11 Atmospheric Oxidation of Acid Rain Precursors

Prof. K.S. Gupta

epgp books

 

 

Contents

  1. Introduction
  2. The Oxidation Pathways
  3. Homogeneous Gas Phase Oxidation SO2
  4. Gas Phasee Oxidation of NO2
  5. Aqueous Phase Autoxidation of SO2
  6. Aqueous Phase SO2 – Oxidation Pathways
  7. Uncatalysed Oxidation of Sulfur(IV) by O2
  8. Homogenous Metal Ion Catalysed Autoxidation
  9. Iron (III) – Catalysed Oxidation of Dissolved Sulfur Dioxide
  10. SO2-Control Strategies
  11. Control of NOX
  12. Acid Rain Modelling
  13. References

Introduction

   Main acid rain precursors responsible for acidification of rain water and other aqueous systems are sulfur dioxide, SO2, nitrogen oxides, NO andNO2, and organic acids – formic and acetic acid in particular. Other sulfur compouonds like H2S and CH3SCH3 also contribute but through oxidation to SO2 first. The atmospheric oxidation of these acid precursors occurs both in gas and aqueous phases. These precursor gases by direct dissolution in aqueous phase shall not to lead to any significant acidification because the acids generated, namely, H2SO3 and HNO2, are weak acids having low dissociation constants. So the concentration of hydrogen ions produced shall be quite low having only a small impact on pH.

 

   There are several chemical pathways, which result in the oxidation of these precursor gases into very strong acids, such as sulfuric acid, H2SO4, and nitric acid , HNO3. These acids due to very high dissociation constants are

   almost fully dissociated in aqueous solutions leading to production oh high [H+] and consequently low pH. In sum, the chemistry of acidification of rain water is basically the chemistry of oxidation of SO2, NOx and other acid rain precursors.

 

The Oxidation Pathways

   The oxidation of acid rain precursors occur in both gas and aqueous phases homogeneously as well as heterogeneously. In the gas phase, the oxidation of SO2 , NOx and other acid precursors is brought about by radicals, such as hydroxyl radical, OH, hydroperoxy radical, HO2, organic radicals, etc., and moecules like ozone, oxygen and hydrogen peroxide. As discussed in another Module, the radicals are formed by photchemical reactions and the further reactions of these radicals produce molecules such as ozone, hydrogen peroxide, etc. The most important oxidants in aqueous phase

 

(a) Uncatalyzed autoxidation of aqueous SO2

(b) Catalyzed autoxidation of aqueous SO2 – These catalyzed reactions are of two types:

                       (i) Homogeneous metal ion catalyzed

                       (ii) Heterogeneous particle surface catalyzed

 

Homogeneous Gas Phase Oxidation SO2

   All available evidences suggest that the most important atmospheric oxidant of SO2 in gas phase is OH radical and this accounts for most of sulfate production in troposphere. It may be pointed out thar the reactions of excited sulfur dioxide molecules, formed by the adsorption of

 

The rate constants for the oxidation reactions of SO2 with other atmospheric oxidants are included in Table 2.

Table 2. The second order rate constant for the gas phase oxidation of 25OC (Finlayson -Pitts and Pitts,1986 )

 

Calculation of Atmoshperic Gas Phase Conversion Rate

   In the reaction(12), M is third body, whose function is to remove excess energy. M can be any species present, which does not react with OH and SO2. Its concentration in atmosphere can be taken as constant. Assuming that the concentration of OH also remains constant, due to its regeneration by reaction(11) as reported by Stockwell and Calvert (1983), then the rate of reaction would be first order in SO2 only (Finlayson-Pitts and Pitts, 1986). Therefore, the rate Eq. (13) on integration will yield Eq.(14).

 

Gas Phasee Oxidation of NO2

   The most important fate of NO in atmosphere is oxidation to NO2 mostly by ozone and some radicals. Rate constants of some of these oxidation reactions are given in Table 3. The oxidation of NO by O2 is very slow. The subsequent gas phase oxidaiton of NO2 by OH (Eq. (22) is a major source of

Aqueous Phase Autoxidation of SO2

Speciation of Dissolved SO2

The reactivities of different dissolved SO2  species are different, therefore an understanding

 

   These all species are collectively called sulfur(IV) because the oxidation number of sulfur in each of these species is +4. The concentrations of species in Eq.(31) are governed by equilibria (28-30). Using equilibrium law for Eqs.(28-29), [S(IV)] is given by equation (31). According to equiation (31), the solubility of SO2 in water increases with increase in pH.

Fig. 2 pH- dependence of solubilityof SO2 in water.

 

   This reaction is catalyzed by hydrogen ions, and hence with increase in pH the rate decreases. Based on the nature of [H+] – dependence of rate, the reactive species appear to be HSO3- ion. This ion is preponderent in the pH range 4- 6. When the pH is increased beyond this range, concentration of unreactive sulfite ions, SO32- increases and so the rate of oxidation of sulfur(IV) by H2O2 decreases. However, in atmospheric conditions the rate decrease on increasing pH is compensated by increases in solubility of SO2 in terms of equation (31). The following

 

   where HA is H+ or some other acid. In modelling of nigttime chemistry, the oxidation of sulphur(IV) by H 2 O2 has been found to be a major reaction, in particular, when pH < 5.

    A comparison of the oxidation rates of sulfur dioxide by OH radicals and H2O2 is important . Jacob(1999) has discussed this issue as follows. Based on OH concentration and its rate constant for SO2 oxidation, a life time of 1-2 weeks for SO2 is calculated.This is almost twice its residence time. Experimental studies have pointed that atmospheric oxidation of SO2 largely takes place in cloudwater and in raindrops. In these aqueous media, sulfur(IV) largely exists as reactive HSO3- which is rapidly oxidized by H2O2. This explanation is true for those regions where rainwater/cloudwater pH is less than~ 6. In Indian continent, rainwater pH often exceeds 7 and so the sulfur(IV) exists almost exclusively as SO32- , an unreactive species with regard to reaction with H2O2.Thus, the role of H2O2 – sulfur(IV) oxidation in acidification of rainwater shall be limited when pH is high.

 

Oxidation of Aqueous SO2 by Ozone

   The transition metal ions, which can exist in two oxidation states, act as catalysts.This reaction is inhibited by free radical scavengers such as volatile organic compounds, ammonia and ammonium ions. This provides an strong evidence for the operation of a radical mechanism involving oxysulfur radicals, viz., sulfite ion radical, SO3-, sulfate ion radical, SO4-, and peroxomonosualfate ion radical,HSO5 / SO5-. The reaction is believed to be initiated the through the one –electron oxidation of S(IV) by trace metal ion impurities, such as iron(III), Mn(III), Cu(II), etc. A general current version of the mechanism is as follows(Dhayal et al., 2014).

Homogenous Metal Ion Catalysed Autoxidation

   The most important metals ions from atmospheric point of view are iron, manganese and copper. The anothropogenic activities and soil are the major contributor of metals. Metal oxides are the major constitutent of anthropogenic emissions and the fly ashes. In the aquated aersol particles, the leaching of soluble metal compounds is responsible for introducing metal ions in aqueous phase.

 

Iron (III) – Catalysed Oxidation of Dissolved Sulfur Dioxide

   From atmospheric view point, iron is the most important trace metal present in all aqueous media such as rainwater, cloudwater, fog and mist. Because of its atmospheric importance, iron(III) – catalyzed reaction has been the subject of a large number of studies. The speciation of iron(III) changes drastiObviously, the iron(III) catalysis is very complicated. Above pH 2.5, hydrolysis of Fe3+ is very significant due to formation of Fe(OH) 2 / Fe(OH)2 /dimeric/polymeric forms. In fact, above pH 4, rate becomes independent of total iron(III) concentration in atmospheric systems. At low pH (< 3) the rate of oxidation, Robs, is defined by Eq.(50).

Manganese (II)-Catalysts

   This is another metal-catalyzed reaction, which contributes significantly to aqueous oxidation of sulphur(IV) in atmosphere. The kinetics and mechansim of this reaction are also very complicated. In this reaction, there is evidence to implicate manganese(III) as a reactive intermediate, which is formed by the oxidation of manganese(II) by radicals. Manganese(III) initiates the reaction by reaction with sulfur(IV) to produce SO3- radical as in Eq. (42).

 

SO2-Control Strategies

   To control the acid rain, it is necessary to take steps to reduce the emission of acid rain precursors in atmosphere. Baird (1998) has described the techniques for the abatement of SO2 and NOx pollution. A brief description of these techniques as follows.

 

Flue-Gas Desulfurization

   In case of power plants, the emitted SO2 is dilute and hence oxidation processes remove it. The reaction with limestone (CaCO3) or lime(CaO) in the form of wet crushed solid, are as in equations (60) and (61).

 

Catalysis by Aerosol Particles

   The suspended particulate matter(SPM) is an inhomogeneous mixture of large number of substances such as silica(SiO2), mineral and rock powders(e. g., sandstone ,limestone), fly ash, soil particles, metal oxides, etc. Many of these substances catalyze the oxidation of SO2 due to their incorporation in atmospheric water through leaching of metal ions and/or particle surface..

Coal Cleaning

   Pre-combustion cleaning is done for the removal of sulfur present as inclusion, which is mostly inorganic in nature and largely FeS2.

 

   In the combustion cleaning technique, the modified combustion processes are used to check the formation of the pollutants and/or to capture the pollutants formed by introduction of the absorbing substances. In fluidized bed combustion, pulverized coal and limestone are mixed and suspended(fluidized) on jets of compressed air in the combustion chamber and SO2 is captured before it can escape. This procedure requires a much reduced combustion temperature. So the formation of NOx is also reduced significantly.

 

   The post combustion cleaning processes utilize the techniques such as dry/wet scrubbers. The former uses fine grains of CaO and the latter uses slurries of CaCO3/MgO/Na2SO3. In the SNOX process, cooled flue gases and ammonia are mixed with to remove the nitric oxide by its catalytic reduction to N2. By reheating the remaining gas, SO2 is catalytically oxidized to SO3, which is converted to H2SO4.

 

Coal conversion

    The coal is gasified by reaction with steam, and the gas mixture is made free of   pollutants. Electricity is generated by burning the pollutant free gas turbine.

 

Control of NOX

  1. The formation of NO can be decreased by lowering the combustion temperature.
  2. Catalytic converters are used in automobiles to control emission of NO, CO and unburnt hydrocarbons. In a catalytic converter, a surface impregnated with a rhodium catalyst reduces NO into N2 and O2, and then a platinum or palladium catalyst oxidizes CO / hydrocarbons into CO2.
  3. There are several other technologies based on reduction in peak temperature, chemical reduction of NO, use of a sorbent, etc.

Acid Rain Modelling

   The modelling of any atmospheric chemical and physical phenomenon is extremely complex. For example, for a reliable modelling of acid rain, Calvert et al., 1986 have listed the requirement of the following information with a spatial resolution of a few tens of kilometers.

  1. Inventory, geographical distribution and composition of both the natural and anothropogenic emissions of SO 2 , NOx , hydrocarbons, aldeydes, and acid neutralising compounds, e.g., NH3 and other emissions involved in the acid generating tropospheric chemical reactions.
  2. The complex transport processes,which distribute the emissions and their reaction products.
  3. The mechanisms  and  the  rates  of  relevant  tropospheric  depositions  of  the  acid precursors and initial products of the chemical reactions, which occur in the troposphere.
  4. The different chemical pathways responsible for acid generation in the gas and variety of aqueous phases, and solid aerosols or their aqueous suspensions.
  5. Description of physical processes, which occur in cloud and result in mixing and redistribution of the pollutants etc.

   Coming to acid rain in particular, the sequence of steps (Finlayson – Pitts and Pitts, 1986) leading to the conversion of gas phase SO2 into aqueous phase sulphate is as follows.

  1. Transport of the gas to the surface of the dropiet.
  2. Transfer of the gas across the air-liquid surface.
  3. Establishment of the acid-base equilibria of the dissollved species.
  4. Transport of the dissolved species from the to the bulk aqueous phase of droplet.
  5. Reaction in the droplet.

   A large number of modelling studies on acid raein have been carried out. The simpleast one is based on only rates and mechanisms of chemical reaction of acid genertaing reactions, Several equilibrium specieation models have been developed by using data collected on the concentrations of chemical constituents in fog, cloud and rain water.

you can view video on Atmospheric Oxidation of Acid Rain Precursors

References

  1. H. A, Bridgeman (1990), Global Air Pollution: Problems for 1990s, CBS Publishers, New Delhi.
  2. D. J Spedding, (1974), Air Pollution, Oxford University Press, London.
  3. D. J. Jacob (1999), Introduction to Atmospheric Chemistry, Princeton University Press, Princeton, NJ, USA.
  4. J. H. Seinfeld and S.N. Pandis, (1998), Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York.
  5. J. Finlalyson-Pitts and J.N. Pitts, Jr. (1986), Atmospheric Chemistry: Fundamentals and Experimental Techniques, John Wiley, New York
  6. R. P.Wayne (2000), Chemistry of Atmospheres , Oxford University Press, Oxford, UK.
  7. P. V. Hobbs, (2000), Introduction to Atmospheric Chemistry, Cambridge University Press, Cambridge.P. Brimblecombe, (1996), Air Composition and Chemistry, Cambridge University Press, Cambridge. UK.
  8. P. Brimblecombe, (1996), Air Composition and Chemistry, Cambridge University Press, Cambridge. UK.
  9. C. Baird, (1998), Environmental Chemistry, W.H. Freeman, New York.
  10. J. Feichter, E. Kjellstrom, H. rodhe, F. Dentener, J. Lelieved and G. Roelofs(1996), Simulation of the tropospheric sulfur cycle in a global climate model, Atmospheric Environment, 30, 1693-1707
  11. Dhayal, C. P. S. Chandel and K. S. Gupta, , Role of some organic inhibitors on the oxidation of dissolved sulfur dioxide by oxygen in rainwater medium, Environmental Science and Pollution Research, 2014, 21, 3474–3483
  12. S.V. Manoj, C. D. Mishra, M. Sharma, A. Rani, R. Jain, S. P. Bansal, S. P. and K. S. Gupta(2000), Iron, manganese and copper concentrations in wet precipitations and kinetics of the oxidation of SO2 in rain water at two urban sites, Jaipur and Kota, in western India. Atmospheric Environment. 34, 4479-4486.