Effect Of Pyrolysis Temperature On Characteristics Of Heavy Metals,polycyclic Aromatic Hydrocarbons And Dissolved Organic Matter In Different Macroalgal Biochars

The resource utilization of macroalgae has gradually come into people’s attention,and its preparation into biochar can be used as a potential important use.Despite the increasing attention to the application of macroalgal biochars,there are few reports on the potential risks of macroalgae biochar.In this study,Sargassum fusiforme,Sargassum thunbergii,Sargassum vachellianum of Phaeophyta,Grateloupia turuturu and Chondria crassicaulis of Rhodophyta,Ulva pertusa of Chlorophyta were used as raw materials of biochar.The contents of heavy metals,polycyclic aromatic hydrocarbons(PAHs)and dissolved organic matter(DOM)in macroalgal biochars prepared at different pyrolysis temperatures were analyzed,and the correlation between pyrolysis temperature and the characteristics of heavy metals,PAHs and DOM in different macroalgal biochars was discussed,in order to provide technical support for the application of macroalgal biochars in the environment.This paper mainly focuses on three aspects,as follows:(1)Macroalgal biochars were produced from Sargassum fusforme,Sargassum thunbergii,Sargassum vachellianum,Grateloupia turuturu,Chondria crassicaulis and Ulva pertusa at different pyrolysis temperatures(200,300,400,500 and 600℃)under limited-oxygen conditions.The effects of pyrolysis temperature on the yield,heavy metal(Fe,Mn,Zn,Cu,As,Cr,Cd,Pb and Hg)contents and their relative enrichment factors of different macroalgal biochars were investigated.The results showed that the yield of macroalgal biochar decreased with the increase of pyrolysis temperature.The contents of Fe,Zn,Mn,Cu,Cr and Pb in macroalgal biochars were significantly higher than those in their respective biomass.The As content in macroalgal biochars decreased first and subsequently increased with the increase of pyrolysis temperature,and the As content was not higher than that of raw biomass.The change trend of Cd content with the increase of pyrolysis temperature was related to macroalgae biomass.Hg wasn’t detected in the biochars.The contents of Zn,Cu,Cr,Pb and Hg in macroalgal biochars were all better than the basic quality grade stipulated by the European Biochar Foundation,but the contents of Cd and As in the partial biochars exceeded the threshold.In general,pyrolysis may promote the volatile migration of heavy metals in the macroalgal biochar.Furthermore,the correlation analysis showed that the heavy metal contents in the macroalgal biochars were critically dependent on the biomass and pyrolysis temperature.Cluster analysis demonstrated that the prepared biochars from the special macroalgae biomass could be approximately classified into the same cluster,which effectively reflected the differences in the characteristics of heavy metals in different macroalgal biochars.(2)PAHs were extracted from macroalgal biochars by Soxhlet extraction,and their contents were determined by gas chromatography-mass spectrometry(GC-MS).The toxicity characteristics of biochar were evaluated.The results showed that the amount of PAHs in macroalgal biochars first increased and then decreased with the increase of pyrolysis temperature.The total amount of PAHs in the biochar prepared at 600℃ was the lowest with a mass fraction of 78.2 μg/kg,while the total amount of PAHs in the biochar prepared at 300℃ was the highest with a mass fraction of 2243.1 μg/kg.It is easier to produce 2-ring and 3-ring PAHs in macroalgal biochars at low temperature,and it is easier to produce 4-ring and 5-ring PAHs at high temperature.When the pyrolysis temperature is 200-500℃,the macroalgal biochars were mainly composed of 2-ring and 3-ring PAHs.At the pyrolysis temperature of 600℃,PAHs in macroalgal biochars were mainly 4-ring PAHs,and 6-ring PAHs were not detected.In the pyrolysis temperature range of 200-600℃,the toxic equivalent concentration of benzo[a]pyrene(TEQBap)of macroalgal biochars first increased and then decreased with the pyrolysis temperature,and the TEQBaP of Chondria crassicaulis biochar prepared at 600℃ was the lowest(0.1959 μg/kg).While TEQBaP of Sargassum vachellianum biochar prepared at 500℃ was the highest(2.3336μg/kg).Among the biochar prepared at different temperatures,4-ring and 5-ring PAHs contributed significantly to TEQBAP.Then the pyrolysis temperature is 600 ℃,the content of PAHs in macroalgal biochar is low,and the equivalent concentration of PAHs is small.In this study the macroalgal biochar all belongs to the safe and high-quality biochar stipulated by the European Biochar Foundation,Therefore,macroalgal biochar at the most appropriate temperature can be selected according to different application fields and applied in practice as soil amendments and wastewater adsorbents.(3)The effects of pyrolysis temperature and pH on DOM content and spectral characteristics of macroalgal biochars under sterile conditions were studied by using organic carbon analysis,three-dimensional fluorescence spectroscopy(EEM)and PARAFAC technology.The results showed that the concentration of dissolved organic carbon(DOC)of macroalgal biochar was related to biomass feedstock,pyrolysis temperature and solution pH.In the pyrolysis temperature range of 200-500℃,DOC concentrations of all macroalgal biochars decreased with the increase of pyrolysis temperature.The EEM spectra of biochar DOM were analyzed by PARAFAC,and there were six main fluorescence components,including five humus components and one protein component.The fluorescence intensity and distribution of the six components depend on biomass,pyrolysis temperature and solution pH.