S Rakovsky - Fields of ozone applications - страница 1

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CHEMISTRY & CHEMICAL TECHNOLOGY

Vol. 3, No. 2, 2009 Chemical Technology

Slavcho Rakovsky1, Metody Anachkov1 and Gennady Zaikov2

FIELDS OF OZONE APPLICATIONS

11nstitute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria 2 N. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, 119334 Moscow, Russia; chembio@sky.chph.ras.ru

Received: July 08, 2008

© Rakovsky S., Anachkov M., Zaikov G. 2009

Abstract. The powerful oxidative action of ozone provides basis for development of novel or improved technologies which are widely used in ecology and industry. Special attention is paid to purification of waste gases, water, and soils. The disinfection and cleaning of drinking and process water are considered in detail. Applications of ozone in chemical, pharmaceutical and perfume industries, cosmetics, cellulose, paper and sugar industries, flotation, microelectronics, and many others are also examined in the review.

Key words: ozone, ecology, industry, water, gases, organic synthesis.

1. Introduction

Ozone is a pale blue poisonous gas with a sharp, cold, irritating odor. Most people can detect about 0.01 ppm in air. At 161 K it forms a dark blue liquid. At temperatures below 83 K it forms a violet-black solid. Its molecule contains three oxygen atoms:

©

has C2v symmetry, 0-0 bond lengths 1.278 A°, 0-0-0 angle 116.780 and IUPAC name trioxygen.

Ozone forms protective absorbed layer on the altitude of 25-30 km in the atmosphere, where it is synthesized by the reactions: O2 + hv = 2 O 02 + 0   = °,

302 =  2 03        Л H at 101 kPa is +284.5 kJ-mol1

Sunlight with wavelength until 240 nm brakes the O-O bond in oxygen molecule and forms atomic oxygen, which recombined with a molecule of oxygen forms ozone.

The appeared ozone absorbs the sun light until 320 nm and thereby preserves the life on the earth. The ozone concentration in the ozone layer can reach 50 ppm. The problems with ozone hole are the subject of many special issues but not of this paper.

Exposure to 0.1 to 1 ppm produces headaches, burning eyes, and irritation to the respiratory passages. On the earth ozone is a part of the photochemical smog, and it is produced by photochemical reactions in the air of conifer forests and sea coasts where its concentration varies from 0.005 to 0.05 ppm. In higher concentrations ozone is formed around all devices using radiation from gamma to UV-light and high voltage and sparks.

The allowable ozone concentration in the air of the working places is 0.1 ppm (vol). The olfactory threshold of the ozone concentration is 0.02 ppm (vol).

Ground-level exposure of 0.1 to 1 ppm ozone produces headaches, burning eyes, and irritation to the respiratory passages.

Ozone is not stabile and can not be stored. Its half-life at ordinary temperature in the air is 16-20 h and in water - 15-30 min at solubility of 570 mg/l.

Ozone is a very strong oxidizing agent due to its high oxidation potencial of 2.07 V. By this index it is ordered on the place after fluorine (3.06), hydroxil radicals (2.80), and atomic oxygen (2.42). It oxidizes all metals (except Pt, Au and Ir), non-metals to the highest extents of oxidations, and organic and inorganic compounds in the mild conditions and is extremely activity against viruses, bacterias, microbes, spores, fungi, etc. The powerful oxidative characteristics of ozone make it very attractive for industrial usage [1], wide application in ecology [2], chemical, pharmaceutical, and perfume industries, cosmetics [3, 4], cellulose, paper, and sugar industries, flotation, microelectronics, veterinary and human medicine, agriculture, foodstuff industry, and many others [5-8].

The ozone reacts in three ways:

1. Direct oxidation on the substrate by the process of ozonolysis;

2. Direct oxidation on a substrate for the oxygen atom loss;

3. Catalytic oxidation due to the oxygen present in ozonized air.

Corona discharge in a dry process gas containing oxygen is presently the most widely used method of ozone generation for water treatment. A classical production line is composed of the following units: gas source (compressors or liquefied gas), dust filters, gas dryers, ozone generators, contacting units, and off gas destruction.

It is of utmost importance that a dry process gas is applied to the corona discharge. Limiting nitric acid formation is also important in order to protect the generators and to increase the efficiency of the generation process. In normal operation of properly designed systems, a maximum of 3 to 5 g nitric acid is obtained per kilogram ozone produced with air. If increased amounts of water vapor are present, larger quantities of nitrogen oxides are formed when spark discharges occur. Also, hydroxyl radicals are formed, that combine with oxygen radicals as well as ozone. Both reactions reduce the ozone generation efficiency. Consequently, the dryness of the process gas is of relevant importance to obtain a yield of ozone. Moreover, with air, nitrogen oxides can form nitric acid, which can cause corrosion. The presence of organic impurities in the feed gas should be avoided, including impurities arising from engine exhaust, leakages in cooling groups, or leakages in electrode cooling systems.

Oxidation of organic materials by ozone is selective and incomplete at the concentrations and pH values of aqueous ozonation. Unsaturated and aromatic compounds are oxidized and split at the double bonds, producing carboxylic acids and ketones as products. Because of the high reactivity of ozone, oxidation of organic matter in the aqueous environment, whether potable water or wastewater, will consume ozone in varying amounts. Therefore, one of the most significant parameters for evaluating ozone is the determination of the immediate ozone demand. Oxidation of the (organic) material is usually incomplete. It is estimated that the reduction in TOC may be only 10-20 percent although decreases in COD and BOD are generally greater, ranging up to 50 percent of COD reduction.

There are instances where COD has appeared to increase, resulting from conversion to more readily oxidized compounds. Ozone also causes the formation of assimilable organic carbon (AOC). AOCs are compounds that are more readily digested by bacteria.

Ozone is an effective bleaching agent against organic compounds that contribute to color in wastewater and potable water. The ability to attack these compounds, including humates and fulvates, makes ozone a perfect wastewater polishing agent.

The ability of ozone to destroy taste forming phenolic compounds is an important contribution to the field of potable water treatment. Ozone is capable of destroying other taste forming compounds of unknown origin. There are two major mechanisms by which ozone reacts with organic material. The first one is a direct additive attack in which ozonides and ultimately peroxides are formed together with the organic molecule splitting.

Another mechanism is an accompaniment to the decomposition of ozone. This decomposition proceeds by the formation of the free radicals OH, HO2, and HO3. These free radicals, especially the hydroxy radical, are highly reactive against organic material and may lead to auto-oxidation of a wide variety of organic matter, particularly substances present in wastewater effluents. The free radical auto-oxidation mechanism may be well involved in the disappearance of residual ozone after the initial rapid demand has been satisfied.

Low levels of ozone have been advertised to be of some disinfectant use in residential homes, however the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency. Humidity control can vastly improve both the killing power of the ozone, and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Industrially, ozone is used to:

disinfect laundry in hospitals, food factories, care homes, etc.;

• disinfect water in place of chlorine;

deodorize air and objects, e.g. after a fire; this process is extensively used in fabric restoration;

• kill bacteria on food or on contact surfaces;

• ozone swimming pools and for spas sanitation;

• scrub yeast and mold spores from the air in food processing plants;

• wash fresh fruits and vegetables to kill yeast, mold and bacteria;

• chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");

• aid flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);

• manufacture chemical compounds via chemical synthesis [2];

• clean and bleach fabrics (the former use is utilized in fabric restoration) (the latter use is patented);

• assist in processing plastics to allow adhesion of

inks;

• age rubber samples to determine the useful life of a batch of rubber;

• clean hospital operating rooms where air and surfaces needs to be sterile;

eradicate water borne parasites such as Giardia and Cryptosporidium in surface water treatment plants.

Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.

Ozone will oxidize metals (except gold, platinum, and iridium) to metals oxides in their highest oxidation state:

2 Cu2+ + 2 H+ + O3 ® 2 Cu3+ + H2O + O2 Ozone converts oxides to peroxides:

SO2 + O3 ®       + O2 It also oxidizes oxides to oxides of higher oxidation number:

NO + O3 ® NO2 + O2 The   above  reaction  is   accompanied by chemiluminescence. The NO2 can be further oxidized:

The NO3 formed can react with NO2 to form N2O5:

NO2 + NO3 ® N2O5

Ozone reacts with carbon to form carbon dioxide, even at room temperature:

C + 2 O3 ® CO2 + 2 O2 Ozone does not react with ammonium salts but it reacts with ammonia to form ammonium nitrate: 2 NH3 + 4 O3 ® NH4NO3 + 4 O2 + H2O Ozone reacts with sulfides to make sulfates:

PbS + 4 O3 ® PbSO4 + 4 O2 Sulfuric acid can be produced from ozone, either starting from elemental sulfur or from sulfur dioxide: S + H2O + O3 ® H2SO4 3 SO2 + 3 H2O + O3 ® 3 H2SO4 All three atoms of ozone may also react, as in the reaction with tin (II) chloride and hydrochloric acid:

3 SnCl2 + 6 HCl + O3 ® 3 SnCl4 + 3 H2O

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:

H2S + O3 ® SO2 + H2O

In an aqueous solution, however, two competing simultaneous reactions occur, one with the formation elemental sulfur, and another with the formation of sulfuric acid:

H2S + O3 ® S + O2 + H2O 3 H2S + 4 O3 ® 3 H2SO4 Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone: I2 + 6 HClO4 + O3 ® 2 I(ClO4)3 + 3 H2O Solid nitryl perchlorate can be made from NO2, ClO2, and O3 gases:

2 NO2 + 2 ClO2 2 O3 ® 2 NO2ClO4 + O2 Ozone can be used for combustion reactions and combusting gases in ozone provide higher temperatures than combusting in dioxygen (O2). The following is the carbon subnitride combustion reaction:

3 C4N2 + 4 O3 ® 12 CO + 3 N2 Ozone can react at cryogenic temperatures. At 77 K atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical, which dimerizes [9]:

H + O3 ® HO2 + O 2 HO2 ® H2O4 Ozonides can be formed, which contain the ozonide anion, O3-. These compounds are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO3, RbO3, and CsO3 can be prepared from their respective superoxides:

Although KO3 can be formed as above, it can also be formed from potassium hydroxide and ozone [10]: 2 KOH + 5 O3 ® 2 KO3 + 5 O2 + H2O

NaO3 and LiO3 must be prepared by action of CsO3 in liquid NH3 on an ion exchange resin containing Na+ or

Li+ ions [11]:

CsO3 + Na+ ® Cs+ + NaO3 Treatment of calcium dissolved in ammonia with ozone leads to ammonium ozonide not calcium ozonide

[12]:

3 Ca + 10 NH3 + 6 O3 ® Ca«6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2 Ozone can be used to remove manganese from water, forming a precipitate which can be filtered: 2 Mn2+ + 2 O3 + 4 H2O ® 2 MnO(OH)2 (s) + 2 O2 + 4 H+ Ozone will also turn cyanides to one thousand times less toxic cyanates:

CN- + O3 ® CNO- + O2

Finally, ozone will also completely decompose urea

[13]:

(NH2)2CO + O3 ® N2 + CO2 + 2 H2O

2. Ecology

2.1. Waste Gases

More than 100 contaminants of the atmospheric air have been identified. Among them SO2, CO2, nitrogen oxides, various hydrocarbons, and dust constitute 85 %.

The main sources of harmful substances emissions, i.e. dust, SO2, and CO2 in the air are coke, briquettes, and coals processing plants, thermal power stations, air, water, and road transport [9].The exhaust gases contain also CO, organic and inorganic compounds, etc. [10]. Dust, sulfur dioxide, carbon dioxide, nitrogen oxides, and organic compounds, etc. are the main pollutants released in the environment from metallurgy, fertilizers industry, and petrochemistry. The most typical air pollutants emitted by some chemical productions are listed in Table 1.

The technologies with ozone participation are very promising for SO2 utilization as sulfate, CO as carbonate, and nitrogen oxides as nitrates.

Scrubbers used previously for removing only acids, for example HCl and HF etc., now can be used for SO2

Table 1

Main air contaminants emitted by chemical productions

No.

Production

Pollutants

No.

Production

Pollutants

1.

Nitric acid

NO, NOx, NH3

12.

Ammonium nitrate

CO NH3, HNO2, NH4NO3 - dust

2.

Sulfuric acid:

a) Nitroso

b) contact

NO, NOx, SOx, H2SO4, Fe2O3, dust

13.

Superphosphate

H2SO4, HF, dust

3.

Hydrochloric acid

HCl, Cl2

14.

Ammonia

NH3, CO

4.

Ocsalic acid

NO, NOx, C2H2O2, dust

15.

Calcium chloride

HCl, H2SO4, dust

5.

Sulfamidic acid

NH3, H2SO4, NH(SO3NH4)2,

16.

Chlor

HCl, Cl2, Hg

6.

Phosphor

P2O3, H3PO4, HF, Ca5F4(PO4)2, dust

17.

Caprolactam

NO, NO2, SO2, H2S, CO

7.

Phosphoric acid

P2O3, H3PO4, HF, Ca5F4(PO4)2, dust

18.

PVC

Hg, HgCl2, NH3

8.

Acetic acid

CH3CHO, CH3OH

19.

Artificial fibers

H2S, CS2

9.

Nitrogen fertilizers

NO2, NO, NH3, HF, H2SO4, HNO3

20.

Mineral pigments

Fe2O3, FeSO4

10.

Carbamide

NH3, CO, (NH2)2CO, dust

21.

Electrolysis of NaCl

Cl2, NaOH

separation via the addition of an oxidizing agent into the water intended for gas treatment.

Ozone as compared with the conventional oxidizers such as hydroperoxide, chlorine, sodium hypochloride, perchlorate, shows appreciably higher oxidizing efficiency [11].

The rate constant of ozone reaction with NO is about 1010 cm7(mol-s), and with NO2 it is ~ 107 cm3/ (mol-s) whereby the oxidation of NO by ozone in the liquid phase is characterized by a higher absorption rate and rise in the concentration of the obtained HNO3. Moreover the oxidation of NO can be accomplished in the exhaust gases or in the course of absorption [12].

The CO oxidation by ozone is carried out in the presence of Fe, Ni, Co, and Mn oxides. In most cases ozonation is more appropriate than the conventional methods and sometimes it appears the only possible way for its preparation [13].

The purification of exhaust gases from burners working on liquid and gas fuel is accomplished by using oxidation catalysts of ABO3 perovskite-type oxides combined with ozonation. Ozone is injected prior to the waste gases flow. A may be La, Pr, or other alkali earth element; B may be Co, Mn, or other transition metal. A may be also partially substituted by Sr, Ca, or any other alkali earth element. A catalyst deposited on ceramic support of honeycomb type may contain SrCo0.3Mn0.7O3. It is oxidized 24 % CO in the absence of ozone and 80 % in ozone atmosphere [14].

The removal of nitrogen oxides (NOx) from waste gases is carried out through ozone oxidation in a charged vertical column. The lower part of the column is loaded by 5-15 % KMnO4, and ozone is blown through the lower and upper part of the column and the waste gases are fed to the center of the column charge. The inlet concentration of 1000 ppm NOx in the waste gas is reduced to 50 ppm in the outlet gas flow. Another method for cleaning of exhaust gases from NOx involves the application of plasma generator and ozonator [15] which practically leads to the complete removal of nitrogen oxides.

Sulfur containing compounds are removed from gas mixtures by silicon oil scrubbing and ozone oxidation of absorbed pollutants. A model system providing that the gas mixture contains methandiol and deimethylsulphide is fed at a rate of 9-120 m3/h into silicon oil charged scrubber followed by ozone oxidation of the absorbate. The oxidates are extracted with water and the silicon oil is regenerated. Thus the purified gas mixtures practically do not contain any sulfur [16].

A special apparatus is designed for decomposing acyclic halogenated hydrocarbons in gases. It includes a chamber for mixing of the waste gases with ozone coupled with UV-radiation, ozonator, and inlet and outlet units. This method is very appropriate for application in semiconducting industry whereby acyclic and halogenated hydrocarbons are used as cleaning agents [17].

The mechanisms of ozonolysis of volatile organic compounds such as alkenes and dienes are discussed and the products output is determined by matrix isolation FT-IR spectroscopy [18].

The synthesis of a material from zeolite via pulverization, granulation, and drying at 773-973 K,

cooling to 373-473 K, electromagnetic radiation or ozone treatment is described in [19]. The material obtained is suitable for air deodorization, drying and sterilization.

The purification of gases containing condensable organic pollutants can be carried out by gas treatment with finely dispersed carbon, TiO2, Al2O3, Fe2O3, SiO2 and H2O2 and ozone as oxidizers [20].

A wet-scrubbing process for removing total reduced sulfur compounds such as H2S and mercaptans, as well as the accompanying paramagnetic particles from industrial waste gases is proposed [21]. For this purpose a water-absorbing clay containing MnO2 is used. The collected clay is regenerated by ozone oxidation.

The removal of mercury from waste gases is carried out by catalytic ozonation [22]. The used gases mixed with ozone are blown over a zeolite supported Ni/NiO catalyst. This procedure results in 87 % conversion of mercury into mercuric oxide which is isolated by filtration. Pt and CuOx/HgO system is proposed as another catalyst for this process [22].

2.2. Waste Water

In contrast to the cleaning of natural water by ozonation, which is experimentally confirmed as the most appropriate, the purification of waste water by ozone is still an area of intensive future research. This could be explained by the great diversity of pollutants in the used water and the necessity of specific approach for each definite case.

The recycling of water from cyanide waste water (copper cyanide contaminant) is accomplished by ozone oxidation combined with UV radiation and ion exchange method [23]. Further, the oxidate is passed through two consecutive columns charged with cationite and anionite resins. The ozonolysis priority over the conventional chlorinating method is demonstrated by the fact that ozonation fails to yield chlorides whose removal requires additional treatment.

The ozonolysis of CN ions leading to CNO- ion formation combined with UV-radiation proceeds at threefold higher rate as compared with that without radiation employment. Upon exchange in NaOCN, the Na+ is substituted by H+ followed by its decomposition to CO2 and NH4+ via hydrolysis in acid medium. The ammonium ion is absorbed by the cation-exchange resin. The solution electroconductivity on the anion-exchange resin becomes lower than 10 mu-S/cm and cyanides, cyanates or copper ions have not been monitored. On the basis of these experimental results, a recycling method and apparatus for detoxification of solutions containing cyanides is proposed. The recycled water from the cyanide waste water may be re-used as deionization water in gold-plating. The process ion-exchange resins are regenerated after conventional methods. Actually, this method is not accompanied by the formation of any solid pollutants [23].

The waste water from the electrostatic and galvanic coatings containing CN- -ions and heavy metals is treated consecutively by ozone and CO2. The cyanides and carbonates accumulated in the precipitate are already biodegradable [24].

Waste water containing Cr(III) is oxidized by ozone combined with UV-radiation [25]. The radiation reduces the oxidation time about three-fold. The oxidate is consecutively passed through columns charged with cationite and anionite resulting in the obtaining of waste water with electroconductivity below 20 mu-S/cm. This deionized water can be successfully used for washing of coated articles. The Cr(IV) concentrated solution from the anionite regeneration can be further treated by appropriate ion-exchange method yielding a high purity

Cr(IV) solution [25].

Purification of waste water containing ClCH2COOH and phenol is carried out by ozone treatment and UV-radiation in the presence of immobilized photocatalyst. Thus the rate of decomposition is higher than the rate obtained only in the presence of ozone and UV-radiation [26].

Used water containing small amounts of propyleneglycol nitrate or nitrotoluenes is a subject to combined oxidation with ozone and H2O2, under pressure and heating to the supercritical point. Thus a nominal conversion of the contaminants higher than 96.7 % is achieved [27].

In laboratory experiments for reduction in residual COD in biologically treated paper mill effluents it is subject to ozonation or combined ozonation and UV radiation at various temperatures and pressures [28]. At a ratio of the absorbed ozone to COD less than 2.5:1 g/g, the elimination level with respect to COD and DOC is up to 82 % and 64 %, respectively. The ozone consumption is essentially higher in the case of the UV combined ozonation at pH>9 and elevated temperature to 313 K.

The decolourization and destruction of waste water containing surface active substances is performed by ozone treatment at ozone concentration of about 80 mg/l. It has been found that for waste water which has undergone partial biological treatment by fluent filters or anaerobe stages up to COD concentration > 500 mg/l and ratio 30D5/COD>0.2 ozonolysis with ozone concentrations up to 1.8 g/l does not change CBR, while after the complete biological treatment at COD<500 mg/l ozonation results in the rise of the 30D5/COD ratio from <0.05 to 0.37. This evidences the increase of CBR [28].

The purification of waste water from paper manufacture characterized by enhanced biodegradability, as well as the removal of COD and halogenated compounds is carried out by a method based on ozonation and biofiltration [29].

Waste water from the paper pulp production is cleaned up by the treatment with ozone and activated acid tar. The high values of pH favor the lignin decomposition

and carboxylic acids formation since in this case the •OH - radicals and not ozone are the oxidizing agent. The application of activated acid tar has a positive effect on the dynamics of the microbiological growth, substrate consumption, and CBR of organic acids. For example, maleic and oxalic acids are decomposed completely at ozonation of waste water in the presence of activated acid tar. For immobilization of biological culture the use of polyurethane foams appear to be very suitable [30].

The used water in collectors of a plant for paper manufacture is ozonized in a foamy barbotating contactor. The results indicate that ozone is very efficient in the oxidation of coloring and halogenated compounds. Its activity is proportional to the amount of absorbed ozone regardless of the variations in the gas flow rate, the inlet ozone concentration and contactor design. The amount of consumed ozone depends on the operating conditions and waste water characteristics. The absorption rate rises in the presence of stages including ozone decomposition, particularly at higher rates [31].

In biological granular activated carbon (GAC) columns the effect of pretreated ozonation on the biodegradability of atrazine is investigated. The metabolism of isopropyl-14C-atrazine gives higher amount of 14CO2 than the ring UL-14C - atrazine which shows higher rate of dealkylation than the process of ring cleavage. The pretreatment with ozone increases the mineralization of the ring UL-14C - atrazine and consequently raises the GAC-columns capacity. 62 % of the inlet atrazine is transformed into 14CO2 in columns charged by ozonized atrazine and water. However, in columns with nonozonized atrazine and water only 50 % of the inlet atrazine is converted into 14CO2 and in columns supplied only with nonozonized atrazine only 38 % of it are converted to 14CO2 [32].

The waste water from petrochemical industry contains substantial amounts of phenol. The phenols mixtures appear to be more toxic than phenol itself and possess a synergetic effect. Their decomposition to nontoxic products is an acute problem. The oxidation of phenols such as p-cresol, pyrocatechin, rezorzine, and hydroquinone by O3 in aqueous medium appears to be a promising method for their degradation. At concentrations of 100 mg/l and pH = 11.5 their complete oxidation is performed for 30 min.

The phenol content in used water is reduced from 145-706 mg/l to 2.5 mg/l at ozone consumption of 1.1­2.6 g/l. In real conditions the ozone consumption varies from 5 to 10 g/l per g phenol (Table 2).

The treatment of concentrated aqueous solutions of phenol (1.0 g/l) with ozone causes the appearance of a yellowish coloring after absorption of 1 mol O3 per 1 mol phenol which gradually fades away. In this case the higher phenols undergo slight oxidation and H2O2 is identified in the water after the treatment.

Table 2

Results of the ozonation of waste water from coke processing. [O3] = 5 mg/l, flow rate 6 l/min and treatment time - 4 h

Pollutant, mg/l

Before ozonation

After ozonation

Monophenols

710

0.8

Polyphenols

380

198

Cyanides

3

29

Thiocyanides

384

0

Thiosulfates

538

0

Sulfides

43

2

H2O2

0

12

Bases

114

27

The tests demonstrate that upon varying content of phenols in the used water the ozonation is one of the most promising method for their removal. The ozonation was found to be applicable for the decomposition of rhodanide in neutral and weak acid medium in the temperature range of 282-298 K. The ozone consumption in this case is 2 mg/mg. The ozone uptake for the oxidation of the cyanide ion (CN-) is 1.8 mg/mg.

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