An Introduction to Surface Chemistry

1. Volcanogenic products from the eruption: Volcanic gasses, pyroclasts , aerosol particles derived from reactions of volcanogenic and atmospheric materials.

2. Products from the ambient atmosphere, such as H₂0 and gaseous species and various particles from the land and sea including wind blown silicates, sea salt and others. The volcanogenic components make the clouds distinctive, and they can be tracked by satellite sensors for periods that range from minutes to weeks (Bluth et al, 1997; Schneider et al, 1995). During this time the volcanogenic particles mix and interact with meteorological and hydrospheric particles.

Volcanogenic particles in volcanic clouds consist of fine pyroclasts , salts and acids in aerosol form.

The particles consist of two main types: (Reference to: http://www.geo.mtu.edu/~raman/GE4170.volcanicclouds.html)

1. Silicate pyroclasts representing fragments of the magma. These are glassy pyroclasts and minerals, which represent the crystalline fraction of the magma. Their shape is angular, and basaltic and andesitic eruptions give rise to particles that have moderate aspect ratios (Riley et al, 1999), while rhyolitic eruptions can generate an abundance of glassy pyroclasts with a platy geometry and extreme aspect ratios (Rose and Chesner, 1987). The diameters of silicate pyroclasts generated during explosive eruptions range from meters to microns. Those in volcanic clouds are smaller, generally less than about 50 μm. The mass proportions of silicate particles with diameters less than about 1 μm are very small (Rose et al 1980).

2. Non-silicate particles which are related to reactions among the constituents of the volcanic gases. These particles are generally smaller than the silicates, usually less than 1 micron in diameter. The commonest composition for these is sulfate, especially H2SO4, which forms as submicron spherical droplets which also contain H20 (typically about 25% by volume--Zhao et al, 1995). A total of at least 28 different phases have been observed also including native sulfur, sulfates, haloids, metallic oxides, and such exotic species as silver sulfide and even native gold (Meeker et al, 1985). Overall the analogy between the observed phases and fumarolic incrustations and sublimates at gas vents (Stoiber and Rose, 1974; Bernard, 1985; Symonds et al, 1987) suggests that these phases originate from reactions among the volcanic gases, sometimes involving the atmosphere and volcanic silicates.

Besides these two broad types, a wide variety of other, unexplained materials have been observed in volcanic clouds. They consist largely of phases that are amorphous and have uncertain compositions (Chuan et al, 1987). Many or most of these particles are likely to be non-volcanic in origin, and represent accidental material of surficial or extraterrestrial origin.

Constituents of volcanic gas

(Reference: http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html)

1. Types of volcanic gases:
   • The most abundant gas released into the atmosphere from volcanic systems is water vapor (H20), carbon dioxide (CO₂) and sulfur dioxide (SO₂). Volcanoes also release smaller amounts of others gases, including hydrogen sulfide (H₂S), hydrogen (H₂), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He).

   • Examples of volcanic gas compositions, in volume percent concentrations
     (from Symonds et. al., 1994)
2. Potential effects of volcanic gases
   • The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano. Globally, large explosive eruptions that inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer. Because carbon dioxide gas is heavier than air, the gas may flow into in low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation. A few historic eruptions have released sufficient fluorine-compounds to deform or kill animals that grazed on vegetation coated with volcanic ash; fluorine compounds tend to become concentrated on fine-grained ash particles, which can be ingested by animals.

Gas adsorption onto the volcanic ash

1. Adsorption theories

   Adsorption is a combination of physical and chemical processes in which a substance accumulates on a solid surface (cf. Mihelcic et al., 1997). When gas molecules are present near a solid surface, a vast number of collisions occur each second. Molecules striking the surface will either rebound or adsorb to the surface. As molecules strike repeatedly upon the solid surface and linger for a time, a higher concentration of gas molecules develops at the surface than in the bulk gas phase. This phenomenon is called adsorption (Boer, 1953).
   
   

   From a thermodynamic viewpoint, molecules always prefer to be in a lower energy state (cf. Mihelcic et al., 1997). An adsorbed molecule has traded a higher energy state (that of a gas molecule) for a lower energy state (that of an adsorbed molecule). The difference in the two energy states is called the heat of adsorption (Q) and this is the quantity of energy liberated when the molecule is adsorbed (Boer, 1953). Attraction of a molecule to a surface can be caused by physical and chemical forces. Electrostatic forces are the basic physical forces in most adsorption reactions. These forces include dipole-dipole interactions, dispersion interactions or London-van der Waals force, and hydrogen bonding (Mihelcic et al., 1997). In contrast to physical adsorption, chemisorption involves the formation of strong chemical bonds between adsorbate molecules and specific surface locations known as chemically active site (Hoffmann, 1988).
   
   

   Physical adsorption is characterized by relatively small heats of adsorption-about 5 kcal per mole or less. All gases exhibit physical adsorption but some also exhibit chemisorption, which involves forces and energies about 10 to 100 kcal per mole. Although physical adsorption occurs in all cases, chemisorption is observed only when some kind of chemical interaction is possible (Boer, 1953).
   
   

   When a surface is initially exposed to a gas, the average number of molecules on the surface increases with time. After some time, the rate at which molecules are leaving the surface (desorption) will equal the rate at which molecules are being adsorbed and the average number of molecules per unit area of surface per interval of time will become constant. This condition is called dynamic equilibrium (Boer, 1953).

2. Adsorption models

   There are many types of adsorption models. The classical adsorption models for isothermal conditions are the Langmuir, Fruendlich, and Brunauer, Emmett and Teller (BET).
   
   

   1. Langmuir:
      

      (Reference: http://dom3.newcastle.edu.au/sample/5a6d7a7.htm)

      
      The earliest model of gas adsorption wassuggested by Langmuir (1916). The classical Langmuir model is limited to monolayer adsorption. It is assumed that gas molecules striking the surface have a given probability of adsorbing. Molecules already adsorbed similarly have a given probability of desorbing. At equilibrium, equal numbers of molecules desorb and adsorb at any time. The probabilities are related to the strength of the interaction between the adsorbent surface and the adsorbate gas (cf. Swinkels, 1999).
      
      The Langmuir model is usually expressed as:
      
      

      Where:

      V= volume of gas adsorbed at pressure P;

      V_m = volume of gas which could cover the entire adsorbing surface with a monomolecular layer;

      P₀ = saturation pressure of the gas, i.e., the pressure of the gas in an equilibrium with bulk liquid at the temperature of the measurement;

      x = P/P₀ = relative pressure (0£ x£ 1);

      C = a constant for the gas/solid combination.

      The Langmuir model is applicable when there is a strong specific interaction between the surface and the adsorbate so that a single adsorbed layer forms and no multi-layer adsorption occurs (cf. Swinkels, 1999).

   2. Fruendlich:
      
      The Fruendlich isotherm model is valid for heterogeneous surfaces, monolayer coverage (cf. Hand, 1997). The Fruendlich isotherm equation can be expressed as (Valsaraj et al., 1992):
      
      

      

      Where:

      a and n are experimentally determined parameters.

      When 1/n =1, the reaction is linear and called "partitioning". When 1/n<1, the reaction is said to be "favorable" as the incremental change in amount sorbed decreases with increasing concentrations. While 1/n>1 is called "unfavorable" because the reverse is true. Most natural adsorbents exhibit either linear or favorable adsorption. The Langmuir and the Fruendlich model for 1/n<1 are concave downwards, so both models can be calibrated to similar data..

   3. BET:
      

      (Reference: http://dom3.newcastle.edu.au/sample/5a6d7d2.htm"

      
      Brunauer, Emmett and Teller (BET) developed several models for gas adsorption on solids which have become the effective standard for surface area measurements. The models are valid for multiple layers on homogeneous surfaces (cf, Hand, 1997).
      
      The assumptions underlying the simplest BET isotherm are:
      • Gas adsorbs on a flat, uniform surface of the solid with a uniform heat of adsorption due to van der Waals forces between the gas and the solid.
        
        
      • There is no lateral interaction between the adsorbed molecules.
        
        
      • After the surface has become partially covered by adsorbed gas molecules, additional gas can adsorb either on the remaining free surface or on top of the already adsorbed layer. The adsorption of the second and subsequent layers occurs with a heat of adsorption equal to the heat of liquefaction of the gas (Swinkels, 1999).

      The single-solute BET isotherm is:

      

      Where: C is analogous to the C parameter for Langmuir. The BET simplifies to the Langmuir when relative pressure x < 0.01 and C>100 (Valsaraj et al., 1992).