regenerated humic acids obtained by the air oxidation of south ...

Zdeněk Cihlář, Jiří Kučerík/Petroleum & Coal 52 (4) 254-260, 2010 257shown in Figure 1. It can be seen two (or three) pronounced separated peaks generallyattributed to the thermo-oxidative degradation of labile (~250–440 o C) and stable (~445–510 o C)humic acids constituents. The resulting heat is proportional to the peak area and it is aresult of both degradation and recombination reactions [26] .Table 2 The summary of results obtained from DSC and TGA (*recalculated for dry part).TGA results Δm [%]Samples Combustion heat [kJ g –1 ] moisture content 1. step* 2. step* ash content*HA 11.3 6.65 77.8 21.1 1.09RHA 1 10.8 6.44 75.9 23.5 0.58RHA 2 10.7 6.61 70.0 29.2 0.78RHA 3 10.3 6.82 75.6 23.8 0.62RHA 4 11.4 7.87 73.3 25.6 1.18RHA 5 12.7 7.44 75.5 23.4 1.10RHA 6 11.8 7.23 79.9 18.9 1.24RHA 7 11.7 7.16 80.5 18.5 1.00As can be seen in Figure 2 all the humic samples gave similar DSC records. Nevertheless,some differences appeared comparing the degradation onsets, peak temperatures andpeak area (proportional to combustion heat). It seems that increased amount of labileparts as detected by TGA caused the increase in the DSC peaks areas (Figure 2). It canbe seen that the peak area around 250–400 o C increased confirming the results from TGA.However, it is noteworthy, that the peak increase from 400–500 o C occurred as well, but,this region is attributed to the degradation of stable part which was, according to TGAresults, partly decomposed by regeneration. Therefore, it implies that the processes inthe first step are mostly exothermal recombination of labile part than its degradation.Fig. 1. DSC record of thermooxidativedegradation of RHA 4Fig. 2. DSC record of thermooxidativedegradation of samples RHA 4–7Fig. 3 shows a typical TGA record including the first derivative (DTG). In all samples,the progressive mass loss which occurred up to 125 o C was caused mainly due to the lossof adsorbed water. In general, the first derivative of thermogravimetric curves showedtwo degradation rate peaks. One peak occurred between 300 and 440 o C and is associatedwith a mass loss from 70 to 80 % of the total sample mass. A second peak occurredbetween 480 and 520 o C and gave 18 to 30% of mass loss. Thermogravimetric resultsconformed results obtained by DSC measurements.The parameters obtained from the DTG, TGA and DSC measurements are summarizedin Table 2. HA analyzed in this paper were intensively purified by HCl and HF to reducethe ash content. As it can be seen, the ash content was relatively low, in all cases it wasdetermined about 1%. Moisture content was determined in the interval from 6.6 to 7.9%.It seems that the greater moisture content had samples prepared in the static reactor(RHA 4–7). This increase can be associated with the physical and chemical changes inhumic samples as a result of structure alteration which occurred in the course of oxidationperiods. TGA measurement of humic samples further showed two mass loss steps. Thefirst step can be related to decomposition of labile parts and the second one to the stableparts of humic acids. Comparison of the values from samples prepared in the static reactor

Zdeněk Cihlář, Jiří Kučerík/Petroleum & Coal 52 (4) 254-260, 2010 258(RHA 4–7) indicates an increase in labile component content and simultaneously a decreaseof stable fraction. Evidently, during oxidation the partial decomposition of stable humicpart occurred and therefore the RHA extracted from oxidized lignite contained larger poolof labile molecules. Further decomposition is then easier which can be related to the changeof stabile/labile parts ratio. Nevertheless, these results are not confirmed by DSC measurement.Total heat evolved was obtained by the DSC peak integration (Fig. 1). The largest valueof 12.7 kJ g –1 was observed for sample RHA 5. In this case, no significant correlation wasobserved.Fig. 3. TGA and DTG record of RHA 4Fig 4. Dependence of the loss of mass in thefirst step of RHA 4–7 on the time of oxidationFigure 4 shows the correlation between the mass loss of humic acids in the first stepand the time of respective lignite oxidation. It seems that amount of mass decompositionin the TGA first step increases with the time of oxidation. Increasing of labile parts in RHAis also confirmed by DSC measurement as mentioned above. As it can be seen the increasereaches the limit which implies that at approximately 240 hours of oxidation the lignitestructure was completely changed a no further conversion can be obtained by extendedperiod of oxidation. Apparently, that statement is valid only for conditions used in thiswork, i.e. for South Moravian lignite and oxidation by air at 85 o C.3.2 Moisture uptakeSome differences in chemical character among samples have already been revealedusing FTIR which indicated a small increase in the number of ester bonds in RHA [15] . Resultsof HPSEC showed the increase in the apparent molecular weight with increasing time oflignite oxidation of respective samples prepared in the fluid reactor [15] . Thermal analysistechniques such as DSC and TGA confirmed the difference in physical structure. In orderto evaluate the changes in detail, the capacity of surface area to uptake moisture was testedusing moisturizing container.Table 3 Moisture content in RHA samplesSample Δm – moisture content [%]HA 12.9RHA 1 12.9RHA 2 11.8RHA 3 13.5RHA 4 10.0RHA 5 10.8RHA 6 11.2RHA 7 13.1Fig. 5. Evaporation of moisture from samplesRHA 4–7Fig 5 shows selected TGA curves, which resulted from the evaporation of moisture adsorbedon the surface of samples RHA 4–7, placed into the moisturizing chamber for 21 days. As

Zdeněk Cihlář, Jiří Kučerík/Petroleum & Coal 52 (4) 254-260, 2010 259it can be seen all the records gave similar curve shapes which imply similar mechanismsof moisture release. Water content was determined as a mass loss recorded up to 125 o C.The results regarding the determination of moisture content in RHA samples are summarizedin Table 3. Amount of evaporated moisture was determined in the range from 10 to 13%.In fact, water content increases with increasing time of lignite oxidation. The evaporationof water molecules from RHA is governed by the strength of polar groups on the surfaceof humic material. The greatest rate of evaporation occurred at the beginning of the experimentwhere the records had steepest slopes, while at approximately 100 o C the kinetics decreasedand at between 150 and 200 o C degradation started.As could be seen, moisture content was higher in samples which were prepared at lowertemperatures and also increased at longer period of oxidation at 85 o C. This trend corroboratewith yield of humic substances extracted from oxidized and parental lignite publishedrecently [15] and can be probably attributed to the content of COOH and phenolic OH groupsin humic material.4. ConclusionThermal analysis was used to study properties of humic acids and RHA produced fromlignite oxidized by air under different conditions. The influence of these conditions on moistureuptake and thermal stability were studied. Our recent results indicated an increase in numberof ester bonds in RHA [15] . Further, by means of HPSEC it was demonstrated the increasein the apparent molecular weight with increasing time of oxidation for samples preparedin static and the fluid reactor.It has been verified that higher temperature during the oxidation supports the productionof more “cross-linked” RHA. Moisture content in samples placed into the moisturizingchamber for 21 days (relative humidity is 100%) increased with increasing time of parentallignite oxidation. Comparison of the TGA values from samples prepared in the static reactor(RHA 4–7) indicates an increase of labile component content, and, simultaneously, adecrease of a stable fraction. The increase reached the limit which implies that at approximatelyafter 240 hours of oxidation the structure was completely altered a no further transformationof humic matter took place by further oxidation (for conditions used in this work, oxidation byair at 85 o C). No significant correlation was observed for samples extracted from ligniteoxidized in fluid reactor. Increasing content of labile pool in RHA was also confirmed by aDSC measurement.AcknowledgementsThe work was financially supported by the project MSM 0021630501.References[1] Stevenson, F.J.: Humus Chemistry. Genesis, Composition, reactions. John Wileyand Sons, Inc., NY. 2nd edition. 1994.[2] Simón, M., Garcia, I., Gil, C., Polo, A. M.: Geoderma. 1994, 61, 119.[3] Rashid, M. A.: Geochemistry of Marine Humic Compounds. Springer-Verlag. NY, 1985.[4] Thurman, E. M.: Organic Geochemistry of natural Waters. Dordrecht, BostonHingham. Massachusetts, 1985.[5] Schulten, H. R., Schnitzer, M.: Naturwissenschaften. 1995, 82, 487.[6] Aiken, G. R., McKnight, D. M., Wershaw, R. L., McCarthy, P.: Humic substances inSoil, Sediments and Water: geochemistry, isolation and characterization. Wiley,NY. 1985.[7] Klavins, M.: Aquatic Humic Substances: Characterization, Structure and Genesis,Riga, LU. 1998, 286.[8] Piccolo, A.: Adv. Agron. 2002, 75, p. 57.[9] Piccolo, A.: Humic substances in terrestrial ecosystems. Elsevier, Amsterdam, 1996, 225.[10] Stevenson, F. J., Cole, M. A.: The cycles of soils, 2nd edition, Wiley, NY, 1999.[11] Hayes, M. H. B., Clapp, C. E.: Soil Science. 2001, 166, 723.[12] Rausa, R., Girardi, E., Calemna, V.: In Humic Substances in the Global Envinronmentand Implications on Human Health (N. Senesi and T.M. Miano eds). 1994, Elsevier.[13] Van Krevelen D. W.: Coal: Typology – Physics – Chemistry – Constitution. Elsevier, 1981.[14] Calemna, V., Rausa, R., Girardi, E.: Proc. of the Int. Conf. on Coal Sci., Tokyo, Japan,1989, 232.[15] Kučerik, J., Cihlář, Z., Vlčková, Z., Drastík, M.: Petroleum and Coal. 2008, 50 (3), 49[16] Kučerík, J., Kovář, J., Pekař, M.: J. Therm. Anal. Cal. 2004, 76, 55.

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