd was measured according to the US-EPA Method3050B (US Environmental Protection Agency, 1996). In order to determine the plant available form of trace elements, extraction was done with 0.005 M DTPA and the metal concentrations in the extracts were measured using atomic absorption (AA, model Perkin Elmer A200) (Lindsay and Norvell, 1978).
Physical analysis Particle density (PD) of biochar and sewage sludge samples was determined using the method of Gupta et al. (2002). In this method, the volume of oven-dried material was determined using a conventional pycnometer (density bottle) by volume displacement with kerosene. Bulk density (BD) of sewage sludge and biochars was determined using the core method (Grossman and Reinsch, 2002). The organic samples were poured into a stainless cylinder with a diameter of 5 cm and a height of 7.8 cm, and then the BD of loose materials (BDloose) was calculated. It is generally believed that the BD values of different soils under a similar loading condition are easily comparable (Havaee et al., 2014; Keller and Håkansson, 2010). We applied this concept to our experiment with organic materials: organic samples were put in the stainless cylinder, and loaded with a confined axial stress of 100 kPa using a uni-axial compression apparatus. The BD of
f = 1 − ( BD / PD ) (2)
Water-holding capacity (i.e. saturated water content) of sewage sludge and biochars was measured. Fifteen grams of dried sewage sludge and biochars were poured into stainless cylinders with a diameter of 2 cm and a height of 4 cm. Then, the cylinders were saturated with distilled water from beneath to avoid air entrapment. After 24 h when the water absorption was completed (i.e. equilibrated), the samples were weighed and the gravimetric water content at saturation (θs) was calculated. Persistence of water repellency of sewage sludge and biochars was determined using the water droplet penetration time (WDPT) test. Three droplets of distilled water were placed on the flat surface of oven-dried samples by a medical syringe. The average time for penetration of water droplets into the samples was recorded as WDPT (Letey et al., 2003). Water repellency of sewage sludge and biochars was determined according to the molarity of ethanol droplet (MED) test (King, 1981; Watson and Letey, 1970). The MED test determines the ethanol concentration at which a droplet will infiltrate the surface of the sample within 10 s, which provides an index of water repellence for sample. A series of ethanol concentration with 0.2 M increment was prepared from a 96% ethanol stock solution. Since water repellency depends on the media wetness, temperature and surface roughness, all the water repellency tests were done on oven-dried samples with flat surface at 25°C (Roy and McGill, 2002). Water repellency rating (WRR) and water repellence severity (WRS) were determined from the MED values according to Table 1, adapted from King (1981). The MED is an index of water repellency but is not a fundamental physical-chemical property of the solid–liquid interaction. Therefore the MED value was used to calculate fundamental wetting properties of media solids, including 90° surface tension (γND), surface tension of the solution for which there is transition from penetration to settling on the surface, solid–air surface tension (γs) and solid–water contact angle (β) using the following equations (Letey et al., 2003):
(
γ ND = 58.49 − ( 6.846 × MED ) + 0.512 × MED 2
(
)
)
− 0.0139 × MED + (13.88 × expp(−MED)) 3
(3)
γs =
γ ND (4) 4
cosβ = (
γ ND ) − 1 (5) γs
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Table 1. Water repellency ranking (severity and rating) according to molarity of ethanol (MED) values (adapted from King, 1981). Molarity of ethanol (MED)
Water repellency severity
Water repellency rating
0a 0a 0.0–0.3 0.3–0.7 0.7–1.1 1.1–1.5 1.5–1.9 1.9–2.3 2.3–2.7 2.7–3.1 3.1–3.5 3.5–3.9 >3.9
Never apparent Very low
1 2 3 4 5 6 7 8 9 10 11 12
aThe
Low
Moderate
Severe Very severe
categories “never apparent” and “very low” can be distinguished using the water droplet penetration time (Watson and Letey, 1970).
Figure 2. Yields of produced phases of sewage sludge biochar at different pyrolysis temperatures. The bars on the columns indicate standard deviations.
Statistical analysis All the measurements were set up in a completely randomized design (organic material type or pyrolysis temperature) with three replicates. Results were analysed using analysis of variance (ANOVA) procedures and means were separated using the protected least significant difference (LSD) test at the 0.05 probability level. Pearson correlation analysis between different physical properties and parameters (i.e. MED, γND, β, WDPT, and θs) was examined (SAS Institute, 2000).
Results and discussion Products yield Biochar yield significantly decreased with increasing pyrolysis temperature from 72.5% at 300°C to 52.9% at 700°C (Figure 2). The decrease in the biochar yield could be due to greater decomposition of raw materials at higher temperatures (Fu et al., 2011). The gas yield significantly increased from 9.7% at 300°C to 27.5% at 700°C (Figure 2). Liquid yield slightly increased with increasing pyrolysis temperature, but its change was less than
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Khanmohammadi et al. Table 2. The pH and electrical conductivity (EC) in sewage sludge and its biochars produced at different temperatures and their liquid phase. Phase
Property
Solid phase (1:10 solution) Liquid phase
pH EC (dS m–1) pH EC (dS m–1)
Sewage lsludge
Biochar produced at temperature (°C) 300
400
500
600
700
6.8f (±0.2) 2.2a (±0.05) – –
8.2e (±0.1) 0.5e (±0.05) 7.8e (±0.06) 43.2e (±0.5)
9.2d (±0.2) 0.8c (±0.04) 8.5c (±0.08) 48.0d (±1.0)
9.7c (±0.2) 0.7d (±0.03) 9.0a (±0.07) 50.5c (±0.5)
11.0b (±0.3) 0.6d (±0.04) 8.7b (±0.04) 55.5b (±0.3)
12.0a (±0.2) 1.9b (±0.05) 8.3d (±0.05) 63.6a (±0.6)
Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test.
Table 3. Chemical properties of sewage sludge and its biochars produced at different pyrolysis temperatures. Property
TN TOC C/N ratio P K Na CCE
Unit
g 100g–1 g 100g–1 – g 100g–1 g 100g–1 g 100g–1 g 100g–1
Sewage sludge
3.3a (±0.033) 40.8a (±0.6) 12.4c (±0.2) 1.1c (±0.05) 0.20d (±0.015) 0.20e (±0.007) 16.9d (±0.3)
Biochar produced at temperature (°C) 300
400
500
600
700
2.7b (±0.066) 34.0b (±0.1) 12.8c (±0.3) 1.7b (±0.12) 0.26c (±0.033) 0.24d (±0.015) 23.0c (±0.9)
2.1c (±0.038) 33.0c (±0.7) 15.4b (±0.5) 1.8b (±0.18) 0.29c (±0.035) 0.27c (±0.013) 24.5b (±0.4)
2.0d (±0.033) 32.7c (±0.1) 16.0b (±0.3) 2.20a (±0.01) 0.36b (±0.026) 0.30b (±0.002) 27.5a (±0.5)
1.7e (±0.004) 26.9d (±0.3) 15.6b (±0.2) 2.1a (±0.1) 0.41b (±0.023) 0.32b (±0.018) 24.9b (±0.3)
1.1f (±0.013) 25.1e (±0.3) 23.3a (±0.3) 2.3a (±0.2) 0.47a (±0.014) 0.35a (±0.011) 24.4b (±0.4)
TN: total nitrogen content; TOC: total organic carbon content; P: total phosphorous content; K: total potassium content; Na: total sodium content; CCE: calcium carbonate equivalent. Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test.
those for solid and gas phases. The minimum liquid yield was produced at 300°C and there was no significant change between 500 and 600°C. Wei et al. (2006) found that fast secondary reactions in the gas phase, following by over-cracking of pyrolysis vapours may result in gas yield increment at high temperatures. Besides, increase in pyrolysis temperature through secondary decomposition of char could produce non-condensable gases and consequently increased the gas yield (Fu et al., 2011).
pH and EC The sewage sludge pH was 6.8 and increased with pyrolysis (Table 2). The biochar pH significantly increased with increasing pyrolysis temperature. Yuan et al. (2011) also reported similar results. The liquid phase pH increased from 7.8 at 300°C to 9.0 at 500°C, and then decreased to 8.3 at 700°C. EC of sewage sludge was 2.2 dS m–1 and significantly decreased after pyrolysis (Table 2). The EC increased with increasing temperature and biochar produced at 300°C had the lowest EC among the pyrolysis temperatures.The EC of solid-phase suspension and liquid phase of the produced biochar increased with increasing pyrolysis temperature (Table 2). The liquid-phase EC was very high, indicating that soluble ions were mainly separated by the effluent from the solid phase (biochar). This led to a low concentration of soluble ions in the biochar solid-phase suspension compared to sewage sludge. The
liquid-phase EC increased with increasing pyrolysis temperature from 43.2 dS m–1 at 300°C to 63.6 dS m–1 at 700°C.
Chemical analysis Total N in sewage sludge significantly decreased with biochar production and rising pyrolysis temperature (Table 3).The biochar TN decreased by 59.7% with increasing temperature from 300 to 700°C. Probably emission of different nitrogen groups, such as NH4–N or NO3–N, at low temperatures and pyridine at high temperatures (>600°C) caused TN decline (Bagreev et al., 2001). The pyrolysis process decreased the TOC of sewage sludge as well (Table 3). The TOC of biochars decreased with increasing pyrolysis temperature; however, there was no significant difference between the TOC of biochars produced at 400 and 500°C. It seems organic C transferred from solid phase into liquid or gas phases during pyrolysis. There was no significant difference in the C/N ratio of sewage sludge and biochar produced at 300°C. As the pyrolysis temperature rose, C/N ratio increased and the maximum C/N ratio was found at 700°C. Yuan et al. (2011) reported C/N ratio increment during pyrolysis as a result of the N depletion. Our results showed the rate of TN decline was greater than TOC decrease rate, which resulted in a higher C/N ratio. There was no significant difference in C/N ratio of the biochars produced at 400, 500 and 600°C (Table 3). The biochars had
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Table 4. Total and diethylene triamine penta acetic acid (DTPA)-extractable concentrations of heavy metals in sewage sludge and its biochars produced at different temperatures. Heavy metal
Sewage sludge
Biochars produced at temperature (°C) 300
400
500
600
700
17,730d (±225) 1350b (±20) 362ab (±15) 380ab (±18) 62b (±3) 100b (±26) 116bc (±14) ND ND
18,890c (±265) 1430a (±45) 396a (±28) 400a (±21) 68ab (±5) 178a (±14) 142ab (±10) ND ND
19,490b (±80) 1470a (±25) 386a (±24) 364b (±12) 70a (±6) 96b (±8) 132b (±8) ND ND
20,930a (±50) 1470a (±30) 414a (±20) 392a (±10) 76a (±4) 104b (±26) 152a (±10) ND ND
53.2c (±8) 6.7c (±0.7) 1.6d (±0.8) 5.5c (±0.8) 0.3c (±0.1) 1.3b (±0.1) 6.2d (±0.4) ND ND
68.0c (±12) 8.0c (±1.0) 6.2c (±0.1) 5.7c (±0.5) 0.3c (±0.2) 1.5ab (±0.2) 10.9b (±0.8) ND ND
248.0b (±11) 13.3b (±1.2) 15.3b (±0.9) 11.5b (±0.6) 1.2b (±0.1) 1.8a (±0.2) 8.6c (±0.7) ND ND
Total concentration (mg kg–1) Fe Zn Cu Mn Ni Cr Pb Co Cd
12,210f (±225) 910d (±60) 256c (±30) 298c (±22) 60b (±7) 92b (±22) 107c (±11) ND ND
DTPA-extractable concentration (mg kg–1)
Fe Zn Cu Mn Ni Cr Pb Co Cd
288.0a (±22) 130.0a (±4.0) 20.5a (±1.5) 23.3a (±0.6) 3.6a (±0.3) 1.2b (±0.2) 13.2a (±1.0) ND ND
16,330e (±190) 1150c (±30) 352b (±22) 386a (±8) 72a (±3) 88b (±16) 107c (±9) ND ND 27.6d (±13) 2.5d (±0.5) 0.5d (±0.3) 2.0d (±0.4) ND 1.3b (±0.2) 1.4e (±0.0) ND ND
54.6c (±16) 7.0c (±1.0) 0.8d (±0.1) 6.7c (±0.7) 0.3c (±0.1) 1.3b (±0.1) 6.2d (±0.2) ND ND
Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test. ND: Non-detectable by atomic absorption spectrophotometer.
higher total P contents compared to sewage sludge (Table 3). Liu et al. (2014) and Yuan et al. (2011) reported similar results. Total P in the biochar increased with increasing pyrolysis temperature, but there were no significant differences between 500, 600 and 700°C. Greater P content of biochars produced at high temperatures is attributable to the relation between P and the sewage sludge inorganic part. In addition, sewage sludge pyrolysis concentrated the P components in the biochars (Yuan et al., 2011). The K and Na increased with increasing pyrolysis temperature, indicating K and Na in sewage sludge is mainly in the inorganic part (Hossain et al., 2011). Pyrolysis increased the carbonate content (CCE%) compared to sewage sludge. It seems that elevating the mineral part ratio may cause the CCE increment upon pyrolysis. The pyrolysis temperature slightly affected the CCE of the biochar. The CCE% decreased at temperatures above 500°C, presumably due to destruction of carbonates. Table 4 shows the total and DTPA-extractable concentrations of HMs in sewage sludge and biochars. The HMs concentration in the sewage sludge was below the upper critical limits suggested by the USEPA (US Environmental Protection Agency, 1993). Total concentrations of Fe, Zn, Cu, Mn, Ni, Cr and Pb in the biochars were higher than those in the original sewage sludge (Table 4). Probably dissociation of organic compounds and some minerals, such as carbonates, with pyrolysis temperature participated in the increased HMs concentrations. For example, HMs chlorides and sulphide-chloride compounds are easily vapourized forms but their sulphide compounds are
more resistant to vapourization (Yau and Naruse, 2009). The greatest concentration of Mn and Cr in the biochar was found at 500°C. Concentration of Cr significantly decreased with increasing temperature above 500°C. In the case of Ni and Pb, the highest concentrations were obtained at 700°C (Table 4). The concentrations of Co and Cd in the sewage sludge and all of the biochars were below the detection limit of atomic absorption spectroscopy (AAS). The DTPA-extractable concentration of HMs was regarded as their available form for plant uptake (Soltanpour and Schwab, 1977). The DTPA-extractable concentrations of Fe, Zn, Cu, Mn, Ni, and Pb decreased with pyrolysis (Table 4). The higher pH of the biochars compared to sewage sludge probably prevented the HMs release. Therefore, pyrolysis reduced the soluble forms into more insoluble forms. For example, pyrolysis could transform exchangeable compounds of Zn and Pb to oxide and sulphide forms, which are less soluble (Liu et al., 2014). However, rising pyrolysis temperature increased the DTPAextractable concentrations of Fe, Zn, Cu, Mn, Ni and Pb. The highest availability of Pb in biochars was found at 600°C. The DTPA-extractable concentration of Cr increased with biochar production and increasing pyrolysis temperature, but the increase was only significant at 700°C (Table 4). There was no significant difference in the DTPA-extractable concentrations of Fe, Zn, Mn and Ni between 400, 500 and 600°C. The DTPAextractable concentration of Cu did not differ significantly between 300, 400 and 500°C (Table 4).
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Table 5. Particle density, bulk density, porosity, saturated water content and water repellency indices of sewage sludge and its biochars produced at different pyrolysis temperatures. Property
PD BDloose ƒloose BD100kPa ƒ100kPa θs WDPT MED γND β WRR WRS
Unit
Mg m–3 Mg m–3 m3 m–3 Mg m–3 m3 m–3 g 100g–1 min M dynes cm–1 ° – –
Sewage sludge
1.53e (±0.033) 0.62a (±0.04) 0.59d (±0.013) 0.70a (±0.06) 0.54c (±0.02) 130.4a (±1.58) 26e (±1.00) 5.4a (±0.10) 51d (±0.31) 125.5a (±0.08) 12 Very severe
Biochars produced at temperature (°C) 300
400
500
600
700
1.81d (±0.056) 0.56ab (±0.02) 0.69c (±0.018) 0.62ab (±0.03) 0.66b (±0.02) 95.7e (±0.44) 201c (±07.64) 5.2b (±0.05) 51.6dc (±0.33) 125.3a (±0.09) 12 Very severe
1.96c (±0.051) 0.58ab (±0.03) 0.70c (±0.013) 0.63ab (±0.02) 0.68b (±0.01) 97.2d (±0.38) 220ab (±4.36) 2.2c (±0.05) 67.6c (±0.93) 121.1b (±0.22) 8 Moderate
1.99bc (±0.054) 0.56ab (±0.02) 0.72bc (±0.016) 0.64ab (±0.06) 0.68b (±0.01) 97.1d (±0.70) 230a (±14.00) 0.5d (±0.05) 157.8b (±17.16) 105.1c (±2.54) 4 Low
2.07ab (±0.039) 0.53b (±0.03) 0.74ab (±0.011) 0.62ab (±0.04) 0.70ab (±0.01) 103.8c (±0.62) 214bc (±7.50) 0.5d (±0.05) 157.8b (±17.16) 105.1c (±2.54) 4 Low
2.16a (±0.051) 0.52b (±0.04) 0.76a (±0.019) 0.58b (±0.04) 0.73a (±0.03) 105.4b (±0.12) 43d (±5.70) 0.2e (±0.00) 815a (±0.00) 47.0d (±0.00) 3 Low
PD: particle density; BDloose and ƒloose: bulk density and porosity at loose state, respectively; BD100kPa and ƒ100kPa: bulk density and porosity under confined axial stress 100 kPa, respectively; θs: saturated water content; WDPT: water droplet penetration time; MED: molarity of ethanol droplet; γND: 90° surface tension; β: solid–water contact angle; WRR: water repellency rating; WRS: water repellency severity. Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p
research-article2014
WMR0010.1177/0734242X14565210Waste Management & ResearchKhanmohammadi et al.
Original Article
Effect of pyrolysis temperature on chemical and physical properties of sewage sludge biochar
Waste Management & Research 1–9 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14565210 wmr.sagepub.com
Zahra Khanmohammadi, Majid Afyuni and Mohammad Reza Mosaddeghi
Abstract The objective of this study was to evaluate the effects of pyrolysis temperatures (300, 400, 500, 600 and 700°C) on properties of biochar produced from an urban sewage sludge. Biochar yield significantly decreased from 72.5% at 300°C to 52.9% at 700°C, whereas an increase in temperature increased the gas yield. Biochar pH and electrical conductivity increased by 3.8 and 1.4 dS m–1, proportionally to the increment of temperature. Biochar produced at low temperatures had higher total nitrogen and total organic carbon content but a lower C/N ratio, calcium carbonate equivalent, and total P, K and Na contents. Total and diethylene triamine penta acetic acid (DTPA)-extractable concentrations of Fe, Zn, Cu, Mn, Ni, Cr and Pb increased with increment of temperature. Lower DTPA-extractable concentrations of Fe, Zn, Cu, Mn, Ni and Pb were found in biochars compared to the sewage sludge. Pyrolysis decreased bulk density, whereas particle density and porosity increment was observed upon pyrolysis with increment of temperature. Sewage sludge saturated water content (θs) was 130.4 g 100g–1 and significantly greater than biochar, but biochar θs significantly increased with temperature (95.7 versus 105.4 g 100g–1 at 300 and 700°C, respectively). Pyrolysis decreased the biochar’s water repellency, assessed by molarity of ethanol droplet (MED), compared to the sewage sludge. The lowest MED of 0.2 and water repellency rating of 3 were found for the biochar produced at 700°C. Based on our results and considering the energy consumption, pyrolysis temperature in the range of 300–400°C may be suggested for sewage sludge pyrolysis. Keywords Pyrolysis, sewage sludge biochar, heavy metals, bulk density, water repellency, molarity of ethanol droplet
Introduction Population growth and economic developments have resulted in increasing sewage sludge production. The role of sewage sludge utilization on agricultural lands as a fertilizer is known to result in the recycling of valuable components, including organic matter, N, P, K and micronutrients (Singh and Agrawal, 2007). However, high concentrations of heavy metals (HMs), such as Zn, Cu, Cr, Ni, Cd, and high amounts of pathogens in sewage sludge led to concerns regarding soil and groundwater pollution (Gascó et al., 2005; Wang et al., 2008). In addition, high concentrations of N and P in sewage sludge have adverse environmental effects (Sumner, 2000). Sewage sludge effect on soil organic matter (SOM) and cation exchange capacity (CEC), improving plant water availability and uptake of micronutrients such as Zn and Cu in calcareous soils have been reported (Karami et al., 2008; Yeganeh et al., 2010). Pyrolysis of organic wastes, such as sewage sludge, crop residues or wood, involves the thermal transformation of biomass under partial or no oxygen supply to solid (charcoal or biochar), liquid (bio-oil) and gas phases (Laird, 2008; Lehmann, 2007). When biochar is applied to the soils, it remains in an essentially permanent form and leads to net removal of carbon from the
atmosphere (Glaser et al., 2009; Laird, 2008; Lehmann, 2007). In fact, the polycyclic aromatic structure of biochar is the reason for its stability and carbon sequestration in the soil (Verheijen et al., 2010). In addition, incorporation of biochar can improve soil physical, chemical and biological properties (Lehmann and Joseph, 2009). Biochar application to the soil increases the soil aeration (Laird, 2008), porosity and specific surface area (Laird et al., 2010; Steiner et al., 2007), water-holding capacity and drainage condition (Ibrahim et al., 2013; Steiner et al., 2007), soil fertility and following plant yield responses (Major et al., 2010; Park et al., 2011). Pyrolysis conditions and feedstock characteristics largely control the physical and chemical properties (e.g. composition, particle and pore size distribution) of the biochar, which in turn Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Corresponding author: Zahra Khanmohammadi, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran. Emails: [email protected]
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Figure 1. Schematic diagram of pyrolysis apparatus used in this study.
determine its suitability for a given application (Chan and Xu, 2009). Fu et al. (2011) reported that increasing pyrolysis temperature decreases biochar and liquid yield but raises gas yield. Yuan et al. (2011) studied the forms of alkalis of the biochars produced from the straws of canola, corn, soybean and peanut at different pyrolysis temperatures. They reported that the alkalinity and pH of biochars increased with raising pyrolysis temperature. Hossain et al. (2011) found that biochar yield of wastewater sludge decreased from 72.3% at 300°C to 52.4% at 700°C. Besides, pH of the produced biochars increased with increasing pyrolysis temperature from 300 to 700°C but the electrical conductivity (EC) of the biochars had a decreasing trend with temperature. Liu et al. (2014) investigated the nutrients and HMs in biochar produced at 450°C from sewage sludge. They found that total and available N contents of biochar were lower, whereas total and available P and K contents were higher when compared to sewage sludge. Total concentrations of HMs were greater in the biochar than in sewage sludge, but their diethylene triamine penta acetic acid (DTPA)-extractable concentrations were lower in the biochar. In a recent study, Claoston et al. (2014) reported that porosity and surface areas (determined by the Brunauer–Emmett–Teller method) of empty fruit bunch and rice husk biochars increased with pyrolysis temperature increment. Therefore, sewage sludge pyrolysis can be an acceptable method for its management (Hwang et al., 2007). Pyrolysis removes sewage sludge pathogens, reduces its volume and transport costs (Caballero et al., 1997), and has positive effects on soil properties and agricultural outputs. Various studies have been conducted on the pyrolysis of biomass as a potential carbon sequestrator (Glaser et al., 2009; Laird, 2008; Verheijen et al., 2010). Although some chemical and physical properties of sewage sludge biochar, including nutrient, HMs, specific surface area, pore volume and its morphology, have been studied, less attention has been given to the effect of pyrolysis temperatures
on the water repellency and water-holding capacity of sewage sludge biochar. The objective of this study was to investigate the effects of pyrolysis temperature on chemical and physical properties of urban sewage sludge’s biochar with emphasis on physical properties, such as water repellency and water-holding capacity.
Materials and methods Pyrolysis experimental setup and procedure A secondary anaerobically digested sewage sludge was collected from Isfahan wastewater treatment plant (32° 48’ 21” N 51° 35’ 21” E). The sewage sludge was oven-dried at 40°C for 24 h. The sewage sludge was then grinded and sieved to obtain particle sizes less than 2 mm, then stored in air-tight plastic bags until it was pyrolized. The sample was pyrolized under controlled conditions to ensure uniform heating. A modified electrical furnace was used to produce the limiting-oxygen condition of pyrolysis (Figure 1). A cubic iron encasement with dimensions of 16 cm in diameter and 45 cm in length with three shelves was designed and put in the electrical furnace. The door of the enclosure had an inlet for inert gas and an outlet to pass the produced liquid phase and gases. Sewage sludge samples were placed on the encasement shelves, and then the encasement was put in the chamber of the electrical furnace (Figure 1). Sewage sludge pyrolysis was done at 300, 400, 500, 600 and 700°C at a rate of 3°C min–1 and using a continuous inflow of argon (inert gas) in three replicates. Therefore, 15 samples were pyrolized (5 temperatures × 3 replicates). Approximately 1.4 kg sewage sludge was used for pyrolysis for each temperature and replicate. The argon inflow pushed the air out of the encasement to maintain the limiting-oxygen condition during pyrolysis. The final temperature was maintained for 2 h. Then the furnace was
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Khanmohammadi et al. cooled down to 100°C slowly. As shown in Figure 1, the liquid phase (effluent) flowing through the outlet tube was collected in a container. The percentage of biochar yield at various temperatures was calculated using Equation (1): mass of Biochar yield = biochar
loaded samples (BD100kPa) was also determined, which is better comparable among the treatments due to the well-defined compaction condition. The porosity (ƒ) of samples was calculated at loose (ƒloose) and 100 kPa (ƒ100kPa) conditions as follows:
−1
mass of original ×100 (1) × sewage sludge
Liquid phase yield was calculated similar to the biochar yield. The gas yield percentage was obtained with subtraction of biochar and liquid yields from 100.
Chemical analysis The pH and EC of sewage sludge and biochars were measured in 0.01M CaCl2 solution (1:10 ratio) and 1:10 biochar:deionized water ratio, respectively, based on the method of Blakemore et al. (1987). The pH and EC in the liquid phase were measured directly in the effluent by a pH meter (Metrohm) and an EC meter (Metrohm), respectively. Total organic carbon (TOC) and total nitrogen (TN) were determined with a TOC analyser (CS22 SKALAR) and the Kjeldahl method, respectively. To determine total P, K and Na, sewage sludge and biochars were first combusted at 550°C and the resulting ash was extracted with 2 M HCl. Then the total P in the solution was determined using ascorbic acid–NH4–molybdate blue colorimetry at 880 nm. Total K and Na were measured by flame photometry. Calcium carbonate equivalent (CCE) was determined by the back-titration method with NaOH (Sims, 1996). Total concentration of Fe, Zn, Cu, Mn, Ni, Cr, Pb, Co, and Cd was measured according to the US-EPA Method3050B (US Environmental Protection Agency, 1996). In order to determine the plant available form of trace elements, extraction was done with 0.005 M DTPA and the metal concentrations in the extracts were measured using atomic absorption (AA, model Perkin Elmer A200) (Lindsay and Norvell, 1978).
Physical analysis Particle density (PD) of biochar and sewage sludge samples was determined using the method of Gupta et al. (2002). In this method, the volume of oven-dried material was determined using a conventional pycnometer (density bottle) by volume displacement with kerosene. Bulk density (BD) of sewage sludge and biochars was determined using the core method (Grossman and Reinsch, 2002). The organic samples were poured into a stainless cylinder with a diameter of 5 cm and a height of 7.8 cm, and then the BD of loose materials (BDloose) was calculated. It is generally believed that the BD values of different soils under a similar loading condition are easily comparable (Havaee et al., 2014; Keller and Håkansson, 2010). We applied this concept to our experiment with organic materials: organic samples were put in the stainless cylinder, and loaded with a confined axial stress of 100 kPa using a uni-axial compression apparatus. The BD of
f = 1 − ( BD / PD ) (2)
Water-holding capacity (i.e. saturated water content) of sewage sludge and biochars was measured. Fifteen grams of dried sewage sludge and biochars were poured into stainless cylinders with a diameter of 2 cm and a height of 4 cm. Then, the cylinders were saturated with distilled water from beneath to avoid air entrapment. After 24 h when the water absorption was completed (i.e. equilibrated), the samples were weighed and the gravimetric water content at saturation (θs) was calculated. Persistence of water repellency of sewage sludge and biochars was determined using the water droplet penetration time (WDPT) test. Three droplets of distilled water were placed on the flat surface of oven-dried samples by a medical syringe. The average time for penetration of water droplets into the samples was recorded as WDPT (Letey et al., 2003). Water repellency of sewage sludge and biochars was determined according to the molarity of ethanol droplet (MED) test (King, 1981; Watson and Letey, 1970). The MED test determines the ethanol concentration at which a droplet will infiltrate the surface of the sample within 10 s, which provides an index of water repellence for sample. A series of ethanol concentration with 0.2 M increment was prepared from a 96% ethanol stock solution. Since water repellency depends on the media wetness, temperature and surface roughness, all the water repellency tests were done on oven-dried samples with flat surface at 25°C (Roy and McGill, 2002). Water repellency rating (WRR) and water repellence severity (WRS) were determined from the MED values according to Table 1, adapted from King (1981). The MED is an index of water repellency but is not a fundamental physical-chemical property of the solid–liquid interaction. Therefore the MED value was used to calculate fundamental wetting properties of media solids, including 90° surface tension (γND), surface tension of the solution for which there is transition from penetration to settling on the surface, solid–air surface tension (γs) and solid–water contact angle (β) using the following equations (Letey et al., 2003):
(
γ ND = 58.49 − ( 6.846 × MED ) + 0.512 × MED 2
(
)
)
− 0.0139 × MED + (13.88 × expp(−MED)) 3
(3)
γs =
γ ND (4) 4
cosβ = (
γ ND ) − 1 (5) γs
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Table 1. Water repellency ranking (severity and rating) according to molarity of ethanol (MED) values (adapted from King, 1981). Molarity of ethanol (MED)
Water repellency severity
Water repellency rating
0a 0a 0.0–0.3 0.3–0.7 0.7–1.1 1.1–1.5 1.5–1.9 1.9–2.3 2.3–2.7 2.7–3.1 3.1–3.5 3.5–3.9 >3.9
Never apparent Very low
1 2 3 4 5 6 7 8 9 10 11 12
aThe
Low
Moderate
Severe Very severe
categories “never apparent” and “very low” can be distinguished using the water droplet penetration time (Watson and Letey, 1970).
Figure 2. Yields of produced phases of sewage sludge biochar at different pyrolysis temperatures. The bars on the columns indicate standard deviations.
Statistical analysis All the measurements were set up in a completely randomized design (organic material type or pyrolysis temperature) with three replicates. Results were analysed using analysis of variance (ANOVA) procedures and means were separated using the protected least significant difference (LSD) test at the 0.05 probability level. Pearson correlation analysis between different physical properties and parameters (i.e. MED, γND, β, WDPT, and θs) was examined (SAS Institute, 2000).
Results and discussion Products yield Biochar yield significantly decreased with increasing pyrolysis temperature from 72.5% at 300°C to 52.9% at 700°C (Figure 2). The decrease in the biochar yield could be due to greater decomposition of raw materials at higher temperatures (Fu et al., 2011). The gas yield significantly increased from 9.7% at 300°C to 27.5% at 700°C (Figure 2). Liquid yield slightly increased with increasing pyrolysis temperature, but its change was less than
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Khanmohammadi et al. Table 2. The pH and electrical conductivity (EC) in sewage sludge and its biochars produced at different temperatures and their liquid phase. Phase
Property
Solid phase (1:10 solution) Liquid phase
pH EC (dS m–1) pH EC (dS m–1)
Sewage lsludge
Biochar produced at temperature (°C) 300
400
500
600
700
6.8f (±0.2) 2.2a (±0.05) – –
8.2e (±0.1) 0.5e (±0.05) 7.8e (±0.06) 43.2e (±0.5)
9.2d (±0.2) 0.8c (±0.04) 8.5c (±0.08) 48.0d (±1.0)
9.7c (±0.2) 0.7d (±0.03) 9.0a (±0.07) 50.5c (±0.5)
11.0b (±0.3) 0.6d (±0.04) 8.7b (±0.04) 55.5b (±0.3)
12.0a (±0.2) 1.9b (±0.05) 8.3d (±0.05) 63.6a (±0.6)
Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test.
Table 3. Chemical properties of sewage sludge and its biochars produced at different pyrolysis temperatures. Property
TN TOC C/N ratio P K Na CCE
Unit
g 100g–1 g 100g–1 – g 100g–1 g 100g–1 g 100g–1 g 100g–1
Sewage sludge
3.3a (±0.033) 40.8a (±0.6) 12.4c (±0.2) 1.1c (±0.05) 0.20d (±0.015) 0.20e (±0.007) 16.9d (±0.3)
Biochar produced at temperature (°C) 300
400
500
600
700
2.7b (±0.066) 34.0b (±0.1) 12.8c (±0.3) 1.7b (±0.12) 0.26c (±0.033) 0.24d (±0.015) 23.0c (±0.9)
2.1c (±0.038) 33.0c (±0.7) 15.4b (±0.5) 1.8b (±0.18) 0.29c (±0.035) 0.27c (±0.013) 24.5b (±0.4)
2.0d (±0.033) 32.7c (±0.1) 16.0b (±0.3) 2.20a (±0.01) 0.36b (±0.026) 0.30b (±0.002) 27.5a (±0.5)
1.7e (±0.004) 26.9d (±0.3) 15.6b (±0.2) 2.1a (±0.1) 0.41b (±0.023) 0.32b (±0.018) 24.9b (±0.3)
1.1f (±0.013) 25.1e (±0.3) 23.3a (±0.3) 2.3a (±0.2) 0.47a (±0.014) 0.35a (±0.011) 24.4b (±0.4)
TN: total nitrogen content; TOC: total organic carbon content; P: total phosphorous content; K: total potassium content; Na: total sodium content; CCE: calcium carbonate equivalent. Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test.
those for solid and gas phases. The minimum liquid yield was produced at 300°C and there was no significant change between 500 and 600°C. Wei et al. (2006) found that fast secondary reactions in the gas phase, following by over-cracking of pyrolysis vapours may result in gas yield increment at high temperatures. Besides, increase in pyrolysis temperature through secondary decomposition of char could produce non-condensable gases and consequently increased the gas yield (Fu et al., 2011).
pH and EC The sewage sludge pH was 6.8 and increased with pyrolysis (Table 2). The biochar pH significantly increased with increasing pyrolysis temperature. Yuan et al. (2011) also reported similar results. The liquid phase pH increased from 7.8 at 300°C to 9.0 at 500°C, and then decreased to 8.3 at 700°C. EC of sewage sludge was 2.2 dS m–1 and significantly decreased after pyrolysis (Table 2). The EC increased with increasing temperature and biochar produced at 300°C had the lowest EC among the pyrolysis temperatures.The EC of solid-phase suspension and liquid phase of the produced biochar increased with increasing pyrolysis temperature (Table 2). The liquid-phase EC was very high, indicating that soluble ions were mainly separated by the effluent from the solid phase (biochar). This led to a low concentration of soluble ions in the biochar solid-phase suspension compared to sewage sludge. The
liquid-phase EC increased with increasing pyrolysis temperature from 43.2 dS m–1 at 300°C to 63.6 dS m–1 at 700°C.
Chemical analysis Total N in sewage sludge significantly decreased with biochar production and rising pyrolysis temperature (Table 3).The biochar TN decreased by 59.7% with increasing temperature from 300 to 700°C. Probably emission of different nitrogen groups, such as NH4–N or NO3–N, at low temperatures and pyridine at high temperatures (>600°C) caused TN decline (Bagreev et al., 2001). The pyrolysis process decreased the TOC of sewage sludge as well (Table 3). The TOC of biochars decreased with increasing pyrolysis temperature; however, there was no significant difference between the TOC of biochars produced at 400 and 500°C. It seems organic C transferred from solid phase into liquid or gas phases during pyrolysis. There was no significant difference in the C/N ratio of sewage sludge and biochar produced at 300°C. As the pyrolysis temperature rose, C/N ratio increased and the maximum C/N ratio was found at 700°C. Yuan et al. (2011) reported C/N ratio increment during pyrolysis as a result of the N depletion. Our results showed the rate of TN decline was greater than TOC decrease rate, which resulted in a higher C/N ratio. There was no significant difference in C/N ratio of the biochars produced at 400, 500 and 600°C (Table 3). The biochars had
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Table 4. Total and diethylene triamine penta acetic acid (DTPA)-extractable concentrations of heavy metals in sewage sludge and its biochars produced at different temperatures. Heavy metal
Sewage sludge
Biochars produced at temperature (°C) 300
400
500
600
700
17,730d (±225) 1350b (±20) 362ab (±15) 380ab (±18) 62b (±3) 100b (±26) 116bc (±14) ND ND
18,890c (±265) 1430a (±45) 396a (±28) 400a (±21) 68ab (±5) 178a (±14) 142ab (±10) ND ND
19,490b (±80) 1470a (±25) 386a (±24) 364b (±12) 70a (±6) 96b (±8) 132b (±8) ND ND
20,930a (±50) 1470a (±30) 414a (±20) 392a (±10) 76a (±4) 104b (±26) 152a (±10) ND ND
53.2c (±8) 6.7c (±0.7) 1.6d (±0.8) 5.5c (±0.8) 0.3c (±0.1) 1.3b (±0.1) 6.2d (±0.4) ND ND
68.0c (±12) 8.0c (±1.0) 6.2c (±0.1) 5.7c (±0.5) 0.3c (±0.2) 1.5ab (±0.2) 10.9b (±0.8) ND ND
248.0b (±11) 13.3b (±1.2) 15.3b (±0.9) 11.5b (±0.6) 1.2b (±0.1) 1.8a (±0.2) 8.6c (±0.7) ND ND
Total concentration (mg kg–1) Fe Zn Cu Mn Ni Cr Pb Co Cd
12,210f (±225) 910d (±60) 256c (±30) 298c (±22) 60b (±7) 92b (±22) 107c (±11) ND ND
DTPA-extractable concentration (mg kg–1)
Fe Zn Cu Mn Ni Cr Pb Co Cd
288.0a (±22) 130.0a (±4.0) 20.5a (±1.5) 23.3a (±0.6) 3.6a (±0.3) 1.2b (±0.2) 13.2a (±1.0) ND ND
16,330e (±190) 1150c (±30) 352b (±22) 386a (±8) 72a (±3) 88b (±16) 107c (±9) ND ND 27.6d (±13) 2.5d (±0.5) 0.5d (±0.3) 2.0d (±0.4) ND 1.3b (±0.2) 1.4e (±0.0) ND ND
54.6c (±16) 7.0c (±1.0) 0.8d (±0.1) 6.7c (±0.7) 0.3c (±0.1) 1.3b (±0.1) 6.2d (±0.2) ND ND
Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p < 0.05 according to the least significant difference test. ND: Non-detectable by atomic absorption spectrophotometer.
higher total P contents compared to sewage sludge (Table 3). Liu et al. (2014) and Yuan et al. (2011) reported similar results. Total P in the biochar increased with increasing pyrolysis temperature, but there were no significant differences between 500, 600 and 700°C. Greater P content of biochars produced at high temperatures is attributable to the relation between P and the sewage sludge inorganic part. In addition, sewage sludge pyrolysis concentrated the P components in the biochars (Yuan et al., 2011). The K and Na increased with increasing pyrolysis temperature, indicating K and Na in sewage sludge is mainly in the inorganic part (Hossain et al., 2011). Pyrolysis increased the carbonate content (CCE%) compared to sewage sludge. It seems that elevating the mineral part ratio may cause the CCE increment upon pyrolysis. The pyrolysis temperature slightly affected the CCE of the biochar. The CCE% decreased at temperatures above 500°C, presumably due to destruction of carbonates. Table 4 shows the total and DTPA-extractable concentrations of HMs in sewage sludge and biochars. The HMs concentration in the sewage sludge was below the upper critical limits suggested by the USEPA (US Environmental Protection Agency, 1993). Total concentrations of Fe, Zn, Cu, Mn, Ni, Cr and Pb in the biochars were higher than those in the original sewage sludge (Table 4). Probably dissociation of organic compounds and some minerals, such as carbonates, with pyrolysis temperature participated in the increased HMs concentrations. For example, HMs chlorides and sulphide-chloride compounds are easily vapourized forms but their sulphide compounds are
more resistant to vapourization (Yau and Naruse, 2009). The greatest concentration of Mn and Cr in the biochar was found at 500°C. Concentration of Cr significantly decreased with increasing temperature above 500°C. In the case of Ni and Pb, the highest concentrations were obtained at 700°C (Table 4). The concentrations of Co and Cd in the sewage sludge and all of the biochars were below the detection limit of atomic absorption spectroscopy (AAS). The DTPA-extractable concentration of HMs was regarded as their available form for plant uptake (Soltanpour and Schwab, 1977). The DTPA-extractable concentrations of Fe, Zn, Cu, Mn, Ni, and Pb decreased with pyrolysis (Table 4). The higher pH of the biochars compared to sewage sludge probably prevented the HMs release. Therefore, pyrolysis reduced the soluble forms into more insoluble forms. For example, pyrolysis could transform exchangeable compounds of Zn and Pb to oxide and sulphide forms, which are less soluble (Liu et al., 2014). However, rising pyrolysis temperature increased the DTPAextractable concentrations of Fe, Zn, Cu, Mn, Ni and Pb. The highest availability of Pb in biochars was found at 600°C. The DTPA-extractable concentration of Cr increased with biochar production and increasing pyrolysis temperature, but the increase was only significant at 700°C (Table 4). There was no significant difference in the DTPA-extractable concentrations of Fe, Zn, Mn and Ni between 400, 500 and 600°C. The DTPAextractable concentration of Cu did not differ significantly between 300, 400 and 500°C (Table 4).
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Table 5. Particle density, bulk density, porosity, saturated water content and water repellency indices of sewage sludge and its biochars produced at different pyrolysis temperatures. Property
PD BDloose ƒloose BD100kPa ƒ100kPa θs WDPT MED γND β WRR WRS
Unit
Mg m–3 Mg m–3 m3 m–3 Mg m–3 m3 m–3 g 100g–1 min M dynes cm–1 ° – –
Sewage sludge
1.53e (±0.033) 0.62a (±0.04) 0.59d (±0.013) 0.70a (±0.06) 0.54c (±0.02) 130.4a (±1.58) 26e (±1.00) 5.4a (±0.10) 51d (±0.31) 125.5a (±0.08) 12 Very severe
Biochars produced at temperature (°C) 300
400
500
600
700
1.81d (±0.056) 0.56ab (±0.02) 0.69c (±0.018) 0.62ab (±0.03) 0.66b (±0.02) 95.7e (±0.44) 201c (±07.64) 5.2b (±0.05) 51.6dc (±0.33) 125.3a (±0.09) 12 Very severe
1.96c (±0.051) 0.58ab (±0.03) 0.70c (±0.013) 0.63ab (±0.02) 0.68b (±0.01) 97.2d (±0.38) 220ab (±4.36) 2.2c (±0.05) 67.6c (±0.93) 121.1b (±0.22) 8 Moderate
1.99bc (±0.054) 0.56ab (±0.02) 0.72bc (±0.016) 0.64ab (±0.06) 0.68b (±0.01) 97.1d (±0.70) 230a (±14.00) 0.5d (±0.05) 157.8b (±17.16) 105.1c (±2.54) 4 Low
2.07ab (±0.039) 0.53b (±0.03) 0.74ab (±0.011) 0.62ab (±0.04) 0.70ab (±0.01) 103.8c (±0.62) 214bc (±7.50) 0.5d (±0.05) 157.8b (±17.16) 105.1c (±2.54) 4 Low
2.16a (±0.051) 0.52b (±0.04) 0.76a (±0.019) 0.58b (±0.04) 0.73a (±0.03) 105.4b (±0.12) 43d (±5.70) 0.2e (±0.00) 815a (±0.00) 47.0d (±0.00) 3 Low
PD: particle density; BDloose and ƒloose: bulk density and porosity at loose state, respectively; BD100kPa and ƒ100kPa: bulk density and porosity under confined axial stress 100 kPa, respectively; θs: saturated water content; WDPT: water droplet penetration time; MED: molarity of ethanol droplet; γND: 90° surface tension; β: solid–water contact angle; WRR: water repellency rating; WRS: water repellency severity. Note: Each value represents the mean of three replicates; figures in the parentheses are standard deviations; in each row, different letters stand for significant difference at p
