Polycyclic Aromatic Hydrocarbons In Biochar Research Articles

Occurrence, distribution, and toxicity assessment of polycyclic aromatic hydrocarbons in biochar, biocrude, and biogas obtained from pyrolysis of agricultural residues

Occurrence, distribution, and toxicity assessment of polycyclic aromatic hydrocarbons (PAHs) in pyrolysis steams (biochar, biocrude, and biogas) of three agricultural residues was investigated at pyrolysis temperatures of 400–800 °C. Increasing PAHs formation was observed in the narrow temperature range (500–600 °C) in all feedstocks due to temperature-induced dehydration, decarboxylation, and dehydrogenation reactions. Low molecular weight PAHs (naphthalene, phenanthrene) were dominant in all product streams while high molecular weight PAHs were found in negligible concentrations. Leaching studies showed that pyrolyzed biochars produced at lower temperatures are more prone to leaching due to the presence of hydrophilic amorphous uncarbonized structures, while the presence of hydrophobic carbonized matrix with denser and stronger polymetallic complex prevents the leaching of PAHs in the high temperature pyrolyzed biochar. Low leaching potential, low toxic equivalency, and permissible total PAHs values in biochar derived from all three feedstocks warrant the broader application and ensure ecological safety.

Pyrolysis processes affecting polycyclic aromatic hydrocarbon profile of pineapple leaf biochar exemplified by atmosphere/temperature and heteroatom doping

The content of polycyclic aromatic hydrocarbons in pineapple leaf biochar was examined as a function of pyrolysis atmosphere (CO2 or N2), pyrolysis temperature (300–900 °C), and heteroatom (N, B, O, P, NP, or NS) doping. Without doping, the polycyclic aromatic hydrocarbon production was maximal (1332 ± 27 ng/g) in CO2 at 300 °C and minimal (157 ± 2 ng/g) in N2 at 700 °C. The main components naphthalene and acenaphthylene accounted for about 91% of the total polycyclic aromatic hydrocarbon in the biochar prepared under CO2 at 300 °C. Under the maximal polycyclic aromatic hydrocarbon production conditions (CO2, 300 °C), doping decreased the total hydrocarbon content by 49% (N), 61% (B), 73% (O), 92% (P), 93% (NB), and 96% (NS). The results shed new light on the management of polycyclic aromatic hydrocarbons in BC production by controlling the pyrolysis atmosphere and temperature in addition to heteroatom doping. Results significantly contributed to the development of circular bioeconomy.

Composition of PAHs in Biochar and Implications for Biochar Production.

The content of polycyclic aromatic hydrocarbons (PAHs) in biochar has been studied extensively; however, the links between biomass feedstock, production process parameters, and the speciation of PAHs in biochar are understudied. Such an understanding is crucial, as the health effects of individual PAHs vary greatly. Naphthalene (NAP) is the least toxic of the 16 US EPA PAHs but comprises the highest proportion of PAHs in biochar. Therefore, we investigate which parameters favor high levels of non-NAP PAHs (∑16 US EPA PAHs without NAP) in a set of 73 biochars. On average, the content of non-NAP PAHs was 9 ± 29 mg kg–1 (median 0.9 mg kg–1). Importantly, during the production of the biochars with the highest non-NAP PAH contents, the conditions in the post-pyrolysis area, where pyrolysis vapors and biochar are separated, favored condensation and deposition of PAHs on biochar. Under these conditions, NAP condensed to a lower degree because of its high vapor pressure. In biochars not contaminated through this process, the average non-NAP content was only 2 ± 3 mg kg–1 (median 0.5 mg kg–1). Uneven heat distribution and vapor trapping during pyrolysis and cool zones in the post-pyrolysis area need to be avoided. This demonstrates that the most important factor yielding high contents of toxic PAHs in biochar was neither a specific pyrolysis parameter nor the feedstock but the pyrolysis unit design, which can be modified to produce clean and safe biochar.

Co-composting of biochar and nitrogen-poor organic residues: Nitrogen losses and fate of polycyclic aromatic hydrocarbons

Composting recycles nutrients and biodegrades organic pollutants, but often results in N leaching. Biochar can enhance the composting process and reduce N losses. Research, however, has focused on composting N-rich residues; also, information on the fate of biochar polycyclic aromatic hydrocarbons (PAHs) during composting is scarce. We explored the composting of biochar with N-poor organic residues as a strategy to reduce N losses and biochar PAHs. A small-scale composting experiment was performed with three treatments: 100% yard residues and two mixtures of 85% yard residues and 15% gasification- or pyrolysis-derived biochar. Temperatures were recorded daily during composting and Nlosses and changes in PAHs loads were calculated. Results across all treatments showed overall low N losses, likely caused by low temperatures and N contents, circumneutral pH values, and absence of leachate, and simultaneous immobilization and mineralization of PAH contents. Treatments with biochar showed a slower release of inorganic N (NO3--N and NH4+-N), although they also had overall lower inorganic N contents. This slower release of inorganic N may relate to biochar’s high surface area. We conclude that biochar provides valuable benefits for N-poor composting, and that composting should be further explored as a promising strategy to reduce the contents of PAHs in biochar.

Microbial Ecotoxicity of Biochars in Agricultural Soil and Interactions with Linear Alkylbenzene Sulfonates

Large-scale application of biochar on agricultural land offers the prospect of soil improvement and carbon sequestration for climate-change mitigation. However, negative side-effects on the soil microbial ecosystem are poorly understood, notably in relation to the functions of native microbiomes under realistic routes of biochar exposure. Due to divergent properties, different biochars might interact with soil in complex ways. This might result in decreased or increased ecotoxicity from biochar contaminants, such as heavy metals and polycyclic aromatic hydrocarbons (PAHs). Using five biochars produced from straw and wood under contrasting pyrolysis conditions, we traced their ecotoxicological dose-effect using a bioassay for potential ammonia oxidation (PAO), through microorganisms that are sensitive stress indicators. Assays were made after soil/biochar interaction for up to 3 weeks, where straw biochar with the lowest PAH content (<0.5 mg kg−1) showed the most pronounced dose-effects to PAO, corresponding to a 10% effect concentration (EC10) of 4.6% (dry weight biochar/dry weight soil). In comparison, straw biochar with the highest PAH content was least ecotoxic (EC10, 15.2% after 3 weeks) and wood biochars pyrolysed at high temperature (700–725 °C) showed no ecotoxicity to PAO. Interactions between biochars and anionic surfactants, i.e., linear alkylbenzene sulfonates, which are common soil pollutants, resulted in varying effects on PAO, but the effects were small and of limited ecological importance for soil-amended biochars. In conclusion, the results showed that short-term microbial side-effects of biochar in the soil ecosystem were minor at relevant field application rates (such as <30 Mg ha−1 mixed into a plough layer of 20 cm), and that inherent PAHs in biochar were not a likely source of short-term ecotoxicity. However, there were notable differences in the effects eventually observed at very high biochar rates, stressing that individual biochars need specific ecotoxicological assessment before their safe application at large scale in agricultural soils.

Transformation characteristics of polycyclic aromatic hydrocarbons during hydrothermal liquefaction of sewage sludge

Transformation characteristics of polycyclic aromatic hydrocarbons (PAHs) during hydrothermal liquefaction (HTL) of sewage sludge are explored in this study. PAHs are generated in the range of 7.3–13.8 mg/kg during HTL of sludge. The increase of reaction time (0–60 min) favors the formation of PAHs; however, the reaction temperature and solid–liquid ratio exhibit a critical value (300 °C and 0.1 g/mL, respectively), at which, the amounts of PAHs synthesized are the lowest. Most PAHs (> 89%) are distributed in bio-oil. Compared with those of raw sludge, the total/leachable contents and toxic equivalent quantity (TEQ) of PAHs in biochar are all reduced, suggesting a lower environmental risk for biochar in view of PAH hazard. Notably, the total contents of PAHs in bio-oil reach 55.0–106.6 mg/kg, with high TEQ values of 6.1–8.6 mg/kg. The liquefaction process should be further improved to reduce the content and toxicity of PAHs in bio-oil.

Suppressed formation of polycyclic aromatic hydrocarbons (PAHs) during pyrolytic production of Fe-enriched composite biochar

The pyrolytic production of Fe-enriched composite biochar is receiving increasing attention. However, understanding of the environmental risk from the polycyclic aromatic hydrocarbons (PAHs) potentially generated during composite biochar production is lacking. This study investigated the formation of PAHs from the pyrolysis of barley straw impregnated with FeCl3 or Fe(NO3)3 at 350 °C, 500 °C, and 650 °C. The total amount of PAHs formation increased with increasing heating temperature. Most of the PAHs were concentrated in bio-oil (72.7–94.6%), with only a small fraction retained in biochar (1.7–11.1%) and in biogas (2.2–16.2%). Preloading FeCl3 or Fe(NO3)3 onto the biomass greatly reduced PAH formation by up to 33% and 21%, respectively, compared to that obtained with biomass alone. The suppressed formation of PAHs was due to the generation of more reductive forms of Fe, such as Fe0 and FeO, in the O2-starved pyrolysis atmosphere, which reduced C2H2 and C6H5OH, two important PAH precursors in hydrogen abstraction acetylene addition reactions. Although Fe loading reduced the amounts of PAHs in biochar, the toxic equivalent value increased because Fe induced more accumulation of high-molecular-weight PAHs in the biochar. This study proved that Fe loading suppresses PAH generation during biomass pyrolysis, which can guide the design of composite biochar production.

Effect of pyrolysis conditions on the total contents of polycyclic aromatic hydrocarbons in biochars produced from organic residues: Assessment of their hazard potential

The interest of using biochar, the solid byproduct from organic waste pyrolysis, as soil conditioner is significantly increasing. Nevertheless, persistent organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), are formed during pyrolysis due to the incomplete combustion of organic matter. Consequently, these pollutants may enter the environment when biochar is incorporated into soil and cause adverse ecological effects. In this study, we examined the content of the 16 United States Environmental Protection Agency (USEPA) PAHs in biochars produced from rice husk, wood, wheat and sewage sludge residues using three different pyrolytic reactors and temperatures (400, 500 and 600 °C). The total concentration of PAHs (∑PAH) ranged from 799 to 6364 μg kg−1, being naphthalene, phenanthrene and anthracene the most abundant PAHs in all the biochars. The maximum amount of PAHs was observed for the rice husk biochar produced in the batch reactor at 400 °C, which decreased with increasing temperature. The ∑PAH value of the wood biochar produced via traditional kilns doubled compared with the wood biochar produced using the other pyrolytic reactors (5330 μg kg−1 in Kiln; 2737 μg kg−1 in batch and 1942 μg kg−1 in the rotary reactor). Looking for a more reliable risk assessment of the potential exposure of PAHs in biochar, the total toxic equivalent concentrations (TTEC) of the 14 produced biochars were calculated. When comparing the same feedstock and temperature, TTEC values indicated that the rotary reactor produced the safest biochars. In contrast, the biochars produced using the batch reactor at 400 and 500 °C have the greatest hazard potential. Our results provide valuable information on the potential risk of biochar application for human and animal health, as well as for the environment due to PAHs contamination.

Polycyclic aromatic hydrocarbons and volatile organic compounds in biochar and biochar‐amended soil: a review

AbstractResidual pollutants including polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), and carbon (aceous) nanoparticles are inevitably generated during the pyrolysis of waste biomass and remain on the solid coproduct called biochar. Such pollutants could have adverse effects on the plant growth as well as microbial community in soil. Although biochar has been proposed as a ‘carbon negative strategy’ to mitigate the greenhouse gas emissions, the impacts of its application with respect to long‐term persistence and bioavailability of hazardous components are not clear. Moreover, the co‐occurrence of low molecular weight VOCs with PAHs in biochar may exert further phytotoxic effects. This review describes the basic need to unravel key mechanisms driving the storage vs. emission of these organics and the dynamics between the sorbent (biochar) and soil microbes. Moreover, there is an urgent need for standardized methods for quantitative analysis of PAHs and VOCs in biochar under environmentally relevant conditions. This review is also extended to cover current research gaps including the influence of biochar application on the short‐ and long‐term fate of PAHs and VOCs and the proper control tactics for biochar quality and associated risk.

High concentrations of polycyclic aromatic hydrocarbons (naphthalene, phenanthrene and pyrene) failed to explain biochar's capacity to reduce soil nitrous oxide emissions

The presence of polycyclic aromatic hydrocarbons (PAHs) has been postulated as a mechanism by which biochar might mitigate N2O emissions. We studied whether and to what extent N2O emissions were influenced by the three most abundant PAHs in biochar: naphthalene, phenanthrene and pyrene. We hypothesised that biochars contaminated with PAHs would show a larger N2O mitigation capacity and that increasing PAH concentrations in biochar would lead to higher mitigation potentials. Our results demonstrate that the high-temperature biochar (550 °C) had a higher capacity to mitigate soil N2O emissions than the low-temperature biochar (350 °C). At low PAH concentrations, PAHs do not significantly contribute to the reductions in soil N2O emissions; while biochar stimulated soil N2O emissions when it was spiked with high concentrations of PAHs. This study suggests that the impact of biochar on soil N2O emissions is due to other compositional and/or structural properties of biochar rather than to PAH concentration.

Environmental contextualisation of potential toxic elements and polycyclic aromatic hydrocarbons in biochar

Nine dissimilar biochars, produced from varying feedstock at different pyrolysis temperatures, are appraised with respect to concentrations of potentially toxic elements, specifically, metals, metalloids and polycyclic aromatic hydrocarbons (PAHs). Concentrations of the metals and metalloids varied with the following ranges (mg kg−1): 0.02–0.94, Cd; 0.12–6.48, Cr; 0.04–13.2, Cu; 0.1–1.37, Ni; 0.06–3.87, Pb; 0.94–207, Zn and 0.03–0.27, As. Σ16PAH concentrations (16 Environmental Protection Agency (EPA) PAHs) range between 0.08 mg kg−1 to 8.7 mg kg−1. Subsequent comparison with background soil concentrations, concentration applied to the regulation of composted materials (Publicly Available Specification (PAS 100)) and European Union (EU) regulations relating to the application of sewage sludge to agricultural land suggest low risk associated with the concentrations of PTEs observed in biochar. Collectively, results suggest that environmental impacts attributable to metals, metalloids and PAHs associated with biochar following its application to soil are likely to be minimal.

Quantitative Determination of PAHs in Biochar: A Prerequisite To Ensure Its Quality and Safe Application

Biochar is increasingly promoted as a beneficial soil conditioner. However, it may contain residues of polycyclic aromatic hydrocarbons (PAHs) as a result of its production by pyrolysis. To date, analytical methods to analyze PAHs in biochar quantitatively are hardly available. This study presents an optimized and validated procedure to quantify the 16 U.S. EPA PAHs in biochar. PAHs were best extracted with Soxhlet for 36 h using 100% toluene. Average absolute recoveries of isotope labeled internal standards used for each analyte from three different biochars ranged from 42% to 72%, and relative recoveries were between 71% and 105%. The limits of detection were biochar-dependent, but on average a factor of >50 lower than quantified PAH concentrations (9-355 mg kg(dry weight)(-1)). The established method prepares the ground for a harmonized protocol for PAH analysis of biochars, a necessity for biochar quality control, registration, and legislation.