Barrier Calcium carbonate

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Examples of reversible breakdown of structure have been reported for procaine penicillin dispersions (7), for model systems of calcium carbonate in polybutene ( ), and for numerous other systems. During shear the particles are forced into contact with each other with sufficient kinetic energy to overcome any natural barrier against their displacement of a lyosphere around each individual particle. A dispersion which is inherently stable can thus be forced by shear into a condition of instability. 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

On the opposite end of the spectrum, thermodynamics cannot explain why some PIC can sink through undersaturated waters without dissolving to accumulate on the seafloor. This is a widespread phenomenon as evidenced by the spatial %CaCOj gradients seen in the surface sediments (Figure 15.5). If the saturation horizon dictated the survival of sinking and accumulating PIC, a sharp depth cutoff should exist below which calcium carbonate is absent from the surface sediment. The importance of this kinetic barrier to dissolution is also seen in the relatively high fraction of surfece-water PIC (20 to 25%) that accumulates in the sediments as compared to the low fraction of surfece-water POC (1%). 

In practice, at the time, it was usual to destroy most or all of the alkalinity and reduce the pH to below 7.0 and often to as low as 6.0 because of the effectiveness of chromate inhibitor barrier films, it was not considered necessary to additionally practice a controlled calcium carbonate deposition program. 

If for some reason the primary system failed, a leak of acidic compounds would occur that should be treated to prevent formation of toxic clouds. The second barrier against toxic clouds proposed here is a chemical mitigation system, which uses a calcium carbonate dilution as a neutralisation agent. 

Plastic pipes are polymeric in nature (e.g., polyvinyl chloride). Within the pipe are traces of the monomers used in the manufacture of the pipe (e.g., vinyl chloride). In addition, there are a variety of other chemicals added during the manufacture of the pipe as lubricants to facilitate their manufacture or stabilizers to prevent the breakdown of the pipe. In Europe, lead has been used as the stabilizer for pipes, whereas various organic tin compounds have been utilized in the United States. Lead is widely recognized as being toxic. Inorganic tin has a very limited toxicity, but this is not the form of tin that is used. Some of the organic tin compounds are potent nervous system toxins (e.g., trimethyl or triethyl tin), while others appear to adversely affect the immune system (dioctyl tin). The forms of tin used in polyvinyl chloride pipe, however, are primarily monomethyl and dimethyl tin, which are much less active as neurotoxins than the trimethyl tin. There will be some extraction of all these chemicals from the pipe when it is first put into service. However, the concentrations that are found in the water decrease sharply with continued use of the pipe. This is only partially due to the depletion of the chemical from the pipe because continuous water flow will form an impermeable barrier (e.g., calcium carbonate) on the interior of the pipe that minimizes leaching from its surface. 

As the amount of CO2 stored increases, it becomes progressively more difficult to guarantee a physical barrier that prevents CO2 from returning to the atmosphere. Chemical conversion to a thermodynamically lower state would thus be desirable and is indeed possible. CO2 is the anhydrous form of carbonic acid and therefore can be used to displace weaker acids such as silicic acid. The formation of carbonates from silicates is well known as geological weathering. Thermodynamically, CO2 can be bound as a carbonate. In many instances, these carbonates dissolve in water, but some, such as magnesium and calcium carbonates, are remarkably stable as solids. Thus, mineral sequestration would provide a means of storing CO2. 

Cathodic precipitates increase cathodic site passivity with precipitation of insoluble compounds. Frequendy used cathodic precipitation inhibitors are CaCOs, MgCOs, or zinc sulfates that precipitate as Zn(OH)2. The efficiency of these inhibitors is only controlled by pH adjustment. Calcium carbonate (limestone) dissolves in water as calcium bicarbonate Ca(HC03)2- Carefid pH control forms smooth and hard calcium carbonate barrier films. Once the precipitate is formed, pH must be carefully controlled to avoid film dissolution at lower pH values ... 

Calcium carbonates (CaCOs), with a density of 2.7 g/cm, are commonly used filler materials for packing applications [57, 58]. CaCO filler improves flex modulus, impact strength, stiffness, tear strength, gas and water barrier properties, and printability. PE/carbonates are commonly used in the food packaging industry. 

Over time, the stony corals build up large networks of calcium carbonate upon which a reef is built. The size of such structures can be immense, as illustrated by the Great Barrier Reef. 

More recently nanoscale fillers such as clay platelets, silica, nano-calcium carbonate, titanium dioxide, and carbon nanotube nanoparticles have been used extensively to achieve reinforcement, improve barrier properties, flame retardancy and thermal stability, as well as synthesize electrically conductive composites. In contrast to micron-size fillers, the desired effects can be usually achieved through addihon of very small amounts (a few weight percent) of nanofillers [4]. For example, it has been reported that the addition of 5 wt% of nanoclays to a thermoplastic matrix provides the same degree of reinforcement as 20 wt% of talc [5]. The dispersion and/or exfoliahon of nanofillers have been identified as a critical factor in order to reach optimum performance. Techniques such as filler modification and matrix functionalization have been employed to facilitate the breakup of filler agglomerates and to improve their interactions with the polymeric matrix. 

The two parameters that control corrosivity of soft waters are the pH and the dissolved oxygen concentration. In hard waters, however, the natural deposition on the metal surface of a thin diffusion-barrier film composed largely of calcium carbonate (CaCOs) protects the underlying metal. This film retards diffusion of dissolved oxygen to cathodic areas, supplementing the natural corrosion barrier of Fe(OH)2 mentioned earlier (Section 7.2.3). In soft water, no such protective film of CaCOs can form. But hardness alone is not the only factor that determines whether a protective film is possible. Ability of CaCOs to precipitate on the metal surface also depends on total acidity or alkalinity, pH, and concentration of dissolved solids in the water. For given values of hardness, alkalinity, and total dissolved salt concentration, a value of pH, given the symbol pHs, exists at which the water is in equilibrium with solid CaCOs. When pH > pHs, the deposition of CaCOs is thermodynamically possible. 

Research and development is being targeted at calcium carbonates that produce tougher film and makes mouldings without reducing their tensile strength or resistance to tearing. There is also a desire to improve the barrier properties of calcium carbonate-filled LLDPE films. 

Applications include multilayer barrier films with PA6 (polyamide 6)/ethylene-vinyl alcohol (EVOH)/ PA6 for the highest barrier applications especially for medical products, co-extruded cast films based on PP or PE with fillers such as calcium carbonate monoaxially stretched to achieve breathability to provide a water barrier but enabling water vapour to permeate the film, used in domestic wrapping, roof underlay applications, hygiene products, for example, nappies and feminine care. 

Detergents in oils form micelles, which are aggregates of surfactant molecules. Polar molecules are made of a polar part (hydrophilic head) and a nonpolar hydrophobic part. According to the nature of the solvent (polarity), they can form micelles or reverse micelles. Hydrocarbon chains create a steric barrier, preventing the particles from aggregation. Overbased micelles have a mineral core made, for example, of calcium carbonate CaCOa (Figure 1.2). 

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