Scholarly article on topic 'Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene'

Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene Academic research paper on "Environmental engineering"

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Academic research paper on topic "Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene"

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SUBJECT AREAS: GEOCHEMISTRY ENVIRONMENTAL CHEMISTRY

Received 8 January 2014

Accepted 27 May 2014

Published 13 June 2014

Correspondence and requests for materials should be addressed to K.S. (sunke@bnu.edu.

Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene

Mengyi Qiu1, Ke Sun1, Jie Jin1, Bo Gao2, Yu Yan1, Lanfang Han1, Fengchang Wu3 & Baoshan Xing4

1State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China, 2State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China, 3State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 10001 2, China, 4Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States.

The properties of plant residue-derived biochars (PLABs) and animal waste-derived biochars (ANIBs) obtained at low and high heating treatment temperatures (300 and 450 ° C) as well as their sorption of dibutyl phthalate (DBP) and phenanthrene (PHE) were investigated in this study. The higher C content of PLABs could explain that CO2-surface area (CO2-SA) of PLABs was remarkably high relative to ANIBs. OC and aromatic C were two key factors influencing the CO2-SA of the biochars. Much higher surface C content of the ANIBs than bulk C likely explained that the ANIBs exhibited higher sorption of DBP and PHE compared to the PLABs. H-bonding should govern the adsorption of DBP by most of the tested biochars and p-p interaction play an important role in the adsorption of PHE by biochars. High CO2-SA (>200 m2 g"1) demonstrated that abundant nanopores of OC existed within the biochars obtained 450°C (HTBs), which likely result in high and nonlinear sorption of PHE by HTBs.

Biochar is a byproduct from pyrolysis of carbon-rich biomass like grass, forestry products and animal manures1. The roles of biochars in biogeochemical cycles have received significant attention in recent years, since they could be used as soil amendments for helping biomass waste management2, improving soil fertility and crop yield3,4, and sequestrating carbon to mitigate climate change5. The properties and sorption strength of biochars have been reported to be affected by many factors, such as heating treatment temperatures (HTTs) and feedstock sources6-9. According to the HTTs, four distinct categories based on changes in the physical and chemical properties of the biochars could be identified: transition chars, amorphous chars, composite chars and turbostratic chars10. It has been reported that biochars obtained by pyrolyzing biomass at relative high temperature (HTT $ 500°C) generally have simple structures, composed of large amounts of condensed aromatic C, and the sorption mechanisms are mainly adsorption-controlled. By contrast, biochars produced at relative low temperature (HTT # 500°C) are partially carbonized with high yields and complicated compositions, and contain diversified characteristics1,6,11,12. Previous study demonstrated that the sorption of hydrophobic organic compounds (HOCs) to biochars prepared at relatively low HTTs is composed of both adsorption on carbonized fractions and concurrent partition into the residual noncarbonized fractions9. Therefore, this study mainly focuses on the sorption of HOCs to low HTT biochars due to the complicacy in their sorption mechanisms stemmed from their complex chemical structures. Moreover, extensive studies reported that biochars could sorb lots of hazardous organic compounds, such as estrogens13, pesticides14, polychlorinated biphenyl (PCBs)15, and polycyclic aromatic hydrocarbons (PAHs)16,17. However, the information about the sorption of phthalic acid esters (PAEs), as one kind of the emerging organic contaminants18, by soils and sediments as well as biochars is still very limited7,19,20. In recent years, high production and wide application of PAEs in industry and agriculture created a big environmental problem. Humans use PAEs as plasticizers to induce workability and flexibility of polymeric materials. PAEs could enter soil, sediments, or even food through different pathways21-23. Unfortunately, this type of chemical has been suspected to have estrogenic effect as endocrine disruption compounds (EDCs) and has a potential tendency for bioaccumulation24,25. Some PAEs are suggested to be

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Table 1 | Bulk elemental composition, ash content, atomic ratio, and CO2-SA of biochars

biochars C(%%) O(%%) H(%%) N (%) Ash (%) H/C O/C N/C (O + N)/C CO2-SA'

C0300 67.1 20.7 4.6 1.3 6.3 0.83 0.23 0.02 0.25 243.6

P0300 59.1 19.7 5.2 4.0 12.1 1.05 0.25 0.06 0.31 103.4

LE300 62.2 16.9 5.4 4.8 10.7 1.04 0.20 0.07 0.27 90.3

RI300 55.3 19.5 3.7 0.8 20.6 0.81 0.26 0.01 0.28 188.5

WH300 63.3 24.0 4.4 0.5 7.8 0.84 0.28 0.01 0.29 162.3

MA300 63.8 25.3 5.1 0.8 5.1 0.95 0.30 0.01 0.31 188.3

NU300 60.9 29.2 5.4 0.2 4.3 1.06 0.36 0.00 0.36 40.5

WD300 65.4 27.3 5.3 0.0 2.0 0.97 0.31 0.00 0.31 155.0

CH300 10.0 4.6 0.9 0.6 83.9 1.10 0.34 0.05 0.40 31.4

SW300 36.5 14.9 3.6 3.2 41.8 1.18 0.31 0.08 0.38 89.6

CO450 71.6 13.3 3.9 1.2 10.1 0.65 0.14 0.01 0.15 367.1

PO450 61.9 14.3 3.7 3.7 16.4 0.72 0.17 0.05 0.22 222.0

LE450 63.4 12.6 3.7 5.0 15.2 0.70 0.15 0.07 0.22 248.6

RI450 57.9 11.8 3.3 0.8 26.2 0.69 0.15 0.01 0.16 293.4

WH450 70.2 12.9 4.3 0.5 12.2 0.73 0.14 0.01 0.14 349.7

MA450 74.4 11.8 3.8 1.0 9.1 0.61 0.12 0.01 0.13 388.3

NU450 78.4 11.8 3.6 0.3 6.0 0.54 0.11 0.00 0.12 394.1

WD450 75.9 16.7 3.7 0.1 3.7 0.58 0.16 0.00 0.17 601.6

CH450 9.8 3.6 0.9 0.5 85.2 1.12 0.28 0.05 0.33 33.2

SW450 33.7 10.2 2.6 2.6 50.9 0.91 0.23 0.07 0.29 162.0

"Calculated from nonlocal density functional theory (NLDFT) and grand canonical Monte Carlo simulation (GCMC) using CO2 adsorption ((m2g2,|. CO, PO, LE, RI, WH, MA, NU, WD, CH and SW refer to the biochars produced from plant residues of cotton straw, potato straw, leaf, rice straw, wheat straw, maize straw, nut, wood dust (PLABs) and manures of chicken and swine (ANIBs). The numbers of 300 and 450 were represented to the heating treatment temperatures of the biochars (e.g., 300 and 450°C).

mutagens, hepatotoxic agents and carcinogens, which could cause toxic effects to human and wildlife26. Therefore, the environmental behavior of PAEs is becoming a hot area of research. Recently, Yang et al. reported that the soil carbon content has a crucial influence on sorption of PAEs by three soils19. If biochars are added to soil to improve fertility and store C: there is a potential for biochars and PAEs to interact. Therefore, our previous study about sorption of PAEs by grass- and wood-derived biochars has been carried out and proposed that amorphous biochars exhibit the greatest sorption capacity7. Also Sun et al. concluded that aliphatic and polar domains in biochars derived from only two plant-residues (e.g., low-mineral biochar) controlled the sorption of PAEs7. In order to confirm the conclusion, much more biochars derived from different feedstock sources (e.g., wood, grass, animal or poultry wastes) need to be further investigated. Additionally, it is worth noting that most sorption studies, to date, have used low-mineral biochars produced from relatively pure plant residues (e.g., plant residue-derived biochars (PLABs))27, the sorption properties of HOCs to the high-mineral biochars (e.g., animal waste-derived biochars (ANIBs)) are rarely reported28. Although some of these contain considerable ash (e.g. grass biochar that contains substantial silica in the form of phyto-liths)7, the sorption properties of the ash component have not generally been considered. Especially, the recent studies demonstrated that the minerals within both biochars and soils have influence on the spatial arrangement or conformation of their organic matter (OM), in turn, which impacts on the sorption of phenanthrene (PHE) by the biochars or soils28-31. It is hypothesized that the minerals of biochars should play different role in the sorption between PAEs and PHE due to their different chemical properties, and also expected that the properties of ANIBs and PLABs should have different influence on sorption of PAEs. Some information about the sorption of PAEs and PHE by the six high-temperature biochars (HTBs) including ANIBs and PLABs produced at only high HTT (450°C) has been reported accompanying in our previous study20, whose objective was to test the importance of the p-p electron donor —acceptor (EDA) interactions in the overall sorption of PHE and PAEs, respectively, onto the high-temperature biochars via competitive sorption of PHE, Dibutyl phthalate (DBP), and Cd2120, however, the comprehensive sorption behavior of PAEs by both low-temperature biochars (LTBs) and HTBs and how the properties of these biochars would influence

the sorption of PAEs were poorly understand, which would be very important to precisely predict the fate of PAEs in the soils amended by the different kinds of biochars.

Therefore, the primary objectives of this study were to investigate how the properties of biochars influenced on the sorption of PAEs to a large number of LTBs and HTBs including the low-mineral PLABs and the high-mineral ANIBs, and further identify the different sorption mechanism between PAEs and PHE via analyzing the correlations of their sorption parameters to the bulk and surface structural characteristics of the biochars. We expect that this study could provide important information about agricultural and environmental application of biochars on immobilizing PAEs and PAHs. To meet these objectives, both ANIBs and PLABs were produced at low and high HTTs (300 and 450°C, respectively) to represent the high-mineral biochars and low-mineral biochars, respectively. The biochars produced at 300 and 450°C, respectively, were referred to LTBs and HTBs. DBP was chosen as a model of PAEs because the octanol-water partition coefficients of DBP and PHE are comparable. Furthermore, PHE, a typical representative of PAHs, was selected as a contrasting sorbate to explore dissimilar sorption mechanism between PAHs and PAEs to biochars due to their different molecular structure.

Results

Characterization of biochars. The bulk elemental composition, atomic ratio, ash content, and surface area determined by CO2 (CO2-SA) of 20 biochars produced at two different HTTs (300 and 450°C) are listed in Table 1. The C contents of PLABs were higher than ANIBs (Table 1), because the feedstocks of ANIBs had relatively more minerals than PLABs. However, their H/C ratios exhibited a reverse trend (Fig. 1a), suggesting that the higher degree of aromatization of organic matter (OM) associated with PLABs relative to OM within ANIBs. The relatively high H/C ratio of ANIB also indicated a great amount of original organic residues9. As H is primarily associated with OM, the H/C atomic ratios could be described as an indicator of carbonization9. In addition, ANIBs produced at 450°C had the remarkable high bulk (O + N)/C ratio (Fig. 1b), representing the presence of the polar functional groups in OM of the high HTTs ANIBs. The C contents increased with increasing HTT except for ANIBs (Table 1) and the values of

Figure 1 | Comparison of H/C atomic ratio (a), bulk polarity ((O 1 N)/C) (b), O/C atomic ratio (c), ash content (d), N content (e), surface polarity ((O 1 N)/C) (f), surface area determined by CO2 adsorption (CO2-SA) (g), and aromaticity (h) between the low-temperature biochars (LTBs) and high-temperature biochars (HTBs). CO, PO, LE, RI, WH, MA, NU, WD, CH and SW refer to the biochars produced from cotton straw, potato straw, leaf, rice straw, wheat straw, maize straw, nut, wood dust and manures of chicken and swine, respectively. Plant-residues and animal-waste derived biochars were named by PLABs and ANIBs, respectively. LTB and HTB represent the biochars obtained under pyrolysis or heat treatment temperature (HTT) at 300°C and 450°C, respectively.

polarity indexes (e.g., O/C and (O + N)/C) of LTBs were higher than those of HTBs (Fig. 1b and c), implying the enhancement of hydrophobicity1 and reduction of polar functional groups with increasing HTT. However, no obvious reduction of O/C and (O + N)/C was observed for ANIBs compared to the remarkable reduction of O/C and (O 1 N)/C for PLABs (Fig. 1b and c), suggesting the minerals likely have a potential role in protecting the polar functional groups of OM within ANIBs from being removed during pyrolysis. The result was consistent with the previous studies that biochars produced at low HTTs were polar1. Both N and ash contents in the biochars were mostly constant with increasing HTT (Fig. 1d and e), indicating no distinct change of both N and ash during the charring process of these feedstocks.

The surface elemental composition and functional groups of the investigated biochars from X-ray photoelectron spectroscopy are

presented in Table S1. The surface (O 1 N)/C values of ANIBs were obviously higher than PLABs (Table S1, Fig. 1f), which was consistent with the ash contents (Fig. 1d). Moreover, the ash content of the biochars were closely associated with the surface polarity ((O 1 N)/ C) of the biochars (R = 0.96, p < 0.01, Fig. 2a). However, no significant positive correlation was observed between the ash contents and bulk polarity of the biochars (Fig. 2a). Therefore, it could be concluded that the minerals of the tested biochars could contribute to the high content of surface O of these biochars and their bulk O was mainly from the O of OM in these biochars.

Fourier transform infrared (FTIR) spectroscopy spectra of the 20 biochars as a function of feedstock sources and HTTs are shown in Fig. S1. References for the band assignments are provided in Table S2. The intense bands for aliphatic CH2 disappear abruptly in the spectra of the HTBs and the disappearance of C= C ring stretching

Figure 2 | Relationships between the bulk and surface polarity ((O 1 N)/C) of low-temperature biochars (LTBs) and high-temperature biochars (HTBs) and the ash content (a); between CO2-surface area (CO2-SA, determined using CO2 adsorption) of LTBs and HTBs and their organic carbon (OC) content (b); between the CO2-SA of the biochars and ash content (c); between the OC-normalized CO2-SA (CO2-SA/OC) and contents of aromatic C (d) and aliphatic C (e), respectively; and between logX"d (Ce = 0.01SW) of DBP and PHE and the OC contents of LTBs and HTBs (f).

vibration of lignin and plane bending of phenolic -OH related to syringyl units of lignin indicates the heat-induced decomposition of the feedstocks of PLABs. The polar functional groups (e.g.,-OH and C-O) are significantly reduced upon the HTTs. The band intensities for ester C=O, aromatic CO- and phenolic -OH groups of biochars diminished when the HTT reached to 450°C. Whereas C=C and C=O stretching vibrations in the aromatic rings were present when HTTs were elevated from 300 to 450°C. Three bands (885, 815, and 750 cm1) of out-of-plane deformations of aromatic -CH in the spectra increased with the elevated HTTs, indicating the formation of more condensed aromatic C. The 13C-NMR spectra of the biochars and their integrated data of biochars are shown in Fig.S2 and Table S3, respectively. The quality of ANIBs 13C-NMR spectra were affected by their high abundance of mineral and low C content (Fig. S2). The band intensity of aliphatic -CH2-, carboxylic or car-bonylic C and polar functional groups were significantly reduced with increasing HTT, which was consistent with the result of the FTIR spectra. The aromaticities of the investigated biochars were more than 55% (Table S3), indicating these biochars were mainly characterized with highly aromatic (aryl-dominated) structures.

CO2-SA of PLABs (40.5-601.6 m2 g1) was remarkably higher than those of ANIBs (31.4-162.0 m2 g21) (Table 1). CO2-SA of the biochars was elevated distinctly with increasing HTT (Fig. 1g). A significantly positive relationship between CO2-SAs and organic carbon (OC) contents of (R = 0.86, p < 0.01, Fig. 2b) as well as a good negative correlation between CO2-SAs and ash contents (R = —0.84, p < 0.01, Fig. 2c) was observed for the HTBs, suggesting that OC is a major contributor to CO2-SAs of HTBs. Therefore, the higher C content of the PLABs compared to ANIBs could explain that the CO2-SA of PLABs was much higher than ANIBs (Fig. 1g). As for the LTBs, besides the OC, the molecular composition of OC should be another factor in dominating the CO2-SA. The CO2-SA of biochars normalized by OC (CO2-SA/OC) was correlated with the contents of the functional groups as indicated by the 13C-NMR spectra of the biochars in order to find out other factors influencing the CO2-SA of biochars. The CO2-SA/OC of the biochars excluding the CO450 and PO450 was positively related to the aromatic C content (93-165 ppm) (R = 0.87, p < 0.01, Fig. 2d) and negatively related to the aliphatic C content (0-93 ppm) (R = —0.83, p < 0.01, Fig. 2e). Therefore, the increase of CO2-SA of biochars with increasing HTT

Figure 3 | Comparison of Freundlich nonlinearity coefficients (n) of DBP and PHE sorption isotherms (a), sorption capacity as indicated by the organic carbon (OC)-normalized distribution coefficient (logKOo mL g"1) (b), and the KHW-normalized KOC values (KHW, hexadecane-water partition coefficient) of DBP and PHE between the LTBs and HTBs (c). CO, PO, LE, RI, WH, MA, NU, WD, CH and SW refer to the biochars produced from cotton straw, potato straw, leaf, rice straw, wheat straw, maize straw, nut, wood dust and manures of chicken and swine, respectively. LTB and HTB represent the biochars obtained under pyrolysis or heat treatment temperature (HTT) at 300 and 450°C, respectively.

could be attributed to the elevated aromatic C of the biochars (Fig. 1g and h).

Adsorption of DBP and PHE by biochars. The sorption of PHE and

DBP onto these 20 biochars is presented in Fig. S3, and was fitted well with Freundlich equation (R2 > 0.962). The model parameters are listed in Table S4 and S5. The nonlinearity coefficients (n) derived from Freundlich model were significantly less than 1 (p < 0.01) (Fig. 3a). The low n values of biochars indicate heterogeneous glassy, rigid or condensed sorption domain and a wide site energy distribution in the biochars32. The majority of n values of DBP by the LTBs were lower than PHE by the LTBs, however, the opposite result was observed for the HTBs (Fig. 3a). Additionally, LTBs had lower isotherm nonlinearity (n) of DBP than HTBs, however, an inverse trend was observed for PHE (Fig. 3a). The significant and positive correlation of DBP sorption distribution coefficients (Kd) values of biochars to their OC contents was observed (R = 0.97, p < 0.01, Fig. 2f), suggesting that OC was one key factor in regulating the PAEs sorption by these biochars, consistent with the previous result that soil OC has been identified as a predominant sorbent for HOCs in soils and sediments as long as the total OC is >0.1%33. The OC

normalized concentration-specific sorption distribution coefficients (KOC) values of biochars (Table S4 and S5) could be obtained from multiplying OC-normalized sorption capacity coefficient (KF) by Cen—1. The logKOC values of DBP or PHE varied with the different equilibrium concentrations due to their nonlinear sorption isotherm. ANIBs generally demonstrated the relatively high logKOC values of DBP and PHE by both LTBs and HTBs (Fig. 3b). The higher surface C content of ANIBs than bulk C indicated that OM of ANIBs is likely distributed on the surface of their particles due to their abundant minerals, which thus let the solutes be easily accessible to the sorption domain of OM associated with the highminerals ANIBs. As a result, this could likely account for that ANIBs exhibited relatively high sorption capacity (KOC) of DBP and PHE compared to PLABs (Fig. 3b).

Discussion

In order to investigate how the properties of PLABs affect the sorption of DBP and PHE, the following discussion excluded the ANIBs to avoid the interference of the difference in the OM source between ANIBs and PLATs. The all correlation between the logKoc values and

Figure 4 | Correlations between the organic carbon (OC)-normalized sorption distribution coefficient (logKOC, Ce = 0.01SW (the solubility of DBP or PHE in water at 20°C)) of DBP and PHE by the plant-derived biochars at 300°C (PLABs-300) and the content of bulk O (a) and bulk atomic ratio of O/C (b) as determined by the elemental analysis, carbohydrate-C (c) according to the 13C-NMR results, the content of surface N (d) as indicated by XPS results, the bulk atomic ratio of N/C (e), the content of alkyl-C (f), the content of surface polar group of C=O (g), and the ash content (h) of PLABs-300.

properties of the PLABs produced at 300 and 450°C (PLABs-300 and PLABs-450), respectively, are presented in Fig. 4. For PLABs-300, the negative relationships between the logKoc values and the contents of bulk O (R # —0.89, p < 0.01, Fig. 4a), the atomic ratio of O/C (R # — 0.86,p < 0.01, Fig. 4b) and carbohydrate-C (R # —0.78 p < 0.05, Fig. 4c) were observed, implying that the O-containing polar groups of OM associated with the PLABs-300 may block the sorption of DBP and PHE. Moreover, DBP and PHE had the similar correlations between their logKOC values and the parameters of the properties of the PLABs-300, demonstrating that DBP and PHE had the similar sorption mechanism during their sorption by the low-temperature PLABs containing the abundant O-containing polar groups. On the other hand, the logKOC values of DBP and PHE were positively related to the content of the surface N (R $ 0.78, p < 0.05,

Fig. 4d), the atomic ratio of bulk N/C (R $ 0.72, p < 0.05, Fig. 4e), contents of alkyl-C according to the 13C-NMR results (R $ 0.78, p < 0.05, Fig. 4f) and the polar group of C=O as measured by the XPS results (R $ 0.95, p < 0.01, Fig. 4g), and ash contents (R $ 0.75, p < 0.05, Fig. 4h), suggesting that the minerals, alkyl-C, N-containing polar groups, and surface C=O associated with PLABs-300 could facilitate the sorption of both DBP and PHE, and that the H-bonding involving the N-containing polar groups of PLABs-300 should play the important role in the sorption. Additionally, for PLABs-450, the atomic ratios of O/C and H/C increased with the logKoc values of DBP and PHE (R $ 0.77, p < 0.05, Fig. 5a and b), indicating that the O-containing groups of OM within the PLABs-450 could benefit the sorption of DBP and PHE. The N-containing polar groups of low-and high-temperature PLABs played the same role in their DBP

Figure 5 | Correlations between organic carbon (OC)-normalized sorption distribution coefficient (logK"Oo Q = 0.01Sw (the solubility of DBP or PHE in water at 20° C)) of DBP and PHE by plant-derived biochars at 450° C (PLABs-450) and the atomic ratios of H/C (a) and O/C (b), the percentages of COOH (c) determined by XPS results, and surface content of N (d).

sorption, however, N-containing polar groups of PLABs-450 had no positive impact on the sorption of PHE, suggesting DBP and PHE had different adsorption mechanisms. The sorption ofboth DBP and PHE by PLABs-300 decreased with their increasing bulk atomic ratio of O/C (R # —0.86, p < 0.05, Fig. 4b) while the corresponding sorption of DBP and PHE by PLABs-450 increased generally with the elevated the values of O/C (R $ 0.78, p < 0.05, Fig. 5b), suggesting the most of O-containing polar groups of PLABs-300 likely blocked the sorption of both DBP and PHE because O functionality preferentially sorbs water molecules which could competitively block sorption of organic solutes proposed in other previous studies34,35, however, these polar groups in PLABs-450 should play the positive role in the sorption ofboth DBP and PHE due to their involvement in the p—p EDA interactions36,37.

The logKoc values of PHE of all biochars were obviously higher than those of DBP (Fig. 3b), which was similar to the order of their hydrophobicity as indicated by the hexadecane-water partition coefficient (KHW) (logKHW = 3.81 for DBP38 and logKHW = 4.74 for PHE39). As hexadecane is incapable of interactions such as p-p EDA interaction,polar interaction, and H-bonding, the KOC values were normalized with hexadecane-water partition coefficient to exclude the hydrophobic effect38 for investigating other sorption mechanisms apart from hydrophobic effect. After KHW normalization, the KOC/ KHW values of DBP and PHE by LTBs had no significant difference (p = 0.446, t-test) (Fig. 3c), while the significant difference in the KOC/ KHW values between DBP and PHE were observed for HTBs (p = 0.000, t-test) and for HTBs KOC/KHW values of PHE were generally higher than DBP (Fig. 3c), suggesting that DBP and PHE have the same chance to be involved in the adsorption by LTBs, however, PHE should be more readily to reach the adsorption sites of the HTBs compared to DBP due to PHE being a planar molecule.

The Os of the ester groups on DBP are able to act as H-bonding acceptors. It could form H-bonds through the water molecules40. It has been reported that H of the hydroxyl group (H-bonding donors) on biochars can directly form the H-bonds with the DBP molecule33. So DBP (hydrogen bond-forming ability: HB = 8.041) could be strongly sorbed on biochars through bridging water molecules or on H-donating functional groups. Additionally, it was reported that

apolar PHE as an aromatic hydrocarbon might have some ability to act as a weak H-acceptor39,42,43 with low energies as a result of its weak H-bonding ability (HB = 0.244). Therefore, H-bonding of DBP/PHE may act in addition to hydrophobic interaction. The significant positive correlations between logKOC (Ce = 0.01Sw) values of both DBP and PHE and surface N of PLABs (r $ 0.78, p < 0.05, Fig. 4d and R = 0.80, p < 0.05, Fig. 5d) along with the higher KOC of ANIBs (containing high polarity) than PLABs were observed (Fig. 3b), both supporting that the above conclusion that the H-bonding interaction should partly play the role in the overall sorption of DBP and PHE by these tested biochars. If H-bonding interaction regulated the overall sorption of DBP and PHE on biochars, the n values of their isotherms by the biochars should be negatively correlated with the polarity. Coincidentally, this negative correlation was only observed for the DBP sorption by the tested biochars excluding NU300 and CH450 (R # —0.77p < 0.05, Fig. 6a), however, the reverse relationship between the isotherm n values of PHE by the biochars excluding LE300 and the polarity was detected (R $ 0.58, p # 0.01, Fig. 6a), suggesting that H-bonding interaction could affect the overall adsorption of DBP by most of the tested biochars, but not the major specific interactions between PHE and these biochars. It was also noted that the significant and negative correlation between n of PHE and aromaticity of the biochars was obtained (R = —0.75, p < 0.01, Fig. 6b) combined with the significant and positive relationship between logKOC values of PHE and the surface polarity of PLABs-450 (R $ 0.78, p < 0.05, Fig. 5b), which was attributed to the enhanced p acceptor ability of biochars with increasing electron-withdrawing polar moieties on biochar surface7, suggesting that the major specific interactions between PHE (p-donor) and biochars should be the p-p interaction because recent studies suggest that PHE could sever as a p-donor to interact with the p-acceptor moieties in soil humic substances45. Additionally, enhancement of the logKOC values (Fig. 3b) and reduction of n values for PHE from LTBs to HTBs (Fig. 3a) as a result of the increasing of aromaticity (Fig. 1h) and decreasing N- or O-containing polar groups (Fig. 1b and c), and the opposite trend in the change of both logKOC values and DBP isotherm n values (Fig. 3) also both support that the p-p interaction should mainly contribute to the specific interactions between PHE

Figure 6 | Correlations between the Freundlich nonlinearity coefficients (n) of DBP and PHE sorption isotherms of DBP and PHE by the biochars and the polarity ((N 1 O)/C and O/C) (a), and contents of the aromaticity (b), methoxyl-C (c), and carbohydrate-C (d) obtained by 13C-NMR, and organic carbon (OC)-normalized surface area as determined by CO2 adsorption (CO2-SA/OC) (e) of the biochars.

and biochars while the H-bonding likely regulate the adsorption of DBP by the biochars.

Besides the specific adsorption mechanism, pore-filling was concluded to be the most important sorption mechanism for PAHs with flat aromatic sorbent surfaces46. Aromatic C increased with increasing HTT, leading to the elevated SAs of biochars (Fig. 2d). Moreover, from LTBs to HTBs, the increased CO2-SA values were consistent with the enhancement of the logKOC values and reduction of n values for PHE (Fig. 3a and b). Hence, it could be concluded that pore-filling mechanism may play an important role in PHE adsorption by most of the tested biochars. Especially for HTBs, the remarkably high CO2-SA (>200 m2 g—1, Table 1) demonstrated that more nanopores existed within the biochars (Fig. S4), which could be responsible for the strong nonlinearity of sorbates, further supporting that PHE sorption by HTBs had generally stronger nonlinear and higher sorption capacity than LTBs (Fig. 3b).

Methods

Chemicals. DBP (991%) and PHE (981%) were purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany) and Sigma-Aldrich Chemical Co, respectively. DBP and PHE were individually dissolved in methanol as stock solution, and stored at 4°C in the dark. The water solubilities (Sw) of DBP and PHE are 11.2 mgL—1 and 1.12 mg L—1, respectively47,48.

Biochars preparation. A total of 20 biochars were used in this study. Detailed information about the biochars preparation was reported elsewhere28. Briefly, two kinds of feedstock materials, including straws of cotton, potato, rice, wheat and maize, leaves, nutshells, wood dust and animal manure wastes of chicken and swine, were selected to obtain the PLABs and ANIBs, respectively. Here, biochars were referenced according to feedstock source materials (plant residues: cotton straw, potato straw, leaf, rice straw, wheat straw, maize straw, nut and wood dust; animal-waste: manures of chicken and swine) (i.e., CO, PO, LE, RI, WH, MA, NU, WD, CH and SW) and HTTs (300 and 450°C) (i.e., CO300, CO450, PO300, PO450, LE300, LE450, RI300, RI450, WH300, WH450, MA300, MA450, NU300, NU450, WD300, WD450, CH300, CH450, SW300 and SW450). PLABs could be classified as grass-like biochars (GBs) including the biochars produced from CO, PO, LE, RI, WH, and MA and

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wood-like biochars (WBs) including the biochars obtained from the NU and WD. The biochars produced at low and high temperature (300 and 450°C, respectively) were named as LTBs and HTBs, respectively.

Characterization of biochars. The bulk C, H, N and O contents of the biochars were determined using an Elementar Vario ELIII elemental analyzer (Elementar Company, Germany). Ash content was determined by heating biochars at 750°C for 4 h. The microporosity and cumulative surface area (CO2-SA) of biochars were obtained by Autosorb-1 gas analyzer (Quantachrome Instrument Corp., Boynton Beach, FL) using CO2 isotherm at 273 K49. Surface elemental composition (e.g., C, O, N, and Si) as well as the carbon-based functional group distribution ofbiochars were examined by using X-ray photoelectron spectroscopy (XPS) with a Kratos Axis Ultra electron spectrometer using monochromated Al Ka source operated at 225 W, including an elemental survey scan and a high-resolution scan at the C1s edge50. Fourier transform infrared (FTIR) spectroscopy spectra were recorded by a Nexus 670 FTIR spectrophotometer (Thermo Nicolet Corporation, US). The 13C nuclear magnetic resonance (13C-NMR) spectra of biochars were obtained by using CPMAS (solid-state cross-polarization magic angle spinning) technique. More detailed information about the characterization procedures of FTIR, 13C-NMR, and XPS as well as the chemical shift assignments of 13C-NMR was described in detail elsewhere14.

Sorption experiments of DBP and PHE by biochars. A batch equilibration technique was used to obtain the sorption isotherms ofDBP and PHE to 20 biochars. The stock solution was diluted sequentially in background solution which containing 200 mg L"1 NaN3 (as bio-inhibitor) and 0.01 M CaCl2 (as ionic strength adjuster) to 10 different concentrations (100-10,0000 mg L"1 for DBP, and 2-1100 mg L"1 for PHE). The sorption experiments were conducted using 15 ml and 40 ml glass vials with Teflon-lined screw caps. After shaking at 100 rpm for about 1 h, the test solution of DBP or PHE was added to vials with appropriate amount of biochars until the head space was kept minimal to avoid any potential vapor loss. The amount of biochars was selected to obtain 20-80% uptake of initially added DBP or PHE at equilibrium. All vials were sealed and continuously shaken at room temperature (23°C) for 7 d (DBP) and 10 d (PHE), respectively, according to the preliminary test. All experiments including the blanks were conducted in duplicates. In addition, blank experiments showed that the loss of DBP and PHE in both systems were insignificant. Hence, uptakes of DBP and PHE by biochars were calculated by mass balance.

Detection of solutes. After being shaken on the rotary shaker for 7 d or 10 d at room temperature, the vials were centrifuged at 3000 rpm for 30 min, then 2 mL supernatant was transferred from each sample and added to the 2 mL vial for analyzing the sorbate concentration in solution phase by HPLC (1260 Series, Agilent Technolohies, Santa Clara, CA) equipped with reversed-phase C18 analytical column (5 mm, 250 mm X 4.6 mm). DBP was detected by a UV detector with the detection wavelength set at 228 nm. The mobile phase consisted of 80% acetonitrile and 20% ultrapure water at a flow rate of 1 mL min"1. Low concentration of PHE was analyzed with a fluorescence detector with excitation wavelength set at 250 nm and emission wavelength set at 364 nm. UV detector at 250 nm was applied to determine the concentrations which were higher than 50 mg L"151. The mobile phase consisted of 90% methanol and 10% ultrapure water at a flow rate of0.8 mL min"1. The injection volume was 20 mL, and temperature of the column was 35°C.

Statistical analysis. The detected and calculated biochars-sorbed DBP/PHE (qe) and freely-dissolved DBP/PHE (Ce) concentrations could form sorption curves and the sorption curves were fitted with Freundlich model (logarithmic form):

log qe = log Kf Z n log Ce (1)

where qe (mg g"1) is the solid-phase concentration and Ce (mg L"1) represents liquidphase free concentration. Kf ((mg g"1)/(mg L"1)n) is the sorption affinity-related coefficient, and n is the nonlinearity factor. The quality of the fitting was determined using SigmaPlot 10.0 and statistical analysis (Pearson correlation analysis and t-test) was performed using SPSS 18.0.

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Acknowledgments

This research was supported by National Natural Science Foundation of China (41273106), USDA Hatch Program (MAS 00982), Beijing Higher Education Young Elite Teacher Project, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Author contributions

K.S., J.J. and M.Q. designed the experiments, J.J. and Z.W. performed the sorption experiments, J.J., L.H. and Y.Y. conducted characterization, M.Q., B.G. and K.S. analyzed the data, M.Q., K.S., F.W. and B.X. co-wrote the paper. All authors reviewed the manuscript.

Additional information

Supplementary information accompanies this paper at http://www.nature.com/ scientificreports

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Qiu, M.Y. et al. Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene. Sci Rep. 4, 5295; DOI:10.1038/srep05295 (2014).

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