ELECTROCHEMICAL OXIDATION OF ORGANIC POLLUTANTS

Electrochemical Oxidation Of Organic Pollutants

NUME:Stefan Andreea

GRUPA:SIPOL

Electrochemical Oxidation Of Organic PollutantsAbstract

The main aim this paper is to use "Electrochemical Technology for removal of Organic Pollutants from Waste Water". Today hazardous waste management has become one of the most challenging tasks to this technological world because tons of organic pollutants including various carcinogens are being exposed without the sufficient treatment prescribed as per the "ENVIRONMENTAL PROTECTION AGENCY" norms. Also in the current scenario, the Indian government has made it mandatory to treat wastewater before discharging it.

Electrochemical depollution is a very efficient and economic method, suitable when the wastewater contains toxic or non-biodegradable organic pollutants. Wherever a high standard of living is to be secured in the long-term and environmental resources protected, essential requirements are that wastewater is treated in a proper manner.

Thus by means of electrochemical technology, which is cost and safety effective, we can help reduce concentrations of organic pollutants to an extent as minimum as possible and maintain the present status of the chemical world.Keywords : Electrochemical oxidation, organic pollutants, electrochemical cells. IntroductionWater has a number of unique properties that are essential to life and that determine its environmental chemical behaviour. Many of these properties are due to waters polar molecular structure and its ability to form hydrogen bonds. Water also has the highest dielectric constant of any common liquid, a maximum density as a liquid at 40 C and a higher heat capacity than any other liquid except amonnia. But most of the important chemical phenomena associated with water do not occur in solution but rather through interaction of solutes in water with other phases. For example the oxidation -reduction reactions catalyzed by bacteria occur in bacterial cells. Many organic hazardous wastes are carried through water as emulsions of very small particles suspended in water. Some hazardous wastes are deposited in sediments in bodies of water from which they may later enter the water through chemical or physical processes and cause severe pollution effects which need to be eliminateed as much as possible.

The wastewaters are usualy oxidated with ozone, which is a powerful oxidant but the total organic carbon removal was no more than 30% and the same results were obtained using hydrogen peroxide in the presence of Fe+2 as a catalyst. In general by a chemical oxidation the organic pollutants are almost completely eliminated but the removal of total organic carbon still remains a problem. These are the reasons why the electrooxidation of the hazardous pollutants has been the subject of extensive studies during recent years.

The electrochemical method of depollution presents many important advantages because it does not need auxiliary chemicals, it is applicable on a large range of pollutants and does not need high pressures and temperatures.

There has been new research on the electrochemical oxidation of organic compounds from the wastewaters due to its great efficiency and the rigorous control which it allows. The electrochemical oxidation of pollutants from wastewaters has been studied using anodes made from different materials. This is attributed to the oxidation of the adsorbed organic compounds to carbon dioxide. It has also been proven that under the same conditions the electrooxidation index obtained on the SnO2 anode was higher than on the Pt anode, indicating a higher degree of phenol oxidation.

Wastewater Treatment :An Overview

As indicated above, industrial wastewater contains a vast array of pollutants: insoluble, colloidal, and particulate forms, both inorganic and organic. In addition, the required effluent standards are also diverse, varying with the industrial and pollutant class. Consequently, there can be no standard design for industrial water pollution control; rather, each site requires a customized design to achieve optimum performance. However, each of the many proven processes for industrial waste treatment is able to remove more than one type of pollutant and is in general applicable to more than one industry. Generally, a combination of several processes is utilized to achieve the degree of treatment required at the least cost. Much of the experience and data from wastewater treatment has been gained from municipal treatment plants. Industrial liquid waste is similar to wastewater but differs in significant ways. Thus, typical design parameters and standards developed for municipal wastewater operations must not be blindly utilized for industrial wastewater. It is best to run laboratory and small pilot tests with the specific industrial wastewater as part of the design process. It is most important to understand the temporal variations in industrial wastewater strength flow, and waste components and their effect on the performance of various treatment processes. Industry personnel, in an effort to reduce cost, often neglect laboratory and pilot studies and depend on waste characteristics from similar plants. This strategy often results in failure, delay, and increased costs. Careful studies on the actual waste at a plant site cannot be overemphasized.

Figure 1: Traditional overall treatment of wastewater

Organic Hazard In Asia

Life-threatening poisons such as DDT, aldrin, chlordane, dieldrin and heptachlor -all of which are either severely restricted or banned in most countries- continue to be manufactured, stored, used and traded freely in South Asia, according to an investigative report released by the international environmental group Greenpeace. The report titled "Toxic Legacies; Poisoned Futures: Persistent Organic Pollutants in Asia" reveals a story of potentially widespread contamination caused by irresponsible corporate behavior, shortsighted lending agencies and misguided government policies.

"Asia faces a frightening scenario of historic, current and potential poisoning by the most dangerous variety of persistent poisons. This situation is a result of existing stockpiles of obsolete PCBs, the continuing production of organophenols and other chemical pesticides and the unmitigated expansion of dirty chlorine-based industries in the region," Focusing on a class of poisonous chemicals called Persistent Organic Pollutants or POPs, which are now targeted for elimination by ongoing international negotiations under the United Nations Environment Program (UNEP), Greenpeace investigations conducted between April and August 2001 in seven Asian countries, including Bangladesh, India, Nepal and Pakistan, revealed that stocks of 5000 metric tons or more of obsolete pesticides, including POP chemicals, are stored in extremely hazardous conditions in more than a thousand sites in Pakistan and Nepal. A sizeable portion of these pesticides are reported to have arrived as part of aid packages from Western countries, and almost all the pesticides were exported by developed nations and India to Pakistan and Nepal.

Chemical corporations whose products were identified in stockpiles in Pakistan and Nepal by Greenpeace investigators include: Bayer and Hoechst (Germany); DuPont, Dow Chemicals, Diamond Shamrock and Velsicol (USA); Shell (Netherlands); Sumitomo Chemical and Takeda Chemical (Japan); Rhone Poulenc (France); Sandoz (Switzerland); ICI (UK); Bharat Pulverising Mills (India).

India is among the three remaining known manufacturers of DDT (10,000 mt capacity) in the world, the other two being Mexico and China;

India exports nearly 800,000 kilograms of POP pesticides including aldrin, DDT, BHC, and chlordane to a long list of countries, including countries where their usage is banned. Exports of pesticides that could be branded POPs in the near future such as endosulfan, sodiumpentachlorophenate, 2,4-D, and lindane total more than two million tons. Some pesticides such as aldrin are illegal to manufacture in India.

In Pakistan, India, Nepal and Bangladesh, locally banned or severely restricted pesticides are freely available. Greenpeace found DDT, BHC, Dieldrin and Heptachlor openly sold in vegetable markets in Karachi. Hardware stores in New Delhi stock the deadly pesticide aldrin, whose registration was withdrawn more than two years ago.

POPs are a class of synthetic toxic chemicals that cause severe and long-term effects on wildlife, ecosystems and human health. POPs have been implicated in the rising incidence of certain cancers (e.g. breast, prostate, endometriosis, etc.), reproductive deficits such as infertility and sex-linked disorders, declining sperm counts, fetal malformations, neurobehavioral impairment, and immune system dysfunction. Because of major threats to human health, the UNEP process has shortlisted an initial twelve substances for elimination which include organochlorine pesticides (DDT, chlordane, mirex, hexachlorobenzene, endrin, aldrin, toxaphene, heptachlor), industrial chemicals like cancer-causing PCBs (polychlorinated biphenyls), and the super-toxic dioxins and furans.

In line with the emerging requirements of the UNEP POPs process, Greenpeace also urges governments in the region to take action now by taking inventory of all sources of POPs in their countries and preventing the expansion of POP-producing technologies such as incinerators, PVC manufacturing, pesticide production facilities, and pulp and paper mills using chlorine bleaching processes.

"It is unfortunate that while governments in the region are still grappling for ways to dispose of their stockpiles of obsolete imported pesticides, the continuing production and trade of these chemicals goes on unabated. This could only lead to an endless cycle of poisoning whose unwitting and eventual victims are communities and future generations," "Governments should aim for an eventual phase-out of such polluting practices and push for international cooperation in developing viable and sustainable non-chemical alternatives.""While governments in the region are responsible for taking action against POPs pollution, the liabilities associated with such action must always fall on the polluter -the corporations and international lending agencies- and not on the citizens who have long-endured the polluter the corporation- and international consequences of toxic pollution, added

Jayaraman.

Advances in wastewater treatment

Several effective, affordable, and environmentally sound waste disposal options are available; others are within scientific reach. Separating waste categories can go a long way in addressing waste problems. By some estimates, intensive reuse and recycling systems could take care of nearly 80 percent of municipal waste. Current data suggest much lower rates of recycling in most locations: 8 percent in the UK, 28 percent in the US, and so on. Conversion to clean production technologies can both help save business dollars and protect the environment. For toxic industrial and medical wastes, non-incineration technologies such as the gas phase thermo-chemical reduction process achieve virtually 100 percent efficiency in POPs destruction and capture all residues and releases. Other emerging technologies include electrochemical oxidation, molten metal technology, solvated electron process, and supercritical water oxidation.

Advanced technologies (non-combustion) available for wastewater treatment are listed in table 1.

Why Only Electrochemical Oxidation?

In electrochemical oxidation (EO), an electrochemical cell, operating at 50 to 60oC (120o to 140oF) and atmospheric pressure, is used to generate an oxidizing species (a mediated metal ion) at the anode (the negative electrode) in an acidic solution. As indicated in the flowsheet, this is accomplished by supplying a voltage across two electrodes immersed in an acidic solution containing the oxidizing species in its reduced, more natural state.

What are electrochemical cellsMany oxidation-reduction reactions occur spontaneously, giving off energy. An example involves the spontaneous reaction that occurs when zinc metal is placed in a solution of copper ions as described by the net ionic equation shown below.Cu+2 (aq) + Zn (s) Cu(s) + Zn+2 (aq)

The zinc metal slowly "dissolves" as its oxidation produces zinc ions, which enter into solution. At the same time, the copper ions gain electrons and are converted into copper atoms, which coats the zinc metal, or sediments to the bottom of the container.

List of advanced technologies (non-combustion) available

for wastewater treatmentTechnologyNon combustion destruction technologyIntrinsic PCDD/F formation Capable of containing all process streamsCapable of reprocessing all process streamsDemonstrated high DE

Incineration

NoyesNoNono

GPCR - EcologicYesnoYesYesyes

Base Catalysed DechlorinationYesnoYesYesyes

Sodium reduction process(es)Yesno??no

Solvated electron processYesnoYesYesyes

Super Critical Water OxidationYes?Yesyes?yes

Electrochemical oxidationYesnoYesYesYes

VitrificationNoyesNoNoNo

Ball millingYesnoYesyes?No

Molten salt?????

Molten metal

?????

Catalytic hydrogenationYesno??Yes

Solvent washingNonoN/AN/ANo

Landfill/burialNononoN/ANo

Solidification/

StabilizationNononoN/ANo

Land spreadingNononoN/ANo

Deep-well injectionNononoN/ANo

TechnologyNon combustion destruction technologyIntrinsic PCDD/F formation Capable of containing all process streamsCapable of reprocessing all process streamsDemonstrated high DE

Incineration

NoyesnoNoNo

GPCR - EcologicYesnoyesYesYes

List of technologies that meet initial screening

TechnologyCommercial scale Countries where licensed and/or used for commercial treatment

Gas Phase Chemical ReductionfullAustralia, Canada, USA, Japan (Argentina?)

Sodium reduction process(es)fullFrance, Germany, UK, Netherlands, South Africa, Australia, USA, Saudi Arabia, Japan, New Zealand

Base Catalysed DechlorinationfullAustralia, USA, Mexico, Spain, New Zealand

Solvated electron processfullUSA

Electrochemical oxidationlimited USA

Catalytic hydrogenationlimitedAustralia

Super-critical water oxidationlimitedUSA

Ball millinglimited/demoGermany

Molten saltdemoN/A

The energy produced in this reaction is quickly dissipated as heat, but it can be made to do useful work by a device called, an Electrochemical cell. This is done in the following way.

An Electrochemical cell is composed of two compartments or half-cells, each in turn composed of an electrode dipped in a solution of electrolyte. These half-cells are designed to contain the oxidation half-reaction and reduction half-reaction separately as shown below.

The half-cell, called the anode, is the site at which the oxidation of zinc occurs as shown below.

Zn (s) Zn+2 (aq) +2e-

During the oxidation of zinc, the zinc electrode will slowly dissolve to produce zinc ions (Zn+2), which enter into the solution containing Zn+2 (aq) and SO4-2 (aq) ions.

The half-cell, called the cathode, is the site at which reduction of copper occurs as shown below.

Cu+2 (aq) + 2e - Cu (s)

When the reduction of copper ions (Cu+2) occurs, copper atoms accumulate on the surface of the solid copper electrode.

The reaction in each half-cell does not occur unless the two half cells are connected to each other.

Recall that in order for oxidation to occur, there must be a corresponding reduction reaction that is linked or "coupled" with it. Moreover, in an isolated oxidation or reduction half-cell, an imbalance of electrical charge would occur, the anode would become more positive as zinc cations are produced, and the cathode would become more negative as copper cations are removed from solution. Using a salt bridge connecting the two cells as shown in the diagram below can solve this problem. A "salt bridge" is a porous barrier, which prevents the spontaneous mixing of the aqueous solutions in each compartment, but allows the migration of ions in both directions to maintain electrical neutrality. As the oxidation-reduction reaction occurs, cations (Zn+2) from the anode migrate via the salt bridge to the cathode, while the anion, (SO4)-2, migrates in the opposite direction to maintain electrical neutrality.

The two half-cells are also connected externally. In this arrangement, electrons provided by the oxidation reaction are forced to travel via an external circuit to the site of the reduction reaction. The fact that the reaction occurs spontaneously once these half-cells are connected indicates that there is a difference in potential energy. This difference in potential energy is called an electromotive force (emf) and is measured in terms of volts. The zinc/copper cell has an emf of about 1.1 volts under standard conditions.

Any electrical device can be "spliced" into the external circuit to utilize this potential energy produced by the cell for useful work. Although the energy available from a single cell is relatively small, electrochemical cells can be linked in series to boost their energy output. A common and useful application of this characteristic is the "battery". An example is the lead-acid battery used in automobiles. In the lead-acid battery, each cell has a lead metal anode and lead (IV) oxide (lead dioxide) cathode both of which are immersed in a solution of sulfuric acid. This single Electrochemical cell produces about 2 volts. Linking 6 of these cells in series produces the 12-volt battery found in most cars today. One disadvantage of these "wet cells" such as the lead-acid battery is that it is very heavy and bulky. However, like many other "wet cells", the oxidation-reduction reaction, which occurs, can be readily reversed via an external current such as that provided by an automobile's alternator. This prolongs the lifetime and usefulness of such devices as an energy source.

Improtance of removing organic pollutants and carcinogens

Phenol is human poison by inhalation, ingestion, and skin absorption. It is a severe irritant to the eyes, skin and respiratory system. Human mutation data have been reported for phenol. Also it is a questionable carcinogen in animals and a suspected carcinogen in humans, although the data are inconclusive. Phenol is a general protoplasmic poison that is corrosive to any living tissue it contacts. Phenol is highly soluble in water. Concentrations of 1000 milligrams and more will mix with a cubic decimeter of water. About 26.3% of phenol will eventually end up in air, approximately 73.3% in water, and about 0.2% in terrestrial soil and aquatic sediments. It is used to manufacture various phenolic and epoxy resins, for refining lubricating oils, a fuel-oil sludge inhibitor, and as a reagent in chemical analysis. It is also used in the production or manufacture of a large variety of aromatic compounds including fertilizers, illuminating gases, coke, explosives, lampblack, paints, paint removers, asbestos goods, wood preservatives, textiles, perfumes, bakelite, rubber, and other plastics. Many industrial processes generate wastewater flows with a high concentration of phenols and other related compounds. These compounds are known to betoxic, even at low concentrations, and their treatment is very important.

The removal of phenols from wastewaters is, therefore, an important problem and electrochemical oxidation technologies offer the prospect of relatively simple equipment, environmental friendliness, and the possibility of high-energy efficiency by using Electrochemical oxidation techniques.Benefits of electrochemical over othersThe advantages of biological treatmentsare very well known, but also their limitations either for a high COD (Chemical Oxygen Demand) value or the presence of very toxic compounds. The possible presence of inorganic compounds, such as heavy metals, may cause a drop of the bacterial count. On the other hand, the incineration of organic compounds can originate the formation of toxic products that are dragged at the same time by the combustion gases; also, the presence of corrosive agents can cause problems in the stability of the materials of the incineratorElectrochemical degradation (direct and indirect Electrochemical oxidation) and electrocatalisys of hazardous waste water have several advantages compared with incineration and biological treatment.Electrochemical treatment is able to treat very toxic wastes.

This process can operate at room temperature and atmospheric pressure It is a environmentally friendly technology because it uses only electricity. The energy consumption depends on COD

The Electrochemical treatment can be stopped simply by switching off the power. Cost and safety effective switching off the power. Cost and safety effective.

Comparison of various electrodes

CompoundAnodeCathodeConducting oxideNon con oxide

4-chloro phenolTitaniumLead dioxide

4-catechecolTio2---Indium oxide

PhenolicsTitaniumStrontium oxideRuo2Tio2

PhenolicsTitaniumStrontium oxideIro2Zr02

PhenolicsTitanium Strontium oxidePtoxTa205

PhenolicsSno2-----

PhenolicsStainless steelDiamond thin film----

PhenolicsTitaniumLead dioxide----

PhenolicsTitaniumOxide

PhenolTitaniumStrontium oxide

PhenolTitaniumRuo

PhenolTitaniumIro

Other Applications Of Electrochemical TechnologyThe PCBs degradation In-situ chlorine production Ozone generation

Destruction of cyanides and nitrites

Purification of wastewater using oxidising agents and in general as a method for the reduction of COD from any effluent. Elimination of phenol.Elimination of tensioactives compounds and dyesInnovative aspects of electrochemical technologyElectrochemical technology is able to treat toxic wastewater with high concentrations of organic compounds.Suitable when traditional treatment methods are not effective due to: non-biodegradable materials, heavy metals, hazardous compounds are not completely degraded It is an environmentally friendly technology. It avoid the problem of dropping of the bacterial count on the biological treatments It is a cost and safety effective technology

Major Drawbacks

The major drawbacks of this technology mainly include:

1) passivation

2) destabilisation of electrode material.

3) Passivation: The Passivstion of electrodes is due to building of sucessive layers of blocking films of high molecular weight unreactive materials thus leading to the decrease in rate of reaction and after sometime complete end of reaction.

4) Destabalisation of material: The destabilisation of electrode material widely confines to the corrison problem. Due to improper selection of electrode pair sometimes the organic pollutant in long reaction time corrodes the electrode material leading to destabilisationPresent State Of Electrohcemical TechnologyAt present Electrochemical technology is applicable for following treatment units:

Elimination of lead and lead oxides from waste waters.

Recovery of NaCl by electrodialysis.

Textile wastewater treatment using Electrochemical technology.

Conclusion

From the overall study of Electrochemical oxidation of organic pollutants and Electrochemical technology we can conclude that this electrochemical degradation proved to be faster than the usual treatments such as biodegradation, photoxidation. It is a clean technique, it does not need any chemical reagent which may be harmful or expensive, it can be easily operated (needs no complex controls) and also optimum safety condition prevails since the oxidizing agents are generated in-situ.

The only drawback of this technology is thr high operating cost of the reactor.

This suggests that in the future this kind of treatment should be prefferentially employed for the preliminary treatment of organic wastewater constituents.References

1) Janetm M . K esselman, Nathans Lewis & Michaelr. Hoffmann, Photoelectrochemical Degradation of 4-Chlorocatechol at TiO2 Electrodes, W. M. Keck Laboratories, California Institute of Technolog2) L. Gherardini a , Ch. Comninellis b, N. Vatistas a,a , Electrochemical oxidation of waste water in acid medias using ti-sno2 electrodes, Chemical Engineering Department, University of Pisa3) Subramanian Tamilmani Srini Raghavan Wayne Huang, Electrochemical Treatment of WaSTE Water Using Boron Doped Diamond Electrode (BDD), Materials Science and Engineering Department University of Arizona

4) P. Caizares, F. Martnez, M. Daz, Electrochemical oxidation azqueos waste using active and non-active electrodes, Department of Chemical Engineering. Universidad de Castilla SPAIN

5) Edison A. Laurindo, Nerilso Bocchi and Romeu C. Rocha-Filho, Production and Characterization of Ti/PbO2 Electrodes by a Thermal-Electrochemical Method, Departamento de Qumica, Universidade - SP, Brazil6) Jiangning Wu, Oxidative Polymerization of Phenol in Wastewater: An Assessment of Ozonation and Enzymatic Processes, School of Chemical Engineering, Ryerson Polytechnic University, Toronto, Ontario, Canada 7) William O'Grady , Paul Natishan, Brian StonE and Patrick Hagans, Electrochemical Oxidation of Phenol Using Boron-Doped Diamond Electrodes, Chemistry Division, Naval Research Laboratory, Washington, DC

8) A.Savall, N. Belhadj Tahar, C. Manaranche,, Kinetics and mechanisitc aspects of electrochemical oxidation of organic pollutants, France

9) L. Gherardini a , Ch. Comninellis b , N. Vatistas a, electrochemical oxidation of waste waters, Chemical Engineering Department, University of Pisa.

TRADUCEREA ARTUCOLULUI

Oxidarea electrochimica a poluanilor organiciAbstract

Scopul principal al acestei lucrri este de a folosi "tehnologia electrochimiei pentru ndeprtarea poluanilor organici din apele reziduale". Astzi, gestionarea deeurilor periculoase a devenit una dintre sarcinile cele mai dificile n aceast lume tehnologic, deoarece tone de poluani organici, inclusiv diferite substante cancerigene sunt expuse fr tratament suficient prescris conform normelor "Agentiei pentru Protectia Mediului". De asemenea, n contextul actual, guvernul indian a fcut obligatorie tratarea apelor uzate nainte de evacuarea acesteia.

Depoluarea electrochimica este o metod foarte eficient i economic, potrivita atunci cnd apele uzate cuprind poluanii organici toxici sau non-biodegradabili.

Astfel, prin intermediul tehnologiei electrochimice, care este sgura si ieftina, putem ajuta la reducerea concentraiilor de poluani organici ntr-o msur minima i menine starea actual a lumii chimice.

Cuvinte cheie: Oxidare electrochimic, poluanii organici, celule electrochimice.

IntroducereApa are o serie de proprieti unice, care sunt eseniale pentru viaa i care determin comportamentul chimic al mediului. Multe dintre aceste proprieti se datoreaz structurii moleculare polare ale apei i capacitatii de a forma legturi de hidrogen. Apa are, de asemenea, cea mai mare constanta dielectric , o densitate maxim n stare lichid la temperatura de 40 C i o capacitate termic mai mare dect n alte lichide cu excepia amonniacului. Dar cele mai importante fenomene chimice asociate cu apa nu au loc n soluie, ci mai degrab printr-o interaciune a substanelor dizolvate n ap cu alte faze. De exemplu, reaciile de oxido-reducere catalizate de bacterii apar n celulele bacteriene. Multe deeuri periculoase organice sunt efectuate prin intermediul apei, emulsii de foarte mici particule n suspensie n ap. Unele deeuri periculoase sunt depozitate n sedimente n corpurile de ap prin care poate intra mai trziu apa prin procese chimice sau fizice i provoca efecte grave de poluare care trebuie s fie eliminate ct mai mult posibil.

Apele uzate sunt de obicei, oxidat cu ozon, care este un oxidant puternic, dar ndeprtarea carbonului organic total a fost de nu mai mult de 30% i aceleai rezultate au fost obinute cu ajutorul peroxidului de hidrogen n prezen de Fe +2 drept catalizator. n general, printr-o oxidare chimic poluanii organici sunt aproape complet eliminati, dar ndeprtarea carbonului organic total rmne o problem. Acestea sunt motivele pentru care electrooxidarea poluanilor a fost subiectul unor studii extinse n ultimii ani.

Metoda electrochimic de depoluare prezint mai multe avantaje importante, pentru c nu are nevoie de produse chimice auxiliare, acesta este aplicabil pe o gam larg de poluani i nu are nevoie de presiuni si temperaturi ridicate.

Oxidarea electrochimic a poluanilor din ape reziduale a fost studiat folosind anozi realizati din diferite materiale. Acest lucru este atribuit oxidarii compuilor organici adsorbii la dioxid de carbon. De asemenea, sa dovedit c, n aceleai condiii indicele electrooxidare obinut pe anod SnO2 a fost mai mare dect la catod Pt, indicnd un grad mai ridicat de fenol oxidare.

Tratarea apelor uzate: O privire de ansamblu

Aa cum sa indicat mai sus, apele uzate industriale conin o gam larg de poluani: insolubil, coloidali, i forme de particule, att anorganice si organice. n plus, standardele sunt, de asemenea, diverse, variind cu clasa industrial i poluant. Prin urmare, nu poate exista nici o varianta standard pentru controlul polurii apelor industriale. Cu toate acestea, fiecare dintre multiplele procese pentru tratarea deeurilor industriale sunt capabile de a elimina mai mult decat un tip de poluant i este n general aplicabil la mai mult de o industrie. n general, o combinaie de mai multe procese este utilizat pentru a atinge gradul de tratament necesar la cel mai mic cost. O mare parte a experienei i a datelor de epurare a apelor uzate a fost ctigat de la instalaiile de tratare municipale. Deeurile lichide industriale sunt similare cu a apelor uzate, dar difer n mod semnificativ. Astfel, parametrii de proiectare tipici i standarde elaborate pentru operaiunile de evacuare a apelor reziduale municipale nu trebuie s fie utilizate orbete pentru ape uzate industriale. Cel mai bine este de a rula i testele de laborator pilot mici, cu apele uzate industriale specific ca parte a procesului de proiectare. Este cel mai important s se neleag variaiile temporale n fluxul industrial putere a apelor uzate, precum i a componentelor de deeuri i efectul lor asupra performanei diferitelor procese de tratare. Personalul din Industrie, ntr-un efort de a reduce costurile, de multe ori faci studii pilot care depind de caracteristicile deeurilor de la instalaii similare. Aceast strategie de multe ori duce la eec, ntrziere, i creterea costurilor.

Hazard Organic n Asia

Otrvurile pun viata in pericol, cum ar fi DDT, aldrin, clordan, dieldrin i-heptaclor toate care sunt fie sever restricionate sau interzise n majoritatea rilor, continu s fie fabricate, depozitate, utilizate i comercializate n mod liber n Asia de Sud, potrivit unui raport publicat de investigaie de ctre grupul internaional de mediu Greenpeace. Raportul intitulat "moteniri toxice; otrvuri Futures: poluanii organici persisteni n Asia", dezvluie o poveste de contaminare pe o scar larg cauzat de un comportament corporativ iresponsabil, ageniile de miopi de creditare i de politicile guvernamentale greite.

"Asia se confrunt cu un scenariu nfricotor de otrviri istorice, actuale i poteniale de cea mai periculoasa varietate de otrvuri persistente. Aceast situaie este o urmare a stocurilor existente de PCB-uri nvechite, continuarea produciei de organophenols i alte pesticide chimice i extinderea fr cruare de clor murdar industriile din regiune pe baza de "Concentrandu-se pe o clasa de substante chimice toxice numite poluani organici persisteni sau POP, care sunt n prezent vizate pentru eliminarea de negocierile internaionale n curs de desfurare n cadrul Programului Naiunilor Unite pentru Mediu (UNEP), investigaii Greenpeace desfurate ntre aprilie i august 2001 n apte ri din Asia, inclusiv Bangladesh, India, Nepal i Pakistan, a relevat faptul c stocurile de 5000 de tone sau mai mult de pesticide inutilizabile i interzise, inclusiv POP, substane chimice sunt depozitate n condiii extrem de periculoase n mai mult de o mie de site-uri din Pakistan i Nepal. O parte important din aceste pesticide sunt raportate pentru a fi ajuns, ca parte a pachetelor de ajutor din rile occidentale, i aproape toate pesticidele au fost exportate de ctre rile dezvoltate i India pentru Pakistan i Nepal.

Corporaii chimice ale cror produse au fost identificate n stocurilor din Pakistan i Nepal de anchetatori Greenpeace includ: Bayer si Hoechst (Germania), DuPont, Dow Chemicals, Diamond Shamrock i Velsicol (SUA), Shell (Olanda), Sumitomo Chemical i Takeda Chemical (Japonia) , Rhone Poulenc (Frana), Sandoz (Elvetia), ICI (Marea Britanie); Bharat pulverizarea Mills (India).

India este printre cei trei productori de DDT (10.000 tone capacitate) din lume, celelalte dou fiind Mexic i China;

India export aproape 800.000 de kilograme de pesticide POP, inclusiv Aldrin, DDT, BHC, i clordan pentru o list lung de ri, inclusiv ri n care folosirea lor este interzis. Exporturile de pesticide, care ar putea fi de marc POP n viitorul apropiat, cum ar fi endosulfan, sodiumpentachlorophenate, 2,4-D, i lindan total de mai mult de dou milioane de tone. Unele pesticide, cum ar fi aldrin sunt ilegale pentru fabricarea n India.

n Pakistan, India, Nepal i Bangladesh, pesticide interzise la nivel local sau sever restricionate sunt disponibile n mod liber. Greenpeace a gsit DDT, BHC, Dieldrin Heptaclor i vndut n mod deschis n pieele de legume din Karachi. Magazine de hardware n New Delhi stoc aldrin pesticide de moarte, a crui nregistrare a fost retras n urm cu mai mult de doi ani.

POP sunt o clas de substane chimice toxice sintetice care produc efecte asupra vieii slbatice, ecosistemelor i sntii umane pe termen lung grave i. POP au fost implicate n creterea incidenei anumitor tipuri de cancer (de exemplu, de sn, de prostat, endometrioz, etc), deficite de reproducere, cum ar fi infertilitate si tulburari legata de sex, n scdere numrului de spermatozoizi, malformaii fetale, afectare neuro-comportamentale, i disfuncie a sistemului imunitar. Din cauza unor ameninri majore la adresa sntii umane, procesul de UNEP a nominalizat un iniiale de dousprezece substane de eliminare, care includ pesticidele organoclorurate (DDT, clordan, mirex, hexaclorbenzenul, endrin, aldrin, toxafen, heptaclor), produse chimice industriale, cum ar fi cauzatoare de cancer, PCB (policlorurai bifenili) i a dioxinelor i furanilor super-toxice.

n conformitate cu cerinele emergente ale procesului de UNEP POP, Greenpeace solicit, de asemenea, guvernele din regiune s ia msuri acum prin luarea inventar al tuturor surselor de POP-uri n rile lor i a preveni extinderea de tehnologii productoare de POP, cum ar fi incineratoare, de fabricaie PVC, faciliti de producie de pesticide, i celuloz i hrtie mori folosind procese cu clor.

"Este regretabil faptul c n timp ce guvernele din regiune sunt nc se lupt pentru modaliti de a dispune de stocuri lor de pesticide inutilizabile i interzise importate, continuarea produciei i comerului cu aceste substane chimice va continua,. Acest lucru ar putea duce doar la un ciclu nesfrit de otrvire a cror netiutor i eventualele victime sunt comunitile i generaiile viitoare "," Guvernele ar trebui s urmreasc pentru o eventual faz-out din astfel de practici poluante i mpinge pentru cooperarea internaional n dezvoltarea de alternative non-chimice viabile i durabile. "

"n timp ce guvernele din regiune sunt responsabile pentru luarea de msuri mpotriva polurii cu POP, datoriile asociate cu o astfel de aciune trebuie s fie ntotdeauna cdea pe poluatorul corporaiile i ageniile internaionale de creditare i nu asupra cetenilor care au lung a suferit poluatorul consecinelor internaionale ale polurii toxice-profit i, "adaug

Jayaraman.

Progrese in tratarea apelor uzate

Mai multe efective, la preuri accesibile, i ecologice opiunile de eliminare a deeurilor sunt disponibile, altele sunt la indemana tiinifice. Separarea categorii de deeuri poate merge un drum lung n abordarea problemelor deeurilor. Dupa unele estimari, sistemele de refolosire i de reciclare intensiv ar putea avea grij de aproape 80 la sut din deeurile municipale. Datele actuale sugereaza rate mult mai mici de reciclare n cele mai multe locaii: 8 la sut n Marea Britanie, 28 la suta din SUA, i aa mai departe. Conversie pentru a cura tehnologii de producie poate att ajuta la salvarea de dolari de afaceri i de a proteja mediul nconjurtor. De deeuri industriale i medicale toxice, tehnologii non-incinerare, cum ar fi procesul de reducere a termo-chimic n faz de gaz atinge aproape 100 la suta eficienta in distrugerea POP i captura toate reziduurile i comunicate. Alte tehnologii emergente includ oxidarea electrochimic, tehnologia de metal topit, procesul de electroni solvatat, i oxidare n ap supercritic.

Tehnologii avansate (fr combustie) disponibile pentru tratarea apelor uzate sunt prezentate n tabelul 1.

De ce electrochimia de oxidare?

La oxidarea electrochimic (EO), o celul electrochimic, care funcioneaz la 50 pn la 60 C (120 la 140 F) i la presiune atmosferic, este utilizat pentru a genera o specie de oxidare (un ion metalic mediat) la anod (electrodul negativ) ntr-o soluie acid. Dup cum se indic n schema tehnologic, aceasta se realizeaz prin furnizarea de o tensiune peste doi electrozi cufundai ntr-o soluie acid care conine speciile de oxidare n redus, stare mai natural.

Ce sunt celule electrochimiceMulte reacii de oxido-reducere apar spontan, oferindu-off de energie. Un exemplu implic reacia spontan care apare atunci cand zincului metalic este plasat ntr-o soluie de ioni de cupru aa cum este descris de ecuaia ionic net se arat mai jos.

Cu +2 (aq) + Zn (uri) Cu (s) + Zn +2 (aq)

Zincul metalic ncet "dizolv" ca oxidarea acestuia produce ioni de zinc, care intr n soluie. n acelai timp, ionii de cupru obine electroni i se transform n atomi de cupru, care acoper metalul de zinc, sau sedimente la fundul recipientului.

Lista de tehnologii avansate (non-ardere) disponibile

pentru tratarea apelor uzate

TehnologieTehnologie non distrugere de ardereFormarea PCDD intrinsec / FCapabil de a conine toate fluxurile de procesCapabil de reprocesare tuturor fluxurilor de procesA demonstrat de nalt DE

Incinerare [1]NudaNuNunu

GPCR - EcologicDanuDaDada

Catalizat de baz declorareDanuDaDada

Proces de reducere de sodiu (ES)Danu??nu

Procesul de electroni solvatDanuDaDada

Oxidarea cu ap super-criticDa?Dada?da

Oxidare electrochimicaDanuDaDaDa

VitrificareNudaNuNuNu

Bile de frezatDanuDada?Nu

Sare topit?????

De metal topit [2]?????

Hidrogenarea cataliticDanu??Da

Splat solventNunuN / AN / ANu

Depozitele de deeuri / nmormntareNununuN / ANu

Solidificare /

StabilizareNununuN / ANu

Teren de raspandireNununuN / ANu

Injecie Deep-bineNununuN / ANu

TehnologieTehnologie non distrugere de ardereFormarea PCDD intrinsec / FCapabil de a conine toate fluxurile de procesCapabil de reprocesare tuturor fluxurilor de procesA demonstrat de nalt DE

Incinerare [3]NudanuNuNu

GPCR - EcologicDanudaDaDa

Lista de tehnologii care ndeplinesc screening-ul initialTehnologieScar comercialrile n care liceniai i / sau utilizate pentru tratarea comercial

Faza de gaz de reducere chimiccompletAustralia, Canada, Statele Unite ale Americii, Japonia (Argentina?)

Proces de reducere de sodiu (ES)completFrana, Germania, Marea Britanie, Olanda, Africa de Sud, Australia, Statele Unite ale Americii, Arabia Saudit, Japonia, Noua Zeeland

Catalizat de baz declorarecompletAustralia, Statele Unite ale Americii, Mexic, Spania, Noua Zeeland

Procesul de electroni solvatcompletStatele Unite ale Americii

Oxidare electrochimicalimitatStatele Unite ale Americii

Hidrogenarea cataliticlimitatAustralia

Oxidare super-critic de aplimitatStatele Unite ale Americii

Bile de frezatlimitat / demoGermania

Sare topitdemoN / A

Energia produs n aceast reacie este rapid disipat sub form de cldur, dar se poate face pentru a face o munc util de ctre un dispozitiv numit, o celul electrochimic. Acest lucru se face n felul urmtor.

O celul electrochimic este compus din dou compartimente sau jumti de celule, fiecare la rndul lor, compuse dintr-un electrod nmuiat ntr-o soluie de electrolit. Aceste jumti de celule sunt proiectate s conin oxidare jumtate de reacie i reducerea jumtate de reacie separat, aa cum se arat mai jos.

Jumtate de celul, numit anod, este locul la care oxidarea zincului are loc aa cum se arat mai jos.

Zn (uri) Zn +2 (aq) 2 e-

In timpul oxidarea zinc, electrodul de zinc se dizolv ncet pentru a produce ioni de zinc (Zn +2), care intr n soluia care conine Zn +2 (aq) i SO4-2 (aq) ioni.

Jumtate de celul, numit catod, este locul la care reducerea cuprului are loc aa cum se arat mai jos.

Cu +2 (aq) + 2e - Cu (s)

Atunci cnd are loc reducerea ionilor de cupru (Cu +2), atomii de cupru se acumuleaz pe suprafaa electrodului de cupru solid.

Reacia la fiecare jumtate de celul nu are loc dect dac cele dou celule jumti sunt conectate ntre ele.

S ne amintim c, pentru ca s se oxideze, trebuie s existe o reacie de reducere corespunztor, care este legat sau "cuplat" cu ea. Mai mult dect att, ntr-o oxidare sau reducere izolat jumtate de celul, s-ar produce un dezechilibru de sarcin electric, anodul ar deveni mai pozitiv ca cationi de zinc sunt produse, iar catodul ar deveni mai negativ ca cationii de cupru sunt eliminate din soluie. Folosind o "punte de sare", care leag cele dou celule aa cum se arat n diagrama de mai jos se poate rezolva aceast problem. O "punte de sare" este o barier poroas, care mpiedic amestecarea spontan a soluiilor apoase din fiecare compartiment, dar permite migrarea ionilor n ambele direcii pentru a menine neutralitatea electric. Dup cum se produce reacia de oxidare-reducere, cationi (Zn +2) de la anod migra prin puntea de sare la catod, n timp ce anion, (SO4) -2, migreaz n direcia opus pentru a menine neutralitatea electric.

Cele dou jumti de celule sunt, de asemenea, conectat extern. n acest aranjament, electronii furnizate de reacia de oxidare sunt obligate s se deplaseze printr-un circuit exterior la locul de reacia de reducere. Faptul c reacia are loc n mod spontan o dat aceste jumti de celule sunt conectate indic faptul c exist o diferen de potenial energetic. Aceast diferen de potenial de energie se numete o for electromotoare (EMF) i este msurat n termeni de voli. Celula de zinc / cupru are o emf de aproximativ 1,1 voli n condiii standard.

Orice dispozitiv electric poate fi "mbinat" n circuitul extern pentru a utiliza aceast energie potenial produs de celula de lucru util. Dei energia disponibil dintr-o singur celul este relativ mic, celule electrochimice pot fi legate n serie pentru a stimula producia de energie. O aplicaie comun i util de aceast caracteristic este "baterie". Un exemplu este bateria plumb-acid folosite n automobile. In bateria plumb-acid, fiecare celul are un anod metalic i plumb (IV) oxid (dioxid de plumb) catod de plumb ambele fiind scufundate ntr-o soluie de acid sulfuric. Acest singur celul electrochimic produce aproximativ 2 voli. Legarea 6 din aceste celule n serie produce baterie de 12 voli a gsit n cele mai multe masini de astzi. Un dezavantaj al acestor celule "ude", cum ar fi bateria plumb-acid este faptul c este foarte grele i voluminoase. Cu toate acestea, ca i multe alte celule "ude", reacia de oxido-reducere, care are loc, poate fi uor inversat prin intermediul unui curent extern cum ar fi cele furnizate de alternator un automobil lui. Acest lucru prelungete durata de via i utilitatea de dispozitive, cum ar fi o surs de energie.

Improtanta de ndeprtare a poluanilor organici si agenti cancerigeni

Fenolul este o otrav care prin inhalare, ingestie, i absorbtie prin piele duce la moartea oamenilor. Este un iritant sever la ochi, piele i sistemul respirator. Datele mutatie umane au fost raportate de fenol. De asemenea, aceasta este o substan cancerigen discutabil la animale i un carcinogen la om, dei datele nu sunt concludente. Fenol este o otrav protoplasmic general, care este coroziv pentru orice tesut-l contacte vii. Fenolul este foarte solubil n ap. Concentraii de 1,000 miligrame i mai mult se va amesteca cu un decimetru cub de ap. Aproximativ 26,3% din fenol se va ncheia n cele din urm n aer, aproximativ 73,3% n ap, i aproximativ 0,2% n sol terestru i sedimentele acvatice. Este folosit pentru fabricarea diferitelor rini fenolice i epoxidice, pentru uleiuri lubrifiante de rafinare, o inhibitor de nmol combustibil-ulei, i ca reactiv n analiza chimic. Acesta este, de asemenea, utilizat n producerea sau fabricarea de o mare varietate de compui aromatici, inclusiv ngrminte, gaze de iluminat, cocs, explozivi, negru de fum, vopsele, de ndeprtare a vopselei, produse de azbest, de conservare a lemnului, textile, parfumuri, bachelit, cauciuc i alte materiale plastice . Multe procese industriale genera fluxuri de ape uzate, cu o concentraie mare de fenoli i ali compui conexe. Aceti compui sunt cunoscui betoxic, chiar i la concentraii sczute, iar tratamentul lor este foarte important.

ndeprtarea fenolilor din ape reziduale este, prin urmare, o problem important i tehnologii electrochimice de oxidare ofera perspectiva echipament relativ simplu, respectarea mediului, precum i posibilitatea de eficien nalt energie, prin utilizarea tehnicilor de oxidare electrochimic.

Beneficiile electrochimiei n detrimentul altor tehnologii

Avantajele tratamente biologice sunt foarte bine cunoscute, dar, de asemenea, limitele lor, fie pentru o mare valoare COD (Cererea de oxigen chimic) sau prezena unor compui foarte toxici. Posibila prezen de compui anorganici, cum ar fi metalele grele, poate cauza o scdere a numrului de bacterii. Pe de alt parte, incinerarea compui organici pot originea formrii produilor toxici care sunt trase n acelai timp, prin arderea gaze; asemenea, prezena unor ageni corozivi pot cauza probleme n stabilitatea materiale de incinerator

Degradare electrochimica (directe i indirecte oxidare electrochimic) i electrocatalisys de ap deeuri periculoase au mai multe avantaje n comparaie cu incinerarea i tratarea biologic.

Tratamentul electrochimic este capabil de a trata deeuri foarte toxice.

Acest proces poate funciona la temperatura camerei i presiune atmosferic

Este o tehnologie ecologic, deoarece folosete doar energie electric.

Consumul de energie depinde COD

Tratamentul electrochimice poate fi oprit pur i simplu prin oprirea putere.

Cost i siguran eficiente oprirea alimentrii.Compararea diferitilor electrozi

CompusAnodCatodEfectuarea oxidOxid non con

4-clor fenolTitanDioxid de plumb

4-catechecolTiO2---Oxid de indiu

FenoliTitanOxid de stroniuRuo2TiO2

FenoliTitanOxid de stroniuIro2Zr02

FenoliTitanOxid de stroniuPtoxTa205

FenoliSnO2-----

FenoliInoxDiamant film subire--

FenoliTitanDioxid de plumb--

FenoliTitanOxid

FenolTitanOxid de stroniu

FenolTitanRuo

FenolTitanIro

Alte aplicaii ale tehnologiei electrochimice

Degradarea PCB

In-situ pentru producerea clorului

Generaie ozon

Distrugerea cianurilor i nitrii

Purificarea apelor uzate utiliznd ageni de oxidare i, n general, ca o metod de reducere a CCO de la orice efluent.

Eliminarea fenol.

Eliminarea compuilor tensioactivi i vopsele

Aspcte inovative ale tehnologiei electrochimice

Tehnologia electrochimica este capabil de a trata apelor uzate toxice cu concentraii mari de compui organici.

Potrivit atunci cnd metodele de tratament traditionale nu sunt eficiente din cauza: materiale non-biodegradabile, metale grele, compui periculoi nu sunt complet degradate

Este o tehnologie ecologic.

Evita problema de scdere a numrul de bacterii pe tratamentele biologice

Este o tehnologie eficient de cost i de siguran

Dezavantajele majoreCele mai importante dezavantaje ale acestei tehnologii includ n principal:

1) pasivizare

2) destabilizare a materialului de electrod.

3) Pasivizarea: Passivstion de electrozi se datoreaz construirea de straturi sucessive de blocare filme de greutate materiale redusa molecular mare conducnd astfel la scderea ratei de reacie i dup sfritul cndva complet de reacie.

4) Destabalisation de material: Destabilizarea de material electrod limiteaz foarte mult la problema corrison. Datorit selecie necorespunztoare de perechi de electrozi, uneori poluant organic n timpul de reacie timp corodeaz materialul de electrod care duce la destabilizarea

Stadiul actual al tehnologiei Electrochimice

n prezent tehnologia electrochimic se aplic pentru urmtoarele uniti de tratament:

Eliminarea de plumb i plumb oxizi din apele uzate.

Recuperarea de NaCl de electrodializ.

Textile epurare a apelor uzate folosind tehnologia electrochimica.

Concluzie

Din studiul de ansamblu a "oxidarii electrochimice a poluanilor organici" prin tehnologie electrochimica, putem concluziona c aceast degradare electrochimica s-au dovedit a fi mai rapida dect tratamentele uzuale, cum ar fi de biodegradare, photoxidation. Este o tehnica de curat, ea nu are nevoie de nici un reactiv chimic care poate fi duntoare sau scump, acesta poate fi utilizat cu uurin (nu are nevoie de controale complexe) i, de asemenea, condiii optime de siguran prevaleaz, deoarece agenii de oxidare sunt generate in situ.

Singurul dezavantaj al acestei tehnologii este thr costuri mari de operare a reactorului.

Acest lucru sugereaz c, n viitor, acest tip de tratament ar trebui s fie folosite prefferentially pentru tratamentul preliminar de constitueni organici a apelor uzate.

Referine

1) J anetm M. K esselman, N Athans Lewis &

M ichaelr. H offmann, Degradarea fotoelectrochimica de 4-Chlorocatechol la TiO2 Electrozi, WM Keck Laboratories, California Institute of Technolog

2) L. Gherardini o, Ch. Comninellis b, N. Vatistas o, o, E oxidare lectrochemical a apelor uzate n mass-media de acid utiliznd electrozi ti-SnO2, Departamentul de Inginerie Chimica, Universitatea din Pisa

3) Subramanian Tamilmani Srini Raghavan Wayne Huang, Tratamentul electrochimic a apelor uzate folosind bor dopate Diamond electrod (BDD), Stiinta si Ingineria Materialelor Departament Universitatea din Arizona

4) P. Caizares, F. Martnez, M. Daz, E azqueos oxidare lectrochemical pierde folosind electrozi active i inactive, Departamentul de Inginerie Chimica. Universidad de Castilla SPANIA

5) Edison A. Laurindo, Nerilso Bocchi i Romeu C. Rocha-Filho, producie i caracterizarea Ti/PbO2 electrozi de un termic Metoda electrochimic, Departamento de Qumica, Universidade - SP, Brazilia

6) Jiangning Wu, oxidativ Polimerizarea de fenol n apelor uzate: O evaluare de ozonare i enzimatice Procese, Scoala de Inginerie Chimica, Ryerson Politehnic University, Toronto, Ontario, Canada

7) William O'Grady, Paul Natishan, Brian Stone si Patrick Hagans, electrochimice de oxidare de fenol folosind bor dopate Diamond electrozi, Divizia de Chimie, Naval Research Laboratory, Washington, DC

8) A.Savall, N. Belhadj Tahar, C. Manaranche,, Netics ki i aspecte mechanisitc de oxidare electrochimic de poluani organici, Frana

9) L. Gherardini o, Ch. Comninellis b, N. Vatistas o, oxidare electrochimica a apelor uzate, Departamentul de Inginerie Chimica, Universitatea din Pisa.

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