Comparative study on the relative significance of low-/high-condensation aromatic moieties in biochar to organic contaminant sorption.

Aromatic moieties of biochar are considered as key components for immobilizing hydrophobic organic contaminants in the environment. However, the relative importance of different aromatic moieties such as low-/high-condensation components in sorption has not been comprehensively investigated. In this study, biochar was produced from flue-cured tobacco straw (TB) and pine wood sawdust (WB) at various pyrolysis temperatures (200-600 °C). Aromatic moieties were characterized via elemental analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and benzene polycarboxylic acid molecular markers (BPCAs). The significance of different aromatic moieties in the sorption of phenanthrene (PHE) and bisphenol A (BPA) was assessed based on the individual BPCA patterns. The results indicated that aromaticity and aromatic moiety contents increased with increasing pyrolysis temperature. Biochar at 200 °C produced lower mellitic acid (B6CA) contents (18.7-27.9%) than the others. When the pyrolysis temperature was increased to 600 °C, the B6CA contents representing high-condensation aromatic moieties accounted for 55.4-60.9% of all the aromatic moieties. The unitary linear regressions between the individual BPCA distribution patterns and the n values and log Kd suggested that the high-condensation aromatic moieties played a more significant role than the low-condensation aromatic moieties (represented by B3CA-B5CA) in facilitating sorption nonlinearity (for PHE and BPA) and sorption capacity (for PHE). The elevated sorption of PHE can be attributed to the increased specific surface area and hydrophobicity of the newly formed aromatic moieties. Hydrogen bonds and π-π electron-donor-acceptor were the main mechanisms of BPA sorption. Because the WB biochar contained more aromatic moieties and more O-containing groups on the surface of the TB biochar, the WB exhibited a higher sorption for PHE; however, slightly elevated sorption was observed on the TB for BPA. This research may provide a new perspective in understanding the behavior of biochar aromatic moieties in sorption of organic contaminants.

Black carbon accrual during 2000 years of paddy-rice and non-paddy cropping in the Yangtze River Delta, China.

Rice straw burning has accompanied paddy management for millennia, introducing black carbon (BC) into soil as the residue of incomplete combustion. This study examined the contribution of BC to soil organic matter and the rate at which it accumulates in paddy soils as a result of prolonged paddy management. Soil depth profiles were sampled along a chronosequence of 0-2000 years of rice-wheat rotation systems and adjacent non-paddy systems (50-700 years) in the Bay of Hangzhou (Zhejiang province, China). The soil BC content and its degree of condensation were assessed using benzene-polycarboxylic acids (BPCAs) as geochemical markers. The results showed that despite regular long term BC input, BC only contributed 7-11% of total soil organic carbon (SOC) in the topsoil horizons. Nevertheless, along with SOC, paddy soils accumulated BC with increasing duration of management until 297 years to reach a steady-state of 13 t BC ha(-1). This was 1.8 times more than in non-paddy soils. The fate of BC in paddy soils (0-1 m) could be modeled revealing an average annual input of 44 kg ha(-1) yr(-1), and a mean residence time of 303 years. The subsoils contributed at least 50% to overall BC stocks, which likely derived from periods prior to land embankment and episodic burial of ancient topsoil, as also indicated by BPCA pattern changes. We conclude that there is a significant but limited accumulation of C in charred forms upon prolonged paddy management. The final contribution of BC to total SOC in paddy soils was similar to that in other aerobic ecosystems of the world.

Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment.

Pyrogenic organic matter (PyOM) decomposes on centennial timescale in soils, but the processes regulating its decay are poorly understood. We conducted one of the first studies of PyOM and wood decomposition in a temperate forest using isotopically labeled organic substrate, and quantified microbial incorporation and physico-chemical transformations of PyOM in situ. Stable-isotope (¹³C and ¹5N) enriched PyOM and its precursor wood were added to the soil at 2 cm depth at ambient (N0) and increased (N+) levels of nitrogen fertilization. The carbon (C) and nitrogen (N) of added PyOM or wood were tracked through soil to 15 cm depth, in physically separated soil density fractions and in benzene polycarboxylic acids (BPCA) molecular markers. After 10 months in situ, more PyOM-derived C (>99% of initial 13C-PyOM) and N (90% of initial ¹5N-PyOM) was recovered than wood derived C (48% of 13C-wood) and N(89% under N0 and 48% under N+). PyOM-C and wood-C migrated at the rate of 126 mm yr ⁻¹ with 3-4% of PyOMC and 4-8% of wood-C recovered below the application depth. Most PyOM C was recovered in the free light fraction(fLF) (74%), with 20% in aggregate-occluded and 6% in mineral associated fractions ­ fractions that typically have much slower turnover times. In contrast, wood C was recovered mainly in occluded (33%) or dense fraction (27%).PyOM addition induced loss of native C from soil (priming effect), particularly in fLF (13%). The total BPCA-C content did not change but after 10 months the degree of aromatic condensation of PyOM decreased, as determined by relative contribution of benzene hexa-carboxylic acid (B6CA) to the total BPCA C. Soil microbial biomass assimilated 6-10% of C from the wood, while PyOM contributions was negligible (0.14­0.18%). The addition of N had no effect on the dynamics of PyOM while limited effect on wood.

Characteristics of dissolved black carbon in riverine surface microlayer.

Black carbon (BC) is produced by the incomplete combustion of biomass and fossil fuels. The dissolved form of BC (DBC), which is transported through rivers into the oceans, is of great significance for the carbon cycling on the planet due to its refractory features. However, the characteristics and sources of DBC in riverine water are poorly constrained. Here, we analyzed DBC contents and stable carbon isotope (δ13C) signatures in surface microlayer (SML) from the upper, middle and lower reaches of Pearl River (PR) in the first study of its kind. The DBC contents (100.9 to 166.6 µg L-1) in SML were lower than the global average for riverine water following a trend of upper > middle > lower reaches in PR. The molecular markers of DBC (BPCAs) and their δ13C values showed no statistical differences between the sampling sites (p > 0.05), suggesting biomass burning as the dominant source.

Embedding of biochar in soil mineral fractions: Evidence from benzene polycarboxylic acids molecular biomarkers.

Investigators are debating on the positive and negative priming effects of biochar on native soil organic carbon (SOC), which is largely attributed to the technical barrier of identifying biochar contribution to the apparently measured SOC or mineralized CO2. We combined benzene polycarboxylic acids (BPCAs) molecular biomarkers and soil particle density fractionation to identify biochar contributions to the carbon content in three representative allitic soils in Yunnan. The soil-biochar mixture was incubated for one-month to avoid significant biodegradation of biochar. The results showed that BPCAs were mainly distributed in free light fractions (fLF) up to 87 % of the total BPCAs contents after one month incubation. Recognition of BPCAs in occluded light fractions (oLF) and heavy fractions (HF) suggested a significant interaction between biochar and soil mineral particles. In addition, the percentage of B6CA is comparable or even higher in HF than in fLF or oLF. Thus, biochar-mineral interactions may be an additional stabilization mechanism besides the condensed aromatic structures in biochar. The apparently measured carbon contents increased after biochar application, and both positive and negative priming effects to native SOC were observed after deducting biochar contents based an accurate calculation from BPCAs. The most native SOC depletion (positive priming effects) was noted for the soil with the most favored biochar embedding in soil mineral compositions. This study emphasized that combining BPCAs molecular biomarkers and soil particle density fractionation could accurately quantify different carbon pools, and thus facilitate a comprehensive understanding on the stabilization and turnover of biochar in soils.

Benzene polycarboxylic acids as molecular markers of black carbon: Progresses and challenges.

Black carbon (BC) is generated as a result of the pyrolysis of biomass and fossil fuels. Different approaches have been taken to analyse BC in the environment, including thermal, optical and chemical methods. The chemical approach which uses benzene polycarboxylic acids (BPCAs) as molecular markers of BC has gained popularity within the scientific community recently. These pyrogenic molecular markers can be used to reconstruct ancient fire history and human presence. Here we review the development of the BPCA protocols for the analysis of BC and the previous studies that have used these methods. Additionally, this review explores the biogeochemical factors that influence the content and composition of BPCAs, which in turn affect the sources attributed to BC. These factors include the generation temperature of char, photodegradation, biodegradation and the interference of non-pyrogenic organic matter (OM) in BPCA-BC analysis. Different combustion temperatures can yield charred BC with varying degrees of aromatic condensation throughout the BC continuum, while aged soot-BC undergoes photochemical degradation, causing the loss of its original condensed aromatic structure. Photodegradation reduces the degree of BC condensation by preferentially breaking down the most condensed forms, whereas biodegradation primarily mineralizes the smaller and more biolabile BC. Non-pyrogenic sources, such as humic acids (HAs), have been found to contribute up to 25% of BPCA-BC in soil, and their presence can lead to overestimations of BC. Future research should focus on calibrating contemporary BPCA protocols using known reference materials and investigating the role of non-pyrogenic OM in BPCA-BC analysis.

The molecular markers provide complementary information for biochar characterization before and after HNO3/H2SO4 oxidation.

Biochar inevitably goes through long-term aging under biotic and abiotic processes in the environment, which results in various changes in its physicochemical properties. However, the traditional characterization methods based on particle separation cannot effectively monitor biochar in complex matrixes. Molecular markers, especially benzene polycarboxylic acids (BPCAs), can be used to directly identify the source and properties of biochar. In this study, biochars were prepared using corn straw (CS) and pinewood (PW) and were oxidized with HNO3/H2SO4 to simulate the aging processes. Molecular markers of lignin-derived phenols showed that PW has more vanillyl unit and thus more stable than CS. The overall BPCAs content and the relative content of mellitic acid (B6CA) both increased with pyrolysis temperature, indicating increased aromatic condensation/aromaticity. The pristine CS biochar has a higher BPCAs content compared to PW biochar. HNO3/H2SO4 treatment greatly decreased the lignin components and more vanillyl and cinnamyl units were removed from CS biochar than PW biochar. In addition, BPCAs contents decreased by 41-60 mg/g for CS biochar, while increased by 86-133 mg/g for PW biochar after HNO3/H2SO4 oxidation. This is owing to the release of the condensed aromatic structures in CS biochars, but the concentration of the condensed aromatic structures in PW biochars after oxidation. These results showed that PW biochars are more stable than CS biochars. The application of the molecular markers can help understanding the dynamic change of biochar in the environment.

Distribution of black carbon and PAHs in sediments of Peninsular Malaysia.

Concentrations, sources and interactions between black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) were investigated in 42 sediment samples collected from riverine, coastal and shelf areas in Peninsular Malaysia. The concentrations of BC measured by benzene polycarboxylic acid (BPCA) method and PAHs showed broad spatial variations between the relatively pristine environment of the East coast and developed environment of the West and South coast ranging from 0.02 to 0.36% dw and 57.7 ng g-1 dw to 19,300 ng g-1 dw, respectively. Among diagnostic ratios of PAHs, the ratios of Ant/(Ant+Phe) and LMW/HMW drew the clearest distinctions between the East coast versus the West and South coast sediments indicating the predominance of petrogenic sources in the former versus pyrogenic sources in the latter. PAHs significantly correlated with BC and total organic carbon (TOC) in the sediments (p < 0.05) having similar correlation coefficients. BC accounted for 6.06 to 30.6% of TOC in sediments.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids.

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

Preparative field flow fractionation for complex environmental samples: online detection by inductively coupled plasma mass spectrometry and offline detection by gas chromatography with flame ionization.

Asymmetric flow field flow fractionation (AF4) in particular online with elemental detection via inductively coupled plasma mass spectrometry (ICP-MS) has been developed as powerful and flexible separation technique for suspensions of nano- and micro-particles covering a broad range of applications including environmental water samples and soil extracts. However, for challenging applications, such as particulate phosphorus determination in non-contaminated water samples at levels close to the limit of detection the throughput of the analytical field flow fractionation (FFF) is not sufficient. The same holds true for more specific identification and quantification of black carbon (BC) which needs a subsequent complex multi-step analysis using the well-established benzene polycarboxylic acids (BPCA) method. To overcome these limitations, the performance of a commercially available preparative AF4 channel, which has rarely been applied, yet, was investigated in this study. Using the example of an extract from charcoal spiked soil, method development for the preparative channel was performed and the results from six replicate fractionations with multi-element online detection by ICP-MS were compared to the results from the analytical channel for the same extracts. A similar fractionation pattern was achieved and the quantitative results agreed well for most of the particulate fractions (ratio 1.7 with standard deviation (SD) 0.2 for fraction 1, ratio 0.81 with SD 0.14 for fraction 2 and ratio 1.1 with SD 0.2 for fraction 3). Relative standard deviations were in the range of 9% to 18% for the preparative channel and between 3% and 17% for the analytical channel. Transferability of the separation parameters between both channels is discussed as well as the operational challenges of the preparative channel. As proof of principle, preparative fractionation of an extract from charcoal spiked soil was performed with fraction collection and subsequent quantification of BC via the BPCA method including derivatization, cation exchange pre-cleaning and finally gas chromatographic separation and quantification via flame ionization detection. The results indicated the majority of detected BC in the often so-called dissolved fraction was bound to nanoparticles (48%) and colloids (27%). Only 25% was detected in the cross flow (truly dissolved fraction). This successful example opens new possibilities for hyphenation of FFF separation with multiple detection techniques for improved characterization of particulate matter in challenging applications.

Combining bulk characterization and benzene polycarboxylic acid molecular markers to describe biochar properties.

The physicochemical properties of biochar determined its sorption of organic contaminations, and the environmental aging process changed the biochar properties. However, the correlation between biochar heterogeneous properties and their sorption characteristics is unclear. In this study, peanut shell biochars were produced at 200-700 °C, and HNO3/H2SO4 was used to oxidize 400 °C biochar for 2-10 h to simulate the enhanced aging process of biochar in the environment. Benzene polycarboxylic acid (BPCA) molecular markers, and bulk characterization were analyzed to describe biochar physicochemical properties and to further predict the sorption characteristics to bisphenol A (BPA). For pristine biochars, the mellitic acid/BPCAs (B6CA/BPCAs) increased with the raise of pyrolysis temperature and the H/C atomic ratio was positively correlated with benzenepentacarboxylic acid/B6CA (B5CA/B6CA) (P < 0.01), which indicated the increased aromatic condensation. After HNO3/H2SO4 treatment, the aromaticity (H/C ratio) decreased while the highly condensed components in biochars were enriched (increased B6CA/BPCAs values). Multiple regression models were adopted to establish a quantitative relationship between biochar heterogeneous properties and their sorption of BPA. Both nonlinearity coefficient N values (N = 0.08 + 0.103 B5CA/B6CA + 0.721 (O + N)/C, R2 = 0.985) and single-point sorption coefficients log Kd (log Kd = 1.236 + 0.006 BPCAs + 1.449 (O + N)/C, R2 = 0.936) could be estimated combining molecular markers and polarity parameters for biochars.

[Determination and Source Apportionment of Aromatic Acids in PM2.5 from the Northern Suburb of Nanjing in Winter].

Atmospheric particulate samples of PM2.5 were collected from the northern suburb of Nanjing in December, 2014, and a LC-MS method was optimized for the determination of aromatic acids in PM2.5; The concentrations of major water-soluble ions, organic carbon and elemental carbon were also determined. The quantification results showed that the average total concentration of five aromatic acids we have determined was (50.01±16.05) ng·m-3, and the average concentrations of terephthalic acid, phthalic acid, trimellitic acid, 4-methylphthalic acid and iso-phthalic acid were (34.54±12.79)、(8.14±3.34)、(2.27±1.39)、(1.68±0.77) and (1.08±0.43) ng·m-3, respectively. The different source apportionments of atmospheric particulate were analyzed by principal component analysis/absolute principal component scores (PCA/APCS) receptor model. The results of source apportionment showed that the main sources of Phthalic acid, Trimellitic acid and 4-methylphthalic acid were mainly secondary transformation, and primary emissions such as vehicle exhaust contributed less to Trimellitic acid; Secondary transformation and biomass burning made the most significant contributions to iso-Phthalic acid and vehicle exhaust contributed less; The sources of Terephthalic acid were primary emissions such as biomass burning and vehicle exhaust.

Pyrogenic molecular markers: linking PAH with BPCA analysis.

Molecular characterization of pyrogenic organic matter (PyOM) is of great interest to understand the formation and behavior of these increasingly abundant materials in the environment. Two molecular marker methods have often been used to characterize and trace PyOM: polycyclic aromatic hydrocarbon (PAH) and benzenepolycarboxylic acid (BPCA) analysis. Since both methods target pyrogenic polycyclic compounds, we investigated the linkages between the two approaches using chars that were produced under controlled conditions. Rye and maize straws and their analogues charred at 300, 400 and 500 °C, respectively, were thus analyzed with both methods. Moreover, we also measured BPCAs directly on the lipid extracts, on which PAHs were analyzed, and on the respective extraction residues, too. Both methods revealed important features of the chars, in particular the increasing degree of aromatic condensation with increasing highest heating temperature (HTT). The overlap between the two methods was identified in the lipid fraction, where the proportion of benzenetricarboxylic acids (B3CAs) correlated with PAH abundance. The results confirmed the validity and complementarity of the two molecular marker methods, which will likely continue to play a crucial role in PyOM research due to the recent developments of compound-specific PAH and BPCA stable carbon (δ(13)C) and radiocarbon ((14)C) isotope methods.