Article pubs.acs.org/JAFC
Mapping Thermomechanical Pulp Sludge (TMPS) Biochar Characteristics for Greenhouse Produce Safety Ataullah Khan,*,† Mohyuddin Mirza,§ Brian Fahlman,‡ Ryan Rybchuk,‡ Jian Yang,† Don Harfield,† and Anthony O. Anyia† †
Alberta Innovates Technology Futures − Bio-Resource Technologies, Highway 16A and 75 Street, Vegreville, Alberta, Canada T9C 1T4 § Alberta Greenhouse Growers Association, 10331 178 Street, Edmonton, Alberta, Canada T5S 1R5 ‡ Alberta Innovates Technology Futures − Environmental Analytical Services, Highway 16A and 75 Street, Vegreville, Alberta, Canada T9C 1T4 ABSTRACT: This study evaluates the existence of toxic compounds in thermomechanical pulp sludge (TMPS) derived biochars obtained through a slow pyrolysis process and establishes the criteria for manufacturing benign-quality biochar for safe greenhouse-based food production. Accordingly, nine TMPS biochars generated at different temperatures (450, 500, 550 °C) and residence times (30, 60, 120 min) were investigated. Depending on the production conditions, the polycyclic aromatic hydrocarbons (PAHs) sum varied from 0.4 to 236 μg/g biochar. Interestingly, correlations between the PAH content, toxicity, and process conditions were derived in the form of process toxicity relationships (PTRs). On the basis of the learning garnered in this study, it is recommended that TMPS feedstock will yield benign quality biochar when processed at a minimum 500 °C temperature for an optimum residence time of 30 min. KEYWORDS: biochar, pulp sludge, PAHs, bio-assay, toxicity, greenhouse food safety
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INTRODUCTION AND BACKGROUND Biochar is a carbonaceous solid product derived from biomass via a pyrolysis process carried out in oxygen-deficient ambience. As per the International Biochar Initiative (IBI) definition, biochar differs from charcoal in the fact that it is primarily produced for soil applications and can potentially improve soil fertility, water/ nutrient-holding capacity, and carbon content and lead to improved productivity. The role of biochar as a tool of permanent carbon capture and storage has attracted the utmost attention in the recent past.1 Apart from the above, biochar also finds use in the areas of hydroponics, soil amendments, reclamation/remediation, oil sand tailing treatment, lake water de-eutrophication, etc.2−5 Biochar can be potentially manufactured from a wide variety of unprocessed feedstocks such as agriculture residue (straw), forestry residue (saw dust, pulp sludge, wood), and livestock residue (chicken litter, manure) and from processed feedstocks such as municipal residue (treated wood, construction/demolition waste). Furthermore, a wide range of process conditions (temperature, time, rate, environment) are employed during biochar production. The variation in the feedstock and process conditions leads to a wide variation in the chemical composition of the end product, which is broadly termed “biochar”.6 The availability of multiple feedstocks with different quality grades and the wide variation in the process conditions employed in biochar production create a quality control challenge. In view of this IBI has formulated biochar product standardization and testing guidelines for soil application.7 Accordingly, IBI identifies two mandatory categories of tests, namely, basic and toxicant testing. The basic test classifies biochar into three classes on the basis of its organic carbon (Corg) content and limits the volatile matter content in © 2015 American Chemical Society
biochar with the help of the H:C atomic ratio, which is required to be ≤0.7.7 The toxicant tests evaluate the existence of toxic compounds [polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs)] and put a maximum allowed threshold limit on them. IBI biochar standards relate to the physicochemical characteristics of biochar product only and do not prescribe production methods or specific feedstocks.7 Thus, there is a need for correlating the biochar production parameters to the resultant biochar characteristics to demarcate the slow pyrolysis process boundaries for obtaining benign quality biochars. Biochar could potentially contain two types of contaminants:8,9 (a) those present in the feedstock itself (e.g., heavy metals, chlorine, sulfur) and (b) those produced during pyrolysis (e.g., PAHs, PCDDs, PCDFs, PCBs). In most cases heavy metal, chlorine, and sulfur contaminants are introduced into the biochar from the feedstock itself. Careful selection of feedstock is necessary to avoid or minimize the first category of contaminants, although some could be separated and removed during biochar production. The second category of contaminants is strictly feedstock-independent and likely formed during thermochemical processing (pyrolysis). The formation of PCDDs, PCDFs, and PCBs has been linked to the presence of high amounts of chlorine in the feedstock. The formation of PAHs can be minimized by appropriate selection of operating Received: Revised: Accepted: Published: 1648
October 9, 2014 January 12, 2015 January 21, 2015 January 21, 2015 DOI: 10.1021/jf502556t J. Agric. Food Chem. 2015, 63, 1648−1657
Article
Journal of Agricultural and Food Chemistry
Figure 1. Slow pyrolysis experimental setup. Feedstock Preprocessing. TMPS was manually delumped to improve heat transfer efficiency during drying and carbonization. It was dried at 110 °C overnight to obtain bone-dry homogeneous material, which was subsequently characterized and used in biochar production. Feedstock Characterization. Proximate (volatile, ash, and fixed carbon), ultimate (CHNSO), and trace metal (36 element scan) analyses of TMPS were performed using the standard American Standard for Testing Materials (ASTM) and U.S. Environmental Protection Agency (EPA) methods.11−20 All of the results and discussion in this study are presented on a dry weight basis (db). Biochar Production. The laboratory-scale experimental setup employed to generate slow pyrolysis biochars from the TMPS feedstock is depicted in Figure 1. It comprises three components: (1) a tube furnace;,(2) a 4 in. stainless steel reactor, and (3) a three-stage programmable temperature controller. To maintain uniform process conditions all through the TMPS feedstock, about 800 g of bone-dry TMPS was packed between two perforated metallic plates in the center of a tube reactor, which created a 10 in. void space on both/either ends of the reactor. The sealed reactor was inspected for leaks and placed in a split hinge tubular furnace. The tube furnace is equipped with three programmable temperature zones (left, center, right), which are regulated with the help of three thermocouples (TCs). In addition, a calibrated TC was placed directly into the center of the sludge sample, which was employed to measure the actual material temperature and was used to determine the process temperature and residence time in all of the experiments. In a typical run, the furnace was heated to a desired (material) temperature and maintained at that desired temperature for a desired time (residence time). Nine different kinds of biochars were made from TMPS by treating it at three different material temperatures (450, 500, 550 °C) and for three different residence times (30, 60, 120 min) under anoxic conditions (CO2 purge). At the end of each experiment the tubular reactor was removed from the furnace and quenched rapidly (20 min) to cool the biochar to room temperature. Subsequently, the biochar was collected and weighed to calculate its yields. Biochar Characterization. A composite aliquot of each biochar generated in this study was first ground and sieved to 0.25 mm size and then dried in the oven at 105 °C overnight. The dried samples were characterized by various techniques using standard ASTM and EPA methods,11−23 with the exception of bioassays,21 wherein as-produced biochars were employed. PAHs in biochar were extracted by Soxhlet extraction following EPA method 3540C. Extracts were further cleaned up using silica gel column according to EPA method 3630C prior to analysis by GC-MS operated in SIM as described in EPA methods 625 and 8270.22 Soxhlet extraction employed 1 g of biochar and 10 g of sodium sulfate with hexane/DCM overnight (18 h). The extracts were concentrated to 1 mL and analyzed GC-MS (SIM) to determine PAH concentrations. Analyte recovery was determined by the addition of surrogate standards [2-fluorobiphenyl, 4-terphenyl, benzo(g,h,i)-
conditions, specifically temperature range and residence time, to obtain biochars with negligible PAH content. The impetus behind this work is the prospect of the existence and bioavailability of PAHs in the biochar, which are pyrolytic compounds produced during pyrolysis. This valid concern is hindering biochar’s market acceptance with greenhouse growers and cooperators. This study will provide the applied knowledge required to ensure confidence (of cooperators/greenhouse growers) in biochar as a consistent, high-quality, welldocumented, locally made greenhouse growing medium (substrate). This aspect has not been sufficiently studied or documented before, and the existing knowledge gap in the information is vital for commercial adoption of biochar as a benign growing medium for the greenhouse industry. The results of the proposed study will help fill the gap of assuring consistency in the quality and safety of biochar use for food production in greenhouses. The overall objective of this project is to make recommendations to the biochar producers and biochar end users (greenhouse growers) on what criteria biochars derived from thermomechanical pulp sludge (TMPS) can be considered safe for use in greenhouses for food production. For this study the TMPS feedstock was sourced from Alberta Newsprint Co. (ANC), which was considering installing a 5000 tonne/year commercial biochar production facility at White Court, Alberta, Canada. ANC employs a thermomechanical pulping (TMP) process, wherein the wood chips are heated using steam to mechanically separate fibers (pulp) in a refiner. Wastewater generated from the pulping process is directed to a wastewater treatment system, where the remnant solids, termed sludge (TMPS), are separated. TMPS is essentially a combination of primary and secondary sludges. The primary sludge consists mainly of cellulose, hemicellulose, and lignin, whereas the secondary sludge is mainly composed of proteins, cellulose fibers, and lignin. Approximately 12000 oven-dried (OD) tonnes of sludge is produced by ANC annually. The main recycling and disposal routes for sludge are land-spreading or incineration. The conversion of TMPS, a negative-value effluent, into biochar, a high-value greenhouse growing medium, would be economically favorable to ANC.
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MATERIALS AND METHODS
Feedstock. One hundred liters of TMPS was supplied by ANC, White Court, AB, Canada. 1649
DOI: 10.1021/jf502556t J. Agric. Food Chem. 2015, 63, 1648−1657
Article
Journal of Agricultural and Food Chemistry perylene] to the biochar sample prior to the Soxhlet extraction; typically recovery varies from 60 to 120%. Due to variability of the sample matrix the MDL was set at 1 ng/g to provide a high level of confidence in the data. The biochar exhibiting highest PAH content was submitted to PCDDs/PCDFs analyses, which was performed in accordance with EPA methods 1613 and 1668.23 Bioassay of Biochar Phytotoxicity and Effect on Plant Performance. A 5 day germination bioassay was performed to compare phytotoxicity differences on radish seeds between the various carbonized TMPS biochars, uncarbonized (raw) coconut coir (greenhouse industry standard growing medium), Sunshine (SS) commercial potting mix, and quartz sand by adapting the standard Organisation for Economic Co-operation and Development (OECD) methodology (1984).21 Radish seeds (11) were plated on the substrate in a Petri plate, and each sample was tested in three replicated plates that were incubated in a humidity chamber at room temperature with 12 h of light for 5 days. Germination rate and root/shoot length were recorded for each treatment. To assess the biochar’s phytotoxicity and its effects on plant performance and human health, a growth chamber trial was conducted on a rapid-growing lettuce vegetable crop from July 31 to August 30, 2013. Six different biochar samples, coconut coir, and SS mix (controls) with six replicates each were employed in a 30 day growth chamber trial. Lettuce seeds (Burgia variety) were sown in 1 in. rockwool cubes and incubated in a growth chamber programed at 20/16 °C, day/night, and 16 h of light for 2 weeks. Seedlings were transplanted to biochar or coconut coir or SS mix in 4 in. plastic pots, one seedling per pot and six pots per treatment, watered with nutrient solution daily, and incubated under the same conditions for 30 days. The pH and EC of leachates were monitored and measured once a week. The foliage biomass yield was recorded, and lettuce tissue samples were subjected to Soxhlet extraction, cleanup, and PAH analysis as per EPA methods 3540C, 3630C, 8270, and 625, respectively (GC-MS analyses).23
Table 1. TMPS Feedstock Basic Analyses feedstock
a
proximate analysis moisture content wt % (wb) volatile matter (VM) content wt % (db) ash content wt % (db) fixed carbon (FC) content wt % (db) (by difference) ultimate analysis C wt % (db) H wt % (db) N wt % (db) S wt % (db) O wt % (db) (by difference)
TMPS
typical clean woodb
64.5 82.5
18.2 85.4
3.0 14.5
0.4 14.2
52.2 6.2 1.6 0.3 36.7
49.6 6.4 0.15 0.1 43.75
hemicellulose24
39.05 5.73 0.003 55.22
a
db, dry basis; wb, wet basis. FC was computed by the following equation: FC (%) = 100 − [VM (%) + ash (%)]. Oxygen was computed by the following equation: O (%) = 100 − [N (%) + C (%) + H (%) + S (%) + ash (%)]. bFor comparison, proximate/ultimate analyses of typical wood feedstock were referenced from The Handbook of Biomass Combustion & Co-firing, edited by Sjaak van Loo and Jaap Koppejan in 2008.
clean biomass commonly employed in combustion and co-firing applications.25 On the basis of the above finding TMPS can be designated a clean feedstock suitable for multiple applications ranging from land spreading to thermochemical processing (torrefaction, pyrolysis, combustion) applications. A recent study by Parikh’s group26 investigated trends in the biochar properties in relation to its feedstock material to develop guidelines for biochar use in agronomic systems. Special emphasis was given to ash content, C/N ratio, and surface area measurements. The overall exercise was intended to guide end users with feedstock selection tool for specific soil functions.26 Biochar Production. Nine different biochars were generated by carbonizing the TMPS at three different material temperatures for three different residence (retention) times. The observed biochar yields are shown in Figure 2. As noted in Figure 2, biochar yields decrease with increasing carbonization (material) temperature and residence (material retention) time on account of a higher degree of devolatilization. The results reveal that the temperature increment has a more pronounced effect on the degree of devolatilization and associated weight loss than residence time. Generally, in the demonstration-scale continuous operation, the feedstock is conveyed using augers or rakes and gravity and is under constant agitation and could also experience air penetration, which would potentially raise the material temperatures to ≥600 °C. In the current study, laboratory-scale batch equipment was selected for producing biochars at specific temperatures and residence times as it offers more accurate control over the process parameter than demonstration-scale equipment operated in continuous mode. In the current investigation, the biochar material temperatures were precisely regulated within ±5 °C tolerance and air infiltration/penetration was prevented by regulating a continuous CO2 purge through the tube furnace. Biochar Basic Characterization. All of the biochars generated in this study were analyzed for gross and elemental compositions by ASTM methods. The measurements obtained are presented in Table 3. As noted, the volatile matter decreases, whereas fixed carbon and elemental carbon contents increase
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RESULTS AND DISCUSSION Feedstock Characterization. As-received TMP sludge contained about 65 wt % moisture content. It was manually delumped and dried overnight at 110 °C to obtain bone-dry TMPS feedstock, which was analyzed to evaluate its quality and was employed in the biochar production runs. The proximate composition, ultimate composition, and inorganic constituents of the TMPS are shown in Tables 1 and 2, respectively. In the TMP process, chipped soft woods (spruce, fir, pine) are pulped thermomechanically to extract cellulose fibers (pulp). During pulping most of the hemicellulose fraction of the wood is extracted into the liquid phase. The concentration of chemical compounds in TMP mill waste streams is usually rather low and constitutes a complex mixture of various substances such as hemicelluloses, extractives, inorganic ions, and suspended matter. These waste waters are directed to the waste treatment plant, where the remnant solids are separated, termed sludge, which is predominantly lignin, hemicellulose, protein, and cellulose remnants.10 Table 1 compares the proximate and ultimate analyses values of typical clean wood and TMPS feedstock. As noted, it is evident that TMPS has characteristics similar to those of wood, except for high ash and lower oxygen contents. Removal of hemicellulose with significantly high oxygen content24 during pulping results in lowering the oxygen content of TMPS, as evidenced from Table 1.24 As noted from Table 2, the concentrations of specified metals such as As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Se, and Zn were far below the stipulated guideline for maximum permissible metal concentration in biomass ashes, processed sewage, compost, and biosolids for land applications as regulated by European Union (EU) and Canadian Food Inspection Agency (CFIA) guidelines.25 Furthermore, the TMPS also meets the specifications of 1650
DOI: 10.1021/jf502556t J. Agric. Food Chem. 2015, 63, 1648−1657
Article
Journal of Agricultural and Food Chemistry Table 2. TMPS Feedstock Trace Metal Analyses ref25
current study
recommended guidelines (μg/g)25 max permissible heavy metal concn in biosolids for land application in Canada and USA (compiled from MENV, 2002; anonymous, 1998d
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
element
concn (μg/g)
aluminum antimony arsenic barium beryllium bismuth boron cadmium calcium chlorine chromium cobalt copper iron lead lithium magnesium manganese mercury molybdenum nickel phosphorus potassium selenium silicon silver sodium strontium sulfur thallium thorium tin titanium uranium vanadium zinc
460 0.0653 0.243 43.8 0.0083 0.0133 21.5 0.822 6290 297 3.36 0.364 6.97 414 0.249 0.172 285 498 0.0229 1.17 1.76 1870 378 0.13 340 0.325 243 15.2 3480 0.0285 0.0476 0.534 14.4 0.093 0.685 73.5
typical clean wooda (μg/g)
EU guideline for max concn of heavy metals in biomass ashes added to agricultural landb
