Wastewater treatment by photocatalytic oxidation of Nano-ZnO 1

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					           Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.


            Wastewater treatment by photocatalytic oxidation of Nano-ZnO

                                                  Zhao Meng*            Zhang Juan*


                                                                Abstract
     Zinc Oxide is an excellent photocatalytic oxidation material. It has been widely used to deal with
wastewater, such as pharmacy wastewater, printing and dyeing wastes, papermaking wastewater, and so
on. The catalytic activity of nano-ZnO is much better than normal materials. It can also absorbs the light
in more wide spectrum. Its catalytic activity is mostly affected by the dosage of the catalyzer, the original
concentration of reactants, illumination time, intensity of illumination, pH value and atmosphere(oxygen)
flow, etc..
Key words: Zinc Oxide, photocatalytic oxidation, degradation, wastewater treatment, nano-ZnO,


                                                                     nanophase structure and characteristics, it gets the
                                                                     surface effect and bulk effect and gains some
                                                                     special capabilities(on magnetism, light, electric
                     1. Introduction
                                                                     and sensitivity etc.) and many new usages. The
                                                                     nano-zinc oxide grains’ latent importance is
      Most in the last few years, making use of the                  indubitable, some developed nations all throw
semi-conductor material photocatalytic oxidation                     great deal of funds to the research work, many
to oxidize poisonous pollutant is valued                             local academies and institutes, universities also
increasingly. A lot of organic matter can be                         organizes research strength, opening the research
decomposed into inorganic and low toxicity                           work of the nanomaterials(Yu(1995)).This paper
smaller compound. Because this kind of reaction                      handles the mechanism of waste water treatment
needs only light, catalyst and air, the processing                   by ZnO photocatalytic oxidation, the nano-ZnO
cost is lower, having become a kind of new                           synthesizes and the research situation and
promising method of the liquid waste                                 development trend on processing wastewater
processing(Legrini(1993), Hu(1995) ). Matthews                       through ZnO photocatalytic oxidation.
made use of TiO2, ZnO to deal with aromatic
hydrocarbons containing phosphorus, sulphur,
nitrogen and halogen by photocatalytic oxidation,                               2 principles of photocatalytic
confirming the finally production are CO2, H2O,
PO43-, SO42-, NO3-, and X(Matthews(1988)).                               oxidation(Cui(2001), Lei(2001))
     In recent years, nanophase materials gain
                                                                          Photocatalyst particles’ band structure is
more attention because of its special physical and
                                                                     composed of a low energy valence band filled
chemical features, and is applied in each realm
                                                                     with electrons and an empty high energy
gradually(Jiu(2002)). The nano-zinc oxide granule
                                                                     conduction band, which are divided by forbidden
size lies in the 1-100 nms, because of having
                                                                     band.When use energy bigger than the width of
   * Kunming University of Science and Technology, China
   I would like to thank Professor Hiroshige Tanaka and anonymous referee for their very helpful comments.
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.


the forbidden band to illuminate the catalyst,                 means the valence band electric potential
valence band electrons on the surface may absorb               changes more positive, otherwise the
the light energy to transit to the conduction band.            conduction electric potential changes
Then the conduction band gets electrons with                   more        negative.      Then      the
reducibility, yet the valence band produces holes              oxidation-reduction capability of the
possessing oxidizability. These electrons and                  electrons and holes is enhanced, and the
holes can move to the surface of the catalyst under            oxidation activity of nano-ZnO is
light energy, and can react with other                         improved;
substances(oxidate or reduce). In addition, they               2) Bigger specific surface area: There
can react with deliquescent oxygen and water in                are more atoms on the surface, it
suspended liquid and produce hydroxyls with high               improves the adsorption capability of
chemical activity, which can oxide the organic                 ZnO to organic pollutants;
pollutants into CO2, H2O and N2, and attain a
purpose of reducing the COD of waste water.
                                                             The efficiency of photocatalysis is related
     The basic reactions of above-mentioned            with the time spent by electrons and holes to get to
process are as follows:                                the surface of the particles. To the nano particles,
                  ZnO + hv →h+ + e-                    the particle diameter is very small, so it’s very
                                                       easy for the carriers moving from the inside to the
                    H2O→H+ + OH-                       surface, and causing oxidation or reduction
                   H+ + 0H- →H O•                      reaction. The smaller of the particle diameter, the
                                                       shorter time may be spent by carriers diffusing
                     h+ + H2O→H+ + •OH                 from inside to the surface. It can get higher
                     e- + O2 →•02-                     separated efficiency, and lower probability of
                                                       electrons and holes’ combination. As a result, the
                    02- 十 H+→H02•                      higher photocatalytic activity can be got.Therefore,
                                                       the nano-zinc oxide has more photocatalysis
                    2H02•→•02 + H2O2
                                                       efficiency than the common zinc oxide.
                 H2O2 + •02-→•OH+ OH-+ O2

                   H2O2 + hv → 2•OH                                  3    Synthesization of
    Organ+•OH 十 O2       → CO2 + H20 + other                             Nano-ZnO
productions
         The light reaction velocity is subjected to         There are so many methods to product
the restriction of combination of electrons and        nano-ZnO, such as solid-phase method,
holes. Adsorbed oxygen on grain surface is an          liquid-phase method, and gas-phase method
electron captor, which can restrain the                divided through materials; dry method and moist
combination of electron and hole. The nanoscale        method divided by state of reactants; physical
catalyst has more higher photocatalytic activity       method(sedimentation), chemical method, and
than normal material(Zhang(2001)). The reasons         comprehensive method divided by production
are:                                                   measure. The physical method is mainly a milling
         1) Quantum size effect: when the              process method, but we can get particles under 1
         particle diameter become less than a          micron. The production methods and their features
         certain critical value, the valence band      are summarized as follows (Shen(2004),
         and conduction band change into discrete      Cheng(2000)) in the table 1.
         level, the energy gap change broader,              As a conclusion, there are so many making
                                                       2
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.


methods of nano-zinc oxide. We can choose a                        economic technique request.
method according to the different usage and the

                                           Table 1 Production methods of nano-ZnO

                         classification                        process explanation

         Solid-p            comminution process                Conventional milling process method
    hase method
                          Solid-phase reaction method         Make solid-phase precursor first, then nano-ZnO can be got by its
                                                          thermal decomposition under certain temperature(Shen(2000)).

         Gas-ph        Gas-phase oxidation                     Under the high temperature, zinc powder react with oxygen, taking
    ase method                                            nitrogen as he carrier.

                       Spray pyrolysis                         Acetate or nitrate aqua fog of zinc change into tiny liquid drop,
                                                          then the liquid drop become product granules in the reactor through
                                                          the evaporation, desiccation, pyrogenation, agglomeration processes
                                                          etc.

                       laser induced chemical reaction         Gas molecules can be pyrolyzed or react by absorbing articular
                                                          wave-length laser beam, reacted crystals bring together into a nucleus
                                                          and then form nano powder. Or make use of a high energy laser beam
                                                          irradiates upon a metal slice surface to get nano powder by heating,
                                                          gasification and evaporation, oxidization(Zhu(1993)).

    Liquid-phas      chemical          Direct                  After adding precipitator to the dissoluble salt aqua, the precipitate
    e method       precipitation   precipitation          separates out under certain condition from the aqua, then the nano-ZnO
                      method       method                 can be got by precipitate’s thermolysis.

                                   precipitation   from         The precipitator doesn’t react to bring out the precipitate directly.
                                   homogeneous            It separates out from the solvent by reaction slowly and uniformly.
                                   solution

                       Sol-gel method                         Using inorganic salt or metallic alcohol salt as precursor,nano
                                                          powder can be gained through gelation process by hydrolytic
                                                          polycondensation and some post-treatment process.

                       hydrothermal synthesis                  Means a synthetic method that original reactants reacting under
                                                          certain temperature and water pressure in a closed system, using water
                                                          as solvent(Wang(1997)).

                       emulsion reaction                       Using some surfactants to produce inverse micelles in non-polar
                                                          solvent spontaneously. Those inverse micelles provide nanoreactor
                                                          energetics controllable. Then the nano granules can be
                                                          got(Zhang(1993)).

                                                                   aniline(Hu(1998)). The experiments confirmed
                   4    Application of                             that the photocatalytic reaction on the ZnO
                                                                   catalyst can be divided into 3 stages: the
      photocatalytic oxidation in                                  hydroxylation of benzene ring, production of
                                                                   carboxylic acid, oxidization to the carbon dioxide
          wastewater treatment                                     completely. The aniline degradation matches first
                                                                   order kinetics regulation. Under the condition of
    4.1 Degradation of aniline through                             oxygen shortage, the velocity of degradation
photocatalytic oxidation                                           declines. The main intermediate products are
                                                                   o-aminophenol, p-aminophenol and carboxylic
      Hu Chun using sunlight, xenon lamp as a                      acid compounds. The organic acids can be
light source, ZnO as photocatalyst, researched the                 oxidized into carbon dioxide. The different
photocatalytic     solution     behaviour       of                 carboxylic acids can be gained under different

                                                                  3
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.
conditions, with pH value under 6 or over 6,                   Superoxide anion may be changed into
because of the different degradation routes of           hydrogen peroxide through protonation:
aniline.
                                                                        O-*2+H+→HO·2
     The degradation reaction of aniline
 accelerates with the increasing of the ZnO
                                                                         HO·2+HO·2→H2O2+O2
 dosage, reaching the maximum at last. The
 dosage of ZnO arrives at 1g/l. This value is                            O-*2+HO·2→O2+HO-2
 corresponded to the entirely absorption of all
 photons, and changes with the different light                              HO-2+H+→H2O2
 intensity.
      There are more OH- adsorbed in the ZnO                     Then the hydroxyl free radical is
surface in the alkaline aqua. The aniline                brought out: H2O2→2OH.
degradation velocity is larger in this
situation..Claire(1991) studied the oxidation                     So O-*2 is one source of OH·, the oxygen
                                                       is the oxidant of hydroxylation procedure at the
product in the procedure of organic compounds
‘photocatalysis, confirmed that the free hydroxyl      same time.
radicals is the only oxidizing agent in the pH=11
                                                        4 . 2 Treatment of paper making wastewater
alkaline aquas, and its oxidation function is more
                                                       through photocatalytic oxidation
efficient than in the acid or neutral aqua greatly.
So in the pH>9 solution, along with the pH                  Zhu(2005) dealed with the lime grass pulp
increment, the aniline degradation speed increases.    paper- making wastewater by photocatalytic
In the acid, neuter and alkalescence aqua, the         oxidation, investigating the affection of ZnO
quantity of OH- adsorbed on the ZnO is smaller.        dosage, hydrogen peroxide quantity, pH value of
So the degradation velocity of aniline is slower.      wastewater, illumination time and light intensity
Also the reaction speed difference occurrence can      to the removal rate of COD in wastewater.
also be caused by the production’s difference in             The removal rate of COD is increasing
the acid and alkali solution. The spectrum of          gradually with enhancing the ZnO quantity, and
xenon light is continuous spectrum, same as the        reaches the maximum value with ZnO is 3g/l.
sun light. The degradation regulation is same in       Then the removal rate descends gradually with the
two kinds of light sources, but because of valid       addition of more ZnO over 3g/l. This is because
light intensity dissimilarity, the degradation speed   when the ZnO quantity is less, the holes number is
under sunlight is quicker.                             less, it is going against to OH· formation. And on
     The function of oxygen in the degradation of      the other hand, the excessive ZnO grains will
aniline. The reaction rate constant is 4.30×10-3       produce dispersion of light, which may affect the
min-1 under anaerobic situation. It is 8.64×10-3       transmittance of the solution, then decline the
min-1 under oxygen concentration as 20%. It is         reaction rate.
confirmed that the photocatalysis reaction rate is           Adding appropriate H202 in the liquid, can
limited of the combined of electrons and holes         accelerate the photocatalytic reaction obviously.
from the experiment conditions at present. The         This is because that H202 is a strong oxidizing
adsorbed oxygen is electrons’ capturer which can       agent, it can capture electrons and lower the
restrain the combination of electrons and holes.       recombination of surface electrons and holes, and
                                                       it can form •OH under the violet illumination itself.
                   O2(ads.)+e-→O-*2                    But as increasing the amount of H202, the COD
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.
removal rate becomes descending. Because high           catalysis center. But when there are excessive
H202 density may cause a great deal of H202             ZnO grains, suspended ZnO grain have certain
adsorbed in the surface of catalyst, and may            effect to the light intensity, by reflecting or
obstruct organic matter in water to be adsorbed.        sheltering the light. Then the removal rate is
                                                        affected.l
     The COD removal rate is the highest when
the waste water pH is 10. The photocatalytic                  The removal rate of 4-nitrochlorobenzene can
reaction mechanism thinks that OH- can be used as       reach 98% when the illumination time is 100 mins.
the capturer of holes(h'+OH-→•OH), when the pH          It is increased with the augment of illumination
is low, there is no enough •OH, it is not conducive     time.
to the degradation reaction. With the increasing of         The highest removal rate can be got when the
pH, the OH- concentration on the surface of ZnO         pH value of the solution is within 7 to 9.
granules, it is conducive to the form of •OH, then
accelerates the reaction rate. On the other hand,           4.4 Treatment of methyl orange through
the high H+ concentration is go against the form        photocatalytic oxidation
of •OH(h++H20→ •OH+H+) . But with the higher                  Hong Ruoyu(2005) made nano-ZnO by
pH, ions produced from organic matter                   precipitation from homogeneous solution. The
degradation may replace some •OH on the surface         product particles are balls, the diameter is about
of the catalyst, it is not good to the degradation      30nm. The structure is hexagonal system
reaction. So there is a best pH value in the reaction   examined by XRD. The research result shows the
system. In the meantime, the ZnO is an                  photocatalysis reaction of ZnO and methyl orange
amphoteric oxide, the waste water pH will               is first order reaction. The best efficiency can be
influence the stability of the catalyst.                got when the catalyst dosage is 3.3-5.0g/l with the
     The COD removal rate increases with the            air flow of 45-55ml/min. The violet can be took
exaltation of the light intensity. When the             place when there is certain sunlight intensity. The
low-pressure mercury light power is 50 W,               research also shows that the efficiency can be
shining for 8 h, the COD removal rate can reach         enhanced if adding certain Ag+ in the nano-ZnO,
80% above, at this time water COD is 154 mg/L.          the best quantity if the Ag+ dosage is 1%(mass
The method is effective to water treatment, and         ratio) .
has no secondary pollution, provided a new                  4 . 5 Treatment of printing and dyeing
method for the processing of lime method deckle         wastewater through photocatalytic oxidation
waste water.
                                                              High chroma printing and dyeing wastewater
     4 . 3 Treatment of 4-Nitrochlorobenzene            has serious pollution hazard. Its treatment is very
through photocatalytic oxidation                        difficult because that its constituent is very
     Yu(2005) made nano-ZnO with the                    complex and its high chroma. The dye itself is a
evaporation oxidation method. The product has           photosensitizer and can form singlet state oxygen
high purity, has no miscellaneous things. The ZnO       under light, which can accelerate the
crystal size is 75 nms. The experiments using the       photocatalytic oxidation. So the photocatalytic
product processing the 4-nitrochlorobenzene             oxidation is suitable to dyeing wastewater. Ma
confirms that the highest removal rate can be get       Zhengxian’s(Ma(2002), Ma(2005)) research using
when the mass concentration of ZnO is 0.5 g/l.          nano-ZnO with average diameter 15nms finds the
Because of the reaction taking place on the surface     dosage of catalyst has obvious effect to the COD
of the ZnO grain, the removal rate is low when the      removal rate. The removal rate increases with the
catalyst ZnO is of a little amount, with less           augment of catalyst until the maximum 0.3 %
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.
-0.4 %. Continuous increasing has reverse effect.        reaction conditions and less secondary pollution.
So the dosage of ZnO is 0.3% in the experiment of            Xie Jie researched the treatment of
illumination time.                                       semisynthesis wastewater of cephalosporin using
      The COD removal rate increases with the            the photocatalytic oxidation. The experiments
illumination time prolonged. The relationship is         show(Xie(2005)):
linearity. So the reaction rate of Reactive Yellow           1)There is a best dosage of catalyst. The best
K-RN’s photocatalytic degradation answeres for           dosage can be declined by improving the mixture
first order reaction.                                    conditions of the reactor.
      Zhou Xiaohong researched the effect of zinc            2)The original pH value of the wastewater
oxide      to     6    kinds     of     water-soluble     has large affection to the efficiency of COD
dyestuffs(Zhou(2003)).The experiment expresses            removal.
that the decolor rate takes direct ratio relation to
the illumination time, the more time, the larger              3)The COD removal rate is increased with
decolor rate. The photocatalytic decolor efficiency      the enhanced illumination time. At last, it can get
is different because of the dyestuff structure           a stable efficiency.
dissimilarity. As to malachite green(arylmethane               4)The original concentration of wastewater
dye), the catalyst efficiency is the best, the decolor   has obvious affection to the COD removal rate. So
rate is 99.8 %. The efficiency of reactive brilliant     the treatment efficiency can be enhanced by
blue X-BR(anthraquinone dye) decolor rate is the         diluting the wastewater.
worst, is 45.82%. The decolor rate of azo dyestuff
                                                               5)The treatment efficiency of semisynthesis
(such as reactive yellow) is mediacy. The decolor
                                                         wastewater of cephalosporin is similar, regardless
rate sequence of different dyes is arylmethane
                                                         using either ZnO or TiO2 as catalyst. The
dyes> azo dyestuff> anthraquinone dyes. The
                                                         efficiency can be better if using both two kinds of
research shows that the relationship of the
                                                         catalysts.
concentration of dyestuff and illumination time is
the first order reaction.                                    4.7 Treatment of landfill leachate through
                                                         photocatalytic oxidation
     4 . 6 Treatment of pharmacy wastewater
through photocatalytic oxidation                              The leachate of landfill is an organic
                                                         wastewater with complex compositions. It has
      The pharmacy wastewater has many varieties,
                                                         many organic pollutants and over 10 kinds of
their composition is very complex, and the
                                                         metal ions. Zhou Changwei(2003) researched the
pollutant concentration is high. So it is very
                                                         processing efficiency of leachate with TiO2 and
difficulty to treat those wastewater. In the
                                                         ZnO in different conditions. The results show that
pharmacy industry, the most wastewater is
                                                         the TiO2 is better than ZnO. The pH value has no
discharged from the cephalosporin and penicillin
                                                         obvious affection. The best efficiency can be got
production. In biotreatment processes, the special
                                                         under illumination for 60 minutes.
pollutants    can     restrain   or     kill    the
microorganisms(Fan(2002)). So the degradation
efficiency can not be ensured in the activated                              5 Conclusions
sludge systems. The special pollutants can also get
into the environment and accumulate there, and               The photocatalytic oxidation reactions show
make more serious affection to the ecosystem. The        some properties as follows: Most of the reactions
photocatalytic oxidation is a useful method with         are first order dynamics or quasi first order
simple operation, low energy consumption, balmy          dynamics; there is a best reaction dosage of
          Global Environmental Policy in Japan,No.12(April, 2008) Internet Version,1-9.
oxidant and a best original reactant concentration;       References
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