Control of Gaseous Emissions

Sulfides

The generation of offensive odors in wastewaters, domestic sewage and other installations of water treatment is mainly due to the action of reductive bacteria, which act anaerobically over sulfates present in the medium. In oil refineries and terminals, the waters of production and processes are contaminated due to the natural existence of sulfides in oil. 

Problems caused by sulfides: 

Odor 
The odor of rotten eggs of sulfides is well known and is perceptible even in concentrations below 0.3 ppm. In elevated concentrations, the sulfidric gas inhibits the olfactory system, eliminating a factor that would serve as alarm in a danger situation. 

Toxicity 

Sulfides constitute an environmental threat for being poisonous to aquatic life in general.

The sulfidric gas is equally toxic and, in concentrations over 1000 ppm in the air, causes death in a few minutes. Eye and respiratory tract irritation, headaches and fatigue sensation are symptoms of an exposition to concentrations over 5 ppm.

Due to its low solubility and elevated volatility, the danger represented by the sulfidric gas is of the same order of the cyanide gas.

The presence of sulfidric gas in the working environment in tanneries depends mainly of the processing step that the hides go through and air circulation inside the factory.

Corrosion
It also corrodes piping, pumps and installations, even concrete parts. 

Unfavorable action over biological treatment 
In elevated concentrations, sulfides are toxic to the biological treatment, reducing the efficiency of the process and inhibiting microbial activity. They also favor the growing of filamentous bacteria in the processes of treatment by activated sludge. To avoid disturbances of the active biomass, sulfide concentration must not be superior to 25 mg/l, and should be kept constant to avoid shocks that hamper the biological activity in the processes. 

Oxidation of sulfides by hydrogen peroxide: 
 

Our hydrogen peroxide products are easily handled and their application doesn’t involve large increases in cost. They provide partial oxidizing of sulfides to intermediate compounds that do not exhale bad smell, and that can be treated by aeration, efficiently and at low cost.

Sulfides are formed as follows:

H⁺ + HS^- ↔ H₂S (Neutral or acid medium)
HS^- ↔ H⁺ + S^2- (Basic medium)

In a basic medium, there are species of low corrosive power in equilibrium: S2- and HS-, with a low concentration of H2S (less than 1% of dissolved sulfur). However, in this pH range, nor domestic sewage nor industrial effluents could be dumped over rivers. They are dumped with neutral pH, which is when S2- and HS- ions are converted into the volatile, toxic and corrosive H2S.

The sulfide oxidation by hydrogen peroxide occurs in two ways:

In an acid or neutral solution prevails the following reaction:

H₂S + H₂O₂ → S(s) + 2 H₂O

Where the biggest part of sulfide is transformed into elemental sulfur. The rest is constituted of different soluble compounds of sulfur, and according to their structure might be oxidized later. 

The reaction is relatively slow in acid medium, but can be catalyzed through ions of transition metals. After addition of dissolved iron (such as iron (III) sulfate), the reaction completes in few minutes, even at room temperature.

In alkaline reactions, the oxidation occurs according to the following equation: 

S^2- + 4 H₂O₂ → SO₄^2- + 4 H₂O

In that case, the reaction is considered faster than in acid medium. At ambient temperature it concludes itself in few minutes, even without catalystaddition. To avoid byproducts, the proportion of hydrogen peroxide that must be used should be above the stoichiometric.

There are mainly four practical and economically viable ways to oxidize the sulfides with our products:

• Complete oxidation
• Auxiliary Oxidation
• Preventive Oxidation
• Polishing

Complete Oxidation 

The complete oxidation of sulfides to sulfates by hydrogen peroxide is used preferably by factories with a great wastewater with sulfides flow and small area for treatment, or by those who are still constructing their wastewater treatment.

It can also be used in case of accidental spill, momentary inoperability of the treatment station or emergency cases.

The speed of implanting the dosage system, the operational easiness and reaction effectiveness in a very short time are inherent characteristics of the hydrogen peroxide.

Auxiliary Oxidation 
To the companies that already oxidize its wastewaters by aeration or oxygen injection, the hydrogen peroxide is a valuable partner, since the use in large enough amounts for a partial oxidation of the wastewater eliminates the bad smell immediately and accelerates the operation of the subsequent aeration, guaranteeing an increase in efficiency of the process with energy economy.
 

Preventive Oxidation
When used as a preventive measure to the development of sulfides, hydrogen peroxide is primarily used as source of oxygen. 

H₂O₂ → H₂O + ½ O₂

The decomposition level to oxygen will depend of the presence of contaminants in the wastewater, particularly transition metals. The concentration of hydrogen peroxide to keep the aerobic conditions is lower than those necessary of molecular oxygen.
 

Polishing 
The polishing is an adequation of the wastewater to the conditions required by the environmental control organs. In case of the wastewater still having sulfide content above the allowed emission levels, it can be adequated by adding a small dosage of hydrogen peroxide. 

Disinfection 
In some countries, for the disposal of sewage in rivers and the sea, where is foreseen use for recreation of primary contact, it is mandatory by law to disinfect it to eliminate the thermo tolerant coliforms. For this application, the oxidizing action of the hydrogen peroxide works as a first step of disinfection, reducing the peracetic acid necessary for complete elimination of pathogenic microorganisms.

Gases containing SO2 (sulfur dioxide) e SO3 (sulfur trioxide) are generated in many pyrometallurgical and fuel burning operations (coal and oil), being noticeably generators of acid rain, thus the necessity of having their emissions controlled. 

Hydrogen Peroxide is most efficiently used for the treatment of these gases in scrubbers, converting the SO2 in usable sulfuric acid – with the important characteristic of not generating solid residues.

The process reaction is simple and quite fast – even at ambient temperature 

SO₂ (g) + H₂O₂ (aq) → H₂SO₄ (aq) 

The emissions of nitrous oxides are noticeably toxic, formers of acid rain and contributors to the greenhouse effect, thus the necessity of its control. The control strategies are: suppression at the source or treatment in scrubbing columns.

The suppression of NOx at the source can be achieved through addition of hydrogen peroxide to the baths of superficial league treatment that use HNO3 nitric acid, so that the gaseous NOx generated as treatment product can be reconverted to HNO3 within the same bath, saving nitric acid and eliminating the necessity of gas treatment.

The scrubbing constitutes in absorbing and oxidizing of the nitrous gases in an aqueous solution in which hydrogen peroxide is added, which quickly and efficiently oxidizes NO (nitrogen oxide) to NO2 (nitrogen dioxide), producing an effluent with HNO3 that can be reused or easily neutralized.

The relevant reactions are:
 

2 NO₂ (g) + H₂O₂ (aq) → 2 HNO₃ (aq) 

NO (g) + NO₂ (g) + 2 H₂O₂ (aq) → 2 HNO₃ (aq) + H₂O (aq) 

2 NO (g) + 3 H₂O₂ (aq) → 2 HNO₃ (aq) + 2 H₂O (aq)