CC BY
15Journal of Siberian Federal University. Chemistry 2 (2011 4) 111-120
УДК 661.183.2:66.094.37
Interaction of Hydrogen Peroxide with Nanoporous Material Prepared by Alkaline Activation of the Brown Coal
Alexandra S. Gribanova, Vladimir A. Kucherenko, Tatyana G. Shendrik and Yuliya V. Tamarkina*
L.M. Litvinenko Institute of Physical-Organic and Coal Chemistry, NAS of Ukraine 70 R. Luxemburg st., Donetsk, Ukraine 83114 1
Received 3.06.2011, received in revised form 10.06.2011, accepted 17.06.2011
The work analyzes applications of carbon adsorbents in catalytic wet peroxide oxidation (CWPO) being a variant of Advanced Oxidation Processes (AOPs). Under CWPO condition (concentration [Н2О2] < 30 %, 20±2 °C) we have studied the activity of nanoporous adsorbent АС-К prepared by КОН-activation (800 °C, 1 h) of brown coal. We have compared АС-К with solid product of thermolysis (SPT) of brown coal formed under the same conditions without КОН. АС-К, which has a high adsorbtion activity, catalyzes decomposition ofН2О2 to form ОН-radicals. This allows to combine two environmentally important processes: concentration of organic pollutants on the surface of adsorbent and their further decomposition by ОН-radicals.
Decomposition of Н2О2 in presence of АС-К or SPT is described by kinetic first-order equation and runs 20-30 times faster in contact with АС-К. Rate constants vary within the range of0.053-0.28 min-1 (АС-К) and 0.002-0.012 min-1 (SPT) and grow under [Н2О2] increasing. Oxidative modification of АС-К and SPT surfaces under CWPO conditions has been studied. The dependence between content of ОН groups in modified АС-К (24 h) samples and [Н2О2] is described as a curve with a maximum at [HflJ = 10 %, where the maximum modifying effect and the highest increse in ОН-groups content (from 1.00 to 1.55 mmole/g) are observed. Modification level is negligible; only 1 % of oxidant reacts to form functional groups.
Keywords: nanoporous carbon, hydrogen peroxide, decomposition kinetics, surface modification.
Research of Advanced Oxidation Processes (AOPs) is a relatively new and rapidly progressing fields [1]. The beginning of AOPs was the discovery of radical oxidation of organic substances by hydrogen peroxide in the presence of ferrum ions (Fenton's reaction) [2]. Such processes are applied to
* Corresponding author E-mail address: y_tamarkina@rambler.ru
1 © Siberian Federal University. All rights reserved
remove organic pollutants (OP) from air and water. Their intensive development was driven by strict regulations set forth for recycle water quality in many countries.
AOPs apply four main oxidants: oxygen,ozon, air and hydrogen peroxide as a reactant of catalytic wet peroxide oxidation (CWPO) [1]. There exist lots of oxidative methods, including those applying ultrasound [3] and UV-irradiation [4], catalytic agent [O-O), supercritical water [10] and many oSfertcchniquns [U, 12]- Theoemethrds ate especially effieienr to decompose toxic substances or biodegradation resistaat po[lutants ur aqueous mcdra, as wrU ao substances which are hardly decomposed,e.g-aromaticcompounds [r [ 3,14],phrxoland itr dxrivativos []_[-20], oil components [1, 4], pestiordes ond herdicidcs [3, 4, 9, SO, 2S,22]gilyes [323]. Triebigaest number of works is focused on destructionof suet compoundsar: l_)iritigf)^[ino]^i^, whichaoe qoitereslstanttobiological decomposition as they aie ioxis io micro tDrrtoroiiia^s, esneciatly oh gltos^ir^ii] 2) chk)rophenol, in particular
pentachkiropteno1 - lughfy toxkiantisepSkiwti1 ani^fungus and bosieriiidal effect; 3) dyes of different structure,pesSicider and herbkMes.
Oxydestenclion durlngAOPi runs sri^l^ pcrtiuipation of OHrradlcaSs, which decompose OP to low-moleculascompounds.For instunoe,2,4- drchidrophennlisooiOizodto low-molecular non-toxic compounds ns siiownlni below [1]
CL
+5h2o + 6 o2 ch+ + 6hcor + 2Ci"
CL
This p^c^d^essi^ib^l so failed minera1izatignoS organiocompoundi, beeause they are transformed into simpk nonlOrgamcsubstadcc i [1)
(ZnHmX+ cs nCO( + C,5(m-z)H2O + zHX.
Carbon in orgrnie componndr is cenverteO isilo Ctatoms if moleculed and ions CO2, H2CO3, HCO3", C032_, white X-atoms benome 00x1 of msnrrfl ¡sc^ds, wh(re HX=HCl, HBr, HNO3, HNO2, H2SO4 (tc. Usi+g ap+4opriatu AOPs evas OP molsculei whh cyanidngroups can be oxidized to essentMfy lasr toxic rompxundt.
Thcbaric mechanirm o!1 oxiraisve reactions, wldch baslcadly imltaies natural photochemical processes inthe tsiiei^rh nlmosnhere, iucinXeo, inparlicular, formelion of highly reactive short-living oxygen-containing intermediates, namely, OH-radicals. Hydroxyl radical is a powerful highly reactive nonselective oxidant with electrophilic properties. Its reactions have high rate constants and are often controlled by a diffusion [13].
AOPs group also includes processes of OP oxydestruction in the presence of activated carbon (AC), for instance, air oxidation of phenol [18] or trinitrophenol [19], ozonation of aromatic S-containing compounds [8], OP decomposition by hydrogen peroxide in AC presence (CWPO-process) [6,7,9,12,15,2024]. CWPO-process generates OH-radicals during hydrogen peroxide decomposition, which is promoted (or catalyzed) by activated carbon. This process is demonstrated by the equation below
Cn + H2O2 ^ Cn+^ + OH + OH" (1)
which is formally similar to Fenton's reaction [2].
- 112 -
Another process reveals in AC-H202-H20 systems. Oxidant can modify AC surface by forming different oxygen-containing functional groups (OFG), though as hydrogen peroxide decomposes fast its modification efficiency is low, i.e. this causes a minor increase in surface groups concentration [22, 25-31].
On the other hand, OP molecules get sorbed on AC surface. When adsorbed they are decomposed by OH-radicals slower. According to studies[6, 15], H2O2 is decomposed by activated carbon much more slowly in the presence of 4-chlorophenole, which was explained by the adsorbate shielding a number of surface active sites catalytically active in the decomposition of H2O2. Nonetheless, the research has registered quite a high conversion rate of 4-chlorophenol: 25 % for 26 min when a solution with a 1 g/dm3 concentration flows through the column with AC [6]. Oxidation of methyl-tret-butyl-ether, trichloroethylene and 2,4,5- trichlorophenol by hydrogen peroxide in the presence of AC shows [7] that adsorbed adsorbate molecules are low-reactive, i.e. they are protected from attacks by OH-radicals. OP adsorbtion causes an inverse effect - adsorbed molecules are more protected to oxidation by radicals, though Off molecules are placed on AC surface near by OP.
AOPs are hard to apply to weak aqueous solutions, where OP concentrations make 1-100 mg/m3. To resolve the issue, we find it reasonable to combine two processes: 1) OP adsorbtion on AC for them to be concentrated in small space; 2) OP decomposition by hydroxyl radicals generated from decomposition of hydrogen peroxide catalized by AC. The main obstacle here - adsorbed OP decomposes slower. This might bring to nothing technological advantage obtained through preconcentration or might cause a need to add another stage to desorb OP from adsorbent surface, e.g. through ultrasound irradiation [16].
Earlier, we obtained brown coal-derived nanoporous material with highly-developed surface, substantial portion (<70 %) of micro pores and high adsorption activity towards organic substances in aqueous media, in particular, towards phenol and methylene blue [32-35]. We deemed it reasonable to test this material in AOPs with hydrogen peroxide.
The first stage of the present work focuses on H2O2 decomposition kinetics in the presence of AC-K and SPT, as well as seeks to asses oxidative modification of their surface as it strongly affects AC catalytic activity and adsorption power [22, 24].
Experimental
To conduct this work we have prepared two carbonaceous materials from the brown coal of Alexandria coal deposit with properties as described below [33].
1. Solid product of thermolysis (SPT) of the brown coal in argon at 800°C for 1 h (yield ~45 %) [32]. SPT has a specific surface area SBET=200 cm2/g, total pores volume Vz=0.17 cm3/g, adsorptive capacities in iodine AI=560 mg/g and methylene blue AMB=50 mg/g. OH-acid groups content is [-OH]=0.14 mmole/g [36].
2. Activated coal (AC-K) is a solid product of alkaline activation, which includes brown coal impregnation by potassium hydroxide, thermolysis (800°C, 1 h, argon), cooling, washing off alkaline and drying (yield ~30 %) [33, 36]. AC-K product has the following properties: SBET=1100 cm2/g, Vz=0.66 cm3/g, Ai=1000 mg/g, Amb=200 mg/g, [-OH]=1.00 mmole/g.
Reaction of hydrogen peroxide with SPT or AC-K is carried out as follows. A sample (2.0g) dried at 105°C for 2 h was brought in contact with H2O2 aqueous solution of preset concentration ([H2O2]=5-
30 %) and stirred at room temperature. Solution volume was selected in such a way to ensure the preset mole ratio H2O2/coal carbon (R = 0.5 or 1.0). Residual concentration of H2O2 was measured at times t. For this purpose, 0.2 cm3 of aqueous phase was put into a flask with 10 cm3 of sulphuric acid solution (5N), and titrated with potassium permanganate solution (0,1N). After that, the remaining reaction mixture was mixed with 200 cm3 of water, stirred, filtered out solid product. This solid was dried at 105°C for 2 h.
In order to determine the content of OH-acid groups ([-OH], mmole/g), barium hydroxide was used by technique described below [37]. A sample (0.5g) was stirred with 20 cm3 of Ba(OH)2 water solution (0.05 N) for 24 h, then the aliquote (5 cm3) was titrated with 0.01 N HCl. Experimentally measured error for [-OH] value was 4 %.
Results and discussion
Kinetics. In contact with SPT and AC-K the content of hydrogen peroxide decreases (Fig.1) and reagent fully decomposes for 24 h. As for AC-K, the process of decomposition is much faster. With lower ratio R=0.5, [H2O2] concentration decreases faster, especially during the first 30 minutes of the process. A similar picture is observed for both products and H2O2 solutions of any concentration within [H2O2]<30 % range. Hydrogen peroxide solutions are stable (Fig.1, line 1): within measured errors, [H2O2] concentration remains unchanged during 24 h.
Kinetics of H2O2 decomposition in the presence of AC-K or SPT is adequately described by the first order equation (Fig.2) and is shown as follows: R = 0.988 exp(-0.053T) (for AC-K product at [H2O2]=10 %); R = 0.913 exp(-0.0022T) (for STP with [H2O2]=10 %); R = 0,897exp(-0.0048T) (STP, [H2O2]=20 %); R = 0.977 exp(-0,0076T) (STP, [^O2]=30 %). For all equations correlation coefficients range from 0.96 to 0.99
With equal initial mole ration H2O2/carbon coal (R=1.0), the rate of hydrogen peroxide decomposition depends on its initial concentration (Fig. 2) and is drastically different for SPT and AC-K. In case of AC-K, H2O2 decomposes much faster: at [H2O2] = 30% the reagent decomposes violently with a strong exothermic effect. Effective rate constants k of H2O2 decomposition in STP presence range from 0.002 to 0.012 min-1, and their values demonstrate a liner growth as the concentration of reagent raises (Fig. 3). In case of AC-K, rate constants also grow with a raise in oxidizer concentration and range from k = 0.053 to 0.28 min-1.
Modification. While contacting with H2O2, weight of SPT remains unchanged or tends to decrease (Fig.4). The OH-groups content also remains practically unchanged and ranges [OH]=0.12-0.16 mmole/g. This value is close to content of phenol groups [OH]=0.14 mmole/g determined for solid thermolysis product of the brown coal at 800°C [36].
In contact with hydrogen peroxide, the changes in composition of the AC-K surface functional groups are more visible, which is apparently down to substantially bigger number (as compared to SPT) of active centers, the concentration of which is directly proportional to the specific surface area.
In longer contact of concentrated H2O2 (30 %) with AC-K, the content of OH-groups remians unchanged (Fig.4, line b) and makes 1.01±0.04 mmole/g for 24 h. If low concentrated (10 %) solutions are used, the content of acid groups grows monotonously (Fig.4, line 6) up to 1.55±0.04 mmole/g at 24 h contact.
time, min
Fig.1 Variation in hydrogen peroxide concentration in contact with SPT and AC-K: 1 - H2O2; 2 - SPT, R = 0.5; 3 - SPT, R = 1,0, 4 - AC-K,R = 1.0
time, min
Fig.2 lnR paramct^j^ij^ ^jndiEixonEBiscifljuiSiBiBnii'■
2- [H2O2]=20 %; ai8B^^injM-ell8l!iiittlmnE!l II I 111 I
[H2O2],%
Fig.3. The hydrogenperoxidedecomposition rateconstants vs concentration: 1- AC-K;2 -SPT
120
2,0
1,5
1,0
o
0,5
0,0
jn hfy Egg I sjl l^saj n»Q9
rOgSBnpi
Figll%(ra)flBmi)H BgmTgV cfflta№ffl||l|QUi323SDl|llHji^B
if treated with hydrogen peroxide with concentration of 10 %(□), 20 %(A), 30 %(o); 2- OH-groups content in ll^jjg^j^SlngiieilS^^: 4- OH-groups content in SPT at
[H2O2] = 30 % I
[h|02], %
Fig.5 OH-groups content in AC-K products modified with hydrogen peroxide solution of different concentration
(T=24h« I III I
Dependence of OH-groups content in modified AC-K samples (at 24 h contact) on hydrogen peroxide concentration is a curve with a maximum at [H2O2] = 10 % (Fig.5). Solution with such concentration displays the biggest modification effect, if we consider the growth in content of OH-acid groups. In this case, H2O2 is used to maximum efficiency, though the portion of oxidant used to form phenol groups is small: just 0.7 moles H2O2 of 100 moles reagent are used to form hydroxyl groups. H2O2 solutions of higher concentration are less efficient. A possible reason may be high rate of hydrogen peroxide decomposition in concentrated solutions, which essentially exceeds rate of modification reaction.
Thus, the first stage of our research showed, that H202 intensively decomposes in contact with brown coal thermolysis products obtained both in the presence of potassium hydroxide (AC-K product) and without it (SPT). Carbon material Cn acts as electron donor, which reduces OH-radicals according the reaction [23]
Cn + H2O2 ^ + OH + OH" (2)
By analogy with radical reactions in H2O2-Fe(II)-H 2O systems, we can assume the following interaction of hydroxyl radical to form a less active HO/-radical and its reaction with coal surface
Off + H2O2 ^ H2O + HO/ (3)
HO2^ + ^ Cn + O2 + H+ (4)
H+ + OH- ^ H2O (5)
In accepted mild conditions, modifying capacity of hydrogen peroxide is quite low. If treated with SPT, it does not form any functional groups or forms just a few of them (below sensitivity threshold of the method) (Fig.4). Also, reactions to form and further transform OH-acid groups are likely to have similar rates and stationary concentration of OH-groups remains unchanged. In this case, decomposition of hydrogen peroxide is a dominating process.
If AC-K is used, H2O2 decomposes faster, but modification is more visible and is demonstrated by increased content of OH-acid groups (Fig.5), which can be formed during reactions between coal arene fragments with hydroxyl radicals. Also, peroxide functional groups of active carbon can be formed through attachement of HO/-radical to cation-radical Cn+^ generated in reaction (2). When low concentration solutions are used, hydrogen peroxide decomposes slower and the contribution of modification reactions grows (Fig.5).
The second stage of this work will focus on an influence of organic adsorbates on AC-K behavior in CWPO. Changes in H2O2 decomposition kinetics in the presence of AC-K following adsorption of phenol and chlorophenols will be the subject of our further researches.
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Взаимодействие пероксида водорода с нанопористым материалом, полученным щелочной активацией бурого угля
А.С. Грибанова, В.А. Кучеренко, Т.Г. Шендрик, Ю.В. Тамаркина
Институт физико-органической химии и углехимии им. Л.М. Литвиненко НАН Украины Украина 83114, Донецк, ул. Р.Люксембург, 70
Рассмотрены применения углеродных адсорбентов в catalytic wet peroxide oxidation (CWPO) как варианте Advanced Oxidation Processes (AOPs). В условиях CWPO (концентрация [Н2О] < 30 %, 20±2 °C) изучено поведение нанопористого адсорбента АУ-К, полученного КОН-активацией (800 °C, 1 ч) бурого угля. Проведено сравнение АУ-К с твердым продуктом термолиза (ТПТ) бурого угля, полученного в тех же условиях без КОН. АУ-К, обладающий высокой адсорбционной активностью, является катализатором разложения Н2О2 с образованием ОН-радикалов, что даёт возможность объединить два экологически значимых процесса: концентрирование органических экотоксикантов на поверхности адсорбента и их последующее расщепление ОН-радикалами.
Разложение Н2О2 в присутствии АУ-К и ТПТ описывается уравнением кинетики I-го порядка и в контакте с АУ-К протекает в 20-30 раз быстрее. Константы скорости варьируются в интервалах 0,053-0,28 мин-1 (АУ-К) и 0,002-0,012 мин-1 (ТПТ) и с увеличением [Н2О2] возрастают. В условиях CWPO изучена окислительная модификация поверхности АУ-К и ТПТ. Зависимость содержания ОН-кислотных групп модифицированных образцов АУ-К (24 ч) от [Н2О2] передается кривой с максимумом при [Н2О2] = 10 %, где наблюдается максимальный модифицирующий эффект и наибольший прирост содержания ОН-групп (с 1,00 до 1,55ммоль/г). Уровень модификации мал; не более 1 % окислителя идет на образование функциональных групп.
Ключевые слова: нанопористый углерод; перекись водорода; кинетика разложения; модификация поверхности.
