Encyclopedia of Renewable Energy. James G. Speight

      The acid-producing bacteria, involved in the second step, convert the intermediates of fermenting bacteria into acetic acid (CH₃COOH), hydrogen (H₂) and carbon dioxide (CO₂). These bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic acid, the bacteria need oxygen and carbon. For this, they use the oxygen solved in the solution or bounded-oxygen whereby the acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms. Moreover, these bacteria reduce the compounds with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide, hydrogen sulfide and traces of methane.

      From a chemical standpoint, this process is partially endergonic (i.e., only possible with energy input), since bacteria alone are not capable of sustaining that type of reaction. An endergonic reaction is a chemical reaction in which total amount of energy is a loss – it takes more energy to initiate the reaction than the energy produced by the reaction and, thus, the total energy is a negative net result.

      An acetogen is a microorganism that generates acetate (CH₃COO⁻) as an end product of anaerobic respiration (fermentation) and can produce, in most cases, acetate as the end product) from two molecules of carbon dioxide (CO₂) and four molecules of molecular hydrogen (H₂). This process (acetogenesis) is different from acetate fermentation, although both occur in the absence of molecular oxygen (O₂) and produce acetate.

      Acetogens are found in a variety of habitats, generally those that are anaerobic (lack oxygen). Thus, acetogenesis is a process through which acetate is produced from carbon dioxide and an electron source (such as hydrogen and carbon monoxide) by anaerobic bacteria. In this reaction, carbon dioxide is reduced to carbon monoxide and formic acid (HCO₂H) or directly into a formyl group, the formyl group is reduced to a methyl group and then combined with the carbon monoxide and Coenzyme A produce acetyl-CoA. Two specific enzymes participate on the carbon monoxide side of the pathway: (i) CO-dehydrogenase, which catalyzes the reduction of the carbon dioxide and (ii) acetyl CoA synthase, which combines the resulting carbon monoxide with a methyl group to give acetyl-CoA.

      The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide to carbon monoxide and the attachment of the carbon monoxide to a methyl group. The first process is catalyzed by specific enzymes (carbon monoxide dehydrogenase enzymes) and the coupling of the methyl group (provided by methylcobalamin) and the carbon monoxide is catalyzed by acetyl CoA synthetase.

      The accumulation of hydrogen can inhibit the metabolism of the acetogenic bacteria and present knowledge suggests that hydrogen may be a limiting feedstock for methanogens. This assumption is based on the fact that addition of hydrogen-producing bacteria to the natural biogas-producing consortium increases the daily biogas production. At the end of the degradation chain, two groups of methanogenic bacteria produce methane from acetate or hydrogen and carbon dioxide. These bacteria are strict anaerobes and require a lower redox potential for growth than most other anaerobic bacteria.

      See also: Acidogenesis, Anaerobic Digestion, Methanogenesis.

Acid Catalyst

      A catalyst alters the rate of a reaction without changing the reaction thermodynamic parameters through another route. Normally, the one in the energy of the transition state is lower. There are two types of catalysts which can affect the reaction pathway: (i) homogeneous catalysis and (ii) heterogeneous catalysts.

      Since the acidity of the catalyst support reflects the reaction pathway and plays a key role in the catalyst performance, it is possible to differentiate between homogeneous and heterogeneous catalyst. The homogeneous catalyst has lower ability of acidic sites rather than heterogeneous catalysts and it requires higher temperature in its reaction system, whereas the solid structure and the surface of a heterogeneous catalyst give its ability for higher strength and locations for acidic sites.

      Many mineral acid catalysts that are active in homogeneous catalysis can be made suitable for heterogeneous catalysis by supporting the catalyst on an inorganic oxide. Strongly acidic heterogeneous catalysts are prepared by supporting Brønsted acids such as trifluoro-sulfonic acid, sulfuric acid, phosphoric acid, and Lewis acids (such as such as boron trifluoride, BF₃, and antimony pentafluoride, SbF₅) on high surface area oxides such as silica, SiO₂, alumina, Al₂O₃, and zirconia, ZrO₂). In particular, supported phosphoric acid on silica is still widely used, and BF₃–γ-Al₂O₃ and H₂SO₄–ZrO₂ possess acidic sites that enable them to perform reactions that other solids are not strong enough acids to catalyze. Supported acids are difficult to characterize and are highly dependent on the methods and materials used in their preparation but do offer a suitable alternative for reactions that require strong and even super acidity.

      See also: Catalysts.

      Acid Deposition

      Acid deposition is the scientific term used to describe acid rain (which includes including acid fog, acid sleet, and acid snow).

      Acid deposition (acid rain) occurs when sulfur dioxide (SO₂) and, to a lesser extent, NO_x emissions are transformed in the atmosphere and return to the earth as dry deposition or in rain, fog, or snow. Acid rain is another environmental problem that affects many industrialized area of the world resulting in damage crops, forests, wildlife populations, and causing respiratory and other illnesses in humans.

      When atmospheric pollutants such as sulfur dioxide and nitrogen oxides mix with water vapors in the air, they are converted to sulfuric and nitric acids.

      Sulfur dioxide with water in produce sulfurous acid:

      In the gas phase, sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:

      In the presence of water sulfur trioxide (SO₃) is converted rapidly to sulfuric acid:

      Nitric acid is formed by the reaction of water with nitrogen dioxide and by the reaction of carbon dioxide with water:

Although carbonic acid is a weaker acid the nitric acid, sulfurous acid, and sulfuric acid, it does, however, contribute to acid rain which, in high concentrations, can cause damage to natural environments including forests and freshwater lakes.

      Acid deposition can be classified as wet deposition such as acid rain, snow, sleet and fog or dry deposition such as deposition as particulate matter even less than PM_2.5. Effects of acid rain can be either chronic or episodic.

      Chronic acidification is a long-term effect due to years of acid rain. Episodic acidification is due to heavy rain storms; it also occurs in spring as concentrated nitrate and sulphate in lower layer of snow pack get released when snow melts. A second method of acid deposition is known as dry deposition. Whilst wet deposition involves the precipitation of acids, dry deposition occurs when the acids are first transformed chemically into gases and salts, before falling under the influence of gravity back to Earth. Sulfur dioxide, for example, is deposited as a gas and as a salt.

      The gases present in acid deposition are found to occur naturally in the environment. They are given off from a number of sources including volcanic