Ammoxidation of Straw-PulpAlkaline Lignin by HydrogenPeroxideJiang Qi-Pei,

a,b

Zhang Xiao-Yong,a

Mo Hai-Tao,a

and Li Zuo-Hu,a

a

National Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353,Beiertiao 11, Zongguancun, Haidian District, Beijing, 100080, P. R. Chinab

Graduate School of the Chinese Academy of Sciences, Beijing, 100080, P.R. China; cellulose420@163.com (for correspondence)

Published online 2 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10148

A novel method that used hydrogen peroxide as anoxidizer to ammoxidize straw-pulp alkaline lignin wasstudied. The purpose of this research was to find a moreefficient way to utilize technical lignin, which is amajor pollutant in China. The ammoxidation processwas employed to nitrogen-modify straw-pulp lignin us-ing ammonia liquor and hydrogen peroxide. At thedesirable conditions, such as 120°C, 90 min reactiontime, 20% hydrogen peroxide charge, and 20% ammo-nia charge, the nitrogen content of the ammoxidizedproduct reached 11.62% and the ammonia nitrogenwas 4.12%. The study showed that hydrogen peroxidecould completely substitute for oxygen as an oxidizerfor ammoxidizing alkaline lignin. Straw-pulp alkalinelignin can be effectively converted into slow-releasefertilizer. © 2006 American Institute of Chemical EngineersEnviron Prog, 25: 251–256, 2006

Keywords: straw pulp, alkaline lignin, hydrogenperoxide, ammoxidation

INTRODUCTIONNon-wood materials, such as wheat straw and vari-

ous other agricultural residues, are abundant and as afiber source attractive for pulp-making, especially inChina, which has minimal wood resources but a dra-matically growing demand for paper. The major prob-lem with alkaline straw pulping is disposal of the blackliquor that is produced by straw digestion. Straw-pulplignin from the spent liquor is difficult to combust torecover alkali due to its high silicate content and highviscosity. Pollution from the black liquor has been the

most serious factor hindering development of the pulpand paper industry in China [1].

Lignin ranks as the secondary reusable organic mat-ter in the natural biomass resource category, just be-hind cellulose. The main pollutant of the spent liquorfrom paper mills is usually called technical lignin. Tech-nical lignin is a major source of BOD and COD in blackliquor, with more than 2.5 million tons being producedeach year in China. However, lignin is an importantprecursor for the formation of humic substances in soils[1]. Its biological degradation is very slow due to thecomplex three-dimensional network structure and itshigh C/N ratio [1]. Lignin can act as an inert support forslow-release fertilizer because it can be degraded bymicroorganisms slowly to release the nutrients. Thefertilizing effect of N-modified lignin is well recog-nized, and it has been described elsewhere [2–8].

Conventional ammoxidation of lignin is normallyconducted using oxygen at high pressure and hightemperature. However, it is difficult to use such a pro-cess in small-scale paper mills in China. Hydrogenperoxide as an oxidizer offers a novel approach toammoxidize lignin at normal pressure and lower tem-perature. In this article, the effect of variables, such ashydrogen peroxide and ammonia charge, reaction timeand reaction temperature on ammoxidation, are re-ported.

MATERIALS AND METHODS

MaterialsBlack liquor was supplied for this research by Wu-

gang paper mill in HeNan Province, China. At thisplant, wheat straw was cooked with sodium hydroxide,© 2006 American Institute of Chemical Engineers

Environmental Progress (Vol.25, No.3) October 2006 251

which was the only chemical added. The straw blackliquor, produced in this process, had a solid contentranging between 5 and 10% by weight. Followingcooking, the black liquor was processed by dry frac-tionation to form feedstocks. The main process stepswere acid precipitation, washing, drying, grinding, andscreening. The main components of the feedstockwere: lignin 49.44%, water 4.85%, and ash 12.85%; theelemental analysis was: C 51.38%, H 5.54%, N 1.69%.

Ammoxidation of Wheat Straw Alkaline Lignin byHydrogen Peroxide

Ammoxidizing experiments were carried out in a 2-Lbatch cylindrical autoclave digester that was heatedelectrically. Powdered alkaline lignin (30 g), a presetamount of hydrogen peroxide (30%, 60 ml), and am-monia liquor (25�28%, 60 ml) were fed into the reac-tor. The temperature was increased from room temper-ature to the preset maximum temperature and thenkept constant for a given period of time. After thereaction was complete, the products of the ammoxida-tion process were spray-dried and the nitrogen contentdetermined.

Analytical MethodsThe nitrogen content of the lignin was measured

using the micro-Kjeldahl method. The ammonia nitro-gen content was measured before the sample was di-gested (depicted as A) and the total nitrogen afterdigestion. Ammonia nitrogen subtracted from total ni-trogen is shown as 1-A, which is an important indexwith which the extent of ammoxidation was evaluated.Organic nitrogen is an important index for evaluatingammoxidation due to the covalent bond between car-bon and nitrogen, but a uniform standard is not estab-lished in the determination of organic nitrogen. Conse-quently, residual nitrogen, except ammonia nitrogenexpressed as 1-A, should be the most important guide-line to evaluate ammoxidation in this experiment becauseits composition is mainly organic-compound nitrogen.

In this work, FTIR spectra analysis of ammoxidizedlignin was performed using a Perkin-Elmer System2000 FTIR. An element assay was performed using anElement Analysis Instrument (Flash EA1112, ThermoElectron SPA).

RESULTS AND DISCUSSION

Hydrogen Peroxide ChargeAn acid-precipitation lignin 30 g, a preset amount of

hydrogen peroxide (v/v: 30%), and 60 mL of ammonialiquor (v/v: 25�28%) were fed into the autoclave di-gester. Distilled water was added to bring the volumeto 300 mL. Reaction conditions were: 90 min, 100°C,and stirring rate 600 rpm. The nitrogen content of theresulting product was determined after the ammoxi-dized products were spray-dried.

The variation of the nitrogen content with varioushydrogen peroxide charges is shown in Figure 1. Theresults demonstrated the hydrogen peroxide chargeplayed a vital role in the ammoxidation process. Onlysmall amounts of nitrogen introduced into the lignin

macromolecule illustrated that it was difficult to reactlignin in ammonia liquor without the use of hydrogenperoxide as an oxidizing agent. Nitrogen from ammon-ification might have come from the combination oforganic acids in the feedstock and the added ammonia.The nitrogen content in the ammoxidized product no-tably increased with the addition of hydrogen perox-ide. When the hydrogen peroxide charge was 20%, thetotal nitrogen, ammonia nitrogen, and 1-A were maxi-mized. By comparison, the total nitrogen contentreached 11.34% and 1-A 7.26%, while the total nitrogenwas 14�16% when oxygen was used as an oxidizer [1].It can be concluded from these experiments that lignincan be ammoxidized when hydrogen peroxide wasemployed as a replacement for oxygen as an oxidizer.

Reaction TimeOperating conditions were as described above, with

other variables kept constant. The reaction time wasvaried from 30 min to 180 min. The reaction time in thisarticle refers to the time at the maximum temperatureafter the time to the maximum temperature was approx-imately 25 min. The nitrogen content was determinedafter the ammoxidized products were spray-dried.

The reaction time is one of the most important pa-rameters on the ammoxidizing process. As shown inFigure 2, the total nitrogen and 1-A increased sharplyduring the first 30 min of the reaction and then moreslowly. The maximum content was not identical for thethree kinds of nitrogen entities. The maximum totalnitrogen content, 11.34%, was obtained at 90 min,while the ammonia nitrogen maximized at 75 min.However, prolongation of the reaction time exceeding90 min had no effect on the nitrogen content. Thisresult indicated that too long a reaction time couldincrease degradation of ammoxidized lignin at highertemperatures. Consequently, 90 min was selected forthis experiment as 1-A was maximized at this point.

Figure 1. Effects of hydrogen peroxide charge on thenitrogen content of the lignin.

252 October 2006 Environmental Progress (Vol.25, No.3)

Reaction Temperature

Operating conditions were as described above, withother variables kept constant. The reaction temperaturewas in a range from 30°C to 180°C. The nitrogen con-tent was determined after the ammoxidized productswere spray-dried.

An increase in temperature makes it possible tocomplete the reaction more quickly. At the same time,the increase in temperature speeds up degradation ofhydrogen peroxide and slows down ammoxidation.Figure 3 shows that the total nitrogen, the ammonianitrogen, and 1-A were maximized at 120°C. The totalnitrogen content reached 11.62%.

Ammonia ChargeAnother set of experiments with variation in the

ammonia charge were performed. The ammoxidizedprocess was carried out according to the conditionsspecified above. The pH was measured before reac-tion; the RAU (ratio of ammonia utilization) was calcu-lated after the completion of reaction.

As indicated in Figure 4, the ammonia charge had asignificant effect on the nitrogen content of lignin.However, greater than a 15% ammonia charge had aminor impact on the nitrogen content. Meanwhile, thetotal nitrogen and 1-A fell gradually when the ammoniaconcentration was more than 20%. The increase in pHcould be responsible for this result. In this experiment,20% ammonia was a suitable charge. pH varied from8.8 to 11.2 with the increases of ammonia added. Con-sequently, pH � 10 should be a suitable value forammoxidation. It was logical that the RAU (ratio ofammonia utilization) decreased with the increase ofammonia charge. The RAU was 50.7% at optimizedammonia charge. The ensuing experiments that con-cerned pH are discussed below.

The results shown in Figure 5 were obtained using asimilar procedure as with the other dependent vari-ables. Only the initial pH values of the reaction systemwere adjusted by adding caustic soda and phosphoricacid to predetermined values.

Figure 5 shows that the ammonia nitrogen contentdecreases significantly as the pH increases. This resultoffered a new approach for increasing nitrogen contentof the product by adding acid. A decrease in pH ofammoxidized product would lead to an increase in thenitrogen content. A pH value between 8 and 10 wasbest for 1-A in this experiment; the results shown inFigure 4 also were supported by this result.

Concentration of Lignin/Water ChargeWith other parameters kept constant, various

amounts of acid-precipitation lignin were fed into thedigester to determine its optimum concentration.

Figure 2. Effects of reaction time on the nitrogencontent of the lignin.

Figure 3. Effect of reaction temperature on thenitrogen content of the lignin.

Figure 4. Effect of ammonia charge on the nitrogencontent of the lignin.

Environmental Progress (Vol.25, No.3) October 2006 253

As shown in Figure 6, the effect of lignin concentra-tion on the nitrogen content was not significant. Thenitrogen content decreased when the concentration oflignin was more than 25% due to incomplete reaction.Hence, 10% lignin, the black liquor concentration, wasutilized in the experiments.

Metal IonsZuniga [7] reported that compounds of transitional

metals such as iron, molybdenum, copper, and man-ganese had a catalytic effect on the ammoxidationprocess. In this experiment, the effect of four metal ionson the ammoxidation process was tested. The condi-tions were described previously; Fe2�, Mo6�, Mn2�,and Cu2� at 10 mg/L concentration were injected into

the digester in separate tests. Products were deter-mined after spray-drying.

Data reported in Figure 7 showed that transitionalmetal ions played a minor role on the ammoxidationprocess. Compared to the control experiments, no in-crease in the nitrogen content of lignin was found for10 mg/L Mn2� and Cu2�. However, 10 mg/L Fe2� andMo6� had a slight catalytic impact on the ammoxidizingreaction. The nitrogen was increased only 2.88% and3.04% using Fe2� and Mo6�, respectively. Unlike thecase of using oxygen as an oxidizer, transitional metalions had little effect on the ammoxidation processwhen hydrogen peroxide was used.

Optimum Ammoxidizing Conditions Using HydrogenPeroxide as an Oxidizer

Our work has shown that the optimum conditionsfor ammoxidation of straw-pulp alkaline lignin usinghydrogen peroxide as the oxidizer were: reaction tem-

Figure 5. Effect of pH on the nitrogen content of thelignin.

Figure 6. Effect of concentration of lignin on thenitrogen content of the lignin.

Figure 7. Effect of transitional metal ions on theammoxidizing reaction.

Figure 8. FTIR spectra of black liquor and ligninresulting from ammoxidation.

254 October 2006 Environmental Progress (Vol.25, No.3)

perature 120°C, reaction time 90 min, ammonia charge20%, hydrogen peroxide charge 20%, optimal pH range8�10, and lignin concentration 10%. The nitrogen con-tent of the ammoxidized product (N-lignin) is: totalnitrogen content 11.62%, ammonia nitrogen content(A) 4.12%, residual nitrogen content (mainly organiccompounds) 7.5%. Compared with the case of ammoxi-dation using oxygen as an oxidizer, the total nitrogen(10�12%) was slightly lower (14�16% when oxygenwas used as the oxidizer [1]) and the ammonia contentwas close (approximately 4%). But for being a finalproduct of N-fertilizer, the nitrogen content was con-sidered somehow low. However, AOL (ammoxidizedlignin) had also been reported to be an urease andnitration inhibitor [3]; therefore, AOL mixed with chem-ical NPK fertilizer can reduce fixation by soil of P-fertilizer and inhibit eluviation of K-fertilizer [3]. Thiscan result in fertilizer nutrients being more slowly re-leased. Therefore, AOL might be well suited as a hu-mification aid combined with chemical fertilizer.

pH Adjustment Using Phosphoric Acid on IncreasingNitrogen Content of Product and Rate of AmmoniaUtilization

During ammoxidation, ammonia liquor cannot com-pletely react with the lignin. Consequently, residualammonia without reaction was lost in the course ofspray-drying. There should be, however, an effectiveway of absorbing residual ammonia using acid to retain

nitrogen. If the pH of the product of the ammoxidationprocess (before spray-drying) was adjusted using phos-phoric acid at the end of reaction, the nitrogen contentwould be increased.

Table 1 shows the variation of the nitrogen contentas a result of pH adjustment by the addition of variousamounts of 85% phosphoric acid to the reacted liquor.The ammonia nitrogen increased sharply and the totalnitrogen increased, 1-A decreased slightly and RAUincreased from 46.63% to 68.66%.

FTIR Spectra Analysis of Ammoxidized LigninFTIR spectra were determined using a Perkin-Elmer

System 2000 FTIR instrument, employing the KBr pellettechnique.

Compared with black liquor’s lignin, the spectra ofammoxidized lignin indicated the degradation of thearomatic structures as the intensities of the aromaticskeletal vibration at 1515 cm-1. Additionally, the guai-acyl rings at 1285 cm�1 weakened. The growing bandat 1462 cm�1 was a C-N stretching in primary amides[1]. The emergence of a band at 3197 cm�1 N-H wasworth noting. IR spectra proved that the ammoxidationby hydrogen peroxide is the same process as by oxy-gen [1] in the inner part of the lignin molecule. Theresult is shown in Figure 8.

Product of Ammoxidized LigninThe products of ammoxidized lignin and acid-ad-

justed ammoxidized lignin were brown polishing pow-der. All products had good water solubility. Compari-son of the feedstocks product of ammoxidation andproduct of adjusted pH was made using an ElementAnalysis (Flash EA1112, Thermo Electron SPA). Resultsof the work are shown in Table 2.

The results of Element Analysis shown in Table 2were basically identical to the assay by the micro-Kjeldahl method. The C/N ratio of ammoxidized andacid-adjusted lignin decreased to that microbial degra-dation [1]. It can be concluded that the product ofammoxidized lignin by hydrogen peroxide should bean excellent humus fertilizer.

CONCLUSIONS LT1. As an oxidizer replacement for oxygen, hydrogen

peroxide could favorably improve the lignin am-moxidation process. lt

2. Optimal parameters of ammoxidation by hydrogenperoxide were as follows: reaction time 90 min,reaction temperature 120°C, ammonia charge 20%,

Table 1. Result of pH adjustment on the nitrogencontent of product and the rate of ammoniautilization.

pH

Totalnitrogen

(%)

Ammonianitrogen

(%)

Organicnitrogen(1-A%) RAU

11.5 10.88 3.53 7.25 46.63%11 11.21 4.02 7.19 48.04%10 11.82 4.37 7.45 50.66%9 12.16 4.98 7.18 52.11%8 12.98 5.87 7.12 55.63%7 13.65 6.62 7.03 58.50%6 14.29 7.52 6.77 61.24%5 15.11 8.59 6.52 64.76%4 16.02 9.67 6.35 68.66%

RAU � nitrogen in ammoxidized product/nitrogen inammonia liquor added.

Table 2. Comparison of feedstocks ammoxidized product and ammo-product of adjusted pH.

C(%)

N(%)

H(%) C/N pH*

Ammonianitrogen (%)

Feedstock 51.38 1.69 5.54 30.4 6.5 0.38Ammoxidized 39.12 11.62 5.37 3.37 10.12 4.12Acid-adjusted 38.66 16.02 6.18 2.41 4 9.67

*pH was measured by diluting 1:10 with distilled water.

Environmental Progress (Vol.25, No.3) October 2006 255

optimum pH 8�10, hydrogen peroxide charge 20%,and lignin concentration 10%. The products of am-moxidation were: the total nitrogen 11.62%, the am-monia nitrogen content 4.12%, and 1-A 7.5%.

3. If the pH of the ammoxidation process was adjustedto pH using phosphoric acid at the end of reaction,the nitrogen content could be increased to 16%.

4. FTIR spectra indicated ammoxidized lignin had ob-vious C-N stretching and an N-H band. The aromaticskeletal vibration was degraded due to introducingnitrogen.

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Paper Industry, Journal of Cleaner Production, 6,349–355.

2. Meier, D., & Schiene, R. (1994). Conversion of Tech-nical Lignins into Slow-Release Nitrogenous Fertil-izer by Ammoxidation in Liquid Phase, BioresourceTechnology, 49, 121–128.

3. Zhu, Z.-H., & Wang, D. (2001). Fertilizer Efficiencyof Ammonia-Oxidized Lignin (AOL) a Modified Lig-nin from Wastewater in Paper-Making Used as a

Slow-Released Nitrogen Fertilizer, Chinese Journalof Agro-Environmental Protection, 20, 98–100.

4. Chen, Q., & Mu, H.-Z. (2003). Development of Lig-nin Fertilizer and Its Effect on Availabilities of NFertilizer and P Fertilizer, Chinese Journal of Agro-Environmental Science, 22, 41–43.

5. Zhang, X.-Y., & Zhang, J. (1999). Chemical Ammon-ification of Straw Pulp Lignin. Chinese Journal ofEngineering Chemistry & Metallurgy, 20, 215–219.

6. Ramırez-Cano, F., & Ramos-Quirarte, A. (2003).Slow-Release Effect of N-Functionalized Kraft LigninTested With Sorghum Over Two Growth Periods.Bioresource Technology, 76, 71–73.

7. Zuniga, V., Martınez, A., Delgado, E. G. & Coca, P. J.,& Camacho, A. (1993). Ammoxidation of LignaceousMaterials in a Fluidized Bed Reactor. Proceedings ofthe 2nd Brazil Symposium on Chemistry of Ligninand Other Wood Components, Campinas, Brazil,Sept. 2–4, 1992.

8. Ramırez, F., & Gonzalez, V. (1997). AmmoxidizedKraft Lignin as a Slow-Release Fertilizer Tested onSorghum Vulgare, Bioresource Technology, 61, 43–46.

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