Sulphur compounds

Table of Contents

7.1. Sulphur oxides (SOx)

7.1.1. Reaction sulphur dioxide in the atmosphere

7.2. Hydrogen sulphide (H₂S)

7.3. Carbonyl sulphide (COS)

7.4 Carbon disulfide (CS₂)

There are several sulphuric compounds in the atmosphere: sulphur dioxide (SO₂), sulphur trioxide (SO₃) – we can group these two chemical species as SO_x – hydrogen sulphide (H₂S), carbonyl sulphide (COS), carbon disulfide (CS₂), and sulphates (SO₄²⁻) (Figure 7.1–7.6).

Figure 7.1: Chemical structure of sulphur compounds – sulphur dioxide (Source: Wikipedia).

Figure 7.2: Chemical structure of sulphur compounds – sulphur trioxide (Source: Wikipedia).

Figure 7.3: Chemical structure of sulphur compounds – hydrogen sulphide (Source: Wikipedia).

Figure 7.4: Chemical structure of sulphur compounds – carbonyl sulphide (Source: Wikipedia).

Figure 7.5: Chemical structure of sulphur compounds – carbon disulfide (Source: Wikipedia).

Figure 7.6: Chemical structure of sulphur compounds – sulphate ion (Source: Wikipedia).

Sulphur is an essential element of living matter (amino acids and proteins). Sulphur exists in various forms in the Earth such as elemental sulphur, sulphides, sulphites, sulphates and other species. Sulphur mainly exists in the form of elemental sulphur, sulphides, and sulphates (Seinfeld and Pandis, 2006). Combustion of fossil fuels, emission from oceans due to activity of microorganisms, volcanic eruptions, and decomposition of living matters are the main sources of sulphur in the atmosphere. Sulphur dioxide and hydrogen sulphide are mostly emitted into the troposphere. H₂S undergoes oxidation producing SO₂ followed by oxidation of SO₂ to sulphuric acid (H₂SO₄ or SO₄²⁻). Sulphuric acid dissolves in water, thus the sulphur content is carried back to soil. Sulphate ion can be absorbed by plant and incorporated into organic compounds (amino acids). The sulphur bounded in amino acids is transferred from the producers (plants) to consumer (animals, humans). Excretion and the death of consumers carry sulphur back to the soil and water bodies, where it can be used by special bacteria. The –SH (thiol group or a sulphhydryl group) of amino acids (e.g., cysteine) gets separated as H₂S. Several groups of bacteria can use hydrogen sulphide as fuel, oxidizing it to elemental sulphur or to sulphate by using dissolved oxygen, metal oxides (e.g., Fe oxyhydroxides and Mn oxides) or nitrate as oxidant. Hydrogen sulphide can be oxidized in an aerobic condition to SO₄²⁻ by bacteria adapted to perform this change

,

(R7.1)

and the formed sulphate ion can be used again by autotrophs (Figure 7.7). In anaerobic condition some photosynthetic bacteria can use hydrogen sulphide to form carbohydrates and oxidize it to elemental sulphur or sulphate

,	(R7.2)
.	(R7.3)

Other bacteria can transform elemental sulphur to sulphate. This reaction is fast in the presence of molecular oxygen

.

(R7.4)

Figure 7.7: The sulphur cycle.

7.1. Sulphur oxides (SOx)

SO₂ is one of the most important contributors to air pollution. Sulphur in low concentration is essential for living organisms, but it becomes harmful when its concentration is increased. There are two sources: natural and anthropogenic. Natural sources (e.g., volcanoes) provide about two thirds of the sulphur oxides pollution on the Earth, while the smaller amount id from anthropogenic sources. Among anthropogenic sources, fossil fuel combustion accounts for ~ 75% of the total SO_x emission.

7.1.1. Reaction sulphur dioxide in the atmosphere

In the atmosphere, SO₂ does not remain in the gas phase for a long time. It reacts with water in the atmosphere in the presence of radiation to form sulphuric acid, which finally can deposit (wash out) with rain water causing acid rain. Sulphuric acid production from SO₂ oxidation proceeds via a series of radical reactions. First, sulphur dioxide reacts with an OH radical, which is normally produced from water vapour via reaction with electronically excited atomic oxygen, which in turn is formed from ozone photolysis

.

(R7.5)

The HSO₃ radical then rapidly reacts with molecular oxygen to yield either SO₃ and HO₂ or an intermediate complex HSO₅ (Kurten et al., 2009):

,	(R7.6)
,	(R7.7)

where M denotes a collision partner (typically molecular nitrogen or oxygen). The HSO₅ radical may also decompose to SO₃ and HO₂, or it may react with other compounds.

(R7.8)

The produced SO₃ reacts with water (catalyzed by another water molecule) to yield sulphuric acid:

.

(R7.9)

In the daytime, the photochemically generated radicals react as catalyst in these reactions. Sunny weather enhances the rate of conversion to acid, and this rate of conversion to acid decreases fast after sunset.
SO₂ undergoes several chemical reactions in the atmosphere forming particulate matter and aerosols, which are scavenged from the air. Sulphur dioxide and trioxide are transformed and washed out from the atmosphere in the form of sulphuric acid. Increasing concentration of sulphuric acid in the troposphere involves the occurrence of acid rain. A number of factors such as temperature, radiation intensity, and humidity can affect these reactions. Sulphur dioxide reacts through several ways in the atmosphere: (i) Photochemical reactions; (ii) Chemical reactions in the presence of NO_x and hydrocarbons; (iii) Chemical reactions in water phase and on solid aerosol particles. If fuel (e.g., coal) contains sulphur, its burning forms SO₂

.

(R7.10)

Sulphuric acid in the atmosphere reacts with ammonia (NH₃) and metal salts (e.g., NaCl) producing sulphates

,	(R7.11)
.	(R7.12)

In relatively humid conditions sulphur dioxide may be oxidized by reactions taking place inside the water phase in aerosol particles. These reactions proceed faster in the presence of ammonia and catalyst such as Fe(II), Ni(II), Mn(II), and Cu(II)

,	(R7.13)
.	(R7.14)

Oxidation rate of sulphur dioxide is more intense in a photochemically active air containing sulphur dioxide, VOCs, and NO_x, and this process results in more pronounce formation of aerosol particles. Sulphur dioxide, aerosols, soot particles, ammonium sulphate and water are the major components of the London smog. This smog has reductive chemical nature. SO₂ is a strongly irritating gas, which has effects on both living and man-made environment. For humans this gas is very dangerous because causes irritation (~ 5 ppm) and increases airway resistance in the respiratory tract. Sulphur dioxide damages crops and plant growth due to killing leaf tissues causing necrosis.