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Environmentally Assisted Cracking in Metals
Published in T.L. Anderson, Fracture Mechanics, 2017
To illustrate the electrochemical processes that lead to occluded chemistry, consider a steel component with a surface crack exposed to aerated seawater. Assuming the seawater is neutral (pH 7), the overall reaction consists of the dissolution of iron (Equation 11.3) and the reduction of oxygen to hydroxide ions (Equation 11.8). Charge conservation is maintained in both steel and water. Each electron that is produced during the oxidation of iron is immediately consumed by the oxygen reduction reaction. Over time, oxygen in the crack is depleted, and the oxygen reduction reaction ceases. However, the oxidation of iron continues, which results in an excess of positive iron ions. To maintain the charge balance, chloride ions migrate into the crack. The iron and chloride ions react with water to form ferric hydroxide. Hydrogen ions react with chloride ions to form hydrochloric acid. A crack or crevice exposed to a neutral dilute sodium chloride solution may contain as much as 10 times the bulk concentration of chlorine and have a pH in the range of 2–3.
Acid mine drainage in karst terranes: Geochemical considerations and field observations
Published in Barry F. Beck, Felicity M. Pearson, Karst Geohazards, 2018
Ira D. Sasowsky, William B. White, John A. Webb
Reactions [2] and [3] take place along the flow path of the acid mine water. First the dissolved ferrous iron is oxidized to ferric iron which is highly insoluble at pH > 2. Ferric iron hydrolyses to precipitate as ferric hydroxide (ferrihydrite). The oxidation of Fe2+ consumes one mole of hydrogen ion for each mole of iron while the hydrolysis reaction releases three moles of hydrogen ion. The decomposition of pyrite is among the most acidic of all weathering reactions, because the oxidation of one mole of pyrite releases ultimately four moles of H+.
Role of Nanostructures in the Process of Water Treatment by Coagulation
Published in Victor M. Starov, Nanoscience, 2010
Flocs of ferric hydroxide are produced intensively at pH 5–7, with optimum lying within pH = 6.1–6.5. The isoelectrical point of ferric hydroxide corresponds to pH 6.5. So, we see that flocculation of ferric hydroxide, in contrast to aluminum hydroxide, runs within materially wider range of pH values. It was found by x-ray measurements that one and the same modification of ferric hydroxide, namely, goethite [α-FeO(OH)], arises at hydrolysis offerrous and ferric salts in hydrocarbonate-chloride and hydrocarbonate–sulfate media.
Olive mill wastewater pretreatment by combination of filtration on olive stone filters and coagulation–flocculation
Published in Environmental Technology, 2019
Ghizlane Enaime, Abdelaziz Baçaoui, Abdelrani Yaacoubi, Marc Wichern, Manfred Lübken
As mentioned above, the efficiency of coagulation–flocculation is significantly controlled by the variation of pH values. A highly acidic and alkaline pH reduce coagulation–flocculation process efficiency [43]. This can be explained by the fact that the pH plays a predominant role in the formation of coagulant species in solution. In an earlier study performed by Black and Chen [44], the authors demonstrated that destabilization of colloids with AS at pH 3 was due to electrical double layer depression by unhydrolyzed species. At pH 5, the destabilization was due to adsorption of positively charged hydrolysis products. Increasing the pH value between 6 and 8 favors the formation of amorphous solid-state , because the aluminum ions required sufficient alkalinity to form it. In this range of pH, the destabilization mechanism consists of a combined action of charge neutralization and precipitate enmeshment with species [45]. This range of pH was reported as optimal for an effective coagulation using AS [46]. For ferric ions, lower values of pH (<3) are marked with a predominance of the hydrated aquo metal ions. Therefore, the mechanism of destabilization is almost totally due to double layer repression by ionic strength considerations. At pH > 4.5, the ferric ions can be hydrolyzed and precipitated as ferric hydroxide, insoluble in a large range of pH, and so there is no particular upper pH limit for ferric coagulation [47]. However, as reported by Genovese et al. [48] FC functions effectively over a pH range from 4.5 to 5.5 where both TP and COD removals get their maximum level.
Removal of flocculated TiO2 nanoparticles by settling or dissolved air flotation
Published in Environmental Technology, 2021
H. A. Oliveira, A. Azevedo, J. Rubio
Ferric hydroxide (after hydrolysis of ferric salts) is commonly employed because it is a very effective and cheap coagulant. The peculiar structure of the colloidal precipitates (open and strong flocs) is responsible for the enmeshment and sweeping down effect of most of the dispersed particles while settling, a technique widely used in drinking water secondary treatment [16,54].