Water Treatment

Ozonation

Ozone is an oxidant and a disinfectant which is widely used for drinking water treatment. Ozone causes oxidation of compounds responsible for bad taste and odor, natural organic matter, oxidation of manganese, toxically organic compounds…. In the field of wastewater treatment, ozone is also used to decolorize industrial effluents, increase the biodegradability of effluents or decrease the toxicity of effluents before biological treatment. From the 1980’s, AOP’s involving ozone with hydrogen peroxide [9] or with UV irradiation [10] have begun to be studied and used industrially. More recently, heterogeneous catalytic ozonation has been developed. The mechanism of action of ozone in aqueous solution shown is shown in Figure 2. The main reactive species in ozone cycle are O2·-/HO2· radicals. These radicals can be recombination to form H2O2 in solution aqueous.

Figure 4: Decomposition of ozone in weakly basic solution according to literature [12].

The mechanisms of ozone reactions with organic compounds in aqueous solution have been studied since the early 1970’s. These studies have shown that ozone can react with organic compounds and minerals along two pathways: a mode involving direct attack of ozone on the molecular compounds, and an indirect method involving hydroxyl radicals formed during the decomposition of ozone in water. Studies have also shown that the addition of hydroxyl radicals quenched in the water, such as bicarbonate and carbonate ions, leads to reduced rates of oxidation of organic compounds. Many studies have shown a high efficiency of ozone oxidation of organic compounds, e.g.  removal of pesticides, haloorganic compounds, etc. [11-14]. Mechanistic research have been followed by investigations of the kinetics of ozonation [15-19], e.g. to predict the influence of experimental parameters such as pH, concentration of radical quenching, and….. on the auto-decomposition rate of ozone in aqueous solution or the oxidation rates of model compounds. Many reactions are common to these kinetics models, but usually the models differ with respect to formulation of one or more specific reactions. Studies of ozonation kinetics in presence of bicarbonate – which are ubiquitous in natural waters – are scarce.

In this part of the project, ozone reaction with selected pesticides and antibiotic compounds used in agriculture and aquaculture in Vietnam will be studied – the selection of compounds will be based partly on the results from WP1. These data are needed for proper operation of the pilot water treatment plant. Also, the kinetics of decomposition of ozone in the matrices used will be quantified and modeled in order to better describe the mechanism of ozonation in the flooding water treatment systems.

Photo-Fenton degradation

The Fenton reaction been used for over a century for oxidation of organic compounds. The potentials for using Fenton’s reagent in water cleaning have been studies during these last twenty years. The mechanisms of decomposition of H2O2 by iron – ferrous and ferric iron –  in aqueous solution and the mechanisms of oxidation of organic compounds by Fe(II)/H2O2 and Fe(III)/H2O2 have been the subject of a considerable number of works. Most current studies are based on the mechanisms initially proposed by Haber and Weiss [20] then reviewed by Barber et al. [21,22] and Walling [23]. In case of oxidation of rather persistent compounds (e.g. as some polyaromatic, aliphatic compounds) quite high doses of reagents are needed [26]. In addition, regeneration of Fe(II) in the reaction medium also limits the reaction rate. To improve the performance of the method, different approaches have been proposed or are currently operating such as Photo-Fenton, Electro-Fenton, etc.

Under irradiation, the reduction of ferric iron in aqueous solution can be achieved according to the following equation:

Fe(III)aq + hn + H2O ® Fe(II)aq + OH· + H+

The rate of photoreduction of Fe(III) and the rate of production of •OH radicals depends on the wavelength of irradiation and pH as each species of ferric iron does not present the same photo-reactivity. The values of quantum yields for different species of ferric iron are shown in Table 1.

l (nm)

Species

Quantum yields (f)

References

254

313

360

Fe3+

Fe(OH)2+

Fe(OH)2+

0.065

0.14

0.017±0.003

Langford and Carey [24]

Faust et Hoigné [25]

Faust et Hoigné [25]

Several authors have studied the efficiency of degradation of organic compounds by the photo-Fenton process. According to previous studies, this method requires smaller doses of iron than the conventioanl Fenton process and  lower radiation dose than H2O2/UV processes to completely mineralise organic pollutants [26-29].

However, the presence of common inorganic anions (such as chlorides, sulfates, nitrates, bicarbonates) which are found in all waters to be treated, can influence the performance of this process. These anions can also be introduced to the water to be treated during the implementation of pollution control processes (eg., as coagulation-flocculation reagents, acids and bases for neutralization), or even be formed during the mineralization of halogenated organic compounds, sulfur or nitrogen. We have recently shown that the performances of Fenton’s reagent for oxidation of organic compounds can markedly decrease in the presence of these anions [30], but much more need to be learnt. Furthermore, complexation of Fe(III) with chloride or sulphate ions may lead to the generation of other radicals (Cl, Cl2•-, SO4•-, …) in the solution via direct photoreduction of Fe(III) complexes or by secondary reactions of OH with Cl- or SO42-.

Because of uncertainties and lack of data on quantum yields in photoreduction of Fe(III) complexes, our study will aim to estimate such values, and furthermore to investigate the influence of chloride and sulfate anions on the efficiency of photo-Fenton process on the oxidation of an organic compounds. In addition we want to identify and quantify production of reaction intermediates. As model compounds we will select pesticides or antibiotic compounds common in flloding water, cf. WP1.  It should also be noted that most of the results from these photochemical works will add to a better understanding of the chemistry of aquatic environments.

 

 

Electro-Fenton processes

Recently, a new method for regeneration of ferrous iron  in Fenton reagent’s has been proposed based on electro-reduction of ferric iron and oxygen at surfaces of working electrodes. The advantages of this method are its simplicity of implementation and in a controlled production of hydroxyl radicals without introducing an oxidant, or a large amount of catalyst. Hence, electrochemistry offers a practical solution to eliminate the limitation of the Fenton process [31]. The main advantage of this method is the electro-catalytic production of both chemical reagents: H2O2 and Fe2+. Hydrogen peroxide is generated at the cathode of the electrochemical cell from the O2 in the air at low potential (-0.26 V/NHE):

O2 + 2H+ + 2e- ® H2O2

Ferrous ions are produced simultaneously at the same potential and at the same electrode from a small catalytic amount of ferric iron initially introduced into the solution:

Fe3+ + e-  ® Fe2+

With generation of Fe2+ and H2O2 hydroxyl radicals are formed. The in situ electrochemical production of H2O2 reduces reagent consumption, and chemical electro-catalytic generation of ferrous ions can limit the dose of a homogeneous catalyst and thus reduce the formation of iron hydroxide sludge.

The electrochemical reaction that takes place at the anode of the electrochemical cell, is simply the oxidation of water O2

2H2O ® O2 + 4H+ + 4e-

In practical, the application of a low cathodic potential (E1= -0.26V) using a DC power supply (industrial application) or a potentiostat (laboratory) can initiate this chain of reactions. The cathodic potential is more than sufficient to reduce ferric ions (E2 = 0.77 V) continuously with reduction of O2. Our recent work has shown that the concentration of  H2O2 increases continuously during operation of the cell, and that the H2O2 concentration depends on oxygen concentration in the electrochemical cell. The optimal potential for production of H2O2 was determinate as -0.5V (vs.Ag/AgCl). [32]

The objective of this research task is to develop and optimize a process for remediation of flooding water from agriculture and aquaculture containing toxic and refractory organic pollutants, particularly pesticides and their degradation products. The realization of this research part will take place in four major steps:

-          Comparison of electro-Fenton oxidation of organic compounds compared with other AOP’s (2a and 2b).

-          Studies of the effect of various parameters (e.g. apply potential, current, electrolyte) on the process efficiency

-          Investigation on the mechanism and reaction kinetics of electro-Fenton processes

-          Optimization of electro-Fenton processes at laboratory scale

 

Disinfection

The disinfection is indispensable and last step in all water treatment system. The disinfection step to kill any pathogens in water as viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoa that form cysts in water (Giardia lamblia and Cryptosporidium, both of which are pathogenic). The use of chloramine is becoming more common as a disinfectant. Although chloramines is not as strong an oxidant, it does provide a longer-lasting residual than free chlorine and it won’t form THMs or haloacetic acids, a by-products of chlorination.

Ultra violet light is very effective at inactivating cysts, as long as the water has a low level of colour so the UV can pass through without being absorbed. Under irradiation, the microorganism will be destroy by scrambling its DNA structure, the cell are rendered sterile and cannot reproduce or dead and no longer a threat. With exposed to dosage of UV light about 15000 µW/cm2, almost common destroys viruses, bacteria, fungi, algae and protozoa will be destoy. Recently, the use of chloramines in combination with ultraviolet light to improve capacity of disinfection as well as longer-lasting residual of oxidant in water.

During or after flooding period, to get the drinking water, people use the traditional method as coagulation by alum and disinfected by di- or tri-chloramines. The water can be use after 30 minute of disinfection. Inorganic chloramines (NH2Cl, NHCl2, NCl3) absorb UV radiation in the range of wavelengths between 200 and 350nm. The exposure to UV irradiation or sunlight cause decomposition of chloramines or chlorine in water [33,34]. The decomposition of chloramines lead to the formation of radical Cl and NH2which can be react with trace organic compound to form toxically organochlorine or nitrite and nitrate ions, both of them are carcinogenic and affect the human health.

Some recently studies have shown the disinfection capacity of AOPs processes [35]. The combination of AOPs and chloramines may be a good method for efficient disinfection and reduce the using amount of chloramines. This task will focus on the microorganism destroy capacity and optimization of AOPs/Chlorine or Chloramines processes.

Updated: August 6, 2014 — 12:59 am
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