pyrolysis and gasification are thermochemical processes that treat biomass feedstock 
 with heat 
 to produce intermediates that can be converted to liquid fuels with 
 the aid of catalysts . in 
 addition to these intermediates , the trace elements in biomass , 
 such as potassium , sulfur , phosphorus , and chlorine , are also partitioned 
 between gas and solid ( biochar and ash ) phases . biochar has been reported 
 to act as a carbon sink , as a sorbent to remove contaminants , and as a way to improve crop production . in fact , a primary reason for studying the forms of sulfur in biochars 
 is that there is significant interest in using biochar for soil augmentation , 
 and because plants need sulfur , an understanding of the quantity and 
 speciation of sulfur in biochar is of interest to the agricultural 
 and environmental community . sulfur is an essential , growth - limiting 
 plant nutrient ; it is required 
 for protein synthesis , occurs in the form of sulfolipids in photosynthetic 
 membranes , and affects the use efficiency 
 of other plant nutrients . numerous studies have shown indirectly that the addition of biochars 
 to soils could increase soil sulfur bioavailability . however , direct information or direct studies on the impact 
 of biochar on sulfur availability and transformations in soils is 
 scarce . a recent study on sulfur mineralization 
 of plant litter and corn stover biochar by blum et al . found that 
 speciation rather than total sulfur is likely the major factor that 
 influences sulfur mineralization in soil . this research finding indicates the need for quantitative determination 
 of sulfur speciation in biochars produced from different feedstocks 
 under different conditions . there is empirical evidence that 
 the temperature at which the biochar 
 material is formed affects its potential nutrient contribution or 
 its function as a soil conditioner and driver of nutrient transformation . this may be related to changes in the composition 
 and speciation of the biochar as the temperature of biochar production 
 changes . for example , knudsen et al . found that , as the temperature 
 of thermal conversions of agricultural residues increased from 500 
 to 800 c , the biochar that is produced contains less sulfate 
 but more sulfur that is in a form that can not be extracted with acid . they hypothesized that the fraction that is 
 not extractable is organically associated , i.e. , bound to carbon in 
 the biochar matrix . another main 
 reason for our interest in direct measurement of sulfur 
 speciation in biochar is because gas - phase sulfur that is released 
 during biomass conversion is often in the form of hydrogen sulfide and organosulfur that can deactivate catalysts 
 used for downstream biofuel production . the factors that control the 
 relative partitioning of sulfur in gas versus biochar are not well - understood . consequently , an understanding of biochar speciation may suggest a 
 potential sulfur transformation pathway , which may in turn support 
 the purposeful development of strategies to sequester as much of the 
 sulfur in the solid biochar form as possible . x - ray absorption 
 near - edge structure ( xanes ) is an element - specific 
 method that can provide direct speciation information , including the 
 oxidation state and the chemical environment around a specific element 
 of interest . it has been used for determining the speciation of sulfur 
 in petroleum , soil , and natural environments . in our previous study to determine the speciation of sulfur in deactivated 
 catalyst , sulfur - xanes was able to differentiate 
 sulfur species in samples where sulfur may not have existed in crystalline 
 form to have x - ray diffraction patterns or where there was insufficient 
 surface sulfur for x - ray photoelectron spectroscopy characterization . in this research , biochars produced from different feedstocks at 
 different thermochemical conversion conditions were quantitatively 
 analyzed and then studied with xanes . where applicable , wet chemical 
 measurements of sulfate content and scanning electron microscopy energy - dispersive 
 spectroscopy ( sem  eds ) imaging were also used to provide a 
 second method to determine sulfur speciation and spatial distribution 
 of sulfur in the biochar . pyrolysis and gasification experiments 
 were conducted in a pilot - scale facility at the national renewable 
 energy laboratory ( nrel ) . the details of the fluid bed gasification 
 experiments were described in a previous publication . briefly , pelletized biomass ( 0.25 in . in diameter and 0.253 
 in . in length ) was fed through a crusher to reduce the particle size 
 to less than 2.3 mm in diameter . the resulting feed particles were 
 transported directly into an entrained flow reactor , where they were 
 either pyrolyzed in nitrogen at 500600 c or gasified 
 in steam at 850 c . the oak 850 c gasification experiments 
 were conducted in a slightly different manner : the oak particles were 
 fed to a 20.3 cm ( 8 in . ) olivine sand 
 in the reactor was fluidized by steam and kept at 700 c . the 
 vapors and biochar produced from the fluidized - bed reactor flowed 
 through an entrained flow reactor for secondary cracking and gasification 
 at 850 c . the ash and biochar produced were separated 
 from the pyrolysis vapor or syngas stream by cyclonic separators and 
 then cooled , collected , and weighed . to obtain representative samples , resulting samples were then 
 ground and analyzed for sulfur content and speciation . for simplicity , in the rest of this 
 paper , the biochars will be named with the feedstock followed by the 
 temperature in which it was produced . total sulfur contents 
 of the feedstocks and biochars were measured with a leco truspec sulfur 
 add - on module . the measurement procedure involves combustion of the 
 material at 1350 c in 99.5% oxygen ( to break down all sulfur - bearing 
 material and to oxidize the sulfur to so2 ) followed by 
 infrared measurement of so2 . we used a 
 wet chemical method similar to that described in astm d2492 for sulfate 
 analysis . astm method d2492 is a method used for determining sulfate 
 and sulfide forms of sulfur in coal . the 
 sulfate portion of the sample is extracted with hot dilute hydrochloric 
 acid because sulfates are soluble in hydrochloric acid , while sulfidic 
 and organic sulfur are not . afterward , the sulfur in the extract is 
 determined gravimetrically as a baso4 precipitate . because 
 the method does not assume any sequestration agent for sulfate in 
 the sample , we concluded that the sulfate portion of the method is 
 applicable for biochar . we did not use astm method d2492 for 
 sulfide determination because the determination of the sulfide form 
 of sulfur in the method is achieved by extraction with dilute nitric 
 acid , followed by determination of the iron content of the extract . in other words , the inherent assumption in the 
 method is that sulfide in coal exists as pyrite ; therefore , the concentration 
 of sulfide is dependent upon the iron content . because the concentrations 
 of major elements are significantly different between biochar and 
 coal ( e.g. , coal generally has higher fe but much lower potassium than the biochars of this study ) , the assumption 
 of  all sulfide exists as pyrite  most likely does not 
 hold for biochar . sulfur k - edge xanes of biochar 
 was performed 
 at beamline 4 - 3 of the stanford synchrotron radiation lightsource . the spectra were collected in the energy range of 24402600 
 ev using a four - element si drift detector . step size was 0.1 ev for 
 the energy range of 24602483 and 2 ev outside of that range . energy calibration was conducted using the first inflection of sodium 
 thiosulfate , with the first white line of thiosulfate accurately determined 
 as 2472.02 ev in a previous study . merging 
 of individual scans and spectra normalization were performed with 
 sam s interface for xas package ( sixpack ) . we used minisem evex 
 to conduct sem 
 and evex nanoanalysis module to conduct eds . for data collection , 
 a biochar sample was mounted on carbon tape and observed under magnifications 
 varying from 500 to 15000. surface structure images of 
 the biochar were captured using sem . the nine elements chosen for 
 mapping are sulfur , carbon , oxygen , magnesium , aluminum , silicon , 
 calcium , potassium , and chlorine . the eds in our equipment has 
 a detection limit of approximately 
 1000 ppm . because of the low amount of sulfur in the samples , eds 
 is useful only for the biochar containing the highest amount of sulfur , 
 which is the corn stover 500 c sample . table 1 shows total sulfur contents as well as proximate , 
 ultimate , and elemental analyses of feedstocks and biochars . the proximate 
 and elemental analysis data are not available for oak 850 c 
 biochar because that biochar is mixed with olivine . during gasification , fine olivine dust ( derived from the olivine fluidized - bed material ) 
 is occasionally elutriated or blown out into the cyclones and mixes 
 with biochars . because olivine is a magnesium iron silicate , the resultant 
 ash analysis has unusually high magnesium and iron and the elemental 
 analyses of the ash are not accurate . however , we have conducted xanes 
 analysis of the olivine and confirmed that it has practically zero 
 sulfur content and does not affect the sulfur speciation results . we have also used x - ray diffraction ( see figure 
 s2 of the supporting information ) to measure the biochars and 
 detected olivine peaks only in the oak 850 c biochar . the parts per 
 million ( ppm ) values 
 reported are equivalent to mg / kg . the weight percentage of ash in 
 the proximate analysis ( fifth column from the left side of the table ) 
 is in the oxide form , which is standard practice . however , the s , 
 fe , ca , mg , k , and si analysis results are expressed in ppm or percentage 
 of the elemental form . in 
 comparison of the two feedstocks , corn stover contains approximately 
 400 times more silicon and 20 times more ash , sulfur , magnesium , and 
 potassium than oak . in comparison of biochars derived from the two 
 feedstocks , the biochars from corn stover contain much higher ash , 
 sulfur , and other inorganic elements than the biochars from oak . for 
 example , the oak and corn stover biochars contain 150170 and 
 600800 ppm total sulfur , respectively . our results demonstrate 
 feedstock as the primary determinant of the total sulfur content of 
 the resultant biochars . in comparison of mineral contents in 
 the biochars to those in the 
 originating feedstocks , the average sulfur content of oak biochars 
 ( 160 ppm ) is much higher than the oak feedstock , indicating 
 an enrichment of sulfur in the solid phase during the biomass conversion 
 process . the sulfur contents of corn stover biochars , however , are 
 not statistically different from that in corn stover feedstock . we 
 will provide our hypotheses for this lack of sulfur enrichment in 
 the discussion . from examination of 
 the effect of the temperature for corn stover 
 biochars , the sulfur content in the 850 c gasification sample 
 ( 610 ppm ) is lower than the 500 c pyrolysis sample ( 800 ppm ) 
 by 24% , which is more than our measurement uncertainties . the result 
 for corn stover suggests the temperature and biomass conversion conditions 
 as a secondary determinant of the total sulfur content in the biochar . the difference in the sulfur contents of the oak biochar samples is 
 not as reliable because of the presence of fine olivine in the oak 
 850 c biochar . reference spectra 
 of sulfur model compounds 
 with different oxidation states and bonding environments were collected 
 for qualitative comparison and quantitative fitting of the sample / unknown 
 spectra . a list of the model compounds used in this study and their 
 xanes spectra figures are in table s1 and figure 
 s1 of the supporting information . the xanes results of 
 the model compounds show that there are distinct spectra for sulfur 
 atoms in different oxidation states and coordination environments , 
 in agreement with other works examining xanes of sulfur . as the oxidation state of sulfur in the molecule or crystal increases , 
 the white line or absorption edge of sulfur increases , providing a 
 unique signature to determine oxidation states of sulfur in the biochars . the xanes spectra for the oak and corn stover biochar samples are 
 shown in panels a and b of figure 1 , respectively . on the basis of visual inspection , there are two dominant peaks at 
 2474 and 2482.5 ev ( middle and right dashed lines ) that vary in intensity 
 depending upon the samples . the spectrum of the 600 c oak biochar 
 has a distinct shoulder at 2472.5 ev ( left dashed line in figure 1a ) . a less distinct shoulder can also be seen in 
 the corn stover 500 c biochar spectrum ( left dashed line in 
 figure 1b ) . xanes spectra of biochar generated from 
 ( a ) oak and ( b ) corn stover 
 at different pyrolysis ( 500 and 600 c in nitrogen ) and gasification 
 ( 850 c in steam ) conditions . table 2 shows linear combination fitting 
 results using sixpack . for several of 
 the samples , linear combination fitting using athena s  fit 
 all combinations the percentage values are a fraction 
 percentage of the total sulfur . in the case of xanes , the fraction 
 percentage was obtained directly from linear combination fitting of 
 the spectra . in the case of d2492 , the direct result of the method 
 is the sulfate concentration in mg / kg . the sulfate fraction percentage 
 in the table was derived from dividing the sulfate concentration by 
 the total sulfur concentration . most of the samples have two different sulfur species , except the 
 oak 600 c sample , which is best fitted with three , and the oak 
 850 c sample , which is only fitted with one species . the spectra of four sulfur - containing organic compounds , namely 
 those of dibenzyldisulfide , l - cysteine , dibenzothiophene , 
 and thianthrene , were used individually and in combinations of two 
 to determine the structure and identity of the peak that centers around 
 2474 ev . in every sample , dibenzothiophene provides the best fit . 
 in the corn stover 850 c biochar , fitting the 2474 ev peak with 
 both dibenzothiophene ( 59% of total sulfur ) and dibenzyldisulfide 
 ( 14% of total sulfur ) improves the goodness of fit by 6.5% . this is 
 consistent with an interpretation that the organosulfur in biochar 
 is a mixture of sulfur bound to carbon in different structures . however , 
 for simplicity , table 2 shows best fits obtained 
 without including the second organosulfur component ; i.e. , all of 
 the organically bound sulfur is grouped as one . the oak 600 
 c and corn stover 500 c biochar samples 
 both show a small peak or shoulder in the 2472.5 ev energy area that 
 indicates the presence of sulfide . three different sulfide spectra 
 were tried in the linear combination fitting ; they are the sulfide 
 in nis that we collected for this research and the pyrite and marcasite 
 spectra in a database named id21 maintained by the european synchrotron 
 radiation facility . however , the linear combination 
 fitting of corn stover 500 c biochar shows negligible improvements 
 when sulfide is included in the fit . biochar is inherently heterogeneous ; 
 therefore , we also checked 
 for range of values in the speciation distribution . to this end , four 
 different xanes data sets of different portions of the ground , riffled 
 corn stover 500 c biochar were collected . when the spectra were 
 fitted , we found 77 , 85 , 98 , and 100% sulfate and 23 , 15 , 3 , and 0% 
 organosulfur in the four samples , respectively . consequently , the 
 percentages of sulfate and organosulfur for that sample are reported 
 in table 2 as 77100 and 023% , 
 respectively . the result indicates that there is more sulfate in the 
 corn stover 500 c sample than the corn stover 850 c sample 
 even when accounting for sample heterogeneity . the xanes fitting 
 method shows that both the sulfate and sulfide 
 fractions decrease with the temperature , while the organosulfur fraction 
 increases with the temperature . this trend is true for both oak- and 
 corn - stover - derived biochars , even though the total sulfur contents 
 of these two biochars differ by 34 times . these results show 
 that the temperature and conversion conditions are the most important 
 factors in determining sulfur speciation . the results also suggest 
 that there may be some similarities in the sulfur transformation pathway , 
 at least in the 500850 c temperature region . the detection limit of sulfate for the astm method 
 d2492 is 200 ppm or 0.02% . this method is potentially suitable for 
 corn stover biochars but not suitable for oak biochars , which only 
 have total sulfur content of 160 ppm and , consequently , will not have 
 sufficient sulfate for the measurement even if all of the sulfur in 
 oak biochars was in the sulfate form . we detected 63 and 17% 
 sulfate species for the corn stover biochar produced at 500 and 850 
 c , respectively ( table 2 ) . the so4 contents measured by astm d2492 agree well with those determined 
 with xanes analysis , differing by 1014% , which is within experimental 
 uncertainty . figure 2 shows sem pictures of the oak 500 c ( panels 
 a and b of figure 2 ) and corn stover 500 c 
 ( panels c and d of 
 figure 2 ) biochars . in comparison of sem pictures 
 of similar scale ( panels b and c of figure 2 , where both show structure on the order of 100 m or slightly 
 larger ) , it is clear that the corn stover biochar is less fibrous / woody 
 than oak biochar . there are many fine particles on both the outer surfaces 
 of the  foam  and trapped within  channels  
 in the foam ( highlighted by black and blue arrows in figure 2d ) . sem and eds images of ( a and b ) oak and ( c  e ) corn 
 stover 
 500 c biochars . the sem image in panel d represents the square 
 area enclosed by red dashed lines in panel c. the elemental maps in 
 panel e show element concentrations in an area enclosed by dashed 
 purple lines in panel d. there is a particle in the area that is enriched 
 with sulfur and potassium but depleted with carbon and oxygen . the 
 other fine particles ( two of which are shown with black arrows ) in 
 panel d are enriched with potassium and chlorine . because we detected multiple fine particles on the biochar , we also 
 conducted detailed elemental analyses specifically of the particles 
 to determine their elemental composition . almost all of the fine particles 
 turn out to be rich in potassium and chlorine , which we interpreted 
 to be kcl particles ( two of them are indicated by black arrows in 
 figure 2d ) . of the mineral particles 
 shown in figure 2d , only one was found to have 
 high sulfur levels ( indicated by a 
 blue arrow in figure 2d ) . eds of the particle 
 collected over the energy range of nine different elements shows that 
 it is also enriched in potassium but comparatively low in oxygen and 
 carbon ( four of the elemental maps are shown in figure 2e ) . this result is in agreement with the observation of a small 
 shoulder at approximately 2472 ev , which is an absorption peak corresponding 
 with sulfide , for the corn stover 500 c biochar . because the 
 imaging method only selectively images some but not all particles 
 in the sample , it is not surprising that an individual particle has 
 a composition that turns out to be a minority species detected by 
 xanes , which is a method that measures the bulk composition . table 1 shows that , while 
 oak biochar is enriched in sulfur , corn stover biochar is not . one 
 possible explanation for this observation lies with the very high 
 concentration of silicon in corn stover ( table 1 ) . the elements in biomass that are likely bound to sulfur in sulfate 
 form are ca , k , mg , and na . according to thermodynamic calculations 
 published in the literature , silicate may compete with sulfate for 
 these cations ; therefore , even though 
 there are high concentrations of metals , such as potassium and calcium 
 in corn stover , they may not be able to capture much of the sulfur . this would result in more sulfur being released to the syngas , a phenomenon 
 often reported for herbaceous feedstocks . because the stability of silicate is higher than sulfate at 
 higher temperatures , silicate is able to sequester even more of the 
 inorganic cations , resulting in a lower sulfur concentration in the 
 corn stover 850 c biochar than in the corn stover 500 c 
 biochar . the above reasoning suggests that the h2s concentrations 
 in the syngas derived from corn stover gasification may be higher 
 at higher temperatures , which is consistent with literature findings . it should be noted that another factor 
 that determines sulfur concentrations 
 in the gas phase is the extent of conversion from feedstock to gas . thermochemical conversion at a higher temperature ( e.g. , 850 c ) 
 generally produces a higher gas yield and less biochar than conversion 
 at a lower temperature . consequently , for oak , even though the concentration of sulfur in the biochar is approximately 
 independent of the temperature , the quantity of biochar and total 
 sulfur sequestered in biochar decrease with the temperature , while 
 the concentration and quantity of h2s in the syngas increase 
 with the temperature . although 
 the mass closure for carbon is 8090% in many biomass 
 conversion studies , we do not have good 
 mass balance for trace elements , such as sulfur . traditionally , carbon 
 ( and hydrogen ) mass balance during biomass conversion is obtained 
 by accounting for all of the carbon ( and hydrogen ) in biochar , tar 
 ( another byproduct of gasification ) , and gases ( or bio - oil ) produced . 
 however , although the biofuel community generally monitors sulfur 
 concentrations in syngas and in biochar , the amount of sulfur in tar 
 and pyrolysis vapor is usually not measured . the finding of 
 organosulfur in biochar suggests that there may 
 also be organosulfur trapped in tars and pyrolysis vapors . elliott had also measured sulfur in oak , pine , and switchgrass 
 pyrolysis oils , although no speciation information was reported . he also concluded that the sulfur and other 
 inorganics are distributed in both the oil phase and biochar particulates 
 suspended in the oil . recent publications 
 further suggest that there may be mineral deposits on the surfaces 
 of the fluidized - bed material used during thermochemical biomass conversion . in future studies , measurements of sulfur in 
 tars , pyrolysis vapors , and bed materials , in addition to conventional 
 measurements of sulfur in syngas and biochar , the detection of the high sulfate content in biochars formed under 
 lower temperature regions ( 500600 c ) and high organosulfur 
 content in biochars that were formed under high temperatures ( table 2 ) confirms the hypothesis by knudsen et al . while showing a similar trend as previous studies 
 of sulfur speciation in coal . for example , telfer and zhang found 
 that , as sulfatic sulfur decomposes in coal , the fraction of organosulfur 
 increases . in a review of sulfur 
 in coal , attar suggested that transformation of solid - state inorganic 
 sulfide to organosulfur can occur via the reactions of hydrocarbon , 
 such as c2h2 , with fes2 or could 
 be a two - step process , where the metallic sulfide reacts with h2 and forms h2s in the process and h2s then reacts with organic groups to form organosulfur . once formed , some of the organosulfur ( e.g. , 
 thiophenic compounds ) are kinetically slow to decompose . in other research , cleyle et al . used 
 sem  eds and showed 
 that , after pyrolysis , additional sulfur was observed in the organic 
 matrix surrounding pyrite particles in coal . other proof supporting the hypothesis that , in coal , sulfur is transformed 
 from sulfidic sulfur to organic sulfur came from radioactive s tracer experiments . on the basis of literature reports of changes in biomass morphology 
 and the above - cited reports on sulfur transformation in coals , a tentative 
 pathway for sulfur transformation is proposed . in the early phase 
 of heating , the biomass feedstock is devolatilized , while the lignocellulosic material undergoes a glass transition 
 phase . the tissue and macropore expand 
 and then collapse , and eventually , the 
 biomass forms a foam - like biochar . it has been further hypothesized that , during the early devolatilization 
 process , dehydrated ions precipitate out as inorganic salts or become 
 ion - exchanged to functional groups in the biochar matrix . it is highly likely that the foam - like biochar 
 can trap some of the inorganic salt . our eds images show that at least 
 some of the sulfur exists in the form of small particles trapped within 
 the biochar matrix . as the temperature of the biomass particles 
 approaches the reactor 
 temperature , some of the sulfur is converted from sulfate and sulfide 
 to organosulfur when hydrocarbon molecules ( present in pyrolysis vapors 
 and syngas ) and/or h2 ( present in syngas ) react with the 
 inorganic salt . this hypothesized reaction pathway that occurs at 
 a higher temperature with biomass feed material is parallel to the 
 reaction pathways proposed for coal described above . even though 
 there are a number of studies indicating the large 
 impact of the temperature of biochar production on their properties , 
 such as cation - exchange capacity , aromaticity , and sorption capability , there have been few studies on the effect of the temperature and 
 feedstocks on the speciation of sulfur and other trace components 
 of the biochar . it has been reported that most soil sulfur exists 
 in an organic state , such as ester sulfate or other carbon - bonded 
 sulfur , and must be oxidized to so4 prior to plant uptake , indicating the importance of sulfur speciation 
 for mineralization . a previous study by blum et al our results provide important 
 biochar sulfur speciation information as a function of biomass conversion 
 conditions . this research shows that there is a much higher fraction 
 of sulfur in organosulfur form for biochar produced from biomass gasification 
 ( 850 c ) than biochar produced from pyrolysis ( 500600 
 c ) . this result suggests the sulfate - rich biochar produced from 
 pyrolysis may be more easily mineralized , although actual sulfur mineralization 
 field studies with biochar amendment to soils will be necessary to 
 test the hypothesis . further studies are also required to determine 
 whether the sulfur speciation trend observed in this study holds for 
 other feedstocks .
the 
 effects of feedstock type and biomass conversion conditions 
 on the speciation of sulfur in biochars are not well - known . in this 
 study , 
 the sulfur content and speciation in biochars generated from 
 pyrolysis and gasification of oak and corn stover were determined . 
 we found the primary determinant of the total sulfur content of biomass 
 to be the feedstock from which the biochar is generated , with oak 
 and corn stover biochars containing 160 and 600800 ppm sulfur , 
 respectively . 
 in contrast , for sulfur speciation , we found the primary 
 determinant to be the temperature combined with the thermochemical 
 conversion method . 
 the speciation of sulfur in biochars was determined 
 using x - ray absorption near - edge structure ( xanes ) , astm method d2492 , 
 and scanning electron microscopy 
 energy - dispersive spectroscopy 
 ( sem  eds ) . 
 biochars produced under pyrolysis conditions at 
 500600 c contain sulfate , organosulfur , and sulfide . 
 in some cases 
 , the sulfate contents are up to 77100% . 
 biochars 
 produced in gasification conditions at 850 c contain 73100% 
 organosulfur . 
 the increase of the organosulfur content as the temperature 
 of biochar production increases suggests a similar sulfur transformation 
 mechanism as that in coal , where inorganic sulfur reacts with hydrocarbon 
 and/or h2 to form organosulfur when the coal is heated . 
 eds mapping of a biochar produced from corn stover pyrolysis shows 
 individual sulfur - containing mineral particles in addition to the 
 sulfur that is distributed throughout the organic matrix .