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Co-Pyrolysis of Biomass with Phosphate Fertilizers to Improve Biochar Carbon Retention, Slow Nutrient Release, and Stabilize Heavy Metals in Soil Ling Zhao, Xinde Cao, Wei Zheng, John W Scott, B. K. Sharma, and Xiang Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01570 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016
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ACS Sustainable Chemistry & Engineering
Co-Pyrolysis of Biomass with Phosphate Fertilizers to Improve Biochar Carbon Retention, Slow Nutrient Release, and Stabilize Heavy Metals in Soil
Ling Zhao a, Xinde Cao a,*, Wei Zheng b,*, John W Scott b, Brajendra K Sharma b, Xiang Chen a
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
b
Illinois Sustainable Technology Center, University of Illinois at Urbana-Champaign, One Hazelwood Dr., Champaign, Illinois 61820, USA
*
Corresponding authors:
Tel: +86-21-54743926; Fax: +86-21-54740825; e-mail:
[email protected] (Xinde Cao) Tel: +1-217-3337276; Fax: +1-217-3338944; e-mail:
[email protected] (Wei Zheng)
Notes The authors declare no competing financial interest.
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ABSTRACT: Two phosphate fertilizers, triple superphosphate (TSP) and bone meal (BM), were pre-mixed with sawdust and switchgrass biomass for pyrolytic biochar formation. Carbon retention, P release kinetics, and capacity of biochar for stabilizing heavy metals in soil were evaluated. Results show that TSP and BM pretreatment increased carbon retention from 53.5–55.0% to 68.4–74.7% and 58.5–59.2%, respectively. The rate constants (k2) of P release from the TSP- and BM-composite biochars are 0.0012−0.0024 and 0.89−0.91, respectively, being much lower than TSP and BM themselves (0.012 and 1.79, respectively). Co-pyrolysis with phosphate fertilizers enhanced biochar capability for stabilizing metals in soil significantly, especially the BM-composite biochar which increased Pb, Cu, and Cd stabilization rates by up to about 4, 2, and 1 times, compared to the pristine biochars. During pyrolysis process, Ca(H2PO4)2 in TSP converted to Ca2P2O7 and reacted with biomass carbon to form C–O–PO3 or C–P, leading to greater carbon retention and lower P release. PO43- in both composite biochars could precipitate with heavy metals, resulting heavy metal immobilization in soil. This study indicates that co-pyrolysis of biomass with P-containing fertilizers could obtain multiple environmental benefits.
KEYWORDS: Biochar, Triple superphosphate, Bone meal, Carbon retention, Phosphate release, Heavy metals stabilization
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■ INTRODUCTION Pyrolytic conversion of biomass to biochar and its soil amendment is regarded as a strategy of carbon sequestration. Many studies have proved that biochar incorporated into soil can provide a long-term carbon sink, 1-3 and meanwhile, biochar has additional agricultural and environmental benefits for soil properties and quality such as soil amelioration, 4, 5 filtration of percolating soil water, 6 and nutrient retention. 7 There is also much work showing the potential of biochar in stabilizing contaminants in soil due to its favorable physical/chemical surface characteristics. 8 In spite of these benefits, there are still several barriers that limit the wide practical application of biochars. Firstly, biochar production is a carbonization process in which the biomass carbon is rearranged towards stable and aromatic structures, accompanied by a release of volatile compounds. In general, almost half of biomass carbon is immediately released during pyrolysis, and only the remaining about half of carbon is converted to biochar. 9, 10 Thus, the process of biomass pyrolysis to produce biochar initially releases more carbon compared to biomass decomposing in soil. Secondly, a common biochar cannot usually be regarded as a fertilizer because P and K are not present in substantial quantities. 11 Manure and sludge biochars contain high P, and crop waste biochars bear a relatively high content of K. 12 However, these nutrients generally release quickly in the initial days after biochar is applied to soil. 13 If exogenous P was adsorbed on biochar, it could be used as an effective slow P-release fertilizer. 14 The third barrier is that routine biochar has limited ability to adsorb and stabilize inorganic contaminants,15, 16 and stabilization efficiency is not consistent and is even questionable for certain metals. For example, Beesley (2011) pointed out that biochar could stabilize Zn and Cd, but increase As mobility. 17 Amendment of sewage sludge biochar reduced the bioaccumulation of As, Cr, Cu, and Pb, but increased that of Cd and Zn. 18
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Therefore, more attention should be paid to generation of the designed biochar with multiple functions to overcome the barriers for wide biochar application. Our previous works found that co-pyrolysis of biomass with additives, especially with P-containing materials such as Ca(H2PO4)2, could increase carbon retention by about 30%. 19, 20
It has been reported that H3PO4 pretreatment of sawdust improved the oxidation resistance
of derived carbonaceous materials such as activated carbon and graphite. 21, 22 A study revealed that P could react with biomass during pyrolysis, forming protection layers on biochar’s surface. 23 In addition, biomass pretreatment with P-containing materials may enhance the fertility of biochar, while simultaneously strengthening its capability to sorb heavy metals. 24 Cao et al. (2011) found that dairy manure biochar effectively immobilized Pb in contaminated soil and they attributed this effectiveness to the formation of stable Pb phosphate precipitates due to the high P content in the dairy manure biochar. 25 Therefore, we assumed that co-pyrolysis of biomass with P fertilizers could obtain composite biochar with multiple environmental benefits. In this study, we pyrolyzed sawdust (Pinus spp.) and switchgrass (Panicum virgatum) with two different types of phosphate fertilizer, i.e., triple superphosphate (TSP) and bone meal (BM). TSP is highly soluble in P, and its direct application often results in excessive P leaching into the surrounding water body. Phosphorus in BM is more stable than in TSP, while its direct use often provide poor fertility due to the less soluble P. 26 The objectives were to investigate (1) the effect of biomass pretreatment with two types of phosphate fertilizer on carbon retention in biochar; (2) the fertility supply and P release of the composite biochars; and (3) their capability of stabilizing heavy metals in soil. The interaction mechanisms of biomass and fertilizers during pyrolysis were also explored.
■ MATERIALS AND METHODS 4
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Biomass and phosphate fertilizer. Two common types of plant-based biomass, pine tree sawdust (Pinus spp.) and switchgrass (Panicum virgatum), were chosen for this study because they represent biomass composed primarily of lignin and cellulose, respectively. These biomasses were collected from the University of Illinois Energy Farm located in Urbana, Illinois, USA. Two common types of phosphate fertilizer, triple superphosphate (TSP) and bone meal (BM), were purchased from a local fertilizer company. All biomass and phosphate fertilizer were air-dried, and then ground into particle sizes smaller than 0.3 mm. The physicochemical properties of these materials are presented in Table S1 in Supporting Information (SI). Biochar production. Biochar production was conducted in a laboratory-scale pyrolysis unit comprised of a tube reactor equipped with a programmable temperature controller and a cooling system to collect condensed vapors of gas and bio-oil. Prior to being loaded into the pyrolysis reactor, the biomass and fertilizer were completely mixed at a 4:1 (w/w) of biomass to additive. Pyrolysis was performed by raising the temperature to 500°C at a rate of 15°C·min-1 under a 2 L·min-1 N2 flow and then maintaining that temperature for 2 h. All experiments were carried out in duplicate. Biochars derived from sawdust and switchgrass mixed with TSP and BM are henceforth referred to as BCsaw-TSP, BCsaw-BM, BCswi-TSP, and BCswi-BM, respectively. In order to further examine the reaction process of biomass-fertilizer composite during pyrolysis, we determined the biomass weight loss and gas composition alteration using thermogravimetric-mass spectrometric (TG-MS) analysis, simulating the pyrolysis process. The materials were heated from 25°C to 650°C at 15°C·min-1 under the same N2 flow with biochar production (TGA/DSC 1, METLER, USA). Calculation of carbon retention in biochar. Carbon retention was calculated based on the biochar carbon and the total carbon amounts in the original feedstocks. 5
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Carbon retention (%) =
∗ ∗ ∗
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………..(1)
where Cbiochar, Cbiomass, and Cadditive are carbon contents (%) and Wbiochar, Wbiomass, and Wadditive are dry-weight (g) of the biochar, biomass, and additive, respectively. Biochar characterization. Biochar pH was measured with a solid-to-liquid ratio of 1:20 (w/v) after 18 h equilibrium. C, H, and N contents in biochar were measured using a CHN/O elemental analyzer (PerkinElmer 2400 Series II, USA). Metal concentration was measured using an ICP-MS instrument (PerkinElmer Elan DRCe ICP-MS, USA) after digestion (USEPA, 1986). Minerals in biochar were analyzed using a X-ray diffractometer (Siemens D5000 theta/theta XRD system, Germany), which was operated at 40 kV and 30 mA; data was collected over the 2θ range from 5° to 60° using Cu Kα radiation with a scan speed of 1° per minute. Surface functional group was determined by FTIR spectroscopy (Thermo-Nicolet Nexus 670, USA). Biochar surface morphology was observed with a scanning electron microscope (Hitachi S 4800 High Resolution SEM, Japan). The bonding energies of P, C and O atoms on the surface of biochar were determined using X-ray photoelectron spectroscopy (PHI 5400 XPS, Perkin-Elmer, USA). Phosphorus release from biochar. To evaluate the P stability in the fertilizer-amended biochar and its possible P supply to the soil, the release kinetics of P from the composite biochar was determined through a batch leaching experiment. Biochar was mixed with distilled water at a solid: liquid ratio of 1:50 (g: mL) and then agitated at 25°C for 120 h. At intermittent times, the leachate was collected and filtered using a 0.45-µm filter membrane, and the P concentration in the leachate was measured using ICP-MS (PerkinElmer Elan DRCe ICP-MS, USA). The change in P concentration as a function of time was fitted using kinetic models such as pseudo first, pseudo second, Elovich, power function, and parabolic diffusion models. 27, 28 Capacity of biochar for stabilizing soil metals. Contaminated soil was made artificially 6
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by spiking the soil with Pb(NO3)2, Cu(NO3)2, and Cd(NO3)2. The obtained concentration of Pb, Cu, and Cd was about 1300, 800, and 6.00 mg·kg-1, respectively, which represents 2.5 times the level III environmental quality standard for soils in China (GB15618–1995) (Table S2). The contaminated soil was treated with biochar at a rate of 3%. After equilibration for 3 weeks at room temperature and 65% of field moisture capacity, the TCLP extraction method was used to evaluate the metals stabilization effectiveness (USEPA 1992). 29 Briefly, the leaching fluid was prepared with diluted glacial acetic acid with pH of 2.88 ± 0.05. The ratio of soil/leaching fluid was 1/20 (w/v). After being agitated at 30 rpm at room temperature for 18 h, the supernatant was filtered through a 0.45-µm Millipore filter and its metals concentration was measured using ICP (PQ-EXCELL ICP/MS, Thermo Elemental, USA). The stabilization rate of metals by biochar was calculated by eq. 2. Stabilization rate (%) = (1 −
%&'()&* +&,'-. (%) /0,) 102()'3 '**0,024 %&'()&* +&,'-. (%) /0,)25, 102()'3
) × 100% ….(2)
Statistical analysis. Differences in carbon retention and stabilization rates of heavy metals were conducted with SPSS 22.0 (one-way ANOVA). Note, Pb, Cu, and Cd were analyzed separately.
■ RESULTS AND DISCUSSION Properties of biomass, fertilizer, and the produced biochars. The two biomasses used in this study, sawdust and switchgrass, had similar C contents, about 45% (Table S1). The main components of TSP and BM were soluble Ca(H2PO4)2 and less soluble Ca3(PO4)2, respectively (Table S1). The pristine biochars were of high alkalinity (pH 8.56−9.56), and co-pyrolyzed with BM and TSP reduced their pH to 8.45−8.67 and 4.96−5.12, respectively (Table 1). Low pH values in TSP-composite biochars resulted from the high acidity of TSP. Ash contents (21.6−27.0%) of the composite biochars were much higher than those of the
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pristine biochars (3.19−14.0%) due to the input of the additives. Accordingly, the C contents in the former (53.4−62.8%) were less than those in the latter (77.7−82.5%). Please note that they are biochar C contents and not biomass C retention. Addition of TSP and BM increased P content from 0.14−0.15% in pristine biochars to 4.82−5.82% in the composite biochars. The contents of heavy metals in all biochars are not high and safe to soil (Table 1). Carbon retention after co-pyrolysis of biomass with phosphate fertilizers. Biochars generated from sawdust and switchgrass without additives retained 53.5% and 55.0% of biomass carbon, respectively (Fig. 1). The similar carbon retention between the two biochars might be attributed to the similar carbon structure in their original biomass (Table S1). Addition of TSP and BM increased biomass carbon retention in sawdust biochar to 68.4% and 59.2%, respectively, representing an increase of 27.9% and 10.7% compared to the pristine biochar. For switchgrass biochar, the carbon retentions were increased to 74.7% (TSP) and 58.5% (BM), with an increase of 35.8% and 6.36%, respectively. The carbon retention was significant (p
